Nanoparticle Assemblies and Superstructures 1st Edition Nicholas A. Kotov
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- 4. DK3136_title 6/6/05 11:37 AM Page 1 Nanoparticle Assemblies and Superstructures Edited by Nicholas A. Kotov Boca Raton London New York Singapore A CRC title, part of the Taylor & Francis imprint, a member of the Taylor & Francis Group, the academic division of T&F Informa plc. © 2006 by Taylor & Francis Group, LLC
- 5. Published in 2006 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2006 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-10: 0-8247-2524-7 (Hardcover) International Standard Book Number-13: 978-0-8247-2524-2 (Hardcover) Library of Congress Card Number 2005041260 This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC) 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Nanoparticle assemblies and superstructures / edited by Nicholas A. Kotov. p. cm. Includes bibliographical references and index. ISBN 0-8247-2524-7 1. Nanoscience. 2. Nanostructures. 3. Nanostructures materials--Electric properties. 4. Nanotechnology. 5. Nanoparticles. I. Kotov, Nicholas A., 1965- QC176.8.N35N353 2005 620'.5--dc22 2005041260 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Taylor & Francis Group is the Academic Division of T&F Informa plc. DK3136_Discl.fm Page 1 Wednesday, June 15, 2005 12:31 PM © 2006 by Taylor & Francis Group, LLC
- 6. Dedication Dedicated to my mother and father — one chemist and one physicist — who represent the cosmos in the miniature. DK3136_book.fm Page 5 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 7. Preface Surveying the current nanoscience literature, one can see a wide variety of shapes and morphologies of nanoscale particles that can be produced now. In addition to semiconductor and metal rods, wires, and core-shell particles that have already been extensively studied, the latest synthetic protocols demonstrate the possibility of making rings, cubes, tetrapods, triangular prisms, and many other exotic shapes. They include nanoacorns, nanocentipedes, nanoshells, nanowhiskers, and many other examples. Now the question is, What are we going to do with this nanocollection? The variety of available nanoscale objects can be considered building blocks of larger and more complex systems. Therefore, the present challenge of nanoscale science is to shift from making certain building blocks to organizing them in one-, two-, and three-dimensional structures. Such assemblies and superstructures are the next logical step in the development of nanoscience and nanotechnology. In this respect, one needs to pose the following questions: 1. What are the methods of organization of nanocolloids in more complex structures? 2. What kind of structures do we need? 3. What are the new properties appearing in the nanocolloid superstructures? This book is the first attempt to answer these questions. It starts with two reviews assessing the current status of nanoparticle assemblies and the requirements for different applications of organized nanomaterials. The chapters in the second part of the book address changes in various properties of individual particles when they form agglomerates and simple assemblies. After that, different methods of organi- zation of particles in the complex nanostructured superstructures are described. They include techniques involving biological ligands, force fields such as the magnetic field, layering protocols, and methods based on self-organization. The field of nano- scale assemblies and superstructures is developing very rapidly, and this list cannot be absolutely complete. There is no other way to make this book happen than to draw the line at some point and present a snapshot of the work in progress. I strongly believe that the answers to the questions mentioned above hold many interesting discoveries and surprising phenomena. This book is just the initial step toward these new scientific and technological advances that can bring a profound change to many areas of our lives. In conclusion, I thank all the contributors of this volume. It is their hard work and excitement that give an edge to this book. DK3136_book.fm Page 7 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 8. Editor Nicholas A. Kotov was born in Moscow, Russia, in 1965. He graduated with honors from the Chemistry Department of Moscow State University in 1987. He subse- quently received his Ph.D. degree in 1990 (advisor, Prof. M. Kuzmin) for research on photoinduced ion transfer processes at the liquid–liquid interface. In 1992, he joined the group of Prof. J. Fendler in the Chemistry Department of Syracuse University as a postdoctoral associate, where he started working on the synthesis of nanoparticles and layer-by-layer assembly of nanostructured materials. Nicholas Kotov moved to Oklahoma State University to take a position as assistant professor in 1996 and was promoted to associate professor in 2001. Currently, Nicholas Kotov is an associate professor in the Department of Chem- ical Engineering at the University of Michigan in Ann Arbor, sharing this appoint- ment with the Departments of Materials Science and Biomedical Engineering. His research interests in the field of nanostructured material include synthesis of new nanocolloids, their organization in functional assemblies, layer-by-layer assembled nanocomposites, computer modeling of self-organization processes, ultrastrong materials from organized nanocolloids, nanowire-based nanodevices, biosensing applications of nanomaterials, interface of nanomaterials with living cells, and cancer treatment and diagnostics with nanoparticles. He has received several state, national, and international awards for his research on nanomaterials, among which are the Mendeleev stipend, Humboldt fellowship, and CAREER award. DK3136_book.fm Page 9 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 9. Contributors L. Amirav Department of Chemistry and Solid State Institute Technion University Haifa, Israel M. Bashouti Department of Chemistry and Solid State Institute Technion University Haifa, Israel S. Berger Materials Engineering Technion University Haifa, Israel Shaowei Chen Department of Chemistry and Biochemistry Southern Illinois University Carbondale, Illinois Jinwoo Cheon Department of Chemistry Yonsei University Seoul, Korea George Chumanov Department of Chemistry Clemson University Clemson, South Carolina Jeffery L. Coffer Department of Chemistry Texas Christian University Fort Worth, Texas Helmut Cölfen Department of Colloid Chemistry Max Planck Institute of Colloids and Interfaces MPI Research Campus Golm Potsdam, Germany Herwig Döllefeld Institute of Physical Chemistry University of Hamburg Hamburg, Germany Karen J. Edler Department of Chemistry University of Bath Bath, United Kingdom M. Eisen Department of Chemistry Technion University Haifa, Israel Alexander Eychmüller Institute of Physical Chemistry University of Hamburg Hamburg, Germany Latha A. Gearheart Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina Sara Ghannoum Department of Chemistry American University of Beirut Beirut, Lebanon DK3136_book.fm Page 11 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 10. Michael Giersig caesar research center (center of advanced european studies and research) Bonn, Germany Christian D. Grant Department of Chemistry Rutgers, the State University of New Jersey Piscataway, New Jersey Lara Halaoui Department of Chemistry American University of Beirut Beirut, Lebanon Li Han Department of Chemistry State University of New York at Binghamton Binghamton, New York J.W. Harrell Center for Materials for Information Technology University of Alabama Tuscaloosa, Alabama Gregory V. Hartland Department of Chemistry and Biochemistry University of Notre Dame Notre Dame, Indiana Michael Hilgendorff caesar research center (center of advanced european studies and research) Bonn, Germany Min Hu Department of Chemistry and Biochemistry University of Notre Dame Notre Dame, Indiana Jad Jaber Department of Chemistry American University of Beirut Beirut, Lebanon Nikhil R. Jana Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina Christopher J. Johnson School of Chemistry University of Bristol Bristol, United Kingdom Young-wook Jun Department of Chemistry Yonsei University Seoul, Korea Shishou Kang Center for Materials for Information Technology University of Alabama Tuscaloosa, Alabama Nancy Kariuki Department of Chemistry State University of New York at Binghamton Binghamton, New York Keisaku Kimura Department of Material Science Graduate School of Science Himeji Institute of Technology Hyogo, Japan Nicholas Kotov Department of Chemical Engineering University of Michigan Ann Arbor, Michigan DK3136_book.fm Page 12 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 11. Nina I. Kovtyukhova Department of Chemistry Pennsylvania State University University Park, Pennsylvania and Institute of Surface Chemistry N.A.S.U. Kiev, Ukraine M. Krueger Department of Physics and Solid State Institute Technion University Haifa, Israel Seung Jin Ko Department of Chemistry Yonsei University Seoul, Korea R.D. Levine The Fritz Haber Research Center for Molecular Dynamics Hebrew University of Jerusalem Jerusalem, Israel and Department of Chemistry and Biochemistry University of California–Los Angeles Los Angeles, California E. Lifshitz Department of Chemistry and Solid State Institute Technion University Haifa, Israel Luis M. Liz-Marzán Departamento de Química Física Universidade de Vigo Vigo, Spain Jin Luo Department of Chemistry State University of New York at Binghamton Binghamton, New York Thomas E. Mallouk Department of Chemistry Pennsylvania State University University Park, Pennsylvania Serhiy Z. Malynych Department of Chemistry Clemson University Clemson, South Carolina Stephen Mann School of Chemistry University of Bristol Bristol, United Kingdom Mariezabel Markarian Department of Chemistry American University of Beirut Beirut, Lebanon Mathew M. Maye Department of Chemistry State University of New York at Binghamton Binghamton, New York Alf Mews Institut für Physikalische Chemie Universität Mainz Mainz, Germany Paul Mulvaney School of Chemistry The University of Melbourne Melbourne, Australia Catherine J. Murphy Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina Christof M. Niemeyer Universität Dortmund, Fachbereich Chemie Biologisch-Chemische Mikrostrukturtechnik Dortmund, Germany DK3136_book.fm Page 13 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 12. David E. Nikles Center for Materials for Information Technology University of Alabama Tuscaloosa, Alabama Thaddeus J. Norman, Jr. Lawrence Livermore National Laboratory Livermore, California Sherine O. Obare Department of Chemistry and Biochemistry University of South Carolina Columbia, South Carolina Isabel Pastoriza-Santos Departamento de Química Física Universidade de Vigo Vigo, Spain F. Remacle The Fritz Haber Research Center for Molecular Dynamics Hebrew University of Jerusalem Jerusalem, Israel and Département de Chimie Université de Liège Liège, Belgium Andrey L. Rogach Institute of Physical Chemistry University of Hamburg Hamburg, Germany Sarah K. St. Angelo Department of Chemistry Pennsylvania State University University Park, Pennsylvania A. Sashchiuk Department of Chemistry and Solid State Institute Technion University Haifa, Israel Seiichi Sato Department of Material Science Graduate School of Science Himeji Institute of Technology Hyogo, Japan Elena V. Shevchenko Institute of Physical Chemistry University of Hamburg Hamburg, Germany U. Sivan Department of Physics and Solid State Institute Technion University Haifa, Israel Xiangcheng Sun Center for Materials for Information Technology University of Alabama Tuscaloosa, Alabama Dmitri V. Talapin Institute of Physical Chemistry University of Hamburg Hamburg, Germany Zhiyong Tang Department of Chemical Engineering University of Michigan Ann Arbor, Michigan Horst Weller Institute of Physical Chemistry University of Hamburg Hamburg, Germany Yan Xin National High-Magnetic Field Laboratory Florida State University Tallahassee, Florida DK3136_book.fm Page 14 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 13. Hiroshi Yao Department of Material Science Graduate School of Science Himeji Institute of Technology Hyogo, Japan Shu-Hong Yu Hefei National Laboratory for Physical Sciences at Microscale Department of Materials Science and Engineering and Structural Research Laboratory of CAS University of Science and Technology of China Hefei, People’s Republic of China Jin Z. Zhang Department of Chemistry University of California Santa Cruz, California Chuan-Jian Zhong Department of Chemistry State University of New York at Binghamton Binghamton, New York DK3136_book.fm Page 15 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 14. Contents Nanoscale Superstructures: Current Status Chapter 1 Organization of Nanoparticles and Nanowires in Electronic Devices: Challenges, Methods, and Perspectives................................3 Nicholas Kotov and Zhiyong Tang Chapter 2 Colloidal Inorganic Nanocrystal Building Blocks ............................75 Young-wook Jun, Seung Jin Ko, and Jinwoo Cheon Electronic Properties of Nanoparticle Materials: From Isolated Particles to Assemblies Chapter 3 Fluorescence Microscopy and Spectroscopy of Individual Semiconductor Nanocrystals............................................................103 Alf Mews Chapter 4 Coherent Excitation of Vibrational Modes of Gold Nanorods .......125 Gregory V. Hartland, Min Hu, and Paul Mulvaney Chapter 5 Fabricating Nanophase Erbium-Doped Silicon into Dots, Wires, and Extended Architectures..................................................139 Jeffery L. Coffer Chapter 6 Conductance Spectroscopy of Low-Lying Electronic States of Arrays of Metallic Quantum Dots: A Computational Study......153 F. Remacle and R.D. Levine Chapter 7 Spectroscopy on Semiconductor Nanoparticle Assemblies ............179 Herwig Döllefeld and Alexander Eychmüller Chapter 8 Optical and Dynamic Properties of Gold Metal Nanomaterials: From Isolated Nanoparticles to Assemblies....................................193 Thaddeus J. Norman, Jr., Christian D. Grant, and Jin Z. Zhang DK3136_book.fm Page 17 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 15. Chapter 9 Synthesis and Characterization of PbSe Nanocrystal Assemblies......207 M. Bashouti, A. Sashchiuk, L. Amirav, S. Berger, M. Eisen, M. Krueger, U. Sivan, and E. Lifshitz Biological Methods of Nanoparticle Organization Chapter 10 Biomolecular Functionalization and Organization of Nanoparticles ...............................................................................227 Christof M. Niemeyer Chapter 11 Nature-Inspired Templated Nanoparticle Superstructures...............269 Shu-Hong Yu and Helmut Cölfen Assembly of Magnetic Particles Chapter 12 Magnetic Nanocrystals and Their Superstructures..........................341 Elena V. Shevchenko, Dmitri V. Talapin, Andrey L. Rogach, and Horst Weller Chapter 13 Synthesis, Self-Assembly, and Phase Transformation of FePt Magnetic Nanoparticles ...................................................................369 Shishou Kang, Xiangcheng Sun, J.W. Harrell, and David E. Nikles Chapter 14 Assemblies of Magnetic Particles: Properties and Applications ....385 Michael Hilgendorff and Michael Giersig Layered Nanoparticle Assemblies Chapter 15 Template Synthesis and Assembly of Metal Nanowires for Electronic Applications..............................................................413 Sarah K. St. Angelo and Thomas E. Mallouk Chapter 16 Heteronanostructures of CdS and Pt Nanoparticles in Polyelectrolytes: Factors Governing the Self-Assembly and Light-Induced Charge Transfer and Transport Processes ........437 Sara Ghannoum, Jad Jaber, Mariezabel Markarian, Yan Xin, and Lara I. Halaoui DK3136_book.fm Page 18 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 16. Chapter 17 Layer-by-Layer Assembly Approach to Templated Synthesis of Functional Nanostructures...........................................................463 Nina I. Kovtyukhova Chapter 18 Coherent Plasmon Coupling and Cooperative Interactions in the Two-Dimensional Array of Silver Nanoparticles..................487 George Chumanov and Serhiy Z. Malynych Self-Assembly of Nanoscale Colloids Chapter 19 Self-Organization of Metallic Nanorods into Liquid Crystalline Arrays ............................................................................515 Catherine J. Murphy, Nikhil R. Jana, Latha A. Gearheart, Sherine O. Obare, Stephen Mann, Christopher J. Johnson, and Karen J. Edler Chapter 20 Tailoring the Morphology and Assembly of Silver Nanoparticles Formed in DMF........................................................525 Isabel Pastoriza-Santos and Luis M. Liz-Marzán Chapter 21 Interparticle Structural and Spatial Properties of Molecularly Mediated Assembly of Nanoparticles..............................................551 Chuan-Jian Zhong, Li Han, Nancy Kariuki, Mathew M. Maye, and Jin Luo Chapter 22 Langmuir–Blodgett Thin Films of Gold Nanoparticle Molecules: Fabrication of Cross-Linked Networks and Interfacial Dynamics........................................................................577 Shaowei Chen Chapter 23 Self-Assembling of Gold Nanoparticles at an Air–Water Interface............................................................................................601 Hiroshi Yao, Seiichi Sato, and Keisaku Kimura DK3136_book.fm Page 19 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 17. Nanoscale Superstructures: Current Status DK3136_book.fm Page 1 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 18. 3 1 Organization of Nanoparticles and Nanowires in Electronic Devices: Challenges, Methods, and Perspectives Nicholas Kotov and Zhiyong Tang CONTENTS 1.1 Introduction ......................................................................................................4 1.2 Prototype Devices Based on Single NPs and NWs ........................................5 1.2.1 Nanodevices Based on NPs .................................................................5 1.2.2 Nanodevices Based on Semiconductor NWs ......................................7 1.3 Prototype Devices from Thin Films of Nanocolloids...................................11 1.3.1 Devices Based on Thin Films of NPs ...............................................11 1.3.1.1 Single-Electron Charging ...................................................12 1.3.1.2 Photoelectronic Devices from NP Thin Films...................12 1.3.1.3 Electroluminescence Devices .............................................13 1.3.1.4 Photovoltaic Devices ..........................................................15 1.3.1.5 Electrochromic Devices......................................................18 1.3.1.6 Sensors and Biosensors ......................................................21 1.3.2 NWs as Molecular Electronic Devices..............................................23 1.3.2.1 Nanocircuits ........................................................................24 1.3.2.2 Electroluminescence Devices .............................................27 1.3.2.3 Sensors................................................................................28 1.4 Strategies of NP and NW Assembly into More Complex Structures...........30 1.4.1 Assembly of NPs ...............................................................................31 1.4.1.1 One-Dimensional Assemblies of NPs ................................31 1.4.1.2 Two-Dimensional Assemblies of NPs................................37 1.4.1.3 Three-Dimensional Assemblies of NPs .............................41 DK3136_book.fm Page 3 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 19. 4 Nanoparticle Assemblies and Superstructures 1.4.2 Assemblies of Single NWs................................................................47 1.4.2.1 Physical Method of Aligning NWs....................................48 1.4.2.2 Chemical Method of NW Alignment.................................50 1.4.2.3 Biological Method of NW Alignment................................52 1.4.3 Superstructures of NWs and NPs......................................................52 1.5 Prospects.........................................................................................................53 References................................................................................................................54 1.1 INTRODUCTION Physical dimensions of electronic devices are approaching the limit of manufacturing technologies. The elements in new integrated circuitry reached the critical size of 60 nm, which is beyond the limitation of conventional photolithography. Although some methods of lithography, such as electron beam, x-ray, and scanning probe microscopy lithography, are capable of overcoming this limitation,1 high cost and slow processing impede their practical application for mass production. One of the possible alternative ways to circumvent the bottleneck of low dimen- sions is self-organization of 60-nm and smaller structures from nanoscale building blocks. Although it is difficult to imagine an intricate electronic circuit forming from small pieces of semiconductor and metal spontaneously, the self-assembled super- structures can serve as vital elements in electronic circuits, which are interfaced to other devices by traditional lithographic means.2–4 This approach assumes, first, the availability of nanoscale elements with dimensions from 1 to 50 nm, such as nano- particles (NPs),5 nanowires (NWs), and nanotubes,6 and molecular compounds,7 and, second, the availability of methods of their organization and self-organization into more complex structures. These methods of structural design are the primary subject of this review. The problem of organization at the nanoscale level actually emerged long before the appearance of nanoscience as a separate discipline. Research in this area started about 30 years ago, when pioneering theoretical calculation by Aviram and Ratner showed the possibility of using single molecules as electronic elements.8 This study opened the field of molecular electronics, which tackles very similar problems because molecules whose lengths often exceed 1 nm need to be properly positioned with respect to each other and electrodes to function as electronic devices. Molecular electronics and nanoscience also share the same problems when it comes to prospects of device manufacturing. The inevitable intrinsic defects and disorder of molecular assemblies will be a significant obstacle for reproducible molecular and nanoscale electronics.9,10 Regardless of the many challenges lying on the way to nanocomput- ers, understanding the processes of self-organization of nanoscale elements will be exceptionally beneficial both for practical applications and for fundamental nanos- cale science. This article surveys current research on NPs and NWs as device elements and materials components for photonics and electronics. Since the synthesis and appli- cation of carbon nanotubes (CNTs) have been extensively reviewed before,11–14 they are excluded from the scope of this paper. Some examples of CNTs are nonetheless mentioned here, because several experimental techniques used for organization of DK3136_book.fm Page 4 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 20. Organization of Nanoparticles and Nanowires in Electronic Devices 5 CNTs are also applicable to NWs. This material is organized into three parts: (1) basic electronic properties of single NPs and NWs as functional elements of high- tech devices; (2) demonstrated applications of NP and NW assemblies in electronics, photonics, and sensors; and (3) methods of structural design of NP- and NW-based materials. We hope that such composition gives the reader a fairly complete view of the subject matter. Even though some of the important papers highly relevant to the problem of organization at the nanoscale level and superstructures from nano- colloids will come out after this book’s publication, the fundamental approach taken here will still be useful for anyone interested in critical assessments of prospects of NPs and NWs for photonics and electronics. 1.2 PROTOTYPE DEVICES BASED ON SINGLE NPs AND NWs 1.2.1 NANODEVICES BASED ON NPS Many prototype devices with NPs are related to the single-electron tunneling (SET) effect. Theoretically predicted to occur in quantized systems,15,16 SET was first experimentally observed for microscopic systems. Fulton and Dolan observed that the charging effects in bulk metal–insulator–nanoparticle–insulator–bulk metal (MINIM) junctions could be modulated at 1 K by the gate voltage.17 The discreteness of charge cannot be observed in conventional electronics at room temperature, because large thermal energy KT (~26 meV) overcompensates the energy of single- electron tunneling. From the equation W = Q2/2C, where Q is the total charge stored on the central particle and C is the junction capacity equation, one can see that if C is small (<10 to 18 F) and the resistance is high, the work energy is large enough to resist the thermal drift at room temperature. Small capacity and large resistance of junction require the dimensions of the device to be within a few nanometers. Colloidal synthesis of metal or semiconductor NPs made a strong impact on nanoscale electronics.5 SET-based analogs of field effect transistors (FETs) can be smaller and faster than conventional devices but require atomic-scale precision in NP placement between the electrodes. Scanning tunneling microscopy (STM) and corresponding scanning tunneling spectroscopy (STS) are convenient experimental tools to investigate SET and to position the particles at the right distance from the source and drain electrodes. The gaps on each side of NP (Figure 1.1A) act as double-insulator tunnel junctions with low capacity and high resistance, as required for SET to occur at room temperatures. When bias is applied between STM tips and substrates, the current discretely increases with a step of e/2C. Andres et al.18 and, later, Taleb et al.19 observed predicted SET phenomena at room temperature in STS spectra of Au or Ag NPs adsorbed to thiol-modified Au substrates. The crystal structure of NPs influences the SET behavior as a result of its effect on the ladder of their electronic levels. For example, additional peaks in i-V curves were observed for crystalline, compared to amorphous, Pd NPs.20,21 They were attributed to a certain band gap state associated with small-size metal NPs. The dependence of SET on the band gap is more prominent for semiconductor InAs NPs (Figure 1.1B).22,23 On the side of positive bias, which corresponded to the conduction DK3136_book.fm Page 5 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 21. 6 Nanoparticle Assemblies and Superstructures band (CB), two groups of peaks were identified. Two closely spaced peaks appeared after current onset, followed by a larger gap and a group of six peaks with equal spacing between them of about 110 mV, which arose from charging energies of SET. The first group of peaks had 1Se characteristics, and thus doublets were observed. The six peaks in the second group stemmed from the degeneracy of the 1Pe state. It was also noticed that the energy separation was smaller for negative bias corre- sponding to the valence band (VB). This is consistent spin–orbit coupling and split- off24 in the VB electronic levels. As expected, SET decreased with the increase of NP sizes, and the values of energy gaps between ground and excitation states from STS measurements were virtually identical to those from optical spectroscopy. SET effect in single NPs was used in single-electron transistors.25–27 Klein et al. designed a pair of source and drain electrodes with a space distance of only about 10 nm (Figure 1.2). Since nanoparticles had a much stronger Van der Waals (VDW) attraction at the place of steps or grooves than flat substrates,27 the NPs intended to shift on the electrodes and were trapped in the nanogap between source and drain electrodes. By applying gate voltage, the number of electron transfer events can be accurately controlled. FIGURE 1.1 STM (A) and STS (B) of a single InAs nanocrystal 32 Å in radius, acquired at 4.2 K. (From Banin, U. et al., Nature, 400, 542–544, 1999. With permission.) −2 −1 1 2 −2 10 8 6 4 2 0 0 1.0 0.5 −0.5 −1.0 0.0 Tip QD DT Au ∆VB + Ec ∆CB + Ec Eg + Ec Ec Tunnelling Current (nA) dl/dv (a.u.) A B Bias (V) DK3136_book.fm Page 6 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 22. Organization of Nanoparticles and Nanowires in Electronic Devices 7 Amlani and Orlov28,29 produced an NP logic gate consisting of four quantum dots connected in a ring by tunnel junctions, and two single-dot electrometers. In such quantum dot cellular automata, the digital data are encoded in the positions of only two electrons. The device is operated by applying inputs to the gate of the cell. The logic AND and OR operations are successfully realized as the electrometer outputs. 1.2.2 NANODEVICES BASED ON SEMICONDUCTOR NWS Compared to CNTs, semiconductor NWs represent a more recent development in the field of nanoscale electronics and have a substantially greater range of chemical properties and, thus, more practical applications and methods of organization. The chemical methods of NW synthesis were recently reviewed by Wu et al.,6 Xia et al.,30 and Rao et al.31 Since the preparations strongly affect the approaches to subsequent functionalization, they will be briefly mentioned here. The NW prepa- rations involve two main types: wet colloidal and gas phase synthesis. The first group includes template-directed,32 solution–liquid–solid,33–35 supercritical fluid–liq- uid–solid,36 thermal decomposition of metallorganic precursors,37 and self-assembly from single NPs.38 Gas phase methods include vapor–liquid–solid,39 oxide-assisted,40 and vapor–solid.41 Wet methods are generally more amenable to subsequent modi- fication reactions and therefore assembly coding. At the present moment, however, the complexity of the superstructures obtained from gas phase–produced wires is somewhat higher. Cui et al.42 achieved an important milestone for this area, i.e., successful doping of silicon NWs by adding reaction precursors of boron (B) or phosphorus (P) to the vapor source. Analogous to the conventional bulk semiconductor doping, B-doped Si NWs exhibit characteristic p-type behavior, and the P-doped Si NWs show n- type behavior. As an alternative route, the NWs with different electrical properties can also be produced through choosing appropriate semiconductor materials.43,44 For instance, group III-V InP and group II-VI CdS NWs are good n-type semiconductors, and IV group Si NWs exhibit p-type characteristics. FIGURE 1.2 Scheme of single-electron transistor of CdSe NP. (From Klein, D.L. et al., Nature, 389, 699–701, 1997. With permission.) V Vg Nanocrystals Au Leads Linker Molecules SiO2 Insulating Layer Doped Si Substrate I DK3136_book.fm Page 7 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 23. 8 Nanoparticle Assemblies and Superstructures In order to fabricate NW FETs, NWs are placed on silicon substrates with a surface oxide layer, and then both conductive source and drain electrodes are sput- tered on the ends of NWs (Figure 1.3). As with a conventional FET device (Figure 1.3A), only when positive potential is applied to the gate electrode is the current allowed to pass through the n-type NWs between source and drain electrodes. Although the structures and work principles are identical, NW FETs exhibit several advantages compared to traditional FETs. Besides the small size and high integration density, the transconductance of NW FETs can be as high as 3000 nAV–1, which is 10 times higher than that of conventional metal oxide FETs.45 This feature should proportionally increase the speed of the nanoscale computers. At the same time, one needs to mention that the precision of assembly of NW FET should be in the angstrom scale. Otherwise, fault-tolerant computational schemes must be used, which can operate with less reproducible behavior of elementary transistors.46 This eventually will reduce the computational speed. Other types of electrical characteristics of NWs can be achieved by their surface modification. Note that there are many examples of CNT superstructures with various nanoscale entities.47–50 Reports on surface modification of semiconductor and metal NWs are significantly less abundant. Lauhon et al.51 prepared a variety of coaxial NWs with p–n junction. The i-Ge/p-Si core-shell, i-Si/SiOx/p-Si core-doubleshell, or p-Si/i-Ge/SiOx/p-Ge core-multishell structures were produced by sequential chemical vapor deposition (i and p denote intrinsic and p-type conductivities, respec- tively). These structures can be convenient for the further minimization of NW FET FIGURE 1.3 Scheme of conventional (A) and NW (B) FETs. Metal p-Type Substrate Drain Electric Field View from above Gate gate drain source Standard Symbol Source Channel A Source Gate Oxide Drain n-type nanowires B DK3136_book.fm Page 8 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 24. Organization of Nanoparticles and Nanowires in Electronic Devices 9 by integrating all drain, source, and gate electrodes onto the single NW. Electrical measurements showed that the transconductance of coaxial structures was as high as 1500 nAV–1 for source–drain bias as low as 1 V. This property can be optimized depending on the device specification taking advantage of variable types of NWs.52 Coaxial NW structures of a different type — with an insulating layer — can be equally useful and possibly even more common than those with n–p junctions. The influence of external conditions and surrounding molecules on NW properties is beneficial for sensing53 but must be tightly regulated when NWs are employed for other purposes. For instance, field effects and temperature-dependent surface adsorp- tion/desorption equilibrium are likely to produce strong cross talk and operational errors. Their influence in nanocircuits is expected to be much stronger than in conventional ones. Insulation of NWs with a layer of inert material may prevent or reduce the strong noise in nanocircuits. Such a layer can protect the surface from adsorption of unwanted species, prevent charge injection, and partially screen the external fields when necessary. For sensor applications, a thin insulation layer can also be used to improve the environmental stability of the devices.54–56 One of the materials most suitable for NW protection is silica. It has a high-voltage breakdown potential and dielectric constant,57 considerable mechanical strength, and exceptional resilience to environmental factors. Moreover, silicon oxide processes are common for the modern electronic industry and can be transferred to the new generation of devices. Ag, SiC, Si, and other NWs have been coated with a shell of silica to meet different requirements.58–60 All of these NWs except Ag are processed in nonaqueous solvents or solid-state materials via vapor phase or high-temperature routes. Very recently, subsequent deposition of silica and metal in the porous alumina templates was explored to prepare insulating Au nanowires.61 Besides silica, some polymers can also be considered for making the insulating layer. As such, CdSe/poly(vinyl acetate)62 and Au/polystyrene63 core-shell one-dimensional structures with small aspect ratios were produced. The actual insulating capabilities of the organic or inorganic shell strongly depend on its quality and remain unknown for both types of coatings. Liang et al.64 used the sol-gel method to make coaxial SiO2-on-CdTe NWs with controlled thickness of the silica layer in the range of 10 to 66 nm. Importantly, the silica layer on the ends of the NWs was consistently thinner than on its sides. This afforded the preparation of first examples of open-ended coaxial NWs with electrical accessibility of the semiconductor core (Figure 1.4A). Conducting atomic force microscopy measurements (Figure 1.4B) demonstrated that the SiO2 coating was indeed insulating, with bias voltage reaching as high as 12 V for any thickness when the coating remained uniform (>10 nm). Taking advantage of the same technique, the current through the naked NWs was found to be mediated by mid-band-gap states. Interestingly enough, CdTe NWs stabilized with mercaptosuccinic acid pro- duce coaxial SiO2 composites with rather unusual morphologym, resembling nano- scale centipedes (Figure 1.5).65 The thickness of the silica layer in the ends still remains thinner than on the NW sides. Besides the distinctive three-dimensional configuration, such geometry provides exceptionally high surface area and a struc- tural foundation for highly branched nanocolloid superstructures. In addition, it will guarantee superb matrix connectivity in the nanowire–polymer composites for opti- cal materials with stringent requirements to mechanical properties.66 DK3136_book.fm Page 9 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 25. 10 Nanoparticle Assemblies and Superstructures Speaking of strength, a few words need to be said comparing devices made from NWs and CNTs because both of them share the same niche in the domain of applications. CNTs were used as field effect transistors, complementary inverters, and logic gates.67 The advantages of nanotubes for nanoscale electronics are in exceptionally high mobility of charges, high sustainable currents, light weights, and excellent stiffness and other mechanical properties. Unfortunately, both multiwall and single-wall CNTs have a number of problems not related to the challenges of their organization. First of all, despite great success in designing methods of selective growth and separation of different CNTs,68–72 the starting material for related devices remains a fairly complex mixture. Additionally, the modification of CNTs, which should allow for their self-assembly, also remains an unsolved issue.73,74 Their FIGURE 1.4 (A) TEM images of silica-coated NWs. Scale in the insert is 150 nm. (B) Conductive AFM i-V curves for As-doped silicon wafer (1), naked CdTe NW (2), and silica- coated NW samples (3). The bias in conductive AFM measurements is applied to the substrate while the probe is grounded. (From Liang, X. et al., Langmuir, 20, 1016–1020, 2004. With permission.) 200 nm A 3 −3 −2 −1 0 Sample Bias (V) B 3 2 1 1 2 Current (nA) −12 −8 −4 0 4 8 1. Si 2. CdTe NW/Si 3. CdTe NW/SiO2/Si 12 DK3136_book.fm Page 10 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 26. Organization of Nanoparticles and Nanowires in Electronic Devices 11 exceptional properties gradually disappear with progressively higher degrees of graphene sheet functionalization. In addition, environmentally stable doping of CNTs is equally difficult.75 Unlike CNTs,76 the inorganic semiconductor and metal NWs provide greater flexibility in chemical and electronic properties. Simulta- neously, they give rise to greater dependence of NW properties on surface passivation methods. The role of surface states in the electronic properties of NWs needs to be investigated in much greater detail. As such, the dependence of transconductance in semiconductor NWs on the surface composition and thermal activation of charge carried in them can play a significant role in their function as electronic devices. 1.3 PROTOTYPE DEVICES FROM THIN FILMS OF NANOCOLLOIDS Taking a step up in respect to system complexity from electronic circuits based on a single NP or NW, one needs to consider devices based on thin films of nanocolloids. In addition to the control of NP/NW electrode positioning or contact, one also needs to tune the NP–NP, NP–NW, or NW–NW gaps. Despite the increase of complexity, this task might actually be easier to accomplish than it is for single-element devices because of the large number of particles involved, and these distances become averaged in nature. Consequently, lower precision in particle positioning is required, which makes it more suitable for the current experimental tools and also more practical. 1.3.1 DEVICES BASED ON THIN FILMS OF NPS The best progress in this field has been achieved for NP thin films largely because they have a longer history than coatings from NWs. Thus, the overview of this subject will be given first. FIGURE 1.5 TEM images of mercaptosuccinic acid–stabilized CdTe/SiO2 core-shell struc- tures obtained with different amounts of sol-gel agents. (From Wang, Y. et al., Nano Lett., 4, 225–231, 2004. With permission.) 200 nm A 10 nm 150 nm B 100 nm C DK3136_book.fm Page 11 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 27. 12 Nanoparticle Assemblies and Superstructures 1.3.1.1 Single-Electron Charging Similar to single NPs placed between electrodes (Figure 1.2), discrete electron transport phenomena occur in NP assemblies as long as they are separated by a tunneling gap. However, the particle and gap size distributions blur the SET peaks and make them less pronounced than those of single NPs. This problem can be alleviated by the exploration of monodispersed gold clusters protected by uniform monolayers from, say, thiols, which narrows the spread of both parameters essential for SET. Under these conditions, SET can be observed for the solution of noble metal NPs by traditional electrochemical techniques.77–80 The discrete single-electron charging peaks, also called the ensemble Coulomb staircase, were observed in the double-layer capacitances zone of differentialpulse voltammograms (DPVs) (Figure 1.6). One can observe up to 13 SET peaks in the Au NP solution by this method.81 As expected, the amplitude and distance between individual SET peaks decreased with the increase of sizes of NPs from 1.1 to 1.9 nm (8- to 38-kDa core mass).82 The practical application requests that the NP films register in the form of thin film, which has secondary importance as long as NPs are in a closely packed or otherwise controlled spatial arrangement.83,84 The SET peaks of adsorbed Au NPs were also found to be very sensitive to the type of ions in the solution,85 which indicated that the SET phenomena could be used in the thin-film ion sensors. This effect can be further enhanced by molecular design of the coating around the NPs or the matrix in which they are impeded. Selective expansion of NP gaps in response to the presence of specific analytes can be predicted.86 1.3.1.2 Photoelectronic Devices from NP Thin Films Electrical and optical properties of NPs can be combined for practical purposes in three different modalities: FIGURE 1.6 Differential pulse voltammogram for hexanethiolate Au NPs in 0.1 M Bu4NPF6 /CH2Cl2 at Pt electrode. (From Hicks, J.F. et al., J. Am. Chem. Soc., 124, 13322– 13328, 2002. With permission.) 1500 1000 1 µA 500 Potential (mV) vs. Ag Wire Pseudoreference 0 −500 DK3136_book.fm Page 12 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 28. Organization of Nanoparticles and Nanowires in Electronic Devices 13 1. When potential is applied to semiconductor NPs, the electrons can be excited to the conduction band, and subsequent radiative recombination between hole and electron releases the stored energy as a photon, giving rise to electroluminescence. 2. Conversely, the irradiation with UV-visible light produces electron–hole pairs, which may generate photocurrent via the effective routes of charge separation. 3. Electrochromism of NP materials results from the change of optical absorption upon applying electrical potential. For instance, the electrons excited to the 1Se excitation state in CB can further absorb the energy in the infrared (IR) part of the spectrum and go to a higher 1Pe state.87–89 Let us consider this combination of electrical and optical processes in NP materials and corresponding devices in the framework of NP organization. 1.3.1.3 Electroluminescence Devices The goal of structural optimization in electroluminescent devices is to maximize radiative recombination of electrons and holes injected from both sides of an NP film sandwiched between the cathode and anode. Colvin et al.90 and Schlamp et al.91 were the first to report on the NP light-emitting diode (LED) made by building up binary layers of poly(p-phenylenevinylene) (PPV) polymer and CdSe or CdSe/CdS core-shell NPs onto the surface of indium tin oxide (ITO) electrodes (Figure 1.7A). The conductive polymer acts as a hole transport layer resting on an ITO hole injection electrode. The top coatings of Ag and Mg facilitated the injection of negative charge in the electron transport layer made from NPs. The radiative recombination on the PPV and NP interface gave rise to the strong luminescence near the band edge of CdSe NPs (Figure 1.7C). For the forward bias (positive bias was applied at the ITO electrode; electrons and holes are injected into the n- and p-conductive layers, respectively), the external quantum efficiency was as high as 0.1%. The reverse bias gave only little light, and the spectrum of the emission showed the additional peak from PPV emission (Figure 1.7D). The NP layer is probably not the best electron transport layer because of the layer of stabilizer coating the semiconductor cores. Additionally, injection of charge in the NPs prevents efficient emission from them, stimulating other processes such as Auger recombination manifesting, for instance, as emission blinking.92,93 Rational spatial organization of the layers of the device can significantly improve its perfor- mance. A recent study by Coe et al.94 showed that when the CdSe/CdS NP monolayer was placed exactly at the interface of the electron–hole transport layer made from different polymers, the light-emitting efficiency increased 25 times, and brightness and external quantum efficiency were up to 1.6 cd A–1 and 0.6%, respectively. Currently, NP and organic LEDs have approximately similar performance char- acteristics with some lead of the latter in longevity and commercial implementation. NP devices, however, have intrinsically higher hue purity and can easily be adjusted to emit in different parts of the spectrum from ultraviolet to visible to infrared.95–100 DK3136_book.fm Page 13 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 29. 14 Nanoparticle Assemblies and Superstructures Incorporation of other nanocolloids such as NWs or nanorods, which can improve electron (or hole) transport in the films, can be expected to boost their performance.101 Thus, optimization of the layer sequence shall remain one of the primary sources of the performance improvements; for example, Achermann et al.102 reported the considerable enhancement of emission color of CdSe NPs placed proximal to the expitaxial quantum well via energy transfer pumping. As with polymer LEDs, one of the future directions of further development of NP devices with electroluminescence (or with a similar process called electrogen- erated chemiluminescence103–105) will be thin-film lasers. Observation of optical gain for semiconductor NPs, despite the strong Auger recombination process in them,106 raises hopes that an appropriate thin-film structure acting as an optical resonator can FIGURE 1.7 (A) Layer sequence in a typical light-emitting diode. (B) Dependence of exter- nal quantum efficiency vs. current density of the device on the left. Positive current values correspond to ITO positively biased with respect to Mg. Inset: Spectra of light emitted in forward (C) and reverse (D) bias. (From Schlamp, M.C. et al., J. Appl. Phys., 82, 5837–5842, 1997. With permission.) A Ag Mg PPV ITO Glass CdSe or CdSe/CdS 10−4 10−3 10−2 QEext 10−1 50 25 0 Current Density (mA/cm2 ) B D 480 580 Wavelength (nm) 680 480 580 Wavelength (nm) 680 C −25 −50 EL EL DK3136_book.fm Page 14 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 30. Organization of Nanoparticles and Nanowires in Electronic Devices 15 lead to electrically excited NP lasers. Unconventional optical schemes, including photonic crystals,107 can become quite beneficial for this purpose. 1.3.1.4 Photovoltaic Devices The fairly broad UV-visible spectrum of NPs and the high oscillation strength of VB-to-CB transitions make them exceptional candidates for photovoltaic devices. Among various types of semiconductors, thin films from TiO2 colloids and similar metal oxide NPs have probably been the most explored for this purpose, because of high photocorrosion stability.108–110 They are typically used in conjunction with hole scavengers and oxidation-resistant dyes to prevent fast photobleaching. Metal and metal chalcogenides are often used with semiconducting polymers, which cannot withstand the attack of free radicals generated on the TiO2 surface upon illumination. NPs added to the matrix of polymers cause dissociation of excitons or, in other words, physical separation of the positive and negative charges.108 In such systems, the hole conductivity is primarily associated with a polymer component and electrical transport is typically taking place in the part containing NPs. This is true for the majority of systems, with the exception of probably CdTe and some other semicon- ductors with quite low redox potential.111–114 Material-specific charge transport rep- resents the fundamental similarity between light-emitting and photovoltaic systems. As well, the thickness of the films should be kept relatively small for both applica- tions to avoid losses during the transport to or from the electrode. The major difference between a solar cell and LED is that the former should maximize elec- tron–hole separation and optical density rather than light emission. The major chal- lenges of photovoltaic devices include improving efficiency of the charge separation and charge collection, which can be answered by selecting an appropriate nanocol- loid and by the intelligent design of the film structure. The numerous works on Gratzel’s solar cells demonstrated that a molecular (i.e., Angstrom scale) arrange- ment of the photosensitizer and the charge transport layer is critical for high-output photovoltaic elements.115 In this part, we shall focus on the effect of nanoscale organization of thin films. Initial charge separation at the polymer–NP interface is typically very fast (ca. 800 fsec).116,117 However, the back electron transfer, i.e., electron–hole recombina- tion, is also quite fast and presents the major limitation for efficient charge separa- tion.118 This problem can be overcome by combining two different NPs in the photoactive centers.119–123 Nasr et al.120 investigated several coupled semiconductor systems, such as SnO2/TiO2 and SnO2/CdSe, and found that fast withdrawal of the electron or hole in a separate but tightly connected NP significantly improved the element performance, resulting in the open-circuit voltage of 0.5 to 0.6 V and a power conversion efficiency of ca. 2.25%. Greenham et al.124 and Ginger and Greenham125 demonstrated that the mixture of CdSe or CdS NPs with the conjugated polymer poly(2-methoxy,5-(2′-ethyl)-hexyloxy-p-phenylenevinylene) (MEH-PPV) can effectively separate charges from the hole and electron, and the external quantum efficiency of such a device was 12%. The combination of semiconductor NPs with metal NPs in the film was also investigated for the enhancement of light absorption due to energy/charge transfer DK3136_book.fm Page 15 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 31. 16 Nanoparticle Assemblies and Superstructures and surface plasmon effect on the surface of Au or Ag. Strong electrical fields generated in the vicinity of illuminated noble metal NPs can further increase the transition dipole moment in the adjacent semiconductor NP. In turn, thinner, more light-absorbing films can be produced from such a composite, which will eventually increase the light/charge collection efficiency.126–128 Of equal importance, since the energy position of the Fermi level of the noble metal is lower than that of the conduction band of the semiconductor, the electron flow toward metal NPs can effectively realize charge separation after photoexcitation of semiconductor NPs. Although this approach demonstrated some improvement in photovoltaic perfor- mance, it also left a lot of room for improving the distribution and ordering of both particles within the film.129,130 One of the greatest problems for all of the thin-film NP devices is the facilitation of charge transport through the film and across the electrode–polymer interface.131,132 If there is a direct lattice contact between the crystallites, for instance, in ZnO or TiO2 electrodes, the electron/hole hopping from NP to NP can be rather fast.131 Studies by Noack et al.133,134 revealed that the photocurrent in this case was a result of a fast initial charge transport, a slow transport via deep trap states, and a transport via conduction band states or shallow traps. When an NP solid is assembled from an NP bearing a coat of organic stabilizer, the necessity to tunnel through this gap makes the charge transport in nanostructured systems very slow, with resistivities more than 1014 ohms/cm.135 Also, there is a strong Coulomb correlation between electron occupancy in an NP solid. The films become more resistive as more charge is injected, and the charge in them can be stored for a long time (several hundred seconds). A possible explanation for these facts is the buildup of the charge in traps near the source electrode, which reduced the field emission when a certain level was reached.136 Morgan et al.135 argued that the bottleneck of the charge transport was not the injecton of charges but rather the extraction of charges from the space charge region, which formed near the contact due to the slow hopping rates. The length of the space charge region is estimated to be less than 100 nm. Hikmet et al.137 showed that the current in CdSe/ZnS composites could also be explained by space charge– limited current in the presence of defects and rationalized the role of traps. At sufficiently high voltages the traps can be filled and a trap-free space charge-limited current observed. The characteristic trap depth was estimated to be about 0.15 eV. As can be seen, there is significant progress in the understanding of charge transport in NP films and development of new techniques for charge transport assessment such as imaging.138 It also becomes clear that essential problems of photovoltaic devices can be addressed by proper organization of the nanocompos- ites.132 As applied to photovoltaics, significant achievements in this direction were attained by using rod-like nanocolloids, which (1) can channel the charge to the electrode and (2) eliminate energy-demanding interparticle hopping.139 Huynh et al.140 utilized CdSe nanorods in a mixture with poly(3-hexylthiophene) (P3HT), which was spin-casted onto an ITO glass electrode coated with poly(ethylene diox- ythiophene) doped with polystyrene sulfonic acid (Figure 1.8). When CdSe NPs (aspect ratio = 1) were used, the maximum external quantum efficiency was only about 20%. It increased to 55% when CdSe nanorods with an aspect ratio of 8.6 were embedded in the conductive matrix. Nanorods eliminated a large number of DK3136_book.fm Page 16 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 32. Organization of Nanoparticles and Nanowires in Electronic Devices 17 interparticle hops, which were necessary for charge transport in the film, and there- fore reduced the electron–hole recombination.141 This concept was confirmed by replacing CdSe nanorods with branched CdSe nanocrystals with a shape of tetrapods. They further enhanced the photocurrent because the tetrapods can always be pointed in the direction perpendicular to the electrode plane, which shortens the transport path.141,142 The data for nanorods can also be compared with data obtained by Arici et al.143 for CuInS2 NPs in a very similar polymer matrix. Other studies also indicate the potential of structural optimization for the improvement of photovoltaic performance.144,145 Combination of two NPs of different materials not only may reduce charge carrier recombination but also can eliminate the charge traps.146,147 Arango et al.148 emphasized the necessity of a layered approach to the preparation of photovoltaic films rather than simple blending. In this respect, layer-by-layer (LBL) assembly, a new technique of thin-film deposition applicable to most aqueous nanocolloids, presents unique opportunities. It was initially utilized for NP photovoltaic elements from CdS and TiO2 by Kotov et al.119 and later was used for other systems.149–158 This technique for photovoltaic applications is reviewed in greater detail elsewhere. For instance, LBL affords preparation of graded semi- conductor films from NP and other components,159–162 which are known to enhance the charge transport within the film due to the intrinsic gradient of the electron and hole potential.163 Sheeney-Haj-Ichia et al.129 used a similar principle to enhance charge transfer in hybrid NP–4,4′ bipyridinium photovoltaic element and demonstrated that thin-film architecture can be a powerful tool for controlling the direction of charge FIGURE 1.8 (A) The chemical structure of P3HT. (B) Charge separation scheme in P3HT– CdSe interface. (C) Schematics of the CdSe–P3HT photovoltaic device with a hole transport layer made from poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEPOT-PSS). (From Huynh, W.U. et al., Science, 295, 2425–2427, 2002. With permission.) A S S S S Al CdSe/P3HT Blend PEDOT:PSS Substrate ITO External Circuit C B P3HT CdSe n/4 h+ e− Regioregular P3HT DK3136_book.fm Page 17 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 33. 18 Nanoparticle Assemblies and Superstructures flow. It was also established that NWs in LBL films can self-assemble and have surprisingly high levels of organization.164 Inclusion of intricately intertwined carbon nanotubes in these films66 can be an important milestone toward improving collection efficiency of photoelements. However, the common use of insulating polyelectrolytes as LBL partners inevitably decreases the photovoltaic parameters (quantum effi- ciency did not exceed 7.2% for TiO2 films, so far),149 necessitating more active utilization of water-soluble conductive components for this purpose.165 1.3.1.5 Electrochromic Devices Thin films that change color under bias probably showed the most promising results from a practical point of view.166 The industrial prototypes of rearview mirrors and display signs have already been produced with characteristics exceeding sometimes more common liquid-crystal displays (Figure 1.9). Most commonly, NP electrochro- mic devices are made from metal oxide systems such as WO3, NiO, MoO3, IrO2, CeO2, Fe2O3, or Co2O3. The color change occurs due to the formation of colored intercalation compounds (WO3), charge storage on the particle (SnO2:Sb), or coupled redox reactions on their surface (TiO2). Organic materials are often necessary to obtain the color change as either redox (for instance, Prussian Blue or viologens) or charge transport (polyaniline, polypyrrole, or polythiophene) agents. The most important parameters of electrochromic devices are switching time, depth of color change, and environmental stability. zum Felde et al.89 and Coleman et al.167 studied the ionic and electronic processes in photochromic effect in SnO2:Sb. They found that the switching time is controlled by the rate of ionic diffusion into the porous film of cations, which counters the negative charge of electrons injected into the NPs. The charge trapped in the Sb centers is very stable and produces up to 1 optical density unit (O.D.) change in color in the visible and IR parts of the spectrum. The response times were <10 msec,89 which was faster than those for TiO2–Prussian Blue films, with switching times of about 1 sec, and still significantly better than for organic polymers devices.168,169 The diameter of the counterions was essential for maintaining the electrical and optical characteristics. When the bulky tetrabutyl ammonium cation was replaced with lithium, the photochromic effect in the visible region increased with the concomitant decrease in the response time. However, the photochromic response in the IR part of the spectrum was not affected. Polyoxometalates (POMs) attract special interests of researchers working with electrochromic devices.170 The sizes of POM nanocrystals can be easily controlled from several angstroms to 10 nm by simple chemical synthesis. More importantly, unlike common NPs, POMs’ nanocrystals are intrinsically monodispersed and have uniform composition. Liu et al.171,172 have demonstrated that POMs could be inte- grated into ultrathin films upon LBL assembly. For thin films with 20 monolayers of Eu-(H2O)P5W30O110, contrast changes up to 92% were observed within 7 min of turning on the bias.173 Metal oxide NPs are attractive for electrochromic applications due to their high environmental stability. As many as 1 million cycles of color switching were per- formed on SnO2:Sb films.89 However, metal chalcogenides became their strong competitors when narrow on/off absorption bands were required. Wang et al.87,88 DK3136_book.fm Page 18 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 34. Organization of Nanoparticles and Nanowires in Electronic Devices 19 explored the electrochromic effect of CdSe NPs in solution. Upon applying negative potential, a new IR absorption peak appeared for both CdSe NP solutions with different sizes (Figure 1.10). The appearance of an IR absorption peak implied that the electrons injected at negative bias go to the 1S state of CdSe NPs. Concomitantly, the lowest interband excitation transition is bleached. The new IR peak corresponds to the transition from the 1S state in the CB to higher electronic levels in the same band. The appearance and magnitude of the IR peak were strongly dependent on FIGURE 1.9 (A) Electrochromic sign based on Sb-doped Sn oxide produced by Monsanto. (From Coleman, J.P. et al., Displays, 20, 145–154, 1999. With permission.) (B, C) Prototype electrochromic window from sintered TiO2 nanocrystalline film modified with Prussian Blue in clear (B) and colored (C) forms. (From Bonhote, P. et al., Displays, 20, 137–144, 1999. With permission.) DK3136_book.fm Page 19 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 35. 20 Nanoparticle Assemblies and Superstructures the sizes of NPs (for example, IR peaks for 5.4-nm CdSe NPs were quite small). A decrease of NP diameter also resulted in a high-threshold voltage for electron injection, which was consistent with the wider band gap in small NPs. In addition to CdSe, identical IR peaks were observed for PbSe NPs,174 which demonstrated the versatility of potential-induced IR absorption for semiconductor NPs. The effect of the particle organization in the electrochromic films of NP can be as profound as for LED and photovoltaic devices.175 The image contrast and response time are determined by the rate of charge transport between NPs, which can be controlled by the thin-film deposition process. Sintered spin-coated films appear to be the most common substrates for the studies of photochromism. Room temperature processing such as electrodeposition176 or layer-by-layer assembly of nanoparticles119 can lead to a more complex and sophisticated structure with excellent photochromic characteristics. For instance, the addition of Au NPs to Fe2O3 film greatly improves the film conductance and photochromic color depth and response time.177–180 One such endeavor can be the improvement of the longevity of the chalcogenide NPs and acceleration of ion diffusion within the film. Recently, the photochromic effect with NPs made directly from Prussian Blue 5 to 10 nm in diameter has been reported by DeLongchamp and Hammond.181 The films were made by LBL assembly, which affords several performance improvements compared to other deposition methods (Figure 1.11). Poly(ethyleneimine), a weak polyelectrolyte used as an LBL partner of Prussian Blue nanoparticles, can intrin- sically ionize in the vicinity of the NPs, which facilitate the switch in the redox state of the particles responsible for the color change. It also provides greater thermody- namic stability for the reduced state of Prussian Blue and, therefore, better longevity. Nanoporosity of the film supports fast diffusion of potassium counterions neutral- izing the negative charge stored in the inorganic component, although with switching time in the range of seconds. A superior contrast of 77% was achieved due to eliminated reflection and scatter in the transparent state and the small size of the FIGURE 1.10 IR and UV-visible spectra of 5.4-nm CdSe nanocrystals (A) and 7.0-nm CdSe nanocrystals (B) at different potentials. (From Wang, C. et al., Science, 291, 2390–2392, 2001. With permission.) 2.5 2 1.5 1 0.5 0 2.5 2 1.5 1 0.5 0 Absorbance 0 V 1.5 V ×10 0 0.2 0.4 0.6 0.8 1.6 1.8 2 2.2 2.4 Energy (eV) 2 1.5 1 0.5 0 2 1.5 1 0.5 0 Absorbance 0 V 1.2 V 0 0.2 0.4 0.6 1.6 1.8 2 2.2 Energy (eV) A B DK3136_book.fm Page 20 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 36. Organization of Nanoparticles and Nanowires in Electronic Devices 21 Prussian Blue particles. Their uniform dispersion within the polymer matrix allowed for greater bleaching because the potassium ion intercalation distance is limited by the nanoparticle diameter. 1.3.1.6 Sensors and Biosensors Metal or semiconductor NPs can also offer new transduction techniques of both an optical and electrical nature.182 As was already mentioned, the charge transport between NPs in thin films is exceptionally sensitive to tunneling conditions, which can be altered by a variety of means, from incorporation of analytes between the NPs to their excitation by electromagnetic fields of appropriate energy. Interparticle gaps become exceptionally important for sensing applications and have to be tuned FIGURE 1.11 (A) The dependence of the absorption spectrum on the applied potential. (B) Color change of Prussian Blue films for different applied voltages. (From DeLongchamp, D.M. and Hammond, P.T., Adv. Funct. Mater., 14, 224–232, 2004. With permission.) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Absorbance (a.u.) 400 500 600 700 800 Wavelength (nm) 0 0.0 10 20 30 40 50 60 1.0 0.8 0.6 0.4 0.2 700 nm abc at D.D.V layer pair number step 0.1 V 10.6 V −0.2 V A (LPEI/PB)50 oxidation reduction oxidation reduction oxidation reduction −0.2 V (clear) 0.6 V (cyan) 1.0 V (green) 1.5 V (yellow) B DK3136_book.fm Page 21 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 37. 22 Nanoparticle Assemblies and Superstructures for sensitivity and selectivity. This can be done typically by variation of the chemical structure of the stabilizer layer around NPs and subsequent preparation of a closely packed structure.183–185 This approach to the control of particle organization in thin films is relatively universal and was taken advantage of in both optical and electrical sensors.186 The dependence of the corresponding properties on the relative degree of order in NP solids was described by Beverly et al.187,188 The theoretical aspects of the charge transport between the NPs in two- and three-dimensional solids were extensively investigated by Remacle et al.189 and are presented in Chapter 6. The comparison with experimental data obtained for compressed Langmuir–Blodgett (LB) films of Ag NPs strongly supports the Mott–Hubbard band model of charge transport.190–192 This makes the system of metal NPs one of the best examples of developing collective properties in organized NP systems.193 Typically, the transduction mechanism in nanostructured films precipitated by the presence of an analyte is related to the change in the gap distance between the NPs due to swelling or shrinking. Some other processes making possible signaling in NP sensors include pressure-induced enhancement of photoluminescence intensity of CdS(Se) NPs,194 reversible reduction of the crystal lattice of the nanocrystals,195 and the magnetotunneling effect.196 One of the most representative examples of transduction based on swelling is given by Zamborini et al.84 and can be applied to a fairly large set of analytes. A thin film of alkanethiolate and ω-carboxylate alkanethiolate-protected Au NPs was deposited on the interdigitated array electrodes by cross-linking them with carboxylate–Cu2+–carboxylate bridges (Figure 1.12A). Because a small constant bias of –0.2 V was applied to the film, an analyte-dependent current was registered. When the device was exposed to ethanol vapors, the current sharply decreased with the increase of C2H5OH partial pressure (Figure 1.12B). This effect stems from the high affinity of ethanol and the Au–thiol–carboxylate com- posite, resulting in film swelling. Correspondingly, the gap between Au NPs increases and the current displays a pronounced drop. The conductivity of such sensors will change with the chemical structure of the vapor and its concentration. The swelling effect is relatively fast and reversible, with a response time within 50 sec. Gas sensors are probably the most abundant among the NP thin films,183–185,195,197–199 with some examples of field/force sensors194,196 and biosensors. The applications of NP films to biosensors were recently reviewed by Shipway and Willner200 and Willner et al.201 and hence will not be dwelled upon now. As a brief remark, we need to mention that the development of the latter is strongly accelerated by intense cross-fertilization between biology and nanotechnology, which can be noticed in different aspects of organized NP assemblies and superstructures presented in other chapters. Similar to light-emitting, photovoltaic, and electrochromic devices, the major challenge for NP sensors is the accurate tuning of the two- and three-dimensional film structure to improve the sensitivity or selectivity of the device.202,203 Quite often, the use of highly monodispersed particles is sufficient.86 However, the preparation of more elaborate sandwich structures is necessary to fine-tune the analyte response. For instance, the work of Stich et al.204 showed that complex layered structures are required to attain parallel screening of proteins in order to overcome some limitations of fluorescence, enzyme labels, and colloid techniques. The combination of excellent DK3136_book.fm Page 22 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 38. Organization of Nanoparticles and Nanowires in Electronic Devices 23 two-dimensional ordering and tight control over the geometry of individual NPs obtained by nanosphere lithography (see Chapter 14) afforded an unprecedented level of wavelength agility of the sensor throughout the visible, near-IR, and mid- IR regions of the electromagnetic spectrum.205,206 1.3.2 NWS AS MOLECULAR ELECTRONIC DEVICES The most common view of NWs is as interconnects and functional elements of nanocircuits, such as leads and electronic diodes. In addition, NWs also have great potential as elements for electroluminescence devices and sensors, which will be FIGURE 1.12 (A) Scheme of gas sensors of Au nanoparticles films. (B) Current changes are monitored over time. Films were alternately exposed to pure nitrogen flow and ethanol vapor in an increased sequence of fractions, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0, of the saturated ethanol vapor pressure. (From Zamborini, F.P. et al., J. Am. Chem. Soc., 122, 4514–4515, 2000. With permission.) Au Core e− e− Carboxylate “Linker” Site Alkanethiolate “Non-linker” MPC Assembled Film Cu2+ A Au IDA Finger Au IDA Finger 15 µm gap Au IDA Finger 0.1 µm Glass Current (µA) 0.0 0.5 1.0 1.5 2.0 8000 6000 4000 Time (s) 2000 0 DK3136_book.fm Page 23 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 39. 24 Nanoparticle Assemblies and Superstructures discussed in this part of the review. They can also be used as voltage-driven actua- tors.207 Unfortunately, the latter remains a relatively unexplored area. 1.3.2.1 Nanocircuits Similar to conventional circuits, the integration of NWs in predesigned patterns extends their application from single electronic elements to functional devices. Prototype electronic elements on their basis have been successfully demonstrated and include p–n diodes,43 bipolar junction transistors,208 field effect transistors,45,208,209 and complementary inverters.210 It was also shown that NWs could perform basic functions of complex logical gates, such as OR, AND, and NOR gates using p-type NWs from Si (green in Figure 1.13) and n-type NWs from GaN (red in Figure 1.13).210 Since the understanding of potential computational schemes is important for the future development of NW assemblies, examples of gate operation are given here in some detail. Assuming that the nanoscale circuit will be operating in the binary regime similarly to the conven- tional circuits, one can assign logic 0 and logic 1 to low (0 V) and high (±5 V) voltage in the output, respectively. For an OR gate, two inputs were made from p- NW; one n-NW was placed over them and served as an output (Figure 1.13A). Since FIGURE 1.13 Schemes of logic gates from p-Si (green) and n-GaN (red) NWs for OR (A–C), AND (D–F), and NOR (G–I) operations. (From Huang, Y. et al., Science, 294, 1313–1317, 2001. With permission.) 5 4 3 V 0 (V) V 0 (V) 2 1 0 0 1 2 V1(V) 3 4 5 5 4 3 V 0 (V) 2 1 0 1 2 V1 (V) 3 4 5 5 4 3 V 0 (V) 2 1 0 0 1 2 V1 (V) 3 4 5 B C OR V11(V) 0.0(0) 0.0(0) 5.0(1) 5.0(1) 0.0(0) 5.0(1) 0.0(0) 5.0(1) 0.00(0) 4.58(1) 4.57(1) 4.79(1) V12(V) V0(V) F AND V11(V) 0.0(0) 0.0(0) 5.0(1) 5.0(1) 0.0(0) 5.0(1) 0.0(0) 5.0(1) 0.71(0) 0.90(0) 0.88(0) 4.96(1) V12(V) V0(V) I NOR V11(V) 0.0(0) 0.0(0) 5.0(1) 5.0(1) 0.0(0) 5.0(1) 0.0(0) 5.0(1) 4.85(1) 0.02(0) 0.06(0) 0.01(1) V12(V) V0(V) E H A D G 0.1 5 4 3 2 1 0 V 0 (V) 5 4 3 2 1 0 V 0 (V) 5 4 3 2 1 0.0 0.0 0.1 1.0 1.1 0.1 1.0 1.1 1.0 0.0 OR Address Level AND Address Level NOR Address Level 1.1 V?? V11 V12 ? ? ? ? V?? V11 V?? V?? V?? V?? V?? V?? V?? V?? V?? V?? V?? V V?? V?? V?? V?? V?? V?? V?? V?? V?? V?? V?? V? V? ? ? ? ? V Silicon oxide Silicon oxide Silicon oxide V12 V0 V?? V?? OR AND NOR R R DK3136_book.fm Page 24 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 40. Organization of Nanoparticles and Nanowires in Electronic Devices 25 the NW crossings function as common diodes, logic 0 or logic 1 on the output wire appeared when at least one of the inputs had low or high voltage, respectively (Figure 1.13B and C). For a more complex AND gate, the output was assigned to a p-type NW under bias of 5 V, whereas two of n-type NWs were used as inputs (Figure 1.13D). A gate electrode was also crossing the p-type NW with a constant voltage to deplete a portion of it. Only when both input voltages were high enough to reduce the voltage drop across the constant NW resistor was the high voltage obtained at the output (logic 1). Otherwise, logic 0 appeared (Figure 1.13E and F). A NOR gate could be made when the NW crossings functioned as FETs. Two n-type NWs were connected as FET series, whereas a bias of 2.5 V was applied to the third n-NW FET to create a constant resistance of about 100 megaohms (Figure 1.13G). When either or both input voltages were high, the transistors were in the off state, i.e., had a higher resistance with respect to the constant resistor. Thus, most voltage dropped across the transistors (logic 0). Only when both input voltages were close to 0 V was the high-output voltage observed (logic 1) (Figure 1.13H and I). The fabrication of nanoscale logic gates is an important milestone toward the practical application of NWs as nanocircuits.211 Whereas some of the logic operations were previously demonstrated for organic molecules of rotaxane in thin films212 and carbon nanotubes,213,214 the inorganic NWs or carbon nanotubes are more likely to succeed as long-term, cost-effective nanodevices. Futhermore, the easy surface mod- ification of inorganic NWs gives us numerous opportunities to produce functional electronics; for instance, recent studies showed In2O3 NWs can be used as data storage materials after adsoption of porphyrin or Bis(terpyridine)-Fe molecules.215,216 Significant advances are also being made in the manufacturing of the NW crossbar systems, grids, and other networks. Most common current approaches are based on solution chemistry or external fields217,218 and can be almost equally well applied to carbon nanotubes.219 Melosh et al.220 demonstrated an elegant method of manufacturing of planar systems with aligned NWs, taking advantage of well- developed methods of thin-film deposition. The thickness of the films in molecular beam epitaxy can be controlled to the angstrom level. If the sandwich structure were turned on a side, etched, and partially filled with a different material, then parallel NWs could be stamped onto an appropriate substrate (Figure 1.14). Record junction densities as high as 1011 cm–2 were obtained with pitch (center-to-center distance) as small as 8 nm, which is substantially better than the NW arrangements obtained by electron beam lithography. The successful demonstration of the basic devices from NWs and new advances in their manufacturing should not camouflage an important problem of a paradigm change that must happen upon transition from a conventional lithographic technology of chip production capable of organizing structures up to 100 nm in scale to the true nanometer scale devices. One cannot and should not replicate the architecture of the computer chips from NWs or NPs by simply reducing their size. As one can see, the accuracy of positioning of circuit elements cannot be totally mistake-proof. The variability in the device performance will also be quite high, especially for a large number of nanoscale p–n junctions or SET gaps. The same issues are to be expected for carbon nanotube electronics compounded by their intrinsic variability in properties.12,221 Therefore, one DK3136_book.fm Page 25 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 41. 26 Nanoparticle Assemblies and Superstructures needs to redesign the computing algorithm to be significantly more tolerant to defects in the circuit structure. This can be the only way to a functional and viable nanoscale circuit, which can take advantage of exceptionally high levels of integration. Although this area of research and, in particular, NW + NP superstructures support- ing the defect-tolerant computing are still waiting for the active development of NW superstructure preparation, an important step in this direction was made by the creation of the Teramac computer at Hewlett-Packard.46,222 Heath et al.46 demon- strated that a fast and powerful computer can be made fairly inexpensively from elements prone to manufacturing and interconnect defects on the basis of a new addressing scheme that can reroute the signal around malfunctioning elements. In this respect, it is important to develop manufacturing techniques for new geometrical arrangements of the NWs topologically different from the classical crossbar scheme (Figure 1.14). Recent developments on the synthesis of branched NWs can provide more design room for devices.223–225 One interesting example is regular dendritic NW arrays with multiple side branches from ZnO (Figure 1.15A and B), reported by Yan et al.226 In addition to interesting electronic properties, the high refractive index of ZnO and observed waveguiding phenomena in these NWs (Figure 1.15C) make hybrid optoelectronic computing schemes potentially possible. One can notice that the research on CNT circuits goes largely in parallel with that on NWs and NPs. Similar milestones have been achieved in both fields, such as preparation of FET,227 logic gates,213,214 transistor arrays,12 photodiodes,228 and light emitters.229 Although there is a lot of overlap in projected applications, many performance parameters in NW devices and CNT devices are complementary; for instance, the wavelengths in light emission and different types of transistors or sensor functions. Therefore, it would be important to evaluate potential technologies to integrate the different families of the devices. Currently, this field is only in the beginning of its development, largely because of different chemical/physical approaches to device assembly. FIGURE 1.14 SEM images of NW arrays prepared by superlattice NW pattern transfer. (A) SEM image of silicon NWs deposited on SiO2. The 19 NWs on the left are 18 nm wide and 30 nm apart (center to center). The 40 NWs on the right are 20 nm wide and 40 nm apart. (B) Model crossbar circuit made from Pt NWs with pitches from 20 to 80 nm. Scale bars, 500 nm. (From Melosh, N.A. et al., Science, 300, 112–115, 2003. With permission.) A 500 nm B DK3136_book.fm Page 26 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 42. Organization of Nanoparticles and Nanowires in Electronic Devices 27 1.3.2.2 Electroluminescence Devices The charge transport and electron–hole recombination phenomena underlying the operation of p–n junctions of electroluminescent devices from NWs are identical to those made as thin films. The challenge here is in obtaining a crossbar arrangement of the p- and n-type NWs, which was realized for p-type Zn-doped InP and n-type Te-doped InP NWs (Figure 1.16B).44,230 Analogous to conventional bulk semicon- ductor p–n junctions, NW crossing exhibits characteristic current rectification in the forward bias, with a sharp current onset at 1.5 V. As expected, the NW intersection with the p–n junction serves as the light-emitting point where electrons and holes recombine upon electrical excitation, while both NWs in the device light up for the optical excitation mode (Figure 1.16A, insert). The intensity of the electrolumines- cent point follows the i-V curve of the diode (Figure 1.16B), with a threshold voltage as low as 1.7 V. The wavelength of the emitted light can be easily adjusted by using InP NWs of different diameters. As such, NW junctions with light emission peaks at 820 and 680 nm have been demonstrated (Figure 1.16C and D). The device in Figure 1.16 is the smallest light-emitting diode currently made. Miniaturization, though, may not be the only direction for further development of FIGURE 1.15 (A, B) SEM images of comb structures made from ZnO nanowires. (C) Far- field optical image of light emission with spatially resolved emission from individual NWs. (From Yan, H. et al., J. Am. Chem. Soc., 125, 4728–4729, 2003. With permission.) A 1 µm B 1 µm C DK3136_book.fm Page 27 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 43. 28 Nanoparticle Assemblies and Superstructures the electroluminescent devices from single NWs. Nanoscale semiconductor rods can serve as optical resonators, which was demonstrated by Johnson et al.231 for optically pumped single-GaN NWs. They can also be periodically doped to produce one or many p–n junctions directly on a single NW.232 These facts indicate the prospects of electrically pumped laser on single NW, which was recently confirmed by the report by Duan et al.233 on optical and electrical measurements made on single- crystal CdS NWs acting as Fabry–Perot optical cavities, with mode spacing inversely related to the nanowire length. Electrically driven nanowire lasers may also be assembled in arrays capable of emitting a wide range of colors. 1.3.2.3 Sensors The optical and electronic devices described above demonstrate the state of the art of the field. At the same time, their practical applications in nanoscale electronics may be quite distant. This is not the case for various sensors that can be designed FIGURE 1.16 (A) Electroluminescence (EL) image of the light emitted from a forward- biased NW p–n junction at 2.5 V. Inset: PL image of the junction. Scale bars, 5 µm. (B) EL intensity vs. voltage. Inset: i-V characteristics. Inset in this inset: Field-emission SEM image of the junction itself. Scale bar, 5 µm. (C, D) The spectrum peaks at 820 and 680 nm. (From Duan, X. et al., Nature, 409, 66–69, 2001. With permission.) A 1.6 1.2 0.8 0.4 0.0 1.6 2.0 2.4 2.8 3.2 Forward Bias (V) −2 0 2 Voltage (V) 4 3 2 1 0 Current (µA) Intensity (counts) B 700 800 900 Wavelength (nm) 1,600 1,200 800 400 0.0 Intensity (counts) C 600 700 800 Wavelength (nm) 250 200 150 100 50 0 Intensity (counts) D DK3136_book.fm Page 28 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 44. Organization of Nanoparticles and Nanowires in Electronic Devices 29 on the basis of NWs, which can significantly outperform and become commercially viable alternatives for thin-film sensors quite soon.234 They exhibit exceptional sen- sitivity due to the strength of surface effects on the electrical properties in nanometer scale objects, whereas surface modification makes possible manipulations with their selectivity. The research on semiconductor and metal NW sensors was largely inspired by the success of similar applications of carbon nanotube,235 for which chemical sensors,236 biosensors,237,238 and gas sensors were developed.239 The surface chemistry of CNTs and semiconductor NWs is quite different, which predetermines different sensor functions. Metal oxide NWs have specific interactions with many gases. Adsorption of a redox gas on the NW surface results in trapping or injection of charge carriers, whose concentration in a small volume of the wire drastically changes. The conductivity of NWs follows the charge carrier concentra- tion and typically decreases upon exposure to the analyte, which offers a convenient transduction mechanism for gas detection.53 This mechanism of conductivity modulation is perceived to be advantageous to the percolation of charge in NP arrays (see above) because of better stability, signal- to-noise ratio, and faster response times.240 NWs made of a variety of semiconducting oxides such as ZnO, SnO2, In2O3, TiO2, and Si have been explored to be a gas sensor for O3, O2, Cl2, NO2, NH3, CO, H2, H2O, and EtOH.240–246 The response time, for instance, for hydrogen sensing in mesoscopic Pd NWs was as short as 75 msec,247 which was much faster than in sensors based on NP thin films (see above). The sensitivity for NW sensors based on ZnO nanobelts was as high as a few parts per billion, which is probably not yet the theoretical limit.248 Besides electrical sensors, optical transduction schemes with NWs by, for instance, surface-enhanced Raman scattering are also being explored.249 However, they require conversion to the thin-film sensor modality, although the act of sensing still occurs on a single NW. Preparation of NW bioconjugates for biosensors has relevance not only to life sciences but also to nanotechnology in general. Decoration with biomacromolecules can be used for organization of nanocolloids taking advantage of highly specific interactions between them. The conductivity changes in NW–protein or NW–DNA constructs are affected by the electrical field around the complementary biomolecules attaching to the protein or DNA already present on the NW surface.250,251 When Si NWs were functionalized by a monolayer of biotin (Figure 1.17A),252 the conduc- tance of biotin-modified Si NWs increased rapidly to a constant value upon addition of a streptavidin solution, whereas it remained unchanged after the addition of pure buffer solution (Figure 1.17B). The plateau value linearly depended on the concen- tration of monoclonal antibiotin (Figure 1.17C). To some extent, the transduction mechanism in NW biosensors is similar to the operation of NW FETs. The increase of the conductance can be understood as chemical gating of charge when negatively charged streptavidin binds to the p-type Si NW surface. Field sensors are exemplified by optical sensors from ZnO NWs (which can also be considered optical switches253) and magnetic field sensors with Ni NWs.254 In the latter example, an array of highly parallel NWs defined by ion-track technology was embedded in polyimide plastic. On the top and on the bottom of the membrane, they were connected to two lithographically structured surface layers. The sensing DK3136_book.fm Page 29 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 45. 30 Nanoparticle Assemblies and Superstructures mechanism was based on magnetoresistance and was critically dependent on the parallel organization of the NWs in the matrix between planar electrodes. The collaboration between Sony and the University of Hamburg resulted in a similar optical switch with aligned ZnO rods with a strong photoresponse at 366 nm.255 The ZnO nanorods with a diameter of 15 to 30 nm and a length of 200 to 300 nm were directed into 200- to 800-nm-wide electrode gaps by using alternating electrical fields at frequencies between 1 and 10 kHz and field strengths between 106 and 107 V/m. The nanorods aligned parallel to the field lines and made contact with theAu electrodes. The i-V characteristics of the aligned rods were strongly nonlinear and asymmetric, showing rectifying, diode- like behavior and asymmetry factors up to 25 at 3-V bias. 1.4 STRATEGIES OF NP AND NW ASSEMBLY INTO MORE COMPLEX STRUCTURES In order to improve the functions and simplify the manufacturing of the nanoscale devices, one needs to learn how to organize nanocolloids in the space of commen- surate dimensions. Here we assume that it is the spatial organization of NPs, NWs, and CNTs — i.e., what one has in the end, not how it was made — that determines the functionality of the superstructure. This may not be entirely true in the practical sense because methods of processing affect the surface of nanocollids. Even the FIGURE 1.17 (A) Scheme of a biotin-modified Si NW (left) and subsequent binding of streptavidin to the Si NW surface (right). (B) Plot of conductance vs. time for a biotin-modified Si NW, where region 1 corresponds to buffer solution, region 2 corresponds to the addition of 250 nM streptavidin, and region 3 corresponds to pure buffer solution. (C) Plot of the conductance change of a biotin-modified Si NW vs. m-antibiotin concentration; the dashed line is a linear fit to the four low concentration data points. (From Cui, Y. et al., Science, 293, 1289–1292, 2001. With permission.) SiNW SiNW A 1700 1650 1600 1550 0 100 200 1 2 3 Conductance (nS) B C 0 10 20 30 40 150 100 50 0 Conductance change (nS) DK3136_book.fm Page 30 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 46. Organization of Nanoparticles and Nanowires in Electronic Devices 31 most advanced methods of NP/NW characterization afford only limited information about their surface, so the apparent similarity of superstructures made by different methods can be deceiving. Regardless of this caveat, it is scientifically interesting and practically important to find simpler and more efficient methods of constructing more complex structures from nanocolloids as building blocks. The strategies of organization of NPs and NWs are different in comparison to those of molecules, polymers, and biomolecules. This section will give you a fairly detailed account of current approaches to production of organized nanoscale structures. 1.4.1 ASSEMBLY OF NPS In general, the assemblies of NPs can be divided into one-, two-, and three-dimen- sional systems. Methods of their preparation differ vastly, depending on the actual type of NP material. There is hardly any universal method that could be applied for most of the NPs. 1.4.1.1 One-Dimensional Assemblies of NPs The oldest approach to the preparation of one-dimensional assemblies of NPs is the use of a linear template. This is a versatile method of preparation of all kinds of linear NPs assemblies. If necessary, after completing its role as a structural agent, the linear template may be removed. CNTs or semiconductor NWs are probably the most logical structure-directing matrix for the preparation of one-dimensional NP assemblies. For instance, multiwall CNTs were used as templates for spontaneous assembly of Au on them, which subsequently merged into complex coaxial (albeit granular) NWs.256 In this review, we will discuss CNT-NP and NW-NP assemblies separately below, because NP-NW superstructures have special relevance to nanos- cale electronics and to nanocolloid constructs (discussed above). Among other tem- plates, linear pores and channels inside polymers, alumina, and silica templates can be used to make linear agglomerates.257 In addition to them, one-dimensional assem- blies of metallic NP can be made at the edge of lattice plane terraces by chemical and electrochemical reduction of corresponding metal ions.258 Hutchinson et al.259 demonstrated that ditches of the corrugated carbon surface act as nucleation sites for the synthesis of aligned Au NPs. Fort et al.260 applied the same idea to the preparation of Ag NP chains in faceted grooves of alumina. Biomolecules, including DNAs, proteins, and sometimes even more complex structures, are rapidly becoming a very common template for one-dimensional NP assemblies. The recent popularity of this idea can be attributed to the exceptional selectivity of the biomolecular assembly, versatility of experimental methods for manipulations of their structure, and excellent intellectual and instrumental founda- tion for such studies developed by Life Sciences. The possibility of self-assembly of very complex structures following the amino acid or peptide code is very attractive to many scientists. Among biotemplates, DNA is probably the most frequently used class of mol- ecules owing to its strong electrostatic and coordination interaction with NPs. Dif- ferent kinds of NPs, such as Ag,261,262 Pd,263 Au,264,265 and Pt266 have been organized DK3136_book.fm Page 31 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 47. 32 Nanoparticle Assemblies and Superstructures by this technique. As such, Braun et al.261 initially stretched DNA molecules between two Au electrodes with a separation of about 15 µm. After that, Ag+ was ion exchanged and complexed with amino groups present on the surface of DNA. Further chemical reduction transformed Ag+ to Ag NPs. After several cycles of ion exchange and reduction of silver, one-dimensional Ag NP chains along the DNA molecular template were prepared (Figure 1.18A). FIGURE 1.18 (A) Scheme of construction of a silver NP wire between two electrodes. (B) Experimentally observed i-V curves. (From Braun, E. et al., Nature, 391, 775–778, 1998. With permission.) 100 µm 12–16 µm 50 µm Oligo A Oligo B ‘R-S-S-R ‘R-S-S-R -S-R -R-S -R-S -R-S -R-S -S-R -S-R -S-R -S-R -R-S -R-S -R-S -R-S -S-R -S-R -S-R DNA DNA a b c d e Ag+/OH− Conductive silver wire 2Ag+ + Hq 2Ag0 + Bq + 2H+ Ag+ ions + Hydroquinone/H+ Hydroquinone/OH− A 400 B 200 20 10 0 0 −200 −400 −20 −10 Bias (V) Current (nA) DK3136_book.fm Page 32 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 48. Organization of Nanoparticles and Nanowires in Electronic Devices 33 Most of the research in this field is done with premade NPs absorbing on the template. However, several groups took advantage of electrostatic interaction of the metal or semiconductor NPs with DNA; for instance, Torimoto et al.267 and Jin et al.268 made 3-nm-wide chains from CdS. Raman and x-ray photoelectron energy spectroscopy confirmed that the adsorption sites of CdS NPs were phosphate acid groups of DNA. Warner et al.269 showed that the ribbon-like and even branched Au NP assemblies could be prepared on DNA templates. Fu et al.270 obtained double-helical arrays by assembling Au and Pd NPs’ peptide nanofibrils at different pH values. Biotemplates of higher levels of complexity are also employed.271 Dujardin et al.,272 Fowler et al.,273 and Shenton et al.274 used the tobacco mosaic virus with a shape of a linear tube for assembly of various kinds of NPs inside or outside the tubes. Djalali et al.,275,276 Banerjee et al.,277 and Yu et al.278 assembled Au NPs onto the surface of polypeptide nanotubes, and their assembly position on the biomole- cules could be controlled by specific affinity of polypeptide sequences. Het- erodimeric tubulin was used as a template to assemble Pd NPs.279 Mao et al.280,283 and Lee et al.281,282 utilized bacteriophage to obtain oriented CdS, ZnS, CoPt, and FtPt NWs. Au and Ag NWs were also obtained by depositing Au and Ag solution onto the surface of yeast Saccharomyces cerevisiae Sup35P. Their results showed that after coating, the resistance of nanofibers decreased from 1014 to 86 ohms.284 There is a growing realization in the research community that the templates may not be necessary for the NP to form one-dimensional structures at all because under certain conditions they may self-assemble due to inherent anisotropy of NP–NP interactions. This statement refers not only to magnetic particles, for which chain formation and anisotropy was known a long time ago, but also to numerous non- magnetic colloids in the absence of external fields. In one of the early experiments, Korgel and Fitzmaurice demonstrated that the prolate Ag NPs self-assembled into NWs during solvent evaporation.285 The shape anisotropy was implicated in the formation of NWs, but the details of the process were not disclosed. Pacholski et al.286 observed a similar process in solution for ZnO NPs (Figure 1.19A). It was found that the self-orientation of the particles and aligning of the crystal lattices occurred prior to the formation of ZnO nanorods.287 In the case of stabilizer-depleted CdTe, not only rods or NWs but also an intermediate stage, NP chains, was observed by Tang et al.38 Analysis of the assembly process demonstrated that dipole–dipole interaction between the NPs was one of the primary forces in the self-assembly of the CdTe nanocolloid. When methanol partially stripped the TGA layer, the electro- static repulsion preventing NP association decreased. The intrinsic dipolar moments of NPs resulted in their assembly in chains. Later, analogous processes of the self-assembly of NP in one-dimensional struc- tures were also observed for a variety of other colloids. By selective desorption of Trizma ligands from the (001) crystal plane, Polleux et al.288 successfully prepared the TiO2 NWs via the self-assembly of NPs along the (001) direction (Figure 1.19B). One-dimensional assembly was also reported by Chang et al.289 and Liao et al.290 for metal silver and gold NPs, respectively. Chapter 20 also provides interesting experimental data on this subject. A similar process was also observed for Ag2S by Gao et al.291 In some cases, linear stabilizers with two functional groups (EDTA, diamine) were believed to be the reason for the self-organization. In light of data DK3136_book.fm Page 33 Sunday, June 19, 2005 11:40 AM © 2006 by Taylor & Francis Group, LLC
- 49. Discovering Diverse Content Through Random Scribd Documents
- 50. and lastly came Mengs, bringing with him a spirit wholly distinct from that of the French, a style erudite and academic which was not sufficiently powerful to create an artistic output of any importance in Spain, but which possessed much destructive power, although that was limited as regards time to about a century, during which period the national production was weak, despite the number of artists, of whom those most worthy to be mentioned are Maella, the Bayeus and Paret. Such was the condition of Spanish painting when, without precedent, reason or motive, appeared in the province of Aragon, a region which years afterwards came to typify the resistance to foreign invasion, a figure of great significance in Spanish art, and worthy of comparison with the greatest masters of the preceding centuries—Francisco de Goya. . . . . . . . . . . . . . . . The long life of Goya coincides with an epoch which divides two ages. The critic is somewhat at a loss how to place his work and personality, to conclude whether he is the last of the old masters or the first of the moderns. His greatness is so obvious, his performance so vast and its gradual evolution so manifest, that we may be justified in holding that the first portion of his effort belongs to the old order of things, while the second must be associated with the origins of modern painting. In his advance, in the manner and development of it, it is noticeable—as we have already said in certain of our works which deal with Goya—that he substituted for the picturesque, agreeable and suggestive note of his younger days, another more intense and more embracive. It would seem that the French invasion of the Peninsula, the horrors of which he experienced and depicted, influenced him profoundly in the alteration of his style. There is a Goya of the eighteenth century and a Goya of the nineteenth. But this is not entirely due to variation in technique, to mere artistic development, it is more justly to be traced to a change in creative outlook, in character, in view-point, which underwent a rude and violent transformation. Compare the subjects of his tapestries or of his festive canvases, joyful and gallant, facile in conception and at times almost trivial, with the tragic and macabre scenes of his old age, and with the drawings of this period and the compositions known as “The Disasters of War.”
- 51. His spirit was fortified and nourished by the warmth of his imagination, and assisted by an adequate technique, marvellously suited to the expression of his ideas, he produced the colossal art of his later years. If his performance is studied with reference to the vicissitudes and the adventures of which it is eloquent, the influence upon his works of the times in which they were created is obvious. The changes in his life, the transference from those gay and tranquil years to others full of the horrors of blood and fire, of shame and banishment, tended, without doubt, to discipline his spirit and excite his intelligence. His natural bias to the fantastic and his tendency to adapt the world to his visions seized upon the propitious occasion in a time of invasion and war to exalt itself, or, as he himself expressed it, “the dream of reason produces prodigies.” An artist and creator more as regards expression than form, especially in the second phase of his work, unequal in achievement and at times inaccurate, he sacrificed much to divest himself of these faults. He deliberately set himself to discipline his ideas and develop that degree of boldness with which he longed to infuse them. But he was not quite able to subject himself to reality, and, as he was forgetful and indolent, that which naturally dominated him began to show itself in quite other productions of consummate mastery. This art, imaginative in expression and idea, is more striking as regards its individual and original qualities, than for any degree of discipline which it shows. To follow Goya throughout the vicissitudes of his long life is not a matter of difficulty. The man to whom modern Spanish art owes its being was born in the little village of Fuendetodos and lived whilst a child at Saragossa. He came to Madrid at an early age, and before his thirtieth year went to Rome with the object of perfecting himself in his art. But he failed to obtain much direction at the academies in Parma, and having but little enthusiasm for the Italian masters of that time, returned to Spain, settling at Madrid. Until this time the artist had not evinced any exceptional gifts. Goya was not precocious. The first works to assist his reputation were a series of cartoons for tapestries to be woven at the Royal Factory. They were destined for the walls of the royal palaces of Aranjuez, the Escurial and the Prado, which Carlos IV desired to renovate according to the fashion of the time. These works, which brought fame to Goya, showed two distinctive qualities. One of them evinces the originality of his subjects, in which appear gallants, blacksmiths, beggars, labourers, popular types in short, who for the first time
- 52. appeared in the decoration of Spanish palaces and castles, which, until then, had known only religious paintings, military scenes, the portraits of the Royal Family and stately hidalgos. Goya, in this sense, democratized art. The other note to be observed in his work is a certain distinction of craftsmanship, the alertness which it reveals, which is, perhaps, due to the lightness of his colouring. On canvases prepared with tones of a light red hue, which he retained as the basis of his picture, he sketched his figures and backgrounds with light brushes and velatures, retaining, where possible, the tone of the ground. This light touch, rendered necessary by the extensive character of the design and the rapidity with which it had to be executed, gave to the artist a freedom and quickness in all he drew, and from it his later works, much more important than these early essays though they were, profited not a little. Already during these earlier years he had commenced to paint portraits which did much to enhance his reputation, and shortly afterwards he entered the royal service as first painter to the Court, where he addressed himself to the execution of that vast collection of works of all kinds which arouse such interest to-day. The list is interminable and embraces the portraits of Carlos IV and of the Queen Maria Louisa, those of the members of the Royal Family, of all the aristocracy, of the Albas, Osunas, Benaventes, Montellanos, Pignatellis, Fernán-Núñezs, the greatest wits and intellectuals of the day, especially those of Jovellanos, Moratin, and Meléndez Valdés, three men who profoundly influenced the thought of Goya in a progressive and almost revolutionary manner, in spite of his connection with the Court and the aristocracy. He also painted many portraits of popular persons, both men and women, among whom may be mentioned La Tirana, the bookseller of the Calle de Carretas, and that most mysterious and adventurous of femmes galantes of whom, now clothed, now nude, the artist has bequeathed to us those souvenirs which hang on the walls of the Prado Museum. In these the artist has for all time fixed and immortalized the finest physical type of Spanish womanhood, in which an occasional lack of perfect proportion is compensated for by elegance, grace, and unexaggerated curve and figure, without doubt one of the most exquisite feminine types which has been produced by any race. Besides these, the artist produced many lesser canvases containing tiny figures full of wonderful grace and gallantry, and having rural backgrounds, frequently of the banks of the Manzanares, and others of larger proportions and scope, among the most excellent of which is
- 53. that of the family of Carlos IV, treasured in the Prado Museum as one of its most precious jewels. Along with The Burial of the Count of Orgaz (Plate V.) and Las Meninas (Plate X.), this picture may be regarded as the most complete and astonishing which Spanish art has given us. It is not a “picture” in the ordinary sense of the word, but an absolute solution of the problem of how colour harmonies are to be attained, and a most striking essay in impressionism, in which an infinity of bold and varied shades and colours blend in a magnificent symphony. Goya, triumphant and rejoicing in a life ample and satisfying, received on all sides the flatteries of the great, and, caressed by reigning beauties, lived in the tranquil pursuit of his art, which, though intense, was yet graceful and gallant, and, as we have said, still adhered to the manner of the eighteenth century, when a profound shock agitated the national life—the war with Napoleon and the French invasion. The first painter to the Court of Carlos IV, a fugitive, deaf, and already old, life, as he then experienced it, might have seemed to him a happy dream with a terrible awakening. His possessions, his pictures, and his models were dispersed and maltreated; the Court seemed to have finished its career, for his royal master was banished by force, many of the nobility were condemned to death, and Countesses, Duchesses and Maids of Honour vanished like the easy and enjoyable existence he had known. Above all, Saragossa, that heroic city, beleaguered on every side, was closed to him; a depleted army defended the strategical points of the Peninsula, and the people—the people whom Goya loved and who had so often served him as models for his damsels, his bull-fighters, his wenches, his little children—were wandering over the length and breadth of Spain, only to be shot as guerillas and stone-throwers by the soldiers of Napoleon. It was at this moment that the true development of the artist began. The painter, like his race, was not to be conquered. The old Goya remained, strong in the creation of a lofty art. The last twenty years of his life were full indeed, and represented its most vigorous phase, the most energetic in the whole course of his achievement. Scenes of war and disaster occupied almost the whole of this important period, full of a profound pessimism, which still does not lack a certain graceful style, and displays unceasingly some of the saddest thoughts which man has ever known. These works of Goya are not of any party, are not political nor sectarian. They are simply human. For his greatness is all-embracive and his might enduring. Typical of his work in this last respect are The Fusiliers, of 1808, and his
- 54. lesser efforts, those scenes of brigandage, madness, plague and famine which occur so frequently in his paintings during the years which followed the war. We do not mean to make any hard and fast assertion that Goya would not have developed in intensity of feeling if he had not personally experienced and suffered the horrors of the invasion, but merely to indicate that it was this which brought about the revulsion within him and powerfully exalted him. His last years in Madrid, and afterwards in Bordeaux, where he died, were always characterized by the note of pessimism, and at times, of horror, as is shown in the paintings which once decorated his house and are now preserved in the Prado Museum. Not a few portraits of these years also show that the artist gained in intensity and in individual style. It is precisely these works, so advanced for their time and so progressive, that provided inspiration to painters like Manet, who achieved such progress in the nineteenth century, and who were enamoured of the visions of Goya, of his technique and his methods, naturalistic, perhaps, but always replete with observation and individual expression. We must not forget to mention that Goya produced a decorative masterpiece of extraordinary distinction and supreme originality—the mural painting of the Chapel of St. Antonio of Florida, in Madrid. Nor is it less fitting to record his fecundity in the art of etching, in which, as in his painting, it is easy to observe the development of their author from a style gallant and spirited to an interpretation of deep intensity, such as is to be witnessed in the collection of “The Caprices” and “The Follies,” if these are compared with the so-called “Proverbs” and especially with “The Disasters of War.” The pictures representing Goya at Burlington House were composed of some twenty works. Among those which belonged to his first period were the portraits of the Marchioness of Lazan, the Duchess of Alba, lent by the Duke of Alba, “La Tirana,” from the Academy of St. Fernando, the Countess of Haro, belonging to the Duchess of San Carlos, four of the smaller paintings of rural scenes, the property of the Duke of Montellano, and An Amorous Parley (“Coloquio Galante”), the property of the Marquis de la Romana, the prototype of the Spanish feeling for gallantry in the eighteenth century. As representative of the second phase, of that which holds a note intense and pessimistic, may be taken A Pest House, lent by the Marquis de la Romana, and those truly dramatic scenes, the property of the Marquis of Villagonzalo.
- 55. Of portraits of the artist by himself two were exhibited, one small in size painted in his youth (Plate XXVI.), in which the full figure is shown, and the other a head, done in 1815, which gives us a good idea of the expression and temperament of this extraordinary man. The influence of the art of Goya was not immediate. A contemporary of his is to be remembered in Esteve, who assisted him and copied from him. Later, an artist of considerable talent, Leonardo Alenza, who died very young and had no time to develop his art, was happily inspired by him. With regard to Lucas, a well-known painter whose production was very large, and who flourished many years later, and is now known to have followed Goya, he can scarcely be considered as one of his continuators, but rather as an imitator—by no means the same thing. For he imitated Goya, as, on other occasions, he imitated Velázquez and other artists. Lucas is much more praiseworthy when he follows his own instincts and does original work. His picture The Auto de Fé, the property of M. Labat, which was shown at the London exhibition in the room dedicated to artists of the nineteenth century, is one of the best that we know of from his brush. . . . . . . . . . . . . . . . If the eighteenth century was for Spanish painting an epoch of external influences, the nineteenth century, especially its second half, must be characterized as one which sought for foreign direction. During this period the greater number of painters of talent sought for inspiration from foreign masters. This was a grave mistake, not because in Spain there were artists of much ability or even good instructors, but because this exodus of Spanish painters was a sign that they had lost faith and confidence in themselves and were strangers to that native force which in the end triumphs in painting as in everything else. First Paris, then Rome, the two most important centres of the art of this period, were undoubtedly centres of a lamentable distortion of Spanish art. The organizing committee did not wish the London exhibition to be lacking in examples of this period of prolific production, to which they dedicated a room in which were shown examples of the painters of the nineteenth century. We mention some of the many artists of talent of the Spain of those days, and indicate their individual characteristics; but we are unable to allude
- 56. to their general outlook and the characterization of their schools, which we do not think existed among them to any great extent. The most famous painter who succeeded Goya was Vincente López, better known for his portraits than for his other canvases, a skilful artist with a perfect knowledge of technique, conscientious, fecund, minute in detail, who has left us the reflection of a whole generation. Classicism arrived in Spain with all the lustre of the triumphs of Louis David, under whose direction José de Madrazo placed himself, the first of those artists of this type to maintain a position of dignity throughout three artistic generations. He held an important place among contemporary painters at a difficult time during which, in consequence of the political disorder which reigned, the commissions usually given by the churches and religious communities ceased, private persons acquired few paintings, and the academies decreased in the number of their students. It was a time in which art offered but little wherewithal to its votaries. But this period of paralysis was of short duration. The pictorial temperament, which inalienably belongs to Spain, and the appearance of romanticism, with a tendency conformable to the spirit of Spain, and which had for a long time given a brilliant impulse to her men of letters, revived painting, which forgot its period of exhaustion. The frigid classicism, ill- suited to the national genius, now passed away. José de Madrazo was succeeded in prestige and surpassed in ability by his son Federico de Madrazo. By his portraits he has bequeathed to us faithful renderings of all the personages of his day, which compete with those of the greater foreign portrait painters among his contemporaries. Studying at first under classical influences, but regarded as romantics in their later development, were remarkable portrait painters like Esquivel and Gutiérrez de la Vega, and a landscape painter of especial interest, Pérez Villamil, who may in a manner be compared to the great English landscape painter Turner, though he had no opportunities for coming in contact with him or any knowledge of his work. Both men, each in his own environment, breathed the same atmosphere; and, although reared in lands remote from one another, thought in a like manner because they both reflected the period in which they lived. Becquer and others adequately maintained the descriptive note which now entered into the making of popular subjects.
- 57. Such was the condition of painting in Spain when there appeared the fruitful and extraordinarily popular genre of historical painting. In its origin it was not Spanish but was introduced from other countries, especially from France; but its Spanish affinities are manifest in its examples, most of which are canvases of great size, imposing, dramatic, and, in general, effective. . . . . . . . . . . . . . . . In this period culture, which in Spain had formerly been the preserve of a limited class, now spread itself more widely, and in the sphere of art was greatly fostered by exhibitions of painting, open to all and sundry, without distinction of social status. Pictures and sculpture, which in other times had been dedicated solely to art and to religious piety, the possessions of kings and grandees, now came into public view, were alluded to in publications of all kinds, and the people, enthusiastic and critical, were brought face to face with their native art. Many artists, perceiving this, hoped to gain popular applause, and consequently worked upon their subjects as seemed most agreeable to the masses. The historical picture in such circumstances seemed to offer the greatest possibilities for achieving a popular reputation. Gisbert painted the popular heroes of the past and was regarded as the representative of those revolutionary tendencies in art which were to triumph several years later. Alisal, Mercade, Palmaroli, Luis Alvarez, careful and excellent artists, painted both historical and genre pictures. From this group arose a most remarkable figure who died whilst still very young, but who has left us a most striking example of his workmanship. This was Eduardo Rosales, the painter of The Death of Isabel the Catholic. Rosales represented the Spanish tradition in painting. Averse to foreign influences, he studied and found in the great masters the sources of his art, and his works, both in Spain and beyond it, excited the greatest interest in his time. The picture above mentioned, sober and simple in style, though it must be classed as genre painting, has still many admirable and enduring qualities. The pity is that this group of artists did not follow him; for, flattered by the public acclamation, they entered upon the second period of historical painting, less effective than the first and always conventional, which lasted many years, indeed almost to the present time. For an atmosphere inimical to the traditions of Spanish painting arose, in which this type of historical composition flourished at a time when it had been condemned and forgotten
- 58. in other countries, where it was forced to give place to those tendencies in which modern painting had its origin. Rigurosamente, a contemporary of Rosales, was another exceptional artist of unusual gifts, likewise Mariano Fortuny, who unfortunately died in his youth. Fortuny, though he may appear quite otherwise to-day, was in his own time considered a progressive innovator. When he visited Madrid for the first time, drawn thither by youthful enthusiasm, he did so with no other idea than that of copying from Velázquez. But seeing in the Prado Museum the works of Goya, which were totally new to him, he received a revelation. He copied from Goya, and later, going to Africa, he painted many studies and pictures replete with light. Light as a pictorial factor, as an element in a picture, the study of light, the reflection of it in his own works—that is the progressive element which we find in Fortuny. The rapid success of his first works, their triumph in Paris and Rome, was due to an agreeable style, gracious in touch, suggestive, which appealed to collectors and dealers. At the same time we do not believe this to have been altogether his ideal, since a few years before his death, which took place in his thirty-seventh year, we see him betaking himself to the shores of Italy, where he made new studies of light and air. Was it reserved to Fortuny to be one of those of whom it will be said that he assisted the development of the study of atmosphere and light? We firmly believe this to be so, but the work of the critic has nothing to do with prophecy, and we must deal only with that which Fortuny has left us, which is indeed sufficient. It must not be forgotten in judging his work to-day that its defects, or what seem to be its defects, were those of his time and were not personal, and that what is personal to him was his good taste, his mastery, and a series of innovations and bold essays in colour obvious to those who study his works. Fortuny was not a Spanish painter in the sense that he did not preserve the traditions of our School. He certainly took the elements of his palette from Goya, but his traits of manner show no sign of the typical qualities of Spanish painting. It is fitting to allude here to artists of different types and talents in some of the cities of Spain, and others living abroad, who laboured during the last years of the nineteenth century—the Madrazos, Raimundo and Ricardo, sons of Don Federico de Madrazo, who studied under the direction of Fortuny; Plasencia, Domínguez and Ferrán, who distinguished themselves in work of a decorative character in the Church of Saint Francisca the Great in Madrid; Pradilla and Villegas, who have obtained the greatest triumphs during a long
- 59. career; the brothers Mélida, Enrique and Arturo, the first working in Paris for many years, and the second a famous decorative artist; Egusquiza, painter and engraver; Moreno Carbonero, who, more a historical and portrait painter, found a popularity for his pictures inspired by episodes in literature, especially those of Quixote, in which he has coincided with Jiménez Aranda. We may also mention a group of artists, all of Valencia, a city which in times past, as in the present, enjoyed notable artistic prosperity: Sala, Muñoz Degrain, Pinazo Camarlench, José Benlliure and many others. Nearly all of them were represented at the Exhibition at Burlington House in the Salon set apart for the painters of this epoch. . . . . . . . . . . . . . . . In the second half of the nineteenth century the study of nature in the form of landscape arose as a creed, the artist coming face to face with the scene which he desired to transfer to his canvas. It has been said “what the landscape is, so is he who praises it.” Until then the landscape had been nothing but a background for a composition or figure, and those who called themselves landscape painters, when they undertook to paint a scene used it as a peg on which to hang poetical ideas, embellishing it, but never treating it as a true rendering of nature. Now the artist came to the country, felt the influence of nature, and faithfully copied it. The object of his work was to be as natural as possible, without embellishing or poetizing his subject, but to portray it, as one might say. This was a new idea to the painters of the time. Pérez Villamil, a follower of romanticism in painting, also practised landscape art in Spain until it underwent the change mentioned above through the arrival of a Belgian, Charles de Haes, who succeeded Pérez Villamil as professor of landscape at the School of Painting. Haes broke with tradition. He would have no conventionalisms, no studied compositions, nor preconceptions. He took his pupils to the country and there told them to copy Nature herself, leaving them without any further inspiration than that with which God had endowed them. To-day the studies of this master and of his disciples, generally executed in strong contrasts of light, seeking, doubtless, the effectiveness thus produced, appear to us, although they have a sense of luminosity, poor in colour, obscure and hard. But what progress is represented in them in comparison with all former art! And it is clear that
- 60. they express the tendency which, modern in that time, everywhere governed the advance of art. Shortly afterwards a Spanish landscape painter, not a disciple of Haes, Martín Rico, a companion of Fortuny, but who, having lived longer than he and reached a more mature age, advanced a further step in the art of landscape painting. If the chief aim of this painter had not been the rapid translation of his gifts into money, and had he not striven to please the public, he might have achieved lasting fame. Casimiro Saiz, Muñoz Degrain—whom we have mentioned already as a painter of the figure—Urgell, Gomar and others devoted themselves to landscape; but the most salient examples of Spanish landscape painting are to be found in the work of three artists who developed with the rapid evolution of their time—Beruete, Regoyos and Rusiñol. Of these three sincere and individual painters, Beruete, in his youth a disciple of Haes, and later of Rico, evinced a very decided modern tendency. He devoted the years of his maturity to the making of a large number of pictures of Spanish cities, especially of Castile, paintings truthful and sincere in character, and revealing a very personal outlook. Regoyos was influenced by impressionism, to which he was strongly attracted, and in the North of Spain he inspired many by his numerous works. Rusiñol is, perhaps, more a poet than a painter. He still lives and works. He used to find in the gloomy and deserted gardens of Spain subjects for his pictures. One of the most remarkable figures in Catalonia to-day, both as a litterateur and painter, he has also sought inspiration in the scenes and countryside of this, his native province. . . . . . . . . . . . . . . . Spanish painting was completely modernized during the last years of the nineteenth century. Three great international events took place during that period—the three exhibitions in Paris of the years 1878, 1889 and 1900. At these Spanish painting was fully represented. At the first was shown a varied collection of the works of Fortuny—one of the most famous artists of his time—who had died shortly before. In the second we experienced a rebuff, for a number of historical paintings of enormous proportions, full of the inspiration of the past, were not admitted, nor, indeed, were some of these worthy to hang in the exhibition. But in the years between 1889 and 1900
- 61. the development of Spanish painting was most marked, and in the last of the exhibitions alluded to the Spanish salons revealed a high level of excellence and a significant modernity. Moreover, there emerged the personality of a young painter, hitherto unknown, who by unanimous consent was regarded as well-nigh qualifying for the highest honours. This was a man whose name shortly afterwards became famous throughout the world—Joaquín Sorolla, one of those personalities who from time to time arise in Spain quite unexpectedly. Sorolla, who was of humble origin, was born in Valencia, and in his youth was naturally influenced by the paintings of the old masters in his native city. He went to Madrid, later to Italy, and finally to Paris, where his work of a wholly realistic character was admired, for actuality was to this painter as the breath of life. A French advocate of naturalism has said “one rule alone guides the art of painting, the law of values, the manner in which the light plays upon an object, in which the light distributes colour over it; the light, and only the light is that which fixes the position of each object; it is the life of every scene reproduced in painting.” This statement Sorolla seems to have taken greatly to heart, even while he was still under the influence of old traditions and standards of thought. Possessing a temperament of much forcefulness, and of great productive exuberance, enthusiastic about the scenery of the Mediterranean, and especially enamoured of the richness of colour of his native soil, the ruddy earth planted with orange-trees, the blue sea and the dazzling sky, Sorolla, oblivious of what he had done before, felt a powerful impulse to paint that which was rich in colour, so greatly was he moved by the eastern spirit. The coasts of Valencia, the lives of the fishermen, those children of the sea, the bullocks drawing the boats, the scenes beneath the cliffs and other analogous subjects, painted in full sunlight—the sunlight of July and August for preference—these are the subjects on which Sorolla laboured for several years, producing canvas after canvas, now famous both in Europe and America. We do not say that this outlook is ideal, but the study of light and atmosphere was a contribution to the history of modern art, and was among the elements which will be handed down to posterity as the original note of the painters of the last years of the nineteenth century. Of these Sorolla was one of the most forceful, and we lay stress upon his work, as in our judgment
- 62. its importance demands especial notice. We have not alluded to his great talent as a portrait painter, nor to the decorative works which he has dedicated to the Hispanic Society of America in New York, and which, although they are completed, are not yet installed in place. Some few years after the appearance of Sorolla, there arose almost simultaneously two Spanish painters of other tendencies, equally noteworthy, and whose names are universally known—Zuloaga and Anglada. Zuloaga must be regarded in a very different manner from Sorolla. In no sense does he go to nature merely to copy it in the manner in which it presents itself to our vision, but he seeks, both in nature and humanity, for types, for characteristic figures of a representative and realistic kind. His work has developed with robustness and force, and attracts the attention of the modern critic eager for characteristic and singular qualities. To his reception in the universal world of art it is not necessary to allude here. The reviews and periodicals of all countries have commented with praise upon the achievements of this master, who is still busily at work, constantly engaged in the representation of popular types in the characteristic costume of many regions, especially his own people, the Basques, and the Castilians, for whom he appears to have a special predilection. Those landscapes which he takes for the backgrounds of his pictures also seem to be inspired by that love of character which animates all his productions. In his latest phase, too, he has executed numerous portraits of people of different social categories. In technique it is noticeable that Zuloaga strives to preserve those tonalities which characterize the Spanish School; and the study he has made of the works of Velázquez and Goya is manifested in the lively reminiscences of these masterpieces displayed at times in his pictures, which exhibit, nevertheless, a relative modernity. Anglada is, in our view, completely distinct from Sorolla and Zuloaga. Enamoured of the charm of colour, his work has no connection with schools or traditions. Aloof from every influence, he aspires to nothing so much as rich colour-schemes and harmonies, and seeks inspiration in night-bound gardens, brightly illuminated, in subjects which reflect electric light, and in figures which appear all the more distinct as the background is often the sea beneath the radiance of the Mediterranean light. These unusual sources of inspiration appear strange at first sight; but it is noticeable that they manifest on the part of the painter always the same idea of seeking for rich colouring. We must regard Anglada as one of the most remarkable and most original of
- 63. modern painters. It is a great pity that he was not represented at Burlington House. His absence, like that of Sert, the great decorative painter, Beltran, Miguel Nieto and others, was accounted for by the fact that the pictures were received too late to be included in the Exhibition. . . . . . . . . . . . . . . . The salons set apart for modern painting at the London Exhibition seem to us to have been disposed and arranged with care. There were shown in the first of these rooms works by Sorolla, his disciple Benedito, one of the most esteemed portrait painters in Madrid, Zaragoza, Moisés, Carlos Vázquez, and some landscapes by Rusiñol. The second room was in complete harmony with the first, and in it we observed the works of artists, some of whom are still young, but nevertheless masters of strong propensity and perfect equilibrium; the great composition by Gonzalo Bilbao, The Cigar- makers (Plate XXXVII.); the striking portraits of Chicharro and Sotomayor; the unmistakably Spanish canvases of Mezquita and Rodriguez Acosta; and the picturesque and suggestive note of the Valencian figures by Pinazo Martinez. The neighbouring room was dedicated to those who may be called painters of character, for such was the exclusive note of all the works shown there. It would not be easy to say who occupied the place of honour here, Zuloaga, Romero de Torres, an artist of Cordova, who has tried to create a type of female beauty famous throughout Spain, the brothers Zubiaurre, peculiarly Basque in feeling, and now well known everywhere, Salaverria, Ortiz Echagüe, Arrúe, Juan Luis y Arteta, a delicate and emotional painter who has found on the Basque shores subjects for pictures unusually simple, in which is displayed a delicacy of technical expression together with the significance of an idea, inspired, like his subjects, by a simple poetry. Following these, in still other rooms, were hung works similar in type, but bolder, perhaps, such as those of Solana, whose three canvases, painted in low tones, were of great interest and excited much remark in the exhibition; Vázquez Díaz, so various in his subjects, but always individual; Maeztu, the consistent exponent of a colossal and decorative style; Castelucho, Urgell, Guezala; and Astruc y Sancha, who combines caricature of consummate mastery with the painting of landscapes of manifest originality.
- 64. In another room were exhibited smaller landscapes. These included examples of Rusiñol, Beruete, Regoyos, Meifren, Forns, Raurich, Colom, Grosso and Mir. Among the work of other young painters of promise but as yet little known, we must mention the seascapes of Verdugo Landi and Nogue. The next salon, known as the Lecture Room, formed a kind of overflow for the last, and contained pictures by Hermoso, Garnelo, Simonet, Morera, Marin Bagües, Canals, Cardona, Villegas Brieva, Oroz, Madrazo-Ochoa, Covarsi, Bermejo, and many other artists, a list of whom would be much too extensive for inclusion here. . . . . . . . . . . . . . . . We do not think that the assertion that Spanish painting has been a powerful factor in the history and development of universal art will be regarded by anyone as a discovery, nor will such a statement appear as a result of patriotic enthusiasm. Spanish painting to-day follows its brilliant traditions; and although we believe this present period to be one of gestation, it occasionally reveals qualities of splendour and greatness. It is indubitably lacking in marked and decided outlook, but it is, nevertheless, universally respected and suffers, at the most, merely from the exigencies of the time. Moreover, not a few critics of distinction in the Peninsula, who concern themselves with the study of particular movements, see in it a tendency to the formation of regional groups. The central one naturally has its focus in Madrid, and radiates thence over the whole of Spain; but a large output is always forthcoming from the cities of Seville and Valencia, which appear, by the light of tradition, as the most brilliant centres of pictorial art. There are, moreover, two other regions which have produced rich and flourishing art— Catalonia and the Basque provinces, with their two capital cities, Barcelona and Bilbao. Catalan art is no new thing in Spanish tradition, and is in a measure descended from that which was formerly the art of the Kingdom of Aragon before the national union. The Catalans have confined it entirely to their territory, have cultivated it with enthusiasm, and have created a Catalan school of Spanish Art. It is a great pity that they have not tried to preserve a more national spirit and have frequently sought inspiration from foreign
- 65. sources, especially from France. But, this notwithstanding, Catalan achievement is indeed most worthy of praise. The artistic production of the Basque provinces is forcible and original. The Basques, with a scanty pictorial tradition, have shrewdly sought for inspiration in the Spanish sphere without distinction of locality, and have produced an art of undoubted interest. But apart from this there exists at the present time a movement of worldwide character, which seems to have a literary origin and which may, perhaps, be called, for want of a better name, the new spirit. Though still in a chaotic state, this movement, varied in its aspects, may in all lands be identified by an underlying intention to revolutionize everything, creating a new æsthetic code and turning its back on the past and on all tradition. It is not our intention to deal with this movement or to discuss its importance. Spain does not appear to be the country best fitted to lead it. Its history seems to show that while it is ready of acceptance, it is not to be hurried in its advance; nor is it eager to seize upon radical ideas. But this notwithstanding, it has painters who understand and cultivate art of this kind, and it must not be forgotten that one of the outstanding figures in the ultramodern movement is the Spaniard Picasso, who has shown once more that in all phases of artistic effort the Spanish temperament significantly reveals itself. A. de Beruete y Moret. (Translated by Lewis Spence)
- 66. PLATE I
- 68. PLATE I YAÑEZ DE LAALMEDINA (Collection of the Marquis de Casa-Arquedin, Madrid) “ANTA CATALINA” (“SAINT CATHERINE”)
- 69. PLATE II
- 71. PLATE II PANTOJA DE LA CRUZ (Collection of H.M. The King of Spain) “PHILIP II”
- 72. PLATE III
- 73. PLATE III EL GRECO (Collection of H.M. The King of Spain) “LA GLORIA DE FELIPE II” (“THE ‘GLORY’ OF PHILIP II”)
- 75. EL GRECO (Provincial Museum, Toledo) “SAN PABLO” (“SAINT PAUL”)
- 76. PLATE V PLATE V EL GRECO
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