Overview of nanomaterials - Nanoscience and nanotechnologies

NANOYOU Teachers Training Kit in Nanotechnologies

Chapter 5‐ Overview
of Nanomaterials
Module 1‐ Fundamental concepts in nanoscience and nanotechnologies

Written by Luisa Filipponi and Duncan Sutherland
Interdisciplinary Nanoscience Centre
Aarhus University, Denmark
January 2010

Creative Commons Attribution ShareAlike 3.0 unless indicated in text or figure captions

NANOYOU Teachers Training Kit – Module 1– Chapter 5

Contents
Biomimetic nanomaterials .............................................................................................................................................
5

Gecko‐inspired adhesive (or Bio‐rubber) ...................................................................................................................
5

Self‐healing adhesives ...............................................................................................................................................
6

Biomimetic membranes, capsules and bioreactors ...................................................................................................
6

Biomimetic energy nanomaterials .............................................................................................................................
7

Self‐assembled nanomaterials .......................................................................................................................................
7

Liquid crystals ............................................................................................................................................................
8

Liquid crystals´ phases and their properties ........................................................................................................ 0
1

What is special about liquid crystals ....................................................................... Error! Bookmark not defined.

LCs applications ................................................................................................................................................... 1
1

Nanostructured Metals and Alloys .............................................................................................................................. 2
1

Metal nanoparticles ................................................................................................................................................. 2
1

Plasmonic structures ............................................................................................................................................ 2
1

Reinforcements .................................................................................................................................................... 4
1

Environmental applications ................................................................................................................................. 4
1

Nanocrystalline metals ............................................................................................................................................ 4
1

Ferrofluids ................................................................................................................................................................ 5
1

Polymers ...................................................................................................................................................................... 6
1

Conductive Polymers ............................................................................................................................................... 6
. 1

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The research leading to these results has received funding from the European Community's Seventh Framework Programme
(FP7/2007‐2013) under grant agreement n° 233433


Block Copolymers ..................................................................................................................................................... 6
1

Responsive “smart” polymers .............................................................................................................................. 8
1

Biomedical applications ....................................................................................................................................... 8
1

Nano‐reactors ...................................................................................................................................................... 8
1

Artificial moving parts ......................................................................................................................................... 9
1

Other applications ............................................................................................................................................... 9
1

Polymeric nanofibres ............................................................................................................................................... 9
1

Semiconductors ........................................................................................................................................................... 0
2

Quantum dots .......................................................................................................................................................... 1
2

Semiconducting oxides ............................................................................................................................................ 1
2

Titanium dioxide .................................................................................................................................................. 1
2

Zinc dioxide .......................................................................................................................................................... 2
2

ITO .......................................................................................................................... Error! Bookmark not defined.

Photonic crystals ...................................................................................................................................................... 4
2

Ceramic and glassy materials ...................................................................................................................................... 5
2

Porous alumina ........................................................................................................................................................ 5
2

Zeolites .................................................................................................................................................................... 5
. 2

Aerogels ................................................................................................................................................................... 6
2

Carbon‐based materials............................................................................................................................................... 6
2

Carbon nanotubes ................................................................................................................................................... 8
2

Mechanical properties ......................................................................................................................................... 9
2

Electrical and thermal properties ........................................................................................................................ 0
3
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Chemical reactivity .............................................................................................................................................. 1
3

Applications ......................................................................................................................................................... 1
3

Composites .................................................................................................................................................................. 1
3

Inorganic nano composites ...................................................................................................................................... 2
3

Structural nanocomposites .................................................................................................................................. 3
3

Nanocomposites with enhanced magnetic properties ........................................................................................ 4
3

Polymer Nanocomposites ........................................................................................................................................ 5
3

Nanoparticle‐ polymer nanocomposites .............................................................................................................. 7
3

Carbon nanotubes in polymer composites .......................................................................................................... 8
. 3

Polymer‐clay nanocomposites ............................................................................................................................. 9
3

Nanocoatings ............................................................................................................................................................... 1
4

Tribological nanocoatings ....................................................................................................................................... 2
. 4

Responsive nanocoatings ........................................................................................................................................ 3
4

Self‐cleaning surfaces .......................................................................................................................................... 4
4

This document has been created in the context of the NANOYOU project (WP4 Task 4.1). All information is
provided “as is” and no guarantee or warranty is given that the information is fit for any particular purpose. The
user thereof uses the information at its sole risk and liability. The document reflects solely the views of its

authors. The European Commission is not liable for any use that may be made of the information contained

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Chapter 5 Overview of Nanomaterials
This chapter provides an overview of nanomaterials, their properties and functions. The application
areas of the cited nanomaterials are also mentioned. The second Module of this Teachers Training Kit is
devoted to applications of nanotechnologies.

Biomimetic nanomaterials
The first class of engineered nanomaterials that we review is Biomimetic materials. As mentioned in
Chapter 2 of Module 1 (“Natural Nanomaterials”), Nature is the best nanotechnology platform.
Through thousands of years of evolution Nature has developed an enormous array of materials, ranging
from feathers, to shells, wood, bone, and many more that have intricate hierarchical structures at the
nano, micron and macro level which confer specific properties to the material, such as strength,
lightweight, permeability, colour etc. Natural materials provide an amazing platform for inspiring
material engineers to fabricate advanced materials that posses specific functions. As a matter of fact
numerous “macro” materials we use today were developed after inspiration from natural materials. One
example is Velcro, which was developed in 1948 by a Swiss engineer named George de Mistral who got
inspired by the clinging mechanism of cockleburs to dog hair and to fabric. Often the nanostructure in
the natural material plays a crucial role, which inspires scientists to mimic them starting from a
molecular level (molecular biomimetics). This is also called biomimetic nanotechnology. Here we
provide some examples of those biomimetic nanomaterials.

Geckoinspired adhesive (or Biorubber)
In Chapter 2 we discussed the adhesive properties of the Gecko foot, and how these are not related to a
glue in the foot, but rather to van der Waals and capillary forces exerted by million of nanostructures
(called saete) that make up the foot. This allows the animal to walk upside down, against gravity, on
many different surfaces, even on wet ones. Moreover a Gecko can walk on a dirty surface without losing
adherence, since its feet is also self‐cleaning. A truly amazing material! Scientists have been inspired by
this animal to design and fabricate adhesives for numerous applications. For instance a group of
researchers at the University of California, Berkeley, has developed adhesive gecko‐like foot surfaces for
use in climbing robots. The adhesive is made of patches of microfiber arrays with 42 million
polypropylene microfibers per cm2. The patches can support up to 9 N cm‐2: a patch 2 cm2 can support a
load of 400 g. This result is very close to the loads supported by a gecko, which are about 10 N cm‐2. This
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gecko‐like adhesive is very similar in functionality to the natural gecko foot, yet not as good. Researchers
still have to make it topography independent (capable of attaching to any surface) and self‐cleaning. In
another university a biomimetic gecko tape has been produced using polymer surfaces covered with
carbon nanotubes hairs, which can stick and un‐stick on a variety of surfaces, including Teflon.

Other gecko‐inspired nanomaterials are under development for medical applications, which
will be covered in Chapter 1 of Module 2 (“Applications of Nanotechnologies: Medicine &
Healthcare”).

Selfhealing adhesives
Diatoms are a type of algae with nanostructured amorphous silica surfaces. Some diatom species have
evolved strong, self‐healing underwater adhesives. Some are free‐floating; others have adhesive
properties in water: for instance diatoms in the Antarctic seas can attach to ice. Others secrete viscous
mucilage which binds colonies together while protecting the silica shells from wear as they rub against
each other. Molluscs are another species of animals that have adhesive properties under water. Both
diatoms and molluscs have strong underwater glues that can also resist stress and self‐heal if necessary.
For these reasons they serve as a biomimetic model for self‐healing materials. Researchers have studied
those natural adhesives and found that their self‐healing properties are due to the properties of the
proteins that compose them. These proteins have “sacrificial” bonds that allow the molecule to be
reversibly stretched by their breaking and re‐bonding. The sacrificial bond behaviour has been observed
in many other materials such as wool. The detailed analysis of these natural materials is inspiring new
nano‐adhesives with self‐healing properties.

Biomimetic membranes, capsules and bioreactors
The bilipid membrane has served as a biomimetic model for decades. A simple example is liposomes
(lipid vesicles) which are easily formed by shaking vigorously oil in water. Planar supported bilayers are
also inspired by the lipid membrane and are formed by simply “dipping” a suitable substrate inside an
organic aqueous phase.

Inside the cell there are numerous self‐assembled structures that encapsulate specific
biomolecules and release them as a consequence of molecular signalling. These structures
are inspiring a wide range of nanocapsules for application in drug delivery. In some cases
bacteria or viruses are used as templates for fabricating these nanocapsules. These

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nanomaterials are described in more details in Chapter 1 of Module 2 (“Applications of
Nanotechnologies: Medicine & Healthcare”).

Biomimetic energy nanomaterials
Many challenges that we are now facing in the energy area (improvements needed in solar panels,
hydrogen fuel cells, rechargeable batteries etc.) can be solved through the use of nano‐engineered
materials. Some of these materials are developed through direct inspiration from Nature, such as the
new types of solar photovoltaic cells, which try to imitate the natural nanomachinery of photosynthesis.
Another interesting example is that of using battery electrodes with self‐assembling nanostructures
grown by genetic engineered viruses.

Both examples will be described in more details in Chapter 3 of Module 2 (“Applications of
Nanotechnologies: Energy”)

Selfassembled nanomaterials
The concept of self‐assembly derives from observing that, in natural biological processes, molecules self‐
assemble to create complex structures with nanoscale precision. Examples are the formation of the DNA
duple helix or the formation of the membrane cell from phospholipids. In self‐assembly, sub units
spontaneously organize and aggregate into stable, well defined structures through non covalent
interaction. This process is guided by information that is coded into the characteristics of the sub units
and the final structure is reached by equilibrating to the form of the lowest free energy. An external
factor, such as a change in temperature or a change in pH, can disrupt this organization. For instance, a
protein self‐assembles in a specific structure, but if exposed to conditions such as high heat or high
acidity, it can denature, which means that its structure is damaged, and the protein unfolds. This means
that the protein looses its function as its structure is damaged. So in Nature self‐organized structures
have specific functions.

Molecules in Nature change conformation and move from one self‐organized structure into another as
they bind to certain ions or atoms. A lot of examples can be given, like hemoglobulin (that captures and
releases an iron ion), or the potassium‐sodium pump, chlorophyll etc.

The use of self‐assembly to create new materials is a bottom‐up approach to nanofabrication, and is
thus an essential tool in nanotechnology (see Chapter 7 of Module 1 “Fabrication techniques”).  Instead

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of carving nanostructures out of larger materials (which is the typical top‐down approach, like
micromachining and microlithography, used to fabricate integrated electronic circuits), nanostructures
are created from the bottom, from atomic building blocks that self‐assemble in larger structures. In the
laboratory scientists can make use of this self‐organization of matter as a way of programming the
building of novel structures with specific functions.

Liquid crystals
A liquid crystal is a fourth state of matter: it has
property between those of a conventional liquid
and those of a solid crystal. Like a liquid, it flows,
and like a crystal, it can display long‐range
molecular order (Figure 1). In terms of
classifications, LCs (together with polymers and
colloids), are often classified as “soft matter” Figure 1. The liquid crystal phase is in between a solid
and a liquid phase (Image credit: L. Filipponi, iNANO,
(Figure 2) and treated under the branch of
Aarhus University, Creative Commons Attribution
physical chemistry of condensed matter.
ShareAlike 3.0).

Figure 2. Diagram showing
where the branch of "liquid
crystals" fit into the
classification of physical
chemistry of condensed
matter.



A fascinating and characteristic feature of liquid‐crystalline systems is that they change their molecular
and supermolecular organization drastically as an effect of very small external perturbations: The
molecules in liquid crystal displays for instance are reoriented by relatively weak electrical fields.  If one

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dissolves a small amount of chiral molecules in an achiral liquid‐crystalline host phase, this results in
remarkable macroscopic chirality effects, ranging from helical superstructures to the appearance of
ferroelectricity.

Liquid crystal selfassembly
Liquid crystals are partly ordered materials, somewhere between their solid and liquid phases. This
means that LCs combine the fluidity of ordinary liquids with the interesting electrical and optical
properties of crystalline solids.

The molecules of liquid crystals are often shaped like rods or plates or some other forms that encourage
them to align collectively along a certain direction (Figure 5).

Figure 5. Example of the self‐organization of
anisometric (i.e., with asymmetrical parts)
molecules in liquid‐crystalline phases. On the
left: rod‐like molecules form a nematic liquid,
in which the longitudinal axes of the
molecules are parallel aligned to a common
preferred direction ("director"). On the right:
disc‐like (discotic) molecules arrange to
molecule‐stacks (columns), in which the
longitudinal axes are also aligned parallel to
the director. As a result of their orientational
order, liquid crystals exhibit anisotropic
physical properties, just like crystals.
(Reprinted from: http://www.ipc.uni‐
stuttgart.de/~giesselm/AG_Giesselmann/Fors

chung/Fluessigkristalle/Fluessigkristalle.html).

Liquid crystals are temperature sensitive since they turn into solid if it is too cold and into liquid if it is
too hot. This phenomenon can, for instance, be observed on laptop screens when it is very hot or very
cold.

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Liquid crystals´ phases and their properties
A liquid crystal is formed by the self‐assembly of molecules in ordered structures, or phases. An external
perturbation, such as a change in temperature or magnetic field, even very small, can induce the LC to
assume a different phase. Different phases can be distinguished by their different optical properties
(Figure 3).

Figure 3. Schematic representation of molecules in a solid
(left, molecules are well organized), in a liquid crystal
(centre) molecules have a long range distance order) and in
a liquid (right) molecules are not ordered. (Image credit:
Copyright IPSE Educational Resources, University of
Wisconsin Madison)



Liquid crystals are divided in three groups:

‐ Thermotropic LC: they consist of organic molecules, typically having coupled double bonds, and exhibit
a phase transition as temperature is changed (Figure 4, left)

‐ Lyotropic LC: they consist of organic molecules, typically amphiphilic (water‐loving) and exhibit a phase
transition as a function of both temperature and concentration of the LC molecules in a solvent
(typically water) (Figure 4, right)

‐ Metallotropic LC: they are composed of both organic and inorganic molecules, and their LC transition
depend not only on temperature and concentration, but also on the organic‐inorganic composition
ration.

Figure 4. (Left) chemical structure
of N‐(4‐Methoxybenzylidene)‐4‐
butylaniline (MBBA); (Right):
Structure of lyotropic liquid crystal:
1 is a bilayer and 2 is a micelle. The
red heads of surfactant molecules

are in contact with water, whereas
the tails are immersed in oil (blue).
Page 10 of 44
(Image credit: Wiki commons,
Creative Commons Attribution
ShareAlike 3.0).


       Lyotropic liquid‐crystalline phases are abundant in living systems, such as biological membranes,
cell membranes, many proteins (like the protein solution extruded by a spider to generate silk), as well
as tobacco mosaic virus. Soap is another well know material which is in fact a lyotropic LC. The thickness
of the soap bubbles determines the colours of light they reflect.

LCs applications
The order of liquid crystals can be manipulated with mechanical, magnetic or electric forces. What is
interesting is that this change of order can be obtained with very small variations of these forces. The
properties of liquid crystals are useful in many applications. The colour of some liquid crystals depends
on the orientation of its molecules, so any perturbation that disturbs this orientation (e.g., a difference
in magnetic or electric field; a difference in temperature; or the presence of certain chemicals) can be
detected with a colour change.

The EXPERIMENT B in the Experiment Module deals with synthesizing and testing
different temperature sensitive liquid crystals. In this experiment students test how
different LCs are sensitive to different temperature ranges and build a LC thermometer.

The details of how a thermotropic LC works are described in the background document for
the EXPERIMENT B in the Experiment Module.

Liquid crystals are routinely used in displays for cell phones, cameras, laptop computer and other
electronics. In these displays, an electric field changes the orientation of the molecules in the liquid
crystal, and affects the polarization of light passing through them. ‐ Because of their sensitivity to
temperature, and the property of changing colour, they are also used in thermometers. In miniaturized
sensors, liquid crystals can detect certain chemicals, electric fields and changes in temperature.

Liquid crystal displays (LCDs) are discussed in Chapter 4 of Module 2 “Applications of
Nanotechnologies: ICT” of Module 2.

In the future LCs might have very interesting applications. Recently, it has been found that the inclusion
of specific molecules, often in the order of nanometres, in the liquid crystal can lead to new electrical
and optical properties in the LC. For instance, LC materials have been modified to induce new photonic
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functions, for application to waveguide and optical storage (ICT application area). Another class of LCs
under development is photoconductive LCs, with the aim of having new materials for PVs cells and light
emitting diodes (Energy application area). Another area of study is to modify LCs in ways that allow
using them in stimuli‐responsive materials and as templates for nanoporous polymers (potential
nanomedicine application area).

Nanostructured Metals and Alloys
This section covers metal nanoparticles; nanocrystalline materials consisting of a single metal or of alloys
made of two or more metals and nanostructured metal surfaces.

Metal nanoparticles
Metal nanoparticles are a clear example of how at the nanometre scale the properties of matter can
change. For instance metal gold is notably yellow in colour and used for jewellery. As the noblest of all
metals, gold is very stable (for instance, it does not react with oxygen or sulphur). However, if gold is
shrunk to a nanoparticle, it changes colour, becoming red if it is spherical (Figure 6) and even colourless
if it is shaped in a ring. Moreover, gold nanoparticles become very reactive and can be used as new
catalysts (this will be discussed in Chapter 2 “Applications of nanotechnologies: Environment” of
Module 2).

Figure 6. Gold colloid is ruby red, not
golden! (Image credit: L. Filipponi,
iNANO, Aarhus University, Creative
Commons Attribution ShareAlike
3.0).

Plasmonic structures
Noble metal nanoparticles, meaning gold, silver, platinum and palladium nanoparticles show localized
surface plasmon‐resonances (LSPR), an effect that was described in Chapter 4 of Module 2
(“Fundamental nano‐effects”). The energy of the LSPR depends on the particle shape, size,
composition, inter‐particle spacing, and dielectric environment. The surface of the nanoparticles can
be functionalised with numerous chemical and biochemical molecules enabling specific binding of
organic molecules such as antibodies, making them useful in sensors .
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For this reason they are of special interest for optical detection and sensing in analytical chemistry and
molecular biology. The refractive index can be used as the sensing parameter: changes in the local
dielectric environment, induced by the sensing process, are used for detecting the binding of molecules
in the particle nano‐environment. The change in aggregation between the nanoparticles as a result of
analyte attachment affects the LSPR energy, so this effect can be used for highly miniaturized sensors.
Figure 7 schematically illustrates those two LSPR‐based detection methods.

Figure 7. (Top) LSPR sensor based on a change of
refractive index induced by analyte attachment on a
functionalized surface; (Bottom) LSPR sensor based on a
change in nanoparticle aggregation induced by the
attachment on functionalized nanoparticles.

Localised surface plasmons have been explored in a range of nanoparticles shapes such as disks,
triangles, spheres and stars. Also more complex structures have been studied, such as holes in thin
metal film, nanoshells and nanorings. Based on nanostructured metallic surfaces a variety of optical
applications are possible. An important feature of plasmonic structures is that they allow label free
detection, which is important in optical sensing. Currently plasmonic components are investigated with
respect to future applications in, cancer therapy, solar cells, waveguides, optical interconnect, cameras
LEDs, OLEDs and more.

The application of metal nanoparticles in medical diagnostics and targeted treatment is
discussed in details in Chapter 1 of Module 2 “Applications of Nanotechnologies: Medicine
and Healthcare”.

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Reinforcements
Metal nanoparticles are used as reinforcements in alloys for application in lightweight construction
within the aerospace sector and increasingly in the automotive sector. The method is used for instance
to harden steel. For instance, titanium nanoparticles are used as alloy compound to steel, and the
resulting material shows improved properties with respect to robustness, ductility, corrosion and
temperature resistance. Particles of iron carbide are also precipitated into steel to make it harder. The
nanoparticles block the movement of the dislocations in the crystalline material (this effect was
described in details in Chapter 4 of Module 1 “Fundamental “nano‐effects”) increasing hardness. The
trade off between steel strength and ductility is an important issue, since modern construction requires
high strength whereas safety and stress redistribution require high ductility. The presence of hard
nanoparticles in the steel matrix could lead to a material with a combination of these properties,
effectively matching high strength with exceptional ductility.

Environmental applications
Some forms of metal nanoparticles have important environment applications.

Zero‐valent (Fe0) iron nanoparticles are under investigation for the remediation of contaminated
groundwater and soil. Iron when exposed to air oxidizes easily to rust; however, when it oxidizes
around contaminants such as trichloroethylene (TCE), carbon tetrachloride, dioxins, or PCBs, these
organic molecules are broken down into simple, far less toxic carbon compounds. Iron nanoparticles
are more effective than conventional ‘iron powder’, which is already used to clean up industrial
wastes. Iron nanoparticles are 10 to 1000 times more reactive then commonly used iron powders.

Silver nanoparticles have a strong anti‐bacterial capacity. They are used in numerous products to
prevent or reduce the adherence of bacteria to surfaces.

Metal nanoparticles and their use in environment applications are covered in the Chapter 2
of Module 2  “Applications of Nanotechnologies: Environment”.

Nanocrystalline metals
Nanocrystalline metals are classical metals and alloys that have an ultra‐fine crystalline structure below
100 nm. They exhibit extraordinary mechanical and physical properties which makes them interesting

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for many applications. Examples of nanocrystalline metal materials are Al, Mg, and Al‐Mg alloys which
offer high strength and are lightweight. Other examples are Ti‐ and Ti‐Al‐alloys.

Some crystalline metals offer exceptional magnetic properties. An example is the “Finemet”
nanocrystalline soft magnetic alloys, which consist of melt‐spun Fe‐Si‐B alloys containing small amounts
of Nb and Cu.

The application of nanomaterials in the field of magnetic materials is very important and promising, with
applications in magnetic recording, giant‐magneto resistance, magnetic refrigeration and magnetic
sensing. These novel materials are composed of magnetic nanoparticles dispersed in a magnetic or
nonmagnetic matrix (particle‐dispersed nanocomposites) or of stacked magnetic thin films (magnetic
multilayer nanocomposites).

We will cover the application of nanostructured magnetic materials for memory storage in
the Chapter 4 of Module 2  “Applications of Nanotechnologies: ICT”.

Ferrofluids
Ferrofluids are colloidal mixtures composed of ferromagnetic or
ferrimagnetic nanoparticles (such as magnetite) suspended in a carrier
fluid, usually an organic solvent or water. The ferromagnetic nano‐
particles are coated with a surfactant to prevent their agglomeration
(due to van der Waals and magnetic forces). Although the name may
suggest otherwise, ferrofluids do not display ferromagnetism, since they
do not retain magnetization in the absence of an externally applied
Figure 8 A ferrofluid with a
magnet underneath. (Image
field. Rather they display bulk‐like paramagnetism, and are often
referred to as “superparamagnetic” materials due to their large
credit: G. F. Maxwell, Wiki
Commons, GNU magnetic susceptibility. When a paramagnetic fluid is subjected to a
Free
documentation licence).   sufficiently strong vertical magnetic field, the surface spontaneously
forms a regular pattern of corrugations; this effect is known as the normal‐field instability (Figure 8).

Ferrofluids have numerous applications: in mechanical engineering, particularly for vehicle suspension
and breaking systems, due to their low‐friction properties; in the ICT area, as liquid seals (ferrofluidic
seals) around the spinning drive shafts in hard disks; and in the biomedical sector as drug carriers.
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Polymers
A polymer is a large molecule made of a chain of individual basic units called monomers joined together
in sequence. A copolymer is a macromolecule containing two or more types of monomers. When the
polymer is a good conductor of electricity, it is refereed to as conductive polymer (or organic metal). In
this section nanostructuring of polymers is described and the effect this can have on polymers´
properties. The focus is on co‐polymers since these are extremely useful in nanotechnologies.

Conductive Polymers
Polymers that are good conductors of electricity are called conductive polymers and include
polyacetylene, polyaniline, polypyrrole, polythiophene, and many more have been synthesised. These
polymers are characterized by having alternating double‐single chemical bonds, so they are π‐
coniugated. The π ‐coniugation of the carbon bonds along the oriented polymer chains provides a
pathway for the flow of conduction electrons, and is thus responsible for the good electrical conduction
properties of the material. A detailed SEM analysis of conductive polymers has revealed that these are
made of a sequence of metallic nanoparticles about 10nm in diameter. The high conductivity of
polymers such as polyacetylene and polyaniline is related to the nanostructure of the polymer.
Polyaniline and its analogous change colour when a suitable voltage is applied, or when reacting with
specific chemicals (electrochromic and chemochromic). For this reason they are promising for use in
light‐emitting diodes (LEDs). Other applications are the surface finish of printed‐circuit boards, corrosion
protection of metal surfaces, semitransparent antistatic coatings for electronic products, polymeric
batteries, and electromagnetic shielding.

Block Copolymers
A copolymer is a macromolecule containing two or more type of monomers, and a block copolymer has
these basic units or monomer types joined together in long individual sequences called blocks. An
example is the diblock polymer (A)m(B)n, which is made of a linear sequence of m monomers of type A
joined together to a linear sequence of n monomers of type B. A transition section joints the two blocks:

[end group]‐[polyA]m‐[transition member]‐[polyB]n‐[end group]

Often, block copolymers are made of a hydrophilic (water‐attracted) block, and a hydrophobic (water‐
repellent) block. In general, macromolecules having hydrophilic and hydrophobic regions, such as lipids,
self‐assembled in ordered structures when in water: the hydrophobic region packs together, avoiding

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the water molecules, leaving the hydrophilic
molecules to the exterior of the structure. In
the same way, block copolymers made of
hydrophilic and hydrophobic blocks when
mixed in a selective solvent, like water, can
self‐assemble into ordered architectures in
the nanoscale regime.

The geometry and degree of order of these
structures depends on the concentration and
the volume ratio between insoluble and
soluble blocks. Depending on these
Figure 9. Different geometries formed by block copolymers in
parameters, the block copolymer can form
selective solvent conditions. Image adapted from Smart et al.
spherical micelles (nanospheres), cylindrical
Nano Today (2008), 3 (3‐4), 38‐45, Copyright (2008) with
permission from Elsevier. micelles, and membranes. Both cylindrical
and spherical micelles consist of a nonsoluble
(hydrophobic) core surrounded by a soluble corona. Membranes are made of two monolayers of block
copolymer aligned so to form a sandwich‐like membrane: soluble block‐insoluble block‐soluble block.
Molecules that at low concentration form spherical aggregates will assemble into cylindrical and
eventually membrane‐like structures, as the concentration is increased (Figure 9).

Among spherical micelles, if the lengths of the projections formed by the hydrophilic corona are short
compared to the sphere diameter, the nanostructure is called “hairy nanosphere”, whereas if the sphere
is small and the projections long, it is called “star polymer” (Figure 10).

Figure 10. Sketch of a star polymer and of a hairy polymer.
(Image credit: L. Filipponi, iNANO, Aarhus University,
Creative Commons Attribution ShareAlike 3.0).

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Responsive “smart” polymers
Block copolymers form nanostructures that are very sensitive to external fields. For instance moderate
electrical fields or shear stimulation can trigger macroscopic re‐arrangements in specific directions. This
is a versatile property for making materials that respond and change on demand. The intrinsic
macromolecular nature of these copolymers leads to very slow and kinetically controlled phase
transitions. Thus metastable or intermediate phases have longer lifetimes, which is desirable in
applications that want to exploit the phase transition properties of these materials.

Biomedical applications
The ability of block copolymers to form nanoparticles and nanostructures in aqueous solutions makes
them particularly useful for biomedical applications, such as therapeutics delivery, tissue engineering
and medical imaging. In the field of therapeutic delivery, materials that can encapsulate and release
drugs are needed. Hydrogels are very useful for the controlled release of drugs and block copolymer
hydrogels are particularly advantageous for the possibility of conferring some stimuli‐activated
properties, such as temperature‐sensitivity. Block copolymers form nanostructures with both
hydrophilic and hydrophobic areas, so they can form vesicles that can encapsulate and carry both
hydrophobic and hydrophilic therapeutic agents. Micelles formed using block copolymers have a
hydrophilic corona that makes them more resistant to the interaction of proteins, in particular plasma
proteins, therefore these types of micelles exhibit long circulation times in vivo. Insoluble domains can
also be engineered so to exploit the sensitivity of specific hydrophobic polymers to external stimuli such
as pH, oxidative species, temperature and hydrolytic degradation.

Block copolymers are also of interest for preparing scaffolds for tissue engineering. For instance very
long micelles that mimic the natural extracellular matrix were recently prepared exploiting the self‐
assembly properties of a peptide copolymer.

Nanoreactors
The ability to generate compartmentalized volumes at the nanometre level is one of the fundamental
mechanisms used by cells in synthesising biomolecules and performing the biochemical processes
necessary for their function. This motif is now reassembled using block copolymer micelles and vesicles
as nanoreactors. This approach has been employed for carrying enzymatic reactions in nanosized
compartments.

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It has been shown that this approach can be expanded also into a nonaqueous solvent. For instance a
recent work reports of block polymer nanostructures in ionic liquid and the ability of micelles to shuttle
from an aqueous solvent to an ionic liquid as a function of temperature. This opens the way to use block
copolymer technology in more sophisticated synthetic routes.

Artificial moving parts
Block copolymers structures can also be used to mimic the ability of biomolecules to convert chemical
energy into mechanical energy, for instance by using a pH sensitive block copolymer whose micelles
swell based on pH variation. Thus these materials are investigated to create artificial muscles or moving
nanostructures.

Other applications
The application of block copolymers is not limited to the biomedical field. They can be used in
conjunction with other materials to form block copolymers nanocomposites. For instance star polymers
are used in industry to improve metal strength.

Finally, block copolymers can form nanoporous membranes for applications in filtering systems and fuel
cell technology.

Polymeric nanofibres
Nanostructured fibrous materials, or nanofibres, are an important class of nanomaterials, now readily
available thanks to recent developments in electrospinning and related fabrication technologies.
Differently from conventional woven fabrics, that have the typical structure illustrate in Figure 11.

Nanofibres have some unique properties: they are highly porous, i.e.,
they have a large interconnected void volume in the range of 50% or
even grater then 90% and possess a very high surface‐to‐volume
ratio. It is possible to increase the mechanical stability of nanofibrous
structures by annealing the fabric so to join together the crossing
points of those fibres.

Figure 11. A typical sample of electruspun polystyrene. (Image credit: T. Uyar, Interdisciplinary Nanoscience Canter
(iNANO), Aarhus University. Copyright 2008. Reprint not permitted).

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These properties make nanofibrous scaffolds useful for many biomedical and industrial applications.

In addition, researchers have succeeded in making coaxial nanofibres composed on two different
polymers or composite coaxial fibres. Researchers also try to make aligned nanofibres. These types of
materials, particularly if made of conducting polymers, could have important application for electronic
and medical devices.

Nanofibres have a broad spectrum of
applications as schematized in Figure 12.

Figure 12. Overview of polymer nanofibres
applications.

The two areas that have so far received most attention are the medical field (e.g., wound
healing membranes) and filtration applications. These will be covered in the Chapter 1
“Applications of nanotechnologies: Medicine and healthcare” and Chapter 2
“Applications of nanotechnologies: Environment”), respectively. Nanofibres are also considered for
composite material reinforcement. As nanofibres are a relatively recent type of nanomaterial it is
expected that in the future many more applications will be explored.

Semiconductors
Semiconductors, unlike metals, have a band gap. The band gap is between the valence band and the
conduction band. In intrinsic semiconductors which posses no impurities (like boron, silicon, germanium,
indium) there are no electronic states in the band gap. The properties of semiconductors, in particular
the band gap, are manipulated by addition of dopants: impurities able to donate charge carriers in the
form of electrons (n‐type) or holes (p‐type).

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Like for metals, the reduction of the size of the semiconductor triggers the insurgence of novel physical
properties. The most evident example is that of quantum dots.

Quantum dots
Quantum dots are made of semiconductor materials such as CdSe, ZnSe, CdTe, about 10 nm in size. Like
in the case of metal nanoparticles, electrons in quantum dots are localized in space. A QD has a discrete,
quantized energy spectrum, therefore they can absorb a specific wavelength, and emit a
monochromatic colour. Depending on their size, QD emit different colours, as shown in Figure 13. The
reason for this was described in Chapter 4 of Module 2 (“Fundamental nano‐effects”): the width of the
bandgap is related to the size of the semiconductor, smaller sizes lead to a blue shift in the emission
spectra.
Figure 13. Ten distinguishable emission colours of ZnS‐capped CdSe
QDs excited with a near‐UV lamp. From left to right (blue to red), the
emission maxima are located at 443, 473, 481, 500, 518, 543, 565,
587, 610, and 655 nm. (Reprinted by permission from Macmillan
Publishers from Macmillan Publishers Ltd: Nature Biotech. (2001) 19,
631‐635, Copyright 2001).

QD like metal nanoparticles hold great promise for nano‐enhanced imaging which will bring
progress to fields like environmental monitoring, medical diagnostic and treatment. QD are
also investigated as novel light sources to improve LED technology and in solar cell
technology. These applications of QDs in the energy and ICT sector will be reviewed in
Chapter 3 “Applications of Nanotechnologies: Energy” and Chapter 4 “Application of
Nanotechnologies: ICT” of Module 2.

Semiconducting oxides
Semiconducting oxides like TiO2 and ZnO in bulk (macro) form are widely used in industry in many
products. When they are in a nanoscale form, they display interesting physical properties that allow
designing new materials and improve old ones. Below is a short description of these properties.

Titanium dioxide
Titanium dioxide (TiO2) is a mineral mainly found in two forms, rutile and anatase. Titanium dioxide is
the most widely used white pigment because of its brightness (white colour) and very high refractive

Page 21 of 44



index (n=2.4). It is used in paints, plastics, toothpastes, papers, inks, foods and medicines. In sunscreens
with a physical blocker, titanium dioxide is used both because of its high refractive index and its
resistance to discolouration under ultraviolet light. This is because TiO2 is a UV filter, it adsorbs UV light.
TiO2, particularly in the anatase form, can be employed also as a photocatalyst under UV light. It oxidizes
water to create hydroxyl (OH) radicals, and it can also oxidize oxygen or organic materials directly. For
this reason TiO2 is added to confer sterilizing, deodorizing and anti‐fouling properties to paints, cements,
windows, and tiles.

Titanium nanoparticles (30‐50nm, often referred to as nano‐TiO2) are at the centre of much attention
due to their optical and catalytic properties: they retain the ability to adsorb UV light but light scattering
is dramatically reduced, so that TiO2 goes from appearing white to transparent (a detailed explanation of
this effect is given in Chapter 4 of Module 1 (“Fundamental nano‐effects”). Nano‐TiO2 is thus suitable
for transparent coatings, and for new generation sunscreens, which are characterised by a high
protective factor but transparent appearance.  The catalytic properties of TiO2 when in a nano‐size are
also greatly enhanced due to the large surface‐to‐volume ration. This property is increasingly used for
chemical catalysis applications such as photo‐catalytic purification of water and air to decompose
organic pollutants (solar photocatalytic remediation). Thin films of TiO2 are used in windows to confer
self‐cleaning properties to the glass (this of application of nano‐TiO2 nanoparticles is reviewed in the
“nanocoatings” section of this document).

Another important application of nano‐TiO2 is in organic solar cells, like the Grätzel solar
cell, where nano‐TiO2 is used as the catalyst to promote light‐induced electrical charge
separation. The use of titanium dioxide in this type of solar cells is covered in more detail in
Chapter 2 “Applications of Nanotechnologies: Environment” of Module 2.

One limitation of using TiO2 as a photocatalist is that this material only adsorbs UV light which
represents about 5% of the solar spectrum. In this context, nanotechnology could bring an improvement
in the form of nanoparticles with surfaces modified with organic or inorganic dyes to increase the
photoresponse window of TiO2 from UV to visible light.

Zinc dioxide
Zinc oxide (ZnO) has some similar properties to TiO2, i.e., its nanoparticles scatter light so it can be used
for transparent UV filters, in creams or coatings. Like TiO2 it is used for solar photocatalytic remediation

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but compared to TiO2 it has a weaker photocatalytic effect. ZnO also suffers from the same limitation of
adsorbing only a fraction of the solar spectrum so research is underway to increase its photoresponse.

A peculiarity of ZnO is that it has a tendency to grow in self‐organised nanostructures. By controlling
crystal growth conditions, a variety of crystal shapes are possible. Researchers have been able to grow
nanoscale wires, rods, rings etc. ZnO nanocolumns are of particular interest since low‐temperature
photoluminescence measurements have revealed intense and detailed ultraviolet light emission near
the optical band gap of ZnO at 3.37 eV. ZnO thus can act as an optical amplification medium and as a
laser resonator.

NANOYOU VIRTUAL GAME: Students can try to synthesise ZnO nanowires through a virtual lab excersise.
Details are found at www.nanoyou.eu/virtual‐lab.html

ZnO wires arrayed on a surface are also being investigated as piezoelectric elements for miniaturised
power sources. This would allow creating flexible, portable power sources that could be included in
textiles so that they are able to scavenge energy from body motion, light wind, air flow etc .

Figure 14. Patterned ZnO grown as vertical
pillars in an arrayed format. (Reprinted with
permission from Xu et al., Journal of American
Chemical Society 2008, 130, 14958‐14959.
Copyright 2008 American Chemical Society.

The use of ZnO  for the development of miniaturized power sources is described in details
in Chapter 3 of Module 2 “Applications of Nanotechnologies: Energy”.

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ITO
ITO is a semiconducting material whose main feature is the combination of electrical conductivity and
optical transparency. ITO is made of Indium(III)‐oxide (In2O3) to around 90% and Tin(V)‐oxide (SnO2) to
around 10%. It is widely used in its thin‐film form as transparent electrodes in liquid crystal displays,
touch screens, LEDs, thin‐film solar cells, semiconducting sensors etc. ITO is an infrared absorber thus it
is now used as a thermal insulation coating on window glass. Its anti‐static properties make it
additionally useful in applications like packaging and storing of electronic equipment. Since the material
is very expensive alternative materials, such as fluor tin oxides and aluminium zinc oxides are
considered. The use of ITO in “smart coatings” is further discussed in the “Nanocoatings” section of this
document.

Photonic crystals
A photonic crystal consists of a lattice of periodic dielectric or metal‐dielectric nanostructures that
affects the propagation of electromagnetic waves. Essentially, photonic crystals contain regularly
repeating internal regions of high and low dielectric constant. Photons (behaving as waves) propagate
through this structure ‐ or not ‐ depending on their wavelength. The periodicity of the photonic crystal
structure has to be of the same length‐scale as half the wavelength of the electromagnetic waves, i.e.
~200 nm (blue) to 350 nm (red) for photonic crystals operating in the visible part of the spectrum. Such
crystals have to be artificially fabricated by methods such as electron‐beam lithography and X‐ray
lithography.

Photonic crystals exist in Nature. For instance in Chapter 2 of Module 1 “Natural Nanomaterials” it was
shown how the beautiful blue wings of some butterflies own their colour to their internal nanostructure
which is in fact a photonic crystal structure.

Photonic crystals are now receiving much attention because of their potentials in particular
in the optical‐communication industry. Researches are considering using light and
photonic crystals (in alternative to electrons travelling in wires) for the new generation
integrated circuits. Light can travel much faster in a dielectric medium than an electron in a wire, and it
can carry a larger amount of information per second. Given the impact that semiconductor materials
have had on every sector of society, photonic crystals could play an even greater role in the 21st
century. Photonics and their impact in the ICT sector are analysed in Chapter 4 of Module 2
(“Application of Nanotechnologies: ICT”)

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Ceramic and glassy materials
Ceramic materials by definition are ionically bound; they are hard materials, both electrically and
thermally very stable. In this category are included for instance Al2O3, Si3N4, MgO, SiO2, Na2O, CaO, and
ZrO2. Ceramics are characterised by being hard yet brittle, therefore in many cases they are used as
composites where they are mixed with other materials (e.g., metals) to increase their mechanical
performance. Composite hard nanocoatings are particularly important for cutting tools (see the section
“Inorganic composites” in this document).

Ceramic exist both in a crystalline and non‐crystalline form (for instance the broad family of “glass”).
Nanostructures within the material have important consequences for their properties as discussed in
the next sessions.

Porous alumina
Porous alumina membranes produced by anodic aluminium oxidation are characterized by having
hexagonally close‐packed channels with diameters ranging from 10nm to 250 nm or grater. This
material is often used as a template for the synthesis of other materials.

Zeolites
Zeolites are natural crystalline materials with pores having regular arrangements. They are also often
used in the template synthesis of nanomaterial. They can also be used to prepare organized structures
of a certain material to confer new optical properties to it. For instance, selenium
can be incorporated in the channels of
Figure 15. A schematic
mordenite, a natural zeolite. The difference of a
representation of a zeolite.
mordenite‐selenium crystal compared to a
(Image credit: Wikimedia
natural selenium crystal is noteworthy: the Commons, image for public
optical absorption spectrum is considerably domain).
shifted to higher energies for the mordenite‐
selenium crystal.

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Aerogels
Silicon dioxide (SiO2) is the main fraction of quartz. It is chemically robust and
finds widespread applications.  In commercial products it appears as additive in
rubber products for vehicle tires but it is also the component of new types of
aerogels. Generally speaking an aerogel is a solid with up to 95% of the volume
consisting of nanoscale pores. Aerogels Figure 16. The thermal insulating
are manufactured with a sol‐gel properties of a silica aerogel. (Image
technique and can be made of carbon, credit: Wikimedia Commons, image for
metal oxide, polymers or silicates.   public domain).

Due to their high porosity aerogels have an extremely high surface area and very low thermal
conductivity. Thus they are suitable for thermal insulation and as filter materials (Figure 16). Another
prominent property is their low specific weight, making them interesting for lightweight construction.
Aerogels are also interesting for their optical properties, namely high optical transparencies. Silica
aerogels are made of pores of about 10 nm arranged in distances between 10 and 100nm. They are
resistant and chemically inert to liquid metals, heat resistant up to 1200 ºC and non‐toxic, thus they
have also biomedical applications, such as substrates for cell growth and analysis. One of the problems
of SiO2 aerogels that needs to be addressed is that this material needs to be costly protected against
humidity, since it is not water resistant and suffers from a loss of stability and thermal conductivity once
it gets wet.

Carbonbased materials
In Nature there are some pure materials that have striking different properties even though they are
made of the same atoms. For instance, graphite and diamond (Figure 17): two very popular materials,
one used conventionally in pencils and the other in jewellery. These two materials could not be more
different: graphite is soft, light, flexible, and conducts electricity while diamond is extremely strong, hard
and does not conduct electricity. Both materials are made of atoms of carbon linked through strong
bindings (covalent), but in graphite each carbon uses three out of his four electrons to form single bonds
with its neighbours, forming a linear sheet, whereas in diamonds each carbon uses all its four electrons
to form four single bonds, resulting in a 3‐D structure. The different properties of graphite and diamond
are a consequence of the different way the carbon atoms in the materials are bonded together.

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Figure 17. Two allotropes of carbon and their respective
chemical structure: (left, top and bottom): diamond; right
(top and bottom): graphite.(Image credit: Wiki Commons,
Creative Commons Attribution ShareAlike 3.0)

Graphite and diamond are two pure forms (allotropes) of
carbon. In 1985 a new allotrope of carbon was discovered
formed of 60 atoms of carbons linked together though
single covalent bonds arranged in a highly symmetrical,
closed shell that resembles a soccer ball. This material was
officially named Buckminster fullerene and is often referred to as buckyball, fullerene or simply C60.
Since its discovery, fullerenes with 70, 80 and even more carbons were discovered.

Figure 18. Eight allotropes of carbon: (a) diamond,
(b) graphite, (c) lonsdaleite, (d) C60, (e) C540, (f)
C70, (g) amorphous carbon and (h) a carbon
nanotube. Image by Michael Ströc, (GNU Free
Documentation License).

In the early 1990s, an incredibly new carbon form
was discovered, carbon nanotubes. These appear
like graphite sheets rolled up with fullerene‐type
end caps, but have totally different properties
compared to graphite. Figure 18 shows different
forms of carbon allotropes (image d and h are
structures of a C60 and a nanotube, respectively).

It is now known that fullerenes and carbon nanotubes form naturally in common places like flames
(produced by burning methane, ethylene and benzene) and in soot. Scientists have now developed
methods to synthesize these nanomaterials with control over their final properties.

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Carbon nanotubes
Carbon nanotubes can be as single‐wall nanotubes (SWNTs), with a diameter of approximately 1.4nm, or
multi‐wall nanotubes (MWNTs), consisting of 2 to 30 concentric tubes with an outer diameter of 30‐
50nm. The structure of a SWNT can be conceptualized by wrapping a one‐atom‐thick layer of graphite
called graphene into a cylinder. To complete the nanotube, imagine adding two half fullerenes on each
end of the nanotube.

Carbon nanotubes can range in length from a few tens of nanometres to several micrometers, and can
have metallic properties (comparable to, or even better than copper) or can be semiconductors (like
silicon in transistors), depending on their structure. These nanomaterials are truly amazing and have
great potential in numerous fields as illustrated in Figure 19.

Figure 19. Overview of application areas of carbon nanotubes. (Reprinted from: M.A.H. Hyder, Master
Thesis, Technical University of Hamburg‐Harburg, Germany, Copyright M. A. H. Hyder 2003).

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The application of CNTs in the energy, environment, medicine and ICT sectors are covered
in the respective application chapters in Module 2.

Mechanical properties
The chemical bonding of nanotubes is composed entirely of sp2 bonds (carbon double bonds), similar to
those of graphite (whereas in diamonds all bonds are sp3). This bonding structure, which is stronger than
the sp3 bonds found in diamonds, provides the molecules with their unique strength. Nanotubes
naturally align themselves into "ropes" held together by Van der Waals forces.

Theoretical predictions say that carbon nanotubes are 100 times stronger than steel but have
one sixth of its weight. Therefore they are ideal in lightweight construction, for instance for the
automotive and aviation industry. Carbon nanotubes are already used in some consumer
products to add strength (without compromising weight) such as tennis rackets.

The mechanical properties of carbon nanotubes are summarised in the table below. Young modulus (Y)
is a measure of how stiff, or elastic, a material is. The higher the value is, the less a material deforms
when a force is applied. The tensile strength describes the maximum force that can be applied per unit
area before the material snaps or breaks. A third interesting measure of a material is its density, which
gives an idea of how light a material is. From the table below it can be seen that wood is very light but
weak, while nanotubes are many times stronger than steel and yet much lighter.

Material Young´s Tensile Density
Modulus (GPa) Strenght (GPa) (g/cm3)
Single‐wall 800 >30 1.8
nanotube
Multi‐wall 800 >30 2.6
nanotube
Diamond 1140 >20 3.52
Graphite 8 0.2 2.25
Steel 208 0.4 7.8
Wood 16 0.008 0.6
Table 1

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Because of their mechanical properties carbon nanotubes are very interesting as fillers in
polymeric and inorganic composites.

Electrical and thermal properties
The electrical properties of a material are based on the movement of electrons and the spaces or
“holes” they leave behind. These properties are based on the chemical and physical structure of the
material. In nanoscale materials some interesting electrical properties appear. Carbon nanotubes are
the best example of this effect at the nanoscale. If one considers “building” a nanotube by rolling up a
graphene sheet, the resulting nanotube can be conductive (and indeed, very conductive!) or
semiconductive with relatively large band gaps. The electrical properties of the nanotube depend on the
way it was “rolled up” (better known as chirality). If it is rolled up so that its hexagons line up straight
along the tube’s axis, the nanotube acts as a metal (conductive). If it is rolled up on the diagonal, so the
hexagons spiral along the axis, it acts as a semiconductor.

Why is it so? Graphene (that is, one layer of graphite) is not an insulator, but neither a metal nor a
semiconductor; it has electrical properties somehow in between, it is called a “semimetal”. When rolled
up, it leads to a structure that is either metallic or semiconducting. On the other hand diamond has a
tetrahedral structure (derived from the fact that carbons are hybridized sp3 rather than sp2 like in
graphene) and is insulator.

One interesting property found in single walled carbon nanotubes (SWCNT) is that electric conductance
within them is ballistic (which means that all electrons in the conductor go into one end come out of the
other end without scattering, regardless of how far they need to travel).

Researchers are also investigating if nanotubes can be superconductors near room temperature,
meaning ballistic conductors that also exhibit a resistance of zero. A superconductor can transport an
enormous amount of current flow at tiny voltages. At present known superconductors work at very low
temperatures. This field of research is very important since if a material was superconductive at room
temperature, it would carry current with no resistance at room temperature, with no energy lost as
heat. This could lead to faster, lower‐power electronics and the ability to carry electricity long distances
with 100 per cent efficiency.

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Nanotubes as superconductors, and their applications, is further discussed Chapter 3
“Applications of Nanotechnologies: Energy” of Module 2.

In terms of thermal properties, carbon nanotubes dissipate heat better than any other known material
and are excellent thermal conductors.

Chemical reactivity
Carbon nanotubes are very stable; they can withstand the attack of numerous chemicals and resist
exposure to a large temperature range. However, their chemical structure can be changed by the
addition of specific ligands with functional groups that allow the interaction with different chemicals.
This allows using them in sensors.

Applications
Carbon nanotubes are very promising materials for numerous applications (these will be described in
more details in Module 2). Applications include: nanomedicine (drug deliver), environment (chemical
sensors), energy (supercapacitors, hydrogen storage materials, solar cells), ICT (integrated circuits,
electronic paper), advanced materials for construction, transport, sport equipment, and more.

Scientists have now developed methods to control the synthesis of carbon nanotubes to obtain regular
structures with specific properties. To date, thought, the synthetic processes lead to moderate amount
of CNTs and, mostly, very limited length (on the order of millimetres). Often these processes lead to
CNTs which are not totally pure (traces of the catalyst used in the synthesis are present in the product)
and this has been associated with toxicity issues of CNTs.

The cost associated to the production of carbon nanotubes is extremely high. In the future this cost
must be considerably reduced to allow large‐scale production and use of CNTs.

Composites
The idea behind nanocomposites is to use building blocks with dimensions in the nanometre range to
design and create new materials with unprecedented flexibility and improvements in their physical
properties. This concept is exemplified in many naturally occurring materials, like the bone, which is a
hierarchical nanocomposite built from ceramic tablets and organic binders (see Chapter 2 of Module 1
“natural Nanomaterials”). When designing the nanocomposite, scientists can chose constituents with

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different structures and composition and hence properties, so that materials built from them can be
multifunctional.

As a general definition a nanocomposite is a conventional material reinforced by nanoscale particles or
nanostructures which are dispersed through the bulk material. The base material itself generally consists
of non‐nanoscale matrices. Nanocomposite typically consists of an inorganic (host) solid containing an
organic component or vice versa or two (or more) inorganic/organic phases in some combinatorial form.
At least one component must be in the nanosize. In general, nanocomposite materials can demonstrate
different mechanical, electrical, optical, electrochemical, catalytic, and structural properties which are
different from that of the individual components. Apart from the properties of the individual
components, interfaces in a nanocomposite play an important role in determining the overall properties
of the material. Due to the high surface area of nanostructures, nanocomposites present many
interfaces between the intermixed phases, and often the special properties of the nanocomposite are a
consequence of the interaction of its phases at interfaces. In comparison, the interfaces in conventional
composites constitute a much smaller volume fraction of the bulk material.

In this chapter, nanocomposites are sub‐divided in two main groups: inorganic nanocomposites, which
are characterised by an inorganic matrix (e.g., ceramic) reinforced by nanoscale particles or
nanostructures of inorganic (e.g., metal) or organic (e.g., carbon‐based) nature; and polymer
nanocomposites, which are characterised by an organic matrix (e.g., polymer) reinforced by nanoscale
particles or nanostructures by inorganic (e.g., clay) or organic nature.

Inorganic nano composites
High‐performance ceramics are sought in many applications, like highly efficient gas turbines, aerospace
materials, automobiles etc. The field of ceramics that focuses on improving their mechanical properties
is referred to as structural ceramics.

Nanocomposite technology is also applicable to functional ceramics such as ferroelectric, piezoelectric,
varistor and ion‐conducting materials. In this case the properties of these nanocomposites relate to the
dynamic behaviour of ionic and electronic species in electro‐ceramic materials. Among these materials
here we limit the review to nanocomposite with enhanced magnetic properties.

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Structural nanocomposites
Presently even the best processed ceramics pose many unsolved problems such as poor resistance to
creep, fatigue and thermal shock; degradation of mechanical properties at high temperatures; and low
fracture toughness and strength. To solve these problems one approach has been incorporating a
second phase such as particulates, platelets, whiskers, and fibres in the micron‐size range at the matrix
grain boundaries. However, the results obtained with these methods have been generally disappointing.
Recently, the concept of nanocomposites have been considered, where nanometre‐size second phase
dispersions are inserted into ceramic matrices. Large improvements in both the fracture toughness and
the strength of a ceramic can often be achieved with nanometre‐range particles embedded in a matrix
of larger grains at their grain boundaries. These can involve the incorporation of a nano‐ceramic in a
bulk ceramic, a nano‐metal in a ceramic, or a nano‐ceramic in a metal. Another possibility is the
incorporation of a polymer in a ceramic. Without going too much into the details, we list here some
examples of inorganic nanocomposites that have improved structural properties:

The incorporation of fine SiC and Si3N4 nanoparticles in an alumina matrix (Al2O3, a structural
ceramic material) first demonstrated the concept of structural nanocomposites. The dispersion
of these particles have shown to improve the fracture toughness from 3 to 4.8 MPa m1/2 and
the strength from 350 to 1050 MPa at only 5 vol. % additions of SiC.

One possibility for the fabrication of advanced structural ceramics is the dispersion of metallic
second‐phase particles into ceramics which improves their mechanical properties, such as
fracture toughness, and influences other properties including magnetic and optical properties.
Nanomposites of this type are Al2O3/W or MgO/Fe. Granular films can also be made with a
ceramic phase embedded with nanosize metal granules (such as Fe/Al2O3, Fe/SiO2). Such films
often show unusual or enhanced transport, optical and magnetic properties. The inclusion of
nanosized metals in a thin ceramic film can transform it from an insulator to a conductive film.

Another possibility is to add fine, rigid ceramic reinforcements to a ductile metal or alloy
matrix (called metal matrix composites, or MMC). The reinforcement can be either in the form
of particles (e.g., silicon carbide, aluminium oxide), fibres (e.g., silicon oxide, carbon) or a
mixture of both (hybrid reinforcement). Materials produced with this method are particularly
useful in the aerospace, automotive and aircraft industry. The advantage of MMCs is that they
combine metallic properties (ductility and toughness) with ceramic characteristics (high
strength and modulus), leading to materials having greater strength to shear and compression
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and to higher working temperature capabilities. The properties of MMCs are controlled by the
size and volume fraction of the reinforcements as well as by the nature of the
matrix/reinforcement interface. The attention is now oriented towards the incorporation of
nanoparticles and nanotubes for structural applications, since these materials exhibit even
greater improvements in the physical, mechanical and tribological properties compared to MMC
with micron‐size reinforcements.

Nanoscale ceramic powders with carbon nanotubes provide another method for creating dense
ceramic‐matrix composites with enhanced mechanical properties. For instance hot pressed α‐
alumina with mixed carbon nanotubes results in lightweight composites with enhanced
strength and fracture toughness compared to polycrystalline alumina. The processing
conditions greatly influence the properties of this material, though. In metal matrix composites
(MMC) the incorporation of carbon nanotubes is considered very promising since these
materials have higher strength, stiffness and electrical conductivity compared to conventional
metals.

Nanocomposites with enhanced magnetic properties
Materials with outstanding magnet properties are of high demand as these are employed in nearly all
important technical fields, e.g., electrical power, mechanical power, high‐power electromotors,
miniature motors, computer elements, magnetic high density recording, telecommunication, navigation,
aviation and space operations, medicine, sensor techniques, magnetic refrigeration, materials testing
and household applications. Recent developments in the field of magnetic materials have involved
materials with a nanocrystalline structure or, in the case of thin‐films, layers of nanometre thickness.

Nanostructuring of bulk magnetic materials leads to soft or hard magnets with improved properties.

An example is the “Finemet” nanocrystalline soft magnetic alloys, which consist of melt‐spun Fe‐Si‐B
alloys containing small amounts of Nb and Cu. When annealing at temperatures above the crystallization
temperature, the Fe‐Si‐B‐Nb‐Cu amorphous phase transforms to a crystalline solid with grain sized of
about 10nm. These alloys have excellent magnetic induction and large permeability, and a very small
coercive field.

Nanosized magnetic powders can have extreme properties and have no hysteresis at any temperature.
These materials are called superparamagnetic and one example is nanosized powders of a Ni‐Fe‐Co
alloy.
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Nanostructuring has also been studied in the context of hard magnets (permanent magnets). The
strongest known magnets are made of neodymium (Nd), iron and boron (e.g., Nd2Fe14B). In these
materials is has been found that the coercive field decreases significantly below ~40 nm and the
remnant magnetization increases. Another approach to improve the magnetization curve of permanent
magnets has been to make nanocomposites made of hard magnetic phases, such as Nd2Fe14B and
Sm2Fe17N3, within soft magnetic matrices (e.g., soft α‐phase of iron). The effect of the inclusion of a soft
iron mixed in with a hard material is to increase the remnant field.

The size of the grain size of the material also influences the magnetization saturation point. For
instance the magnetization of ferrite increases significantly below a grain size of 20nm. Thus reducing
the size of the grains in the magnet increases the energy product (which is the product of
magnetization and coercivity). The coercivity also increases with decreasing the grain sizes. In the case
of nanocomposite magnetic films this is true if the grains are isolated (no interaction), but when the
grains start contacting and exchange interaction kicks in, the coercivity falls rapidly with grain size. The
coercivity is thus highest at percolation, when the grains just start touching each other. This effect is
important in the context of designing thin film nanocomposites (magnetic multilayer nanocomposites)
for instance for high magnetic density recording.

Magnetic multilayer nanocomposites are becoming an essential component of the new
generation data storage devices like Magnetoresistive Random Access Memory (MRAM).
This and other nanostructured devices made of nanomagnets with application in the ICT
sector are discussed in Chapter 4 of Module 2 (“Applications of Nanotechnologies: ICT”).

Polymer Nanocomposites
Polymer composites are materials where a polymer is filled with an inorganic synthetic and/or natural
compound in order to increase several properties, like heat resistance or mechanical strength, or to
decrease other properties, like electrical conductivity or permeability for gases like oxygen or water
vapour. Materials with synergistic properties are used to prepare composites with tailored
characteristics; for instance high‐modulus but brittle carbon fibres are added to low‐modulus polymers
to create a stiff, light‐weight polymer composite with some degree of toughness. Current polymer
composites are really filled polymers, since these materials lack an intense interaction at the interface
between the two mixed partners. Progress in this field has involved moving from macro scale fillers, to
micron‐scale fillers, to even smaller fillers.

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In recent years scientists have started to explore a new approach to the production of polymer
composites with the use of nanoscale fillers, in which the filler is below 100nm in at least one
dimension. Nanoscale fillers include: nanoparticles;
nanotubes; and layered (also called “plate‐like”) Figure 20. Schematic of nanoscale fillers. make
inorganic materials such as clays (Figure 20). own copy. (Image credit: L. Filipponi, iNANO,
Aarhus University, Creative Commons Attribution
ShareAlike 3.0).



Although some nanofilled composites have been used for more then a century, such as carbon black
and fumed silica filled polymers, only recently researchers have started to systematically produce and
study those materials. The motivation has been the realization of the exceptional combined properties
that have been observed in some polymer nanocomposites. This, together with substantial development
in the chemical processing of nanoparticles and in the in situ processing of nanocomposites has led to
unprecedented control over the morphology of these materials.

One of the most common reasons for adding fillers to polymers is to improve their mechanical
performance. In traditional composites this often compromises the ductility of the polymer and in some
cases impacts negatively its strength, because of stress concentration caused by the fillers. Well
dispersed nanofillers, such as nanoparticles or nanotubes, can improve the modulus and the strength
and maintain (or even improve) ductility because their small size does not create large stress
concentration. For all nanofillers, a key requirement is the homogeneous dispersion of the filler within
the polymer matrix. As it will be discussed in the next sessions, this is a challenge in many cases, and a
topic of intense research.

Although the scientific community has made remarkable progress in this field in the last years, polymer
nanocomposites have just started to be explored, and many research question still need to be
addressed. What is clear so far is that the use of nanoscale fillers opens the doors for the development
of materials with exceptional properties. For instance, nanoparticles do not scatter light significantly,
thus it is possible to make polymer composites with altered electrical or mechanical properties that
remain optically clear. Nanoparticles are also of interest not just for their small size, but for their
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inherent unique properties. Carbon nanotubes for instance display the so far highest values of elastic
modulus, as high as 1 TPa, and strengths that can be as high as 500 GPa. This could allow for instance
the fabrication of polymeric composites with exceptional strength and flexibility.

Another outstanding property that the use of nanoscale fillers confers to the nanocomposites is an
exceptionally large interfacial area. The increase of surface area below 100nm is dramatic. The interface
controls the degree of interaction between the filler and the polymer, and is thus responsible for the
composite properties. Thus the largest challenge in nanocomposite science is learning to control the
interface.

Nanoscale fillers can potentially allow creating a vast variety of different polymeric materials with
advanced properties. In general macroscopic reinforcement elements have the limitation of containing
always imperfections, but as the reinforcements become smaller and smaller, structural perfection
could be reached. The ideal reinforcements would have atomic or molecular dimensions and be
intimately connected with the polymer. The use of nanoscale fillers, though, also introduces a series of
fabrication challenges. Because of their small size and high surface area, nanoscale fillers such as
nanoparticles have a strong tendency to agglomerate rather to disperse homogeneously in a matrix. This
leads to particle‐matrix mixtures with high viscosities, which can make the processing of those materials
quite challenging. The result is that even the most exciting polymer nanocomposites have very low
fractions of particle content, and relatively weak mechanical properties when compared with those
predicted in theory. Therefore polymer nanocomposites are a type of exciting advanced materials that
hold great promise in many application, but which are still mainly in a development stage. The intense
research efforts in this area though suggest that these materials will become more readily accessible in
the near future.

Nanoparticle polymer nanocomposites
Nanoparticles are a type of nanofillers that offer the opportunity to develop polymers with new or
advanced properties. As discussed before the size of a nanoparticle affects its properties; for instance Au
nanoparticles have different optical absorption spectrum depending on the particle size. One of the
advantages of using nanoparticles in polymer composites is that the particle size and distribution can be
tuned. Materials that cannot be grown easily as single crystals can be used at the nanoscale and
dispersed in a polymer to take advantage of the single‐crystal properties.

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In general, nanoparticle‐filled polymer display better mechanical properties, at least at low volume
fractions and if well dispersed in the polymer. The reason is that nanoparticles are much smaller than
the critical crack size for polymers and thus don’t initiate failure. Thus nanoparticles provide a way for
simultaneously toughening and strengthening polymers. Proper dispersion is critical, though, for
achieving this.

The size scale of nanoparticle/polymer composites ranges from hybrid nanocomposites, in which the
polymer matrix and the filler are so intimately mixed that they are no longer truly distinct, to discrete
particles in a continuous matrix.  Hybrid nanocomposites often arise by the use of block copolymers, and
this is discussed in a separate section of this chapter (see the section “Polymers”).

In terms of properties, the use of nanoparticle in filling polymers not only can influence the polymer
mechanical properties, but also the polymer mobility and relaxation behaviour, which in turn are
connected with the glass transition temperature of the polymer (Tg), that is, the temperature at which a
polymer becomes brittle on cooling or soft on heating. In general, adding well dispersed, exfoliated
nanofiller increases the Tg of the polymer. Nanoparticle‐filled polymers show also an increase in the
abrasion resistance of the composite.

One of the most exciting opportunities of using nanoparticles in polymer composites is to create
composites with combined functionalities, such as electrically conducting composites with good wear
properties that are optically clear. This is possible since nanoparticles do not scatter light.

Carbon nanotubes in polymer composites
Carbon nanotubes have very distinct properties compared to graphite, as summarized in Table 1 (page
29). In the context of nanocomposites, SWNT are the most promising nanotube fillers. Some properties
are particularly interesting, in particular their flexibility under mechanical stress, their behaviour under
high temperature conditions and their electrical properties. As with other applications that make use of
carbon nanotubes, in composites it has been observed that the processing conditions ultimately affect
nanotubes properties, and as a consequence the composite, as well as the nanotubes purity. Carbon
nanotubes can also be doped, for instance with nitrogen and boron, which changes their surface
reactivity. For instance nitrogen atoms inserted in the lattice of nanotubes makes them more dispersed
is solution (carbon nanotubes are insoluble in water). Modified nanotubes present different electrical
and optical properties and hence their use could lead to composites with novel properties.

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It should be noted that the inclusion of carbon nanotubes in a polymer does not necessarily improve
its mechanical properties. Although in theory the modulus of nanotubes is much higher then any
graphite fibre, and hence of the composite should have outstanding mechanical properties, it has been
demonstrated that a number variables influence this outcome. For SWNT composites, the SWNT are in a
bundle; until SWNT are isolated from the bundles or the bundles are crosslinked, the modulus of
composites made from these materials will be limited. For this reason researchers are concentrating on
developing processing methods that allow obtaining significant volume fractions of exfoliated
nanotubes. As it will be also discussed later for clay‐polymer nanocomposites, the structural
arrangement of the nanofiller within the polymer is also important: if the nanotubes are not straight
when placed in the composite, then the modulus of the composite significantly decreases.

Apart from mechanical enhancement of polymers, nanotubes are also of interest for their electrical
properties. Carbon nanotubes are inherently more conductive then graphite. It has been found for
instance that nanotube/PPV composites show large increase in electrical conductivity compared to
simple PPV, of nearly 8 orders of magnitude. Recently an improvement of 4.5 orders of magnitude in the
electrical conductivity of nanotube/PVA nanocomposites has been reported.

Finally, nanotube/polymer composites are promising in the context of light‐emitting devices. These
devices were developed after the discovery of electroluminescence from conjugated polymer materials
(such as PPV). The practical advantages of polymer‐based LEDs are low cost, low operating voltage, ease
of fabrication and flexibility. Small loadings of nanotubes in these polymer systems are used to tune the
colour of emitted light from organic LEDs.

Polymerclay nanocomposites
In the late 1980s it was discovered that adding 5% in weight of nano‐sized clays to Nylon 6, a synthetic
polymer, greatly increased its mechanical and thermal properties. Since then, polymer‐clay
nanocomposites have been widely studied and many commercial products are available. These hybrid
materials are made of organic polymer matrices and clay fillers. Clays are a type of layered silicates that
are characterized by a fine 2D crystal structure; among these, mica has been the most studied. Mica is
made up of large sheets of silicate held together by relatively strong bonds. Smectic clays, such as
montmorillonite, have relatively weak bonds between layers. Each layer consists of two sheets of silica
held together by cations such as Li+, Na+, K+ and Ca2+. The presence of the cations is necessary for
compensating the overall negative charge of the single layers. The layers are 20‐200 nm in diameter
laterally and come into aggregates called tactoids, which can be about 1 nm or more thick. Naturally
Page 39 of 44



occurring clays include montmorillonite (MMT), hecrite, and heir synthetic equivalents are saponite and
laponite, respectively.

For these layered silicates to be useful as filler in nanocomposites, the layers must be separated, and
the clay throughout mixed in the polymer matrix. This is not trivial as silicate clays are inherently
hydrophilic, whereas polymers tend to be hydrophobic. The solution is to exchange the cations that
keep together the layers in the silicate with larger inorganic ions that can thus open the galleries
between the layers (intercalation). When the silicate layers get completely separated, the material is
called exfoliated. In the case of intercalation, extended polymer chains are present between clay layers,
resulting in a multilayer structure with alternated polymer/clay phases at repeated distances of few
nanometres; in the exfoliated state the silicate layers are totally separated and dispersed in a
continuous polymer matrix. Those two states are schematized in Figure 21.

Figure 21. Principle of formation of clay monolayer
containing nanocomposites with enhanced
mechanical and barrier properties. (Reprinted with
permission from: Weiss, J.; Takhistov, P.;
McClements, D. J., “Functional Materials in Food
Nanotechnology”. In Journal of Food Science 2006;
Vol. 71, pp R107‐R116. Copyright 2006 Wiley‐
Blackwell).

As mentioned before, the fabrication of a polymer‐clay nanocomposite requires mixing two
components that are intrinsically non‐compatible. Surfactants are ionic, and thus interact well with the
clay, but not with the polymer. An ideal solution is the use of “macro‐surfactants” like block copolymers
combining hydrophilic and hydrophobic blocks that can interact with the clay and the polymer,
respectively. For instance poly(ethylene oxide), or PEO, is an excellent intercalation material.

Although homogenous dispersion of the filler in the polymer is an important parameter, another
important aspect is the packing (or alignment) of the filler in the polymer. To understand this concept it
is useful to consider a natural nanocomposite, bone. The unique properties of bone are a list of

Page 40 of 44



apparent contradictions: Rigid, but flexible; lightweight, but solid enough to support tissue; mechanically
strong, but porous. In order to meet these different demands, bone has a hierarchical structure that
extends from the nanoscale to the macroscopic length scale. The hierarchical structure of bone is
responsible for its load transfer ability (see Chapter 2 of Module 1 “Natural Nanomaterials” for details
on the structure of bone). Nanoparticle‐filled polymer composites have mechanical properties that are
actually disappointing compared to theoretical predictions, and this is due to the difficulty in obtaining
well‐dispersed large volume fractions of the reinforcing nanomaterial and a lack of structural control.
Taking this into consideration, recently some scientists have reported the fabrication of ultrastrong and
stiff layered polymer nanocomposites. In this work bottom‐up assembly of clay/polymer nanocomposite
allowed the preparation of a homogeneous, transparent material, where the clay nanosheets are
oriented plenary. It was found that the stiffness and tensile strength of these multilayer nanocomposites
are one order of magnitude grater then those of analogous nanocomposites.

Nanocalys are used as fillers in polymers also to increase the thermal stability of polymers. This
property was first demonstrated in the late 1960s for montmorillonite/PMMA composites. The
dispersion of the clays is critical to increasing the thermal stability (that is, increase the degradation
temperature) of the polymer.

In addition to thermal stability, the flammability properties of many clay/polymer nanocomposites are
also improved. Combining traditional flame retardants with intercalated or, better, exfoliated clays can
result in further improvements in flame retardance.

Finally, polysaccharide/clay nanocomposites are a class of materials that are important especially for the
food industry. These composite make use of naturally occurring polymers, such as starch, mixed with
clay to make biopolymer film with enhanced properties, particularly permeability to water vapour.

Nanocoatings
Nanocoatings are a type of nanocomposites.  The layer thickness of a nanocoating is usually in the 1‐100
nm range. Nanocomposite films include multilayer thin‐films, in which the phases are separated along
the thickness of the film, or granular films, in which the different phases are distributed within each
plane of the film (Figure 22).

Page 41 of 44



Figure 22. Schematic of possible structures for
nanostructured coatings. (Left) multilayeres
microstructure (e.g., TiN/TiC) for
nanocomposite coatings; (right)
homogeneous nanostructured coating (e.g.,
NiCoCrAlY alloy). (Image credit: L. Filipponi,
iNANO, Aarhus University, Creative Commons
Attribution ShareAlike 3.0).

Nanocoatings allow to change properties of some materials, for instance change the transmission of
visible and IR radiation in glass, or to introduce new properties such as “self‐cleaning” effects. In this
document this first type of nanocoating will be discussed under the umbrella term “responsive
nanocoatings”.

Another important area of application of nanocoatings is tribological coatings. Tribology is the science
and technology of interacting surfaces in relative motion. Tribological properties include friction,
lubrication and wear. Tribological coatings are those coatings that are applied to the surface of a
component in order to control its friction and wear. In this area the term “thin films” is often employed
in alternative of nanocoatings due to the fact that it is an area of innovation that started years ago and
has now reached the nanoscale.

Tribological nanocoatings
Tribological coatings play a key role in the performance of internal mechanical components of a vehicle,
such as the engine and power train. They are also key elements in cutting tools and machinery in
general. By reducing wear and friction these coatings increase the lifetime of the working material while
also reducing the dissipation of energy as heat, thus increasing the efficiency of the moving part. When
applied to machinery and tools, tribological coatings can reduce (or eliminate) the need for lubricants,
increase cutting speed, increase the rate of material removal, reduce maintenance costs or reduce
processing cycle times.

Traditional materials used in coatings for tribological applications are carbides, cemented carbides,
metal ceramic oxides, nitrides and carbon‐based coatings. Since the microstructure controls many of the
physical properties of the coating, having a nano‐scaled microstructure may lead to significant
improvements of the coatings´ mechanical (e.g., hardness), chemical (e.g., corrosion resistance) and
Page 42 of 44



electrical properties. Thus nanocomposite coatings are presently investigated in alternative to the
traditional approach of using specific alloying elements in single‐phase coating materials to improve, for
instance, properties such as hardness. One type of nanocomposite coatings are multilayered thin‐films
made of different layers in the orders on nanometres. These films are usually used for their enhanced
hardness and elastic moduli, which is higher in multilayered films than in homogeneous thin films of
either component, and wear properties. Commercial multilayer coatings made with multilayer periods in
the nanoscale range already exist, such as WC/C coatings used in the cutting tool industry. Other
examples are films of alternating layers of TiN and NbN, or TiAlN/CrN multilayers which are more
efficient than TiAlN films.

Responsive nanocoatings
Responsive nanocoatings are those where the properties of the material in the coating react to
environment conditions, such as light or heat, either in a passive or an active way. These coatings allow
changing the properties of some materials, such as glass, by conferring new or improved properties.

The use of glass in modern building is very common, since it allows the construction of transparent and
seemingly lightweight structures. However the relative high transmittance of visible and infrared (IR)
light is a major disadvantage, since this leads to a large heat transfer which is particularly undesirable in
summer. The problem is reversed in winter, when heat is dispersed through the glass. In order to
address these problems various types of nano‐coatings that modulate light transmission in glass are
under investigation and commercialised. The aim is to reduce indoor heating in summer, so less air‐
conditioning is required to keep the ambient cool, with consequent energy saving. One type of coating is
referred to as “low‐e” meaning “low emissivity”, which is based on a thin silver film, about 10nm thick,
surrounded by dielectric layers. Metallic layers have been widely used to increase the reflectivity of light
(and reduce transmittance) for years, but they have the disadvantage of giving a mirror‐like appearance.
Silver looses its metallic appearance when scaled to a nano‐film therefore eliminating this problem. Such
a coating is commercialised by Von Ardenne.

“Low‐e” coatings are a type of passive nano‐coatings, since the properties of the layers are
unperturbed during its operation. Another class of coatings used in glasses are those often indicated as
dynamic or “smart coatings”. In this case the environmental conditions, such as radiation intensity or
temperature, induce a change in the properties of the coating (e.g., darkening of windows). When the
effect is a change in the colour (meaning also transparency) they are called “chromogenic smart
materials”. The change can be induced actively, by pressing a button. This is the case of electrochromic
Page 43 of 44



coatings where applying a small voltage induces a change in transmittance, and in the case of
gaschromic coatings, which change their transmittance at the presence of specific gases. Gaschromic
glazing makes use of the properties of tungsten oxide thin films (WO3) which go from colourless to blue
in the presence of hydrogen with a suitable catalyst. Gaschromic windows follow a double pan model:
on one pane a film of WO3 is deposited with a thin layer of catalysts on top. Hydrogen gas is fed into the
gap producing coloration (windows colour with 1.1‐10 % hydrogen which is below the flammability
concentration). To switch the colour of the window back, another gas is purged in (oxygen). Smart
coatings can also be passive in the sense of changing their optical properties due to a change of external
temperature (thermochromic) or light incidence (photochromic). Another example of nanotechnology
applied to smart coatings is the use of a family of wavelength‐selective films for manufacturing ‘heat
mirrors’. One of these materials is indium‐tin‐oxide (ITO), an infrared absorber. A 300 nm ITO coating on
glass provides more then 80% transmission for the wavelengths predominant in sunlight. The
transmission properties of the window can be varied by changing the thickness and material
composition of the coating, so that a combination of materials could be used to produce smart windows
that reflect solar energy in summer but transmit solar energy in winter.

Selfcleaning surfaces
Another example of functional nanocoating is photocatalytic coatings (commercialised as “self‐
cleaning” glass) which use the catalytic properties of titanium dioxide (TiO2). Pilkington Activ™ Self
Cleaning Glass is a commercial example of a glass with a photocatalytic coating that renders the material
easier to clean. Superhydrophobic coatings have surfaces that mimic the one found in the Lotus leaf
(see Chapter 2 “Natural Nanomaterials” for details) and are developed for many applications that
require resistance to dirt and easiness in cleaning. Both materials are covered in Chapter 2 of Module 2
“Application of nanotechnologies: Environment”.

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Overview of nanomaterials - Nanoscience and nanotechnologies

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