MESOSCOPIC STRUCTURE.pptx
- 2. MESOSCOPIC STRUCTURE Meso-scopic structure deals with materials of an intermediate length. The structures which have a size between the macroscopic world and the microscopic or atomic one are called meso-scopic structure. These structures have size usually range from a few nanometres to about 100 nm. The dimensions (size) of the solid is of the order of, or smaller than these characteristics lengths, the material might show new properties.
- 3. characteristics lengths construct semiconductor nanostructures with one or two of their dimensions of the order of, or smaller than B. (ii) Mean free path The distance travelled by the electron between two inelastic collisions is usually called the mean free path le of the electron in the solid. (iii) Diffusion length (i) de Broglie wavelength mean free path le is much larger than L, the particle moves throughout the structure without scattering. This is the so-called ballistic transport if le le is less than L, transport can be explained as a diffusion process.
- 4. COHERENT TRANSPORT • The distance travelled by the electron without the carrier wave changing its phase is defined as phase coherence LФ • In mesoscopic systems (ballistic regime) electrons are practically unscatterred - LФ should be a length similar to the inelastic scattering mean free path le. Coherent states can evidently show interference effects – Phasing • once the coherent states loose their coherence, by inelastic scattering, the corresponding waves cannot be superposed and cannot show interference. The loss of coherence is usually called dephasing. • show interference effects over distances smaller than LФ.
- 5. CONDUCTANCE FLUCTUATIONS • electrical conductance will vary from sample to sample, mainly due to inhomogeneous scattering sites. LФ larger than Lm
- 6. Factors influence Conductance fluctuation • At zero temperature without de-coherence • Factors: the symmetry and the shape of the sample. anisotropy of Fermi surface. In diffusive samples with many impurities, these changes in interference ( due to magnetic field or gate voltage). larger structures, the interference contributions from different phase-coherent units of size LФ, are statistically averaged.
- 7. QUANTUM INTERFERENCE EFFECT • Magnetic fields can produce and control interference effects between the electrons in solids. • The phase coherence length LФ is the distance travelled by an electron without changing its phase. The phase of an electron wave is generally destroyed when electrons interact inelastically with defects in the lattice. • ballistic electrons with a mean free path show the interference effects when le >> L
- 9. QUANTUM INTERFERENCE EFFECT • phase difference between the waves travelling around upper and lower paths ∆ɵ The intensity of the interference of the waves Applications of Quantum Interference Effect Superconducting Quantum Interference Device (SQUID). quantum cryptography. quantum computing and quantum interference transistor.
- 10. Quantum Interference Transistor (QUIT) • Electrons are made to propagate through two arms of the quantum wire ring. • an electron wave enters the ring from left to right. The wave entering through “A” gets split up into two partial waves. A constructive interference can be expected to occur at“B”. • constructive interference - reduces the resistance of the ring. • destructive interference - increases the resistance of the ring.
- 11. MAGNETIC SEMICONDUCTORS The semiconducting materials which exhibit both ferromagnetism and useful semiconductor properties. Magnetic semiconductors can also allow control of quantum spin state (up or down). spin polarization which is an important property for spintronics applications (Ex: spin transistors)
- 12. Dilute Magnetic Semiconductors (DMS) • Based on traditional semiconductors, but they are doped with transition metals instead of, or in addition to, electronically active elements.
- 13. SPINTRONICS
- 14. SPINTRONICS • The ‘spin’ of the electron can be used rather than its charge to create a remarkable new generation. These are smaller, more versatile and more robust. • spintronic devices act according to the simple scheme. 1. Information is stored (written) into spins as a particular spin orientation (up or down). 2. The spins, being associated to mobile electrons, carry the information along a wire. 3. The information is read at a terminal.
- 15. Spin-FET • Spin polarized Field Effect Transistor. • a non magnetic layer is used for transmitting and controlling the spin polarized electrons from source to drain and it plays a crucial role.
- 16. SPINFET
- 17. • first the spins have to be injected from source into this non-magnetic layer and then transmitted to the collector. • The injected spins which are transmitted through this layer start precessing ,before they reach the collector due to the spin-orbit coupling effect. • When Vg is zero - injected spins which are transmitted through the 2DEG (2-Dimensional Electron Gas) layer starts precessing before they reach the collector, thereby reducing the net spin polarization.
- 18. Carbon
- 19. Different structures of carbon
- 20. Carbon nanotubes • Hexagonal lattice of carbon is simply graphite • Single layer is called graphene • It consists of a graphene layer which is rolled upto into cylindrical shape • CNT is hollow cylinders of extremely thin diameter, 10000 times smaller than a human hair
- 22. Types of CNT structures • Rolling a graphite sheet with different orientation about the axis Three types • Armchair structure When the axis of the tube parallelto C-C bonds of the carbon hexagons • Zig zag structure Tube axis perpendicular to C-C bonds • Chiral Structure C-C bond inclined to tube axis
- 24. Classification of CNT • Based on number of layers i) Single walled CNT (SWCNT) Single graphene cylinder ii) Multi walled CNT (MWCNT) Several concentric graphene cylinders
- 29. Properties • Electrical properties Metal / semiconductor - depends on dia and chirality bandgap decreases with increase of dia bandgap varies along tube axis due to extra energy states at the ends conduction occurs through discrete electronic states
- 30. • Mechanical properties High strength due to strong C-C bond Young’s modulus 10 times grater than that of steel Able to withstand extreme strain recover from structural distortions
- 31. Physical properties high strength – weight ratio higher surface area than graphite Chemical properties highly resistant to chemical reactions difficult to oxidize Thermal properties high thermal conductivity conductivity increases with decrease in diameter
- 32. Applications of CNT Flat panel display due to high field emission Vacuum tube lamp, FET device, chips because of low resistance Computer switching device Charges are stored Hydrogen can be stored Gas sensor Catalysts Increase the tensile strength of steel Shielding material for electromagnetic radiation
- 37. Applications of Nanophase materials Materials Technology Synthesis harder metals Flexible / dense ceramics and insulator Nano Polymers used as filters Unusual color paints Nano transistors, ceramic capacitors Information Technology Data storage used as read write heads and sensors Bio medicals Controlled drug delivery finds the applications as an implant material
- 38. Energy Storage ionic batteries magnetic refrigeration hydrogen storage device Optical device semiconducting laser sunscreens coatings for eye glasses to product scratch/ breakage Signal processing elements Integrated circuits, mass and pressure sensors Robotic machines on a molecular scale Under water nano sensors