An Introduction to Nano-Science & Nano-TechnologyDr. Abdul Waheed Anwar Nanotechnology Research Center Department of Physics UET Lahore
Nano-materials What is “nano-material” and why we areinterested in it? Optical and electronic properties of nano-materials
Nano-materials Definition: low dimension structures includingquantum wells, quantum wires, and quantum dots Expecting different behavior of electrons in theirtransport (for electronic devices) and correlation (foroptoelectronic devices) from conventional bulk material
Nano-materialsElectron behavior:Quantum well – 1D confined and in parallel plane 2D Bloch waveQuantum wire – in cross-sectional plane 2D confined and 1D Bloch waveQuantum dot – all 3D confined
Nano-materialsBulk semiconductor – plane wave like with effective mass, two different type of electrons identified with opposite sign of their effective mass, i.e., electrons and holes – parabolic band dispersion (E~k) relation – density of states in terms of E: continues square root dependence, with different parameters for electrons/holes in different band
Nano-materials• Quantum well – discrete energy levels in 1D for both electrons and holes – different effective masses in 2D parallel plane for electrons and holes – dispersion (E~k) relation: parabolic bands with discrete states inside the stop- band – density of states in terms of E: additive staircase functions, with different parameters for electrons/holes in different band• Quantum wire – discrete energy levels in 2D cross-sectional plane for both electrons and holes – different effective masses in 1D for electrons and holes – dispersion (E~k) relation: parabolic bands with discrete states inside the stop- band – density of states in terms of E: additive staircase decayed functions, with different parameters for electrons/holes in different band
Nano-materials• Quantum dot – discrete energy levels for both electrons and holes – discrete energy states only – density of states in terms of E: -functions for electrons/holes
Nano-materials Electrons in semiconductors: easily controllable and accessible Electrons in atomic systems: hardly controllable or accessible
Nano-materials Geometrical dimensions in the artificial structure can be tuned to change the confinement of electrons and holes, hence to tailor the correlations (e.g., excitations, transitions and recombinations) The reduced probability of inelastic and elastic collisions (much expected for quantum computing, could be a drawback for light emitting devices) Definite polarization (spin of photons are regulated) (Coulomb) binding between electron and hole is increased due to the localization
Nano-materials• Current technologies – Top-down approach: patterning etching re- growth – Bottom-top approach: patterning etching selective-growth – Uneven substrate growth: edge overgrowth, V- shape growth, interface QD, etc. – Self-organized growth: most successful approach so far
Carbon NanotubesCarbon nanotubes: an important 1-Dmaterial in Nanoscience and nanotechnologywith exceptional properties such as Properties SWNT Comparison Very low density 1.33-1.44 Al 2.7g/cm3 g/cm3 High tensile Upto 63 GPa Steel <2GPa strength High current ≈109 A/cm2 Cu <106 A/cm2 density Excellent heat ≈ 4000 W/mK Diamond ≈ 4000 transmission W/mK Strong temperature Upto 2800 oC Metal in stability in vaccum μchips <1000 oC
Carbon NanotubesWide range of applications: Molecular Electronics Fibres and Fabrics Conductive Plastics Field Emission Conductive Adhesives Sensors DNA-Functionalized Thermal Materials CNT-FET for Medical diagnostic Chemical Sensing medical treatment Cristian Staii et al, Nano Letters, 5, 1774 (2005)
Carbon Nanotubes CNT is a tubular form of two dimensional graphene Zigzag (n1,0) Armchair (n1,n1) Chiral indices n1(4),n2(2) Chiral (n1,n2) Chiral vector Ch=n1a1+n2a2 Chiral angle θ: between Ch and a1
Carbon Nanotubes C1 Metallic SWCNT M Constant DOS at E11 Fermi level V1Semiconducting SWCNT C1 SC E11 Zero DOS at V1 Fermi level The band structure Densities of statesSWCNT Optical properties depend on theallowed electronic transitions between vanHove singularities (vHs)
Carbon Nanotubes Metallic :(n1-n2)mod3=0 Semiconducting:(n1-n2)mod3=1 or :(n1-n2)mod3=2 SWCNT optoelectronic properties depend on chiral indices & diameterwww.sustainability.rit.edu/nanopower/rcn.html
Gold, Silver and Platinum Nano-materials Metals are unique in their physical and chemicalproperties as compared to other compound materialssuch as metal oxides, sulphides and nitrides. Metals have ductility, malleability, luster, highdensity, fewer defects and are generally crystalline innature.
Nano-materialsGold is one of the few metallic elements that can be used innano scale system and devices due to its resistance tooxidation.More over gold has some additional properties at nano scale
Gold, Silver and Platinum Nano-materials The coloring nature of Au and Ag nano-particles was fundamental identification for their nano-particle colloid formation. Making use of this, they have been used as coloring agents in decorative glasses and clothing. This is due to light-absorbing nature of the surface of Au and Ag nano-particles because of the surface plasmon resonance.
What Is Nano? Pt nano-particles are catalytically active for oxidation andreduction reactions. As a result, these nano-materials find applications for catalyticuse. Since Au, Ag and Pt nano-particles have considerable stabilityas compared to other metals, they have gained importance.However, in the near future, all metals will be possibly shapedin nano size by using suitable stabilizing agents and medium.
Electronic Properties• Ballistic transport – a result of much reduced electron-phonon scattering, low temperature mobility in QW (in-plane direction) reaches a rather absurd value ~107cm2/s-V, with corresponding mean free path over 100m• Resulted effect – electrons can be steered, deflected and focused in a manner very similar to optics, as an example, Young’s double slit diffraction was demonstrated on such platform
Electronic Properties• If excitation (charging) itself is also quantized (through, e.g., Coulomb blockade), interaction between the excitation quantization and the quantized eigen states (i.e., the discrete energy levels in nano-structure) brings us into a completely discrete regime• Resulted effect – a possible platform to manipulate single electron to realize various functionalities, e.g., single electron transistor (SET) for logical gate or memory cell
Optical Properties• Discretization of energy levels increases the density of states• Resulted effect – enhances narrow band correlation, such as electron-hole recombination.
Optical Properties• Discretization of energy levels reduces broadband correlation• Resulted effect –reduces temperature dependence; which is very much needed in quantum computing and reduces device performance temperature dependence
Optical Properties• Quantized energy level dependence on size (geometric dimension)• Resulted effect – tuning of optical gain/absorption spectrum
Quantum DotQuantum dots are semiconductor very small nanocrystalswhich can be considered as dimensionless.Quantum dots range from 2-10 nanometers (10-50 nm) indiametersAn exciton pair is defined as an electron and hole pair.An exciton Bohr radius is the distance in an electron holepair The size of QD is of the same order as the radius of excitonBohr radius