nano tech lecture2 dr. a. waheed anwar
TRANSCRIPT
An Introduction to Nano-Science & Nano-Technology
Nanotechnology Research CenterDepartment of Physics UET Lahore
Dr. Abdul Waheed Anwar
Nano-materials
What is “nano-material” and why we are interested in it?
Optical and electronic properties of nano-materials
Nano-materials
Definition: low dimension structures including quantum wells, quantum wires, and quantum dots
Expecting different behavior of electrons in their transport (for electronic devices) and correlation (for optoelectronic devices) from conventional bulk material
Nano-materials
Electron behavior:
Quantum well – 1D confined and in parallel plane 2D Bloch wave
Quantum wire – in cross-sectional plane 2D confined and 1D Bloch wave
Quantum dot – all 3D confined
Nano-materials
Bulk 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 Nanotubes
Properties SWNT Comparison
Very low density 1.33-1.44 g/cm3
Al 2.7g/cm3
High tensile strength
Upto 63 GPa Steel <2GPa
High current density
≈109 A/cm2 Cu <106 A/cm2
Excellent heat transmission
≈ 4000 W/mK Diamond ≈ 4000 W/mK
Strong temperature stability
Upto 2800 oC in vaccum
Metal in μchips <1000 oC
Carbon nanotubes: an important 1-D material in Nanoscience and nanotechnology with exceptional properties such as
Carbon Nanotubes
Wide range of applications:
Molecular Electronics
Fibres and Fabrics Conductive Plastics Field Emission Conductive Adhesives Sensors Thermal Materials Medical diagnostic medical treatment
DNA-Functionalized CNT-FET for Chemical Sensing
Cristian Staii et al, Nano Letters, 5, 1774 (2005)
Carbon Nanotubes
Zigzag (n1,0) Armchair (n1,n1)Chiral (n1,n2)
CNT is a tubular form of two dimensional graphene
Chiral indices n1(4),n2(2)
Chiral vector Ch=n1a1+n2a2
Chiral angle θ: between Ch and a1
Carbon Nanotubes
http://www.nano-lab.com/nanotube-image3.html
10 nm
DWCNT CCVD Peapods C60@SWCNT Arc Discharge
Nanoscience and Technology,2005,Part III,203-224
MWCNT
DWCNT
diameter : 1.2 nmSWCNT
http://www.almaden.ibm.com/st/past_projects/nanotubes/?page3
Carbon Nanotubes
C1
V1
C1
V1
EM
11
The band structure Densities of states
Metallic SWCNT
Semiconducting SWCNT
SWCNT Optical properties depend on the allowed electronic transitions between van Hove singularities (vHs)
ESC
11
Constant DOS at Fermi level
Zero DOS atFermi level
Carbon Nanotubes
www.sustainability.rit.edu/nanopower/rcn.html
Metallic :(n1-n2)mod3=0
Semiconducting:(n1-n2)mod3=1 or :(n1-n2)mod3=2
SWCNT optoelectronic properties depend on chiral indices & diameter
Gold, Silver and Platinum Nano-materials
Metals are unique in their physical and chemical properties as compared to other compound materials such as metal oxides, sulphides and nitrides.
Metals have ductility, malleability, luster, high density, fewer defects and are generally crystalline in nature.
Gold, Silver and Platinum Nano-materials
Nano-materials
Gold is one of the few metallic elements that can be used in nano scale system and devices due to its resistance to oxidation.
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 coloringagents in decorative glasses and clothing.
This is due to light-absorbing nature of thesurface of Au and Ag nano-particles because of the surface plasmon resonance.
What Is Nano?
Pt nano-particles are catalytically active for oxidation and reduction reactions.
As a result, these nano-materials find applications for catalytic use.
Since Au, Ag and Pt nano-particles have considerable stability as compared to other metals, they have gained importance.
However, in the near future, all metals will be possibly shaped in 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 Dot
Quantum dots are semiconductor very small nano crystalswhich can be considered as dimensionless.
Quantum dots range from 2-10 nanometers (10-50 nm) in diameters
An exciton pair is defined as an electron and hole pair.
An exciton Bohr radius is the distance in an electron hole pair
The size of QD is of the same order as the radius of exciton Bohr radius
What Is Nano?