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?