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Different Electronic Materials. Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …) Insulators: SiO 2 , Al 2 O 3 , Si 3 N 4 , SiO x N y , ... Conductors: Al, Au, Cu, W, silicide, ...

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different electronic materials
Different Electronic Materials
  • Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …)
  • Insulators: SiO2, Al2O3, Si3N4, SiOxNy, ...
  • Conductors: Al, Au, Cu, W, silicide, ...
  • Organic and polymer: liquid crystal, insulator, semiconductor, conductor, superconductor
  • Composite materials: multi-layer structures, nano-materials, photonic crystals, ...
  • More: magnetic, bio, …
insulators conductors semiconductors inorganic materials
Insulators, Conductors, SemiconductorsInorganic Materials






conduction band













Eg < 5eV

Eg > 5eV




valence band


Insulator Semiconductor Conductor

Si: Eg = 1.1 eV

Ge: Eg = 0.75 eV

GaAs: Eg = 1.42 eV

SiO2: Eg = 9 eV

electronic properties device function of molecules
Electronic properties & device function of molecules
  • Electrons in molecule occupy discrete energy levels---molecular orbitals
  • Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are most important to electronic applications

Bandgap of molecule: Eg = E(LUMO) - E(HOMO)

  • Organic moleculeswith carbon-based covalent bonds, with occupied bond states ( band) as HOMO and empty antibonding states (* band) as LUMO

Lower energy by delocalization:

 Benzene Biphenyl

Conducting Polymers

Polyacetylene: Eg ~ 1.7 eV

 ~ 104 S cm-1

Polysulphur nitride (SN)n

 ~ 103-106 S cm-1

Poly(phenylene-vinylene) (PPV)

High luminescence efficiency

diodes and nonlinear devices molecule with d a structure c 16 h 33 q 3cnq
Diodes and nonlinear devicesMolecule with D--A structure C16H33Q-3CNQ



Highly conductive zwitterionic D+--A- state at 1-2V forward bias Reverse conduction state D---A+ requires bias of 9V

I-V curve of Al/4-ML C16H33Q-3CNQ LB film/Al structure


Negative differential resistance (NDR): electronic structural change under applied bias, showing peak conductance


Self-assembled layer between Au electrodes

NDR peak-to-valley ratio ~ 1000


Molecular FET and logic gates

Molecular single-electron transistor:

Could achieve switching frequency > 1 THz


Assembly of molecule-based electronic devices

“Alligator clips” of


Attaching functional atoms

S for effective contact to Au

High conductance through leads but surface of body is insulating


Self-assembled Molecular (SAM) Layers

Carene on Si(100)

Simulated STM images

for (c)

for (a)

0.1 ML 1-nitronaphthalene adsorbed on Au(111) at 65 K

Ordered 2-D clusters


Conventional Organic Electronic Devices

Organic Thin Film Transistors (OTFT)

Organic Light Emitting Diode (OLED)

For large-area flat-panel displays, circuit on plastic sheet



Soft-lithographic process in fabrication of organic electronic circuits


Unique electronic & opto-electronic properties of nanostructures

  • DOS of reduced dimensionality (spectra lines are normally much narrower)
  • Spatial localization
  • Adjustable emission wavelength
  • Surface/interface states

Effective bandgap blue-shifted, and adjustable by size-control


Optical properties of quantum dot systems

Excitons in bulk semiconductors

An e-h pair bound by Coulomb potential

H-atom like states of exciton in effective-mass approximation:

M = me*+ mh*, ħK: CM momentum

 = me*mh*/(me*+ mh*) reduced mass

Bohr radius of the exciton:

Bohr radius of electron or hole:

(a0 = 0.529 Å)

aB = ae + ah


In GaAs (me*= 0.067m0, mhh*= 0.62m0, r = 13.2)

Binding energy (n = 1): 4.7 meV, aB = 115 Å

Generally, binding energy in meV range, Bohr radius 50-400 Å

Excitons in QDs

Bohr radius is comparable or even much larger than QD size R

Weak-confinement regime:

R >> aB, the picture of H atom-like exciton is still largely valid:


Strong confinement regime (R << ae and ah): model of H atom-like exciton is not valid, confinement potential of QD is more important.

Lowest energy e-h pair state {1s, 1s}:


Production of uniform size spherical QDs

Controlled nucleation & growth in supersaturated solution

All clusters nucleate at basically same moment, QD size distribution < 15%

QDs of certain average size are obtained by removing them out of solution after a specific growth period

Further size-selective processing to narrow the distribution to  5%


Similar nucleation and growth processes of QDs also occur in glass (mixture of SiO2 and other oxides) and polymer matrices

Ion implantation into glass + annealing

Mono-dispersed nanocrystals of many semiconductors, such as CdS, CdSe, CdTe, ZnO, CuCl, and Si, are fabricated this way

Optimal performance of QDs for semiconductor laser active layers requires 3D ordered arrays of QDs with uniform size

In wet chemical QDs fabrication: proper control of solvent composition and speed of separation


In SK growth of QDs: strain-mediated intra- and inter-layer interactions between the QDs

Aligned array of GaN QDs in AlN


Passive optic devices with nanostructures:Photonic Crystal

An optical medium with periodic dielectric parameter r that generates a bandgap in transmission spectrum


Luminescence from Si-based nanostructures

Luminescence efficiency of porous Si (PSi) and Si QDs embedded in SiO2 ~ 104 times higher than crystalline Si

Fabrication of PSi: electrochemical etching in HF solution, positive voltage is applied to Si wafer (anodization)

Sizes of porous holes: from nm to m, depending on the doping type and level


Nano-finger model of PSi: from Si quantum wires to pure SiO2 finger with increasing oxidation

Emission spectrum of PSi: from infrared to the whole visible range


Remarkable increase in luminescence efficiency also observed in porous GaP, SiC

Precise control of PSi properties not easy

Si-based light emitting materials and devices

Digital Display


Atomic structures of carbon nanotubes

Stable bulk crystal of carbon Graphite

Layer structure: strong intra-layer atomic bonding, weak inter-layer bonding

3.4 Å

1.42 Å


Enclosed structures: such as fullerene balls (e.g., C60, C70) or nanotubes are more stable than a small graphite sheet

Trade-off: curving of the bonds raises strain energy, e.g., binding energy per C atom in C60 is ~ 0.7 eV less than in graphite

MWNT, layer spacing ~ 3.4 Å



Index of Single-wall Carbon Nanotubes (SWNT)

Armchair (n, n)

Zigzag (n, 0)

General (m, n)


Synthesis of CNTs by Laser vaporization: Pulsed laser ablation of compound target (1.2% at. Co-Ni + 98.8% C)

High yield (~70%) of SWNT ropes


Carbon arc discharge: ~500 Torr He, 20-25 V across 1-mm gap between 2 carbon rods

Plasma T > 3000C, CNT bundles deposited on negative electrode

With catalyst (Co, Ni, Fe, Y, Gd, Fe/Ni, Co/Ni, Co/Pt)


Without catalyst



Vapor-phase synthesis: similar to CVD

Substrate at ~ 700-1500C decorated with catalyst (Co, Ni or Fe) particles, exposed to hydrocarbon (e.g. CH4, C6H6) and H2

Aligned CNTs grow continuously atop of catalyst particles

Regular CNT arrays on catalyst pattern

Useful for flat panel display


Growth mechanisms of C nanotubes

1) C2 dimer addition model: C2 dimer inserted near pentagons at cap

2) Carbon addition at open ends: attach C2 at armchair sites and C3 at zigzag sites

Functions of catalyst clusters: stabilizing terminators, cracking of hydrocarbons

Fit the controlled CVD process, the open-end is terminated by a catalyst cluster


Structural identification of nanotubes: with TEM, electron diffraction, STM

HRTEM: number of shells, diameter

STM: diameter, helicity of nanotube out-shell, electronic structure


Electronic properties of SWNTs

SWNTs: 1D crystal

If m - n = 3qmetallic

Otherwise semiconductor

Zigzag, dt = 1.6nm

=18, dt = 1.7nm

Bandgap of semiconducting SWNTs:

=21, dt = 1.5nm

=11, dt = 1.8nm

Armchair, dt = 1.4nm

= 1.42 Å,  5.4 eV, overlap integral

STM I-V spectroscopy


Junctions between SWNTs:homojunctions, heterojunctions, Schottky junctions, but how to connect and dope?

SWNT connections: insert pentagons and heptagons

Natural SWNT Junctions


Doping of semiconductor SWNTs

N, K atoms  n-type; B atoms, oxygen  p-type

SWNT CMOS inverter & its characteristics


Other nanotubes and nanowires

GaN nanowires

BN nanotubes

p-Si/n-GaN nanowire junction

Si nanowires