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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, ...

Organic and polymer:

conductor, superconductor liquid crystal, insulator, semiconductor,

Composite materials:

photonic crystals, ...

multi-layer structures, nano-materials,

More:

magnetic, bio, …

Insulators, Conductors, Semiconductors

Inorganic Materials

E conduction band empty Forbidden region Band gap

E g >

5eV valence band filled Insulator SiO 2 : E g = 9 eV E E conduction band Band gap

E g <

5eV + valence band

electron hole

partially-filled band Semiconductor Si: E g Ge: E g = 1.1 eV = 0.75 eV GaAs: E g = 1.42 eV Conductor

Electronic properties & device function of molecules

 Electrons in molecule occupy discrete energy levels---

molecular orbital

s  Highest occupied molecular orbital ( HOMO ) and lowest unoccupied molecular orbital ( LUMO ) are most important to electronic applications Bandgap of molecule:

E

g =

E

(LUMO) -

E

(HOMO) 

Organic molecules

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

Lower energy by

delocalization

: Benzene Conducting Polymers Polyacetylene: E g  ~ 10 4 ~ 1.7 eV S cm -1  Polysulphur nitride (SN) n ~ 10 3 -10 6 S cm -1 Poly(phenylene-vinylene) (PPV) High luminescence efficiency Biphenyl

Diodes and nonlinear devices Molecule with D-



-A structure C

16

H

33

Q-3CNQ

D

 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 C 16 H 33 Q-3CNQ LB film/Al structure

A

Negative differential resistance (NDR) : electronic structural change under applied bias, showing peak conductance 2’-amino-4-ethynylphenyl-4’ethynylphenyl-5’-nitro-1-benzennthiol Self-assembled layer between Au electrodes NDR peak-to-valley ratio ~ 1000

Molecular FET and logic gates Molecular single-electron transistor: Could achieve frequency > 1 THz switching

Assembly of molecule-based electronic devices

“ Alligator clips ” of molecules: 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

Self-assembled patterns of

trans

-BCTBPP on Au(111) at 63 K

Interlocking with CN groups

Conventional

Organic Electronic Devices

Organic Thin Film Transistors (OTFT) Organic Light Emitting Diode (OLED) For large-area flat-panel displays, circuit on plastic sheet

Printing:

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:

E



E g

  2 K 2 2

M

 13 .6

n 2  m 0 

r

2 (eV) M =

m

e * +

m

h * , ħ

K

: CM momentum  =

m

e *

m

h * /(

m

e * +

m

h * ) reduced mass

Bohr radius a B

 

r m

0  of the exciton:

a

0 (

a

0 = 0.529 Å) Bohr radius of electron or hole:

a e

,

h

 

r m

0

m

*

e

,

h a

0

a

B =

a

e +

a

h

In GaAs (

m

e * = 0.067

m

0 ,

m

hh * = 0.62

m

0 ,  r = 13.2) Binding energy (n = 1): 4.7 meV,

a

B = 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

>>

a

B , the picture of H atom-like exciton is still largely valid:

E



E g

  2  2 2

MR

2  13 .6

 n 2 m 0 

r

2 (eV)

Strong confinement regime

(

R

<<

a

e and

a

h ): model of H atom-like exciton is not valid, confinement potential of QD is more important. Lowest energy e-h pair state {1s, 1s}:

E

(

R

) 

E g

  2  2

R

2 2     1

m

*

e

 1

m

*

h

     4 1 .

8

e

 0  2

r R

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 SiO 2 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 generates a bandgap in transmission spectrum that

Luminescence from Si-based nanostructures

Luminescence efficiency of porous Si (PSi) and Si QDs embedded in SiO 2 ~ 10 4 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 SiO 2 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., C 60 , C 70 ) 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 C 60 is ~ 0.7 eV less than in graphite MWNT, layer spacing ~ 3.4 Å SWNT

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)  SWNTs Without catalyst  MWNTs

Vapor-phase synthesis: similar to CVD

Substrate at ~ 700-1500  C decorated with catalyst (Co, Ni or Fe) particles, exposed to hydrocarbon (e.g. CH 4 , C 6 H 6 ) and H 2 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) C 2 dimer addition model : C 2 dimer inserted near pentagons at cap 2) Carbon addition at open ends: attach C 2 sites and C 3 at armchair 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

= 3

q



metallic

Otherwise 

semiconductor

Zigzag,

d

t = 1.6nm

 =18  ,

d

t = 1.7nm

 =21  ,

d

t = 1.5nm

 =11  ,

d

t = 1.8nm

Bandgap of semiconducting SWNTs:

E g



t a C



C d t

Armchair,

d

t = 1.4nm

STM I-V spectroscopy

a C



C t

 5.4 eV, overlap integral

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

BN nanotubes GaN nanowires Si nanowires p-Si/n-GaN nanowire junction