Molecular ElectronicsMolecular Electronics

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, SemiconductorsInorganic Materials

E

valence band filled

conduction band empty

Forbiddenregion Eg > 5eV

Bandgap

E

conduction band

Eg < 5eVBandgap

+

-electronhole

E

valence band

partially-filledband

Insulator Semiconductor ConductorSi: Eg = 1.1 eVGe: Eg = 0.75 eVGaAs: Eg = 1.42 eV

SiO2: Eg = 9 eV

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

Lower energy by delocalization:

Benzene Biphenyl

Conducting Polymers

Polyacetylene: Eg ~ 1.7 eV

σ ~ 104 S cm-1

Polysulphur nitride (SN)n

σ ~ 103-106 S cm-1

Poly(phenylene-vinylene) (PPV)

High luminescence efficiency

Diodes and nonlinear devices

Molecule with D-σ-A structure C16H33Q-3CNQ

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

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

film/Al structure

AσD

Self-assembled layer between Au electrodes

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

2’-amino-4-ethynylphenyl-4’ethynylphenyl-5’-nitro-1-benzennthiol

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

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

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

Ordered 2-D clusters

Carene on Si(100)

Simulated STM images

for (c)

for (a)

Self-assembled patterns of trans-BCTBPP on Au(111) at 63 K

Interlocking with CN groups

Organic Thin Film

Transistors (OTFT)Organic Light Emitting Diode

(OLED)

Conventional Organic Electronic Devices

For large-area flat-panel displays,

circuit on plastic sheet

Printing:

Soft-lithographic

process in

fabrication of

organic electronic

circuits

Production of uniform size spherical QDs

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%

Controlled nucleation & growth in supersaturated solution

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

Passive optic devices with nanostructures: Photonic Crystal

An optical medium with periodic dielectric parameter εr 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 µm, 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 SWNT

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

Electronic properties of SWNTs

SWNTs: 1D crystal

If m - n = 3q → metallic

Otherwise → semiconductor

Zigzag, dt = 1.6nm

θ=18°, dt = 1.7nm

θ=21°, dt = 1.5nm

θ=11°, dt = 1.8nm

Armchair, dt = 1.4nm

STM I-V spectroscopy

Bandgap of semiconducting SWNTs:

tdCCat

gE −=

= 1.42 Å, ≈ 5.4 eV, overlap integral

t