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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