We are conducting a comprehensive simulation of the
electrical, optical, structural, and transport properties of
various nanowires, with the focus on their size dependence.
The goal is to make use of the computational capabilities
provided by today's information technology to perform
theoretical modeling of materials that may play a key role in
the hardware development for tomorrow's information
technology. Issues being examined include stability and
growth, electronic structure, vibrational modes, conductance,
and nanocontacts.
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Insulating broken nanowire |
Conducting H2-welded nanowire |
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Band-by-band charge distribution in Si nanowire |
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 Project
Summary
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Motivated by recent fabrication developments, the initial
focus will be on quantum wires, namely, one-dimensional
nanostructures.
A comprehensive study of the electronic, optical,
structural, and transport properties of various semiconductor
nanowires.
The goal is to make use of the computational capabilities
provided by today's information technology to perform
theoretical modeling of materials that may play a key role in
the hardware development for tomorrow's information
technology.
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Physical
Issues
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Stability and growth
- Catalyst assisted vapor-liquid-solid growth:
size-dependent orientation
- Thermal evaporation synthesis of nanobelts of
semiconducting oxides: rectangular cross sections,
atomically flat surfaces, unique orientation
Electronic Structure: band gaps, excitons, and optical
properties
Electrons and holes are confined in two out of three
dimensions.
- Gap dependent on the diameter AND orientation of the
wire
- Excitons with larger binding energies and oscillator
strengths
- Quantization effect in collective electronic excitations
(plasmons)
"Phonons" in Nanostructures
Thermal properties important for heat conduction and power
dissipation
Confinement effect
- broadening and shift of peaks
- acoustic phonon dispersion and group velocity modified
- phonon distribution modified by boundary scattering
Size and shape dependence
Device Simulations (conductance, contacts, etc.)
Conductance spectra exhibit size-dependent oscillations due
to interference resonance from scattering with the contacts
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Computational Methods
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- First-principles molecular dynamics simulations within
density functional theory with pseudopotentials and plane
waves
- Stability, growth, energetics, electronic wave functions,
vibrational modes, etc.
- Quantum Monte Carlo methods (variational, VMC and
diffusion, DMC)
Energy gap, excitation energies, algorithm development (linear
scaling with nonorthogonal Wannier functions, calculation of
optical transition strength using DMC to obtain the
imaginary-time correlation function)
- Many-body perturbation theory
GW quasiparticle energies, optical excitations including
exciton effects (Bethe-Salpeter equation, evaluate the Coulomb
scattering matrix in real space using Wannier functions)
- First-principles calculation of conductance
Recursion-transfer-matrix method to solve the coupled
differential equation involving reflected and transmitted
waves (Hirose and Tsukada) and eigenchannel analysis for the
transmission
(Brandbyge et al.)
- The Green's function method and the self-consistent
Lippmann-Schwinger equation with scattering boundary condition
(Lang et al.)
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Educational
and Outreach Activities
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- Train students (undergraduate and graduate) and postdocs
in computational techniques for materials simulations
- Involve undergraduate students in materials research
through the existing REU program at Georgia Tech
- Partnership between Georgia Tech and Clark Atlanta
University (a Historically Black University): co-advising
Ph.D. students; regular exchange visits of faculty and
students; joint seminars; joint courses; joint workshops
- Minority students in the project: Alexis Nduwimana
(Georgia Tech), Damian Cupid (Clark Atlanta), Anthony
Cochran (Clark Atlanta), Carmen Robinson (Clark Atlanta),
Robert Easley, Jr. (Clark Atlanta)
2003 Activities
- Information Technology Research Seminars
- Mini-workshop on Quantum Approximate Methods for Novel
Materials (Clark Atlanta University, October 2003); all
participants are minority students
- Special course "Physics of Small Systems" taught by
Landman
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