Predicting the Electronic Properties of 3D, Million-Atom Semiconductor
Nanostructure Architectures.

Abstract of the proposal

As recently as five years ago, a respectable theoretical challenge in the area
of quantum nanostructures was the prediction of energy levels and structural
properties of ~100-atom clusters. While there certainly remain some important
unsolved computational problems in this size regime, recent experimental
advances in nanoscience and nanotechnology now pose staggeringly more difficult
challenges to theory and simulation efforts. The challenges are in three areas:
(i) the need to address large single nanostructures, in the range of 105-106
atoms, (ii) the complexity of the underlying electronic phenomena, involving
quantum entanglement, excitonic complexes, exchange-induced fine-structure,
Coulomb and spin blockades, unusual decay mechanisms of excited states, etc.,
and, (iii) the emergence of nanostructure systems, involving combined 3D
architectures of quantum dots, wires and wells (0D, 1D, 2D, respectively),
as the main vehicle for nano devices and nano technology. Indeed, experiments
seem to currently be far more advanced than theory in these areas of quantum
dots, with the main conferences [ NANO-7  in Sweden,  MSS-10  in Austria,
Excitons in Confined Systems  in Montpelier,  Nanoscience  in Japan, the
International Conference on the Physics of Semiconductors  (ICPS) in Scotland,
as well as the American APS, MRS and EMC] carrying well over 90% experimental
talks, with most theory talks based on highly simplified, continuum
effective-mass models.

The challenges now posed to theory and simulations of nanostructures by
contemporary experiments call for a significant and crucial involvement of
applied math and computational science. Even though many of the methods we use
for quantum nanostructures (FFT, iterative-diagonalizations; multi-variable
optimization; genetic algorithms; determinant many-body expansions, etc.) are
commonly used in quantum-mechanical modeling, these approaches were never
before pushed to the limit of ~106 atom systems. Our proposal represents a
unique opportunity to test and expand the existing algorithms and applied math
methods in the limit of million-atom systems demanded by current experimental
advances in nanoscience.

This proposal assembles a world-class team of physicists, computer scientists
and mathematicians from three national laboratories, a University and support
from NERSC to develop the blue print for the next generation methodology of
theory of million-atom nanostructure architectures. NREL's newly established
Computational Sciences Center (featuring, among others, K. Kim and W. Jones,
our collaborators in this proposal) is an important addition, committed to the
success of this endeavor. The active participation of expert mathematicians
(J. Dongarra and O. Marques) collaborating with the physicists who developed
the pioneering basic techniques in nanostructure simulations (A. Zunger,
L.-W. Wang and A. Franceschetti), will ensure fruitful synergism. We will build
on the methodology we have developed over the past five years, which is
currently the only quantum mechanical approach capable of atomistically
describing the properties of large nanostructures.

We propose to embark on four novel directions: (a) replace the previous,
empirical pseudopotential approach by ab-initio methodology, (b) address the
complex nanoscience phenomenology of entanglement, exciton complexes, Coulomb
and spin-blockade noted above, (c) address nanostructure systems made of 0D,
1D and 2D building blocks, and (d) offer design capabilities, via the  inverse
band structure  approach, i.e., predict the structure of systems that have
prescribed electronic properties.