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Our research project focused on accurate theoretical prediction of the structural, electronic, and optical properties of core-shell semiconductor nanocrystals composed of wide band gap semiconductors, such as ZnSe, ZnTe, CdSe, and CdTe. The study was carried out using first principles density functional and time-dependent density functional methods implemented on a parallel computational platform. The aim of this research project was to expand the knowledge of chemistry and physics of semiconductor core-shell heterostructures and facilitate the development of composite materials for efficient conversion of solar energy.

Inorganic and organic composites containing luminescent wide-gap semiconductor nanocrystals have recently emerged as a promising new class of photovoltaic materials that can be utilized in the next generation of solar cells. These materials represent an attractive alternative to traditional light harvesting assemblies made from bulk semiconductors. Semiconductor nanocrystals exhibit a size-dependent variation of the absorption and emission optical gaps due to quantum confinement effects. The ability to tune the size of the optical gap in these nanocrystals makes it possible to design photovoltaic systems that can capture a wider portion of the solar spectrum than bulk semiconductors. However, the use of semiconductor nanoparticles in photovoltaic cells has so far been hindered by fast nonradiative Auger recombination. Recent studies demonstrated that the rate of Auger recombination can be substantially suppressed in multicomponent core-shell semiconductor nanocrystals due to the spatial separation of the electron and hole wave functions in these structures. This makes core-shell semiconductor nanoparticles highly attractive for use in tunable lasers and nanocomposite solar cells.

During the second year of the project, our research was focused on the development of computational methodology and software tools for studying the properties of core-shell semiconductor nanoparticles, the selection and testing of an efficient surface passivation scheme for group II-VI semiconductor nanocrystals, and theoretical calculations of the structures, electronic characteristics, and absorption gaps of traditional and inverted core-shell semiconductor nanocrystals.

Theoretical modeling of core-shell nanoparticles presents significant challenges to computational methods employed in quantum chemistry and condensed matter physics. The structural complexity and large size of these systems necessitates the use of efficient numerical techniques in conjunction with parallel computational algorithms. To address these challenges, we developed a massively-parallel version of the time-dependent density functional code and implemented it in the framework of the PARSEC (Pseudopotential Algorithm for Real-Space Electronic Calculations) electronic structure program. The developed code uses symmetry operations to reduce the size of the Hamiltonian matrix and the number of matrix elements calculated within the time-dependent density functional linear response algorithm. The accuracy of time-dependent density functional calculations was improved by using asymptotically correct exchange-correlation functionals. This work was done in collaboration with the Center for Computational Materials at the University of Texas at Austin. The accuracy and performance of the developed code was tested on several different types of nanostructures, including small, medium-sized, and large nanoparticles. The results of these studies were reported in an invited chapter for "Handbook of Nanophysics" published by CRC press, and in a review article published in the Journal of Molecular Structure. We have also developed a novel computational approach for calculating the dielectric properties of nanoscale structures. The new approach substantially improved the accuracy of theoretical calculations without a significant increase in computational cost. Our findings were presented in an article published in Physical Review A.

The studies of wide-gap semiconductor nanocrystals have been hampered in the past by the lack a simple surface passivation scheme for group II-VI semiconductors that could remove dangling bonds from the nanocrystalline surfaces and render them chemically inert. The 'standard' hydrogen passivation technique tends to produce localized surface states situated within the optical gap of group III-V and II-VI semiconductor nanocrystals, which makes it unsuitable for studying confinement effects in nanoparticles and quantum dots composed of wide-gap semiconductors. In our study, we implemented a recently developed passivation technique based on the use of fictitious partially charged hydrogen atoms. The value of fractional charge was selected according to the type of covalent bond. We found that this approach completely removed the electronic states associated with the surface hydrogen atoms from the gap of semiconductor nanocrystals. These results were reported in an invited paper published in Proceedings of SPIE.

The computational methods developed in the framework of this project were applied to study the electronic and optical properties of group II-VI core-shell semiconductor nanocrystals. The outer surfaces of nanocrystals were passivated using fractionally charged hydrogen atoms. Using this approach, we calculated the structures, densities of states, electronic gaps, and optical absorption spectra of traditional CdSe/ZnSe and CdTe/ZnTe and inverted ZnSe/CdSe and ZnTe/CdTe core-shell semiconductor nanocrystals containing up to several hundred atoms. The results of our calculations were presented as an invited talk at the 2010 SPIE Nanoscience and Engineering Conference on Physical Chemistry of Interfaces and Nanomaterials in San Diego, California. A paper based on the results of this study was submitted for publication in Physical Review B.

Our theoretical studies were carried out in collaboration with the Theoretical Chemistry Group at Los Alamos National Laboratory and the Center for Computational Materials at the Institute for Computational Engineering and Sciences and the University of Texas at Austin. The collaborative partnerships created through this project helped advance the PI's research program at New Mexico State University (NMSU). The research conducted in connection with this project has generated several peer-reviewed publications in high-ranked U.S. and international physical and chemical journals. The results of this study will help attract additional external research funding to NMSU.

This research project has also benefited graduate students at NMSU. The major part of the project budget was used to support graduate students at the Physics Department of NMSU through stipends and benefits. The research carried out under this project provided a basis of Ph.D. dissertations of two graduate students.