Calculations of electron-energy-loss spectra from specific atoms at a model Si-SiO2 interface with and without suboxide bonds (Si-Si bonds on the oxide side of the interface). Comparison of such curves with experimental spectra leads to a confirmation of the existence of suboxide bonds.

John Cooke, Oak Ridge National Laboratory

Research Objectives
This research investigates atomic-scale structures and electronic properties in interfaces, grain boundaries, and catalytic surfaces using self-consistent ab initio density functional calculations. Specific projects include (1) the high-efficiency and long-term performance of semiconductor-nanocrystallite catalysts in fixation of CO₂ and conversion to organic compounds; (2) the microscopic origin of the grain boundary potential barrier in yttrium barium copper oxide (YBCO); (3) the relation between quantum yield of perovskite photocatalysts and their microstructure; (4) the structures of Si/SiO₂ and SiC/SiO₂ interfaces; and (5) the growing process of the single-wall nantotube from the Ni-C solution.

Computational Approach
Density functional theory with the local density approximation, pseudopotentials with plane waves or full potential linearized augmented plane waves (FLAPW), and supercells constitute the method of choice for solid state calculations. We have a variety of codes which have been proven successful for our study, including the well-known codes VASP and WIEN97.

Accomplishments
Calculations of the surfaces of wurtzite CdSe and zinc-blende CdSe demonstrate that the neutral CO₂ molecules are first chemisorbed into a Se vacancy, where they attract an excess electron. They are then re-emitted taking that electron with them, becoming negatively charged and thus more reactive. The barrier for chemisorption and desorption is small, so the process goes on back and forth and all the CO₂ molecules become charged and reactive. These results account for the observation that CdSe nanocrystals must be Cd-rich (i.e., have Se surface vacancies) to be good catalysts, and for the fact that flat surfaces are not good catalysts.

We have showed that non-stoichiometry is essential for the formation of the grain boundary barrier in SrTiO₃, which is a role model for perovskite materials. This non-stoichiometry is confirmed by experiment and is explained by the differences in oxygen vacancy formation energies at the grain boundaries and in bulk as well as by the grain boundary core structure itself.

Using VASP ab initio code, we have calculated the momentum angular dependent projected density of states (PDOS) in Si, SiO₂ and SiC materials, and have shown that PDOS of the appropriate symmetry is a good approximation for electron energy loss spectra. These results, together with similar, more exact calculations performed with all-electron LAPW (Wien) code, have resolved a long-standing controversy regarding the role of core excitons in x-ray absorption and electron energy loss spectroscopy.

Significance
An understanding of atomic-scale structures and electronic properties in interfaces, grain boundaries, and catalytic surfaces is important for both technological and environmental issues, including carbon sequestration; the development of high-temperature superconductors; efficient decomposition of water into H₂ and O₂ to produce energy; building microelectronic devices with oxide thickness less than 30 Å, where the interface dominates the electrical behavior; and carbon nanotubes applications ranging from nano-electronics to super-strong structural materials.

Publications
R. Buczko, S. J. Pennycook, and S. T. Pantelides, “Bonding arrangements at the Si-SiO₂ and SiC-SiO₂ interfaces and the origin of their contrasting properties,” Phys. Rev. Lett. 84, 943 (2000).

M. Kim, G. Duscher, N. D. Browning, S. J. Pennycook, K. Sohlberg, and S. T. Pantelides, “Non-stoichiometry and the electric activity of grain  boundaries in SrTiO₃,” Phys. Rev. Lett. (submitted).

L. G. Wang, S. J. Pennycook, and S. T. Pantelides, “Mechanism for the catalytic activity of CdSe nanocrystals in CO₂ fixation,” Phys. Rev. Lett. (in preparation).