James R. Shoemaker

An Ab Initio Cluster Study of the Structure of the Si(001) Surface

Ab initio calculations, employing double zeta plus polarization (DZP) basis sets and generalized valence bond (GVB) wave functions, have been performed on clusters of varying size, to investigate the utility of such clusters as prototypes for the study of silicon surfaces, and to investigate the effect of the level of theory used on predicted results. This work builds on landmark papers by Goddard in 1982 and Paulus in 1998 that demonstrate that a single reference wave function description of the silicon dimer bond is incorrect, and that a multireference description results in a symmetric dimer in a silicon cluster containing one dimer. In this work, it is shown that the imposition of arbitrary geometrical constraints (fixing subsurface atoms at lattice positions) on cluster models of the Si(100) surface can also lead to nonphysical results. Calculations on the largest clusters, without geometrical constraints, reveal that surface rearrangement due to dimer bond formation is “felt” several layers into the...

Electronic structure and properties of strained polymers: 2. Rigid-rod PBI, PBO and PBZT

Abstract A theoretical study of single-chain moduli employing the molecular-orbital semi-empirical AM1 (Austin model 1) approach is presented for the poly( p -phenylene benzobisimidazole) (PBI), poly( p -phenylene benzobisoxazole) (PBO) and poly( p -phenylene benzobisthiazole) (PBZT) rigid-rod polymers. This computational study is important for comparisons of mechanical properties for the series of related polymers. An analysis of molecular deformation on tension and compression of these rigid-rod polymers offers an insight into tensile and compressive processes at the molecular level, allowing their relative ordering by theoretical tensile and/or compressive strength and moduli. Furthermore, an examination of the electronic structures of these molecular systems containing various hetero atoms provides insight into the charge distribution and bond order changes on the application of strain. This approach will be useful for the relative prediction of the properties of polymers that have not yet been synthesized.

High-temperature photoluminescence in sol-gel silica containing SiC/C nanostructures

Silicon carbide and carbon nanostructures were produced by pyrolysis of organosilane or aromatic compounds in nanoporous sol-gel silica glasses. Intense photoluminescence was observed in the visible and the near infrared regions, depending on material processing. Emission bands at 2.97, 2.67, 2.53, 2.41, 2.24, 2.09, 1.93, 1.13, 1.00, and 0.85 eV were observed in samples prepared at temperatures between 870 and 1220 K. Phosphorescence emission showed two lifetime components at 300 K: a 0.03 s component and a very long component of 0.5–4 s that depends on the precursors and sample processing. These lifetimes approximately doubled at 77 K. The visible emission increased significantly as the temperature was elevated from 77 to 950 K, suggesting thermally assisted light emission from sites in the silica glasses containing SiC/C nanostructures. Surface SiC vacancy defects modeled using integrated ab initio quantum mechanics/molecular mechanics calculations suggest phosphorescence may originate from C vacancy (S...

Electronic structure and properties of strained polymers: 1. Methods for polyethylene

Abstract The modulus and strain-dependent frequency spectrum of polyethylene have been computed from a cluster of C 6 H 12 units using the semi-empirical AM1 Hamiltonian and unrestricted Hartree—Fock wavefunctions. The computed modulus substantially exceeds experimental values, a result which derives from neglect of electron correlation in the calculations. Good qualitative and quantitive agreement with reported strain-dependent frequencies is found, suggesting that the method describes cubic anharmonicity in the bonding while systematically overestimating components of the Hessian.

Response to “Comment on ‘An ab initio cluster study of the structure of the Si(001) surface’ ” [J. Chem. Phys. 113, 9353 (2000)]

Given the agreement among TCSCF, MP2, and MRMP methods, it is concluded that the structure of Si9H12 is symmetric, that dynamic correlation does not play a major role in determining the structure, and that DFT incorrectly predicts Si9H12 to be buckled.

Energetics and configurations of lattice defects in CuTi

The energies and configurations of interstitials and vacancies in the ordered compound CuTi were calculated using atomistic simulation. Vacancies created by the removal of either a Cu for Ti atom resulted in a vacant Cu site, with and antisite defect in the latter case. The vacancy at the Cu site was found to be very mobile within two adjacent (001) Cu planes, resulting in two dimensional migration. Interstitials created by inserting either a Cu or Ti atom had complicated configurations containing one or more antisite defects. 11 refs., 1 fig., 1 tab.