Larry W. Burggraf

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...

Photoacoustic studies of complexation of copper(II) with an ethylenediamine analog immobilized on silica gel

Abstract Photoacoustic spectroscopy and solid-solution distribution measurements are used to study complexation of copper(II) with N-2-aminoethyl-3-aminopropyltrimethoxysilane immobilized on silica gel. Nearly consecutive formation of bis and mono complexes occurs on the surface with increasing copper(II) concentration. Evidence for conversion of bis sites to mono sites with increasing copper(II) concentration was not found. Heterogeneous distribution measurements fit well with a model which assumes two independent types of binding sites. An approximately 2.5:1 ratio of mono to bis sites was found.

Photothermal spectroscopy using multilayer cantilever for chemical detection

Photothermal spectroscopy is demonstrated using a high-aspect-ratio multilayer cantilever to measure adsorbed dimethyl methylphosphonate, which has an optical absorbance of ∼10−5 in the near infrared range. Detection sensitivity was 160 pW for a reed of 6 mm in length, 2 mm in width, and 10 μm. Sensitivity of the cantilever is compared to a thermal diffusion model that accounts for conduction loss through the cantilever clamp and to air along the length of the cantilever surface.

The structure and stability of neutral pentacoordinated silicon compounds

Abstract Ab initio and semi-empirical wavefunctions are used to examine the stability of neutral pentacoordinated compounds with the formula H 3 NSiXY 3 , where X and Y are the axial and equatorial ligands, respectively, and both ligands can be H, F, Cl. It is found from both levels of theory that a relatively weakly bound ligand (e.g. Cl) in the axial position facilitates complexation of the Si with NH 3 due to a more balanced three-center bond. Very electronegative ligands (e.g. F) are also stabilizing, since they increase the positive charge on Si.

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...

Photoelectron spectroscopy of Si2C3− and quantum chemistry of the linear Si2C3 cluster and its isomers

The 364 nm photoelectron spectrum of Si2C3− is reported, together with high level ab initio calculations of the linear anion, and six linear and eight nonlinear structures of the neutral Si2C3. The adiabatic electron affinity of Si2C3, corresponding to the transition from the linear anion to the lowest electronic state of the linear singlet neutral, is found to be 1.766±0.012 eV. Theoretical results were essential for interpreting the spectrum. The level of theory necessary to accurately describe the electronic structure of Si2C3 cluster isomers is presented and discussed. Several vibration frequencies for the neutral linear structure are obtained from the spectra and compared to results from different levels of theory.

Nondestructive inspection using Compton scatter tomography

A method for non-invasively generating tomographic images of electron density in materials using Compton scattered gamma rays is investigated. Electron density is an indicator of density or composition changes in a material. In Compton scatter tomography, the energy distribution of monoenergetic gamma rays is measured after scattering from a material target. Measuring a scattered gamma rays energy localizes the scattering position to a definable region of the sample. We develop an analytic computational model to study the image quality of a realistic system. In particular, the deleterious effects of finite source and detector size, Compton broadening, and Poisson noise are investigated. A backprojection algorithm is presented to demonstrate the impact of these effects on image reconstruction and contrast recovery. Detection of corrosion in low-Z materials is an application of interest, and is used to study the Compton scatter tomography technique.

Modeling positrons in molecular electronic structure calculations with the nuclear-electronic orbital method.

The nuclear-electronic orbital (NEO) method was modified and extended to positron systems for studying mixed positronic-electronic wavefunctions, replacing the mass of the proton with the mass of the positron. Within the modified NEO framework, the NEO-HF (Hartree-Fock) method provides the energy corresponding to the single-configuration mixed positronic-electronic wavefunction, minimized with respect to the molecular orbitals expressed as linear combinations of Gaussian basis functions. The electron-electron and electron-positron correlation can be treated in the NEO framework with second-order perturbation theory (NEO-MP2) or multiconfigurational methods such as the full configuration interaction (NEO-FCI) and complete active space self-consistent-field (NEO-CASSCF) methods. In addition to implementing these methods for positronic systems, strategies for calculating electron-positron annihilation rates using NEO-HF, NEO-MP2, and NEO-FCI wavefunctions were also developed. To apply the NEO method to the positronium hydride (PsH) system, positronic and electronic basis sets were optimized at the NEO-FCI level and used to compute NEO-MP2 and NEO-FCI energies and annihilation rates. The effects of basis set size on NEO-MP2 and NEO-FCI correlation energies and annihilation rates were compared. Even-tempered electronic and positronic basis sets were also optimized for the e+LiH molecule at the NEO-MP2 level and used to compute the equilibrium bond length and vibrational energy.

Controlled patterning of polymer films using an AFM tip as a nano-hammer

A force-modulated atomic force microscope (AFM) was used to produce controlled two-dimensional nano-structure arrays in polymer films. Unmodulated AFM methods have not achieved the registry control necessary to generate multi-dimensional structures. Registry control was achieved by generating optimally modulated stress waves using a scanning AFM cantilever tip as a nano-hammer. Ordered nano-dots in selected two-dimensional lattice structures were constructed by reshaping the surface of polystyrene thin films using the force-modulated AFM tip. Selecting the wavelength and orientation of the stress distribution in polymer films enabled control of the periodicity and symmetry of nano-arrays. The formation and growth of the nano-structures are attributed to the stress-induced reconstruction in the thin polymer films. Surface reconstruction models based on the stress-induced instability near polymer surfaces may provide further insight into structures induced in polymers by a scanning tip.

Influence of radionuclide adsorption on detection efficiency and energy resolution for flow-cell radiation detectors

Flow-cell and batch test experiments were performed to determine the uranyl ion adsorption onto and/or complexation with CaF/sub 2/:Eu scintillator as a function of pH. The flow-cell experiments were modeled with an energy dispersive Monte Carlo algorithm. At pH 2, the flow-cell and batch tests gave consistent results, detection efficiency /spl sim/60% and distribution coefficient (K/sub d/) /spl sim/0.3-0.7 mL/g, with the model. At pH 8, the comparison was not as good and assumed to be due to variation in scintillator particle size realized in the experiment. From the flow-cell experiments performed at pH 8, the detection efficiency was determined to be 68% which correlated reasonably well with the model (60%). Where the experiments and model differed greatly was with K/sub d/ which was determined to be 102 and 60 for the flow-cell and batch experiments, respectively, and 0.43 from the model.