Scientific Publications 2006
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2006. "Structure, Vibrational Spectra and Ring Puckering Barrier of Cyclobutane." Journal of Physical Chemistry A 110:10487-10494. doi:10.1021/jp062472r Abstract We present the results of high level ab initio calculations on the structure and puckering barrier of cyclobutane in an effort to establish the minimum theoretical requirements needed for their accurate description. Our best computed value for the puckering angle is 29.68o. Furthermore we found that accurate estimates for the barrier between the minimum (D2d) and transition state (D4h) configurations require both higher levels of electron correlation [MP4, CCSD(T)] and basis sets of quadruple-z quality or larger. By performing CCSD(T) calculations with basis sets as large as cc-pV5Z we obtained a complete basis set (CBS) estimate of 498 cm-1 for the puckering barrier. Our estimate for the barrier is within 10 cm-1 to the value proposed originally, but it lies ~50 cm-1 higher than the one obtained more recently, therefore revisiting the analysis of the experimental data might be warranted. The results of the current study can serve as a guide for calculations on the substituted four member ring compounds. To this end we present a method for estimating the barrier height at higher levels of electron correlation [MP4, CCSD(T)] from the MP2 results.
2006. "Adsorption of alkali metals on Ge(001)(2×1) surface." Chemical Physics Letters 417(1-3):6-10. doi:10.1016/j.cplett.2005.09.103 Abstract Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(001)(2×1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge-Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect become more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.
2006. "Atomic and electronic structures of rubidium adsorption on Si(001)(2 x 1) surface: Comparison with Cs/Si(001) surface." Chemical Physics 323(2-3):383?390. doi:10.1016/j.chemphys.2005.10.003 Abstract First-principles calculations based on DFT-GGA method have been performed on rubidium adsorption on Si(001)(2×1) surface. The atomic and electronic structures of Si(001)(2×1)-Rb have been calculated and compared with those of Cs adsorption (J.Chem. Phys.122 (2005) 174704). It turns out that the saturation coverage of Rb is one monolayer rather than half a monolayer, similar to that of Cs adsorption. Comparison of Rb on Si(001)(2×1) with Cs adsorption showed that at saturation coverage larger alkali metal (AM) atom leads to stronger AM-AM interaction and weaker AM-Si interaction. However, for low coverage of 0.25 and 0.5 ML the Rb-Si interaction is surprisingly weaker than Cs-Si interaction. Further detailed analysis suggested that this is a consequence of depolarization effect with decreasing AM size below 1 ML coverage. For the saturation coverage the dispersion curves show that the surface is of semi-conducting character. This result does not support the direct and inverse angle-resolved photoemission investigation where a metallization is observed at saturation coverage.
2006. "Excited Carrier Dynamics of α-Cr2O3/α-Fe2O3 Core-Shell Nanostructures." Journal of Physical Chemistry B 110(34):16937-16940. doi:10.1021/jp062507n S1520-6106(06)02507-7 Abstract In this work α-Cr2O3/α-Fe2O3 core-shell polycrystalline nanostructures were synthesized using α-Cr2O3 nanoparticles as seed crystals during aqueous nucleation. The formation of α-Fe2O3 polycrystallites on α-Cr2O3 surfaces were confirmed by x-ray diffraction, transmission electron microscopy, and energy dispersive x-ray analysis. The excited state relaxation dynamics of as-grown core-shell structures and \pure" α-Fe2O3 particles of the same size were measured using femtosecond transient absorption spectroscopy. The results show the carrier lifetimes decay within a few picoseconds regardless of sample. This is likely due to fast recombination/trapping of carriers to defects and iron d-states.
2006. "Laser-Induced Oxygen Vacancy Formation and Diffusion on TiO2(110) Surfaces Probed by Photoemission Electron Microscopy." Physica Status Solidi. C 3(10):3598-3602. Abstract Photoemission electron microscopy is used to probe photon-induced oxygen vacancies generated on TiO2 (110)-(1×2) surfaces. An increased oxygen vacancy concentration within the irradiated region leads to an increase of local photoelectron emission. The local oxygen deficient region can be compensated by exposing the surface to molecular oxygen at 1×10-5 Torr, or via surface diffusion at 450 K in vacuum. The surface diffusion coefficient was estimated to be on the order of 10-12 m2/s. Photoemission electron microscopy allows in situ studies of surface electronic defect formation and removal.
2006. "In Situ Photoelectron Emission Microscopy of a Thermally Induced Martensitic Transformation in a CuZnAI Shape Memory Alloy." Applied Physics Letters 88(9):Art. No. 091910. Abstract Photoemission electron microscopy, in conjunction with photoemission spectroscopy, reflectivity, and surface roughness measurements, is used to study the thermally-induced martensitic transformation in a CuZnAI shape memory alloy. Real-time phase transformation is observed as a nearly instantaneous change of photoelectron intensity, accompanied by microstructural deformation and displacement due to the shape memory effect. The difference in the photoelectron intensity before and after the phase transformation is attributed to the concomitant change of work function as measured by photoelectron spectroscopy. Photoemission electron microscopy is shown to be a valuable new technique facilitating the study of phase transformations in shape memory alloys, and provides real-time information on microstructural changes and phase-dependent electronic properties.
2006. "Introduction to Photoelectron Emission Microscopy: Principles and Applications." Journal of Chinese Electron Microscopy Society 25(1):15-25. Abstract In the past decade, photoelectron emission microscopy (PEEM) has undergone major instrument development and become commercially available. PEEM probes photoelectrons in a high-contrast imaging technique that is sensitive to the surface electronic structure. In this paper we illustrate the principles of PEEM and analyze important PEEM contrast mechanisms. We briefly summarize the applications of PEEM to areas such as surface structure analysis, surface chemistry, magnetism, and semiconductor device characterization. Two important new directions in PEEM development are multiphoton and time-resolved PEEM. Multiphoton PEEM is capable of imaging materials with work functions greater than the incident photon energy, while time-resloved PEEM enables study of fast relaxation dynamics of surface intermediate states. We discuss our recent progress on implementing femtosecond time-solved PEEM and multiphoton PEEM to investigate the silver nanostructured film coated on silicon. The multiphoton images consist of some “hot spots” with far greater photonelectron intensity than is observed in single-photon derived images. We surmise that this is due to the highly-selective excitation of surface localized plasmon of silver. To illustrate the utility of PEEM, we also describe an in-situ thermal-induced structural phase transformation of CuZnA1 shape memory alloy.
2006. "High-Affinity Binding and Direct Electron Transfer to Solid Metals by the Shewanella oneidensis MR-1 Outer Membrane c-type Cytochrome OmcA." Journal of the American Chemical Society 128(43):13978-13979. doi:10.1021/ja063526d Abstract The identification of electron transfer proteins that couple soluble redox carriers to electrode surfaces has great potential in permitting the development of scalable bioreactors. In this respect, the 85 kDa outer membrane decaheme cytochrome OmcA (SO1779) from Shewanella oneidensis MR-1 can reduce soluble Fe(III) chelates, and has previously been suggested to function in concert with other membrane proteins as one of the terminal electron donors in the metal reductase protein complex of Shewanella oneidensis MR-1.1 Shewanella is a facultative anaerobe that can reduce a range of different metal oxides, including iron [Fe(III)], manganese [Mn(III/IV)], chromium [Cr(IV)] and uranium [U(VI)]2, and whose metabolic diversity has considerable promise for both the bioremediation of organic and metal contaminants as well as in the design of microbial fuel cells3. Biofuel cells offer a potential means to couple the breakdown of bio-wastes to generate power4. Miniaturization of these fuel cells is dependent on the elimination of the currently necessary membrane between cathode and anode compartments.5 It has been demonstrated that the immobilization of redox active proteins, such as glucose oxidase, on electrodes with redox active polymer coatings renders the membrane unnecessary.6 The identification of a purified metal-reducing enzyme able to densely bind and directly donate electrons to iron-oxide coated electrodes (commonly used to increase electron transfer efficiencies7) has great potential to contribute to fuel cell design. To identify the terminal electron donors in the S. oneidensis metal reductase system, and to explore whether isolated proteins can directly bind and mediate electron transfer reactions to reduce solid metals, we have purified OmcA and measured its ability to bind and transfer electrons to solid Fe2O3 in the mineral hematite.
2006. "Pd Diffusion on MgO(100): The Role of Defects and Small Cluster Mobilit." Surface Science 600:1351-1362. Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(10 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D–3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 · 1014 s!1 for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at P600 K during Pd vapor deposition on MgO(100) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].
