Scientific Publications 2010
2010. "N-TiO2 nanoparticles embedded in silica prepared by Ti ion implantation and annealing in nitrogen." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 268(9):1440-1445. Abstract Room temperature Ti ion implantation and subsequent thermal annealing in N2 ambience have been used to fabricate the anatase and rutile structured N-doped TiO2 particles embedded in the surface region of fused silica. The Stopping and Range of Ions in Matter (SRIM) code simulation indicates a Gaussian distribution of implanted Ti, with a projected range of 74.4 nm and straggling of 16.5 nm. However, Rutherford backscattering spectrometry and transmission electron microscopy results show a much shallower distribution peaked at ~ 30 nm. Significant sputtering loss of silica substrates has occurred during implantation. Nanoparticles with size of 10-20 nm in diameter have formed after implantation. X-ray photoelectron spectroscopy indicates the coexistence of TiO2 and metallic Ti in the as-implanted samples. Metallic Ti is oxidized to anatase TiO2 after annealing at 600ºC, while rutile TiO2 forms by phase transformation after annealing at 900ºC. At the same time, N-Ti-O, Ti-O-N and/or Ti-N-O linkages have formed in the lattice of TiO2. A red shift of 0.34 eV in the absorption edge is obtained for N-doped anatase TiO2 after annealing at 600 ºC for 6 h. The absorbance increases in the ultraviolet and visible waveband.
2010. "Threshold displacement energies and defect formation energies in Y2Ti2O7." Journal of Physics. Condensed Matter 22(41):Article No.: 415801. doi:10.1088/0953-8984/22/41/415801 Abstract Ab initio molecular dynamics simulations have been carried out to determine both the threshold displacement energies Ed and corresponding defect configurations, and ab initio methods have been used to accurately determine the formation energies in Y2Ti2O7. The minimum Ed is found to be 27 eV for a Y recoil along the <100> direction, 31.5 eV for Ti atoms along the <100> direction, 14.5 eV for O48f atoms along the <110> direction and 13 eV for O8b atoms along the <111> direction. The average Ed value along three directions determined is 35.1, 35.4, 17.0 and 16.2 eV for yttrium, titanium, O48f and O8b atoms, respectively. Cation interstitials at vacant 8a sites, which are generally occupied by oxygen anions, and at the bridge sites between two neighboring cations along the <010> direction are observed after low energy recoil events. A systematic study of the defect formation energies suggests that cation interstitials, which are located at 8a sites and bridge sites along the <010> direction, and in split configurations along the <010>, <110> or <111> direction, are all stable in these configurations. It is suggested that the relative stability of cation interstitials may provide a pathway of driving ion-irradiation induced amorphization in Y2Ti2O7.
2010. "Zirconate pyrochlores under high pressure." Physical Chemistry Chemical Physics. PCCP 12(39):12472-12477. doi:10.1039/c0cp00278j Abstract Ab initio total-energy calculations and x-ray diffraction measurements have been combined to study the phase stability of zirconate pyrochlores (A2Zr2O7; A=La, Nd and Sm) under pressures up to 50 GPa. Phase transformations to the defect-cotunnite structure are theoretically predicted at pressures of 22, 20 and 18 GPa, in excellent agreement with the experimentally determined values of 21, 22 and 18 GPa for La2Zr2O7, Nd2Zr2O7 and Sm2Zr2O7, respectively. Analysis of the elastic properties indicates that elastic anisotropy may be one of the driving forces for the pressure-induced cubic-to-noncubic phase transformation.
2010. "Photoelectron Imaging of Doubly Charged Anions, -O2C(CH2)nCO2- (n=2-8): Observation of Near 0 eV Electrons Due to Secondary Dissociative Autodetachment." Journal of Physical Chemistry A 114(13):4524-4530 . doi:10.1021/jp1011523 Abstract The hallmark of multiply charged anions is the repulsive Coulomb barrier (RCB), which prevents low energy electrons from being emitted in photodetachment experiments. However, using photoelectron imaging, we have observed persistent near zero-eV electrons during photodetachment of doubly charged dicarboxylate anions, -O2C(CH2)nCO2- (Dn2-, n = 2-8). Here we show that these low energy electron signals are well structured and are independent of the detachment photon fluxes or energies. The relative intensities of these signals are dependent on n, with maxima at n = 2, 4, 6. These near zero-eV electrons cannot come from direct photodetachment of the dianions and are proposed to come from decarboxylation of the product radical anions upon photodetachment of the parent dianions [•O2C(CH2)nCO2- → CO2 + •(CH2)nCO2-], followed by dissociative autodetachment [•(CH2)nCO2- → (CH2)n + CO2 + e] or hydrogen-transfer-induced electron detachment [•(CH2)nCO2- → CH2=CH(CH2)n-2CO2H + e]. Energetic considerations suggest that these processes are exothermic. It is further observed that solvation by one water molecule quenches the low energy electron signals in the spectra of Dn2-(H2O), consistent with the proposed mechanisms. These indirect dissociative autodetachment processes are expected to involve cyclic transition states for n > 2, in agreement with the dependence on the chain length due to the anticipated strains in the intermediate steps. The quenching of the low energy electron signals by one water molecule demonstrates the importance of solvation on chemical reactions. ________________________
2010. "Materials Applications of Photoelectron Emission Microscopy." JOM. The Journal of the Minerals, Metals and Materials Society 62(12):90-93. doi:10.1007/s11837-010-0189-1 Abstract Photoelectron emission microscopy (PEEM) is a versatile technique that can image a variety of materials including metals, semiconductors and even insulators. Under favorable conditions the most advanced aberration corrected instruments have a spatial resolution approaching 2 nm. Although PEEM cannot compete with transmission or scanning electron microscopies for ultimate resolution, the technique is much more gentle and has the unique advantage of imaging structure as well as electronic and magnetic states on the nanoscale. Since the image contrast is derived from spatial variations in electron photoemission intensity, PEEM is ideal for interrogating both static and dynamic electronic properties of complex nanostructured materials. PEEM can be performed using a variety of photoexcitation sources including synchrotron emission, femtosecond laser pulses and conventional UV lamp emission. Each source has advantages, for example, fs laser excitation enables time-resolved imaging for study of ultrafast dynamics of surface intermediate states while tunable synchrotron sources allow chemically specific excitation. Even more detail can be extracted from energy resolved PEEM. Here, we review the key principles and contrast mechanisms of PEEM and briefly summarize materials applications of PEEM with examples of a thermally-induced structural phase transformation in barium titanate, inter-diffusion between thin metal copper and ruthenium layers, and multiphoton imaging of polystyrene nanoparticles on a silver coated substrate.
2010. "Study of Highly Selective and Efficient Thiol Derivatization using Selenium Reagents by Mass Spectrometry." Analytical Chemistry 82(16):6926-6932. doi:10.1021/ac1011602 Abstract Biological thiols are critical physiological components and their detection often involves derivatization. This paper reports a systemic mass spectrometry (MS) investigation of the cleavage of Se-N bond by thiol to form a new Se-S bond, the new selenium chemistry for thiol labeling. Our data shows that the reaction is highly selective, rapid, reversible and efficient. For instance, among twenty amino acids, only cysteine was found to be reactive with Se-N containing reagents and the reaction takes place in seconds. By adding dithiothreitol (DTT), the newly formed Se-S bond of peptides/proteins can be reduced back to free thiol. The high selectivity and excellent reversibility of the reaction provide potential of using this chemistry for selective identification of thiol compounds or enriching and purifying thiol peptides/proteins. In addition, the derivatized thiol peptides have interesting dissociation behavior, which is tunable using different selenium reagents. For example, by introducing an adjacent nucleophilic group into the selenium reagent in the case of using ebselen, the reaction product of ebselen with glutathione (GSH) is easy to lose the selenium tag upon collision-induced dissociation (CID), which is useful to "fish out" those peptides containing free cysteine residues by precursor ion scan. By contrast, the selenium tag of N-(phenylseleno) phthalimide reagent can be stable and survive in CID process, which would be of value in pinpointing thiol location using a top-down proteomic approach. Also, the high conversion yield of the reaction allows the counting of total number of thiol in proteins. We believe that ebselen or N-(phenylseleno) phthalimide as tagging thiol-protein reagents will have important applications in both qualitative and quantitative analysis of different thiol-proteins derived from living cells by MS method.