EMSL: Wayne P Hess's Publications

Govind N, PV Sushko, WP Hess, M Valiev, and K Kowalski. 2009. "Excitons in Potassium Bromide: A Study using Embedded Time-dependent Density Functional Theory and Equation-of-Motion Coupled Cluster Methods." Chemical Physics Letters 470(4-6):353-357. doi:10.1016/j.cplett.2009.01.073 Abstract We present a study of the electronic excitations in insulating materials using an embedded- cluster method. The excited states of the embedded cluster are studied systematically using time-dependent density functional theory (TDDFT) and high-level equation-of-motion coupled cluster (EOMCC) methods. In particular, we have used EOMCC models with singles and doubles (EOMCCSD) and two approaches which account for the e®ect of triply excited con¯gurations in non-iterative and iterative fashions. We present calculations of the lowest surface excitations of the well-studied potassium bromide (KBr) system and compare our results with experiment. The bulk-surface exciton shift is also calculated at the TDDFT level and compared with experiment.

Khan EH, SC Langford, JT Dickinson, LA Boatner, and WP Hess. 2009. "Photoinduced Formation of Zinc Nanoparticles by UV Laser Irradiation of ZnO." Langmuir 25(4):1930-1933. Abstract Simple exposure of single-crystal ZnO to 193 11m excimer laser radiation at room temperature results in unexpected coloration. The gray to nearly black colored material. seen principally in the irradiated laser spot, is superficial. We present unambiguous evidence that this coloration is due to high densities of metallic Zn nanoparticles growing on the exposed surface of the crystal. Higher fluence laser exposure generates accumulated surface metal just outside of the irradiated spot. We suggest that the near surface bulk is photodecomposing; thermally driven diffusion leads to surface Zn metal aggregation.

Trevisanutto PE, PV Sushko, KM Beck, AG Joly, WP Hess, and AL Shluger. 2009. "Excitation, Ionization, and Desorption: How Sub-band gap Photons Modify the Structure of Oxide Nanoparticles." Journal of Physical Chemistry C 113(4):1274-1279. Abstract Nanoparticles of wide-band-gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies up to ~ 0.4 eV. We use ab initio embedded cluster methods to study theoretically a variety of elementary photoinduced processes at both ideal and defect-containing surfaces of these nanoparticles and develop a mechanism for the desorption process. The proposed mechanism includes multiple local photoexcitations resulting in sequential formation of localized excitons, their ionization, and further excitations. It is suggested that judicious choice of sub-band-gap photon energies can be used to selectively modify surfaces of nanomaterials.

Beck KM, AG Joly, O Diwald, S Stankic, PE Trevisanutto, PV Sushko, AL Shluger, and WP Hess. 2008. "Energy and Site Selectivity in O-Atom Photodesorption from Nanostructured MgO." Surface Science 602(11):1968-1973. doi:10.1016/j.susc.2008.03.046 Abstract Electronic excitation of wide gap ionic solids can induce desorption of neutral atoms with distinct hyperthermal and thermal kinetic energy distributions. Hyperthermal atomic desorption results from electronic surface excitation while thermal desorption is initiated primarily by bulk excitation. Calculations indicate that surface-localized transitions can be excited independently from bulk transitions using selected photon energies. The photon energy required to excite specific surface sites depends upon the site coordination with successively lower energies required to excite terrace, step, and corner sites. Here, we excite low-coordinated surface sites of nanostructured MgO samples using 4.7 eV UV laser pulses and observe dominant hyperthermal O-atom emission. We then selectively excite bulk sites of nanostructured MgO, using a 7.9 eV laser, and observe dominant thermal O-atom desorption. These results are analyzed in terms of laser desorption models developed previously for alkali halide crystals. We propose a multi-step mechanism for hyperthermal O-atom desorption, under surface selective excitation, based on hole trapping at 3C (corner) O-atom sites followed by exciton decomposition. The proposed “hole plus exciton” model has similarities to the surface exciton desorption model, established for alkali halides, but is more complex and requires more steps. Nonetheless, the principle of site-specific photoreaction, established for alkali halide crystals, is clearly extendable to a prototypical metal oxide.

Joly AG, KM Beck, and WP Hess. 2008. "Electronic Energy Transfer on CaO Surfaces." Journal of Chemical Physics 129(12):124704. doi:10.1063/1.2980049 Abstract We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following either direct corner excitation or diffusion and localization of excitons from higher coordinated surface sites.

Cai M, SC Langford, JT Dickinson, G Xiong, T Droubay, AG Joly, KM Beck, and WP Hess. 2007. "An In Situ Study of the Martensitic Transformation in Shape Memory Alloys Using Photoemission Electron Microscopy." Journal of Nuclear Materials 361(2-3):306-312. doi:10.1016/j.jnucmat.2006.12.008 Abstract Thermally-induced martensitic phase transformations in polycrystalline CuZnAl and thin-film NiTiCu shape memory alloys were probed using photoemission electron microscopy (PEEM). Ultra-violet photoelectron spectroscopy shows a reversible change in the apparent work function during transformation, presumably due to the contrasting surface electronic structures of the martensite and austenite phases. In situ PEEM images provide information on the spatial distribution of these phases and the evolution of the surface microstructure during transformation. PEEM offers considerable potential for improving our understanding of martensitic transformations in shape memory alloys in real time.

Cai M, SC Langford, MJ Wu, WM Huang, G Xiong, T Droubay, AG Joly, K Beck, WP Hess, and JT Dickinson. 2007. "Study of Martensitic Phase transformation in a NiTiCu Thin Film Shape Memory Alloy Using Photoelectron Emission Microscopy." Advanced Functional Materials 17(1):161-167. doi:10.1002/adfm.200600611 Abstract The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.

Joly AG, G Xiong, CM Wang, DE McCready, KM Beck, and WP Hess. 2007. "Synthesis and Photoexcited Charge Carrier Dynamics of beta-FeOOH Nanorods." Applied Physics Letters 90(10):Art. No. 103504. doi:10.1063/1.2711395 Abstract Akaganeite(B-FeOOH) nanorods of dimensions 15 nm diameter and 200 nm length were prepared by aqueous synthesis. Charge carrier dynamics following femtosecond excitation displays three timescales. The first is a sub-picosecond decay of initially excited carriers to the band edge followed by trapping or nonradiative decay within 2 ps. The trapped electrons and holes persist for significantly longer times (at least tens-of-ps), similar to previous results from a-Fe2O3 materials. The short carrier lifetimes in these materials are attributed to fast trapping to Fe d-d and midgap states.

Wei W, SL Parker, Y Sun, JM White, G Xiong, AG Joly, KM Beck, and WP Hess. 2007. "Study of Copper Diffusion Through Ruthenium Thin Film by Photoemission Electron Microscopy." Applied Physics Letters 90:111906. doi:10.1063/1.2712832 Abstract Photoemission electron microscopy (PEEM) is employed to study Cu diffusion in real time through a Ru barrier in a Cu/Ru bilayer system. The PEEM images display large contrast between Cu and Ru due of the differences in work function between the two metals, making PEEM an ideal methodology to study diffusion in real time. At low temperature (175-290 °C), Cu mainly diffuses through the defective sites in the Ru film. Uniform diffusion of Cu through a Ru thin film occurs at approximately 300 °C. The results are confirmed by X-ray photoemission spectroscopy (XPS) depth profiling and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis.

Xiong G, R Shao, T Droubay, AG Joly, KM Beck, SA Chambers, and WP Hess. 2007. "Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals." Advanced Functional Materials 17(13):2133-2138. doi:10.1002/adfm.200700146 Abstract Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.