EMSL: Kenneth M. Beck's Publications

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.

Lu CY, PJ Shamberger, EN Yitamben, KM Beck, AG Joly, MA Olmstead, and FS Ohuchi. 2008. "Laser and Electrical Current Induced Phase Transformation of In2Se3 Semiconductor thin film on Si(111) ." Applied Physics A, Materials Science and Processing 93(1):93-98. doi:10.1007/s00339-008-4776-8 Abstract Phase transformation of thin film (~30 nm) In2Se3/Si(111) (amorphous  crystalline) was performed by resistive annealing and the reverse transformation (crystalline  amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous InxSey was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (6.4 Å). Upon resistive annealing to 380°C, the film was transformed into a y-In2Se3 single crystal with its {0001} planes parallel to the Si (111) substrate and parallel to Si , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and x-ray diffraction. Laser annealing with 20 nanosecond pulses (0.1 milliJoules/pulse) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous InxSey film.

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.

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.

Beck KM, M Henyk, CM Wang, PE Trevisanutto, P Sushko, WP Hess, and AL Shluger. 2006. "Site-Specific Laser Modification of MgO nanoclusters: Towards Atomic-Scale Surface Structuring." Physical Review. B, Condensed Matter 74:045404 (5 pages). Abstract Atomic emission from MgO nanostructures is induced using laser light tuned to excite specific surface sites at energies well below the excitation threshold of the bulk material. We find that near UV excitation of MgO nancrystalline films and nanocube samples desorbs neutral Mg-atoms with hyper-thermal kinetic energies in the range of 0.1-0.4 eV. Our ab initio calculations suggest that metal atom emission is induced predominantly by electron trapping at surface 3-coordinated Mg sites followed by electronic excitation at these sites. The proposed general mechanism can be used to control atomic scale modification of insulating surfaces.

Joly AG, M Henyk, KM Beck, PE Trevisanutto, PV Sushko, WP Hess, and AL Shluger. 2006. "Probing Electron Transfer Dynamics at MgO Surfaces by Mg-Atom Desorption." Journal of Physical Chemistry B 110(37):18093-18096. doi:10.1021/jp064092b Abstract Desorption of a weakly bound adsorbate from a porous solid was studied for the case of N2 on amorphous solid water (ASW). Porous ASW films of different thickness were grown on Pt(111) by ballistic deposition. N2 adsorption and desorption kinetics were monitored mass-spectrometrically. Temperature programmed desorption spectra show that with the increasing film thickness, the N2 desorption peak systematically shifts to higher temperatures. The results are explained and quantitatively reproduced by a simple model, which assumes that the N2 transport within the film is faster than the depletion rate to vacuum. The local coverage at the pore mouth determines the desorption rate. For thick ASW films (>1 μm), the assumption of the fast equilibration within the film is shown to be no longer valid due to diffusion limitations. The mechanisms of the adsorbate transport are discussed.

Xiong G, AG Joly, GR Holtom, CM Wang, DE McCready, KM Beck, and WP Hess. 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.