EMSL: Timothy C Droubay's Publications

Droubay T, TC Kaspar, BP Kaspar, and SA Chambers. 2009. "Cation dopant distributions in nanostructures of transition-metal doped ZnO:Monte Carlo simulations." Physical Review. B, Condensed Matter and Materials Physics 79(7):Art. No. 075324. Abstract The path from trace doping to solid solution formation involves an intermediate regime in which the doping level is a few to several atomic percent. In this regime, dopant-dopant interactions, which are driven by the spatial arrangement of dopants, are critical factors in determining the resulting properties. Conventional wisdom counts on simple probabilistic methods for predicting dopant distributions. Here, we use Monte Carlo simulations to show that widely used, straightforward statistical models, such as that of Behringer1, are accurate only in the limit of infinitesimally small surface–to-volume ratio. For epitaxial films and nanoparticles, where much of the current interest resides, dopant distributions depend strongly on the surface-to-volume ratio. We present empirical expressions that accurately predict dopant bonding configurations as a function of film or particle size, shape and dopant concentration for doped ZnO, a material of particular interest in semiconductor spintronics.

Heald SM, TC Kaspar, T Droubay, V Shutthanandan, SA Chambers, A Mokhtari, AJ Behan, HJ Blythe, JR Neal, M Fox, and G Gehring. 2009. "X-ray absorption fine structure and magnetization characterization of the metallic Co component in Co-doped ZnO thin films ." Physical Review. B, Condensed Matter 79(7):Art. No. 075202. Abstract X-ray absorption fine structure (XAFS) measurements have been used to characterize a series of Co doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional Co2+. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take account of both types of Co. Results are reported for two types of films from two different groups that show distinctly different behavior. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as hcp or fcc Co. Another set of films were annealed in Zn vapor to induce magnetism. These also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. Using bulk magnetic values, there are some discrepancies with room temperature magnetic measurements. The 2 magnetic properties of the small metal particles are likely changed by their surroundings and by superparamagnetism. Low temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50K.

Lower BH, R Yongsunthon, L Shi, L Wildling, HJ Gruber, NS Wigginton, CL Reardon, GE Pinchuk, T Droubay, JF Boily, and SK Lower. 2009. "Antibody recognition force microscopy shows that outer membrane cytochromes OmcA and MtrC are expressed on the exterior surface of Shewanella oneidensis MR-1." Applied and Environmental Microbiology 75(9):2931-2935. Abstract Antibody-recognition force microscopy showed that OmcA and MtrC are expressed on the exterior surface of living Shewanella oneidensis MR-1 cells during anaerobic growth, when Fe(III) served as the terminal electron acceptor. OmcA was localized to the interface with hematite, while MtrC was more uniformly displayed on the bacterium’s exterior cell surface. Both cytochromes were also found associated with extracellular material.

Eggleston CM, J Voros, L Shi, BH Lower, T Droubay, and PJ Colberg. 2008. "Binding and Direct Electrochemistry of OmcA, an Outer-Membrane Cytochrome from an Iron Reducing Bacterium, with Oxide Electrodes: A Candidate Biofuel Cell System." Inorganica Chimica Acta 361(3):769-777. doi:10.1016/j.ica.2007.07.015 Abstract Dissimilatory iron-reducing bacteria transfer electrons to solid ferric respiratory electron acceptors. Outer-membrane cytochromes expressed by these organisms are of interest in both microbial fuel cells and biofuel cells. We use optical waveguide lightmode spectroscopy (OWLS) to show that OmcA, an 85 kDa decaheme outer-membrane c-type cytochrome from Shewanella oneidensis MR-1, adsorbs to isostructural Al2O3 and Fe2O3 in similar amounts. Adsorption is ionic-strength and pH dependent (peak adsorption at pH 6.5–7.0). The thickness of the OmcA layer on Al2O3 at pH 7.0 [5.8 ± 1.1 (2r) nm] from OWLS is similar, within error, to that observed using atomic force microscopy (4.8 ± 2 nm). The highest adsorption density observed was 334 ng cm 2 (2.4 · 1012 molecules cm 2), corresponding to a monolayer or 9.9 nm diameter spheres or submonolayer coverage by smaller molecules. Direct electrochemistry of OmcA on Fe2O3 electrodes was observed using cyclic voltammetry, with cathodic peak potentials of 380 to 320 mV versus Ag/AgCl. Variations in the cathodic peak positions are speculatively attributed to redox-linked conformation change or changes in molecular orientation. OmcA can exchange electrons with ITO electrodes at higher current densities than with Fe2O3. Overall, OmcA can bind to and exchange electrons with several oxides, and thus its utility in fuel cells is not restricted to Fe2O3.

Kaspar TC, T Droubay, SM Heald, MH Engelhard, P Nachimuthu, and SA Chambers. 2008. "Hidden Ferromagnetic Secondary Phases in Cobalt-doped ZnO Epitaxial Thin Films." Physical Review. B, Condensed Matter 77(20):201303. doi:10.1103/PhysRevB.77.201303 Abstract The quest to discover a dilute magnetic semiconductor which is ferromagnetic at room temperature has led to extensive research on doped semiconducting oxides. However, the wide range of reported properties has raised doubts regarding the presence of intrinsic ferromagnetism in these materials. Here we explore the origin of ferromagnetism in epitaxial Co:ZnO thin films, which are paramagnetic but become weakly ferromagnetic (~0.05 μB/Co) after annealing in Zn vapor to introduce interstitial Zn. Conventional bulk materials characterization techniques indicate no phase segregation or Co reduction has occurred. However, x-ray photoelectron spectroscopy sputter depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption spectroscopy is utilized to identify the secondary phase as ferromagnetic CoZn (1.5 μB/Co, TC ~ 400 – 450 K). This work demonstrates that the potential for ferromagnetic secondary phases in doped oxides must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.

Kaspar TC, T Droubay, SM Heald, P Nachimuthu, CM Wang, V Shutthanandan, CA Johnson, DR Gamelin, and SA Chambers. 2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.

Li Y, TC Kaspar, T Droubay, AG Joly, P Nachimuthu, Z Zhu, V Shutthanandan, and SA Chambers. 2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.

Li Y, TC Kaspar, T Droubay, Z Zhu, V Shutthanandan, P Nachimuthu, and SA Chambers. 2008. "Electronic properties of H and D doped ZnO epitaxial films." Applied Physics Letters 92(15):Art. No. 152105. Abstract ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2, or bulk single crystals annealed in H2. These include temperature-independent resistivities of ~0.1 -cm, carrier (electron) concentrations in the 1018 cm-3 range, mobilities of 20-40 cm2/V-sec, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultra-shallow donor or conduction band states not achievable by post-growth annealing in H2.

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.