EMSL: Shuttha Shutthanandan's Publications

Aiken A, D Salcedo, MJ Cubison, J Huffman, P DeCarlo, IM Ulbrich, KS Docherty, DT Sueper, J Kimmel, DR Worsnop, A Trimborn, M Northway, EA Stone, JJ Schauer, RM Volkamer, E Fortner, B de Foy, J Wang, A Laskin, V Shutthanandan, J Zheng, R Zhang, JS Gaffney, NA Marley, GL Paredes-Miranda, WP Arnott, LT Molina, G Sosa, and JL Jimenez. 2009. "Mexico City Aerosol Analysis during MILAGRO using High Resolution Aerosol Mass Spectrometry at the Urban Supersite (T0). Part 1: Fine Particle Composition and Organic Source Apportionment." Atmospheric Chemistry and Physics 9(17):6633-6653. Abstract Submicron aerosol was analyzed during the MILAGRO field campaign in March 2006 at the T0 urban supersite in Mexico City with a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) and complementary instrumentation. Mass concentrations, diurnal cycles, and size distributions of inorganic and organic species are similar to results from the CENICA supersite in April 2003 with organic aerosol (OA) comprising about half of the fine PM mass. Positive Matrix Factorization (PMF) analysis of the high resolution OA spectra identifies three major components: chemically-reduced urban primary emissions (hydrocarbon-like OA, HOA), oxygenated OA (OOA, mostly secondary OA or SOA), and biomass burning OA (BBOA) that correlates with levoglucosan and acetonitrile. BBOA includes several very large plumes from regional fires and likely also some refuse burning.

Chang C, S Sankaranarayanan, MH Engelhard, V Shutthanandan, and S Ramanathan. 2009. "On the relationship between non-stoichiometry and passivity breakdown in ultra-thin oxides: combined depth-dependent spectroscopy, Mott-Schottky analysis and molecular dynamics simulation studies." Journal of Physical Chemistry C 113(9):3502-3511. Abstract Understanding the relationship between non-stoichiometry and physical properties of ultra-thin oxides is of great importance from both scientific and technological aspects. A specific example includes the onset of passivity breakdown in an ultra-thin oxide film in aqueous medium leading to the onset of corrosion. In this work, using the model system of ultra-thin oxide of alumina on aluminum synthesized by natural oxidation and photon-assisted oxidation processes; we demonstrate a direct correlation between passivity and quality of the oxide film quantitatively. Depth-dependent high resolution X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nuclear reaction analysis (NRA) have been performed to characterize the physical and chemical properties of the oxide films, while detailed impedance measurements and Mott-Schottky studies have been performed to understand electronic transport. Combined NRA and TEM analysis reveal an 18% increase in oxygen density (for oxide films with near identical thicknesses ~3.8nm) in case of photon-assisted oxidation. The denser oxide film results in a ~34% more blockage of chloride ions transport as indicated by XPS analysis. Mott Schottky measurements on these oxide films indicates a 43% reduction of defect levels for UV-synthesized alumina when compared to native one, suggestive of chloride ion transport via oxygen vacancies. Additionally, molecular dynamics simulations have been performed to provide insights into the structure of the oxides at the atomic level to correlate with the experimental measurements. These simulations employ dynamic charge transfer between atoms and are used to investigate nanoscale oxides grown on Al (100) surfaces due to atomic and molecular oxygen. Oxidation using molecular and atomic oxygen resulted in an amorphous oxide scale with self limiting thickness of ~ 16 and 22 Å, respectively at 300 K. Structural and dynamic correlations indicate significant charge transfer to exist in the oxide film in both the cases. The oxide growth in both the cases occurs due to the inward oxygen and outward cation diffusion. The calculated in-plane and out-of-plane atomic diffusivities are 40-70% higher in case of atomic oxidation. In the presence of atomic oxygen, the O/Al ratio is more uniform and varies from 1.37 at the oxide-gas interface to 1.30 at metal-oxide interface whereas that formed by natural oxidation was sub-stoichiometric and oxygen deficient with O/Al values varying from 1.27 (oxide-gas interface) to 1.05 (metal-oxide interface) at room temperature. The simulation results are consistent with the reported experimental investigations.

Gupta S, SVNT Kuchibhatla, MH Engelhard, V Shutthanandan, P Nachimuthu, W Jiang, LV Saraf, S Thevuthasan, and S Prasad. 2009. "Influence of samaria doping on the resistance of ceria thin films and its implications to the planar oxygen sensing devices." Sensors and Actuators. B, Chemical 139(2):380-386. doi:10.1016/j.snb.2009.03.021 Abstract In order to evaluate and analyze the effect of samarium (Sm) doping on the resistance of cerium oxide, we have grown highly oriented samaria doped ceria (SDC) thin films on sapphire, Al2O3 (0001) substrates by using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE). The film growth was monitored using reflection high-energy electron diffraction (RHEED) which shows two-dimensional growth throughout the deposition. Following growth, the thin films were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution X-ray diffraction (HRXRD), and Rutherford backscattering spectrometry (RBS). XPS depth-profile shows Sm atoms are uniformly distributed in ceria lattice throughout the bulk of the film. The valence states of Ce and Sm in doped thin films are found to be Ce4+ and Sm3+, respectively. HRXRD shows the samaria doped ceria films on Al2O3(0001) exhibit (111) preferred orientation. Ion-channeling in RBS measurements confirms high quality of the thin films. The resistance of the samaria doped ceria films, obtained by two probe measurement capability under various oxygen pressure (1mTorr-100Torr) and temperatures (623K to 973K), is significantly lower than that of pure ceria under same conditions. The 6Sm% doped ceria film is the optimum composition for highest conductivity. This is attributed to the increased oxygen vacant sites in fluorite crystal structure of the epitaxial thin films which facilitate faster oxygen diffusion through hopping process.

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.

Hlaing Oo WM, LV Saraf, MH Engelhard, V Shutthanandan, L Bergman, J Huso, and MD Mccluskey. 2009. "Suppression of conductivity in Mn-Doped ZnO Thin Films." Journal of Applied Physics 105(1):013715. doi:10.1063/1.3063730 Abstract We studied the dopant concentration distribution and conductivity in ZnO:Mn films grown by metalorganic chemical vapor deposition (MOCVD). The ion beam, surface and microstructural properties of undoped ZnO films were compared with Mn-doped ZnO films. Suppression of ZnO conductivity was noticed up to ~ 4.5 atom% Mn doping. The presence of Mn2+, confirmed by X-ray photoelectron spectroscopy (XPS), is correlated with the reduction in conductivity. No major change in the activation energy (~40 meV) and a reduction in the Zn/O ratio as a function of Mn concentration in highly sensitive proton induced X-ray emission (PIXE) technique also support this hypothesis. We discuss our results from a view point of homogeneous Mn distribution, elemental XPS ratio offsets and secondary phase formations in ZnO films.

Johnson BR, BJ Riley, SK Sundaram, JV Crum, CH Henager, Jr, Y Zhang, V Shutthanandan, CE Seifert, RM Van Ginhoven, CE Chamberlin, A Rockett, D Hebert, and A Aquino. 2009. "Synthesis and Characterization of Bulk Vitreous Cadmium Germanium Arsenide." Journal of the American Ceramic Society 92(6):1236-1243. doi:10.1111/j.1551-2916.2009.03001.x Abstract Abstract Cadmium-germanium-diarsenide (“CGA”) glasses were synthesized in bulk form (~2.4 cm3) using the procedures adapted from the literature. Several issues involved in the fabrication and quenching of amorphous CdGexAs2 (x = 0.45, 0.65, 0.85, 1.00) are described. An innovative processing route is presented to enable quenching of vitreous, crack-free ingots with sizes up to 10 mm diameter, and 30 – 40 mm long. Specimens from selected ingots were characterized using thermal analysis, optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, particle-induced X-ray emission, Rutherford backscattering, secondary ion mass spectrometry, X-ray diffraction, density, and optical spectroscopy. Variations in properties as a function of processing conditions and composition are described. Results show that the density of defect states in the middle of the band gap and near the band edges can be decreased three ways: through suitable control of the processing conditions, by doping the material with hydrogen, and by increasing the concentration of Ge in the glass.

Kuchibhatla SVNT, P Nachimuthu, F Gao, W Jiang, V Shutthanandan, MH Engelhard, S Seal, and S Thevuthasan. 2009. "Growth-Rate Induced Epitaxial Orientation of CeO2 on Al2O3(0001)." Applied Physics Letters 94(20):204101:1-3. doi:10.1063/1.3139073 Abstract High-quality ceria (CeO2) films were grown on sapphire (Al2O3) (0001) substrates using oxygen plasma-assisted molecular beam epitaxy. The epitaxial orientation of the ceria films has been found to be (100) and (111) at low (< 8 Å/min) and higher growth rates (up to ~30 Å/min), respectively. Evidence shows that CeO2 (100) film grows as three-dimensional islands, while CeO2 (111) proceeds with layered growth. Three in-plane domains at 30° to each other are observed in the CeO2 (100), which is attributed to the close match of the oxygen sub-lattices in the film and substrate that has a three-fold symmetry. Molecular dynamic simulations have further confirmed that the CeO2 film retains (100) orientation on the Al2O3 (0001) substrate.

Kuchibhatla SVNT, SY Hu, Z Yu, V Shutthanandan, Y Li, P Nachimuthu, W Jiang, S Thevuthasan, CH Henager, Jr, and SK Sundaram. 2009. "Morphology, Orientation Relationship and Stability Analysis of Cu2O nanoclusters on SrTiO3 (100) ." Applied Physics Letters 95(5):Art. No. 053111. doi:10.1063/1.3193530 Abstract Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM) and theoretical studies based on classical nucleation theory have been used to understand the morphology, orientation relationship and stability of Cu2O nanoclusters on SrTiO3 (100) (STO). We propose that the competing interfacial and elastic energies facilitate an in-plane rotation of the Cu2O clusters by 45o with respect to the STO substrate and stabilize Cu2O clusters on STO(100) with an orientation relationship of (001) Cu 2o //(001) SrTiO3 and <100> Cu 2o //<110> SrTiO3. Preliminary theoretical analysis also suggests that this particular orientation results in smaller critical nucleus sizes and lower nucleation barriers. The study also indicates a chemical potential (growth rate) dependence of the orientation relationship.

Nachimuthu P, YJ Kim, SVNT Kuchibhatla, Z Yu, W Jiang, MH Engelhard, V Shutthanandan, J Szanyi, and S Thevuthasan. 2009. "Growth and characterization of barium oxide nanoclusters on YSZ(111)." Journal of Physical Chemistry C 113(32):14324-14328. doi:10.1021/jp9020068 Abstract Barium oxide (BaO) was grown on YSZ(111) substrate by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). In-situ reflection high-energy electron diffraction, ex-situ x-ray diffraction, atomic force microscopy and x-ray photoelectron spectroscopy have confirmed that the BaO grows as clusters on YSZ(111). During and following the growth under UHV conditions, BaO remains in single phase. When exposed to ambient conditions, the clusters transformed to BaCO3 and/or Ba(OH)2 H2O. However, in a few attempts of BaO growth, XRD results show a fairly single phase cubic BaO with a lattice constant of 0.5418(1) nm. XPS results show that exposing BaO clusters to ambient conditions results in the formation BaCO3 on the surface and partly Ba(OH)2 throughout in the bulk. Based on the observations, it is concluded that the BaO nanoclusters grown on YSZ(111) are highly reactive in ambient conditions. The variation in the reactivity of BaO between different attempts of the growth is attributed to the cluster size.

Ohsawa T, I Lyubinetsky, Y Du, MA Henderson, V Shutthanandan, and SA Chambers. 2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.