EMSL: Theva Thevuthasan's Publications

Baer DR, MH Engelhard, AR Felmy, JJ Ford, JZ Hu, AS Lea, P Nachimuthu, LV Saraf, JA Sears, and S Thevuthasan. 2009. "New Approaches for Characterizing Sensor and Other Modern Complex Materials." ECS Transactions 19(6):137-148. doi:10.1149/1.3118546 Abstract Advances in understanding of sensor and other modern complex materials are often enabled by new research tools. This paper highlights three capability development themes used to identify new research tools to be provided to users of the U. S. Department of Energy’s Environmental Molecular Sciences Laboratory. These capability development directions address the importance of dynamic measurements in realistic environments, the need for increased resolution in three dimensional analyses as well as the importance of linking theory and experiment. Capability development involves expanding the range of operation for a number of important techniques, developing and applying new capabilities, and advancing methods of data processing. Examples of current developments are provided including those related to magnetic resonance, x-ray diffraction, application of a focused beam capability to fuel cell aging, and near real time analysis of XPS spectra.

Devanathan R, S Thevuthasan, and JD Gale. 2009. "Defect Interactions and Ionic Transport in Scandia Stabilized Zirconia." Physical Chemistry Chemical Physics. PCCP 11(26):5506-5511. Abstract Atomistic simulation has been used to study ionic transport in scandia-stabilized zirconia, as well as scandia and yttria-co-doped zirconia, as a function of temperature and composition. The oxygen diffusion coefficient shows a peak at a composition of 6 mole % Sc2O3. Oxygen vacancies prefer to be second nearest neighbours to yttrium ions, but have little preference between first and second neighbour positions with respect to scandium ions. The Sc-O bond length is about 2.17 Å compared to 2.28 Å for the Y-O bond. Oxygen migration between cation tetrahedra is impeded less effectively by Sc-Sc edges than by Y-Y edges. A neutral cluster of two scandium ions with an oxygen vacancy in the common first neighbour position has a binding energy of -0.56 eV. The formation of such clusters may contribute to conductivity degradation of stabilized zirconia at elevated temperature.

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.

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.

Wang CM, Z Yang, S Thevuthasan, J Liu, DR Baer, D Choi, D Wang, J Zhang, LV Saraf, and Z Nie. 2009. "Crystal and Electronic Structure of Lithiated Nanosized RutileTiO2 by Electron Diffraction and Electron Energy-loss Spectroscopy." Applied Physics Letters 94(23):Art. No.: 233116. doi:10.1063/1.3152783 Abstract The electronic structure of the nanosized rutile TiO2 before and after mechanical lithiation were studied using TEM and EELS. EELS reveals the Li K-edge at the energy-loss position of ~ 61 eV. After lithiation, the separation of the t2g-eg crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts towards a higher energy-loss position and the separation between the pre-edge peak and main peak on the O K-edge decreases. These results suggest that the lithiation of rutile TiO2 was accompanied by the reduction of Ti ion and a charge transfer from Li to Ti.

Wang Y, L An, LV Saraf, CM Wang, V Shutthanandan, DE Mccready, and S Thevuthasan. 2009. "Microstructure and ionic-conductivity of alternating-multilayer structured Gd-doped ceria and zirconia thin films." Journal of Materials Science 44(8):2021-2026. Abstract Multilayer thin-film of consisting of alternating Gd-doped ceria and zirconia have been grown by sputter-deposition on -Al2O3 (0001) substrates. The films were characterized using x-ray diffraction (XRD), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). The Gd-doped ceria and zirconia layers had the fluorite structure and are highly textured such that the (111) plane of the films parallel to the (0001) plane of the -Al2O3. The epitaxial relationship can be written as (111)ZrO2/CeO2//(0001)Al2O3 and [11-2]ZrO2/CeO2//[-2110]Al2O3.. The absence of Ce3+ features in the XPS spectra indicates that the Gd-doped ceria films are completely oxidized. The ionic conductivity of this structure shows great improvement as compared with that of the bulk crystalline material. This research provides insight on designing of material for low-temperature electrolyte applications.

Bera D, SVNT Kuchibhatla, S Azad, LV Saraf, CM Wang, V Shutthanandan, P Nachimuthu, DE Mccready, MH Engelhard, OA Marina, DR Baer, S Seal, and S Thevuthasan. 2008. "Growth and characterization of highly oriented gadolinia-doped ceria (111) thin films on zirconia (111)/sapphire (0001) substrates." Thin Solid Films 516(18):6088-6094. doi:10.1016/j.tsf.2007.11.007 Abstract Highly-oriented pure and gadolinia-doped ceria thin films have been grown on pure and ZrO2 (111)-buffered Al2O3 (0001) substrates using oxygen plasma-assisted molecular beam epitaxy (OPA-MBE) to understand the oxygen ionic transport processes in ceria based oxide thin films. Gadolinia-doped ceria films grown on pure Al2O3(0001) substrate show polycrystalline features due to structural deformations resulting from the large lattice mismatch between the Al2O3(0001) substrate and the films. However, the films, grown on a thin layer of ZrO2(111) buffered Al2O3 (0001) substrate, appears to be highly oriented. These films were characterized using high resolution transmission electron microscopy (HRTEM) and x-ray photoelectron spectroscopy (XPS) depth profiling. Oxygen ionic conductivity in gadolinia-doped ceria films was measured as a function of Gd concentration and these results were compared with the ion conductance data of the polycrystalline and single crystalline yttria-stabilized zirconia (YSZ).

Gan J, TR Allen, RC Birtcher, V Shutthanandan, and S Thevuthasan. 2008. "Radiation Effects on the Microstructure of a 9Cr-ODS Alloy." JOM. The Journal of the Minerals, Metals and Materials Society 60(1):24-28. doi:10.1007/s11837-008-0003-5 Abstract Oxide dispersion strengthened (ODS) steels are a prime candidate for high temperature, high dose cladding in advanced nuclear reactors. A 9Cr-ODS alloy was irradiated with 5 MeV Ni ions at temperatures of 500, 600 and 700ºC to doses up to 150 dpa. There was no significant change in the dislocation arrangement. For oxide particles, there is a small shrinkage in size and increase in density with increasing irradiation dose. The atom probe analysis on the unirradiated and the Ni ion irradiated 9Cr-ODS at 700ºC to 150 dpa shows similar spatial configuration of oxide particles. Radiation-induced void swelling was not identified following Ni ion irradiation. Irradiation with 1.0 MeV Kr ions at temperatures of 500, 600, 700 and 800ºC to doses of 30-50 dpa reveals the presence of voids in all the irradiation conditions and it is believed to be due to Kr ions trapped in the materials. This work confirms that oxide particles and the microstructure of the 9Cr- ODS show minimal and acceptable changes under irradiation at temperature up to 700ºC and doses up to 150 dpa.