EMSL: Lax Saraf'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.

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.

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.

Saraf LV, Z Zhu, CM Wang, and MH Engelhard. 2009. "Microstructure and Secondary Phase Segregation Correlation in Epitaxial/Oriented ZnO Films with Unfavorable Cr Dopant." Journal of Materials Research 24(2):506-515. doi:DOI: 10.1557/JMR.2009.0054 Abstract We discuss the effect of microstructure on the secondary phase segregation region and mobility of carbon impurities in case of poorly soluble Cr as a dopant in ZnO thin films. Thin films of Cr:ZnO ~50 nm in thickness were grown by metal organic chemical vapor deposition (MOCVD) of Zn(TMHD) and Cr(TMHD) precursors in reactive oxygen partial pressure environment. For an accurate comparison among the differences among the grain-boundary density and degree of orientation on the secondary phase segregation and impurity mobility, simultaneous thin film growths were carried out on single crystals of Si (100), c-plane oriented Al2O3 (c-ALO) and r-plane oriented Al2O3 (r-ALO) substrates. High-resolution transmission electron microscopy (HRTEM) measurements across the film substrate interface indicate that growths on Si(100) and c-ALO resulted in highly oriented Cr:ZnO films whereas a good epitaxial growth was observed on r-ALO. The trace carbon impurity detection, secondary phase formation and their mobility properties were studied by sensitive x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). We have observed that secondary phase segregation regions occur near the surface for Cr:ZnO films grown on ALO whereas the region moves near the interface for the growth on Si. Considering the presence of grain boundaries in Cr:ZnO grown on c-ALO and Si, it appears to be a weak relationship between grain boundary density and unfavorable dopant mobility as well as preferred segregation region. A near uniform stress distribution observed at r-ALO/ZnO interface indicates good epitaxial growth by domain matching epitaxy process. We also observe that low carbon impurity distribution in the studied thickness regime remains more or less uniform inside Cr:ZnO. This gives strong evidence that trace amount of carbon is soluble in the Cr:ZnO system as a direct result of oxygen vacancy defects.

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.

Wang D, D Choi, J Li, Z Yang, Z Nie, R Kou, D Hu, CM Wang, LV Saraf, J Zhang, IA Aksay, and J Liu. 2009. "Self-assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-ion Insertion ." ACS Nano 3(4):907-914. Abstract We used anionic sulfate surfactants to assist the stabilization of graphene in aqueous solutions and facilitate the self-assembly of in-situ grown nanocrystalline TiO2, rutile and anatase, with graphene. These nanostructured TiO2-graphene hybrid materials were used for investigation of Li-ion insertion properties. The hybrid materials showed significantly enhanced Li-ion insertion/extraction in TiO2. The specific capacity was more than doubled at high charge rates, as compared with the pure TiO2 phase. The improved capacity at high charge-discharge rate may be attributed to increased electrode conductivity in presence of a percolated graphene network embedded into the metal oxide electrodes.

Zhang Q, LV Saraf, JR Smith, P Jha, and F Hua. 2009. "An Invisible Bend Sensor Based on Porous Crosslinked Polyelectrolyte Film." Sensors and Actuators. A, Physical 151(2):154-158. doi:10.1016/j.sna.2009.02.034 Abstract This paper describes the fabrication of a porous cross-linked polyelectrolyte membrane and the characterization of its humidity sensitivity performance. Electrostatic self-assembly, combined with acid treatment, and post-deposition annealing produced the membrane. The fabrication process offers the ability to control the thickness of the membrane, as well as enabling the engineering of the humidity sensitivity properties. A transparent humidity sensor was fabricated by integrating the membrane into a capacitive structure. In order to improve the moisture absorption and diffusion, both the polyelectrolyte layer and the electrode were made porous. The membrane was cross-linked to enhance the durability in high humid environments. Such a polyelectrolyte membrane showed high sensitivity to relative humidity variation over a range of 25-99%. The see-through property of the structure adds extra features and benefits to the sensor.

Zhang Y, J Lian, Z Zhu, WD Bennett, LV Saraf, JL Rausch, CA Hendricks, RC Ewing, and WJ Weber. 2009. "Response of Strontium Titanate to Ion and Electron Irradiation." Journal of Nuclear Materials 389(2):303-310. doi:10.1016/j.jnucmat.2009.02.014 Abstract Response of strontium titanate (SrTiO3) to ion and electron irradiation is studied at room temperature. For an accurate energy to depth conversion and a better determination of ion-induced disorder profile from Rutherford backscattering spectrometry measurement, a detailed iterative procedure is described and applied to ion channeling spectra to determine the dechanneling yield and the disorder profiles for the Sr and Ti sublattices. The result shows a large underestimation in disorder depth, ~ 40% at the damage peak, which indicates a large overestimation of the electronic stopping power for 1.0 MeV Au ions in SrTiO3 predicted by the SRIM (Stopping and Range of Ions in Matter) code. Overestimation of heavy ion stopping power may lead to an overestimation of the critical dose for amorphization. The current study also demonstrates possible ionization effects in SrTiO3 under ion and electron irradiation. Pre-amorphized SrTiO3 exhibits strong ionization-induced epitaxial recovery at the amorphous/crystalline interface under electron irradiation.

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).