Photosynthesis converts carbon dioxide (CO2) into hydrocarbons that can be utilized as food and fuel. We have developed a synthetic photosynthesis system that combines plant pigments and a titanium dioxide (TiO2) photocatalyst to convert CO2 into hydrocarbons using a broader range of solar energy. Synthetic pigment components were assembled by partially replacing magnesium ions in chlorophyll with TiO2. The TiO2 photocatalyst utilizes both UV and green light, changing the landscape of photosynthesis by allowing plants to scavenge additional wavelengths. TiO2 nanowires and nanotubes also resemble the natural antenna systems formed by plant pigments that concentrate light energy. Hybrid pigment structures were formed by self-assembly on silk protein substrates and analyzed using UV-Vis absorption spectroscopy, tensile strain, and structure analysis. The hybrid pigment structures were also confirmed for biocompatibility and physiology compatibility. Water and CO2 were converted to hydrocarbons using a microfluidic and capillary approach under simulated light and atmospheric and compressed gas extraction pressure. The synthetic photosynthesis system efficiently produces hydrocarbons not possible in natural plants with unprecedented efficiency and applicability.

Photolytic water splitting as a method of hydrogen production has attracted a lot of attention since Honda et al. first demonstrated this concept with TiO2 in 1972 [1]. TiO2 has since been used as a reference material in numerous experimental as well as in theoretical studies in the field of solar hydrogen production[2-4]. The most promising performance is observed with nanostructured films consisting of predominantly the anatase TiO2 phase[2]. The atomic scale structure of such films is not well understood, and could be essential to further the understanding of the mechanisms involved in this complex process. Being able to predict which anatase surfaces are likely to be exposed in experimental conditions is, therefore, essential to achieve this goal. With the aim of gaining a better understanding of anatase surfaces, Li et al studied a vicinal surface of anatase[5]. The experimental techniques used did not allow for the determination of the atomic structure of the surface or distinguish between the (514) and (516) planes. In this work we present hybrid-exchange density functional theory calculations of low-index anatase surfaces as well as the vicinal surfaces possibly observed by Li et al.[5]. We show that the (516) surface is very stable and, in fact, has a surface formation energy comparable to the (101) surface. The (514) surface is also relatively stable with respect to other low-index surfaces and both could provide ways of forming low energy steps. These could be important when forming nanostrucured films. Electronic structure calculations were used to simulate STM images (in the constant current mode) to allow for the comparison of the computed structures with experimentally obtained images. References 1. Fujishima K. and Honda, A. Nature, 1972 238, 37. 2. Graetzel, M. Nature, 2001, 414, 338-344 3. Lazzeri, M. and Selloni, A. Phys. Rev.Lett. 2001, 87, 266105 4. Labat, F., Baraneka, P. Domain, C., Minot, C., Adamo, C. J. Chem. Phys. 2007 126, 154703 5. Li, S.C., Dulub, O. and Diebold, U., J.Phys.Chem.C Lett. 2008 112, 16166-16170

TaON is considered as a potential candidate as a visible-light responsive photocatalyst. We report the results of ab initio studies of electronic structure of TaON alloys. Specifically, we show that the position of conduction and valence band can be modified by varying the oxygen and nitrogen concentrations in TaO1-xN1+x. We find that the band gap decreases monotonically with the increase of N/O ratio. The band gap energy is decreased in monoclinic TaON from near 2.7eV to just over 1.1eV (i.e. by 230%) when N/O ratio is reduced from 1/3 to 3/1. The band gap reduction is mostly associated with the change in the position of the valence band due to the hybridization of N 2p states, while the conduction band consisting mostly of Ta 5d-states is not sensitive to N content. The calculated optical absorption spectra show reduction in the optical band gap with increasing N/O ratio. Our calculations show that the band gap reduces as function of the N/O ratio in a series of experimentally fabricated alloys ZrTa3O5N3->TaON->YTa7O7N8 where the latter has the cubic, and the formers have the baddeleyite crystal structure.

Photoelectrochemical water splitting using semiconductor electrodes offers a promising route towards harvesting solar energy and storing it in chemical bonds. However, efficient performance requires several traits which one single material cannot itself satisfy, foremost being the generation of a photovoltage sufficient to drive both the reduction and oxidation half-reactions of electrolysis (1.23 V plus overpotentials). For instance, the metal oxide hematite (α Fe₂O₃) has a band gap suitable for substantial solar absorption, yet with a conduction band too positive to achieve water reduction, hematite-only electrodes require applied bias to achieve water splitting. No degree of catalyst treatments, doping, or nanostructuring can solve this fundamental problem of band misalignment. We have therefore performed studies on the under-investigated aspect of hematite electrode energetics, using combinations of materials design and high-quality synthesis by atomic layer deposition (ALD). Both n-p homojunction hematite and silicon nanowires / hematite heterojunction devices produced enhanced photovoltages, manifested as cathodic shifts in the photocurrent onset potentials signifying a decreased requirement for applied anodic bias. These results represent some of the lowest turn-on potentials observed on hematite devices and were achieved without hematite doping, catalysts, or surface treatments, pointing towards an important new direction of study for enhancing the efficiency of metal oxide photoelectrodes.

The anticipated production of fuels as H2 or CH-compounds with solar light would provide a sustainable and secure primary energy source by producing a storable and transportable fuel. For this reason oxide photocatalysts to mimic photosynthesis have intensively been studied in the past. However, the systems identified so far are limited in their conversion efficiencies. Advanced heterostructure photocatalysts are suggested in this presentation based on nanosized Janus type structures. We suggest the combination of a wide band gap light absorber (bandgap 2-3eV) and adapted contact materials (co-catalysts) in a photovoltaic arrangement which breaks the spherical symmetry of the absorber and provide the conditions for quantum efficiencies approaching 100%. Central part is the (nanoscale) synthesis of Janus type heterostructures either by combining two different semiconductor materials with a well defined mis-alignment of band edges (type II alignment) or by forming asymmetric contacts by different metals providing different work functions. Efficient charge carrier separation is provided by a well-defined vectorial separation of electron-hole pairs and transport of electrons and holes to the different co-catalyst particles ideally deposited on opposite sides of the semiconductor. The additionally needed minimization of photovoltage (chemical potential) losses can be expected for isoenergetic electronic coupling of the highly efficient HER/OER catalyst to the conduction/valence band of the semiconductor. Possible ways to manufacture such Janus structures will be presented.

Current fuel cell technology requires the use of large amounts of platinum in the cathode for the oxygen reduction reaction (ORR). A major roadblock to the commercialization of fuel cells is the high cost of Pt. Current efforts to reduce the cost of producing fuel cells have focused on the minimization of the amount of Pt, thus bringing down the overall cost. Prior work has demonstrated the creation of stable conductive titanium oxide films grown using atomic layer deposition (ALD). ALD is a self-limiting technique used to grow single layers of a film, or well dispersed particles of a material. These ALD-grown supports have demonstrated themselves to be stably conductive in oxidizing environments. This allows for noncontinuous coatings of Pt catalysts for the ORR. Current work focuses on ALD Pt on titanium oxide grown in a multi-chamber ALD cluster tool. Pt was deposited using methylcyclopentadienyltrimethyl platinum (IV) (MeCpMe3Pt) as the precursor with NH3 and H2 as reactant gases, and a recipe was developed to optimize the the Pt with respect to growth rate and conductivity. In addition, post-deposition annealing in a reducing atmosphere was used to increase conductivity of ALD Pt films. Films were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), Rutherford backscattering spectrometry (RBS), four point probe analysis, and cyclic voltammetry (CV). It was found that Pt nanoparticle size could be controlled by adjusting the number of ALD growth cycles. In the first 50 cycles of Pt deposition, particles in the 3-5 nm regime are visible. After 200 cycles, the particles have grown and coalesced into full films. After this point no further increase in catalytic activity is expected.

Cuprous oxide (Cu₂O) has emerged as a promising p-type thin film semiconductor for solar cells as its bandgap is well matched to the solar spectrum. However, as it is not possible to stably dope the material n-type, it is necessary to use heterojunction cells rather than homojunctions. There are several published reports that heterojunctions of Cu₂O deposited onto ZnO have on/off current rectification ratios of over 100 [Gershon, T., et al., Sol. Energy Mater. Sol. Cells, 96, 148-154 (2012); Mittiga, A., et al., Appl. Phys. Lett., 88, 163502 (2006)]. In these previous studies, a number of deposition techniques have been employed, including sputtering, thermal oxidation and electrodeposition. In all cases, the junction has been exposed to atmosphere between the deposition of the two layers. In this work, ZnO/Cu₂O heterojunctions were fabricated using a low temperature sputtering technique (HiTUS) which allows the ZnO and Cu₂O thin films to be deposited consecutively without the need to break vacuum [Li, F.M., et al., Thin Solid Films, 520, 1278-1284 (2011)]. These heterojunctions were found to have a rectification ratio of 10, which compares poorly to cells published in literature fabricated with atmospheric exposure to the heterojunction interface. Hence, a series of experiments were performed to investigate this apparent effect. It is known that ZnO is prone to adsorption of both oxygen and water vapour. Therefore this study aimed to understand if either or both of these were responsible for the improved cell rectification. It was found that when the ZnO thin film was exposed to atmospheric conditions briefly (8 minutes) before the Cu₂O layer was deposited, the resulting heterojucntions had improved rectification ratios of up to 300. This improvement came from a reduction in the reverse saturation current, suggesting that the ZnO/Cu₂O interface defect density had been reduced by the exposure. Further devices were also fabricated with prolonged exposure to atmosphere, but this did not noticeably improve the junction quality. In order to investigate whether H₂O or O₂ was the likely cause of the improvement, cells were fabricated where the ZnO layer was dipped in water before the Cu₂O deposition. These were found to produce diodes with a rectification ratio of over 28000. Control cells were also fabricated with exposure to atmosphere for 362 hours followed by annealing at 393 K for 15 minutes and this resulted in the rectification ratio reducing once more to 10. These results demonstrate for the first time that exposure to H₂O is important for controlling interface states in the ZnO/Cu₂O heterojunction. This result has implications for the fabrication of ZnO based thin film devices where the active interface is exposed to atmosphere during processing.

Transparent conducting oxides (TCOs) with tunable morphology and tunable work functions are of general interest in a range of optoelectronic systems. In this study, we have examined variation of opto-electronic properties such as crystallinity, transparency, conductivity, work functions and surface roughness of Indium Zinc Tin Oxide (IZTO) thin films. Specifically we have focused on controlled radio frequency magnetron sputter deposition from a fixed composition In0.5 Zn0.25Sn0.25Ox target. Our focus was to evaluate the effect(s) of various regions of empirical parameter space during deposition of the thin films including: power density applied to the target (ρrf); substrate temperature (Ts); and process gas pressure (P). The results indicate that films grown at room temperature (30 oC) are largely amorphous while those grown at elevated temperature of 150 oC and 250 oC show varying crystallinity which is multivariate dependent upon other deposition conditions. Cross-sectional TEM data shows the amorphous IZTO (a-IZTO) films obtained for a higher temperature of 250 oC suffered some degree of phase segregation. Crystalline IZTO (c-IZTO) films have x-ray diffraction with the bixbyite In2O3 structure suggesting it is primarily In2O3 with SnO2 and ZnO as co-substitutional dopant. The films grown with increased substrate temperature yielded more efficient doping in both a-IZTO and c-IZTO resulting in high conductivity films. The highest σ to date for this composition of c-IZTO and a-IZTO film are respectively 2260 ± 30 S/cm and 1470 ± 20 S/cm. Higher temperature deposited TCOs have broader optical gap due to a Burstein-Moss shift resulting from the increase in carrier concentration and improved doping efficiency. Films have work functions ranging from -5.6 eV to -6.1 eV and optical transparency of >80% in the visible is observed for both high conductivity a-IZTO and c-IZTO films. Both c-IZTO and a-IZTO with similar conductivity show significant variation in surface conductivity as measured by conductive AFM. The impact of these changes to surface conductivity is examined in the context of organic photovoltaic devices in which we demonstrate the use of IZTO as a transparent contact. Acknowledgements: Materials development including oxide deposition and characterization equipment was provided by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory. Support for characterization and analysis work was provided as part of the Center for Interface Science: Solar-Electric Materials (CIS:SEM), an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award Number DE-SC0001084

It has been recently demonstrated that intentionally doped cadmium oxide (CdO) can have very high mobility of around 300 cm²/Vs at electron concentration of 3x10²⁰ cm^-3. These properties, combined with a wide transmission window extending from 400 nm to above 1500 nm, make CdO a promising transparent conductive material for solar cell application as well as for photodiodes and gas sensors [1]. It should be noted that these exceptional electrical properties were obtained in polycrystalline CdO synthesized by variety of methods including pulsed laser deposition (PLD) and sputtering. Here, we use thermal annealing as a tool to better understand the influence of native defects on the electrical and optical properties of undoped and intentionally doped CdO films. Samples were synthesized by pulsed laser deposition and sputtering techniques at growth temperatures ranging from 25-320 °C. The films were treated with rapid thermal annealing (RTA) for 30 seconds at 300-600 °C in nitrogen and oxygen atmospheres. The electron concentrations and mobilities were determined by Hall effect measurements and the optical characteristics were measured using optical absorption and photoluminescence (PL). We find that both the annealing temperature and atmosphere affect the properties of the films. For nominally undoped samples that received RTA treatments in an oxygen atmosphere, a decrease in the electron concentration and an increase in the mobility is observed at RTA temperatures up to 400 °C, with the electron concentration and mobility reaching saturation values of n~5x10¹⁹ cm^-3 (as grown: n~2x10²⁰ cm^-3) and µ~200 cm²/Vs (as grown µ~75 cm²/Vs). Such trends are not observed in samples annealed in nitrogen atmosphere. Collectively, these results can be explained within the model that native defects serve as donors within CdO: with increasing RTA temperature in an oxygen ambient, the carrier concentration decreases as oxygen vacancies are healed. Moreover, the removal of native defects reduces ionized impurity scattering and leads to an increase in the mobility. We will also present the results of the absorption and PL measurements as well as our results of the effects of thermal annealing on the properties of intentionally doped CdO. [1] K.M Yu, et al., J. Appl. Phys. 111 123505 (2012) This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Inverted organic solar cells have attracted great attention due to their exceptional environmental stability compared to the conventional solar cell architecture.1 Metal-oxides are commonly employed in inverted solar cells as electron collector/injector layers. Inverted solar cells are subjected to losses, in particular in photo-voltage, due to imperfect injection levels and interfacial charge traps at the metal-oxide interfaces. Here, we demonstrate a 35% enhancement in the efficiency of inverted solar cells as a result of increased open-circuit voltage and fill factor by adsorbing an ultrathin layer of a ruthenium dye N719 on an aluminum-doped zinc oxide (ZnO-Al) electron collecting interfacial layer. Interface characterization using ultraviolet photoemission spectroscopy shows that the interface modification with N719 results into modification of charge injection levels. The efficiency of inverted solar cells with a bulk heterojunction photo-active film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester has increased from 2.80 to 3.80 percent upon employing the modified electrodes.2 References: 1. Y. Sun, J. H. Seo, C. J. Takacs, J. Seifter, and A. J. Heeger, Adv. Mater. 23, 1679 (2011). 2. A. Gadisa, Y. Liu, E. T. Samulski, and R. Lopez, Appl. Phys. Lett. 100, 253903 (2012). # This work is supported by the NSF SOLAR Grant (DMR-0934433).

Indium tin oxide (ITO)-based transparent conducting electrodes have been widely used in electronic devices. However, such brittle oxide electrodes suffer from poor mechanical durability, which poses significant challenge to their successful application in large area flexible electronics, such as paper like displays and flexible solar cells. Inspired by recent development of inorganic/organic hybrid permeation barriers for flexible electronics, we design and fabricate ITO-based multilayer electrodes with enhanced electro-mechanical durability. In situ electro-mechanical experiments of five structural designs of ITO-based multilayer electrodes are performed to investigate the evolution of crack density and the corresponding variance of electrical resistance of such electrodes. A coherent mechanics model is established to determine the driving force for crack propagation in the ITO layer in these electrodes. The mechanics model suggests that a top protective polymeric coating above and an intermediate polymeric layer below the ITO layer can effectively enhance the mechanical durability of the ITO electrodes by reducing the crack driving force up to ten folds. The modeling results offer mechanistic understanding of the in situ experimental measurements of the critical fracture strains of the five types of ITO-based multilayer electrodes. The findings in this work provide quantitative guidance for the material selection and structural optimization of ITO-based multilayer transparent electrodes of high mechanical durability.

Application of the economical metal oxide thin-film photovoltaic devices is hindered by the poor energy efficiency and complexity of device fabrication approach. This paper investigates the metal-insulator-semiconductor (MIS) type photovoltaic cell with an ultrathin tantalum oxide (TaOx) tunnel barrier, formed by the plasma oxidation of a pre-deposited tantalum (Ta) film. These ~3 nm TaOx tunnel barriers showed approximately 160 mV open circuit voltage and 3-5% energy efficiency, for varying light intensity. The ultrathin TaOx (~3 nm) could absorb approximately 12% of the incident light radiation in 400-1000 nm wavelength range; this strong light absorbing capability was found to be associated with the dramatically large extinction coefficient. Spectroscopic ellipsometry revealed that extinction coefficient of 3 nm TaOx was ~0.2, two orders higher than that of the stochiometeric Ta2O5. Interestingly, refractive index of this 3 nm thick TaOx was comparable with that of stochiometeric Ta2O5. However, heating and prolonged high-intensity light exposure deteriorated the photovoltaic effect in TaOx junctions. This study provides the basis to explore the photovoltaic effect in a highly economical and easily manufactured ultrathin metal oxide tunnel barrier or analogous systems.

Energy from the sun can provide sufficient power for all of our energy needs if it can be efficiently harvested. An elegant and potentially efficient route to storing solar energy is to convert light into chemical energy in the form of chemical bonds, which is a form of an artificial photosynthesis process. Considering the abundance of H2O on the planet, water splitting is a natural pathway for artificial photosynthesis. Hematite (α-Fe2O3) is a candidate material to be used as a photoanode for water splitting due to intrinsic properties such as suitable band gap (2.0-2.2 eV), chemical and photoelectrochemistry stability, abundance and low cost. By combining high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) with analytical capability, we investigated the nanostructure of a textured hematite photoanode with columnar grains obtained by the colloidal deposition of magnetite nanocrystals. This initial report describes in detail the structure and chemistry of the α-Fe2O3/FTO interface by identifying semi-coherent and incoherent interfaces as well as a localized inter-diffusion layer of Sn and Fe at the interface (~100 nm in length). Our study indicates unintentional doping by tin at a high sintering temperature is not significant in enhancing hematite photoanode performance for water oxidation. The correlation of nanoscale morphology with photoelectrochemical characterization facilitated the identification of the beneficial effect of a preferential growth direction of a hematite film along the [110] axis for water-splitting efficiency. In addition, the colloidal process combined with high sintering temperature resulted in achieving a photocurrent of 1.85 mA.cm-2 at 1.23 VRHE which is one of the best photoelectrochemical performances reported to date in the literature for pure hematite.

The band offset between two semiconductor materials is essential for the behavior of the charges at the heterointerface. As known from Minemoto et al. [1] it affects the electron transport in device applications. Using photoelectron spectroscopy (XPS) we investigate the band offsets of the heterointerfaces Cu₂O/ZnO and Cu₂O/GaN. For the first one we found a conduction band offset (CBO) value of 0.97 and for the other one of 0.24 eV. Out of this information one can see, that the large CBO between ZnO and Cu₂O will very likely result in low photovoltaic power conversion efficiencies as is the current status of Cu₂O/ZnO solar cells. For good photovoltaic performance a low conduction band offset is necessary. Thus gallium nitride seems to be a more suitable candidate for the front contact of Cu₂O based solar cells. [1] Minemoto, T. et al., Solar Energy Materials and Solar Cells, 2001, 67(1-4):83-88.

Oxygen ion conductors are critical components in many important energy conversion devices, including solid oxide fuel cells (SOFCs), oxygen separation membranes, and oxidation catalysts. Enhancements in performance may be possible using thin film growth to produce properties not seen in bulk samples, such as using epitaxy or strain to grow metastable phases. One model ionic conductor is the high temperature phase, δ-Bi₂O₃, which exhibits the highest oxygen ion conductivity of any oxide material but is stable only in a very narrow temperature range, 730-825°C. The study of δ-Bi₂O₃ thin films can provide insight into the key structural factors for high ionic conductivity as well as the challenges in enlarging the stability region of such a material. In our research, we have stabilized δ-Bi₂O₃ to room temperature via synthesis on (001) perovskite single crystal surfaces. Growth by magnetron sputtering using a Bi₂O₃ target in an Ar/O₂ gas mixture at substrate temperatures higher than 300°C produced (001)-oriented δ-Bi₂O₃ nanoislands coherently strained to the (001) substrates. Synchrotron x-ray scattering observations at controlled temperatures and oxygen partial pressures revealed that the δ-Bi₂O₃ nanostructures exhibit a superstructure that may arise from ordering of the vacant oxygen sites. Continuous (001)-oriented single crystal films were achieved by sputtering at substrate temperatures of 300°C and below in a 100% oxygen environment. Synchrotron studies show the continuous films do not exhibit the same superstructure as the nanoislands grown at higher temperature. The potential for achieving high ionic conductivities at low temperatures in δ-Bi₂O₃ films and for elucidating the origin of superionic conductivity in oxide materials in general will also be discussed.

In addition to clean and renewable energy generation, the subsequent storage and delivery of such energy is likewise a critical facet to achieving long-term sustainable energy practices. An emerging technology known as electrochemical capacitors (ECs) has exhibited efficient, high-power energy storage in both the commercial and academic realm. ECs are physically analogous to traditional capacitors (i.e. charge storage occurs in opposing electrodes), but couple an organic/aqueous electrolyte with high surface area electrodes to enhance energy storage by means of electrical double layer phenomena. Pseudocapacitors (PCs) are a subcategory of ECs that strive to further enhance energy storage by depositing or substituting redox active materials onto the electrode surfaces, thereby permitting the aqueous/organic electrolyte to interact with the electrodes via redox processes that yield greater net charge transfer. Ruthenium dioxide, RuO₂, has been the most promising candidate for PC electrode materials, but the implementation of RuO₂-based PCs has been hampered by its high market price. Manganese dioxide, MnO₂, has displayed auspicious redox characteristics, particularly with respect to its market cost, but suffers from long-term redox instabilities and generally prohibitive electrical resistance. The goal of this research is to explore the use of thin-film manganese dioxide for incorporation into composite PC anode materials. This desired morphology promotes redox activity with an increased active surface area while simultaneously reducing the electron diffusion paths for rapid power delivery. We report the facile synthesis of birnessite K_0.15MnO₂ nanosheets, which were fully characterized by transmission electron microscopy (TEM), scanning TEM (STEM), high-resolution TEM (HRTEM), X-ray diffraction spectroscopy (XRD), energy dispersive spectroscopy (EDS), and thermogravimetric analysis (TGA). A K_0.15MnO₂ composite material was tested in a three-electrode cell with Na₂SO₄ electrolyte, resulting in a specific capacitance in excess of 300 F/g that significantly increased baseline carbonaceous materials. To further elucidate the properties of the evolving Mn(III/IV) valence state during discharge processes, we constructed an in situ X-ray absorption near edge spectroscopy (XANES) cell, which permitted for transmission and fluorescence analyses while simultaneously applying an external electrical potential. These results suggested a region of principal Mn(III/IV) oxidation/reduction for optimal operating voltage ranges of MnO₂-based PCs.

N-type oxide semiconductors are of great technological importance, mainly when these oxides show photoactivity for water oxidation via photoeletrochemistry cell (PEC). Among the n-type semiconductors oxides, the tungsten oxide (VI) shows a promising behavior for photoanode, due to the maximum theoretical efficiency conversion of 8% considering a band gap of 2.6 eV. Here we report an approach to prepare tungsten oxide (VI) photoanode by a non-hydrolytic route where tungsten oxide was synthesized in oleylalcohol at 150 oC. The colloidal stability in hexane was achieved by addition of oleylamine, which have the ability to bind strongly to the acid sites on the surface of the oxide. In this way, the tungsten oxide colloid was obtained in hexane, forming a yellowish solution in a single synthetic step. Herein this stable precursor was easily deposited on an FTO glass substrate. The thin film prepared was thermally treated at different temperatures, in order to study the effect of temperature in the grain growth and consequently on the WO3 photoresponse. The relevant result obtained in the process was a photocurrent (at 1.23 VRHE) of 1.09 mA cm-2 in a standard condition of AM 1.5 G illumination (in H2SO4 electrolyte - 1.0 mol). The morphology and thickness was characterized with scanning electron microscopy (SEM). Besides, the cross-sectional TEM image was acquired with sample prepared with Focus Ion Beam (FIB-SEM) that reveled a mesoporous thin film and excellent interface. The film microstructure and the photocurrent showed clear sintering temperature dependence.

Nanoparticles having core-shell morphology have found use in disparate applications including improved biocompatibility for magnetic nanoparticles, reducing agglomeration, or chemical functionalization. In this study, BaTiO3 nanoparticles were coated with a crystallizable glass shell (29.6% BaO-7.4% SiO-37% TiO2-8% SiO2-16% B2O3-2% Al2O3) by pulsed laser deposition (PLD) and were used as improved dielectric starting materials for capacitors. Utilizing the extremely high dielectric constant of ferroelectric titanates and the high breakdown strength of alkali-free glass, the core-shell morphology further increases the composite dielectric homogeneity and the hermetic seal formed at the interface. 50 nm diameter BaTiO3 particles were coated with amorphous thin film glass by a KrF excimer laser at 0.24 J/cm2 for 15 minutes for a desired 6nm layer. The core-shell particles were then sintered at 1200C for 4 hrs to form a pellet for testing. The resulting composite material consisted of a glass matrix surrounding the core ferroelectric nanoparticles. The composite mixture was further improved for capacitive energy storage because the glass stoichiometry used would crystallize into BaTiO3, i.e., BaTiO3 in the glass was then crystallized at 1000C for 2 hrs, adding to the original core particle by heterogeneous nucleation and increasing the dielectric constant of the overall composite material. The remaining glass increased the breakdown strength of the material, allowing for the creation of especially high energy density capacitors. Electron microscopy was used to characterize the core-shell structure of the nanoparticles.

The mixed ionic-electronic conducting (MIEC) oxides are effective catalysts towards oxygen reduction reaction (ORR) and therefore they have been widely studied as potential solid oxide fuel cell cathode materials [1-3]. In this study, we show that the oxygen permeation flux of Au-short-circuited oxygen ion conductor membranes is dramatically enhanced (approximately 2 orders of magnitude) by decorating the surface with porous MIEC La_0.8Sr_0.2CoO_3-δ (LSC₁₁₃) or LaSrCoO_4±δ (LSC₂₁₄) thin films prepared by pulse laser deposition (PLD). The oxygen permeation performance of these membranes is further improved by changing the short-circuiting material from Au to Ag. Crystal structure and surface morphology were studied by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Using gas chromatography (GC) measurements the oxygen permeation properties were examined. Based on the feed and permeate side oxygen partial pressure dependence of the oxygen permeation flux, the feed side ORR is determined to be rate-limiting. The aforementioned enhancement of the oxygen permeation flux is then believed to be mainly due to the accelerated ORR at the feed side. Consequently further enhancement can be achieved by varying the morphology and/or the composition of the decoration MIEC oxide thin films. [1] S. B. Adler, Chem. Rev. 104 (2004) 4791. [2] A. Tarancon, M. Burriel, J. Santiso, S. J. Skinner, J. A. Kilner, J. Mater. Chem. 20 (2010) 3799. [3] G. J. la O', S. J. Ahn, E. Crumlin, Y. Orikasa, M. D. Biegalski, H. M. Christen, Y. Shao-Horn, Angew. Chem. Int. Ed. 49 (2010) 5344.

Thermochromic properties of nanolayers of titanium and vanadium oxides and their alloys have been investigated. The ultra thin oxides layer was deposited by reactive magnetron sputtering from pure titanium and vanadium targets. The sputtering was done in Argon-Oxygen mixture at elevated substrate temperatures. The development of “smart window” materials for efficient energy consumption in buildings and automobiles are of interest in this project. The optical and electrical properties of the thin films as a function of temperatures are reported. The composition and thickness of the films was determined with Rutherford Backscattering Spectroscopy (RBS) and Energy Dispersive Spectrometer (EDS) data. The surface morphology of the films from SEM and AFM are reported also.

A pyroelectric crystal develops a spontaneous electrical polarization when its temperature changes. It is possible to cycle the temperature and electric field to drive the crystal through an order/disorder phase transition to convert between heat and electric energies. The phenomena known as the electrocaloric effect (ECE), in which electrical energy is used to induce a temperature change, is directly related to this thermodynamic cycle. The ECE holds great potential for future technologies such as solid-state refrigeration. In this presentation I will demonstrate the calculation of the pyroelectric properties of a perovskite crystal using an effective Hamiltonian model within a molecular dynamics framework. In this presentation we focus on the perovskite BaTiO3 (BTO) compound for a variety of reasons: it is a well studied archetypical perovskite crystal that is relatively easy to produce, it exhibits an good pyroelectric response, it does not contain the toxin lead, and its ferroelectric properties have the potential to be tailored by alloying or the creation of superlattice structures. The ECE in BTO has been observed and reported for a variety of geometries including thin and thick films, nanostructures, and surprisingly off-the-shelf BTO multilayer capacitors. Previous theoretical studies of BTO have largely relied on thermodynamic models, for example the Ginzburg-Landau-Devonshire model. In this presentation we directly calculate the pyroelectric response using a molecular dynamics approach and study the impact of epitaxial strain on the magnitude of the ECE. The results presented here demonstrate that BTO can exhibit a moderate sized pyroelectric and ECE response, with a ΔT around 5-6 K, for a relatively small electric field gradient, less than 100 kV/cm. Unlike the Pb-containing alloys that have been the focus of most giant-ECE studies, it is unnecessary to apply a large electric field gradient because there is no antiferroelectric ground state that must be avoided through the use of large fields.

Cerium oxide (CeO2, ceria) is a rare earth metal-oxide which is scientifically important because of its unique properties and various applications, in particular as a redox active material for two-step thermochemical cycling. In this study, nanostructured cerium oxide films are grown on Si(100) and quartz substrates by pulsed DC magnetron sputtering from a cerium oxide target. The influence of oxygen partial pressure in both the sputtering chamber and post-deposition annealing on the film properties are studied. The structural and optical properties of the films are examined using X-ray diffractometry (XRD) and UV-visible (UV-Vis) absorption spectroscopy, respectively. XRD results show that the films have preferential orientation along the [111] direction and the crystalline quality of the films, as measured by grazing incidence XRD, can be improved by post-deposition annealing in an O2 ambient. Morphological studies of the films using atomic force microscopy (AFM) indicated grain formation and an increase in surface roughness as a result of the annealing process. The chemical nature of the cerium oxide films is determined using secondary ion mass spectrometry (SIMS) and the electrochemical charge storage properties of the films are examined using cyclic voltammetry experiments.

ZnO has attracted significant attention for applications in transparent electrodes for solar cells. Since a ZnO crystal has a wurzite-type structure, it shows spontaneous polarization along the c-axis. Therefore, ZnO has polar surfaces corresponding to the c(+)-face and c(-)-face. It is well known that various properties of ZnO depend on its polarity, such as chemical stability of the surface, incorporation effciency of nitrogen and growth rate. Therefore, it is important for device applications of ZnO to investigate the effects of crystal poarlity on poroperies of ZnO films. In this study, we investigated the crystal polarity and electorical properties of heavily doped ZnO films. We found that the films grown at a low temperature by pulsed laser deposition using a heavily Al-doped ZnO target had a c(-)-face, whereas the films prepared at a high temperature had a c(+)-face. The room-temperature resistivities of the films were 4.50×10^-4 and 1.94×10^-3Omega;cm, respectively. More details will be presented at the conference site.

The possibility of splitting water via light irradiated TiO2 reported by Fujishima and Honda in 1972 [1] has tremendously boosted the interest of researchers for this very "intriguing" material. Its relevance in many industrial and technological applications, in catalysis, in the field of solar-to-energy conversion, in photocatalysis (and in many others), is testified by the ever increasing number of published scientific reports. Nowadays, the availability of more accurate experimental techniques has lead to the synthesis of nanostructured TiO2-based materials: their surface area enhancement and the inherent improvement of their photochemical activity make them subject of further deep analysis. Among all the possible morphological shapes [2], the study of (001)-oriented nanosheets is of wide interest for the reported improved performances of systems with such facet exposure [3]. In particular, a double morphological nature characterizes such layered structures. The first is the “ordinary” reconstruction obtained from the "cut" of bulk anatase along [001]. The second is derived from the previous which, after the gliding of the upper part of the film over the lower along the Ti-O-Ti chain direction, evolves towards the so-called lepidocrocite [4] (barrierless path due to stress reduction). Such nanosheets are precursors of titania nanotubes (NTs) which in turn can be used for further assembling of nanostructured materials with different morphologies and applications [5]. Despite the strong interest, due to different preparation methods and chemical environments, conflicting reports about their photo-excited properties exist and even their crystalline structures are still under debate. Here we show by means of first-principles simulations, the unambiguous relation among atomic structure, electronic bandstructure and optical properties of several TiO2-based nanosheet. Results of GW/BSE calculations on top DFT simulations on the electronic and optical properties of TiO2-based nanosheets reveal that the inclusion of many-body effects in the theoretical description is of primary importance to get a comparison between experiment and theory. The excitonic nature of the main optical peaks clearly comes out from the analysis of the theoretical spectra of isolated nanosheets. The packing of layers influences both the electronic and optical gaps of such nanosystems, while the inclusion of lattice vibrations in the optical spectra calculation provides a clear evidence of the Stokes-shifts experimentally observed. The optical spectra excitonic nature of the 2D TiO2-based nanostructures is confirmed via finite-temperature (300 K) simulations. [1]Honda, K. et al., Nature, 1972, 238, 37. [2] See for example Chen, X. et al. Chem. Rev. 2007, 107, 2891 and refs. therein. [3] Yang H.G. et al., J. Am. Chem. Soc., 2009, 131, 3152. [4] Orzali T., et al. Phys. Rev. Lett., 2006, 97, 156101. [5] Pradhan, S. K. Chem. Mater. 2007, 19, 61.

The conversion of solar energy into chemical fuels, which can be easily stored and transported, is being pursued via a wide range of processes. In particular, photoelectrochemical cell (PEC), which produces hydrogen under illumination by splitting water, has attracted enormous attention. However, the reported efficiency is still rather low mainly due to low reaction kinetics at room temperature. Here, we propose to develop a PEC of new design that can work at temperatures significantly above room temperature. This can be achieved by creating a heterostructure consisting of a photon absorber and a solid electrolyte. As the temperature increases, the rate of thermally-activated chemical reactions is substantially enhanced over that at room temperature, reducing or eliminating the need for expensive catalysts. A device-level model is created to describe the thermal enhancement mechanisms in processes ranging from light absorption to surface reaction. Finally, proof-of-concept devices are fabricated and characterized with and without illumination under controlled gas environment and temperature.

Transparent conducting oxide (TCO) films such as tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO) and aluminum-doped zinc oxide (AZO) have high transmittance in the visible region of the electromagnetic spectrum, combined with reasonable electrical conductivity. TCO films are therefore used as components in opto-electronic devices for solar cell applications, light emitting diodes and flat panel displays. In a dye-sensitized solar cell (DSC), a TCO film is coated with a nanoporous oxide semiconductor (photo-anode) which is sensitized with an organic or inorganic dye. The role of the TCO is to allow significant light transmittance into the DSC and also to “collect” injected electrons from the sensitized photo-anode into the external circuit. Therefore, the TCO and the sensitized photo-anode play crucial roles in the performance of the solar cell. In this work, we have optimized the deposition conditions of AZO films by radio frequency (rf) magnetron sputtering using ZnO ceramic target in pure argon gas with different aluminum concentrations. The optimized AZO film (Al concentration of 4.3 atomic %) was used as a TCO in a DSC consisting of a ZnO photo-anode coated with an indoline dye (D-358, organic dye) and its photovoltaic properties were measured. For comparison, a DSC consisting of ZnO photo-anode coated with the same D-358 dye on a commercial FTO was also made. The porous ZnO on the two transparent substrates (ZnO/AZO and ZnO/FTO) were prepared by spray pyrolysis technique and the resultant films were heated in air at 500 degree celsius for 30 minutes. X-Ray diffraction (XRD) patterns of the ZnO/AZO showed dominant (002) orientation similar to those of AZO films, while the ZnO/FTO pattern showed dominant (100) and (101) peaks. The two DSCs were prepared (active area 0.25 cm2) by sandwiching the dye-coated (D-358) photo-anode with a platinum-coated glass plate, and the intervening space was filled with an electrolyte. Photocurrent action spectra from the two DSCs indicated broader spectra and higher light harvesting of ZnO/AZO DSC than the ZnO/FTO DSC. Under AM 1.5 irradiation (1000 W m-2 simulated sunlight), conversion efficiencies of 7.3 and 4.4 % were recorded for the ZnO/AZO and ZnO/FTO DSCs, respectively; but both efficiencies were reduced with time. For current-voltage parameters; ZnO/AZO DSC photocurrent density of 20.8 mAcm-2 (17.05 mAcm-2 for ZnO/FTO), voltage of 0.64 V (0.53 V for ZnO/FTO) and 55 % fill factor (53 % for ZnO/FTO) were recorded. The higher conversion efficiency of 7.3 % and the high current-voltage parameters recorded for the ZnO/AZO DSC are attributed mainly to higher light harvesting, better crystallographic properties and charge transport exhibited by the porous ZnO on AZO after the thermal treatment. The importance of TCOs in opto-electronic devices where thermal treatments are necessary during the fabrication process is highlighted in this work.

Hydrogen is envisioned as “future” fuel and considered as a leading candidate for renewable and environmentally benign energy carrier. Photoelectrochemical (PEC) splitting of water is one of the most attractive, economical and environment friendly method for hydrogen generation. There has been extensive investigation on various metal-oxide semiconductors, for their use as photoelectrode, but with very limited efficiency. In search for efficient photocatalysts for water splitting process, various attempts have been made to advance the design, fabrication and modifications of semiconductor nanostructured materials. Bilayered systems are the recent strategy to achieve absorption of large part of the solar spectrum for increasing the efficiency of the PEC process. Bilayered photoelectrodes consisting of two semiconductors possessing different energy levels for their corresponding conduction and valence bands. The small/mid band gap semiconductor is primarily responsible for visible light absorption and sensitizing the large band gap semiconductor through electron-hole injection. The fine control of the interfaces by synthetic techniques and the synergistic effect of the two different band gap semiconductors may lead to superior properties of bilayered system than that of parent materials. This study presents an investigation on the PEC properties of sol-gel spin-coated SrTiO₃ / WO₃ bilayered thin films deposited on ITO substrate. These bilayered films were used as photoelectrode and their PEC properties were studied by recording current-voltage characteristics. Bilayered thin films of SrTiO₃ /WO₃ exhibited enhanced PEC response, as compared to alone SrTiO₃ and WO₃ thin films. The Structural, Morphological and Optical properties are studied using XRD, SEM and UV-Visible Optical Spectroscopy. The significant enhancement in photoelectrochemical properties of bilayered photoelectrodes can be attributed to the extended absorption in the visible region and enhanced charge separation at the heterojunction resulting in decrease of recombination of charge carriers. Results would be discussed in detail.

We show that pure rutile TiO2 can be photo-responsive even under low energy visible light after annealing in vacuum where we surmise the point defects play an important role. Epitaxial rutile/sapphire(001) heterostructures were grown by pulsed laser deposition technique where formation of atomically sharp interfaces and the epitaxial growth were ascertained by HAADF-STEM imaging. Using phi-scan and 2theta;-scan XRD methods, the epitaxial relationship between the rutile film and c-sapphire substrate was determined to be (100)[010]R||(0001)[1-210]S. The films were annealed under different pressures ranging from 5×10-6 to 5×10+1 Torr. Based on the XPS, UV-Vis and PL spectroscopy results, it was found that the defect concentration increased after annealing under lower pressures, e.g. 5×10-6 Torr. Morphology of the films was also investigated employing AFM technique. It was observed that increasing the annealing pressure results in formation of larger grains. The 4-chlorophenol was selected as a model material and decomposed by the annealed TiO2 films where maximum photocatalytic reaction rate constants were determined as 0.0107 and 0.0072 min.-1 under UV and visible illuminations.

Current proton exchange membrane fuel cell (PEMFC) technology most commonly utilizes Pt deposited on colloidal carbon black as the cathode material. While effective, this current electrode technology suffers from a couple well-known challenges, specifically, carbon corrosion, and electrode flooding. Carbon corrosion, which occurs when the colloidal carbon catalyst support oxidizes over time under typical fuel cell operating conditions, causes the catalyst particles (e.g., platinum) to be released from the carbon support material. This results in agglomeration which limits the active catalytic surface area. Electrode flooding is a result of the accumulation of water at catalyst sites in the cathode (blocking active Pt sites). This accumulation occurs due to inadequate gas transport pathways in a tightly packed colloidal carbon based cathode. These two commonly occurring effects lead to degradation of the overall fuel cell efficiency. The current work investigates strategies for overcoming these two challenges. It is suggested that changing the type and structure of the catalyst support material will result in improved fuel cell performance. Substoichiometric TiO₂ (TiO_x) has been suggested as a replacement PEMFC catalyst support material because of its conductivity and corrosion resistance. Atomic layer deposition of TiO_x using half reactions of TiCl₄ and H₂O over anodic aluminum oxide (AAO) and silicon nanowire (SiNW) templates enables high aspect ratio nanostructures to be formed. Using a scanning electron microscope (SEM) 100:1 aspect ratio structures were observed. These high aspect ratio catalyst support structures may help to increase gas transport pathways in the PEMFC cathode and thereby decrease the probability of electrode flooding. Four point probe measurements of TiO_x films indicate a large increase in conductivity with only a slight reduction in oxygen following a post deposition anneal in hydrogen, as measured by x-ray photoelectron spectroscopy. Liquid phase deposited Pt and plasma enhanced atomic layer deposition (PEALD) of Pt were used for the metallization of TiO_x structures. Electrochemical measurements of the Pt on TiO_x catalyst material were performed using cyclic voltammetry (CV) and rotating disk electrode (RDE) analysis. Electrochemical data shows promising result for Pt on TiO_x catalyst materials when compared to the typical Pt on carbon materials.

WO3 is an n-type semiconductor which has also been recognized as an active visible-light-driven photoanode material for photoelectrochemical (PEC) water-splitting since 1976. The properties that have made WO3 imperative in current PEC research include good stability in water (< pH 4), photostability, suitable band gap energy (around 2.6 eV) for considerable absorption within the solar spectrum, energetically favorable valence band position for water oxidation and excellent electron transport property along with hole diffusion length around 150 nm. Morphology control of metal oxides has been a focus of recent research interest because the morphology of the nanostructures has a significant effect on their properties and applications. The morphology of the particles is greatly influenced by the method of preparation, the initial precursor, type of the chemicals used and control of the experimental parameters. The syntheses of morphology controlled WO3 and WO3 hydrates nanostructures, as for examples, nanowires, nanorods, nanotubes and hexagonal-shaped nanodiscs are reported in the literature. They have been prepared by high temperature evaporation, precipitation, hydrothermal reaction, electrochemical or template assisted routes. Hydrothermal synthesis is a facile, cost-effective, single step low temperature approach which can successfully be used in morphology-tailored synthesis by introducing suitable structure directing mediator under the controlled reaction conditions. Some of these WO3 or WO3 hydrates materials with controlled morphologies have been demonstrated for their photocatalytic activities. Recently, few reports have been published on the photooxidation of water using hydrothermally grown WO3 nanoparticles. However, there is a scope to investigate the photocatalytic activity of WO3 with different morphologies. The WO3 hydrates are the parent materials of WO3. Generally, the WO3 hydrates are produced first in liquid-phase synthesis routes and subsequently annealed to obtain the desired crystal phase of WOx. Herein, we demonstrate a simple hydrothermal condition to realize the morphology controllable synthesis of the crystalline tungsten oxide hydrates which convert to hexagonal WO3 with nanosize dimension upon subsequent thermal treatment. We report for the first time the synthesis of different morphologies of tungsten oxide hydrates from ammonium metatungstate precursor employing different organic acids such as citric, oxalic, and tartaric acid as structure directing agents. Detailed characterization of the crystal structure, particle morphology of the synthesized powders has been done in this work. The photoactivity of the synthesized powders in dehydrated form assembled on F: SnO2 (FTO) coated glass substrate through film casting has been compared for photoelectronchemical water oxidation under the standard condition air mass 1.5 G.

Electrochemical supercapacitors (ESs) are attractive energy storage devices because of their potential role in electric and fuel cell vehicles. ESs can be of two types: electric double layer capacitors (EDLCs) and redox supercapacitors. Activated carbon materials are predominantly used for EDLCs because of their relatively low cost and high surface area. The redox supercapacitor makes use of a reversible redox reaction in order to store charges. The behavior of redox supercapacitors is typically termed “pseudo-capacitance” and resembles a re-chargeable battery more than a traditional capacitor. Ruthenium oxide has found success as a pseudo-capacitor exhibiting specific capacitance values up to 750F/g. However, cost is prohibitive to large-scale commercial production. Manganese and its oxides have received much attention as alternatives to ruthenium oxides in ESs applications because of their relatively low cost and toxicity. It is reported that compounds of mixed oxide composites, such as Ni-Mn, Ni-Co oxides, have superior capacitive performance to single transition metal oxides. Technique for producing binary metal oxides for supercapacitor materials includes physical vapor deposition, sol-gel method, and chemical precipitation. However, the use of oxide powders for fabricating electrodes (synthesizing powder, mixing with a conducting agent and a binder, and making an electrode) is complicated and inconvenient. A more effective method to prepare manganese oxide electrodes for ESs application is electrodeposition. In this work, electrodeposition and electrochemical analysis of binary metal (Ni-Mn, Co-Mn, Fe-Mn) oxides were investigated. Binary metal oxides were prepared carbon sheet and Ni foil by anodic electrodeposition. The electrochemical characteristics of binary metal oxides were evaluated by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance measurements. The morphology and crystal structure of the film have been investigated by scanning electron microscopy and X-ray diffraction, respectively.

Sol-gel derived metal oxide has been widely used as the carrier transport layer in fabricating polymer solar cells because it can be made into different morphologies and nanostructures to optimize their optoelectronic characteristics. Herein we demonstrate a facile and cost-effective method of forming the mesoporous zinc oxide (ZnO) without complicated synthesis and high temperature process. The mesoporous ZnO layer was prepared using a conventionally in-situ sol-gel approach and simply removed the unreacted precursors and organic residual by solvent extraction. Compared with flat ZnO film, by using the mesoporous ZnO as an electron transport layer, ~ 8% photocurrent enhancement can be achieved in an inverted poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester solar cell.

Charge transport in nanoparticle-based materials underlies many emerging energy conversion technologies, yet assessing the impact of nanometer-scale interfacial structure on charge transport across micron-scale distances remains a challenge. Here we develop an approach for bridging these length scales by correlating the spatial distribution of crystalline domains with the size, shape, and charge transport characteristics of the electronic percolation networks in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe₂O₃ electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometer resolution across micron-scale distances, we identify the structural characteristics within these nanoparticle aggregates that are most responsible for their high water splitting performance: in particular, we find that grain boundaries obstruct charge transport and that their removal provides charge percolation, resulting in the highest photocurrent of any metal oxide photoanode for photoelectrochemical water splitting under 100 mW cm^-2 air mass 1.5 global sunlight.

The ability to make use of on-site energy resources in the field is of importance to the U.S. Army, allowing for the ability to recharge electrical systems, and thus reducing the amount of stored energy (typically in the form of batteries) that needs to be carried into theater. Metal-insulator-metal (MIM) diodes rely on quantum-mechanical tunneling for current generation and are one of the few device structures capable of rectifying radiation absorbed by an antenna designed in the gigahertz and terahertz frequency range (visible and infrared wavelengths). Desired current-voltage characteristics that a MIM device for electrical rectification should exhibit includes high asymmetry, non-linearity, relatively fast responsivity, low hysteresis, low impedance, and a turn-on bias close to 0 V. These characteristics depend primarily on the materials that make up the MIM stack. Important factors of the materials selected for producing a MIM diode with these preferred characteristics include having a high work function difference between the two metals, and having a relatively low insulator barrier height. This study surveys planar MIM tunnel diodes based on niobium and niobium oxide thin films, with varying top metals to determine possible material systems that show the desired electrical characteristics for use in a possible rectenna. The niobium/niobium oxide combination was chosen due to the ability to create a high quality interface between the two materials using an anodic oxidation process, niobium's relatively low work function (and thus, making possible a relatively high work function difference), and niobium oxide's relatively low insulator barrier height compared to other ALD oxides available in the facility. Top metals, including Ag, Au, Cu, and Pt, were chosen based on their relatively high work function differences compared to Nb, and based on findings from previous works 1. A Nb top metal was used as a control. A photolithographically-patterned planar diode structure was used due to its manufacturability and simplicity. Point contact MIM diode devices have been fabricated and characterized using similar material systems in the past, but such devices are not practical from a large-scale manufacturing stand-point due to their very low reproducibility and repeatability. Planar MIM structures are scalable, and can be integrated into standard CMOS processes, as well as other electronic fabrication processes. Initial results show that Nb/Nb2O5/Pt and Nb/Nb2O5/Au MIM diodes have promise, with an asymmetric ratio of greater than 10,000 at +/-1 V, relatively high non-linearity and relatively good responsivity. On the other hand, as expected, the Nb/Nb2O5/Nb system yielded a symmetric I-V response. Based on this survey, the potential MIM systems (Nb/Nb2O5/Au and Pt) for future high frequency studies are identified. (1) Periasamy, P., et al., Adv Mater 2011, 23, 3080-3085.

Metal-Insulator-Metal (MIM) structures are attractive candidates for high-frequency rectification applications such as THz imaging and sensors, infrared/visible energy harvesting (rectenna) devices. In this work, we have developed a first-of-its kind MIM materials space diagram that correlates materials properties to rectification performance. The materials space diagram was generated based on systematic experimental studies to explore the role of metals and insulator on MIM device performance by evaluating the current-voltage response. A novel modified point contact geometry is developed to examine a number of MIM material combinations. Material properties such as work function (Phi;_M) of the metals and electron affinity (chi;) of the insulator, as well as the thermodynamic stability of the interface are identified as crucial elements for MIM materials selection. Investigation performed to identify the role of metals revealed that it is sufficient to choose the metals such that their ΔPhi; is > k_BT (~ 0.25 eV at RT) to achieve desired rectification characteristics (high asymmetry and nonlinearity). Using the Nb/Nb₂O₅ bilayer as the model system, the asymmetry and the nonlinearity was found to be weakly dependent on ΔPhi; above ~0.4 eV. This result is crucial for the MIM community because it overturns a general misconception that it is necessary to choose metals such that they have as high a ΔPhi; as possible to achieve high asymmetry and nonlinearity. By suggesting that any metal combination with ΔPhi;> 0.25 eV may be workable, our study opens up a larger number of possible metal combinations for MIM design consideration. A hypothesis was developed and tested that guides the insulator selection criteria. The proposed hypothesis states, “To minimize the turn-on voltage and maximize asymmetry and nonlinearity, the electron affinity of the insulator should be close to one of the metal work function values so as to produce a low barrier height”. A variety of insulator candidates including Al₂O₃, TiO₂, ZrO₂, MgO and Nb/Nb₂O₅ were chosen to yield a range of barrier height values from~0 to 5 eV. Although the study validated the hypothesis across the material systems studied, preliminary experiments on two additional high potential MIM systems (Hf/TiO₂/Pt and Sm/ZrO₂/Pt) unexpectedly yielded much lower asymmetry and nonlinearity than predicted by the hypothesis. Thermodynamic and TEM cross-sectional analysis on these systems (Hf/TiO₂ and Sm/ZrO₂) revealed a critical observation that these interfaces are reactive even at RT and resulted in an interfacial compound (~ 3 nm thick). It is speculated that this reaction layer adversely influences the rectification performance. Thus it is proposed that in addition to choosing the materials based on their work function and electron affinity it is important to consider the thermodynamic stability of these interfaces as well.

Heterogeneous catalysis plays a crucial role in the society today, both as the means for environmental protection and as the backbone technology for most of the chemical industries. The development of new catalysts is given a very high priority since they facilitate a much better utilization of our scarce energy reserves and it can drive the concept of waste-free ‘green&’ chemistry and the development of a sustainable energy sector. Metal oxides like Al₂O₃ play major roles in heterogeneous catalysis as supports for catalytically active nanoclusters because of their excellent mechanical and thermal stability. Of all the transitional aluminas, γ-Al₂O₃ is the most important in catalysis, but so far surface science studies have been unable to address the surface structure of this insulating material in detail. A better understanding of the surface structure of support materials seems to be a prerequisite for the synthesis of more sintering stable catalysts and the realization of nanocatalysts implementing catalyst particles with a tailored size and morphology. Benefitting from a nearly perfect structural match between the (100) surface of MgAl₂O₄ and γ-Al₂O₃, we show that we can use MgAl₂O₄(100) as a template to grow thermodynamically stable and crystalline alumina films with γ-Al₂O₃-like properties. Previous atom-resolved nc-AFM studies have shown that the MgAl₂O₄(100) surfaces terminates by an Al-O rich termination, which turns out to be ideal for the continually grown alumina films [1]. Here we use an interplay between atom-resolved non-contact atomic force microscopy (nc-AFM) and X-ray photoelectron spectroscopy to characterize the formation of an epitaxially grown Al₂O₃ film on MgAl₂O₄(100) synthesised by in-situ oxidation of Al deposited on the surface. The characterization reveals that the transition alumina film adopts the spinel structure and we have successfully grown stable γ-Al₂O₃ films up to a thickness of 3ML. Atom-resolved nc-AFM images reveal a periodic square lattice, which is likely to represent the distribution of tetrahedrally coordinated Al atoms on bulk truncated γ-Al₂O₃(100) surface. Atomic defects observed in the atom-resolved NC-AFM images possibly reflect Al vacancies in tetrahedral Al positions, which are part of the theoretically predicted γ-Al₂O₃ structure. The stable γ-Al₂O₃ (100) films make it possible to open up a whole new range of fundamental studies of the metal/support interaction for catalytic systems based on surfaces with real γ-Al₂O₃ properties. The synthesis of stable well-defined alumina films with transition alumina characteristics will allow us to address important catalytic properties such as acid-based properties, cluster adhesion strength and sintering scenarios for model systems of supported nanoclusters that incorporate the real oxide support structure. 1. Rasmussen, M., et al., Stable Cation Inversion at the MgAl₂O₄(100) Surface. Physical Review Letters, 2011. 107(3): p. 036102.

(Ba_0.6Sr_0.4)TiO₃ (BST) thin films were deposited on (La_0.5Sr_0.5)CoO₃ (LSCO) buffered Ti substrates. Both BST and LSCO films were prepared by the sol-gel method. X-ray diffraction and scanning electron microscopy analysis were used to investigate the effect of LSCO sol concentration on the crystallinity and surface morphology of BST films. With the increase of LSCO sol concentration, BST films show variation of the structure and dielectric properties. BST films for LSCO of 0.2 mol/L exhibit a better crystallinity and improved dielectric properties, with the tunability, dielectric constant and tanδ of 30% , 420 and 0.028 respectively.

Engineering procedures governing the development of inkjet printable nanostructured metal oxides nanoparticles (nPs) for chromic, photovoltaic, photocatalytic and power storage applications are the main objectives of presented studies. The focus is given on how we can control both, the process in which nPs are created, and the formulation of printable dispersion which enable us to deposit nPs as functional thin films for high performance electrochemical applications. Since different nPs nanomorphologies can be obtained in a range of crystal phases via various material synthesis techniques, almost unlimited selection of nanocrystalline content for ink formulation is possible. Thus, controlled synthesis of nPs opens new opportunities for ion intercalation with significantly enhanced electrochemical response. In the presented case-study, inkjet printable nanostructured tungsten oxide particles in monoclinic (m-WO3) and orthorhombic hydrate (ortho-WO3x0.33H2O) polymorphs were successfully synthesized via hydrothermal processes using pure or acidified aqueous sol-gel precursor. Based on proposed scheme, the structure and morphology of nPs were tailored to assure their desired performance and printability. The effect of various reaction parameters on nPs properties leading to variations in electrochromic performance of dual-phase films is described. Moreover, we propose here a mechanism of nanostructured WOX nPs growth from aqueous peroxopolytungstic acid precursor under hydrothermal conditions, with predictable and controlled properties, while maintaining their good processability via Inkjet Printing Technology.

Despite its promise as an earth-abundant semiconductor and 90 years of research history, cuprous oxide (Cu2O) solar cells have reached a maximum efficiency of only ~4%. Given the Shockley-Queisser efficiency limit of 20% for this material, it is necessary to identify the key performance-limiting loss mechanisms that lead to such low efficiencies. The present work elucidates these loss mechanisms by fitting SCAPS-1D simulations to existing device data. Cu2O thin film devices are produced through RF sputtering and electrodeposition techniques and are characterized through current-voltage, capacitance-voltage, and quantum efficiency measurements, as well as optical and x-ray photoelectron spectroscopy. Specific simulation results include studying the impacts of: (1) optical and charge collection losses on short circuit current; (2) poor p-n heterojunction band alignment on open circuit voltage; (3) bulk and interface defects on open-circuit voltage and fill factor; and (4) materials considerations for minimizing contact resistance. The present simulations also provide prescriptions for the material and device properties necessary to achieve >10% efficiencies. Finally, lessons from simulation are implemented in real devices to demonstrate the applications of this method.

In order to achieve high efficiency and low cost for thin-film solar cells, we developed the nanostructure on the transparent substrates. The substrates with nanostructure had higher transmittance than flat one in a wide range. Moreover, the nanostructure can increase the optical length of incoming light and enhance the extraction of photocarriers in the active layers. In this study, we fabricated nanostructure on quartz by the self-assembling of nanospheres and reactive ion etch (RIE). The advantages of this texturing method were controllable surface morphology and periodic structure. The periodic arrangement was applicable in plasmonic effect for thin-film solar cell. The texturing method included the coating of nano-sphere masks and reactive ion etch (RIE). First, the quartz substrates and silicon wafer were boiled in mixed solutions (H2SO4/H2O2, or NH4OH/H2O2/H2O) for the hydrophilic property on the surface. Second, the 2.5wt% monodisperse polystyrene (PS) nanosphere solution was diluted with equal amount of anhydrous ethanol. The PS sphere masks were spread on silicon wafer by spin-coating, then silicon wafer were slowly immersed into the glass vessel filled water to lift-off nano-sphere masks. Immediately, we picked up the closed-packed nano-sphere array with quartz. The RIE process was operated at frequency of 13.56 MHz and power of 200 W. CF4 and Ar served as precursor and mixed under the pressure of 40 mTorr in the chamber. The morphologies of samples were examined by SEM. By controlling etching time, the nanostructure profiles can be truncated cone, parabola cone and nanocone. The spacing of periodic array was 220nm. The height of truncated cone, parabola cone and nanocone were separately 130, 220 and 60nm. We demonstrated that the parabola-cone structure had very high optical transmittance around 96 % in 300-1100 nm, obviously higher than the 93.57% transmittance of flat quartz substrates. Hence, we speculated that the parabola-cone structure can significantly enhance the short-circuit current in thin-film solar cells.

Copper oxide (CuO) is a p-type metal-oxide semiconductor, with an indirect band gap ~1.2 -1.9 eV. In addition, it has high refractive index and low absorption coefficient in visible spectrum which enables it to be used in various electronic and optoelectronic applications such as photovoltaic and gas sensors. For gas sensing applications, CuO offers good chemical stability at high temperatures even in harsh environments. Furthermore, the interaction between CuO and gas is influenced by its crystallography, porosity, roughness, and thickness of the sensor&’s sensitive layer. However, detailed characterization of the microstructure and the porosity of such films are difficu< Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) are generally used to characterize the crystal and roughness size but these techniques are not well suited to provide quantitative information about the films porosity. Spectroscopic Ellipsometry (SE) measurements and simulation enable the determination of thin film parameters including film structure (e.g., nucleation layers, composition gradients, surface roughness) and corresponding optical functions (n, k) or (ε1,ε2) which provide information on their density/porosity. Here, we report the use of SE (1.5- 6.5 eV) to obtain detailed information about the thin film porosity on a series of CuO films prepared by radio-frequency (RF) magnetron sputtering on SiO2/Si substrates with different deposition time resulting in films of different thicknesses not only (probably different structures also). All as-prepared films were annealed in atmospheric conditions at 400 C for 30 min. The SE data analysis of the annealed films was realized using a structural model consisting of roughness (Bruggeman effective medium approximation with 50% void) and a bulk film with optical properties modeled using a 5 components generalized oscillator model. It was found that such model could be used to satisfactory fit the SE experimental data only when two additional layers with distinctive porosity where added at the substrate-film and film-roughness interface. The grain size and roughness measured by SEM and AFM are in correspondence with calculated values from SE from the model. The void volume fraction of the thinnest film studied here (30s deposition time) was found to be the lowest indicating optimum CuO thin films for sensor application in accordance with previous experience for ozone sensor applications as it will be reported. Additional SE studies, for example real-time measurements during annealing, are expected to provide useful insight into sensor device optimization. Keywords: CuO, thin films Microstructure, Sputtering, Ellipsometry, Optical properties.

Hematite (α-Fe2O3) is one of the most promising materials for anodes in the photo-assisted splitting of water. Interestingly, recent studies have shown that the photo-electrochemical performance of hematite nanostructured thin films, produced by different methods, depend significantly on the temperature of prior thermal annealing processes. These variations on performance upon thermal annealing were mainly attributed to changes in morphology [1,2]. Although these results clearly demonstrate that thermal annealing at high temperatures (>500°C) can result in materials with improved transport properties, this effect is not well understood so far. In this work, we perform a comprehensive investigation on the optical properties of different hematite thin films and materials. We focus on the dependence of the optical properties on thermal annealing treatments in an attempt to elucidate the effect of such treatments. Two sets of hematite thin films were produced and characterized. The first set was obtained by chemical vapor deposition using ferrocene as precursor and the other was produce by a solution method based on Ref [2]. The morphologies of the films were determined by scanning electron and atomic force microscopies. X-ray diffraction and electron dispersive spectroscopy were employed to identify their structure and chemical composition. The optical characterization was carried out by spectroscopic ellipsometry and UV-VIS spectroscopy (in transmission and/or diffuse reflectance mode). By combining both techniques we are able to determine the refractive index and extinction coefficient (n,k) spectra in a wide spectral range. In addition, in order to provide a reference, we have measured the optical spectra of a natural single-crystal hematite sample, as well as commercial hematite powder. Currently, we are performing photo-electrochemical characterization of the films produced and we expect to provide a correlation between the photo-electrochemical and transport properties of the films with their optical properties. References: [1] K.Sivula et al. J. Am. Chem. Soc. 132, 21, 2010 [2]G.Wang et al.NanoLetters, 2011. dx.doi.org/10.1021/nl202316j

Thin film photovoltaic materials especially new emerging technologies such as CZTS and OPV present new and unique problems for the development of highly functional transparent contacts. This is true made even more interesting for Organic Photovoltaics (OPV) which offer the promise of low-cost high-performance photovoltaics but have a diverse materials set. While efficiencies have now surpassed 10% it is clear that both the absorber and contact materials need to be further optimized. The very large possible number of organic materials and that the contacts will have to be tailored to the organic materials suggests the need for a new approach to optimizing materials and interfaces. To this end we have begun to approach each of these areas by coupling high-throughput theory and experiment to develop both new organic and inorganic materials. We will discuss an example of this approach: where we are developing predictive tools to allow for the computation of new contact materials and TCOs. We will highlight in this area development of selective contacts that improve efficiency and lifetime for OPV devices, specifically inorganic hole-transport layers (HTL) and the related electron-transport layers (ETL). Most of the studied materials belong to the general class of wide-bandgap p-type oxide semiconductors. Coupled to the “conventional TCO&’s” the pairing of new materials by design for the HTL and ETL potentially be tailored to a particular bulk heterojunction to improve efficiency and stability tailored to a particular bulk heterojunction. How we can begin to design such materials and then realize them experimentally is the topic of the talk. Potential candidates suitable for HTL applications include SnO, NiO, MO3, Cu2O (and related CuAlO2, CuCrO2, SrCu2O4 etc) and Co3O4 (and related ZnCo2O4, NiCo2O4, MgCo2O4 etc.). Materials have been optimized by high-throughput combinatorial approaches. The thin films were deposited by RF sputtering and pulsed laser deposition at ambient and elevated temperatures. Performance of the inorganic HTLs and that of the reference organic PEDOT:PSS HTL were compared by measuring the power conversion efficiencies and spectral responses of the P3HT/PCBM- and PCDTBT/PCBM-based OPV devices showing performance comparable to PEDOT:PSS. This work then shows that materials for solar energy conversion can be much more rapidly developed using a materials by design approach driven by theory and continuously iterated with experiment. We will discuss the potential long term applicability of the approach to other thin film PV systems. Acknowledgement We gratefully acknowledge funding from U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory for OPV device development and the . Center for Inverse Design (CID), an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award Number DE-SC0001084

Organic photovoltaic (OPV) devices offer the promise of low cost, lightweight energy harvesting capabilities with flexible form factors. However, exposure to ambient atmosphere induces oxidation of active layer constituents and electrode interfaces, rapidly diminishing device performance. Thus, successful OPV implementation is contingent on providing an effective encapsulation barrier to prevent oxidative breakdown mechanisms by inhibiting atmospheric diffusion. Ideally, the encapsulation barrier should further act as a damage-resistant protective coating to the soft matter constituents while maintaining the lightweight, flexible nature of OPVs. To date, encapsulation of OPV devices has typically fallen to two methodologies: (1) coating an existing device with an adhesion layer before affixing a protective overlayer or alternatively, (2) in-situ workup of the protective layer from the device surface. While the former methodology remains the more explored option, preserving device flexibility and reducing additional packaging weight remain significant challenges. In contrast, in-situ fabrication of the protective layer directly onto the device surface provides the option of encapsulation using ultra-thin layers (le; 100 nm), providing significant reductions in additional device packaging weight. In particular, atomic layer deposition (ALD) has shown exceptional promise for extending device lifetime [1] while maintaining both optical clarity and flexibility [2]. Recent studies have shown that ALD recipe parameters (temperature, cycling time, and oxidative constituent) strongly influence film development thereby altering atmosphere permeability, optical and mechanical properties of the film [1b-c,2,3]. Furthermore, though ALD allows low temperature (le; 200 °C) film deposition, thermal degradation of OPV device performance is highly dependent on ultimate temperature and exposure time making encapsulation process optimization necessary to balance initial device performance with device longevity. In this work, ultra-thin alumina layers are deposited by ALD to encapsulate pre-fabricated OPV devices. A snapshot of the effects of ALD recipe (temperature, cycling time, and number of cycles) on film properties and device lifetime is presented. Cross-sectional microscopy further elucidates the conformal nature of the thin film oxide coating. From these results, an optimized recipe emerges, which extends device lifetime by two orders of magnitude while simultaneously enhancing the abrasive resistance of the polymer based device. References [1] (a) Potscavage, W.J. et al. Appl. Phys. Lett. 90, 253511 (2007); (b) Chang, C.-Y. et al. Org. Electronics 10, 1300 (2009); (c) Sarkar, S. et al. Org. Electronics 11, 1896 (2010). [2] (a) Aslan, M.M. et al. Thin Solid Films 518, 4935 (2010); (b) Jen, S.-H. et al. J. Appl. Phys. 109, 084305 (2011). [3] (a) Dameron, A.A. et al. J. Phys. Chem. C 112, 4573 (2008); Groner, M.D. et al. Chem. Mater 16, 639 (2004).

Deposition of metal oxide transparent conducting oxides generally requires temperatures greater than 350 °C which make them unsuitable for use as top contacts in organic optoelectronic devices. Here we demonstrate the use of pulsed laser deposition (PLD) as a method for depositing highly crystalline, highly conducting (<50 Omega;/sq) and transparent (>90 %) aluminium-doped ZnO and indium tin oxide (ITO) at low temperatures (<200 °C) directly on to an functional organic layer, under conditions that do not degrade the optical or electronic properties of the organic materials. The technique allows precise control of the oxide film thickness, orientation and stoichiometry. Microstructural and morphological data of the oxide and organic layers are presented alongside photovoltaic device data with transparent metal oxide top contacts. This deposition technique and flexibility of the materials available opens possibilities into unexplored photovoltaic architectures - specifically the incorporation of transparent interlayers and the ability to fabricate devices from metal electrode up.

Transparent conductors (TCs) are critical components in applications such as photovoltaics, light-emitting diodes, and flexible screens. The TC of choice for laboratory studies and the commercial market is tin-doped indium oxide (ITO), chosen because of its conductivity and transparency in the visible range of the spectrum. However, the application space available to ITO is limited by strong absorption in the ultraviolet and infrared; furthermore, thin-film fabrication of ITO involves complicated high-vacuum deposition techniques with poor precursor utilization efficiency. Alternative TCs are required that are easier to deposit using atom-efficient processes, are transparent throughout a broader spectral range, and exhibit comparable or higher electronic conductivity. We study a nontraditional transparent conductive oxide composed of ultrathin RuO2 films (~10 nm) deposited by an easy, bench-top protocol at sub-ambient temperature [1]. The process uses a commercially available precursor, RuO4, and yields nanoscale skins of RuO2 that exhibit high conductivity and are transparent from ultraviolet to microwave wavelengths, far exceeding the range of ITO [2]. Also, the deposition protocol allows for conformal, non-line-of-sight coating of structured 3D substrates. In this presentation, the optical, electrical and spectroelectrochemical properties of RuO2 thin-film TCO will be presented, and structure-property correlations, discussed. [1] C.N. Chervin, A.M. Lubers, K.A. Pettigrew, J.W. Long, M.A. Westgate, J.J. Fontanella, D.R. Rolison, Nano Lett. 9 (2009) 2316-2321. [2] J.W. Long, J.C. Owrutsky, C.N. Chervin, D.R. Rolison, J.S. Melinger, U.S. Patent Application 20110091723.

Today, transparent conductive coatings are an essential building block of many applications for solar energy conversion. The most common approach is based on transparent conducting oxides, mainly indium-tin oxide (ITO). Due to the scarcity of Indium, the long term availability of ITO at reasonable cost is unclear and ITO alternatives required. Recently, metal (Ag or Cu) nanowires have been introduced as promising approach [1, 2]. To improve mechanical and electrical properties composites of NW and conducting polymers (e.g. PEDOT:PSS) have been used [3,4]. In this work, composite layers of Ag NWs and conductive metal oxides are presented. Specifically, room-temperature sol-gel processed SnOx or ZnO deposited by low-temperature atomic layer deposition (T=100°C) were used as overcoat, respectively. The conformal coating by ALD even allows for surface functionalization of the NW and at the same time can serve as a gas permeation barrier on polymer substrates [5]. Ag NWs with a diameter of 90 nm and a length of 20-60 microns are coated from an alcohol-dispersion. The layers are studied by electron microscopy and the electrical connection of the NWs with each other is analyzed by voltage contrast scanning electron microscopy. Depending on the NW concentration, as deposited layers of Ag-NWs reproducibly show a low sheet resistance of 11-90 Ohm/sq and a corresponding average diffusive transmittance of about 90% (spectral range 380-800 nm). The main drawback of pristine layers of NWs is their poor adhesion to the substrate and their overall limited integrity. Therefore, the subsequent metal-oxide coating is intended to fuse the wires together and also to ‘glue&’ them to the substrate. As a result of the fusion a reduction of the sheet resistance down to 8 Ohm/sq. is found. For comparison, sputtered ITO processed at low temperatures shows a sheet resistance of 80 Ohm/sq at a comparable average transmittance of 78%. The adhesion of the NWs to the substrate is significantly improved and the resulting composited withstand the scotch tape peeling test without loss in conductivity. The composite of Ag NWs and ALD-ZnO even withstands storage at 80°C/80%rH without dramatic change in electrical and optical characteristics. Owing to the low processing temperatures, our concept allows for highly robust, highly conductive and transparent coatings even on top of temperature sensitive objects, e.g. polymer foils, organic devices. [1] L. Hu, H. Wu, Y. Cui, MRS Bulletin, 36, 760 (2011). [2] L. Hu, H. S. Kim, J.-Y. Lee, P. Peumans, Y. Cui, ACS Nano 4, 30 (2010). [3] W. Gaynor, G. F. Burkhard , M. D. McGehee , P. Peumans, Adv. Mater. 23, 2905 (2011). [4] R. Z., C.-H. Chung, K. C. Cha, W. Yang, Y. B. Zheng, H. Zhou,T.-B. Song, C.-C. Chen, P. S. Weiss, G. Li, Y. Yang, ACS Nano 5, 9877 (2011). [5] J. Meyer, P. Görrn, F. Bertram, S. Hamwi, T. Winkler, H.-H. Johannes, T. Weimann, P. Hinze, T. Riedl, W. Kowalsky, Adv. Mater. 21, 1845 (2009).

Transparent conducting oxides (TCOs) are the key components in flat panel displays, photovoltaic cells, organic light emitting diodes, thin film transistors, smart windows and optical waveguides. Current research on TCOs is focused to obtain high mobility, which increases the electrical conductivity without sacrificing the optical transmittance in visible to near infrared (NIR) region. In this paper we present some results concerning the production at room temperature by rf magnetron sputtering of high mobility and high NIR tranaprency of indium based and indium free TCOs.

Chromium oxide and N-doped chromium oxide have been recently reported as holes transporting layer in organic solar cell [1, 2]. The effect of nitrogen in chromium oxide was to enhance the optical properties, while poor conductivities were reported in both cases. Despite the poor electrical properties, it was demonstrated that amorphous chromium oxide can substitute PEDOT:PSS as buffer layer between, in these cases, FTO and the organic layer, with comparable or higher device performance. In the second contribution, N-doped films were used. Nitrogen improves the optical properties of chromium oxide with limited effect on the electrical properties. In comparison with undoped amorphous chromium oxide a further improvement of the solar cell performance was reported. Recently we reported on the possibility of improving both optical and electrical properties of chromium oxide by co-doping with Mg and N. Doping with Mg has a major impact on enhancing the conductivity, while both optical and electrical properties can simultaneously been improved by co-doping with N [3]. These films were deposited by spray pyrolysis and the morphology was too poor in order to use them as buffer layer in organic solar cells. In order to improve the morphology and the properties, films have been deposited by PLD on different substrate. The effect of the substrate and deposition parameters will be presented. [1]. Qin, P.L. et al., Organic solar cells with p-type amorphous chromium oxide thin film as hole-transporting layer. Thin Solid Films 2011, 519, (13), 4334-4341. [2]. Qin, P. L. et al, Nitrogen doped amorphous chromium oxide: Stability improvement and application for the hole-transporting layer of organic solar cells. Solar Energy Materials and Solar Cells 2011, 95, (3), 1005-1010. [3]. Arca, E.; Fleischer, K.; Shvets, I. V., Magnesium, nitrogen codoped Cr2O3: A p-type transparent conducting oxide. Applied Physics Letters 2011, 99, (11), 111910-1, 111910-3.

P-type oxide materials are under active refinement for their application to energy harvesting devices. In this work we demonstrate that Zn-Ni-Co-O thin films deposited at ambient temperature have high p-type conductivity (30 S/cm), high work function (5.3-5.8 eV) and reasonable transparency. These values nearly match the performance of their high-temperature counterparts, which makes these thin films promising as top contacts to temperature-sensitive photovoltaic devices. Materials with p-type conduction are of interest for renewable energy harvesting applications. For example, water splitting and photovoltaics require p-type materials with defined work functions to enable more efficient device engineering. Ambient temperature deposition allows for a wider range of application to devices that have restrictive processing conditions and reduces the potential cost of using the material. Ambient temperature processing also provides the ability to deposit amorphous films that may have advantages over crystalline films such as lower surface roughness, better impermeability and facile large-area deposition while maintaining high conductivity. We will outline our effort on p-type oxide films with different Zn, Co and Ni cation ratios that were deposited at ambient temperature. Combinatorial RF sputtering was used to study almost the entire ternary phase space between ZnO, NiO and Co₃O₄. We utilize a 44-point measurement grid to compare the results of mapping measurements of film composition (XRF), crystal structure (XRD), sheet resistance (four point probe), work function (Kelvin probe), as well as UV-Vis-NIR transmission and reflection. As mentioned above, the results of this study show that the performance of Zn-Ni-Co-O thin films deposited at ambient temperature are comparable to the performance obtained by their high-temperature counterparts. The highest transmission is found in the Zn-rich oxides whereas the highest conductivity is found in the Ni-rich and Co-rich oxides. Therefore, a trade-off between the conductivity and transparency of the films exists. Furthermore we found small amorphous regions at the Co_3-xZn_xO_y and Co_3-xNi_xO_y binary tie-lines that did not continue through the center of the ternary phase diagram. A complete summary of the results and expected ramifications on ternary Zn-Ni-Co oxide deposition processes will be presented.

The long term target for the world wide installation of photovoltaic systems is far beyond 10TW. A sustainable development asks for photovoltaics based on unlimitedly available elements. Silicon based materials already fulfill these requirements but significantly increasing efficiencies and cost reduction can only be achieved by multi-band gap thin film solar cells. We have investigated copper oxide based nanoparticles for new absorber material in thin film solar cells. Raman, photoluminescence (PL) and optical absorption by photothermal deflection spectroscopy (PDS) are used accompanied by microstructure investigation with TEM and XRD. The nanoparticles have been modified by thermal annealing for 30 min in N2 atmosphere to improve their electronic properties. We present results on layers made of commercially available CuO nanoparticles. The nanoparticles are not uniform and exhibit a large variation in size and shape. According to TEM sizes around 30 nm and larger particle and agglomerates have been found. In as-prepared CuO nanoparticles the PL spectra obtained at room temperature exhibit peak at about 1.28 eV and a width of about 0.4eV with an asymmetric shape, i.e. a slightly stronger decay towards high energies. The onset of the PL is at about 1.6 eV in very good agreement to the band edge determined by PDS. PDS spectra show significant sub gap absorption extending to low energies indication a high defect concentration in the initial state. By stepwise annealing in N2 atmosphere up to 700 C the sub gap absorption decreases tremendously and the PL intensity increases by more than one order of magnitude without changing its position. At an annealing temperature of 750 C the phase changes partially to Cu2O which is indicated by a shift of the absorption edge, an additional PL band at about 2 eV and characteristic Raman modes. By further annealing up to 900 C the PL intensity increases by more than two orders of magnitude and exhibits a fine structure possibly related to excitonic transitions. Raman and PDS measurements indicate the coexistence of CuO and Cu2O phase also at high annealing temperature (>850 C) but from the absorption data a maximum contribution of 5% CuO phase can be estimated. As Raman and XRD indicate a pure CuO phase at low annealing temperature the surprisingly high PL intensity at 1.28 eV is interpreted as band edge emission of the CuO phase. At high annealing temperature the well-known band edge emission of the Cu2O phase is observed with a high intensity accompanied by weakly pronounced defect bands of oxygen and copper vacancies. The high PL intensity demonstrates the high quality of the CuO and Cu2O nanoparticles that can be achieved by proper annealing and indicates the suitability of this material for photovoltaic applications.

Amorphous transparent conducting oxides (a-TCOs) have several potential advantages include ambient-temperature deposition, intrinsic damp-heat resistance, smooth surfaces, flexibility, conformal coating of rough surfaces, temperature stability up to 600 °C and excellent optical transparency. Here we report the materials growth, properties and PV application testing of two prototypical a-TCOs, a-InZnO (a-IZO) and a-ZnSnO (a-ZTO). First, co-sputtering composition-spread combinatorial methods are used to survey the overall all amorphous alloy space. Then, selected compositions are further optimized using DC, RF and RF-superimposed DC sputtering. Finally, these a-TCO materials are further developed by actual testing in PV devices including organic photovoltaics (OPV), Cu(In,Ga)Se₂ and Film Si Heterojunctions. For InZnO, DC co-sputtering from angled In₂O₃ and ZnO targets has established that In_xZn_1-xO is amorphous for 0.55 < x < 0.85 for ambient temperature depositions with a maximum conductivity σ_MAX asymp; 3000 S/cm for In_0.8Zn_0.2O_x films. Once deposited, a-InZnO remains amorphous up to 500 - 600 °C. However, for DC sputtering, deposition conditions which yields optimized a-IZO for a 200 nm thick film yield grey 1000 nm thick films. The use of RF-superimposed DC sputtering eliminates this problem and a-In_0.8Zn_0.2O_x films with σ = 3200 S/cm and R_S < 4 Omega;/sq. with VIS> = 88 % (vs glass) were grown. a-InZnO is electrically and optically stable for 40 days in damp heat testing at 85 °C and 85 % RH. Initial results on reactive sputtering of a-InZnO from In:Zn metal alloy targets has demonstrated transparent films with σ asymp; 700 S/cm. For ZnSnO, ambient temperature co-sputtering from ceramic targets yields amorphous films for 10 - 75 at.% Zn but with σ_MAX asymp; 10 S/cm. Increasing the deposition temperature to 400 °C improves the conductivity but only to σ asymp; 250 S/cm, about 10 times lower than that for ambient temperature deposited a-InZnO. Reactive sputtering from metallic targets yields similar results. Accordingly, a-ZnSnO is not conducting enough to be used the primary TCO. However, in a-ZnSnO the work function can be tuned from 4.3 - 5.2 eV making it an attractive interfacial charge transport layer. In PV testing, a-IZO-finished CIGS solar cells have demonstrated 16.4% efficiency, essentially equal to equivalent CIGS cells finished with Al-doped ZnO. For Si heterojunction testing, we fabricated epitaxial film c-Si/a-Si heterojunction solar cells on heavily doped, electronically dead silicon wafers using both a-IZO and reactive evaporated ITO as the transparent conducting oxide (TCO). The best a-IZO and ITO reference solar cells have similar efficiencies, 6.2% for the a-IZO device and 6.0% with ITO. For a-ZTO used in place of the intrinsic ZnO layer in CIGS solar cells, preliminary results gave a 9.5% average efficiency using ZTO from a Zn:Sn target with a 55%:45% composition, compared to 11.6% using conventional ZnO.

In the last years, a rapidly growing attention is being directed towards the investigation of the ionic conducting properties of oxide thin films and hetero-structures, with the main aim of the development of micro-solid oxide fuel cells. Experimental evidence has been reported showing that interfacial phenomena at hetero-phase interfaces, including film-substrate interfaces, give rise to faster ion conduction pathways than the bulk or homo-phase interfaces. This has been ascribed either to the building up of space charge regions at the interfaces or to interfacial strains derived from the lattice mismatch between the two adjacent materials. However, controversial results have been reported: in some cases the nature of the ionic carriers has been questioned, or the presence of strains did not enhance the ionic conductivity. These findings show the need of a deeper understanding of the interface transport properties to unravel the interfacial conduction mechanisms. This talk will present the recent efforts performed in our lab towards the fabrication by pulsed laser deposition (PLD) of ionic (both proton or oxygen-ion) conducting oxide thin films and superlattices, and their electrochemical characterization to clarify the causes for enhanced ionic conductivity at oxide hetero-interfaces. One example will be the PLD fabrication of superlattices based on ceria and zirconia, doped and undoped, on (001)-oriented MgO single-crystal wafers. To allow an epitaxial growth of the hetero-structures, a thin buffer layer of SrTiO₃ (STO) deposited by PLD was used. The growth of each layer was monitored in-situ by reflection high energy electron diffraction (RHEED). Structural analysis showed the high quality of the deposited interfaces: very well ordered super-lattices were obtained even for samples with a thickness of each layer as small as 5 unit cells. A computational density functional theory study of the structural and electronic properties of the (100) and (111) ZrO₂-CeO₂ interfaces was performed to support experimental findings.

The growing demand for miniaturized systems in energy conversion and storage has prompted extensive research aimed at fabricating solid-state ionic devices in thin-film form such as micro-solid oxide fuel cells (mu;-SOFCs). These devices can readily be miniaturized using thin film deposition technologies such as Pulsed Laser Deposition (PLD) [1]. Oxide materials with fluorite structure, such as gadolinium-doped-ceria (20%) (CGO) or yttria stabilized zirconia (ZrO2:8 mol% Y2O3) are particularly promising as electrolytes for mu;-SOFCs [1-6]. High quality, dense epitaxial thin film of fluorite is crucial to obtain physical stability in reducing/oxidizing atmospheres in the temperature range 400-800C [3]. In this work we have studied the growth of thin epitaxial fluorite electrolyte films as candidate materials for low temperature solid oxide fuel cell (LT-SOFC) applications. The grown films were characterized using various ex-situ techniques to understand their structure, crystalline quality, chemical composition and the electrochemical properties. In order to improve the quality of the films, a thin buffer layer (a few nm thick) is introduced to facilitate the epitaxial growth of the fluorite film [4-6]. [1] U. P. Muecke, D. Beckel, A. Bernard, A. Bieberle-Hütter, S. Graf, A. Infortuna, P. Müller, J. L. M. Rupp, J. Schneider, L. J. Gauckler, Adv. Funct. Mater., 2008, 18, 3158. [2] V. Esposito, E. Traversa, J. Am. Ceram. Soc., 2008, 91, 1037. [3] S. Sanna , V. Esposito , D. Pergolesi , A. Orsini , A. Tebano , S. Licoccia , G. Balestrino , E. Traversa , Adv. Funct. Mater., 2009 , 19, 1713. [4] S. Sanna, V. Esposito, A. Tebano, S. Licoccia, E. Traversa, and G. Balestrino, Small, 2010, 6, No. 17, 1863. [5] K. Mohan Kant, V. Esposito, and N. Pryds, App. Phys. Lett., 2010, 97, 143110. [6] E. Wachsman, K.T. Lee, Science, 2011, 334, 935.

A topic of great interest in recent years is the study of the ionic conduction in oxide thin film heterostructures. Evidence has been reported of significant enhancement in electrical conduction in these materials (1-8 orders of magnitude, eg. refs [1-3]) while the cause of this has remained ambiguous. In order to investigate the true nature of the conduction mechanism in systems such as these, we have developed a novel experimental technique for directly studying the oxygen diffusion behaviour. An exchange anneal in an atmosphere of high oxygen-18 concentration is first performed on a sample with a surface oxygen-blocking layer with a single region exposed. Time of flight SIMS is then performed in an area containing both exposed and covered heterostructure material. The 3-dimensional nature of the analyses performed by ToF-SIMS allows the extraction of individual diffusion coefficients for individual layers down to approximately 10 nm thickness. Systems consisting of repeated layer units of alternating doped and undoped ceria (reported in [4]), yttrium-stabilised zirconia (YSZ) and undoped ceria, and samarium-doped ceria and praseodymium nickel copper gallate (PNCG) all grown on MgO by PLD by several collaborators were studied by this method. The diffusion coefficients obtained can be used to calculate ionic contributions to the total electrical conductivity however these are never seen to be greater than that expected for the appropriate single crystal lattice component. This work has been made possible through collaborations with ORNL, USA, NIMS, Japan and Kyushu University, Japan. [1] J. Garcia-Barriocanal et al., “Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures,” Science, vol. 321, no. 5889, p. 676, Aug. 2008. [2] M. Sillassen et al., “Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized Zirconia,” Advanced Functional Materials, vol. 20, no. 13, pp. 2071-2076, 2010. [3] I. Kosacki et al., “Nanoscale effects on the ionic conductivity in highly textured YSZ thin films,” Solid State Ionics, vol. 176, no. 13-14, pp. 1319-1326, 2005. [4] J. M. Perkins et al., “Anomalous Oxidation States in Multilayers for Fuel Cell Applications,” Advanced Functional Materials, vol. 20, no. 16, pp. 2664-2674, Aug. 2010.

The recently reported fast oxygen reduction (OR) kinetics at the interface of (La,Sr)CoO3-δ (LSC113) and (La,Sr)2CoO4+δ (LSC214) phases opened up new questions for the potential role of dissimilar interfaces in advanced cathodes for solid oxide fuel cells (SOFCs). Obtaining a microscopic level understanding for such behavior is important for designing novel interfaces for very high-performance SOFC cathodes. In this work, in order to probe the governing mechanism of such high OR kinetics, we implemented a novel combination of in-situ scanning tunneling spectroscopy and focused ion beam milling and performed density functional theory calculations. In particular, we investigated the OR activity on the basis of local electronic structure and quantitatively assessed the oxygen incorporation mechanism near the interfaces of model (La0.8Sr0.2)CoO3-δ/(La0.5Sr0.5)2CoO4+δ superlattices. The strongly anisotropic oxygen adsorption and dissociation on LSC214 and the lattice strain in the vicinity of the LSC113/214 interface are computationally found to contribute to the OR activity enhancement significantly, providing 4×10^2 times faster oxygen incorporation kinetics at 500 °C. Furthermore, in situ tunneling spectroscopy measurements demonstrated that, starting at 200-300 °C, the LSC214 layers are electronically activated through interface coupling with LSC113. Such electronic activation is expected to facilitate charge transfer from the surface to the oxygen adsorbates in the reduction process, providing about 10^3 times faster OR kinetics. These results show that the electronic activation of LSC214 in concert with its anisotropically fast oxygen incorporation kinetics are likely the key mechanisms governing the very fast oxygen reduction kinetics observed near the LSC113/214 interfaces. Insights gained from our results provide a new understanding of oxide hetero-interfaces at high temperatures and point towards electronically coupled oxide structures as novel cathodes.

The interface between the perovskite La_0.8Sr_0.2CoO_3-δ (LSC-113) and the K₂NiF₄-type (La_0.5Sr_0.5)₂CoO_4-δ (LSC-214) heterostructure was recently reported to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest for developing better functioning cathodes for electrochemical devices such as solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ~0.9 eV energy gain for exchanging a Sr from LSC-113(25%Sr) with a La from LSC-214(50%Sr). We report an even larger (~1.3 eV) energy gain for exchanging a Sr from LSC-113(40%Sr) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that, in a typical experimental temperature range of 500-600 °C, the Sr concentrations in the LSC-214 phase in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr) may be enriched to about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase such Sr enrichment is expected to enhance the oxygen vacancy concentration by 2-2.5 orders of magnitude under typical experimental conditions. Such an increased vacancy concentration in LSC-214 near the interface can potentially explain most of the enhanced oxygen kinetics observed up until now in these heterostructures. The results of this work have been published in M. J. Gadre, Y.-L. Lee, D. Morgan, Physical Chemistry Chemical Physics, 14, 2606-2616 (2012).

The kinetics of electrochemical electrode reactions strongly affects the performance of electrochemical cells such as solid oxide fuel cells (SOFCs) or solid oxide electrolysis cells (SOECs). The corresponding polarization resistances may depend on many parameters including electronic and ionic conductivity of the electrode, grain size and porosity, surface chemistry of electrode and electrolyte, etc. A deconvolution into the effects of each individual parameter is highly sophisticated, particularly for porous electrodes. Therefore thin film electrodes became very popular in mechanistic studies. Still, it is far from trivial to relate a measured overall polarization resistance of, for example, a mixed conducting thin film electrode to the atomistic processes, i.e. to the electrochemical reaction at the surface, ion transport through the electrode, ion transfer across the electrode/electrolyte interface, or electron conduction. The latter may be particularly relevant in H2/H2O atmosphere since most of the commonly used acceptor-doped mixed conductors exhibit low conductivity under reducing conditions. Accordingly, there is urgent need for experiments and tools revealing additional information on the mechanistic origin of polarization resistances. In this contribution, several approaches are discussed that help identifying and separating the processes determining the polarization resistance of electrode reactions in SOFC/SOEC related materials. Even though impedance spectroscopic studies may allow separation of different resistive contributions, their interpretation is often ambiguous. Combination with oxygen tracer experiments as a second tool for probing the electrode kinetics yields much less ambiguous results. This is exemplified for (La,Sr)CoO3-x electrodes on YSZ. It is also shown that film preparation strongly affects the importance of surface exchange and ion diffusion and that a clear correlation of different properties with local structure (analyzed by TEM) and cation mobility exists. As a second tool we introduce a novel method of impedance studies on mixed conducting electrodes: Two interdigitally arranged Pt electrodes were embedded into a Sr(Ti,Fe)O3 thin film microelectrode and two different types of impedance measurements were performed in H2/H2O atmosphere: one between the two Pt electrodes (in-electrode), and one in a conventional arrangement versus a counter electrode on the solid electrolyte. By fitting both spectra simultaneously it is possible to determine up to six materials parameters including ionic and electronic film conductivity and surface reaction rate. As a third approach leading to additional information we employed switching between two electrode reaction for a given material, i.e. instead of “only” varying the oxygen partial pressure we analyzed (La,Sr)FeO3 microelectrodes in H2/H2O as well as in oxygen containing atmosphere. This revealed further details on the parameters affecting the polarization resistance.

A solid oxide fuel cell (SOFC) is a type of fuel cell which is comprised of oxide-based electrolyte and electrodes. SOFCs have been actively studied as promising next-generation energy conversion devices because these have high energy efficiency and can utilize any hydrocarbon fuel currently available, other than pure hydrogen. Traditional SOFCs operate at high temperatures (ge; 800 oC) to secure oxygen ion conductivity and catalytic activity of oxide components. This high temperature operation induces problems, such as chemical reactions between the cell components and microstructural degradation. When minimizing the problems associated with high temperature operation, one of the key issues in the SOFC research is the reduction of the operating temperature without compromising performance. Low temperature operation is of significant technical importance to both conventional large-capacity and portable miniaturized SOFCs. It ensures reliability and cost-effectiveness, providing a pathway to commercialization of the former system, and facilitates thermal management, resulting in reduction of size of the latter system. Therefore, the attempt of improving the low temperature performance of SOFCs by introducing thin electrolytes and nano-structure electrodes surged during the past decade. As a result, thin-film and nano-technologies, which have been relatively unfamiliar techniques to the SOFC in the past, are actively studied nowadays. However, as a consequence of overlooking the fatal issues related to the usage of the thin films and nano-structures at elevated operating temperatures, tremendous research efforts turned into vain in attaining targeted performances and thermomechanical reliabilities for thin-film and nano-structure based SOFCs (TF-SOFC). Among many issues, the thermomechanical vulnerability of the ultra-thin electrolyte membrane and the fast degradation of nano-porous metal electrodes, even in the low operation temperature regime of SOFCs (le; 500 °C), have been the main limitations. During the last several years, we have reported our achievements of both high-performance at low operating temperatures and improved thermomechanical reliability in the TF-SOFC. In our approach, the structural stability of thin-film electrolytes was secured by realizing nano-porous supporting structure. In terms of the nano-structure electrode, fabricating nano-composite electrodes and employing structurally supporting templates significantly mitigated the degradation in long-term operation. In the current presentation, the overview of the KIST TF-SOFC research and recent advances will be introduced. This presentation will provide insights into how to break the limitation of the usage of thin-film and nano-structure materials for high-temperature operating devices including SOFCs, leading to the critical performance possibly achievable.

Cathode materials have been receiving a great deal of attention because oxygen incorporation at the cathode is often considered as the key factor limiting development of intermediate temperature SOFCs (IT-SOFCs). In order to identify the important factors governing oxygen reduction kinetics at the cathode, our group has introduced the Sr(Ti,Fe)O3-δ (STF) system as a model mixed conducting cathode material. The STF cathode was observed to exhibit typical mixed ionic-electronic conducting (MIEC) behavior with the electrode reaction occurring over the full electrode surface area rather than being limited to the triple phase boundary. The surface oxygen exchange reaction was confirmed to be the rate determining step (RDS), and values for the surface exchange coefficient, k, were found to be comparable in magnitude to those exhibited by other popular MIEC oxides such as (La,Sr)(Co,Fe)O3. Interestingly, the surface oxygen exchange rate of STF was found to be only weakly correlated with the transport properties of the majority electronic and ionic carriers (σ_el and σ_ion), in contradiction to common expectations. Instead, charge transfer from the electrode surface to oxygen molecules, mediated by the availability of minority electrons (STF is a p-type conductor), was found to control the overall reaction kinetics at high pO2. In this work, we extend our studies to further examine this correlation between k and minority charge carrier density (n for STF) by controlling the band gap energy (Eg) and the reduction enthalpy (ΔHred). We do so by introducing Ba on the A site of STF i.e., (Ba,Sr)(Ti,Fe)O3-δ (BSTF). We have shown in preliminary work that Ba2+ substitution for smaller Sr2+ leads to an increase in lattice constant of BSTF, with a corresponding decrease in reduction enthalpy, band gap, and activation energy for oxygen surface exchange, presumably due to weaker bonds. To further understand this behavior, we have extended our studies of oxygen nonstoichiometry of bulk BSTF by thermogravimetric analysis (TGA). In addition, BSTF thin film cathodes, deposited onto single crystal yttria stabilized zirconia by pulsed laser deposition, with well defined area and thickness, are examined by electrochemical impedance spectroscopy as a function of electrode geometry, temperature and pO2. Special care is taken to understand the role of surface reaction products (e.g. carbonates) and surface segregation of the alkaline earth elements. Preliminary defect, transport and oxygen exchange models are presented.

To improve the efficiency of solid oxide fuel cells, a more fundamental understanding of the oxygen reduction reaction at the cathode is needed. The surface exchange coefficient (k_chem) for oxygen incorporation into a cathode can be measured by changes in the oxygen vacancy concentration in response to the electrochemical potential. Since k_chem could vary due to the surface crystallographic orientation or grain boundaries, it is advantageous to control the cathode microstructure by using thin films. Our approach is to use in situ X-ray diffraction on pulsed laser deposited thin films to indirectly measure the oxygen vacancy concentration. The advantage of this method is that it can be used to study the effects of applied potential on the thin films, which is not possible for electrical conductivity measurements. Our samples were La_0.6Sr_0.4Fe_1-xCo_xO₃ (x=0,0.2,0.95) on yttria stabilized zirconia (YSZ) single crystal substrates, running in a half cell configuration between 680-890K in air. Although the 3 different types of PLD films had different degrees of epitaxy with the YSZ(111) substrate, we found that chemical expansion occurred under cathodic potentials in all of the films. We also monitored for Sr segregation during our experiment and found that it starts to occur at 890K. Changes in the chemical expansion in response to the applied potential as the Sr segregation develops will be discussed.

The wide spread application of the current solid oxide fuel cells devices is limited by the high operation temperatures. An strong research effort is being directed to increase the conductivity of the electrolyte to minimize ohmic losses at lower temperatures. Interface effects in epitaxial ionic conducting heterostructures appear as a promising pathway towards novel artificial electrolytes for cooler fuel cells or other electrochemical devices. Heterostructures combining transition metal oxides, as compared to other materials, are able to accommodate very large amounts of epitaxial strain without breaking into islands or structural domains. Coherently strained interfaces are an interesting playground for the search of materials with enhanced ion diffusivities, of interest in devices for energy generation and storage. In this regard, it has been proposed that the coherent growth of strained interfaces in heterostructures combining materials with different degrees of lattice mismatch may promote ion diffusivity and thus, these heterostructures may play an important role in the optimization of materials for energy generation and storage. This is the case of the Y2O3- ZrO2 / SrTiO3 (YSZ/STO) superlattices where different structures (fluorite vs. perovskite) are combined with a large lattice mismatch of 7%. The interface between highly dissimilar structures stabilizes a disordered oxygen sublattice with an increased number of oxygen vacancies which promote oxygen diffusion. In this talk we will highlight the importance of the interface structure of highly strained YSZ/PMO superlattices alternating YSZ and transition metal oxides with a perovskite related structure (PMO), in determining changes of their ionic conductivity. We will show results of different PMO systems including SrTiO3, LaAlO3, YAlO3, where epitaxial strain can be consistently modified. We will discuss the role of growth orientation in controlling the structure and morphology of the interface. Results of density functional theory calculations are discussed, showing that the incompatibility of the oxygen positions at the interface planes plays a key role in stabilizing the high values of ionic conductivities.

In the last decade some studies in heteroepitaxial growth of oxide ionic conducting YSZ films on different substrates along with multilayered structures combining YSZ with other materials (mostly non ionic conducting compounds) have revealed a significant dependence of ionic conductivity with film microstructure and epitaxial strain. Particularly, for the controversial case of epitaxial SrTiO3-YSZ heterostructures, some experimental studies on SrTiO3-YSZ (1nm thick) multilayers have claimed up to 7-8 order of magnitude enhancement in ionic conductivity [1]. In this direction it has been reported through theoretical calculations that coherent films up to nominal +7% tensile strain may show several orders of magnitude enhanced ionic conductivity provided the fluorite structure is preserved up to such high strain [2]. However, some other studies in YSZ structure at the interface with SrTiO3 under tensile strain have revealed a certain instability of fluorite phase [3]. In this study we confirm that the structure of YSZ nanolayers (of a nominal thickness of a few nanometres) sandwiched between SrTiO3 slabs may be substantially modified to form strained tetragonal and monoclinic zirconia interfaces, and in some cases to form a modified structure, not compatible with the fluorite, which resembles that of a perovskite-type Zr-O containing layer. Depending on film orientation, either deposited on 100 or 110 SrTiO3 substrates, as well as on the substrate chemical termination, either SrO or TiO2, the YSZ film growth can be controlled to form continuous YSZ slabs with fluorite structure. However, in all cases the YSZ grows almost fully relaxed, as proven by XRD measurements. We also provide evidence that, despite the disturbed microstructure and the unstrained state of the YSZ, the electrical conductivity of the films might be enhanced by about 6-7 orders of magnitude (if one attributes the conductivity changes only to the YSZ material). However, the pO2 power dependence of the conductance with exponent +1/4 measured for these films is an indication of the p-type electronic conductivity in the system. This was attributed to the SrTiO3 conductance enhancement at the interface [4]. Isotopic 18O tracer diffusion experiments did not show any evidence of ionic diffusion along the multilayer structure parallel to the interfaces. Heating-cooling cycles in air at different rates reveal a strong dependence of the electrical properties of the layers. This is explained in terms of interface phenomena associated with oxygen diffusion between YSZ and SrTiO3 and changes in SrTiO3 oxygen stoichiometry. This results in a large span of metastable states of the samples depending on their history. [1] J. Garcia-Barriocanal et al, Science 321 (2008) 676 [2] A. Kushima and B. Yildiz, J. Mater. Chem. 20 (2010) 4809-4819 [3] W. L. Cheah and M. W. Finnis J Mater Sci (2012) 47:1631-1640 [4] A. Cavallaro et al. Solid State Ionics 181(2010) 592-601

In the recent years, there has been an emerging interest towards the use of thin-film technology for miniaturization of solid oxide fuel cells (SOFCs) with thinner components (particularly the electrolyte) that can potentially reduce the operating temperature of the cell while delivering enhanced performance. Moreover, development of novel nano-architectures not only has led to enhancing the properties of individual components at the nano-scale, but also has made the down-scaling of the cells to smaller dimensions possible for development of micro-solid oxide fuel cells (µSOFCs) as well as for other applications. Toward these goals, we present a novel design and fabrication process of a two-phase one-dimensional hetero-nanostructured electrolyte architecture grown on silicon substrate. The developed hetero-nanostructure is composed of a backbone vertically-oriented nanotubes array of strontium titanate SrTiO3 (STO) that has been synthesized for the first time on silicon substrate using the electrochemical anodization and hydrothermal processes. Then the nanotubes are coated with yttria-stabilized zirconia (Y2O3)x(ZrO2)1-x (YSZ) using metal-organic chemical vapor deposition (MOCVD) to form a core-shell nanotubular structure. Analysis of the crystal lattice strain and the defect chemistry of the YSZ/STO hetero-interface is performed using a field emission scanning electron microscopy (FESEM), high-temperature glazing angle X-ray diffraction (HT-GAXRD), and high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). Local atomic/electronic structure of the YSZ coating is also obtained around Y and Zr centers using the X-Ray Absorption Fine Structure (XAFS) measurements. Results obtained from the preliminary cell testing shows promise toward the use of the developed YSZ/STO nanotubes structure as an electrolyte membrane of µSOFCs. Correlations between the charge transfer characteristics and the microstructure of the developed nano-composite at the interface are also described. The developed approach proposes a cost-effective method for design and fabrication of hetero-nanostructured electrolyte membranes for µSOFCs and a potential route to develop silicon-based energy devices for portable applications.

Oxide ionic conductivity in the laminated film with nm thickness is attracting much interest. In this study, layered thin film using Cu- and Ga-doped Pr2NiO4 and Sm-doped CeO2 (PNCG/SDC) was fabricated by pulsed laser deposition (PLD) method, and its change in oxide ion conductivity was studied in details. It was confirmed that prepared film was successfully deposited by PLD method, and its electrical conductivity was improved with decreasing the thickness of SDC layer. In order to identify the charge carrier, ion blocking method was applied. From the results on PO2 dependency, laminated film shows p-type dependence in high oxygen partial pressure and slope for dependency is close to that of PNCG bulk material, suggesting that hole conductivity in laminated film is assigned to the hole in PNCG layer. On the other hand, in wide PO2 range, conductivity is independent of oxygen partial pressure suggesting dominated oxide ion conductivity. This result might suggest that SDC layer is reduced state and formation of electron is suppressed in a laminated film. As a result, improved conductivity observed in laminated film could be assigned to oxide ionic conductivity. The estimated transport number by ion blocking method is also close to unity and the ionic transport number increased with decreasing SDC film thickness and increasing number of interface.

Innovation in advanced materials and devices for embeddable power sources is crucial to the realization of autonomous systems operable in complex and dynamic environment. In this general framework, I will discuss our on-going efforts to realize ultra-thin solid oxide fuel cells with electrolytes at the electron tunneling limit. Such studies allow one to probe rigorously electronic-ionic transport in two-dimensional oxide membranes that are self-supported and proximal to free surfaces. Special instrumentation designed to probe transport and thermo-mechanical stability real-time in such structures at extreme chemical potential gradient and elevated temperatures will be described. Experimental techniques to synthesize atomically tailored designer materials for electrolyte versus electrode functionality in thin film form in suspended structures will be considered. Experimental realization of high performance two-dimensional solid oxide fuel cells operable in a variety of fuels will be presented. The ability to operate such SOFCs at low temperatures (< 500 deg.C) creates an intriguing opportunity: the exploration of charge ordered complex oxides as fuel cell components while retaining memory of the electronic complexity. I will consider this briefly as well.

The power output of a solid oxide fuel cell (SOFC) quickly decreases to zero when the fuel supply is interrupted. Materials that could store energy during the fuel cell operation and restitute this energy when the fuel supply is interrupted would enable new modalities in SOFCs. This could also be assisted by the on-going research trend to reduce the operating temperature of solid oxide fuel cells without significant loss of power density. One such strategy we have explored is to use compositionally complex oxide anodes that can rapidly change valence states but also possess sufficiently high electronic conductivity to enable mixed conduction pathways. We synthesized vanadium oxide thin films by RF magnetron sputtering for thin film solid oxide fuel cell anodes. The electrolyte was nanostructured yttria stabilized zirconia (YSZ) deposited by RF magnetron sputtering and the cathode was porous platinum deposited by DC sputtering at high Ar pressure. It is well known that the oxidation state of vanadium oxide can vary depending on the ambient conditions, thus providing the ability to store energy electrochemically. Furthermore, hydrogen insertion has been reported in the various vanadium oxides, providing an additional pathway for energy storage. Compared to reference porous platinum anode thin film solid oxide fuel cells (Pt/YSZ/Pt SOFCs), the vanadium oxide anode SOFCs (VOx/YSZ/Pt) provide energy much longer once the fuel supply is interrupted. The time during which the Pt/YSZ/Pt fuel cells delivered energy was always about 15 seconds once the fuel supply was interrupted. Our vanadium oxide anode SOFCs delivered energy up to 210 seconds after the fuel supply was interrupted, depending on the current density and the vanadium oxide thickness. Regarding their performance during regular fuel cell operation, the VOx/YSZ/Pt SOFCs had lower open circuit potentials and maximum power densities compared to the reference Pt/YSZ/Pt SOFCs. This is thought to be due to the poorer catalytic properties of vanadium oxide for hydrogen oxidation compared to platinum and clearly points out new research vectors. We will discuss these results in detail along with efforts to explore the concept of multi-functional electrode materials for emerging fuel cell applications. The authors thank Mr. Kian Kerman for assistance in manufacturing the thin film solid oxide fuel cells.

Layered cathode materials with mixed ionic and electronic transport properties, such as those with double perovskite and K2NiF4-type structures, are widely studied as promising materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The intrinsic anisotropic structure of these families of compounds enables high oxygen diffusivity in a preferential direction due to the presence of fast diffusion pathways or interstitial sites. Moreover, the oxygen surface exchange could also be highly enhanced for a particular orientation due to differences in the outermost surface structure and composition, which would result in different pathways for the oxygen incorporation process. With this in mind we have selected two compounds within these families: GdBaCo2O5+δ (GBCO) double perovskite and (La0.5Sr0.5)2CoO4 (LSC214) with K2NiF4-type structure. Both GBCO and LSC214 have been deposited as epitaxial thin films on two different substrates, giving us the opportunity of studying them as model materials with defined orientations. The oxygen diffusion and exchange properties were analysed by isotopic 18O exchange depth profiling (IEDP) and Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) along longitudinal and transverse directions using a particular methodology which was developed for thin films. In the first study we deposited GBCO films with different orientations, either pure c-axis or a-axis oriented, on SrTiO3 (001) and NdGaO3 (110) single crystals, respectively, and exchanged the samples using different temperatures and exposure times. As a result we found that both longitudinal diffusion coefficients (Dc* and Da*) increase following a thermally activated dependence, but with different apparent activation energies for c- and a-axis oriented films. In this case the corresponding oxygen surface exchange rates (k*) do not show any evidence of anisotropy with very similar values within 20-30% differences for films with different orientations. The second study is focused on the study of LSC214 thin films grown on single-crystal SrTiO3 (001) and LaSrAlO4 (100),respectively. We have studied and compared the k* and D* values along the (001) and (100) directions of LSC214, with a particular emphasis in the different surface activities between the two orientations. Finally, we have also studied the effect of surface cation chemistries on the exchange coefficient of these films by gentle HCl etching. We show how this work involving the combination of two layered model materials, each of them grown with two different orientations, has helped us acquire a deeper and better understanding of the effect of crystallographic orientation on the oxygen exchange and diffusion kinetics of layered epitaxial thin film cathodes.

As the ABO3 perovskite oxide compositions show their intrinsic oxygen reduction reaction (ORR) limitations, there is a need to search for alternative types of mixed ionic and electronic conductors to enhance solid oxide fuel cells efficiencies. The Ruddlesden-Popper La2NiO4+δ (LNO214) is of particular interest(1, 2) due to their ability to incorporate excess oxygen and their anisotropic oxygen transport properties. Recently, Bassat et al.(3), have reported that the activation energy of the surface exchange reaction measured in the (a,b) plane is lower than that in the c plane. The molecular dynamics simulation of oxygen interstitial migration mechanism of LNO214(4) showed that the apical ion takes up residence in an adjacent interstitial site. Despite these efforts suggesting oxygen surface exchange dependence on orientation, this relationship is not well understood. We here examine the ORR activity of a-axis normal epitaxial growth of LNO214 thin films possessing varying amounts of strain on (001)-oriented single-crystal yttria-stabilized zirconia (YSZ), which can provide insight into ORR mechanisms of Ruddlesden-Popper type materials. Pulsed laser deposition (PLD) was utilized to deposit a-axis normal epitaxial thin LNO214 films (50 nm, 120 nm, 210 nm, corresponding to -3.4 %, -5 %, and -5.8% c-lattice mismatch and 1%, 0.32%, 0.3% a-lattice mismatch) on YSZ with gadolinium-doped ceria (GDC) as the buffer layer. Crystallinity, crystallographic relationships, and strains of the films were analyzed using 4-circle X-ray diffraction. Using electrochemical impedance spectroscopy (EIS) measurements conducted on patterned micro-electrodes ORR activity of epitaxial LNO214 films of different amounts of strain was examined. Interestingly, the surface exchange rate (kq) of a-axis normal epitaxial growth of LNO214 thin films strongly depends on c-lattice mismatch which may be caused by the change in chemical composition of LNO. These findings suggest that c-lattice mismatch may play an important role in controlling the formation of oxygen interstitial which can be contributed to ORR activity. The mechanism of ORR activity on Ruddlesden-Popper La2NiO4+δ thin films will be discussed. References 1. E. Boehm, J. M. Bassat, P. Dordor, F. Mauvy, J. C. Grenier and P. Stevens, Solid State Ion., 176, 2717 (2005). 2. V. V. Kharton, A. A. Yaremchenko, A. L. Shaula, M. V. Patrakeev, E. N. Naumovich, D. I. Loginovich, J. R. Frade and F. M. B. Marques, J. Solid State Chem., 177, 26 (2004). 3. J. M. Bassat, P. Odier, A. Villesuzanne, C. Marin and M. Pouchard, Solid State Ion., 167, 341 (2004). 4. A. Chroneos, D. Parfitt, J. A. Kilner and R. W. Grimes, J. Mater. Chem., 20, 266.

Thin-film solid oxide fuel cells (SOFCs), or micro-SOFCs, are under intense study due to their high energy conversion efficiency and relatively low operation temperature (<500 ^oC). Electrolyte materials with high ionic conductivity and long-term stability are a key component for successful operation. It is of great interest to develop a high-throughput methodology to screen candidate materials with high reliability and resolution. In this paper, we developed a high-throughput methodology to investigate ionic conductivity in oxygen-ion conductors. Yttria stabilized zirconia (YSZ) composition-spread thin films with nanometer-size grains were prepared by 90^o off-axis reactive RF co-sputtering. The morphology and microstructure were characterized by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The ionic conductivity of the YSZ composition spreads was evaluated using electrochemical impedance spectroscopy (EIS) measurements. We compare results for two electrode configurations, namely, out-of-plane (parallel plate) and in-plane (planar interdigitated electrode) and find that the contribution from the intragrain conductivity in YSZ thin films (150 nm) is more explicit in the latter configuration because it greatly diminishes electrode effects. The intragrain oxygen ion conductivity of thin film YSZ was systematically measured as a function of yttria concentration over the range 2 mol% to 12 mol%. The results show that the measured conductivity of the YSZ thin films is close to that of corresponding bulk materials with a peak value around 3 x 10^-4 S cm^-1 at 440 °C at the optimum Y₂O₃ concentration of 8 mol%. Validation of this technique means that it can be applied to novel chemical systems for which systematic bulk measurements have not been attempted.

Thermoelectric materials convert heat into electricity (and vice versa) through Peltier and Seebeck effects and can be used for refrigeration, cooling microelectronic devices, and providing an energy source from waste heat. The thermoelectric properties (Seebeck coefficient for example) of materials are strongly dependent on the grain size, defects and doping. In this work, we have deposit structured thermoelectric oxides to tune Seebeck coefficient and electrical conductivity. Alumina ceramics, made by Sprak Plasma Sintering, have been thusly prepared to serve as substrates and Ca3Co4O9 thin films were grow using the pulsed laser deposition technique on top of them. Using Electron Backscattering Diffraction technique, the synthesis of Al2O3 will be correlated to the grain sizes, texture and grain distributions. Structural characterisation and thermoelectric measurements will also be presented, and the properties Ca3Co4O9 will be correlated with the microstructure and nanostructure of alumina. Other examples tested will also be shown.

Single crystals of NaxCoO2 show very promising thermoelectric properties and a power factor of 8.1mW/K2cm was reported, however the ZT values are currently limited by the high thermal conductivity of these NaxCoO2 single crystals. An effective approach to reduce the phonon contribution of the thermal conductivity is the fabrication of nanostructured NaxCoO2-based materials, such as superlattices. Previously single-phase growth of NaxCoO2 thin films was already shown, although a study of the intrinsic properties was not possible due to stability issues at ambient conditions. This lack of chemical stability hindered further characterization and also limits its use as thermoelectric material or as building blocks for superlattice structures. We present a method to obtain chemically stable, single-phase NaxCoO2 thin films by pulsed laser deposition, due to the in-situ deposition of an amorphous AlOx capping layer. The suggested reactions of sodium with moisture and CO2 from air are completely prevented with this capping layer and the thin films remain stable for at least several months. This enables detailed characterization of the intrinsic properties of these thin films for the first time. We demonstrate a controlled epitaxial growth of single-phase NaxCoO2 thin films on hexagonal as well as on cubic substrates with a layer thickness ranging from 5nm to 200nm. We show a detailed study of these NaxCoO2 thin films, which relates the structural and thermoelectric properties, resulting in optimized samples with a resistivity and thermopower of respectively 1.3mOmega;cm and 103mu;V/K at room temperature. Finally, we present temperature dependent X-Ray diffraction, thermopower and resistivity measurements, revealing the high temperature stability and thermoelectric potential of NaxCoO2 thin films.

Oblique angle deposition (OAD) is a self organizing physical vapor deposition (PVD) technique that has been used to grow sculpted 3D nanostructures. These nanostructures include helices, slanted rods, and zigzag structures; among others. The OAD structures can be fabricated from virtually any material that can be deposited using PVD including: polymers, metals, semiconductors, oxides, nitrides, etc. The control over the nano-scale structural anisotropy of these OAD materials allows one to tailor their electrical, magnetic, mechanical, crystalline, and optical properties. Through the careful design of the OAD structure and material selection these OAD materials can form photonic materials (1D, 2D, and 3D). We will discuss ongoing work using OAD to develop oxide thin film interference filters that can withstand extreme temperatures (800-1000 C) at mTorr vacuum levels. These filters are being developed for thermal photovoltaic applications.

Due to their well-defined orientation and homogeneous surface, epitaxial thin films can act as a model system to improve fundamental understanding of the surface exchange kinetics on oxide catalysts. However, thin films of oxides are known to have physical properties that can dramatically differ from those of bulk materials.^1,2 Recent studies exploring the oxygen reduction reaction (ORR) on epitaxial films of La_0.8Sr_0.2CoO_3-δ observed markedly increased catalytic activity in comparison to bulk ceramics.³ This is in contrast to previous studies on other transition metal oxides that demonstrate reduced ORR activity.^4,5 We present an investigation of the catalytic activity of La_1-xSr_xCoO_3-δ epitaxial thin films (x = 0.0, 0.2, 0.4, 0.6) on (001)-oriented yttria-stabilized zirconia (YSZ), as well as surface decorations (< 2 nm) of La_1-xSr_xCoO_3-δ (x = 0.0, 0.2, 0.4, 0.6) on La_0.8Sr_0.2CoO_3-δ thin films. Electrochemical impedance spectroscopy (EIS) was used to characterize the ORR activity on films of different composition and those including secondary composition decorations. The trends observed for the enhancement and reduction of activity will be presented and a hypothesis on the physical origin for their deviation from bulk will be discussed. 1. J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D. G. Schlom, . V. Waghmare, N. A. Spaldin, K. M. Rabe, M. Wuttig, R. Ramesh. Science 299, 1719 (2003). 2. J. Garcia-Barriocanal, A. Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S. J. Pennycook, J. Santamaria. Science 321, 676 (2008). 3. G. J. la O&’, S.-J. Ahn, E. Crumlin, Y. Orikasa, M. D. Biegalski, H. M. Christen, Y. Shao-Horn. Angew. Chem. Int. Ed. 49, 5344 (2010). 4. G. Kim, S. Wang, A. J. Jacobson, Z. Yuan, W. Donner, C. L. Chen, L. Reimus, P. Brodersen, C. A. Mims. Appl. Phys. Lett. 88, 3 (2006). 5. A. Yamada, Y. Suzuki, K. Saka, M. Uehara, D. Mori, R. Kanno, T. Kiguchi, F. Mauvy, J. C. Grenier. Adv. Materi. 20, 4124 (2008).

Pulsed laser deposited (PLD) TiO2 was compared to similarly fabricated Ta doped TiO2. These were then tested as photoanode material for dye sensitized solar cells (DSSC). The photoanodes were characterized using scanning electron microscopy (SEM), optical absorption spectroscopy (UV-Vis), incident-photon-to-current efficiency (IPCE), current density- voltage (JV curve) measurements as well as electrochemical impedance spectroscopy. Photophysical behaviour of the dye-sensitized solar cells fabricated from the tantalum doped photoanode exhibited larger photocurrents, comparable open circuit voltages and improved overall efficiencies compared to undoped TiO2. The enhanced performance maybe attributed to a combination of increased electron concentration and a reduced electron recombination rate.

Pyrochlore A2B2O7 compounds are excellent candidates for photo-catalysis due to the possibility of electronic band structure modulation and band gap tuning through hybridization between A and B cations, and O anion. Bi2Sn2O7 is a particularly attractive option due to the reasonable mobility of photon-induced charges, resulting from the reduced band gap and band dispersion from the Bi 6s + O 2p valence band maximum (VBM), and the Bi 6p + Sn 5s + O 2p for conduction band minimum (CBM). Our approach to improve the photocatalytic efficiency driven by the solar spectrum is the use of heteroepitaxial oxide thin films grown by molecular-beam epitaxy (MBE) technique. As a first trial, we grew epitaxial pyrochlore Bi2Sn2O7 thin films on (111) Y: ZrO2 (YSZ) substrates in the adsorption-controlled growth regime.[4] We used commercially available fluorite YSZ substrate due to the structural similarity with regards to the pyrochlore A2B2O7 structure. From the in-situ reflection high-energy electron diffraction (RHEED) and ex-situ four-circle x-ray diffraction (XRD) as a function of growth temperature, we found stoichiometric and (111)-oriented epitaxial Bi2Sn2O7 growth regime between 570 and 620 °C substrate temperature when we utilized an over-pressure Bi-flux (1×10^14 [atoms/cm2 sec]), and Bi/Sn flux ratio more than 30. The 2theta;/omega; XRD revealed clear thickness fringes in the main 222 Bi2Sn2O7 film peak with the omega; rocking curve width less than 0.08°. phi;-scan of off-axis 400 Bi2Sn2O7 peak showed 3-fold in-plane symmetry, indicating high crystalline quality and in-plane aligned epitaxial growth with respect to the (111) YSZ substrate. Cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) verified the entire interfacial relation, i.e. (222) Bi2Sn2O7//(111) and YSZ, [1-10] Bi2Sn2O7//[1-10] YSZ. In addition, selected area electron diffraction pattern (SAED) of the Bi2Sn2O7 film taken along with three different zone-axes [01-1], [11-2], and [1-41] lattice revealed super-refection corresponding to the Bi, and Sn cation ordering in the pyrochlore Bi2Sn2O7 structure. UV-VIS transmittance measurements indicated that the Bi2Sn2O7 films have a direct optical band gap of 3.15eV. To get the highest quality of Bi2Sn2O7 films, we are trying to grow Bi2Sn2O7 on iso-structural pyrochlore Sm2Ti2O7 substrates, which we have prepared as high quality single crystals for this purpose. There are no commercially available pyrochlore substrates so we are developing Ln2Ti2O7 (Ln=Sm, Eu, and Gd) substrates for the study of systematical strain in the films, reactivity, and surface structure of the photocatylic pyrochlore phase. Their structural, optical and photo-catalytic properties will be discussed in the future.

Semiconducting oxides have the potential the play an important role in the next generation of electronic and energy conversion devices including photodetectors, photovoltaics, and photocatalysts. Among semiconducting ABO3 perovskite oxides, the La1-xSrxFeO3 system is promising for band gap engineering as the gap energy of LaFeO3 is ~2.1 eV while SeFeO3 is a metal. In this study, thin films of La1-xSrxFeO3 (LSFO) (xle;0.4) were deposited on single crystal (001) strontium titanate (STO) substrates using molecular beam epitaxy (MBE). Extensive x-ray diffraction and reflectivity were used to confirm the quality and composition of the films. To investigate the changes in the optical properties with substitution of mixed valence A-site cations, variable angle spectroscopic ellipsometry was employed. Several systematic changes in the optical properties of these films were observed with increasing Sr content including a red-shifting of the absorption edge that corresponds to the band gap of pure LaFeO3. Additionally, Sr doping leads to the formation of a new state in the gap of LaFeO3, with increasing sub-gap absorption observed with increasing Sr content. The nature of these electronic transitions will be discussed and compared to previously reported x-ray-based spectroscopic studies.

Bismuth vanadate (BiVO4) has attracted increasing attention as a photoanode candidate for photoelectrochemical water splitting for the past years. It has a direct band gap about 2.4-2.5 eV as well as desirable onset potential and valence band position for oxygen evolution. However, among all of the reports, there lacks a scalable thin film synthesis approach for this promising photoanode material. In this study, we are going to report the synthesis of BiVO4 thin films by reactive sputtering for the first time. From the synthesis results, the conductivity and phase composition was highly dependant on the stoichiometry of BiVO4. The structural characterization and optical measurement confirmed the formation of photo-active monoclinic phase. The sputtered BiVO4 thin films were also compared with hydrothermal synthesized samples in powder form in terms of photoelectrochemical properties. In addition, the doping effect of W and Mo was also explored and reported in the study.

Wide bandgap oxides have been extensively used as a transparent electrode of various applications such as photovoltaics, photoelectrochemical cells, and flat panel displays. Especially indium tin oxide (ITO) and Al-doped ZnO (AZO) have been in the main focus due to its excellent electrical conductivity and optical transparency. Recently, there have been increasing demands of depositing thin TCO films by atomic layer deposition (ALD). ALD is a variation of the conventional chemical vapor deposition and it uses alternate supply of precursor and reactant molecules. These surface-saturated and self-limiting reaction mechanisms guarantee an extremely conformal deposition of TCO materials onto large surface area or complex three dimensional substrates. Even though the ALD process has long been excluded for a mass production owing to its very low growth rate, recent emergence of nanostructured solar cells and organic light emitting diodes and development of high-speed ALD tools have accelerated an investigation of ALD-TCO materials. In this presentation, we report structural, electrical, and optical properties of ALD-ZnO films doped with various foreign elements. Specifically, it was found that a layer-by-layer doping manner takes a critical role of determining material properties that are deviated from conventional homogeneous TCOs. For instance, our recent reports have revealed two major findings; 1) ALD-AZO film has a unique heterolayered structure consisting of a ZnO matrix and AlOx dopant layers, 2) the amount of dopants incorporated within every atomic-doping layer determines an electron density and a doping efficiency of this film [1,2]. In this work, we intended to extend our contribution into verifying an optimum dopant of ALD-ZnO and understanding an underlying carrier generation mechanism. In order to elucidate those motivations, various dopants, including group III elements (Al, B, In) and transition metals (Hf, Zr, Ti), have been comparatively tested into the ZnO as a transparent electrode material. It is remarkable that Ti exhibited the lowest resistivity and the highest mobility among the tested dopants due to the inherent surface-saturated nature of the ALD process. This finding is extraordinary compared to the conventional homogeneous AZO. If time allows, our recent results on optical properties of these materials will be also covered. [1] D.-J. Lee et al., Adv. Funct. Mater. 21, 448 (2011). [2] D.-J. Lee et al., J. Electrochem. Soc. 158, D277 (2011).

Radio Frequency (RF) plasma source atomic spectroscopy and mass spectrometry have been developed in our laboratory to characterize various transparent conducting oxides (TCOs) such as Al doped ZnO film (AZO), Sn doped In2O3 (ITO) and Sb or F doped SnO2 (ATO and FTO). The films, used in photovoltaic, thermoelectric, and piezoelectric applications, can be analyzed for elemental composition, spatial distribution, and trace impurities. The specific techniques that were studied included glow discharge optical emission spectroscopy (RF GD-OES) and laser ablation ICP mass spectrometry (LA ICP-MS). By utilizing RF plasma or laser ablation for material sputtering, excitation or ionization, many intrinsic limitations associated with traditional electron, ion or x-ray techniques are avoided in characterizing surface, interface and bulk TCO thin films. RF GD-OES has been used to profile surface, near surface and interface of TCO thin films with nm depth resolution and simultaneous multi-element profiling capability. The depth profiles obtained have been successfully used to examine film uniformity and study dopant vertical distribution in an effort to ultimately obtain a high transmittance and low resistivity TCO film. Dopant concentration and film stoichiometry can also be verified against NIST traceable elemental standards developed in our laboratory. LA ICP-MS has been used for microscopic defect identification and quantitative determination of trace residue impurities in any TCO films and sputtering targets used to deposit these films. The impurity information obtained has been useful to help optimize material purification process, facilitate target fabrication and selection, and reduce cross-contamination during plasma deposition. The signal intensities produced by RF GD-OES and LA ICP-MS all have simple and well-defined mathematical (linear) relationships with elemental concentrations in the material. A wide linear dynamic range (over six orders of magnitudes) possessed by these techniques coupled with availability of various NIST traceable material standards have made accurate and precise surface and bulk TCO film analyses possible.

We have studied the microstructure and defects of heavily Ga-doped ZnO (GZO) thin films deposited by plasma-enhanced molecular beam epitaxy. Films were grown under metal-rich, stoichiometric, and oxygen-rich conditions on undoped ZnO seed layers deposited on a-plane sapphire substrates and on GaN templates on sapphire. Characterization was primarily by aberration-corrected STEM imaging and microanalysis. On sapphire, Ga doping under metal-rich conditions leads to local switching of the polarity, creating inversion domain boundaries and voids. The other primary defects are low-angle grain boundaries. Under oxygen-rich conditions, the film structure switches from wurzite to zinblende at a thickness of ~150 nm. GZO on GaN has two rows of misfit dislocations, one at the GaN / seed layer interface and another at the seed layer / GZO interface. The only threading defects are low-angle grain boundaries. The influence of the microstructure and defects on the electrical properties of the films (reported in [1]) will be discussed. [1] H. Y. Liu, V. Avrutin, N. Izyumskaya, A. B. Yankovich, A. V. Kvit, P. M. Voyles, H. Morkoc, J. Appl. Phys. 111, 103713 (2012).

Solid state micro-batteries prepared by vapor deposition of oxide, oxynitride, phosphate, and metal films have been extensively investigated at ORNL and are now manufactured commercially. The thin film architecture offers very high power and energy density when the battery is integrated with electronic or energy harvesting devices. These thin film batteries are also ideal for fundamental studies of battery materials, their interfaces and film growth processes because of the simple geometry and compositions free of binders and additives. For renewable energy applications requiring batteries with liquid electrolytes or dual solid and liquid electrolytes, thin films may be sputter deposited onto membranes or powders that form the electrode-electrolyte interface. Such protective electrolyte coatings may act as synthetic SEI (solid electrolyte interphase) layers to help stabilize the interface, promote longer cycle life and access higher cell voltage by inhibiting dendrite formation and side reactions. Acknowledgement: Research sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy.

The recent technological development of miniaturized electronic systems has induced a strong demand for developing efficient and compact power sources such as thin film batteries. As their capacity does not exceed 200 µAh.cm-2.µm-1, intensive research effort is led to develop positive electrodes exhibiting better performances. Conversion materials which allows the insertion of more than one lithium per formula unit, are promising candidates. Among these materials, copper oxide has attracted our attention due to its high theoretical volumetric capacity (426 µAh.cm-2.µm-1), its working potential (0.5 V - 2.5 V vs Li+/Li), and its ability to be deposited by sputtering which is commonly used in microelectronics. This study deals with the preparation of copper oxide thin films by rf magnetron sputtering from a copper target under a mixed atmosphere (Ar + O2) and their thorough physico-chemical and electrochemical characterizations with a liquid and a solid electrolyte. In all these samples, 2 Li+ ions can be inserted during the first discharge, which corresponds to a capacity close to 360 µAh.cm-2.µm-1. The reversibility of the lithium insertion remains nevertheless strongly dependent on the physico-chemical properties of the films. Finally, efficient thin film electrodes able to reversibly exchange up to 1.6 Li/CuO (290 µAh.cm-2.µm-1) were obtained under optimized sputtering conditions.

Electrolytes composed of lithium-ion conducting oxides have potential advantages over non-oxide solid electrolytes with respect to chemical and thermal robustness. Since the discovery of its high lithium-ionic conductivity (~10^-3 S cm^-1 at room temperature) by Inaguma et al.,^[1] lanthanum lithium titanate La_2/3-xLi_3xTiO₃ (0.03<x<0.167, LLT hereafter) with a double perovskite structure has attracted much attention as a very promising candidate for lithium-ion conducting solid electrolytes. However, the lithium-ion conductivity of LLT remains insufficient due to its high grain boundary interfacial resistance. Here we show fabrication and lithium-ion conductivity of single crystalline LLT thin films with atomically flat surfaces.^[2] LLT thin films are heteroepitaxially grown on (001) SrTiO₃ (STO) single crystal plates by PLD. The HR-XRD, HAADF-STEM, and AFM analyses of the resultant LLT films indicate that they are heteroepitaxially grown with atomically flat surfaces. Films are considered to be mostly single crystalline LLT, but 90^o-rotated multi domains (~5 nm) with epitaxial relationships of (100) [001] LLT || (001) [100] STO and (100) [010] LLT || (001) [100] STO are present. The bulk conductivity of the LLT (x=0.1) film is 2.5×10^-2 S cm^-1 at 190^oC, which is consistent with that of a bulk single crystal LLT. These films should aid in clarifying the interfacial lithium-ion conductivity. Additionally, a solid electrolyte with an atomically flat surface is essential to fabricate planar heterostructures on the nanometer scale. [1] Y. Inaguma et al., Solid State Commun.86, 689 (1993). [2] H. Ohta et al., Appl. Phys. Lett.100, 173107 (2012).

Oxide thin films have been a favorable system for energy storage application. In lithium-ion battery research, they draw even more attention due to the prospects of thin film batteries. However, the stress evolution of oxide thin films during Li-ion intercalation/de-intercalation process remains relatively less known which severely limits further improvement of the storage capability of oxide thin films. Large and irreversible stress (volume change) can be catastrophic for the electrode in terms of cycling capacity and even cause severe safety problems. To gain a better insight into solving this problem, we used vanadium pentoxide thin film as the model system to study the non-stoichiometry, i.e. oxygen deficiency, effect on the stress and electrochemical properties. Vanadium oxide thin films of 50 nm and 100 nm were deposited onto Au coated quartz wafer by reactive sputtering. The films were then annealed in air at 550 C for 3 hours to fully oxidize; oxygen deficiency was created by further annealing the fully oxidized vanadium pentoxide film in reducing forming gas (5 percent hydrogen and 95 percent nitrogen) at 500 C for another 3 hours. The crystallized vanadium oxide film after reducing treatment possessed oxygen-deficiency as confirmed by x-ray photoelectron spectroscopy (XPS). The stress evolution of both fully oxidized and reduced vanadium oxide films during Li-ion intercalation/de-intercalation was studied in-situ by multi-beam optical stress sensor (MOSS). It is noticed in the electrochemical test that reduced vanadium oxide exhibited noticeably better Li-ion intercalation/de-intercalation stability than fully oxidized vanadium oxide. In-situ stress study provides hints for this improved cyclic behavior: while having a larger compressive stress as compared with fully oxidized V2O5 (0.6 Gpa vs. 0.4 Gpa), the tensile stress of reduced vanadium oxide film is fully reversible in long cycles. To the opposite, fully oxidized V2O5 film has a much higher irreversible stress which accumulates over cycles and causes faster capacity degradation of the electrode. The higher compressive stress of reduced vanadium oxide film could be attributed to the extra stress generated by space-charge layer on the thin film surface which was overlooked in the stress study of Li-ion battery electrodes. Oxygen deficiency environment of reduced vanadium oxide film facilitates surface charge transfer process which significantly increases the stress reversibility and thus alleviates the damage to electrode structure due to irreversible stress/volume change. This work has demonstrated the critical role of surface chemistry in stress evolution and electrochemical performance of vanadium oxide as Li-ion storage electrode. Oxide thin films with appropriately designed surface chemistry could exhibit highly reversible stress/volume change; accordingly the electrochemical properties could be significantly improved.

As high performance portable electronic systems are becoming vastly popular, development of power sources and sensors together with micro-electromechanical (MEM) components and other active electronics on the same silicon wafer seem to be very promising for use in portable electronic devices. In this work, the fabrication process and the growth mechanism of single and multiple layers of electrochemically grown self-organized TiO2 nanotubes arrays on silicon and conductive glass substrates is reported. Electrochemical anodization process is used to grow the nanotubes from thin-film titanium sputtered on silicon substrate at various anodization conditions. Vertically-oriented nanotube arrays with minimum average diameter of 25 nm and 100nm length are synthesized at low potentials and growth time. It is shown that the diameter, length and the crystallinity of the nanotubes can be controlled via anodization parameters as well as in the post-annealing process. The microstructure, crystallinity and chemical composition of as-prepared and heat-treated nanotubes were characterized by field emission scanning electron microscopy (FESEM), high-temperature glazing angle X-ray diffraction (HT-GAXRD), and X-ray photoelectron spectroscopy (XPS). Preliminary results on the use of such thin-film nanotubes for photoelectrochemical applications and the fabrication of micro-patterned nanotube arrays on silicon will be discussed showing promise toward the use of the proposed approach for the development and integration of metal-oxide nanostructures with micro-electronic and integrated-circuit (IC) devices.

We will introduce our approach to prepare and investigate thin film materials for application in all solid state batteries by using integrated UHV preparation facilities. For thin film synthesis different thin film deposition techniques have been applied as magnetron sputtering, chemical vapor deposition/synthesis or physical vacuum deposition. The thin films have been studied using photoelectron spectroscopy (XPS, UPS, Resonant Photoemission (ResPES)) for investigating the occupied valence band states and X-ray absorption near edge spectroscopy (XANES) for investigating unoccupied conduction band states. The experimental data will be compared to theoretical calculations and are correlated to electrochemical data. In addition, the electrochemical performance of the battery materials have been investigated mostly addressing chemical surface reactions and contact formation. Such investigations allow a better insight into the thermodynamics of intercalation but also into the mechanisms of concurrent reactions which may lead to the capacity loss of batteries with cycling.

Transition metal oxides (Eg. MoO3, WoO3) have many interesting applications in catalysis, smart windows (excellent electro-chromic behavior), gas sensors, lubricants and so on. We have come up with an efficient way of making thin films (under 10 nm) of these oxides where we begin with the exfoliation of the layered transition metal dichalcogenides like MoS2 and WS2. We reported a solution-processable method for the fabrication of single layer MoS2 [1] and WS2 films, which are annealed in the presence of oxygen under controlled conditions to form layered MoO3 and WO3. X-ray photoelectron spectroscopy (XPS) confirmed the composition of the material and characterization techniques like Raman spectroscopy and X-ray diffraction confirmed the crystalline structure which was similar to commercially available metal oxide powders. The electronic band structure was also measured using ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPS) and the bang energy values calculated were close to the values reported in literature. The Electrochromic property of these films was studied using cyclic voltammetry (CV) measurements and coloration and bleaching states were observed for reduction and oxidation states. Also, these films were used as Photocatalysts for oxygen reduction reactions, as electrode material for supercapacitors and as hole transport layer for organic photovoltaic devices. Material synthesis, characterization and applications will be discussed. 1. Eda, G. et al. Photoluminescence from Chemically exfoliated MoS2. Nano Lett. 11, 5111-5116 (2011).

Titanium(IV) oxide (TiO2), acting as a stable support for photocatalysts and sensitizers, has applications in catalysis and energy science. Utilizing ambient pressure synchrotron x-ray photoelectron and absorption spectroscopies, we explore the properties of TiO2 thin films and ordered linear arrays of TiO2 nanoparticles under in situ water vapor exposure and heating. Our nondestructive depth profiles (obtained by varying the photoelectron kinetic energy) of electronic and surface structures, combined with density-functional theory calculations, indicate an enhancement of the density of states (DOS) near the Fermi level due to surface Ti3+ and oxygen vacancies. Introducing water on the interface suppresses this DOS enhancement. The Ti L-edge and O K-edge absorption spectra, in combination with atomic multiplet calculations, provide information on crystal field effects and multiplet interactions, helping to determine the phases of the TiO2 particles. Our in situ studies suggest that isolated TiO2 nanoparticles may enhance solar absorption efficiency, and the TiO2 band gap can be tuned reversibly under water exposure and heating.

Recent advances in studies of transition metal oxides (TMO) have not only shed light on the fundamental physics governing their behavior, but also enriched, among others, a wide spectrum of opto-electronic applications, including memristive and neuromorphic electronics, smart glasses, charge-writing, solid oxide fuel cells, etc. The suitability of TMO for these applications relies on the complex interplay between the electronic and ionic transport, magnetic and ferroelectric properties that, in turn, are controlled through the tuning of the transition metal oxidation state and concentration of oxygen vacancies. These latter can be modified experimentally, in particular, by external electric field that shifts the electrochemical state of TMO. However, the precise tailoring of the material&’s functionality requires local control over these phenomena at the scale of individual defects and correlation lengths. Electrochemical Strain Microscopy (ESM) technique (based on voltage-strain coupling) addresses these issues and yet is blind to the origin of the strain, being unable to distinguish between ionic and ferroelectric polarization contributions. Therefore we have developed a simple SPM method for probing local ionic dynamics in mixed electron-ionic conductors based on the first-order reversal curve current-voltage measurements (FORC-IV). Here we report on the principles of the patent-pending FORC-IV technique, as well as demonstrate its advantages using Ca-doped BiFeO3 (CaBFO) as an example of an electrochemically-active TMO. The FORC-IV method relies on the appearance of hysteresis in the IV curves recorded at the tip-surface junction due to the onset of electrochemical processes above a certain voltage threshold. Thus, it elegantly distinguishes between the local electronic and ionic transport and supplements standard current-AFM with information on the electrochemical activity at the nanoscale. It also allows for mapping of the surface in terms of ionic dynamics, which extends the technique to 2D spectroscopic imaging. We further show how FORC-IV applies to studies of CaBFO thin film, helping discriminate between the ferroelectric and ionic properties, imaging natural and artificially-created defects. Variable temperature FORC-IV also reveals information on the interplay between the thermodynamics (activation energy barriers) and kinetics (relaxation processes) in this material. Acquired data are discussed in the light of the recent publications on CaBFO. This study demonstrates a new technique to explore and understand the local dynamics in electrochemically-active materials.Research was supported (S.V.K., Y.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the CNMS (S.V.K., S.J.), which is sponsored at ORNL by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE.

Overcoming the intermittency of solar radiation using energy storage methods is a major challenge for adoption of solar energy at very large scale. Synthesis of fuels from sunlight is one potential storage approach, providing the need for optimized photoelectrochemical devices and materials. We have recently reported on atomic layer deposition (ALD) of ultrathin TiO2 tunnel oxides that can stabilize highly efficient silicon anodes for photocatalytic water splitting [1]. Pin-hole free films of ~ 2 nm thickness have been found to offer substantial protection of underlying n-Si anodes during water oxidation. Recent electrical and electrochemical characterization indicates that a nanocomposite Ir/TiO2/SiO2/Si architecture behaves like a Schottky diode but without the Fermi-level pinning observed in many metal-silicon systems. With 2 nm TiO2 thickness, these Schottky tunnel junctions produce an inferred photovoltage of up to 550 mV under one sun. A survey of differing ALD-oxide thicknesses and surface catalyst layers allows the determination of various junction properties key to understanding device operation. This presentation will describe the effects of tunnel oxide thickness, water oxidation catalyst layer and related design parameters on the photovoltage, overpotential for water oxidation, and the facility of hole transport between the silicon anode and water. [1] YW Chen, J.D. Prange, et al. Nat. Mater. 2011, 10, 539-44.

Wide bandgap oxide semiconductors are widely used in flat-panel displays, touch panels, and solar cells. In general, these semiconductors, such as,indium tin oxide (ITO) and ZnO:Al, exhibit n-type conductivity. NiO is a promising candidate because of its p-type conductivity and wide bandgap (3.7 eV). NiO films have been deposited by various methods, including sputtering, pulsed laser deposition, and spray pyrolysis. Among these, sputtering is the most suitable method because it enables a large-area film with a well-controlled composition to be deposited economically. In this study, NiO films were prepared by an RF sputtering method to fabricate visible-light-transparent solar cells. Visible-light-transparent solar cells are much more attractive because their optical transparency permits more flexibility in installation location; therefore, the total power generated can be increased. In addition, they have the advantage of absorbing ultraviolet light, which is harmful to humans. NiO-related solar cells have reportedly been fabricated by sputtering without intentional heating, making the process safe and suitable for application to flexible substrates. From the technological viewpoint, a highly transparent undoped NiO film is an insulator; therefore, its electrical properties have not been clarified. In this study, we prepared NiO film, investigated its carrier density, and fabricated NiO-related solar cells. Approximately 200-500-nm-thick NiO films were grown by RF reactive sputtering on soda lime glass (SLG) substrates. Ar and O2 were used as the sputtering and source gases, respectively. The fabricated NiO-related solar cells generated electricity under illumination. Optical transmittance of >70% was obtained in the wavelength range of 500-800 nm. This result indicated their possible effectiveness in visible-light-transparent solar cells.

Fluorinated SnO2 is one of the most common TCO materials in use to today. Under the Pilkington brand name it is typically referred to as TEC series glass. Its primary use is in architecture glass as a heat reflector due to its low emissivity (Low-E) characteristics. Another large application is as an inexpensive substrate for CdTe solar cells and this material is under exploration for many energy production technologies. This SnO2:F film is typically deposited on soda lime glass in a layer from a few hundred Å to 1µm in thickness depending on the application. In our work on laser scribing CdTe solar cells we have found what appears to be an unpublished laser material interaction that allows precise laser etching of the film to an arbitrary thickness with high uniformity. This precise and efficient laser etching mechanism allows arbitrary reduction of the film thickness in a controlled manner on the scale of a few nm. Such precise etching control raises the possibility of several applications. As one example it is possible to control the apparent color of the glass through thin film interference by controlling the film thickness. Other examples could include direct writing of holographic images, writing passive electronic components, and creating structures for active transparent electronic devices and the like. In this work we propose a theoretical basis for this selective laser etching mechanism and demonstrate several applications from decorative glass marking to direct writing of phase gratings.

Piezoelectric films are widely used in variety of applications, including ultrasonic sensor, etc.¹⁾ Recently, the energy harvesting using piezoelectric films is attracted as one of the method to resolve the energy issues, especially the isolated situation without power supply network. The piezoelectric generator prefers the use of environmental friendly piezoelectric materials on flexible substrate by low-cost manufacturing process. Pb(Zr,Ti)O₃ has been widely investigated for this purpose, but the toxicity of lead is recognized to be a serious problem. Therefore, various lead-free materials have been investigated. Piezoelectric (K,Na)NbO₃ is attended because the property is superior to other lead-free materials.²⁾ We reported on the successful growth of the epitaxial (K,Na)NbO₃ films on (100)_cSrRuO₃//(100)SrTiO₃ single crystal substrates at 240 °C by hydrothermal method.³⁾ This method is the wet process and it is possible to prepare films below 300°C. The low temperature process indicates the low energy consumption process with low cost. For the energy harvesting applications, we deposit (K,Na)NbO₃ films on flexible substrates along (100) orientation because the piezoelectric properties strongly depended on the crystal orientation. We succeeded in the growth of {100} one-axis oriented (K,Na)NbO₃ films on flexible metal substrates using the LaNiO₃ buffer layer for orientation control. LaNiO₃ has a self (100) orientation characteristics.⁴⁾ The ferroelectric and piezoelectric properties were enhanced by the orientation control as well as the deposition rate. Furthermore, (K,Na)NbO₃ films were also deposited on flexible organic substrates using same buffer layer. In our presentation, we also report on the energy harvesting property of (K,Na)NbO₃ films on flexible substrates on that day. 1) N. Settter et al., J. Appl. Phys. 100 (2006) 051606 1-46. 2) Y. Saito et al., Nature 432 (2004) 84-87. 3) T. Shiraishi et al., Jpn. J. Appl. Phys. 50 (2011) 09ND11-1-4. 4) K. Takahashi et al., Mater. Res. Soc. Symp. Proc. 833 (2005) G1.9.1-G1.9.6

Thin-film solar cells are promising for renewable energy applications due to their low material usage compatible with terawatts-level deployment, and inexpensive manufacturing potential. Cuprous oxide (Cu₂O) is a promising earth-abundant semiconductor with ~20% single-junction theoretical maximum photovoltaic energy conversion efficiency. However, due to non-ideal junction formation with unfavorable band alignments and interfacial defects, only ~4% efficiency has been achieved so far. The non-ideal junction formation induces a significant energy loss by interfacial recombination. This problem is common to most non-epitaxial heterojunction devices, including conventional CIGS and CdTe solar cells. In this contribution, we develop a high-efficiency all-metal-oxide (Cu₂O-ZnO) thin-film solar cell. We design and synthesize a novel buffer layer that can mitigate the interfacial recombination problem by tailoring its bandgap and relative conduction-band energy level to its neighboring layers. This novel oxide-based buffer layer is deposited by using atomic layer deposition technique in between the Cu₂O and ZnO layers. The bandgap is tuned (3.4 - 4.3 eV) by controlling atomic composition of the buffer layer. The atomic composition of the layer is measured by Rutherford backscattering spectroscopy and the band-alignment is characterized by X-ray photoelectron spectroscopy. We study the effect of the buffer layer by measuring the I-V characteristics and internal quantum efficiency of the devices and performing optical simulations of Cu₂O devices with the buffer layer. We report the strong enhancement of energy conversion efficiency by increased open-circuit voltage and fill-factor.

Many applications require materials with specific structure dependent properties, e.g. for battery storage, metastable hexagonal phases of tungsten trioxide and molybdenum trioxide are more effective than the stable orthorhombic phase[1-3]; orthorhombic phase of MoO3 has high selective and sensitive to ammonia[4] whereas monoclinic WO3 is a good acetone sensor[5]. Such structural changes occur due to polymorphic transitions in binary metal oxides. Thus it is essential to theoretically predict the conditions of polymorphic transitions so that materials can be affectively used in engineering applications. Temperature and pressure are the two main factors affecting the bulk state phase transformation of materials. For nanomaterials it has been observed that particle size and temperature are the main factors affecting the phase transformation, e.g. γ-Fe2O3 to α-Fe2O3[6,7], monoclinic to orthorhombic transformation in MoO3[8], anatase to rutile transformation in Titania[9], γ to α Alumina transformation[10]. We compile from literature the main factors which affect the phase stability of a nanocrystalline binary metal oxide. A heuristic approach to formulate particle size is put forth. Factors like surface energy[11], surface tension[12] and particle shape[13] are considered. The model fits well with the experimental results for nanocrystalline alumina and titania. Such an approach can be applied to predict the particle size dependent stability of a phase at known temperature range. Knowing the particle size and temperature range of phase stability of a polymorph would increase the predictability and reliability of materials for energy applications. References [1] Jimei Song, Xiong Wang, Xiaomin Ni, Huagui Zheng, Zude Zhang, Mingrong Ji, Tao Shen, Xingwei Wang. Mat. Res. Bull. 40(10), 1751-1756(2005) [2] S.H. Lee, M.J. Seong, C.E. Tracy, A. Mascarenhas, J.R. Pitts, S.K. Deb. Sol. St. Ion., 147, 129(2002) [3] K.P. Reis, A. Ramanan, M.S. Whittingham, J. Sol. St. Chem. 96, 31-47(1992) [4] A.K. Prasad, D. Kubinski, and P. I. Gouma. Sensors & Actuators B 9, 25-30(2003) [5] L. Wang, A. Teleki, S.E. Pratsinis, and P.I. Gouma. Chem. Mater., 20(15), 4794-4796(2008) [6] Fu Su Yen, Wei Chien Chen, Janne Min Yang, and Chen Tsung Hong. Nano Letters 2(3), 245-252(2002) [7] Ozden Ozdemir and Subir K. Banerjee. Geophysics research letters 11(3), 161-164(1983) [8] A. K. Prasad. Phd thesis, Stony brook university, 2005 [9] P. I. Gouma and M. J. Mills. J. Am. Ceram. Soc., 84(3), 619-622(2001) [10] Shuxue Zhou, Markus Antonietti, and Markus Niederberger. Small 3(5), 763(2007) [11] Hengzhong Zhang and Jillian F. Banfield. J. Mater. Chem. 8, 2073(1998). [12] Varghese Swamy, Alexei Kuznetsov, Leonid S. Dubrovinsky, Rachel A. Caruso, Dmitry G. Shchukin, and Barry C. Muddle. Phys. Rev. B 71, 184302(2005) [13] Michel Wautelet and Aram S. Shirinyan. Pure Appl. Chem. 81(10), 1921-1930(2009)

Through the use of high dielectric constant (~10,000) and high breakdown field (~3 MV/cm) ceramic glass composites, we have demonstrated low cost, large scale, and high gravimetric energy storage densities for use in renewable power applications. The ceramic material systems explored include the titanates and lead-based relaxors and each were combined with alkali free glasses. Both of these systems possess unique properties for large-scale capacitive energy storage. For example, the addition of glasses in a core-shell geometry provides for especially uniform multilayer coatings and without extrinsic defects at the ceramic interface that would cause cascading shorts. By using specific stoichiometry glasses, it is possible to heterogeneously nucleate the glass as a ceramic on the ceramic surface, know as glass-ceramics, providing the improved dielectric packing (composite dielectric constant) beyond random closed packed as well as improving as well as maintaining many of the properties of glass alone, including the low processing temperature and passivation of dangling bonds. With these combinations, we were able to demonstrate graviemetric energy storage densities approach Pb acid batteries.

In this work we will describe a new method to analyse oxygen surface exchange kinetics in oxide materials in the form of epitaxial thin films by exploring subtle cell parameters variations induced by changes in oxygen stoichiometry of the material. The method consists of continuously analysing the X-ray diffraction pattern of particular film reflections by using a linear X-ray fast detector in static position while exposing the sample to sudden changes in the pO2 of the atmosphere at elevated temperatures. With this method we have been able to follow cell volume changes as small as 10 ppm in time intervals as short as 10 sec in La2NiO4+δ epitaxial films. This method provides a simpler and complementary tool to existing Electric Conductivity Relaxation (ECR) and Isotopic Exchange depth profiling (IEDP) experiments currently used for analyzing oxygen surface exchange kinetics and diffusion in transition metal oxide compounds. Variations of the out-of-plane cell parameter of epitaxial films of c-axis oriented La2NiO4+δ deposited by PLD on SrTiO3(100) substrates have been explored at different temperatures, and extracted the exchange rates k, as well as corresponding activation energies. These values are compared to those obtained from ECR measurements on the same type of samples. Besides, the ability of this method to selectively analyse the volume changes of individual components of the film structure enables to study the oxygen exchange not only at the surface but also at the interface between two solid materials in bi-layers without the coupling that normally exists in conductivity measurements.

Oxide materials are often used as catalysts for redox reactions, but progress in this field has in part been hindered by our inability to connect macroscopic reaction kinetics with details of the surface structure. Single crystal model systems, including epitaxial thin films, are sometimes used in order to simplify the structure, however the small active area limits sensitivity to reaction products. To overcome this challenge, we have developed experiments that are conducted with a small volume, in-operando reaction chamber, mounted to a six-circle diffractometer at the Advanced Photon Source. This setup allows simultaneous x-ray scattering / spectroscopy and monitoring of the catalytic products by gas chromatography - mass spectrometry. We have studied the CO oxidation reaction on coherently-strained epitaxial BaTiO₃ / (001) SrTiO₃ heterostructures, which are compared with resonant x-ray scattering conducted at the Ba K-edge. Results indicate that variations in the oxygen partial pressure alter the magnitude of ferroelectric polarization (cation-anion displacements) likely through interactions with screening charges at the film surface. The implications of this finding on (001) BaTiO₃ surface reactivity will be discussed. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357 (BES-DMSE).

Different p-type doped organic/inorganic semiconductors have widely been explored as hole transporting layer in solar photovoltaic cells. Developing materials with tunable optical bandgap and carrier mobility is essential to achieve PV high performance. Layered semiconductors such as MoS₂ and WS₂ (typically, n-type doped) are potential candidates for solar energy-harvesting application. Graphene oxide, a p-type doped layered semiconductor, can be synthesized through chemical routes. We investigate oxygen plasma processing of chemical vapor deposition assembled multilayer graphene (ML-G) to convert into p-type doped multilayer graphene oxide (ML-GO) semiconductor with an optical bandgap of ~3.6 eV. A heterojunction solar cell employing CdSe quantum-dot-coated TiO₂ as an electron transport layer and ML-GO as a hole transporting layer is presented in this work. The heterojunction solar cell having active layer stack of ITO/TiO2/CdSe/ML-GO/Pt was studied by electrical and optical characterizations (I-V and EQE), suggesting that ML-GO as p-type doped semiconductor exhibits effective hole transport. The electronic bonds between carbon and oxygen are covalent in ML-GO in which presence of both sp² and sp³ hybridizations makes the film heterogeneous. Lateral carrier transport occurs in GO film between localized states dominated by hopping transport process. In our study, the low-power oxygen plasma treatment oxidizes ML-G film and resulted in ML-GO. However, we expect that oxidation was not incurred uniformly in ML-G film, and hence a significant number of localized electronic states present in the film. The effect of ML-GO thickness and oxygen content on electron transport, carrier recombination, and overall photovoltaic performance will be discussed.

Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometer interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles in the 0.5 V to 1.4 V range, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3 %, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.

Hematite has serious drawbacks to be used as photoelectrochemical (PEC) electrode materials because of its poor electronic conductivity and extremely short hole diffusion length. In order to effectively improve the PEC efficiency of hematite, one has to design the morphology in such a way that most of the photo-generated holes can reach the interfaces with the electrolyte, and at the same time, the photo-generated electrons can be readily transferred to the electrode. Here, we sought the solutions by forming mesoporous thin films of hematite (MHF) with the wall thickness thinner than or close to the hole diffusion length and introducing an appropriate underlayer between the MHF and the FTO electrode. All of the underlayer materials studied so far are limited to large band gap insulators with the conduction band minima (CBMs) higher than that of hematite. This approach is workable only under the premise that the amount of photo-generated electrons from the hematite is small. In other words, if the photo-generated holes can be efficiently removed so that there is a large flow of electrons from hematite to FTO, the underlayer material has to be able transmit the electrons efficiently; new underlayer materials with low-lying CBMs need to be found. We considered that SnO2 could be fully competent for doing this function due to its wide band gap to transmit photons with suitable band position and similar chemical composition with FTO. Therefore, the heterojunction SnO2/Fe2O3 film was designed for effective charge carrier separation and charge carrier transfer. Our results demonstrated nearly 300% enhancement in PEC response has been achieved while exploiting a SnO2 layer sandwiched between MHF and FTO.

We present a novel method for femtosecond-laser doping of titanium dioxide (TiO2) for above bandgap absorptance by irradiating titanium metal in the presence of oxygen and metal dopants. With a bandgap of 3.2 eV for the anatase crystalline phase, TiO2 most strongly absorbs in the UV range (lambda; < 387 nm). However, doping with metals has been shown theoretically to create intermediate states in the bandgap. Using femtosecond laser doping on titanium in oxygen, we produce laser-induced periodic surface structures. We present compositional data from x-ray photoelectron and Raman spectroscopy and structural data from scanning electron microscopy. Using a three-electrode setup, we present photoelectrochemical results and show enhanced oxidation under illumination. Our research presents an innovative approach using laser scanning techniques to alter the structure of TiO2 and generate a new material for visible-light photocatalysis.

Water splitting using photoelectrochemical cell based on semiconducting materials can be an effective way for harvesting solar energy which is stored in the form of H₂ bond. Hematite is a promising photoanode material for its appropriate band edge position for water oxidation, excellent electrochemical stability, and ability to absorb visible light due to its low band gap (2.1 eV). However, hematite has limited photoactivity due to the low conductivity, and external doping with electron donor e.g., Ti, Si, Sn, etc. has been tried to enhance the conductivity. Intrinsic defects for nonstoichiometric metal oxide, i.e., oxygen vacancy, can also be an electron donor, so it can enhance the photoactivity. We investigated the effect of intrinsic defects as a doping on photocurrent density in hematite by controlling the oxygen partial pressure during annealing. Various oxygen partial pressures can change the concentration of oxygen vacancy which can act as donor centers. Hematite film was fabricated on FTO glass substrate by spray pyrolysis with 10 mM FeCl₃ precursor solution on 400 ^oC hot plate. To make oxygen-deficient and iron-rich hematite, we developed the two step annealing process; firstly hematite films were annealed at 550 ^oC, for 1 hour with high vacuum to reduce the hematite films, and then oxygen was flowed into the chamber with controlled oxygen partial pressure (pO2, 0.05 mTorr, 5 mTorr, 50mTorr, 500mTorr, Air ambient) for 2 hours to oxidize reduced films. The post annealing process which involves reduction step is quite effective for increasing carriers in hematite by increasing oxygen vacancy. Hematite annealed with ambient air showed extremely low photocurrent density, but with the new process, it showed dramatically increased value with 8 times or more. In the annealing process that does not involve reduction step, initial state is hematite (Fe₂O₃), but with reduction step, initial state becomes magnetite (Fe₃O₄), which made hematite films more oxygen deficit even when the films were oxidized. In controlling oxygen partial pressure, there was clear tendency that lower partial pressure made higher photoactivity. Especially, annealed at 0.05 mTorr showed photocurrent density of 1.2 mA/cm², whereas annealed at ambient air showed that of 0.76 mA/cm². XPS analysis showed that the area of Fe²⁺ peak increased with decreasing oxygen partial pressure. Lower oxygen partial pressure induced smaller oxidation number of Fe that made Fe²⁺ more favorable. Since the conduction mechanism of hematite is small polaron hopping through Fe³⁺/Fe²⁺ valence exchange, abundance of Fe²⁺ phase can enhance the conductivity.

TiO2/TiN multi-layered thin film stacks were fabricated by RF reactive sputtering with the number of layers changing from 18 to 36. The morphological, compositional, electronic, optical, and photoactive performance for each of the films was measured and compared. It was shown that several parameters greatly affected the photoactive performance of the films such as the number of layers, the thickness of the layers, and the ratio of TiN to TiO2. In particular, visible light activity was achieved for many of these films, indicating that the interface between the two layers may act as a pseudo-nitrogen doped TiO2 layer, allowing for visible light absorption. Furthermore, when the light source was turned off, several of the films held a positive current for up to 60 seconds in the dark. The long charge relaxation time is increased when the thickness of the TiN is increased in the multi-layer stacks, indicating that there is appreciable charge separation due to the heterojunction between TiO2 and TiN, caused by appreciable differences in their electronic structures. Therefore, this novel thin film configuration is able to improve on two key challenges in photocatalysis; visible light absorption and activity, and extended charge carrier lifetime. The unique ability to hold charge in the dark can have far reaching applications in many optoelectronic devices.

Visible-light-responsive titanium dioxide (TiO2) photocatalysts films were fabricated by applying a low pressure H2-plasma treatment method on Sol-gel TiO2 films. As compared to the pristine TiO2 films, the treated samples exhibit enhanced visible light absorption and excellent photocatalytic activity for degradation of methylene blue and reduction of CO2, especially under visible light (>400nm). As revealed by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) measurements, the H2-plasma treated samples do not exhibit any phase transition, or changes in particle size and chemical composition compared to the original TiO2 films. It is proposed that the H2-plasma treatment-induced defects states at the TiO2 particles surface layer contribute to the enhanced visible light absorption, thereby greatly improving the photocatalytic activity.

Hydrogen production from water using semiconductor materials is one of the most promising methods for future clean energy and chemical industry. The reaction that requires photo excitation of electrons from the valence band to the conduction has many limitations due to multiple competing processes including a fast electron-hole recombination rate, a slow rate of electron injection into the valence band and a fast rate of reverse reaction (to water) on the metal/semiconductor interface [1]. Among the many materials used and proposed for this reaction semiconductor photonic band gap (PBG) materials hold a considerable potential. PBG materials are materials where light is forbidden to propagate at their band gap [2]. The design of semiconductor PBG materials with their electronic bang gap (EBG) coinciding with the PBG is poised to dramatically increase their photo-reaction activity because of the expected suppression of the electron-hole recombination. In this work we have prepared, characterized and tested TiO2 PBG materials on which nano-particles of Au are deposited. Over 2 wt % Au/TiO2 with a PBG of 360 nm in air (theta;= 0o) and in water (theta;= 60o) a high rate of hydrogen is produced from water/ethanol (volume ratio = 99/1). Equivalent materials but with larger PBG show weaker activity; similar to that observed for Au/TiO2 anatase nano-particle powder [3]. References 1. K. A. Connelly and H. Idriss, Green Chemistry, 14 , 260-280 (2012). 2. E. Yablonovitch, Phys. Rev. Lett. 58, 2059-2062 (2012) 3. M. Murdoch, G.W.N. Waterhouse, M.A. Nadeem, M.A. Keane, R.F. Howe, J. Llorca, H. Idriss, Nature Chemistry, 3, 489-492 (2011)