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 fro