Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications
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Transition metal oxides, TiO2 and Ta2O5, are two of the most extensively studied wide bandgap semiconductor materials (with high work functions). Due to their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation, their application in heterogeneous photocatalysis has attracted a lot of attention. These oxides are also great components in the field of electronic devices such as field effect transistors, solar cells, and more recently advanced memory devices. Here, we focus on ultrasmall nanoclusters (< 5 nm) and nanocrystalline thin films of defect-rich TiO2 and Ta2O5 and their applications as high-performance photocatalysts in photoelectrochemical water splitting reactions and as resistive switching materials in memory applications. The present work is divided into two main parts. In the first part, ultrasmall nanoclusters (below 10 nm) of defect-rich TiO2 and Ta2O5 are synthesized using a gas phase aggregation technique in a nanocluster generation source based on DC magnetron sputtering. With a careful optimization of the deposition parameters such as aggregation zone length (condensation volume), Ar gas flow rate, deposition temperature and source power, we are able to produce metal/metal oxide nanoclusters with a narrow size distribution. As most of these as-grown nanoclusters are negatively charged, it is possible to conduct size-selection according to their mass-to-charge ratio. Using a quadrupole mass filter (directly coupled to the magnetron source), we achieve precise size-selection of nanoclusters, with the size distribution reduced to below 2% mass resolution. The nearly monosized nanoclusters so produced are deposited onto appropriate substrates to serve as the photoanodes for photoelectrochemical water splitting application. We demonstrate, for the first time, that the precisely size-selected TiO2 nanoclusters can be deposited on H-terminated Si(001) in a soft-landing condition and they can be used as highperformance photocatalysts for solar harvesting, with greater enhancement in the photoconversion efficiency. Three different sizes of TiO2 nanoclusters (4, 6 and 8 nm) are synthesized with appropriate combinations of aggregation length and Ar flow rate. Despite the low amount of material loading (of ~20% of substrate coverage), these supported TiO2 nanoclusters exhibit remarkable photocatalytic activities during photoelectrochemical water splitting reaction under simulated sunlight (50 mW/cm^2). Higher photocurrent densities (up to 0.8 mA/cm^2) and photoconversion efficiencies (up to 1%) with decreasing nanocluster size (at the applied voltage of –0.22 V vs Ag/AgCl) are observed. We attribute this enhancement to the presence of surface defects, providing a large amount of active surface sites, in the amorphous TiO2 nanoclusters as-grown at room temperature. We have further shown that the incorporation of metallic nanoclusters with the semiconductor photocatalysts can enhance the photoconversion efficiency. In this work, we have co-deposited surface oxygen deficient Ta2O5 or TaOx nanoclusters along with Pt nanoclusters of similar nanocluster size (~5 nm), the latter used as a promoter. The electron-hole pairs generated in the water splitting reaction can be effectively separated and stored with the presence of Pt nanoclusters, while the increase in Pt loading as a promoter can enhance the reaction by providing a large number of electrons for H2 evolution. However, loading too much Pt nanoclusters could actually reduce the photoresponse, which is due to blocking of photosensitive TaOx surface by excess Pt nanoclusters. In both cases, the photoconversion efficiency could potentially be enhanced at least 5 times by increasing the amount of nanocluster loading from 20% coverage to a monolayer coverage (e.g., by increasing the amount of deposition time for TiO2 and TaOx nanoclusters). Even higher photoconversion efficiency can be obtained with multiple layers of nanoclusters and by employing nanoclusters with even smaller size and/or with modification by chemical functionalization. These potential improvements could dramatically increase the photoconversion efficiency, making these nanocluster samples to be among the top photoelectrochemical catalysis performers. In the second part of the present work, we employ defect-rich nanocrystalline TiOx and TaOx thin films as active materials for resistive switching for memory application. Based on resistive switching principle, memristive devices (or memristors) provide the unique capability of multistep information storage. The development of memristors has often been hailed as the next evolution in non-volatile memories, low-power remote sensing, and adaptive intelligent prototypes including neuromorphic and biological systems. One major obstacle in achieving high switching performance is the irreversible electroforming step that is required to create oxygen vacancies for resistive switching. Using magnetron sputtering film deposition technique, we have fabricated the heterojunction memristor devices based on nanocrystalline TiOx and TaOx thin films (10-60 nm thick) with a high density of built-in oxygen vacancies, sandwiched between a pair of metallic Pt electrodes (30 nm thick). To avoid the destructive electroforming process and to achieve a high switching performance in the memristor device, we carefully manipulate the chamber pressure and ambient in deposition chamber during deposition to generate the required highly oxygen deficient semiconducting films. The films, as-deposited at room temperature, exhibit a crystallite size of 4-5 nm. In the fabricated Pt/TiOx/Pt memristors, a high electric field gradient can be generated in the TiOx film at a much lower electroforming voltage of +1.5 V, due in part to the nanocrystalline nature, which causes localization of this electric field and consequently enhanced reproducibility and repeatability in the device performance. After the first switching, consecutive 250 switching cycles can be achieved with a low programing voltage of ±1.0 V, along with a high ON/OFF current ratio, and long retention (up to 10^5 s). We further improve this TiOx memristor device by totally removing the electroforming step by fabricating an electroforming-free memristive device based on a heterojunction interface of TiOx and TaOx layers. In the Pt/TiOx/TaOx/Pt architecture structure (with Pt serving as the top and bottom electrodes), a high-κ dielectric TaOx layer is used to facilitate trapping and release of the electronic carriers, while a TiOx layer provides low-bias rectification as an additional oxygen vacancy source. With the incorporation of TaOx layer, the need for the electroforming step can be eliminated. More importantly, the resistance states of the device can be tuned such that switching between the high resistance state and the low resistance state can be achieved even smaller programming voltage of +0.8 V. With the low leakage current properties of TaOx, the high endurance (10^4 repeated cycles) and high retention capabilities (up to 10^8 s) can be enhanced manifold with highly stable ON/OFF current ratio. In both memristor devices, four different junction sizes (5×5, 10×10, 20×20 and 50×50 μm^2) have been evaluated according to their ON/OFF current ratio. We observe that the smaller is the junction size is, the higher is the current ratio. For the Pt/TiOx/TaOx/Pt memristor, we have also analyzed the thickness dependent effect of the switching behavior of devices with four different TaOx layer thicknesses (10, 20, 40 and 60 nm) and a TiOx layer thickness constant at 10 nm. The device with 10 nm thick TaOx (being amorphous in nature) shows unipolar switching with two SETs and two RESETs in one sweep cycle. This is in contrast to the bipolar resistive switching found in devices with the thicker TaOx films with a SET in the positive sweep and a RESET during the negative sweep. We further demonstrate that resistive switching can also occur at very low programming voltage (~50 mV), thus qualifying it as an ultralow power consumption device (~nW). The stable non-volatile bipolar switching characteristics with high ON/OFF current ratio and low power consumption make our devices best suitable for various analog and discrete programmable electric pulses. With the simplicity in the construction, the performance achieved for our memristors represents the best reported to date. This new class of defect-rich metal oxides nanomaterials with an ultrananocrystalline nature shows solid promises for various catalytic and electronic applications and, also, the simple, scalable roomtemperature device fabrication process makes this approach easily migratable further to transparent and/or flexible devices.
Cite this version of the work
Saurabh Srivastava (2016). Defect-Rich Size-Selected Nanoclusters and Nanocrystalline Films of Titanium (IV) Oxide and Tantalum (IV) Oxide for Efficient Photocatalyst and Electroforming-Free Memristor Applications. UWSpace. http://hdl.handle.net/10012/10487