Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Niobium and tantalum oxides as model materials for resistive switching effect

Celem współczesnej nauki jest znalezienie nowych rozwiązań dla wciąż zmieniającego się świata. Jednym z wyzwań jakie stawiają sobie naukowcy jest znalezienie nowego materiału dla pamięci nieulotnych o dużej gęstości zapisu, który w coraz to mniejszej scali, nanoskali, pozwoli na zapisanie coraz większej ilości danych. Takimi materiałami mogą być tlenki metali przejściowych, które bazując na reakcji redoks, wykazują zdolność do zmiany oporu pod wpływem przyłożonego pola elektrycznego. Jednakże wiedza o fizycznych podstawach tego zjawiska jest wciąż ograniczona. Do tej pory nie zostało jasno i klarownie przedstawione wyjaśnienie natury zjawiska przełączania rezystywnego, a co za tym idzie jego aplikacja w urządzeniach elektronicznych, może być nadal problematyczna. Niniejsza praca doktorska została poświęcona tlenkom metali przejściowych jakim są tlenki niobu i tantalu. Chociaż jak się często podkreśla są to materiały wciąż badane od wielu lat i wydaje się, że posiadamy już duży zasób wiedzy na ich temat, to nadal są miejsca, gdzie materiały te potrafią nas zaskoczyć. Praca ta została podzielona na dwie części. Pierwsza została poświęcona monokryształowi Nb₂O₅ natomiast w drugiej badania były skoncentrowane na cienkich warstwach Nb-O i Ta-O. W pracy przedstawiono wyniki badań podstawowych właściwości fizykochemicznych materiału przed oraz po redukcji termicznej. Temperatury od 800°C -1000°C znacząco redukują monokryształ Nb₂O₅. Natomiast w cienkich warstwach amorficznych Nb-O czy Ta-O o złożonej strukturze wewnętrznej, w której skład wchodzą warstwy pięciotlenków, zaobserwowano ten efekt w znacznie niższych temperaturach. Nawet niewielka zmiana temperatury (300°C dla Nb-O i 600°C dla Ta-O) wpływa na stopień redukcji warstwy. Wpływ temperatury ma również silenie znaczenia na przewodnictwo. W cienkich warstwach zaobserwowano przełączanie oporności typu bipolarnego. Natomiast w krysztale ten sam efekt również był zauważalny, lecz znacznie słabszy. Można było, podobnie jak w cienkich warstwach, zmodyfikować jego powierzchnie w celu zapisania informacji przy pomocy igły z mikroskopu sił atomowych. Reasumując przedstawione wyniki badań w tej pracy pozwalają w szerszy sposób spojrzeć na problem chemicznej i strukturalnej niestabilności tlenków metali przejściowych (Nb, Ta) w krysztale jak i cienkich warstwach. Pokazują wpływ tych zmian na ich przewodnictwo, które może być lokalnie kontrolowane, pozwalając na łatwiejszą aplikacje tych materiałów

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Robust Simulation of a TaO Memristor Model

This work presents a continuous and differentiable approximation of a Tantalum oxide memristor model which is suited for robust numerical simulations in software. The original model was recently developed at Hewlett Packard labs on the basis of experiments carried out on a memristor manufactured in house. The Hewlett Packard model of the nano-scale device is accurate and may be taken as reference for a deep investigation of the capabilities of the memristor based on Tantalum oxide. However, the model contains discontinuous and piecewise differentiable functions respectively in state equation and Ohm's based law. Numerical integration of the differential algebraic equation set may be significantly facilitated under substitution of these functions with appropriate continuous and differentiable approximations. A detailed investigation of classes of possible continuous and differentiable kernels for the approximation of the discontinuous and piecewise differentiable functions in the original model led to the choice of near optimal candidates. The resulting continuous and differentiable DAE set captures accurately the dynamics of the original model, delivers well-behaved numerical solutions in software, and may be integrated into a commercially-available circuit simulator

A spatial and economic analysis for land conservation planning

Rapid loss of biodiversity has been mainly attributed to the loss of habitat and habitat fragmentation. Conservation efforts have increased over the years but the rate of biodiversity loss has yet to slow down. Biologists attribute this to setting conservation targets without considering scientific recommendations. Practitioners on the other hand claim scientific recommendations to ignore socioeconomic costs and therefore are unrealistic. In the United States, protected areas primarily include public lands but conservation easements have become increasingly more popular. However, the effectiveness of conservation easements have been debatable and has garnered much discussion. Three related issues in land conservation planning is examined. The first essay includes a cost-effective theoretical framework for selecting conservation areas that considers the persistence probabilities of the wildlife species. The theoretical model presented in the essay emphasized the need for location-based conservation targets that would simultaneously answer where to conserve and how much? A simplistic case study is presented to show the implementation of the framework in the Cacapon River Watershed portion of Morgan County. The model presented for conservation area selection provides a guideline for an optimal allocation of conservation areas considering both the biological and the economic recommendations simultaneously. The second essay employs a spatial hedonic pricing model to determine if different open space types: permanently preserved public areas, conservation easements, and developable open space have different price premiums associated with them for the Cacapon River Watershed portion of Morgan County, West Virginia. A spatial hedonic model is used to account for the spatial dependence in the residential property values. A spatial Durbin model (SDM) was used to estimate the hedonic model. The SDM allows calculating the feedback and spillover effects not possible in non-spatial models. Findings for the study area showed that in heavily forested residential area like the Cacapon River Watershed portion of Morgan County, the proximity to open space may not hold a price premium. The vegetation within the residential property itself could substitute the need for the proximity to open space types. The third and final essay assessed the current private land conservation efforts of land trusts in West Virginia. Data from a survey of individual conserved properties by land trusts operating within West Virginia was used to assess characteristics of the conserved properties. A negative binomial model was employed to assess the factors that determined the distribution of conservation efforts by land trusts. The findings showed that the conserved properties were located in areas high in species richness; however, the conserved properties were not in proximity to public protected areas. The dissertation considered multiple facets of the land conservation planning problem and attempted to provide insights into conservation planning. The study contributes to existing literature related to the optimal area selection problem, hedonic property valuation, and conservation easement assessment

Anodic HfO2 crossbar arrays for hydroxide-based memristive sensing in liquids

The development of miniaturized and portable sensing devices is crucial to meeting the high processing capacity demands of contemporary computing systems. Hence, the conceptualization of memristive sensors for hydroxide-containing liquids is proposed in this study. Metal-insulator-metal (MIM) structures were formed on electrochemically anodized Hf thin films with Pt patterned as top electrodes. These MIM memristive structures were integrated into a crossbar array, allowing the investigation of a high number of potential memristor sensors. The MIM structures have demonstrated sensing possibilities in the detection of the hydroxyl ion in D-glucose, used as a standard solution. The sensing method was based on the resistive state ratio extracted from I-U sweeps measurements. Analytical characterization of the memristor sensor was done based on the resistive state ratio in relation to different concentrations of a standard solution drop cast directly on the surface of the device. Linearity was found for D-glucose concentrations ranging from 10 mM to 80 mM with a reasonable corresponding correlation factor (R2=0.96809). Additionally, D-glucose incorporation in anodic oxide was studied by XPS to investigate its effect on conductive filaments formation. A carbon bonded by a single covalent bond to oxygen (O-C-O) was detected, confirming the proposed sensing mechanism defined by the glucose penetrating the oxide/electrode interface

NASA Tech Briefs Index, 1977, volume 2, numbers 1-4

Announcements of new technology derived from the research and development activities of NASA are presented. Abstracts, and indexes for subject, personal author, originating center, and Tech Brief number are presented for 1977

Filamentary Threshold Switching In Niobium Oxides

Two-terminal metal/oxide/metal (MOM) structures exhibit characteristic resistance changes, including non-volatile memory and volatile threshold switching responses when subjected to electrical stress (i.e., voltage or current stimuli), which are of interest as active elements in non-volatile memory arrays and neuromorphic computing. Recently, the threshold switching response in MOM devices based on vanadium oxides and niobium oxides have attracted particular attention due to their simple structure and reliability. Interestingly, specific phases of these oxides (e.g., VO2, NbO2 etc.) exhibit a metal-insulator transition (MIT) which causes dramatic changes in their intrinsic properties, including electrical and thermal conductivities, and often arguably reported as the dominant cause of the observed threshold switching response. While this response has been extensively studied for VO2, but the low transition temperature (~ 340K) limits their use only to low temperature microelectronics applications. In contrast, NbO2 has a much higher transition temperature ~ 1070 K, and NbO2 and other NbOx phases have drawn recent attention due to their reliable threshold switching characteristics. The resistance changes in MOM structures are often initiated by a one-step electroforming process that forms a filamentary conduction path. Knowledge about the structure, composition and spatial distribution of these filaments is essential for a full understanding of filamentary resistive/threshold-switching and for effective modelling and optimisation of associated devices. Additionally, NbOx-based devices exhibit a wide range of resistive and threshold switching responses that critically depend on operating condition, composition and device geometry. Thus, a proper understanding of these factors is important for achieving reliable switching with desired characteristics. This thesis focuses on understanding the electroforming process and subsequent threshold switching responses in NbOx by employing different techniques, including electrical testing, and thermo-reflectance imaging. At first, a simple means of detecting and spatially mapping conductive filaments in metal/oxide/metal cross-point devices is introduced and the utility of this technique is demonstrated to identify distinct modes of electroforming in low- and high-conductivity NbOx films. After that, the role of metal/oxide interface reactions on the post-forming characteristics of reactive-metal/Nb2O5/Pt devices is demonstrated. Specifically, devices are shown to exhibit stable threshold switching under negative bias but the response under positive bias depends on the choice of metal. Then, the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 are compared. In particular it is shown that doping offers an effective means of engineering the device response. Based on temperature dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature. Finally, the physical origin of the discontinuous 'snapback' NDR is investigated. Specifically, it is shown that the snapback response is a direct consequence of current localisation and redistribution within the oxide film. Furthermore, it is demonstrated that material and device dependencies are consistent with predictions of a two-zone parallel memristor model of NDR which is based on a non-uniform current distribution after electroforming. These results advance the current understanding of threshold switching response in amorphous NbOx films, and provide a strong basis for engineering devices with specific NDR characteristics. Significantly, these results also resolve a long-standing controversy about the origin of the snapback response which has been a subject of considerable debate

Synthesis and Characterization of NbOx Thin Film

Niobium Oxides (NbOx) thin films have been deposited on silicon (100) and quartz substrates by magnetron sputtering using metallic Nb target in an optimized argon-oxygen atmosphere. To date, suitable deposition conditions for crystalline NbOx thin films by reactive sputtering techniques have not been established. Therefore, this work investigates the dependence of structure-property relations on the key deposition parameters towards establishing optimum deposition conditions for the growth of NbOx crystalline films. It is found that a substrate temperature of 720 °C, low gas pressures of 8 mtorr and a target to substrate distance of 45 mm gives NbOx thin films with good homogeneity and a high degree of crystallinity. X-Ray Diffraction (XRD) and Raman spectroscopy confirmed the tetragonal phase of NbO2 and orthorhombic phase of Nb2O5 for similar deposition temperatures. Scanning Electron Microscopy (SEM) observations indicate that NbO2 has a unique nanoslice structure while Nb2O5 has a flake-like structure. Effect of post deposition annealing on the structural properties are also investigated in this dissertation. It is found that structural changes in NbO2 samples are prominent after annealing in air or Ar atmosphere. In contrast, well crystalline Nb2O5 samples do not show significant changes after annealing in air atmosphere. However, amorphous Nb2O5 can be transformed to tetragonal NbO2 after annealing in Ar atmosphere (reduction). The electrical conductivity and optical transmittance of the films have been investigated and found to be dependent on the oxygen gas content; the conductivity increases, and the optical transmittance decreases with increasing O2 gas content. Optical constants of the films were calculated by fitting model calculations to experimental transmittance data using the modified Swanepoel technique. The nanohardness and stress in the films were measured by nanoindentation and an optical profilometer, respectively. Nano hardness and stress in the film show no large dependence on the oxygen gas content except at high oxygen gas content. V being one of the prominent transition metals, is doped with NbO2 to evaluate the structural variations and the optical transmittance. It is found that higher sputtering power of V is required to be doped with NbO2 and V contents have less effect on optical transmittance of NbO2