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Neuromorphic computing approaches become increasingly important as we address future needs for efficiently processing massive amounts of data. The unique attributes of quantum materials can help address these needs by enabling new energy-efficient device concepts that implement neuromorphic ideas at the hardware level. In particular, strong correlations give rise to highly non-linear responses, such as conductive phase transitions that can be harnessed for short- and long-term plasticity. Similarly, magnetization dynamics are strongly non-linear and can be utilized for data classification. This Perspective discusses select examples of these approaches and provides an outlook on the current opportunities and challenges for assembling quantum-material-based devices for neuromorphic functionalities into larger emergent complex network systems. [ABSTRACT FROM AUTHOR]

VO2 thin films were grown on conducting oxide underlayer SrRuO3 buffered SrTiO3 (111) and Si/SiO2 substrates, respectively, using sputtering. X-ray diffraction phi-scans revealed the epitaxial nature of the VO2 films grown on SrRuO3 buffered SrTiO3 and polycrystalline structure for films grown on SrRuO3 buffered Si/SiO2. X-ray photoelectron spectroscopy confirms a dominant presence of V4+ in both films and establishes a high-quality growth of single-phase VO2 films. Temperature and electric-field driven metal-insulator-transition in both the in-plane and out-of-plane configurations were investigated. Depending on the configuration, the resistance change across the metal-insulator-transition varies from a factor of 1.57–3. The measured resistance in each state as well as the magnitude of resistance change were similar during temperature and electric-field driven metal-insulator-transition. To shed light on the suppressed metal-insulator-transition characteristics due to the current shunting effect from conducting SrRuO3 bottom electrode, a distributed resistance network model is proposed and benchmarked against reports from the literature. The results demonstrate the growth of high-quality VO2 on conducting SrRuO3 layers and their electrical behavior, which is of particular interest for all-oxide electronic devices utilizing phase transitions such as resistive memory and neuromorphic oscillators. [ABSTRACT FROM AUTHOR]

We present a method for nanoscale thermal imaging of insulating thin films using atomic force microscopy (AFM), and we demonstrate its utility on VO2. We sweep the applied voltage V to a conducting AFM tip in contact mode and measure the local current I through the film. By fitting the IV curves to a Poole–Frenkel conduction model at low V, we calculate the local temperature with spatial resolution better than 50 nm using only fundamental constants and known film properties. Our thermometry technique enables local temperature measurement of any insulating film dominated by the Poole–Frenkel conduction mechanism and can be extended to insulators that display other conduction mechanisms. [ABSTRACT FROM AUTHOR]

The intriguing physics of vanadium dioxide (VO2) makes it not only a fascinating object of study for fundamental research on solid-state physics but also an attractive means to actively modify the properties of integrated devices. In particular, the exceptionally large complex refractive index variation produced by the insulator-to-metal transition of this material opens up interesting opportunities to dynamically tune optical systems. This Perspective reviews some of the exciting work done on VO2 for nanophotonics in the last decade and suggests promising directions to explore for this burgeoning field. [ABSTRACT FROM AUTHOR]

We report on experimental realization of p-n heterojunctions based on p-type GaN, and an n-type correlated oxide, VO2. The band offsets are evaluated by current-voltage and capacitance-voltage measurements at various temperatures. A band diagram based on the conventional band bending picture is proposed to explain the evolution of the apparent barrier height from electrical measurements and it suggests that the work function of VO2 decreases by ∼0.2 eV when it goes through the insulator to metal transtion, in qualitative agreement with Kelvin force microscopy measurements reported in literature. The frequency-dependent capacitance measurements allow us to differentiate the miniority carrier effect from the interface states and series resistance contributions, and estimate the minority carrier lifetime in insulating phase of VO2 to be of the order of few microseconds. The nitride-oxide based p-n heterojunctions provide a new dimension to study correlated-electron systems and could be of relevance to emerging electronic devices that exploit collective phenomena. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) is a correlated electron system that features a metal-insulator phase transition (MIT) above room temperature and is of interest in high speed switching devices. Here, we integrate VO2 into two-terminal coplanar waveguides and demonstrate a large resistance modulation of the same magnitude (>103) in both electrically (i.e., by bias voltage, referred to as E-MIT) and thermally (T-MIT) driven transitions. We examine transient switching characteristics of the E-MIT and observe two distinguishable time scales for switching. We find an abrupt jump in conductivity with a rise time of the order of 10 ns followed by an oscillatory damping to steady state on the order of several μs. We characterize the RF power response in the On state and find that high RF input power drives VO2 further into the metallic phase, indicating that electromagnetic radiation-switching of the phase transition may be possible. We measure S-parameter RF properties up to 13.5 GHz. Insertion loss is markedly flat at 2.95 dB across the frequency range in the On state, and sufficient isolation of over 25 dB is observed in the Off state. We are able to simulate the RF response accurately using both lumped element and 3D electromagnetic models. Extrapolation of our results suggests that optimizing device geometry can reduce insertion loss further and maintain broadband flatness up to 40 GHz. [ABSTRACT FROM AUTHOR]

The elementary basis of intelligence in organisms with a central nervous system includes neurons and synapses and their complex interconnections forming neural circuits. In non-neural organisms such as slime mold with gel-like media, viscosity modulation enables adaptation to changing environments. At a larger scale, collective intelligence emerges via social interactions and feedback in animal colonies. Learning and memory are therefore multi-scale features that evolve as a result of constant interactions with the environment. There is growing interest in emulating such features of intelligence in computing machines and autonomous systems. Materials that can respond to their environment in a manner similar to organisms (referred to as "organismic materials") therefore may be of interest as hardware components in artificial intelligence machines. In this brief review, we present a class of semiconductors called correlated oxides as candidates for learning machines. The term "correlated" refers to the fact that electrons in such lattices strongly interact and the ground state is not what is predicted by classical band theory. Such materials can undergo insulator–metal transitions at near ambient conditions under external stimuli such as thermal or electrical fields, strain, and chemical doping. Depending on the mechanism driving the transition, intermediate states can be metastable with different volatilities, and the time scales of phase change can be controlled over many orders of magnitude. The change in electronic properties can be sharp or gradual, leading to digital or analog behavior. These properties enable the realization of artificial neurons and synapses and emulate the associative and non-associative learning characteristics found in various organisms. We examine microscopic properties concerning electronic and structural transitions leading to collective behavior and theoretical treatments of the ground state and dynamical response, showcasing VO2 as a model system. Next, we briefly review algorithms designed from the plasticity demonstrated by phase changing systems. We conclude the brief review with suggestions for future research toward realizing non-von Neumann machines. [ABSTRACT FROM AUTHOR]

We demonstrate that a metal-insulator phase transition can be electrothermally actuated in the correlated complex oxide SmNiO3 (SNO) above room temperature from current-voltage measurements on thin film two-terminal devices. We simulate the internal temperature of SmNiO3 as a function of applied dc power by a Joule heating mechanism with substrate/electrode dissipation and find good agreement with experiment and device scaling. The results are relevant towards integrating correlated oxide phase transition functionality into semiconductor electronic/optoelectronic platforms. [ABSTRACT FROM AUTHOR]

Oxide semiconductor systems are often electrostatically doped with ionic liquids as gate insulators towards modulating carrier density and inducing phase transitions, while simultaneously serving as a means to probe their electronic phase diagram. The electronic and electrochemical properties of ionic liquid/correlated oxide interfaces are, therefore, important in interpreting such field-effect phenomena. Here, we use DEME-TFSI and VO2 as a model system to investigate the interface properties, slow relaxations, as well as field-effect in electric double layer transistor geometry. The stability of these interfaces is probed by combination of current-voltage measurements, x-ray photoelectron spectroscopy, impedance spectroscopy, and constant current charging. Three-terminal field effect transistor-type devices fabricated by photolithography are used to investigate kinetics of channel resistance modulations under varying gate bias polarity. Bias regions for reversible modulation of channel conductance have been determined. A time-dependent transconductance effect and as large as 20× increase in conductance are observed. [ABSTRACT FROM AUTHOR]

We present a study of electrostatic gating of VO2 thin films in ionic-liquid-based electric double-layer transistor geometry. Devices were fabricated by lithographic patterning of VO2 thin films as channel on sapphire substrates, ionic liquid as gate dielectric, and Au as gate/source/drain electrode, respectively. A significant unipolar increase in channel conductance at room temperature is observed. The VO2 channel resistance decreases ∼50% at + 2 V gate bias, whereas it increases slightly under negative bias. The polarity dependence of resistance modulation suggests electrons to be a dominant carrier, which is consistent with Hall measurements. In the high-temperature metallic state of VO2, no gating effect is observed. The effect of multiple transition cycles on the channel resistance change under bias is discussed. The study contributes to on-going efforts to realize room-temperature field-effect switches with correlated oxides. [ABSTRACT FROM AUTHOR]

Correlated oxides that exhibit metal-insulator phase transitions are emerging as potential candidates for switching devices. One such material is SmNiO3, which has a transition temperature above room temperature (∼400 K in bulk crystals). In this work, we study temperature- and bias-dependent conduction mechanisms in epitaxial and polycrystalline SmNiO3 thin films. In both cases, at low electric field we observe thermally assisted hopping conduction through defect states with activation energies of ∼170 meV and ∼270 meV, respectively. At high electric field the conduction transitions to a space-charge limited regime controlled by an exponential trap distribution. The power law exponents are ∼3 in epitaxial films and ∼8-14 in polycrystalline films. The trap decay parameter in epitaxial films does not have the expected 1/T temperature dependence, which may be a signature of bandgap narrowing at high temperature because of the insulator-to-metal transition. The larger activation energy and power law dependency in polycrystalline films are consistent with additional defect density from extraneous phases. In polycrystalline films, current-voltage data measured perpendicular to the film surface are rectifying because of asymmetry in electrode work functions with a ratio of 104 at ± 1 V. We find that whereas the space-charge limited conduction for positive bias is bulk limited, the negative bias conduction is injection limited. [ABSTRACT FROM AUTHOR]

Novel information processing techniques are being actively explored to overcome fundamental limitations associated with CMOS scaling. A new paradigm of adaptive electronic devices is emerging that may reshape the frontiers of electronics and enable new modalities. Creating systems that can learn and adapt to various inputs has generally been a complex algorithm problem in information science, albeit with wide-ranging and powerful applications from medical diagnosis to control systems. Recent work in oxide electronics suggests that it may be plausible to implement such systems at the device level, thereby drastically increasing computational density and power efficiency and expanding the potential for electronics beyond Boolean computation. Intriguing possibilities of adaptive electronics include fabrication of devices that mimic human brain functionality: the strengthening and weakening of synapses emulated by electrically, magnetically, thermally, or optically tunable properties of materials.In this review, we detail materials and device physics studies on functional metal oxides that may be utilized for adaptive electronics. It has been shown that properties, such as resistivity, polarization, and magnetization, of many oxides can be modified electrically in a non-volatile manner, suggesting that these materials respond to electrical stimulus similarly as a neural synapse. We discuss what device characteristics will likely be relevant for integration into adaptive platforms and then survey a variety of oxides with respect to these properties, such as, but not limited to, TaOx, SrTiO3, and Bi4-xLaxTi3O12. The physical mechanisms in each case are detailed and analyzed within the framework of adaptive electronics. We then review theoretically formulated and current experimentally realized adaptive devices with functional oxides, such as self-programmable logic and neuromorphic circuits. Finally, we speculate on what advances in materials physics and engineering may be needed to realize the full potential of adaptive oxide electronics. [ABSTRACT FROM AUTHOR]

We investigate the electrically triggered metal-insulator transition (E-MIT) in VO2 thin films at temperatures far below the structural phase transition temperature (∼340 K). At 77 K, the maximum current jump observed across the E-MIT is nearly 300×. The threshold voltage for E-MIT decreases slightly from ∼2.0 V at 77 K to ∼1.1 V at 300 K across ∼200 nm thick films, which scales weakly over the temperature range of 77-300 K with an activation energy of ∼5 meV. The phase transition properties are found to be stable after over one thousand scans, indicating reproducible measurements. Analysis of the scaling behavior suggests that the observed weak temperature-dependence of the threshold voltages for E-MIT is smaller than that predicted for a purely current induced Joule heating effect and may include contribution from field effect or carrier injection under applied bias. The results are of potential relevance to the field of phase transition oxide electronics and further understanding of the transition mechanisms. [ABSTRACT FROM AUTHOR]

Corrosion is a fundamental problem in electrochemistry and represents a mode of failure of technologically important materials. Understanding the basic mechanism of aqueous corrosion of metals such as Cu in presence of halide ions is hence essential. Using molecular dynamics simulations incorporating reactive force-field (ReaxFF), the interaction of copper substrates and chlorine under aqueous conditions has been investigated. These simulations incorporate effects of proton transfer in the aqueous media and are suitable for modeling the bond formation and bond breakage phenomenon that is associated with complex aqueous corrosion phenomena. Systematic investigation of the corrosion process has been carried out by simulating different chlorine concentration and solution states. The structural and morphological differences associated with metal dissolution in the presence of chloride ions are evaluated using dynamical correlation functions. The simulated atomic trajectories are used to analyze the charged states, molecular structure and ion density distribution which are utilized to understand the atomic scale mechanism of corrosion of copper substrates under aqueous conditions. Increased concentration of chlorine and higher ambient temperature were found to expedite the corrosion of copper. In order to study the effect of solution states on the corrosion resistance of Cu, partial fractions of proton or hydroxide in water were configured, and higher corrosion rate at partial fraction hydroxide environment was observed. When the Cl- concentration is low, oxygen or hydroxide ion adsorption onto Cu surface has been confirmed in partial fraction hydroxide environment. Our study provides new atomic scale insights into the early stages of aqueous corrosion of metals such as copper. [ABSTRACT FROM AUTHOR]

SmNiO3 (SNO) thin films were deposited on LaAlO3 (LAO), SrTiO3, SrLaAlO4, Si, and Al2O3 (sapphire) substrates by RF magnetron sputtering and studies were conducted to understand how film structure and composition influence the insulator-metal transition properties. It is observed that the compressive strain induces the insulator to metal transition (MIT), while tensile strain suppresses it. In the case of non-epitaxial films, semiconducting behavior is obtained on sapphire over a broad temperature range, while on heavily-doped Si substrate; an MIT is seen in out-of-plane resistance measurement. In addition, thickness dependence on the resistance behavior and nickel oxidation state has been examined for epitaxial SNO films on LAO substrates. Fine control of the MIT by modifications to the mismatch strain and thickness provides insights to enhance the performance and the functionality of these films for emerging electron devices. [ABSTRACT FROM AUTHOR]

VO2 undergoes a sharp metal-insulator transition at ∼67 °C with several orders of change in conductivity and optical transmittance. Understanding and control of the properties of vanadium oxide layers grown on technologically relevant substrates such as Si (100) single crystals is therefore of great interest. In this work, we show tunability of metal-insulator transition temperature as well as recoverable stress in VO2 thin films grown on Si substrate by introducing nanoscale atomic layer deposited HfO2 interfacial layers with no degradation in the resistance ratio. For a confined VO2 film, the metal-insulator transition temperature is suppressed by ∼16 °C and the recoverable stress is 150 MPa, compared to 400 MPa for a bare film. These observations are further correlated with in situ variable temperature measurement of stress changes occurring during the phase transition. Structural and microstructural studies on the various samples have been carried out by x ray diffraction and cross-sectional transmission electron microscopy. The strategy of tuning the metal-insulator transition characteristics by nanoscale interfacial dielectrics is of broader relevance in design of programmable materials and integration into solid state devices for electronics. [ABSTRACT FROM AUTHOR]

We present an atomistic simulation study on the size dependence of dopant distribution and the influence of nanoscale film thickness on carrier transport properties of the model oxide-ion conductor yttria stabilized zirconia (YSZ). Simulated amorphization and recrystallization approach was utilized to generate YSZ films with varying thicknesses (3-9 nm) on insulating MgO substrates. The atomic trajectories generated in the molecular dynamics simulations are used to study the structural evolution of the YSZ thin films and correlate the resulting microstructure with ionic transport properties at the nanoscale. The interfacial conductivity increases by 2 orders of magnitude as the YSZ film size decreases from 9 to 3 nm owing to a decrease in activation energy barrier from 0.54 to 0.35 eV in the 1200-2000 K temperature range. Analysis of dopant distribution indicates surface enrichment, the extent of which depends on the film thickness. The mechanisms of oxygen conductivity for the various film thicknesses at the nanoscale are discussed in detail and comparisons with experimental and other modeling studies are presented where possible. The study offers insights into mesoscopic ion conduction mechanisms in low-dimensional solid oxide electrolytes. [ABSTRACT FROM AUTHOR]

We report on oxygen surface exchange studies in ∼450-nm-thick nanocrystalline titania films with an average grain size of ∼13 nm by electrical conductivity relaxation along with the conductivity measurements at varying temperatures and oxygen partial pressures (pO2s). By electrochemical impedance spectroscopy technique, the high temperature conductivity was measured in the pO2 range from ∼10-16 to ∼10-6 Pa at temperatures from 973 to 1223 K and activation energy, ΔEa, for conduction was estimated as ∼3.23 eV at pO2 ∼10-11 Pa. Under reducing atmosphere (pO2 < 10-6 Pa), two distinct n-type conduction regimes were observed and corresponding predominant defects are discussed while, at high pO2 regime (pO2 >10-6 Pa), ionic conduction appears dominant leading to a conductivity plateau. The surface relaxation was observed to have two independent time constants likely originating from microstructural effects. The surface exchange coefficients are measured as ∼10-8-10-7 m/s and ∼10-9-10-8 m/s for each contribution with ΔEas of 2.79 and 1.82 eV, respectively, without much pO2 dependence across several orders of pO2 range of ∼10-16-10-6 Pa in the temperature range between 973 and 1223 K. The results are of potential relevance to understanding the near-surface chemical phenomena in nanocrystalline titania which is of great interest for energy and environmental studies. [ABSTRACT FROM AUTHOR]

We have successfully synthesized the colossal dielectric constant oxide La2-xSrxNiO4 in thin film form by reactive cosputtering from metallic targets and careful annealing protocols. Composition and phase purity was determined through energy dispersive spectra and x-ray diffraction, respectively. The dielectric constant exceeds values of over 20 000 up to 1 kHz and the activation energy for the frequency-independent conductivity plateau was extracted to be approximately 155 meV from 300 to 473 K, both in agreement with measurements conducted on bulk single crystals. However, unlike in single crystals, we observe early onset of relaxation in thin films indicating the crucial role of grain boundaries in influencing the dielectric response. ac conductivity at varying temperatures is analyzed within the framework of the universal dielectric law leading to an exponent of approximately 0.3, dependent on the electrode material. Impedance spectroscopy with electrodes of different work function (Pt, Pd, and Ag) was further carried out as a function of temperature and applied bias to provide mechanistic insights into the nature of the dielectric response. [ABSTRACT FROM AUTHOR]

The ability to controllably tune cation valence state and resulting electrical conductivity of transition metal-oxides such as NiO is of great interest for a range of solid state electronic and energy devices and more recently in understanding electron correlation phenomena at complex oxide interfaces. Here, we demonstrate that it is possible to enhance electrical conductivity of NiO thin films by one order of magnitude by photoexcitation and three orders of magnitude by ozone treatment at as low as 310 K. The change occurs within nearly 2000 s and, thereafter, reaches a self-limiting value. A surprising difference is seen at 400 K: ultraviolet photon and ozone treatments cause only a marginal reduction in resistance in the first few minutes and, then, the resistance begins to increase and recovers its original value. This unusual reversal is explained by considering metastable incorporation of oxygen in NiO and oxygen equilibration with the environment. Variation in nickel valence state prior to and after photoexcitation and ozone treatment, investigated by x-ray photoelectron spectroscopy, provides mechanistic insights into resistance trends. This study demonstrates photon-assisted and ozone oxidation as effective low temperature routes to tune the electrical properties as well as metastably incorporate oxygen into oxides with direct influence on electrical conduction properties. [ABSTRACT FROM AUTHOR]

The electrical characteristics of reactive metal–oxide interfaces are largely dependent on the thermodynamics and kinetics of interfacial reactions that may occur during synthesis and further processing conditions. Such reactions may lead to undesirable properties such as increased loss tangent, frequency dispersion, or asymmetric capacitance. Metals such as Ni are being explored as electrodes in various electronic packaging as well as on-chip decoupling capacitors and this presents a challenge to advance processing routes that render utilization of high-dielectric constant oxide thin films. In this work, we demonstrate that photon-assisted oxidation enables a unique low temperature processing route to synthesize reduced loss (Ba,Sr)TiO3 thin film based capacitors with Ni bottom electrodes. Our results show a decrease in the dielectric loss tangent in photon-assisted oxidized films compared to conventionally re-oxidized at an applied field of 0.33 MV/cm, tan δ of 0.08 versus 0.63, respectively. Leakage currents showed a significant decrease, 7.8 A/cm2 for conventional re-oxidation versus 0.046 A/cm2 for photon assisted, at an applied field of 0.33 MV/cm. The results are of relevance to processing reactive materials wherein athermal routes are necessary to avoid interfacial reactions. [ABSTRACT FROM AUTHOR]

Electrostatic control of the metal-insulator transition (MIT) in an oxide semiconductor could potentially impact the emerging field of oxide electronics. Vanadium dioxide (VO2) is of particular interest due to the fact that the MIT happens in the vicinity of room temperature and it is considered to exhibit the Mott transition. We present a detailed account of our experimental investigation into three-terminal field effect transistor-like devices using thin film VO2 as the channel layer. The gate is separated from the channel through an insulating gate oxide layer, enabling true probing of the field effect with minimal or no interference from large leakage currents flowing directly from the electrode. The influence of the fabrication of multiple components of the device, including the gate oxide deposition, on the VO2 film characteristics is discussed. Further, we discuss the effect of the gate voltage on the device response, point out some of the unusual characteristics including temporal dependence. A reversible unipolar modulation of the channel resistance upon the gate voltage is demonstrated for the first time in optimally engineered devices. The results presented in this work are of relevance toward interpreting gate voltage response in such oxides as well as addressing challenges in advancing gate stack processing for oxide semiconductors. [ABSTRACT FROM AUTHOR]

The structural phase transition (SPT) of VO2 films epitaxially grown on c-Al2O3 substrates by pulsed laser deposition and reactive sputtering was investigated by in situ temperature-controlled x-ray diffraction (XRD) across metal-insulator transition (MIT). An intermediate insulator phase was observed between room temperature monoclinic phase and high temperature metallic tetragonal phase. Gaussian curve fittings for measured XRD patterns revealed a significant contribution of the intermediate phase covering wide temperature range across MIT. Polycrystalline VO2 films grown on Si substrates revealed pure rutile phase after MIT in contrast with the epitaxial films on c-Al2O3 substrates. Strained structure of the VO2 films grown on c-Al2O3 substrates could be a mechanism for the formation of such intermediate phase known as monoclinic M2 phase. Presence of the intermediate insulator phase in highly oriented VO2 films may be an important factor in understanding the relation between MIT and SPT. [ABSTRACT FROM AUTHOR]

200 nm diameter Au contacts were fabricated by e-beam lithography on sputtered thin film vanadium oxide grown on conducting substrates and current perpendicular to plane electron transport measurements were performed with a conducting tip atomic force microscope. Sharp jumps in electric current were observed in the I-V characteristics of the nano-VO2 junctions and were attributed to the manifestation of the metal-insulator transition. The critical field and dielectric constant were estimated from quantitative analysis of the current-voltage relationship and compared with reported values on micrometer and larger size scale devices. These results are of potential relevance to novel oxide electronics utilizing metal-insulator transitions. [ABSTRACT FROM AUTHOR]

Utilizing metal-vanadium oxide (VO2)-semiconductor capacitor device structures, we have investigated the temperature- and frequency-dependent dielectric and ac conductance responses of vanadium oxide thin films that undergo metal-insulator transition (MIT). In both metallic and insulating regimes, VO2-based devices showed large tunabilities as high as ∼95% and ∼42%–54%, respectively. The frequency dependence of capacitance and ac conductance displays power-law behavior with respect to temperature and applied voltage over a broad range. Low-frequency dispersion in dielectric properties was also observed and their onset frequency varies across the MIT from ∼0.5 MHz in insulating state to ∼50 kHz in metallic state. The results are of potential relevance to utilizing functional oxides in electronic devices. [ABSTRACT FROM AUTHOR]

Microstructure evolution and electrical conductivity relaxation kinetics in highly textured and nanocrystalline dense ceria thin films (∼65 nm) are reported in this paper. Highly textured films were grown on sapphire c-plane substrates by molecular beam synthesis (MBS) with orientation relationship (111)CeO2∥(0001)Al2O3 and [110]CeO2∥[1210]Al2O3. No significant structural changes were observed in highly textured films even after extensive annealing at high temperature. In contrast to MBS grown films, ceria films grown by electron beam evaporation at room temperature had polycrystalline structure with ∼10 nm grains, which grew to ∼30 nm upon annealing at 1173 K. Grain growth kinetics was self-limiting and the out-of-plane orientation was found to be substrate dependent. From conductivity relaxation measurements, oxygen exchange rate in highly textured thin films was found to be much slower than that in polycrystalline films. The response time for highly textured films to changes in P(O2) from 1.07×10-12 to 5.43×10-10 Pa at 1148 K was 0.65 s, whereas that for polycrystalline films was 0.13 s under identical conditions. From temperature dependent experiments, activation energy for relaxation time was found to be similar, suggesting similar rate-limiting mechanisms in polycrystalline and highly textured films. The results highlight the importance of near-surface defects in controlling kinetics of oxygen incorporation into nanostructured oxides. In a broader context, the results maybe of relevance to designing catalytic surfaces in solid state ionic devices such as fuel cells. [ABSTRACT FROM AUTHOR]

There is a tremendous interest in understanding mechanisms governing ion and electron transport along interfaces in low-dimensional systems. We report on high temperature ion transport using a model superlattice system consisting of oxygen-ion-conducting yttria-doped zirconia (YDZ) and insulating layers (silica and alumina) of varying conducting layer thickness in the range of 3–33 nm. Total in-plane conductivity shows nearly fivefold increase in conductivity when the YDZ layer thickness decreases from 33 to 3 nm while the total film thickness is held constant. Analysis of the conductivity data shows that volume contributions are dominant when film thickness is ≥210 nm. Activation energies for volume and interfacial oxygen-ion transport have been estimated. [ABSTRACT FROM AUTHOR]

The study of metal-insulator transition (MIT) in VO2 thin films synthesized by means of rf sputtering from a VO2 target is presented. A comparison with conventional reactive sputtering from a V target is also given. Detailed x-ray diffraction analysis, electrical resistance switching, and infrared optical reflectance measurements confirm that our sputtering technique yields high-quality VO2 films. We discuss in depth how synthesis conditions affect MIT parameters derived from temperature dependence of electrical resistance. Sharp MIT is observed in films sputtered on technologically important Si substrates. The choice of Si (or sapphire) substrates results in the transition temperature above (below) the values obtained for single VO2 crystals. The MIT becomes narrower and stronger in thinner films. This is consistent with the assumption that the increased width of the MIT in thin films with respect to single crystals is the result of averaging of the transition parameters over a distribution of crystallites in the film. The measurements of MIT in VO2 patterned into devices do not reveal a noticeable lateral size effect down to 20 μm devices, encouraging use of the phase transition in switching electronic devices. The effect of substrate temperature and ambient during the sputtering on MIT is discussed. While VO2 films are found to be stable in ambient environment with time, the additional exposure to UV radiation near room temperature is shown to enhance the oxidation kinetics and produce changes in film resistance. Using UV radiation as an additional tool to control the oxidation process during VO2 synthesis may allow the synthesis temperature to be lowered and an improvement in material quality. We anticipate these results may be of relevance to synthesizing functional oxide films with potential applications in electronics and sensors. [ABSTRACT FROM AUTHOR]

The authors present a hybrid Brownian dynamics/Monte Carlo algorithm for simulating solutions of highly entangled semiflexible polymers or filaments. The algorithm combines a Brownian dynamics time-stepping approach with an efficient scheme for rejecting moves that cause chains to cross or that lead to excluded volume overlaps. The algorithm allows simulation of the limit of infinitely thin but uncrossable threads, and is suitable for simulating the conditions obtained in experiments on solutions of long actin protein filaments. [ABSTRACT FROM AUTHOR]

Zirconia films of varying thickness (ranging from 20-55 Å) have been grown by the method of UV ozone oxidation at room temperature. The electrical properties of these films have been studied in detail by capacitance-voltage (C-V) and I-V measurements. Capacitors were subjected to various anneals in differing ambient to study their effects on C-V hysteresis, dispersion and charge trapping. It was found that annealing in nitrogen followed by forming gas resulted in C-V curves with negligible hysteresis. The effects of different underlayers on the electrical properties of zirconia films have also been studied and are briefly discussed. It was found that zirconia films grown on UV-ozone grown SiO[sub 2] had lower hysteresis and lower interface trap density compared to zirconia films grown on chemical oxide. The effect of oxidation time and oxygen pressure have been investigated; in particular, detailed electrical studies have been performed on partially oxidized zirconia. Defective oxides are shown to have significant frequency dispersion in both the accumulation and depletion regions of the C-V curves along with very high loss tangent factor compared to stoichiometric oxides. A physical mechanism based on Maxwell-Wagner interfacial polarization is presented here for a model system of ZrO[sub 2]-Zr to explain the experimental data qualitatively. Finally, we have attempted to correlate the C-V hysteresis to the presence of traps in the zirconia film using temperature-dependent current-voltage measurements. The leakage current was found to be nearly independent of temperature at low voltages, suggesting a tunneling mechanism, while at higher voltages the data can be modeled using the Poole-Frenkel conduction mechanism. It is suggested that C-V hysteresis in zirconia films possibly arises from electrical traps in the film and can be identified by modeling their I-V characteristics. [ABSTRACT FROM AUTHOR]

We fabricate a heterostructure of SmNiO3 and VO2 on silicon wherein both correlated oxides undergo a metal-insulator transition at different threshold temperatures. It is demonstrated that the threshold voltage for electrically switching the VO2 from insulating to metallic can be tuned depending on the conducting state of the SmNiO3 layer. An electrically triggered SmNiO3 phase transition induces a resistance modification in the layer, which alters the field distribution and shifts the effective threshold voltage for VO2 switching in a non-volatile manner. The ability to tune the switching characteristics in such heterostructures could be relevant for emerging correlated oxide adaptive electron devices. [ABSTRACT FROM AUTHOR]

The influence of successive thermally induced metal-insulator transition (MIT) on electrical transition characteristics in high quality vanadium dioxide (VO2) thin films synthesized by reactive sputtering was investigated in conjunction with structural studies. We show that the transition quality of VO2 thin films was largely unaffected up to ∼100 heating-cooling cycles with respect to hysteresis width, resistivity ratio, and MIT onset temperature. The hysteresis width decreased by ∼3% along with an enhancement in film texture, while no reduction in resistivity ratio or MIT temperature was observed, implying that the stoichiometry of overall VO2 thin film did not degrade under multiple thermally induced transitions. The results are of potential relevance to solid state sensing elements utilizing vanadium oxide. [ABSTRACT FROM AUTHOR]

Metal-insulator transitions in strongly correlated oxides such as vanadium oxide (VO2) are of great scientific and technological interest. Due to the presence of multiple oxidation states, synthesis of high-quality VO2 films on substrates with the desired phase transition characteristics such as large jumps in phase transition resistance is a challenge. We show that the resistance ratio across the metal-insulator transition as well as the resistance of thin film VO2 can be modulated at relatively low temperatures by the use of ultraviolet irradiation. The enhanced oxygen incorporation due to creation of excited oxygen species enables controllably tunable stoichiometry. [ABSTRACT FROM AUTHOR]

We present a study of co-sputtered VO2-SiO2 nanocomposite dielectric thin-film media possessing continuous temperature tunability of the dielectric constant. The smooth thermal tunability is a result of the insulator-metal transition in the VO2 inclusions dispersed within an insulating matrix. We present a detailed comparison of the dielectric characteristics of this nanocomposite with those of a VO2 control layer and of VO2/SiO2 laminate multilayers of comparable overall thickness. We demonstrated a nanocomposite capacitor that has a thermal capacitance tunability of ∼60% between 25 °C and 100 °C at 1 MHz, with low leakage current. Such thermally tunable capacitors could find potential use in applications such as sensing, thermal cloaks, and phase-change energy storage devices. [ABSTRACT FROM AUTHOR]

Electron doping by hydrogenation can reversibly modify the electrical properties of complex oxides. We show that in order to realize large, fast, and reversible response to hydrogen, it is important to consider both the electron configuration on the transition metal 3d orbitals, as well as the thermodynamic stability in nickelates. Specifically, large doping-induced resistivity modulations ranging several orders of magnitude change are only observed for rare earth nickelates with small ionic radii on the A-site, in which case both electron correlation effects and the meta-stability of Ni3+ are important considerations. Charge doping via metastable incorporation of ionic dopants is of relevance to correlated oxide-based devices where advancing approaches to modify the ground state electronic properties is an important problem. [ABSTRACT FROM AUTHOR]

Vanadium dioxide (VO2) is a correlated electron material which undergoes an insulator-metal transition proximal to room temperature. The large change of optical properties across this phase transition is promising for tunable optical and optoelectronic devices especially at infrared frequencies. We demonstrate the ability to locally tune the optical properties on the micron scale through a simple design consisting of two electrodes patterned on a VO2 thin film. By current injection between the electrodes, a localized conducting path (metallic phase) can be formed within the insulating background. The width of the conducting path can be controlled by varying the applied current. Fourier transform infrared imaging shows that this current-modulated reflectance changes significantly over a distance on the order of the wavelength in the mid-infrared spectral range. [ABSTRACT FROM AUTHOR]

The authors present an overview of the fundamentals and representative examples of the growth of epitaxial complex oxide thin films on structurally dissimilar substrates. The authors will delineate how the details of particular crystal structures and symmetry of different oxide surfaces can be employed for a rational approach to the synthesis of nonisostructural epitaxial heterostructures. The concept of oxygen eutaxy can be widely applied. Materials combinations will be split into three categories, and in all cases the films and substrates occur in different crystal structures: (1) common translational and rotational symmetry between the film and substrate planes; (2) translational symmetry mismatch between the substrates and films that is distinct from a simple mismatch in lattice parameters; and (3) rotational symmetry mismatch. In case (1), in principle single-crystalline thin films can be attained despite the films and substrates possessing different crystal structures. In case (2), antiphase boundaries will be prevalent in the thin films. In case (3), thin-film rotational variants that are joined by tilt boundaries will be present. Diffraction techniques to determine crystallographic alignment and epitaxial variants are discussed, and transmission electron microscopy studies to investigate extended defects in the thin films will also be reviewed. The authors end with open problems in this field regarding the structure of oxide interfaces that can be topics for future research. [ABSTRACT FROM AUTHOR]

We investigated the dependence of absolute SnS band-edge energies on surface orientation using density functional theory and GW method for all surfaces with Miller indices -3 ≤ h; k; l ≤ 3 and found variations as large as 0.9 eV as a function of (hkl). Variations of this magnitude may affect significantly the performance of photovoltaic devices based on polycrystalline SnS thin-films and, in particular, may contribute to the relatively low measured open circuit voltage of SnS solar cells. X-ray diffraction measurements confirm that our thermally evaporated SnS films exhibit a wide distribution of different grain orientations, and the results of Kelvin force microscopy support the theoretically predicted variations of the absolute band-edge energies. [ABSTRACT FROM AUTHOR]

We present a metal-free tunable anisotropic metamaterial where the iso-frequency surface is tuned from elliptical to hyperbolic dispersion by exploiting the metal-insulator phase transition in the correlated material vanadium dioxide (VO2). Using VO2-TiO2 heterostructures, we demonstrate the transition in the effective dielectric constant parallel to the layers to undergo a sign change from positive to negative as the VO2 undergoes the phase transition. The possibility to tune the iso-frequency surface in real time using external perturbations such as temperature, voltage, or optical pulses creates new avenues for controlling light-matter interaction. [ABSTRACT FROM AUTHOR]

The correlated electron system SmNiO3 exhibits a metal-insulator phase transition at 130 °C. Using an ionic liquid as an electric double layer (EDL) gate on three-terminal ultrathin SmNiO3 devices, we investigate gate control of the channel resistance and transition temperature. Resistance reduction is reproducibly observed across both insulating and metallic phases with ∼25% modulation at room temperature. We show that resistance modulation is predominantly due to electrostatic charge accumulation and not electrochemical doping by control experiments in inert and air environments. We model the resistance behavior and estimate the accumulated sheet density (∼1-2 × 1014 cm-2) and EDL capacitance (∼12 μF/cm2). [ABSTRACT FROM AUTHOR]

Utilizing focused ion beam (FIB) milling, we have fabricated thin membranes (also referred to as nanowalls) of TixOy of 100-300 nm thickness starting from rutile titania bulk single crystals. Low probe currents (∼80 pA) in conjunction with XeF2-assisted milling enables minimal contamination during the fabrication process. Transmission electron microscopy studies indicated polycrystallinity and presence of nano-twins in the FIB-milled nanowalls. Formation of such nanoscale twinned structures may be related to high degree of nonstoichiometry, i.e., reduction as a result of milling in TiO2 that is consistent with observations in other oxides in the literature. Compositional analysis in the transmission electron microscope also showed reduced content of oxygen, confirming nonstoichiometry after milling. The authors have studied the temperature dependence of the electrical conductivity behavior in such ultra-thin walls in the temperature range of 300 K < T <520 K as a first effort. Temperature dependence of the electrical resistivity of the nanowall showed semiconducting behavior with an activation energy different from that corresponding to TiO2 single crystal and was attributed to formation of reduced TixOy phases after FIB processing. The results suggest a pathway to fabricate oxide structures for probing mesoscopic conduction phenomena. [ABSTRACT FROM AUTHOR]

The authors investigate capacitance-voltage characteristics of HfO2/VOx/HfO2/n-Si metal-oxide semiconductor devices that include vanadium oxide (VOx) films of different composition. Temperature dependent capacitance-voltage measurements are reported spanning the metal-insulator transition boundary of VOx films. The measured trends in dielectric properties are cross-correlated with resistance ratio change and oxidation state in identical films. The results could be of relevance to advancing synthesis of correlated oxide films on dielectric layers and further utilizing capacitance spectroscopy as a way to probe oxide stoichiometry in gated heterostructures. [ABSTRACT FROM AUTHOR]

We report on synthesis and phase transition characteristics of VO2 films grown on various single crystal substrates Al2O3 (0001), TiO2 (101), TiO2 (001), and MgF2 (001). An epitaxial orientation relationship was established for films on Al2O3 and TiO2 from x-ray 2θ-ω coupled scans and φ scans. Films grown on these substrates exhibit a metal to insulator transition below that for bulk single crystals with accompanying resistance change of 3 to 4 orders of magnitude. Trends in phase transition characteristics with substrate physical properties are analyzed. Postdeposition treatment studies in oxygen and ozone at low temperatures demonstrate that epitaxial VO2 films on TiO2 can tolerate oxidation environment better than the films on Al2O3. The studies could be of relevance toward further advancing structure-functional property relations in this important material system. [ABSTRACT FROM AUTHOR]

Nanoelectromechanical oscillators are very attractive as sensing devices because of their low power requirements and high resolution, especially at low pressures. While many experimental studies of such systems are available in the literature, a fundamental theoretical understanding over the entire range of operating conditions is lacking. In this article, we use our newly developed Bhatnagar-Gross-Krook based low Mach number direct simulation Monte Carlo method to study the noncontinuum drag force acting on a cylinder oscillating normal to a wall. We explore quasisteady flows in which ωτf<1 as well as unsteady flows for which ωτf=O(1). Here ω is the oscillation frequency and τf is the characteristic time for the development of the gas flow. The drag force per unit length acting on a long cylindrical wire is studied as a function of the Knudsen number, defined in terms of the mean free path λ and the radius of the cylinder R as Kn=λ/R. For quasisteady flows, we also present theoretical calculations for the slip regime, Kn<1, and the free molecular flow regime, Kn>1. Simulations of unsteady gas flow around a sinusoidally oscillating cylinder near a wall indicate that the drag force per unit length nondimensionalized by 4πμU approaches constant values for ωτf<1 (quasisteady flow) and for ωτf>1. Here μ is the gas viscosity and U is the maximum value of the nanowire velocity. The simulation results are compared with experimental measurements in the quasisteady regime. [ABSTRACT FROM AUTHOR]

Some recent observations on carrier (electronic and ionic) transport in thin film and nanostructured metal oxides and superlattices, where interfaces play a dominant role in overall conductivity response, are reviewed. Conduction in nominally pure oxides is compared to heavily aliovalently doped oxides (that correspond to changes in the Debye length over few orders of magnitude). The role of dopant segregation and their spatial variation in affecting transport is pointed out. The ability to utilize interfaces in enabling ultrafast-ion conduction as well as the creation of novel interfacial materials and implications in energy technologies as well as some scientific questions that may warrant further attention are highlighted. [ABSTRACT FROM AUTHOR]

The direct simulation Monte Carlo (DSMC) method is the preferred approach for simulating rarefied gas flows in complex geometries. However, the standard DSMC method becomes inefficient in the limit [formula] when the thermal velocity fluctuations which scale with the speed of sound are much larger than characteristic ensemble-averaged flow speed U. In this paper, we propose a modified DSMC algorithm which simulates the linearized Bhatnagar–Gross–Krook (BGK) approximation to the Boltzmann equation. The particles in this method are uniformly distributed in space and have a rest-state, equilibrium Maxwellian velocity distribution. The deviations from the equilibrium state are captured by weightings, indicating that each particle represents a noninteger expectation for finding gas molecules with the simulation particle’s velocity in the spatial cell where it is found. The BGK approximation makes the implementation of such a method simple and efficient because the BGK collision rule can be implemented by adjusting particle weightings without the need to create new particles during the simulation. The method can be incorporated into existing DSMC simulation programs with minimal changes. We apply the new algorithm to two test problems—pressure-driven flow through a nanochannel and flow around an isolated sphere. Results obtained with the new method are in excellent agreement with previous theories and simulations. [ABSTRACT FROM AUTHOR]

We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor., Engineering and Applied Sciences

We demonstrate controlled local phase switching of a VO[sub]2 film using a biased conducting atomic force microscope tip. After application of an initial, higher ‘training’ voltage, the resistance transition is hysteretic with IV loops converging upon repeated voltage sweep. The threshold Vset to initiate the insulator-to-metal transition is on order ∼ 5 V at room temperature, and increases at low temperature. We image large variations in Vset from grain to grain. Our imaging technique opens up the possibility for an understanding of the microscopic mechanism of phase transition in VO[sub]2 as well as its potential relevance to solid state devices., Engineering and Applied Sciences, Physics

Transport and Kelvin probe force microscopy measurements were simultaneously conducted on epitaxial VO2 thin films. The sample's work function abruptly dropped from 4.88 eV to 4.70 eV during heating from 333 K to 353 K, suggesting a significant change in its electronic band structure spanning the metal insulator transition. The work function showed nearly no statistical deviation across the film's surface during the transition, likely due to band bending at the boundaries of the small domains. Resistance profiles confirmed that the local work function corresponded closely to the resistance of the corresponding area. [ABSTRACT FROM AUTHOR]

We report on measurements of variation in metal-insulator transition characteristics through thickness in VO2 film grown on model SiO2 gate insulator by nanometric scale controlled etching followed by electrical and compositional measurements. The phase transition magnitude defined as ratio of resistivity at 25 °C to that at 100 °C of VO2 decreases from ∼159 at the surface to ∼14 at ∼10 nm away from a VO2/SiO2 interface, showing a difference of >10 times, while that for a VO2 thin film grown with identical conditions on single crystal sapphire only shows ∼3 times difference. The off-stoichiometric composition near the VO2/SiO2 interface induced by unoriented growth on amorphous SiO2 is likely responsible for the dramatic change in transition characteristics. [ABSTRACT FROM AUTHOR]