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ABSTRACT: The correlated electron system SmNiO3 exhibits a metal-insulator phase
transition at 130 {\deg}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 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 en-vironments. We model the
resistance behavior and estimate the accumulated sheet density (~1-2 x 10^14
cm^-2) and EDL capacitance (~12 {\mu}F/cm^2).
Applied Physics Letters 05/2013; 102(18):183102. · 3.84 Impact Factor
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ABSTRACT: The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as
unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable
insulator-metal transition that are subjects of active research. Here we
present temperature-dependent transport measurements of the resistivity,
magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial
SmNiO3 thin films with varying oxygen stoichiometry. We find that from room
temperature through the high temperature insulator-metal transition, the Hall
coefficient is hole-like and the Seebeck coefficient is electron-like. At low
temperature the N\'eel transition induces a crossover in the sign of RH to
electron-like, similar to the effects of spin density wave formation in
metallic systems but here arising in an insulating phase ~200 K below the
insulator-metal transition. We propose that antiferromagnetism can be
stabilized by bandstructure even in insulating phases of correlated oxides,
such as RNiO3, that fall between the limits of strong and weak electron
correlation.
01/2013;
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ABSTRACT: Oxidation of iron surfaces and oxide growth mechanisms have been studied using reactive molecular dynamics. Oxide growth kinetics on Fe(100), (110), and (111) surface orientations has been investigated at various temperatures and/or an external electric field. The oxide growth kinetics decreases in the order of (110), (111), and (100) surfaces at 300 K over 1 ns timescale while higher temperature increases the oxidation rate. The oxidation rate shows a transition after an initial high rate, implying that the oxide formation mechanism evolves, with iron cation re-ordering. In early stages of surface oxide growth, oxygen transport through iron interstitial sites is dominant, yielding non-stoichiometric wüstite characteristics. The dominant oxygen inward transport decreases as the oxide thickens, evolving into more stoichiometric oxide phases such as wüstite or hematite. This also suggests that cation outward transport increases correspondingly. In addition to oxidation kinetics simulations, formed oxide layers have been relaxed in the range of 600-1500 K to investigate diffusion characteristics, fitting these results into an Arrhenius relation. The activation energy of oxygen diffusion in oxide layers formed on Fe(100), (110), and (111) surfaces was estimated to be 0.32, 0.26, and 0.28 eV, respectively. Comparison between our modeling results and literature data is then discussed. An external electric field (10 MV cm(-1)) facilitates initial oxidation kinetics by promoting oxygen transport through iron lattice interstitial sites, but reaches self-limiting thickness, showing that similar oxide formation stages are maintained when cation transport increases. The effect of the external electric field on iron oxide structure, composition, and oxide activation energy is found to be minimal, whereas cation outward migration is slightly promoted.
Physical Chemistry Chemical Physics 12/2012; · 3.57 Impact Factor
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ABSTRACT: The perovskite SrTiO(3) is arguably one of the most important oxide systems in condensed matter research. In this study, we report measurement of the orientation dependence of oxygen exchange on SrTiO(3) single crystal surfaces by dynamic conductivity measurements under electrochemical perturbations. Activation energy for electrical conduction in the 923-1223 K range at an oxygen partial pressure of ∼10(-11) Pa of (100), (111), and (110) single crystals was found to be 2.6 eV, 2.7 eV, and 3.1 eV, respectively. The equilibration kinetics show profound dependence on the surface orientation and are modelled using a heterogeneous relaxation process. All surfaces show similar cationic sub-lattice limited rate behavior with (111), (100), and (110) having the fastest, intermediate, and slowest rates, respectively. We discuss the orientation dependence and its relation to local atomic structure in light of previous experimental and theoretical studies.
Physical Chemistry Chemical Physics 07/2012; 14(34):11953-60. · 3.57 Impact Factor
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ABSTRACT: Vanadium dioxide (VO2) thin films have been shown to undergo a rapid electronic phase transition near 70°C from a semiconductor to a metal, making
it an interesting candidate for exploring potential application in high speed electronic devices such as optical switches,
tunable capacitors, and field effect transistors. A critical aspect of lithographic fabrication in devices utilizing electric
field effects in VO2 is the ability to grow VO2 over thin dielectric films. In this article, we study the properties of VO2 grown on thin films of Yttria-Stabilized Zirconia (YSZ). Near room temperature, YSZ is a good insulator with a high dielectric
constant ($\epsilon _{\rm r} > 25$\epsilon _{\rm r} > 25). We demonstrate the sputter growth of polycrystalline VO2 on YSZ thin films, showing a three order resistivity transition near 70°C with transition and hysteresis widths of approximately
7°C each. We examine the relationship between chemical composition and transition characteristics of mixed phase vanadium
oxide films. We investigate changes in composition induced by low temperature post-deposition annealing in oxidizing and reducing
atmospheres, and report their effects on electronic properties.
Journal of Materials Science 04/2012; 46(17):5768-5774. · 2.02 Impact Factor
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ABSTRACT: Vanadium dioxide (VO2) has been shown to undergo an abrupt electronic phase transition near 70°C from a semiconductor to a metal, with an increase
in dc conductivity of over three orders of magnitude, making it an interesting candidate for advanced electronics as well
as fundamental research in understanding correlated electron systems. Recent experiments suggest that this transition can
be manifested independent of a structural phase transition in the system, and that it can be triggered by the application
of an electric field across the VO2 thin film. Several experiments that have studied this behavior, however, also involve a heating of the VO2 channel by leakage currents, raising doubts about the underlying mechanism behind the transition. To address the important
question of thermal effects due to the applied field, we report the results of electro-thermal simulations on a number of
experimentally realized device geometries, showing the extent of heating caused by the leakage current in the “off” state
of the VO2 device. The simulations suggest that in a majority of the cases considered, Joule heating is insufficient to trigger the
transition by itself, resulting in a typical temperature rise of less than 10K. However, the heating following a field-induced
transition often also induces the structural transition. Nevertheless, for certain devices, we identify the possibility of
maintaining the field-induced high conductivity phase without causing the structural phase transition: an important requirement
for the prospect of making high-speed switching devices based on VO2 thin film structures. Such electronically driven transitions may also lead to novel device functionalities including ultra-fast
sensors or gated switches incorporating ferroelectrics.
Journal of Materials Science 04/2012; 44(19):5345-5353. · 2.02 Impact Factor
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ABSTRACT: Atomistic simulations employing dynamic charge transfer between atoms are used to investigate ultra-thin oxide growth on Al(100) metal substrates in the presence of an ac electric field. In the range of 1-10 GHz frequencies, the enhancement in oxidation kinetics by ∼12% over natural oxidation can be explained by the Cabrera-Mott mechanism. At field frequencies approaching 0.1-1 THz, however, we observe a dramatic lowering of the kinetics of oxygen incorporation by ∼35% compared to the maximum oxidation achieved, which results in oxygen non-stoichiometry near the oxide-gas interface (O/Al ≈ 1.0). This is attributed to oxygen desorption from the oxide surface. These results suggest a general strategy to tune oxygen concentration at oxide surfaces using ac electric fields that could be of interest in diverse fields related to surface chemistry and applications such as tunnel barriers, thin dielectrics and oxide interfaces.
Physical Chemistry Chemical Physics 03/2012; 14(10):3360-8. · 3.57 Impact Factor
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ABSTRACT: Using reactive force-field (ReaxFF) and molecular dynamics simulation, we study atomistic scale chloride ion adsorption and transport through copper oxide thin films under aqueous conditions. The surface condition of passive oxide film plays a key role in chloride ion adsorption and facilitates initial adsorption when surface corrosion resistance is low. Using implemented surface defects, the structural evolution of the copper oxide film from thinning to breakdown is investigated. In addition to chemical thinning of passive film, extended defects in the metal substrate are observed, at high concentration of adsorbed chloride ions. The initial stage of breakdown is associated with rapid depletion of adjacent chloride ions, which creates a locally deficient environment of chloride ions in the solution. The dissolved copper cations gain higher charge upon interaction with chloride ions. Owing to the increased Coulomb interactions resulted from dissolved copper ions and locally low density of chloride ions, far-field chloride ions would diffuse into the local corrosion sites, thereby promoting further corrosion.
ACS Applied Materials & Interfaces 03/2012; 4(3):1225-32. · 4.53 Impact Factor
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ABSTRACT: We present a study of electrostatic gating of VO 2 thin films in ionic-liquid-based electric double-layer transistor geometry. Devices were fabricated by lithographic patterning of VO 2 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 VO 2 channel resistance decreases #x223c;50% at #x2009;+ #x2009;2 #x2009;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 VO 2 , 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.
Journal of Applied Physics. 01/2012; 111(1).
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ABSTRACT: We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300 K and 600 K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600 K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300 K but the highest oxidation is observed at 600 K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600 K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible.
Philosophical Magazine. 11/2011; 91(32):4073-4088.
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ABSTRACT: 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.
Applied Physics Letters 10/2011; 99(15):151910-151910-3. · 3.84 Impact Factor
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ABSTRACT: 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.
Journal of Applied Physics 10/2011; 110(7):071101-071101-20. · 2.17 Impact Factor
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Advanced Materials 09/2011; 23(39):4521-5. · 13.88 Impact Factor
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ABSTRACT: Vanadium dioxide (VO(2)) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of vanadium dioxide thin films across the phase transition using variable temperature Kelvin force microscopy (KFM). The work function is estimated to be ∼5.15 eV in the insulating phase and increases by ∼0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. This change in work function is distinct from bulk resistance-versus temperature trends commonly used to evaluate synthesis protocols for such vanadium oxide films and optimize stoichiometry. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid-state devices incorporating complex oxides containing multivalence cations.
ACS Applied Materials & Interfaces 08/2011; 3(9):3396-401. · 4.53 Impact Factor
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ABSTRACT: A one-step wet chemistry route has been explored to synthesize hollow hydroxyl titanium oxalate nanoscale spheres under mild experimental conditions. The hollow spheres were ∼200 nm in diameter, with a shell thickness of ∼30 nm. The nanospheres were formed by smaller aggregated colloidal subunits. The influence of temperature and solvent on the structure of the nanospheres was investigated. The formation of hollow interiors in the nanospheres may be rationalized by Ostwald ripening mechanism. Simple thermal treatment topotactically transformed the chemical composition into anatase TiO2. The high-order hollow porous spherical structure was preserved, with smaller crystalline anatase TiO2 nanoparticles as building units. Dense hydroxyl titanium oxalate nanospheres and their corresponding non-hollow porous anatase TiO2 nanospheres were also successfully achieved in suitable reaction conditions. The method and procedure reported herein may be extended in principle for the fabrication of other functional materials.
Journal of Materials Research. 06/2011; 26(12):1545 - 1551.
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ABSTRACT: 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.
Journal of Applied Physics 06/2011; 109(12):124110-124110-6. · 2.17 Impact Factor
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ABSTRACT: 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.
The Journal of chemical physics 06/2011; 134(23):234706. · 3.09 Impact Factor
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ABSTRACT: We report on in situ stress relaxation behavior of vanadium dioxide thin films across the thermally driven metal–insulator transition (MIT) and size effects. Although the residual stress follows an inverse relationship with film thickness, the metal–insulator phase transition-induced stress varies nonmonotonically with increase in film thickness and grain size. Maximum transformation stress of −447 MPa is observed across the MIT for ∼170-nm-thick film with an average grain size of ∼70 nm. The interplay between constraint effects and nanostructure leads to nontrivial stress relaxation trends and provides insights into design of phase transition materials for switching devices.
Journal of Materials Research. 06/2011; 26(11):1384 - 1387.
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ABSTRACT: We present results on terahertz (THz) spectroscopy on epitaxial vanadium dioxide (VO(2)) films grown on sapphire across the metal-insulator transition. X-ray diffraction indicates the VO(2) film is highly oriented with the crystallographic relationship: (002)(film)//(0006)(sub) and [010](film)//[2 ̅1 ̅10](sub). THz studies measuring the change in transmission as a function of temperature demonstrate an 85% reduction in transmission as the thin film completes its phase transition to the conducting phase, which is much greater than the previous observation on polycrystalline films. This indicates the crucial role of microstructure and phase homogeneity in influencing THz properties.
Optics Letters 05/2011; 36(10):1927-9. · 3.40 Impact Factor
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ABSTRACT: 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.
Applied Physics Letters 05/2011; 98(19):192113-192113-3. · 3.84 Impact Factor
