The remarkable functionality and epitaxial compatibility of complex oxides provides many opportunities for new physics and applications in oxide heterostructures. Perovskite cobaltites and manganites provide excellent examples, being of interest for oxygen separation membranes, solid oxide fuel cells, ferroelectric RAM, catalysis, gas sensing, and resistive switching memory. From the magnetism perspective they have high conduction electron spin polarization, and a variety of functional ground states, attractive properties for oxide spintronics. However, the same delicate balance between phases that provides such diverse functionality also leads to a serious problem - the difficulty of maintaining desired properties (e.g. high spin polarization and conductivity) close to the interface with other oxides. This is exemplified by manganite magnetic tunnel junctions for example, where the interface spin polarization is suppressed and drops rapidly with temperature. Although this general problem is widespread, and could present a significant roadblock to the development of oxide electronics, there is no consensus as to its origin. In this work, using SrTiO₃(001)/La_1-xSr_xCoO₃ as a model system, we have combined epitaxial growth via high pressure oxygen sputtering [1] with high resolution x-ray diffraction, atomic resolution electron microscopy and spectroscopy, and detailed magnetic, transport, and neutron scattering measurements, to determine the fundamental origin of the deterioration in interfacial transport and magnetism. The effect is found to be due to nanoscopic magnetic phase separation in the near-interface region driven by depletion in interfacial hole doping due to accumulation of O vacancies [2,3]. This occurs due to a novel mechanism for accommodation of lattice mismatch with the substrate based on formation and long-range ordering of O vacancies [4], thus providing a fundamental link between strain state and O vacancy density. With this link understood we will demonstrate how interfacial magnetic and electronic properties can be controllably fine-tuned by manipulating oxygen vacancy ordering via heteroepitaxial strain. Work at UMN supported by DoE (neutron scattering) and NSF. Work at ORNL supported by DMS&E, DoE. Work at UCM supported by the European Research Council. [1] Torija, Sharma, Fitzsimmons, Varela, Wu and Leighton, J. Appl. Phys. 104 023901 (2008); Sharma, Gazquez, Varela, Schmitt and Leighton, J. Vac. Sci. Technol. 29 051511 (2011). [2] Torija, Sharma, Gazquez, Varela, He, Schmitt, Borchers, Laver, El-Khatib, Maranville and Leighton, Adv. Mater. 23 2711 (2011). [3] Sharma, Gazquez, Varela, Schmitt and Leighton, Phys. Rev. B. 84 024417 (2011). [4] Gazquez, Luo, Oxley, Prange, Torija, Sharma, Leighton, Pantiledes, Pennycook and Varela, Nano. Lett. 11 973 (2011).

Resistance switching (RS) memory effect, which occurs within a metal/oxide/metal junction, so called “Memristors”, have attracted much attention due to the potential applications not only for next generation non-volatile memories alternative to current flash memory technology but also for artificial neural computing systems beyond Boolean computing. Although the importance of nanoscale physical events on RS has been highlighted in previous works using thin film RS devices, investigating the occurrence of RS at nanoscale beyond the limitation of current lithographic length scales and extracting the exact nanoscale RS mechanisms is still a challenging issue. Self-assembled oxide nanowire-based RS offers an alternative approach not only to reduce the size of the cells beyond the limitation of current lithographic length scales but also to extract the underlying nanoscale RS mechanisms. Here we report “Nanowire Memristor” based on i) the fabrication of well-defined oxide nanowires via VLS mechanisms, ii) the construction of heterostructured oxide nanowires and iii) a single oxide nanowire junction down to 10 nm scale. First, we have examined the principle to fabricate an oxide nanowire by utilizing vapor-liquid-solid mechanism. We succeeded to fabricate single crystalline NiO and CoO nanowires by newly developed formation technique. [1-2] By using these novel oxide nanowires, we constructed a single oxide nanowire junction and discover the existence of memristive switching behavior at the 10 nm scale. [3-6] We confirmed the endurance at least up to 8th power of 10 and the existence of multistate memory effects. In addition, information as to the memristive switching including the carrier type for memristive switching and the spatial switching location, have been successfully extracted by the present open-top nanowire memristor, which had been buried in conventional capacitor-type memristors. [5] We also performed the first principle calculation to identify the origin of carriers by considering the presence of cation and anion defects in the calculations. [7] References [1] J. Am. Chem. Soc., 130, 5378 (2008), [2] J. Am. Chem. Soc., 131, 3434 (2009), [3] Nano Lett., 10, 1359 (2010), [4] J. Am. Chem. Soc., 132, 6634 (2010), [5] Nano Lett., 11, 2114 (2011), [6] J. Am. Chem. Soc., 133, 12482 (2011), [7] J. Am. Chem. Soc., 134, 134567 (2012).

Hysteretic metal-insulator transitions (MIT) mediated by ionic dynamics or ferroic phase transitions underpin emergent applications for non-volatile memories and logic devices. In virtually all cases to date, the MITs are controlled by applied electric or magnetic fields, giving rise to a broad set of existent and emergent information technology applications. However, multiple classes of MIT are intrinsically coupled to strain and thus will be affected by mechanical forces. Here, we explore local pressure-induced MIT in oxides and demonstrate creation of remanent conductive and non-conductive states. To demonstrate this effect, we have chosen NiO and La0.7Ca0.3MnO3 thin films as model systems, resistive switching in which closely resembles MIT. Scanning the surfaces with an electrically grounded tip significantly alters their electronic conductance changing it to high-resistance state in both thin films. In particular, the changes in the surface potential far exceed variations of the same across the MIT. This strongly suggests that the local pressure modifies the chemical composition of the sample surface. Furthermore, scanning the surfaces with a grounded tip at different pressure levels reveals that both surface potential and current are controlled by the contact force. The additional theoretical results indicate that the mechanical stimuli will give rise to changes in local ionic concentration. Even though there can be a couple of different mechanisms, e.g. triboelectric and electrostatic effects, for the observed phenomena, these effects can be excluded based on the experimental results. As a result, we have predicted and experimentally observed the pressure-induced changes in transition metal oxides. Since the coupling between strain and electrochemical reactions can be used to tune material properties, the present pressure-controlled physics of transition metal oxides can open up new opportunities for device applications. Research was supported (S.V.K., Y.K.) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences (S.V.K., S.J., M.D.B.), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work was also supported by WCU program through the NRF funded by the Korea MEST (Grant No. R31-2008-000-10057-0) and Basic Science Research Program through the NRF funded by the Korea MEST (Grant No. 2011-0025607).

The correlated rare-earth nickelate SmNiO₃ is an insulator at room temperature and exhibits an insulator-to-metal phase transition (MIT) as a function of temperature at ~400 K in the bulk. The phase transition in SmNiO₃ is of particular interest for room temperature electronics because the transition temperature is sufficiently above typical junction temperatures of CMOS devices (~350 K). Therefore, the MIT of SmNiO₃ should remain accessible if integrated into existing electronics platforms. With a focus on application, in this work we demonstrate that the phase transition in epitaxial SmNiO₃ thin films can be driven electrically in two-terminal devices in ambient conditions. SmNiO₃ films are grown by rf magnetron sputtering in relatively high background pressure (~300 mTorr) onto (001)-oriented LaAlO₃. High pressure sputtering enables stabilization and reversibility of the MIT in SmNiO₃. Current-voltage measurements of patterned two-terminal devices exhibit a signature of the MIT at high voltages. The static resistance V/I initially decreases as a function of voltage before turning up and increasing at high voltages in an analogous manner as the resistivity as a function of temperature. Indeed, V/I plotted as a function of applied power V×I agrees very well with the two-terminal resistance-temperature curve, suggesting that the applied power in I-V measurements induces the MIT in SmNiO₃ similarly as temperature. We are able to model this effect using a Joule heating mechanism, similar to chalcogenide phase change memory, and find good agreement with theoretical fitting parameters. We also show that the SmNiO₃ phase transition characteristics can be modified electrostatically in three-terminal devices. Using a gated ionic liquid as an electric double layer, we measure resistivity-temperature curves as a function of gate bias and observe systematic, reversible changes in MIT properties with applied gate voltage. Our results are potentially significant for realization of correlated oxide electronics for information processing.

The ability to tune the electrical properties of memristive devices is crucial if these circuit elements are to be exploited in diverse applications. In particular, large-scale practical memristive elements are challenging to achieve using current approaches, and power systems applications will require realization of memristors with high load tolerance. To date, little attention has been paid to engineering materials parameters to achieve tunable memristive behavior. Here, we present the fabrication and characterization of nickel-oxide/porous silicon (NiO/PSi) nanocomposite thin films as a material system that facilitates the tunability of memristive device properties. We demonstrate that by tuning the average pore diameter of PSi by only 22 nm, the minimum instantaneous resistance of composite devices can be varied over three orders of magnitude. Importantly, the applied voltage necessary to initiate memristive behavior drops by more than three orders of magnitude when NiO is intercalated into the PSi matrix compared to a bulk NiO film. NiO/PSi films were fabricated by sol-gel deposition of NiO into 1 µm thick n+ PSi films and subsequent annealing. Three different average PSi pore diameters were utilized: 19, 27, and 41 nm. Two-terminal devices were fabricated by conventional photolithography. Materials characterization by energy dispersive x-ray spectroscopy and x-ray diffraction was performed to verify NiO infiltration throughout the PSi matrix and the cubic rock salt (B1) crystal phase of NiO. Electrical characterization of devices was carried out by applying a sinusoidally varying voltage (10 V, 0.25 Hz) and simultaneously measuring the current through the device. The presence of a pinched hysteresis loop in the current-voltage curves confirmed that the nanocomposite films function as memristive devices. The smallest pore diameter films exhibit a minimum instantaneous resistance of 165 kOmega; while the largest pore diameter films exhibit a minimum instantaneous resistance of 962 MOmega;. For comparison, analogous devices composed of pure NiO on planar silicon were highly resistive (>5 GOmega;) and exhibited no observable hysteresis loop over the voltage range examined. Devices demonstrated state persistence above 1000 s; meaning that state retention/recovery was observed for more than 10,000 s when 20 V, 1 s pulses were applied every 1000 s. The aforementioned experimental results along with a proposed model explaining the memristive phenomena in the NiO/PSi films will be presented. A discussion of the trends in memristive properties linked to the film parameters will also be included. Specifically, we will address the roles of 1) NiO:Si compositional ratio, 2) ionic defect transit time differences due to pore size, and 3) varying defect mobilities due to nanocrystallite size differences between the films.

Observations of hysteretic current-voltage behavior and resistive switching have renewed interest in transition-metal oxide-based two-terminal metal/insulator/metal devices. The multiple accessible oxidation states, i.e. chemical complexity, of many transition-metal cations make ionic diffusion and solid-state reactions critical in these devices. In systems in which changes in cation valence state of the insulator are responsible for resistive switching, it is unknown whether crystallinity and extended crystalline defects are necessary for switching. Our work is focused on two-terminal vertical devices based on structurally disordered (amorphous) VOx films grown at room temperature. Both electrochemically active and inert electrodes are studied. Vanadium is known take on many stable oxidation states, as demonstrated by its many stoichiometric oxides; therefore, vanadium oxide films are susceptible to valence changes during device operation. However, since the films are amorphous, there are no extended crystalline defects. As-grown devices show a hysteretic current-voltage response pinched at the origin, a region of negative differential resistance, and bipolar characteristics. After electrical forming, abrupt voltage-induced resistive switching of over four orders of magnitude is achieved. The possible roles of both valence change in the oxide and electrochemical dissolution and drift of the cations from the top electrode on the electrical characteristics of as-grown devices and formed junctions will be discussed.

Resistive random access memory (RRAM) has shown great potential as a leading candidate for next-generation nonvolatile memory technology.^{1, 2} RRAM devices integrated in passive crossbar arrays have advantages in low energy consumption and excellent scalability. However, challenges arise when one tries to read a specific memory cell in the passive crossbar array due to the so-called sneak path problem, which is caused by the parasitic current flowing through undesignated devices at ON-state in the crossbar network. Recently the concept of complementary resistive switch (CRS) has attracted significant research interest as a potential solution for the sneak path problem.³ A typical CRS is composed of two bipolar memory cells that are connected anti-serially. Here we report a tantalum-oxide based resistive memory that achieves the complementary switching functionality within a single RRAM cell.⁴ The complementary switching effect is accompanied by switching polarity reversal in different voltage bias regimes within the same cell. These effects were explained by the redistribution of oxygen vacancies inside the tantalum-oxide layers. In particular, we show complementary is in fact more natural than conventional bipolar switching in general when the cell does not have strong asymmetry, and bipolar switching regimes correspond to a subset of the internal states with the presence of symmetry breaking. Because the CRS states correspond to different internal configurations of the cell instead of the overall high and low conductance states, both “1” and “0” states in CRS can produce either a high or a low read current depending on the polarity of the read pulses, distinct from normal resistive memory effects. References: 1. R. Waser, R. Dittmann, G. Staikov, K. Szot, Adv. Mater. 21, 2632-2663 (2009). 2. Y. Yang, P. Gao, S. Gaba, T. Chang, X. Pan, and W. Lu, Nat. Commun. 3, 732 (2012). 3. E. Linn, R. Rosezin, C. Kügeler, and R. Waser, Nat. Mater. 9, 403 (2010). 4. Y. Yang, P. Sheridan, and W. Lu, Appl. Phys. Lett. 100, 203112 (2012).

Bipolar resistance random access memories (ReRAMs) have recently been proposed as a new class of non-volatile switches. Even for the first reported ReRAMs based on metal-oxide-metal structures such as Pt-TiO2-x-Pt, the mechanisms that govern the non-linear resistive switching remain unclear. A plausible working hypothesis suggests that the non-linear resistive switching characteristics result from shifts in the Schottky barrier height at the metal-metal oxide-metal interfaces induced by accumulation of ions and/or electrons under bias, resulting in modifications in the space charge at the interfaces. The device properties then depend upon carrier flux and redistribution of space charge potentials upon change in bias polarity. State-of-the-art redox-based ReRAMs consist of metal-oxide-metal structures with a single thin oxide film. However, one may benefit by replacing the single oxide film by a multilayer oxide film structure. One may repeat, for example, two oxide materials whereby one is a wide band gap semiconductor (SrTiO3) and the second a small band gap semiconductor (La0.7Sr0.3MnO3) with up to 20 repetitions of each layer being 10 nm thick as done in this study. One thereby artificially increases the number of interfaces and space charge regions at the oxide interfaces, in addition to the ones at the metal electrode-oxide interfaces. The multilayer structures show stabilization in the RON to ROFF ratios compared to single small band gap oxide film-based ReRAM structures of equal film thickness. The single La0.7Sr0.3MnO3 oxide films exhibit unstable cycling characteristics for given conditions and show variations in the shape of the hysteresis curve relative to cycling and time, especially close to the initial set voltage. Moreover, a lowered conductivity is measured relative to the multilayer structure at a characteristic activation energy of 0.21 eV. The impedance characteristics of RRAM`s with SrTiO3-La0.7Sr0.3MnO3 multilayers vs those for single films of SrTiO3 or La0.7Sr0.3MnO3 are compared and discussed. Space charges at the multilayer internal interfaces are suspected to be advantageous in stabilizing the RON to ROFF ratio.

One of the newly emerging properties on oxide materials is resistive switching (RS) phenomenon. Oxide RS materials exhibit stable resistance states depending on external voltage conditions [1-4], so they have drawn remarkable attention as a promising candidate for the ultimate nonvolatile memory devices. Basically, oxide RS materials show two types of resistance switching modes: unipolar RS (URS) and bipolar RS (BRS). Voltage-polarity-independent URS is understood by the formation and rupture of a conducting filament by dielectric breakdown and Joule heating [1,2]. On the other hand, despite the great significance of voltage-polarity-dependent BRS for high scalability of applications, due to its phenomenological diversity depending on oxide materials [3,4], it has been difficult to clarify and understand the BRS mechanism. Recently, the defect concept, i.e. oxygen vacancy, was introduced to explain the BRS mechanism. However, the usual oxygen vacancy model, which only considers oxygen vacancy migration as a major source of RS, is not sufficient to describe various physical situations for specific BRS depending on oxide system. In this presentation, we will explain BRS mechanisms on the reduced SrTiO3 (001) thin film and doped-SrTiO3 (001) single crystal using angle-resolved spectroscopy (ARPES). Based on the ARPES results and density functional calculations, we demonstrated that two types of BRS in nonstoichiometric strontium titanate thin films are originated from the migration of anion and cation vacancy and formation of vacancy-cluster depending on the applied voltage conditions. In addition, we observed that an infinitesimal change of electron density at the Fermi level causes BRS on the doped-SrTiO3 (001) single crystal. [1] S. C. Chae et al, Adv. Mater. 20, 1154 (2008). [2] J. S. Lee et al, Phys. Rev. Lett. 105, 205701 (2010). [3] K. Shibuya et al, Adv. Mater. 22, 411 (2010). [4] F. Miao et al, Nanotechnology 22, 254007 (2011).

Non-volatile memories in production today are based mainly in using charge storage as the memory mechanism, however this technology is approaching their technical and physical limits and a new generation of memory devices is currently under development. One of the possibilities for replacing this charge storage based memory devices is using the resistive switching (RS) phenomena, i.e., the change of resistance by using a pulse of current or voltage. RS have been observed in a variety of materials [1,2] however, the observed switching behavior seems to differ depending on the material. In the case of manganites RS was initially associated to the existence of phase separation phenomena [3], but later it was also observed in manganites without phase separation such as the case of La2/3Sr1/3MnO3 (LSMO). Nevertheless, in spite of the work already done the driving mechanism behind the RS phenomena in manganites is still to be unveiled. In this work we report on the measurement of I(V) characteristic curves in LSMO epitaxial thin films prepared by RF sputtering. I(V) curves have been measured in LSMO lithographied microbridges. With this topology the metal-insulator-metal geometry typically used for measuring RS phenomena is avoided and we also minimize interface-related effects. We have analyzed I(V) curves as a function of the base temperature and magnetic field. I(V) are non linear and they exhibit a jump from a low resistance state (LRS) to a high resistance state (HRS) which, in principle, precludes electroforming effects. However I(V) curves exhibit origin symmetry and they are almost reversible except for a small irreversibility around the jump from the LRS to the HRS. All these features of I(V) curves strongly suggest that Joule self-heating effects are of relevance in our samples. To check this possibility we have microfabricated a Pt thermometer on top of the LSMO bridge to have access to the actual temperature of the samples while measuring the I(V) curve. Our results make evident that a remarkable increase of the temperature of the samples takes place when current increases. However Joule self-heating effects alone cannot explain the observed variation of the resistance and the jump between the LRS and HRS states. The possible driving mechanisms for this electroresistance effect will be discussed. [1] R. Waser et al., Adv. Mater. 21, 2632 (2009). [2] A. Sawa, Materials Today. 11, 28 (2008) [3] A. Asamitsu et al. Nature. 388, 50 (1997)

The rare earth nickelates exhibit a metal-insulator transition, which can be tuned through changes in physical structure. Using a combination of synchrotron x-ray diffraction and first principles theory, we identify the key structural differences in LaNiO3 and NdNiO3 thin films and superlattices, and relate the differences in physical structure with the measured electronic transport properties. We find thickness and temperature dependent changes involving sub-nanometer distortions of the oxygen octahedra that modify the Ni-O bond overlap. We have also found that these key structural parameters can be controlled when the conducting layers are incorporated into oxide heterostructure form.

Transition metal oxides are known to display a wide range of physical properties arising from the interplay between spin, charge, orbital and lattice degrees of freedom. Strain, field effect, reduced dimensionality and / or interfacial interactions may allow access to hidden phases and possibly to new properties and functionalities. Here, we study nickelate thin films and heterostructures. We show that strain, field effect and light allows the metal-insulator transition to be controlled in NdNiO3 thin films [1,2]. In LaNiO3, the only member in the perovskite nickelates family that, in bulk form, lacks any ordering phenomena remaining paramagnetic and metallic at all temperatures, we found that ultrathin films undergo a metal-insulator transition as their thickness is reduced to just a few unit cells. Magnetotransport in the vicinity of the transition can be explained by 2D weak localization in the presence of significant spin fluctuations [3]. We also studied high quality superlattices that combine LaNiO3 with ferromagnetic LaMnO3, grown along the [111] direction. Surprisingly, exchange bias is observed in the magnetic properties of these superlattices, indicating a strong interfacial coupling between the Mn and Ni cations and that magnetic order develops in the LaNiO3 layers. First-principles calculations point towards a spin-density wave as the most plausible type of magnetic order in LaNiO3 [4]. [1] R. Scherwitzl et al. Adv. Mat. 22, 5517 (2010). [2] A. Caviglia et al. Phys. Rev. Lett. 108, 136601 (2012). [3] R. Scherwitzl et al., Phys. Rev. Lett. 106, 246403 (2011). [4] M. Gibert et al, Nature Materials 11, 195, (2012).

Strongly correlated electron systems display exotic phenomena not seen in materials that are well-described by conventional band theory. One such example are the rare-earth nickelates, which undergo a paramagnetic-antiferromagnetic and metal-insulator transition that depends on the size of the rare-earth atom in the material. In this work, we aim to controllably tune the electronic properties and dimensionality of this system. Though metallic in the bulk, thin films of LaNiO3 below ~4 unit cells in thickness have been shown to be insulating below room temperature, and in NdNiO3 the metal-insulator transition gives way to a fully insulating state below a thickness of ~5 unit cells. Here, we show the ability to tune the metal-insulator transition in ultrathin nickelate films by direct chemical doping of the rare-earth site using molecular beam epitaxy. Further, by confining the films in superlattices with insulating barriers, we are able to observe metallicity in nickelate layers as thin as two unit cells. This work demonstrates a method for controlling electron correlations and achieving 2D conduction in the rare earth nickelates.

The abrupt onset of a highly resistive, charge ordered state in La_1/3Sr_2/3FeO₃ (LSFO) offers a unique opportunity to develop new types of electronic devices. In order to obtain the epitaxial thin films necessary for gating devices, we used molecular beam epitaxy to fabricate epitaxial thin films of LSFO on SrTiO₃. Growth conditions were optimized to produce epitaxial thin films with an out of plane lattice constant c = 3.86 Å and room temperature resistivities on the order of 0.01 ohm-cm. Resistivity measurements show an abrupt increase in resistivity at 180 K. Synchrotron x-ray diffraction measurements indicate that n/3(111) type reflections appear below 180 K, confirming that the large resistivity is caused by a charge-ordered state. We have also measured the charge ordering temperature of LSFO as a function of film thickness and epitaxial strain. This work is supported by the Office of Naval Research (N00014-11-1-0664).

The metal-insulator transition (MIT) in VO₂ has been a subject of debate for several decades and whose origin has presented an important problem for condensed matter physics. The change from high temperature metallic rutile phase to low temperature insulating monoclinic occurs abruptly at 360 K for bulk VO₂. The origin of the MIT, whether structural (i.e. Peierls-like transition due to V-V dimerization and tilting along the c_R axis) or electronic (i.e. Mott-Hubbard transition due to strong electron correlation effects) or some combination of the two still remains a matter of debate. Moreover, nano-scale thin films (le; 40 nm) of VO₂ can tailor the c_R lattice constant and severely alter the temperature of the MIT.[1] It has been shown that varying strain and thickness of the VO₂ film can affect the transition temperature and the abruptness of the MIT. Because of these effects, an abrupt MIT for films grown < 5nm has proved difficult to show.[2] Here, we report our angular- and temperature-dependent O K-edge (1s→2p*) and V L-edge (2p→3d*) x-ray absorption spectroscopy (XAS) of ultra-thin VO₂. The VO₂ was epitaxially grown to 3.85 nm in thickness on a TiO₂ (001) substrate, with an abrupt MIT centered at ~300 K with width < 20 K and a resistance (R) change of ΔR/R = 4670. Data was taken in both, electron yield mode (surface sensitive) and fluorescent yield mode (bulk sensitive) in order to compare surface contributions with bulk. From our data we can observe an angular contribution in the O K-edge region only in the fluorescent yield mode associated with the TiO₂ substrate. We find no evidence of vanadium reduction in either set of spectra, with both modes indicating a V⁴⁺ charge state. In the O K-edge region of the VO₂ only two peaks are observed associated with the π* and σ* bonding orbitals in the high temperature phase (i.e. 320 K). Upon cooling to well-below the MIT temperature (i.e. 230 K), the O K-edge revealed a third peak with strong angular dependence consistent with the splitting of the d_|| orbital.[3], The energetic splitting between the π* and d_||* is in agreement with 40 nm thick samples under the same strain as reported by Laverock et al.[4] This indicates clear evidence of the transition to the monoclinic insulating phase for these ultra-thin films and will be discussed further. [1] Y. Muraoka and Z. Hiroi, Appl. Phys. Lett. 80, 583 (2002) [2] K. Nagashima et al., J. Appl. Phys. 101, 026103 (2007) [3] M. Abbate et al., Phys. Rev. B 43, 7263 (1991) [4] J. Laverock, L. F. J. Piper et al., Phys. Rev. B 85, 081104(R) (2012)

The properties of perovskite oxide (ABO3) thin films can be tuned by controlling the lattice parameters and the BO6 octahedral tilts through the formation of heterostructures. Understanding the detailed structural changes at heterojunction interfaces and its connection to the property changes of the material is important for future device applications. We use position averaged convergent beam electron diffraction (PACBED) in scanning transmission electron microscopy (STEM) to measure the strain and the octahedral tilts in single-layer LaNiO3 films and [SrTiO3(3 u.c.)/LaNiO3(4 u.c.)]n superlattices grown on LSAT substrates. PACBED has unit cell resolution and can characterize the individual layers of the superlattices on the nanometer scale. PACBED shows that, unlike in bulk LaNiO3, which has octahedral tilts about all 3 Cartesian axes of 5.2°, the single layer LaNiO3 film has larger out-of-plane tilts and smaller in-plane tilts, as a result of the in-plane tensile strain. In the SrTiO3/LaNiO3 superlattices, the out-of-plane tilts in LaNiO3 are limited by the SrTiO3 layers. However, the in-plane tilts are not restricted and are observed to increase compared to the bulk case, along with a unit cell expansion in the z (growth) direction, despite the tensile strain from the LSAT substrate. We will relate the octahedral tilts to the transport and optical properties of LaNiO3 layers and superlattices, specifically the large decrease in resistance of ultrathin LaNiO3 layers embedded in superlattices, and observed changes in bandwidth and mass.

A fundamental characteristic of ABO3 perovskite oxides is a strong coupling between atomic and electronic structures. Of particular importance are the rotations and distortions of the BO6 octahedra, which couple to an array of physical properties including metal-insulator transitions, magnetism, and ferroelectricity. Therefore, a detailed understanding of how octahedral behavior can be engineered in oxide heterostructures is a crucial step toward realizing new phenomena in complex oxides. I will present recent work utilizing synchrotron x-ray diffraction to demonstrate how epitaxial strain and the formation of superlattices can be used to tune octahedral behavior in oxide heterostructures. The resultant effect on macroscopic properties such as electronic transport, charge disproportionation, and carrier dynamics will be discussed. This work has been performed in collaboration with P. J. Ryan, A. Bhattacharya, J.-W. Kim, J. M. Rondinelli, E. Karapetrova, R. J. Sichel-Tissot, and C. R. Smith. This work has been supported by the U.S. DOE under contract no. DE-AC02-06CH11357 and the Office of Naval Research under grant no. N00014-11-1-0664.

LaCoO3 (LCO) films have received great recent attention due to its unexpected ferromagnetic ordering, which is distinctly different from the bulk counterpart. Although the exact origin has not yet been understood, previous studies have speculated that the epitaxial strain should play a major role in creating a ferromagnetic state. In this work, we report an unconventional strain relaxation behavior in LCO epitaxial thin films. In particular, we show that the ferromagnetism in LCO films is directly coupled to a locally-ordered microstructure, which is formed to relieve the epitaxial strain. We used pulsed laser epitaxy to deposit epitaxial LCO thin films on various substrates in order to impose different strain states. X-ray diffraction and x-ray absorption spectroscopy (XAS) studies showed that the high quality epitaxial films were coherently grown, without any secondary phases. Moreover, the valence state and electronic structure of the films were robust from the XAS and ellipsometry study. Even though the macroscopic structural properties were not influenced by the degree of strain, a clear strain dependent atomic ordering was observed by Z-contrast scanning transmission electron microscopy (STEM). With increasing degree of tensile strain, increasing number of dark vertical stripes between the La atoms manifested the strain dependent local atomic ordering. We suggest the microscopic strain relaxation triggered the nanostructural domain formation as the origin of the atomic ordering and the dark vertical stripes observed from STEM. Furthermore, the average magnetic moment for a Co atom in the ferromagnetic LCO film corresponded to the stripe density, indicating that the magnetic moments within the nanostructural domain boundary were ferromagnetically ordered. Thus, our study provides a valuable insight in studying the link between the microscopic structure and macroscopic physical properties in oxide heterostructures. The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory.

Today, few p-type transparent conducting oxides (TCO) are known that exhibit the desired properties. Finding a single compound that demonstrates optical transparency, high hole concentration, and high hole conductivity remains challenging. The inverse design method helps focus attention on possible candidate materials by screening a large set of compounds by means of a set of design criteria. The successful candidate material must follow the five design criteria for p-type conducting oxide i) easy formation of hole producing defects, ii) unfavorable formation of hole-killing defects (such as oxygen vacancies), iii) thermodynamically stable phase, iv) light hole effective masses to ensure high hole mobility and v) an optical band gap of 3.1 eV. From a set of ternary silver oxides, we first select Ag3VO4 as a case study to assess as a candidate p-type TCO. The crystal structures of α-Ag3VO4 and β-Ag3VO4 were previously studied by powder crystal X-ray diffraction. For the first time, we report on the single crystal analysis of β-Ag3VO4. High temperature single crystal analysis was employed to refine the atomic positions of β-Ag3VO4, because the phase transition from α- to β occurs at 110 °C. The compound spontaneously transforms back to α-Ag3VO4 on cooling. Single crystal X-ray diffraction demonstrated the integrity of the single crystals during the reversible transitions between α-Ag3VO4 and β-Ag3VO4. The optical and electrical properties of polycrystalline α-Ag3VO4 were studied by diffuse reflectance spectroscopy and impedance spectroscopy. With an optical gap of 2.1 eV and a low absorption, this material is at the verge of transparency. The total conductivity of the α-phase at room temperature is 10-9 S/cm, but increases from 10-6 S/cm at 110C to 10-3 S/cm at 400C. Thin films were grown by combinatorial sputtering and PLD. Thin films of β-Ag3VO4 were grown, but transformed to α-Ag3VO4 on cooling to room temperature. The experimental conditions that were required to deposit pure α-Ag3VO4 thin films, were found to be sensitive to changes in deposition parameters. As expected, small changes in temperature, composition or oxygen partial pressure, quickly induced secondary phases such as Ag metal, V2O3, V2O5. There was also little flexibility in the stoichiometry of α-Ag3VO4, with small deviations of the V/(V+Ag)=0.25 immediately rendered additional microcrystalline products. The onset of absorption of the thin film was very similar to the results obtained on polycrystalline samples. Electrical properties of the sputtered thin film exhibited conductivity in the range of 10-3 S/cm.

Multiferroic oxide heterostructures of consisting of ferroelectric PbZr0.2Ti0.8O3 (PZT) and ferromagnetic La0.8Sr0.2MnO3 (LSMO) grown on SrTiO3 (STO) undergo a large, charge-driven magnetoelectric coupling at the interface between the ferroelectric and magnetic layers. For this system, it has been shown that the polarization of a ferroelectric strongly couples to the Mn spin at the LSMO-ferroelectric interface. In this work, we use oxide molecular beam epitaxy to grow epitaxial heterostructures of La0.8Sr0.2MnO3/ BaTiO3/La0.8Sr0.2MnO3. The physical and electronic structure of the interface as a function of ferroelectric polarization is determined using scanning transmission electron microscopy combined with electron energy loss spectroscopy (STEM-EELS), atomic force microscopy (AFM), and x-ray diffraction (XRD). We have also used x-ray photoelectron spectroscopy (XPS) to determine how the band alignments between the BaTiO3 and LSMO change as a function of the direction of the ferroelectric polarization.

The energetics of La doping in BaTiO₃ are reported for the both (electronic) donor-doping with the creation of Ti³⁺ cations and ionic doping with the creation of Ti vacancies. Our experiments and simulations demonstrate that ionic doping is the preferred mechanism for all concentrations of La doping. The apparent disagreement with electrical conduction of these ionic doped samples is explained by subsequent oxygen-loss which leads to the creation of Ti³⁺ cations. Our simulations show that oxygen-loss is much more favourable in the ionic-doped system than in pure BaTiO₃ due to the unique local structure created around the defect site. These findings resolve the so-called ‘donor-doping&’ anomaly in BaTiO₃ and explain the source of semiconductivity in positive temperature coefficient of resistance (PTCR) BaTiO₃ thermistors.

For the past decades, mixed valence manganites La1-xAxMnO3 (A=Ca, Sr, or Ba) have attracted lots of attention owing to the fact of their complex spin, charge, lattice and orbital interactions. Among them, La1-xSrxMnO3 ( LSMO) is one of the most-studied compounds of the mixed-valence manganite family for its robust and large MR effects above room temperature. BiFeO3 (BFO), probably the only room temperature multiferroic material, has led to enormous unique functionalities, such as domain engineering through substrates and BFO-based epitaxial devices. The highly anisotropic resistivity in strained manganites is theoretically studied and could be present in a wide range of strained manganites. In this work , we report the epitaxial anisotropic strain provided by 71° ferroelectric domain structure in BiFeO3 would epitaxially lock the perovskite manganites, leading to a preferential percolation of electronic phase domains in LSMO, and therefore, a giant transport anisotropies. Firstly, La1-xSrxMnO3 /BiFeO3 (LSMO/BFO) hetero-epitaxial bilayer samples are deposited by using pulsed laser deposition. The 71° striped BFO ferroelectric domains and epitaxial growth of LSMO are confirmed by piezoelectric force microscopy (PFM) and X-ray rocking curve. HRTEM and X-ray reciprocal space mapping (RSM) have been used to confirm the epitaxial relationships of the two functional layers and in-plane lattice constant of BFO and LSMO (along perpendicular and parallel to domain stripe). The differences of in-plane strain misfit (εxx ne; εyy) enable us to control the preferential percolation of electronic phase domains. Temperature-dependent resistivity measurements using PPMS show substantial differences in the metal-insulator transition temperature and remarkably high anisotropic resistivities along the directions that are perpendicular and parallel to the stripy domains. Magneto-optical Kerr effect (MOKE) and S.Q.U.I.D. have been used to unveil the magnetic properties and anisotropy in the films. X-ray magnetic linear dichroism (XMLD) at the Mn L2, 3 edge revealed an in-plane preferential occupation of the eg(3z2minus;r2) or eg(x2minus;y2) orbitals at room temperature. At last, we also demonstrate a nonvolatile electric field control of anisotropic resistivity switching which opens a gate to combining manganite-multiferroic materials together and paves a way for multi-control devices in the future.

Today&’s efforts to produce commercially viable photochemical cells for water splitting have focused on metal oxides - as they are robust, reliable, cost effective and stable - which must meet the following criteria: a band gap (EG) large enough to straddle the H2O/O2 oxidization and H+/H2 reduction potentials with a high photon-electron conversion efficiency.[1] In other words, a direct band gap material with 1.8 < Eg < 2.2 eV with excellent carrier conduction. This explains the inefficiency of TiO2 in the desired visible region and how Asahi and co-workers increased efficiency by shifting the band gap into the visible region through nitrogen doping 3 i.e. raising the valence band maximum (VBM). However, the immense efforts to improve the efficiency of TiO2 through doping have not produced a suitable material. An alternative approach is to consider SrTiO3, which has a similar band gap to TiO2 but better conduction band minimum (CBM) alignment with the normal hydrogen electrode, and use transition metal doping to modify EG and improve photocatalytic performance. Recent density functional theory (DFT) calculations have revealed that Mn/Fe dopants at the Ti sites provides a means of band gap engineering without introducing undesirable mid-gap levels.[3] Co-doping of Mn and Fe would enable one to not only reduce the band gap but tailor the band edges with respect to the important redox potentials. Here, we report our recent Synchrotron-based soft x-ray spectroscopy results from (SrTiO3)m/(SrFeO3)n superlattices grown by molecular beam epitaxy (MBE). The ability to control the growth of the doped SrTiO3 at the atomic level afforded by MBE superlattices and the element- and orbital-sensitivity of the soft x-ray spectroscopy techniques provides a means to directly evaluate the predictions by Zhou et al.[3] From the Fe L3,2-edge absorption spectra we find evidence for a Fe3+ charge state indicating significant ionic compensation occurring even within these atomically abrupt superlattices. Despite this the energetic separation between the O K-edge emission and absorption spectra indicates a band gap reduction of at least 1 eV for the superlattice ratios of 2:1 and 4:1. However, the band gap reduction is associated with a lowering of CBM in contrast to the expected raising of the VBM from DFT.[3] [1] A. Kudo and Y. Miseki, Chem. Soc. Rev. 38, 253 (2009). [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, Science 293, 269 (2001). [3 ]X. Zhou, J. Y. Shi, and C. Li, Journal of Physical Chemistry C 115, 8305 (2011).