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The development of simple, scalable, and cost-effective methods to prepare Van der Waals materials for thermoelectric applications is a timely research field, whose potential and possibilities are still largely unexplored. In this work, we present a systematic study of ink-jet printing and drop-casting deposition of 2H phase SnSe2 and WSe2 nanoflake assemblies, obtained by liquid phase exfoliation, and their characterization in terms of electronic and thermoelectric properties. The choice of optimal annealing temperature and time is crucial for preserving phase purity and stoichiometry and for removing dry residues of ink solvents at inter-flake boundaries, while maximizing the sintering of nanoflakes. An additional pressing is beneficial to improve nanoflake orientation and packing, thus enhancing electric conductivity. In nanoflake assemblies deposited by drop casting and pressed at 1 GPa, we obtained thermoelectric power factors at room temperature up to 2.2 × 10−4 mW m−1 K−2 for SnSe2 and up to 3.0 × 10−4 mW m−1 K−2 for WSe2.

The development of simple, scalable, and cost-effective methods to prepare Van der Waals materials for thermoelectric applications is a timely research field, whose potential and possibilities are still largely unexplored. In this work, we present a systematic study of ink-jet printing and drop-casting deposition of 2H phase SnSe

The thermoelectric power of the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface is explored below room temperature, in comparison with that of Nb-doped ${\mathrm{SrTiO}}_{3}$ single crystals. For the interface, we find a region below $T=50$ K where thermopower is dominated by phonon drag, whose amplitude is hugely amplified with respect to the corresponding bulk value, reaching values $\ensuremath{\sim}\mathrm{mV}$/K and above. The phonon-drag enhancement at the interface is traced back to the tight carrier confinement of the 2DES, and represents a sharp signature of strong electron-acoustic phonon coupling at the interface.

We use ballistic electron emission microscopy to investigate prototypical Au/Nb-doped SrTiO3 (NSTO) Schottky barrier diodes for different temperatures and doping levels. To this end, ultrathin Au overlayers are thermally evaporated onto TiO2-terminated NSTO single crystal substrates. We show that at room temperature, regardless of the nominal doping, rectification is controlled by a spatially inhomogeneous Schottky barrier height (SBH), which varies on a length scale of tens of nanometers according to a Gaussian distribution with a mean value of 1.29–1.34 eV and the standard deviation in the range of 80–100 meV. At lower temperatures, however, doping effects become relevant. In particular, junctions with a low Nb content of 0.01 and 0.05 wt. % show an ∼300 meV decrease in the mean SBH from room temperature to 80 K, which can be explained by an electrostatic analysis assuming a temperature-dependent dielectric permittivity for NSTO. In contrast, this model fails to predict the weaker temperature dependence of SBH for junctions based on 0.5 wt. % NSTO. Our nanoscale investigation demands to reassess conventional models for the NSTO polarizability in high-intensity electric fields. Furthermore, it contributes to the comprehension and prediction of transport in metal/SrTiO3 junctions and devices.

We report the fabrication and modeling of a microelectromechanical system (MEMS) entirely made of crystalline oxides. A suspended SrTiO3 (001) microcantilever is employed as flexible substrate for the deposition of epitaxial transition-metal oxide (TMO) thin films. Tensile strain is generated at the upper surface of the SrTiO3 cantilever during its bending. Strain is thus transmitted to an overgrown TMO film by epitaxial lock with the SrTiO3 element. A strain-generator device for epitaxial TMO films is thus demonstrated and modeled by finite element methods. Evaluation of the applied strain and its distribution along microcantilevers of rectangular and triangular geometries is given.

Ta and Nb substituted TiO2 are important transparent conducting oxides that have potential for applications in photovoltaics, photocatalysis, and water splitting/CO2 sequestration. In addition to donating electrons, what are the effects of Nb and Ta substitution? Here we observe strong experimental evidence that Ta and Nb substitution induces large and small polarons in anatase TiO2 epitaxial thin films. The degenerate donor electrons (from both Nb and Ta) show a high temperature T3 dependence on electrical resistivity, which confirms the presence of large polarons, along with room temperature metallic transport. This is further confirmed by the enhancement in the electron effective mass, which was estimated from thermopower measurements. Femtosecond transient absorption (fs-TA) reveals the life time of the Ti-t2g and eg levels and the separation of these levels are consistent with the X-ray absorption spectroscopy (XAS) measurement. In addition, fs-TA reveals the presence of small polarons with a life time substantially >1 ns, which arises from defect levels and is a consequence of Ta and Nb substitution. X-ray photoelectron spectroscopy (XPS) provides evidence of Ti3+, which may be identified as the defects responsible for the small polarons. These long-lived small polarons may provide a way to minimize recombination dynamics in TiO2-based electrodes for photo-excited devices.

A series of La-doped (10 at.%) SrTiO3 ceramics with grain size ranging from 6 pm to 24 nm was prepared from nanocrystalline powders using high-pressure field assisted sintering (HOP-FAST). A progressive reduction of thermal conductivity kappa with decreasing grain size was observed. At room temperature, kappa of the ceramic with grain size of 24 nm (1.2 Wm(-1) K-1) is one order of magnitude lower than that of undoped single crystals. The strong suppression of kappa can be ascribed to (i) the high concentration of lattice defects, (ii) the increasing contribution of grain boundaries to phonon scattering when the grain size is decreased to the nanoscale and (iii) a moderate amount (10-15 vol.%) of nanopores. These results demonstrate that nanostructuration can be a successful strategy to attain a considerable reduction of kappa in heavily doped bulk oxide ceramics. The low electrical conductivity of the La:SrTiO3 nanoceramics represents a major obstacle for thermoelectric applications. (C)2013 Elsevier Ltd. All rights reserved.

Using advanced first-principles calculations we predict that the non-polar SrTiO$_3$/SrZrO$_3$ (001) interface, designed as either thin SrZrO$_3$ film deposited on SrTiO$_3$ or short-period (SrTiO$_3$)$_m$/(SrZrO$_3$)$_n$ superlattice, host a 2-dimensionally confined electron gas. Mobile electron charge due to native impurities, field-effect, or modulation doping remains tightly trapped at the interface. Key ingredients for this occurrence are a) the peculiar chemistry of 3d orbitals, b) the large band offset at titanate-zirconate interface.

Possible ferromagnetism induced in otherwise nonmagnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated with magnetic Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon.

We discuss the resistivity, the Hall effect, the Nernst effect, and the magnetization in the mixed state taking the motion of the Abrikosov and Josephson vortices into account. Measurements on a (BiPb)2Sr2Ca2Cu3O10 tape point out how the contribution of the Josephson vortices to the resistivity ρJ prevails at low temperatures and an analytical expression for ρJ is proposed. Above the irreversibility line the Hall effect, the Nernst effect, and the magnetization are less affected by the granularity of the material and, at first approximation, can be considered to be intrinsic properties. Starting from this assumption, we calculate the contribution of the Abrikosov vortices to the resistivity ρA using the Nernst effect and the reversible magnetization data. The ρA thus calculated has an order of magnitude close to the values reported in literature for single crystals, and shows an activated behavior with temperature, whose activation energy value is in strict agreement with that measured in epitaxial thin films of Bi(2223) compound.

A planar side-gate device for field effect with a La0.67Ba0.33MnO3 channel on a SrTiO3 substrate is fabricated by means of the voltage-biased tip of an atomic force microscope. The peculiar geometry and the high dielectric permittivity of the substrate enhance the channel resistance modulation up to 20% at low temperature by a gate voltage of ±40 V. Moreover, a reversible shift by 1.3 K of the metal–insulator transition temperature (TMI) by field effect is observed. The signs of the changes of resistance and TMI both depend on the sign of the gate voltage, as expected for pure field effect; in particular, the TMI is raised (lowered) by accumulating (depleting) holes in the channel.

We combine transport experiments, advanced ab initio calculations, and model analysis to determine the thermoelectric power in the two-dimensional electron gas formed at the paradigmatic oxide interface SrTiO3/LaAlO3. We demonstrate that contrary to popular expectation, quantum confinement does not enhance the thermoelectric power of the electron gas at this interface with respect to its corresponding three-dimensional case. Our analysis directly relates the thermopower behavior to band structure characteristics typical of the oxide heterostructure (i.e., on-site and intersite band splitting), furnishing general interpretive prescriptions to search for oxide heterostructures with improved thermoelectric capabilities.

We present an improved ac method to measure the thermal diffusivity in an extended range of temperatures (15–330 K) and magnetic fields (0–8 T). We solder wires of a selected material directly on the sample: so, the thermal sensor is the junction between the sample and the wire. We have verified that this fast thermocouple works at frequencies high enough to neglect thermal losses also at the highest temperatures. Moreover, the soldered wires allow a concurrent four-wire resistivity measurement on the sample itself.

Magnetization measurements were performed with a SQUID magnetometer on apressed Ag-sheathed BSCCO (2223) tape. The hysteresis loops, analyzed in the framework of the “Critical State model” show clear anomalies. The results are explained invoking granularity of the sample, as confirmed by the analysis of the reverse leg of the magnetization. The irreversibility line, measured in the orientations B//c and B⊥c, is also presented.

El empleo de la terapia de presión negativa es una opción ampliamente extendida dentro del campo de la Cirugía Plástica. Desde su aparición en el mercado se han ido sumando indicaciones de tratamiento de diversa índole, como podemos ver reflejado en la bibliografía existente al respecto. El sistema VAC® promueve la curación de las heridas mediante la aplicación de presión negativa, de manera que el uso de niveles controlados de presión subatmosférica y succión controlada, acelera la resolución de las mismas favoreciendo la vascularización y el desbridamiento. Actualmente, este sistema se considera un arma fundamental en el tratamiento de las heridas complejas, las cuales suponen un reto terapéutico importante y muchas veces acarrean a los pacientes que las padecen un largo y costoso ingreso hospitalario. Gracias a la terapia de vacío se puede disminuir el tamaño de dichas heridas a la par que se protegen de la infección, haciendo innecesario el uso prioritario de colgajos libres y grandes intervenciones. Presentamos una revisión de casos de interés sobre nuestra experiencia clínica con el uso de terapia de vacío en los últimos 6 años, durante la etapa comprendida entre mayo del 2005 y noviembre del 2011, con un total de 41 pacientes.

The observation of metallic behavior at the interface between insulating oxides has triggered worldwide efforts to shed light on the physics of these systems and clarify some still open issues, among which the dimensional character of the conducting system. In order to address this issue, we measure electrical transport (Seebeck effect, Hall effect, and conductivity) in ${\text{LaAlO}}_{3}/{\text{SrTiO}}_{3}$ interfaces and, for comparison, in a doped ${\text{SrTiO}}_{3}$ bulk single crystal. In these experiments, the carrier concentration is tuned, using the field effect in a back-gate geometry. The combined analysis of all experimental data at 77 K indicates that the thickness of the conducting layer is $\ensuremath{\sim}7\text{ }\text{nm}$ and that the Seebeck-effect data are well described by a two-dimensional density of states. We find that the back-gate voltage is effective in varying not only the charge density but also the thickness of the conducting layer, which is found to change by a factor of $\ensuremath{\sim}2$, using an electric field between $\ensuremath{-}4$ and $+4\text{ }\text{MV}/\text{m}$ at 77 K. No enhancement of the Seebeck effect due to the electronic confinement and no evidence for two-dimensional quantization steps are observed at the interfaces.

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and the a-axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

In this paper, we report on field-effect experiments in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ side-gate channels patterned on ultrathin epitaxial films having thickness ranging from 12 to 4 unit cells. Transport mechanisms and competition between phases, under the effect of electric and magnetic fields, as well as of other perturbations such as disorder and proximity to the interface with substrate are explored. We observe, in a 7 unit cells thick sample, a shift of the metal-insulator transition temperature as high as 43 K and a resistivity modulation up to 250% at low temperatures. In striking contrast, the 6--4 unit cells thick samples result to be insulating and almost insensitive to field-effect modulation. Such a finding indicates that for films thinner than 7 unit cells, a strong localization mechanism develops, which cannot be healed by band refilling. On the other hand, our results are compatible with a Mn ${e}_{g}$ orbital rearrangement driven by the broken translational symmetry at the surface and/or interface, which suppresses the double-exchange mechanism and localizes the carriers.

The ability to control the spatial arrangement of molecular nanocrystals is of fundamental importance for the fabrication of nanoscale devices. In this work, we report on experimental procedures to control the formation of 50 nm, organically capped, gold nanoparticles by tuning the deposition conditions. We found the parameters which lead to the formation of different domain structures, from three-dimensional aggregates to two-dimensional structures, to even isolated particles. (c) 2007 Elsevier B.V. All rights reserved.

We perform field-effect experiments on side-gate devices made of underdoped ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ epitaxial thin films. The resistance modulation is small due to the high carrier concentration in this compound; however, it is significantly enhanced by an external magnetic field. We discuss our results in the framework of a double-exchange mechanism and phase separation and suggest that our experimental data together with other complementary data of the literature are a straightforward consequence of the percolative nature of transport in the phase-separation regime of manganites. We demonstrate the possibility of driving the percolation process by field effect.

We report on the fabrication of a novel surface acoustic wave device based on piezoelectric interdigital transducers designed in high quality films of Pb(Zr0.2Ti0.8)O3 grown by RF magnetron sputtering. The approach followed is to replace the conventional metallic interdigital transducers by piezoelectric ones consisting of a series of ferroelectric domains with alternating polarization states. The artificial modification of the ferroelectric domain structure is realized by using an atomic force microscope as a local electric field source. A 1.2 μ m wavelength device has been realized and its transmission properties simulated.

We report a field effect device fully made of strontium titanate (STO). This perovskite-type material is very attractive for oxide electronics both for its notable dielectric properties as well as for its semiconducting properties in the doped state. We exploit both of these properties by developing a field effect device in which oxygen deficient STO acts as a conducting channel and stoichiometric STO as a dielectric barrier. Such a barrier is obtained by electrochemical oxidation of the surface of an oxygen deficient semiconducting STO film, deposited by pulsed laser ablation in ultrahigh vacuum conditions. The channel conductivity is varied by the application of an electric field between the channel itself and a metallic gate deposited onto the dielectric barrier. Modulation capability of more than 60% is achieved by applying potential lower than 1 V. Conductivity changes are due to electrostatic induced variations of the charge carrier density (n). This result is confirmed by Hall effect measurements during gate biasing. The very good agreement of the measured n with the value calculated from the device capacitance proves the electrostatic origin of the effect observed.

We have measured the resistivity and the Seebeck coefficient of a MgB2 sintered sample. The temperature dependence of resistivity is fitted well by a generalized Bloch-Gruneisen equation with a Debye temperature of 1050 K. The Seebeck coefficient is given by the sum of a diffusive and a phonon drag term and the behavior in the temperature region $% Tc, Comment: 9 pages, 3 figures

By applying a negative voltage to the conducting tip of an atomic force microscope, we modify on submicron-scale semiconducting oxygen deficient SrTiO3−δ thin films grown on LaAlO3 substrates. In comparison with the as-grown film, the modified regions present different electrical and structural properties, which can be exploited to realize submicrometer circuits. After a discussion on the mechanisms of the process, we report a prototype of a SrTiO3−δ-based sidegate field-effect transistor, showing a 4% modulation of channel resistivity with gate voltages up to 40 V.

Superconducting thin films of MgB2 were deposited by Pulsed Laser Deposition on magnesium oxide and sapphire substrates. Samples grown at 450C in an argon buffer pressure of about 10-2 mbar by using a magnesium enriched target resulted to be superconducting with a transition temperature of about 25 K. Film deposited from a MgB2 sintered pellet target in ultra high vacuum conditions showed poor metallic or weak semiconducting behavior and they became superconducting only after an ex-situ annealing in Mg vapor atmosphere. Up to now, no difference in the superconducting properties of the films obtained by these two procedures has been evidenced.

Superconducting multilayers, whose growth and structure are determined by the kinetic deposition and not by thermodynamics, are a powerful tool for investigating High T c superconductors (HTSC) properties and for possible future applications. By means of Pulsed Laser Deposition (PLD), we grew such materials alternating infinite layer phases both pure and doped, to supply CuO 2 planes and charge reservoir (CR) blocks respectively. We found out that using CR chemically doped with high concentrations of Ag and Sc substituted on the copper site in infinite layer compounds leads to a semiconducting behavior, whereas oxygen doping seems to be much more effective, since the BaCuO 2 infinite phase, grown in high oxygen partial pressure, does actually bring about a superconducting behavior. We deposited BaCuO 2 /CaCuO 2 superlattices with various periodicities and studied their transport and structural properties. By varying the deposition parameters, we reached both kinetic and thermodynamic growth and we observed semiconducting and superconducting behaviors.

We investigated the room-temperature current-voltage characteristics of Au/Nb:SrTiO3 Schottky junctions under various atmospheres and working pressures. We observed that oxygen partial pressure reversibly modulates junction response, briefly individual specimens behave as high-quality rectifiers in oxygen-rich atmospheres and as bipolar resistive switches in vacuum and inert gases. A two orders of magnitude modulation of resistance switching characterizes samples with the highest content of interfacial oxygen vacancies. We attribute this behavior to oxygen ionosorption and chemical oxidation at the metal-oxide interface. Our results are relevant to oxide devices displaying resistive switching at ambient-exposed interfaces, and might be exploited for gas detection purposes.

The application of local anodic oxidation to conducting perovskite oxides such as SrTiO3−δ thin films causes a local decomposition of the films shown by the formation of mounds at the surface. The size of the patterns is limited by the dimension of the water meniscus which forms under the biased tip of the atomic force microscope (AFM). Due to the instabilities of the water meniscus, biasing with a constant voltage in contact mode does not guarantee a good uniformity of the patterns. In this work, we show and discuss how, by controlling the electrical current through the AFM tip, it is possible to realize lines with uniform widths down to 150 nm over a total length of hundreds of micrometers.

In this letter we present the effect of neutron irradiation up to fluences of 3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the critical temperature is slightly diminished (36 K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K. For larger neutron fluences the critical temperature is suppressed down to 12 K and the superconducting properties come out strongly degraded.

We show that the growth of the heterostructure LaGaO3 /SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3 /SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.