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Two-dimensional (2D) metal dichalcogenides have drawn considerable interest because they offer possibilities for the implementation of emerging electronics. The emerging electronics are moving toward two major directions: vertical expansion of device space and flexibility. However, the development of a synthesis method for 2D metal dichalcogenides that meets all the requirements remains a significant challenge. Here, we propose a promising method for wafer-scale, conformal, and low-temperature (≤240 °C) synthesis of single-phase SnS2 via the atomic layer deposition technique. There is a trade-off relationship between the crystallinity and orientation preference of SnS2, which is efficiently eliminated by the two-step growth occurring at different temperatures. Consequently, the van der Waals layers of the highly crystalline SnS2 are parallel to the substrate. Thin-film transistors (TFTs) comprising the SnS2 layer show reasonable electrical performances (field-effect mobility: ∼0.8 cm2 V-1 s-1 and on/off ratio: ∼106), which are comparable to that of a single-crystal SnS2 flake. Moreover, we demonstrate nonplanar and flexible TFTs to identify the feasibility of the implementation of future electronics. Both the diagonal-structured TFT and flexible TFT fabricated without a transfer process show electrical performances comparable to those of rigid and planar TFTs. Particularly, the flexible TFT does not exhibit substantial degradation even after 2000 bending cycles. Our work would provide decisive opportunities for the implementation of future electronic devices utilizing 2D metal chalcogenides.

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi2Te3 alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of ∼1019/cm3 via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5 × 10−3 W-m−1-K−2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high-temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way.

Chemiresistive electronic nose (CEN) composed of villi-like nanostructures (VLNs) of SnO2 and WO3, and Au-functionalized VLNs was fabricated by applying electron-beam evaporation in a glancing angle deposition mode. The VLNs-based CEN with a back-heater (212 °C) shows high responses with low detection limits of parts per billion (ppb)-levels for NO and NH3 vapors at 80% relative humidity atmosphere. The enhanced sensitivities in a high humidity condition turn out to be attributed to the spillover effect by the Au nanoparticles and a large surface-to-volume ratio in porous VLNs. Employing Au NPs on VLNs leads to the increase of O− ions via the spillover effect which impedes the adsorption of water molecules, maintaining the enhanced responses against environmental humidity. Consequently, high responses for NO and NH3 vapors maintain even in the high humidity condition. Herein, with the principal component analysis (PCA), we demonstrate highly selective detection of NO and NH3 vapors against C2H5OH, CO, C7H8, C6H6, and CH3COCH3 vapors. These results open up wide applications of the VLNs-based CEN as an inexpensive and non-invasive diagnostic tool for asthma and kidney disorder.

Research on two-dimensional (2D) metal dichalcogenides is rapidly expanding owing to their unique characteristics that do not exist in bulk materials. The industrially compatible development of these emerging materials is indispensable to facilitate the transition of 2D metal dichalcogenides from the research stage to the practical industrial application stage. However, an industrially relevant method, i.e., the low-temperature synthesis of wafer-scale, continuous, and orientation-controlled 2D metal dichalcogenides, still remains a significant challenge. Here, we report the low-temperature (≤350 °C) synthesis of uniform and continuous n-type SnS2 thin films via the combination of atomic layer deposition (ALD) of tin oxides and subsequent sulfurization. Well-crystallized and aligned SnS2 layers parallel to the substrate are demonstrated through the phase engineering of the ALD-grown tin oxide and the substrate surface. The additional H2S plasma treatment at 300 °C leads to the formation of stoichiometric SnS2. The formation of conformal SnS2 layers over a three-dimensional undulating hole structure is confirmed, which reveals the potential for applications beyond the planar structured architecture. The present results could be a step toward the realization of 2D metal dichalcogenides in industry.

Plasmonic nanocrystals, which exhibit extraordinary optical properties, are challenging to grow in selective positions with a cost-effective and high-throughput process. We demonstrate that plasmonic isotropic gold nanospheres (AuNSs) can be selectively synthesized on wafer-scale rigid and flexible substrates at room temperature by laser irradiation. First, we prepare gold nanocolumn (AuNC) thin films on sapphire and polydimethylsiloxane substrates with glancing angle deposition (GAD). Then, a KrF excimer laser is exposed at selected positions with a 24 ns pulse duration. Finally, highly isotropic AuNSs as plasmonic nanocrystals are synthesized at the targeted positions. We suggest that the formation of such isotropic AuNSs is caused by reshaping from the top of the AuNCs; this is verified by the temperature distribution in the AuNCs during laser irradiation through finite element method simulations. We further investigate the formation of AuNSs by varying the laser energy density and the kind of substrate. By using a simple mask process, we demonstrate patterning of the letters “KIST” via selectively grown AuNSs on a flexible substrate. The simple laser irradiation process on GAD-grown metal NC thin films is expected to be a promising method for scalable synthesis of plasmonic isotropic NSs at targeted positions with a rapid process and at room temperature.

In this study, we synthesized porous (porosity: ~16%, average pore size: ~60 nm) ZnO nanosheets (thickness: ~80 nm) using a conventional solvothermal method to investigate NO2 gas sensing properties. Porous ZnO nanosheets triggered the detection of NO2 gas with high sensitivity. Responses of 2.93 – 0.5 ppm and 74.68 – 10 ppm NO2 gas at 200 °C were observed in the porous ZnO nanosheet-based gas sensor. In addition, improved sensing properties with high selectivity to NO2 gas, reasonable stability, and high response even in the presence of water vapor molecules were obtained. We found that the enhanced NO2 gas response of the porous ZnO nanosheet-based gas sensor was due to the synergetic effects of the high surface area, ZnO/ZnO homojunctions, and structural defects. We developed a highly sensitive NO2 gas sensor with improved reliability using morphologically engineered ZnO, which was prepared via a simple and scalable chemical-synthesis route.

We employed a hot rolling process to modulate the n-type Bi2Te2.82Se0.18 microstructure. The as-quenched Bi2Te2.82Se0.18 ingot was hot-rolled immediately after heating at 773 K for 1 h. The reduction ratio in the hot rolling process was controlled by changing the rolling gap in a range from 2 mm to 5 mm. With the increase in reduction ratio, the (00 l) planes of Bi2Te2.82Se0.18, which have a higher carrier mobility, were more aligned parallel to the rolling direction and grain refinement was accelerated. Such changes in structural properties led to a drastic enhancement in carrier mobility and increased electron concentration. Consequently, the electrical conductivity of Bi2Te2.82Se0.18 was enhanced by the hot rolling process.

A strategy for a microbolometer operating over a wide range of temperatures up to 100 °C with low nonlinearity and without hysteresis is described, based on a thermally stable VO2(B) film and 5-stack Ti/MgF2 infrared (IR) absorbers. The highly crystalline VO2(B) film formed on an amorphous SrTiO3 layer shows a high temperature coefficient of resistivity and low resistivity without any significant changes up to 100 °C. No significant changes are observed in the two values even after keeping the film at 100 °C for over a month. This outstanding thermostability makes the microbolometers with and without the IR absorber easily capable of operating up to 100 °C. The microbolometers with the 5-stack Ti/MgF2 IR absorber also shows increases in its responsivity by 4 times at 20 °C and by 3 times at 100 °C compared to that without IR absorber due to the increase of IR absorption. These results are achieved without significant increase in the thermal time constants even with the introduction of thermally massive IR absorber.

Two-dimensional (2-D) metal chalcogenides have received great attention because of their unique properties, which are different from bulk materials. A challenge in implementing 2-D metal chalcogenides in emerging devices is to prepare a well-crystallized layer over large areas at temperatures compatible with current fabrication processes. Tin monosulfide, a p-type layered semiconductor with a high hole mobility, is a promising candidate for realizing large-area growth at low temperatures because of its low melting point. However, tin sulfides exist in two notable crystalline phases, SnS and SnS2. Therefore, it is imperative to control the oxidation state of Sn to achieve a pure SnS film. Here, the synthesis of SnS thin films by atomic-layer-deposition (ALD) is demonstrated using bis(1-dimethylamino-2-methyl-2-propoxy)tin(II) and H2S as Sn and S sources, respectively, over a wide temperature window (90–240 °C). Impurities such as carbon, oxygen, and nitrogen were negligibly detected. The morphological evol...

A concentrating photovoltaic (CPV) cell exhibits the highest conversion efficiency among any solar cells. However, the further enhancement of the CPV efficiency is strongly limited by the heat generation at high solar concentrations. Here, we demonstrate a concentrating photovoltaic/thermoelectric hybrid generator using a single-junction, GaAs-based solar cell and a conventional thermoelectric module as a model system. Our hybrid generator gives rise to the conversion efficiency larger than the single CPV cell by ~3% at the solar concentration of 50 suns. Controlling thermal flow in the hybrid generator and the Peltier cooling effect is the key to achieving high efficiency. Our result provides a framework for designing a highly-efficient hybrid generator using both photo-electric and photo-thermal effects for the clean-energy production.

We fabricated polycrystalline yttrium-iron-garnet (Y3Fe5O12, YIG) samples using the sol-gel synthesis method to develop an energy harvesting material based on the spin Seebeck effect. We confirmed that crystallization occurred during calcination at 850 °C and that only the polycrystalline YIG structure was formed. We found that a sintering process at 1400 °C not only increased the size of the YIG particles and the densification of their microstructure but also enhanced their saturation magnetization (Ms) and dramatically reduced their coercivity (Hc). A mechanical pressing process was carried out between the calcination and sintering treatments to prepare a free-standing YIG fillet sample. We found that Ms was enhanced by almost three times without an associated reduction in Hc. We found that mechanical pressing assists the oxidation reaction during the subsequent sintering process by increasing the surface energy and densification before the heat treatment.

A facile method to fabricate a mechanically robust, stretchable solar absorber for stretchable heat generation and an enhanced thermoelectric generator (TEG) is demonstrated. This strategy is very simple: it uses a multilayer film made of titanium and magnesium fluoride optimized by a two-dimensional finite element frequency-domain simulation, followed by the application of mechanical stresses such as bending and stretching to the film. This process produces many microsized sheets with submicron thickness (∼500 nm), showing great adhesion to any substrates such as fabrics and polydimethylsiloxane. It exhibits a quite high light absorption of approximately 85% over a wavelength range of 0.2-4.0 μm. Under 1 sun illumination, the solar absorber on various stretchable substrates increased the substrate temperature to approximately 60 °C, irrespective of various mechanical stresses such as bending, stretching, rubbing, and even washing. The TEG with the absorber on the top surface also showed an enhanced output power of 60%, compared with that without the absorber. With an incident solar radiation flux of 38.3 kW/m

Recent advances in synthesizing complex oxide epitaxial heterostructures with precise control in atomic scale have opened a new era of materials science and engineering research, enabling discoveries of novel physical phenomena even from materials that have been studied for a long time. The exquisite control of high-quality thin films through composition, defects, strain, and microstructure allows us to clearly distinguish intrinsic and extrinsic properties that were obscured by the limitation of sample quality. This is vividly exemplified by the recent research on bismuth ferrite (BiFeO 3 ). Due to the moderately low symmetry of BiFeO 3 with a rhombohedral structure, domain engineering, controlling the configuration of domains and domain walls, plays a critical role not only in understanding fundamental physics of electrical and magnetic properties, but also in inducing novel functionalities such as photovoltaic and photocatalysis. In this review, various ways to control domain structures of BiFeO 3 will be described with the consequent modification in the physical properties of BiFeO 3 . This methodology can be expanded to other low-symmetric thin film materials for designing new functionalities.

Here, we demonstrate high-performance p-type thin film transistors (TFTs) with a SnO channel layer grown by atomic layer deposition (ALD). The performance of the SnO TFTs relies on hole carriers and defects in SnO and near the back-channel surface of SnO as well as the quality of the gate dielectric/SnO interface. The growth of SnO films at a high temperature of 210 °C effectively suppresses the hole carrier concentration, leading to a high on-current/off-current (Ion/Ioff) ratio. In addition, the SnO films grown at 210 °C achieve high field effect mobility (μFE) compared with the SnO films grown at lower temperatures because of their large grain size and lower impurity contents. However, the SnO films grown at 210 °C still contain defects and hole carriers, especially near the back-channel surface. The post-deposition process – back-channel surface passivation with ALD-grown Al2O3 followed by post-deposition annealing at 250 °C – considerably alleviates the defects and hole carriers, resulting in superior TFT performance (Ion/Ioff: 2 × 106, subthreshold swing: 1.8 V dec−1, μFE: ∼1 cm2 V−1 s−1). We expect that the SnO ALD and subsequent process will provide a new opportunity for producing high-performance p-type oxide TFTs.

Interfacial conductivity at the interface between two insulating oxides, that is 2DEG, shows a number of intriguing properties and applications, such as on/off switching with external electric fields, use in nanoscale electronic devices and tunable conductivity. Here, we report the effect of the interfacial conductivity on the kinetic behavior of electron-beam-induced epitaxial crystallization of an oxide amorphous thin film on an SrTiO3 substrate. Epitaxial growth from the interface can occur without direct e-beam irradiation at the interface due to accumulated charge around the beam position in the insulating materials. 2DEG, which acts as a current path delays the crystallization kinetics, thus delicate control of the crystallized pattern shape and size is available. As a result, successful pattern writing with a width of about 5 nm was performed. The present work provides useful guidelines for coherent atomic scale e-beam patterning considering the critical distance of the electron beam from the interface for the epitaxial growth, e-beam dose rate effect on the growth rate and the heterostructure interfacial conductivity.

Flexible thermoelectric modules (TEMs) are often built with a polymer filler as structural support. However, the filler may serve as a thermal bypass, leading to reduced temperature difference across a TEM and degradation of the output. Herein, we present flexible TEMs made of porous PDMS fillers and Bi2Te3-based thermoelectric legs. The heat conduction through the filler is suppressed by leveraging the low thermal conductivity of porous PDMS (0.08 W/m∙K), leading to an increase in the temperature difference across the TEMs. The porous PDMS TEM exhibits 23% enhanced power density and a figure of merit (ZT) of 0.75 around the room temperature. Lastly, the efficacy of the porous PDMS TEM is demonstrated by testing the TEMs with body heat. Our approach would offer a promising insight into the flexible TEM designs.

The thermoelectric figure-of-merit (zT) is a general standard used to evaluate the performance of thermoelectric materials. However, the performance of thermoelectric devices is largely affected by other properties such as electrical conductivity, thermal conductivity, and the power factor, depending on applications and boundary conditions. Therefore, it is beneficial to have a comprehensive map where the full range of thermoelectric properties are presented to design a high performance thermoelectric module optimized for any customized applications. In this study, we explore the thermoelectric properties of the n-type (1-x)Bi2Te3-xBi2Se3 alloys from x = 0.04 to x = 0.20 with various SbI3 doping levels. We used a hot extrusion technique to obtain a textured microstructure of our specimens to take advantage of their strong anisotropic nature. The SbI3 donor dopant enabled the precise control of electron density in the range of ∼1019 cm−3. Using this process, we constructed a thermoelectric property map as a function of the composition of the bismuth tellurium selenides and doping levels, to provide guidance on designing high performance thermoelectric modules for various applications.

Microbial forensics, exploiting bacteria, archaea, and eukaryotes, has been considered as one of the primary fields to trace the postmortem interval from the decaying cadavers. On the other hand, there remain several challenges of laboratory-based analysis for prediction of postmortem interval, including long-time measurement, complicated measuring procedure, and bacterial growth while carrying samples from the scene. Herein, we introduce the Smart Forensic Kit, which consists of a highly sensitive colorimetric gas sensor, a quality control algorithm, and a smartphone-based analysis method, to quantify the bacterial-derived ammonia gas in real-time. As a result, the estimation system of the postmortem interval has a superior selectivity to the ammonia gas with a detection limit of 38.7 ppb, response linearity to the target bacteria (Escherichia coli, Pseudomonas aeruginosa, and Pseudomonas putida), and short measuring time (10 min) with the maximum predicted postmortem interval from the mouse carcass (168 h). Furthermore, thanks to measuring the postmortem interval within 10 min, the negligible increase rate of bacterial concentration was observed. Consequently, the results reflected a high correlation between the ammonia gas emitted from bacteria and the postmortem interval so that we believe the Smart Forensic Kit will be applied for tracing down the decomposition of the cadavers in the near future.

Controlling crystalline phases in polymorphic materials is critical not only for the fundamental understanding of the physics of phase formation but also for the technological application of forbidden, but potentially useful physical properties of the nominally unstable phases. Here, using tin oxide (SnO2) as a model system, we demonstrate a new way to enhance the mechanical hardness of an oxide by stabilizing a high-pressure dense phase through nitrogen integration in the oxide. Pristine SnO2 has a tetragonal structure at the ambient pressure, and undergoes phase transitions to orthorhombic and cubic phases with increasing pressure. Leveraging the enhanced reactivity of nitrogen in plasma, we are able to synthesize tin oxynitride (SnON) thin films with a cubic phase same as the high-pressure phase of SnO2. Such nitrogen-stabilized cubic SnON films exhibit a mechanical hardness of ∼23 ± 4 GPa, significantly higher than even the nitride counterpart (Sn3N4) as the result of the shortened atomic distance of ...

As a field-assisted technique, spark plasma sintering (SPS) enables densification of specimens in a very short period of time compared to other sintering techniques. For high performance thermoelectric material synthesis, SPS is widely used to fabricate nanograin-structured thermoelectric materials by rapidly densifying the nanopowders suppressing grain growth. However, the microstructural evolution behavior of thermoelectric materials by SPS, another important process during sintering, has been rarely studied. Here, we explore SPS as a tool to control the microstructure by long-time SPS. Using p-type (Bi 0.25 Sb 0.75 ) 2 Te 3 thermoelectric materials as a model system, we systematically vary SPS temperature and time to understand the correlations between SPS conditions, microstructural evolution, and the thermoelectric properties. Our results show that the relatively low eutectic temperature (∼420 °C) and the existence of volatile tellurium (Te) are critical factors to determine both microstructure and thermoelectric property. In the liquid-phase sintering regime, rapid evaporation of Te leads to a strong dependence of thermoelectric property on SPS time. On the other hand, in the solid-phase sintering regime, there is a weak dependence on SPS time. The optimum thermoelectric figure-of-merit (Z) of 2.93 × 10 −3 /K is achieved by SPS at 500 °C for 30 min. Our results will provide an insight on the optimization of SPS conditions for materials containing volatile elements with low eutectic temperature.

Frequency-dependent impedance spectroscopy was applied to the 2-dimensioanl conduction transport in the LaAlO 3 /Sr x Ca 1− x TiO 3 /SrTiO 3 system. The 2-dimensional conduction modifies the electrical/dielectric responses of the LaAlO 3 /Sr x Ca 1− x TiO 3 /SrTiO 3 depending on the magnitude of the interfacial 2-dimensional resistance. The high conduction of the 2-dimensional electron gas (2DEG) layer can be described using a metallic resistor in series with two parallel RC circuits. However, the high resistance of the 2-dimensional layer drives the composite system from a finite low resistor in parallel with the surrounding dielectrics composed of LaAlO 3 and SrTiO 3 materials to a dielectric capacitor. This change in the resistance of the 2-dimensional layers modifies the overall impedance enabled by the presence of the interfacial layer due to Sr x Ca 1− x TiO 3 , which alters the charge transport of the 2-dimensional layer from metallic to semiconducting conduction. A noticeable change is observed in the capacitance Bode plots, indicating highly amplified dielectric constants compared with the pristine SrTiO 3 substrates and Sr x Ca 1− x TiO 3 with a greater Ca content.

Flexible piezoelectric generators have attracted considerable attention in recent years on account of their potential applications in mechanical energy scavenging devices using portable, wearable, and implantable electronics. Key issues for realizing a flexible piezoelectric generator include insufficient output power generation and poor efficiency at low frequency. We therefore propose structural approaches to enhancing the output power of the flexible piezoelectric generator using polyvinylidene fluoride. Specifically, we propose the use of a substrate and curved structure, and optimization of the aspect ratio of the generator for maximizing output power density. Through these approaches, induced stress and output voltage of the generator are analyzed by finite element modeling and validated through experiments. Considering these results for generator optimization, we fabricate a multilayered flexible curved generator, which produces ~200 V of the peak output voltage and ~2.7 mA of the peak output current. The output power density of the generator reaches ~17 mW/cm 2 , which is sufficient to drive various commercial electronics as a power source. Furthermore, it is demonstrated that this power source can illuminate 952 LED bulbs. This conceptual technology can provide the groundwork to enhancing the output power of conventional piezoelectric generators, thereby enabling novel approaches to realizing self-powered systems.

Here we report comprehensive study of 2DEG at a-LAO/STO interfaces in comparison with 2DEG at crystalline LaAlO3 (c-LAO)/STO interfaces. We observe that the oxygen deficient environment during the deposition of LAO overlayer is essentially required to create 2DEG at LAO/STO interface regardless of growth temperature from 25°C to 700°C, indicating that the oxygen-poor condition in the system is more important than the crystallinity of LAO layer. The critical thickness (2.6 nm) of 2DEG formation at a-LAO/STO heterostructure is thicker than (1.6 nm) that at c-LAO/STO. Upon ex-situ annealing at 300°C under 300 mTorr of oxygen pressure, 2DEG at a-LAO/STO interface is annihilated, while that in c-LAO/STO interface is still maintained. With combing these findings and scanning transmission electron microscope (STEM) analysis, we suggest that oxygen vacancies at the LAO surface is attributed to the origin of 2DEG formation at the LAO/STO and the crystallinity of the LAO overlayer plays a critical role in the annihilation of 2DEG at a-LAO/STO interface rather than in the formation of 2DEG. This work provides a framework to understand the importance of prohibiting the LAO surface from being oxidized for achieving thermally stable 2DEG at a-LAO/STO interface.

Unlike bulk materials, many physical properties of thin-film materials depend on film thickness due to extrinsic effects such as residual stress and the dead layer. In this work, the effect of thickness on the electrocaloric properties of Pb0.9La0.1(Zr0.65Ti0.35)O3(10/65/35) (PLZT) films grown by the sol–gel method was studied experimentally. In the sol–gel synthesis, the annealing process results in residual stress and the metal–dielectric contact generates a dead layer. These extrinsic effects influence the dielectric and ferroelectric properties of the thin films, and their roles are film thickness dependent. For PLZT of nanometer thickness (from 420 nm to 1080 nm), the permittivity and polarization, and their temperature dependences, showed strong dependence on film thickness. In particular, in the temperature range from 70°C to 350°C, the electrocaloric temperature change showed threefold improvement (from 0.2°C to 0.6°C) as the thickness was increased from 420 nm to 840 nm. This work will aid development of polycrystalline thin films including PLZT for electrocaloric applications.

The LaAlO3/SrTiO3 (LAO/STO) heterostructure has an inherent space inversion asymmetry causing an internal electric field near the interface. The Rashba spin-orbit coupling arising from this structural characteristic has a considerable influence on spin transport. With application of an external magnetic field, we observed conductance change in the LAO/STO interface which depends on the sign and magnitude of the field. Our systematic study revealed that these results come from spin dependent transport, by which we obtained quantitative strength of the Rashba effect. The Rashba strength in this system depends on the temperature: it varies from 2.6 x 10(-12) eVm to negligible value in the temperature range of 1.8 K-12 K. This method for detecting Rashba effect covers a wider temperature range in comparison with those obtained from Shubnikov-de Haas oscillation or weak antilocalization measurements.

The growth behavior and properties of ZnOxNy (ZnON) films grown by atomic layer deposition (ALD) with diethylzinc (DEZ), H2O, and NH3 were investigated. Although no growth of a ZnNx film occurs at 150 °C from DEZ and NH3, the ZnON film thickness is increasingly saturated by increasing ZnNx subcycles in a supercycle up to three successive ZnNx subcycles. The adsorbed NH3 during the injection step of NH3 induces the chemisorption of DEZ on the surface, consequently resulting in the growth enhancement. The optical band gap of the films decreases from 3.25 to 3.0 eV with increasing ZnNx subcycles. The resistivity of the films is tuned in the range from 4 × 10–2 to 1 × 102 Ω·cm by the variation of the ZnNx subcycles. However, the nitrogen concentration in the films is limited to approximately 2 at. % even at very high ZnNx cycles. The low nitrogen concentration is attributed to the exchange reaction of NH3 on the ZnON surface with H2O injected during the following step. These intriguing phenomena are not obser...

The carrier concentration in Bi2Te3-based alloys is a decisive factor in determining their thermoelectric performance. Herein, we propose a novel approach to modulate the carrier concentration via the encapsulation of the alloy precursor powders. Atomic layer deposition (ALD) of ZnO and SnO2 was performed over the Bi2Te2.7Se0.3 powders. After spark plasma sintering at 500 &deg, C for 20 min, the carrier concentration in the ZnO-coated samples decreased, while the carrier concentration in the SnO2-coated samples increased. This trend was more pronounced as the number of ALD cycles increased. This was attributed to the intermixing of the metal ions at the interface. Zn2+ substituted for Bi3+ at the interface acted as an acceptor, while Sn4+ substituted for Bi3+ acted as a donor. This indicates that the carrier concentration can be adjusted depending on the materials deposited with ALD. The use of fine powders changes the carrier concentration more strongly, because the quantity of material deposited increases with the effective surface area. Therefore, the proposed approach would provide opportunities to precisely optimize the carrier concentration for high thermoelectric performance.

Recently, all-solid-state lithium batteries have attracted great attention due to their safe operation without risk of explosion under high operating voltage. However, integrating all solid layers on flexible polymer substrate has been hampered by conflicts between the high crystallization temperature and the endurable temperature of the polymer substrate. In this work, we present a simple fabrication method for all-solid-state flexible lithium batteries that are directly integrated on a polymer substrate. Using excimer laser annealing technique, we are able to selectively crystalize the amorphous cathode layer grown on top of the polymer platform without any damage to the substrate. Our flexible battery is bendable up to a curvature radius of 9 mm without mechanical breakdown, and has an initial capacity of 20 μA h μm−1 cm−2 at a 0.2 C-rate. Our approach using laser annealing provides new opportunities for integrating all-solid-state lithium thin film batteries on a broader range of platforms.

An atomic structure is widely recognized as the key that determines the physical properties of a material. A critical challenge to engineer the atomic structure is that many useful crystals are metastable under ambient conditions and difficult to realize. Here, it is demonstrated that highly metastable atomic arrangements can be synthesized in the isostructural matrix via atomic layer deposition. Studying highly metastable BeO6 octahedra in rocksalt MgO as a model system, it is experimentally and theoretically shown that the single-phase BexMg1-xO thin films adopt rocksalt structure over wurtzite for the composition range x

Despite the wide applicability of oxynitrides from photocatalysis to refractory coatings, our understanding of the materials has been limited in terms of their thermodynamics. The configurational entropy via randomly mixed O/N or via cation vacancies are known to stabilize oxynitrides, despite the positive formation enthalpies. Here, using tin oxynitrides as a model system, we show by ab initio computations that oxynitrides in seemingly charge-unbalanced composition stabilize by forming pernitrides among metal-(O,N)6 octahedra. The nitrogen pernitride dimer, =(N-N)=, results in the effective charge of −4, facilitating the formation of nitrogen-rich oxynitrides. We report that the dimer forms only in structures with corner-sharing octahedra, since the N-N bond formation requires sufficient rotational degrees of freedom among the octahedra. X-ray photoemission spectra of the synthesized tin oxynitride films reveal two distinct nitrogen bonding environments, confirming the computation results. This work opens the search space for a novel kind of oxynitrides stabilized by N dimer formation, with specific structural selection rules.

We experimentally investigated the transport properties near metal electrodes installed on a conducting channel in a LaAlO3/SrTiO3 interface. The local region around the Ti and Al electrodes has a higher electrical conductance than that of other regions, where the upper limits of the temperature and magnetic field can be well defined. Beyond these limits, the conductance abruptly decreases, as in the case of a superconductor. The samples with the Ti- or Al-electrode have an upper-limit temperature of approximately 4 K, which is 10 times higher than the conventional superconducting critical temperature of LaAlO3/SrTiO3 interfaces and delta-doped SrTiO3. This phenomenon is explained by the mechanism of electron transfer between the metal electrodes and electronic d-orbitals in the LaAlO3/SrTiO3 interface. The transferred electrons trigger a phase transition to a superconductor-like state. Our results contribute to the deep understanding of the superconductivity in the LaAlO3/SrTiO3 interface and will be helpful for the development of high-temperature interface superconductors.

We experimentally investigated the transport properties near metal electrodes installed on a conducting channel in a LaAlO3/SrTiO3 interface. The local region around the Ti and Al electrodes has a higher electrical conductance than that of other regions, where the upper limits of the temperature and magnetic field can be well defined. Beyond these limits, the conductance abruptly decreases, as in the case of a superconductor. The samples with the Ti- or Al-electrode have an upper-limit temperature of approximately 4 K, which is 10 times higher than the conventional superconducting critical temperature of LaAlO3/SrTiO3 interfaces and delta-doped SrTiO3. This phenomenon is explained by the mechanism of electron transfer between the metal electrodes and electronic d-orbitals in the LaAlO3/SrTiO3 interface. The transferred electrons trigger a phase transition to a superconductor-like state. Our results contribute to the deep understanding of the superconductivity in the LaAlO3/SrTiO3 interface and will be helpful for the development of high-temperature interface superconductors.

We grew Pt films on TiO2-terminated SrTiO3 (001) by atomic layer deposition, using trimethyl(methylcyclopentadienyl)-platinum(IV) as the Pt source and O2 and O3 as the oxidants. The orientation of the Pt films grown with O2 varied from (111) to (001) as the growth temperature was increased from 220 to 350 °C, while the Pt films grown with O3 have a strong preference for the (111) orientation even at a high growth temperature of 350 °C. The difference in the orientation of the Pt films on SrTiO3 (001) was attributed to changes in the degree of chemical bonding across the Pt/SrTiO3 interface with respect to the oxidant. We observed an increase in Pt–O bonding at the interface between the Pt grown with O3 and the SrTiO3 substrate. The interfacial energy of Pt (111)∥SrTiO3 (001) may have been significantly decreased by the increase in Pt–O bonding at the interface, which eventually led to the strong (111) preference of the Pt grown with O3. The findings provide the possibility of controlling the orientation o...

Thermoelectric devices have attracted a great attention for renewable energy harvesters and solid-state coolers. For practical applications, the mechanical properties of thermoelectric materials become critical for the device reliability, a persistent performance with a long time and high operation cycles. Bi–Te based single-crystals, mostly used in commercial thermoelectric devices, are intrinsically brittle with weak van der Waals bonding, often leading to device failures such as crack and debonding during fabrication and operation. Thus, it is highly desirable to enhance the mechanical property of Bi–Te based alloys as well as the thermoelectric property. Here, we investigate the effect of B 4 C nanoparticles (less than 0.5 wt%) dispersed in p -type Bi 0.4 Sb 1.6 Te 3 matrix on the mechanical properties. X-ray diffraction (XRD) result confirms that B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 has a single phase. We observe that the grain size of Bi 0.4 Sb 1.6 Te 3 becomes decreased with the B 4 C nanoparticle concentration by electron backscatter diffraction (EBSD) technique. Hardness, Young’s modulus, and flexural strength of B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 are enhanced, compared to the B 4 C-free Bi 0.4 Sb 1.6 Te 3 polycrystals. On the other hand, the thermoelectric figure-of-merit of B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 is almost identical to that of the pure Bi 0.4 Sb 1.6 Te 3 . Such enhancements of the mechanical properties of the B 4 C-dispersed Bi 0.4 Sb 1.6 Te 3 are attributed to the grain boundary hardening and second-phase hardening. Beyond thermoelectric materials, our result implies that the grain refinement by nanoparticle dispersion is a simple and promising way to strengthen the mechanical properties of other brittle materials with layered structure.

An accurate and rapid characterization of a thermoelectric module (TEM) is critical to understand the problems in module design and fabrication. We describe an apparatus and a method that directly measure the cooling performance of a TEM such as current for maximum cooling (Imax), maximum cooling power (Qc,max), and maximum temperature difference (ΔTmax). The apparatus is designed based on a finite element model to ensure a simple heat flow measurement. We evaluate the module performance metrics based on differential measurement between the cooling powers with temperature difference across the module under transient conditions. The use of transient data reduces measurement time, and the use of a differential technique enables compensation of the thermal losses. The measured data fit well with conventional theoretical relations for the TEM performance metrics. We test a commercial TEM and validate the results using the Harman method.

The 2-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 heterointerface was analyzed using frequency-dependent impedance spectroscopy. The electrical conduction of 2DEG significantly influences the high-frequency impedance and induces dielectric amplification at low frequency regimes. The impedance responses obtained from the LaAlO3/SrTiO3 oxide was modeled using an equivalent circuit model. The frequency-dependent characterization used here does not necessitate the formation of ohmic contacts between the 2DEG layer and the adjacent electrodes. Through thermal bias-stress tests, the 2DEG conduction mechanism is proposed to partially originate from the oxygen vacancy-controlled defect concepts, indicating the controllability of 2DEG transport.

With the widespread use of wearable electronics, the flexible piezoelectric energy harvesting devices have been extensively studied to efficiently convert the physical motion of the human body into electrical energy. The major obstacles for realizing a flexible piezoelectric generator include the insufficient output power generation and the poor efficiency at the low-frequency regime. Here, we demonstrate a curved piezoelectric generator favorable for wearable applications, generating a high output power. The curved structure plays a key role to improve the power generation, by effectively distributing the applied force across the piezoelectric layer, as well as to allow operation at the low frequency vibration range. Accordingly, this generator produces � 120 V of peak output voltage and � 700 mA of peak output current during a cycle. Furthermore, our generator can operate at low frequencies below 50 Hz, generating � 55 V of output voltage and 250 mA of output current at 35 Hz, and it even works at frequencies as low as 1 Hz. With this generator, we successfully lit up 476 commercial LED bulbs. In addition, we experimentally demonstrate the possibility that the generator can be used in shoes, watches, and clothes as a power source. Our results will provide a framework to enhance the output power of conventional piezoelectric generators, and open a new avenue for realization of self-powered systems, such as wearable electronic devices.

In the present work, 0.01–0.05wt.% Sn-doped Bi2(Te0.9Se0.1)3 alloys were prepared by mechanical deformation followed by hot pressing, and their thermoelectric properties were studied. We observed that the Sn element is a very effective dopant as an acceptor to control the carrier concentration in the n-type Bi2(Te0.9Se0.1)3 alloys to optimize their thermoelectric property. The n-type carrier concentration can be controlled from 4.2 × 1019/cm3 to 2.4 × 1019/cm3 by 0.05wt.% Sn-doping. While the Seebeck coefficient and the electrical resistivity are both increased with doping, the power factor remains the same. Therefore, we found that the thermoelectric figure-of-merit becomes maximized at 0.75 when the thermal conductivity has a minimum value for the 0.03wt.% Sn-doped sample.

The initial growth behavior of ZnO films by atomic layer deposition (ALD) on layer-structured Bi2Te3 was investigated. Despite the lack of adsorption sites on the basal plane of Bi2Te3, negligible incubation in the ALD of ZnO on Bi2Te3 was found in the temperature range from 100 to 160 °C, and even the enhancement of the initial growth was observed at 200 °C. We demonstrate that a ZnTe interfacial layer was formed in the early growth stage by the interaction between diethylzinc and Bi2Te3, which improved the nucleation of ZnO on the basal plane of Bi2Te3. These results indicate that surface modification via the interaction between a precursor and layer-structured materials is an efficient way to achieve fluent and uniform nucleation on layer-structured materials such as Bi2Te3.

SnO 2 thin films were grown by atomic layer deposition (ALD) with dimethylamino-2-methyl-2-propoxy-tin(II) (Sn(dmamp) 2 ) and O 3 in a temperature range of 100–230 °C. The ALD window was found to be in the range of 100–200 °C. The growth per cycle of the films in the ALD window increased with temperature in the range from 0.018 to 0.042 nm/cycle. Above 230 °C, the self-limiting behavior which is a unique characteristic of ALD, was not observed in the growth because of the thermal decomposition of the Sn(dmamp) 2 precursor. The SnO 2 films were amorphous in the ALD window and exhibited quite a smooth surface. Sn ions in all films had a single binding state corresponding to Sn 4+ in SnO 2 . The concentration of carbon and nitrogen in the all SnO 2 films was below the detection limit of the auger electron spectroscopy technique and a very small amount of carbon, nitrogen, and hydrogen was detected by secondary ions mass spectroscopy only. The impurity contents decreased with increasing the growth temperature. This is consistent with the increase in the density of the SnO 2 films with respect to the growth temperature. The ALD process with Sn(dmamp) 2 and O 3 shows excellent conformality on a hole structure with an aspect ratio of ∼9. This demonstrates that the ALD process with Sn(dmamp) 2 and O 3 is promising for growth of robust and highly pure SnO 2 films.

The effect of Sn doping on the thermoelectric properties of p-type Bi0.4Sb1.6Te3 (BST) thin films was studied. Sn-doped BST films were grown on 4° tilted GaAs (001) substrates by metal–organic chemical vapor deposition. To control the Sn ion concentration in the films, we systematically controlled the dose of the Sn precursor by varying the H2 flow rate from 0 sccm to 100 sccm. The hole carrier concentration increased as the H2 flow rate was increased. Interestingly, the Seebeck coefficient of the films simultaneously increased with the carrier concentration when the H2 flow rate was increased up to 60 sccm. This might be attributed to the formation of virtual bound states in the valence band by Sn doping. Consequently, the Sn ion doping contributed to the thermopower enhancement of the BST films.

Two-dimensional electron gas (2DEG) at the complex oxide interfaces have brought about considerable interest for the application of the next-generation multifunctional oxide electronics due to the exotic properties that do not exist in the bulk. In this study, we report the integration of 2DEG into the nonvolatile resistance switching cell as a bottom electrode, where the metal-insulator transition of 2DEG by an external field serves to significantly reduce the OFF-state leakage current while enhancing the on/off ratio. Using the Pt/Ta2O5-y/Ta2O5-x/SrTiO3 heterostructure as a model system, we demonstrate the nonvolatile resistance switching memory cell with a large on/off ratio (10(6)) and a low leakage current at the OFF state (∼10(-13) A). Beyond exploring nonvolatile memory, our work also provides an excellent framework for exploring the fundamental understanding of novel physics in which electronic and ionic processes are coupled in the complex heterostructures.

Perovskite CaCu3Ti4O12 has drawn a great deal of attention for various electronic applications due to its giant dielectric property as well as a strong stability in a wide range of temperature. In this paper, we use an off-axis continuous composition-spread (CCS) sputtering method to investigate the full range dielectric characteristics of calcium copper titanate thin films. The film compositions are continuously distributed by deposition from two targets of CaTiO3 and CuTiO3. A slightly Ca-deficient, Cu- and Ti-rich film, which has a 0.9:3.2:4.3 ratio for Ca:Cu:Ti, demonstrated the best performance by showing a dielectric constant of 781 at 100 kHz. On the other hand, all other films far away from the CaCu3Ti4O12 composition showed suppressed dielectric properties. Analyses by X-ray photon spectroscopy, micro-Raman microscopy, transmission electron microscopy, and Rutherford backscattering spectroscopy reveal that there are three possible origins for such superior performance at off stoichiometric thin films: (1) bulk doping by excessive Cu and Ti ions, (2) chemically modified grain boundary, and (3) the lowered electrode-sample interface resistance. Our result will provide a new insight into engineering the dielectric properties using off-stoichiometric synthesis.

In this work, p-type 20%Bi2Te3–80%Sb2Te3 bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%Bi2Te3–80%Sb2Te3 sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10−3/K and 2.85 × 10−3/K were achieved for samples prepared by MA and MD, respectively.

Control of the ferroelastic and ferroelectric domain structure of BiFeO3 through the use of epitaxial growth on substrates with reduced symmetry is reviewed. The first approach presented utilizes orthoscandate substrates, specifically TbScO3, to reduce the number of possible ferroelastic domains from 4 to 2. Experimental results and phase field simulations are presented which are in agreement with the theory of anisotropic strain relaxation, due to differing in-plane lattice parameters of the orthorhombic substrate, causing a reduction in the possible domains. The second approach that is presented involves the use of miscut cubic substrates, such as SrTiO3, to tailor the domain structure from 4-domain to 2- or single-domain is presented, the former being achieved with a miscut in the [1 0 0] direction and the latter with a miscut in the [1 1 0] direction, assuming a film normal orientation of [0 0 1]. The use of these techniques in understanding the fundamental nature of the ferroelastic and ferroelectric properties in BiFeO3, and the use of these methods in tailoring BiFeO3 to meet the needs of future device applications is discussed.

Accuracy of the Harman measurement largely depends on the heat transfer between the sample and its surroundings, so-called parasitic thermal effects (PTEs). Similar to the material evaluations, measuring thermoelectric modules (TEMs) is also affected by the PTEs especially when measuring under atmospheric condition. Here, we study the correction methods for the Harman measurements with systematically varied samples (both bulk materials and TEMs) at various conditions. Among several PTEs, the heat transfer via electric wires is critical. Thus, we estimate the thermal conductance of the electric wires, and correct the measured properties for a certain sample shape and measuring temperature. The PTEs are responsible for the underestimation of the TEM properties especially under atmospheric conditions (10–35%). This study will be useful to accurately characterize the thermoelectric properties of materials and modules.

A two-dimensional electron gas emerged at a LaAlO3/SrTiO3 interface is an ideal system for “spin–orbitronics” as the structure itself strongly couple the spin and orbital degree of freedom through the Rashba spin–orbit interaction. One of core experiments toward this direction is the nonlocal spin transport measurement, which has remained elusive due to the low spin injection efficiency to this system. Here we bypass the problem by generating a spin current not through the spin injection from outside but instead through the inherent spin Hall effect and demonstrate the nonlocal spin transport. The analysis on the nonlocal spin voltage, confirmed by the signature of a Larmor spin precession and its length dependence, displays that both D’yakonov–Perel’ and Elliott–Yafet mechanisms involve in the spin relaxation at low temperature. Our results show that the oxide heterointerface is highly efficient in spin-charge conversion with exceptionally strong spin Hall coefficient γ ∼ 0.15 ± 0.05 and could be an outs...

Interaction between electrons has long been a focused topic in condensed-matter physics since it has led to the discoveries of astonishing phenomena, for example, high-Tc superconductivity and colossal magnetoresistance (CMR) in strongly-correlated materials. In the study of strongly-correlated perovskite oxides, Nb-doped SrTiO3 (Nb:SrTiO3) has been a workhorse not only as a conducting substrate, but also as a host possessing high carrier mobility. In this work, we report the observations of large linear magnetoresistance (LMR) and the metal-to-insulator transition (MIT) induced by magnetic field in heavily-doped Nb:STO (SrNb0.2Ti0.8O3) epitaxial thin films. These phenomena are associated with the interplay between the large classical MR due to high carrier mobility and the electronic localization effect due to strong spin-orbit coupling, implying that heavily Nb-doped Sr(Nb0.2Ti0.8)O3 is promising for the application in spintronic devices.

The thermoelectric properties of I-doped Bi2Te3 films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi2Te3 films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi2Te3 films was easily controlled by the variation in a flow rate of H2 carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi2Te3 films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi2Te3 films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi2Te3 films. Through doping I ions into Bi2Te3, the thermopower was also enhanced in Bi2Te3, and a high power factor of 5 × 10−3 W K−2 m−1 was achieved.