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Dual-frequency ultrasound has advantages over conventional ultrasound, which operates at a specific frequency band, due to its additional frequency response. Moreover, a tunable frequency from a single transducer enables sonographers to achieve ultrasound images with a large detection area and high resolution. This facilitates the availability of more advanced techniques that require low- and high frequency ultrasound simultaneously such as harmonic imaging and image-guided therapy. In this article, we present a novel method for dual-frequency ultrasound generation from a ferroelectric piezoelectric micromachined ultrasound transducer (PMUT). Uniformly designed transducer arrays can be used for both deep low-resolution imaging and shallow high-resolution imaging. To switch the ultrasound frequency, the only requirement is to tune a DC bias for controlling the polarization state of the ferroelectric film. Flextensional vibration of the PMUT’s membrane strongly depends on the polarization state, producing low- and high-frequency ultrasound from a single excitation frequency. This strategy for dual-frequency ultrasound dispenses with the requirement for either multi-electrode configurations or hetero-designed elements, which are integrated into an array. Consequently, this technique significantly reduces the design complexity of transducer arrays and their associated driving circuits.

Atomic layer deposition of MoS2 reveals stepwise growth – ‘stop and go’ growth of MoS2 – behavior where the surface migration energy of the adsorbates is significantly enhanced.

We demonstrate the successful remote oxygen scavenging of HZO-based capacitors, highlighting the significant enhancement of remanent polarization, switching voltage, endurance, and retention.

Three-dimensional flower-like tungsten oxide nanoplates sequentially doped with Co at different concentrations were fabricated using a single solvothermal process. The response of the nanostructure of the n-type tungsten oxide used for sensing ethanol gas, which is a reducing gas, changed sequentially according to the process temperature and doping concentration of Co. This response cannot be attributed to a single reason, but it can be regarded as a result of the combination of several complicated direct and indirect phenomena such as (1) point, line, and cross-section defects on the surface, (2) change in initial resistance corresponding to doping concentration, (3) catalytic effect of dopant, and (4) difference in energy level at heterojunctions. Therefore, unlike in the case of other gas sensing methods that depend on a main cause or mechanism, it might be meaningful in the case of ethanol gas sensing to analyse each cause and predict the results of different combinations.

The urchin-like nanoparticles of semiconducting oxides have been attracting considerable attention as they can significantly improve reactivity owing to their enlarged surface areas. Herein, we report a simple and scalable synthesis method of ZnO nanomaterials via a microwave-assisted drying process. Commercial ZnO powders (~300 nm) and expanded graphite were used as starting materials to prepare urchin-like ZnO nanoparticles with nanoscale legs (diameter ~20 nm) by triggering the rapid phase transformation, which was induced by a repeated microwave irradiation in an alumina crucible for a few minutes. The possible mechanisms to trigger this morphological and dimensional change are described in detail. The high-yield production and superior reproducibility completed in shortened processing times enable this synthetic route to be widely applicable for the massive production of nanostructured oxide particles.

Vanadium oxides exhibit a broad spectrum of physical properties due to their ability to form various compounds and polymorphs. To utilise a particular property, it is essential to selectively synthesise a desired phase. Herein, we demonstrate a method to selectively and reproducibly grow (00l)-textured VO2(B) thin films using an amorphous SrTiO3 buffer layer by sputtering at −3.5%/K at 25 °C and >−1.5%/K at 95 °C) and low electrical resistivities (∼5 × 10−1 Ω cm at 25 °C and

Grain enlargement is a crucial requirement to synthesizing two-dimensional (2D) metal chalcogenides because it can minimize the effects of carrier scattering at the grain boundaries. To this end, r...

Herein, we have reported domain engineering of epitaxial (001) Bi2Te3 thin films by miscut (100) substrates. On a nominal flat (100) GaAs substrate, two-variant domains that were in-plane rotated by 60°, including the 60° domain boundaries, were formed in the epitaxial Bi2Te3 film, such that the symmetry elements of two-fold rotational and/or mirror symmetries of the GaAs substrate were preserved. The domain variants were successfully reduced to obtain mono-domain Bi2Te3 thin films without any domain boundaries using the 2°-miscut GaAs substrates, where a particular step-and-terrace structure on the vicinal surface macroscopically broke the intrinsic symmetry of GaAs, lowering the number of possible domains. Depending on the miscut directions, the in-plane orientations of the mono-domain Bi2Te3 films were varied with respect to the GaAs substrate. A model was proposed to explain the effect of miscut substrate on the domain structure of Bi2Te3 thin films. Low-temperature Hall measurements revealed that in the intrinsic regime (10 K) the electron concentration of the mono-domain Bi2Te3 films (~2 × 1018 cm−3) was significantly lower than that of the two-domain films (~1019 cm−3). This was attributed to the donor-like effect of the 60° domain boundaries. These results provide an opportunity not only to integrate the single-crystalline, mono-domain, layered-chalcogenides on semiconductor single crystals, but also to manipulate their electronic transport properties by domain engineering.

The ease of Te sublimation from Bi2Te3-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi2Te3-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi2Te2.7Se0.3 powders resulted in significant suppression of the generation of pores in Bi2Te2.7Se0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi2Te2.7Se0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi2Te3-based alloys without sacrificing thermoelectric performance.

Hot extrusion is a promising method for producing high-performance thermoelectric bismuth telluride alloys because of its ability to create textured microstructures. However, hot extrusion is less favourable for scaling-up because of temperature and strain gradients along the radial direction, and only -textured thermoelectric legs can be obtained because of the fibre-like texture. We suggest a way to overcome these disadvantages by implementing an additional spark plasma sintering process on a stack of extrudates. Using this combined process, we demonstrate the fabrication of 12 × 15 × 13 mm3 p-type (Bi0.2Sb0.8)2Te3 samples from extrudates that had originally been 3 mm in diameter. The evolution of sheet-like texture revealed by SEM, XRD, and EBSD allows us to obtain both - and -textured thermoelectric legs from a single specimen that are desirable for low- and high-temperature applications, respectively. Our results demonstrate the combined method as an industry-friendly process for fabricating high-performance thermoelectric materials.

The outstanding multifunctionality of ferroelectric oxides has opened up new fields in microelectronics. However, the high crystallization temperature of the ferroelectric oxides limits their integ...

Controlling the structure of a material over a wide range of scales has been extensively pursued because the structure dictates its function. Here, we explore the formation of 3D structures at almost a millimetre scale using single-crystal complex oxides, which has been challenging because of the brittle nature of oxides. Our scheme is to release epitaxial oxide thin film heterostructures from the rigid substrate in order to utilise the elastic epitaxial strain as a driving force for self-shaping the flexible free-standing membrane in a controlled manner. Using an epitaxial free-standing LaAlO3/SrTiO3 membrane as a model system, we were able to create various 3D forms, such as cylindrically-rolled, spherically-bent, and helically-twisted structures, where the inversion-symmetry is broken by the strain gradient via flexoelectric effects. Our results will provide opportunities not only to broaden the application of functional oxides toward flexible electronics, but also to discover new functionalities driven by 3D architectures at various scales.

The atomic layer deposition (ALD) of SnO 2 films has drawn interest from many researchers as a way to enhance the performance of functional materials for emerging applications such as transistors, sensors, and transparent electrodes. Among the critical issues in the exploitation of SnO 2 ALD in industrial applications is the lack of commercially available Sn precursors. Here, we demonstrate an ALD process for SnO 2 films using commercially available tetraethyltin (TET) as a precursor. H 2 O 2 is used as the oxygen source to overcome the lack of reactivity between TET and H 2 O, consequently the reaction of TET and H 2 O 2 results in the growth of SnO 2 films. The ALD process has a wide ALD window of 250–400 °C. In the range of the growth temperature, the grown films show a high density of ~6.2 g/cm 3 and an optical band gap of 3.7–3.9 eV, which is comparable to that of bulk SnO 2 . Negligible impurities remained in the films grown over the entire temperature range. The crystallization behavior and electrical properties of the SnO 2 films were examined as well. The ability of this ALD process to produce high-quality SnO 2 films with a commercially available tin precursor will allow it to be exploited in various applications.

We investigated the optical response of La0.67Sr0.33MnO3 (LSMO) films which show the metal-insulator transition (MIT) with the film thickness dependence. By using spectroscopic ellipsometric technique we found that the optical spectra below the charge transfer gap exhibited the coherent-incoherent crossover behavior through MIT. The formation of the incoherent mode near 1.5 eV was reminiscent of the polaron absorption which had been widely observed in various manganites. We suggest that the electron-phonon coupling could be enhanced due to low dimensionality in the ultrathin LSMO film in consideration of orbital polarization.

Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring ac...

Conventional artificial muscles induce bending by aligning large-sized ions within the electrolyte upon bias application. Such design, alike many other actuator types, suffer from volatile actuation where the actuated position gets lost upon switch-off. Here, we develop a non-volatile artificial muscle with ion insertion electrode materials. Upon bias application, the inserted ions pose stress on the electrodes that sustain even after power shut-off. The demonstrated actuator consists of lithium germanide (LixGe) thin films deposited on both sides of a flexible polyimide (PI) substrate. The device exhibits 35.2 mm displacement when operated at 2 V and generates the blocking force of 0.67 mN. The observed stress and volume expansion reach 248 MPa and 8.2% for the 284 nm Li3Ge thin films, respectively. The actuated position is maintained against gravity with 12.1% decay in the actuated distance after 10 min. The novel actuator type proves the potential use of lithium insertion materials as actuation materials and shows that non-volatile actuation can be realized with ion-insertion electrodes.

SrTiO3 (STO) is a highly attractive oxide material due to its flexible tunability of electrical properties. It can be designed to exhibit a high mobility with a tunable carrier concentration by creating oxygen vacancies, or by doping with Nb or La, which substitute the Ti and Sr sites, respectively. Here we show a micro-patterned surface doping of STO by using reactive ion etching (RIE). The creation and pattering of a conductive STO surface were achieved by sequential treatments with Ar and O2 plasma. The patterned conductive surface edge was well defined as confirmed by an electrostatic force microscopy. The electronic characteristics of the RIE treated STO surface were probed by a synchrotron radiation photoemission spectroscopy, which shows the emergence of Ti3+, Ti2+, Ti1+ states and metallic states near the Fermi level. The electrical mobility of the conductive STO surface can be increased up to 12000 cm/V s with a typical sheet carrier concentration around 1013–1014 cm−2. Increasing Ar plasma time elongate the depth of the conductive surface, which reflects the change of magnetoresistance behavior at low temperature. The demonstrated control of the STO surface conductivity along with a large area and high precision patterning method can be widely used for a variety of oxide electronic and spintronic devices.

Oxide interfaces such as LaAlO3/SrTiO3 (LAO/STO) are interesting platforms for the investigation of ‘spin–orbitronics’ because of their strongly coupled spin and orbital degrees of freedom due to the inversion asymmetry of the structure. In this investigation, we demonstrate a tunable Rashba spin–orbit field at the LAO/STO interface via the application of an external gate electric field. The strength of the Rashba field was indirectly estimated by measuring the planar angle dependence of the anisotropic magnetoresistance (AMR). The asymmetry of the planar AMR between θ = 0 and π indicates the existence of Rashba spin–orbit fields, which are tunable by adjusting the current density and gate electric field. From the AMR measurements, the effective Rashba field exhibits up to 4 T for the application of an external back-gate voltage of 30 V. This controllable and relatively high Rashba field suggests that the LAO/STO is an attractive 2-D interface for potential spin–orbitronic applications, such as spin-charge converters, spin-FETs, and spin–orbit torque devices.

The electrical resistance of a thermoelectric module is an important parameter as well as the thermoelectric figure-of-merit for the application of both waste energy harvesters and solid-state cooling devices. High resistance causes energy dissipation through Joule heating, leading to the degradation of the device's generating or cooling performance. Therefore, it is highly desirable to reduce the electric resistivity of the thermoelectric materials. Here, we report a hot extrusion technique to fabricate p-type bismuth antimony tellurides that has low electrical resistivity. Extrusion is a promising tool to fabricate a fiber-like texture of the microstructure, where the 001 normal vector of grains is pointing perpendicular to the extrusion direction. Such a textured microstructure can enhance the mobility of electrical carriers along the extrusion direction due to the anisotropic nature of electrical transport in bismuth telluride-based alloys. Beyond bismuth telluride-based thermoelectric materials, we believe that our result will offer a simple tool to manipulate physical properties of any two-dimensional, layered materials by controlling microstructure.

We demonstrate a fabrication of an atomically controlled single-crystal heart-shaped nanostructure using a convergent electron beam in a scanning transmission electron microscope. The delicately controlled e-beam enable epitaxial crystallization of perovskite oxide LaAlO3 grown out of the relative conductive interface (i.e. 2 dimensional electron gas) between amorphous LaAlO3/crystalline SrTiO3.

Deep reactive-ion etching (DRIE) is commonly used for high aspect ratio silicon micromachining. However, scalloping, which is the result of the alternating Bosch process of DRIE, can cause many problems in the subsequent process and degrade device performance. In this work, we propose a simple and effective method to smoothen the scalloping of DRIE trenches. The proposed method utilizes sidewall dry etching by reactive-ion etching (RIE) based sulfur hexafluoride (SF6) plasmas, following the DRIE process. To investigate the effect of the etch parameter on the scallop smoothing effect, the radio frequency (RF) power and gas flow are controlled. After the RIE treatment, the scallop smoothing effects were evaluated by measuring the average scallop depth under each condition. The scallop depth was reduced by 91% after implementing the scallop smoothing technique using RIE. Thus, our smoothening method based on SF6 plasmas would provide broad availabilities and applicability in silicon micromachining with the simple low-temperature process.