We report the twin structure and phase transition of the VO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document} thin film grown on an r-Al2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}O3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_3$$\end{document}(011 & strns;\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{1}$$\end{document}2) substrate. We found that the film is composed of nano-scale grains with their (2 & strns;\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{2}$$\end{document}11) or (200) crystallographic plane-normal close to the substrate-normal direction. The (2 & strns;\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{2}$$\end{document}11) grains exhibited a twin-domain structure separated by the (100)M\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{ extrm{M}}$$\end{document} twin plane, and their (2 & strns;\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{2}$$\end{document}11) plane-normal was tilted away from the substrate normal by 1.45 degrees\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<<^>>\circ$$\end{document} to accommodate the twin formation. In situ 3D RSMs revealed a gradual reduction of the tilt angle from 1.45 degrees\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<<^>>\circ$$\end{document} to 1.25 degrees\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<<^>>\circ$$\end{document} during the monoclinic-to-rutile structural phase transition. In the rutile phase, the tilt angle remained to be finite, indicating that the (100)M\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_{ extrm{M}}$$\end{document} twin plane is preserved even in the rutile phase. A comparison with electrical resistance measurements showed that the structural phase transition (SPT) occurs at a temperature lower than the metal-insulator transition (MIT), while the recovery of the tilt angle upon cooling proceeds gradually contrasting the steep resistance change. These results suggest that the twin structure may influence the kinetics of the structural phase transition in VO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document} films grown on r-plane sapphire substrates.

We report the twin structure and phase transition of the VO _2 thin film grown on an r-Al _2 O _3 (01 1̅ 2) substrate. We found that the film is composed of nano-scale grains with their ( 2̅ 11) or (200) crystallographic plane-normal close to the substrate-normal direction. The ( 2̅ 11) grains exhibited a twin-domain structure separated by the (100) _M twin plane, and their ( 2̅ 11) plane-normal was tilted away from the substrate normal by 1.45 ^∘ to accommodate the twin formation. In situ 3D RSMs revealed a gradual reduction of the tilt angle from 1.45 ^∘ to 1.25 ^∘ during the monoclinic-to-rutile structural phase transition. In the rutile phase, the tilt angle remained to be finite, indicating that the (100) _M twin plane is preserved even in the rutile phase. A comparison with electrical resistance measurements showed that the structural phase transition (SPT) occurs at a temperature lower than the metal–insulator transition (MIT), while the recovery of the tilt angle upon cooling proceeds gradually contrasting the steep resistance change. These results suggest that the twin structure may influence the kinetics of the structural phase transition in VO _2 films grown on r-plane sapphire substrates.

Controlling the metal-insulator transition (MIT) in VO2 thin films requires a fundamental understanding of the physical and chemical properties at the film-substrate interface, such as interfacial mixing, local oxygen stoichiometry, and metastable intermediate polymorphs arising from lattice-level shear/tilt distortions. With threedimensional X-ray reciprocal-space mapping and hard X-ray photoelectron spectroscopy (HAXPES), we explore the structural and electronic evolution of VO2 thin films deposited on LaAlO3(111) during the MIT. We find that diffusion of La atoms from the LaAlO3 substrate generates La-enriched and La-deficient VO2 grains in the film, leading to the stabilization of an intermediate triclinic (T) phase that coexists with the monoclinic (M1) phase. Compared to the M1 phase, the T phase exhibits a higher MIT temperature and a distinct lattice-distortion pathway. HAXPES measurements reveal a distinct chemical state of La-containing VO2 grains in the interfacial region, which nevertheless exhibit typical MIT behaviors. Our findings demonstrate that cation interdiffusion can influence the formation of VOA polymorphs and their structural transition pathways, thereby providing valuable insights into the relationship between phase transition mechanisms and interfacial properties in VOA.

The selective phase growth of Ti-based oxide thin films on sapphire substrates is crucial in controlling the electronic properties, such as the insulator-to-metal transition (IMT). Thin films are generally prepared by pulsed laser deposition under high temperatures, but it is challenging to obtain a smooth surface. In this study, we deposited ultra-smooth epitaxial TiO2 and Ti2O3 thin films with roughnesses below 0.34 Å on sapphire substrates. By controlling the oxygen partial pressure at a relatively low temperature, at 473 K, we obtained highly crystalline thin films with selective growth. The thin films grown at 1 and 10−3 Pa exhibited a rutile-type TiO2 phase, and those grown at 10−6 Pa exhibited a hexagonal Ti2O3 phase. The crystal structures and electronic structures were consistent with the previous reports on TiO2 and Ti2O3 thin films. Moreover, Ti2O3 underwent an IMT, whereas TiO2 was unchanged.

We carried out simultaneous in-situ three-dimensional X-ray reciprocal space mapping and electrical resistance measurements of VO2 films grown on c-cut sapphire to investigate the relationship between the structural phase transition (SPT) and the metal-insulator transition (MIT). The decoupling of the MIT and SPT behaviors was more pronounced in the thinnest film (37 nm), with a difference in transition temperatures (triangle Tc) of approximately 8.3 degrees C. Despite a decrease of over 50% in the M1 fraction, the electrical resistance remained in the insulating phase, indicating a delay in the MIT. This behavior is attributed to the disconnected island morphology, which restricts the formation of continuous conduction pathways. As the film thickness increased, triangle Tcgradually decreased, and eventually both thermal-hysteresis characteristics became similar at a thickness of 360 nm. Furthermore, pronounced six-fold diffuse X-ray scattering was observed around the VO2 M1 (020) Bragg peak, revealing the presence of structural defects and small crystalline domains. This hexagonal pattern originates from three in-plane variants that are rotated by 120 degrees about the film-normal b-axis. Each variant is paired with a 21 screw-axis twin domain, corresponding to a 180 degrees rotation about the same axis. As a result of this combined rotational and twinning symmetry, the diffuse scattering appears along the a* and c* directions of the M1 phase. The correlation length (xi x) of the monoclinic order steadily decreased even in the pre-transition region, exhibiting a trend similar to that of the electrical resistance. When xi x reached approximately 6 nm, the electrical resistance began to decrease rapidly. This correlated behavior suggests a possible interplay between structural disorder, such as defect generation between coherent domains and limited grain connectivity, and charge transport in VO2 films.

With the applications of in situ X-ray diffraction (XRD), electrical I-V measurement, and ambient pressure hard X-ray photoelectron spectroscopy (AP-HAXPES), the characteristics of the topotactic phase transition of LaCoO3 (LCO) thin films are examined. XRD measurements show clear evidence of structural phase transition (SPT) of the LCO thin films from the perovskite (PV) LaCoO3 to the brownmillerite (BM) La2Co2O5 phases through the intermediate La3Co3O8 phase at a temperature of 350 degrees C under high-vacuum conditions, similar to 10(-5) mbar. The reverse SPT from BM to PV phases is also found under ambient pressure (>100 mbar) of air near 100 degrees C. Both observed SPTs in XRD are also identified in the electrical I-V measurements, i.e., the metallic PV phase to the insulating BM phase and vice versa. During the onset of SPTs, the bulk chemical and electronic states of LCO thin films are monitored with AP-HAXPES. The oxidation states in Co 2p spectra indicate that the oxygen vacancies are closely related to the SPT of LCO thin films. Also, the presence of enlarged band gap is observed as the SPT from PV to BM phases takes place, revealing the modified electronic properties of LCO due to the creation of oxygen vacancies. The analysis of valence band structures is further compared to the I-V measurements.

We present a new type of stretchable dichroic film in which Au and Ag alloy nanoparticles (NPs) are dispersed in polydimethylsiloxane (PDMS). The alloy NPs are synthesized with different atomic compositions and sizes to modulate their plasmonic resonance frequencies and absorption and scattering cross sections. The PDMS dichroic film in which 100 nm alloy NPs with a Au/Ag ratio of 7:3 are dispersed shows exotic optical properties under tensile strain. When 40% tensile strain is applied, the film exhibits a strain-sensitive transmission and straininsensitive reflection behavior in which the transmittance is increased up to 2.6 times, whereas the reflectance remains unchanged. Moreover, we demonstrate a stretchable anticounterfeiting film and a flexible dichroic sculpture fabricated with the PDMS composite. This work demonstrates a new type of plasmonic application that has great potential in various applications, such as special-purpose optical films, security patterns, and smart windows.

We report on the behavior of the monoclinic order near the metal-insulator phase transition (MIT) in vanadium dioxide (VO2) films grown on c-plane sapphire investigated by three-dimensional x-ray reciprocal space mapping (RSM). In the plane perpendicular to the film normal monoclinic b-axis [010] direction, pronounced diffuse scattering was observed in six specific directions whose origin was attributed to the rutile-like planar defects separating ordered monoclinic domains. The correlated region of the monoclinic domains was thin elliptical disk shaped with the disk normal along the monoclinic [001] direction. As the MIT was approached, the diffuse peaks first moved away from the (020) Bragg peak progressively and then disappeared rapidly, which suggests that the monoclinic-to-rutile structural phase transition accompanying the MIT was progressed in two stages. In the first pre-transitional stage, the rutile-like defect boundaries separating monoclinic domains are nucleated and grow progressively, which crosses over to the transitional stage where the rutile phase grows rapidly within the domains leading to the transition.

In this study, the conceptual design and performance of a multimodal X-ray probe station recently installed at the 9C coherent X-ray scattering beamline of the Pohang Light Source-II are presented. The purpose of this apparatus is to measure coherent X-ray diffraction, X-ray fluorescence and electrical properties simultaneously. A miniature vacuum probe station equipped with a four-point probe was mounted on a six-axis motion hexapod. This can be used to study the structural and chemical evolution of thin films or nanostructures, as well as device performance including electronic transport properties. This probe station also provides the capability of varying sample environments such as gas atmosphere using a mass-flow-control system and sample temperatures up to 600°C using a pyrolytic boron nitride heater. The in situ annealing of ZnO thin films and the performance of ZnO nanostructure-based X-ray photodetectors are discussed. These results demonstrate that a multimodal X-ray probe station can be used for performing in situ and operando experiments to investigate structural phase transitions involving electrical resistivity switching.

We report a graphene oxide (GO)-based composite, featuring GO/cross-linking agent (CA) nanoparticles, inspired by a nacre-like hierarchical structure present in nature. The as-prepared GO/CA composite was powdered to nanoscale particles and then mixed with pure GO to be GO/CA/GO (GCG) composite forming hierarchical GO/CA nanoasperities on the GO surface. The strength and toughness of the nacre-inspired GCG composite films were simultaneously improved by adjusting the nanoparticle concentration and hierarchical level of the GO-based films. Compared to pristine GO films and GO/CA composites, which exhibit a low level of hierarchy in their structures, the tensile strength and toughness of the GCG composites with higher hierarchy were enhanced 3.1 and 1.6 times and 47.6 and 10.9 times, respectively. Furthermore, a plausible mechanism of increasing mechanical properties based on nanoscale asperities and homogeneous interactions between GO and CA has been discussed.

We report a metal/polymer layered structure to control the transparency and surface morphology by applying strain. After a thin metal layer is coated on a PDMS substrate, the applied strain causes the micropattern on the surface to buckle owing to the difference in the elastic modulus between the metal and PDMS. When we start stretching the layered structure, the micropattern on the substrate is formed perpendicular to the direction of strain. As the substrate is stretched, the micropattern becomes wider and the substrate changes from opaque to transparent. When we stretched the substrate further by approximately 80% or more, the second buckling caused an additional micropattern in the direction of the strain. Consequently, the substrate became opaque again. We determined the mechanism based on the micropattern image and light diffraction analysis. These micropatterns change the sliding behavior of a water droplet on a slanted substrate. Moreover, the sliding speed of the water droplets was determined by the elongation rate of the substrate. We demonstrate a smart window to control the transparency and sliding speed of a water droplet using our bilayer substrate along with stretching.

We report on the synthesis and structure of 'V' shape twinned vanadium dioxide (VO2) nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)M twin plane played a key role in determining the morphology of VO2 nanocrystals. The growth of VO2 nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (aM- axis) direction resulting in 'V' shape twinned crystals with the angle between the sides of approximately 115.4 degrees. At a relatively low growth temperature of 900 degrees C, the growth of the sides of 'V' was limited producing 'coffee-bean' shape crystals in which flat crystal facet regions are connected with rounded edges. The twinned VO2 nanocrystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (bM-axis) normal to the substrate. In the in-plane direction, the (001)M planes of the VO2 twin crystals were aligned to the direction +/- 2.3 degrees away from the sapphire (1120) plane. The sides of 'V' exhibit a rectangular cross-section truncated by the substrate. In-situ synchrotron x-ray diffraction measurement across the metal-insulator transition of the twinned nanocrystals implies that the phase transition of the coffeebean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown Vshape nanocrystals.

We report on the structural transformation of VO2 particles from the insulating monoclinic to the metallic rutile phase using in-situ synchrotron X-ray diffraction (XRD) in a grain-orientation-specific manner using a two-dimensional X-ray pixel detector. The XRD data averaged in all grain orientations corresponding to a typical powder XRD profile show that the transition occurred over a broad temperature range of about 8.9 K. However, we found that the transition temperature and range of individual grains vary greatly. We attribute the broadness of the transition in the averaged profile to this variance of the transition temperature among grains. Consistently, the transition was much sharper in the nanoparticle specimen with much fewer grains than the bulk powder specimen. To understand the intrinsic physical characteristics of VO2, it is necessary to carefully analyze the data of the bulk system and it is preferable to study the VO2 single crystal.

Recently, wearable triboelectric sensors capable of self-powering, which can be widely used in artificial skin and robotics, have received much attention. Herein, we develop a stretchable triboelectric pressure sensor with a new pattern by superimposing two patterns using both polystyrene beads and UV-ozone treatment. This patterned structure works more sensitively to pressure than a general planar and one-kind patterned structure. The sensor is constructed by sandwiching styrene butadiene rubber (SBR) and poly(dimethylsiloxane) (PDMS). It demonstrates a high sensitivity of 0.078 kPa-1 (0-20 kPa), a low detection limit (1.2 kPa), and pressure sensitivity maintained under 40% strain. The detection behavior of the strain-insensitive triboelectric sensor against pressure is very consistent with the simulation based on the theory. In applications, we successfully detect various human motions, not only small motions such as bending fingers but also large motions such as standing up and raising arms.

Monolayer graphene grown by chemical vapor deposition has been intensively studied for applications such as transparent conductive films, electronic devices, sensors, molecular barriers, and electrodes. However, technology to transfer monolayer graphene from metal film must be improved through environmentally friendly and non-defective methods on arbitrary target substrates. Here, we report a clean and direct method for transferring monolayer graphene from Cu foil without defects and over a large area. In a water bath at 90-95 degrees C, we floated Cu foil with graphene on the water surface. After 5 h, a Cu2O layer formed uniformly at the interface between the graphene and the Cu. Subsequently, the monolayer graphene on the Cu foil was delaminated from the thermal release tape, releasing the graphene to the target substrate (SiO2/Si and PET). The Cu2O formation and defect changes were monitored at each step via various characterization methods. The Cu2O layer was uniformly established and no defects were generated after the transfer. Finally, we fabricated a graphene field effect transistor that exhibited an excellent electronic performance.

Aesthetically appealing photovoltaic (PV) panels with colorful layers are used in numerous applications involving color matching with the surroundings. To develop a colored film for a PV system, appropriate optical properties such as high transparency and low angle sensitivity are necessary because the colored layers can reduce the efficiency of the PV system by causing variations in the transmittance and angle of incidence. Herein, we propose a facile fabrication method for bioinspired three-dimensional (3D) photonic crystal (PC) films that exhibit broad angle-insensitive transmission and reflection, for application in colorful PV. This structure, patterned on a sequentially stacked 11-layer film of SiO2 and TiO2, is fabricated via nanoimprint lithography and a one-step dry-etching process, without using a metal mask. The changes in transmission and reflection are observed via ultraviolet-visible spectroscopy and from the reflected images obtained under various angles. The transmittance dips of the 3D PC film shift by less than 10 nm in wavelength, for angles from 0 to 45°, indicating low angle dependency. In addition, the change in the observed color, with respect to the viewing position, is less in the fabricated film. Once the 3D PC film was added to a commercial PV cell, it exhibited a higher efficiency (approximately 6% upper) when compared to a cell with a one-dimensional PC film, during the duration of the experiment, from 0 to 30°. Thus, the proposed method demonstrates excellent potential for developing structural color films for achieving aesthetically appealing PV cells.

We report on the pulsed laser-induced dewetting (LID) process of Co/sapphire(0001) thin films in water environment. In contrast to LID in a vacuum, where only Co nanoparticles (NPs) are formed, a 12 nm thick Co film was dewetted into a mixture of crystalline CoO NPs and nanowalls (NWs). The formation of the CoO NPs can be explained by the grooving model occurring concurrent with the oxidation of Co to CoO. Possible scenarios for the formation of NWs are proposed. The dissolution of Co into water occurred near the hole area. Upon subsequent laser irradiation, Co2+ and O2- ions abundant in water facilitate the laser-induced hydrothermal process (LIHP) producing CoO NWs. Furthermore, we found that the combination of dissolution and LIHP is energetically more favorable over dewetting in a 4.8 nm thick sample, where only CoO NWs were observed. Finally, the reduction of sapphire substrate is revealed by the formation of Al-rich NPs. (c) 2020 The Japan Society of Applied Physics

ABO3 perovskite materials and their derivatives have inherent structural flexibility due to the corner sharing network of the BO6 octahedron, and the large variety of possible structural distortions and strong coupling between lattice and charge/spin degrees of freedom have led to the emergence of intriguing properties, such as high-temperature superconductivity, colossal magnetoresistance, and improper ferroelectricity. Here, an unprecedented polar ferromagnetic metal phase in SrRuO3 (SRO) thin films is presented, arising from the strain-controlled oxygen octahedral rotation (OOR) pattern. For compressively strained SRO films grown on SrTiO3 substrate, oxygen octahedral network relaxation is accompanied by structural phase separation into strained tetragonal and bulk-like orthorhombic phases, and the asymmetric OOR evolution across the phase boundary allows formation of the polar phase, while bulk metallic and ferromagnetic properties are maintained. From the results, it is expected that other oxide perovskite thin films will also yield similar structural environments with variation of OOR patterns, and thereby provide promising opportunities for atomic scale control of material properties through strain engineering.

One of the major challenges in the field of nanotechnology is the facile and inexpensive fabrication of < 10-nm nanostructures with a defect-free and precise pattern over a large area. Nanoimprint lithography (NIL) is a nanopatterning method that allows low-cost, fast production, and large-scale fabrication, but it needs improvement with regard to achieving a high resolution over a large area, processing on soft substrates, and nanomaterial patterning. Herein, it is demonstrated 10-nm patterning on soft and hard substrates via an advanced nanoimprint process. With a 100-nm master mold, both soft and hard molds with reduced pattern sizes (50, 20, 10, and 5 nm) are first fabricated using atomic layer deposition (ALD) of Al2O3. After surface functionalization of the mold, nanoscale patterns are imprinted on both hard (Si) and soft polyethylene terephthalate substrates, which result in structures with a pattern size of 10 nm. Using a hybrid supporting layer poly(methyl methacrylate) (PMMA)/poly(vinyl alcohol) (PVA) and ion sputter etching, a well-defined and clean sub-10 nm nanostructure is achieved for the nanomaterial on the substrate after lift-off and thermal annealing processes. Using this method, a graphene nanoribbon 10 nm wide is fabricated. Our approach is suitable for the fabrication of devices and structures with a 10-nm-scale pattern over a large area.

We report on the graphene oxide/cross-linking agent (GO/CA) composite inspired by the nacre structure. Based on the "brick-and-mortar" concept of nacre, graphene oxide and cross-linking agent are covalently conjugated in the form of nacre. The mechanical characteristics of the nacre-mimetic GO/CA composite film can be controlled by adjusting the preparation method, degree of cross-linking, and cross-linking times. As a result, the cross-linking strategy can drastically enhance the tensile strength (142.9 ± 6.4 MPa (~ 2.3-fold)), modulus (4.7 ± 0.36 GPa (~ 15.7-fold)), and hardness (917.4 ± 85.7 MPa (~ 9.0-fold)), which are superior to those of pristine materials. The cross-linking agent-based chemical bonding method for mechanically improved integration is mainly attributed to the formation of strong cross-linked networks between the GO-based 2D interfaces and CA. The facile fabrication process provides many opportunities to design advanced, robust, and integrated nacre-like GO/CA composites, which can be applied to future aerospace utilizations, electronic protectors, robotic elements, and permeable membranes.