We investigated the crystallization behavior and structural evolution of amorphous Ga2O3 thin films deposited on sapphire (0001) substrates via solid phase epitaxy (SPE) using a synchrotron-based in-situ multimodal X-ray probe station. Real-time X-ray diffraction (XRD) and electrical conductance measurements during post-annealing revealed that crystallization into the alpha-Ga2O3 phase begins at approximately 500 degrees C, accompanied by a marked increase in conductance owing to thermally activated carrier transport. High-resolution XRD confirmed the formation of a high-quality alpha-Ga2O3 film with the c-axis oriented along the surface normal and fully relaxed lattice parameters in the in-plane and out-of-plane directions. Azimuthal angle scans revealed six-fold symmetry, confirming epitaxial in-plane alignment between the alpha-Ga2O3 film and sapphire (0001) substrate. Rocking curve analysis yielded dislocation densities of similar to 1.07 x 10(8) (screw) and similar to 4.34 x 10(9) cm(-2) (edge), corresponding to a total threading dislocation density of approximately 4.45 x 10(9) cm(-2), which is comparable to previously reported values for directly grown alpha-Ga2O3 films. These findings demonstrate that SPE is a promising alternative to conventional epitaxy for producing high-quality alpha-Ga2O3 thin films. This approach enables phase-selective crystallization with excellent structural quality, without the need for high-temperature deposition or complex substrate engineering, thereby offering a viable pathway for integration into thermally sensitive device platforms.

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.

The crystallization of the liquid Sn tip following vapor–liquid–solid (VLS) growth of Sn–ITO nanostructures, presents an intriguing problem due to the confinement by the bottom substrate and side ITO wall. We investigated the crystalline structure and morphology of the Sn tips grown by VLS on c-plane sapphire using Bragg coherent diffraction imaging (BCDI) with a nanoscale spatial resolution in three dimensions. BCDI reconstructions of individual Sn tips reveal that each one is a single crystal with a high degree of crystalline order. Each tip maintains an overall spherical morphology—reminiscent of a liquid droplet—truncated at its base and one side. Despite this, distinct crystalline facets are identified on the crystal surface. Furthermore, a defective region was observed near the tip–substrate interface, characterized by high strain and reduced crystalline order.

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.

We present a 3D reciprocal-space mapping (RSM) method using a pink self-amplified spontaneous emission (SASE) X-ray free-electron laser beam. The energy of each specific pulse in a SASE beam can be determined using the diffraction pattern of a specimen excited by pumping itself as a spectroscopic reference. A thin slab of RSM, whose thickness corresponds to the energy bandwidth of the pink beam, is successfully reconstructed using the proposed method. By rocking a sample in a few steps, we obtained a 3D RSM covering both the diffuse scattering and the Bragg rod in NiO thin films during a pump-probe X-ray diffraction measurement.

Formation of a eutectic AuSi compound was observed during the solid state dewetting and agglomeration in a Ni-Au bilayer thin film grown on a Si3N4 substrate using coherent X-ray diffractive imaging, X-ray diffraction, and scanning electron microscopy. During rapid thermal annealing at 850 °C in vacuum, AuNi films first separate into islands that are composed of a mixture of Au-rich and Ni-rich phases. As the dewetting proceeds, Si and nitrogen dissociate due to the catalytic action of Ni, which resulted in the formation of NiN, NiSi, and AuSi. With increasing time of the annealing process, nitrogen atoms in NiN are gradually evaporated by forming N2. The eutectic phenomenon in AuSi alloys results in the migration of Au atoms to form Au5Si2 with a composition near the eutectic point, 18.6 at% of Si. Our findings indicate that nucleation of the Au5Si2 alloy formation does not primarily occur through bulk interdiffusion, but instead initiates through grain boundary diffusion of Au atoms.

Formation of a eutectic AuSi compound was observed during the solid state dewetting and agglomeration in a Ni–Au bilayer thin film grown on a Si 3 N 4 substrate using coherent X-ray diffractive imaging, X-ray diffraction, and SEM.

We investigated the consequences of discrete Fourier transformation in coherent diffraction imaging (CDI). The object density reconstructed from the discretely sampled diffraction data within a truncated range is inherently aliased, blurred, and further aggravated in phase retrieval process. We devised a preprocessing procedure to correct input Fourier constraints using a convolution kernel and to exclude erroneous Fourier constraints. By applying the proposed preprocessing to both simulated and experimental data, we demonstrated that image reconstruction was substantially improved, effectively suppressing physically unsound fluctuations in the retrieved images. This procedure could improve the fidelity of the quantitative object density retrieved by CDI.

Ultrafast laser excitation can drive materials into exotic states beyond thermodynamic limits, offering alternative ways to control how matter stores and releases energy. Yet, whether light can actively steer energy-relaxation pathways during structural transitions remains unclear due to the lack of direct experimental evidence. Here we show, using single-pulse time-resolved X-ray imaging of gold nanorods, that photoinduced localized surface plasmons control ultrafast energy relaxation into distinct deformation modes, transverse or longitudinal deformation modes, each accompanied by characteristic plasmon-induced oscillatory distortions depending on the laser fluence. Numerical simulations further confirm that localized surface plasmons dictate ultrafast energy relaxation process from photoexcited hot electrons to anharmonic nanocrystal deformations. Our results provide direct evidence that surface plasmon-mediated interactions enable ultrafast, nanoscale control of materials' energetics, opening a pathway for tailoring energy-transfer processes with femtosecond laser fields. This approach lays the foundation for customizing nonequilibrium phase dynamics at the nanoscale and provides a route to tailoring energy-transfer processes using femtosecond laser fields.

The polymorphic solid phase epitaxy of amorphous tin oxide (SnO2) thin films deposited on sapphire(0001) substrates through radio-frequency powder sputtering was investigated. A multimodal X-ray probe station was utilized to simultaneously investigate the crystallization and electronic conductivity changes during the in situ annealing of amorphous SnO2 thin films under vacuum conditions. Crystallization was initiated at 200 degrees C, and most of the amorphous phase was converted to the crystalline phase upon annealing to 500 degrees C. This trend was consistent with the temperature-dependent electrical conductance variation, demonstrating metal-like behavior. Off-specular X-ray diffraction results confirmed the polymorphism of the SnO2 thin films, i.e., the coexistence of orthorhombic columbite (C-SnO2) and tetragonal rutile (R-SnO2) phases and their epitaxial relationship with the sapphire(0001) substrate. In the 35-nm-thick sample, both C-SnO2 and R-SnO2 phases were formed epitaxially, while an additional polycrystalline R-SnO2 phase was observed with increasing film thickness. This indicated that the formation of the metastable C-SnO2 phase is determined by the strain field induced by the sapphire (0001) substrate. The result also revealed that the volume effect in the absence of interfacial strain favors the formation of a polycrystalline R-SnO2 phase in the bulk region of amorphous thin films.

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.

In this study, synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods were combined to identify polymorphs in SnO2 thin films deposited on sapphire (0001) substrates using radio-frequency powder sputtering. A wide range of off-specular Bragg peaks-including higher-order reflections-were examined via high-resolution, in-plane XRD analyses for precise phase identification. Evidently, the orthorhombic columbite (C-SnO2) and tetragonal rutile (R-SnO2) phases coexisted in the as-deposited films. Because both the phases were aligned with their (200) planes along the surface normal, their out-of-plane Q(z) components appeared nearly identical. However, the in-plane Q(x) and Q(y) components were distinguishable. The lattice constants were estimated from the in-plane Bragg peak positions, and the corresponding strain states in ultrathin films (<10 nm) were determined. In the early stage of growth, the C-SnO2 and R-SnO2 domains exhibited opposing strains-compressive and tensile strains, respectively-because of extended domain matching epitaxy, which accommodated lattice mismatch and governed the stabilization of each polymorph. The coexistence of the two phases at the atomic scale was further supported by cross-sectional high-resolution TEM analysis. These findings provide new insights into the strain-driven stabilization of polymorphs and the structural evolution of epitaxial SnO2 thin films on symmetry-mismatched substrates.

Ultrafast photoinduced melting provides an essential platform for studying nonequilibrium phase transitions by linking the kinetics of electron dynamics to ionic motions. Knowledge of dynamic balance in their energetics is essential to understanding how the ionic reaction is influenced by femtosecond photoexcited electrons with notable time lag depending on reaction mechanisms. Here, by directly imaging fluctuating density distributions and evaluating the ionic pressure and Gibbs free energy from two-temperature molecular dynamics that verified experimental results, we uncovered that transient ionic pressure, triggered by photoexcited electrons, controls the overall melting kinetics. In particular, ultrafast nonequilibrium melting can be described by the reverse nucleation process with voids as nucleation seeds. The strongly driven solid-to-liquid transition of metallic gold is successfully explained by void nucleation facilitated by photoexcited electron–initiated ionic pressure, establishing a solid knowledge base for understanding ultrafast nonequilibrium kinetics.

Photoinduced ultrafast phenomena in materials exhibiting nonequilibrium behavior can lead to the emergence of exotic phases beyond the limits of thermodynamics, presenting opportunities for femtosecond photoexcitation. Despite extensive research, the ability to actively control quantum materials remains elusive owing to the lack of clear evidence demonstrating the explicit control of phase-changing kinetics through light-matter interactions. To address this drawback, we leveraged single-pulse time-resolved X-ray imaging of Au nanorods undergoing photoinduced melting to showcase control over the solid-to-liquid transition process through the use of localized surface plasmons. Our study uncovers transverse or longitudinal melting processes accompanied by characteristic oscillatory distortions at different laser intensities. Numerical simulations confirm that the localized surface plasmons, excited by polarized laser fields, dictate the melting modes through anharmonic lattice deformations. These results provide direct evidence of photoinduced surface plasmon-mediated ultrafast control of matter, establishing a foundation for the customization of material kinetics using femtosecond laser fields.

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.

SrTi0.5Fe0.5 O3-delta (STF) has emerged as a promising cathode material for solid oxide fuel cells due to its excellent high ionic and electronic conductivity. We investigated the strain-state in epitaxial STF thin films on LaAlO3 (001) using synchrotron x-ray reciprocal space mapping measurement. The STF layer consists of two groups of compressively strained domains mixed horizontally in the film plane with a strain difference of about 1.4 %. The stress-free relaxed unit cell of more compressively strained domains, where a large degree of Sr segregation is expected, is relatively larger indicating that Sr segregation causes unit cell expansion. The lattice strain becomes gradually relaxed as the film thickness increases and the in-plane domain size decreases. We conjecture that the segregated Sr from compressively strained domains diffuse to the surface to form a layer of SrO(x )islands.

Bonding orbital dynamics in ultrafast melting have been directly observed using femtosecond time-resolved resonant X-ray scattering at the Pohang Accelerator Laboratory X-ray Free-Electron Laser, and smoking-gun evidence on direct linkage between bonding orbitals and lattice stability unifying thermal-to-nonthermal reactions to explicate photo-induced phase transitions is provided.

Femtosecond laser pulses drive nonequilibrium phase transitions via reaction paths hidden in thermal equilibrium. This stimulates interest to understand photoinduced ultrafast melting processes, which remains incomplete due to challenges in resolving accompanied kinetics at the relevant space-time resolution. Here, by newly establishing a multiplexing femtosecond X-ray probe, we have successfully revealed ultrafast energy transfer processes in confined Au nanospheres. Real-time images of electron density distributions with the corresponding lattice structures elucidate that the energy transfer begins with subpicosecond melting at the specimen boundary earlier than the lattice thermalization, and proceeds by forming voids. Two temperature molecular dynamics simulations uncovered the presence of both heterogeneous melting with the melting front propagation from surface and grain boundaries and homogeneous melting with random melting seeds and nanoscale voids. Supported by experimental and theoretical results, we provide a comprehensive atomic-scale picture that accounts for the ultrafast laser-induced melting and evaporation kinetics.

We report a spontaneous phase separation in indium gallium oxide thin films with a nominal composition of (In0.006Ga0.994)2O3 grown on sapphire (0001) substrates using a powder sputtering method in a reducing atmosphere. The In-rich (In0.24Ga0.76)2O3-x domains are non-stoichiometric and located underneath the surface islands, whereas the In-poor domains are nearly stoichiometric Ga2O3, forming a continuous flat surface, though both domains have a monoclinic crystal structure. Furthermore, the In-rich domains exhibit a short-range positional order with a correlation length of approximately 1 & mu;m because the phase separation occurs along the binodal decomposition accompanying periodic concentration profiling of In atoms. The analysis of X-ray photoelectron spectroscopy confirmed that the non-stoichiometry of the thin films originated from the formation of Ga sub-oxide (Ga2O) and metallic Ga, whereas the chemical state of the In atoms was close to In2O3 rather than metallic In. Our results indicate that the spontaneous phase separation occurs in (InxGa1-x)2O3 thin films even at an In content of 0.6 at%. Also, the non-stoichiometry associated with oxygen vacancies is an important parameter for causing spontaneous phase separation via the segregation of In atoms.