Abstract Near-total reflection geometry in X-ray-excited experiments gives rise to several effects connected to the refractive behavior of X-rays spanning from a finely tunable penetration depth to the formation of a strong X-ray standing wave and the propagation of an X-ray evanescent wave. These effects are often hard to quantify due to the uncertainties in experimental geometry, sample morphology, and optical properties of the studied materials. Here, we present a multimodal approach exploiting these X-ray optical effects and obtaining detailed chemical and structural information on a nanoparticle/substrate system under reaction conditions. Using the combination of standing-wave ambient-pressure photoemission spectroscopy (SWAPPS) and grazing incidence X-ray scattering (GIXS) techniques, we investigated the oxidation process of poly(ethylene glycol)-capped Au nanoparticles (NPs) on a p-doped Si substrate. SWAPPS results indicate that the organic capping layer is decomposed by the radical oxidizers generated from X-ray irradiation of O2 molecules, resulting in the dislocation of Au NPs as ∼2 nm closer to the interface with the substrate. Meanwhile, the scattering contrast is enhanced due to the decomposition of the organic capping layer, and the variation of X-ray scattering features caused by changes in the particle positions is also observed. Based on these multimodal results, an extensive X-ray optical and photoemission modeling of the system was performed with sub-nm accuracy using the PyXRO package. This work demonstrates a state-of-the-art methodology for studying morphologically complex heterogeneous interfaces both structurally and chemically with high (<1 nm) depth accuracy.

As an operando model system study for CO2 catalyst, PtGa(111) single crystal alloy is chosen to explore the CO2 adsorption properties of the PtGa alloy surface under elevated pressure of CO2 using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). During surface cleaning process under UHV, i.e., sputtering and high temperature annealing process, considerable surface compositions changes are observed. When the PtGa alloy is exposed to CO2 gas, the CO2 adsorption-derived species (CO2δ-, HCOO–) start to appear only after the CO2 gas pressure reaches 0.02 mbar, indicating a distinct pressure gap in CO2 adsorption/dissociation on PtGa alloy. As surface temperature increases to 400–500 K under 0.2 mbar, both CO2δ- and HCOO– species start to diminish while surface GaOx starts to increase. The concurrent reduction in surface work function likely stems from the dynamic interactions among surface GaOx, CO2δ–, and adsorbed C–H at their optimal coverage. Throughout the CO2 reaction, only Ga is oxidized while Pt remains metallic states, implying the important role of Ga oxide in CO2 reaction.

We investigate oxygen-induced surface segregation and chemical state variations in a polycrystalline Pt3V alloy using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS), complemented by in situ scanning tunneling microscopy (STM). Upon exposure to sub-millibar O2, vanadium (V) undergoes pronounced surface segregation and selective oxidation, while platinum (Pt) progressively loses chemical bonding with V, particularly at elevated temperatures. Non-destructive, quantitative depth profiling reveals V enrichment at the surface accompanied by V depletion in the subsurface. The competition between V segregation and oxygen uptake produces distinct depth-dependent variations in V oxide evolution pathways across the near-surface region. At elevated temperatures, enhanced V segregation stabilizes lower-valence V oxides at the outermost surface despite increasing oxygen content. In contrast, new higher-valence V oxides continue to emerge in the V-depleted subsurface due to increased oxygen coordination around the remaining V species, despite the less oxygen content compared with the surface. This surface–subsurface divergence in V oxide evolution pathways shows that segregation-driven compositional redistribution can override simple thermodynamic oxidation trends under reactive conditions. These findings provide insight into the interplay among surface segregation, gas–solid interfacial reactions, and alloy oxidation, and establish a mechanistic framework for understanding chemical state variations in alloy systems such as Pt-based catalysts.

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 report the performance of ultraviolet (UV) photodetectors based on non-stoichiometric amorphous Ga2O3-delta thin films deposited using the radio-frequency powder sputtering method. At a substrate temperature of 25 degrees C, the Ga2O3-delta film grew with an amorphous phase on a sapphire (0001) substrate. Hard X-ray photoelectron spectroscopy analysis revealed that the chemical composition of the as-deposited thin film was highly non-stoichiometric owing to the oxygen deficiency associated with sub-oxide Ga2O and metallic Ga species. Metal-semiconductor-metal photodetectors were fabricated, and their photoresponse properties under UV exposure were investigated. The photo-to-dark current ratio was estimated to be 2.91 x 10(5). The photoresponsivity and specific detectivity were calculated to be similar to 29.54 A/W and 8.95 x 10(14) Jones, respectively, at an applied bias of 10 V and a wavelength of 250 nm. Our results indicate that non-stoichiometric amorphous Ga2O3-delta thin films with thicknesses less than 100 nm are suitable for the fabrication of solar-blind photodetectors with self-powered characteristics.

Highly efficient semiconductor metrology and inspection techniques have played important roles for the early detection of defects in the semiconductor-fabrication industry. We here report a new voltage contrast inspection technique based on X-ray photoelectron spectroscopy (XPS) and an electron beam (XPS/EB-VCI). This technique detects an electrical defect (in particular, an electrically open defect) on a semiconductor wafer while simultaneously analyzing the chemical state of the wafer. In XPS/EB-VCI, the existence of an electrical defect is identified in a wide area on the sub-millimeter scale, and the location of the defect can be specified with micrometer-level precision. Furthermore, with the application of auger electron spectroscopy, the identified electrical defect area can be further mapped on the nanometer scale. We believe that XPS/EB-VCI could become a viable assessment tool for the semiconductor inspection process, along with a current inspection technique based on scanning electron microscopy.

Proton conducting electrochemical cells (PCECs) are efficient and clean intermediate-temperature energy conversion devices. The proton concentration across the PCECs is often nonuniform, and characterizing the distribution of proton concentration can help to locate the position of rate-limiting reactions. However, the determination of the local proton concentration under operating conditions remains challenging. Here, we employed in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate an Au/BaZr0.9Y0.1O3-δ/Au symmetric cell with DC bias of 1 V applied between the working and counter electrodes (CE). The relative intensity of hydroxyl groups, deconvoluted from the O 1s XPS spectra, reveals the distribution of proton concentration across the electrolyte. The applied electric field induces proton accumulation at the counter electrode, imposing binding energy shifts of the surface components for metal elements relative to their lattice components. Combined XPS and impedance analysis suggests that the accumulation layer of protons is much thicker at 500 K compared to that at 670 K, as a result of a larger amount of hydroxyl groups at the lower temperature. This nonuniform distribution of proton concentration affects the chemical environment of metal elements, and the local electrical potential, as revealed by the in situ XPS. This work demonstrates in situ NAP-XPS as a tool to probe the distribution of proton concentration and its impact on the defect chemistry and local electrical potential of PCECs, thereby advancing the understanding of the impact of proton defect chemistry and the performance improvement of PCECs.

The design of oxygen evolution reaction (OER) electrocatalysts demands a delicate balance between activity and stability. In this study, we present a rational design approach that leverages catalyst-support interactions to enhance both the intrinsic activity and durability of Ir-based catalysts. Our study reveals that while Mo doping energetically promotes the formation of high-valent Ir species, enhancing intrinsic catalytic activity, it also leads to a reduction in electrical conductivity. These findings emphasize that supporting doping can introduce both beneficial and limiting effects, highlighting the need for a carefully balanced design strategy to optimize the overall OER performance. Simultaneously, in situ analytical techniques and comparative evaluation reveal the crucial role of oxide supports in stabilizing the catalyst. These findings highlight the pivotal role of interface engineering in maintaining catalyst integrity and the need for support materials that balance dopant-driven electronic promotion with structural and electrochemical robustness. These interconnected degradation pathways highlight the need to move beyond a catalyst-centric view and instead adopt a system-level understanding of the stability. Our approach offers a strong foundation for the rational design and evaluation of high-performance OER electrocatalysts for electrochemical energy applications.

We report on the performance of ultraviolet (UV) photodetectors based on nonstoichiometric beta-Ga2O3-delta thin films with a thickness of 65 nm. The epitaxial Ga2O3-delta films were deposited on sapphire (0001) substrates by radio-frequency powder sputtering at 500 degrees C. The oxygen-deficient, nonstoichiometric nature of the films was confirmed by hard X-ray photoelectron spectroscopy. The fabricated metal-semiconductor-metal photodetectors exhibited a photo-to-dark-current ratio of 2.17 x 10(2), photoresponsivity of 136.15 A W-1, and specific detectivity of 5.06 x 10(13) Jones at 10 V under 254 nm UV illumination. Self-powered operation was achieved, yielding a photocurrent of similar to 0.6 nA at a nominal zero bias. Biexponential fitting of the time-resolved response revealed slow decay components attributed to oxygen-vacancy-related traps. These results indicate that the oxygen vacancies play a dual role by enhancing the photocurrent through free-carrier generation and contributing to persistent photoconductivity. Our findings demonstrate promising potential of sub-100 nm nonstoichiometric beta-Ga2O3-delta thin films for high-performance UV photodetectors. (c) 2025 The Japan Society of Applied Physics. All rights, including for text and data mining, AI training, and similar technologies, are reserved

The manifestation of giant persistent photoconductivity (GPPC) is demonstrated with a fs (femtosecond) Ti:sapphire laser pulse that has a duration of 40 fs and a central wavelength of 400 nm. The femtosecond laser pulse was irradiated on a two-terminal VO2 device fabricated on a corning glass substrate. Under the applied voltages of 9–12 V, the GPPC takes place within 8.6–15 μs after the laser irradiation. The photocurrent from the GPPC in the VO2 device remains stable with the current decreasing slope of ∼0.003%/minute. With one-dimensional thermal model, the temperature (TIR) of the irradiated area is estimated as a function of time, indicating that TIR is above the insulation-to-metal transition temperature of VO2 thin film prior to the onset of GPPC. The ultrafast onset of GPPC of VO2 device can be utilized for ultrafast optoelectronic switch and memory device.

Depth profiling is an essential method to investigate the physical and chemical properties of a solid electrolyte and electrolyte/electrode interface. In conventional depth profiling, various spectroscopic tools such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) are utilized to monitor the chemical states along with ion bombardment to etch a sample. Nevertheless, the ion bombardment during depth profiling results in an inevitable systematic error, i.e., the accumulation of mobile ions at the electrolyte/electrode interface, known as the ion pile-up phenomenon. Here, we propose a novel method using bias potential, the substrate-bias method, to prevent the ion pile-up phenomena during depth profiling of a solid electrolyte. When the positive bias potential is applied on the substrate (electrode), the number of accumulating ions at the electrolyte/electrode interface is significantly reduced. The in-depth XPS analysis with the biased electrode reveals not only the suppression of the ion pile-up phenomena but also the altered chemical states at the interfacial region between the electrolyte and electrode depending on the bias. The proposed substrate-bias method can be a good alternative scheme for an efficient yet precise depth profiling technique for a solid electrolyte.

Defect formation through the fragmented formamidinium cation in lead iodide perovskite leads to the widening charge inversion layer and limited solar cell performance.

The adsorption properties of CO2 on the SrTiO3(001) surface were investigated using ambient pressure X-ray photoelectron spectroscopy under elevated pressure and temperature conditions. On the Nb-doped TiO2-enriched (1 x 1) SrTiO3 surface, CO2 adsorption, i.e., the formation of CO3 surface species, occurs first at the oxygen lattice site under 10(-6) mbar CO2 at room temperature. The interaction of CO2 molecules with oxygen vacancies begins when the CO2 pressure increases to 0.25 mbar. The adsorbed CO3 species on the Nb-doped SrTiO3 surface increases continuously as the pressure increases but starts to leave the surface as the surface temperature increases, which occurs at approximately 373 K on the defect-free surface. On the undoped TiO2-enriched (1 x 1) SrTiO3 surface, CO2 adsorption also occurs first at the lattice oxygen sites. Both the doped and undoped SrTiO3 surfaces exhibit an enhancement of the CO3 species with the presence of oxygen vacancies, thus indicating the important role of oxygen vacancies in CO2 dissociation. When OH species are removed from the undoped SrTiO3 surface, the CO3 species begin to form under 10(-6) mbar at 573 K, thus indicating the critical role of OH in preventing CO2 adsorption. The observed CO2 adsorption properties of the various SrTiO3 surfaces provide valuable information for designing SrTiO3-based CO2 catalysts.

Based on the giant persistent photoconductivity (GPPC) characteristic of VO2, we report a remote-control switch that allows high-speed switching on/off functionality. The switch, composed of two VO2 film devices connected in parallel and two semiconductor lasers, can operate multiple electrical devices at near room temperature. The estimated switching-on (off) time of the remote switch is 73 mu s (64 mu s) with laser intensity of 1.1 x 10(7) W/m(2) at the ambient temperature of 30 degrees C. The switching-on/off mechanisms can be understood by alternative generation of GPPC at the parallelly connected VO2 devices and the current division rule. Our high-speed remote VO2 switch can be applied to many diverse electronic devices such as safety switches, optical computing, and sensors.

During Ar + sputtering and UHV annealing processes on undoped SrTiO 3 (001) surfaces, the interactions between SrO and impurity ions can be witnessed from their Sr 3d spectra.

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.

Using ambient pressure X-ray photoelectron spectroscopy(APXPS)and mass spectrometry (MS) while varying the temperature, we wereable to directly understand how the sequence of exposing reactantgases, CO2 and H-2, influenced the surface speciesand reverse water-gas shift reaction product formation on Ni(100).When first dosing CO2 at room temperature (RT), the Ni(100)surface shows a significant amount of surface oxide due to CO2 spontaneously dissociating into CO and O and this is nearlyunchanged upon introducing H-2. However, when H-2 is dosed first to the nickel surface and CO2 is subsequentlyintroduced, the spontaneous dissociation of CO2 still continuesyet it forms less surface oxide due to the pre-adsorbed hydrogen onthe surface. Interestingly, the major product of CO2 dissociationfrom both reactions is a surface oxide, while only a small amountof OH species is observed in either exposure case. This observationconfirms that the dominant pathway of CO2 dissociationon Ni(100) follows the redox mechanism. As the temperature increases,the adsorbed CO further dissociates into atomic carbon and oxygen.As the temperature continues to increase, the influence of the reactantgas exposure sequence no longer exists: the desorption of CO as CO(g)takes place in both cases. It is to note that the surface oxide ispresent on a nickel surface even at high temperatures over 400 & DEG;C,indicating kinetic rate differences between the adsorbed CO desorptionand surface oxide hydrogenation to H2O. Our results revealhow the sequence of dosing gases affects the surface reactivity ofthe catalyst, providing important insight into the behavior of thecatalyst surfaces and their product formation.

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO2) activation. Despite extensive reactivity benchmarks, a correlation between CO2 conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO2(g). The CO2 dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO2 conversion to renewable energy sources during C1 chemical reactions.

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.