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Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties.

Summary The electrosynthesis of H2O2 via the 2e− oxygen reduction reaction (ORR) is an attractive method for the clean and continuous on-site production of H2O2, for which the development of active and selective electrocatalysts remains a significant challenge. Although carbon nanomaterials have demonstrated promising performance for H2O2 production, the lack of understanding of the active sites and key structural factors has impeded their development. In this work, we have prepared carbon-based model catalysts to investigate the active oxygen functional groups and structural factor. We have identified that the carboxyl group at the edge sites of graphitic carbons is the major active site for the 2e− ORR, and the carbonyl group is a secondary active site. The nanoporous carbon catalyst with abundant active edge sites and optimized structure exhibited the highest H2O2 electrosynthesis activity among the carbon-based catalysts reported to date and excellent long-term stability (168 h) with 99% H2O2 faradic efficiency.

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab-initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

We thank UNIST Central Research Facilities (UCRF) and Y. K. Kim for the cross-sectional high-resolution TEM images. This work was supported by Samsung Electronics (Samsung-SKKU Graphene/2D Center), the research fund (NRF-2017R1E1A1A01074493 and NRF-2019R1A4A1027934), the IBS (IBS-R-019-D1) and a grant (CASE-2013M3A6A5073173) from the Centre for Advanced Soft Electronics under the Global Frontier Research Program via the National Research Foundation of the Ministry of Science and ICT, South Korea. The NEXAFS experiments performed at the 4D, 6D and 10A2 beamlines of the Pohang Accelerator Laboratory (PAL) were supported in part by the Ministry of Science and ICT, POSTECH and UNIST. M.C. acknowledges support from Leverhulme Trust Research Grant RPG-2019-227. S.R., A.A. and M.C. acknowledge the European Union Horizon 2020 research and innovation programme for grant number 785219 and 881603 (Graphene Flagship). A.A. is supported by Project MECHANIC (PCI2018-093120) funded by Ministerio de Ciencia, Innovación y Universidades. The Catalan Institute of Nanoscience and Nanotechnology is funded by the CERCA Programme/Generalitat de Catalunya and supported by the Severo Ochoa Centres of Excellence programme, funded by the Spanish Research Agency (grant number SEV-2017-0706).

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF

The introduction of ferromagnetic order in topological insulators in general breaks the time-reversal symmetry and a gap is opened in topological surface bands. Various studies have focused on gap-opened magnetic topological insulators, because such modified band structures provide a promising platform for observing exotic quantum physics. However, the role of antiferromagnetic order in topological insulators is still controversial. In this report, we demonstrate that it is possible to restore the topological surface states by effectively reducing the antiferromagnetic ordering in Gd-substituted Bi2Te3. We successfully control the magnetic impurities via thermal treatments in ultra-high vacuum condition and observe apparent restoration of topological surface band dispersions. The microscopic mechanism of atomic rearrangements and the restoration process of topological surface states are unraveled by the combination of scanning tunneling microscopy measurements and density functional theory calculations. This work provides an effective way to control the magnetic impurities which is strongly correlated with topological surface states.

Soluble aromatic polyimides and polyvinyls were prepared by incorporating pyridine moiety and its derivatives in the backbone and the side groups, respectively: 6F-Py-i polymers based on the polyimide backbone (6F-Py-1 to 6F-Py-7) and PVPy-i polymers based on the polyvinyl backbone (PVPy-1 to PVPy-4). All polymers were found to be amorphous. The 6F-Py-i polymers were thermally stable up to 511–545 °C; the PVPy-i polymers were stable up to 362–376 °C. Their glass transitions, thin film densities, molecular orbitals, and band gaps were determined. The electrical devices fabricated with the polymers in an electrode/polymer/electrode structure revealed p-type unipolar write-once-read-many times (namely, permanent) or dynamic random access memory or dielectric behavior, depending on the substituents of the pyridine unit and the film thicknesses. In particular, such digital memory characteristics were found to originate from the pyridine moieties possessing a high charge affinity in the polymers. However, the pyridine moieties were found to still need at least two or more aromatic substituents to get enough power to stabilize charges via utilizing the resonance effects provided by the substituents. Overall, this study demonstrated that the pyridine unit conjugated with two or more aromatic substituents is a very useful component to design and synthesize digital memory materials based on thermally stable polyimides and other high performance polymers. The 6F-Py-i polymers have potential for the low-cost mass production of high-performance programmable unipolar permanent memory devices with very low power consumption.

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI3 perovskite solar cells (PeSCs) as a function of temperature under air-free conditions (no moisture and oxygen). Through a comprehensive approach that combines in-situ grazing-incidence wide-angle X-ray diffraction (GIWAXD) and high-resolution X-ray photoelectron spectroscopy (HR-XPS) measurements, we confirm that the surface structure of MAPbI3 perovskite film changes to an intermediate phase and decomposes to CH3I, NH3, and PbI2 after both a short (20 min) exposure to heat stress at 100 °C and a long exposure (>1 hour) at 80 °C. Moreover, we observe clearly the changes in the orientation of CH3NH3+ organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI3 perovskites. These results provide important progress towards improved understanding of the thermal degradation mechanisms in perovskite materials and will facilitate improvements in the design and fabrication of perovskite solar cells with better thermal stability.

The polarity of oxide surfaces can dramatically impact their surface reactivity, in particular, with polar molecules such as water. The surface species that result from this interaction change the oxide electronic structure and chemical reactivity in applications such as photoelectrochemistry but are challenging to probe experimentally. Here, we report a detailed study of the surface chemistry and electronic structure of the perovskite LaFeO3 in humid conditions using ambient-pressure X-ray photoelectron spectroscopy. Comparing the two possible terminations of the polar (001)-oriented surface, we find that the LaO-terminated surface is more reactive toward water, forming hydroxyl species and adsorbing molecular water at lower relative humidity than its FeO2-terminated counterpart. However, the FeO2-terminated surface forms more hydroxyl species during water adsorption at higher humidity, suggesting that adsorbate-adsorbate interactions may impact reactivity. Our results demonstrate how the termination of a complex oxide can dramatically impact its reactivity, providing insight that can aid in the design of catalyst materials.

Hydrogen spillover is a catalytic process that occurs by surface reaction and subsequent diffusion to reversibly provide a massive amount of hydrogen dopants in correlated oxides, but the mechanism at the surface of correlated oxides with metal catalyst are not well understood. Here we show that a significant amount of oxygen is released from the surface of correlated VO2 films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO2 films. By using ambient-pressure X-ray photoelectron spectroscopy, we prove that the formation of surface oxygen vacancies is a consequence of a favorable reaction for the generation of weakly adsorbed H2O from surface O atoms that have low coordination and weak binding strength. Our results reveal the importance of in situ characterization to prove the dynamic change during redox reaction and present an opportunity to control intrinsic defects at the surface.

We firstly report an experimental visualization of a cycloaddition reaction on RT frozen asymmetric Si dimers. The frozen Si dimers with a local c(4 × 2) order were prepared by pinning flip-flopping Si dimers by using molecules. This RT pristine c(4 × 2) structure was used to determine what Si atom of an asymmetric Si dimer bonds to a molecule at the initial stage of the RT cycloaddition reaction, which has been a long-standing puzzling issue. This made it possible to compare directly experimental cycloaddition reactions with theoretical ones. As a prototype for the experiment, a 1,3-butadiene molecule adsorbed between Si dimer rows was used. The 1,3-butadiene molecule was found to prefer a symmetric Si pair on the frozen Si dimers, i.e., two electrophilic lower atoms of asymmetric Si dimers. This result is consistent with the theoretical prediction that a 1,3-diene molecule prefers a symmetric Si pair on the Si(001)c(4 × 2) surface. This experimental approach can also be applied to other studies for the adsorption of a molecule on a Si(001) surface at room temperature.

We present a new method for transferring chemical vapor deposition (CVD)-grown graphene onto the surfaces of organic materials directly. Raman and near edge X-ray absorption fine structure measurements prove that high-quality and single layer graphene/organic thin films can be obtained with minimized impurity introduction. In-situ synchrotron radiation photoemission spectroscopy combined with ultraviolet photoelectron spectroscopy experiments demonstrate that the inserted graphene can not only act as a buffer layer to reduce the interfacial chemical reactions between the deposited Al and organic materials, but also tune the metal/organic interface electronic structure significantly. This new graphene transfer technique may have a great potential in the application of engineering the metal–organic interface properties, which is one of the key technologies for the optimal design and fabrication of organic electronic and optoelectronic devices.

We investigated the surface property change of indium tin oxide (ITO) with the various cleaning methods, using photoemission spectroscopy (PES), work function change and near-edge X-ray adsorption ne structure (NEXAFS). The detergent treatments of ITO with acetone (C3H6O), methanol (CH4O), 2-propanol (C3H8O), and deionized (D) water yielded work function shifts as high as 0.99 eV compared to the no-treatment ITO. In contrast, work function shifts of 0.10 0.35 eV were measured after other treatments. The lowest C 1s concentration was obtained after O2 plasma treatment of ITO. There was no big change in the peak position and the shape of In 3d, and Sn 3d2 after each treatment. But C 1s and O 1s spectra shows a peak shift and shape change. Presence of carbon can be minimized by NaOH treatment combined with oxygen plasma. In NEXAFS spectra of C K-edge on each sample, we found two π-binding states whose positions are 284.5 and 287.9 eV, C=C and C=O, respectively.

We report catalytic decomposition of few-layer graphene on an Au/SiOx/Si surface wherein oxygen is supplied by dissociation of the native SiOx layer at a relatively low temperature of 400 °C. The detailed chemical evolution of the graphene covered SiOx/Si surface with and without gold during the catalytic process is investigated using a spatially resolved photoelectron emission method. The oxygen atoms from the native SiOx layer activate the gold-mediated catalytic decomposition of the entire graphene layer, resulting in the formation of direct contact between the Au and the Si substrate. The notably low contact resistivity found in this system suggests that the catalytic depletion of a SiOx layer could realize a new way to micromanufacture high-quality electrical contact.

Graphene has shown great application opportunities in future nanoelectronic devices because of its outstanding electronic properties. Moreover, its impressive optical properties have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we report the photovoltaic response of graphene–semiconductor junctions and the controlled open-circuit voltage (Voc) with varying numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs by means of in situ annealing, prepared by layer-by-layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite random interlayer-stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces. The surface photovoltaic effects observed here show an electronic realignment in the depth direction in the FLG heterojunction systems, indicating future potential toward solar devices utilizing the excellent transparency and flexibility of FLG. Copyright © 2011 John Wiley & Sons, Ltd.

Benzaldimine monolayer was exposed to soft X-rays, and the involved chemical transformation was investigated using X-ray photoelectron spectra and near-edge X-ray absorption fine structure spectroscopy. The spectroscopy indicated that irradiation of soft X-ray (550 eV)-induced selective transformation of the imine group into a nonhydrolyzable one, i.e., the amine group. Utilizing the selective chemical transformation of the imine group with the soft X-ray irradiation, we were able to generate a micropattern. AFM images showed that the patterning with alternating surface topology was effective. The patterned monolayer was further modified with biotin and Cy3-tagged Streptavidin sequentially. Fluorescence images showed that the above molecules were selectively immobilized onto the amine-terminated region of the patterned surface. The current system is found to be more efficient than the predecessor, 4-nitrobenzaldimine monolayer.

A new nanopatterning system based on a soft X-ray induced chemical transformation of a nitro-substituted aromatic imine monolayer has been developed. The molecular layer was exposed to soft X-rays, and the involved chemical transformation on the molecular layer was analyzed by using Fourier transform infrared reflection−absorption spectroscopy. As a result, we could confirm that the nitro group on the imine monolayer was cleaved upon the soft X-ray irradiation, leaving the hydrophobic phenyl unit intact on the monolayer surface, while the imine functionality was transformed into a new nonhydrolyzable one. However, the source of the hydrogen atom for the reduction and the final functionality at the para position of the aromatic group are unknown yet. Whereas, we could restore the hydrophilic amine functionality from the unexposed imine monolayer through hydrolysis. These phenomena were applied to the patterning of self-assembled monolayers featuring alternating height, chemical reactivity, and wettability....

The surface properties of Al thin films (500 A) modified by irradiation with high-energy electron beams (0.3, 1, or 2 MeV with a flux of 7 × 10 electrons·cm−2·s−1) have been investigated using synchrotron-radiation photoelectron emission spectroscopy. No morphological changes were observed, but increased oxidation of the Al films was observed when they were irradiated for longer than 40 min. Selective oxidation of the irradiated regions of the Al films was achieved using shadow masking. A scanning photoemission electron microscopy image at the O l s core level clearly showed spatially resolved oxidation contrast in the Al layer.

The insertion of LiF for an interlayer material between the Al cathode and tris-(8-hydroxyquinoline) aluminum (Alq3) in the organic light-emitting diodes (OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq3 layer.

We suggest that there is a novel force which is generated by the mass of relatively moving particles. The new force which we named Mirinae Force is a counterpart of the magnetic force operating between electrically charged moving particles. Instead of using the conventional dark matter, we applied the mirinae force to a particular model system of the spiral galaxy in which most of the galaxy's mass is located within the central region where some portion of the inner mass is in revolving motion at a relativistic speed. The calculation yielded three important results that illustrate the existence of mirinae force and validate the proposed model: First, the mirinae force in this model explains why most of the matters in the galactic disk are in the circular motion which is similar to cycloid. Second, the mirinae force well explains not only the flat rotation curve but also the varied slope of the rotation curve observed in the spiral galaxies. Third, at the flat velocity of 220 Km/s, the inner mass of the Milky Way calculated by using the proposed model is 6.0\times10^11 M\odot, which is very close to 5.5\times10^11 M\odot (r

In the gas phase, electron beam irradiation of a furoxan molecule results in the production of two NO molecules and concomitant generation of a triple bond. In this study, we examined whether the selective cleavage of furoxan occurs on the surface of silicon wafers. A furoxan-substituted imine layer was prepared by the reaction of aminosilylated silicon wafers with 4-furoxancarbaldehyde. Formation of the imine layer was confirmed by UV-vis spectroscopy, contact angle goniometry, ellipsometry, and XPS. XPS spectroscopic monitoring of the electron beam (400 eV) induced reaction of the modified silicon wafers showed that two of the furoxan ring nitrogen atoms were lost. To determine if a carbon-carbon triple bond had been generated in the surface product of this reaction, FT-IR spectroscopy and NEXAFS (Near Edge X-ray Absorption Fine Structure) were performed. A weak absorption at 2203 cm -1 was observed in the FT-IR spectrum, reflecting the presence of a triple bond. The carbon K-edge NEXAFS spectrum contained a π*(C≡C) peak at 286.5 eV. Based on these results, we conclude that electron beam irradiation of the furoxan, incorporated on a silicon wafer surface, results in the release of nitrogen oxide and the formation of a triple bond containing product.

Impressive optical properties of graphene have been attracting the interest of researchers, and, recently, the photovoltaic effects of a heterojunction structure embedded with few layer graphene (FLG) have been demonstrated. Here, we show the direct dependence of open-circuit voltage (Voc) on numbers of graphene layers. After unavoidably adsorbed contaminants were removed from the FLGs, by means of in situ annealing, prepared by layer-by-layer transfer of the chemically grown graphene layer, the work functions of FLGs showed a sequential increase as the graphene layers increase, despite of random interlayer-stacking, resulting in the modulation of photovoltaic behaviors of FLGs/Si interfaces.

The characteristic feature of metal contacts with gold on organics is deterioration of the organic crystals during the contact formation. The unveiled key challenge is to probe dynamic details of the microscopic evolution of the organic crystal when the atomic Au is introduced. Here, we report how the collapse of the pentacene crystal is initiated even by a few Au atoms. Our results indicate that the gentle decoupling of intra and intermolecular π-π interactions causes the localization of the lowest unoccupied molecular orbital as well as the removal of cohesive forces between molecules, leading to the subsequent crystal collapse.

Anomalies of Au film formed on the pentacene surface are investigated as a counterpart of pentacene/Au structure. The Au film is found to contain pentacene derivatives originated from the pentacene layers, and it is composed of grains of various sizes formed as the Au thickness increases. The authors suggest that it is this abundance of peculiarities of the Au film that accounts for the attenuated density of states in the valence band. Deformation of Au grains is accompanied by the lift of the pentacene layers, which, in turn, brings about the device failure.

To investigate the effects of metal penetration into organic semiconductors on the electrical properties of organic thin film transistors, gold was deposited onto pentacene films at various deposition rates. The sharp interface between the gold electrode and the pentacene film that results from a fast deposition rate was found to produce lower contact resistance and an increase in the field-effect mobility.

It has recently been observed that benzoic acid strongly reacts with OH group on the silicon surface. Here, by defining the area in which OH group is adsorbed on the Si surface, the selective adsorption of benzoic acid species was attempted. The patterned OH∕Si surface was prepared by irradiating the zeroth order beam from the bending magnet of the synchrotron facility through the gold mesh placed in front of the OH∕Si sample. For discerning the selectively adsorbed molecule by x-ray photoelectron emission microscopy (X-PEEM) at N k edge, 4-nitrobenzoic acid was utilized instead of benzoic acid. Near edge x-ray absorption fine structure spectra at carbon and oxygen k edges were in good accord with the previous results obtained from the benzoic acid system. The X-PEEM images around N k edge clearly showed that the molecules adsorb only on the area in which OH groups remain.

We have investigated the effects of a buffer layer insertion on the performance of the pentacene based thin film transistor with a bottom contact structure. When the pentacene molecules have a standing up coordination on the Au surface that is modified by the benzenethiol or methanethiol, the transition region in the pentacene thin film is removed along the boundary between the Au and silicon oxide region, and the hole-injection barrier decreases by 0.4eV. Pentacene on various surfaces showed that the highly occupied molecular level is 0.2–0.4eV lower in the standing up coordination than in the lying down coordination.

The electronic structures of the interfaces between Au and poly(3-hexylthiophene) (P3HT) films with two different molecular orientations and orderings were investigated using synchrotron radiation photoemission spectroscopy. We found that, depending on whether thermal treatment was used, the P3HT thin film adopts two different molecular orientations, parallel and perpendicular to the silicon substrate, which result in different values of the vacuum level shift and hole-injection barrier. Thus, the molecular orientation and ordering of the P3HT material strongly affect the energy level alignment at the P3HT/Au interface.