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Abstract: Water is one of the obstacles for prolonged life-time of organic based field-effect-transistors (OFETs). Here, we show how H2O molecules adsorb and affect pentacene crystal by using photoelectron spectroscopic methods. Diffusion into the crystal is accompanied with accumulation of H2O onto the crystal. Valence band, core level, and X-ray absorption spectra show that H2O molecules physisorb on the pentacene surface via oxygen, resulting in the increase of the hole-injection barrier. The diffused molecules results in the deteriorated crystallinity, and they reflect the weakening inter-molecular interactions of pentacene crystal. [Copyright &y& Elsevier]

Abstract: The chemical and geometrical characteristics of pyrrole on Si(100)-2×1 were studied by using the photoemission spectroscopy (PES) and the near edge X-ray absorption fine structure (NEXAFS). A pyrrole layer was formed by exposing 20L pyrrole molecules on the Si(100)-2×1 surface at 300K. PES results confirmed that pyrrole reacts with the surface through the breakage of Ne:glyph name="sbnd" />H groups, forming Sind Sietween the Si surface and the pyrrole ring. NEXAFS studies showed that π-bonding state of pyrrole has angle dependence with respect to the surface normal. Thus, pyrrole was dissociatively adsorbed by [2+2] cycloaddition reaction with ∼16.5° tilt angle with respect to the surface normal direction at 300K. [Copyright &y& Elsevier]

Transition metal dichalcogenides (TMDs) benefit electrical devices with spin-orbit coupling and valley- and topology-related properties. However, TMD-based devices suffer from traps arising from defect sites inside the channel and the gate oxide interface. Deactivating them requires independent treatments, because the origins are dissimilar. This study introduces a single treatment to passivate defects in a multilayer MoS 2 FET. By applying back-gate bias, protons from an H-TFSI droplet are injected into the MoS 2 , penetrating deeply enough to reach the SiO 2 gate oxide. The characterizations employing low-temperature transport and deep-level transient spectroscopy (DLTS) studies reveal that the trap density of S vacancies in MoS 2 drops to the lowest detection level. The temperature-dependent mobility plot on the SiO 2 substrate resembles that of the h-BN substrate, implying that dangling bonds in SiO 2 are passivated. The carrier mobility on the SiO 2 substrate is enhanced by approximately 2200% after the injection.

Carbon hydrogasification is the slowest reaction among all carbon-involved small-molecule transformations. Here, we demonstrate a mechanochemical method that results in both a faster reaction rate and a new synthesis route. The reaction rate was dramatically enhanced by up to 4 orders of magnitude compared to the traditional thermal method. Simultaneously, the reaction exhibited very high selectivity (99.8 % CH 4 , versus 80 % under thermal conditions) with a cobalt catalyst. Our study demonstrated that this extreme increase in reaction rate originates from the continuous activation of reactive carbon species via mechanochemistry. The high selectivity is intimately related to the activation at low temperature, at which higher hydrocarbons are difficult to form. This work is expected to advance studies of carbon hydrogasification, and other solid-gas reactions., (© 2022 Wiley-VCH GmbH.)

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 2 -mediated surface oxidation of 2H-MoTe 2 (alpha phase MoTe 2 ). MoTe 2 exposed to XeF 2 gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO x ) on MoTe 2 regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe 2 , which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe 2 field-effect transistors (FETs) with a MoO x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm 2 V -1 s -1 with a high on/off current ratio of ∼10 5 than that of the MoTe 2 device with conventional metal contacts (0.07 cm 2 V -1 s -1 ). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe 2 devices, applicable for 2D electronics., Competing Interests: The authors declare no competing financial interest., (This journal is © The Royal Society of Chemistry.)

The electronic structure of an atomic-layer-deposited MoS 2 monolayer on SiO 2 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 MoS 2 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 MoS 2 and adjacent SiO 2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS 2 -SiO 2 interface.

van der Waals (vdW) materials have shown unique electrical and optical properties depending on the thickness due to strong interlayer interaction and symmetry breaking at the monolayer level. In contrast, the study of electrical and tribological properties and their thickness-insensitivity of van der Waals oxides are lacking due to difficulties in the fabrication of high quality two-dimensional oxides and the investigation of nanoscale properties. Here we investigated various tribological and electrical properties, such as, friction, adhesion, work function, tunnel current, and dielectric constant, of the single-crystal α-MoO 3 nanosheets epitaxially grown on graphite by using atomic force microscopy. The friction of atomically smooth MoO 3 is rapidly saturated within a few layers. The thickness insensitivity of friction is due to very weak mechanical interlayer interaction. Similarly, work function (4.73 eV for 2 layers (hereafter denoted as L)) and dielectric constant (6 for 2L and 10.5-11 for >3L) of MoO 3 in MoO 3 showed thickness insensitivity due to weak interlayer coupling. Tunnel current measurements by conductive atomic force microscopy showed that even 2L MoO 3 of 1.4 nm is resistant to tunneling with a high dielectric strength of 14 MV/cm. The thickness-indifferent electrical properties of high dielectric constant and tunnel resistance by weak interlayer coupling and high crystallinity show a promise in the use of MoO 3 nanosheets for nanodevice applications.

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 VO 2 films during hydrogen spillover, contrary to the well-established observation of the formation of hydrogen interstitials in the bulk part of VO 2 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 H 2 O 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.

In this study, we employ a combination of various in-situ surface analysis techniques to investigate the thermally induced degradation processes in MAPbI 3 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 MAPbI 3 perovskite film changes to an intermediate phase and decomposes to CH 3 I, NH 3 , and PbI 2 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 CH 3 NH 3 + organic cations with respect to the substrate in the intermediate phase, which might be linked directly to the thermal degradation processes in MAPbI 3 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 LaFeO 3 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 FeO 2 -terminated counterpart. However, the FeO 2 -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.

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.