Electronic properties and degradation upon VUV irradiation of sodium chloride on Ag 111 studied by photoelectron spectroscopy

The growth as well as vacuum ultraviolet VUV radiation induced degradation of sodium chloride NaCl on Ag 111 is investigated by ultraviolet and x ray photoelectron spectroscopy. In line with previous scanning tunneling microscopy studies, our results confirm that NaCl grows initially as a bilayer before island growth starts. Simple spectroscopic methods for calibrating the closure of the NaCl bilayer are further presented. In addition, the energy level alignment is studied as a function of NaCl film thickness and VUV light intensity. When measuring with ultra low photon flux, a sharp interface dipole lowers the sample work function by 0.65 eV upon adsorption of the first bilayer, which is followed by vacuum level alignment for subsequently deposited layers. In contrast, measurements performed with standard photon fluxes, such as those provided by commercial He discharge lamps, shows downward band bending like characteristics in the NaCl films. Upon extended exposure time to the standard VUV intensity, photoemission measurements further reveal that strong modifications of the electronic properties of the NaCl surface occur. These are likely correlated with halogen emission, eventually resulting in the formation of Na clusters promoting low work function of parts of the sample surface. This study provides general guidelines for obtaining reliable spectroscopic measurements on alkali halide thin films on metal

Energy Level Alignment at the C60 Monolayer WS2 Interface on Insulating and Conductive Substrates

Combining a transition metal dichalcogenide monolayer ML and molecular semiconductors is an attractive route for forming nanoscale hybrid van der Waals heterostructures with potentially novel opto electronic properties. The energy level alignment at the hybrid interface governs these properties, but precise determination of the interfacial electronic structure is challenging due to the pronounced excitonic nature of both components and the non trivial band structure of the inorganic ML. For instance, dielectric screening by the supporting substrate of such a heterostructure may impact the energy levels, but very few experiments have attended to this important issue to date. Here, it is shown how photoelectron spectroscopy can be used to unravel the energy level line up at the C60 ML WS2 interface supported on an insulating sapphire and a semi metallic graphite substrates. On both substrates, an almost identical staggered type II level alignment is determined. However, C60 ML WS2 exhibits stronger n type characteristics on sapphire, which is suggested to be due to native donor type defects of ML WS2. While these remain occupied and active on the insulating substrate, they are emptied into the charge reservoir of the conductive substrate. These insights should be considered in the future design of functional heterostructures of inorganic ML and molecular semiconductor material

Infection by the castrating parasitic nematode <i>Sphaerularia bombi </i>changes gene expression in <i>Bombus terrestris </i>bumblebee queens

Parasitism can result in dramatic changes in host phenotype, which are themselves underpinned by genes and their expression. Understanding how hosts respond at the molecular level to parasites can therefore reveal the molecular architecture of an altered host phenotype. The entomoparasitic nematode Sphaerularia bombi is a parasite of bumblebee (Bombus) hosts where it induces complex behavioural changes and host castration. To examine this interaction at the molecular level, we performed genome-wide transcriptional profiling using RNA-Seq of S. bombi-infected Bombus terrestris queens at two critical time-points: during and just after overwintering diapause. We found that infection by S. bombi affects the transcription of genes underlying host biological processes associated with energy usage, translation, and circadian rhythm. We also found that the parasite affects the expression of immune genes, including members of the Toll signaling pathway providing evidence for a novel interaction between the parasite and the host immune response. Taken together, our results identify host biological processes and genes affected by an entomoparasitic nematode providing the first steps towards a molecular understanding of this ecologically important host-parasite interaction

Position locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite

The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH(3)NH(3)PbI(3) perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI(2) and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N(2) and ambient air (relative humidity 20%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH(3)NH(3)PbI(3) films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N(2) or air. We conclude that the light-induced decomposition reaction of CH(3)NH(3)PbI(3), leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently “locking” the position of perovskite components at the surface or an interface should enhance the overall photostability

One pot synthesis of high capacity silicon anodes via on copper growth of a semiconducting, porous polymer

Silicon based anodes with lithium ions as charge carriers have the highest predicted theoretical specific capacity of 3579 mA h g amp; 8722;1 for Li15Si4 . Contemporary electrodes do not achieve this theoretical value largely because conventional production paradigms rely on the mixing of weakly coordinated components. In this paper, a semiconductive triazine based graphdiyne polymer network is grown around silicon nanoparticles directly on the current collector, a copper sheet. The porous, semiconducting organic framework 1 adheres to the current collector on which it grows via cooperative van der Waals interactions, 2 acts effectively as conductor for electrical charges and binder of silicon nanoparticles via conjugated, covalent bonds, and 3 enables selective transport of electrolyte and Li ions through pores of defined size. The resulting anode shows extraordinarily high capacity at the theoretical limit of fully lithiated silicon. Finally, we combine our anodes in proof of concept battery assemblies using a conventional layered Ni rich oxide cathod

The Schottky Mott Rule Expanded for Two Dimensional Semiconductors Influence of Substrate Dielectric Screening

A comprehensive understanding of the energy level alignment mechanisms between two dimensional 2D semiconductors and electrodes is currently lacking, but it is a prerequisite for tailoring the interface electronic properties to the requirements of device applications. Here, we use angle resolved direct and inverse photoelectron spectroscopy to unravel the key factors that determine the level alignment at interfaces between a monolayer of the prototypical 2D semiconductor MoS2 and conductor, semiconductor, and insulator substrates. For substrate work function amp; 934;sub values below 4.5 eV we find that Fermi level pinning occurs, involving electron transfer to native MoS2 gap states below the conduction band. For amp; 934;sub above 4.5 eV, vacuum level alignment prevails but the charge injection barriers do not strictly follow the changes of amp; 934;sub as expected from the Schottky Mott rule. Notably, even the trends of the injection barriers for holes and electrons are different. This is caused by the band gap renormalization of monolayer MoS2 by dielectric screening, which depends on the dielectric constant amp; 949;r of the substrate. Based on these observations, we introduce an expanded Schottky Mott rule that accounts for band gap renormalization by amp; 949;r dependent screening and show that it can accurately predict charge injection barriers for monolayer MoS2. It is proposed that the formalism of the expanded Schottky Mott rule should be universally applicable for 2D semiconductors, provided that material specific experimental benchmark data are availabl

A Multifunctional Interlayer for Solution Processed High Performance Indium Oxide Transistors

International audienceMultiple functionality of tungsten polyoxometalate (POM) has been achieved applying it as interfacial layer for solution processed high performance In 2 O 3 thin film transistors, which results in overall improvement of device performance. This approach not only reduces off-current of the device by more than two orders of magnitude, but also leads to a threshold voltage reduction, as well as significantly enhances the mobility through facilitated charge injection from the electrode to the active layer. Such a mechanism has been elucidated through morphological and spectroscopic studies