X-ray spectroscopy of organic materials

The radiation-induced decomposition of glycine is studied using a combination of Near Edge X-ray Absorption Fine Structure (NEXAFS) measurements and density functional theory calculations. Principal Component Analysis was used to determine the number of distinct molecules that were needed to explain the observed changes in the measured spectra, and the emerging absorption features are assigned to various product molecules through comparison with simulated spectra of several model compounds. It is clear from the experiment that the major effect of soft X-ray irradiation is the fragmentation of the molecule, primarily at the carbonyl sites. Peptide formation is shown to occur under irradiation; a condensation reaction initiated by the removal of a carbonyl oxygen is the proposed mechanism. This study utilizes a novel approach to the study of radiation damage that can occur during measurements, and suggests that it may be possible to use simulated model spectra to correct for these effects in measured spectra. A study of oligothiophene-based molecular semiconductor materials with potential applications in light-emitting and photovoltaic devices is undertaken. Angle-resolved NEXAFS measurements of the star-shaped 4(HPBT) molecules on an amorphous indium surface show a strong dichroic signal indicating a well-ordered, uniformly upright arrangement of planar molecules. The X-ray excited optical luminescence (XEOL) measurements showed several sharp features associated with vibronic splitting of the LUMO-HOMO luminescent transition. The HOMO-LUMO gap determined from the XEOL measurements is 2.28 eV; this value is in agreement with previously published optical measurements as well as with the value that is estimated from the combination of NEXAFS and X-ray emission spectroscopy (XES) measurements. Films formed from blended solutions of 4(HPBT) and the hole-transporting molecular semiconductor PCBM are shown to form a bilayer structure with the PCBM adjacent to the substrate. Annealing causes desorption of the 4(HPBT) from the surface

Impact of Catalysis-Relevant Oxidation and Annealing Treatments on Nanostructured GaRh Alloys

In this study, we examine the surface-derived electronic and chemical structures of nanostructured GaRh alloys as a model system for supported catalytically active liquid metal solutions (SCALMS), a novel catalyst candidate for dehydrogenation reactions that are important for the petrochemical and hydrogen energy industry. It is reported that under ambient conditions, SCALMS tends to form a gallium oxide shell, which can be removed by an activation treatment at elevated temperatures and hydrogen flow to enhance the catalytic reactivity. We prepared a 7 at. % Rh containing the GaRh sample and interrogated the evolution of the surface chemical and electronic structure by photoelectron spectroscopy (complemented by scanning electron microscopy) upon performing surface oxidation and (activation treatment mimicking) annealing treatments in ultrahigh vacuum conditions. The initially pronounced Rh 4d and Fermi level-derived states in the valence band spectra disappear upon oxidation (due to formation of a GaOx shell) but reemerge upon annealing, especially for temperatures of 600 °C and above, i.e., when the GaOx shell is efficiently being removed and the Ga matrix is expected to be liquid. At the same temperature, new spectroscopic features at both the high and low binding energy sides of the Rh 3d5/2 spectra are observed, which we attribute to new GaRh species with depleted and enriched Rh contents, respectively. A liquefied and GaOx-free surface is also expected for GaRh SCALMS at reaction conditions, and thus the revealed high-temperature properties of the GaRh alloy provide insights about respective catalysts at work

Polycapillary-boosted instrument performance in the extreme ultraviolet regime for inverse photoemission spectroscopy

A collimating polycapillary half lens, traditionally used in the medium and hard X-ray band, is operated at a photon energy of 36 eV for the first time. While the transmission still exceeds 50%, the measured and simulated spatial resolution and angular divergence approach 0.4 mm or less and at most 20 mrad, respectively. This unexpected, superior performance of the polycapillary optic in the extreme Ultraviolet could enable the design of an e cient, versatile and compact spectrometer for inverse photoemission spectroscopy (IPES): Its wavelength-dispersive component, a customized reflection zone plate, can maintain an energy resolution of 0.3 eV, whereas the sensitivity may be enhanced by more than one order of magnitude, compared to conventional spectrometers. Furthermore, the overall length of 0.9 m would allow for an eased alignment and evacuation. We see a significant potential for numerous polycapillary-based XUV / soft X-ray instruments in the future, in particular after further optimization for this long wavelength regime

Unraveling the Effect of Rh Isolation on Shallow d States of Gallium–Rhodium Alloys

In this study, we report the electronic and chemical structure of supported GaRh alloys as model systems for the active phase in supported catalytically active liquid metal solutions (SCALMS). We prepared a series of gallium–rhodium samples with different Rh contents and tracked the evolution of the sample topography and surface electronic structure via photoemission spectroscopy in combination with ab initio calculations and electron microscopy. Our results reveal a characteristic shift of the Rh 3d core levels and narrowing and shifting of the Rh 4d derived band with decreasing Rh content. Calculations show that these spectroscopic observations can be explained by the coexistence of isolated Rh atoms in random GaRh alloys and GaRh intermetallic compounds (IMCs). These results contribute to an enhancement of the fundamental understanding of the electronic surface structure of GaRh alloys, which is crucially required for apprehending and thus further exploiting the improved catalytic activity of GaRh SCALMS

Zn-Se-Cd-S Interlayer Formation at the CdS/Cu₂ZnSnSe₄ Thin-Film Solar Cell Interface

The chemical structure of the CdS/Cu2ZnSnSe4 (CZTSe) interface was studied by a combination of electron and X-ray spectroscopies with varying surface sensitivity. We find the CdS chemical bath deposition causes a "redistribution" of elements in the proximity of the CdS/CZTSe interface. In detail, our data suggest that Zn and Se from the Zn-terminated CZTSe absorber and Cd and S from the buffer layer form a Zn-Se-Cd-S interlayer. We find direct indications for the presence of Cd-S, Cd-Se, and Cd-Se-Zn bonds at the buffer/absorber interface. Thus, we propose the formation of a mixed Cd(S,Se)-(Cd,Zn)Se interlayer. We suggest the underlying chemical mechanism is an ion exchange mediated by the amine complexes present in the chemical bath

Origin of Interface Limitation in Zn(O,S)/CuInS2‑Based Solar Cells

Copper indium disulfide CuInS2 grown under Cu rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable cliff like conduction band alignment at the buffer CuInS2 interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath deposited Zn O,S co evaporated CuInS2 based devices. Temperature dependent current voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn O,S CuInS2 interface. However, photoelectron spectroscopy measurements indicate a small 0.1 eV spike like conduction band offset at the Zn O,S CuInS2 interface, excluding an unfavorable energy level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi level pinning not to be the main reason, leaving near interface defects as recently observed in Cu rich CuInSe2 as the likely reason for the performance limiting interface recombinatio

Isolated Rh atoms in dehydrogenation catalysis

AbstractIsolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga9Rh2 phase and the hitherto not-studied Ga3Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems. </jats:p

Lawson criterion for ignition exceeded in an inertial fusion experiment

For more than half a century, researchers around the world have been engaged in attempts to achieve fusion ignition as a proof of principle of various fusion concepts. Following the Lawson criterion, an ignited plasma is one where the fusion heating power is high enough to overcome all the physical processes that cool the fusion plasma, creating a positive thermodynamic feedback loop with rapidly increasing temperature. In inertially confined fusion, ignition is a state where the fusion plasma can begin "burn propagation" into surrounding cold fuel, enabling the possibility of high energy gain. While "scientific breakeven" (i.e., unity target gain) has not yet been achieved (here target gain is 0.72, 1.37 MJ of fusion for 1.92 MJ of laser energy), this Letter reports the first controlled fusion experiment, using laser indirect drive, on the National Ignition Facility to produce capsule gain (here 5.8) and reach ignition by nine different formulations of the Lawson criterion