A unified secondary electron cut-off presentation and common mistakes in photoelectron spectroscopy

Photoelectron spectroscopy is a powerful surface analysis technique that can differentiate different bonding environments and directly determine the absolute work function of a sample. Despite its ever-easier accessibility—or perhaps precisely because of it—some common mistakes or bad habits are often found in the literature when it comes to the evaluation or presentation of photoelectron spectroscopy data. Here we address some of these issues and give suggestions for best practice, i.e., a proper presentation of the secondary electron cut-off used for work function determination, correct binding energy referencing and some tips for appropriate peak fitting, as well as valuable literature references to more detailed tutorials. Finally, we present a concise step-by-step guide on how to conduct a complete x-ray photoelectron spectroscopy analysis of an unknown sample.Peer Reviewe

Modulation of the Work Function by the Atomic Structure of Strong Organic Electron Acceptors on H-Si(111)

Advances in hybrid organic/inorganic architectures for optoelectronics can be achieved by understanding how the atomic and electronic degrees of freedom cooperate or compete to yield the desired functional properties. Here we show how work-function changes are modulated by the structure of the organic components in model hybrid systems. We consider two cyano-quinodimethane derivatives (F4-TCNQ and F6-TCNNQ), which are strong electron-acceptor molecules, adsorbed on H-Si(111). From systematic structure searches employing range-separated hybrid HSE06 functional including many body van der Waals contributions, we predict that despite their similar composition, these molecules adsorb with significantly different densely-packed geometries in the first layer, due to strong intermolecular interaction. F6-TCNNQ shows a much stronger intralayer interaction (primarily due to van der Waals contributions) than F4-TCNQ in multilayered structures. The densely-packed geometries induce a large interface-charge rearrangement that result in a work-function increase of 1.11 and 1.76 eV for F4-TCNQ and F6-TCNNQ, respectively. Nuclear fluctuations at room temperature produce a wide distribution of work-function values, well modeled by a normal distribution with {\sigma}=0.17 eV. We corroborate our findings with experimental evidence of pronounced island formation for F6-TCNNQ on H-Si(111) and with the agreement of trends between predicted and measured work-function changes

Mechanisms within the Parietal Cortex Correlate with the Benefits of Random Practice in Motor Adaptation

The motor learning literature shows an increased retest or transfer performance after practicing under unstable (random) conditions. This random practice effect (also known as contextual interference effect) is frequently investigated on the behavioral level and discussed in the context of mechanisms of the dorsolateral prefrontal cortex and increased cognitive efforts during movement planning. However, there is a lack of studies examining the random practice effect in motor adaptation tasks and, in general, the underlying neural processes of the random practice effect are not fully understood. We tested 24 right-handed human subjects performing a reaching task using a robotic manipulandum. Subjects learned to adapt either to a blocked or a random schedule of different force field perturbations while subjects’ electroencephalography (EEG) was recorded. The behavioral results showed a distinct random practice effect in terms of a more stabilized retest performance of the random compared to the blocked practicing group. Further analyses showed that this effect correlates with changes in the alpha band power in electrodes over parietal areas. We conclude that the random practice effect in this study is facilitated by mechanisms within the parietal cortex during movement execution which might reflect online feedback mechanisms

Atomic Layer Deposition of MoS2 Decorated TiO2 Nanotubes for Photoelectrochemical Water Splitting

A thermal atomic layer deposition (ALD) process to fabricate MoS2 thin films is successfully demonstrated by using cycloheptatriene molybdenum tricarbonyl (C7H8Mo(CO)3) and H2S as precursors at an ALD temperature below 300 °C. The process is systematically investigated, showing a typical self‐limiting characteristic within an ALD temperature window of 225–285 °C and a high growth‐per‐cycle of 0.11 nm. The as‐deposited films are amorphous while they can be crystallized in situ by sulfurization with H2S at a low temperature of 300 °C. A prototypical application of the developed ALD process is demonstrated by constructing a MoS2/TiO2 heterostructure through depositing MoS2 onto anodized TiO2 nanotubes for photoelectrochemical water splitting. The MoS2/TiO2 heterostructures exhibit approximately three times superior photoelectrochemical performance than the pristine TiO2 nanotubes. This is attributed to an enhanced visible light‐harvesting ability of MoS2 and an improved separation of the photo‐generated charge carriers at the heterostructure interface, which is affirmed by a staggering gap (type II) between MoS2 and TiO2 as probed by ultraviolet photoelectron spectroscopy.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Peer Reviewe

Ultra-stable self-standing Au nanowires/TiO2TiO_2 nanoporous membrane system for high-performance photoelectrochemical water splitting cells

We introduce for the first time a core shell structure composed of nanostructured self standing titania nanotubes TNT, light absorber filled with Au nanowire AuNW array electrons collector applied to the photoelectrocatalytic water splitting. Its activity is four times higher than that of reference TNT Ti obtained with the same anodizing conditions. The composite photoanode brings a distinct photocurrent generation 8 mA cm amp; 8722;2 at 1.65 V vs. RHE , and a high incident photon to current efficiency of 35 obtained under UV light illumination. Moreover, the full system concept of selected constitutional materials, based on Au noble metal and the very stable semiconductor TiO2, ensures a stable performance over a long time range with no photocurrent loss during 100 on off cycles of light illumination, after 12 h constant illumination and after one month storage in air. We provide experimental evidence by photoelectron spectroscopy measurements, confirming that the electronic structure of TNT AuNW is rectifying for electrons and ohmic for holes, while the electrochemical characterization confirms that the specific architecture of the photoanode supports electron separation due to the presence of a Schottky type contact and fast electron transport through the Au nanowires. Although the composite material shows an unchanged electrochemical band gap, typical for plain TiO2, we find this material to be an innovative platform for efficient photoelectrochemical water splitting under UV light illumination, with significant potential for further modifications, for example extension into the visible light regim

Dual Doping of MoP with M(Mn,Fe) and S to Achieve High Hydrogen Evolution Reaction Activity in Both Acidic and Alkaline Media

Rational design of cost‐effective, high performance and stable hydrogen evolution reaction (HER) electrocatalysts in both acidic and alkaline media holds the key to the future hydrogen‐based economy. Herein, we introduce an effective approach of simultaneous non‐metal (S) and metal (Fe or Mn) doping of MoP to achieve excellent HER performance at different pH. The catalysts show remarkable overpotentials at −10 mA cm−2 of only 65 and 68 mV in 0.5 M H2SO4, and 50 and 51 mV in 1.0 M KOH, respectively, as well as much higher turnover frequencies compared to undoped MoP. Furthermore, the catalysts exhibit outstanding long‐term stability at a fixed current of −10 mA cm−2 for 40 h. The effects of both dopants, such as electronic structure modification and enhancement of the intrinsic activity, increase of the electrochemically active surface area, and formation of coordinatively unsaturated edge sites, act cooperatively to accelerate the HER at both pH media. Additionally, the presence of oxophilic Mn and Fe at the surface results in Mn or Fe oxide/hydroxide species that promote the dissociation of water molecules in alkaline electrolyte. This work introduces a facile and effective design principle that could pave the way towards engineering highly active HER catalysts for a wide pH range.Metal (Mn or Fe) and non‐metal (S) dual doped MoP catalysts were synthesiszed by reductive pyrolysis of the corresponding Mn,Mo‐ and Fe,Mo‐phosphonates precursors, in the presence of elemental S. The derived catalysts showed remarkable hydrogen evolution reaction (HER) activity in acidic and alkaline media. The dual doping process endowed MoP with proper hydrogen binding energy thus enhancing the HER in acidic media. In addition, Mn and Fe acted as surface oxides species in alkaline medium, which facilitated the water dissociation step. imageYousef Jameel Scholarship FundPeer Reviewe

Enabling Aqueous Processing of Ni‐Rich Layered Oxide Cathode Materials by Addition of Lithium Sulfate

Aqueous processing of Ni-rich layered oxide cathode materials is a promising approach to simultaneously decrease electrode manufacturing costs, while bringing environmental benefits by substituting the state-of-the-art (often toxic and costly) organic processing solvents. However, an aqueous environment remains challenging due to the high reactivity of Ni-rich layered oxides towards moisture, leading to lithium leaching and Al current collector corrosion because of the resulting high pH value of the aqueous electrode paste. Herein, a facile method was developed to enable aqueous processing of LiNi0.8Co0.1Mn0.1O2 (NCM811) by the addition of lithium sulfate (Li2SO4) during electrode paste dispersion. The aqueously processed electrodes retained 80 % of their initial capacity after 400 cycles in NCM811||graphite full cells, while electrodes processed without the addition of Li2SO4 reached 80 % of their capacity after only 200 cycles. Furthermore, with regard to electrochemical performance, aqueously processed electrodes using carbon-coated Al current collector outperformed reference electrodes based on state-of-the-art production processes involving N-methyl-2-pyrrolidone as processing solvent and fluorinated binders. The positive impact on cycle life by the addition of Li2SO4 stemmed from a formed sulfate coating as well as different surface species, protecting the NCM811 surface against degradation. Results reported herein open a new avenue for the processing of Ni-rich NCM electrodes using more sustainable aqueous routes.European Union http://dx.doi.org/10.13039/501100000780European Union's Horizon 2020 research and innovation programPeer Reviewe

Electronic properties of hybrid organic/inorganic semiconductor pn-junctions

Hybrid inorganic/organic semiconductor heterojunctions are candidates to expand the scope of purely organic or inorganic junctions in electronic and optoelectronic devices. Comprehensive understanding of bulk and interface doping on the junction’s electronic properties is therefore desirable. In this work, we elucidate the energy level alignment and its mechanisms at a prototypical hybrid pn-junction comprising ZnO (n-type) and p-doped N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) as semiconductors, using photoelectron spectroscopy. The level alignment can be quantitatively described by the interplay of contact-induced band and energy level bending in the inorganic and organic component away from the interface, and an interface dipole due to the push-back effect. By adjusting the dopant concentration in α-NPD, the position of the frontier energy levels of ZnO can be varied by over 0.5 eV and that of α-NPD by over 1 eV. The tunability of this pn-junction’s energy levels evidences the substantial potential of the hybrid approach for enhancing device functionality.Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Peer Reviewe