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Rational design of efficient pH-universal hydrogen evolution reaction (HER) catalysts to enable large-scale hydrogen production via electrochemical water splitting is of great significance, yet a challenging task. Herein, Ru atoms in the Ru2P structure were replaced with M=Co, Ni, or Mo to produce M2-xRuxP nanocrystals. The metals show strong site preference, with Co and Ni occupying the tetrahedral sites and Ru the square pyramidal sites of the CoRuP and NiRuP Ru2P-type structures. The presence of Co or Ni in the tetrahedral sites leads to charge redistribution for Ru and, according to density functional theory (DFT) calculations, to a significant increase in the Ru d-band centers. As a result, the intrinsic activity of CoRuP and 1 NiRuP increases considerably compared to Ru2P in both acidic and alkaline media. The effect is not observed for MoRuP, in which Mo prefers to occupy the pyramidal sites. In particular, CoRuP shows state-of-the-art activity, outperforming Ru2P with Pt-like activity in 0.5 M H2SO4 (η10=12.3 mV; η100=52 mV; turnover frequency (TOF)=4.7 s-1). It remains extraordinarily active in alkaline conditions (η10=12.9 mV; η100=43.5 mV) with a TOF of 4.5 s-1, which is 4x higher than that of Ru2P and 10x that of Pt/C. Further increase of the Co content does not lead to drastic loss of activity, especially in alkaline medium, where for example the TOF of Co1.9Ru0.1P remains comparable to that of Ru2P and higher than Pt/C, highlighting the viability of the adopted approach to prepare cost efficient catalysts.

Graphdiyne polymers have interesting electronic properties due to their π-conjugated structure and modular composition. Most of the known synthetic pathways for graphdiyne polymers yield amorphous solids because the irreversible formation of carbon–carbon bonds proceeds under kinetic control and because of defects introduced by the inherent chemical lability of terminal alkyne bonds in the monomers. Here, we present a one-pot surface-assisted deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes over a copper surface starting with stable trimethylsilylated alkyne monomers. In comparison to conventional polymerisation protocols, our method yields large-area crystalline thin graphdiyne films and, at the same time, minimises detrimental effects on the monomers like oxidation or cyclotrimerisation side reactions typically associated with terminal alkynes. A detailed study of the reaction mechanism reveals that the deprotection and polymerisation of the monomer is promoted by Cu(ii) oxide/hydroxide species on the as-received copper surface. These findings pave the way for the scalable synthesis of crystalline graphdiyne-based materials as cohesive thin films., We present a one-pot deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes on top of a copper surface starting with stable trimethylsilylated alkyne monomers.

Tailor-made electrode work functions are indispensable to control energy-level offsets at the interfaces of (opto-)electronic devices. We show by means of photoelectron spectroscopy that several na...

Coordination polymers (CPs) and metal-organic frameworks (MOFs) have emerged as versatile precursors for transition-metal phosphides catalysts. However, the controlled synthesis of CPs-derived bime...

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 are materials for multifarious applications such as sensing, catalysis, and energy storage. Due to their peculiar charge-transport properties, it is always desired to control their morphologies from vertical nanostructures to horizontal basal-plane oriented smooth layers. In this work, we established a low-temperature ALD process for MoS2 deposition using bis(t-butylimino)bis(dimethylamino)molybdenum(VI) and H2S precursors. The ALD reaction parameters, including reaction temperature and precursor pulse times, are systematically investigated and optimized. Polycrystalline MoS2 is conformally deposited on carbon nanotubes, Si-wafers, and glass substrates. Moreover, the morphologies of the deposited MoS2 films are tuned from smooth film to vertically grown flakes, and to nano-dots, by controlling the reaction parameters/conditions. It is noticed that our MoS2 nanostructures showed morphology-dependent optical and electrocatalytic properties, allowing us to choose the required morphology for a targeted application.

The characterization of the active structure of water-splitting catalysts is crucial to evolve to a sustainable energy future based on hydrogen. Such information can only be obtained by operando methods. We present a diffuse reflectance UV-vis (DRUV) spectroelectrochemical study that allows tracking the changes of solid oxygen evolution catalysts under working conditions. The versatility of our approach is demonstrated on two cobalt-containing catalysts, Zn0.35Co0.65O and CoAl2O4. The changes the catalysts undergo during the oxygen evolution reaction can be tracked by probing the electronic structure using UV-vis spectroscopy. These findings are compared to ex situ analyses, which support the assignments of the structures stabilized under different potentials. Thus, structure–activity correlations can be proposed, and deeper insights into the catalytically active structures can be obtained.

Heterostructures made from semiconducting metal oxides (SMOX) are fundamental for the development of high-performance gas sensors. Yet, despite the recognition of their importance in real applications, the understanding of the transduction mechanism either related to the heterojunction, or simply to the core and shell materials is still lacking. A better understanding of the sensing response of heterostructured nanomaterials requires the engineering of heterojunctions with well-defined core and shell layers. Here, we introduce a series of prototypes nSMOX-CNT, pSMOX-CNT, and pSMOX-nSMOX-CNT and nSMOX-pSMOX-CNT hierarchical core-shell heterostructures (CSHS) permitting us to directly relate the sensing response to the SMOX shell, or to the p-n heterojunction. The carbon nanotubes are here used as highly conductive substrates permitting to operate the devices at relatively low temperature and are not involved in the sensing response. NiO and SnO2 are selected as representative p- and n-type SMOX, respectively, and the response of a set of samples is studied toward hydrogen considered as model analyte. The n,pSMOX-CNT CSHS exhibit response related to the n,pSMOX-shell layer. On the other hand, the pSMOX-nSMOX-CNT and nSMOX-pSMOX-CNT CSHS show sensing responses, which in certain cases are governed by the heterojunctions between nSMOX and pSMOX and strongly depends on the thickness of the SMOX layers. Due to the fundamental nature of this study, these findings are important for the development of next generation gas sensing devices.

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 available

Triazine‐based graphitic carbon nitride (TGCN) is the most recent addition to the family of graphene‐type, two‐dimensional and metal‐free materials. Although hailed as a promising low‐bandgap semiconductor for electronic applications, so far, only its structure and optical properties have been known. Here, we combine direction‐dependent electrical measurements and time‐resolved optical spectroscopy to determine macroscopic conductivity and microscopic charge carrier mobilities in this layered material “beyond graphene”. Electrical conductivity along the basal plane of TGCN is 65‐times lower than through the stacked layers; as opposed to graphite. Furthermore, we develop a model for this charge transport behavior based on observed carrier dynamics and random‐walk simulations. Our combined methods provide a path towards intrinsic charge transport in a direction‐dependent, layered semi‐conductor for applications in field‐effect transistors (FETs) and sensors.

Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH3NH3PbBr3 and CH3NH3PbI3), we reveal the band dispersion in two high-symmetry directions and identify the global valence band maxima. With these benchmark data, we construct "standard" photoemission spectra from polycrystalline thin film samples and resolve challenges discussed in the literature for determining the valence band onset with high reliability. Within the framework laid out here, the consistency of relating the energy level alignment in perovskite-based photovoltaic and optoelectronic devices with their functional parameters is substantially enhanced.

We report experimental and theoretical evidence of strong electron-plasmon interaction in $n$-doped single-layer ${\mathrm{MoS}}_{2}$. Angle-resolved photoemission spectroscopy measurements reveal the emergence of distinctive signatures of polaronic coupling in the electron spectral function. Calculations based on many-body perturbation theory illustrate that electronic coupling to two-dimensional carrier plasmons provides an exhaustive explanation of the experimental spectral features and their energies. These results constitute compelling evidence of the formation of plasmon-induced polaronic quasiparticles, suggesting that highly doped transition-metal dichalcogenides may provide a new platform to explore strong-coupling phenomena between electrons and plasmons in two dimensions.

Silicon-based anodes with lithium ions as charge carriers have the highest predicted charge density of 3579 mA h g-1 (for Li15Si4) while being comparatively safe. Contemporary electrodes do not achieve these theoretical values largely because production paradigms remained unchanged since their inception and rely on the mixing of weakly coordinated, multiple components. In this paper, we present the one-pot synthesis of high-performance anodes that reach the theoretical capacity of the fully lithiated state of silicon. Here, a semi-conductive triazine-based graphdiyne polymer network is grown around silicon nanoparticles directly on the current collector, a copper sheet. The current collector (Cu) acts as the catalyst for the formation of a semi-conductive triazine-based graphdiyne polymer network that grows around the inorganic, active material (Si). In comparison to established electrode assemblies, this process (i) omits any steps related to curing, drying, and annealing, (ii) does away with binders and conductivity-enhancing additives that decrease volumetric and gravimetric capacity, and (iii) cancels out the detrimental effects on performance, chemical and physical stability of conventional, three-component anodes (Si, binder, carbon black). This is because, the porous, semi-conducting organic framework (i) adheres to the current collector on which it grows via cooperative van der Waals interactions, (ii) acts effectively as conductor for electrical charges and binder of silicon nanoparticles via conjugated, covalent bonds, and (iii) enables selective transport of mass and charge-carriers (electrolyte and Li-ions) through pores of defined size. As a result, the anode shows extraordinarily high capacity at the theoretical limit of fully lithiated silicon, excellent performances in terms of cycling (exceeding 70% capacity retention after 100 cycles), and high mechanical and thermal stability. These high-performance anodes pave the way for use in flexible, wearable electronics and in environmentally demanding applications.

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 materials. Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 are materials for multifarious applications such as sensing, catalysis, and energy storage. Due to their peculiar charge-transport properties, it is always desired to control their morphologies from vertical nanostructures to horizontal basal-plane oriented smooth layers. In this work, we established a low-temperature ALD process for MoS2 deposition using bis(t-butylimino)bis(dimethylamino)molybdenum(vi) and H2S precursors. The ALD reaction parameters, including reaction temperature and precursor pulse times, are systematically investigated and optimized. Polycrystalline MoS2 is conformally deposited on carbon nanotubes, Si-wafers, and glass substrates. Moreover, the morphologies of the deposited MoS2 films are tuned from smooth film to vertically grown flakes, and to nano-dots, by controlling the reaction parameters/conditions. It is noticed that our MoS2 nanostructures showed morphology-dependent optical and electrocatalytic properties, allowing us to choose the required morphology for a targeted application.

The rational combination of tetracene Tc with crystalline silicon c Si could greatly enhance c Si solar cell efficiencies via singlet fission. The Tc c Si energy level alignment ELA is thought to be central to controlling the required interface transfer processes. We modified hydrogen terminated c Si H Si with 2,2 amp; 8242; perfluoronaphthalene 2,6 diylidene dimalononitrile F6TCNNQ , C60, or NF3 and probed the effect on the c Si surface chemistry, the Tc c Si ELA, the Tc morphology, and solar cell characteristics using ultraviolet and X ray photoelectron spectroscopy, atomic force microscopy, X ray diffraction, photoluminescence transients, device measurements, and transfer matrix optical modeling. Submonolayer interlayers of F6TCNNQ shifted the Tc H Si 111 ELA by up to 0.55 eV. C60 showed no notable effect on the ELA and proved detrimental for the Tc film morphology and solar cell performance. Neither F6TCNNQ nor C60 improved the Tc related photocurrent significantly. NF3 CVD substituted the H termination of H Si 100 with more electronegative species and resulted in work functions as high as 6 eV. This changed the Tc H Si 100 ELA by up to 0.45 eV. NF3 plasma from a remote source caused pronounced c Si oxidation and a diminished c Si photoluminescence lifetime, which was not observed for NF3 plasma created in close proximity to the c Si surface or neutral NF3. We discuss possible reasons for why the improved ELA does not lead to an improved singlet fission harvest

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 CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 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 N2 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 CH3NH3PbI3 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 N2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, 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.

Ni-based compounds have been widely used for catalytic water splitting in alkaline media. NiO-based nanostructures supported on carbon nanotubes (CNTs) were synthesized by atomic layer deposition (ALD) using NiCp2 and O3 as precursors. NiO-coated CNTs display enhanced catalytic activity for the oxygen evolution reaction. The recorded overpotential of 315 mV at the current density of 10 mA cm–2 in Ar-saturated 1 M KOH is lower than any reported values so far for NiO-based catalysts. Moreover, the overpotential is constant for 12 h electrolysis at current density of 10 mA cm–2, highlighting the stability of our materials. The Tafel slopes of 50 mV dec–1 are also lower than other Ni-based nanostructures previously reported. The study of CVs in the redox region in Fe(III)-free and Fe(III)-saturated 1 M KOH solutions proves that the high catalytic activity is intrinsic of the NiO/carbon composite and is not influenced by iron precipitation. The enhanced catalytic activities could be attributed to the conformal...

Transition metal oxides have been proposed as a possible replacement of Pt for the electrocatalytic oxygen reduction reaction (ORR), however the low intrinsic conductivity makes their application in electrocatalysis challenging. In this work, we demonstrate that atomic layer deposition (ALD) is capable to overcome this problem by coating conductive carbon nanotubes substrates with a conformal Mn3O4 layer of just few‐nm in thickness. The deposition parameters have been optimized in terms of thickness and crystallite sizes in order to produce a material exhibiting catalytic efficiency close to the one of carbon‐supported Pt particles and low polarization costs. The current densities recorded in linear sweep voltammetry prove that the Mn3O4 coating leads to a substantial increase of the catalytic efficiency, compared to uncoated carbon nanotubes, and was also higher than other manganese‐based catalysts reported so far. The sample prepared from only 50 ALD cycles (e.g. coating thickness of ~2 nm) shows the best compromise between catalytic efficiency, with an onset potential at 0.867 V (vs. RHE), and good conductivity of the electrode materials minimizing polarization. Indeed, the Tafel plots exhibit a similar slope than Pt/C demonstrating that the Mn3O4/CNTs reduce oxygen in a one‐step four electrons mechanism and with a similar kinetics as Ptbased electrocatalysts. Moreover, the current density keeps at 80% even after 12 h at 0.58 V displaying therefore a higher stability than Pt-based catalysts. These findings are attributed to the few nm-thick conformal and catalytic active coating obtained by atomic layer deposition, which also protects the underneath CNT substrate from corrosion.

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 metals.

In article number 2100215, Soohyung Park, Sylke Blumstengel, Norbert Koch, and co‐workers demonstrate a key mechanism for opto‐electronic properties enhancement of an organic/inorganic van der Waals semiconductor heterostructure in the 2D limit. A straddling type‐I energy level alignment enables sub‐picosecond resonance energy transfer from organic molecules to a 2D inorganic semiconductor monolayer, with enhanced luminescence yield from the inorganic component. [Image: see text]

Substantial variations in the electronic structure and thus possibly conflicting energetics at interfaces between hybrid perovskites and charge transport layers in solar cells have been reported by the research community. In an attempt to unravel the origin of these variations and enable reliable device design, we demonstrate that donor-like surface states stemming from reduced lead (Pb0) directly impact the energy level alignment at perovskite (CH3NH3PbI3–xClx) and molecular electron acceptor layer interfaces using photoelectron spectroscopy. When forming the interfaces, it is found that electron transfer from surface states to acceptor molecules occurs, leading to a strong decrease in the density of ionized surface states. As a consequence, for perovskite samples with low surface state density, the initial band bending at the pristine perovskite surface can be flattened upon interface formation. In contrast, for perovskites with a high surface state density, the Fermi level is strongly pinned at the con...

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. Here, we demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, we reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor 1,3,4,5,7,8-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) and a MoS2 monolayer. Our results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the design of advanced heterostructures, exhibiting tailored electronic properties in (opto-)electronic devices based on two-dimensional semiconductors. Doping of 2D semiconductors by electron acceptor or donor molecules has been suggested as a method to optimize their properties for electronic devices. Here, the authors identify three distinct charge transfer mechanisms that depend on the substrate and yield different doping efficiencies.

This is a complete self-standing study on operando diffuse reflectance UV-vis spectroelectrochemistry for investigating oxygen evolution electrocatalysts.

The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites’ photophysical properties.

Photovoltaic cells based on halide perovskites, possessing remarkably high power conversion efficiencies have been reported. To push the development of such devices further, a comprehensive and reliable understanding of their electronic properties is essential but presently not available. To provide a solid foundation for understanding the electronic properties of polycrystalline thin films, we employ single-crystal band structure data from angle-resolved photoemission measurements. For two prototypical perovskites (CH

Generally, the lack of long-range order in materials prevents from experimentally addressing their electronic band dispersion by angle-resolved photoelectron spectroscopy (ARPES), limiting such assessment to single crystalline samples. Here we demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide (TMDC) monolayers with 2 H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. We exemplify this by analyzing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. A robust base for investigating TMDC monolayers significantly beyond single crystal samples is thus established. The complex nature of polycrystalline materials mean that characteristics such as their electronic band structure can be more challenging to interpret than their single crystal counterparts. Here, the authors present a framework based on angle-resolved photoemission spectroscopy to reveal the band dispersions for azimuthally disordered transition metal dichalcogenide polycrystalline monolayers.

International audience; To arrive to sustainable hydrogen-based energy solutions, the understanding of water-splitting catalysts plays the most crucial role. Herein, we combined state-of-the-art hypotheses on electrocatalytic active metal sites towards the oxygen evolution reaction (OER) to develop a highly efficient catalyst based on earth-abundant cobalt and zinc oxides. The precursor catalyst Zn 0.35 Co 0.65 O was synthesized via a fast microwave-assisted approach at low temperatures. Subsequent, it transformed in situ from the Wurtzite structure to the layered γ-Co(O)OH, while most of its zinc leaches out. This material shows outstanding catalytic performance and stability towards the OER in 1 M KOH (overpotential at 10 mA cm-2 η initial = 306 mV, η 98 h = 318 mV). By comparing the electrochemical results and ex situ analyses to today's literature, we were able to identify clear structure-activity correlations. Our findings suggest that coordinately unsaturated cobalt octahedra on the surface are indeed the active centers for the OER.

Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of states distribution of the components governs the amount of charge transfer, but a notable dependence on temperature has not yet been considered, particularly for weakly interacting systems. Here, we experimentally observe that the amount of ground state charge transfer in a van der Waals heterostructure formed by monolayer MoS2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of three when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that account for nuclear thermal fluctuations reveal intra-component electron-phonon coupling and inter-component electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multi-component van der Waals heterostructures.

WOS: 000439102400002, PubMed: 30026501, Multiple functionality of tungsten polyoxometalate (POM) has been achieved applying it as interfacial layer for solution processed high performance In2O3 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., Aquitaine regional grant "SMOLED" [2014-1R60306]; DFGGerman Research Foundation (DFG) [SFB951, AM 419/1-1]; China Scholarship Council (CSC)China Scholarship Council; ANR as part of the "Investissements d'avenir" programFrench National Research Agency (ANR) [ANR-10-EQPX-28-01/Equipex ELORPrintTec], The project is supported by Aquitaine regional grant "SMOLED" (2014-1R60306). The work in Berlin was supported by the DFG (SFB951 and AM 419/1-1). A. Ablat thanks the financial support of China Scholarship Council (CSC). Authors are thankful to the ANR as part of the "Investissements d'avenir" program (reference: ANR-10-EQPX-28-01/Equipex ELORPrintTec) and technical support of Dr. Roland Lefevre in XPS measurement.

A novel self-terminating chemical approach for the deposition of WS2 by atomic layer deposition based on chemisorption of bis(t-butylimino)bis(dimethylamino)tungsten(VI) followed by sulfurization b...

Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of these two energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Eg resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams determined at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron–hole pair, i.e. the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function. Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659

Controlling the electronic properties of interfaces including organic semiconductors is key for developing organic devices with increased efficiency and, potentially, new functions. Important progress has been made towards a comprehensive understanding of both weakly and strongly interacting organic interfaces over the past years. Here, we aim at incorporating this progress into a global framework, which will hopefully help refine design strategies for these systems in the future. Specifically, we first present the key concepts of energy-level alignment and related charge transfer mechanisms at play when organic semiconductors form interfaces with solid surfaces. We then underline the central role of electronic equilibrium and electrostatics by reviewing excellent agreement between experiment and emergent theoretical models. Lastly, we provide an overview over the underlying microscopic processes, notably the question of integer versus partial charge transfer, the impact of (inter-site and on-site) Coulomb interaction, the screening and broadening of molecular states via coupling to substrate electrons, and molecular distortions at surfaces.

The electronic properties of interfaces play a decisive role in determining the electrical characteristics of organic photovoltaic cells. In this paper, it is proposed that the mechanism responsible for the energy level alignment observed upon interface formation involves charge transfer between the electrode and the organic semiconductor. The amount of charges transferred is determined by occupying the density of states of the semiconductor according to Fermi–Dirac statistics and the observed energy level shifts are then governed by electrostatics. Within the proposed framework, we also discuss and numerically model the impact of the coupling strength between the molecular and the metal states. Photoelectron spectroscopy studies on two types of technologically relevant model interfaces, namely a metal/insulator/organic semiconductor heterojunction and a donor/acceptor heterojunction supported on a metal electrode, are then presented. The experimental observations, ranging from band bending to interface dipole formation, are shown to emerge naturally from the same theoretical framework. These results unambiguously demonstrate the essential role of the metal electrode in governing the energy level alignment, even significantly away from the metal surface.

Understanding the nature of polarons – the fundamental charge carriers in molecular semiconductors – is indispensable for rational material design that targets superior (opto-) electronic device functionality. The traditionally conceived picture of the corresponding energy levels invokes singly occupied molecular states within the energy gap of the semiconductor. Here, by employing a combined theoretical and multi-technique experimental approach, we show that this picture needs to be revised. Upon introducing an excess electron or hole into the material, the respective frontier molecular level is split by strong on-site Coulomb repulsion into an upper unoccupied and a lower occupied sub-level, only one of which is located within the semiconductor gap. By including also inter-site Coulomb interaction between molecular ions and circumjacent neutral molecules, we provide a complete picture for the electronic structure of molecular semiconductors in the presence of excess charges. With this understanding, a critical re-examination of previous results is called for, and future investigations of the properties and dynamics of polarons in weakly interacting molecular systems are put on sound footing.

Two-dimensional (2D) semiconductors offer a convenient platform to study 2D physics, for example, to understand doping in an atomically thin semiconductor. Here, we demonstrate the fabrication and unravel the electronic properties of a lateral doped/intrinsic heterojunction in a single-layer (SL) tungsten diselenide (WSe2), a prototype semiconducting transition metal dichalcogenide (TMD), partially covered with a molecular acceptor layer, on a graphite substrate. With combined experiments and theoretical modeling, we reveal the fundamental acceptor-induced p-doping mechanism for SL-WSe2. At the 1D border between the doped and undoped SL-WSe2 regions, we observe band bending and explain it by Thomas–Fermi screening. Using atomically resolved scanning tunneling microscopy and spectroscopy, the screening length is determined to be in the few nanometer range, and we assess the carrier density of intrinsic SL-WSe2. These findings are of fundamental and technological importance for understanding and employing s...

Transition-metal phosphides (TMPs) have recently emerged as efficient and inexpensive electrocatalysts for electrochemical water splitting. The synthesis of nanostructured phosphides often involves highly reactive and hazardous phosphorous-containing compounds. Herein, we report the synthesis of nickel phosphides through thermal treatment under H2(5%)/Ar of layered nickel phenylphosphonate (NiPh) or methylphosphonate (NiMe) that act as single-source precursors. Ni12P5, Ni12P5-Ni2P, and Ni2P nanoparticles (NPs) with sizes of ca. 15–45 nm coated with a thin shell of carbonaceous material were produced. Thermogravimetric analysis coupled with mass spectrometry (TG–MS) showed that H2, H2O, P2, and —C6H5 are the main compounds formed during the transformation of the precursor under argon and no hazard phosphorous-containing compounds are created, making this a simple and relatively safe route for fabricating nanostructured TMPs. The H2 most likely reacts with the —PO3 groups of the precursor to form H2O and P2...

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A series of merocyanine dyes with a wide range of molecular dipole moments were deposited on metal oxides covering a wide work function (Φ) range (2.3 to 7.0 eV), and the energy level alignment at these interfaces was studied with photoelectron spectroscopy. We find that a preferential orientation of the merocyanines and their dipoles in the monolayer systematically lowers Φ of the oxides such that Fermi-level (EF) pinning at the highest occupied molecular level of the merocyanines only occurs for very high Φ oxides (≥6 eV). Correspondingly, pinning at the electron affinity level can readily be achieved also with moderate oxide Φ, e.g., for indium tin oxide, and electron transfer between the merocyanines to these oxides can proceed readily. Noteworthy, the EF-pinning behavior and the associated Φ values seem independent of the molecular dipole moment magnitude, most likely due to the self-limiting effect of Φ as soon as the pinning regime is reached.

Through silicon deposition onto the silver (1 1 0) surface, we have fabricated in a one-step process a highly perfect nanoscale template consisting of a self-assembled Si nanostripe array with a pitch of 2 nm, covering uniformly the entire surface. Scanning tunneling microscopy investigations show that this system can be used, in a very simple way, as a template for the growth of identical highly ordered one-dimensional nanostructures. The adsorption of Co at room temperature and C60 at 190 °C gives rise to the growth of self-organized one-dimensional nanostructures reproducing the one-dimensional pattern of the Si template.

A novel atomic layer deposition process of manganese oxide from methylcyclopentadienyl manganese tricarbonyl and ozone has been developed. It is used to coat vertically aligned carbon nanotubes (VACNTs) grown on a conductive Inconel substrate in order to produce a 3D array of metal oxide-carbon nanotube electrode. Electron microscopy studies show that a conformal polycrystalline film can be deposited on the VACNTs arrays. X-ray diffraction, small area electron diffraction, and X-ray photoelectron spectroscopy analysis determine the formation of hausmannite (Mn3O4). The electrochemical properties of the as-prepared VACNTs/Mn3O4 nano-composite electrodes are studied using cyclic voltammetry, galvanostatic charge and discharge cycling in 1 m Na2SO4 aqueous electrolyte. Capacitances as high as 78.62 mF cm(-2) at 5 mV s(-1) are demonstrated which is one order of magnitude higher than that of pristine VACNTs. In addition, the as-prepared VACNTs/Mn3O4 nanocomposite electrode showed a good reversibility and cycling stability.