Spatially selective reversible charge carrier density tuning in WS_2 monolayers via photochlorination

Chlorine-doped tungsten disulfide monolayer (1L-WS_2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal's electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron spectroscopy (XPS) are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS_2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers

3D-to-2D morphology manipulation of sputter-deposited nanoscale silver films on weakly interacting substrates via selective nitrogen deployment for multifunctional metal contacts

The ability to reverse the inherent tendency of noble metals to grow in an uncontrolled three-dimensional (3D) fashion on weakly interacting substrates, including two-dimensional (2D) materials and oxides, is essential for the fabrication of high-quality multifunctional metal contacts in key enabling devices. In this study, we show that this can be effectively achieved by deploying nitrogen (N2) gas with high temporal precision during magnetron sputtering of nanoscale silver (Ag) islands and layers on silicon dioxide (SiO2) substrates. We employ real-time in situ film growth monitoring using spectroscopic ellipsometry, along with optical modeling in the framework of the finite-difference time-domain method, and establish that localized surface plasmon resonance (LSPR) from nanoscale Ag islands can be used to gauge the evolution of surface morphology of discontinuous layers up to a SiO2 substrate area coverage of ∼70%. Such analysis, in combination with data on the evolution of room-temperature resistivity of electrically conductive layers, reveals that presence of N2 in the sputtering gas atmosphere throughout all film-formation stages: (i) promotes 2D growth and smooth film surfaces and (ii) leads to an increase of the continuous-layer electrical resistivity by ∼30% compared to Ag films grown in a pure argon (Ar) ambient atmosphere. Detailed ex situ nanoscale structural analyses suggest that N2 favors 2D morphology by suppressing island coalescence rates during initial growth stages, while it causes interruption of local epitaxial growth on Ag crystals. Using these insights, we deposit Ag layers by deploying N2 selectively, either during the early precoalescence growth stages or after coalescence completion. We show that early N2 deployment leads to 2D morphology without affecting the Ag-layer resistivity, while postcoalescence introduction of N2 in the gas atmosphere further promotes formation of three-dimensional (3D) nanostructures and roughness at the film growth front. In a broader context this study generates knowledge that is relevant for the development of (i) single-step growth manipulation strategies based on selective deployment of surfactant species and (ii) real-time methodologies for tracking film and nanostructure morphological evolution using LSPR

Silver nanoparticle array on weakly interacting epitaxial graphene substrate as catalyst for hydrogen evolution reaction under neutral conditions

Publisher Copyright: © 2021 Author(s).The paucity of research on hydrogen evolution reaction (HER) under neutral conditions, which is a more sustainable way to produce H-2 compared to acidic and alkaline HER, encourages the development of efficient catalytic materials and devices and deeper investigation of the mechanisms behind neutral HER. We present an electrode concept for facilitating HER under neutral conditions. The concept entails the use of annealing-reshaped silver (Ag) nanoparticle array on monolayer epitaxial graphene (MEG) on 4H-SiC. Measurements of HER performance show more positive onset potential of the cathodic HER for Ag-decorated MEG compared to that for pristine MEG, indicating improved water dissociation at Ag/MEG electrodes. Complementary morphological characterization, absorption measurements, and Raman mapping analysis enable us to ascribe the enhanced catalytic performance of electrodes decorated with 2 nm thick annealed Ag on the synergetic effect originating from simultaneous water reduction on circular Ag nanoparticles of 31 nm in diameter and on compressively strained Ag-free graphene regions. The overall results pave the way toward development of stable van der Waals heterostructure electrodes with a tunable metal-carbon interaction for fast HER under neutral conditions. (C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Energetic bombardment and defect generation during magnetron-sputter-deposition of metal layers on graphene

In the present work, we elucidate the interplay among energetic bombardment effects in magnetron sputtering and defect generation in two-dimensional (2D) materials. Using deposition of gold (Au) layers on single-layer graphene (SLG) as a model system, we study the effect of pressure-distance (pd) product during magnetron sputtering on the pristine SLG properties. Raman spectroscopy, complemented by X-ray photoelectron spectroscopy, shows that for pd = 8.2 Pa center dot cm, Au layer deposition causes defects in the SLG layer, which gradually diminish and eventually disappear with increasing pd to 82.5 Pa center dot cm. Stochastic and deterministic simulations of the sputtering process, the gas-phase transport, and the interaction of sputtered and plasma species with the substrate surface suggest that defects in SLG primarily emanate from ballistic damage caused by backscattered Ar atoms with energies above 100 eV. With increasing pd, and thereby gas-phase scattering, such high energy Ar species become thermalized and hence incapable of causing atomic displacements in the SLG layer. The overall results of our study suggest that control of backscattered Ar energy is a potential path toward enabling magnetron sputtering for fabrication of multifunctional metal contacts in devices founded upon 2D materials.Peer reviewe

Spatial non-uniformity in exfoliated WS<inf>2</inf> single layers

Monolayers of transition metal dichalcogenides (TMDs) are atomically thin two-dimensional crystals with attractive optoelectronic properties, which are promising for emerging applications in nanophotonics. Here, we report on the extraordinary spatial non-uniformity of the photoluminescence (PL) and strain properties of exfoliated WS2 monolayers. Specifically, it is shown that the edges of such monolayers exhibit remarkably enhanced PL intensity compared to their respective central area. A comprehensive analysis of the recombination channels involved in the PL process demonstrates a spatial non-uniformity across the monolayer's surface and reflects on the non-uniformity of the intrinsic electron density across the monolayer. Auger electron imaging and spectroscopy studies complemented with PL measurements in different environments indicate that oxygen chemisorption and physisorption are the two fundamental mechanisms responsible for the observed non-uniformity. At the same time Raman spectroscopy analysis shows remarkable strain variations among the different locations of an individual monolayer, however such variations cannot be strictly correlated with the non-uniform PL emission. Our results shed light on the role of the chemical bonding in the competition between exciton complexes in monolayer WS2, providing a method of engineering new nanophotonic functions using WS2 monolayers. It is therefore envisaged that our findings could find diverse applications towards the development of TMD-based optoelectronic devices

Spatially selective reversible charge carrier density tuning in WS<inf>2</inf> monolayers via photochlorination

Chlorine-doped tungsten disulfide monolayer (1L-WS2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal's electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron spectroscopy (XPS) are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers

Exploring the Leidenfrost Effect for the Deposition of High-Quality In<inf>2</inf>O<inf>3</inf> Layers via Spray Pyrolysis at Low Temperatures and Their Application in High Electron Mobility Transistors

The growth mechanism of indium oxide (In2O3) layers processed via spray pyrolysis of an aqueous precursor solution in the temperature range of 100–300 °C and the impact on their electron transporting properties are studied. Analysis of the droplet impingement sites on the substrate's surface as a function of its temperature reveals that Leidenfrost effect dominated boiling plays a crucial role in the growth of smooth, continuous, and highly crystalline In2O3 layers via a vapor phase-like process. By careful optimization of the precursor formulation, deposition conditions, and choice of substrate, this effect is exploited and ultrathin and exceptionally smooth layers of In2O3 are grown over large area substrates at temperatures as low as 252 °C. Thin-film transistors (TFTs) fabricated using these optimized In2O3 layers exhibit superior electron transport characteristics with the electron mobility reaching up to 40 cm2 V−1 s−1, a value amongst the highest reported to date for solution-processed In2O3 TFTs. The present work contributes enormously to the basic understanding of spray pyrolysis and highlights its tremendous potential for large-volume manufacturing of high-performance metal oxide thin-film transistor electronics