Spectral Tuning of a Nanoparticle-on-Mirror System by Graphene Doping and Gap Control with Nitric Acid.

Nanoparticle-on-mirror systems are a stable, robust, and reproducible method of squeezing light into sub-nanometer volumes. Graphene is a particularly interesting material to use as a spacer in such systems as it is the thinnest possible 2D material and can be doped both chemically and electrically to modulate the plasmonic modes. We investigate a simple nanoparticle-on-mirror system, consisting of a Au nanosphere on top of an Au mirror, separated by a monolayer of graphene.

Identification of Ubiquitously Present Polymeric Adlayers on 2D Transition Metal Dichalcogenides.

The interest in 2D materials continues to grow across numerous scientific disciplines as compounds with unique electrical, optical, chemical, and thermal characteristics are being discovered. All these properties are governed by an all-surface nature and nanoscale confinement, which can easily be altered by extrinsic influences, such as defects, dopants or strain, adsorbed molecules, and contaminants. Here, we report on the ubiquitous presence of polymeric adlayers on top of layered transition metal dichalcogenides (TMDs).

Demonstrating the source of inherent instability in NiFe LDH-based OER electrocatalysts.

Nickel-iron layered double hydroxides are known to be one of the most highly active catalysts for the oxygen evolution reaction in alkaline conditions. The high electrocatalytic activity of the material however cannot be sustained within the active voltage window on timescales consistent with commercial requirements. The goal of this work is to identify and prove the source of inherent catalyst instability by tracking changes in the material during OER activity.

Understanding the effect of MXene in a TMO/MXene hybrid catalyst for the oxygen evolution reaction.

Very recently, it has been reported that mixed transition metal oxide (TMO)/MXene catalysts show improved performance over TMO only catalysts for the oxygen evolution reaction (OER). However, the reasoning behind this observation is unknown. In this work mixed Co(OH)/TiCT were prepared and characterized for the OER using ex situ and operando spectroscopy techniques in order to initiate the understanding of why mixed TMO/MXene materials show better performances compared to TMO only catalysts.

Ion-Induced Surface Charge Dynamics in Freestanding Monolayers of Graphene and MoS_{2} Probed by the Emission of Electrons.

We compare the ion-induced electron emission from freestanding monolayers of graphene and MoS_{2} to find a sixfold higher number of emitted electrons for graphene even though both materials have similar work functions. An effective single-band Hubbard model explains this finding by a charge-up in MoS_{2} that prevents low energy electrons from escaping the surface within a period of a few femtoseconds after ion impact. We support these results by measuring the electron energy distribution for correlated pairs of electrons and transmitted ions.

Synthesis and characterisation of thin-film platinum disulfide and platinum sulfide.

Group-10 transition metal dichalcogenides (TMDs) are rising in prominence within the highly innovative field of 2D materials. While PtS has been investigated for potential electronic applications, due to its high charge-carrier mobility and strongly layer-dependent bandgap, it has proven to be one of the more difficult TMDs to synthesise. In contrast to most TMDs, Pt has a significantly more stable monosulfide, the non-layered PtS.

Layered PtSe for Sensing, Photonic, and (Opto-)Electronic Applications.

Since the first experimental discovery of graphene 16 years ago, many other 2D layered nanomaterials have been reported. However, the majority of 2D nanostructures suffer from relatively complicated fabrication processes that have bottlenecked their development and their uptake by industry for practical applications. Here, the recent progress in sensing, photonic, and (opto-)electronic applications of PtSe , a 2D layered material that is likely to be used in industries benefiting from its high air-stability and semiconductor-technology-compatible fabrication methods, is reviewed.

Organic Electrochemical Transistors (OECTs) Toward Flexible and Wearable Bioelectronics.

Organic electronics have emerged as a fascinating area of research and technology in the past two decades and are anticipated to replace classic inorganic semiconductors in many applications. Research on organic light-emitting diodes, organic photovoltaics, and organic thin-film transistors is already in an advanced stage, and the derived devices are commercially available. A more recent case is the organic electrochemical transistors (OECTs), whose core component is a conductive polymer in contact with ions and solvent molecules of an electrolyte, thus allowing it to simultaneously regulate electron and ion transport.

Electronic and structural characterisation of polycrystalline platinum disulfide thin films.

We employ a combination of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to investigate the properties of layered PtS, synthesised thermally assisted conversion (TAC) of a metallic Pt thin film. STM measurements reveal the 1T crystal structure of PtS, and the lattice constant is determined to be 3.58 ± 0.

Influence of Gold Nano-Bipyramid Dimensions on Strong Coupling with Excitons of Monolayer MoS.

Rabi splitting between the longitudinal plasmon of a gold nano-bipyramid and the A exciton of monolayer MoS is observed at room temperature. The dependence of the Rabi splitting on the physical dimensions of the nano-bipyramid is reported. The impact of bipyramid length, aspect ratio, and tip radius on the coupling strength is investigated.

Atomic-Scale Carving of Nanopores into a van der Waals Heterostructure with Slow Highly Charged Ions.

The growing family of 2D materials led not long ago to combining different 2D layers and building artificial systems in the form of van der Waals heterostructures. Tailoring of heterostructure properties postgrowth would greatly benefit from a modification technique with a monolayer precision. However, appropriate techniques for material modification with this precision are still missing.

Low-temperature synthesis and electrocatalytic application of large-area PtTe thin films.

The synthesis of transition metal dichalcogenides (TMDs) has been a primary focus for 2D nanomaterial research over the last 10 years, however, only a small fraction of this research has been concentrated on transition metal ditellurides. In particular, nanoscale platinum ditelluride (PtTe) has rarely been investigated, despite its potential applications in catalysis, photonics and spintronics. Of the reports published, the majority examine mechanically-exfoliated flakes from chemical vapor transport (CVT) grown crystals.

Calibration of Nonstationary Gas Sensors Based on Two-Dimensional Materials.

Two-dimensional materials (2DMs) have high potential in gas sensing, due to their large surface-to-volume ratio. However, most sensors based on 2DMs suffer from the lack of a steady state during gas exposure, hampering sensor calibration. Here, we demonstrate that analysis of the time differential of the signal output enables the calibration of chemiresistors based on platinum or tungsten diselenide (PtSe, WSe) and molybdenum disulfide (MoS), which present nonstationary behavior.

Insights into Multilevel Resistive Switching in Monolayer MoS.

The advent of two-dimensional materials has opened a plethora of opportunities in accessing ultrascaled device dimensions for future logic and memory applications. In this work, we demonstrate that a single layer of large-area chemical vapor deposition-grown molybdenum disulfide (MoS) sandwiched between two metal electrodes can be tuned to show multilevel nonvolatile resistive memory states with resistance values separated by 5 orders of magnitude. The switching process is unipolar and thermochemically driven requiring significant Joule heating in the reset process.

MoS Memtransistors Fabricated by Localized Helium Ion Beam Irradiation.

Two-dimensional (2D) layered semiconductors have recently emerged as attractive building blocks for next-generation low-power nonvolatile memories. However, challenges remain in the controllable fabrication of bipolar resistive switching circuit components from these materials. Here, the experimental realization of lateral memtransistors from monolayer single-crystal molybdenum disulfide (MoS) utilizing a focused helium ion beam is reported.

Surface-State Assisted Carrier Recombination and Optical Nonlinearities in Bulk to 2D Nonlayered PtS.

Cooperite, or platinum sulfide (PtS), is a rare mineral that generally exists as microscale, irregularly shaped crystallites. The presence of impurities, in both naturally occurring and synthesized samples, has hindered the study of its optical properties in the past. In this work, we prepare large-scale, uniform PtS films in bulk to two-dimensional form through the thermally assisted conversion method.

Effects of Annealing Temperature and Ambient on Metal/PtSe Contact Alloy Formation.

Forming gas annealing is a common process step used to improve the performance of devices based on transition-metal dichalcogenides (TMDs). Here, the impact of forming gas anneal is investigated for PtSe-based devices. A range of annealing temperatures (150, 250, and 350 °C) were used both in inert (0/100% H/N) and forming gas (5/95% H/N) environments to separate the contribution of temperature and ambient.

Ultrafast Carrier Dynamics and Bandgap Renormalization in Layered PtSe.

Carrier interactions in 2D nanostructures are of central importance not only in condensed-matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe ) through ultrafast time-resolved pump-probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe .

Additive-free MXene inks and direct printing of micro-supercapacitors.

Direct printing of functional inks is critical for applications in diverse areas including electrochemical energy storage, smart electronics and healthcare. However, the available printable ink formulations are far from ideal. Either surfactants/additives are typically involved or the ink concentration is low, which add complexity to the manufacturing and compromises the printing resolution.

High capacity silicon anodes enabled by MXene viscous aqueous ink.

The ever-increasing demands for advanced lithium-ion batteries have greatly stimulated the quest for robust electrodes with a high areal capacity. Producing thick electrodes from a high-performance active material would maximize this parameter. However, above a critical thickness, solution-processed films typically encounter electrical/mechanical problems, limiting the achievable areal capacity and rate performance as a result.

Perforating Freestanding Molybdenum Disulfide Monolayers with Highly Charged Ions.

Porous single-layer molybdenum disulfide (MoS) is a promising material for applications such as DNA sequencing and water desalination. In this work, we introduce irradiation with highly charged ions (HCIs) as a new technique to fabricate well-defined pores in MoS. Surprisingly, we find a linear increase of the pore creation efficiency over a broad range of potential energies.

A WSe vertical field emission transistor.

We report the first observation of a gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on a SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.

Environmental Effects on the Electrical Characteristics of Back-Gated WSe₂ Field-Effect Transistors.

We study the effect of polymer coating, pressure, temperature, and light on the electrical characteristics of monolayer WSe 2 back-gated transistors with Ni / Au contacts. Our investigation shows that the removal of a layer of poly(methyl methacrylate) (PMMA) or a decrease of the pressure change the device conductivity from p- to n-type. From the temperature behavior of the transistor transfer characteristics, a gate-tunable Schottky barrier at the contacts is demonstrated and a barrier height of ~ 70 meV in the flat-band condition is measured.

In Situ Formed Protective Barrier Enabled by Sulfur@Titanium Carbide (MXene) Ink for Achieving High-Capacity, Long Lifetime Li-S Batteries.

Sulfur (S) is an attractive cathode material with advantages including high theoretical capacity and low cost. However, issues such as the lithium polysulfide shuttle effect and its insulating properties greatly limit the future applications of lithium-sulfur (Li-S) batteries. Here, a viscous aqueous ink with nanoscale S uniformly decorated on the polar, metallically conductive titanium carbide MXene nanosheets (S@TiCT ) is reported to address these issues.

Optimized single-layer MoS field-effect transistors by non-covalent functionalisation.

Field-effect transistors (FETs) with non-covalently functionalised molybdenum disulfide (MoS2) channels grown by chemical vapour deposition (CVD) on SiO2 are reported. The dangling-bond-free surface of MoS2 was functionalised with a perylene bisimide derivative to allow for the deposition of Al2O3 dielectric. This allowed the fabrication of top-gated, fully encapsulated MoS2 FETs.