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. The majority of emitted primary electrons have an energy below 10 eV and are therefore subject to the dynamic charge-up effects at surfaces.

Low-temperature synthesis and electrocatalytic application of large-area PtTe2 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 (PtTe2) 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. This method produces high quality-crystals, ideal for fundamental studies. However, it is very resource intensive and difficult to scale up meaning there are significant obstacles to implementation in large-scale applications. In this report, the synthesis of thin films of PtTe2 through the reaction of solid-phase precursor films is described. This offers a production method for large-area, thickness-controlled PtTe2, potentially suitable for a number of applications. These polycrystalline PtTe2 films were grown at temperatures as low as 450 °C, significantly below the typical temperatures used in the CVT synthesis methods. Adjusting the growth parameters allowed the surface coverage and morphology of the films to be controlled. Analysis with scanning electron- and scanning tunneling microscopy indicated grain sizes of above 1 µm could be achieved, comparing favorably with typical values of ∼50 nm for polycrystalline films. To investigate their potential applicability, these films were examined as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The films showed promising catalytic behavior, however, the PtTe2 was found to undergo chemical transformation to a substoichiometric chalcogenide compound under ORR conditions. This study shows while PtTe2 is stable and highly useful for in HER, this property does not apply to ORR, which undergoes a fundamentally different mechanism. This study broadens our knowledge on the electrocatalysis of TMDs.

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. OECTs are very effective in ion-to-electron transduction and sensor signal amplification. The use of synthetically tunable, biocompatible, and depositable organic materials in OECTs makes them specially interesting for biological applications and printable devices. In this review, we provide an overview of the history of OECTs, their physical characterization, and their operation mechanism. We analyze OECT performance improvements obtained by geometry design and active material selection (i.e., conductive polymers and small molecules) and conclude with their broad range of applications from biological sensors to wearable devices.

Ultrafast Carrier Dynamics and Bandgap Renormalization in Layered PtSe2.

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 (PtSe2 ) 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 PtSe2 . These results imply that few-layer PtSe2 has a semiconductor-like carrier relaxation instead of a metal-like one. The relaxation follows a triple-exponential decay process and exhibits thickness-dependent relaxation times. This occurs along with a band-filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many-body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

Wide Spectral Photoresponse of Layered Platinum Diselenide-Based Photodiodes.

Platinum diselenide (PtSe2) is a group-10 transition metal dichalcogenide (TMD) that has unique electronic properties, in particular a semimetal-to-semiconductor transition when going from bulk to monolayer form. We report on vertical hybrid Schottky barrier diodes (SBDs) of two-dimensional (2D) PtSe2 thin films on crystalline n-type silicon. The diodes have been fabricated by transferring large-scale layered PtSe2 films, synthesized by thermally assisted conversion of predeposited Pt films at back-end-of-the-line CMOS compatible temperatures, onto SiO2/Si substrates. The diodes exhibit obvious rectifying behavior with a photoresponse under illumination. Spectral response analysis reveals a maximum responsivity of 490 mA/W at photon energies above the Si bandgap and relatively weak responsivity, in the range of 0.1-1.5 mA/W, at photon energies below the Si bandgap. In particular, the photoresponsivity of PtSe2 in infrared allows PtSe2 to be utilized as an absorber of infrared light with tunable sensitivity. The results of our study indicate that PtSe2 is a promising option for the development of infrared absorbers and detectors for optoelectronics applications with low-temperature processing conditions.

Highly Sensitive Electromechanical Piezoresistive Pressure Sensors Based on Large-Area Layered PtSe2 Films.

Two-dimensional (2D) layered materials are ideal for micro- and nanoelectromechanical systems (MEMS/NEMS) due to their ultimate thinness. Platinum diselenide (PtSe2), an exciting and unexplored 2D transition metal dichalcogenide material, is particularly interesting because its low temperature growth process is scalable and compatible with silicon technology. Here, we report the potential of thin PtSe2 films as electromechanical piezoresistive sensors. All experiments have been conducted with semimetallic PtSe2 films grown by thermally assisted conversion of platinum at a complementary metal-oxide-semiconductor (CMOS)-compatible temperature of 400 °C. We report high negative gauge factors of up to -85 obtained experimentally from PtSe2 strain gauges in a bending cantilever beam setup. Integrated NEMS piezoresistive pressure sensors with freestanding PMMA/PtSe2 membranes confirm the negative gauge factor and exhibit very high sensitivity, outperforming previously reported values by orders of magnitude. We employ density functional theory calculations to understand the origin of the measured negative gauge factor. Our results suggest PtSe2 as a very promising candidate for future NEMS applications, including integration into CMOS production lines.

Enabling Flexible Heterostructures for Li-Ion Battery Anodes Based on Nanotube and Liquid-Phase Exfoliated 2D Gallium Chalcogenide Nanosheet Colloidal Solutions.

2D metal chalcogenide (MC) nanosheets (NS) have displayed high capacities as lithium-ion battery (LiB) anodes. Nevertheless, their complicated synthesis routes coupled with low electronic conductivity greatly limit them as promising LiB electrode material. Here, this work reports a facile single-walled carbon nanotube (SWCNT) percolating strategy for efficiently maximizing the electrochemical performances of gallium chalcogenide (GaX, X = S or Se). Multiscaled flexible GaX NS/SWCNT heterostructures with abundant voids for Li+ diffusion are fabricated by embedding the liquid-exfoliated GaX NS matrix within a SWCNT-percolated network; the latter improves the electron transport and ion diffusion kinetics as well as maintains the mechanical flexibility. Consequently, high capacities (i.e., 838 mAh g-1 per gallium (II) sulfide (GaS) NS/SWCNT mass and 1107 mAh g-1 per GaS mass; the latter is close to the theoretical value) and good rate capabilities are achieved, which can be majorly attributed to the alloying processes of disordered Ga formed after the first irreversible GaX conversion reaction, as monitored by in situ X-ray diffraction. The presented approach, colloidal solution processing of SWCNT and liquid-exfoliated MC NS to produce flexible paper-based electrode, could be generalized for wearable energy storage devices with promising performances.

Dispersion of nonlinear refractive index in layered WS2 and WSe2 semiconductor films induced by two-photon absorption.

Both the nonlinear absorption and nonlinear refraction properties of WS2 and WSe2 semiconductor films have been characterized by using Z-scan technique with femtosecond pulses at the wavelength of 1040 nm. It is found that these films have two-photon absorption response with the nonlinear absorption coefficient of ∼103 cm GW-1, and a dispersion of nonlinear refractive index in the WS2 films that translated from positive in the monolayer to negative in bulk materials.

Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids.

To progress from the laboratory to commercial applications, it will be necessary to develop industrially scalable methods to produce large quantities of defect-free graphene. Here we show that high-shear mixing of graphite in suitable stabilizing liquids results in large-scale exfoliation to give dispersions of graphene nanosheets. X-ray photoelectron spectroscopy and Raman spectroscopy show the exfoliated flakes to be unoxidized and free of basal-plane defects. We have developed a simple model that shows exfoliation to occur once the local shear rate exceeds 10(4) s(-1). By fully characterizing the scaling behaviour of the graphene production rate, we show that exfoliation can be achieved in liquid volumes from hundreds of millilitres up to hundreds of litres and beyond. The graphene produced by this method performs well in applications from composites to conductive coatings. This method can be applied to exfoliate BN, MoS2 and a range of other layered crystals.

Nitrogen-doped reduced graphene oxide electrodes for electrochemical supercapacitors.

Herein we use Nitrogen-doped reduced Graphene Oxide (N-rGO) as the active material in supercapacitor electrodes. Building on a previous work detailing the synthesis of this material, electrodes were fabricated via spray-deposition of aqueous dispersions and the electrochemical charge storage mechanism was investigated. Results indicate that the functionalised graphene displays improved performance compared to non-functionalised graphene. The simplicity of fabrication suggests ease of up-scaling of such electrodes for commercial applications.

Chemically modulated graphene diodes.

We report the manufacture of novel graphene diode sensors (GDS), which are composed of monolayer graphene on silicon substrates, allowing exposure to liquids and gases. Parameter changes in the diode can be correlated with charge transfer from various adsorbates. The GDS allows for investigation and tuning of extrinsic doping of graphene with great reliability. The demonstrated recovery and long-term stability qualifies the GDS as a new platform for gas, environmental, and biocompatible sensors.

Nitrogen-doped pyrolytic carbon films as highly electrochemically active electrodes.

Nitrogen-doped Pyrolytic Carbon (N-PyC) films were employed as an electrode material in electrochemical applications. PyC was grown by via non-catalysed chemical vapour deposition and subsequently functionalised via exposure to ammonia-hydrogen plasma. The electrochemical properties of the N-PyC films were investigated using the ferri/ferro-cyanide and hexaamine ruthenium(III) chloride redox probes. Exceptional electron transfer properties were observed and quantified for the N-PyC compared to the as-grown films. X-ray photoelectron spectroscopy confirmed the presence of nitrogen in edge plane graphitic configurations and the surface of the N-PyC was investigated using scanning electron microscopy and atomic force microscopy. The excellent electrochemical performance of the N-PyC, in addition to its ease of preparation, renders this material ideal for applications in electrochemical sensing.

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. With this system, we demonstrate, with both experiments and numerical simulations, how the doping of the graphene and the control of the gap size can be controlled to tune the plasmonic response of the coupled nanosphere using nitric acid. The coupling of the Au nanosphere and Au thin film reveals multipolar modes which can be tuned by adjusting the gap size or doping an intermediate graphene monolayer. At high doping levels, the interaction between the charge-transfer plasmon and gap plasmon leads to splitting of the plasmon energies. The study provides evidence for the unification of theories proposed by previous works investigating similar systems.

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. By combining in situ and ex situ Raman analyses we elucidate long-term effects on the catalyst performance from a changing crystallographic phase. In particular, we attribute electrochemically stimulated compositional degradation at active sites as the principal cause of the sharp loss of activity from NiFe LDHs shortly after the alkaline cell is turned on. EDX, XPS, and EELS analyses performed after OER also reveal noticeable leaching of Fe metals compared to Ni, principally from highly active edge sites. In addition, post-cycle analysis identified a ferrihydrite by-product formed from the leached Fe. Density functional theory calculations shed light on the thermodynamic driving force for the leaching of Fe metals and propose a dissolution pathway which involves [FeO4]2- removal at relevant OER potentials.

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). The atomically thin layers, not evident from common analytic methods, such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), or scanning electron microscopy (SEM), could be identified with highly resolved time-of-flight secondary ion mass spectrometry (TOF-SIMS). The layers consist of hydrocarbons, which preferentially adsorb to the hydrophobic van der Waals surfaces of TMDs, derived from the most common methods. Fingerprint fragmentation patterns enable us to identify certain polymers and link them to those used during preparation and storage of the TMDs. The ubiquitous presence of polymeric films on 2D materials has wide reaching implications for their investigation, processing, and applications. In this regard, we reveal the nature of polymeric residues after commonly used transfer procedures on MoS2 films and investigate several annealing procedures for their removal.

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)2/Ti3C2Tx 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. This work shows that the improved electrocatalysis for the composite material compared to the TMO only catalyst is due to the presence of higher Co oxide oxidation states at lower OER overpotentials for the mixed TMO/MXene catalysts. Furthermore, the presence of the MXene allows for a more mechanically robust film during OER, making the film more stable. Finally, our results show that small amounts of MXene are more advantageous for the OER during long-term stability measurements, which is linked to the formation of TiO2. The sensitivity of MXene oxidation ultimately limits TMO/MXene composites under alkaline OER conditions, meaning mass fractions must be carefully considered when designing such a catalyst to minimize the residual TiO2 formed during its lifetime.

Carbon-silicon Schottky barrier diodes.

The simple fabrication of high-performance Schottky barrier diodes between silicon and conductive carbon films (C-Films) is reported. By optimizing the interface, ideality factors as low as n = 1.22 for pyrolytic photoresist films (PPF) have been obtained. These remarkable values, which are not far away from those of commercial products are obtained repeatedly on non-optimized substrates with fully scalable processes.

Layered PtSe2 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 PtSe2 , 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. The advantages and disadvantages of a range of synthesis methods for PtSe2 are initially compared, followed by a discussion of its outstanding properties, and industrial and commercial advantages. Research focused on the broadband nonlinear photonic properties of PtSe2 , as well as reports of its use as a saturable absorber in ultrafast lasers, are then reviewed. Additionally, the advances that have been achieved in a range of PtSe2 -based field-effect transistors, photodetectors, and sensors are summarized. Finally, a conclusion on these results along with the outlook for the future is presented.

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 PtS2 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. The existence of two stable platinum sulfides, sometimes within the same sample, has resulted in much confusion between the materials in the literature. Neither of these Pt sulfides have been thoroughly characterised as-of-yet. Here we utilise time-efficient, scalable methods to synthesise high-quality thin films of both Pt sulfides on a variety of substrates. The competing nature of the sulfides and limited thermal stability of these materials is demonstrated. We report peak-fitted X-ray photoelectron spectra, and Raman spectra using a variety of laser wavelengths, for both materials. This systematic characterisation provides a guide to differentiate between the sulfides using relatively simple methods which is essential to enable future work on these interesting materials.

Influence of Gold Nano-Bipyramid Dimensions on Strong Coupling with Excitons of Monolayer MoS2.

Rabi splitting between the longitudinal plasmon of a gold nano-bipyramid and the A exciton of monolayer MoS2 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. The mode volume of the nanoresonator is significantly reduced because of the sharp tips of the bipyramid, and the Rabi splitting increases with tip sharpness. The results also reveal that greater Rabi splitting is observed for larger bipyramids, contrasting with results previously reported for different nanoresonator shapes. This shows, for the first time, how the magnitude of the splitting has a different response for particular nanoresonators when tuning the size, without increasing the number of excitons coupled into the system. The Rabi splitting, at zero energy detuning between plasmon and A exciton, increases from ∼55 meV with a 70 nm-long bipyramid to ∼80 meV with a 100 nm-long bipyramid. The increase in coupling strength with size arises because of increasing confinement of the field enhancement at the bipyramid tip.