Multilayer WS2 for low-power visible and near-infrared phototransistors.

Mechanically exfoliated multilayer WS2 flakes are used as the channel of field effect transistors for low-power photodetection in the visible and near-infrared (NIR) spectral range. The electrical characterization as a function of the temperature reveals devices with n-type conduction and slightly different Schottky barriers at the drain and source contacts. The WS2 phototransistors can be operated in self-powered mode, yielding both a current and a voltage when exposed to light. The spectral photoresponse in the visible and the NIR ranges shows a high responsivity (4.5 µA/W) around 1250 nm, making the devices promising for telecommunication applications.

Defective ZrSe2: a promising candidate for spintronics applications.

The current study presents the electronic and magnetic properties of monolayer ZrSe2nanoribbons. The impact of various point defects in the form of Zr or Se vacancies, and their combinations, on the nanoribbon electronic and magnetic properties are investigated using density functional theory calculations in hydrogen-terminated zigzag and armchair ZrSe2nanoribbons. Although pristine ZrSe2is non-magnetic, all the defective ZrSe2structures exhibit ferromagnetic behavior. Our calculated results also show that the Zr and Se vacancy defects alter the total spin magnetic moment with D6Se,leading to a significant amount of 6.34µB in the zigzag nanoribbon, while the largest magnetic moment of 5.52µB is induced by D2Se-2in the armchair structure, with the spin density predominantly distributed around the Zr atoms near the defect sites. Further, the impact of defects on the performance of the ZrSe2nanoribbon-based devices is investigated. Our carrier transport calculations reveal spin-polarized current-voltage characteristics for both the zigzag and armchair devices, revealing negative differential resistance (NDR) feature. Moreover, the current level in the zigzag-based nanoribbon devices is ∼10 times higher than the armchair devices, while the peak-to-valley ratio is more pronounced in the armchair-based nanoribbon devices. It is also noted that defects increase the current level in the zigzag devices while they lead to multiple NDR peaks with rather negligible change in the current level in the armchair devices. Our results on the defective ZrSe2structures, as opposed to the pristine ones that are previously studied, provide insight into ZrSe2material and device properties as a promising nanomaterial for spintronics applications and can be considered as practical guidance to experimental work.

The Impact of COVID-19 on Farmers' Mental Health: A Case Study of the UK.

OBJECTIVES: In this paper, we use a UK case study to explore how the COVID-19 pandemic affected the mental health (emotional, psychological, social wellbeing) of farmers. We outline the drivers of poor farming mental health, the manifold impacts of the pandemic at a time of policy and environmental change, and identify lessons that can be learned to develop resilience in farming communities against future shocks. METHODS: We undertook a survey answered by 207 farmers across the UK, focusing on drivers of poor mental health and the effect of the COVID-19 pandemic. We also conducted 22 in-depth interviews with individuals in England, Scotland and Wales who provide mental health support to farmers. These explored how and why the COVID-19 pandemic affected the mental health of farmers. These interviews were supplemented by 93 survey responses from a similar group of support providers (UK-wide). RESULTS: We found that the pandemic exacerbated underlying drivers of poor mental health and wellbeing in farming communities. 67% of farmers surveyed reported feeling more stressed, 63% felt more anxious, 38% felt more depressed, and 12% felt more suicidal. The primary drivers of poor mental health identified by farmers during the pandemic included decreased social contact and loneliness, issues with the general public on private land, and moving online for social events. Support providers also highlighted relationship and financial issues, illness, and government inspections as drivers of poor mental health. Some farmers, conversely, outlined positive impacts of the pandemic. CONCLUSION: The COVID-19 pandemic is just one of many potential stressors associated with poor farming mental health and its impacts are likely to be long-lasting and delayed. Multiple stressors affecting farmers at the same time can create a tipping point. Therefore, there is a need for long-term support and ongoing evaluation of the drivers of poor mental health in farming families.

Femtosecond Laser-Induced Crystallization of Amorphous Silicon Thin Films under a Thin Molybdenum Layer.

A new process to crystallize amorphous silicon without melting and the generation of excessive heating of nearby components is presented. We propose the addition of a molybdenum layer to improve the quality of the laser-induced crystallization over that achieved by direct irradiation of silicon alone. The advantages are that it allows the control of crystallite size by varying the applied fluence of a near-infrared femtosecond laser. It offers two fluence regimes for nanocrystallization and polycrystallization with small and large crystallite sizes, respectively. The high repetition rate of the compact femtosecond laser source enables high-quality crystallization over large areas. In this proposed method, a multilayer structure is irradiated with a single femtosecond laser pulse. The multilayer structure includes a substrate, a target amorphous Si layer coated with an additional molybdenum thin film. The Si layer is crystallized by irradiating the Mo layer at different fluence regimes. The transfer of energy from the irradiated Mo layer to the Si film causes the crystallization of amorphous Si at low temperatures (∼700 K). Numerical simulations were carried out to estimate the electron and lattice temperatures for different fluence regimes using a two-temperature model. The roles of direct phonon transport and inelastic electron scattering at the Mo-Si interface were considered in the transfer of energy from the Mo to the Si film. The simulations confirm the experimental evidence that amorphous Si was crystallized in an all-solid-state process at temperatures lower than the melting point of Si, which is consistent with the results from transmission electron microscopy (TEM) and Raman. The formation of crystallized Si with controlled crystallite size after laser treatment can lead to longer mean free paths for carriers and increased electrical conductivity.

Hysteresis in As-Synthesized MoS2 Transistors: Origin and Sensing Perspectives.

Two-dimensional materials, including molybdenum disulfide (MoS2), present promising sensing and detecting capabilities thanks to their extreme sensitivity to changes in the environment. Their reduced thickness also facilitates the electrostatic control of the channel and opens the door to flexible electronic applications. However, these materials still exhibit integration difficulties with complementary-MOS standardized processes and methods. The device reliability is compromised by gate insulator selection and the quality of the metal/semiconductor and semiconductor/insulator interfaces. Despite some improvements regarding mobility, hysteresis and Schottky barriers having been reported thanks to metal engineering, vertically stacked heterostructures with compatible thin-layers (such as hexagonal boron nitride or device encapsulation) variability is still an important constraint to sensor performance. In this work, we fabricated and extensively characterized the reliability of as-synthesized back-gated MoS2 transistors. Under atmospheric and room-temperature conditions, these devices present a wide electrical hysteresis (up to 5 volts) in their transfer characteristics. However, their performance is highly influenced by the temperature, light and pressure conditions. The singular signature in the time response of the devices points to adsorbates and contaminants inducing mobile charges and trapping/detrapping carrier phenomena as the mechanisms responsible for time-dependent current degradation. Far from being only a reliability issue, we demonstrated a method to exploit this device response to perform light, temperature and/or pressure sensors in as-synthesized devices. Two orders of magnitude drain current level differences were demonstrated by comparing device operation under light and dark conditions while a factor up to 105 is observed at vacuum versus atmospheric pressure environments.

Emulating synaptic response in n- and p-channel MoS2 transistors by utilizing charge trapping dynamics.

Brain-inspired, neuromorphic computing aims to address the growing computational complexity and power consumption in modern von-Neumann architectures. Progress in this area has been hindered due to the lack of hardware elements that can mimic neuronal/synaptic behavior which form the fundamental building blocks for spiking neural networks (SNNs). In this work, we leverage the short/long term memory effects due to the electron trapping events in an atomically thin channel transistor that mimic the exchange of neurotransmitters and emulate a synaptic response. Re-doped (n-type) and Nb-doped (p-type) molybdenum di-sulfide (MoS2) field-effect transistors are examined using pulsed-gate measurements, which identify the time scales of electron trapping/de-trapping. The devices demonstrate promising trends for short/long term plasticity in the order of ms/minutes, respectively. Interestingly, pulse paired facilitation (PPF), which quantifies the short-term plasticity, reveal time constants (τ1 = 27.4 ms, τ2 = 725 ms) that closely match those from a biological synapse. Potentiation and depression measurements describe the ability of the synaptic device to traverse several analog states, where at least 50 conductance values are accessed using consecutive pulses of equal height and width. Finally, we demonstrate devices, which can emulate a well-known learning rule, spike time-dependent plasticity (STDP) which codifies the temporal sequence of pre- and post-synaptic neuronal firing into corresponding synaptic weights. These synaptic devices present significant advantages over iontronic counterparts and are envisioned to create new directions in the development of hardware for neuromorphic computing.

Insights into Multilevel Resistive Switching in Monolayer MoS2.

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 (MoS2) 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. Temperature-dependent electrical measurements coupled with semiclassical charge transport models suggest that the transport in these devices varies significantly in the initial (pristine) state, high resistance state, and low resistance state. In the initial state, the transport is a one-step direct tunneling (at low voltage biases) and Fowler Nordeim tunneling (at higher bias) with an effective barrier height of 0.33 eV, which closely matches the Schottky barrier at the MoS2/Au interface. In the high resistive state, trap-assisted tunneling provides a reasonable fit to experimental data for a trap height of 0.82 eV. Density functional theory calculations suggest the possibility of single- and double-sulfur vacancies as the microscopic origins of these trap sites. The temperature-dependent behavior of the set and reset process are explained by invoking the probability of defect (sulfur vacancy) creation and mobility of sulfur ions. Finally, conductive atomic force microscopy measurements confirm that the multifilamentary resistive memory effects are inherent to a single-crystalline MoS2 triangle and not necessarily dependent on grain boundaries. The insights suggested in this work are envisioned to open up possibilities for ultrascaled, multistate, resistive memories for next-generation digital memory and neuromorphic applications.

Effects of Annealing Temperature and Ambient on Metal/PtSe2 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 PtSe2-based devices. A range of annealing temperatures (150, 250, and 350 °C) were used both in inert (0/100% H2/N2) and forming gas (5/95% H2/N2) environments to separate the contribution of temperature and ambient. The samples are electrically characterized by circular transfer length method structures, from which contact resistance and sheet resistance are analyzed. Ti and Ni are used as metal contacts. Ti does not react with PtSe2 at any given annealing step. In contrast to this, Ni reacts with PtSe2, resulting in a contact alloy formation. The results are supported by a combination of X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, and cross-sectional transmission electron microscopy. The work sheds light on the impact of forming gas annealing on TMD-metal interfaces, and on the TMD film itself, which could be of great interest to improve the contact resistance of TMD-based devices.

Impact of Etch Processes on the Chemistry and Surface States of the Topological Insulator Bi2Se3.

The unique properties of topological insulators such as Bi2Se3 are intriguing for their potential implementation in novel device architectures for low power and defect-tolerant logic and memory devices. Recent improvements in the synthesis of Bi2Se3 have positioned researchers to fabricate new devices to probe the limits of these materials. The fabrication of such devices, of course, requires etching of the topological insulator, in addition to other materials including gate oxides and contacts which may impact the topologically protected surface states. In this paper, we study the impact of He+ sputtering and inductively coupled plasma Cl2 and SF6 reactive etch chemistries on the physical, chemical, and electronic properties of Bi2Se3. Chemical analysis by X-ray photoelectron spectroscopy tracks changes in the surface chemistry and Fermi level, showing preferential removal of Se that results in vacancy-induced n-type doping. Chlorine-based chemistry successfully etches Bi2Se3 but with residual Se-Se bonding and interstitial Cl species remaining after the etch. The Se vacancies and residuals can be removed with postetch anneals in a Se environment, repairing Bi2Se3 nearly to the as-grown condition. Critically, in each of these cases, angle-resolved photoemission spectroscopy (ARPES) reveals that the topologically protected surface states remain even after inducing significant surface disorder and chemical changes, demonstrating that topological insulators are quite promising for defect-tolerant electronics. Changes to the ARPES intensity and momentum broadening of the surface states are discussed. Fluorine-based etching aggressively reacts with the film resulting in a relatively thick insulating film of thermodynamically favored BiF3 on the surface, prohibiting the use of SF6-based etching in Bi2Se3 processing.

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.

Probing Interface Defects in Top-Gated MoS2 Transistors with Impedance Spectroscopy.

The electronic properties of the HfO2/MoS2 interface were investigated using multifrequency capacitance-voltage (C-V) and current-voltage characterization of top-gated MoS2 metal-oxide-semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5-10) MoS2 MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO2 gate oxide layers formed by atomic layer deposition after in-situ UV-O3 surface functionalization. The impedance response of the HfO2/MoS2 gate stack indicates the existence of specific defects at the interface, which exhibited either a frequency-dependent distortion similar to conventional Si MOSFETs with unpassivated silicon dangling bonds or a frequency dispersion over the entire voltage range corresponding to depletion of the HfO2/MoS2 surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (Dit) was extracted from the C-V responses by the high-low frequency and the multiple-frequency extraction methods, where a Dit peak value of 1.2 × 1013 cm-2 eV-1 was extracted for a device (7-layer MoS2 and 13 nm HfO2) exhibiting a behavior approximating to a single trap response. The MoS2 MOSFET with 4-layer MoS2 and 8 nm HfO2 gave Dit values ranging from 2 × 1011 to 2 × 1013 cm-2 eV-1 across the energy range corresponding to depletion near the HfO2/MoS2 interface. The gate current was below 10-7 A/cm2 across the full bias sweep for both samples indicating continuous HfO2 films resulting from the combined UV ozone and HfO2 deposition process. The results demonstrated that impedance spectroscopy applied to relatively simple top-gated transistor test structures provides an approach to investigate electrically active defects at the HfO2/MoS2 interface and should be applicable to alternative TMD materials, surface treatments, and gate oxides as an interface defect metrology tool in the development of TMD-based MOSFETs.

Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO2-RuO2 Alloy Schottky Contacts for Silicon Photoanodes.

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions ≥ 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

Engineering Interfacial Silicon Dioxide for Improved Metal-Insulator-Semiconductor Silicon Photoanode Water Splitting Performance.

Silicon photoanodes protected by atomic layer deposited (ALD) TiO2 show promise as components of water splitting devices that may enable the large-scale production of solar fuels and chemicals. Minimizing the resistance of the oxide corrosion protection layer is essential for fabricating efficient devices with good fill factor. Recent literature reports have shown that the interfacial SiO2 layer, interposed between the protective ALD-TiO2 and the Si anode, acts as a tunnel oxide that limits hole conduction from the photoabsorbing substrate to the surface oxygen evolution catalyst. Herein, we report a significant reduction of bilayer resistance, achieved by forming stable, ultrathin (<1.3 nm) SiO2 layers, allowing fabrication of water splitting photoanodes with hole conductances near the maximum achievable with the given catalyst and Si substrate. Three methods for controlling the SiO2 interlayer thickness on the Si(100) surface for ALD-TiO2 protected anodes were employed: (1) TiO2 deposition directly on an HF-etched Si(100) surface, (2) TiO2 deposition after SiO2 atomic layer deposition on an HF-etched Si(100) surface, and (3) oxygen scavenging, post-TiO2 deposition to decompose the SiO2 layer using a Ti overlayer. Each of these methods provides a progressively superior means of reliably thinning the interfacial SiO2 layer, enabling the fabrication of efficient and stable water oxidation silicon anodes.

Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes.

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+)n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

Lightweight versus heavyweight mesh in laparoscopic inguinal hernia repair: a meta-analysis.

BACKGROUND: Reinforcement of inguinal hernia repair with prosthetic mesh is standard practice but can cause considerable pain and stiffness around the groin and affect physical functioning. This has led to various types of mesh being engineered, with a growing interest in lighter-weight mesh. Minimally invasive approaches have also significantly reduced postoperative recovery from inguinal hernia repair. The aim of this systematic review was to compare the outcomes after laparoscopic inguinal repair using new lightweight or traditional heavyweight mesh in published randomised controlled trials. METHODS: Medline, Embase, trial registries, conference proceedings, and reference lists were searched for controlled trials of heavyweight versus lightweight mesh for laparoscopic repair of inguinal hernias. The primary outcomes were recurrence and chronic pain. Secondary outcomes were visual analogue pain score at 7 days postoperatively, seroma formation, and time to return to work. Risk differences were calculated for categorical outcomes and standardised mean differences for continuous outcomes. RESULTS: Eight trials were included in the analysis of 1,667 hernias in 1,592 patients. Mean study follow-up was between 2 and 60 months. There was no effect on recurrence [pooled analysis risk difference 0.00 (95% CI -0.01 to 0.01), p = 0.86] or chronic pain [pooled analysis risk difference -0.02 (95% CI -0.04 to 0.00); p = 0.1]. Lightweight and heavyweight mesh repair had similar outcomes with regard to postoperative pain, seroma development, and time to return to work. CONCLUSION: Both mesh options appear to result in similar long- and short-term postoperative outcomes. Further long-term analysis may guide surgeon selection of mesh weight for laparoscopic inguinal hernia repair.

Fabrication of HfO2 patterns by laser interference nanolithography and selective dry etching for III-V CMOS application.

Nanostructuring of ultrathin HfO2 films deposited on GaAs (001) substrates by high-resolution Lloyd's mirror laser interference nanolithography is described. Pattern transfer to the HfO2 film was carried out by reactive ion beam etching using CF4 and O2 plasmas. A combination of atomic force microscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis was used to characterise the various etching steps of the process and the resulting HfO2/GaAs pattern morphology, structure, and chemical composition. We show that the patterning process can be applied to fabricate uniform arrays of HfO2 mesa stripes with tapered sidewalls and linewidths of 100 nm. The exposed GaAs trenches were found to be residue-free and atomically smooth with a root-mean-square line roughness of 0.18 nm after plasma etching.PACS: Dielectric oxides 77.84.Bw, Nanoscale pattern formation 81.16.Rf, Plasma etching 52.77.Bn, Fabrication of III-V semiconductors 81.05.Ea.

Exploring symmetry-based logic for a synthesis of palau'amine.

A synthetic approach to palau'amine is described that exploits veiled symmetry in the structure. Bis-alkylidenes i have been prepared and found susceptible to halogenative desymmetrization using t-BuOCl. This oxidation forms the imbedded spirocyclopentane motif observed in the natural product. A host of atypical reactions and processes developed during these studies are discussed, as are plans for completing total syntheses of this compound class.

Synthetic studies toward halichlorine: complex azaspirocycle formation with use of an NBS-promoted semipinacol reaction.

The investigations of a synthetic route incorporating a NBS-promoted semipinacol rearrangement to the 6-azaspiro[4.5]decane fragment within halichlorine ( 1) are presented. A convergent approach was pursued, utilizing two chiral, enantiomerically enriched building blocks, 2-trimethylstannyl piperidene 10 and substituted cyclobutanone 19. Noteworthy synthetic operations in this study include the following: (a) a highly diastereoselective NBS-promoted semipinacol reaction that established four stereogenic centers in ketone 25 and (b) the use of a N- p-toluenesulfonyl-2-iodo-2-piperidene as a precursor to a basic organometallic reagent, which was critical to the success of the coupling of fragments 10 and 19.

Synthesis of functionalized 1-azaspirocyclic cyclopentanones using bronsted acid or N-bromosuccinimide promoted ring expansions.

Azaspirocyclic ring systems are present in a variety of alkaloids. Functionalized 1-azaspirocyclopentanones (6, 7, 11, 12) can be efficiently constructed through semipinacol ring expansion reactions of 2-(1-hydroxycyclobutyl)-p-toluenesulfonylenamides (4) promoted by either a Bronsted acid ((S)-(+)-10-camphorsulfonic acid or HCl) or N-bromosuccinimide, an electrophilic bromine source. Reactions promoted by N-bromosuccinimide tend to proceed in higher yields (80-95%) and with greater diastereoselectivity (3:1-1:0) compared to those reactions promoted by a Bronsted acid. In addition, N-bromosuccinimide promoted reactions can produce a complementary stereochemical outcome compared to the reactions using Bronsted acid.