Schottky Barrier Height of Pd/MoS2 Contact by Large Area Photoemission Spectroscopy

Mechanism of Fermi Level Pinning for Metal Contacts on Molybdenum Dichalcogenide

The high contact resistance of transition metal dichalcogenide (TMD)-based devices is receiving considerable attention due to its limitation on electronic performance. The mechanism of Fermi level (EF) pinning, which causes the high contact resistance, is not thoroughly understood to date. In this study, the metal (Ni and Ag)/Mo-TMD surfaces and interfaces are characterized by X-ray photoelectron spectroscopy, atomic force microscopy, scanning tunneling microscopy and spectroscopy, and density functional theory systematically. Ni and Ag form covalent and van der Waals (vdW) interfaces on Mo-TMDs, respectively. Imperfections are detected on Mo-TMDs, which lead to electronic and spatial variations. Gap states appear after the adsorption of single and two metal atoms on Mo-TMDs. The combination of the interface reaction type (covalent or vdW), the imperfection variability of the TMD materials, and the gap states induced by contact metals with different weights are concluded to be the origins of EF pinning.

A perylene diimide electrochemical probe with persulfate as a signal enhancer for dopamine sensing

Dopamine (DA) is an important biomarker related to parkinsonism, schizophrenia and renal disease. Traditional electrochemical sensors for DA were based on the direct electrochemical oxidation of DA. In this paper, we report a new sensing strategy using N,N'-di(trimethylaminoethyl)perylene diimide (TMPDI) as an electrochemical probe and K2S2O8 as a signal enhancer for DA detection between 0 and -0.7 V with the DPV technique. MoS2 nanoflowers prepared by the hydrothermal method were used as a nanocarrier to load TMPDI. The reduction current of TMPDI was found to show a stepwise and significant increase at -0.24 V with the increase of concentration of K2S2O8 due to the continuous cycle of TMPDI molecules' electrochemical reduction and chemical oxidation. The presence of DA caused a large decrease of the reduction current of TMPDI due to the synergistic interaction of the competitive consumption of DA for K2S2O8 and the blocking effect of polyDA adhering to the electrode surface. The decreased current exhibited a linear response for DA from 10 pM to 100 μM with a detection limit of 4.1 pM and the proposed sensor showed high selectivity and excellent feasibility in human urine/serum sample detection.

Origins of Fermi Level Pinning for Ni and Ag Metal Contacts on Tungsten Dichalcogenides

Tungsten transition metal dichalcogenides (W-TMDs) are intriguing due to their properties and potential for application in next-generation electronic devices. However, strong Fermi level (EF) pinning manifests at the metal/W-TMD interfaces, which could tremendously restrain the carrier injection into the channel. In this work, we illustrate the origins of EF pinning for Ni and Ag contacts on W-TMDs by considering interface chemistry, band alignment, impurities, and imperfections of W-TMDs, contact metal adsorption mechanism, and the resultant electronic structure. We conclude that the origins of EF pinning at a covalent contact metal/W-TMD interface, such as Ni/W-TMDs, can be attributed to defects, impurities, and interface reaction products. In contrast, for a van der Waals contact metal/TMD system such as Ag/W-TMDs, the primary factor responsible for EF pinning is the electronic modification of the TMDs resulting from the defects and impurities with the minor impact of metal-induced gap states. The potential strategies for carefully engineering the metal deposition approach are also discussed. This work unveils the origins of EF pinning at metal/TMD interfaces experimentally and theoretically and provides guidance on further enhancing and improving the device performance.

Wafer-Scale Field-Effect Transistor-Type Sensor Using a Carbon Nanotube Film as a Channel for Ppb-Level Hydrogen Sulfide Detection

Sulfur hexafluoride is widely used in power equipment because of its excellent insulation and arc extinguishing properties. However, severe damage to power equipment may be caused and a large-scale collapse of the power grid may occur when SF6 is decomposed into H2S, SOF2, and SO2F2. It is difficult to detect the SF6 concentration as it is a kind of inert gas. Generally, the trace gas decomposed in the early stage of SF6 is detected to achieve the function of early warning. Consequently, it is of great significance to realize the real-time detection of trace gases decomposed from SF6 for the early fault diagnosis of power equipment. In this work, a wafer-scale gate-sensing carbon-based FET gas sensor is fabricated on a four-inch carbon wafer for the detection of H2S, a decomposition product of SF6. The carbon nanotubes with semiconductor properties and the noble metal Pt are respectively used as a channel and a sensing gate of the FET-type gas sensor, and the channel transmission layer and the sensing gate layer each play an independent role and do not interfere with each other by introducing the gate dielectric layer Y2O3, giving full play to their respective advantages to forming an integrated sensor of gas detection and signal amplification. The detection limit of the as-prepared gate-sensing carbon-based FET gas sensor can reach 20 ppb, and its response deviation is not more than 3% for the different batches of gas sensors. This work provides a potentially useful solution for the industrial production of miniaturized and integrated gas sensors.

Carbon Nanotube-Based Field-Effect Transistor-Type Sensor with a Sensing Gate for Ppb-Level Formaldehyde Detection

The detection of harmful trace gases, such as formaldehyde (HCHO), is a technical challenge in the current gas sensor field. The weak electrical signal caused by trace amounts of gases is difficult to be detected and susceptible to other gases. Based on the amplification effect of a field-effect transistor (FET), a carbon-based FET-type gas sensor with a gas-sensing gate is proposed for HCHO detection at the ppb level. Semiconducting carbon nanotubes (s-CNTs) and a catalytic metal are chosen as channel and gate materials, respectively, for the FET-type gas sensor, which makes full use of the respective advantages of the channel transport layer and the sensitive gate layer. The as-prepared carbon-based FET-type gas sensor exhibits a low detection limit toward HCHO up to 20 ppb under room temperature (RT), which can be improved to 10 ppb by a further heating strategy. It also exhibits a remarkable elevated recovery rate from 80 to 97% with almost no baseline drift (2%) compared to the RT condition, revealing excellent reproducibility, stability, and recovery. The role of sensitive function in the FET-type gas sensor is performed by means of an independent gas-sensing gate, that is, the independence of the sensitive gate and the electron transmission channel is the main reason for its high sensitivity detection. We hope our work can provide an instructive approach for designing high-performance formaldehyde sensor chips with on-chip integration potential.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W-1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.

Pd Nanocluster/Monolayer MoS2 Heterojunctions for Light-Induced Room-Temperature Hydrogen Sensing

Developing sensing approaches that can exploit visible light for the detection of low-concentration hydrogen at room temperatures has become increasingly important for the safe use of hydrogen in many applications. In this study, heterostructures composed of monolayer MoS₂ and Pd nanoclusters (Pd/MoS₂) acting as photo- and hydrogen-sensitizers are successfully fabricated in a facile and scalable manner. The uniform deposition of morphologically isotropic Pd nanoclusters (11.5 ± 2.2 nm) on monolayer MoS₂ produces a plethora of active heterojunctions, effectively suppressing charge carrier recombination under light illumination. The dual photo- and hydrogen-sensitizing functionality of Pd/MoS₂ can enable its use as an active sensing layer in optoelectronic hydrogen sensors. Gas-sensing examinations reveal that the sensing performance of Pd/MoS₂ is enhanced three-fold under visible light illumination (17% for 140 ppm of H₂) in comparison with dark light (5% for 140 ppm of H₂). Photoactivation is also found to enable excellent sensing reversibility and reproducibility in the obtained sensor. As a proof-of-concept, the integration of Pd nanoclusters and monolayer MoS₂ can open a new avenue for light-induced hydrogen gas sensing at room temperature.

Contacts for Molybdenum Disulfide: Interface Chemistry and Thermal Stability

In this review on contacts with MoS₂, we consider reports on both interface chemistry and device characteristics. We show that there is considerable disagreement between reported properties, at least some of which may be explained by variability in the properties of geological MoS₂. Furthermore, we highlight that while early experiments using photoemission to study the interface behavior of metal-MoS₂ showed a lack of Fermi-level pinning, device measurements repeatedly confirm that the interface is indeed pinned. Here we suggest that a parallel conduction mechanism enabled by metallic defects in the MoS₂ materials may explain both results. We note that processing conditions during metal depositions on MoS₂ can play a critical role in the interface chemistry, with differences between high vacuum and ultra-high vacuum being particularly important for low work function metals. This can be used to engineer the interfaces by using thin metal-oxide interlayers to protect the MoS₂ from reactions with the metals. We also report on the changes in the interfaces that can occur at high temperature which include enhanced reactions between Ti or Cr and MoS₂, diffusion of Ag into MoS₂, and delamination of Fe. What is clear is that there is a dearth of experimental work that investigates both the interface chemistry and device properties in parallel.

The Modulation Effect of MoS₂ Monolayers on the Nucleation and Growth of Pd Clusters: First-Principles Study

The geometries, electronic structures, adsorption, diffusion, and nucleation behaviors of Pd_n (n = 1–5) clusters on MoS₂ monolayers (MLs) were investigated using first principles calculations to elucidate the initial growth of metal on MoS₂. The results demonstrate that Pd clusters can chemically adsorb on MoS₂ MLs forming strong Pd–S covalent bonds with significant ionic character. We investigated the initial growth mode of Pd clusters on MoS₂ monolayers and found that Pd_n clusters tend to adopt pyramid-like structures for n = 4–5 and planar structures lying on MoS₂ substrates for n = 1–3. It can be explained by the competition between adsorbate–substrate and the intra-clusters’ interactions with the increasing coverage. Compared with pristine MoS₂ MLs, the work function was reduced from 5.01 eV upon adsorption of Pd monomer to 4.38 eV for the case of the Pd₅ clusters due to the charge transfer from Pd clusters to MoS₂ MLs. In addition, our calculations of the nucleation and diffusion behaviors of Pd clusters on MoS₂ MLs predicted that Pd is likely to agglomerate to metal nanotemplates on MoS₂ MLs during the epitaxial stacking process. These findings may provide useful guidance to extend the potential technological applications of MoS₂, including catalysts and production of metal thin films, and the fabrication of nanoelectronic devices.

Ternary semiconductor metal oxide blends grafted Ag@AgCl hybrid as dimensionally stable anode active layer for photoelectrochemical oxidation of organic compounds: Design strategies and photoelectric synergistic mechanism

The development of ultra-efficient, sustainable, and easily accessible anode with relative non-precious semiconducting metal oxides is highly significant for application in the practical treatment of organically polluted water. Herein, we report SnO₂, TiO₂, and Ag₂O ternary semiconductor metal oxide blend grafted Ag@AgCl hybrids, prepared with the one-step sol-gel method and applied as a dimensionally stable anode (DSA)-active layer on a SnO₂-Sb/Ti electrode. Factors affecting crystal formation, including the presence or absence of O₂ during calcination, the calcination temperature, and Ag@AgCl additive dosage were discussed. The micromorphology, phase composition, and photoelectrochemical activity of the newly designed anode were comprehensively characterized. The optimized preparation, which yielded a solid-solution structure with flat and smooth surface and well-crystallized lattice configuration, occurred in the absence of O₂ during calcination at 550 ℃ with an Ag@AgCl additive dosage of 0.2 g in the sol-gel precursor. The newly designed DSA displayed improved electrocatalysis (EC) and photoelectrical catalysis (PEC) capacity. The phenol and its TOC removal efficiency reached 90.65% and 58.17% for 10 mA/cm² current density with a metal halide lamp in 3 h. The lifespan was four times that of SnO₂-Sb/Ti electrode. This proposed DSA construction strategy may support improved EC and PEC reactivities toward the decomposition of organic pollutants.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.