Mobility improvement and temperature dependence in MoSe2 field-effect transistors on parylene-C substrate

Enhance Carrier Diffusion of Monolayer MoSe2 by Interface Engineering

Two-dimensional materials hold great potentials for beyond-CMOS (complementary metal-oxide-semiconductor) electronical and optoelectrical applications, and the development of field effect transistors (FET) with excellent performance using such materials is of particular interest. How to improve the performance of devices thus becomes an urgent issue. The performance of FETs depends greatly on the intrinsic electrical properties of the channel materials, meanwhile the device interface quality, such as extrinsic scattering of charged impurities, charge traps, and substrate surface roughness have a great influence on the performance. In this paper, the impact of the interface quality on the carrier diffusion behaviors of monolayer (ML) MoSe2 has been investigated by using an in situ ultrafast laser technique to avoid the surface contamination during device fabrication process. Two types of self-assembled monolayers (SAMs) are introduced to modify the gate dielectric surface through an interface engineering approach to obtain chemical-stable interfaces. The results showed that the transport properties of ML MoSe2 were enhanced after interface engineering, for example, the carrier mobility of ML MoSe2 was improved from ∼59.4 to ∼166.5 cm2 V-1 s-1 after the SAM modification. Meanwhile, the photocarrier dynamics of ML MoSe2 before and after interfacial engineering were also carefully studied. Our studies provide a feasible method for improving the carrier diffusion behaviors of such materials, and making them suited for application in future integrated circuit.

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Parylene C as a Multipurpose Material for Electronics and Microfluidics

Poly(p-xylylene) derivatives, widely known as Parylenes, have been considerably adopted by the scientific community for several applications, ranging from simple passive coatings to active device components. Here, we explore the thermal, structural, and electrical properties of Parylene C, and further present a variety of electronic devices featuring this polymer: transistors, capacitors, and digital microfluidic (DMF) devices. We evaluate transistors produced with Parylene C as a dielectric, substrate, and encapsulation layer, either semitransparent or fully transparent. Such transistors exhibit steep transfer curves and subthreshold slopes of 0.26 V/dec, negligible gate leak currents, and fair mobilities. Furthermore, we characterize MIM (metal-insulator-metal) structures with Parylene C as a dielectric and demonstrate the functionality of the polymer deposited in single and double layers under temperature and AC signal stimuli, mimicking the DMF stimuli. Applying temperature generally leads to a decrease in the capacitance of the dielectric layer, whereas applying an AC signal leads to an increase in said capacitance for double-layered Parylene C only. By applying the two stimuli, the capacitance seems to suffer from a balanced influence of both the separated stimuli. Lastly, we demonstrate that DMF devices with double-layered Parylene C allow for faster droplet motion and enable long nucleic acid amplification reactions.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Recent progress in 2D hybrid heterostructures from transition metal dichalcogenides and organic layers: properties and applications in energy and optoelectronics fields

Atomically thin transition metal dichalcogenides (TMDs) present extraordinary optoelectronic, electrochemical, and mechanical properties that have not been accessible in bulk semiconducting materials. Recently, a new research field, 2D hybrid heteromaterials, has emerged upon integrating TMDs with molecular systems, including organic molecules, polymers, metal-organic frameworks, and carbonaceous materials, that can tailor the TMD properties and exploit synergetic effects. TMD-based hybrid heterostructures can meet the demands of future optoelectronics, including supporting flexible, transparent, and ultrathin devices, and energy-based applications, offering high energy and power densities with long cycle lives. To realize such applications, it is necessary to understand the interactions between the hybrid components and to develop strategies for exploiting the distinct benefits of each component. Here, we provide an overview of the current understanding of the new phenomena and mechanisms involved in TMD/organic hybrids and potential applications harnessing such valuable materials in an insightful way. We highlight recent discoveries relating to multicomponent hybrid materials. Finally, we conclude this review by discussing challenges related to hybrid heteromaterials and presenting future directions and opportunities in this research field.

Microwave Plasma-Enhanced Parylene–Metal Multilayer Design from Metal Salts

In this paper, a new approach for the synthesis of Parylene–metal multilayers was examined. The metal layers were derived from a metal salt solution in methanol and a post-drying plasma reduction treatment. This process was designed as a one-pot synthesis, which needs a very low amount of resources and energy compared with those using electron beam sputtering processes. The Parylene coatings were obtained after reduction plasma treatments with Parylene C. Therefore, a Parylene coating device with an included plasma microwave generator was used to ensure the character of a one-pot synthesis. This process provided ultra-thin metal salt layers in the range of 1–2 nm for layer thickness and 10–30 nm for larger metal salt agglomerates all over the metal salt layer. The Parylene layers were obtained with thicknesses between approx. 4.5 and 4.7 µm from ellipsometric measurements and 5.7–6.3 µm measured by white light reflectometry. Tensile strength analysis showed an orthogonal pulling stress resistance of around 4500 N. A surface roughness of 4–8 nm for the metal layers, as well as 20–29 nm for the Parylene outer layer, were measured. The wettability for non-polar liquids with a contact angle of 30° was better than for polar liquids, such as water, achieving 87° on the Parylene C surfaces.

Dielectric engineering enable to lateral anti-ambipolar MoTe2heterojunction

Atomically two-dimensional (2D) materials have generated widespread interest for novel electronics and optoelectronics. Specially, owing to atomically thin 2D structure, the electronic bandgap of 2D semiconductors can be engineered by manipulating the surrounding dielectric environment. In this work, we develop an effective and controllable approach to manipulate dielectric properties of h-BN through gallium ions (Ga⁺) implantation for the first time. And the maximum surface potential difference between the intrinsic h-BN (h-BN) and the Ga⁺implanted h-BN (Ga⁺-h-BN) is up to 1.3 V, which is characterized by Kelvin probe force microscopy. More importantly, the MoTe₂transistor stacked on Ga⁺-h-BN exhibits p-type dominated transfer characteristic, while the MoTe₂transistor stacked on the intrinsic h-BN behaves as n-type, which enable to construct MoTe₂heterojunction through dielectric engineering of h-BN. The dielectric engineering also provides good spatial selectivity and allows to build MoTe₂heterojunction based on a single MoTe₂flake. The developed MoTe₂heterojunction shows stable anti-ambipolar behaviour. Furthermore, we preliminarily implemented a ternary inverter based on anti-ambipolar MoTe₂heterojunction. Ga⁺implantation assisted dielectric engineering provides an effective and generic approach to modulate electric bandgap for a wide variety of 2D materials. And the implementation of ternary inverter based on anti-ambipolar transistor could lead to new energy-efficient logical circuit and system designs in semiconductors.

Excitonic transport driven by repulsive dipolar interaction in a van der Waals heterostructure

Dipolar bosonic gases are currently the focus of intensive research due to their interesting many-body physics in the quantum regime. Their experimental embodiments range from Rydberg atoms to GaAs double quantum wells and van der Waals heterostructures built from transition metal dichalcogenides. Although quantum gases are very dilute, mutual interactions between particles could lead to exotic many-body phenomena such as Bose-Einstein condensation and high-temperature superfluidity. Here, we report the effect of repulsive dipolar interactions on the dynamics of interlayer excitons in the dilute regime. By using spatial and time-resolved photoluminescence imaging, we observe the dynamics of exciton transport, enabling a direct estimation of the exciton mobility. The presence of interactions significantly modifies the diffusive transport of excitons, effectively acting as a source of drift force and enhancing the diffusion coefficient by one order of magnitude. The repulsive dipolar interactions combined with the electrical control of interlayer excitons opens up appealing new perspectives for excitonic devices.

Boosting the Optoelectronic Properties of Molybdenum Diselenide by Combining Phase Transition Engineering with Organic Cationic Dye Doping

Two-dimensional layered transition metal dichalcogenides (TMDs) have been investigated intensively as next-generation semiconducting materials. However, conventional TMD-based devices exhibit large contact resistance at the interface between the TMD and the metal electrode because of Fermi level pinning and the Schottky barrier, which results in poor charge injection. Here, we present enhanced charge transport characteristics in molybdenum diselenide (MoSe₂) by means of a sequential engineering process called PESOD-2H/1T (i.e., phase transition engineering combined with surface transfer organic cationic dye doping; 2H and 1T represent the trigonal prismatic and octahedral phases, respectively). Substantial improvements are observed in PESOD-processed MoSe₂ phototransistors, specifically, an approximately 40 000-fold increase in effective carrier mobility and a 100 000-fold increase in photoresponsivity, compared with the mobility and photoresponsivity of intact MoSe₂ phototransistors. Moreover, the PESOD-processed MoSe₂ phototransistor on a flexible substrate maintains its optoelectronic properties under tensile stress, with a bending radius of 5 mm.

Enhanced performance of multilayer MoS2transistors encapsulated with a photoresist

We report the enhanced performance of multilayer MoS₂transistors by AZ^®5214E photoresist encapsulation. The MoS₂transistors with SiO₂bottom-gate dielectrics exhibited an average increase of 4× in the on/off-current ratio and a 50% increase in the field-effect mobility after photoresist encapsulation. The Y-function method further showed a decrease of 87% in the contact resistance and a 30% increase in the intrinsic carrier mobility, suggesting that photoresist encapsulation provides not only n-type doping but also dielectric screening. The Raman spectra of the photoresist-encapsulated MoS₂also suggest n-type doping, which may be due to the electropositive hydroxyl groups in the photoresist. The operation of the photoresist-encapsulated MoS₂transistors remained stable in ambient air for at least one month. These results demonstrate that simple photoresist encapsulation can be an effective performance booster of MoS₂and other transition metal dichalcogenides transistors.

Schottky barrier heights in two-dimensional field-effect transistors: from theory to experiment

Over the past decade, two-dimensional semiconductors (2DSCs) have aroused wide interest due to their extraordinary electronic, magnetic, optical, mechanical, and thermal properties, which hold potential in electronic, optoelectronic, thermoelectric applications, and so forth. The field-effect transistor (FET), a semiconductor gated with at least three terminals, is pervasively exploited as the device geometry for these applications. For lack of effective and stable substitutional doping techniques, direct metal contact is often used in 2DSC FETs to inject carriers. A Schottky barrier (SB) generally exists in the metal-2DSC junction, which significantly affects and even dominates the performance of most 2DSC FETs. Therefore, low SB or Ohmic contact is highly preferred for approaching the intrinsic characteristics of the 2DSC channel. In this review, we systematically introduce the recent progress made in theoretical prediction of the SB height (SBH) in the 2DSC FETs and the efforts made both in theory and experiments to achieve low SB contacts. From the comparison between the theoretical and experimentally observed SBHs, the emerging first-principles quantum transport simulation turns out to be the most powerful theoretical tool to calculate the SBH of a 2DSC FET. Finally, we conclude this review from the viewpoints of state-of-the-art electrode designs for 2DSC FETs.

Evolution of low-dimensional material-based field-effect transistors

Field-effect transistors (FETs) have tremendous applications in the electronics industry due to their outstanding features such as small size, easy fabrication, compatibility with integrated electronics, high sensitivity, rapid detection and easy measuring procedures. However, to meet the increasing demand of the electronics industry, efficient FETs with controlled short channel effects, enhanced surface stability, reduced size, and superior performances based on low-dimensional materials are desirable. In this review, we present the developmental roadmap of FETs from conventional to miniaturized devices and highlight their prospective applications in the field of optoelectronic devices. Initially, a detailed study of the general importance of bulk and low-dimensional materials is presented. Then, recent advances in low-dimensional material heterostructures, classification of FETs, and the applications of low-dimensional materials in field-effect transistors and photodetectors are presented in detail. In addition, we also describe current issues in low-dimensional material-based FETs and propose potential approaches to address these issues, which are crucial for developing electronic and optoelectronic devices. This review will provide guidelines for low-dimensional material-based FETs with high performance and advanced applications in the future.

Low-Temperature 2D/2D Ohmic Contacts in WSe2 Field-Effect Transistors as a Platform for the 2D Metal-Insulator Transition

We report the fabrication of hexagonal-boron-nitride (hBN) encapsulated multiterminal WSe₂ Hall bars with 2D/2D low-temperature Ohmic contacts as a platform for investigating the two-dimensional (2D) metal-insulator transition. We demonstrate that the WSe₂ devices exhibit Ohmic behavior down to 0.25 K and at low enough excitation voltages to avoid current-heating effects. Additionally, the high-quality hBN-encapsulated WSe₂ devices in ideal Hall-bar geometry enable us to accurately determine the carrier density. Measurements of the temperature (T) and density (n_s) dependence of the conductivity σ(T, n_s) demonstrate scaling behavior consistent with a metal-insulator quantum phase transition driven by electron-electron interactions but where disorder-induced local magnetic moments are also present. Our findings pave the way for further studies of the fundamental quantum mechanical properties of 2D transition metal dichalcogenides using the same contact engineering.

Charged Exciton Kinetics in Monolayer MoSe2 near Ferroelectric Domain Walls in Periodically Poled LiNbO3

Monolayer semiconducting transition metal dichalcogenides are a strongly emergent platform for exploring quantum phenomena in condensed matter, building novel optoelectronic devices with enhanced functionalities. Because of their atomic thickness, their excitonic optical response is highly sensitive to their dielectric environment. In this work, we explore the optical properties of monolayer thick MoSe₂ straddling domain wall boundaries in periodically poled LiNbO₃. Spatially resolved photoluminescence experiments reveal spatial sorting of charge and photogenerated neutral and charged excitons across the boundary. Our results reveal evidence for extremely large in-plane electric fields of ≃4000 kV/cm at the domain wall whose effect is manifested in exciton dissociation and routing of free charges and trions toward oppositely poled domains and a nonintuitive spatial intensity dependence. By modeling our result using drift-diffusion and continuity equations, we obtain excellent qualitative agreement with our observations and have explained the observed spatial luminescence modulation using realistic material parameters.

Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe₂-based heterostructures in which heavily p-doped WSe₂ and MoS₂ flakes share an undoped MoSe₂ channel, allowing us to directly compare the optoelectronic properties of MoSe₂-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe₂-WSe₂ and MoSe₂-MoS₂ heterojunctions with a photoresponse time constant of ∼16 μs, surmounting previous MoSe₂-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe₂ or MoS₂) but is likely attributed to the high carrier mobility of 260 cm² V^-1 s^-1 in the undoped MoSe₂ channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe₂-WSe₂/MoS₂ heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼10¹⁴ Jones has been achieved in MoSe₂-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe₂-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

Improved Current Density and Contact Resistance in Bilayer MoSe2 Field Effect Transistors by AlOx Capping

Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS₂, grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe₂ in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe₂ grown by CVD and capped by sub-stoichiometric AlO_x. The 2L channels exhibit ∼20× lower contact resistance (R_C) and ∼30× larger current density compared with 1L channels. R_C is further reduced by >5× with AlO_x capping, which enables improved transistor current density. Overall, 2L AlO_x-capped MoSe₂ transistors (with ∼500 nm channel length) achieve improved current density (∼65 μA/μm at V_DS = 4 V), a good I_on/I_off ratio of >10⁶, and an R_C of ∼60 kΩ·μm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.

Device Architecture for Visible and Near-Infrared Photodetectors Based on Two-Dimensional SnSe2 and MoS2: A Review

While band gap and absorption coefficients are intrinsic properties of a material and determine its spectral range, response time is mainly controlled by the architecture of the device and electron/hole mobility. Further, 2D-layered materials such as transition metal dichalogenides (TMDCs) possess inherent and intriguing properties such as a layer-dependent band gap and are envisaged as alternative materials to replace conventional silicon (Si) and indium gallium arsenide (InGaAs) infrared photodetectors. The most researched 2D material is graphene with a response time between 50 and 100 ps and a responsivity of <10 mA/W across all wavelengths. Conventional Si photodiodes have a response time of about 50 ps with maximum responsivity of about 500 mA/W at 880 nm. Although the responsivity of TMDCs can reach beyond 10⁴ A/W, response times fall short by 3–6 orders of magnitude compared to graphene, commercial Si, and InGaAs photodiodes. Slow response times limit their application in devices requiring high frequency. Here, we highlight some of the recent developments made with visible and near-infrared photodetectors based on two dimensional SnSe₂ and MoS₂ materials and their performance with the main emphasis on the role played by the mobility of the constituency semiconductors to response/recovery times associated with the hetero-structures.

Effect of the Al2O3 Deposition Method on Parylene C: Highlights on a Nanopillar-Shaped Surface

Parylene C (PC) has attracted tremendous attention throughout the past few years due to its extraordinary properties such as high mechanical strength and biocompatibility. When used as a flexible substrate and combined with high-κ dielectrics such as aluminum oxide (Al₂O₃), the Al₂O₃/PC stack becomes very compelling for various applications in fields such as biomedical microsystems and microelectronics. For the latter, the atomic layer deposition of oxides is particularly needed as it allows the deposition of high-quality and nanometer-scale oxide thicknesses. In this work, atomic layer deposition (ALD) and electron beam physical vapor deposition (EBPVD) of Al₂O₃ on a 15 μm-thick PC layer are realized and their effects on the Al₂O₃/PC resulting stack are investigated via X-ray photoelectron spectroscopy combined with atomic force microscopy. An ALD-based Al₂O₃/PC stack is found to result in a nanopillar-shaped surface, while an EBPVD-based Al₂O₃/PC stack yields an expected smooth surface. In both cases, the Al₂O₃/PC stack can be easily peeled off from the reusable SiO₂ substrate, resulting in a flexible Al₂O₃/PC film. These fabrication processes are economic, high yielding, and suitable for mass production. Although ALD is particularly appreciated in the semiconducting industry, EBPVD is here found to be better for the realization of the Al₂O₃/PC flexible substrate for micro- and nanoelectronics.

Asymmetric electrode incorporated 2D GeSe for self-biased and efficient photodetection

2D layered germanium selenide (GeSe) with p-type conductivity is incorporated with asymmetric contact electrode of chromium/Gold (Cr/Au) and Palladium/Gold (Pd/Au) to design a self-biased, high speed and an efficient photodetector. The photoresponse under photovoltaic effect is investigated for the wavelengths of light (i.e. ~220, ~530 and ~850 nm). The device exhibited promising figures of merit required for efficient photodetection, specifically the Schottky barrier diode is highly sensitive to NIR light irradiation at zero voltage with good reproducibility, which is promising for the emergency application of fire detection and night vision. The high responsivity, detectivity, normalized photocurrent to dark current ratio (NPDR), noise equivalent power (NEP) and response time for illumination of light (~850 nm) are calculated to be 280 mA/W, 4.1 × 10⁹ Jones, 3 × 10⁷ W⁻¹, 9.1 × 10⁻¹² WHz^−1/2 and 69 ms respectively. The obtained results suggested that p-GeSe is a novel candidate for SBD optoelectronics-based technologies.

Promoting Energy Efficiency via a Self-Adaptive Evaporative Cooling Hydrogel

High temperature brings adverse impacts on the energy efficiency, and even destroys a semiconductor device. Here, a novel and cost-effective strategy is proposed to boost the energy efficiency of semiconductor devices by using the self-adaptive evaporative cooling of a lithium- and bromine-enriched polyacrylamide hydrogel. Water inside the hydrogel can quickly evaporate to dissipate the waste heat generated by the nugatory carrier transport in the P-N junction. In dormancy, the hydrogel harvests water molecules from the surrounding air to regenerate itself. The hydrogel is demonstrated to low down the operating temperature of a commercial polycrystalline silicon solar cell by 17 °C under one sun condition and enhances its efficiency from 14.5% to 15.5%. It is also capable of increasing the maximum power of a simulated chip by 45% at a fixed operating temperature. The hydrogel is expected to be widely adopted in current semiconductor industry to improve its energy efficiency.

Waterproof molecular monolayers stabilize 2D materials

Two-dimensional van der Waals materials have rich and unique functional properties, but many are susceptible to corrosion under ambient conditions. Here we show that linear alkylamines n-C _m H_2m+1NH₂, with m = 4 through 11, are highly effective in protecting the optoelectronic properties of these materials, such as black phosphorus (BP) and transition-metal dichalcogenides (TMDs: WS₂, 1T'-MoTe₂, WTe₂, WSe₂, TaS₂, and NbSe₂). As a representative example, n-hexylamine (m = 6) can be applied in the form of thin molecular monolayers on BP flakes with less than 2-nm thickness and can prolong BP's lifetime from a few hours to several weeks and even months in ambient environments. Characterizations combined with our theoretical analysis show that the thin monolayers selectively sift out water molecules, forming a drying layer to achieve the passivation of the protected 2D materials. The monolayer coating is also stable in air, H₂ annealing, and organic solvents, but can be removed by certain organic acids.

A parylene coating system specifically designed for producing ultra-thin films for nanoscale device applications

We report on a parylene chemical vapor deposition system custom designed for producing ultrathin parylene films (5-100 nm thickness) for use as an electrical insulator in nanoscale electronic devices, including as the gate insulator in transistors. The system features a small deposition chamber that can be isolated and purged for process termination, a quartz crystal microbalance for monitoring deposition, and a rotating angled stage to increase coating conformity. The system was mostly built from off-the-shelf vacuum fittings allowing for easy modification and reduced cost compared to commercial parylene coating systems. The production of ultrathin parylene films for device applications is a niche not well catered to by commercial coating systems, which are typically designed to give thicker coatings (microns) with high uniformity over much larger areas. An added advantage of our design for nanoscale device applications is that the small deposition chamber is readily removable for transfer to a glovebox to enable parylene deposition onto pristine surfaces prepared in oxygen/water-free environments with minimal contamination.

Metal-Guided Selective Growth of 2D Materials: Demonstration of a Bottom-Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large-area electronics and circuits strongly relies on wafer-scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal-guided selective growth (MGSG), is reported. The success of control over the transition-metal-precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p- and n-type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom-up complementary metal-oxide-semiconductor inverter based on p-type WSe₂ and n-type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.

A Universal Approach toward Light-Responsive Two-Dimensional Electronics: Chemically Tailored Hybrid van der Waals Heterostructures

Stimuli-responsive hybrid van der Waals heterostructures (vdWHs), composed of organic molecular switches superimposed on inorganic 2D materials (2DMs), can combine the outstanding physical properties of the latter components with the virtually infinite variety of tunable functionality of molecules, thereby offering an efficient protocol for the development of high-performance multifunctional materials and devices. The use of light as a remote control to modulate the properties of semiconducting 2DMs when interfaced with photochromic molecules suffers from both the limitation associated with the persistent photoconductivity characterizing the 2DMs and the finite thermal stability of the photochromic molecule in its different states. Here, we have devised a universal approach toward the fabrication of optically switchable electronic devices comprising a few nanometers thick azobenzene (AZO) layer physisorbed on 2D semiconductors supported on a trap-free polymer dielectric. The joint effect of the improved 2D/dielectric interface, the molecule's light-modulated dipolar doping, and the high thermal stability of cis-AZO offers the highest control over the reversible and efficient charge carrier tuning in 2D semiconductors with a preserved high performance in 2D field-effect transistors, as quantified in terms of carrier mobility and I_on/ I_off ratio. The device has the potential to operate as an optical memory with four current levels and long retention time (>15 h). Furthermore, by using a CMOS-compatible micropatterning process, the photoswitchable resistor-diode transition has been achieved on hybrid lateral heterojunction devices. Our approach is of general applicability toward the generation of high-performance hybrid vdWHs for the emergence of functional and responsive devices.

Threshold Voltage Control of Multilayered MoS2 Field-Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement-Mode Logic Gates

In recent past, for next-generation device opportunities such as sub-10 nm channel field-effect transistors (FETs), tunneling FETs, and high-end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂ ) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self-assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field-effect mobility, subthreshold swing, and current on-off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature-dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD . More impressively, quantum dot light-emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^-2 at the gate-to-source voltage of 5 V, exhibiting promising opportunities for future display application.

Formation of an MoTe2 based Schottky junction employing ultra-low and high resistive metal contacts

Schottky-barrier diodes have great importance in power management and mobile communication because of their informal device technology, fast response and small capacitance. In this research, a p-type molybdenum ditelluride (p-MoTe₂) based Schottky barrier diode was fabricated using asymmetric metal contacts. The MoTe₂ nano-flakes were mechanically exfoliated using adhesive tape and with the help of dry transfer techniques, the flakes were transferred onto silicon/silicon dioxide (Si/SiO₂) substrates to form the device. The Schottky-barrier was formed as a result of using ultra-low palladium/gold (Pd/Au) and high resistive chromium/gold (Cr/Au) metal electrodes. The Schottky diode exhibited a clear rectifying behavior with an on/off ratio of ∼10³ and an ideality factor of ∼1.4 at zero gate voltage. In order to check the photovoltaic response, a green laser light was illuminated, which resulted in a responsivity of ∼3.8 × 10³ A W⁻¹. These values are higher than the previously reported results that were obtained using conventional semiconducting materials. Furthermore, the barrier heights for Pd and Cr with a MoTe₂ junction were calculated to be 90 meV and 300 meV, respectively. In addition, the device was used for rectification purposes revealing a stable rectifying behavior.

Schottky-barrier diodes have great importance in power management and mobile communication because of their informal device technology, fast response and small capacitance.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry.

High-Responsivity Multilayer MoSe2 Phototransistors with Fast Response Time

There is a great interest in phototransistors based on transition metal dichalcogenides because of their interesting optoelectronic properties. However, most emphasis has been put on MoS₂ and little attention has been given to MoSe₂, which has higher optical absorbance. Here, we present a compelling case for multilayer MoSe₂ phototransistors fabricated in a bottom-gate thin-film transistor configuration on SiO₂/Si substrates. Under 650-nm-laser, our MoSe₂ phototransistor exhibited the best performance among MoSe₂ phototransistors in literature, including the highest responsivity (1.4 × 10⁵ AW^-1), the highest specific detectivity (5.5 × 10¹³ jones), and the fastest response time (1.7 ms). We also present a qualitative model to describe the device operation based on the combination of photoconductive and photogating effects. These results demonstrate the feasibility of achieving high performance in multilayer MoSe₂ phototransistors, suggesting the possibility of further enhancement in the performance of MoSe₂ phototransistors with proper device engineering.

Using Ultrathin Parylene Films as an Organic Gate Insulator in Nanowire Field-Effect Transistors

We report the development of nanowire field-effect transistors featuring an ultrathin parylene film as a polymer gate insulator. The room temperature, gas-phase deposition of parylene is an attractive alternative to oxide insulators prepared at high temperatures using atomic layer deposition. We discuss our custom-built parylene deposition system, which is designed for reliable and controlled deposition of <100 nm thick parylene films on III-V nanowires standing vertically on a growth substrate or horizontally on a device substrate. The former case gives conformally coated nanowires, which we used to produce functional Ω-gate and gate-all-around structures. These give subthreshold swings as low as 140 mV/dec and on/off ratios exceeding 10³ at room temperature. For the gate-all-around structure, we developed a novel fabrication strategy that overcomes some of the limitations with previous lateral wrap-gate nanowire transistors. Finally, we show that parylene can be deposited over chemically treated nanowire surfaces, a feature generally not possible with oxides produced by atomic layer deposition due to the surface "self-cleaning" effect. Our results highlight the potential for parylene as an alternative ultrathin insulator in nanoscale electronic devices more broadly, with potential applications extending into nanobioelectronics due to parylene's well-established biocompatible properties.

Nano-"Squeegee" for the Creation of Clean 2D Material Interfaces

Two-dimensional (2D) materials exhibit many exciting phenomena that make them promising as materials for future electronic, optoelectronic, and mechanical devices. Because of their atomic thinness, interfaces play a dominant role in determining material behavior. In order to observe and exploit the unique properties of these materials, it is therefore vital to obtain clean and repeatable interfaces. However, the conventional mechanical stacking of atomically thin layers typically leads to trapped contaminants and spatially inhomogeneous interfaces, which obscure the true intrinsic behavior. This work presents a simple and generic approach to create clean 2D material interfaces in mechanically stacked structures. The operating principle is to use an AFM tip to controllably squeeze contaminants out from between 2D layers and their substrates, similar to a "squeegee". This approach leads to drastically improved homogeneity and consistency of 2D material interfaces, as demonstrated by AFM topography and significant reduction of photoluminescence line widths. Also, this approach enables emission from interlayer excitons, demonstrating that the technique enhances interlayer coupling in van der Waals heterostructures. The technique enables repeatable observation of intrinsic 2D material properties, which is crucial for the continued development of these promising materials.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Broadband MoS2 Field-Effect Phototransistors: Ultrasensitive Visible-Light Photoresponse and Negative Infrared Photoresponse

Inverse photoresponse is discovered from phototransistors based on molybdenum disulfide (MoS₂ ). The devices are capable of detecting photons with energy below the bandgap of MoS₂ . Under the illumination of near-infrared (NIR) light at 980 and 1550 nm, negative photoresponses with short response time (50 ms) are observed for the first time. Upon visible-light illumination, the phototransistors exhibit positive photoresponse with ultrahigh responsivity on the order of 10⁴ -10⁵ A W^-1 owing to the photogating effect and charge trapping mechanism. Besides, the phototransistors can detect a weak visible-light signal with effective optical power as low as 17 picowatts (pW). A thermally induced photoresponse mechanism, the bolometric effect, is proposed as the cause of the negative photocurrent in the NIR regime. The thermal energy of the NIR radiation is transferred to the MoS₂ crystal lattice, inducing lattice heating and resistance increase. This model is experimentally confirmed by low-temperature electrical measurements. The bolometric coefficient calculated from the measured transport current change with temperature is -33 nA K^-1 . These findings offer a new approach to develop sub-bandgap photodetectors and other novel optoelectronic devices based on 2D layered materials.

Vertical MoSe2-MoO x p-n heterojunction and its application in optoelectronics

The hybrid n-type 2D transition-metal dichalcogenide (TMD)/p-type oxide van der Waals (vdW) heterojunction nanosheets consist of 2D layered MoSe₂ (the n-type 2D material) and MoO _x (the p-type oxide) which are grown on SiO₂/Si substrates for the first time via chemical vapor deposition technique, displaying the regular hexagon structures with the average length dimension of sides of ∼8 μm. Vertical MoSe₂-MoO _x p-n heterojunctions demonstrate obviously current-rectifying characteristic, and it can be tuned via gate voltage. What is more, the photodetector based on vertical MoSe₂-MoO _x heterojunctions displays optimal photoresponse behavior, generating the responsivity, detectivity, and external quantum efficiency to 3.4 A W^-1, 0.85 × 10⁸ Jones, and 1665.6%, respectively, at V _ds = 5 V with the light wavelength of 254 nm under 0.29 mW cm^-2. These results furnish a building block on investigating the flexible and transparent properties of vdW and further optimizing the structure of the devices for better optoelectronic and electronic performance.

Interface Engineering for Controlling Device Properties of Organic Antiambipolar Transistors

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (V_on) and offset (V_off) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.

CVD growth of molybdenum diselenide surface structures with tailored morphology

Controllable atmospheric pressure CVD has been optimized to grow transition metal dichalcogenide MoSe₂ with tunable morphology at 750 °C on a silicon substrate with a native oxide layer of 250 nm.

Two-dimensional transition metal dichalcogenides: interface and defect engineering

This review summarizes the recent advances in understanding the effects of interface and defect engineering on the electronic and optical properties of TMDCs, as well as their applications in advanced (opto)electronic devices.

n-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors

We explore the contact properties of monolayer InSe transistors and obtain n-type Ohmic/p-type Schottky contacts.

Facile Electrochemical Synthesis of 2D Monolayers for High-Performance Thin-Film Transistors

In this paper, we report high-performance monolayer thin-film transistors (TFTs) based on a variety of two-dimensional layered semiconductors such as MoS₂, WS₂, and MoSe₂ which were obtained from their corresponding bulk counterparts via an anomalous but high-yield and low-cost electrochemical corrosion process, also referred to as electro-ablation (EA), at room temperature. These monolayer TFTs demonstrated current ON-OFF ratios in excess of 10⁷ along with ON currents of 120 μA/μm for MoS₂, 40 μA/μm for WS₂, and 40 μA/μm for MoSe₂ which clearly outperform the existing TFT technologies. We found that these monolayers have larger Schottky barriers for electron injection compared to their multilayer counterparts, which is partially compensated by their superior electrostatics and ultra-thin tunnel barriers. We observed an Anderson type semiconductor-to-metal transition in these monolayers and also discussed possible scattering mechanisms that manifest in the temperature dependence of the electron mobility. Finally, our study suggests superior chemical stability and electronic integrity of monolayers even after being exposed to extreme electro-oxidation and corrosion processes which is promising for the implementation of such TFTs in harsh environment sensing. Overall, the EA process proves to be a facile synthesis route offering higher monolayer yields than mechanical exfoliation and lower cost and complexity than chemical vapor deposition methods.

Graphene-Contacted Ultrashort Channel Monolayer MoS2 Transistors

2D semiconductors are promising channel materials for field-effect transistors (FETs) with potentially strong immunity to short-channel effects (SCEs). In this paper, a grain boundary widening technique is developed to fabricate graphene electrodes for contacting monolayer MoS₂ . FETs with channel lengths scaling down to ≈4 nm can be realized reliably. These graphene-contacted ultrashort channel MoS₂ FETs exhibit superior performances including the nearly Ohmic contacts and excellent immunity to SCEs. This work provides a facile route toward the fabrication of various 2D material-based devices for ultrascaled electronics.

Recent Advances in Electronic and Optoelectronic Devices Based on Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs.

Graphene-Assisted Antioxidation of Tungsten Disulfide Monolayers: Substrate and Electric-Field Effect

Transition metal dichalcogenides (TMDs) have emerged as promising materials to complement graphene for advanced optoelectronics. However, irreversible degradation of chemical vapor deposition-grown monolayer TMDs via oxidation under ambient conditions limits applications of TMD-based devices. Here, the growth of oxidation-resistant tungsten disulfide (WS₂ ) monolayers on graphene is demonstrated, and the mechanism of oxidation of WS₂ on SiO₂ , graphene/SiO₂ , and on graphene suspended in air is elucidated. While WS₂ on a SiO₂ substrate begins oxidation within weeks, epitaxially grown WS₂ on suspended graphene does not show any sign of oxidation, attributed to the screening effect of surface electric field caused by the substrate. The control of a local oxidation of WS₂ on a SiO₂ substrate by a local electric field created using an atomic force microscope tip is also demonstrated.

Optimizing Charge Injection across Transition Metal Dichalcogenide Heterojunctions: Theory and Experiment

In search of an improved strategy to form low-resistance contacts to semiconducting transition metal dichalcogenides, we combine ab initio density functional electronic structure calculations for an NbSe₂/WSe₂ interface with quantum transport measurements of the corresponding heterojunction between a few-layer WSe₂ semiconductor and a metallic NbSe₂ layer. Our theoretical results suggest that, besides a rigid band shift associated with charge transfer, the presence of NbSe₂ does not modify the electronic structure of WSe₂. Since the two transition metal dichalcogenides are structurally similar and display only a small lattice mismatch, their heterojunction can efficiently transfer charge across the interface. These findings are supported by transport measurements for WSe₂ field-effect transistors with NbSe₂ contacts, which exhibit nearly ohmic behavior and phonon-limited mobility in the hole channel, indicating that the contacts to WSe₂ are highly transparent.

High-Electron-Mobility and Air-Stable 2D Layered PtSe2 FETs

The electrical and optical measurements, in combination with density functional theory calculations, show distinct layer-dependent semiconductor-to-semimetal evolution of 2D layered PtSe₂ . The high room-temperature electron mobility and near-infrared photo-response, together with much better air-stability, make PtSe₂ a versatile electronic 2D layered material.

Large-area synthesis of monolayer MoSe2 films on SiO2/Si substrates by atmospheric pressure chemical vapor deposition

We report the synthesis of large-area monolayer MoSe₂ films extended up to a millimeter scale on SiO₂/Si substrates by atmospheric pressure chemical vapor deposition (CVD).

Two-dimensional transition metal dichalcogenide-based counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cell using counter electrode based on transition metal dichalcogenides.

Visible to short wavelength infrared In2Se3-nanoflake photodetector gated by a ferroelectric polymer

Photodetectors based on two-dimensional (2D) transition-metal dichalcogenides have been studied extensively in recent years. However, the detective spectral ranges, dark current and response time are still unsatisfactory, even under high gate and source-drain bias. In this work, the photodetectors of In2Se3 have been fabricated on a ferroelectric field effect transistor structure. Based on this structure, high performance photodetectors have been achieved with a broad photoresponse spectrum (visible to 1550 nm) and quick response (200 μs). Most importantly, with the intrinsic huge electric field derived from the polarization of ferroelectric polymer (P(VDF-TrFE)) gating, a low dark current of the photodetector can be achieved without additional gate bias. These studies present a crucial step for further practical applications for 2D semiconductors.

Thickness-dependent charge transport in few-layer MoS₂ field-effect transistors

Molybdenum disulfide (MoS2) is currently under intensive study because of its exceptional optical and electrical properties in few-layer form. However, how charge transport mechanisms vary with the number of layers in MoS2 flakes remains unclear. Here, exfoliated flakes of MoS2 with various thicknesses were successfully fabricated into field-effect transistors (FETs) to measure the thickness and temperature dependences of electrical mobility. For these MoS2 FETs, measurements at both 295 K and 77 K revealed the maximum mobility for layer thicknesses between 5 layers (∼3.6 nm) and 10 layers (∼7 nm), with ∼70 cm(2) V(-1) s(-1) measured for 5 layer devices at 295 K. Temperature-dependent mobility measurements revealed that the mobility rises with increasing temperature to a maximum. This maximum occurs at increasing temperature with increasing layer thickness, possibly due to strong Coulomb scattering from charge impurities or weakened electron-phonon interactions for thicker devices. Temperature-dependent conductivity measurements for different gate voltages revealed a metal-to-insulator transition for devices thinner than 10 layers, which may enable new memory and switching applications. This study advances the understanding of fundamental charge transport mechanisms in few-layer MoS2, and indicates the promise of few-layer transition metal dichalcogenides as candidates for potential optoelectronic applications.

Optoelectronic properties of atomically thin ReSSe with weak interlayer coupling

Rhenium dichalcogenides, such as ReS2 and ReSe2, have attracted a lot of interests due to the weak interlayered coupling in these materials. Studies of rhenium based dichalcogenide alloys will help us understand the differences between binary rhenium dichalcogenides. They will also extend the applications of two-dimensional (2D) materials through alloying. In this work, we studied the optoelectronic properties of ReSSe with a S and Se ratio of 1 : 1. The band gap of the ReSSe alloy is investigated by optical absorption spectra as well as theoretical calculations. The alloy shows weak interlayered coupling, as evidenced by the Raman spectrum. A field-effect transistor based on ReSSe shows typical n-type behavior with a mobility of about 3 cm(2) V(-1) s(-1) and an on/off ratio of 10(5), together with the in-plane anisotropic conductivity. The device also shows good photoresponse properties, with a photoresponsivity of 8 A W(-1). The results demonstrated here will provide new avenues for the study of 2D materials with weak interlayer interactions and in-plane anisotropy.