Thickness scaling effect on interfacial barrier and electrical contact to two-dimensional MoS2 layers

Ultrafast, Broadband Photodetector Based on MoSe2/Silicon Heterojunction with Vertically Standing Layered Structure Using Graphene as Transparent Electrode

A MoSe2/Si heterojunction photodetector is constructed by depositing MoSe2 film with vertically standing layered structure on Si substrate. Graphene transparent electrode is utilized to further enhance the separation and transport of photogenerated carriers. The device shows excellent performance in terms of wide response spectrum of UV-visible-NIR, high detectivity of 7.13 × 1010 Jones, and ultrafast response speed of ≈270 ns, unveiling the great potential for the heterojunction for high-performance optoelectronic devices.

High Mobility MoS2 Transistor with Low Schottky Barrier Contact by Using Atomic Thick h-BN as a Tunneling Layer

High-performance MoS₂ transistors are developed using atomic hexagonal boron nitride as a tunneling layer to reduce the Schottky barrier and achieve low contact resistance between metal and MoS₂ . Benefiting from the ultrathin tunneling layer within 0.6 nm, the Schottky barrier is significantly reduced from 158 to 31 meV with small tunneling resistance.

Contact and Support Considerations in the Hydrogen Evolution Reaction Activity of Petaled MoS2 Electrodes

Petaled MoS2 electrodes grown hydrothermally from Mo foils are found to have an 800 nm, intermediate, MoSxOy layer. Similar petaled MoS2 films without this intermediate layer are grown on Au. X-ray photoelectron and Raman spectroscopies and transmission electron microscopy indicate the resulting petaled multilayer MoS2 films are frayed and exhibit single-layer, 1T-MoS2 behavior at the edges. We compare the electrocatalytic hydrogen evolution reaction activity via linear sweep voltammetry with Tafel analysis as well as the impedance properties of the electrodes. We find that petaled MoS2/Au and petaled MoS2/Mo exhibit comparable overpotential to 10 mA cm(-2) at -279 vs -242 mV, respectively, and similar Tafel slopes of ∼68 mV/decade indicating a similar rate-determining step. The exchange current normalized to the geometric area of petaled MoS2/Au (0.000921 mA cm(-2)) is 3 times smaller than that of petaled MoS2/Mo (0.00290 mA cm(-2)), and is attributed to the lower petal density on the Au support. However, Au supports increase the turnover frequency per active site of petaled MoS2 to 0.48 H2 Mo(-1) s(-1) from 0.25 H2 Mo(-1) s(-1) on Mo supports. Both petaled MoS2 films have nearly ohmic contacts to their supports with uncompensated resistivity Ru of <2.5 Ω·cm(2).

Coherent Coupling of WS2 Monolayers with Metallic Photonic Nanostructures at Room Temperature

Room temperature strong coupling of WS2 monolayer exciton transitions to metallic Fabry-Pérot and plasmonic optical cavities is demonstrated. A Rabi splitting of 101 meV is observed for the Fabry-Pérot cavity. The enhanced magnitude and visibility of WS2 monolayer strong coupling is attributed to the larger absorption coefficient, the narrower line width of the A exciton transition, and greater spin-orbit coupling. For WS2 coupled to plasmonic arrays, the Rabi splitting still reaches 60 meV despite the less favorable coupling conditions, and displays interesting photoluminescence features. The unambiguous signature of WS2 monolayer strong coupling in easily fabricated metallic resonators at room temperature suggests many possibilities for combining light-matter hybridization with spin and valleytronics.

Atomically Thin Ohmic Edge Contacts Between Two-Dimensional Materials

With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition-metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs, MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 kΩ·μm. The resulting graphene edge contacts show linear current-voltage (I-V) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.

Carrier Polarity Control in α-MoTe2 Schottky Junctions Based on Weak Fermi-Level Pinning

The polarity of the charge carriers injected through Schottky junctions of α-phase molybdenum ditelluride (α-MoTe2) and various metals was characterized. We found that the Fermi-level pinning in the metal/α-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/α-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/α-MoTe2 and Ni/α-MoTe2 junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt-Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.

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.

Coulomb blockade in monolayer MoS2 single electron transistor

Substantial effort has been dedicated to understand the intrinsic electronic properties of molybdenum disulfide (MoS2). However, electron transport study on monolayer MoS2 has been challenging to date, especially at low temperatures due to large metal/semiconductor junction barriers. Herein, we report the fabrication and characterization of the monolayer MoS2 single-electron transistor. High performance devices are obtained through the use of low work function metal (zinc) contact and a rapid thermal annealing step. Coulomb blockade is observed at low temperatures and is attributed to single-electron tunneling via two tunnel junction barriers. The nature of Coulomb blockade is also investigated by temperature-dependent conductance oscillation measurement. Our results hold promise for the study of novel quantum transport phenomena in 2D semiconducting atomic layer crystals.

Self-Limiting Oxides on WSe2 as Controlled Surface Acceptors and Low-Resistance Hole Contacts

Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WOx with x < 3) grown on tungsten diselenide (WSe2) can be used as both controlled charge transfer dopants and low-barrier contacts for p-type WSe2 transistors. Exposure of atomically thin WSe2 transistors to ozone (O3) at 100 °C results in self-limiting oxidation of the WSe2 surfaces to conducting WOx films. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.

Statistical Study on the Schottky Barrier Reduction of Tunneling Contacts to CVD Synthesized MoS2

Creating high-quality, low-resistance contacts is essential for the development of electronic applications using two-dimensional (2D) layered materials. Many previously reported methods for lowering the contact resistance rely on volatile chemistry that either oxidize or degrade in ambient air. Nearly all reported efforts have been conducted on only a few devices with mechanically exfoliated flakes which is not amenable to large scale manufacturing. In this work, Schottky barrier heights of metal-MoS2 contacts to devices fabricated from CVD synthesized MoS2 films were reduced by inserting a thin tunneling Ta2O5 layer between MoS2 and metal contacts. Schottky barrier height reductions directly correlate with exponential reductions in contact resistance. Over two hundred devices were tested and contact resistances extracted for large scale statistical analysis. As compared to metal-MoS2 Schottky contacts without an insulator layer, the specific contact resistivity has been lowered by up to 3 orders of magnitude and current values increased by 2 orders of magnitude over large area (>4 cm(2)) films.

Charge transport and mobility engineering in two-dimensional transition metal chalcogenide semiconductors

This review presents recent progress on charge transport properties, carrier scattering mechanisms, and carrier mobility engineering of two-dimensional transition metal chalcogenides.

Thickness dependence of surface energy and contact angle of water droplets on ultrathin MoS2 films

We have performed a systematic density functional study of surface energy of MoS₂ films as a function of thickness from one to twelve layers with the consideration of van der Waals (vdW) interactions using the vdW-DF and DFT-D2 methods.

Electrical contacts to two-dimensional semiconductors

The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.

Origin of Noise in Layered MoTe₂ Transistors and its Possible Use for Environmental Sensors

Low-frequency current fluctuations are monitored and the mechanism of electric noise investigated in layered 2H-type α-molybdenum ditelluride transistors. The charge transport mechanism of electric noise in atomically thin transition-metal dichalcogenides is studied under different environments; the development of a new sensing functionality may be stimulated.

Piezoresistivity and Strain-induced Band Gap Tuning in Atomically Thin MoS2

Continuous tuning of material properties is highly desirable for a wide range of applications, with strain engineering being an interesting way of achieving it. The tuning range, however, is limited in conventional bulk materials that can suffer from plasticity and low fracture limit due to the presence of defects and dislocations. Atomically thin membranes such as MoS2 on the other hand exhibit high Young's modulus and fracture strength, which makes them viable candidates for modifying their properties via strain. The bandgap of MoS2 is highly strain-tunable, which results in the modulation of its electrical conductivity and manifests itself as the piezoresistive effect, whereas a piezoelectric effect was also observed in odd-layered MoS2 with broken inversion symmetry. This coupling between electrical and mechanical properties makes MoS2 a very promising material for nanoelectromechanical systems (NEMS). Here, we incorporate monolayer, bilayer, and trilayer MoS2 in a nanoelectromechanical membrane configuration. We detect strain-induced band gap tuning via electrical conductivity measurements and demonstrate the emergence of the piezoresistive effect in MoS2. Finite element method (FEM) simulations are used to quantify the band gap change and to obtain a comprehensive picture of the spatially varying bandgap profile on the membrane. The piezoresistive gauge factor is calculated to be -148 ± 19, -224 ± 19, and -43.5 ± 11 for monolayer, bilayer, and trilayer MoS2, respectively, which is comparable to state-of-the-art silicon strain sensors and 2 orders of magnitude higher than in strain sensors based on suspended graphene. Controllable modulation of resistivity in 2D nanomaterials using strain-induced bandgap tuning offers a novel approach for implementing an important class of NEMS transducers, flexible and wearable electronics, tunable photovoltaics, and photodetection.

Synthesis, properties and applications of 2D non-graphene materials

As an emerging class of new materials, two-dimensional (2D) non-graphene materials, including layered and non-layered, and their heterostructures are currently attracting increasing interest due to their promising applications in electronics, optoelectronics and clean energy. In contrast to traditional semiconductors, such as Si, Ge and III-V group materials, 2D materials show significant merits of ultrathin thickness, very high surface-to-volume ratio, and high compatibility with flexible devices. Owing to these unique properties, while scaling down to ultrathin thickness, devices based on these materials as well as artificially synthetic heterostructures exhibit novel and surprising functions and performances. In this review, we aim to provide a summary on the state-of-the-art research activities on 2D non-graphene materials. The scope of the review will cover the preparation of layered and non-layered 2D materials, construction of 2D vertical van der Waals and lateral ultrathin heterostructures, and especially focus on the applications in electronics, optoelectronics and clean energy. Moreover, the review is concluded with some perspectives on the future developments in this field.

Investigation of band-offsets at monolayer-multilayer MoS₂ junctions by scanning photocurrent microscopy

The thickness-dependent band structure of MoS2 implies that discontinuities in energy bands exist at the interface of monolayer (1L) and multilayer (ML) thin films. The characteristics of such heterojunctions are analyzed here using current versus voltage measurements, scanning photocurrent microscopy, and finite element simulations of charge carrier transport. Rectifying I-V curves are consistently observed between contacts on opposite sides of 1L/ML junctions, and a strong bias-dependent photocurrent is observed at the junction. Finite element device simulations with varying carrier concentrations and electron affinities show that a type II band alignment at single layer/multilayer junctions reproduces both the rectifying electrical characteristics and the photocurrent response under bias. However, the zero-bias junction photocurrent and its energy dependence are not explained by conventional photovoltaic and photothermoelectric mechanisms, indicating the contributions of hot carriers.