Probing Distinctive Electron Conduction in Multilayer Rhenium Disulfide

Negative Differential Interlayer Resistance in WSe2 Multilayers via Conducting Channel Migration with Vertical Double-Side Contacts

The inherent interlayer resistance in two-dimensional (2D) van der Waals (vdW) multilayers is expected to significantly influence the carrier density profile along the thickness, provoking spatial modification and separation of the conducting channel inside the multilayers, in conjunction with the thickness-dependent carrier mobility. However, the effect of the interlayer resistance on the variation in the carrier density profile and its direction along the thickness under different electrostatic bias conditions has been elusive. Here, we reveal the presence of a negative differential interlayer resistance (NDIR) in WSe2 multilayers by considering various contact electrode configurations: (i) bottom contact, (ii) top contact, and (iii) vertical double-side contact (VDC). The contact-structure-dependent shape modification of the transconductance clearly manifests the redistribution of carrier density and indicates the direction of the conducting channel migration along the thickness. Furthermore, the distinct characteristic of the electrically tunable NDIR in 2D WSe2 multilayers is revealed by the observed discrepancy between the top- and bottom-channel resistances determined by four-probe measurements with VDC. Our results provide an optimized device layout and further insights into the distinct carrier transport mechanism in 2D vdW multilayers.

Enhanced Interlayer Charge Injection Efficiency in 2D Multilayer ReS2 via Vertical Double-Side Contacts

Two-dimensional (2D) van der Waals (vdW) layered materials have provided novel opportunities to explore interesting physical properties such as thickness-dependent bandgap, moiré excitons, superconductivity, and superfluidity. However, the presence of interlayer resistance along the thickness and Schottky barrier in metal-to-2D vdW semiconducting materials causes a limited interlayer charge injection efficiency, perturbing various intrinsic properties of 2D vdW multilayers. Herein, we report a simple but powerful contact electrode design to enhance interlayer carrier injection efficiency along the thickness by constructing vertical double-side contact (VDC) electrodes. A 2-fold extended contact area of VDC not only strongly limits an interlayer resistance contribution to the field-effect mobility and current density at the metal-to-2D semiconductor interface but also significantly suppresses both current transfer length (≤1 μm) and specific contact resistivity (≤1 mΩ·cm²), manifesting clear benefits of VDC in comparison with those in conventional top-contact and bottom-contact configurations. Our layout for contact electrode configuration may suggest an advanced electronic device platform for high-performing 2D optoelectronic devices.

Cauliflower-like Mn0.2Cd0.8S decorated with ReS2 nanosheets for boosting photocatalytic H2 evolution activity

A novel ReS2/Mn0.2Cd0.8S composite was fabricated and exhibited enhanced H2 evolution activity due to the modification of cauliflower-like Mn0.2Cd0.8S with ReS2 nanosheets.

Visualizing Orbital Content of Electronic Bands in Anisotropic 2D Semiconducting ReSe2

Many properties of layered materials change as they are thinned from their bulk forms down to single layers, with examples including indirect-to-direct band gap transition in 2H semiconducting transition metal dichalcogenides as well as thickness-dependent changes in the valence band structure in post-transition-metal monochalcogenides and black phosphorus. Here, we use angle-resolved photoemission spectroscopy to study the electronic band structure of monolayer ReSe₂, a semiconductor with a distorted 1T structure and in-plane anisotropy. By changing the polarization of incoming photons, we demonstrate that for ReSe₂, in contrast to the 2H materials, the out-of-plane transition metal d_z² and chalcogen p_z orbitals do not contribute significantly to the top of the valence band, which explains the reported weak changes in the electronic structure of this compound as a function of layer number. We estimate a band gap of 1.7 eV in pristine ReSe₂ using scanning tunneling spectroscopy and explore the implications on the gap following surface doping with potassium. A lower bound of 1.4 eV is estimated for the gap in the fully doped case, suggesting that doping-dependent many-body effects significantly affect the electronic properties of ReSe₂. Our results, supported by density functional theory calculations, provide insight into the mechanisms behind polarization-dependent optical properties of rhenium dichalcogenides and highlight their place among two-dimensional crystals.

High-Mobility InSe Transistors: The Nature of Charge Transport

InSe is a high-mobility layered semiconductor with mobility being highly sensitive to any surrounding media that could act as a source of extrinsic scattering. However, little effort has been made to understand electronic transport in thin InSe layers with native surface oxide formed spontaneously upon exposure to an ambient environment. Here, we explore the influence of InO_x/InSe interfacial trap states on electronic transport in thin InSe layers. We show that wet oxidation (processed in an ambient environment) causes massive deep-lying band-tail states, through which electrons conduct via 2D variable-range hopping with a short localization length of 1-3 nm. In contrast, a high-quality InO_x/InSe interface can be formed in dry oxidation (processed in pure oxygen), with a low trap density of 10¹² eV^-1 cm^-2. Metal-insulator transition can be thus observed in the gate sweep of the field-effect transistors (FETs), indicative of band transport predominated by extended states above the mobility edge. A room-temperature band mobility of 10³ cm²/V s is obtained. The profound difference in the transport behavior between the wet and dry InSe FETs suggests that fluctuating Coulomb potential arising from trapped charges at the InO_x/InSe interface is the dominant source of disorders in thin InSe channels.

High-κ Dielectric on ReS₂: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al₂O₃

We report an excellent growth behavior of a high-κ dielectric on ReS₂, a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al₂O₃ thin film on the UV-Ozone pretreated surface of ReS₂ yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al₂O₃ thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al₂O₃ was achieved using a UV-Ozone pretreatment. The ReS₂ substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S⁻O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials.