Graphene-like two-dimensional (2D) materials not only are interesting for their exotic electronic structure and fundamental electronic transport or optical properties but also hold promises for device miniaturization down to atomic thickness. As one material belonging to this category, InSe, a III-VI semiconductor, not only is a promising candidate for optoelectronic devices but also has potential for ultrathin field effect transistor (FET) with high mobility transport. In this work, various substrates such as PMMA, bare silicon oxide, passivated silicon oxide, and silicon nitride were used to fabricate multilayer InSe FET devices. Through back gating and Hall measurement in four-probe configuration, the device's field effect mobility and intrinsic Hall mobility were extracted at various temperatures to study the material's intrinsic transport behavior and the effect of dielectric substrate. The sample's field effect and Hall mobilities over the range of 20-300 K fall in the range of 0.1-2.0 × 10(3) cm(2)/(V s), which are comparable or better than the state of the art FETs made of widely studied 2D transition metal dichalcogenides.

Gate tunable p-type multilayer tin mono-sulfide (SnS) field-effect transistor (FET) devices with SnS thickness between 50 and 100 nm were fabricated and studied to understand their performance. The devices showed anisotropic inplane conductance and room temperature field effect mobilities ∼5-10 cm V s. However, the devices showed an ON-OFF ratio ∼10 at room temperature due to appreciable OFF state conductance. The weak gate tuning behavior and finite OFF state conductance in the depletion regime of SnS devices are explained by the finite carrier screening length effect which causes the existence of a conductive surface layer from defect induced holes in SnS. Through etching and n-type surface doping by CsCO to reduce/compensate the not-gatable holes near the SnS flake's top surface, the devices exhibited an order of magnitude improvement in the ON-OFF ratio, and a hole Hall mobility of ∼100 cm V s at room temperature is observed. This work suggests that in order to obtain effective switching and low OFF state power consumption, two-dimensional (2D) semiconductor based depletion mode FETs should limit their thickness to within the Debye screening length of the carriers in the semiconductor.

Organometal halide perovskite has emerged as a promising material for solar cells and optoelectronics. Although the long diffusion length of photogenerated carriers is believed to be a critical factor responsible for the material's high efficiency in solar cells, a direct study of carrier transport over long distances in organometal halide perovskites is still lacking. We fabricated highly oriented crystalline CHNHPbI (MAPbI) thin-film lateral transport devices with long channel length (∼120 μm). By performing spatially scanned photocurrent imaging measurements with local illumination, we directly show that the perovskite films prepared here have very long transport lengths for photogenerated carriers, with a minority carrier (electron) diffusion length on the order of 10 μm. Our approach of applying scanning photocurrent microscopy to organometal halide perovskites may be further used to elucidate the carrier transport processes and the vastly different carrier diffusion lengths (∼100 nm to 100 μm) in different types of organometal halide perovskites.

Manipulating the electron spin with the aid of spin-orbit coupling (SOC) is an indispensable element of spintronics. Electrostatically gating a material with strong SOC results in an effective magnetic field which can in turn be used to govern the electron spin. In this work, we report the existence and electrostatic tunability of Rashba SOC in multilayer InSe. We observed a gate-voltage-tuned crossover from weak localization (WL) to weak antilocalization (WAL) effect in quantum transport studies of InSe, which suggests an increasing SOC strength. Quantitative analyses of magneto-transport studies and energy band diagram calculations provide strong evidence for the predominance of Rashba SOC in electrostatically gated InSe. Furthermore, we attribute the tendency of the SOC strength to saturate at high gate voltages to the increased electronic density of states-induced saturation of the electric field experienced by the electrons in the InSe layer. This explanation of nonlinear gate voltage control of Rashba SOC can be generalized to other electrostatically gated semiconductor nanomaterials in which a similar tendency of spin-orbit length saturation was observed (e.g., nanowire field effect transistors), and is thus of broad implications in spintronics. Identifying and controlling the Rashba SOC in InSe may serve pivotally in devising III-VI semiconductor-based spintronic devices in the future.

One of the key challenges in condensedmatter physics is to establish a topological superconductor that hosts exotic Majorana fermions. Although various heterostructures consisting of conventional BCS (Bardeen−Cooper−Schrieffer) superconductors as well as doped topological insulators were intensively investigated, no conclusive evidence for Majorana fermions has been provided. This is mainly because of their very low superconducting transition temperatures (T c ) and small superconducting-gap magnitude. Here, we report a possible realization of topological superconductivity at very high temperatures in a hybrid of Bi(110) ultrathin film and copper oxide superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212). Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, we found that three-bilayer-thick Bi(110) on Bi2212 exhibits a proximity-effect-induced s-wave energy gap as large as 7.5 meV which persists up to T c of Bi2212 (85 K). The small Fermi energy and strong spin−orbit coupling of Bi( 110), together with the large pairing gap and high T c , make this system a prime candidate for exploring stable Majorana fermions at very high temperatures.

IV-VI monochalcogenide SnSe or SnS has recently been proposed as a promising two-dimensional (2D) material for valleytronics and thermoelectrics. We report the synthesis of SnSe nanoflakes and nanostructured thin films with chemical vapor deposition method and their thermoelectric properties. As grown SnSe nanostructures are found to be intrinsically p-type and the single SnSe nanoflake field effect transistor was fabricated. By Ag doping, the power factor of SnSe nanostructured thin films can be improved by up to one order of magnitude compared to the “intrinsic” as grown materials. Our work provides an initial step in the pursuit of IV-VI monochalcogenides as novel 2D semiconductors for electronics and thermoelectrics.

VO with a layered van der Waals (vdW) structure has been widely studied because of the material's potential in applications such as battery electrodes. In this work, microelectronic devices were fabricated to study the electrical and optical properties of mechanically exfoliated multilayered VO flakes. Raman spectroscopy was used to determine the crystal structure axes of the nanoflakes and revealed that the intensities of the Raman modes depend strongly on the relative orientation between the crystal axes and the polarization directions of incident/scattered light. Angular dependence of four-probe resistance measured in the van der Pauw (vdP) configuration revealed an in-plane anisotropic resistance ratio of ∼100 between the a and b crystal axes, the largest in-plane transport anisotropy effect experimentally reported for two-dimensional (2D) materials to date. This very large resistance anisotropic ratio is explained by the nonuniform current flow in the vdP measurement and an intrinsic mobility anisotropy ratio of 10 between the a and b crystal axes. Room-temperature electron Hall mobility up to 7 cm/(V s) along the high-mobility direction was obtained. This work demonstrates VO as a layered 2D vdW oxide material with strongly anisotropic optical and electronic properties for novel applications.

The crystal structure of bulk SrTiO3(STO) transitions from cubic to tetragonal at around 105 K. Recent local scanning probe measurements of LaAlO3/SrTiO3 (LAO/STO) interfaces indicated the existence of spatially inhomogeneous electrical current paths and electrostatic potential associated with the structural domain formation in the tetragonal phase of STO. Here we report a study of temperature dependent electronic transport in combination with the polarized light microscopy of structural domains in mesoscopic LAO/STO devices. By reducing the size of the conductive interface to be comparable to that of a single tetragonal domain of STO, the anisotropy of interfacial electron conduction in relationship to the domain wall and its direction was characterized between T = 10–300 K. It was found that the four-point resistance measured with current parallel to the domain wall is larger than the resistance measured perpendicular to the domain wall. This observation is qualitatively consistent with the current diverting effect from a more conductive domain wall within the sample. Among all the samples studied, the maximum resistance ratio found is at least 10 and could be as large as 105 at T = 10 K. This electronic anisotropy may have implications on other oxide hetero-interfaces and the further understanding of electronic/magnetic phenomena found in LAO/STO.