We observe current induced spin transfer torque resonance in permalloy (Py) grown on monolayer MoS2. By passing rf current through the Py/MoS2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Therefore our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

Two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO) material family. Free-standing VO nanomembranes were fabricated by precisely thinning as-grown VO thin films and etching away the sacrificial layer underneath. VO microtubes with a range of controllable diameters were rolled-up from the VO nanomembranes. When a VO nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO material. The magnitude of the compressive strain is determined by the curvature of the VO microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.

Silver dimetal chalcogenides (Ag–V–VI2) are ternary semiconductors that have potential alternative energy applications due to their optimal band gaps and large extinction coefficients. The synthesis of these materials is challenging due to the lack of effective pnictide precursors. We report the use of tris­[N,N-bis­(trimethylsilyl)­amido]antimony (Sb­[N­(SiMe3)2]3) and tris­[N,N-bis­(trimethylsilyl)­amido]bismuth (Bi­[(N­(SiMe3)2]3) to synthesize nanocrystalline AgSbSe2 and AgBiSe2 quantum dots. The use of these reagents results in the creation of high quality nanomaterials with good crystallinity and narrow size distributions. Furthermore, electrical measurements on monolithic pellets of processed AgSbSe2 and AgBiSe2 nanomaterials demonstrate linear current–voltage behavior at room temperature, which indicates potential for use in electrical applications.

Vertical tunneling junctions showing negative differential resistance (NDR) are realized in WS2 homojunction devices. Mono-/multilayered single crystalline WS2 is grown using chemical vapor deposition. NDR is observed through resonant tunneling in Au/bi-layer WS2/Au and Pt/few-layered WS2/Au tunneling junctions by back-gating at room temperature. While two-dimensional materials have been a central focus of materials research during the past decade, exploiting novel properties in diverse layers of these materials is emerging with new designs for electronic devices. Our results pave the way for novel resonant tunneling devices presenting a route to fabricate homojunction WS2 with simple fabrication techniques.

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We report a detailed analysis on the effects of processing parameters for sputtered tungsten trioxide (WO3) thin nanoscale films on their structural, vibrational and electrical properties. The research aims to understand the fundamental aspects of WO3 sputtering at relatively low temperatures and in an oxygen deprived environment targeting applications of temperature and oxygen sensitive substrates. Structural analysis indicates that films deposited at room temperature, or substrate temperatures at or below 400 °C with low oxygen partial pressure are amorphous. Crystallization of the films was observed with distinct Raman peaks when the films were annealed at 300 °C or above using rapid thermal annealing for 10 min. Films revealed monoclinic phases of WO3 with the presence of W–O–W stretching, bending and lattice vibrational modes in the Raman spectra. Interestingly, a change of transport behavior from insulating to semiconducting was observed for as deposited films on post annealing. Annealed films revealed stoichiometric WO3 phases with no external defects detected. The present study adopts a route to intercalate WO3 in a variety of applications from electrochromic coloration to a nanocrystalline thin film for electronic devices sensitive to higher temperatures and gas flow in the sputtering system.

This work reports morphologically alike, high-quality monolayer MoS2 flakes with a similar strain at various growth temperatures (750–900°C) achieved by adjusting sulfur temperature. The growth dynamics of MoS2 are correlated with changes in the photoluminescence (PL) and Raman peak positions. Monolayer MoS2 crystals are synthesized at different growth temperatures from 750°C to 900°C using chemical vapor deposition (CVD). We examined the structural quality and aimed to extract the recombination mechanisms in MoS2 using low-temperature, variable, and low-laser-intensity PL measurements. Our studies of the defect-associated bound exciton emission are well correlated with the blueshift in the A1g mode of Raman spectra, blueshift in PL spectra, and x-ray photoelectron spectroscopy results for crystal grown at 900°C. Our research findings not only shed light on a thorough, non-intrusive method for modifying growth parameters to enhance optical performance, but they also suggest a way to modify the optical characteristics of MoS2 while maintaining the morphology.

We report successful fabrication of high performance ion-gated field-effect transistors (FETs) on hydrogenated diamond surface. Investigations on the hydrogen (H)-terminated diamond by Hall effect measurements shows Hall mobility as high as ∼200 cm2 V−1 s−1. In addition we demonstrate a rapid fabrication scheme for achieving stable high performance devices useful for determining optimal growth and fabrication conditions. We achieved H-termination using hydrogen plasma treatment with a sheet resistivity as low as ∼1.3 kΩ/sq. Conductivity through the FET channel is studied as a function of bias voltage on the liquid ion-gated electrode from −3.0 to 1.5 V. Stability of the H-terminated diamond surface was studied by varying the substrate temperature up to 350 °C. It was demonstrated that the sheet resistance and carrier densities remain stable over 3 weeks in ambient air atmosphere even at substrate temperatures up to 350 °C, whereas increasing temperature beyond this limit has effected hydrogenation. This study opens new avenues for carrying out fundamental research on diamond FET devices with ease of fabrication and high throughput.