Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film.

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO2) in monolayer molybdenum disulfide (MoS2) when interfacing with a VO2 film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS2 directly synthesized on a SiO2/Si substrate by chemical vapor deposition was first transferred onto a VO2/c-Al2O3 substrate in which the VO2 film was prepared by a sputtering method. We compared the MoS2 interfaced with the VO2 film with the as-synthesized MoS2 by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E2g1 and A1g modes in the monolayer MoS2. Specifically, for the as-synthesized MoS2, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E2g1 and A1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E2g1 and A1g modes for the MoS2 transferred onto VO2 were clearly observed across the phase transition of VO2, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS2/VO2 interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.

Ultraviolet Wavelength-Dependent Optoelectronic Properties in Two-Dimensional NbSe2-WSe2 van der Waals Heterojunction-Based Field-Effect Transistors.

Atomically thin two-dimensional (2D) van der Waals (vdW) heterostructures are one of the very important research issues for stacked optoelectronic device applications. In this study, using the transferred and stacked NbSe2-WSe2 films as electrodes and a channel, we fabricated the field-effect transistor (FET) devices based on 2D-2D vdW metal-semiconductor heterojunctions (HJs) and systematically studied their ultraviolet (UV) wavelength-dependent electrical and photoresponse properties. Upon the exposure to UV light with a wavelength of 365 nm, the NbSe2-WSe2 vdW HJFET devices exhibited threshold voltage shift toward positive gate bias direction, a current increase, and a nonlinear photocurrent increase upon applying a gate bias due to the contribution of the photogenerated hole current. In contrast, for the 254 nm UV-irradiated FET devices, the drain current was decreased dramatically and the threshold voltage was negatively shifted. The time-resolved photoresponse properties showed that the device current after turning off the 254 nm UV light was completely and much more rapidly recovered compared with the case of the persistent photocurrent after turning off the 365 nm UV light. Interestingly, we found that the wettability of the WSe2 surface was changed with increasing irradiation time only after 254 nm UV irradiation. The measured wetting behavior on the WSe2 surface provided direct evidence that the experimentally observed UV-wavelength-dependent phenomena was attributed to the UV-induced dissociative adsorption of oxygen and water molecules, leading to the modulation of charge trap states on the photogenerated and intrinsic carriers in the p-type WSe2 channel. This study will help provide an understanding of the influence of environmental and electrical measurement conditions on the electrical and optical properties of 2D-2D vdW HJ devices for a variety of device applications through the stacking of 2D heterostructures.

Metastable state-induced consecutive step-like negative differential resistance behaviors in single crystalline VO2 nanobeams.

We demonstrate the current-dependent consecutive appearance of two different negative differential resistance (NDR) transitions in a single crystalline VO2 nanobeam epitaxially grown on a c-cut sapphire substrate. It is revealed that the first NDR occurs at an approximately constant current level as a result of the carrier injection-induced transition, independent of a thermally induced phase transition. In contrast, it is observed that the second NDR exhibits a temperature-dependent behavior and current values triggering the metal-insulator transition (MIT) are strongly mediated by Joule heating effects in a phase coexisting temperature range. Moreover, we find that the electrically and thermally triggered MIT behavior can be closely related with the alternate occurrence of current-induced multiple insulating and metallic phase coexistence in the nanobeam. These findings indicate that the current density passing through VO2 plays a critical role in both the electrical and structural phase transitions.

The influence of interfacial tensile strain on the charge transport characteristics of MoS2-based vertical heterojunction devices.

We demonstrate the charge transport characteristics of MoS2-based vertical heterojunction devices through the formation of interfacial strain. Atomically thin MoS2 bilayers were directly synthesized on a p-type Si substrate by using chemical vapor deposition to introduce an interfacial tensile strain in the vertical heterojunction diode structure, which was confirmed by Raman, X-ray and ultraviolet photoelectron spectroscopy techniques. The electrical and optoelectronic properties of the heterojunction devices with the as-grown MoS2 (A-MoS2) on p-Si were compared with those of transferred MoS2 (T-MoS2)/p-Si devices. To clearly understand the charge transport characteristics induced by the interfacial tensile strain, the Fowler-Nordheim (FN) analysis of the electrical properties of the diode devices was conducted with the corresponding energy band diagrams. All of the fabricated MoS2-based vertical diodes exhibited clearly rectifying behaviors, but the photoresponse properties of the A-MoS2-based and T-MoS2-based heterojunctions exhibited distinct differences. Interestingly, we found that the tunneling barrier heights of the A-MoS2-based heterojunction devices were relatively higher than those of the T-MoS2-based devices and were almost the same before and after illumination due to the interfacial tensile strain, whereas those of the T-MoS2-based devices were lowered after illumination. Our study will help further understand the charge transport properties of 2D material-based heterojunction devices in the presence of interfacial strain, ultimately enabling the design of electronic and optoelectronic devices with novel functionalities.

Hot carrier-selective chemical reactions on Ag(110).

Here, we show that the pathways, products, and efficiencies of reactions occurring on a metal surface can be spatially modulated by varying the type and energy of hot carriers produced by injecting tunneling electrons or holes from a scanning tunneling microscope tip into the metal surface. Control over the metal surface reactions was demonstrated for the large-scale dissociation reaction of O2 molecules on a Ag(110) surface. Hot electrons (or holes) transported through the metal surface to chemisorbed O2 selectively dissociated the molecule into two oxygen atoms separated along the [110] (or [001]) lattice direction. The reaction selectivity was enhanced compared to the selectivity of a direct reaction involving tunneling carriers.