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.

Direct observation of the conformational transitions of single pyridine molecules on a Ag(110) surface induced by long-range repulsive intermolecular interactions.

The transition between two conformations of pyridine molecules adsorbed on a Ag(110) surface at 13 K was investigated by performing single-molecule manipulation at a very low coverage and the track-imaging of pyridines for various surface coverages using a variable low-temperature scanning tunneling microscope. A single tilted conformer was converted to an upright conformer when another coadsorbed tilted pyridine molecule approached to within ∼2 nm. The conversion probability depends on the molecular separation. The tilted conformers that are prevalent at a very low coverage were converted to upright conformers with an increasing surface coverage. The minimum molecular separation before this transition is induced was determined to be 2.2 nm using molecular track-imaging and statistical analysis of the pyridine separation as a function of the molecular coverage. The conformation transition was attributed to substrate-mediated long-range repulsive interactions between the pyridine molecules, which are produced by charge redistribution that occurs upon pyridine adsorption on the silver surface.

Conductive fiber-based ultrasensitive textile pressure sensor for wearable electronics.

A flexible and sensitive textile-based pressure sensor is developed using highly conductive fibers coated with dielectric rubber materials. The pressure sensor exhibits superior sensitivity, very fast response time, and high stability, compared with previous textile-based pressure sensors. By using a weaving method, the pressure sensor can be applied to make smart gloves and clothes that can control machines wirelessly as human-machine interfaces.