Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors.

Photonic crystal membranes (PCM) provide a versatile planar platform for on-chip implementations of photonic quantum circuits 1-3 . One prominent quantum element is a coupled system consisting of a nanocavity and a single quantum dot (QD) 4-7 which forms a fundamental building block for elaborate quantum information networks 8-10 and a cavity quantum electrodynamic (cQED) system controlled by single photons 3 . So far no fast tuning mechanism is available to achieve control within the system coherence time. Here we demonstrate dynamic tuning by monochromatic coherent acoustic phonons formed by a surface acoustic wave (SAW) with frequencies exceeding 1.7 gigahertz, one order of magnitude faster than alternative approaches 5-7 . We resolve a periodic modulation of the optical mode exceeding eight times its linewidth, preserving both the spatial mode profile and a high quality factor. Since PCMs confine photonic and phononic excitations 11,12 , coupling optical to acoustic frequencies, our technique opens ways towards coherent acoustic control of optomechanical crystals.In basic research SAWs found applications in the investigation of fundamental quantum effects in nanosystems 13-17 , the manipulation of photonic bandgap structures 18 , microcavity and surface plasmon polaritons 19-21 with frequencies spanning from a few megahertz up to a several gigahertz. We electrically generate SAWs by applying a short radio frequency (RF) voltage pulse to interdigital transducer electrodes (IDT) as shown schematically in Fig. 1 (a). As this pulse propagates across the PCM, it dynamically