  We implemented radio frequency-assisted electrostatic force microscopy
(RF-EFM) to investigate the electric field response of biaxially strained
molybdenum disulfide (MoS2) monolayers (MLs) in the form of mesoscopic bubbles,
produced via hydrogen (H)-ion irradiation of the bulk crystal. MoS2 ML, a
semiconducting transition metal dichalcogenide, has recently attracted
significant attention due to its promising optoelectronic properties, further
tunable by strain. Here, we take advantage of the RF excitation to distinguish
the intrinsic quantum capacitance of the strained ML from that due to atomic
scale defects, presumably sulfur vacancies or H-passivated sulfur vacancies. In
fact, at frequencies fRF larger than the inverse defect trapping time, the
defect contribution to the total capacitance and to transport is negligible.
Using RF-EFM at fRF = 300 MHz, we visualize simultaneously the bubble
topography and its quantum capacitance. Our finite-frequency capacitance
imaging technique is non-invasive and nanoscale, and can contribute to the
investigation of time and spatial-dependent phenomena, such as the electron
compressibility in quantum materials, which are difficult to measure by other
methods.
