We report on the near-field imaging of atomically thin layers of two-dimensional (2D) materials using photoinduced force mapping. This is accomplished by modifying a traditional atomic force microscopy setup to detect optical forces between a nanoscale tip and photoexcited sample. Our setup facilitates the imaging of few-layer flakes of MoS 2 or WS 2 and acquire optical force spectra, both in air and vacuum. The evaluated force spectra in both samples, exhibit the characteristic excitonic resonance peaks that are most typically observed in far-field absorption spectroscopy. We also show that nanoscale defect sites and flake edges can be distinguished from the crystalline flakes with high spectral resolution. Our results pave the way towards gaining a wholesome understanding of optical interactions and structure-property correlations in 2D materials and their heterostructures. Two-dimensional (2D) sheets of atomically thin Van der Waals-bonded crystals have recently opened up new avenues for studying light-matter interactions at the nanoscale. A wide array of candidates including graphene and a palette of transition metal dichalcagonides (TMDs), have fueled the tremendous surge of scientific interest in 2D materials, which demonstrate remarkable optoelectronic
We present a specific near-field configuration where an electrostatic force gradient is found to strongly enhance the optomechanical driving of an atomic force microscope cantilever sensor. It is shown that incident photons generate a photothermal effect which couples with electrostatic fields even at tip-surface separations as large as several wavelengths, dominating the cantilever dynamics. The effect is the result of resonant phenomena where the photothermal-induced parametric driving acts conjointly (or against, depending on electric field direction) with a photovoltage generation in the cantilever. The results are achieved experimentally in an atomic force microscope operating in vacuum and explained theoretically through numerical simulations of the equation of motion of the cantilever. Intrinsic electrostatic effects arising from electronic work-function difference of tip and surface are also highlighted. The findings are readily relevant for other opto-micromechanical systems where electrostatic force gradients can be implemented.
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