Jin Myung Kim1,2,Peiwen Ma1,SungWoo Nam1,2
University of California, Irvine1,University of Illinois at Urbana-Champaign2
Jin Myung Kim1,2,Peiwen Ma1,SungWoo Nam1,2
University of California, Irvine1,University of Illinois at Urbana-Champaign2
Atomically-thin van der Waals (vdW) semiconductors, such as transition metal dichalcogenides (TMDs), have garnered significant attentions as a platform for next-generation nano-optics and quantum optoelectronic devices. Strong light-matter interaction and reduced dielectric screening in low dimensionality allow excitons to govern optical responses of two-dimensional (2D) semiconductors. In particular, manipulating nanoscale energy landscape of 2D excitons plays a pivotal role in exciton funneling and confinement at localized energy trap. However, <i>in situ</i> and non-destructive characterizations of nanoscale optical properties persist as a challenge, particularly where optical responses are highly localized at nanoscale regions narrower than optical diffraction-limit. Here, we report nanoscale optical absorption properties of 2D TMDs via photo-induced force microscopy (PiFM). PiFM technique allows us to measure near-field tip-sample dipole interaction induced by coherent laser excitation. First, we employed polarization-controlled visible wavelength laser source to investigate the in-plane excitonic response of 2D vdW semiconductors. Aligning in-plane dipole moment of 2D TMDs with dipole in the metallic tip was achieved by controlling the polarization of incident illumination, which was critical for measuring light-induced dipole interactions. Second, we carried out PiFM mapping and spectroscopy of various monolayer TMDs. Strong resonance absorption peaks of A and B excitons were observed in fixed wavelength PiFM map as well as PiFM spectra with continuous wavelength sweep. Lastly, we analyzed nanoscale strain modulation in wrinkled and buckle delaminated 2D semiconductors. Strain-induced energy shift of A exciton was observed in PiFM hyperspectral line scanning across sub-micron buckle structure, while conventional photoluminescence (PL) scanning was not able to resolve energy shift due to limited spatial resolution. Spatial resolution of the PiFM hyperspectral mapping was revealed to be < 17 nm, indicating its capability of nanoscale optical characterization. These results suggest new opportunities for advanced nano-characterization for nanoscale optical properties of 2D semiconductors beyond optical diffraction limit.