Abstract
Excitons play a dominant role in the optoelectronic properties of atomically thin van der Waals (vdW) semiconductors. These excitons are amenable to on-demand engineering with diverse control knobs, including dielectric screening, interlayer hybridization, and moiré potentials. However, external stimuli frequently yield heterogeneous excitonic responses at the nano- and meso-scales, making their spatial characterization with conventional diffraction-limited optics a formidable task. Here, we use a scattering-type scanning near-field optical microscope (s-SNOM) to acquire exciton spectra in atomically thin transition metal dichalcogenide microcrystals with previously unattainable 20 nm resolution. Our nano-optical data revealed material- and stacking-dependent exciton spectra of MoSe2, WSe2, and their heterostructures. Furthermore, we extracted the complex dielectric function of these prototypical vdW semiconductors. s-SNOM hyperspectral images uncovered how the dielectric screening modifies excitons at length scales as short as few nanometers. This work paves the way towards understanding and manipulation of excitons in atomically thin layers at the nanoscale.
Highlights
Excitons play a dominant role in the optoelectronic properties of atomically thin van der Waals semiconductors
Local absorption spectra acquired by scattering-type scanning near-field optical microscopy (s-SNOM) allow one to obtain the complex dielectric function[20], εðωÞ 1⁄4 ε1ðωÞ þ iε2ðωÞ and make it possible to probe the engineered excitons at the nanoscale
The exciton resonance energy, radiative lifetime, and the damping rate in van der Waals (vdW) monolayers were extracted from the scattering-type scanning near-field optical microscope (s-SNOM) spectra with hitherto unattainable spatial resolution of 20 nm
Summary
The light backscattered from the tip is registered by a silicon detector and demodulated at the high harmonics of the tip-tapping frequency via the pseudo-heterodyne interferometric scheme This demodulation method allows one to isolate the genuine near-field signals from the undesired far-field background[25,26]. In this energy range, the near-field signal from h-BN is frequencyindependent[27] and can be utilized as reference for spectroscopic data for TMD materials. E by setting the excitation photon energy at 1.68 eV At this energy, a strong phase contrast between the WSe2 monolayer and h-BN substrate is observed (Fig. 1e). For quantitative analysis of the WSe2 monolayer excitonic response, we carried out raster-scanned nano-imaging while a c
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