Nano-spectroscopy of excitons in atomically thin transition metal dichalcogenides

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 controls, 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.

Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

“…1b) between the two materials, tip-enhanced Raman and PL spectroscopy are necessary to characterize the junction with subwavelength resolution. TEPL and TERS were carried out at room temperature and the spatial resolution, determined by the tip diameter, is estimated to be ≈ 40 nm, far below the measured excitonic diffusion lengths we observed in the individual materials [66][67][68] . For TEPL (TERS) a 633 nm (532 nm) linearly polarized laser was focused onto the silver tip apex using a long working distance x100 objective (0.7 NA).…”

Tunable interlayer excitons and switchable interlayer trions via dynamic near-field cavity

“…However, in order to enable practical applications of TMD heterostructures using IX and IX-, several major challenges must be overcome, one of which is the large degree of spatial heterogeneity. The underlying processes, e.g., competing interactions of coupling, dephasing, and energy transfer of intra-and inter-layer excitons as well as IX-interconversion, arise at the nanoscale and cannot be understood by diffraction-limited optical approaches, calling for the near-field optical probing [14][15][16][17] . Furthermore, beyond probing, it is highly important to achieve nanoscale control of local IX and IX-properties in TMD heterostructures.…”