Dark-Exciton Driven Energy Funneling into Dielectric Inhomogeneities in Two-Dimensional Semiconductors

Abstract

The optoelectronic and transport properties of two-dimensional transition metal dichalcogenide semiconductors (2D TMDs) are highly susceptible to external perturbation, enabling precise tailoring of material function through post-synthetic modifications. Here we show that nanoscale inhomogeneities known as nanobubbles can be used for both strain and, less invasively, dielectric tuning of exciton transport in bilayer tungsten disulfide (WSe2). We use ultrasensitive spatiotemporally resolved optical scattering microscopy to directly image exciton transport, revealing that dielectric nanobubbles are surprisingly efficient at funneling and trapping excitons at room temperature, even though the energies of the bright excitons are negligibly affected. Our observations suggest that exciton funneling in dielectric inhomogeneities is driven by momentum-indirect (dark) excitons whose energies are more sensitive to dielectric perturbations than bright excitons. These results reveal a new pathway to control exciton transport in 2D semiconductors with exceptional spatial and energetic precision using dielectric engineering of dark state energetic landscapes. Main text:Two-dimensional transition metal dichalcogenide semiconductors (2D TMDs) are van der Waals materials that hold great promise for nanoscale optoelectronics thanks to their strong light-matter interactions even at the atomically-thin limit. The optoelectronic properties of 2D TMDs are in large part governed by their Coulomb-bound electron-hole pairs (excitons), with relatively large binding energies of up to hundreds of milli-electronvolts (meV) due to weak out-of-plane dielectric screening. [1][2][3][4][5][6] Unlike free charges, excitons are charge neutral and are therefore difficult to manipulate with external electric fields in electronic devices. [7][8][9] Therefore, the transport properties of excitons are largely dictated by random, diffusive motion with no long-range directionality, limiting their use as information and energy carriers. Finding new ways to manipulate exciton transport in 2D TMDs without radically altering other material properties would result in excitonic devices that combine strong light-matter interactions and precise control over energy and information flow in atomically thin materials.An attractive route to controlling the properties of 2D TMDs is to leverage their extreme sensitivity to extrinsic factors such as strain, 10-21 and dielectric screening by the environment (Figure 1a), 5,[22][23][24][25][26] enabling post-synthetic tuning of their optoelectronic and transport properties. For example, tensile strain reduces the optical transition energy of 2D TMDs; 16,18,27,28 localized strain regions thus create energy gradients that can funnel and trap excitons in nanoscale low-energy sites, a process that was leveraged to create long-lived quantum emitters. 14,[29][30][31][32][33] Strain engineering, however, is difficult to control over macroscopic scales and can introduce undesired disorder.A less invasive and in principle more contr...

Tip-Enhanced Dark Exciton Nanoimaging and Local Strain Control in Monolayer WSe2

“…This suggests identical or at least similar mechanisms behind bright and dark exciton enhancement as related to the strain-induced modification of the band structure. This behavior is consistent with that of bright exciton funneling toward areas of greater tensile strain but counter to that of dark excitons funneling to areas of lesser strain. − However, previous funneling work was performed on dielectric surfaces. Our creation of a picocavity and the enhanced out-of-plane field from the tip–substrate geometry suggest that the Purcell enhancement may outcompete with funneling in this case.…”

Ultralocalized Optoelectronic Properties of Nanobubbles in 2D Semiconductors

“…This alignment we believe is critical for near-field observation of inter-layer exciton luminescence, which has substantially weaker oscillator strength. We also expect that strain-localized PL signal is enhanced by funneling effects, where excitons are preferentially shuttled towards the lower energy states [34][35][36] Figure 3c shows point spectra at various locations across the nanobubble edges, identified by the colored markers in Fig. 3a and 3d.…”

Detecting, Distinguishing, and Spatiotemporally Tracking Photogenerated Charge and Heat at the Nanoscale

“…In materials with additional distinct electronic species, the combination of measuring at additional pump energies and fluences to tune the relative densities of distinct photoinduced energy carriers could enable them to be distinguished. Furthermore, a continuously tunable probe source could be leveraged to increase the signal-to-noise ratio of photoexcitations that may not appreciably modify the local dielectric function. , …”