Dynamics and efficient conversion of excitons to trions in non-uniformly strained monolayer WS2

Abstract

We investigate the transport of excitons and trions in monolayer semiconductor WS 2 subjected to controlled non-uniform mechanical strain. We actively control and tune the strain profiles with an AFM-based setup in which the monolayer is indented by an AFM tip. Optical spectroscopy is used to reveal the dynamics of the excited carriers. The non-uniform strain configuration locally changes the valence and conduction bands of WS 2 , giving rise to effective forces attracting excitons and trions towards the point of maximum strain underneath the AFM tip. We observe large changes in the photoluminescence spectra of WS 2 under strain, which we interpret using a drift-diffusion model. We show that the transport of neutral excitons, a process that was previously thought to be efficient in non-uniformly strained 2D semiconductors and termed as funneling, is negligible at room temperature in contrast to previous observations. Conversely, we discover that redistribution of free carriers under non-uniform strain profiles leads to highly efficient conversion of excitons to trions. Conversion efficiency reaches 100% even without electrical gating. Our results explain inconsistencies in previous experiments and pave the way towards new types of optoelectronic devices.

Exciton-to-trion conversion as a control mechanism for valley polarization in room-temperature monolayer WS2

“…The stronger exciton emission at wrinkles indicates that the doping is less effective, possibly resulting from the restricted physical access to the monolayer at wrinkles or from a decreased substrate-induced doping due to the increased substrate-monolayer distance. In addition, the exciton and trion formation could be altered at the wrinkles as a result of local strain 31 .…”

Tuning the physical properties of ultrathin transition-metal dichalcogenides via strain engineering

“…Reproduced with permission. 23 Copyright 2020, Nature. (f) Schematic of the thermal-expansion induced strain in monolayer MoS 2 .…”

“…In addition, an AFM-based setup is also an effective approach to introduce non-uniform mechanical strain in ultrathin TMDs by indenting the lm with an AFM tip. [23][24][25][26] As shown in Fig. 1e, the WS 2 monolayer suspended on top of a hole in the substrate membrane is indented from the bottom by an AFM cantilever and being optically interrogated from the top, in this way, the dynamics of the excited carriers in strained monolayer WS 2 can be revealed by optical spectroscopy.…”

“…1e, the WS 2 monolayer suspended on top of a hole in the substrate membrane is indented from the bottom by an AFM cantilever and being optically interrogated from the top, in this way, the dynamics of the excited carriers in strained monolayer WS 2 can be revealed by optical spectroscopy. 23 It is worth mentioning that approaches in Fig. 1a-c can only introduce a uniaxial strain, however, the effect of biaxial strain in modifying the physical properties of ultrathin TMDs is obviously stronger than that of uniaxial strain (a more detailed discussion is presented in Section 2.2), thus, experimental realization of biaxial strain is of great signicant.…”

Tip‐Induced Nano‐Engineering of Strain, Bandgap, and Exciton Funneling in 2D Semiconductors

“…[29,30] In addition, most importantly, a wrinkle can act as a nanoscale regulator controlling the exciton funneling and dynamics, for example, exciton flux and energy conversion, in semiconductors for the realization of highly-efficient optoelectronic devices. [31][32][33][34][35] Here, we present a hyperspectral tip-enhanced photoluminescence (TEPL) nano-imaging approach, combined with nano-optomechanical strain control, to investigate and control the nano-optical and -excitonic properties of naturallyformed wrinkles in a WSe 2 ML. In this approach, the excitation field is highly localized at the plasmonic tip by adopting the near-field wavefront shaping technique [36] and the tip selectively probes Purcell-enhanced TEPL emission at wrinkles with <15 nm spatial resolution.…”