Thermal assisted transport of biexcitons in monolayer WSe2

TL;DR

The paper investigates high-density biexciton transport in a $WSe_2$ monolayer encapsulated by $hBN$, using spatially and temporally resolved photoluminescence and hyperspectral imaging. A hot biexciton gas creates a radial Seebeck current, producing halo diffusion and lattice heating, with biexciton temperature dynamics observable on sub-100 ps timescales. A simple Seebeck-based diffusion model qualitatively reproduces the halos and temperature gradients, linking halo formation to high-energy populations in 2D TMDs. These results underscore the importance of energetic excitations in excitonic transport and have implications for nano-optoelectronic devices employing multi-excitonic states in TMDs.

Abstract

Studies of excitonic transport in transition metal dichalcogenide monolayers have attracted increasing interest in recent years in order to develop nano-optoelectronic devices made with 2D materials. These studies began with low to moderate optical excitation regimes, and more recently have focused on high injection regimes where nonlinear effects appear. This article is focused on the transport of biexcitons by spatially and temporally resolved photoluminescence spectroscopy at high excitation flux. The study is carried out on a high-quality WSe$_2$ monolayer encapsulated in hexagonal boron nitride. The results show that a Seebeck current affects transport in connection with the presence of hot biexcitons. In particular, we observe the formation of spatial rings, also called halos, which have been observed in other excitonic gases. These results tend to generalize the importance of high-energy populations in excitonic transport in TMD, even for complex and heavy excitonic particles.

Figures (11)

• Figure 1: (a) PL spectra of a $WSe_2$ monolayer as a function of excitation power (cw-633nm laser). Spectra are shifted vertically for ease of reading. The dashed line at 1$\mu W$ is a guide for the eyes reproducing a lack of data due to a detector artifact (b) Double logarithmic plot of integrated PL intensity versus cw excitation power for $X^0$ (blue circles), $XX$ (red circles) and $X^D$ (black circles). The lines represent $I^{PL}\propto P^\alpha$ with $\alpha = 2.1$ for $XX$, $\alpha = 1.4$ for $X^0$ and $\alpha = 1.15$ for $X^D$. Note that beyond $500~\mu W$ it is no longer possible to isolate the dark exciton into the PL spectrum. (c) PL spectra as a function of excitation average power (pulsed-700nm laser). Spectra are shifted vertically for ease of reading. (d) Scheme of the excitonic energy levels involved in the radiative recombination process.
• Figure 2: (a) Two-dimensional images of PL after excitation with the pulsed Ti-Sa laser at $700~nm$. Four average excitation power ranges are shown, from $50~\mu W$ to $10~mW$. (b) PL intensity profile normalized to the intensity maximum from a cross-section along the Y axis. Profiles are offset in intensity for clarity. (c) and (d) Time-resolved PL intensity profiles at $0.5~mW$ and $10~mW$. The spatial profiles are normalized for each time step.
• Figure 3: (a) time evolution of the biexciton temperature and (b) time evolution of the biexciton energy shift extracted from time resolved PL spectra of the biexciton for three different excitation powers.
• Figure 4: (a) spatial profiles of the biexciton temperature and (b) spatial profile of the biexciton peak energy shift extracted from energy-resolved biexciton PL profile (hyperspectral image) performed at 3 different excitation powers.
• Figure 5: Evolution of the PL peak energy for the exciton (blue) and the biexciton (red) as a function of the HeNe laser power.