Exciton diffusion and halo effects in monolayer semiconductors

Abstract: We directly monitor exciton propagation in freestanding and SiO2-supported WS2 monolayers through spatially- and time-resolved micro-photoluminescence under ambient conditions. We find highly nonlinear behavior with characteristic, qualitative changes in the spatial profiles of the exciton emission and an effective diffusion coefficient increasing from 0.3 to more than 30 cm2/s, depending on the injected exciton density. Solving the diffusion equation while accounting for Auger recombination allows us to identify and quantitatively understand the main origin of the increase in the observed diffusion coefficient. At elevated excitation densities, the initial Gaussian distribution of the excitons evolves into long-lived halo shapes with micrometer-scale diameter, indicating additional memory effects in the exciton dynamics.

Exciton Diffusion and Halo Effects in Monolayer Semiconductors

The study conducted by Kulig et al. investigates the dynamics of exciton propagation in monolayer WS(_2), an atomically thin semiconductor. Using spatially and time-resolved micro-photoluminescence techniques under ambient conditions, the researchers provide a detailed analysis of exciton transport in both freestanding and SiO(_2)-supported WS(_2) monolayers. Their findings illustrate a highly nonlinear exciton behavior characterized by significant variations in the effective diffusion coefficient and spatial emission profiles, which are contingent upon the exciton densities.

Key Findings and Analysis

1. Nonlinear Exciton Propagation:

   • The diffusion of excitons exhibits nonlinearity, with the diffusion coefficient (D_{\rm eff}) reported to increase from 0.3 to over 30 cm(^2)/s. These variations are associated with the density of injected excitons.
   • At low exciton densities, the measured diffusion coefficient converges around 0.3 cm(^2)/s. This coefficient transitions drastically when carrier densities elevate, suggesting interactions such as Auger recombination as key contributing factors.

2. Auger Recombination Effects:

   • Auger recombination significantly influences the exciton propagation. With increasing exciton density, the nonradiative Auger processes become pronounced, causing fast recombination at the center of the excitation spots. This process leads to flatter or even double-peak profiles in the photoluminescence cross-section, indicating enhanced effective excitonic diffusion.

3. Observations of Halolike Emission:

   • An unconventional pattern arises where the exciton emission evolves into halolike shapes with micrometer-scale diameters. These halos persist for notable durations and exhibit slow spatial expansion.
   • While the primary mechanisms driving halo formation remain speculative, it is plausible that memory effects related to the Auger processes, effectively influencing energy distribution and scattering events in excitons, play pivotal roles.

4. Modeling Exciton Behavior:

   • The study employs a diffusion model that incorporates Auger recombination to simulate and validate the experimental results. With fixed parameters reflecting low exciton densities, simulations show reasonable alignment with the observed nonlinearity and effective diffusion trends in the measured data.

Implications and Future Directions

This research provides critical insights into excitonic behavior in monolayer semiconductors, which are promising candidates for next-generation optoelectronic and quantum devices. Understanding exciton dynamics is essential for manipulating excitonic currents and interactions in these materials. The findings underscore the need for more comprehensive models that might incorporate temperature effects, complex scattering mechanisms, and multi-component interactions to refine our understanding of exciton transport phenomena.

In terms of potential applications, halo formation phenomena offer fascinating possibilities for engineering spatial light emission characteristics in ultrathin materials, paving the way for technological advancements in photonics and polaritonics. Further research into three-dimensional exciton systems, encapsulated heterostructures, or exciton behavior in other TMDCs could yield enhanced control over excitonic properties, stimulating further exploration in the field of condensed matter physics and material science.