image: a, Temperature-dependent efficient exciton diffusion coefficient. b, Temperature-dependent ultrafast exciton relaxation signal. c, Physical diagram of Phonon-exciton scattering and disorder potential dominated exciton diffusion. d, Physical diagram of ultrafast exciton relaxation. view more
Credit: by Zheyu Fang
Integrated optoelectronic devices, with electrons and photons as energy and information carriers, dominate current daily life and industrial development. However, electron- and photon-based devices are facing unprecedented challenges. On one hand, as charged particles, electrons are sensitive to the dielectric environment, resulting in the difficulty to exceed the nanosecond order for electronic devices. On the other hand, although the response speed of the can be greatly improved in the phonon-based elements, the optical diffraction limit hinders the high integration. Exploring the new generation information device is the key to break through the bottleneck of devices processing speed and integration level.
Excitons are hydrogen-like bosonic quasiparticles with Bohr radius of nanometer dimension, combining advantages of electrons and photons. The transition metal dichalcogenide (TMDC) monolayers provide a great platform for manipulating exciton devices at room temperature. Therefore, excitonic functional devices based on TMDC monolayers at room temperature have attracted intense interest in past decades, as they have promising prospects for overcoming the dilemma of response time and integration in the current generation of electron- or/and photon-based systems. The imperative step towards exciton devices is the control of exciton transport dynamics. However, ultra-short lifetimes and low carrier mobility for two-dimensional (2D) exciton in TMDC monolayers give rise to extremely low exciton diffusion length with 1~2 μm (Phys. Rev. Lett., 120, 207401, 2018). Although interlayer excitons equip a long lifetime, the exciton transport length was still limited at 5 μm (Nature, 560, 340, 2018). The remote lightening that the emission region could be far away (up to 14.6 μm) from excitation centre was achieved by utilizing the femtosecond laser with ultrahigh peak power as the excitation source and the edge region with high photoluminescence efficiency as a bright emitter (ACS Nano, 14, 7897, 2020). Nevertheless, how to further extend exciton diffusion length remains challenging on the road to exciton devices. Up to now, the observed exciton diffusion lengths are far below the theoretical limit. Especially to the monolayer TMDC fabricated by chemical vapor deposition growth and mechanical exfoliation methods, the sample disorders induced by grain boundary, defects and strain are unavoidable, which is of great significance for extending exciton diffusion length and thermal management in exciton based system.
To address the above issue, Fang group investigated the exciton diffusion behavior and nonequilibrium exciton kinetics under phonon scattering and disorder potentials in WSe2 monolayer flake based on the ultrasensitive spectral imaging and femtosecond pump-probe technologies. It is proved that the exciton diffusion and relaxation processes are subjected to the competition between exciton localization in disordered potentials and phonon-exciton scattering. Temperature manipulation can highly optimize the competition to achieve the fold increase of the exciton diffusion coefficient, which is crucial for prolonged exciton transport and thermal management.
In the temperature-dependent measurement, exciton diffusion length can be improved by reducing temperature, while it starts to fall as temperature drops to a critical point of ~280 K, where the effective diffusion coefficient was enhanced by > 200 %. With the temperature decreases, the phonon-exciton scattering effect is prominently attenuated, leading to the enhancement of exciton diffusion coefficient. However, at the same time, the unavoidable ununiform strain and defects induced by the TMDC exfoliate or transfer will produce the disordered modulation on excitons, i.e., producing exciton localization. When the temperature decreases to the point that the thermal fluctuation cannot support excitons escape from the disordered potential well, the exciton localization effect will conquer the exciton-phonon scattering effect and dominate the exciton diffusion process, resulting in the decrease of the exciton diffusion coefficient. The 2D exciton diffusion dynamics dominated by the competition of phonon scattering and disorder potentials was revealed by the femtosecond pump-probe measurement. For temporal relaxation, as the temperature decreases, the phonon-exciton scattering induced exciton recombination is weakened, while the exciton localization relaxation channel caused by disorder potential emerges, and the onset of exciton localization brings forward as decreasing temperature. It indicates that disorder potentials are crucial to exciton dynamics of high-quality monolayer flakes at low temperature, where exciton localization and phonon-exciton scattering compete to dominate the exciton diffusion and relaxation. Extending exciton diffusion length and optimizing thermal management in an exciton-based system can be achieved through balance these two effects by rationally manipulate temperature.
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eLight