High-temperature driven inter-valley carrier transfer and significant fluorescence enhancement in multilayer WS2.

It is quite necessary to understand and control the essential carrier dynamic behaviors of two-dimensional WS2, which is regarded as a very promising material for integrated nanoelectronic and optoelectronic devices. Herein, a high-temperature driven carrier transfer process of multilayer WS2 nanoflakes is proposed. The established model, involving the inter-valley transfer process of photocarriers from the Λ/Γ point to the K point, predicts the significant emission enhancement of the K → K direct transition at high temperatures, which is verified by both theoretical calculations and experimental observations. On the one hand, variations in the estimated population ratio of photo-carriers with increasing temperature well support the proposed model. On the other hand, temperature-dependent photoluminescence spectra clearly show that the K → K direct emission intensity is remarkably enhanced by ~300-fold as the temperature is elevated to 760 K. Time-resolved fluorescence spectral studies confirm the occurrence of the inter-valley carrier transfer process in the multilayer WS2. These results provide solid evidence for the proposed inter-valley carrier transfer model. Such a mechanism could not only be applied to improve the luminescence intensity of indirect-band-gap semiconductors, but also be further extended to design optoelectronic devices which have special requirements on controlling the carrier dynamic behavior.