Ultrafast broadband spectroscopy of widely spread excitonic features in WSe2 nanosheets†

Abstract

The performance of an optoelectronic device is largely dependent on the light harvesting properties of the active material as well as the dynamic behaviour of the photoexcited charge carriers upon absorption of light. Recently, atomically thin two-dimensional transition metal dichalcogenides (2D TMDCs) have garnered attention as highly prospective materials for advanced ultrathin solar cells and other optoelectronic applications, owing to their strong interaction with electromagnetic radiation, substantial optical conductivity, and impressive charge carrier mobility. WSe₂ is one such extremely promising solar energy material. It has absorption throughout the UV-Vis-NIR region with the existence of four excitonic features, just like MoS₂ and WS₂. However, stability issues and the absence of any robust synthetic route limit their practical applications. Herein, we have successfully synthesized atomically thin stable WSe₂ nanosheets using a very effective colloidal hot injection method and further studied the optical properties of this material using femtosecond transient absorption spectroscopy. We probed all four excitonic features of WSe₂ spread throughout the visible region. The dynamics of the high-energy excitons were found to be distinctively slower when compared to their band edge counterparts, adding an additional advantage in optoelectronic applications. We delved further into the factors governing exciton dynamics within WSe₂, uncovering the strong influence of the electronic band structure. Importantly, our study highlights the importance of all four excitonic features in a 2D TMDC material, which emerge in the system irrespective of the excitation wavelength and influence each other.