Recently studied two-dimensional systems such as semiconducting transition metal dichalcogenides (TMDCs) with finite bandgaps have been shown to be very promising for ultrathin and flexible energy conversion and storage devices. Due to the parallel bands in their density of states or the so-called band nesting effect, these atomically thin materials present strong optical responses even for excitation energies far exceeding their bandgaps. However, the fundamental photophysical mechanism and detailed processes for the high-energy hot-carrier relaxation and follow-up extraction in single-layer TDMCs have not yet been elucidated, and could be crucial to understand the work mechanisms in related two-dimensional nanophotonic devices.
Prof. Hong-Bo Sun and Hai-Yu Wang’s research group at Center for Ultrafast Optoelectronic Technologies, State Key Laboratory on Integrated Optoelectronics, Jilin University, has unraveled the generation and relaxation mechanism of photo-induced hot carriers in the high-energy C-exciton state, namely C-exciton hot carriers, in a model two- dimensional TMDC system-MoS2 monolayer, and demonstrated the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80% in MoS2 monolayer/graphene heterostructures. Their finding was published in Nature Communications [Nature Commun. 8, 13906 (2017)] entitled “Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer”.
A fundamental photophysical model in MoS2monolayer is established by carefully analyzing the excited-state relaxation processes, especially the hot-carrier cooling and initial extraction for the C-exciton state in the band nesting region. A two-body recombination mechanism for the C-exciton hot-carrier relaxation is proposed. In addition, when using graphene as the electron acceptor, an initial carrier extraction efficiency of 93%, 81%, and 80% for the A-, B-, and C-exciton states, respectively, is evaluated. Furthermore, a two-fold reduction in the exciton binding energy is identified by scanning tunneling spectroscopic analysis. This study facilitates to probe the fundamental photophysics of the hot-carrier cooling and initial extraction in MoS2 monolayers, which could help understand the essential operating mechanisms of related two-dimensional devices.