Sandhaya Koirala

Long radiative lifetimes of excitons in monolayer transition-metal dichalcogenides MX 2 (M = Mo, W; X = S, Se)

We studied the effective exciton radiative lifetimes of monolayer two-dimensional transition-metal dichalcogenides MX 2 (M = Mo, W; X = S, Se). The photoluminescence (PL) quantum yield and PL decay time were measured for monolayer MoS2, MoSe2, WS2, and WSe2 at room temperature. Effective exciton radiative lifetimes of >10 ns were determined from the PL quantum yield of 10−2 to 10−3 and the PL decay time of several hundred picoseconds. The results are explained on the basis of the long radiative lifetime of bright excitons at a low temperature limit, a finite exciton coherence area of several square nanometers, and the population of dark exciton states.

Photoluminescence quantum yield and long exciton radiative lifetime in monolayer two-dimensional transition metal dichalcogenides

Semiconducting transition metal dichalcogenides (TMDs) have attracted great research interests due to both from fundamental physics and their interesting potential applications of optoelectronic devices. It is very crucial to know its photoluminescence (PL) quantum yield, concurrently with the radiative lifetimes of its elementary excitation, exciton (electron-hole pair). We have experimentally evaluated intrinsic exciton radiative lifetime of 2D (two-dimensional) semiconductor TMDs of monolayer WSe2 from static and time-resolved PL spectroscopy at room temperature. In order to calculate the radiative lifetime, we also employed PL quantum yield measurements using relative method with standard reference fluorescent dye. The exciton radiative lifetime of ~40 ns was evaluated from PL quantum yield of ~1 % and PL decay time of several hundred ps. This value of radiative lifetime of several tenth ns suggests the contribution of dark states and finite exciton coherence length (area) of several nm.