Exciton Radiative Lifetimes in Two-Dimensional Transition Metal Dichalcogenides

Abstract

Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show firstprinciples calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX 2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS 2 and in two MX 2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS 2 . The MoS 2 /WS 2 and MoSe 2 /WSe 2 heterobilayers exhibit very long-lived (∼20−30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. KEYWORDS: Monolayer materials, transition metal dichalcogenides, luminescence, radiative lifetime, excitons, optoelectronics T wo-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for ultrathin electronic, optoelectronic, photocatalytic, and photovoltaic devices. 1−8 Out of approximately 40 existing TMDs, 9,10 some have received particular attention due to their semiconducting nature and tunable band gap. In particular, group 6 monolayer TMDs with chemical formula MX 2 (M = Mo, W and X = S, Se) are direct gap semiconductors with relatively intense photoluminescence (PL), while bilayers and thicker multilayers exhibit indirect gap and weaker PL. 1,10−17 The peculiar nature of the excited states has stimulated intense research efforts to investigate exciton dynamics and radiative/nonradiative lifetimes in TMDs. 13,18−24 Recent time-resolved experiments found a range of characteristic times for exciton dynamics in monolayer TMDs, including fast (1−10 ps) recombination attributed to exciton trapping at defects, and slower processes on a 0.1−1 ns time scale interpreted as radiative exciton recombination. 18,20−22 However, the attribution of the observed signals to radiatiave and nonradiative processes can be ambiguous in time-resolved spectroscopies since defects and impurities can modulate the excited state dynamics. In TMDs, the interpretation of time signals and comparison among different experiments is further complicated by the use of micrometer-size flakes where the edges can play a significant role in exciton recombination. This situation has stimulated an ongoing quest for the intrinsic time scale of exciton recombinatio...

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

“…This demonstrates that the low temperature exciton dynamics in treated and non-treated MLs is not dominated by non-radiative recombination on the defects which are suppressed by the acid treatment. This exciton dynamics can be interpeted by a fast radiative decay at low temperatures as a result of the strong exciton oscillator strength in TMDC MLs, which is consistent with recent measurements performed on MoSe 2 and WSe 2 MLs [20], and theoretical predictions [22,23].…”

MoS2/WS2 Heterojunction for Photoelectrochemical Water Oxidation

“…26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 Indeed, it has been observed and predicted that interlayer excitons in bilayer heterostructures present much longer lifetimes, on the orders of nanoseconds. 10 Specifically, in a MoSe 2 /WSe 2 bilayer heterostructure, 58 electrons localized on the MoSe 2 and holes on the WSe 2 monolayer can have life times over ~ 1.8 ns at 20 K in non-bias conditions, which is 10 times longer than lifetimes in defect-free monolayer material. 10 Although these exciton lifetimes are estimated without any bias applied, they provide an indication of charge carrier dynamics.…”

“…10 Specifically, in a MoSe 2 /WSe 2 bilayer heterostructure, 58 electrons localized on the MoSe 2 and holes on the WSe 2 monolayer can have life times over ~ 1.8 ns at 20 K in non-bias conditions, which is 10 times longer than lifetimes in defect-free monolayer material. 10 Although these exciton lifetimes are estimated without any bias applied, they provide an indication of charge carrier dynamics. CA scans of bulk heterojunctions (Figure 3b) show similar features as those of the individual constituents.…”

“…These interactions arise from a combination of their intrinsic two-dimensionality, without dangling-bonds, their delectron orbital character, and their anisotropic structure. 8,9 They absorb 5-10% of incident light in the visible range, [10][11][12] and mechanically exfoliated flakes have shown photovoltaic characteristics. 13,14 A range of optoelectronic devices, predominantly based on isolated flakes have been demonstrated, 15,16 including photodiodes, 12,17 light emitting devices, 18 and photodetectors.…”

Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides

“…The emission linewidth was determined to be about 100-120 µeV for freestanding [84] and supported samples [85][86][87][88]. Luminescence decay times were measured to be in the range of a few ns, comparable to the decay time of the free exciton at room temperature (~4 ns) [89].…”