Probing excitonic dark states in single-layer tungsten disulphide

By Z. Ye, Ting Cao, Kevin J. O'Brien, Hanyu Zhu, Xiaobo Yin, Yuan Wang, Steven G. Louie, Xiang Zhang. Nature, 2014

950 citations recorded by the source corpus.

Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light–matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe–Salpeter equation (GW-BSE) approach, which employed many-body Green’s-function methodology to address electron–electron and electron–hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (∼0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures in computing, communication and bio-sensing.

Subfields: Electrical and Electronic Engineering, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/dd56330ada20654d8aebb297e608cb2e85d43035

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Citations in the corpus

Valley excitons in two-dimensional semiconductors (2015, background): “The calculated 2D Bohr radius is about aB~1 nm [17], and the wave function for the electron-hole relative motion extends over several tens unit cells [17,20].”
Hexagonal boron nitride is an indirect bandgap semiconductor (2015, background): “Two-photon spectroscopy is a powerful technique for accessing information on the bound states of the exciton relative motion [22], which has recently regained attention in carbon nanotubes [38, 39] or transition metal dichalcogenide monolayers [40, 41].”
Excitons in atomically thin 2D semiconductors and their applications (2017, background): “These two-photon states do not appear in the linear one-photon absorption, therefore, “dark”, making it possible to use the two-photon absorption and subsequent bright-state luminescence (two-photon luminescence, TPL) to determine the excitonic structure of TMDCs [11].”
Two-dimensional perovskite materials: From synthesis to energy-related applications (2019, background): “For example, monolayer WS2 has exciton binding energy as large as 700 meV [66].”
2D materials and van der Waals heterostructures (2016, background): “The series of Rydberg exciton states above the 1-s (A) exciton of WS2 reveals an exciton series that deviates considerably from the hydrogen model (28, 29).”