Indirect excitons in van der Waals heterostructures at room temperature

Abstract

Indirect excitons (IXs) are explored both for studying quantum Bose gases in semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III-V and II-VI semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS 2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.

Interlayer Excitons with Large Optical Amplitudes in Layered van der Waals Materials

“…If the IL distance is increased by more than one Angstrom (or alternatively if a single layer of h-BN is inserted between the MoS 2 layers) our ab-initio calculation leads to dark IL excitations. This is in contrast to experiments of Calman et al 23 in which bright IL states have been reported for bilayer MoS 2 with three h-BN spacer layers.…”

“…Such CT excitons have been observed in heterobilayers with a natural Type II band alignment[20][21][22] and in bilayer MoS 2 where the Type II band alignment is imposed by an electric field applied perpendicular to the bilayer 23. While the spatial separation of the electron and hole in the CT exciton leads to long lifetimes it also causes a small optical transition matrix element making it challenging to probe by them in absorption and luminescence experiments.…”

“…17−19 Such CT excitons have been observed in heterobilayers with a natural Type II band alignment 20−22 and in bilayer MoS 2 where the Type II band alignment is imposed by an electric field applied perpendicular to the bilayer. 23 While the spatial separation of the electron and hole in the CT exciton leads to long lifetimes, it also causes a small optical transition matrix element making it challenging to probe by them in absorption and luminescence experiments. However, interlayer excitons with finite optical amplitudes have been recently identified in bulk MoS 2 , MoSe 2 , and MoTe 2 24,25 (by the different sign of the g-factors in magnetic fields compared to intralayer states).…”

Moiré Phonons in Twisted Bilayer MoS2

“…degrees of freedom to engineer their optical and electronic properties. 4,9−14 The vdWHs of transition metal dichalcogenides (TMDs) are particularly exciting for optoelectronic and light-harvesting applications, 14,15 because many monolayer TMDs are direct-bandgap semiconductors with remarkably strong light−matter interactions. 16,17 In the vdWHs, the two constituents with nearly the same lattice constant are overlaid with a relative twist angle of θ, leading to the generation of moireṕ atterns.…”

Ultrafast pseudospin quantum beats in multilayer WSe$_2$ and MoSe$_2$

“…While in heterobilayers the oscillator strength of IX is weak, the situation can be different for homobilayers or multilayers [13]. In MoS 2 bilayers, strong absorption by IX up to room temperature was reported [14][15][16][17][18][19]. In MoSe 2 , the situation is similar to MoS 2 , though the oscillator strength of the IX is smaller [20].…”