Long-Lived Direct and Indirect Interlayer Excitons in van der Waals Heterostructures

Abstract

We investigate the photoluminescence of interlayer excitons in heterostructures consisting of monolayer MoSe2 and WSe2 at low temperatures. Surprisingly, we find a doublet structure for such interlayer excitons. Both peaks exhibit long photoluminescence lifetimes of several ten nanoseconds up to 100 ns at low temperatures, which verifies the interlayer nature of both.The peak energy and linewidth of both show unusual temperature and power dependences.In particular, we observe a blue-shift of their emission energy for increasing excitation powers.At a low excitation power and low temperatures, the energetically higher peak shows several spikes. We explain the findings by two sorts of interlayer excitons; one that is indirect in real space but direct in reciprocal space, and the other one being indirect in both spaces. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions.Keywords: van der Waals heterostructure, indirect excitons, interlayer exciton, photoluminescence, exciton lifetime, transition metal dichalcogenides; 2 Semiconductor heterostructures (HS) are very often the foundation for the observation of novel phenomena in both fundamental science as well as device applications. The electrical and optical properties of such HS can be engineered in a wide range resulting in precisely tailored functionalities. Of particular interest are optically active HS facilitating many device applications such as photo-detectors, solar cells, light-emitting diodes or lasers and fostering the observation of many-body driven quantum phenomena found in systems with reduced dimensionality such as quantum wells or quantum dots. Excitons are electron-hole pairs coupled by attractive Coulomb interaction which results in a ground state with a reduced energy compared to the corresponding single-particle energies. Exciton ensembles exhibit an intriguing interaction driven phase diagram with different classical and quantum phases including quantum liquids and solids [1][2][3]. The bosonic nature of excitons enables these composite particles even to condensate into macroscopic ground state wave functions forming a Bose-Einstein condensate (BEC) [4].Ensembles of indirect a.k.a. interlayer excitons (IXs) are particularly fascinating systems to explore such classical and quantum phases of interacting bosonic ensembles. IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electronhole wave functions resulting in dense IX ensembles that are thermalized to the lattice temperature. Besides IX ensembles in III-V HS [5][6][7][8][9][10][11][12][13][14][15], hetero-bilayers prepared from semiconducting transition metal dichalcogenides (TMDs) exhibit superior potential for studying interacting IX ensembles [3,[16][17][18][19][20][21][22][23][24][25] with intriguing spin-and valley-properties [26][27][28]. At the same time, TMDs are truly two-dimensional (2D) crystals coupled to each other or to substrat...

Interlayer Excitons in Transition‐Metal Dichalcogenide Heterobilayers

“…With increasing ILE density, dipole–dipole interaction between ILE (as shown in the inset of Figure b) blueshifts the ILE energy. This effect has previously been studied in coupled quantum well systems and was also observed by several groups in TMDC heterobilayers . In the limit of low ILE density, it is expected to scale linearly with the density.…”

“…Thus, electron–hole pairs that are optically generated in either of the constituent layers become spatially separated within a few tens of femtoseconds . These spatially separated charge carriers can form interlayer excitons (ILEs) on ultrafast timescales, which have been observed directly in photoluminescence (PL) for several TMDC heterobilayer combinations . Such ILEs have already been studied in conventional semiconductor heterostructures, e.g., coupled quantum wells.…”

“…[21,22] These spatially separated charge carriers can form interlayer excitons (ILEs) on ultrafast timescales, [23] which have been observed directly in photoluminescence (PL) for several TMDC heterobilayer combinations. [24][25][26][27][28][29][30][31][32][33] Such ILEs have already been studied in conventional semiconductor heterostructures, e.g., coupled quantum wells. There, different aspects such as excitonexciton interaction [34,35] and bosonic condensation of excitons [36][37][38][39] were investigated, but only at low temperatures, limited by the weak ILE binding energies in these systems.…”

Ab initio nonadiabatic molecular dynamics investigations on the excited carriers in condensed matter systems

“…For MoS 2 /WS 2 , the major peak appears at around 400 cm −1 whereby it redshifts to 230 cm −1 in MoSe 2 /WSe 2 . The major peak in both systems can be assigned to be the A 1 mode by comparing with the Raman scattering measurements and first‐principles calculations . The redshift of A 1 mode in MoSe 2 /WSe 2 is due to the different chalcogen masses and chemical interactions.…”

Multiphonon Raman Scattering in Transition Metal Dichalcogenide Double Layers

“…This means that these excitons are very stable even at room temperatures, which offer the opportunity to study room‐temperature optical properties and the potential excitonic devices . Another property is that electrons and holes are spatially separated in different layers, which gives rise to the drastic reduction of the overlap between the electron and hole wavefunctions and thus their lifetimes are orders of magnitude longer than intralayer excitons in monolayer TMDs . This property holds the potential applications for creating excitonic signal processing devices and excitonic circuits with sufficient lifetime.…”