Microcavity coupled interlayer excitons in MOSE2-WSE2 heterostructures

Abstract

After the discovery of graphene, two-dimensional (2D) materials gained immense attention due to their exceptional mechanical, optical, and electronic properties. One of the well-known family of 2D materials is transition metal dichalcogenides (TMDs). Their electronic bandgap makes a transition from indirect to direct when the monolayer limit is reached, making them an excellent medium for studying many-body interactions in condensed matter and light-matter interactions. Also, stacking monolayer TMDs on top of each other enables the creation of heterostructures (HS). The vertical van der Waals heterostructures made from monolayer TMDs can host two types of excitons: one is the intralayer excitons consisting of the strongly Coulomb-bound electron-hole pair within the same material, and the other one is the interlayer exciton made up by spatially separated electrons and holes located in different layers. Intralayer excitons in TMDs exhibit higher oscillator strengths due to spatial confinement. Furthermore, coupling intra- and interlayer excitons with optical cavities allows for observing anticrossing phenomena between excitons and cavity photons. Consequently, TMDs and their HSs provide a versatile platform for investigating exciton-polariton formation and their associated photophysical properties. This thesis presents a detailed study of the room- and low-temperature photoluminescence (PL) spectroscopy of interlayer excitons (IEX) in near-hexagonal MoSe2-WSe2 HSs. The studied hexagonal boron nitride (hBN) encapsulated heterostructures were fabricated using a combination of three methods: (i) mechanical exfoliation for cleaving the monolayers from bulk material, (ii) dry transfer technique to stack them vertically to achieve the heterostructure and (iii) edge identification method to adjust the twist angle between monolayers during stacking. Fabry-Pérot planar microcavities were fabricated for both room and low-temperature studies with different cavity modes using plasma-enhanced chemical vapor deposition with the aim of studying the interlayer exciton-polaritons. Fabricated cavities were further characterized by transmission electron microscopy and focused ion beam lithography for imaging the alternating layers of distributed Bragg mirrors (DBRs) and their thicknesses. The PL of IEXs in MoSe2-WSe2 heterostructures were measured both at room and low temperatures. Low-temperature PL measurements were conducted using a closed-cycle cryostat integrated into a home-built micro-PL setup. The results indicate that our heterostructures exhibit a near-hexagonal structure. The intensities of the spin-triplet and the spin-singlet states of the IEXs are prominent in the energy range of approximately 1.35-1.42 eV. The observed splitting between the spin-triplet and spin-singlet IEXs was in the range of 27-34 meV. Further investigations into the temperature and pump power dependence of the IEXs revealed that the intensity of the spin-triplet IEX is highly sensitive to temperature variations between 3.5 K and 50 K, becoming less intense at higher temperatures. Although the intensity of the spin-singlet IEX also decreased, it remained detectable at 50 K. With increasing pump power, a blueshift of 15 meV was observed for the spin-triplet IEX, while the spin-singlet IEX exhibited a blueshift of only 3 meV in the same pump power range. This indicates that the density of the spin-triplet state IEX increases more significantly with higher pump power than the spin-singlet state IEX. Additionally, the lifetime of the spin-singlet IEX was measured using Time-Resolved Photoluminescence (TRPL) spectroscopy. The fast and slow decay components were in the range of a few nanoseconds and a few tens of nanoseconds, respectively. Moreover, an approximately 9 meV splitting on the spin-singlet IEX PL emission was observed in one of the studied emitters, which has the same resonance wavelength as the FP cavity. This splitting might be attributed to the interlayer exciton-polariton formation at k|| equals zero. However, the whole angle-resolved PL spectrum should be measured to ensure this emission belongs to the polariton formation.