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Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time. [ABSTRACT FROM AUTHOR]

Atomic defects associated with vacancies in two-dimensional transition metal dichalcogenide monolayers efficiently trap charged carriers and strongly localize excitons. Defects in semiconducting monolayers are seldomly utilized for enhancing optical phenomena, although they may provide resonant intermediate states within the energy band gap for applications with multiphoton excitations, like highly efficient and thermally robust photon upconversion. In an MoS 2 monolayer encapsulated by hBN with high defect and resident electron densities, we observe an upconversion of localized exciton (X L ) emission with a huge energy gain of up to 290 meV. The upconverted X L emission is robust up to temperatures of about 120 K and exhibits a sublinear or a nearly linear laser power dependence for the energy gain of about 100 meV and above 200 meV, respectively. The upconversion mechanism is explained by a cooperative energy transfer process between the photocreated and resident electrons, in which hybridized pairs of single sulfur vacancies likely act as real intermediate states. Additionally, we find a weak upconversion of the neutral exciton photoluminescence with an energy gain of about 350 meV for quasi-resonant excitation of the X L exciton. It is attributed to a two-step, two-photon absorption., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Transition metal dichalcogenide monolayers represent unique platforms for studying both electronic and phononic interactions as well as intra- and intervalley exciton complexes. Here, we investigate the upconversion of exciton photoluminescence in MoSe 2 monolayers. Within the nominal transparency window of MoSe 2 the exciton emission is enhanced for resonantly addressing the spin-singlet negative trion and neutral biexciton at a few tens of meV below the neutral exciton transition. We identify that the A' 1 optical phonon at the K valley provides the energy gain in the upconversion process at the trion resonance, while ZA(K) phonons with their spin- and valley-switching properties support the biexciton driven upconversion of the exciton emission. Interestingly, the latter upconversion process yields unpolarized exciton photoluminescence, while the former also leads to circularly polarized emission. Our study highlights high-order exciton complexes interacting with optical and acoustic K-valley phonons and upconverting light into the bright exciton.

Semiconducting monolayers of transition-metal dichalcogenides are outstanding platforms to study both electronic and phononic interactions as well as intra- and intervalley excitons and trions. These excitonic complexes are optically either active (bright) or inactive (dark) due to selection rules from spin or momentum conservation. Exploring ways of brightening dark excitons and trions has strongly been pursued in semiconductor physics. Here, we report on a mechanism in which a dark intervalley exciton upconverts light into a bright intravalley exciton in hBN-encapsulated WSe 2 monolayers. Excitation spectra of upconverted photoluminescence reveals resonances at energies 34.5 and 46.0 meV below the neutral exciton in the nominal WSe 2 transparency range. The required energy gains are theoretically explained by cooling of resident electrons or by exciton scattering with Λ- or K -valley phonons. Accordingly, an elevated temperature and a moderate concentration of resident electrons are necessary for observing the upconversion resonances. The interaction process observed between the inter- and intravalley excitons elucidates the importance of dark excitons for the optics of two-dimensional materials.

We have examined the influence of flake-substrate effects that affect one and few layers of MoS 2 in terms of their electrical and optical properties. In the measurements, we used SiO 2 /Si substrates with etched cavities and aluminum electrodes. Suspended areas are easily identifiable both on images depicting the topography and on the surface potential maps measured with the Kelvin probe force microscopy. Compared to the SiO 2 /Si supported material, surface potential decrease is observable at the membrane. The surface potential value of the flakes located on the electrodes is the lowest. PL measurements prove that single MoS 2 monolayer was obtained. Suspended regions are also correlated with maps obtained as a result of Raman spectroscopy.