Single-layer WS$_2$ is a direct-gap semiconductor showing strong excitonic photoluminescence features in the visible spectral range. Here, we present temperature-dependent photoluminescence measurements on mechanically exfoliated single-layer WS$_2$, revealing the existence of neutral and charged excitons at low temperatures as well as at room temperature. By applying a gate voltage, we can electrically control the ratio of excitons and trions and assert a residual n-type doping of our samples. At high excitation densities and low temperatures, an additional peak at energies below the trion dominates the photoluminescence, which we identify as biexciton emission.Comment: 6 pages, 5 figure

Monolayers of transition metal dichalcogenides (TMDCs) feature exceptional optical properties that are dominated by excitons, tightly bound electron-hole pairs. Forming van der Waals heterostructures by deterministically stacking individual monolayers allows to tune various properties via choice of materials [1] and relative orientation of the layers [2, 3]. In these structures, a new type of exciton emerges, where electron and hole are spatially separated. These interlayer excitons [4, 5, 6] allow exploration of many-body quantum phenomena [7, 8] and are ideally suited for valleytronic applications [9]. Mostly, a basic model of fully spatially-separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first principle calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS 2 /WSe 2 heterostructures residing at the Γ and K valleys. We control the emission energy of this new type of momentum-space indirect, yet strongly-bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in TMDC heterostructures and devices.An optical micrograph of a representative MoS 2 /WSe 2 heterobilayer (HB), which was fabricated by deterministic transfer and stacking [10] followed by an annealing procedure, is shown *

We directly monitor exciton propagation in freestanding and SiO_{2}-supported WS_{2} monolayers through spatially and time-resolved microphotoluminescence under ambient conditions. We find a highly nonlinear behavior with characteristic, qualitative changes in the spatial profiles of the exciton emission and an effective diffusion coefficient increasing from 0.3 to more than 30  cm^{2}/s, depending on the injected exciton density. Solving the diffusion equation while accounting for Auger recombination allows us to identify and quantitatively understand the main origin of the increase in the observed diffusion coefficient. At elevated excitation densities, the initial Gaussian distribution of the excitons evolves into long-lived halo shapes with μm-scale diameter, indicating additional memory effects in the exciton dynamics.

In heterostructures consisting of different transition-metal dichalcogenide monolayers, a staggered band alignment can occur, leading to rapid charge separation of optically generated electron-hole pairs into opposite monolayers. These spatially separated electron-hole pairs are Coulomb-coupled and form interlayer excitons. Here, we study these interlayer excitons in a heterostructure consisting of MoSe2 and WSe2 monolayers using photoluminescence spectroscopy. We observe a non-trivial temperature dependence of the linewidth and the peak energy of the interlayer exciton, including an unusually strong initial redshift of the transition with temperature, as well as a pronounced blueshift of the emission energy with increasing excitation power. By combining these observations with time-resolved photoluminescence measurements, we are able to explain the observed behavior as a combination of interlayer exciton diffusion and dipolar, repulsive exciton-exciton interaction.In recent years, two-dimensional crystal structures have garnered a lot of scientific attention. Using simple techniques such as mechanical exfoliation, a plethora of different materials is readily available as a twodimensional sheet [1], including large-gap insulators, superconductors, and semiconductors. Due to quantum confinement effects, the electronic structure of these atomically thin layers can be very different from that of their corresponding bulk crystals. MoS 2 and related transition-metal dichalcogenides (TMDCs) such as WSe 2 and MoSe 2 are among the most promising systems: while they are indirect-gap semiconductors in the bulk, a transition to a direct band gap occurs as they are thinned down to a single layer [2][3][4]. The peculiar band structure of the TMDC monolayers, combined with a strong spin-orbit interaction, leads to a coupling of spin and valley degrees of freedom [5,6], making these materials highly interesting for potential valleytronic applications. Due to the strictly two-dimensional confinement of electrons and holes and the weak dielectric screening, excitons in these monolayer TMDCs are stable even at room temperature and exhibit large binding energies of about 0.5 eV [7][8][9][10][11]. While various TMDCs show qualitatively similar features, they are characterized by different absolute values of band gap and band offsets with respect to the vacuum level [12]. Therefore, several combinations of TMDCs were predicted to yield a staggered band alignment [12,13] when they are combined into a heterostructure, leading to spatial separation of optically generated electron-hole pairs. The development of various transfer techniques [14,15] for building Van der Waals heterostructures [16] by stacking twodimensional crystals made it possible to fabricate proofof-concept devices such as light-emitting diodes and solar cells using TMDCs [17][18][19], and to experimentally verify the predictions regarding band alignment for different TMDC combinations [20][21][22][23]. Remarkably, photoluminescence (PL) spectra of TMDC heterostructures...

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