We present a photoluminescence study of single-layer MoS2 flakes on SiO2 surfaces. We demonstrate that the luminescence peak position of flakes prepared from natural MoS2, which varies by up to 25 meV between individual as-prepared flakes, can be homogenized by annealing in vacuum, which removes adsorbates from the surface. We use HfO2 and Al2O3 layers prepared by atomic layer deposition to cover some of our flakes. We clearly observe a suppression of the low-energy luminescence peak observed for as-prepared flakes at low temperatures, indicating that this peak originates from excitons bound to surface adsorbates. We also observe different temperature-induced shifts of the luminescence peaks for the oxide-covered flakes. This effect stems from the different thermal expansion coefficients of the oxide layers and the MoS2 flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the oxide layers and are therefore strained.Comment: 3 pages, 2 figure

We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar (nn ) or the bipolar (np ) regime. In the latter, pronounced single-and multibarrier Fabry-Pérot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data are highly consistent with a phase coherent ballistic tight-binding calculation and quantum capacitance model, whereas a superlattice effect and modification of band structure can be excluded.

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The dichalcogenide MoS2, which is an indirect‐gap semiconductor in its bulk form, was recently shown to become a direct‐gap material when it is thinned to a single monolayer. Due to its layered crystal structure, few‐layer flakes of MoS2 can be prepared, just like graphene flakes, by mechanical exfoliation. Scanning Raman spectroscopy is a powerful tool to identify such flakes and to determine the number of layers. In their Letter on , Plechinger et al. prepare MoS2 flakes and use scanning Raman spectroscopy to identify single‐layer flakes. Some of these flakes are covered by dielectric layers using atomic layer deposition. The authors further analyze the flakes by performing low‐temperature photoluminescence (PL) measurements. In bare MoS2 flakes, they observe a high‐energy PL peak, associated with free excitons, and a low‐energy PL peak, associated with excitons bound to surface adsorbates. This low‐energy peak is suppressed in flakes that are covered with dielectric layers. From the temperature‐induced shift of the PL peaks, they infer that the dielectric‐covered flakes are strained.

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The dichalcogenide MoS 2 , which is an indirect-gap semiconductor in its bulk form, was recently shown to become an efficient emitter of photoluminescence as it is thinned to a single layer, indicating a transition to a directgap semiconductor due to confinement effects. With its layered structure of weakly coupled, covalently bonded two-dimensional sheets, it can be prepared, just as graphene, using mechanical exfoliation techniques. Here, we present temperature-dependent and time-resolved photoluminescence (PL) studies of single-layer MoS 2 flakes. Some of the flakes are covered with oxide layers prepared by atomic layer deposition (ALD). At low temperatures, we clearly see two PL peaks in the as-prepared flakes without oxide layers, which we may assign to bound and free exciton transitions. The lower-energy, bound exciton PL peak is absent in the oxide-covered flakes. In time-resolved PL measurements, we observe very fast photocarrier recombination on the few-ps timescale at low temperatures, with increasing photocarrier lifetimes at higher temperatures due to exciton-phonon scattering.