We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoSe sub(2) ...monolayer, we measure (ProQuest: Formulae and/or non-USASCII text omitted) = 1.8+ or -0.2 ps at T= 7K that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe sub(2) monolayers. Our detailed analysis suggests the following scenario: at low temperature (T< ~ 50 K), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.
The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so ...far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe2 and WSe2 . In this work, we show that encapsulation of ML MoS2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T=4K . Narrow optical transition linewidths are also observed in encapsulated WS2 , WSe2 , and MoSe2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.
Recently, rhenium disulfide (ReS sub(2)) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ReS sub(2) crystals. Unlike the well-known members of ...transition metal dichalcogenides (TMDs), ReS sub(2) crystallizes in a stable distorted-1T structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ReS sub(2). Experimentally, irradiation with 3-MeV He super(+2) ions was used to break the strong covalent bonds in ReS sub(2) flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic alpha particle irradiation. In order to understand the energetics of possible vacancies in ReS sub(2) we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1-3 mu sub(B). Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.
Abstract
Engineering non-linear hybrid light-matter states in tailored lattices is a central research strategy for the simulation of complex Hamiltonians. Excitons in atomically thin crystals are an ...ideal active medium for such purposes, since they couple strongly with light and bear the potential to harness giant non-linearities and interactions while presenting a simple sample-processing and room temperature operability. We demonstrate lattice polaritons, based on an open, high-quality optical cavity, with an imprinted photonic lattice strongly coupled to excitons in a WS
2
monolayer. We experimentally observe the emergence of the canonical band-structure of particles in a one-dimensional lattice at room temperature, and demonstrate frequency reconfigurability over a spectral window exceeding 85 meV, as well as the systematic variation of the nearest-neighbour coupling, reflected by a tunability in the bandwidth of the p-band polaritons by 7 meV. The technology presented in this work is a critical demonstration towards reconfigurable photonic emulators operated with non-linear photonic fluids, offering a simple experimental implementation and working at ambient conditions.
Using current-voltage (I−V ), capacitance-voltage (C−V ), and electric-field-modulated Raman measurements, we report on the unique physics and promising technical applications associated with the ...formation of Schottky barriers at the interface of a one-atom-thick zero-gap semiconductor (graphene) and conventional semiconductors. When chemical-vapor-deposited graphene is transferred onto n -type Si, GaAs, 4H-SiC, and GaN semiconductor substrates, there is a strong van-der-Waals attraction that is accompanied by charge transfer across the interface and the formation of a rectifying (Schottky) barrier. Thermionic-emission theory in conjunction with the Schottky-Mott model within the context of bond-polarization theory provides a surprisingly good description of the electrical properties. Applications can be made to sensors, where in forward bias there is exponential sensitivity to changes in the Schottky-barrier height due to the presence of absorbates on the graphene, and to analog devices, for which Schottky barriers are integral components. Such applications are promising because of graphene’s mechanical stability, its resistance to diffusion, its robustness at high temperatures, and its demonstrated capability to embrace multiple functionalities.
Following the rise of interest in the properties of transition metal dichalcogenides, many experimental techniques were employed to research them. However, the temperature dependencies of optical ...transitions, especially those related to band nesting, were not analyzed in detail for many of them. Here, we present successful studies utilizing the photoreflectance method, which, due to its derivative and absorption-like character, allows investigating direct optical transitions at the high-symmetry point of the Brillouin zone and band nesting. By studying the mentioned optical transitions with temperature from 20 to 300 K, we tracked changes in the electronic band structure for the common transition metal dichalcogenides (TMDs), namely, MoS2, MoSe2, MoTe2, WS2, and WSe2. Moreover, transmission and photoacoustic spectroscopies were also employed to investigate the indirect gap in these crystals. For all observed optical transitions assigned to specific k-points of the Brillouin zone, their temperature dependencies were analyzed using the Varshni relation and Bose–Einstein expression. It was shown that the temperature energy shift for the transition associated with band nesting is smaller when compared with the one at high-symmetry point, revealing reduced average electron–phonon interaction strength.
We demonstrate the synthesis of layered anisotropic semiconductor GeSe and GeSe2 nanomaterials through low temperature (∼400 °C) and atmospheric pressure chemical vapor deposition using halide based ...precursors. Results show that GeI2 and H2Se precursors successfully react in the gas-phase and nucleate on a variety of target substrates including sapphire, Ge, GaAs, or HOPG. Layer-by-layer growth takes place after nucleation to form layered anisotropic materials. Detailed SEM, EDS, XRD, and Raman spectroscopy measurements together with systematic CVD studies reveal that the substrate temperature, selenium partial pressure, and the substrate type ultimately dictate the resulting stoichiometry and phase of these materials. Results from this work introduce the phase control of Ge and Se based nanomaterials (GeSe and GeSe2) using halide based CVD precursors at ATM pressures and low temperatures. Overall findings also extend our fundamental understanding of their growth by making the first attempt to correlate growth parameters to resulting competing phases of Ge–Se based materials.
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