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Register a new userMicrostructure and Elastic Constants of Transition Metal Dichalcogenide Monolayers from Friction and Shear Force Microscopy
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Optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. Here we demonstrate how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy (LFM) and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS2) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable us to acquire the fourth-order elastic constants of monolayer WS2 experimentally. Our results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs, insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such two-dimensional semiconductors.
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We have investigated the exciton dynamics in transition metal dichalcogenide mono-layers using time-resolved photoluminescence experiments performed with optimized time-resolution. For MoSe2 monolayers, we measure $tau_{rad}=1.8pm0.2$ ps that we interpret as the intrinsic radiative recombination time. Similar values are found for WSe2 mono-layers. Our detailed analysis suggests the following scenario: at low temperature (T $leq$ 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 PL 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 non-thermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and non-radiative 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.
Application of transition metal dichalcogenides (TMDC) in photonic, optoelectronic or valleytronic devices requires the growth of continuous monolayers, heterostructures and alloys of different materials in a single process. We present a facile pulsed thermal deposition method which provides precise control over layer thickness and stoichiometry of two-dimensional systems. The versatility of the method is demonstrated on ternary monolayers of Mo$_{1-x}$W$_{x}$S$_{2}$ and on heterostructures combining metallic TaS$_{2}$ and semiconducting MoS$_{2}$ layers. The fabricated ternary monolayers cover the entire composition range of $x$ = 0...1 without phase separation. Band gap engineering and control over the spin-orbit coupling strength is demonstrated by absorption and photoluminescence spectroscopy. Vertical heterostructures are grown without intermixing. The formation of clean and atomically abrupt interfaces is evidenced by high-resolution transmission electron microscopy. Since both the metal components as well as the chalcogenides are thermally evaporated complex alloys and heterostructures can thus be prepared.
The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the spin-forbidden dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting $delta=0.6 pm 0.1$ meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization (grey exciton). The first determination of the dark neutral exciton lifetime $tau_D$ in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure $tau_D sim 110 pm 10$ ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.
Transition metal dichalcogenide (TMDC) monolayers are newly discovered semiconductors for a wide range of applications in electronics and optoelectronics. Most studies have focused on binary monolayers that share common properties: direct optical bandgap, spin-orbit (SO) splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states of electrons. Studies on alloy-based monolayers are more recent, yet they may not only extend the possibilities for TMDC applications through specific engineering but also help understanding the differences between each binary material. Here, we synthesized highly crystalline Mo$_{(1-x)}$W$_{x}$Se$_2$ to show engineering of the direct optical bandgap and the SO coupling in ternary alloy monolayers. We investigate the impact of the tuning of the SO spin splitting on the optical and polarization properties. We show a non-linear increase of the optically generated valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. We also probe the impact of the tuning of the conduction band SO spin splitting on the bright versus dark state population i.e. PL emission intensity. We show that the MoSe2 PL intensity decreases as a function of temperature by an order of magnitude, whereas for WSe2 we measure surprisingly an order of magnitude increase over the same temperature range (T=4-300K). The ternary material shows a trend between these two extreme behaviors. These results show the strong potential of SO engineering in ternary TMDC alloys for optoelectronics and applications based on electron spin- and valley-control.
Transition metal dichalcogenide (TMDC) monolayers present a remarkable multifunctional material with potential to enable the development of a wide range of novel devices. However, the functionalities observed often fall short of the expectation, which hinders the device development. Here we demonstrate that the optical, catalytic, and thermal functionalities of TMDC monolayers can all be substantially enhanced by up to orders of magnitude with the intercalation of water molecules or small cations (H+ and Li+) between the monolayers and underlying substrates. In contrast, the same molecules or cations adsorbed on top of the monolayers show negligible effects. We also discover two major roles of the intercalated species in the enhancement: doping the monolayers and modifying the interaction of the monolayers with the substrate. The result points out a versatile and convenient strategy of using the intercalation of molecules or ions to enhance the functionalities of TMDC monolayers.
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