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Register a new userElectric field induced strong enhancement of electroluminescence in multi-Layer MoS2
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The layered transition metal dichalcogenides (TMDs) have attracted considerable interest due to their unique electronic and optical properties. Here we report electric field induced strong electroluminescence in multi-layer MoS2 and WSe2. We show that GaN-Al2O3-MoS2 and GaN-Al2O3-MoS2-Al2O3-graphene vertical heterojunctions can be created with excellent rectification behaviour. Electroluminescence studies demonstrate prominent direct bandgap excitonic emission in multi-layer MoS2 over the entire vertical junction area. Importantly, the electroluminescence efficiency observed in multi-layer MoS2 is comparable to or even higher than that in monolayers, corresponding to a relative electroluminescence enhancement factor of >1000 in multi-layer MoS2 when compared to its photoluminescence. This striking enhancement of electroluminescence can be attributed to the high electric field induced carrier redistribution from low energy points (indirect bandgap) to high energy points (direct bandgap) of k-space, arising from the unique band structure of MoS2 with a much higher density of states at high energy points. The electric field induced electroluminescence is general for other TMDs including WSe2, and can provide a fundamental platform to probe the carrier injection, population and recombination in multi-layer TMDs and open up a new pathway toward TMD based optoelectronic devices.
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We detect electroluminescence in single layer molybdenum disulphide (MoS2) field-effect transistors built on transparent glass substrates. By comparing absorption, photoluminescence, and electroluminescence of the same MoS2 layer, we find that they all involve the same excited state at 1.8eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. The results show that single layer MoS2, a direct band gap semiconductor, is promising for novel optoelectronic devices, such as 2-dimensional light detectors and emitters.
Recently emerged layered transition metal dichalcogenides have attracted great interest due to their intriguing fundamental physical properties and potential applications in optoelectronics. Using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical modeling, we study propagating surface waves in the visible spectral range that are excited at sharp edges of layered transition metal dichalcogenides (TMDC) such as molybdenum disulfide and tungsten diselenide. These surface waves form fringes in s-SNOM measurements. By measuring how the fringes change when the sample is rotated with respect to the incident beam, we obtain evidence that exfoliated MoS2 on a silicon substrate supports two types of Zenneck surface waves that are predicted to exist in materials with large real and imaginary parts of the permittivity. We have compared MoS2 interference fringes with those formed on layered insulator such as hexagonal boron nitride where only leaky modes are possible due to its small permittivity. Interpretation of experimental data is supported by theoretical models. Our results could pave the way to the investigation of surface waves on TMDCs and other van der Waals materials and their novel photonics applications.
We study field effect transistor characteristics in etched single layer MoS2 nanoribbon devices of width 50nm with ohmic contacts. We employ a SF6 dry plasma process to etch MoS2 nanoribbons using low etching (RF) power allowing very good control over etching rate. Transconductance measurements reveal a steep sub-threshold slope of 3.5V/dec using a global backgate. Moreover, we measure a high current density of 38 uA/um resulting in high on/off ratio of the order of 10^5. We observe mobility reaching as high as 50 cm^2/V.s with increasing source-drain bias.
Gate-induced superconductivity at the surface of nanolayers of semiconducting transition metal dichalcogenides (TMDs) has attracted a lot of attention in recent years, thanks to the sizeable transition temperature, robustness against in-plane magnetic fields beyond the Pauli limit, and hints to a non-conventional nature of the pairing. A key information necessary to unveil its microscopic origin is the geometry of the Fermi surface hosting the Cooper pairs as a function of field-effect doping, which is dictated by the filling of the inequivalent valleys at the K/K$^{prime}$ and Q/Q$^{prime}$ points of the Brillouin Zone. Here, we achieve this by combining Density Functional Theory calculations of the bandstructure with transport measurements on ion-gated 2H-MoS$_{2}$ nanolayers. We show that, when the number of layers and the amount of strain are set to their experimental values, the Fermi level crosses the bottom of the high-energy valleys at Q/Q$^{prime}$ at doping levels where characteristic kinks in the transconductance are experimentally detected. We also develop a simple 2D model which is able to quantitatively describe the broadening of the kinks observed upon increasing temperature. We demonstrate that this combined approach can be employed to map the dependence of the Fermi surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions, and provide a method to determine the amount of strain and spin-orbit splitting between sub-bands from electric transport measurements in real devices.
State-of-the-art fabrication and characterization techniques have been employed to measure the thermal conductivity of suspended, single-crystalline MoS2 and MoS2/hBN heterostructures. Two-laser Raman scattering thermometry was used combined with real time measurements of the absorbed laser power, which allowed us to determine the thermal conductivities without any assumptions. Measurements on MoS2 layers with thicknesses of 5 and 14 exhibit thermal conductivity in the range between 12 and 24 Wm-1K-1. Additionally, after determining the thermal conductivity of a selected MoS2 sample, an hBN flake was transferred onto it and the effective thermal conductivity of the heterostructure was subsequently measured. Remarkably, despite that the thickness of the hBN layer was less than a third of the thickness of the MoS2 layer, the heterostructure showed an almost eight-fold increase in the thermal conductivity, being able to dissipate more than 10 times the laser power without any visible sign of damage. These results are consistent with a high thermal interface conductance between MoS2 and hBN and an efficient in-plane heat spreading driven by hBN. Indeed, we estimate G 70 MWm-2K-1 which is significantly higher than previously reported values. Our work therefore demonstrates that the insertion of hBN layers in potential MoS2 based devices holds the promise for efficient thermal management.
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