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Register a new userLayer Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultra-flat Metals
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Transition metal dichalcogenides (TMDs) are layered semiconducting van der Waal crystals and promising materials for a wide range of electronic and optoelectronic devices. Realizing practical electrical and optoelectronic device applications requires a connection between a metal junction and a TMD semiconductor. Hence, a complete understanding of electronic band alignments and the potential barrier heights governing the transport through a metal-TMD-metal junction is critical. But, there is a knowledge gap; it is not clear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few layered (1-5) MoS2 immobilized on ultra-flat conducting Au surfaces (root mean square (RMS) surface roughness <0.2 nm) and indium tin oxide (ITO) substrate (RMS surface roughness <0.7 nm) forming a vertical metal (conductive-AFM tip)-semiconductor-metal device. We have observed that the current increases as the number of layers increases up to 5 layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed that the barrier height decreases as the number of layers increases. Using density functional theory (DFT) calculation, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating the TMDs on a transparent ultra-flat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of TMD-metal junction, and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.
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Imperfections in the crystal structure, such as point defects, can strongly modify the optical and transport properties of materials. Here, we study the effect of point defects on the optical and DC conductivities of single layers of semiconducting transition metal dichalcogenides with the form $M$S$_2$, where $M$=Mo or W. The electronic structure is considered within a six bands tight-binding model, which accounts for the relevant combination of $d$ orbitals of the metal $M$ and $p$ orbitals of the chalcogen $S$. We use the Kubo formula for the calculation of the conductivity in samples with different distributions of disorder. We find that $M$ and/or S defects create mid-gap states that localize charge carriers around the defects and which modify the optical and transport properties of the material, in agreement with recent experiments. Furthermore, our results indicate a much higher mobility for $p$-doped WS$_2$ in comparison to MoS$_2$.
The electronic properties of few-layer graphene grown on the carbon-face of silicon carbide (SiC) are found to be strongly dependent on the number of layers. The carrier mobility is larger in thicker graphene because substrate-related scattering is reduced in the higher layers. The carrier density dependence of the mobility is qualitatively different in thin and thick graphene, with the transition occurring at about 2 layers. The mobility increases with carrier density in thick graphene, similar to multi-layer graphene exfoliated from natural graphite, suggesting that the individual layers are still electrically coupled in spite of reports recording non-Bernal stacking order in C-face grown graphene. The Hall coefficient peak value is reduced in thick graphene due to the increased density of states. A reliable and rapid characterization tool for the layer number is therefore highly desirable. To date, AFM height determination and Raman scattering are typically used since the optical contrast of graphene on SiC is weak. However, both methods suffer from low throughput. We show that the scanning electron microscopy (SEM) contrast can give similar results with much higher throughput.
Thermal transport properties at the metal/MoS2 interfaces are analyzed by using an atomistic phonon transport model based on the Landauer formalism and first-principles calculations. The considered structures include chemisorbed Sc(0001)/MoS2 and Ru(0001)/MoS2, physisorbed Au(111)/MoS2, as well as Pd(111)/MoS2 with intermediate characteristics. Calculated results illustrate a distinctive dependence of thermal transfer on the details of interfacial microstructures. More specifically, the chemisorbed case with a stronger bonding exhibits a generally smaller interfacial thermal resistance than the physisorbed. Comparison between metal/MoS2 and metal/graphene systems suggests that metal/MoS2 is significantly more resistive. Further examination of lattice dynamics identifies the presence of multiple distinct atomic planes and bonding patterns at the interface as the key origin of the observed large thermal resistance.
Since the discovery of graphene -a single layer of carbon atoms arranged in a honeycomb lattice - it was clear that this truly is a unique material system with an unprecedented combination of physical properties. Graphene is the thinnest membrane present in nature -just one atom thick- it is the strongest material, it is transparent and it is a very good conductor with room temperature charge mobilities larger than the typical mobilities found in silicon. The significance played by this new material system is even more apparent when considering that graphene is the thinnest member of a larger family: the few-layer graphene materials. Even though several physical properties are shared between graphene and its few-layers, recent theoretical and experimental advances demonstrate that each specific thickness of few-layer graphene is a material with unique physical properties.
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.
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