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Register a new userThermal light emission from monolayer MoS2
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Because of their strong excitonic photoluminescence (PL) and electroluminescence (EL), together with an excellent electronic tunability, transition metal dichalcogenide (TMD) semiconductors are promising candidates for novel optoelectronic devices. In recent years, several concepts for light emission from two-dimensional (2D) materials have been demonstrated. Most of these concepts are based on the recombination of electrons and holes in a pn-junction, either along the lateral direction using split-gate geometries in combination with monolayer TMDs, or by precisely stacking different 2D semiconductors on top of each other, in order to fabricate vertical van der Waals heterostructures, working as light-emitting diodes (LEDs). Further, EL was also observed along the channel of ionic liquid gated field-effect transistors (FETs) under ambipolar carrier injection. Another mechanism, which has been studied extensively in carbon nanotubes (CNTs) and more recently also in graphene, is thermal light emission as a result of Joule heating. Although the resulting efficiencies are smaller than that of LEDs based on ambipolar electron-hole injection, these experiments provide valuable insights into microscopic processes, such as electron-phonon and phonon-phonon interactions, and the behavior of low-dimensional materials under strong bias in general.
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We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap {Delta} of monolayer MoS2 directly from the device transfer curves and find {Delta} $approx$ 2.4-2.7 eV. In the ambipolar injection regime, we observe electroluminescence due to exciton recombination in MoS2, originating from the region close to the hole-injecting contact. Both the observed transport properties and the behavior of the electroluminescence can be consistently understood as due to the presence of defect states at an energy of 250-300 meV above the top of the valence band, acting as deep traps for holes. Our results are of technological relevance, as they show that devices with useful optoelectronic functionality can be realized on large-area MoS2 monolayers produced by controllable and scalable techniques.
First-principles calculations within density functional theory (DFT) have been carried out to investigate the adsorption of various gas molecules including CO, CO2, NH3, NO and NO2 on MoS2 monolayer in order to fully exploit the gas sensing capabilities of MoS2. By including van der Waals (vdW) interactions between gas molecules and MoS2, we find that only NO and NO2 can bind strongly to MoS2 sheet with large adsorption energies, which is in line with experimental observations. The charge transfer and the variation of electronic structures are discussed in view of the density of states and molecular orbitals of the gas molecules. Our results thus provide a theoretical basis for the potential applications of MoS2 monolayer in gas sensing and give an explanation for recent experimental findings.
Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics and energy harvesting. Large-area growth methods are needed to open the way to the applications. While significant progress to this goal was made, control over lattice orientation during growth still remains a challenge. This is needed in order to minimize or even avoid the formation of grain boundaries which can be detrimental to electrical, optical and mechanical properties of MoS2 and other 2D semiconductors. Here, we report on the uniform growth of high-quality centimeter-scale continuous monolayer MoS2 with control over lattice orientation. Using transmission electron microscopy we show that the monolayer film is composed of coalescing single islands that share a predominant lattice orientation due to an epitaxial growth mechanism. Raman and photoluminescence spectra confirm the high quality of the grown material. Optical absorbance spectra acquired over large areas show new features in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment, we can easily transfer the grown material and fabricate field-effect transistors on SiO2 substrates showing mobility superior to the exfoliated material.
We use micro-Raman and photoluminescence (PL) spectroscopy at 300K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate. The initially degenerate E^1_{2g} Raman mode is split into a doublet as a direct consequence of the strain applied to MoS2 through Van der Waals coupling at the sample-substrate interface. We observe a strong shift of the direct band gap of 48meV/(% of strain) for the monolayer and 46meV/% for the bilayer, whose indirect gap shifts by 86meV/%. We find a strong decrease of the PL polarization linked to optical valley initialization for both monolayer and bilayer samples, indicating that scattering to the spin-degenerate Gamma valley plays a key role.
Optical excitation typically enhances electrical conduction and low-frequency radiation absorption in semiconductors. We have, however, observed a pronounced transient decrease of conductivity in doped monolayer molybdenum disulfide (MoS2), a two-dimensional (2D) semiconductor, under femtosecond laser excitation. In particular, the conductivity is reduced dramatically down to only 30% of its equilibrium value with high pump fluence. This anomalous phenomenon arises from the strong many-body interactions in the system, where photoexcited electron-hole pairs join the doping-induced charges to form trions, bound states of two electrons and one hole. The resultant increase of the carrier effective mass substantially diminishes the carrier conductivity.
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