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Register a new userAmbipolar Light-Emitting Transistors on Chemical Vapor Deposited Monolayer MoS2
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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.
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We demonstrated selective gas sensing with MoS2 thin-film transistors using the change in the channel conductance, characteristic transient time and low-frequency current fluctuations as the sensing parameters. The back-gated MoS2 thin-film field-effect transistors were fabricated on Si/SiO2 substrates and intentionally aged for a month to verify reliability and achieve better current stability. The same devices with the channel covered by 10 nm of Al2O3 were used as reference samples. The exposure to ethanol, acetonitrile, toluene, chloroform, and methanol vapors results in drastic changes in the source-drain current. The current can increase or decrease by more than two-orders of magnitude depending on the analyte. The reference devices with coated channel did not show any response. It was established that transient time of the current change and the normalized spectral density of the low-frequency current fluctuations can be used as additional sensing parameters for selective gas detection with thin-film MoS2 transistors.
Monolayer molybdenum disulphide (MoS$_2$) is a promising two-dimensional (2D) material for nanoelectronic and optoelectronic applications. The large-area growth of MoS$_2$ has been demonstrated using chemical vapor deposition (CVD) in a wide range of deposition temperatures from 600 {deg}C to 1000 {deg}C. However, a direct comparison of growth parameters and resulting material properties has not been made so far. Here, we present a systematic experimental and theoretical investigation of optical properties of monolayer MoS$_2$ grown at different temperatures. Micro-Raman and photoluminescence (PL) studies reveal observable inhomogeneities in optical properties of the as-grown single crystalline grains of MoS$_2$. Close examination of the Raman and PL features clearly indicate that growth-induced strain is the main source of distinct optical properties. We carry out density functional theory calculations to describe the interaction of growing MoS$_2$ layers with the growth substrate as the origin of strain. Our work explains the variation of band gap energies of CVD-grown monolayer MoS$_2$, extracted using PL spectroscopy, as a function of deposition temperature. The methodology has general applicability to model and predict the influence of growth conditions on strain in 2D materials.
Integration of graphene with Si microelectronics is very appealing by offering potentially a broad range of new functionalities. New materials to be integrated with Si platform must conform to stringent purity standards. Here, we investigate graphene layers grown on copper foils by chemical vapor deposition and transferred to silicon wafers by wet etch and electrochemical delamination methods with respect to residual sub-monolayer metallic contaminations. Regardless of the transfer method and associated cleaning scheme, time-of-flight secondary ion mass spectrometry and total reflection x-ray fluorescence measurements indicate that the graphene sheets are contaminated with residual metals (copper, iron) with a concentration exceeding 10$^{13}$ atoms/cm$^{2}$. These metal impurities appear to be partly mobile upon thermal treatment as shown by depth profiling and reduction of the minority charge carrier diffusion length in the silicon substrate. As residual metallic impurities can significantly alter electronic and electrochemical properties of graphene and can severely impede the process of integration with silicon microelectronics these results reveal that further progress in synthesis, handling, and cleaning of graphene is required on the way to its advanced electronic and optoelectronic applications.
Monolayer transition metal dichalcogenides (TMD) have numerous potential applications in ultrathin electronics and photonics. The exposure of TMD based devices to light generates photo-carriers resulting in an enhanced conductivity, which can be effectively used, e.g., in photodetectors. If the photo-enhanced conductivity persists after removal of the irradiation, the effect is known as persistent photoconductivity (PPC). Here we show that ultraviolet light (wavelength = 365 nm) exposure induces an extremely long-living giant PPC (GPPC) in monolayer MoS2 (ML-MoS2) field-effect transistors (FET) with a time constant of ~30 days. Furthermore, this effect leads to a large enhancement of the conductivity up to a factor of 107. In contrast to previous studies in which the origin of the PPC was attributed to extrinsic reasons such as trapped charges in the substrate or adsorbates, we unambiguously show that the GPPC arises mainly from the intrinsic properties of ML-MoS2 such as lattice defects that induce a large amount of localized states in the forbidden gap. This finding is supported by a detailed experimental and theoretical study of the electric transport in TMD based FETs as well as by characterization of ML-MoS2 with scanning tunneling spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. The obtained results provide a basis towards the defect-based engineering of the electronic and optical properties of TMDs for device applications.
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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