Spatially Resolved Electronic Properties of Single-Layer WS2 on Transition Metal Oxides (original) (raw)
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Disentangling the effects of doping, strain and disorder in monolayer WS2 by optical spectroscopy
2D Materials, 2019
Monolayers of transition metal dichalcogenides (TMdC) are promising candidates for realization of a new generation of optoelectronic devices. The optical properties of these two-dimensional materials, however, vary from flake to flake, or even across individual flakes, and change over time, all of which makes control of the optoelectronic properties challenging. There are many different perturbations that can alter the optical properties, including charge doping, defects, strain, oxidation, and water intercalation.
We calculate from first principles the electronic structure and optical properties of a number of transition metal dichalcogenide (TMD) bilayer heterostructures consisting of MoS 2 layers sandwiched with WS 2 , MoSe 2 , MoTe 2 , BN, or graphene sheets. Contrary to previous works, the systems are constructed in such a way that the unstrained lattice constants of the constituent incommensurate monolayers are retained. We find strong interaction between the -point states in all TMD/TMD heterostructures, which can lead to an indirect gap. On the other hand, states near the K point remain as in the monolayers. When TMDs are paired with BN or graphene layers, the interaction around the -point is negligible, and the electronic structure resembles that of two independent monolayers. Calculations of optical properties of the MoS 2 /WS 2 system show that, even when the valenceand conduction-band edges are located in different layers, the mixing of optical transitions is minimal, and the optical characteristics of the monolayers are largely retained in these heterostructures. The intensity of interlayer transitions is found to be negligibly small, a discouraging result for engineering the optical gap of TMDs by heterostructuring.
Disentangling the effects of doping, strain and defects in monolayer WS2 by optical spectroscopy
2019
Monolayers of transition metal dichalcogenides (TMdC) are promising candidates for realization of a new generation of optoelectronic devices. The optical properties of these two-dimensional materials, however, vary from flake to flake, or even across individual flakes, and change over time, all of which makes control of the optoelectronic properties challenging. There are many different perturbations that can alter the optical properties, including charge doping, defects, strain, oxidation, and water intercalation. Identifying which perturbations are present is usually not straightforward and requires multiple measurements using multiple experimental modalities, which presents barriers when attempting to optimise preparation of these materials. Here, we apply highresolution photoluminescence and differential reflectance hyperspectral imaging in situ to CVD-grown WS2 monolayers. By combining these two optical measurements and using a statistical correlation analysis we are able to di...
Nano letters, 2016
Using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we examine the electronic structure of transition metal dichalcogenide heterostructures (TMDCHs) composed of monolayers of MoS2 and WS2. STS data are obtained for heterostructures of varying stacking configuration as well as the individual monolayers. Analysis of the tunneling spectra includes the influence of finite sample temperature, yield information about the quasi-particle bandgaps, and the band alignment of MoS2 and WS2. We report the band gaps of MoS2 (2.17 ± 0.04 eV) and WS2 (2.39 ± 0.06 eV) in the individual materials and type II band alignment for the heterostructure with an interfacial band gap of 1.45 ± 0.06 eV.
Nano letters, 2016
In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we...
SYNTHESIS AND CHARACTERIZATION OF TWO- DIMENSIONAL TRANSITION METAL DICHALCOGENIDES
Semiconducting transition metal dichalcogenides (TMDCs) have attracted intense research interest in recent years, due to their many unique electrical, optical, and mechanical properties and their potential for diverse applications. One of the key challenges in 2D TMDC electronics is to synthesize high-quality and large-scale monolayer TMDCs.
ACS Applied Materials & Interfaces
The fabrication process for the uniform large-scale MoS 2 , WS 2 transitionmetal dichalcogenides (TMDCs) monolayers, and their heterostructures has been developed by van der Waals epitaxy (VdWE) through the reaction of MoCl 5 or WCl 6 precursors and the reactive gas H 2 S to form MoS 2 or WS 2 monolayers, respectively. The heterostructures of MoS 2 /WS 2 or WS 2 /MoS 2 can be easily achieved by changing the precursor from WCl 6 to MoCl 5 once the WS 2 monolayer has been fabricated or switching the precursor from MoCl 5 to WCl 6 after the MoS 2 monolayer has been deposited on the substrate. These VdWE-grown MoS 2 , WS 2 monolayers, and their heterostructures have been successfully deposited on Si wafers with 300 nm SiO 2 coating (300 nm SiO 2 /Si), quartz glass, fused silica, and sapphire substrates using the protocol that we have developed. We have characterized these TMDCs materials with a range of tools/techniques including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-Raman analysis, photoluminescence (PL), atomic force microscopy (AFM), transmission electron microscopy (TEM), energydispersive X-ray spectroscopy (EDX), and selected-area electron diffraction (SAED). The band alignment and large-scale uniformity of MoS 2 /WS 2 heterostructures have also been evaluated with PL spectroscopy. This process and resulting large-scale MoS 2 , WS 2 monolayers, and their heterostructures have demonstrated promising solutions for the applications in next-generation nanoelectronics, nanophotonics, and quantum technology.
Atomic-Scale Spectroscopy of Gated Monolayer MoS2
Nano letters, 2016
The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have str...
2D Materials, 2018
Chlorine-doped tungsten disulfide monolayer (1L-WS2) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal's electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this implicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning Auger Microscopy and X-ray photoelectron spectroscopy are used to link the actual charge carrier doping to the different chlorine configurations in the monolayer lattice. The spectroscopic analyses, complemented by density functional theory calculations, reveal that chlorine physisorption is responsible for the carrier density modulation induced by the pulsed laser photochemical reaction process. Such bidirectional control of the Fermi level, coupled with the capability offered by lasers to process at pre-selected locations, can be advantageously used for spatially resolved doping modulation in 1L-WS2 with micrometric resolution. This method can also be extended for the controllable doping of other TMD monolayers. Monolayer (1L) transition metal dichalcogenides (TMDs) are promising materials for future 2D nanoelectronics due to their unique electrical, 1 optical 2,3 and mechanical properties. 4 Indeed, TMDs have been demonstrated to be ideal candidates for field-effect transistors, photovoltaic cells, lightemitting diodes, single-atom storage, molecule sensing, quantum-state metamaterials and electrocatalytic water splitting applications. 5-7 Carrier modulation is an important parameter in the study of the electronic properties of semiconductors and at the heart of many applications in microelectronics. Tuning the charge carrier density, i.e. doping, can be realized chemically, via bonding of foreign atoms to the crystal structure, 8-10 or electrostatically, by inducing a charge accumulation. 11-13 Electrical doping