Transition Metal Dichalcogenide Films Research Articles

Electrodeposited Transition Metal Dichalcogenides for Use in Hydrogen Evolution Electrocatalysts

Hydrogen is a promising alternative to gasoline due to its higher energy density and ability to burn cleanly only producing H2O as a by-product. Electrolytic water splitting is an effective technique for generating molecular hydrogen. However, for hydrogen to be a viable alternative energy source to be produced from water electrolysis, affordable and durable electrocatalysts need to be developed to replace platinum. Transition metal dichalcogenides (TMDs) are a promising alternative since they are abundant, inexpensive, and have a tunable structure. There are various ways to produce TMD films including chemical and mechanical exfoliation, chemical vapor deposition (CVD), and electrodeposition. Exfoliation and CVD techniques often require a transfer of TMDs from the growth substrate to an electrode, which introduces impurities and possible defects to the film. Electrodeposition, however, provides a way to produce TMDs directly onto the electrode with excellent surface coverage. This work uses electrodeposition to produce TMD and TMD bilayer electrodes using sequential electrodeposition for electrocatalytic hydrogen evolution reaction (HER). The results presented include cost-effective deposition techniques along with enhanced proton reduction activity for the sequentially deposited bilayer TMD structure consisting of MoS2 and MoSe2, which suggests the electron transfer kinetics from the conductive glass substrate to the top-layer is enhanced with a MoS2 layer. Furthermore, the bilayer structures synthesized by sequential deposition are characterized via XPS, XPS depth-profiling, and SEM-EDS for enhanced understanding of the fabricated structure.

Unraveling Heat Transport and Dissipation in Suspended MoSe2 from Bulk to Monolayer.

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental-theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe2 . Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in-plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat-carrying modes around ≈0.1THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out-of-plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe2 by an order of magnitude. These results are crucial for the design of (flexible) TMD-based (opto-)electronicapplications.

(INVITED) Opto-electronic properties of solution-synthesized MoS2 metal-semiconductor-metal photodetector

We report a simple fabrication method for metal-semiconductor-metal (MSM) photodetectors using solution-synthesized MoS2 films as the semiconductor channel and pure indium for the metal contacts. Resonance Raman spectroscopy has been used to confirm the growth of ultra-thin MoS2 films. Due to the low work function of indium, the back-to-back indium-MoS2 Schottky junctions formed relatively low Schottky barrier height (0.43 eV), which has been calculated using the thermionic field emission model. Moreover, the MSM device shows good responsivity (22 mA/W) under white light illumination. The MoS2-MSM device possesses higher photoelectrical response in the visible and near IR region compared to UV due to the indirect bandgap of ultra-thin MoS2 film which has been estimated to be 1.38 eV. These findings reveal the significance of using low work function metals like indium as contacts for layered transition metal dichalcogenides (TMDC) films (produced by low cost solution-based techniques), particularly MoS2, in order to design semiconductor devices with efficient opto-electronic performance.

Optical Characterization of Few-Layer PtSe2 Nanosheet Films.

Thin films of transition-metal dichalcogenides are potential materials for optoelectronic applications. However, the application of these materials in practice requires knowledge of their fundamental optical properties. Many existing methods determine optical constants using predefined models. Here, a different approach was used. We determine the sheet conductance and absorption coefficient of few-layer PtSe2 in the infrared and UV–vis ranges without recourse to any particular model for the optical constants. PtSe2 samples with a thickness of about 3–4 layers were prepared by selenization of 0.5 nm thick platinum films on sapphire substrates at different temperatures. Differential reflectance was extracted from transmittance and reflectance measurements from the front and back of the sample. The film thickness, limited to a few atomic layers, allowed a thin-film approximation to calculate the optical conductance and absorption coefficient. The former has a very different energy dependence in the infrared, near-infrared, and visible ranges. The absorption coefficient exhibits a strong power-law dependence on energy with an exponent larger than three in the mid-infrared and near-infrared regions. We have not observed any evidence for a band gap in PtSe2 thin layers down to an energy of 0.4 eV from our optical measurements.

Growth Mechanisms and Morphology Engineering of Atomic Layer-Deposited WS2.

Transition-metal dichalcogenides (TMDs) have attracted intense research interest for a broad range of device applications. Atomic layer deposition (ALD), a CMOS compatible technique, can enable the preparation of high-quality TMD films on 8 to 12 in. wafers for large-scale circuit integration. However, the ALD growth mechanisms are still not fully understood. In this work, we systematically investigated the growth mechanisms for WS2 and found them to be strongly affected by nucleation density and film thickness. Transmission electron microscope imaging reveals the coexistence and competition of lateral and vertical growth mechanisms at different growth stages, and the critical thicknesses for each mechanism are obtained. The in-plane lateral growth mode dominates when the film thickness remains less than 5.6 nm (8 layers), while the vertical growth mode dominates when the thickness is greater than 20 nm. From the resulting understanding of these growth mechanisms, the conditions for film deposition were optimized and a maximum grain size of 108 nm was achieved. WS2-based field-effect transistors were fabricated with electron mobility and on/off current ratio up to 3.21 cm2 V-1 s-1 and 105, respectively. Particularly, this work proves the capability of synthesis of TMD films in a wafer scale with excellent controllability of thickness and morphology, enabling many potential applications other than transistors, such as nanowire- or nanosheet-based supercapacitors, batteries, sensors, and catalysis.

Atomically Thin, Optically Isotropic Films with 3D Nanotopography.

Flat optics aims for the on-chip miniaturization of optical systems for high-speed and low-power operation, with integration of thin and lightweight components. Here, we present atomically thin yet optically isotropic films realized by using three-dimensional (3D) topographic reconstruction of anisotropic two-dimensional (2D) films to balance the out-of-plane and in-plane optical responses on the subwavelength scale. We achieve this by conformal growth of monolayer transition metal dichalcogenide (TMD) films on nanodome-structured substrates. The resulting films show an order-of-magnitude increase in the out-of-plane susceptibility for enhanced angular performance, displaying polarization isotropy in the off-axis absorption, as well as improved photoluminescence emission profiles, compared to their flat-film counterparts. We further show that such 3D geometric programming of optical properties is applicable to different TMD materials, offering spectral generalization over for the entire visible range. Our approach presents a powerful platform for advancing the development of atomically thin flat optics with custom-designed light–matter interactions.

Micro-patterned deposition of MoS2 ultrathin-films by a controlled droplet dragging approach

Micropatterning of transition metal dichalcogenide (TMDC) ultrathin-films and monolayers has been demonstrated by various multi-step approaches. However, directly achieving a patterned growth of TMDC films is still considered to be challenging. Here, we report a solution-based approach for the synthesis of patterned MoS2 layers by dragging a precursor solution droplet with variable velocities across a substrate. Utilizing the pronounced shearing velocity dependence in a Landau-Levich deposition regime, MoS2 films with a spatially modulated thickness with alternating mono/bi- and few-layer regions are obtained after precursor annealing. Generally, the presented facile methodology allows for the direct preparation of micro-structured functional materials, extendable to other TMDC materials and even van der Waals heterostructures.

Growth of PtSe2 few-layer films on NbN superconducting substrate

Few-layer films of transition metal dichalcogenides have emerged as promising candidates for applications in electronics. Within this group of 2D materials, platinum diselenide (PtSe2) was predicted to be a compound with one of the highest charge carrier mobility. Recently, the successful integration of group III–V nitride semiconductors with NbNx-based superconductors was reported with a semiconductor transistor grown directly on a crystalline superconductor. This opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic, and piezoelectric properties of the semiconducting material. Here, we report on the fabrication of a few-layer PtSe2 film on top of an NbN substrate layer by selenization of pre-deposited 3 nm thick Pt layers. We found the selenization parameters preserving the chemical and structural integrity of both the PtSe2 and NbN films. The PtSe2 film alignment can be tuned by varying the nitrogen flow rate through the reaction chamber. The superconducting critical temperature of NbN is only slightly reduced in the optimized samples compared to pristine NbN. The carrier mobility in PtSe2 layers determined from Hall measurements is below 1 cm2/V s.

Highly Efficient Experimental Approach to Evaluate Metal to 2D Semiconductor Interfaces in Vertical Diodes with Asymmetric Metal Contacts.

The energy band alignments and associated material properties at the contacts between metal and two-dimensional (2D) semiconducting transition metal dichalcogenide (SCTMD) films determine the important traits in 2D SCTMD-based electronic and optical device applications. In this work, we realize 2D vertical diodes with asymmetric metal-SCTMD contact areas where currents are dominated by the contact-limited charge flows in the transport regimes of Fowler-Nordheim tunneling and Schottky emission. With straightforward current-voltage characteristics, we can accurately evaluate the interface parameters such as Schottky barrier heights and the vertical effective masses of tunneling charges. In particular, the differing contact areas and resultant current rectifications allow us to address specific Schottky barrier locations with respect to the conduction and valence band edges of 2D semiconducting WSe2, WS2, MoSe2, and MoS2, thereby determining whether p-type holes or n-type electrons become the majority charge carriers in the SCTMD devices. We demonstrate that our experimental and analytical approaches can be utilized as a simple but powerful material metrology to qualitatively and quantitatively evaluate various metal-SCTMD contacts.

Rapid Growth of Monolayer MoSe2 Films for Large-Area Electronics.

The large-scale growth of semiconducting thin films on insulating substrates enables batch fabrication of atomically thin electronic and optoelectronic devices and circuits without film transfer. Here an efficient method to achieve rapid growth of large-area monolayer MoSe2 films based on spin coating of Mo precursor and assisted by NaCl is reported. Uniform monolayer MoSe2 films up to a few inches in size are obtained within a short growth time of 5 min. The as-grown monolayer MoSe2 films are of high quality with large grain size (up to 120 μm). Arrays of field-effect transistors are fabricated from the MoSe2 films through a photolithographic process; the devices exhibit high carrier mobility of ≈27.6 cm2 V-1 s-1 and on/off ratios of ≈105. The findings provide insight into the batch production of uniform thin transition metal dichalcogenide films and promote their large-scale applications.

Intrinsic effects of thickness, surface chemistry and electroactive area on nanostructured MoS2 electrodes with superior stability for hydrogen evolution

Developing inexpensive, earth abundant materials for electrocatalytic and photoelectrochemical water splitting is critical to the decarburization of our energy system. Aerosol-assisted chemical vapor deposition (AACVD) is a potentially scalable, one-step synthesis technique that can produce large area, homogenous edge-aligned films of transition metal dichalcogenides including MoS2. Herein, we report a hydrogen evolution reaction (HER) study on such electrode films under acidic conditions. Using iR compensated near-steady state DC current-potential measurements and linear sweep voltammetry, the HER onset potential is recorded at −175 mV vs. RHE (current density 0.1 mA cm−2), while a current density of 10 mA cm−2 per nominal area is reached at an overpotential of −430 mV vs. RHE. A Tafel slope of 113 mV dec−1 suggests the existence of increased edge-active sites on the surface of 2H-MoS2. The electrodes offered high stability over 5000 cycles, which continued to improve with cycling, with the current density at −350 mV improved by a factor exceeding 20 between the 1st and 5000th cycle. X-ray photoelectron spectroscopy, contact angle measurements and electrochemical impedance spectroscopy were employed to provide thorough insights on the changes in the intrinsic properties of the material, which led to the observed HER activity enhancement with cycling. It is shown that a reduction in the surface oxide layer and increased electrowetting are the reasons for the improved performance. Electrodes of differing thicknesses were prepared and their performance was compared in association with the surface morphology. The best performing electrode had the lowest mass loading of MoS2, which is attributed to improved faradaic efficiency through the resistive 2H-MoS2 electrode.

Recent Advances in Electrical Doping of 2D Semiconductor Materials: Methods, Analyses, and Applications.

Two-dimensional materials have garnered interest from the perspectives of physics, materials, and applied electronics owing to their outstanding physical and chemical properties. Advances in exfoliation and synthesis technologies have enabled preparation and electrical characterization of various atomically thin films of semiconductor transition metal dichalcogenides (TMDs). Their two-dimensional structures and electromagnetic spectra coupled to bandgaps in the visible region indicate their suitability for digital electronics and optoelectronics. To further expand the potential applications of these two-dimensional semiconductor materials, technologies capable of precisely controlling the electrical properties of the material are essential. Doping has been traditionally used to effectively change the electrical and electronic properties of materials through relatively simple processes. To change the electrical properties, substances that can donate or remove electrons are added. Doping of atomically thin two-dimensional semiconductor materials is similar to that used for silicon but has a slightly different mechanism. Three main methods with different characteristics and slightly different principles are generally used. This review presents an overview of various advanced doping techniques based on the substitutional, chemical, and charge transfer molecular doping strategies of graphene and TMDs, which are the representative 2D semiconductor materials.

In Situ Ultrafast and Patterned Growth of Transition Metal Dichalcogenides from Inkjet-Printed Aqueous Precursors.

Chemical vapor deposition (CVD) has been widely used to synthesize high-quality 2D transition-metal dichalcogenides (TMDCs) from different precursors. At present, quantitative control of the precursor with high precision and good repeatability is still challenging. Moreover, the process to synthesize TMDCs with designed patterns is complicated. Here, by using an industrial inkjet-printer, an in situ aqueous precursor with robust usage control at the picogram (10-12 g) level is achieved, and by precisely tuning the inkjet-printing parameters, followed by a rapid heating process, large-area patterned TMDC films with centimeter size and good thickness controllability, as well as heterostructures of the TMDCs, are achieved facilely, and high-quality single-domain monolayer TMDCs with millimeter-size can be easily synthesized within 30 s (corresponding to a growth rate up to 36.4 µm s-1 ). The resulting monolayer MoS2 and MoSe2 exhibits excellent electronic properties with carrier mobility up to 21 and 54 cm2 V-1 s-1 , respectively. The study paves a simple and robust way for the in situ ultrafast and patterned growth of high-quality TMDCs and heterostructures with promising industrialization prospects. Moreover, this ultrafast and green method can be easily used for synthesis of other 2D materials with slight modification.

Interface Transparency and Rashba Spin Torque Enhancement in WSe2 Heterostructures.

Rashba spin current generation emerges in heterostructures of ferromagnets and transition metal dichalcogenides (TMDs) due to an interface polarization and associated inversion symmetry breaking. Recent work exploring the synthesis and transfer of epitaxial films on the top of low layer count 2D materials reveals that atomic potentials from the underlying substrate interface are not completely screened. The extension of this transparency effect to other interfacial phenomena, such as the Rashba effect and associated spin torques, has not yet been demonstrated. Here, we report enhanced spin transfer torques from the Rashba spin current in heterostructures of permalloy (Py) and WSe2. We show that insertion of up to two monolayers of WSe2 enhances the spin transfer torques in a Rashba system by up to 3×, without changing the fieldlike Rashba spin-orbit torque (SOT), a measure of interface polarization. Our results indicate that low layer count TMD films can be used as an interfacial "scattering promoter" in heterostructure interfaces without quenching the original polarization.

Epitaxial Single-Crystal Growth of Transition Metal Dichalcogenide Monolayers via the Atomic Sawtooth Au Surface.

Growth of 2D van der Waals layered single-crystal (SC) films is highly desired not only to manifest the intrinsic physical and chemical properties of materials, but also to enable the development of unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) films has not been established to date. Here, the SC growth of TMdC monolayers on a centimeter scale via the atomic sawtooth gold surface as a universal growth template is reported. The atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission electron microscopy. The anisotropic adsorption energy of the TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2 , WSe2 , MoS2 , the MoSe2 /WSe2 heterostructure, and W1- x Mox S2 alloys. This strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures on a wafer scale, to further facilitate the applications of TMdCs in post-silicon technology.

High-Crystalline Monolayer Transition Metal Dichalcogenides Films for Wafer-Scale Electronics.

Chemical vapor deposition (CVD) using liquid-phase precursors has emerged as a viable technique for synthesizing uniform large-area transition metal dichalcogenide (TMD) thin films. However, the liquid-phase precursor-assisted growth process typically suffers from small-sized grains and unreacted transition metal precursor remainders, resulting in lower-quality TMDs. Moreover, synthesizing large-area TMD films with a monolayer thickness is also quite challenging. Herein, we successfully synthesized high-quality large-area monolayer molybdenum diselenide (MoSe2) with good uniformity via promoter-assisted liquid-phase CVD process using the transition metal-containing precursor homogeneously modified with an alkali metal halide. The formation of a reactive transition metal oxyhalide and reduction of the energy barrier of chalcogenization by the alkali metal promoted the growth rate of the TMDs along the in-plane direction, enabling the full coverage of the monolayer MoSe2 film with negligible few-layer regions. Note that the fully selenized monolayer MoSe2 with high crystallinity exhibited superior electrical transport characteristics compared with those reported in previous works using liquid-phase precursors. We further synthesized various other monolayer TMD films, including molybdenum disulfide, tungsten disulfide, and tungsten diselenide, to demonstrate the broad applicability of the proposed approach.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Second-harmonic generation divergence-a method for domain size evaluation of 2D materials.

Single-atomic-layered materials are important for future electronics. They allow optoelectronic devices to be fabricated at the single-atomic layer level. A single-atomic-layered two-dimensional (2D) transition metal dichalcogenide (TMD) film is usually composed of randomly orientated single-crystalline domains, and the size distribution of the domains on a large-area film has a significant impact on the applications of the film, but the impact is difficult to characterize. We report an approach to evaluate the size of the single-crystalline domains by measuring the second-harmonic generation divergence caused by the domains of different orientations. Using this method, domain size mapping on an 8×8mm2 region of a continuous MoS2 film is achieved. This method provides a fast and efficient way of domain size characterization across a large area in a non-destructive and transfer-free manner for single-atomic-layered TMD films.

Defect-Induced in Situ Atomic Doping in Transition Metal Dichalcogenides via Liquid-Phase Synthesis toward Efficient Electrochemical Activity.

Transition metal dichalcogenides (TMDs), due to their fascinating properties, have emerged as potential next-generation semiconducting nanomaterials across diverse fields of applications. When combined with other material systems, precise control of the intrinsic properties of the TMDs plays a vital role in maximizing their performance. Defect-induced atomic doping through introduction of a chalcogen vacancy into the TMDs lattices is known to be a promising strategy for modulating their characteristic properties. As a result, there is a need to develop tunable and scalable synthesis routes to achieve vacancy-modulated TMDs. Herein, we propose a facile liquid-phase ligand exchange approach for scalable, uniform, and vacancy-tunable synthesis of TMDs films. Varying the relative molar ratio of the chalcogen to transition metal precursors enabled the in situ modulation of the chalcogen vacancy concentrations without necessitating additional post-treatments. When employed as the electrocatalyst in the hydrogen evolution reaction (HER), the vacancy-modulated TMDs, exhibiting a synergetic effect on the energy level matching to the reduction potential of water and optimized free energy differences in the HER pathways, showed a significant enhancement in the hydrogen production via the improved charge transfer kinetics and increased active sites. The proposed approach for synthesizing tunable vacancy-modulated TMDs with wafer-scale synthesis capability is, therefore, promising for better practical applications of TMDs.

Wafer-Scale Growth of Pristine and Doped Monolayer MoS2 Films for Electronic Device Applications

Scalable production and controlled doping of large-area two-dimensional transition-metal dichalcogenide films are fundamental steps toward their applications in electronic devices. Although a variety of methods for preparation of wafer-scale transition-metal dichalcogenide films have been developed, it is still challenging to realize homogeneous doping of the large-area films to modulate their electronic properties. In this paper, we report a new chemical vapor deposition (CVD) method for preparation of wafer-scale pristine and doped monolayer MoS2 films on 2-inch sapphire wafers. The molybdenum precursors are supplied in a "face-to-face" manner from a silica gel plate to the sapphire wafer, which guarantees uniform nucleation and growth of monolayer MoS2. This method can be used to prepare substitutionally doped monolayer MoS2 films. By using ReCl3 as the dopant precursor, we have obtained continuous Re-doped monolayer MoS2 films on sapphire wafers. Elemental analysis confirms successful Re-doping of the MoS2 film. Spherical aberration-corrected scanning transmission electron microscopy characterization reveals that the Re atoms are incorporated at the substitutional Mo sites in the MoS2 lattice. The incorporation of Re atoms leads to n-type doping of MoS2 as evidenced by Kelvin probe force microscope studies. Electrical measurements reveal that the transport properties of the Re-doped monolayer MoS2 is dramatically enhanced as compared with the pristine MoS2. The CVD method developed in this study can be applied to the production of a variety of two-dimensional transition-metal dichalcogenide films suitable for applications in electronic devices.