Continuous Monolayer MoS2 Research Articles

Controllable structure-engineered janus and alloy polymorphic monolayer transition metal dichalcogenides by plasma-assisted selenization process toward high-yield and wafer-scale production

Two-dimensional (2D) materials with an asymmetric structure like Janus transition metal dichalcogenides (TMDCs) exhibit intriguing electronic and optical properties, which can be used for numerous novel electronic and optoelectronic devices application. However, despite tremendous efforts, the synthesis technology of Janus TMDs has not been fully established, especially for high-yield and wafer-scale production. Here, we demonstrate the synthesis of Janus MoSSe by a top-down method for high-yield and wafer-scale production. In particular, we replace the top S atom of monolayer (ML) molybdenum disulfide (MoS2) with Se by controlled vertical plasma-assisted selenization process (PASP). We first synthesized ML Janus MoSSe flakes through PASP. A set of microscopies, spectroscopies and electrical measurements showed that low synthesis temperature (200 °C) leads to the formation of ML Janus MoSSe flake while high temperatures (400 °C and 600 °C) lead to the formation of polymorphic alloy MoSSe flakes. Furthermore, we show that Janus WSSe can likewise be obtained by PASP, indicating the universality of the process. To demonstrate the feasibility of PASP for wafer-scale production of Janus MoSSe, we perform the exact same process but with continuous ML MoS2 on 2-in. sapphire. Our synthesis approach provides a new scalable route to synthesize Janus TMDs toward future electronics.

TỔNG HỢP VẬT LIỆU NANO THẤP CHIỀU LÕI-VỎ MoS2\TiO2 ỨNG DỤNG TRONG XÚC TÁC ĐIỆN HÓA SINH KHÍ H2

In this report, we present the synthesis of low dimensional shell-core (2D\1D) nanostructures, in which the TiO2 nanorods (TNRs) as the core are covered by a continuous monolayer MoS2 (1L-MoS2) as the shell. The obtain 1L-MoS2\TNRs was directly grown on the conduct graphite foil without any transfer process, thus minimizing the charge transfer resistance from the electrode to the outer most working surface. For the first steps, the TiO2 seed layer was designed by the AF sputtering method, then the TNRs were grown by hydrothermal approach. Thereafter, the TNRs were conformally coated by a continuous monolayer MoS2 via a metal-organic chemical vapor deposition technique, resulting in 1L-MoS2\TNRs nanomaterial. The structural, vibrational, and morphological characteristics demonstrated that the samples are high crystallinity. Interestingly, the 1L-MoS2\TNRs showed highly efficient in electrochemical HER activity with the smallest onset overpotential of -140 mV vs RHE and a corresponding Tafel slope of 80 mV per decade, which were much lower compared to the pristine 1L-MoS2 and TNRs.

CVD growth of the centimeter-scale continuous 2D MoS2 film by modulating the release of Mo vapor with adjusting the particle size of Al2O3 microsphere

We report a convenient chemical vapor deposition (CVD) strategy of two-dimensional (2D) MoS2: the release of Mo vapor is modulated by adjusting the particle size of Al2O3 microsphere powder which uniformly cover on MoO3 precursor, so as to achieve controllable synthesis of MoS2. In this CVD system, Al2O3 is inactive, and the particle size of Al2O3 microsphere is the key parameter of affecting the release amount of Mo vapor. When the particle size of Al2O3 microsphere is 100 nm, a centimeter-scale continuous monolayer MoS2 can be obtained. This work provides a feasible approach to controllably synthesize 2D transition metal dichalcogenides (TMDCs).

Enhanced photoluminescence of monolayer MoS2 on stepped gold structure

Different MoS2/Au heterostructures can play an important role in tuning the photoluminescence (PL) and optoelectrical properties of monolayer MoS2. Previous studies of PL of MoS2/Au heterostructures were mainly limited to the PL enhancement by using different Au nanostructures and PL quenching of monolayer MoS2 on flat Au surfaces. Here, we demonstrate the enhanced excitonic PL emissions of monolayer MoS2/Au heterostructures on Si/SiO2 substrates. By transferring the continuous monolayer MoS2 onto a stepped Au structure consisting of 60-nm and 100-nm Au films, the MoS2/Au-60 and MoS2/Au-100 heterostructures exhibit enhanced PL emissions, each with a blue-shifted PL peak in comparison with the MoS2/SiO2. Furthermore, the PL intensity of MoS2/Au-60 is about twice larger than that of MoS2/Au-100. The different enhanced excitonic PL emissions in MoS2/Au heterostructures can be attributed to the different charge transfer effects modified by the stepped Au structure. This work may provide an insight into the excitonic PL and charge transfer effect of MoS2 on Au film and yield novel phenomena in MoS2/Au heterostructures for further study of PL tuning and optoelectrical properties.

Millimeter-Scale Continuous Film of MoS2 Synthesized Using a Mo, Na, and Seeding Promoter-Based Coating as a Solid Precursor.

While the chemical vapor deposition technique can be used to fabricate 2D materials in a larger area, materials like MoS2 have limited controllability due to their lack of self-controlling nature. This article presents a new technique for synthesizing a void-free millimeter-scale continuous monolayer MoS2 film through the diffusion of a well-controlled Mo, Na, and seeding promoter-based coating under a low-pressure N2 atmosphere. Compared to the conventional method, this technique provides precise control of solid precursors, where MoS2 grows next to the coating. At 800 °C, the synthesized MoS2 showed a uniform single-layer MoS2 film; however, a Na-free coating showed nanoscale voids and poor crystal quality, which are attributed to a higher edge-attachment barrier that slows down the MoS2 lateral growth. The synthesized MoS2 with Na-containing solution showed an intense PL peak with a 1.86 eV band gap. Even at the relatively low temperature of 700 °C, compared to the Na-excluded condition, MoS2 showed almost two times higher area coverage with a comparatively larger crystal size. This finding may assist in the future development of MoS2-based electronic and optoelectronic devices such as transistors and photodetectors.

Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production.

As one of the promising low-cost and high-efficiency catalysts for the electrochemical hydrogen evolution reaction (HER), it is well-known that there are both tiny exposed catalytic active edge sites and large-area inert basal planes in two-dimensional MoS2 structures. For enhancing its HER activity, extensive work has been done to activate the inert basal plane of MoS2. In this article, wafer-scale (2 in.) continuous monolayer MoS2 films with substantial in situ generated sulfur vacancies are fabricated by employing the laser molecular beam epitaxy process benefitting from ultrahigh vacuum growth condition and high substrate temperature. The intrinsic sulfur vacancies throughout the wafer-scale basal plane present an ideal electrocatalytic platform for massive hydrogen production. The fabricated vacancy-rich monolayer MoS2 can achieve a current density of -10 mA/cm2 at an overpotential of -256 mV. The wafer-scale fabrications of sulfur vacancy-rich monolayer MoS2 provide great leaps forward in the practical application of MoS2 for massive hydrogen production.

Growth Mechanism of Continuous Monolayer MoS2 Prepared by Chemical Vapor Deposition

Molybdenum disulfide (MoS2) is a cutting-edge layer-dependent two dimensional semiconductor which monolayer is direct-bandgap. Nano-scale monolayer MoS2 has big potential in electronics and optoelectronics devices. In this work we reported the progress in growing continuous single layer MoS2 by ambient pressure chemical vapor deposition (APCVD). Scanning electron microscope (SEM), Raman, photoluminescence spectra (PL) and atomic force microscopy (AFM) disclose that as-grown films are large-area monolayer and of high quality. SEM observations also clearly reveal the growth process of these films. Figuring out the growth mechanism grants growth of large scale continuous MoS2, and lays the foundation for wide device applications in the future.

Controllable growth of continuous monolayer MoS2 by balancing the moles of gaseous precursors via argon flow

Large-area, uniform, and high quality continuous monolayer MoS2 was successfully grown on a SiO2/Si substrate, demonstrated using diverse analytical testing techniques.

Investigation of the Growth Process of Continuous Monolayer MoS2 Films Prepared by Chemical Vapor Deposition

Recently, great efforts have been devoted to study of molybdenum disulfide (MoS2), particularly monolayer MoS2 semiconductor thin films, due to its excellent electrical and optical properties. Direct growth of continuous monolayer MoS2 films by ambient-pressure chemical vapor deposition is reported herein. Optical microscopy, Raman spectroscopy, photoluminescence spectra (PL), atomic force microscopy, x-ray photoelectron spectroscopy (XPS), and high-resolution transmission electronic microscopy were used to characterize the electronic and structural properties of the films, demonstrating that the MoS2 films grown on silicon dioxide/silicon (SiO2/Si) substrate with spatial size on micron scale were high quality, single crystal, continuous, and monolayer. Raman and PL mapping were performed to confirm the uniformity of the monolayer MoS2 films. The morphological variation of the MoS2 films after different reaction times was observed by optical microscopy and scanning electron microscopy, revealing the growth process and thus helping to understand that the growth mechanism during synthesis of continuous large-area films depends on the distribution of the reactive intermediate molybdenum oxide (MoO3−x) due to its lower saturation vapor density. Back-gated transistors based on MoS2 films were fabricated, exhibiting current on/off ratio of ∼ 104 and subthreshold swing (SS) of 0.44 V dec−1. This work contributes to synthesis of large-area continuous films, thus paving the way for future scaled-up fabrication of MoS2 electronic devices.

Large-area flexible photodetector based on atomically thin MoS2/graphene film

In this paper, we demonstrate a large-area flexible photodetector based on vertically stacked atomically thin MoS2/graphene film. The MoS2 film is grown by CVD method. We have realized the growth of several square millimeter continuous monolayer MoS2 film via precisely control the atmosphere and temperature in the tube furnace. In experiment, the MoS2 and graphene film was transferred on PET substrate successively. Polymer electrolyte composed of PEO and LiClO4 was introduced to fabricate the side-gated flexible photodetector. The devices exhibit high performance at low operation voltage. External responsivity is about 3.5 A/W at Vgs = −1 V, Vds = 1 V with incident power of 60 μW (520 nm). Noteworthy is that the transmittance of the heterostructure at 520 nm is only about 10.5%, the internal responsivity even reaches to about 33.3 A/W, which is higher than most of the flexible photodetectors based on nanomaterials. Besides, the devices display high stability during transient on/off test. We believe the MoS2/graphene photodetector would be a promising candidate in flexible electronic system. Besides, such hybrid film design is also portable and meaningful for future photodetector investigation.

Salt-assisted clean transfer of continuous monolayer MoS2 film for hydrogen evolution reaction

The transfer of two-dimensional (2D) materials from one substrate to another is challenging but of great importance for technological applications. Here, we propose a facile etching and residue-free method for transferring a large-area monolayer MoS2 film continuously grown on a SiO2/Si by chemical vapor deposition. Prior to synthesis, the substrate is dropped with water- soluble perylene-3, 4, 9, 10-tetracarboxylic acid tetrapotassium salt (PTAS). The as-grown MoS2 on the substrate is simply dipped in water to quickly dissolve PTAS to yield the MoS2 film floating on the water surface, which is subsequently transferred to the desired substrate. The morphological, optical and X-ray photoelectron spectroscopic results show that our method is useful for fast and clean transfer of the MoS2 film. Specially, we demonstrate that monolayer MoS2 film transferred onto a conducting substrate leads to excellent performance for hydrogen evolution reaction with low overpotential (0.29V vs the reversible hydrogen electrode) and Tafel slope (85.5mV/decade).

(Invited) Transition Metal Dichalcogenide Semiconductor Growth and Large Area Devices for Optoelectronics and Sensing

Atomically-thin transition metal dichalcogenides, such as MoS2, are topological materials with physical characteristics of great interest for the realization of nano-electronic and photonic devices, with promising applications for chemical and biological sensing. We first report here the growth and characterization of continuous monolayer MoS2 films over large areas by chemical vapor deposition on both sapphire and silicon substrates. The effects of growth temperature were studied. The materials were subsequently characterized using Raman spectroscopy, photoluminescence, fluorescence imaging, optical absorption, atomic force microscopy, and electron microscopy. Large area metal-2D semiconductor-metal photodetector and gas sensor devices were then realized by conventional photolithography using an interdigitated pattern. The gas sensor was able to detect CO2 concentrations as low as 200 ppm, while the photodetector exhibited a detectivity of ~8x1011 Jones at room temperature.

Growth mechanism of largescale MoS2 monolayer by sulfurization of MoO3 film

Monolayer two-dimensional transition metal dichalcogenides (TMDCs) such as MoS2 with broken inversion symmetry possesses two degenerate yet inequivalent valleys that can be selectively excited by circularly polarized light. This unique property renders interesting valley physics. The ability to manipulate valley degrees of freedom with light or external field makes them attractive for optoelectronic and spintronic applications. There is great demand for large area monolayer (ML) TMDCs for certain measurements and device applications. Recent reports on large area ML TDMCs focus on chemical vapor deposition growth. In this work, we report a facile approach to grow largescale continuous ML MoS2 nearly free of overgrowth and voids, by sulfurizing evaporated molybdenum trioxide ultrathin films. Photo conductivity scales with device sizes up to 4.5 mm, suggesting excellent film uniformity. The growth mechanism is found to be vaporization, diffusion, sulfurization and lateral growth, all at local micrometer scale. Our approach provides a new pathway for large-area ML TMDC growth and lithography-free device fabrication.