Growth, structure and stability of sputter-deposited MoS2 thin films

Molybdenum(IV) dithiocarboxylates as single-source precursors for AACVD of MoS2 thin films

Tetrakis(dithiocarboxylato)molybdenum(IV) complexes of the type Mo(S2CR)4 (R = Me, Et, iPr, Ph) were synthesized, characterized, and prescreened as precursors for aerosol assisted chemical vapor deposition (AACVD) of MoS2 thin films. The thermal behavior of the complexes as determined by TGA and GC-MS was appropriate for AACVD, although the complexes were not sufficiently volatile for conventional CVD bubbler systems. Thin films of MoS2 were grown by AACVD at 500 °C from solutions of Mo(S2CMe)4 in toluene. The films were characterized by GIXRD diffraction patterns which correspond to a 2H-MoS2 structure in the deposited film. Mo-S bonding in 2H-MoS2 was further confirmed by XPS and EDS. The film morphology, vertically oriented structure, and thickness (2.54 μm) were evaluated by FE-SEM. The Raman E12g and A1g vibrational modes of crystalline 2H-MoS2 were observed. These results demonstrate the use of dithiocarboxylato ligands for the chemical vapor deposition of metal sulfides.

Large-scale few-layered MoS2 as a saturable absorber for Q-switching operation at 2.3 µm

In this Letter, the fabrication of large-scale (50.8 mm in diameter) few-layered MoS₂ with physical vapor deposition on sapphire is described. Open-aperture Z-scan technology with a home-made excitation source at 2275 nm was applied to explore its nonlinear saturable absorption properties. The as-grown few-layered MoS₂ membrane possessed a modulation depth of 17% and a saturable intensity of 1.185 MW cm^-2. As a consequence, the deposited MoS₂ membrane was exploited as a saturable absorber to realize a passively Q-switched Tm:YAP laser for the first time, to the best of our knowledge. Pulses as short as 316 ns were generated with a repetition rate of 228 kHz, corresponding to a peak power of 5.53 W. Results confirmed that the two-dimensional layered MoS₂ could be beneficial for mid-infrared photonic applications.

Recent Progress in the Synthesis of MoS2 Thin Films for Sensing, Photovoltaic and Plasmonic Applications: A Review

In the surge of recent successes of 2D materials following the rise of graphene, molybdenum disulfide (2D-MoS2) has been attracting growing attention from both fundamental and applications viewpoints, owing to the combination of its unique nanoscale properties. For instance, the bandgap of 2D-MoS2, which changes from direct (in the bulk form) to indirect for ultrathin films (few layers), offers new prospects for various applications in optoelectronics. In this review, we present the latest scientific advances in the field of synthesis and characterization of 2D-MoS2 films while highlighting some of their applications in energy harvesting, gas sensing, and plasmonic devices. A survey of the physical and chemical processing routes of 2D-MoS2 is presented first, followed by a detailed description and listing of the most relevant characterization techniques used to study the MoS2 nanomaterial as well as theoretical simulations of its interesting optical properties. Finally, the challenges related to the synthesis of high quality and fairly controllable MoS2 thin films are discussed along with their integration into novel functional devices.

Effects of Deposition and Annealing Temperature on the Structure and Optical Band Gap of MoS2 Films

In this study, molybdenum disulfide (MoS₂) film samples were prepared at different temperatures and annealed through magnetron sputtering technology. The surface morphology, crystal structure, bonding structure, and optical properties of the samples were characterized and analyzed. The surface of the MoS₂ films prepared by radio frequency magnetron sputtering is tightly coupled and well crystallized, the density of the films decreases, and their voids and grain size increase with the increase in deposition temperature. The higher the deposition temperature is, the more stable the MoS₂ films deposited will be, and the 200 °C deposition temperature is an inflection point of the film stability. Annealing temperature affects the structure of the films, which is mainly related to sulfur and the growth mechanism of the films. Further research shows that the optical band gaps of the films deposited at different temperatures range from 0.92 eV to 1.15 eV, showing semiconductor bandgap characteristics. The optical band gap of the films deposited at 200 °C is slightly reduced after annealing in the range of 0.71–0.91 eV. After annealing, the optical band gap of the films decreases because of the two exciton peaks generated by the K point in the Brillouin zone of MoS₂. The blue shift of the K point in the Brillouin zone causes a certain change in the optical band gap of the films.

One-step H2S reactive sputtering for 2D MoS2/Si heterojunction photodetector

A technique for directly growing two-dimensional (2D) materials onto conventional semiconductor substrates, enabling high-throughput and large-area capability, is required to realise competitive 2D transition metal dichalcogenide devices. A reactive sputtering method based on H₂S gas molecules and sequential in situ post-annealing treatment in the same chamber was proposed to compensate for the relatively deficient sulfur atoms in the sputtering of MoS₂ and then applied to a 2D MoS₂/p-Si heterojunction photodevice. X-ray photoelectron, Raman, and UV-visible spectroscopy analysis of the as-deposited Ar/H₂S MoS₂ film were performed, indicating that the stoichiometry and quality of the as-deposited MoS₂ can be further improved compared with the Ar-only MoS₂ sputtering method. For example, Ar/H₂S MoS₂ photodiode has lower defect densities than that of Ar MoS₂. We also determined that the factors affecting photodetector performance can be optimised in the 8-12 nm deposited thickness range.

Fabrication and Properties of Molybdenum Disulfide Films for Electro-Optical Applications

MoS2 films on sapphire and SiO2/Si substrates were fabricated by sulfurization of sputtered Mo precursor films at 900∘C in a two-zone furnace. Structural, morphological, optical and electrical properties of the films were determined. The films of MoS2 were successfully grown on the substrates with polycrystalline structures and proposed to be used in electro-optical devices.

Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis of Porous Ensembles of MoS2 Nanocrystals for High Gain Photodetectors

Ultrafast synthesis of high-quality transition-metal dichalcogenide nanocrystals, such as molybdenum disulfide (MoS₂), is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solid-state synthesis route for MoS₂ using the chemically homogeneous molecular precursor, (NH₄)₂Mo₃S₁₃·H₂O, resulting in nanoparticles with estimated size down to 25 nm only in 10 s at 1000 °C. Despite the extreme nonequilibrium conditions, the resulting porous MoS₂ nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals, which are composed of the MoS₂ nanoparticles. Despite the very rapid escape of massive amounts of NH₃, H₂O, H₂S, and S gases from the (NH₄)₂Mo₃S₁₃·H₂O mm sized crystals, they retain their original shape as they convert to MoS₂ rather than undergo explosive destruction from the rapid escape process of the gases. The obtained pseudocrystals are made of aggregated MoS₂ nanocrystals exhibit a Brunauer-Emmett-Teller surface area of ∼35 m²/g with an adsorption average pore width of ∼160 Å. The nanoporous MoS₂ crystals are solution processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than 2 orders of magnitude higher conductivity (∼6.25 × 10^-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS₂ composite films. The optoelectronic properties of this nanoporous MoS₂ imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.

Atomic-Scale in Situ Observations of Crystallization and Restructuring Processes in Two-Dimensional MoS2 Films

We employ atomically resolved and element-specific scanning transmission electron microscopy (STEM) to visualize in situ and at the atomic scale the crystallization and restructuring processes of two-dimensional (2D) molybdenum disulfide (MoS2) films. To this end, we deposit a model heterostructure of thin amorphous MoS2 films onto freestanding graphene membranes used as high-resolution STEM supports. Notably, during STEM imaging the energy input from the scanning electron beam leads to beam-induced crystallization and restructuring of the amorphous MoS2 into crystalline MoS2 domains, thereby emulating widely used elevated temperature MoS2 synthesis and processing conditions. We thereby directly observe nucleation, growth, crystallization, and restructuring events in the evolving MoS2 films in situ and at the atomic scale. Our observations suggest that during MoS2 processing, various MoS2 polymorphs co-evolve in parallel and that these can dynamically transform into each other. We further highlight transitions from in-plane to out-of-plane crystallization of MoS2 layers, give indication of Mo and S diffusion species, and suggest that, in our system and depending on conditions, MoS2 crystallization can be influenced by a weak MoS2/graphene support epitaxy. Our atomic-scale in situ approach thereby visualizes multiple fundamental processes that underlie the varied MoS2 morphologies observed in previous ex situ growth and processing work. Our work introduces a general approach to in situ visualize at the atomic scale the growth and restructuring mechanisms of 2D transition-metal dichalcogenides and other 2D materials.

Feasible Route for a Large Area Few-Layer MoS₂ with Magnetron Sputtering

In this article, we report continuous and large-area molybdenum disulfide (MoS₂) growth on a SiO₂/Si substrate by radio frequency magnetron sputtering (RFMS) combined with sulfurization. The MoS₂ film was synthesized using a two-step method. In the first step, a thin MoS₂ film was deposited by radio frequency (RF) magnetron sputtering at 400 °C with different sputtering powers. Following, the as-sputtered MoS₂ film was further subjected to the sulfurization process at 600 °C for 60 min. Sputtering combined with sulfurization is a viable route for large-area few-layer MoS₂ by controlling the radio-frequency magnetron sputtering power. A relatively simple growth strategy is demonstrated here that simultaneously enhances thin film quality physically and chemically. Few-layers of MoS₂ are established using Raman spectroscopy, X-ray diffractometer, high-resolution field emission transmission electron microscope, and X-ray photoelectron spectroscopy measurements. Spectroscopic and microscopic results reveal that these MoS₂ layers are of low disorder and well crystallized. Moreover, high quality few-layered MoS₂ on a large-area can be achieved by controlling the radio-frequency magnetron sputtering power.