AACVD of Molybdenum Sulfide and Oxide Thin Films From Molybdenum(V)-based Single-source Precursors**

Substrate-induced strain in molybdenum disulfide grown by aerosol-assisted chemical vapor deposition

Transition metal dichalcogenides have been extensively studied in recent years because of their fascinating optical, electrical, and catalytic properties. However, low-cost, scalable production remains a challenge. Aerosol-assisted chemical vapor deposition (AACVD) provides a new method for scalable thin film growth. In this study, we demonstrate the growth of molybdenum disulfide (MoS2) thin films using AACVD method. This method proves its suitability for low-temperature growth of MoS2thin films on various substrates, such as glass, silicon dioxide, quartz, silicon, hexagonal boron nitride, and highly ordered pyrolytic graphite. The as-grown MoS2shows evidence of substrate-induced strain. The type of strain and the morphology of the as-grown MoS2highly depend on the growth substrate's surface roughness, crystallinity, and chemical reactivity. Moreover, the as-grown MoS2shows the presence of both direct and indirect band gaps, suitable for exploitation in future electronics and optoelectronics.

Aerosol-Assisted Chemical Vapor Deposition of MoS2 with a Thiourea Sulfur Source: Single-Source Precursors vs Coreactant Mixtures

Aerosol-assisted chemical vapor deposition of MoS2 from solutions containing the single-source precursors cis-Mo(CO)4(TMTU)2 and Mo(CO)5(TMTU) in toluene was compared to depositions from the coreactant solution containing Mo(CO)6 and uncoordinated TMTU in toluene. The results were used to assess the significance of ligand precoordination on the properties of the deposited films. Raman spectra and GI-XRD patterns of the films show that the single-source precursors produced more intense and sharper signals for 2H-MoS2 as compared to the coreactant system of Mo(CO)6 and TMTU, which is indicative of improved crystallinity. SEM and XPS were also used to assess morphology and film composition. Thermolysis of cis-Mo(CO)4(TMTU)2 and analysis of the pyrolysis products by GC-MS and 1H NMR suggested a decomposition mechanism of the TMTU ligand where a terminal sulfido is generated on the molybdenum center with loss of a heteroatom stabilized carbene.

Dithiocarbamate Complexes as Single Source Precursors to Nanoscale Binary, Ternary and Quaternary Metal Sulfides

Nanodimensional metal sulfides are a developing class of low-cost materials with potential applications in areas as wide-ranging as energy storage, electrocatalysis, and imaging. An attractive synthetic strategy, which allows careful control over stoichiometry, is the single source precursor (SSP) approach in which well-defined molecular species containing preformed metal-sulfur bonds are heated to decomposition, either in the vapor or solution phase, resulting in facile loss of organics and formation of nanodimensional metal sulfides. By careful control of the precursor, the decomposition environment and addition of surfactants, this approach affords a range of nanocrystalline materials from a library of precursors. Dithiocarbamates (DTCs) are monoanionic chelating ligands that have been known for over a century and find applications in agriculture, medicine, and materials science. They are easily prepared from nontoxic secondary and primary amines and form stable complexes with all elements. Since pioneering work in the late 1980s, the use of DTC complexes as SSPs to a wide range of binary, ternary, and multinary sulfides has been extensively documented. This review maps these developments, from the formation of thin films, often comprised of embedded nanocrystals, to quantum dots coated with organic ligands or shelled by other metal sulfides that show high photoluminescence quantum yields, and a range of other nanomaterials in which both the phase and morphology of the nanocrystals can be engineered, allowing fine-tuning of technologically important physical properties, thus opening up a myriad of potential applications.

Room-Temperature Patterning of Nanoscale MoS2 under an Electron Beam

Molybdenum disulfide (MoS₂) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the shape and size of MoS₂ crystals that can be patterned precisely at required positions. Here, we describe a method of direct nanoscale patterning of MoS₂ at room temperature by exposing a molybdenum thiocubane single-source precursor to a beam of electrons. Molybdenum thiocubanes with various alkylxanthate moieties [Mo₄S₄(ROCS₂)₆, where R = alkyl] were prepared using a "self-assembly" approach. Micro-Raman and micro-FTIR spectroscopic studies suggest that exposure to a relatively smaller dose of electrons results in the breakdown of xanthate moieties, leading to the formation of MoS₂. High-resolution transmission electron micrographs suggest that the growth of MoS₂ most likely happens along (100) planes. An electron-beam-induced chemical transformation of a molybdenum thiocubane resist was exploited to fabricate sub-10 nm MoS₂ lines and dense dots as small as 13 nm with a pitch of 33 nm. Since this technique does not require the liftoff and etching steps, patterning of MoS₂ with interesting shapes, sizes, and thicknesses potentially leading to tunable band gap is possible.

A new metalorganic chemical vapor deposition process for MoS2 with a 1,4-diazabutadienyl stabilized molybdenum precursor and elemental sulfur

Crystalline MoS₂ thin films are deposited via MOCVD using a new molybdenum precursor, 1,4-di-tert-butyl-1,4-diazabutadienyl-bis(tert-butylimido)molybdenum(vi) [Mo(N^tBu)₂(^tBu₂DAD)], and elemental sulfur.

The synthesis and characterization of Cu2ZnSnS4 thin films from melt reactions using xanthate precursors

Kesterite, Cu₂ZnSnS₄ (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu₂ZnSnS₄ (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10⁺¹⁵ vs 4.55 × 10⁺¹⁷ cm^-3) and mobility (87.1 vs 11.1 cm² V^-1 s^-1). The Hall coefficients were 2.36 × 10³ versus 13.7 cm³ C^-1.

Aerosol-assisted chemical vapor deposition of WS2 from the single source precursor WS(S2)(S2CNEt2)2

WS(S₂)(S₂CNEt₂)₂ is a single source precursor for deposition of nanostructured WS₂ above 350 °C.

Solution based CVD of main group materials

This critical review focuses on the solution based chemical vapour deposition (CVD) of main group materials with particular emphasis on their current and potential applications. Deposition of thin films of main group materials, such as metal oxides, sulfides and arsenides, have been researched owing to the array of applications which utilise them including solar cells, transparent conducting oxides (TCOs) and window coatings. Solution based CVD processes, such as aerosol-assisted (AA)CVD have been developed due to their scalability and to overcome the requirement of suitably volatile precursors as the technique relies on the solubility rather than volatility of precursors which vastly extends the range of potentially applicable compounds. An introduction into the applications and precursor requirements of main group materials will be presented first followed by a detailed discussion of their deposition reviewed according to this application. The challenges and prospects for further enabling research in terms of emerging main group materials will be discussed.