Atmospheric Pressure CVD of TiSe2 Thin Films on Glass

Influence of the choice of precursors on the synthesis of two-dimensional transition metal dichalcogenides

The interest in transition metal dichalcogenides (TMDCs; MEy/2; M = transition metal; E = chalcogenide, y = valence of the metal) has grown exponentially across various science and engineering disciplines due to their unique structural chemistry manifested in a two-dimensional lattice that results in extraordinary electronic and transport properties desired for applications in sensors, energy storage and optoelectronic devices. Since the properties of TMDCs can be tailored by changing the stacking sequence of 2D monolayers with similar or dis-similar materials, a number of synthetic routes essentially based on the disintegration of bulk (e.g., chemical exfoliation) or the integration of atomic constituents (e.g., vapor phase growth) have been explored. Despite a large body of data available on the chemical synthesis of TMDCs, experimental strategies with high repeatability of control over film thickness, phase and compositional purity remain elusive, which calls for innovative synthetic concepts offering, for instance, self-limited growth in the z-direction and homogeneous lateral topography. This review summarizes the recent conceptual advancements in the growth of layered van der Waals TMDCs from both mixtures of metal and chalcogen sources (multi-source precursors; MSPs) and from molecular compounds containing metals and chalcogens in one starting material (single-source precursor; SSPs). The critical evaluation of the strengths, limitations and opportunities of MSP and SSP approaches is provided as a guideline for the fabrication of TMDCs from commercial and customized molecular precursors. For example, alternative synthetic pathways using tailored molecular precursors circumvent the challenges of differential nucleation and crystal growth kinetics that are invariably associated with conventional gas phase chemical vapor transport (CVT) and chemical vapor deposition (CVD) of a mixture of components. The aspects of achieving high compositional purity and alternatives to minimize competing reactions or side products are discussed in the context of efficient chemical synthesis of TMDCs. Moreover, a critical analysis of the potential opportunities and existing bottlenecks in the synthesis of TMDCs and their intrinsic properties is provided.

Iron-Intercalated Zirconium Diselenide Thin Films from the Low-Pressure Chemical Vapor Deposition of [Fe(η5-C5H4Se)2Zr(η5-C5H5)2]2

Transition metal chalcogenide thin films of the type Fe x ZrSe2 have applications in electronic devices, but their use is limited by current synthetic techniques. Here, we demonstrate the synthesis and characterization of Fe-intercalated ZrSe2 thin films on quartz substrates using the low-pressure chemical vapor deposition of the single-source precursor [Fe(η5-C5H4Se)2Zr(η5-C5H5)2]2. Powder X-ray diffraction of the film scraping and subsequent Rietveld refinement of the data showed the successful synthesis of the Fe0.14ZrSe2 phase, along with secondary phases of FeSe and ZrO2. Upon intercalation, a small optical band gap enhancement (E g(direct) opt = 1.72 eV) is detected in comparison with that of the host material.

Thioether complexes of WSCl4, WOCl4 and WSCl3 and evaluation of thiochloride complexes as CVD precursors for WS2 thin films

Thioether complexes of WECl₄ and WECl₃ (E = S, O) are prepared and characterised; the dinuclear [(WSCl₄)₂{ⁱPrS(CH₂)₂SⁱPr}] is shown to be an effective single source precursor for low pressure CVD of WS₂ thin films.

Applications of metal selenium/tellurium compounds in materials science

Metal chalcogenides are technologically important materials. Physical, chemical, electrical and mechanical properties of these materials can be fine-tuned by manipulating their shape, size and composition. Although several methods are employed for their synthesis, single-source molecular precursor route has emerged as a versatile strategy for their synthesis and in controlling shape, size and composition of the material under moderate conditions. This chapter gives a brief coverage on the design and development of single-source molecular precursors which have been employed for the preparation of metal selenide/telluride nanocrystals and for deposition of thin films. The discussion includes synthesis of transition-, main group and f-block metal chalcogenolate and/or chalcogenide clusters as precursors and their conversion into metal chalcogenides in the form of thin films and nanostructures. Precursors for ternary metal chalcogenides are also included.

van der Waals Metallic Transition Metal Dichalcogenides

Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and electrical properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics. Some of these also feature the topological manner. Instead of covering the semiconducting properties of transition metal dichalcogenides, which have been extensively revisited and reviewed elsewhere, here we present the structures of metallic transition metal dichalcogenides and their synthetic approaches for not only high-quality wafer-scale samples using conventional methods (e.g., chemical vapor transport, chemical vapor deposition) but also local small areas by a modification of the materials using Li intercalation, electron beam irradiation, light illumination, pressures, and strains. Some representative band structures of metallic transition metal dichalcogenides and their strong layer-dependence are reviewed and updated, both in theoretical calculations and experiments. In addition, we discuss the physical properties of metallic transition metal dichalcogenides such as periodic lattice distortion, magnetoresistance, superconductivity, topological insulator, and Weyl semimetal. Approaches to overcome current challenges related to these materials are also proposed.

Accessing new 2D semiconductors with optical band gap: synthesis of iron-intercalated titanium diselenide thin films via LPCVD

Fe-doped TiSe₂ thin-films were synthesized via low pressure chemical vapor deposition (LPCVD) of a single source precursor: [Fe(η⁵-C₅H₄Se)₂Ti(η⁵-C₅H₅)₂]₂ (1). Samples were heated at 1000 °C for 1–18 h and cooled to room temperature following two different protocols, which promoted the formation of different phases. The resulting films were analyzed by grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and UV/vis spectroscopy. An investigation of the Fe doping limit from a parallel pyrolysis study of Fe_xTiSe₂ powders produced in situ during LPCVD depositions has shown an increase in the Fe–TiSe₂–Fe layer width with Fe at% increase. Powders were analyzed using powder X-ray diffraction (PXRD) involving Rietveld refinement and XPS. UV/vis measurements of the semiconducting thin films show a shift in band gap with iron doping from 0.1 eV (TiSe₂) to 1.46 eV (Fe_0.46TiSe₂).

Fe-doped TiSe₂ thin-films were synthesized via low pressure chemical vapor deposition (LPCVD) of a single source precursor: [Fe(η⁵-C₅H₄Se)₂Ti(η⁵-C₅H₅)₂]₂ (1).

Macrocycles containing 1,1'-ferrocenyldiselenolato ligands on group 4 metallocenes

Macrocyclic [Fe(η5-C5H4Se)2M(η5-C5H4R)2]2 [M = Ti (1), Zr (2), Hf (3), R = H; and M = Zr (4), Hf (5), R = tBu] were prepared and characterized by 77Se NMR spectroscopy and the crystal structures of 1-3 and 5 were determined by single-crystal X-ray diffraction. The crystal structure of 4 is known and the complex is isomorphous with 5. 1-5 form mutually similar macrocyclic tetranuclear complexes in which the alternating Fe(C5H4Se)2 and M(C5H4R)2 centers are linked by selenium bridges. The thermogravimetric analysis (TGA) of 1-3 under a helium atmosphere indicated that the complexes undergo a two-step decomposition upon heating. The final products were identified using powder X-ray diffraction as FexMSe2, indicating their potential as single-source precursors for functional materials.

Chalcogenoether complexes of Nb(v) thio- and seleno-halides as single source precursors for low pressure chemical vapour deposition of NbS2 and NbSe2 thin films

A series of thioether complexes of NbSCl₃ has been prepared and selected examples demonstrated as precursors to 3R-NbS₂ films; deposition of 2H-NbSe₂ from [NbSe₂Cl₃(SeⁿBu₂)] is also reported.

Area Selective Growth of Titanium Diselenide Thin Films into Micropatterned Substrates by Low-Pressure Chemical Vapor Deposition

The neutral, distorted octahedral complex [TiCl₄(Se ⁿ Bu₂)₂] (1), prepared from the reaction of TiCl₄ with the neutral Se ⁿ Bu₂ in a 1:2 ratio and characterized by IR and multinuclear (¹H, ¹³C{¹H}, ⁷⁷Se{¹H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe₂, films onto SiO₂ and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe₂ (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe₂ films is 3.36 ± 0.05 × 10^-3 Ω·cm with a carrier density of 1 × 10²² cm^-3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO₂/TiN patterned substrates. TiSe₂ is selectively deposited within the smallest 2 μm diameter TiN holes of the patterned TiN/SiO₂ substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 μm hole size, whereas their lengths/widths continue to increase.

Preparation and characterisation of super-hydrophobic surfaces

The interest in highly water-repellent surfaces has grown in recent years due to the desire for self-cleaning surfaces. A super-hydrophobic surface is one that achieves a water contact angle of 150 degrees or greater. This article explores the different approaches used to construct super-hydrophobic surfaces and identifies the key properties of each surface that contribute to its hydrophobicity. The models used to describe surface interaction with water are considered, with attention directed to the methods of contact angle analysis. A summary describing the different routes to hydrophobicity is also given.