A Review on the Chemical Vapor Deposition Synthesis of 2D Materials and Their Applications

1. Introduction

Chemical Vapor Deposition (CVD) is a bottom-up method for the synthesis of 2D-hybrid materials nanoparticles. Two-dimensional (2D) hybrid materials are a class of nanomaterials that have two dimensions (XY-Plane) outside of the nano-metric size range and atomic-scale thicknesses (Z-Dimension). The CVD method of synthesis was first used by (Nickl et al., 1972). Later (Goto et al., 1987) used similar method to make monolithic Ti₃SiC₂. The first well-known 2D material was Graphene consisting of a single layer of carbon atoms arranged in a hexagonal lattice. Graphene by having excellent electronic, optical and mechanical properties lacks an electronic bandgap. Thus, ideal Graphene behaves like metallic materials in contrast to the semi-conductive behavior, this is most undesirable for an electronic application. This drawback has stimulated the scientists, researchers and engineers to open up the electronic band of Graphene or search the new 2D atomic crystals nanomaterials with semiconducting nature. The Transition Metal Chalcogenides (TMCs) or Dichalcogenides (TMDs) have been proven a better semiconducting character regarding 2D hybrid material. These 2D crystals exhibit high crystal quality, macroscopic continuity and better surface morphologies which find exhaustive exploitations in catalysis, optoelectronics and sensors based devices. The search for new layered materials does not stop at TMCs and TMDs only moreover, it has open the channel for further new hybrid materials. For instance, a new family of layered 2D hybrid materials has been found, known as MXenes. A family of 2D crystals typically composed of a transition metal paired with a chalcogen in a similar lamella structure resembling that of graphite. Some examples of the typical transition metals are Molybdenum (Mo) and Tungsten (W) whereas the chalcogens are typically Sulphur (S), Selenium (Se) and Tellurium (Te). Among reported TMDs, MoS₂ has a bandgap of 1.8eV, direct bandgap for single layer and 1.2eV, indirect bandgap for bulk layer. The weak interaction in the adjacent layers of MoS₂ helps to obtain its single-layer sheets through exfoliation. Other TMDs like MoTe₂ and WSe₂ being recent are unexplored found to have a high level of Valleytronics control and bandgap opening can be prepared by salt-assisted method using CVD.

CVD principle involves the adsorption of precursor molecules from the vapor phase onto a pre-heated substrate at ambient temperature where they break down to form a desired material by chemical reaction with the help of an inert or carrier gas flow like Argon or N₂. Gases like CH₄, H₂ or H₂S are also used with the carrier gases as elemental ingredients. These precursor species may have already begun thermal breakdown in the vapor phase. The precursor molecules are often volatile and are delivered to the heated substrate by an inert carrier gas. However, reactive gases such as oxygen or ammonia can also be used if oxidizing or reducing conditions are required. Precursors can either be single source, where all the elements of the desired material are contained within one molecule or multi source where different precursor molecules react together to produce the required material. Volatile byproducts matter produced during the reaction by the decomposing precursors are removed by vacuum or carrier gas with appropriate flow rate. Scalability is an advantage of CVD. CVD technique is a powerful bottom-up chemical approach to prepare high quality, uniform thickness films and modified heterostructures with faster growth rate.