2D materials are of significant interest due to their unique physical, chemical, and electronic properties. They consist of atomically thin layers with applications in fields ranging from semiconductors to energy storage. Chemical vapor deposition (CVD) is a critical technique for the controlled synthesis of these materials, allowing for high-quality film deposition and scalability. CVD involves the gas-phase reaction of volatile precursors that react or decompose on a substrate, forming a solid thin film. The method is renowned for its flexibility, precision, and ability to tailor material properties through various process parameters.
Fig.1 Growing 2D materials using a flow CVD reactor[1].
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2D Precursors and Carrier Gases in CVD Growth of Materials
| Precursors | Metal-organic precursors | for example, high quality WSe2 films can be synthesized using metal-organics such as Wo(CO)6 and (CH3)2Se as precursors. |
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| Solid-state precursors | in the growth of metal sulfides, two solid-state precursors, one metal-containing and the other non-metallic, are usually required, which are located in different heating regions. |
| Low melting point precursors | e.g. metal iodides, used for CVD growth of 2D materials at low temperatures. |
| Carrier gas | Hydrogen (H2) | One of the commonly used carrier gases that reacts with certain materials and helps in the deposition process. |
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| Argon (Ar) | As a carrier gas, it helps to minimize the lattice mismatch between the film and the substrate, and in some cases is more favorable to film growth than nitrogen. |
| Nitrogen (N2) | One of the common carrier gases used to maintain the environment in the reaction chamber. |
CVD Growth of Graphene
Graphene, a monolayer of sp2 bonded carbon atoms arranged in a honeycomb lattice, has become one of the most studied 2D materials due to its excellent electrical and mechanical properties. CVD growth of graphene typically involves carbon precursors, such as methane or ethylene, which are pyrolyzed at high temperatures (900-1100°C) on a metal substrate.
The most commonly used carbon sources are gaseous hydrocarbons. Metal substrates such as copper and nickel are preferred for graphene growth due to their catalytic properties. Copper is particularly suitable for producing monolayer graphene via a surface growth mechanism because it has a low carbon solubility. This allows graphene to nucleate and grow on the surface, forming large, high-quality monolayers.
There are two mechanisms for graphene growth on metal substrates: surface growth and carbon dissolution decomposition. Surface growth occurs on metals with low carbon solubility, such as copper, where graphene grows directly on the metal surface. In contrast, metals such as nickel promote carbon dissolution at high temperatures, which then form graphene upon cooling, allowing multilayer growth.
Fig.2 Growth mechanism of graphene: (a) carburization and carbon deposition mechanism and (b) surface growth mechanism[2].
hBN CVD Growth
Hexagonal boron nitride (hBN) is another key 2D material that is often used as a substrate or insulating layer in heterostructures due to its atomically flat surface and excellent thermal stability.
Typical precursors for hBN growth include borazane and ammonia borane. Controlling the ratio of B and N sources is critical to achieve stoichiometric hBN with a 1:1 atomic ratio of boron and nitrogen.
hBN growth requires high temperatures (typically above 1000°C). The choice of metal substrate, pressure, and temperature significantly affects the quality and thickness of the hBN layer. Copper substrates favor the growth of monolayer hBN films, while nickel substrates favor multilayer growth.
Transition Metal Dichalcogenides CVD Growth
Transition metal dichalcogenides (TMDs), such as MoS2, WS2, and MoSe2, are emerging 2D materials with tunable band gaps, making them attractive for electronic and optoelectronic applications.
TMDs are typically synthesized using metal oxide precursors such as MoO3 or WO3 and a chalcogen source such as sulfur or selenium. During the CVD process, the metal oxide is reduced and reacts with the chalcogen to form the TMD layer. Growth temperatures are typically between 600-800°C and the reaction is performed in a controlled atmosphere, using argon or nitrogen as a carrier gas.
The ability to stack TMDs with other 2D materials such as graphene and hBN has enabled the creation of new heterostructures. These heterostructures have unique electronic, optical, and mechanical properties, unlocking new capabilities for advanced devices.
Fig.3 Growth behavior, new materials/structures, properties and applications of 2D TMDs[3].
In asymmetric synthesis, QACs exhibit high selectivity, ensuring the formation of enantiomerically pure compounds. These attributes make QACs highly valuable in pharmaceutical and fine chemical industries, where purity and precision are critical.
CVD Growth of 2D Heterostructures
2D heterostructures involve the vertical stacking of different 2D materials to form systems with combined or even new properties. CVD growth of heterostructures requires careful control of the thickness, orientation, and interface quality of the individual layers.
CVD allows for precise control of the deposition sequence, allowing for the sequential growth of different 2D materials such as graphene, hBN, and TMDs. For example, a heterostructure consisting of graphene and MoS2 can be grown by first depositing a layer of graphene and then growing MoS2 on top of it via CVD. This layered approach allows the integration of materials with different electronic and mechanical properties.
Challenges and Solutions: One of the key challenges in growing 2D heterostructures is managing the lattice mismatch between the different materials. This mismatch induces strain at the interface, which affects the performance of the heterostructure. To mitigate this, substrates such as hBN are often used, which have an almost perfect lattice match with graphene. In addition, the growth temperature and pressure need to be precisely controlled to ensure smooth and defect-free integration of the layers.
References
- Bhowmik S, et al. (2022). "Chemical Vapor Deposition of 2D Materials: A Review of Modeling, Simulation, and Machine Learning Studies." iScience, 25(3), 103832.
- Li XS, et al. (2009). "Evolution of Graphene Growth on Ni and Cu by Carbon Isotope Labeling." Nano Letters, 9(12): 4268-4272.
- Zhang Y, et al. (2019). "Recent Progress in CVD Growth of 2D Transition MetalDichalcogenides and Related Heterostructures." Adv. Mater, 31, 1901694.
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