Practical Route for the Low-Temperature Growth of Large-Area Bilayer Graphene on Polycrystalline Nickel by Cold-Wall Chemical Vapor Deposition

Recent Advances on Pulsed Laser Deposition of Large-Scale Thin Films

2D thin films, possessing atomically thin thickness, are emerging as promising candidates for next-generation electronic devices, due to their novel properties and high performance. In the early years, a wide variety of 2D materials are prepared using several methods (mechanical/liquid exfoliation, chemical vapor deposition, etc.). However, the limited size of 2D flakes hinders their fundamental research and device applications, and hence the effective large-scale preparation of 2D films is still challenging. Recently, pulsed laser deposition (PLD) has appeared to be an impactful method for wafer-scale growth of 2D films, owing to target-maintained stoichiometry, high growth rate, and efficiency. In this review, the recent advances on the PLD preparation of 2D films are summarized, including the growth mechanisms, strategies, and materials classification. First, efficacious strategies of PLD growth are highlighted. Then, the growth, characterization, and device applications of various 2D films are presented, such as graphene, h-BN, MoS2, BP, oxide, perovskite, semi-metal, etc. Finally, the potential challenges and further research directions of PLD technique is envisioned.

Characteristics of Graphene Growth at Different Temperatures from the Benzene Ring Structure in Coal Tar

A large number of aromatic substances can be found in so-called coal tar (containing >10,000 individual compounds), which is a mixture of heavy liquid fractions (dense viscous black liquor, tended to solidification) obtained after the pyrolysis of coal (solid product—coke, gas products, and light liquid products are also produced during the process). Volatile monocyclic aromatic hydrocarbons, which are naturally occurring in coal tar, can be exploited as premium raw materials for the production of graphene by chemical vapor deposition (CVD). Moreover, aromatic chemicals (compounds with benzene rings) can produce graphene at lower temperatures than other small-molecule gas feedstocks (for graphene growth via methane gas, the temperature must be at least 900 °C). The intermediate reaction mechanism involved in the creation of graphene from various temperature ranges of monocyclic aromatic hydrocarbons in benzene ring structures has long been a fascinating enigma. Accordingly, in this paper, we analyze the graphene growth pattern of benzene at different temperatures from 300 to 900 °C. For graphene synthesis in the lower temperature range (300~600 °C), analytical experiments show that benzene rings (almost) do not crack during the gas phase process. Thus, the structure of the benzene ring is directly coupled into graphene in the above temperature range. When benzene is more thoroughly transformed into tiny molecules that are deposited on the surface of copper foil at higher temperatures (700~900 °C), graphene is formed by a complex mixture of carbon sources, including gaseous small molecules (methane and ethane) and benzene. Based on the process above, we provide an alternative solution for the large-scale industrial preparation of graphene, with low energy consumption, via low-temperature synthesis of graphene by the CVD method using the coal tar carbon source at 500 °C, which is the optimal growth temperature of the benzene ring.

Low-Temperature Nitrogen Doping of Nanocrystalline Graphene Films with Tunable Pyridinic-N and Pyrrolic-N by Cold-Wall Plasma-Assisted Chemical Vapor Deposition

We report a viable method to produce nanocrystalline graphene films on polycrystalline nickel (Ni) with enhanced N doping at low temperatures by a cold-wall plasma-assisted chemical vapor deposition (CVD) method. The growth of nanocrystalline graphene films was carried out in a benzene/ammonia/argon (C₆H₆/NH₃/Ar) system, in which the temperature of the substrate heated by Joule heating can be further lowered to 100 °C to achieve a low sheet resistance of 3.3 kΩ sq^-1 at a high optical transmittance of 97.2%. The morphological, structural, and electrical properties and the chemical compositions of the obtained N-doped nanocrystalline graphene films can be tailored by controlling the growth parameters. An increase in the concentration of atomic N from 1.42 to 11.28 atomic percent (at.%) is expected due to the synergetic effects of a high NH₃/Ar ratio and plasma power. The possible growth mechanism of nanocrystalline graphene films is also discussed to understand the basic chemical reactions that occur at such low temperatures with the presence of plasma as well as the formation of pyridinic-N- and pyrrolic-N-dominated nanocrystalline graphene. The realization of nanocrystalline graphene films with enhanced N doping at 100 °C may open great potential in developing future transparent nanodevices.