Kamal P. Sharma

Structure dependent hydrogen induced etching features of graphene crystals

H2 induced etching of graphene is of significant interest to understand graphene growth process as well as to fabricate nanoribbons and various other structures. Here, we demonstrate the structure dependent H2 induced etching behavior of graphene crystals. We synthesized graphene crystals on electro-polished Cu foil by an atmospheric pressure chemical vapor deposition process, where some of the crystals showed hexagonal shaped snowflake-dendritic morphology. Significant differences in H2 induced etching behavior were observed for the snowflake-dendritic and regular graphene crystals by annealing in a gas mixture of H2 and Ar. The regular graphene crystals were etched anisotropically creating hexagonal holes with pronounced edges, while etching of all the dendritic crystals occurred from the branches of lobs creating symmetrical fractal structures. The etching behavior provides important clue of graphene nucleation and growth as well as their selective etching to fabricate well-defined structures for nanoelectronics.

Edge controlled growth of hexagonal boron nitride crystals on copper foil by atmospheric pressure chemical vapor deposition

Most of the chemical vapor deposition (CVD) systems used for hexagonal boron nitride (h-BN) growth employ pyrolytic decomposition of a precursor molecule, such as ammonia borane (AB), at a temperature close to its melting point. So the control of its partial pressure is essential for high quality crystal growth. Here, we report on the edge controlled growth of a h-BN single crystal larger than 25 μm in edge length on purchased Cu foils. The key was the controlled supply of borazine gas generated by the decomposition of AB, and the stepwise decomposition of AB was found to be essential for the growth of regular h-BN crystals. The h-BN growth was mostly governed by the position of the nucleation point rather than Cu orientation as confirmed by electron back-scattered diffraction (EBSD) analysis. It was also demonstrated that the variation in temperature during the growth and cooling processes induced wrinkles larger than 20 nm due to the thermal straining of the Cu surface and a negative expansion coefficient of h-BN. These results provide a detailed understanding of h-BN growth, which will be applicable to other 2D materials.

Switching isotropic and anisotropic graphene growth in a solid source CVD system

Controlling the isotropic and anisotropic graphene growth in a chemical vapor deposition (CVD) process is a critical aspect to understand the growth dynamics for synthesizing large-area single crystals. Here, we reveal the effect of gas flow and controllability on isotropic and anisotropic graphene growth using a solid carbon source-based atmospheric pressure CVD method. It was found that the growth rate of round-shaped crystals (isotropic growth) was much higher than that of hexagonal crystals (anisotropic growth). The average growth speed increased from 0.276 μm min−1 to 1.89 μm min−1 by switching from hexagonal to circular domain growth in the CVD process. It was also found that there was no significant difference in the quality of graphene crystals when switching the growth from anisotropic to isotropic. Understanding the growth rate of round and hexagonal-shaped crystals can be critical to achieve faster growth of large single crystals. Again, the mixed edge structures (armchair and zigzag) in round-shaped graphene crystals without a fixed orientation unlike hexagonal crystals provide a better chance of seamless merging. Our findings can be significant in understanding the formation of isotropic and anisotropic graphene domains, their growth rate and quality for synthesizing large-area single crystals.