Two-dimensional hexagonal boron nitride (h-BN) is a material of significant interest due to the strong ionic bonding of boron and nitrogen atoms that provides unique properties, including the thinnest insulating nanomaterial, exhibiting a bandgap of 5.9 eV, with superior chemical, mechanical, and thermal stability. In addition, h-BN provides an ideal substrate for improving the electrical properties of graphene since the surface is atomically smooth and free of dangling bonds, thereby reducing charge scattering effects resulting in an order of magnitude increase in graphene charge mobility over materials grown on silicon or silicon dioxide. Previously, the method to synthesize monolayer n-BN utilized ultra-high vacuum chemical vapor deposition (UHVCVD) using borazine as a precursor on single crystal transition metal substrates, such as nickel, platinum, or silver, but proved difficult to scale. Polycrystalline metal foils (Ni, Co, Cu, and Pt) were additionally used to grow h-BN using regular chemical vapor deposition (CVD), but the thickness and quality of the films critically depended on surface morphology and crystal orientation of the substrate. High quality h-BN has been synthesized on Pt foils using ammonia borane precursor, yet control of film thickness and domain size remains a challenge for scaling, and the specific growth mechanisms are not well understood.
Recently, Park et.al., reported results from a systematic study for synthesis of large area single layer h-BN films on polycrystalline Pt foils using low pressure CVD comparing borazine and ammonia borane precursors. The authors’ goal was to study the effect of the Pt lattice orientation, the total pressure, and the different cooling rate in order to understand h-BN growth mechanisms. Since nitrogen is not soluble in Pt, the authors’ objective was to confirm the contributions to h-BN growth surface mediated and precipitation processes. The study included analysis of film properties dependence on cooling rate and crystal orientation of the substrate. Their findings demonstrated that film growth was by a surface mediated growth mechanism, facilitated by a catalytic reaction, that produced polycrystalline h-BN monolayers confined by the underlying Pt surface orientation. The thickness of the h-BN films exhibited a dependence on the Pt surface orientation, presumably determined by the available catalytic reaction sites that decompose the borazine precursor, which would exhibit a dependence on crystal orientation.
Improved understanding of h-BN growth mechanisms will potentially lead to methods for controlling the growth of high-quality h-BN films. This further provides the basis for materials and substrates for application in quantum tunneling devices, novel heterostructures, and two-dimensional semiconductors such as molybdenum sulfide and graphene.
Reference: Park J, Park JC, Yun SJ, Kim H, Luong DH, Kim SM, Choi SH, Yang W, Kong J, Kim KK, Lee YH. Large-Area Monolayer Hexagonal Boron Nitride on Pt Foil. ACS Nano. 2014; 8 (8): 8520-852 doi: 10.1021/nn503140y
Image reprinted with permission from American Chemical Society.