W. Kong, H. Li, K. Qiao, J. Grossman, J. Kim
Massachusetts Institute of Technology,
Keywords: graphene, hexagonal boran nitride, transparency, remote epitaxy
Summary:Transparency of two-dimensional (2D) materials to inter-molecular interactions has been an unresolved topic. It was found that water droplets interact with underlying substrates through graphene, as if the graphene is “transparent”. However, graphene’s transparency determined by droplet wetting angle has been controversial. Recently, precise atomic alignment between epitaxial films and substrates through monolayer graphene has been discovered in a GaAs/graphene/GaAs structure. This finding experimentally confirms the existence of remote atomic interaction through graphene. However, the mechanism of remote interaction through 2D materials at atomic-scale and its relationship with the bonding chemistry of 2D materials have not been fully understood. Here, we report that remote atomic interaction through 2D materials is governed by the polarity of atomic bonds both in substrates and 2D materials. Our density functional theory (DFT) calculation reveals that polarization of atomic bonding in substrates enhances the strength of the electrostatic potential penetrated through 2D materials, preserving the information of atomic registry away from the substrate. Such trend is well observed by performing epitaxy of Si, GaAs, GaN, and LiF on their own substrates through graphene, whose ionic bonding character fractions are 0%, 31%, 50%, and 90%, respectively. The results show pure covalent-bonded Si loses its remote atomic interaction across monolayer graphene, leading to polycrystalline formation, while we obtained epitaxially aligned single-crystalline GaAs, GaN, and LiF through monolayer (1ML), bilayer (2ML), and trilayer (3ML) graphene, respectively. More interestingly, we discovered that such field penetration is substantially attenuated through hexagonal boron nitride (hBN) that contains polarity in its bonding. Thus, van der Waals epitaxy seeded from hBN and remote epitaxy seeded from graphene occur simultaneously when we perform epitaxy of GaN through 1ML hBN. Through 3ML hBN, complete transition from remote epitaxy to van der Waals epitaxy occurs. The excellent consistency between the theoretical and experimental findings unequivocally confirms that the ionic character of atomic bonding determines remote atomic interaction through 2D materials. Our work demonstrates that the transparency of 2D materials can be sensitively probed at atomic scale by performing epitaxy through them. This transparency highly depends on the nature of substrate materials, while it can also be tuned by modulating the thickness and bonding chemistry of the 2D materials.