A New Growth Approach—Alignment van der Waals Epitaxy of Graphene

This report is concerned with a new technique for synthesis of 2D materials, where precise epitaxy is realized by the help of alignment mechanism between 2D materials and substrate. Graphene attracts much attention from its superior characteristics for realizing excellent future devices. Graphene is conventionally grown on copper foil by CVD and transferred to a certain substrate to fabricate devices. However, graphene is inevitably damaged and contaminated during the process. Therefore, direct synthesis of graphene on an insulator substrate is demanded. CVD has already been studied on SiO₂, sapphire etc. but the quality of graphene is still inferior. To obtain large-scale single crystalline graphene, there are two courses; one is enlargement of single nucleus, the other is connection of individual nuclei with highly-aligned direction. In this report, the latter is introduced to improve the crystallinity of grpahene.<br/>Graphene was synthesized on m-plane sapphire by low-pressure CVD using 3-hexyne as a carbon source [1]. AFM image shows smooth surface of graphene, and Raman spectra do single layer graphene was successfully obtained over the whole surface. X-ray in-plane diffraction spectra shows not only a clear 6-fold symmetry of graphene but also extremely narrow FWHM, which is as narrow as 12’’. The value is the narrowest ever reported for CVD graphene and almost equals to that of sapphire substrate. Graphene certainly grew under van der Waals epitaxy but the result indicates presence of special alignment mechanism. A similar phenomenon was reported for carbon nanotubes grown on a sapphire substrate [2]. Carbon nanotubes regularly aligned a certain direction on a sapphire surface. In our case, X-ray diffraction measurement indicates not only the alignment of the 6-memberd rings but also the uniaxial deformation of the carbon rings in the direction of sapphire [11-20] axis. Namely, [01] lattice constant of graphene was matched with the 1/4 spacing of sapphire a-planes. A strong interaction possibly existed between graphene and sapphire surface, which should align the crystal orientation of graphene nuclei during the van der Waals epitaxy.<br/>References<br/>[1] Y. Ueda et al., APL 115, 013103 (2019). [2] S. Han et al., JACS comm. 127, 5294 (2006).