Synthesis of large area single-crystal graphene on 2-inch × 2-inch Cu85Ni15 alloy substrate from a single nucleus. Schematic (left) shows local feedstock injection and optical image (right) shows a 1.5-inch single crystal of graphene grown at 1100 ?C in 150 min on Cu85Ni15. Source: Reprinted by permission from Macmillan Publishers Ltd: Nature Materials 15, 43, copyright (2016).The semiconductor industry relies on high quality wafers of single-crystal silicon for electronic devices. But producing similar wafers of graphene has proved trickier. Current state-of-the-art chemical vapor deposition (CVD) approaches, for example, take ∼12 h to produce 1 cm2 areas. Now researchers have speeded up the process and can produce 1.5-inch wafers in just ∼2.5 h [Wu et al., Nature Materials 15 (2016) 43].
The new approach, devised primarily by Xiaoming Xie and Tianru Wu from Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, and Qingkai Yu from Texas State University, relies on making it possible for a single nucleus to grow into a monolayer at a fast rate. The team use a Cu—Ni alloy substrate that combines the ideal, albeit weak, catalytic activity of Cu for monolayer graphene synthesis with the higher catalytic power of Ni. Density functional theory calculations carried out by Feng Ding at Hong Kong Polytechnic University and Qinghong Yuan from East China Normal University provide an understanding of the role of the alloy catalyst and its optimal composition.
‘‘By locally feeding a carbon precursor though a nozzle onto a Cu85Ni15 alloy substrate, local carbon supersaturation is created that outperforms other nucleationdetermining factors, which would normally lead to random and continuous nucleation,’’ explains Xie.
The result is greatly accelerated growth of graphene — at least ten times faster than the previous record — from a single nucleus. The improved growth rate is driven by an isothermal segregation mechanism, which involves the dissolution of carbon in the alloy. Consequently, the proportions of the Cu—Ni alloy make a significant difference to the speed of the process. As Ni content is increased, carbon solubility in the alloy increases, transforming the growth from surface-mediated to isothermal segregation.
‘‘The precursor supply is increased stepwise to ensure sufficient carbon concentration gradient at the growth front of graphene, thus maintaining the high growth rate throughout the whole process,’’ says Xie.
After just 2.5 h at 1100oC, the approach yields 1.5-inch hexagonally-shaped, single-crystal monolayer wafers of graphene. The strategy is also cost effective: both CVD and Cu—Ni alloy are low cost; and the process is very similar to that used in silicon manufacturing.
‘‘Our approach is very practical and easy to scale up in principle,’’ says Xie. He is confident the scalability will be demonstrated when the group try out the approach on professionally designed CVD equipment with better controllability.
Xiangfeng Duan of the University of California, Los Angeles, believes the results represent a breakthrough advancement in the growth of graphene single crystals.
‘‘Previous growth of large single crystals was often achieved by suppressing the nucleation density partly by reducing the precursor supply, which inevitably slows down the growth rate,’’ he points out. ‘‘By locally feeding in the precursor, the authors ensure only one nucleate initiates growth of single crystalline graphene over the entire substrate. This allows direct growth of the largest graphene single crystals on metal foil, and at a much faster growth rate than before.’’
The approach could also be expanded to undertake parallel growth of multiple single crystals in controlled locations on a large-area substrate, suggests Duan, which would be essential for functional integrated electronic applications.
This article was originally published in Nano Today (2016), DOI:10.1016/j.nantod.2016.01.001