Abstract
The processes governing multilayer nucleation in the chemical vapour deposition (CVD) of graphene are important for obtaining high-quality monolayer sheets, but remain poorly understood. Here we show that higher-order carbon species in the gas-phase play a major role in multilayer nucleation, through the use of in-situ ultraviolet (UV) absorption spectroscopy. These species are the volatilized products of reactions between hydrogen and carbon contaminants that have backstreamed into the reaction chamber from downstream system components. Consequently, we observe a dramatic suppression of multilayer nucleation when backstreaming is suppressed. These results point to an important and previously undescribed mechanism for multilayer nucleation, wherein higher-order gas-phase carbon species play an integral role. Our work highlights the importance of gas-phase dynamics in understanding the overall mechanism of graphene growth.
Highlights
Certain aspects of the role of hydrogen still remain unclear
In-situ UV absorption spectroscopy measurements were conducted in a cold-wall chemical vapour deposition (CVD) system according to Fig. 1
The large infrared background noise from the graphite heaters at typical growth temperatures precludes the use of infrared spectroscopy[29], whereas laser-based methods run the risk of inducing photochemical reactions and producing spurious results at high temperatures
Summary
Certain aspects of the role of hydrogen still remain unclear. In particular, while a large hydrogen partial pressure is crucial for obtaining high-quality, hexagonal domains, several studies have reported an increase in multilayer nucleation under such conditions[16, 23,24,25,26]. Through the use of a novel in-situ gas-phase ultraviolet (UV) absorption spectroscopic technique, we provide evidence for an alternate, possibly complementary explanation, wherein large concentrations of hydrogen activate carbon contaminants in the reaction chamber, volatilizing them and making them available for nucleation. These contaminants, which comprise oils, greases, and condensed products of pyrolysis, accumulate in the chamber as a result of backstreaming from downstream system components, such as the exhaust lines, the vacuum pump, and the foreline trap. The results demonstrate that multilayer nucleation can in some cases be wholly ascribed to the volatilized products of reactions between backstreamed contaminants and H2, highlighting the importance of the role of gas-phase carbon species in multilayer nucleation
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