Direct growth of graphene integrated into electronic devices is highly desirable but difficult due to the nominal ~1000 °C chemical vapor deposition (CVD) temperature, which can seriously deteriorate the substrates. Here we report a great reduction of graphene CVD temperature, down to 50 °C on sapphire and 100 °C on polycarbonate, by using dilute methane as the source and molten gallium (Ga) as catalysts. The very low temperature graphene synthesis is made possible by carbon attachment to the island edges of pre-existing graphene nuclei islands, and causes no damages to the substrates. A key benefit of using molten Ga catalyst is the enhanced methane absorption in Ga at lower temperatures; this leads to a surprisingly low apparent reaction barrier of ~0.16 eV below 300 °C. The faster growth kinetics due to a low reaction barrier and a demonstrated low-temperature graphene nuclei transfer protocol can facilitate practical direct graphene synthesis on many kinds of substrates down to 50–100 °C. Our results represent a significant progress in reducing graphene synthesis temperature and understanding its mechanism.

A single layer of graphene with dimensions of 20 mm × 20 mm was grown directly on an insulating substrate by chemical vapor deposition using Ga vapor catalysts. The graphene layer showed highly homogeneous crystal quality over a large area on the insulating substrate. The crystal quality of the graphene was measured by Raman spectroscopy and was found to improve with increasing Ga vapor density on the reaction area. High-resolution transmission electron microscopy observations showed that the synthesized graphene had a perfect atomic-scale crystal structure within its grains, which ranged in size from 50 nm to 200 nm.

Although the formation of graphene nuclei conventionally requires high temperature of ∼1050 °C in the first growth stage, the presence of the graphene nuclei allowed for low-temperature edge growth, and this peripheral area continued growing even when the temperature was reduced to 300 °C in the second growth stage. The graphene grown at the second stage was a single layer with fewer defects. The Raman map showed high homogeneity with a 2D/G ratio >2 over the entire contact area of molten gallium, and the hole mobility was 600 cm2 V−1 s−1.

Single- and double-layer graphene nanoribbons (GNRs) with widths of around 10 nm were synthesized directly onto an insulating substrate by solid-phase graphitization using a gallium vapor catalyst and carbon templates made of amyloid fibrils. Subsequent investigation revealed that the crystallinity, conductivity, and carrier mobility were all improved by increasing the temperature of synthesis. The carrier mobility of the GNR synthesized at 1050 °C was 0.83 cm2/V s, which is lower than that of mechanically exfoliated graphene. This is considered to be most likely due to electron scattering by the defects and edges of the GNRs.

Amyloid fibrils, which are linear proteins with widths of less than 10 nm and lengths of more than 1 μm, were used as an amorphous carbon template for graphene nanoribbons (GNRs) synthesized by solid-phase graphitization using liquid Ga as the catalyst. The crystal quality of the GNRs improved with increasing synthesis temperature. However, the shape of the GNRs synthesized at temperatures higher than 900 °C became broader, losing the original amyloid shape, whereas the GNRs synthesized at 900 °C seemed to maintain the original amyloid shape in the SEM observation. The conducting paths of GNRs synthesized at 900 °C were found to be slightly diffused outside the topography of the GNRs in the conductive atomic force microscopy map. In addition, some of the sapphire terrace edges of the substrate showed conductivity, which indicates that the growth mechanism of graphene on a sapphire substrate might be a step-flow growth mode.