An In-Situ TEM Study on the Formation of Gold Carbide

In this study, a freestanding graphene membrane sheet serves as a quasi transparent substrate for aberration-corrected high-resolution transmission electron microscopy, as an insitu heater, and as carbon supplier. The sheet has been previously decorated with gold nanoislands. During electron irradiation at 80 kV and at temperatures of approximately 1000K, the accumulation of gold atoms has been observed on defective graphene sites or edges as well as at the facets of gold nanocrystals. Both resulted in clustering, forming unusual crystalline structures. Their lattice parameters and surface termination differ significantly from standard gold nanocrystals. We describe the stepwise formation of these little structures starting from irregular periodical arrangements of gold atoms into rectangularly shaped monolayers, then bilayers and entire cuboids of many atomic layers. Our experimental data, supported by electron energy loss spectroscopy and density-functional theory calculations, suggests that isolated gold and carbon atoms form — under conditions of heat and electron irradiation — a novel type of compound crystal, Au-C in zincblende structure.