Atmospheric Pressure, Temperature-Induced Conversion of Organic Monolayers into Nanocrystalline Graphene

Atomically thin free-standing nanomembranes belong to the emerging class of materials with a great promise for basic research of two-dimensional (2D) systems and applications in nanotechnology. However, their synthesis and characterization remain a frontier challenge in chemical and physical research. Here, we demonstrate that atmospheric pressure (Ar/H-2) annealing in the range from 500 to 1000 degrees C of similar to 1 nm thick carbon nanomembranes (CNMs) made of cross-linked aromatic self-assembled monolayers results in their conversion into free-standing sheets of covalently bonded, in-plane oriented graphene nanocrystals. Upon this transformation the electrical characteristics of CNMs evolve from insulating to conducting, which is accompanied by a change of the room temperature sheet resistivity by more than 5 orders of magnitude. We analyze this atmospheric pressure, temperature-induced transformation of CNMs employing various complementary spectroscopic and microscopic techniques, such as X-ray photoelectron spectroscopy, Raman spectroscopy, electron energy loss spectroscopy, optical microscopy, helium ion microscopy, atomic force microscopy, and transmission electron microscopy. In particular we studied the chemical, structural, and electronic properties of CNMs. We provide a comparative analysis of these data with the properties of pristine graphene, graphene oxide, and reduced graphene oxide sheets, which reveals both similarities and differences between these 2D materials.