researchers at the technical university of munich (tum) in germany have developed a method for manufacturing self-supporting graphene membranes and optimized the growth of graphene crystals, according to a report from the university.
visible to the naked eye: a wafer-thin graphene flake obtained via chemical
vapor deposition. the red coloration of the copper substrate appears when the
sample is heated in air. (j. kraus/ tum)
graphene, a two-dimensional, atomic layer of carbon, has a number of properties that make it of interest to the electronics industry, which sees the material as the future of lighter and more stable devices.
current manufacturing process of graphene leave nanoscale defects, as it is difficult to process the material in a pure form. tum researchers used chemical vapor deposition (cvd) to produce a pure form of graphene.
the article explained, “theoretically, it is very easy to produce graphene: all that is needed are a heated glass vessel, a reactor, in which carbon-containing gas such as methane is fed into, as well as copper as a catalyst. at temperatures of around 1,000°c, the methane decomposes on the copper surface to produce hydrogen and carbon.
“while the hydrogen subsequently leaves the copper surface, the carbon atoms collect on the surface of the copper film used during this chemical precipitation from the gaseous state — a process called chemical vapor deposition. here, the atoms cross-link and form graphene "flakes", spot-like two-dimensional structures with the typical honeycombed structure. what remains is the hydrogen, which can be extracted via suction.”
of course, in practice, it is not simple at all. the 2-d crystals are often different sizes because growth begins at multiple locations, so the honeycombs are not oriented in the same directions causing defects at the areas where the structures meet.
tum chemists used minimal amounts of oxygen gas to ensure that the copper was as free of crystallization nuclei as possible. they also analyzed how pressure and temperature affect graphene formation during cvd.
“it is only when all parameters are selected such that growth occurs ‘close enough’ to the thermal equilibrium that highly pure graphene without defects is formed in a crystal lattice,” the article noted.
the graphene flakes were tested at the ring-shaped particle accelerator at the research centre elettra sincrotrone trieste (italy) to characterize the graphene layers.
“the tum researchers' best record for quality so far: graphene flakes measuring one square millimeter containing ten billion precisely aligned carbon atoms,” the article added.
for his work, tum researcher jurgen kraus received the 2017 research prize from evonik industries ag.
the research was recently published in annalen der physik (annals of physics). the abstract stated:
“understanding and controlling the growth kinetics of graphene is a prerequisite to synthesize this highly wanted material by chemical vapor deposition on cu, e.g. for the construction of ultra-stable electron transparent membranes. it is reviewed that cu foils contain a considerable amount of carbon in the bulk which significantly exceeds the expected amount of thermally equilibrated dissolved carbon in cu and that this carbon must be removed before any high-quality graphene may be grown.
“starting with such conditioned cu foils, systematic studies of the graphene growth kinetics in a reactive ch4/h2 atmosphere allow to extract the following meaningful data: prediction of the equilibrium constant of the graphene formation reaction within a precision of a factor of two, the confirmation that the graphene growth proceeds from a c(ad)-phase on cu which is in thermal equilibrium with the reactive gas phase, its apparent activation barrier and finally the prediction of the achievable growth velocity of the growing graphene flakes during chemical vapor deposition.
“as a result of the performed study, growth parameters are identified for the synthesis of high quality monolayer graphene with single crystalline domains of 100–1000 μm in diameter within a reasonable growth time.”