Yaqian Zhang

Role of wrinkles in the corrosion of graphene domain-coated Cu surfaces

We analyzed the protective ability of chemical vapor deposition (CVD) graphene domains against corrosion of Cu surfaces. Fresh graphene domains of various shapes were ideal corrosion-inhibiting layers. However, obvious corrosion was found within graphene domains exposed to the air for over a week. Our work demonstrates that the opportunities for corrosion of CVD graphene were provided by wrinkles but not others, such as Cu grain boundaries and graphene domain boundaries, which are always believed the primary factor for inferior quality of the CVD graphene at present.

Stripe distributions of graphene-coated Cu foils and their effects on the reduction of graphene wrinkles

Hexagonal single-crystal domains of graphene were analyzed and the wrinkle distribution was obtained using thermal hydrogen etching. We observe parallel stripes on some single-crystal domains and these stripes are associated with graphene wrinkles. The etched trenches in graphene are always perpendicular to the stripes, thereby suggesting the suppressed formation of wrinkles along the stripe direction. Results indicate that the stripes help release the internal stress of graphene to reduce its wrinkle density. Furthermore, these stripes are due to Cu surface reconstruction and relate to two main factors, namely, the distribution of Cu grains and the cooling rate after graphene growth. Continuous graphene films which are synthesized by slow cooling exhibit high stripe area coverage and low sheet resistance because of the low wrinkle density.

Effect of Hydrogen in Size-Limited Growth of Graphene by Atmospheric Pressure Chemical Vapor Deposition

Analysis of graphene domain synthesis explains the main graphene growth process. Size-limited graphene growth caused by hydrogen is studied to achieve efficient graphene synthesis. Graphene synthesis on Cu foils via the chemical vapor deposition method using methane as carbon source is limited by high hydrogen concentration. Results indicate that hydrogen affects graphene nucleation, the growth rate, and the final domain size. Considering the role of hydrogen as both activator and etching reagent, we build a model to explain the cause of this low graphene growth rate for high hydrogen partial pressure. A two-step method is proposed to control the graphene nucleation and growth rate separately. Half the time is required to obtain similar domain size compared with single-step synthesis, indicating improved graphene synthesis efficiency. The change of the partial pressure and transmission time between the two steps is a factor that cannot be ignored to control the graphene growth.

Improved carrier mobility of chemical vapor deposition-graphene by counter-doping with hydrazine hydrate

We developed a counter-doping method to tune the electronic properties of chemical vapor deposition (CVD)-grown graphene by varying the concentration and time of graphene exposure to hydrazine hydrate (N2H4·H2O). The shift of G and 2D peaks of Raman spectroscopy is analyzed as a function of N2H4·H2O concentration. The result revealed that N2H4·H2O realized n-type doping on CVD grown graphene. X-ray photoelectron spectroscopy measurement proved the existence of nitrogen, which indicated the adsorption of N2H4 on the surface of graphene. After counter-doping, carrier mobility, which was measured by Hall measurements, increased three fold.

Undulate Cu(111) Substrates: A Unique Surface for CVD Graphene Growth

Cu(111) is a suitable substrate for sixfold graphene domain synthesis, as confirmed theoretically and experimentally. However, an undulate striped structure, where stretched flower-like or approximate diamond-shaped graphene domains had formed, appeared on Cu(111) after annealing and growth in our study. Graphene domains were stretched along the undulate stripes. The Cu surface coated with graphene domains was flatter than the surrounding undulate striped structure. Oxygen plasma was used to remove the graphene coating, and the exposed Cu was also flat. We propose that slight steps formed on Cu(111) in the annealing process. The faster rate of graphene growth along these steps contributed to the stretching domain shape. Furthermore, the release of internal stress or the shrinking of Cu during cooling promotes the expansion step to form an undulate striped structure. However, the coated Cu step motion is limited by graphene. Consequently, the resulting surface is flat, thereby clearly indicating a graphene–Cu interaction.