Yanping Sui

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.

Graphene/Mo2C heterostructure directly grown by chemical vapor deposition

Graphene-based heterostructure is one of the most attractive topics in physics and material sciences due to its intriguing properties and applications. We report the one-step fabrication of a novel graphene/Mo2C heterostructure by using chemical vapor deposition (CVD). The composition and structure of the heterostructure are characterized through energy-dispersive spectrometer, transmission electron microscope, and Raman spectrum. The growth rule analysis of the results shows the flow rate of methane is a main factor in preparing the graphene/Mo2C heterostructure. A schematic diagram of the growth process is also established. Transport measurements are performed to study the superconductivity of the heterostructure which has potential applications in superconducting devices.

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.

High pressure-assisted transfer of ultraclean chemical vapor deposited graphene

We develop a high pressure-assisted (approximately 1000 kPa) transfer method to remove polymer residues and effectively reduce damages on the surface of graphene. By introducing an ethanol pre-dehydration technique and optimizing temperature, the graphene surface becomes nearly free of residues, and the quality of graphene is improved obviously when temperature reaches 140 °C. The graphene obtained using the high pressure-assisted transfer method also exhibits excellent electrical properties with an average sheet resistance of approximately 290 Ω/sq and a mobility of 1210 cm2/V·s at room temperature. Sheet resistance and mobility are considerably improved compared with those of the graphene obtained using the normal wet transfer method (average sheet resistance of approximately 510 ohm/sq and mobility of 750 cm2/V·s).

Reduction of Dislocation Density in HVPE-Grown GaN Epilayers by Using In Situ-Etched Porous Templates

In this paper, a method was demonstrated to reduce the dislocation density of GaN film grown by hydride vapor phase epitaxy (HVPE) on an in situ selective hydrogen-etched GaN/sapphire template. The dislocations regions were etched by hydrogen to form cavities. The porous structure was formed on the GaN template grown by metal organic chemical vapor deposition after in situ hydrogen etching. The etching condition was optimized by modulating the etching temperature, pressure, and etching time. Two-step buffer layer growth and high temperature GaN film deposition were carried on the porous template. The growth parameters were optimized to keep the porous structure unfilled. The dislocations originally located in etched cavities could not propagate to the next layer grown by HVPE. Therefore, the dislocation density could be significantly reduced. High crystal quality of GaN is obtained with a low dislocation density. The full width at half-maximum FWHM of (002) is 35 arcs, and the FWHM of (102) is 48 arcs.

Growth promotion of vertical graphene on SiO2/Si by Ar plasma process in plasma-enhanced chemical vapor deposition

This study investigates the growth promotion of vertically oriented graphene in plasma-enhanced chemical vapor deposition through Ar plasma treatment. Combined with various substrate treatments, including hydrofluoric acid etching and oxidation after Ar plasma treatment, Ar plasma pretreatment promotes vertical growth through the microcavity on the rough substrate surface and the active growth sites. The microcavity affects the strain distribution and defects of as-deposited planar films, which benefit the transition of 2D deposition to 3D vertical growth. A growth model on the effect of Ar plasma pretreatment is proposed.

Stripe distribution on graphene-coated Cu surface and its effect on oxidation and corrosion resistance of graphene

The morphology and distribution of the stripes caused by Cu surface reconstruction were measured, and the effects of stripes on graphene stability were studied by oxidation and corrosion. The results reveal that the stripes are determined by the crystal orientation of both the Cu surface and graphene, which can both change the stripe distribution, and the stripes can also be influenced by the graphene thickness. The stripes would not induce cracks or destruction to the graphene. The oxidation resistance of graphene can be improved by Cu surface reconstruction. The local nonuniform distortion of the stripe area may induce a bigger strain in the graphene which, in turn, may induce structure instability and result in local stability degeneration in the stripe area.

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.