Chapter 1 Research background on two-dimensional (2D) materials growth
1.1 Chemical vapor deposition (CVD) growth of single crystalline graphene on a substrate
1.1.1 Graphene growth via nucleation control
In early stage, metal foils for graphene CVD growth are usually polycrystalline and most researches of high-quality graphene growth are focused on the route (i) -- nucleation control, and here the endeavor along this direction is summarized in Figure 1.3. In 2011, Ruoff’s group adopted Cu enclosures in a low-pressure CVD system as substrates to grow graphene, as shown in Figure 1.3(a).72 It was found that graphene can grow on both sides of the Cu enclosure, a much lower nucleation density and larger graphene domain size on the inside surface is observed and the largest graphene island is up to 0.5 mm (see Figure 1.3(b)). The authors claimed that the low partial pressure of methane and improved growth environment inside the Cu enclosure are the main reasons for the low nucleation density and large graphene size. In 2012, Yan et al. synthesized a monolayer single crystalline graphene domain with an area of ~4.5 mm2 on a polycrystalline Cu foil in a controlled pressure CVD system.71 By optimizing H2/CH4 flow rate, only one graphene nucleus was achieved on the centimeter scale substrate. The pretreatments of Cu foil, such as electrochemical polishing and high-pressure annealing, were also proved to be critical to suppress the nucleation density. The hexagonal shape, straight edges and well-identified 120º corners in Figure 1.3(c) all proved the high quality of the obtained graphene domains. Afterwards, Ruoff’s group realized centimeter scale single crystalline graphene islands by introducing oxygen in the CVD growth process.73 Oxygen is found to be able to passivate the active sites of Cu surface and hence reduce the graphene nucleation density, and it was also found that oxygen can improve the growth rate and shift the growth kinetics from attachment-limited to diffusion-limited
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growth, resulting in a dendritic island shape, as shown in Figure 1.3(d). The electron backscatter diffraction (EBSD) map in Figure 1.3(e) showed a hexagonal graphene island growing across several Cu grains, confirming that a polycrystalline Cu substrate can be used to synthesize WSSC graphene through the nucleation control method.
Figure 1.3 (a) Photo of a Cu foil enclosure.72 (b) Scanning electron microscope (SEM) image of graphene on the inside of Cu enclosure. (c) Optical and SEM images of graphene domains on Cu.71 (d) Optical image of graphene domains on oxygen rich-Cu.73 (e) Superimposed SEM and EBSD images of a graphene domain grown across Cu grains.
In 2016, Braeuninger-Weimer et al. systematically explored the effects of impurity, surface roughness and oxygen on graphene nucleation.74 Figure 1.4 shows the effects of three types of pretreatments on the surface roughness of Cu substrates and on graphene nucleation density, i.e., (i) surface etching of the Cu substrate to remove contaminations, (ii) electropolishing to reduce Cu surface roughness and (iii) oxidization of the backside of the Cu foil. It was found that increasing the surface etching time can effectively remove carbon residues on the Cu substrates, and simultaneously the graphene nucleation density is reduced by ~ 2 orders of magnitude, despite the substrate surface roughness increases from 300 nm to 550 nm. Electropolishing can not only remove the surface contaminations but also reduce the Cu surface roughness. An electropolishing time of 70 s removed the surface contaminations with the surface roughness remained at 300 nm, resulting in a decreasing of the graphene nucleation density by 2 orders of magnitude. Further increasing the electropolishing time and employing chemical mechanically polishing, the Cu substrate surface roughness can be significantly reduced to 3 nm, and the graphene nucleation density is further reduced by 1 order of magnitude.
Oxidization of the backside of the Cu substrate and further annealing in Ar atmosphere were used to investigate the effect of oxygen. After oxidization and annealing, the surface roughness of the Cu
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substrate remains, while graphene nucleation density is significantly reduced to ~ 10-2 mm-2. These results suggest that surface roughness might be not the main factor influencing the graphene nucleation density. Except for passivating active nucleation sites on Cu substrates, oxygen pretreatment was also found to be able to effectively remove carbon residues (mainly amorphous or graphitic carbon) on the Cu substrates and lead to a homogeneous distribution of carbon on the Cu substrate surface.
Figure 1.4 (a) The effects of Cu pretreatments including surface etching (green), electropolishing (blue) and backside oxidization (red) on graphene nucleation density and surface roughness of Cu substrates, where CMP is chemical mechanically polishing. (b) Sketches of the three different Cu pretreatment.74
Despite of many years of effort, the growth of single crystalline graphene islands to a centimeter scale from a single nucleus by traditional CVD process generally takes a very long time, e.g., 12 hours, which is neither efficient nor economic.
In 2016, Wu et al. realized the synthesis of ~1.5-inch single crystalline graphene monolayer on a polycrystalline Cu-Ni alloy substrate within 2.5 hours by using a local feeding technique in CVD process (Figure 1.5(a-c)), which opened the door of synthesizing WSSC graphene.66 Cu-Ni alloy combines the advantages of low carbon solubility of Cu and superior catalytic efficiency of Ni and is proved to be an ideal catalyst for graphene growth. In addition, Ni was also found to facilitate the fast diffusion of C atoms in the substrate and hence improve the growth rate of graphene films. Secondary ion mass spectrometry (SIMS) in Figure 1.5(c) showed that carbon species only concentrate around the feeding center below the nozzle with a radius of about 500-1000 μm, which is the typical distance between neighboring graphene nuclei in traditional CVD experiments, and consequently only one
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graphene nucleus can be realized on the whole substrate in this study. Two years later, the local feeding setup is further improved by adopting a movable substrate, as shown in Figure 1.5(d), which allows for the continuous roll-to-roll graphene growth on both Cu and Cu-Ni alloy polycrystalline substrates.75 Fig. 1.5(e) shows that multi-nucleation of graphene near the graphene growth frontier on a Cu substrate can be fully suppressed when the H2-Ar buffer gas flow rate was increased to 32 cm·s-1, and at this flow rate, single crystalline graphene growth can also be realized on Cu-Ni alloy substrates.
Figure 1.5 (a) Sketch of local feedstock feeding of graphene grown on a Cu-Ni alloy.66 (b) Optical image of grown graphene by local feedstock feeding. (c) SIMS line scan profiles showing the distribution of carbon concentration on the substrate below the nozzle. (d) The set-up for advancing local control CVD used for graphene growth.75 (e) Optical images of graphene nucleation at various buffer gas speeds on Cu and Cu-Ni alloy substrates.
To date, controlling the graphene nucleation density in a traditional CVD system is still challenging, although great progresses have been made and some critical factors that affect the graphene nucleation were found. Local feeding of carbon precursor, on the other hand, is an effective method of controlling the nucleation of graphene. However, the efficiency of this method in growing WSSC graphene is still limited, because of the rather low growth rate of graphene. Usually, a few hours’
experimental time is required to obtain one piece of inch sized single crystalline graphene.
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