Chapter 4. New Mechanisms on Graphene Chemical Vapor Deposition Growth
4.2 Elements modulation on graphene growth
4.2.2 Mechanism of fluorine modulated graphene growth
135
with black arrowheads) and without oxygen (at top with red arrowheads). White, black, orange, and red spheres represent H, C, Cu and O atoms, respectively.
Therefore, the energy barriers for the methane decomposition are reduced to 0.62, 1.29, 0.53, and 1.11 eV, respectively. Besides, because the formation of the product OH is more stable than H on the Cu(100) surface, the reaction energies of the decomposition are decreased to 0.24, 0.53, -0.43, and -0.11 eV, respectively. The reduction of the decomposition barriers could greatly increase the dissociation rate of CH4 by orders of magnitude, and therefore promote the concentration of the carbon precursors. To show a whole picture of oxygen’s effect on methane dissociation, Fig. 4.4 plots the energy profile during the full decomposition process of CH4 with and without oxygen’s assistance. It indicates that, no matter which step of the decomposition is the rate-limiting step, the methane dehydrogenation is accelerated as the entire energy like with oxygen (black line) is under that without oxygen (red line).
Figure 4.4 Energy profile for the full decomposition of CH4 with and without oxygen supply on Cu(100) surface.
So with DFT calculations, it was proved that surface oxygen on Cu foil could decrease the methane decomposition barrier and therefore increase the carbon flux, which leading to the promotion of the graphene growth rate and the change of the growth mode from diffusion limited to attachment limited.
136
and is expected to greatly change the growth behavior of graphene. However, by now, in most of the reports using fluorine, it was only involved in the post-growth treatments. It is because in graphene synthesis, introducing fluorine into the system directly is very dangerous, considering its high reactivity and high toxicity. Quartz tube can be destroyed because of the corrosive hydrogen fluoride.
Thus Liu et al. designed the reaction tube as shown in Fig. 4.5a where fluorine could be confined in a narrow gap of typically 10-20 μm between the Cu foil and a metal fluoride substrate226. At the growth temperature of ~ 1000 ºC, the metal fluoride surface was release fluorine slowly and steadily which did not do any harm to the quartz tube. This release of fluorine was later confirmed by X-ray photoelectron spectroscopy (XPS) before and after annealing.
Figure 4.5 (a) Schematic diagram showing the experimental design for the graphene growth with the local fluorine supply. SEM images of graphene domain at t = (b) 2 s and (c) 5 s. (d) Graphene domain size as a function of growth time. (e) Schematic diagram showing the transient 13CH4 feeding. Isotope- labeled Raman maps of the 2D band of graphene domain growing with a carbon source flux of (f) 2 s.c.c.m. and (g) 15 s.c.c.m. Reproduced from Ref. 226. Copyright@2019, Springer Nature.
The fluorine was released only in a very small amount but was sufficient to modulate the graphene growth. With the design, in a very short time period, circular graphene domains of 1 mm size were observed on the back side of the Cu foil, facing the metal fluoride (Fig. 4.5b-d).
On the front side of the Cu foil there were only star-like shape graphene domains of ~ 20 μm size. To measure the growth rate of individual graphene domains, a C isotope labeling method was used by a new pulse instrument to inject 13C and 12C methane pulse alternatively through a
137
small pipe with a time interval as short as 1 s (Fig. 4.5e). After the growth, by Raman mapping, the growth rate at 15 sccm CH4 flux was about 190 μm/s (Fig. 4.5d and f-g).
This growth rate is so high that it is even one order of magnitude higher than the graphene growth with oxygen supply. How it was realized? Firstly, we noticed that the domain shapes on the front and back side of the Cu foil were the same as that in the graphene growth with oxygen.
As demonstrated in that work, we know that it is because of the high carbon supply that the graphene growth changes from the diffusion-limited to attachment-limited mode. And the high carbon supply comes from the efficient methane decomposition because introducing oxygen will decrease the decomposition barriers. So is it the same mechanism that fluorine also help for methane decomposition on Cu surface?
We therefore calculated all the four reactions of the methane decomposition on Cu(100) surface with the existence of fluorine (F):
𝐶𝐻4+ 𝐹𝐶𝑢→ 𝐶𝐻3+ 𝐻𝐹, (4.9) 𝐶𝐻3+ 𝐹𝐶𝑢→ 𝐶𝐻2+ 𝐻𝐹, (4.10) 𝐶𝐻2+ 𝐹𝐶𝑢→ 𝐶𝐻 + 𝐻𝐹, (4.11)
𝐶𝐻 + 𝐹𝐶𝑢→ 𝐶 + 𝐻𝐹. (4.12) MEP of each dehydrogenation reaction is shown in Fig. 4.6a-d, respectively. In the figure, all the initial, transition, and final structures are optimized configurations, and white, grey, orange, and blue spheres represent H, C, Cu and F atoms, respectively. It can be seen that the reaction processes are very similar to those of the methane decomposition reactions with the assistance of O. For example, for the first step, methane molecule released an H atom to the Cu surface.
In the transition state, F atom could interacted with the released H atom and formed HF molecule. At last we got a CH3 radical attaching to the Cu surface and HF adsorbed too.
But the result of the reaction barriers and energies are quite surprising as shown in Fig. 4.6e. In the figure, the black line shows the energy profile during the methane decomposition on bare Cu surface. The blue line is that with O and the red line with F. It is obvious that F does help to reduce the decomposition barriers, from 1.57, 1.37, and 1.47 eV to 1.24, 1.19, and 1.36 eV for the first, second, and fourth step, respectively. But it seems to raise the reaction energies, from 0.84, 0.75, -0.02, and 0.7 eV to 0.93, 0.97, 0.44, and 0.82 eV, for the four reactions respectively.
Therefore, except for the first energy barrier, the entire energy profile for the decomposition with surface F (red) is above that on bare Cu surface (black), which is even much higher than
138
that with surface O. This means F can’t help to promote methane decomposition on Cu surface, and would actually not participate in the surface reactions. The result is, in a way, understandable, because the product, HF, is actually not stable on the Cu surface and prefers to desorb.
Figure 4.6 MEPs for the (a) first, (b) second, (c) third, and (d) fourth step of the CH4 decomposition on Cu(100) surface with the assistance of a fluorine atom. (e) Energy profile of the full methane decomposition reaction on Cu(100) surface without the participation of other elements (black) or with the assistance of oxygen (blue) or fluorine (red). White, grey, orange, and blue spheres represent H, C, Cu, and F atoms, respectively.
We then proposed another possible way of F helping methane decomposition. As shown in Fig.
4.7 where white, grey, green, and blue spheres represent H, C, Ba, and F atoms respectively, we considered F directly decompose on the metal fluoride surface. Starting with an adsorbed methane molecule on BaF2 surface (Fig. 4.7a-b), there are several routes for the decomposition or the substitution of CH4. One way is that methane directly lose one H atom on BaF2 surface.
In route 2, it kicks off one F atom from the substrate first and forms a HF molecule and CH3
radical attached on this vacancy site. In route 3, one H atom of the methane molecule is
139
substituted by F atom from the surface. But all these three routes show high energy penalty, indicating that it is nearly impossible to react or decompose directly on the metal fluoride surface.
Figure 4.7 Reactions of CH4 molecule on the BaF2 surface. (a) Side and (b) top view of the CH4 molecule on the BaF2 surface. There are three routes for the adsorbed CH4 to decompose or be substituted by F on the BaF2 surface: Route I, 𝐶𝐻4𝐵𝑎𝐹→ 𝐶𝐻2 3+ 𝐻; Route II, 𝐶𝐻4𝐵𝑎𝐹→ 𝐶𝐻2 3+ 𝐻𝐹; and Route III, 𝐶𝐻4𝐵𝑎𝐹→ 𝐶𝐻2 3𝐹 + 𝐻. The top and side view of the atomic structures after the reactions of three routes are shown in (c-d), (e-f), and (g-h), respectively. White, grey, green, and blue spheres represent H, C, Ba, and F atoms, respectively.
Figure 4.8 Reactions on the BaF2 surface. (a) Releasing of F or F2 to the gas phase. (b) Substitution of the H atom in CH4 by the F atom in F2 molecule. (c) Substitution of the H atom in CH4 by the atomic F in the gas phase. White, grey, green, and blue spheres represent H, C, Ba, and F atoms, respectively.
140
Later it occurred to us that with the help of metal fluoride, certain amount of methane can transfer to fluoromethane—CH3F at ~ 800 ºC, as well as a few CH2F2 and CHF3227. In our system, as we already detected the release of F (Fig. 4.8a), it is expected that F in the atmosphere would react with the methane flow (Fig. 4.8b and c). The energy barriers of the substitution reaction of methane with F2 and F are only 0.62 eV, and 0.05 eV, respectively (Fig. 4.8b and c).
This means the substitution can happen easily at the reaction temperature of ~ 1000 ºC. So it indicates that during the graphene growth, in the atmosphere, there would be certain amount of CH3F in the system, as well as a few CH2F2 and CHF3.
Figure 4.9 (a) Atomic structures of the initial, transition, and final state of the CH3F decomposition reaction on Cu(100) surface. (b) Energy profile for the full methane decomposition reaction without the assistance of any other elements (black), with the help of oxygen (blue), or fluorine (red), and that for the full decomposition of CH3F (green). White, grey, orange, and blue spheres represent H, C, Cu, and F atoms, respectively.
So now the question is how fluoromethane could increase the graphene growth. Our further calculation shows that the decomposition of CH3F on the Cu surface is much easier than that of CH4 (Fig. 4.9). The CH3F molecule adsorbed on the Cu surface released the F atom with the help of Cu and the produced CH3 and F radicals on the surface were very stable (Fig. 4.9a). In addition to the decreasing of the energy barrier of the first methane decomposition reaction which is similar to that for O, CH3F more importantly turns the originally endothermic reaction
141
(reaction energy ~ 0.84 eV) to an exothermic one (reaction energy ~ -0.62 eV) (Fig. 4.9b). This is a quite significant reduction. While a reaction with a barrier of 1.24 eV can still occur frequently at ~ 1000 ºC, such a low reaction energy would greatly promote the concentration of the product—CH3 radicals, which could further increase the concentration of the subsequent decomposition reactions.
Figure 4.10 (a) MEP for the substitution of one H atom in the CH3F molecule by the F atom in the F2
molecule. MEPs for the (b) first, and (c) second step of the CH2F2 decomposition on Cu(100) surface.
(d) Energy profile of the full methane decomposition reaction on Cu(100) surface without the participation of other elements (black) or with the assistance of oxygen (blue) or fluorine (red), and that of the CH2F2 full decomposition on Cu(100) surface (green). White, grey, orange, and blue spheres represent H, C, Cu, and F atoms, respectively.
As the further substitution of H atoms in CH3F molecule is also with low barrier and reaction energy, and is easy to happen (Fig. 4.10a), it should be expected that in the gas phase there will also be species like CH2F2 and CHF3 existing. So the decomposition of CH2F2 and CHF3 on the
142
Cu surface were also calculated (Fig. 4.10b-d and Fig. 4.). It shows that, for CH2F2, the reaction is still exothermic and highly favorable since after the first step of the decomposition reactions, the energy profile line is already the lowest (green line) among those of all the other decomposition strategies. With an even lower reaction energy of -0.52 eV for the second step, the concentration of CH2 radicals would be significantly promoted. For the decomposition of CHF3 (Fig. 4.11), though the initial two steps are endothermic, the third step reaction is exothermic with a reaction energy of -1.24 eV. So for the entire decomposition process, the energy profile line is still under that of all other decomposition strategies. And the extremely low reaction energy of the third step would therefore, result in a higher centration of CH radicals.
Figure 4.11 MEPs for the (a) first, (b) second, and (c) third step of the CHF3 decomposition on Cu(100) surface. (d) Energy profile of the full methane decomposition reaction on Cu(100) surface without the participation of other elements (black) or with the assistance of oxygen (blue) or fluorine (red), and that of the CHF3 full decomposition on Cu(100) surface (green). White, grey, orange, and blue spheres represent H, C, Cu, and F atoms, respectively.
So with the substitution of methane in the gas phase and the further decomposition reactions on the Cu substrate, higher concentrations of CH3, CH2, and CH radicals are realized, which could therefore promote the graphene growth. Later we also checked the temperature effect on the barriers and reaction energies for the decomposition of the methane molecule (Fig. 4.12). It was
143
found that at synthesis temperature, though the barriers and reaction energies increase a little, the reactions are still exothermic and could still happen easily.
Figure 4.12 (a) MEP for the substitution of one H atom in the CH4 molecule by the F atom in the F2
molecule, with the energy barrier and the free energy barrier at 1000 K shown. (b) MEPs for CH3F decomposition on Cu(100) surface, with the energy barrier and the free energy barrier at 1000 K shown.
White, grey, orange, and blue spheres represent H, C, Cu, and F atoms, respectively.
Figure 4.13 Schematic diagram showing the mechanism of fluorine modulated graphene growth.
144
So to explain the ultrafast graphene growth with a continuous F supply, we propose our new mechanism (Fig. 4.13): during graphene CVD growth, i) metal fluoride substrate releases F into the confined space between the Cu foil and the metal fluoride; ii) the released F interacts with the methane flow and substitutes the H atom in the methane molecule to form fluoromethane;
iii) the fluoromethane formed in the gas phase lands on the Cu surface and decomposes by releasing the F atom. Therefore, high concentration of CHx species are realized with the help of F and significantly promote the graphene growth.