3. Growth of Graphene at Low Temperature by Using Ultrathin Poly Vinyl butadiyne Thin Film
3.1 Experimental section
3.1.1 Synthesis and polymer preparation
Poly vinyl acetylene(PVA) was synthesized using a modified procedure (Figure 18(a))68. We used 4- TMS-1-buten-3-yne to prepare a Sonogashira reaction between vinyl bromide and TMS-acetylene. This monomer was polymerized in bulk at 60 °C with azobisisobutyronitile (AIBN). Subsequent reaction of TMS groups in tetra-n-butylammonium fluoride (TBAF) and tetrahydrofuran (THF) afforded PVA with
>95% alkynes available for reaction. Poly vinyl butadyine (PVbd) was synthesized by the process of figure 18(b). A solution of dried THF (0.843 mL), 6-trimethylsilyl-1-hexen-3,5-yne (500 mg, 3.37 mmol) and AIBN (51.2 mg, 0.311 mmol) were added to a 25 mL Schlenk flask, and degassed by three times of freeze-pump-thaw method. The resulting mixture was heating at 80 °C under nitrogen for 24h.
Viscous product was dissolved in acetone and precipitated into methanol. Precipitated polymer was filtered and washed with methanol. After adding small amount of water to filtrate, precipitated polymer was filtered and washed again. The resulting yellow polymer was dried under vacuum and 452 mg (82 %) of product was get.
3.1.2 Spin coating onto substrate
Poly vinyl butadyine (PVbd) powder was diluted in chloroform (1.5 g/cm3) solvent to 0.01, 0.005, 0.0025 wt %. Cleaning treatment of substrate is different as the types of substrate: Cu, Co - N2 blowing, Pt – immersing in acetone and O2 plasma treatment for 10 min, SiO2/Si – bath sonication for 15 min in acetone solvent, dipping in IPA, then N2 blowing. Then PVbd solution was coated on the substrate through spin coating. After putting the solution on the substrate for 10 seconds, increasing the spin rate to 5000 rpm for 7s and keeping 5000 rpm for 2 min.
3.1.3 Rearrangement reaction
Cyclization, dehydrogenation and rearrangement reaction were processed by heat treatment using CVD furnace. At first, increasing temperature to 200 oC for 40 min and keeping for 1 hour is important
point at 0.2 torr, not using any gas. Because cyclization and oxidation of PVbd occurred63, 65 at 200 oC.
This process can help polymer to stabilize. Then pressure is lowered by rotary pump to 5.0 x10-3 torr and hydrogen gas of 30 torr is flowed. Increasing temperature slowly to 600 oC with 4 oC /min of rate is processed so as to prohibit vaporization of polymer65, and 20 oC /min of rate is used in increasing temperature up to 700 or 800 oC. At the final temperature such as 600, 700, and 800 oC, PVbd film was put for 1 hour. Another important point is cooling rate of CVD. This influence on the quality of graphene very much. The time of decreasing temperature from final temperature to 350 oC is usually 4 hours with 20 sccm of hydrogen gas.
3.1.4 Transfer method: electrochemical bubbling-based method
For transferring graphene from Pt foil to arbitrary substrates, we used electrochemical delamination technique to recycle Pt foil69. After growing the graphene on the Pt foil, PMMA was spin coated on the graphene/Pt foil. The graphene/Pt foil was then immersed in a 1 M aqueous NaOH solution. During the use of PMMA/graphene/Pt foil as the cathode, a piece of bare Pt foil was used as the anode. Applying a constant current for a few minutes will create a H2 bubble and separate the PMMA/graphene layer from the Pt foil. After the PMMA/graphene layer was washed with deionized (DI) water, the residual NaOH was removed and transferred to the target substrate. Finally, PMMA was removed with acetone to obtain a high quality single layer graphene.
3.1.5 Characterization
The optical image was obtained using a Axio scope.A1 (Carl Zeiss) and the SEM images were obtained with a S-4800 instrument (Hitachi). The AFM images were collected using a Dimension 3100 microscopy (Veeco) under the tapping mode. Raman spectra were measured using an Alpha 300s micro Raman spectrometer (WITec) equipped with 488 nm laser. Infrared detection was carried out on a 670/620 (Varian) spectrometer and thermogravimetric experiments were performed using Q500 instrument (TA Instruments). The NMR spectra were recorded using an Ascend 400 MHz spectrometer (Bruker) using DMSO-d6 as the solvent.
Figure 19. IR and TGA of (a-b) PVA and PVA with TMS, (c-d) PVbd and PVbd with TMS, respectively
saturated CH and CH2 groups. The peak at 3300 cm-1 in PVbd is twice as large as the peak in PVA, which is a result consistent with the molecular formula. The TGA data of figure 19(b) and (d) show the how much polymers remain after heat treatment at 800 oC. TMS group were vaporized around 500 oC and the remaining carbon structures about 50 % of the original precursor rearrange with the help of metal catalysis. Depending on the presence or absence of the TMS group, the weight reduction pattern was different in TGA, which also affected the experimental results. The synthesized polymer is diluted and spin-coated and then heat-treated. The spin-coated thickness is affected by the concentration of the solution used. The polymer mainly diluted to 0.005 wt% was used, and the film thickness was about 1.5 ~ 2.5 nm when it was examined using AFM and ellipsometry.
We mainly use PVbd with TMS group. Because, when using it among four types of polymers, the result is the best. PVbd is better than PVA because the more acetylene group can form aromatic structure easily. We tried to synthesize Poly vinyl hexatriyene(PVht) which has three acetylene functional group, but we cannot synthesize it. Also, It is believed that TMS group plays a role in holding the polymer so that the polymer does not clump and maintain its position in the early stage of heat treatment. To confirm that the polymer is used as a carbon precursor, the empty substrate was heat-treated at 600 and 800 oC, respectively, without coating the polymer, and Raman confirmed that nothing was synthesized.
During the heat treatment process, the parameters observed to control the quality of graphene include temperature, processing time, kinds and thickness of polymer film, substrate, type and quantity of gas, oxidation of polymer, the thickness of polymer film, and the cooling rate. We tried to several substrates such as Cu, Pt, Co, SiO2/Si. Platinum acted as a good catalyst for the breakdown of precursor material into smaller building units that are essential for the synthesis of graphene and for the arrangement of carbon moieties into graphitic domain by high diffusion coefficient. We will explain the changes in graphene quality according to each parameter about CVD condition in detail.
First thing is cooling rate of CVD furnace. In Figure 20, shown Raman spectra represented results about processing heat treatment at 800 oC for 1hr, and cooling by adjusting the cooling rate. When consuming 4 hours to cool from 800 to 350 oC, the quality of the graphene is much improved rather than cooling rapidly, which can be seen that the 2D peak is larger and the D peak is smaller. Figure 20(a) and (b) are experiments conducted on copper and platinum substrates, respectively. This is because carbon adatom move to defect site of graphene during cooling70-71. When cooling rate is too fast, the
Figure 20. Raman spectra after heat treatment of PVbd at 800 oC on each substrates depending on the cooling rate. (a) on Cu, (b) on Pt, and (c) transferred from Pt to SiO2/Si. Each color of spectra represent cooling time. The black, purple, orange, and red mean fast cooling, 1 hr, 2 hr, 4 hr to 350 oC in order.
type of gas used is also important point. We found optimized condition which is 20 sccm of hydrogen gas. When I did experiment without any gas, the defect was large. In the case of using 20 sccm of Ar gas, there was nothing left. Hydrogen gas is called magic gas, which can help growth of graphene. If we increased the amount of hydrogen gas to 30 sccm, the grown graphene has some vacant part, which carbon adatoms seems to be etched by the hydrogen gas.
Next important parameter is a degree of oxidation. Oxidative treatment of poly(vinyl acetylene) before the carbonization step influence on pyrolysis of the polymer strongly63. The oxidative treatment helps the yield of carbon to maximize after pyrolysis up to 900 oC, by inducing stabilization and rearrangement of polymer. They have identified the formation of the -C=O and -O-H functional groups were confirmed by IR spectroscopy. We also check the formation of the functional groups such as C=O (1720) and benzene ring (1600) by IR after heat treatment at 200 oC with oxygen in atmosphere. We controlled the quantity of oxygen gas by the control of pump valve without using any additional gas.
Figure 21 represent the change of graphene quality after heat treatment at 200 oC for 1 hr at each pressure, followed by a heat treatment at 800 oC. According to figure 21, when the sample is at 0.2- 0.001 torr at 200 oC, the quality of graphene is best. But heat treatment at 200 oC in atmospheric pressure, the amount of remaining carbon is relatively small. It is considered that the polymer made of carbon reacted with lot of oxygen gas and evaporated in the form of water. On the contrary, in the case of low pressure which we continue to draw gas in the furnace by using pump, the D peak became larger as defect became bigger. This is because oxidation process could not occur and then thermal stability of polymer decreased. The carbonyls generated during the oxidation process help the cyclization process, and hydroxyl group can help dehydrogenation and cross-linking through dehydration reaction63. Here, the difference in pressure did not act as experimental variables. We did control experiment that adjust pressure of 0.2 torr by using Ar gas and pump, but the intensity ratio of D/G peak is about 1.6 whose result is similar with the case of heat treatment in atmospheric pressure at 200 oC.
We tried to lower the growth temperature to 600 oC. We kept 0.2 torr at 200 oC and lowered the cooling rate, which took 4 hours from reaction temperature (ex. 600, 700, 800 oC) to 350 oC. Then Raman peak was quite improved. Figure 22 represent SEM and corresponding Raman spectra of graphene film after transferring it from Pt foil to SiO2 (300nm)/Si substrate. The growth at low temperature such as 600, 700 oC is more defective when viewed at Raman graph than when grown at
Figure 21. Optic image and its corresponding Raman spectra depending on the degree of oxidation at 200 oC. (a-e) are optic image related with the black, purple, orange, red, and grey color of (f). It means atmospheric pressure, 1.5, 0.2, 0.01 torr, and low pressure (0.005 torr) in order at 200 oC heat treatment. The spectra represent average value of 10 points. Scale bar is 20 um.
Figure 22. SEM image and its Raman spectra by reaction temperature. Heat treatment at (a,d) 600, (b,e) 700, and (c,f) 800 oC.
3.3 Conclusion
In summary, graphene growth was performed at low temperature (~600 oC) using ultrathin polyvinyl butadiyne(PVbd) film. PVbd has high carbon content and high carbon/hydrogen ratio. It is also expected to be graphitized by heat treatment through cyclization and carbonization process. In this conversion, two points are important factor. One is a cooling rate. When the Pt substrate is slowly cooled, the adatoms of the carbon film have enough time to move to the defect site. Another factor is the oxidation degree of PVbd. The carbonyl and hydroxyl group are formed by oxidation reaction at 200 oC, which helps the cyclization and carbonization process. The formation of uniform graphene and improvement of quality are required, but using PVbd show graphene synthesis is possible at low temperature. This is because the polymer can be rearranged and graphitized with lower energy than methane gas.
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