Part II. Effect of Surface Properties on Sensitivity of Gas Sensor Using Carbon Nanotube and Graphene
Chapter 5. Preparation of chemical vapor deposition-synthesized graphene-based gas sensors with high
5.2 Effect of CVD Synthesis Process on Graphene Properties
Although CVD technique offers a single-layer graphene with low-cost and high-quality, but many factors in the graphene synthesis process affect graphene quality. First, there are impurity particles on the surface of pristine Cu [214]. These particles not only prevent the graphene from being fully synthesized into a single layer, but also affect PMMA residue remaining after transfer process
[111, 113, 215-219]. To clean the Cu surface, there is usually a way to anneal Cu in a hydrogen atmosphere at high temperature for 30 minutes before synthesizing graphene. Along with cleaning at high temperature, pre-cleaning with Ni etchant or nitric acid is useful for uniform single layer graphene for high carrier mobilities and low sheet resistance [214]. For high-quality single-layer graphene synthesis, synthesis temperatures close to 1000 °C and low-vacuum conditions are common method in many groups, but the synthesis flow and cooling process conditions vary widely among researchers [10, 108, 220-223]. After the graphene growth process, rapid cooling and flowing additional methane precursor lower the sheet resistance of graphene and increase the transmittance of graphene [224]. This method provide a transparent electrode for organic solar cells [224, 225]. The
Figure 5.1 Synthesis process of graphene. (a) Total 4 type of graphene were synthesized with controlling whether releasing carbon precursor during cooling process or not, and whether PMMA film is removed using acetone or annealing treatment. (b) Detail synthesis parameter for graphene
Ramping Annealing Grow th Cooling H2: 10 sccm
CH4: 30 sccm CH4 :30
oTemperature (C) sccm
Carbon in Growth + Cooling (w/ carbon) carbon in Growth (w/o carbon)
(b) (a)
Cu Graphene growth PMMA coating
Cu foil etching PMMA / Graphene
Attachment on substrate
Graphene on substrate
A. Annealing B. Acetone
1. Carbon in cooling 2. No carbon in cooling
Patterned graphene on substrate Electrode deposition
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PMMA, one of the typical polymer supporters for graphene transfer, is well decomposed by annealing or acetone. The PMMA is not affected by Cu etchant, and thus it is widely used for graphene transfer.
However, the PMMA make a small residue on the graphene surface and affects the electrical or thermal properties of the graphene [111, 219]. Hence, many studies have been reported to reduce the effect of PMMA residues [113, 215-218, 226].
I have synthesized a single layer graphene under various conditions to check how the CVD synthesis process affects graphene quality. When graphene is dropped in Cu etchant, the etchant would affect the analysis of graphene properties. If FeCl3, commonly used for graphene etching, is used for Cu etchant, a small amount of Fe particles is attached to the surface of the graphene [227].
The influence of these particles does not appear in the pristine graphene. When the annealing is performed at high temperature, the D-mode background of the Raman spectra increases. In order to avoid this phenomenon, Cu etching was performed using ammonium persulfate (APS). Figure 5.1 is a scheme for the process of synthesizing graphene by a typical CVD technique. After pre-cleaning using APS, the graphene is synthesized under low-vacuum conditions with CH4 and H2 flowing. Then, PMMA solution is coated on Cu film and etchant is used to remove Cu catalysts. Transfer the graphene/PMMA supporter onto the desired substrate and remove the PMMA to complete the Figure 5.2 Characteristics of four type of graphene. (a) Optical microscopic images of graphene channel on SiO2/Si wafer. (b) SEM images of graphene. The purple arrow points to the PMMA residue. (c) AFM analysis of graphene. The height scale bar is 50 nm.
50 μm w/ carbon & annealing w/ carbon & acetone w/o carbon & annealing w/o carbon & acetone
(a)
2 μm
(b)
3 μm
(c)
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graphene synthesis. In this process, graphene was synthesized by continuously releasing CH4 as a carbon precursor during the cooling process, and another graphene was synthesized without releasing CH4 during the cooling process; this is presented as “w/” or “w/o” carbon. There are two ways to remove PMMA supporter: by low-vacuum annealing method or by acetone method. Controlling two methods of cooling and removal process, four types of graphene were synthesized. For example, if the Figure 5.3 Raman analysis of four type of graphene before and after air annealing. (a) Raman spectra of graphene before air annealing treatment and (b) after air annealing treatment. (c,d) after air annealing treatment, phenomenon of blue shift occurred in G-mode and 2D-mode.
1570 1610
1580 1590 1600
Raman shift (cm-1)
[G-mode]
○After annealing
●Pristine
2670 2700
2680 2690
Raman shift (cm-1)
[2D-mode]
○After annealing
●Pristine
1500 2000 2500 3000
Raman shift (cm-1)
Intensity/arbitrary
1500 2000 2500 3000
Intensity/arbitrary
Raman shift (cm-1)
Before air annealing After air annealing
(a)
(c)
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w/ carbon & annealing
w/o carbon & annealing w/ carbon & acetone
w/o carbon & acetone
w/ carbon
&
annealing
w/o carbon
&
annealing w/ carbon
&
acetone
w/o carbon
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acetone
w/ carbon
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annealing
w/o carbon
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annealing w/ carbon
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acetone
w/o carbon
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acetone
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carbon precursor released together during cooling and the PMMA film was removed using acetone, it was called the w/ carbon & acetone graphene. As another example, if the PMMA film was removed by annealing method and the graphene was synthesized without the carbon precursor in cooling process, it was called the w/o carbon & annealing graphene. After removing unnecessary area of the synthesized graphene and observing the optical image of graphene channel (size: 200 μm x 60 μm) in Figure 5.2, all four types of graphene look clean. Although the ratio of the double layer is different, all four graphene is single layer. I can obtain information about the graphene layer with an optical microscope [11] but, it does not provide the exact surface information. So, when I check four type of graphene using SEM, I obtained clearer information about grain boundary [10] and PMMA residue. In case of w/ carbon & annealing graphene, the grain boundaries were not clearly in SEM image. The contrast of the double layer graphene appears darker than the other samples in SEM image. The w/
carbon & acetone graphene shows clear grain boundary. Double- and triple-layer graphene can be identified by different contrast. The area marked with a purple arrow was a small PMMA residue. In case of the graphene without releasing carbon precursor in cooling process, the w/o carbon &
annealing graphene was almost identical to the w/ carbon & acetone graphene in SEM images. Single-, double- and triple-layer graphene were distinguished by contrast and the grain boundary was also clear. In the case of w/o carbon & acetone graphene, there was a relatively large number of PMMA residues represented by purple arrows compared to other samples. The SEM image provides information on the layer more clearly than the optical microscope image, but information on the exact surface roughness can be found through AFM analysis. Figure 5.2 (c) is AFM analysis images synthesized according to four type of conditions. In the case of w/o carbon & annealing graphene, grain boundaries were not clear in AFM analysis. There was a relatively small amount of PMMA residue on the surface. In the case of w/ carbon & acetone graphene, the grain boundary was clearly visible and there was a small amount of PMMA residue on the surface. Overall, the amount of PMMA residue on the graphene surface was small for graphene with releasing carbon precursor during cooling. For graphene synthesized without the release of carbon precursor in cooling process, there were a large number of PMMA residues on the surface as a whole. The grain boundaries were found in both w/o carbon & annealing graphene and w/ carbon & acetone graphene.
The information obtained from graphene through Raman spectroscopy is three in D-mode, G-mode and 2D-mode. The three vibrations modes give information on the defect and layer of graphene. The D-mode (~ 1350 cm-1) occurs when vibration of hexagonal carbon is asymmetric. The D-mode means hexagonal defect in graphene. The higher the D-mode intensity, the greater defect in graphene. The Raman spectra obtained from the four type of graphene show little D-mode (Figure 5.3). The intensity ratios of the D-band to G-band (1580 cm−1), or ID/IG, obtained from the graphene w/ carbon and annealing, w/ carbon and acetone, w/o carbon and annealing, and w/o carbon and
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acetone are 0.05 ±0.03, 0.05 ± 0.02, 0.03 ± 0.01, and 0.07 ± 0.02, respectively. In four type of graphene, the defect was negligible because of an ID/IG of less than 0.1. The G-mode (1580 cm-1) and the 2D-mode (~2700 cm-1) intensity ratios provide information about the layer of graphene. The I2D/IG intensity ratio of the 2D-band (2700 cm−1) and the G-band provides information on the graphene layers. The I2D/IG of the graphene samples w/ carbon and annealing, w/ carbon and acetone, w/o carbon and annealing, and w/o carbon and acetone were 0.98 ± 0.09, 2.67 ± 0.18, 1.37 ± 0.13, and 1.77 ± 0.19, respectively. The I2D/IG of single layer graphene should be larger than 2 [228, 229].
However, the I2D / IG ratio is affected by heat treatment so information about layer is distorted. When a single-layer graphene is subjected to a high-temperature heat treatment, for example, the intensity of 2D-mode decreases, and the I2D/IG is reduced to near 1. This is well known because hole doping occurs in the annealing process. Hence, the PMMA film in transfer process is removed generally using acetone to prevent the I2D/IG from being distorted by annealing. The w/ carbon & annealing and w/o carbon & annealing graphene were annealed in Ar and H2 atmosphere in a low vacuum to remove PMMA film. Nevertheless, oxygen doping occurred and the intensity of the 2D-mode of the Raman spectra is reduced [230-232]. Repeated annealing in air make intensity of the 2D peak decreased. The reduced intensity is greater than that annealed in an Ar / H2 atmosphere. Figure 5.3 (b) shows Raman spectra before and after annealing at 300 °C for 1 hr in air. Overall, the intensity of 2D-mode was greatly reduced. The four types of graphene were single layer graphene, but I2D/IG was lower than 1.
And four graphene showed that the background of D-mode was slightly increased after air annealing.
The effect of air annealing change to the position of G-mode and 2D-mode peaks. The G-mode position of pristine graphene is 1580 cm-1. In the four types of graphene, only carbon + acetone graphene is only the G-mode to be located at 1580 cm-1. The positions of the G-mode the graphene samples w/ carbon and annealing, w/o carbon and annealing, and w/o carbon and acetone were 1592 cm-1, 1593 cm-1 and 1594 cm-1 respectively. In the three samples, the position of G-mode is over 1590 cm-1. This is due to the PMMA residue on the surface and the annealing effect. After air annealing the four types of graphene, oxygen doping becomes more effective and the G-mode position of all of graphene moved around 1600 cm-1. The blue shift phenomenon, which increases the Raman frequency after annealing, was the same in 2D mode. Before air annealing, the positions of 2D-mode of the graphene samples w/ carbon and annealing, w/ carbon and acetone, w/o carbon and annealing, and w/o carbon and acetone were 2684 cm-1, 2676 cm-1, 2685 cm-1 and 2684 cm-1 respectively. After air annealing, the blue shift phenomenon was observed at 2694 cm-1, 2692 cm-1, 2695 cm-1 and 2692 cm-1 in w/ carbon & annealing, w/ carbon & acetone, w/o carbon & annealing and w/o carbon & acetone graphene respectively. Summarizing the effect of graphene synthesis and annealing on the Raman spectra, peak shift phenomenon occurs due to PMMA residue, and metal particles of etchant cause increase of D-mode background. Also, depending on the degree of annealing, the intensity of the 2D-
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mode is reduced and information about the layer is distorted. Optical, Raman, SEM and AFM analysis showed that w/ carbon & annealing graphene had few PMMA residues, I2D/IG was larger than 2, and the surface was clean, therefore the synthesis condition were the best among the four conditions.