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
5.4 Effect of CVD Synthesis Process on Sensitivity
In this research, I propose a graphene synthesis process that provides uniform responsivity.
There are two factors that have been carefully controlled during the graphene synthesis process using CVD technique. The first is to continuously release the carbon precursor (in here CH4) during cooling process after the graphene growth process. When Cu is used as a catalyst substrate, the carbon precursor is continuously released during cooling. Unlike Ni catalyst substrate where graphene is synthesized by cooling process due to high carbon solubility, graphene growth process begins as soon as methane is added in hot temperature zone because Cu has low carbon solubility. So, the carbon precursor is continuously released during the cooling process in order to protect the already synthesized graphene.
If the carbon precursor is not added during cooling process, PMMA residue will be formed on the graphene surface after the transfer. So, to avoid PMMA residues and to synthesize high-quality graphene, the carbon precursor is constantly flowed during the cooling process. The second factor is the removal method of PMMA. The PMMA film is usually removed with acetone. Although the PMMA film can be removed by annealing treatment, oxygen doping occurs, which interferes with the synthesis of pristine graphene. These two conditions are easily adjustable, but I have confirmed that the quality of the graphene under the control is clearly different. The graphene sensor was fabricated in channel form through the photolithography process and, exposed to 5 ppm DMMP. The response curve defined here as ΔR/R0= (RDMMP – RN2)/RN2 x 100, where RDMMP and RN2 denote the resistance of the sensor by exposure to DMMP and in dry N2. Figure 5.4 (a) is a representative response curve with 5 ppm exposed to four types of graphene. In carbon + annealing graphene, the noise intensity of the signal was large. Also, there was no response for 5 ppm DMMP. In the case of w/ carbon & acetone
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graphene, the intensity of noise was small, but there was no response for 5 ppm DMMP. In the case of w/o carbon & annealing, the noise intensity was low and showed an immediate response to 5 ppm DMMP vapor. The w/o carbon & acetone graphene was also sensitive to 5 ppm DMMP but less sensitive than carbon + annealing graphene. Using four type graphene, 35 samples were exposed to 5 ppm of DMMP in the same way. (Figure 5.4 (b)) The carbon + annealing graphene showed wide
Figure 5.6 Response from same sample before and after air annealing treatment. The response randomly increased after air annealing. The w/o carbon & annealing graphene show enhanced response with uniformity.
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distribution of ΔR/R0 5 ppm DMMP. The direction of response showed a positive in some sample, and it showed a negative in most sample. The dominant response direction is negative. The w/ carbon &
acetone graphene was also no response to 5 ppm DMMP. Some samples show a positive direction of response and most show negative. In the case of the w/o carbon & annealing graphene, all samples were in the negative direction and the distribution of ΔR/R0 was even more uniform than the other prepared three graphene. The w/o carbon & acetone graphene showed mostly low responsivity. The CVD synthesized graphene showed little or no responsivity to 5 ppm DMMP. The direction of the response also appeared randomly, positive and negative. However, in the case of w/o carbon &
annealing graphene, ΔR/R0 was not large but showed a clear sensitivity and had a uniform reaction distribution. Optical microscope, SEM, Raman and AFM analysis showed that the graphene synthesized through the w/ carbon & acetone process had a clean surface, small PMMA residue, showed high I2D/IG. Hence, the w/ carbon & acetone graphene is the best quality. However, after checking the response test, these analyzes is not related with the uniform response of graphene. The w/o carbon & annealing graphene, which was oxygen doped through the annealing process, showed sensitive and homogeneous responsivity despite the PMMA residues were relatively large on the surface. However, the sensitivity of w/o carbon & annealing graphene is very low compared to the sensitivity of exfoliated graphene. The use of synthesized CVD graphene as a sensor requires more sensitive properties.
Some studies have been reported to increase the sensitivity of sensors fabricated using CVD graphene. There is a way to narrow the width of the graphene to about 5 μm in the form of ribbons.
However, in this study, it was mentioned that the sensitivity of CVD graphene was higher than that of exfoliated graphene, and the exfoliated graphene had small responsivity. They have also reported that CVD graphene exhibits high sensitivity when the width is narrow, but there is no study of the response distribution using many samples. Another way to increase sensitivity is to measure response curve with applying gate voltage to the graphene. As the gate voltage is applied, the temperature of the graphene edge on the substrate increases due to the current flow. The higher the gate voltage, the higher the temperature at the edge and the better the sensitivity. When commercial and practical applications are considered, however, a system without gate voltage is required. Another study to improve sensitivity increases sensitivity when exposed in high temperatures. However, it is difficult to apply such a system to actual sensors. Previous studies have clearly suggested ways to improve the sensitivity of the sensors, but they are lacking in practical terms. A way to improve the sensitivity of the sensor without other system is required. In this study, I suggest how to improve the sensitivity of graphene easily with 300 °C air annealing.
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Figure 5.5 shows an improved response curve after 300 °C air annealing. The response curve before and after air annealing was obtained from exactly the same sample in Figure 5.4 (a). After air annealing, the intensity of the noise was significantly reduced for w/ carbon & annealing graphene and showed response to 5 ppm DMMP. For w/ carbon & acetone graphene, it showed dramatically improved ΔR/R0. There was also a reduction in noise intensity. The w/o carbon & annealing graphene also reduced noise and improved ΔR/R0. However, it does not increase more than the w/ carbon &
acetone graphene. The w/o carbon & acetone graphene improved response after air annealing but, did not reduce the intensity of noise. Figure 5.6 shows the response before and after air annealing. It shows the ΔR/R0 value by tracking the response of each sensor after before/after annealing. After air annealing, the response of most sensors increased. However, it was shown that the ΔR/R0 after air annealing was relatively uniform in graphene synthesized under the w/o carbon & annealing conditions. One thing to mention is that some sensors had a positive reactivity by 5 ppm DMMP
before annealing, but all of them showed negative response to 5 ppm DMMP after air annealing. It is seen from the statistics that the response randomly increases.
The typical response curves before and after air annealing is shown in Figure 5.7. In case of top response curve, it showed low sensitivity before annealing and it showed improved response after air annealing. The middle response curve showed no reactivity to 5 ppm DMMP due to the large noise before air annealing. After air annealing, the noise intensity was lowered as with other sensors and
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Figure 5.7 Typical response curves before and after air annealing treatment. Three data are from the same batch, but the reactivity is different for before air annealing. However, after air annealing, the response direction is negative, the intensity of the noise is similar, and the response size is also improved.
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was responsive to 5 ppm DMMP. In the case of the bottom response, it showed a positive for 5 ppm DMMP before annealing. There were not many, but often, samples showing these positives. After air annealing, however, the response was sharply increased and is high among the three responses.
The signal to noise (S/N) ratio, which is the meaning of the signal, should also be reflected to use as a sensor. In Figure 5.8, the x-axis is ΔR/R0 and the Y-axis is S/N. For w/ carbon & annealing graphene, the S/N ratio was mostly low even though there was a high ΔR/R0 value in before air annealing. The reason was high noise intensity. After annealing, S/N and ΔR/R0 increased, but the distribution was large. The w/ carbon & acetone graphene showed the most obvious difference in response and S/N before and after air annealing. There was almost no response before air annealing.
Figure 5.8 Distribution of ΔR/R0 and S/N before and after air annealing treatment. The data were obtained from exactly same sample before and after air annealing treatment.
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The S/N was low and the ΔR/R0 was dominantly distributed to zero. However, after air annealing, ΔR/R0 clearly increased and S/N also increased a lot. The highest S/N ratio was over 1000 and the ΔR/R0 value was close to 1%. The w/o carbon & annealing graphene showed uniform response and S/N even before air annealing. After air annealing, the response was uniformly improved. It also showed that the distribution before and after air annealing didn’t overlap and clearly showed the effect of air annealing. In the case of w/o carbon & acetone graphene, ΔR/R0 increased after air annealing, but the improvement of S/N was not great because noise intensity is not decreased. It is seen that the distribution of ΔR/R0 was also wider.
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