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
5.5 Effect of Air Annealing on Graphene Surface
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Another change that occurs through air annealing is the reduction in resistance. Figure 5.10 (a) shows the ΔR/R0 response to 5 ppm DMMP depending on the air annealing time. The annealing for 60 min reduced resistance by 12% from 1700 ohms to 1480 ohms. The ΔR/R0 increased by four times from -0.1% to -0.4%. Adding additional annealing for 30 minutes increased the resistance slightly, but the ΔR/R0 was -0.8%, which was 8 times higher than the initial sensitivity. And an additional annealing treatment for 30 minutes increased the resistance and decreased the ΔR/R0 value.
It is shown that the optimum response time exists for each sample, and the optimum response is
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Figure 5.10 Annealing effect on ΔR/R0 and resistance. (a) Change of response and resistance of graphene. (b) Corresponding response curve with additional annealing time
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shown after decreasing the resistance. Figure 5.10 (b) is the response curve during air annealing. It is shown that there is an effect of air annealing depending on time, and noise is low. The shape of the response curve does not change with annealing time.
5.6 The Effect of Graphene Edge on Sensor Property
In graphene, edge have unique characteristics [234, 243]. It has abundant non-pair electrons and has high active sites. So, edge area is used for functionalization of graphene [244] and contact resistance can be reduced through edge contact. Study has shown that the sensitivity of graphene increased, when the width of the graphene was narrower [234]. Other study has also been published that currents were mainly flowed in edge area and it make graphene sensitive to molecular adsorption and desorption [237]. Using graphene synthesized under the condition of the w/o carbon & annealing, a uniform graphene gas sensor can be fabricated with 6% error. I tried to understand the properties of graphene sensor according to edge ratio by using uniform graphene sensor. I manufactured a sensor with a width x number of channels of 60 μm x 1, 30 μm x 2, 15 μm x 4, 10 μm x 6, 6 μm x 10, 5 μm x 12 and 3 μm x 20. In other words, the sensor has the same sensing area of 100 μm × 60 μm, but with a different edge ratio. Figure 5.11 shows resistance statistics from 10 sensors for each type of sensor.
Regardless of the different edge ratio, the initial resistance values are similar if the sensing areas are the same. As the ratio of the edge decreased, however, the standard deviation of the initial resistance decreased. When the distribution of ΔR/R0 was confirmed after 300 °C air annealing (Figure 5.12), it was shown that the value of ΔR/R0 didn’t change much even if the edge ratio was different.
Regardless of the different edge ratio, most response was 0.8%. However, the value of S/N increased as the edge ratio decreased. The 3 μm × 20 graphene sensor had the lowest S/N ratio and the 60 μm ×
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Figure 5.11 Resistance by edge ratio. The resistances were obtained before air annealing treatment. The resistance is similar regardless of the different edge ratio
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1 graphene sensor had the highest S/N ratio. The reason why the S/N ratio increased as the edge ratio decreased was that the intensity of the noise decreased as the edge ratio decreases. (Figure 5.13) That is, the edge ratio does not affect ΔR/R0, but it affects the intensity of the noise. In conclusion, the higher the ratio of the edge, the lower the S/N ratio, which is disadvantageous in terms of the S/N ratio. Figure 5.15 shows the response curves for annealing of 5 μm and 60 μm graphene. In optical images, graphene looked same but showed various ΔR/R0 and noise intensities in the before annealing.
There were samples that reduce noise after air annealing, but noise intensity did not decrease in some sample. Compared with 60 μm graphene, there were relatively many of 5 μm graphene that noise intensity didn’t significantly reduce after annealing. This was the reason that the S/N ratio to decrease as the width decreases. Figure 5.15 (b) response curve of 60 μm graphene showed that there is high initial noise in some sample, but after annealing the noise is gradually decreased. When annealing was treated for 120 minutes, noise intensity of 60 μm graphene was lowered in most of the samples. The
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Figure 5.13 Edge effect on response and S/N. The ΔR/R0 is similar despite of different edge ratio. The S/N is affected by edge ratio. The higher the edge ratio, the lower the S/N ratio.
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Figure 5.12 Noise intensity by edge ratio. When the edge ratio of graphene is low, the noise variance also decreases
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wider the width, the more pronounced the effect of noise reduction by annealing.
The following experiment was conducted to examine why noise increased with increasing edge ratio. As in Figure 5.14, AFM analysis was performed using samples with the same edge ratio but with different S/N. For high-noise samples, the edges of the graphene were clearly visible in the AFM analysis. This means that the edges were not in close contact with the substrate. On the other hand, for low-noise samples, the edges were not clear in AFM analysis. In other words, the edges were in close contact with the substrate. According to the experimental results, the edge of the graphene should have a good contact with substrate for low noise intensity. The higher the edge ratio, all edges are less likely to contact with the substrate. The narrower the width of the graphene, the lower the adhesion to the substrate. So, the wider the width of the graphene, the higher the S/N ratio of the sensor. However, the wide width of the graphene does not mean it can produce good sensors. Further study is required whether the graphene width does not affect the ΔR/R0.