V. Part 2. Reduced Graphene Oxide Coated Polymer Films
5.3 Results and Discussion
5.3.1 Reduced Graphene Oxide on SiO 2 Wafer
In order to exploit the strain transfer capability in GO sensors, we have prepared rGOs with and without tight bonds between rGO and SU-8 support films. Fig. 5-1 illustrates the fabrication process of SU-8 films coated with thermally reduced graphene oxide (Tr-GO) and chemically reduced graphene oxide (Hr-GO). The details for GO synthesis, coating of GO layers on the surface of SiO2-coated Si wafer, and substrate preparation have been presented in the previous section. The key fabrication step for the graphene-based strain sensor is the reduction process of the freestanding GO/SU-8 composite film either via a thermal or chemical process, since the reasonable electrical conductivity of GO should be secured for a piezoresistive strain sensor, for which measurable electrical conductivity is imperative.
In order to obtain a Tr-GO/SU-8 film, the GO film coated on the SiO2 wafer was heated/reduced in a furnace at an elevated temperature under H2 and Ar conditions. A SU-8 film was spin-coated on the Tr- GO, and then the 300-nm-thick SiO2 layer was removed using 10% HF solution, leaving a freestanding Tr-GO/SU-8 film. Since a large fraction of epoxy and -COOH functional groups are reduced and thus inactivated, only physical bonds between Tr-GO and SU-8 film are active, resulting in relatively weak adhesion at the interface.
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Figure 5-1 Fabrication procedures for Tr-GO- and Hr-GO-coated SU-8 films
The Hr-GO/SU-8 composite films utilized GO that has been chemically reduced by hydrazine after the event of covalent bonding between SU-8 and GO as illustrated in Fig. 5-2. To fabricate the Hr- GO/SU-8 films, a SU-8 solution was spin-coated over GO and thermally cured at 100C for 10 min, leading to covalent bonds between GO and the SU-8 film. Previous studies reported that epoxy functional groups in SU-8 remain reactive and thus induce crosslinking reactions during heating due to the presence of the acidic curing agent in the films 160. It is therefore expected that the epoxies in GO participate in crosslinking reactions with the epoxy groups in the SU-8, forming tightly attached chemical bonds at the interface between the GO and SU-8 layers. Finally, The Hr-GO/SU-8 composite films are prepared by the hydrazine reduction process under a sealed container.
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Figure 5-2 Schematic diagram showing the preparation of Hr-GO coated SU-8 film and the mechanism of creating covalent bonds between Hr-GO and SU-8.
The SU-8 films coated with varying rGO densities and thicknesses were fabricated to investigate their effects on the piezoresistive response of rGO coated SU-8 films (Fig. 5-3 and Table 5-1). The 6-7 nm thick rGO films with different coating densities were fabricated by controlling the number of spin- coating of the GO solution (GO 2 mL + water 8 mL). One time coating and three time coating films were designated as “starve coated“ and “fully coated“ samples, respectively. In addition to the density control of the rGO films, the coating thickness was varied to 13-14 and 21-23 nm ranges by spin coating high-concentration GO solution (GO 2 mL + water 2 mL) 2 and 4 times, respectively.
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Figure 5-3 (a) Digital photograph of transparent rGO-coated SU-8 film prepared by hydrazine reduction and SEM images of (b) Hr-GO starve coated and (c) Hr-GO fully coated films.
Table 5-2 Comparison of rGO-coated films with various reduction methods, coating densities, and thicknesses.
Coating thickness [nm] a Hydrazine reduction Thermal reduction
6 – 7 Hr-GO
starve coated
Hr-GO
fully coated Tr-GO starve coated
Tr-GO fully coated
13 – 14 Hr-GO 13 (or 14) Tr-GO 13 (or 14)
21 – 23 Hr-GO 21 (or 23) Tr-GO 21 (or 23)
Fig. 5-4 (a) shows the optical transparency of SU-8 substrates coated with 13-nm-thick GO, Hr-GO and Tr-GO on a quartz plate measured by a UV–visible absorption spectrometer. Unlike the GO-coated SU-8 film, which exhibited nearly 97% transmittance, rGO films showed 83% transmittance over the 450-800 nm wavelength range regardless of the reduction method. These transmittance values are in good agreement with values reported on rGO coated devices with similar coating thicknesses.35 However, Hr-GO and Tr-GO coated films show different surfaces resistivity values of 2.84 and 0.83 MΩ/sq, respectively. These results indicate the higher degree of crystallinity and more effectively reduced graphene structure in Tr-GO, which agrees well with the Raman spectra and the C/O ratios calculated from XPS (Fig.5-5). As shown in Fig. 5-4 (b), the transmittance of Hr-GO and surface resistivity decrease from 90 to 65% at 550 nm and from 6.29 to 0.52 MΩ/sq, respectively, as the thickness of the film is increased from 6-7 to 21-23 nm due to the tightly bound interconnected conductive networks formed by thicker coatings.
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Figure 5-4 Transmittance of (a) GO on Si wafer (dotted line), Hr-GO 13 (black line) and Tr-GO 13 (red line) starve coated film and (b) Tr-GO coated films with different coating thickness (6, 13, 21 nm). The inset images show the surface resistivities of GO and rGO films.
Figure 5-5 Deconvoluted XPS spectra of the C1s region of (a) GO, (b) Hr-GO and (c) Tr-GO. (d) Raman spectra of GO (black line) and rGO (Hr-GO; red line, Tr-GO; grey line) with different reduction methods.