Chapter 5 Fabrication and analysis of photodetector devices based on surface modified
5.3. Results and discussions
the annealing temperature of GO samples increase, but the position of the peaks is retained as constant after different heat treatments. The change in the intensity of the Raman signals may be influenced by the presence of oxygen functional groups on GO. Raman spectra are measured on the different GO samples, which are complex to justify the nature of the GO from the intensity of the peak alone.
ID/IG analysis of the Raman signal is one parameter to define the GO samples' quality (Fig. 5.4d). The reduction of ID/IG value indicates the restoration of the sp2 hybridized structure of thermal annealed GO. Due to the removal of oxygen functional groups attached to the graphitic structure and the presence of the functional groups, it leads to a change in the sp2 to sp3. The ID/IG ratio of the annealed sample was negligibly changed from 0.968 to 0.964 for GO- RT to GO-85; it may be because the 85 ˚C annealing cannot remove the oxygen functional groups in GO. At higher temperatures, its value is reduced to 0.95 for GO-120 and 0.91 for GO-230. The reduction of the ID/IG value is shown by removing the loosely bounded oxygen functional groups from the GO, which is supported by the TGA analysis of GO and RGO, analyzed in section 3.2.2 (chapter 3).
The XPS analysis of the thermal annealed GO at 85 ˚C and 230 ˚C is shown in Fig.
5.3e and Fig. 5.3f. The strong presence of C-O and C=O functional groups in the GO-85 is indicated in the 286 eV and 288.9 eV, respectively24. Moreover, the signified oxygen functional group (C-O) at 286 eV is flatted after thermal annealing of GO at 230˚C. This shows that removal of C-O functional groups from GO after thermal annealing at 230 ˚C leads to restoring the GO's graphitic structure.
FTIR spectra of the GO and thermal annealed GO are studied in the presence of oxygen functional groups and graphitic structure (sp3 and sp2 carbon bonds), which is shown in Fig.
5.4. The transmittance peaks of FTIR for GO and annealed GOs are concentrated in the range from 900 to 3600 cm-1. The peaks range from 950 cm-1 to 1480 cm-1 are comprised of the oxygen functional groups. The peaks of C-O (alkoxyl group) at 1057 cm-1, O-C=O at 1228 cm-
1, C-OH at 1391 cm-1, C=C at 1630 cm-1, and C=O at 1722 cm-1 are more prominent in GO- RT, GO-50, and GO-8525,26.
The intensity of the corresponding peaks for the C-O and O-C=O functional groups are suppressed in GO-120 and GO-230 compared to the previous case. The peaks at 3400 cm-1 related to -OH appears in the spectra for both GO and all annealed GO samples except GO- 230; these may be due to the absorption of water from the atmosphere during the preparation of samples. The signature peak at 1630 cm-1 corresponding to C=C bonds is observed on all
the samples. The absorption peak arising from the carbonyl group (C=O) at 1740 cm-1 are present in all the samples, which is very hard to remove even after thermal treatment. From the XPS and the FTIR analyses, it is evident that the oxygen functional groups are reduced in GO- 230 compared to the other GO samples.
Fig. 5.4: FTIR analysis of as-grown GO and different temperature annealed GO.
5.3.2. UV absorption and PL studies of graphene oxide
UV absorption and Photoluminescent (PL) characteristics of GO-RT are shown in Fig.
5.5. UV absorption spectra of the GO-RT are analyzed in Fig. 5.5a. Two notable peaks at 235 nm and 300 nm are observed. A peak at 235 nm can be assigned to π → π* transition or π- plasmon resonance common for extended sp2 conjugated carbon sheet (C═C present within the aromatic sp2 hybridized carbon framework)12. A slight hump-like shoulder is also seen at 300 nm, corresponding to n → π* from oxygen-containing functional groups.
Fig. 5.5: (a) UV absorption of GO RT; and (b) PL analysis of GO RT.
The presence of oxygen-containing functional groups is evident from the absorption spectra of GO. A broader band in the range 400-780 nm is observed in the PL spectrum of GO- RT (Fig. 5.5b). This band consists of three peaks, centered at 480 nm, 571 nm, and 665 nm, arising from the disorder-induced defect states due to the sp3 character of carbon with oxygen- containing functional groups attached to graphene carbons. The presence of oxygen can increase the number of (C─OH) and (C─O─C) bonds due to the rearrangement of oxygen and oxygen-containing functional groups with carbons. This consequently enhances the transfer of resonance energy from O sites to the sp2 clusters in the graphene lattice, contributing to a broad PL emission13.
5.3.3. Analysis of interdigitated electrode pattern
Fabricated 10µm channel IDE studied the microstructure using FESEM and AFM images, shown in Fig. 5.6. In Fig. 5.6a, a low magnification image of the IDE pattern with the large metal pad is observed; this large area (2.5 × 1) mm is sufficient for probing the 0.5 mm diameter probe for the measurement of photo I-V characteristics. The magnified image on the metal fringes are shown in Fig. 5.6b; the clear separation of the metal fringes is observed in the image, and the spacing between the consecutive metal fringes is obtained at ̴ 10 µm. The IDE is fabricated on the clean SiO2/Si substrate to confirm that it is essential for the fabrication and working of a device.
An AFM image is shown in Fig. 5.6c, which is captured between two consecutive metal fringes of the IDE pattern. The gap in the spacing is measured ̴ 10 µm; this is similarly attained from the analysis of FESEM images. The thickness of the deposited metal is measured using the height profile of the AFM image; it is obtained ̴ 30 nm (Fig. 5.6d). This thickness is sufficient for measuring the device's I-V characteristic and sustaining the probing multi-times on the same metal pad.
FESEM image of the GO sheets deposited on the IDE pattern is studied and viewed in Fig. 5.6e and magnified image in Fig. 5.6f. Due to the extremely thin and transparent nature of the GO sheets, the microscopic imaging of a GO sheet is difficult to spot and observe.
However, the presence of wrinkles and folds on the surface of GO determines the GO sheet on the IDE pattern. In Fig. 5.6e, a large number of non-fashioned bright-line spots on the IDE pattern are observed. The spots are spread across the metal fringes, which shows the GO sheet is connected across the metal fringes, or GO sheets connect two electrodes. The magnified image of the GO deposited IDE is monitored, and found the GO sheets are lying on top of the metal fringes and connected to the electrodes.
Fig. 5.6: (a) FESEM image of IDE (a) low magnification image; (b) high magnification image on fringes; (c) AFM image on the two consecutive fringes; (d) Height profile of electrode thickness; (e) GO sheets on IDE; and
(f) GO sheets in between the metal fringes.