Modification of graphene oxide film properties using KrF laser irradiation

Modification of various properties of graphene oxide (GO) films on SiO2/Si substrate under KrF laser radiation was extensively studied. X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and the electrical resistance measurements were employed to correlate the effects of laser irradiation on structural, chemical and electrical properties of GO films under different laser fluences. Raman spectroscopy shows reduced graphene oxide patterns with increased I2D/IG ratios in irradiated samples. X-ray photoelectron spectroscopy shows a high ratio of carbon to oxygen atoms in the reduced graphene oxide (rGO) films compared to the pristine GO films. X-ray diffraction patterns display a significant drop in the diffraction peak intensity after laser irradiation. Finally, the electrical resistance of irradiated GO films reduced by about four orders of magnitudes compared to the unirradiated GO films. Simultaneously, reduction and patterning of GO films display promising fabrication technique that can be useful for many graphene-based devices.


Introduction
Graphene has been the subject of much research because of its unique electrical, thermal and mechanical properties. It is a promising material in a wide variety of elds including optics and electronics, solar cells, light emitting devices, touch screens, at panel displays and photovoltaic applications. Following exciting research on graphene, graphene oxide (GO) and reduced graphene oxide (rGO) have attracted great interest as a replacement of graphene in some aspects, like facile synthesis and potential applications in electronics and optoelectronics, circuits, sensors and supercapacitors.  The Hummers method is the most promising fabrication process in which the precursor commonly undergoes a reduction process for mass production of the graphene-based material. RGO shows completely different chemical and physical properties compared to GO. Although a complete conversion of GO to graphene has proven to be difficult, partial rGO with improved electrical conductivity can be relatively easily obtained. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] In addition to the reduction process, many applications of the material require simultaneous patterning on various substrates. However, the common patterning methods need photoresist and fabrication of various masks to produce the desired patterns. A laser reduction method has been used to produce rGO patterns on GO lms, which eliminates other complex methods such as lithography aer primarily reduction process. 30,31 So far, many studies have investigated laser patterning of graphene, 32-39 graphite oxide 39,40 and GO lms. [41][42][43][44][45][46] In particular, patterning GO lms were performed using different lasers irradiation on various substrates. [41][42][43][44][45][46][47][48][49] In one study, 29 direct writing of conductive microcircuits on GO lms was demonstrated using femtosecond laser irradiation. The removal of oxygen functional groups was conrmed by XPS studies and a reduction of electrical resistance was observed in rGO lms. However, there isn't any evidence of transformation to the graphene-like structures in irradiated GO lms, because no increase of 2D band was observed in Raman spectrum of irradiated GO lm. The increase of the band intensity conrms transformation to the graphene-like structures. Another study 47 of laser irradiation of GO lms on a glass substrate was reported using a femtosecond laser with 800 nm wavelength. Nevertheless, the measured range of Raman spectra was limited just to D and G bands and there was no report of the 2D band at Raman spectra of their irradiated lm. Several other authors have also looked at the interaction of KrF laser and GO lm 48,50,51 due to a close coincidence of the laser wavelength with those of chemical bonds of the material. In one study, 38 producing the transparent circuit of reduced electrical resistance was reported on GO thin lms on glass substrate. Nevertheless, any chemical analysis or structural investigations of irradiated GO lm were not represented in this study. In another report 50 the effect of KrF laser irradiation on GO lms has been studied with a different number of laser pulses and laser uences. The experiments were carried out in a vacuum and in a H 2 atmosphere and the reduction process of GO lms was found to be more efficient under H 2 atmosphere than the vacuum. In a more recent report, 51 laser reduction of GO lms was studied using KrF, Ti:sapphire and CW lasers irradiation. The substrate used in their work was chosen to be polyethylene terephthalate (PET) because of its exibility and suitability for supercapacitors. A comparison of the performance of each laser on a reduction of GO lms at one certain laser uence was reported and then, the effect of pulse duration on how reduction process of GO lms was investigated. Raman and XPS data of GO lms irradiated with KrF laser showed better results than those irradiated with femtoseconds and CW lasers. However, investigation of electrical resistance changes of irradiated GO lms is absent in their study. The laser irradiation of GO lms can result in fabrication of conductive circuits on insulator GO lms that are attractive in microelectronic applications.
In the present study, the effect of KrF laser irradiation is investigated to improve various properties of GO lms under different laser uences in ambient atmosphere. SiO 2 /Si was chosen as a substrate because it is particularly attractive for microelectronics applications. A wide variety of characterization techniques was employed to monitor various properties of irradiated GO lms included Raman spectroscopy, XPS, XRD, and AFM. These characterizations are necessary to completely understand the laser reduction process. More importantly, our study contains the investigation of the electrical resistance change of irradiated GO lms under different laser uence. The resultant rGO patterns of signicantly different properties were obtained compared to the pristine GO lms. We believe that our report is a comprehensive study of the laser irradiation process of GO thin lms that includes characterization of various properties of irradiated lms under similar irradiation conditions and on the same samples.

Experimental details
GO (Graphenea) used in the experiments was prepared via a modied Hummers method with a concentration 4 mg ml À1 in water. Gold electrodes were deposited on a SiO 2 /Si substrate for electrical resistance measurements. GO thin lms were obtained by spin coating GO aqueous suspension on the substrate at 1000 rpm for 60 s and then dried at 60 C. A pulsed KrF excimer laser of 248 nm wavelength and 10 ns pulse duration was used for the patterning of GO lms. A schematic of steps proposed to prepare GO lms and laser processing system is shown in Fig. 1(a and b), respectively. A 75 mm lens focuses the laser beam on the sample. GO lm has been translated perpendicular to the laser beam and 100 mm s À1 scanning speed was chosen for laser treatment of samples. Laser treatment process of GO samples was carried out at ambient and at room temperature.
Raman spectrometer (Thermo Fisher, DXR) with a laser source 532 nm and a spot size 0.7 mm was used to characterize GO lms. Surface topography was measured using atomic force microscope (AFM), model: Anasys NanoIR2, in the contact mode. X-ray diffraction (XRD) patterns were recorded using a Bruker X-ray diffractometer with Cu Ka irradiations operated at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) was employed to determine chemical states of GO lms using a Physical Electronics PHI 5600 spectrometer with an Al Ka radiation source (1486.7 eV) with an energy resolution of FWHM 0.7 eV for sputtered clean Ag foil (Ag3d 5/2 ) at pass energy 11.75 eV. De-convolution of all XPS data were performed using CasaXPS soware with general forms of Gaussian and Lorentzian line shapes. Electrical resistance measurements were conducted using a 4-point probe method (Fig. 1(c)).

Results and discussion
Prepared GO lms were exposed to KrF laser in an air atmosphere at laser uences ranging from 4 to 72 mJ cm À2 . Fig. 2(a) indicates AFM images of two separate areas of GO and irradiated GO at a certain laser uence. The irradiation trace can be clearly observed in Fig. 2(a). Some islands also exist near irradiation trace due to slightly ablated GO layers that covered unirradiated area. AFM 2D and 3D topography of unirradiated and irradiated areas in smaller scan areas were shown in Fig. 2(b) and (c), respectively. AFM images show that the laser irradiation process increases roughness on the surface of irradiated GO lm compared to the unirradiated area which showed a relatively smooth surface.
Raman spectroscopy was used to study laser reduction process of GO lms. The typical Raman spectrum of GO shows the D and G bands around 1350 and 1580 cm À1 . The D band is assigned to the out-of-plane breathing mode of the sp 2 carbon atoms due to defects. The G band of graphene is produced by the in-plane vibration of C atoms and identies the rst order Ramanallowed mode of graphene. 25,47 2D band (2690 cm À1 ) is the second order of D band, however, there is no need for the presence of the defects for its activation. Fig. 3(a) compares Raman spectrum of GO lm with those of GO lms irradiated with laser uences from 18 to 32 mJ cm À2 . All Raman spectra were normalized to G band to more correctly evaluate changes of the 2D band with laser uence: since this peak is known as a ngerprint of single-layer graphene. The Raman spectrum of the pristine GO lm revealed a strong D band with the intensity comparable to the G band and a broad low-intensity 2D band. However, stronger and narrower 2D bands appeared in Raman spectra of irradiated GO lms (Fig. 3(b)). The intensity of the peak increases as laser uence goes up conrming the transformation to the graphene-like structures as a result of the laser irradiation process. It is established that I D /I G ratio shows the degree of defects and I 2D /I G ratio indicates sp 2 C]C bond in graphene structure. In order to a comparison of rGO lms quality produced under various laser uences, the trend of I 2D /I G and I D /I G changes with increasing laser uence was shown in Fig. 3(c). It is clear that increasing laser uence causes a higher I 2D /I G ratio and it was expected that increasing laser uence further results in higher values of I 2D /I G . The increasing I D /I G ratio in the gure also shows producing further defects during the laser irradiation process. Table 1 also summarizes the positions of each band, bands intensity ratios, and FWHM of the 2D band for rGO lms at different laser uences. The 2D band of irradiated GO lms was centered at around 2690 cm À1 . A decrease of FWHM of the 2D band was observed by increasing laser uence as I 2D /I G ratio increased for GO lm irradiated at these laser uences. The least FWHM value 96.36 cm À1 and the highest I 2D /I G ratio 0.28 were calculated for GO lm irradiated at 32 mJ cm À2 laser uence. The lower value of I D /I G ratio shows lesser defects and higher value of I 2D /I G ratio indicates the transformation to the graphene-like structures that results in higher charge mobility.
The chemical states of pristine GO lm and GO lm irradiated at 32 mJ cm À2 laser uence were investigated by the use of XPS. Fig. 4(a) shows XPS results of the lms and covers the carbon and oxygen regions. While pristine GO lm represents C/O ratio of 0.65, the value reaches to about 1 aer laser irradiation. The deconvolution of the C1s peak of the lms is shown in Fig. 4(b). The C1s spectrum composed of reduced carbon species ($284.5 and 285.6 eV), single bond carbon-oxygen components ($286.7 eV) and double bond carbon-oxygen components ($288.3 eV) as labeled in Fig. 4(b). In the XPS data of irradiated GO lm, a strong peak corresponding to sp 2 carbon bonds at 284.5 eV along with a small peak associated with the oxygenated functional groups are observed indicating the signicant removal of oxygen contents. In addition, another lower intensity peak assigned to sp 3 carbon bonds also exists in rGO lm, which was also observed by others researchers. 50 Table 2 represents C/O ratios and the atomic percentage of carbon components respectively derived from the XPS survey and C1s spectra for GO and rGO lms. Based on the results, it seems KrF laser irradiation process mostly eliminates single bond carbon-oxygen components while the sp 2 carbon bonds remain intact and then results in producing rGO patterns with a high C/O ratio.  To more carefully evaluate the reduction process of GO lm on SiO 2 /Si substrate under laser irradiation, XRD patterns were recorded in a range of 2q from 5 to 75 . The XRD pattern of graphite exhibits a peak located at $26.4 . This peak corresponds to a (002) reection of graphite with a thickness of $3.37 A of atomically at graphene sheets. The position of the diffraction peak is shied to a lower angle for GO due to increased interlayer spacing arising from the presence of oxygen functional groups. Fig. 5 compares the XRD pattern of rGO lm at 32 mJ cm À2 laser uence with that of GO lm (only selected 2q range shown in the gure). The XRD patterns show a diffraction peak at 2q ¼ 9.7 which refers to an interlayer spacing of 9.1Å. Another peak also appears at 2q ¼ 69.3 related to Si substrate and was used to normalize the pattern. The XRD patterns of the GO and rGO lms normalized by the Si peak. As observed, the sharp diffraction signal of GO was signicantly reduced in intensity by laser irradiation which indicates removal of the oxygenated functional groups. On the other hand, there is no trace of graphite in GO and rGO lms corresponding to the 2q peak of 26.4 . Therefore, XRD observations are in agreement with the Raman and XPS results, conrming the transfer of GO to rGO. Similar results were obtained in other studies 31,50 where they used glass and quartz as the substrate.
Further evidence of conrming of GO lms reduction by laser irradiation was obtained using monitoring their electrical properties. The electrical behavior of the rGO lms was studied by measuring the electrical resistance of the lms irradiated at various laser uences. Electrical resistance measurements of rGO lms were conducted using a 4-point probe method. However, the electrical resistance of pristine GO lms was measured by common method since they had a primarily high resistance of the order of GU. Fig. 6(a) shows the dependence of electrical resistance of the thin lms on laser uences. As observed, laser irradiation at low laser uences caused a sharp drop of the electrical resistance of GO lms. It was labeled as a reduction region in the gure. Since the electrical resistances of GO lms strongly depend on the oxygen content, this can be explained by a comparison of 248 nm photons energy (5 eV) of the KrF laser with those of C and O bonds in GO. Since C]C and C]O bonds (6.36 and 7.69 eV, respectively) are much stronger than C-C and C-O bonds (3.61 and 3.73 eV, respectively), the energy of the photons of the KrF laser causes breaking C-C and C-O bonds and so electrical resistance signicantly drops. A reduction of resistance by four orders of magnitudes was obtained at laser uence from 14 to 32 mJ cm À2 . However, the higher values of laser uence cause a slight increase in resistance (ablation region). Therefore, laser irradiation at lower laser uences results in reducing GO lms, whereas at slightly higher values GO lm is ablated to some  extent which, in turn, causes the increase of electrical resistance, as were explained by Yung et al. 48 The results of measured I-V characteristics to determine electrical resistances of rGO lms at different laser uences were presented in Fig. 6(b). Linear dependence between current and voltage shows stable conductivity of microcircuits produced under different laser uences. These results show the capability to control the electric properties of GO lms through an insulator to conductor transition by changing laser uence.

Conclusions
In summary, laser irradiation of GO lms was studied under various laser uences in the air environment. Raman spectra of irradiated GO lms showed the sharper and narrower 2D band with increasing laser uence conrming transformation to the graphene-like structures upon laser processing. Moreover, laser irradiation of GO lms resulted in the considerable elimination of the oxygenated functional groups as conrmed by XPS of irradiated GO lms. Finally, KrF laser irradiation process of GO lms resulted in fabrication of conductive rGO microcircuits with the reduced electrical resistances that can be used in microelectronics devices.

Conflicts of interest
There are no conicts to declare.