# Laser Synthesized Graphene and Its Applications
## 2 Laser Synthesis of Graphene  
Before the laser reduction of GO was first reported, the use of a Xenon flash lamp was proposed for GO reduction by [PERSON] et al. in 2009 [32]. The authors observed the loss of oxygen-containing groups by Fourier-transform infrared (FTIR) spectroscopy. A similar approach was reported by [PERSON] et al. [33], who used a Xenon flash lamp to induce GO reduction onto a polyimide (PI) substrate. This study indeed showed through X-ray photoelectron spectroscopy (XPS) that O-containing bonds were lost during the process; however, the reported Raman spectra did not support the formation of graphene monolayers. Nevertheless, the material was used for gas sensing applications, which will be discussed in the next section. The first report of a laser reduction of graphene appeared in 2010 [18]. Here, the authors produced LRG patterns from GO coated onto a substrate using a femtosecond laser at 790 nm wavelength that was programmed for a raster movement (Figure 1a), thus reducing and patterning GO at the same time, with resolution down to a few micrometers. By XPS it was observed that the signal from C-O bonds was significantly reduced in laser-treated areas, while C-C signal was intact (Figure 1b) [18]. Later, [PERSON] et al. [34] investigated the formation of LRG as a function of the energy transferred by a picosecond laser. Using Raman spectroscopy, and evaluating the width of the characteristic spectral features (G- D- and 2D-bands), they determined that order in the material was increased by increasing laser irradiation and dose (Figure 2). Figure 1: (**a**) Schematics of the LS process, with simultaneous GO reduction and patterning (RP). (**b**) XPS data from GO and reduced and patterned (RP) GO. Reproduced with permission from Ref. [18]. Copyright 2010, Elsevier. Only a few years later, the production of LRG was demonstrated using the laser from a DVD source with a simple commercially available LightScribe software [19; 35; 36]. Here, the process was again quite simple, as a GO solution was drop-cast onto a flexible substrate, shaped like a DVD disc, then dried and exposed to the DVD laser. It was clearly observed that the exposed area was significantly modified, as XPS spectra showed that O-containing bonds almost disappeared and the Raman spectrum was modified to confirm the formation of graphene [35]. Electron microscopy also showed that the exposed area was swollen as layers that were initially well-stacked then appeared as exfoliated. The effect of the type of laser for LS was studied by [PERSON] et al. [37], who investigated a femtosecond laser (100 fs pulse, 800 nm wavelength), a nanosecond laser (10 ns, 248 nm), and a continuous laser (LightScribe, 788 nm). It was found that the most effective laser process is with the nanosecond laser, since they observed both oxygen loss and graphenization, while by femtosecond laser the authors observed oxygen removal but no graphenization. By continuous laser irradiation, oxygen was removed but defects were introduced; thus, the final rGO was low quality. This study also proposed a two-step reduction mechanism, where the first step is oxygen removal, which is a photochemical process, and the second step is the reorganization of the lattice from sp\({}^{3}\) to sp\({}^{2}\), which is thermally mediated [37]. The resolution of the pattern is critical for device miniaturization. One approach for significantly improving the resolution of LRG patterns involves the use of a two-beam system [38]. Here, the constructive and destructive interference of two laser beams on the surface is responsible for creating a pattern (Figure 3a), as the reduction takes place only in the periodic regions of the surface where the two beams add constructively, while no reduction happens where the two beams cancel each other. In this way, lines narrower than 1 \(\upmu\)m can be produced (Figure 3b) [38]. While the studies presented so far are relative to GO films that were formed by drop-casting on a substrate, LS can also be achieved from a GO solution [39; 40].