# Laser Synthesized Graphene and Its Applications
## 2 Laser Synthesis of Graphene
This was achieved again using a ns laser at 248 nm [39] or at 532 nm [40], which induces loss of oxygen as observed by XPS, but there is no graphenization as there is no significant change in the Raman spectrum. While this might appear to be in contrast with the results presented above [37], it must be noted that this is a solution process and no direct comparison should be made as the underlying reaction mechanisms could be different. Figure 2: Raman parameters as function of laser irradiation. Reproduced with permission from Ref. [34]. Copyright 2013, Elsevier. LIG can be obtained from a variety of sources. The first report appeared in 2014, as polyimide (PI) was used as source material [21]. Here, the authors used a CO\({}_{2}\) laser (10.6 \(\upmu\)m wavelength) at 3.6 W power to make patterns onto PI. Such patterns showed loss of oxygen by XPS and the formation of graphene by Raman spectroscopy. The process was repeated on polyetherimide (PEI), although rGO quality was lower than on PI [21]. While the IR laser in the latter study could only produce patterns with a resolution limited to the order of several tens of micrometers, this can be reduced by using lasers in the visible spectrum, i.e., at shorter wavelengths [31]. Indeed, using a 405 nm laser, the resolution of the pattern could be reduced to 10-20 \(\upmu\)m. PEI was also used to make nanocomposites with polycarbonate (PC). Here, a blend of the two polymers was exposed to a CO\({}_{2}\) laser and only PEI was converted to graphene, yielding a nanocomposite [41]. The same process was applied to sulfonated polymers to produce sulfur-doped porous graphene structures [42], which is of particular interest as such polymers are widely used in the technology industry. A sulfonated polymer, i.e., sulfonated poly(ether ketone) (SPEEK) was also used by [PERSON] et al. to produce LIG using a CO\({}_{2}\) laser in order to obtain a flexible conductive material, which was then used as an electrode for supercapacitors [43]. Here, the conversion into graphene, not possible in conventional PEEK, is proposed to be mediated by sulfur, through reticulation by sulfonic group degradation and oxidative dehydrogenation by sulfur reaction [43]. A composite based on carboxymethyl cellulose (CMC) was developed by mixing PI powder in water solution and was deposited by vacuum filtration [44]. The mixture was then exposed to laser writing using a CO\({}_{2}\) laser at 10.6 \(\upmu\)m.  
Beside polymers, LIG can be obtained by LS using natural sources. This was reported for the first time by [PERSON] et al. [22], who used a CO\({}_{2}\) laser on different types of wood under a controlled reducing atmosphere rich in H\({}_{2}\) (>50%). The formation of graphene is supported by XPS and Raman spectroscopy. The authors suggest that a high content of lignin in  
Figure 3: (**a**) Schematics and working principle of a dual beam laser system; (**b**) SEM images of rGO pattern obtained by the dual beam laser system. Reproduced with permission from Ref. [38]. Copyright 2012, Elsevier. the starting material is needed to achieve the best graphene formation [22]. If wood is impregnated by a solution containing a metal salt, the LS process, carried out in Ar atmosphere, not only induces the formation of graphene but also reduces the metal ions to form metal nanocrystals, ultimately making a graphene-metal nanocomposite [45], suitable for application in electrocatalysis, as will be discussed in the next section. It was then noted that other natural materials have higher lignin content than wood, and may thus be suitable for the formation of LIG. Indeed, [PERSON] et al. proposed to apply LS to food sources such as potato skins, coconut shells, or other natural materials such as cork [46]. Due to the higher lignin content, the LS process can be carried out in ambient atmosphere.