# Laser-Induced Graphene Film and Its Applications in Flexible Electronics
## 1 Introduction  
Since it was first reported in 2014 [1], laser-induced graphene (LIG) film has gained extensive attention due to its unique characteristics of high throughput, direct patterning, and customizable fabrication. Through direct laser writing on aromatic polymers (e.g., polyimide, polyetherimide) or biomass (e.g., lignin, paper, textiles) in ambient air, a decimeter-sized LIG film with a 3D macro-porous structure can be achieved in one step, which makes it possible to fabricate flexible graphene electrodes with high efficiency, high yield, and low cost. Typically, multiple steps are required to convert the carbon precursors into porous LIG, including the dissociation of precursors, the release of generated gases, and the recombination of carbon atoms [2]. More specifically, a temperature as high as 3000 K can be generated once a matched laser beam irradiates the surface of carbon precursors. Such high temperature will destroy the carbon-oxygen bonds and carbon-nitrogen bonds of the precursor, so that the oxygen atoms are combined with a small amount of carbon and nitrogen atoms to escape in the form of gases, including carbon monoxide and methane, etc. The remaining carbon and nitrogen atoms recombine to form a small number of nitrogen-doped 3D graphene with atomic defects inside and a porous morphology outside. Thus, the sp3-carbon atoms are photothermally converted to sp2-carbon atoms. Moreover, due to the processing characteristics of rapid heating and rapid cooling of the laser, the resulting graphene lattice structure does not present the six-membered ring of intrinsic graphene, but instead becomes a mixed ring composed of five-, six-, and seven-membered rings [3]. To obtain high-quality LIG films, a reactive molecular dynamics model was developed by our group to study the light-material interactions in LIG fabrication processes [4]. The effect of rapid heating and cooling process on the formation of LIG was systematically investigated at the atomic level. Studies have shown that the initial rapid heating process requires a temperature high enough (e.g., 3000 K) for unbound atoms to be absorbed. In the subsequent heat holding process, the formation of carbon clusters needs to ensure along enough duration (>600 ps). In the eventual cooling to room temperature process, the defective bonds in the carbon cluster repair themselves to form a perfect graphene sheet.  
To date, there are many kinds of laser sources employed for the fabrication of LIG films, such as ultraviolet ultrashort pulsed lasers, blue-violet/blue diode lasers, near-infrared pulsed lasers, and mid-infrared pulsed/continuous wave CO\({}_{2}\) lasers. However, not all lasers are suitable for forming LIG films, or even high-quality ones, which requires specific wavelengths, appropriate laser energy density, and sufficient holding duration [2]. Due to the maskless and programmable processing modes and in situ growth properties on flexible substrates, LIG films show several unique advantages over conventional graphene/binder composites, including high flexibility and foldability, good electric/thermal conductivity, and high surface area. Therefore, LIG films have successfully demonstrated their potential in the field of electronic devices, including mechanical/temperature/gas/electrochemical sensors, micro-supercapacitors, actuators, electrocatalysis, solid-state triboelectric nanogenerators, Joule heaters, etc.