Objective In recent years, graphene-based nanomaterials have been widely studied because of their excellent chemical and physical properties. Among other applications, graphene has been successfully used in sensors and catalysis. Graphene can form a three-dimensional porous structure with a high surface area, depending on the method of synthesis. The assembly of graphene oxide (GO) into foam is one of the conventional methods employed to fabricate porous graphene structures. However, this approach needs the preparation of GO precursor via oxidative-acid synthesis route. Porous graphene can be processed via chemical vapor deposition on porous substrates, but high temperature and complex post-processing activities limit its commercialization. Recently, a facile approach to the formation and patterning of porous graphene on polyimide (PI) under ambient conditions using commercial laser scriber was reported. This one-step process of making laser-induced graphene is better than conventional methods for synthesizing porous graphene, and the method is also relatively simple and cheaper. Presently, there are few domestic studies on laser direct writing PI. In this present study, we report the effects of three sets of laser-process parameters on the carbon forming performance of 1064 nm laser direct writing PI films. We expect our methods and findings to provide a reference for the process parameters of carbon forming of PI film written by 1064 nm laser.
Methods Commercial polyimide films were employed in experimental research. First, the 1064 nm fiber laser was used to directly write on the PI film, while the PI film carbonized after absorbing the laser energy. A scanning electron microscope, Raman spectrometer and X-ray photoelectron spectrometer were used to analyze the morphology and chemical composition of laser direct writing PI film. The four-probe and the contact angle measuring instruments were used to measure the conductivity and hydrophilicity of the laser direct writing PI film. The effects of three groups of parameters (spot size and line spacing; scanning speed and pulse frequency; laser power) on the carbon formation of PI film by laser direct writing were studied.
Results and Discussions The Raman spectrum shows that the laser direct writing PI film has three characteristic peaks of carbon: D peak at 1344 cm -1, G peak near 1500 cm -1, 2D peak at 2683 cm -1 (Fig.3). The XPS results of the material show that there are C1s, O1s, and N1s peaks. Carbon atoms exist in four forms (C—C, C—O—C, C—N, and C=O), and the C—C bond is the main component of carbon (Fig.4). The spot size, line spacing, scanning speed, and pulse frequency affect the conductivity of the laser direct writing PI film to certain degrees. When the laser power is low (1.8--2.0 W), the laser leaves some flocculation on the surface of the PI film. With an increase in laser power, holes gradually appear on the PI film, leading to the formation of a three-dimensional porous structure (Fig.7). The contact angle of the laser direct writing PI film is positively correlated with the degree of damage of the PI film surface. By calculating the ID/IG and I2D/IG, it can be deduced that there is an initial decrease in the defect degree of the carbon layer, followed by an increase as the laser power increases (Fig.8).
Conclusions In this study, using 1064 nm fiber laser direct writing PI film, the influence of laser-process parameters on PI film was studied. The PI film absorbs the pulse laser energy and performs a photothermal conversion, and finally forms a three-dimensional porous carbon layer. In the molecular chain of PI, chemical bonds such as C—H, C=O, C—N, etc. are broken and rearranged. The mass fractions of C, N, and O elements in the laser direct writing PI film are 84.84%, 2.02% and 13.14%, respectively. Using different laser processing-technology and parameters, the conductivity of the carbon layer formed by laser direct writing PI film is studied. The best combination of parameters for the conductivity of laser direct writing PI film was obtained: the laser line spacing was 0.001 mm, the spot size was 0.06 mm, the scanning speed was 150 mm/s and the pulse frequency was 40 kHz. With an increase in laser power, the degree of microscopic ablation morphology of the laser direct writing PI film gradually increases, and the surface changes from a small flocculent carbon particle to a three-dimensional porous carbon structure. With a laser power of 2.2 W, the carbon flaw is the lowest and the carbon crystallization rate is the highest. At this laser power, the sheet resistance is also the lowest (55 Ω/sq). In addition, the contact angle of the laser direct writing PI film increases with a gradual increase in laser power. The surface of the laser direct writing PI film shows superhydrophobicity while the laser power is 2.8 W.
