Objective Graphene oxide (GO) is a graphene derivative with oxygen-containing functional group in its graphite structure. It can be reduced by heat, chemical reaction, and laser-induced reduction methods. Laser irradiation can induce reduction of GO in the irradiation area with good flexibility and area selectivity, and no special environment is required. In this paper, we propose a patterning method to process GO using a spatially-shaped femtosecond laser. We use the spatially-shaped laser irradiating method and a Gaussian laser scanning method to fabricate patterns on GO, and we analyze the effects of irradiation time, laser fluence, etc. The characteristic results demonstrate that the spatially-shaped femtosecond laser irradiating method realizes reductive patterning on GO, and the efficiency of patterning is improved. The proposed method also demonstrates good repeatability and flexibility in patterning. It has application potential in fabricating GO-based microcircuits and microdevices.
Methods GO films were prepared by filtering GO dispersion with a cellulose filter membrane and drying. The femtosecond laser has an 800-nm central wavelength, 35-fs pulse duration, and 1-kHz repetition rate. Different holograms were loaded on a spatial light modulator (SLM) to obtain the corresponding optical field at the focus. The Gerchberg-Saxon algorithm was used to calculate the phase distribution (i.e., hologram) according to the targeted complex amplitude distribution. When the specific optical field shape at the back focal plane of lens was obtained, the 4f optical system was used to guarantee that laser propagates without distortion and focuses on the surface of GO by the 5× objective lens. To realize unshaped laser scanning and compare fabrication results of shaped laser irradiating, a plane phase hologram was loaded on the SLM. Under this condition, an unshaped Gaussian optical field was formed at the focus. After reductive patterning, a scanning electron microscope (SEM) was used to characterize the micromorphology of the patterned areas to confirm the patterning effect of the two fabrication procedures. The height distribution and uniformity of the patterns were observed using a white light interferometer (WLI). Raman spectroscopy was used to obtain the Raman spectra of the patterned areas, and the characteristic peaks in the Raman spectra demonstrated whether the patterned area was reduced by the femtosecond laser.
Results and Discussions The two methods were used to fabricate the same four 40mm×40mm square patterns on GO. Sketch maps of the two methods and micromorphologies characterized by SEM and WLI of the squares are showed (Fig. 2). For the proposed irradiating method, the intensity of the light field in the entire irradiating area was not uniform, and we regarded it as uniform when calculating fluence. The results demonstrate that the patterns are divided into many out-of-shape areas by gully-like structures. Areas surrounded by gullies showed hump-like structures (~5μm scale). Some lamellar structures were observed on these humps. These results were generated by the nonuniform intensity of the optical field, gullies were fabricated by a light field with higher intensity, and humps were fabricated by a weaker light field. For the micromorphology of the laser scanning method, humps and gullies were also observed in the areas. However, the generated humps were smaller (~1μm scale). Floccule structures were also observed in the entire area. These floccule structures were generated because gas was released from GO in a short period during laser scanning, which made the GO porous. Regarding fabrication efficiency, the irradiating procedure was performed in 500 ms; however, the scanning procedure required 640 s. We studied the influences of laser fluence and irradiating time on shaped laser irradiating (Fig. 3). Patterns irradiated with the same irradiating time and different fluence are distinguished on the depth and width of gullies. The D-band intensity of the Raman spectra indicates distortion of the graphic structure of GO, and 2D-band of the Raman spectra represents production of the graphene-like sp 2 structure. For unshaped laser scanning, we also adjusted some parameters to obtain diverse micromorphologies and reduction results (Fig. 4). Based on the Raman spectrum results, the maximum reduction in the two series of experiments was similar; however, the time required to perform each method differed significantly. Processing efficiency was improved significantly using shaped laser irradiating when we fabricated the same pattern and obtained the nearly reduction effect. Finally, we applied the proposed spatially-shaped laser irradiating method to fabricate a “THU” shaped pattern (Fig. 5). For this relatively complicated pattern, it is difficult to fabricate it once via irradiation using the shaped laser because the calculation of the hologram would differ. The tailoring and splicing method can simplify this process. In this example, the entire pattern “THU” was difficult to fabricate directly; thus, we divided it into three parts, i.e., “T,” “H,” and “U,” and we calculated their individual holograms. We then loaded the three holograms on the SLM and irradiated GO one after another.
Conclusions In this study, we have proposed a method based on spatial beam shaping technology and the G-S algorithm to realize reductive patterning on GO. The proposed method is compared with the Gaussian laser scanning method. The micromorphologies of patterns fabricated by the two methods are different. Gullies and big humps (~5μm) were observed in areas irradiated by the shaped laser, and lamellar structures were observed on the humps. In areas scanned by the unshaped laser, the humps were smaller (~1μm), and there were many floccule structures in these areas. Morphology observations demonstrate that the spatially-shaped laser irradiating method has good repeatability, and the final pattern does not change if the holograms loaded on the SLM are the same. Raman spectra demonstrate the reduction effect of the two methods. Longer irradiation time and larger fluence lead to better reduction effect in spatially-shaped laser irradiating procedure. Similarly, shorter scanning speed and larger fluence lead to reduction in laser scanning method. By adjusting related parameters, we obtained similar reduction extent in both methods. Here, the efficiency of the shaped laser irradiating was approximately 400 times that of the unshaped laser scanning. Finally, we applied the spatially-shaped laser irradiating method to fabricating more complicated patterns by tailoring and splicing the entire pattern, and the results confirmed the good flexibility of the proposed method. With improved efficiency and good repeatability and flexibility, the proposed method is expected to be applicable to the fabrication of GO-based microcircuits, microdevices, and many other related devices.
