Micropatterning of Transparent, Flexible Graphene Films By Oxygen Plasma
Authors: Kim Ji-hye , Lee Joo Heon , Ko Euna , Pham Xuan-Hung , Huynh-Nguyen Buu-Chau , Thanh-Hai Le , Seong Gi Hun 
Published: February 2020

Graphene, a single-atom-thick and two-dimensional sp2 carbon networking materials, has attracted a great deal of interest owing to its remarkable electrical, mechanical, and thermal properties. These excellent properties make it more suitable for use in transparent and flexible electrodes. The successful implementation of graphene films for various applications requires high quality and high resolution patterning at defined positions, with large-scale control of location and orientation. The plasma etching technique is one of the patterning techniques, which has been used widely for the surface activation of various materials, as well as for removal of carbon-based organic material from substrate surfaces [1]. This method has several advantages over other patterning techniques: (1) it can be scaled-up to produce large quantities for commercial use; (2) it results in high feature resolutions and sharp pattern edges; (3) the processing time is short; and (4) it offers good reproducibility and reliability. Herein, we report a simple patterning method for the production of graphene films on flexible, transparent plastic substrates such as poly(ethylene terephthalate) PET using an O2-plasma technique in a CCP system [2].
For the preparation of graphene, graphene oxide suspension was reduced at 85°C for 24h by employing L-ascorbic acid as a reducing agent. L-ascorbic acid is non-toxic, so this method is eco-friendly compared to other reducing agents [3]. To apply the homogeneous graphene films in a large area, flexible and transparent graphene films were fabricated by vacuum filtration method and then patterned by conventional photolithography and subsequent O2-plasma treatment.

Figure 1 (a) The effect of O2-plasma treatment on the resistivity and transparency of graphene films. Graphene films were treated with O2-plasma under a power ranging from 200 to 500 W between 1 and 5 min. (b) The changes of resistivity and transparency according to plasma treatment time at a power of 200 W.
Figure 1 shows the effect of O2-plasma treatment on the resistivity and transparency of graphene films. The resistivity increased as the plasma power increased. When graphene films were treated at 100 W for 5 min and at 200 W over 3 min, it was impossible to measure resistivity as it was above the measurement range of the instrument (~100 MΩ). In addition, the transparency of graphene films increased as the power increased. As the results, an applied power of 200 W for 5 min was optimal for the patterning of graphene films.

Figure 2Optimal images of the patterned graphene films under 200 W for 5 min. The scale bar represent 1 mm.
The optical microscope images of the patterned graphene films on the PET flexible substrate are shown in Figure 2. The area patterned by O2-plasma treatment is more transparent than the area protected with the photoresist polymer. The clear patterns demonstrated the effectiveness of graphene patterning by this method. For a more detailed characterization, the morphological change of graphene films was investigated by scanning electronmicroscope (SEM) and atomic force microscopy (AFM). X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were used for the investingation of chemical property changes of graphene films.
An electrochemiluminescence (ECL) reaction was carried out the patterned graphene films to confirm that it could function as a transparent electrode. For the ECL reaction, three electrode configuration was employed, consisting of the patterned graphene film serving as a working electrode, Ag/AgCl (3M NaCl) (a reference electrode), and platinum wire (a counter electrode). Red light is emitted on the graphene electrode surface without leakage, as shown in the line-scan of the chemiluminescent signal in Figure 3. This experiment indicates that the O2-plasma treatment can be successfully applied to pattern a graphene transparent electrode.
We expect that this simple and versatile patterning technique will facilitate the fabrication of graphene film-based electronic of optoelectronic devices.

Figure 3 (a) Luminescence image of the patterned graphene electrodes after the ECL reaction. The scale represents 300µm. (b) ECL line scan at the location indicated by the white line.

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