# A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
## 4 Mask-Free Fabrication of Electrodes and Devices  
An optically-induced electrochemical reaction and deposition scheme was presented to enable dynamic, rapid and mask-free fabrication of microelectrodes [75; 76; 77; 78; 79]. When the OEK chip was illuminated by optically-projected images, an electrical field would be produced in the illuminated area due to the creation of electron-hole pairs. Then, only the metal ions of liquid solution in the illuminated area were reduced by trapping electron from a-Si:H when an external AC bias potential was switched on. Furthermore, the reduced metal atoms would be attached to the illuminated a-Si:H surface and formed into metallic microstructures with the same shape as the incident light.  
Compared to the OEK-force-based method, this scheme could dynamically fabricate microelectrodes in 10 secs with a liquid conductivity as high as \(2\times 10^{7}\) S/m. For instance, silver ion-based microelectrodes were reported, as shown in Figure 11[75]. These microelectrodes exhibited more even distribution and lower roughness. Their heights could be adjusted by the linearly increasing deposition time and the solution concentration. In addition, the entire experiment was conducted at room temperature and atmospheric pressure without using conventional photolithographic techniques and metal nanoparticles and/or inks. In addition, integrated CuO/ZnO-based/single-walled-nanotube [76] nanowire-based field-effect transistors were presented, further validating that this scheme could facilitate the manufacturing of integrated nanodevices and offer an alternative to micro/nanosensor fabrication.  
Silver-based nanostructures with various geometrical topographies were synthesized in an OEK chip, as shown in Figure 12[77]. The incident light excited the generation of electron-hole pairs in the a-Si:H layer and then the electrons migrated from the valence band to the conduction band. With the aid of an existing electric double layer, an electrochemical reaction occurred between the electrons and the suspended silver solution in the liquid chamber under given parameters in the solution; meanwhile, crystallization occurred with the silver-based nanostructures. In this case, silver polyhedral nanoparticles, nanoplates in triangle and hexagon patterns, as well as nanobelts were fabricated by projecting various optical patterns. Experimental parameters that affected the fabrication process, including time duration, AC frequency and voltage, were also investigated and optimized. In sum, this OEK-based method opens up a new path to mask-free and rapid synthesis of nanostructures and nanobelts.  
Figure 11: (**a**) Fabricated microelectrodes with respect to deposition time; scale bar: 10 \(\upmu\)m. (**b**) Surface roughness of microelectrode by atomic force microscope (AFM) and SEM. (**c**) Thickness and (**d**) surface roughness of microelectrodes with respect to deposition time and solution concentration (reproduced from Ref. [75]).  
Using this method, MoS\({}_{2}\) with excellent optical and electronic properties was fabricated into thin-film transistors without relying on any conventional microlithography, such as nanoimprint lithography, laser patterning, or photolithography [78]. The MoS\({}_{2}\) material was first loaded into the OEK chip. Then, the Au and Ag electrodes were rapidly fabricated onto the target MoS\({}_{2}\) film. Accordingly, MoS\({}_{2}\) thin-film transistors with Au and Ag electrodes were obtained, respectively, as shown in Figure 13. These transistors performed the best when they were 30-40 nm thick, exhibiting a low subthreshold swing of 0.75 V/decade and high mobility of 41 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\), much better than conventional Si-based thin-film transistors.