# A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
## 2 Optoelectrokinetics (OEK) Chips and Their Working Principles  
The ODEP or OEK chip was firstly invented in 2005 [32]. The chip typically consists of four layers [32]: a top glass layer coated with a transparent and conductive indium tin oxide (ITO) film, which connects with one end of the AC bias potential; a bottom ITO glass layer serving as the bottom electrode that connects with the other end of the AC bias potential; a thin film (50 nm) n+ hydrogenated amorphous silicon (a-Si:H) layer, deposited onto the bottom ITO glass after generation of a 1 \(\upmu\)m undoped a-Si:H layer and a 20 nm silicon nitride layer; a liquid layer that contains the manipulated micro/nanoparticles to function as a \"gap\" for the OEK chip. The doped a-Si:H layer can decrease the contact resistance between the liquid layer and the undoped a-Si:H layer, and the silicon nitride layer serves the purpose of passivation. It is worth noting that the undoped a-Si:H layer is the photosensitive layer from which optically-induced virtual electrodes are generated. In general, the ITO layer is sputtered onto the glass layer; the a-Si:H layer is deposited onto the ITO layer by plasma-enhanced chemical vapor deposition. Figure 1 is an illustration of its mechanism proposed in our group given in [42]. In our study, only a layer of a-Si:H was deposited, which was also classified into an a-Si:H based OEK chip. When the OEK chip is illuminated by the optically-projected patterns shown on a digital micromirror display (DMD), the external AC voltage shifts to the liquid layer with suspended micro/nanoparticles. Then, an optically-induced non-uniform electric field is produced around the illuminated area. Under the action of this electric field, the micro/nanoparticles are polarized, producing an ODEP force. This force is, in turn, exerted onto the micro/nanoparticles, which is defined as [43]  
\[\left\langle\overset{\rightarrow}{F}_{ODEP}\right\rangle=2\pi R^{3}\varepsilon _{m}\text{Re}[K(\omega)]\
abla\left|\overset{+}{E}_{rms}\right|^{2}, \tag{1}\]  
where \(R\) is the radius of the micro/nanoparticles, \(\varepsilon\) is the permittivity of the liquid, \(E\) is the magnitude of the optically-induced non-uniform electric field, \(rms\) means the root-mean-square value, the subscript \(m\) is the liquid, \(\omega\) is the angular frequency of the AC bias potential across the liquid medium, and \(K(\omega)\) is the Clausius-Mossotti (CM) factor, which is further expressed as  
\[K(\omega)=\frac{\varepsilon_{p}^{*}-\varepsilon_{m}^{*}}{\varepsilon_{p}^{*}+2 \varepsilon_{m}^{*}}, \tag{2}\]  
where \(\varepsilon^{*}=\varepsilon-\text{j}\omega/\omega\), with \(\sigma\) representing the conductivity. The subscript \(p\) means the particle. The direction of the ODEP force, both positive and negative, is fully directed by the real part of the CM factor. Specifically, if the value of the real part of the CM factor is higher than zero, a positive ODEP force will be generated and then the micro/nanoparticles will be attracted and moved to the illuminated area. Otherwise, the micro/nanoparticles will be pushed away from the illuminated area. For micro/nanoparticles that vary significantly in their size, a common method to separate them is by making the ODEP magnitudes have three orders of difference. Other OEK forces, including AC electroosmosis (ACEO) and AC electrothermal, were investigated using the finite element analysis (FEA) method [44]. Typically, those two kinds of electrokinetics forces arise from the interaction between the optically-induced non-uniform electric field and liquid solution at a given AC frequency. As an example, under the action of an ACEO force, the fluid flow can rapidly push a large number of micro/nanoparticles towards the illuminated area and assemble them into desired patterns or devices [45; 46]. Recently, various types of photosensitive materials have been explored to manipulate and fabricate micro/nanomaterials in the same way as an a-Si:H based OEK chip works.