## 2 Physical Performance of DNA Thin Film  
The special physicochemical properties enable DNA-lipid complex into various applications in optical-electronic devices. For example, the surface roughness of the DNA complex is very important when serving as the gate dielectric in OTFTs, because the charge transported at the interface between DNA complex and semiconductors [16]. In OLEDs, the excellent optical transparency in the visible range is necessary as it can be placed on either side of the diode. The characteristics of DNA and DNA complex have been investigated and briefly introduced as follows through various techniques including atomic force microscopy (AFM), current-voltage (I-V) performance, UV-vis and circular dichroism (CD) spectroscopy.  
The surface profile of the DNA polymer film is scanned by AFM, as shown in Figure 2b (\(10\times 10\) um area). The DNA molecules (white dots) are distributed on the silicon substrate and are mutually aggregated with each other, i.e., forming the plurality of agglomerates. In comparison, the surface morphology of uniform film of DNA-CTMA complexes is shown in Figure 2c (\(2\times 2\) um). The RMS was estimated to be as low as 0.3 nm, indicating the DNA-CTMA film is smooth enough to be the gate dielectric layer in the OTFTs [16]. The optical properties of DNA complex film are characterized by the UV and CD spectroscopy, as shown in Figure 2d. From the UV spectroscopy, the DNA complex film shows a \(\lambda_{\text{max}}\) at 260 nm in the range between 220 to 300 nm, which could be attributed to the characteristic absorption of the nucleobases [17]. From the CD spectroscopy, one positive cotton effect and two negative cotton effects are found at about 280, 245, and 225 nm, respectively. Such features indicate that the DNA complex film existed in the configuration of B form [18].  
Figure 1: The ion exchange reaction between DNA and cationic surfactant.  
The thermal stability of DNA and DNA-CTMA surfactant complex are given in Figure 2e, showing thermal degradation curves which indicate generally good thermal stability. It is very surprising that the DNA can be stable until 250 \({}^{\circ}\)C, suggesting that the DNA complex can tolerate various fabrications and may be the candidate for the application of temperature sensors. In addition, the resistivity of the DNA-CTMA and DNA alone were estimated, as shown in Figure 2h. And the current densities of DNA and DNA-CTMA were calculated to be 7.0 \(\mu\)A/cm\({}^{2}\) and 2.9 nA/cm\({}^{2}\) at 10 V, respectively. The good insulating properties of DNA-CTMA complex allow it to be the gate insulator layer in the OTFT devices [16].  
Figure 2: **(a)** The fabrication of DNA and DNA-complex thin films on a silica wafer by spin coating the butanol solution. AFM image of the deposited **(b)** DNA and **(c)** DNA-CTMA thin film characterizing the surface roughness; **(d)** UV–vis and CD spectral of DNA-complex film; **(e)** Thermogravimetric (TG) curves of DNA and DNA-complex; **(f)** Mental/insulator/mental (MIM) structure for I–V measurement; **(h)** I–V characteristics of DNA and DNA-complex thin film, respectively (Reproduced with permission from [19], Elsevier, 2016).  
As discussed above, the generally good physicochemical characteristics and insulating properties of DNA and DNA-CTMA complexes would enable the applications in bio-organic electronics. In the following, we will next highlight the classical electronic devices based on DNA thin-films.