Optical Properties of DNA Biomaterial and Application to UV-Photoconductors
Biomaterial DNA, a waste product from the salmon fishing industry, has recently attracted intensive attention of researchers that has resulted in multiple successful applications in photonic and electronic devices. Typically, to obtain high optical quality thin films, the DNA is complexed with cationic surfactant cetyltrimethylammonium-chloride (CTMA); this complex is soluble in polar organic solvents, such as n -butanol. The molecular weight of these DNA-CTMA complexes can be easily modified by sonication. In order to increase the electrical conductivity, a highly conductive polymer, e.g. Clevios P, can be doped into the DNA-CTMA. Along with these opportunities are a large number of challenges associated with the achievement of practical applications of these materials in optoelectronic devices. In this dissertation, we first explore the optical and electrical properties of the biopolymer complexes under specific processing environments, and then investigate the fabrication and characterization of photoconductive devices from the biopolymers. Potential applications of these optoelectronic devices are discussed as well. Chapter 1 discusses the background of the development of the DNA biomaterial and the pertinent device physics of photoconductors. Chapter 2 presents the experimental studies of the optical properties. These studies include absorbance and time-resolved photoluminescence of the DNA samples. Absorption peaks of both DNA-CTMA and DNA-Clevios P-CTMA were located at around 260 nm. Both DNA samples demonstrated absorbance peaks at ~260 nm and photoluminescence at ~470nm. Interestingly, the PL lifetime was observed to decrease with increasing excitation in an air ambient. Specifically, after excitation with a high power ultrafast (~150fs) UV (266nm) laser pulse in air, the lifetime decreased dramatically after a few minutes of exposure. This is most likely due to the photo-oxidation that results in trap creation on the polymer surface and, subsequently, results in an increase in the non-radiative recombination . In order to investigate the photoconductivity, micro-scale metal-biopolymer-metal (MBM) ultraviolet photoconductors with interdigitated electrodes were fabricated by photolithography, as discussed in chapter 3. To further increase conductivity, electron-beam lithography and focused ion beam milling were used to fabricate the nano-scale MBM devices. The responsivity of the resulting DNA-CTMA photoconductor was approximately 0.002 A/W@200V which indicates that it is a highly resistive device. After doping with Clevios P, the responsivity of the DNA-Clevios P-CTMA increased to 0.7 A/W@20V. However, the photocurrent and dark current are still very close on the same order of magnitude. In order to further enhance the responsivity of the DNA-based UV photoconductor, a vertical architecture was explored. This architecture provided an easier fabrication route and was expected to show better performance, as discussed in Chapter 4. Various polymers (P3HT, PCBM, and PMMA) were combined with the DNA biomaterial to investigate the possibility of enhancement of transport and extraction of the photogenerated electron hole pairs. DNA-Clevios P-CTMA:PCBM showed significant photoresponse to UV radiation (0.73 A/W@1V) and demonstrated robust stability in the air ambient. Potential applications of such sensitive UV photoconductors in DNA detection is discussed in Chapter 5. Finally, Chapter 6 provides conclusions and a summary of the work conducted in this dissertation.