Surface Characterization of Organic and Bioinspired Nanoscale Devices
For the past few decades, the research and industrial application of organic semiconducting materials has been very active. Compared to traditional semiconducting materials, facile chemical modification and processing are advantageous properties of organic electronics. This dissertation focuses on two classes of organic semiconducting devices: light-emitting electrochemical cells (LEECs) and bioinspired nanowires. Organic light-emitting diodes (OLEDs) have emerged in display applications, but not lighting due to high fabrication costs. To achieve high OLED performance at low cost, efforts have focused on light-emitting electrochemical cells (LEECs). LEECs, and particularly iridium LEECs, exhibit substantial efficiency, high luminance, and long lifetime in a simple, solution processable device architecture. Performance is facilitated by the redistribution of ions that assists charge injection. However, the physics of iridium LEECs has not been fully explored, particularly brightness enhancement with lithium additives. Scanning Kelvin Probe Microscopy (SKPM) was used to reveal the surface potential profile of iridium LEEC devices and clarify the effect of lithium addition. We found that ions do not pack densely at the cathode in pristine iridium LEECs devices. Li[PF6] addition produced a doubling of the peak electric field at the cathode from an increase of ionic space charge. This work was the first to clarify the nature of iridium device performance and enhancement from lithium salt additives: the additional mobile cations improves space charge accumulation for improved electron injection. The second class of devices concerns nanowires, specifically, the DNA-inspired self-assembly of nanoscale electronic devices. There is a need to fabricate nanoscale electronics with high yield and high purity. We created devices based on 20 nm long DNA nanowires incorporating a perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) derivative, an organic semiconductor with dimensions similar to two DNA bases. We synthesized these nanowires by automated DNA phosphoramidite chemistry and purified these wires by high performance liquid chromatography, thus achieving high control and purity of a nanoscale electronic element. We patterned gold nanogap electrodes and assembled the nanowires by gold-thiol self-assembly. Current voltage characterization revealed that the current of perylene nanowires was enhanced 4.4 fold over conventional DNA nanowires. Temperature dependence revealed that the current increased from room temperature up to 35 °C for each type of wire, and then lowered rapidly, consistent with DNA melting. We performed atomic force microscopy imaging studies to observe an instance of a single nanowire spanning a nanogap. This research provides a new approach to fabricate nanoscale devices with lower cost and high yield. Chapter 1 will serve as an introduction to organic semiconductor fundamentals and applications. Chapter 2 will discuss the state-of-the-art lithographic fabrication methods and the progress of molecular electronics, including research with DNA nanowires. Chapter 3 describes our research in performing surface characterization of iridium light-emitting electrochemical cells and the effect of lithium additives. Finally, the construction of bioinspired nanowire devices with the organic semiconductor perylene and associated electronic, thermal, and surface characterization is reported in Chapter 4.