SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED NANOSCALE DEVICES by Kuo-Yao Lin APPROVED BY SUPERVISORY COMMITTEE: ___________________________________________ Dr. Jason D. Slinker, Chair ___________________________________________ Dr. Mark Lee ___________________________________________ Dr. Majid Minary-Jolandan ___________________________________________ Dr. Anvar A. Zakhidov ___________________________________________ Dr. Fan Zhang Copyright 2018 Kuo-Yao Lin All Rights Reserved To my parents and Whity SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED NANOSCALE DEVICES by KUO-YAO LIN, BS, MS, ME DISSERTATION Presented to the Faculty of The University of Texas at Dallas in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY IN PHYSICS THE UNIVERSITY OF TEXAS AT DALLAS May 2018 ACKNOWLEDGMENTS I really appreciate the help and guidance from my advisor, Dr. Jason Slinker for the past five years. He always provides useful insight during the most tough moments, and gives me a clear path and direction when I take the wrong turn in my research. He spends extra time and effort to assist me in completing my paper and dissertation to make my graduation possible. I am also very grateful to my PhD committee, Dr. Mark Lee, Dr. Anvar Zakhidov, Dr. Fan Zhang, and Dr. Majid Minary, they have given me valuable opinions to advance my research works. I also want to thank to former lab members, Chris Wohlgamuth, Marc McWilliams, and Lyndon D. Bastatas. Chris and Marc assisted me to get started in the cleanroom for chip fabrication. With their help and knowledge, I quickly adapted to the cleanroom work environment. Lyndon helped me to prepare the samples to allow for the completion of my first publication. I also would like to show my gratitude to current lab members Dimithree Kahanda, Kelly Jackson and Nolan King. Dimithree spends a considerable amount of time on the material I need for the research. With her help, all the samples are ready in time for experiments. Nolan provides his software and hardware skills to enable the automation measurements in my second project. I also would like to thank to the collaborators from UTD and other schools for providing their resources to make my research work go smoothly. Finally, I want to thank to all the professors and faculty at UTD for help me to go through this journey. December 2017 v SURFACE CHARACTERIZATION OF ORGANIC AND BIOINSPIRED NANOSCALE DEVICES Kuo-Yao Lin, PhD The University of Texas at Dallas, 2018 ABSTRACT Supervising Professor: Jason D. Slinker 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 vi 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 vii nanowire devices with the organic semiconductor perylene and associated electronic, thermal, and surface characterization is reported in Chapter 4. viii TABLE OF CONTENTS ACKNOWLEDGMENTS………………………………………………………………………...v ABSTRACT……………………………………………………………………………….……..vi LIST OF FIGURES……………………………………………………………………….……...xi CHAPTER 1 INTRODUCTION ...................................................................................................1 1.1 Preface ............................................................................................................................1 1.2 Energy state of organic materials ...................................................................................2 1.3 Polymer structure and charge transport .........................................................................5 1.4 Applications-Organic light emitting diodes ...................................................................5 1.5 Organic Field effect transistors ......................................................................................8 1.6 Organic semiconductor material process .....................................................................10 1.7 Conclusion and dissertation outline .............................................................................11 CHAPTER 2 MOLECULAR ELECTRONICS AND DNA- BRIDGED NANOGAP JUNCTIONS ……………………………………………………………………………………12 2.1 Preface ..........................................................................................................................12 2.2 Current silicon technology and limitation ....................................................................12 2.2.1 Electron beam lithography ............................................................................13 2.2.2 Lithography limitations and next step ...........................................................15 2.3 Molecular electronics ...................................................................................................16 2.3.1 Current development of molecular electronics .............................................16 2.3.2 Molecular junctions ......................................................................................17 2.3.3 Single and ensemble junction .......................................................................18 2.3.4 Type of single molecule devices ...................................................................19 2.3.5 Type of ensemble molecule devices .............................................................21 2.3.6 Interfaces and bonding ..................................................................................21 2.4 DNA based nanowire MMM junction .........................................................................22 2.4.1 DNA nanowire device experiments ..............................................................25 2.5 Conclusions ..................................................................................................................26 ix CHAPTER 3 THE INFLUENCE OF LITHIUM ADDITIVES IN SMALL MOLECULE LIGHT-EMITTING ELECTROCHEMICAL CELLS ..................................................................27 3.1 Preface ..........................................................................................................................28 3.2 Introduction ..................................................................................................................28 3.3 Scanning kelvin probe microscopy ..............................................................................31 3.4 Devices for surface probe study ...................................................................................32 3.5 Potential distribution of steady state