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Correlating Microstructure and Optoelectronic Performance of Carbon-Based Nanomaterials

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Correlating Microstructure and Optoelectronic Performance of Carbon-Based Nanomaterials Correlating Microstructure and Optoelectronic Performance of Carbon-Based Nanomaterials By Caitlin Rochford Submitted to the graduate degree program in Department of Physics and Astronomy and the Graduate Faculty of the University of Kansas in partial fulfillment of the requirements for the degree of Doctor of Philosophy. ________________________________ Dr. Judy Wu, Chairperson ________________________________ Dr. Hsin-Ying Chiu ________________________________ Dr. Siyuan Han ________________________________ Dr. Hui Zhao ________________________________ Dr. Cindy Berrie Date Defended: June 8, 2012 The Dissertation Committee for Caitlin Rochford certifies that this is the approved version of the following dissertation: Correlating Microstructure and Optoelectronic Performance of Carbon-Based Nanomaterials ________________________________ Dr. Judy Wu, Chairperson Date approved: June 8, 2012 ii Abstract There is a great deal of interest in carbon nanostructures such as graphene and various forms of carbon nanotubes due to their exceptional physical, electronic, and optical properties. Many technological applications have been proposed for these nanostructures, but despite the promise many carbon nanostructure-based optoelectronic devices fail to compete with their conventional counterparts. This is often due in large part to a non-optimized material or device microstructure. Factors such as crystallinity, contact quality, defect structure, and device configuration can critically affect device performance due to the high sensitivity and extreme surface to volume ratio of carbon nanostructures. In order for the exceptional intrinsic properties of the nanostructures to be exploited, a clear understanding of the microstructure and its correlation with device-relevant optoelectronic properties is needed. This dissertation presents four projects which demonstrate this principle. First, a TiO2-coated carbon nanofiber is studied in order to optimize its structure for use in a novel dye-sensitized solar cell. Second, the electrode configuration of an individual multiwall carbon nanotube infrared sensor is investigated in order to surpass the limitations of disordered nanotube film-based infrared sensors. Third, the properties of defect structures in large area transferred graphene films grown by chemical vapor deposition are correlated with carrier diffusion in order to understand the film’s low mobility compared to exfoliated graphene. Fourth, the effect of deposition conditions on graphene-metal contact was studied with the goal of achieving sufficiently transparent contacts for investigation of the superconducting proximity effect. All four projects highlight the unique properties of carbon nanostructures as well as the need to correlate their optoelectronic properties with microstructural details in order to achieve the desired device performance. iii Dedicated to Russell Bryant Friedman “There is no end to education. It is not that you read a book, pass an examination, and finish with education. The whole of life, from the moment you are born to the moment you die, is a process of learning.” Jiddu Krishnamurti iv Acknowledgements Over the past five years at the University of Kansas I have been fortunate to have the support of many faculty, staff, and fellow students. This is truly a supportive and collaborative environment, and I had a great time here. First and foremost, I must thank my advisor, Professor Judy Wu, who taught me so much about how to be a scientist. No matter how busy she was, she always made time to discuss both research and life with me, give me sound advice, and provide me with whatever support I needed. Her work ethic and dedication to the success of her students are amazing. I would also like to thank Professor Hsin-Ying Chiu, who breathed new life into my projects and was extremely generous with her time and resources. Her current and future students are very lucky. In addition to Judy and Hsin-Ying, I had the great pleasure of working with many people on a lot of different projects, and I would like to thank all of them for their expertise and support, especially Professor Siyuan Han, Dr. Rongtao Lu, and Dr. Bo Mao on the graphene-Nb project; Professor Hui Zhao and Nardeep Kumar on the CVD graphene defect and diffusion project; and Dr. Zhuang-Zhi Li, Dr. Javier Baca, Dr. Jianwei Liu, and Professor Jun Li on the TiO2/CNF nanowire project. In addition to the scientists mentioned for these projects, I would like to thank many other people for their training, experimental support, or helpful discussions, especially Professor Cindy Berrie, Professor Ron Hui, Guowei Xu, Gary Malek, Alan Elliot, Dr. Brian Ruzicka, Dr. Lalani Werake, Caleb Christianson, Dr. Hui-Chun Chien, Dr. Rose Emergo, Dr. Jonathan Dizon, and Dr. Xiang Wang. I would also like to thank the current and former staff members of the Department of Physics and Astronomy who keep the department running, especially Tess Gratton, Teri Leahy, v Doug Fay, Kim Hubbel, Don Nieto, and John Clune. A special thanks goes out to Jeff Worth, the person we go to for anything and everything instrument related, and Allen Hase, Zach Kessler, and Steve Davies of the Physics Machine Shop, who can build just about anything. I am not sure who in particular to thank for this, but I want to express an appreciation for the opportunity to teach several Duke T.I.P. physics courses for middle and high school kids and more recently a Nanotechnology course for senior citizens at the Osher Lifelong Learning Institute. There are few things more rewarding than teaching people who show up voluntarily with nothing to gain but knowledge. As scientists I believe we have a great responsibility to reach out to non-scientists of all ages. I was delighted to be able to do that through these classes, and I was surprised by how much fun I had. Finally, I owe a great deal of thanks to my friends and family who helped me through a lot of difficult times over the past several years. It would be impossible to say everything I want to say in this short preface, so I will just say that I am incredibly lucky to have the love and support of so many wonderful people. I would, however, especially like to thank my Dad, who has never been one to give much advice but has taught me so much by example. vi Contents List of Figures ………………………………………………………………………………... xi 1 Introduction …………………………………………………………………………. 1 1.1 Introduction to Carbon Nanomaterials ………………………………………………. 1 1.2 Importance of Studying Microstructure in Nanoscience …………………………….. 5 1.3 Overview and Theme of Following Chapters ………………………………………... 6 2 Individual TiO2/Carbon Nanofiber Core-Shell Nanowire Studies for Dye- Sensitized Solar Cell Applications …………………………………………………. 9 2.1 Introduction to Photovoltaics and Dye-Sensitized Solar Cells……………………….. 9 2.2 Nanowire Dye-Sensitized Solar Cells ………………………………………………... 24 2.2.1 Introduction …………………………………………………………………... 24 2.2.2 TiO2/Carbon Nanofiber Core-Shell Nanowire DSSCs ………………………. 26 2.3 Motivation and Experimental Approach ……………………………………………... 29 2.3.1 Introduction …………………………………………………………………... 29 2.3.2 Sample Fabrication …………………………………………………………... 31 2.4 Photoconductivity and Dye Adsorption Studies ……………………………………... 32 2.4.1 Introduction and Microstructural Characterization …………………………... 32 2.4.2 Steady State I-V Characterization ……………………………………………. 33 vii 2.4.3 Spectral and Intensity Dependence …………………………………………... 38 2.4.4 Transient Photoconductivity …………………………………………………. 39 2.4.5 Conclusion …………………………………………………………………… 41 2.5 Thermal Annealing Studies …………………………………………………………... 42 2.51 Introduction and Microstructural Characterization …………………………... 42 2.5.2 Steady State I-V Characterization ……………………………………………. 45 2.5.3 Transient Photoconductivity …………………………………………………. 49 2.5.4 Conclusion …………………………………………………………………… 51 2.6 Summary and Future Work …………………………………………………………... 52 3 Individual Multiwall Carbon Nanotube Infrared Detector ……………………… 56 3.1 Introduction to Infrared Photodetection ……………………………………………… 56 3.2 Carbon Nanotubes for Infrared Detection ……………………………………………. 63 3.2.1 Optical Properties of Carbon Nanotubes …………………………………….. 63 3.2.2 Previous Reports and Motivation …………………………………………….. 65 3.3 Individual Carbon Nanotube Infrared Detector Studies ……………………………… 68 3.3.1 Device Fabrication and Geometry …………………………………………… 68 3.3.2 Photoresponse and I-V Measurements ……………………………………….. 70 3.3.3 Specific Detectivity Calculations ……………………………………………. 76 3.3.4 Conclusion …………………………………………………………………… 79 4 Correlation of Defect Structure and Diffusion in Transferred Graphene Films Grown by Chemical Vapor Deposition ……………………………………............. 80 viii 4.1 Introduction to Graphene …………………………………………………………….. 80 4.2 Motivation and Methodology ………………………………………………………… 83 4.2.1 Graphene in Transparent Electronics ………………………………………… 83 4.2.2 Large Area Growth of Graphene by Chemical Vapor Deposition …………… 86 4.2.3 Methodology …………………………………………………………………. 87 4.3 Defect Characterization by Raman Spectroscopy ……………………………………. 88 4.4 Optical Measurements of Diffusion ………………………………………………….. 94 4.5 Summary and Future Work …………………………………………………………... 100 5 Graphene-Metal Contact for Studying the Superconducting Proximity Effect in Graphene-Nb Junctions …………………………………………………………….. 103 5.1 Introduction …………………………………………………………………………... 103 5.1.1 Introduction to This Chapter …………………………………………………. 103 5.1.2 Brief Introduction to Superconductivity and the Superconducting Proximity Effect………………………………………………………………………….. 104 5.1.3 Introduction to
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