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Electron Transport in Single Molecule Transistors Electron Transport in Single Molecule Transistors by Jiwoong Park B.S. (Seoul National University) 1996 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Paul L. McEuen, Co-chair Professor John Clarke, Co-chair Professor Steven G. Louie Professor A. Paul Alivisatos Fall 2003 Electron Transport in Single Molecule Transistors Copyright © 2003 by Jiwoong Park Abstract Electron Transport in Single Molecule Transistors by Jiwoong Park Doctor of Philosophy in Physics University of California, Berkeley Professor Paul L. McEuen, Co-chair Professor John Clarke, Co-chair Electron transport through single molecules is strongly affected by single-electron charging and the energy level quantization. In this thesis, we investigate electron transport in single molecule transistors made with several different molecules, including fullerene molecules (C60, C70 and C140) and single Co molecules with different lengths. To perform transport measurements on these small (<3 nm) molecules, electrodes with a gap that is 1~2 nm wide are fabricated using the electromigration-junction technique. We also studied single-walled carbon nanotube devices that are fabricated using a more conventional method. At low temperatures, most single molecule devices exhibit Coulomb blockade with discrete conductance peaks that correspond to quantum excitations of the molecule. The origin of the observed quantum excitation varies from molecule to molecule depending on how tunneling electrons interact with various molecular degrees of freedom. Vibrational excitation is the one that is most frequently observed. The most prominent vibrational excitation was identified as the bouncing-ball mode in C60 and C70 transistors, whereas it was assigned to the intercage stretching mode in C140 transistors. Magnetic excitation was also studied, and the spin state of a single Co molecule was determined by analyzing the Zeeman splitting in a magnetic field. The overall conductance of single molecule transistors is determined mainly by the coupling with electrodes. In single Co transistors, the coupling could be controlled by changing the length of insulating handles. With a longer handle, the conductance is lower as the single Co forms a quantum dot. With a shorter handle, the coupling between 1 Co and electrodes as well as the overall conductance becomes large and the Kondo effect was observed. Finally, the conductance of carbon nanotubes was studied in two different temperature regimes. At low temperatures, they form a single quantum dot (p-doped) or a double quantum dot (n-doped) due to a local doping by the electrodes. In room temperature measurements, a highly efficient electrolyte gate was used to investigate the field effect transistor properties of carbon nanotubes, which unveiled excellent device performances. 2 To my parents i Table of Contents Chapter 1: Introduction and Background ………………………… 1 1.1 Introduction: electron transport in nanoscale systems 1.2 Electron transport in a single molecule device 1.3 Single electron transistor theory 1.4 The charging energy and excitation spectrum in a single molecule device 1.5 Examples of single molecule devices 1.6 Summary and outline Chapter 2: The Coulomb Blockade Theory ………………………. 18 2.1 Overview 2.2 Basic concepts of a single electron transistor 2.3 Single-level quantum dots 2.4 Quantum dots with excited levels 2.5 Transport spectroscopy in a multi-level quantum dot 2.6 Summary and other issues Chapter 3: Device Fabrication and Experimental Setup ………… 52 3.1 Introduction: experimental techniques for wiring up molecules 3.2 Fabrication of nanowires and gate electrodes 3.3 Electromigration-induced breaking in nanowires 3.4 Characterization of the tunnel gap after electromigration 3.5 Deposition of a molecule in the gap 3.6 Measurement setup 3.7 Summary Chapter 4: Nano-mechanical Oscillations in a Single C60 Transistor …………………………………………… 82 4.1 Introduction ii 4.2 Sample preparation 4.3 Coulomb blockade in single-C60 transistors 4.4 The center-of-mass vibration (5 meV excitation) in C60 transistors 4.5 Theory of a vibrating quantum dot 4.6 Summary Chapter 5: Vibration-assisted Electron Tunneling in C140 Single-Molecule Transistors ………………………………… 100 5.1 Introduction 5.2 Sample preparation 5.3 Coulomb blockade in C140 transistors 5.4 Observation of the stretching vibrational mode (11 meV) in C140 transistors 5.5 Excitation mechanism of the stretching vibrational mode 5.6 Summary Chapter 6: Coulomb Blockade and the Kondo Effect in Single Atom Transistors ………………………………………….. 112 6.1 Introduction 6.2 The molecules and device preparation 6.3 Coulomb blockade in [Co(tpy-(CH2)5-SH)2] transistors 6.4 The Kondo effect in [Co(tpy-SH)2] transistors 6.5 Summary Chapter 7: Electrical Conductance of Single-Wall Carbon Nanotubes …………..………………………………………. 126 7.1 Overview 7.2 Introduction to carbon nanotubes 7.3 Formation of a p-type quantum dot at the end of an n-type nanotube 7.4 High performance electrolyte-gated carbon nanotube transistors 7.5 Summary iii Chapter 8: Conclusion ……………………………………………… 144 8.1 Summary 8.2 Future directions 8.3 Concluding remarks References ……………………………………………………………. 149 iv Acknowledgements Throughout my graduate work, I have truly enjoyed working with many wonderful people. It is my greatest pleasure to thank all the people who helped me bring this thesis to the light. First, I was incredibly fortunate to be able to work with and learn from Paul, my thesis advisor. Over the years, I came to realize what a great scientist, a great teacher, and a great person he is. His constant pursuit of new ideas and uncompromising attention to important details imprinted in my mind a model of a great scientist to which I will endeavor to come close in coming years. It is embarrassing to realize how many mistakes I had to make before I come to this end. I cannot thank Paul enough for his trust and guidance (academically and personally) throughout my graduate study. I also thank my committee members, Professor Paul Alivisatos, Steve Louie and John Clarke for their advice and comments on this thesis. I was very lucky to be able to work in two very wonderful places during my graduate career – Berkeley and Cornell. It was especially a great experience to witness the dynamic changes of the McEuen group, starting from the old days “on the hill” (Lawrence Berkeley Lab) to the crowded Berkeley campus, then finally to the friendly Clark Hall basement at Cornell. My thanks to all my dear McEuenites for their help and company: Hongkun, Andrew, Michael, Marc, Michael (“Tex”), Ethan, Noah, Philip, Jeff, Adrian, Manu, Chris (so far Berkeley) and Scott, Sami, Marcus, Luke, Ji-Yong, Yuval, Alex, and all the young members (at Cornell). The Ralphians deserve special thanks as well. I thank Dan for giving me his advice and help on numerous occasions. Other Ralphians, especially Abhay and Mandar spent many hours with me, discussing over science and joking about many other things. Many thanks to all the great H-corridor members, who gave me hands whenever I need them. I also thank several chemists who taught me the beauty of chemistry! Prof. Paul Alivisatos (and his students), Jay Groves, Hector Abruna and Jonas, without their help and advice, this thesis must be a lot thinner and much more boring. v Several people deserve additional comments. First, special thanks to Hongkun, who gave me lots of good lessons from the early days till now. I cherish the crazy days (and nights) spent in the lab with him at Berkeley. Also many thanks to Abhay, who, with his cheerful manners and creative energy, kept my days at Clark Hall very pleasant and fun. Many chapters of this thesis are the results of the wonderful collaboration with him. I thank my dear friend Michael as well. He impresses me in many ways inside and outside of the lab with his talent in cooking and singing, just to name a few. I also want to acknowledge some other members of my very special “support system.” I had lots of fun with my old friends who all happened to be living in the Bay Area. Especially, Seokhwan, Eunsun, Haktae and Jinbaek all paid many visits to my place to keep me entertained and happy throughout my days at Berkeley. Special thanks, of course, go to my family. My dear Dad and Mom, with their usual positive advice and unceasing love, gave me the strength to finish this work. Without their support and love, I cannot be here finishing up this thesis. I also thank my cheerful brother Chulwoong, who supported me through his quiet but very warm caring. I am also deeply indebted to EoJean’s family for their strong support and love. I thank my parents-in-law for lending me their wisdom and insight in many occasions. Finally, my warmest thanks to my wife, EoJean. Without her love and support, this thesis cannot exist. Through her creative energy and witty insights, she has made my life full of excitement and joy. Thank you and I love you, EoJean. vi Chapter 1 Introduction and Background 1.1 Introduction: Electron Transport in Nanoscale Systems Electrical conductance of a macroscopic object is described by the well-known Ohm’s law. The conductance (G) of a rectangular conductor is proportional to its width (W) and inversely proportional to its length (L). Namely, σW G = (1.1) L Here σ is the conductivity of the conductor, which is decided mainly by the charge carrier density and the mean free path. As the conductor gets smaller, several effects that are negligible in a macroscopic conductor become increasingly important. In a very small object such as nanostructures and molecules, electron transport usually does not follow Ohm’s law.
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