Single Molecule Spintronics and Friction A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Yang Li May 2018 © 2018 Yang Li. All Rights Reserved. 2 This dissertation titled Single Molecule Spintronics and Friction by YANG LI has been approved for the Department of Physics and Astronomy and the College of Arts and Sciences by Saw-Wai Hla Professor of Physics and Astronomy Robert Frank Dean, College of Arts and Sciences 3 ABSTRACT LI, YANG, Ph.D., May 2018, Physics and Astronomy Single Molecule Spintronics and Friction Director of Dissertation: Saw-Wai Hla This thesis thoroughly investigates spintronic and frictional behaviors of individual magnetic molecules adsorbed on a metal surface and graphene nanoribbons utilizing scanning tunneling microscopy, tunneling spectroscopy and non-contact atomic force microscopy combined with atomic and molecular manipulation schemes in an ultrahigh vacuum environment at low temperatures. The atomic scale studies are realized by using a porphyrin based magnetic molecule, TBrPP-Co, directly adsorbed on Au(111) surface as well as on semiconducting graphene nanoribbons grown on Au(111). On Au(111) surface, the TBrPP-Co molecules exhibit a Kondo resonance due to many-body interactions between the magnetic moment of the molecule and free host electrons from the substrate. The observed Kondo resonance is dependent on three adsorption sites of the molecules on Au(111) surface. The Kondo resonance disappears when the molecules are located on the elbows of the Au(111) surface herringbone reconstructions, and reappears when the molecules are relocated off the elbows. When TBrPP-Co molecules are isolated from the Au(111) surface by forming TBrPP-Co-armchair graphene nanoribbons (AGNRs) – Au(111) heterostructures, the molecules are electronically decoupled from the metal surface as expected. However, surprisingly, a robust Kondo resonance with almost 100% interaction strength as in the case of the molecules directly adsorbed on Au(111) has been discovered. Tunneling 4 spectra of TBrPP-Co molecules between AGNRs and on edges of AGNRs indicate that AGNRs mediate spin interactions between the TBrPP-Co and the free electrons from the substrate. The friction forces of TBrPP-Co molecules on Au(111) and on AGNRs are also measured separately. The comparison between the lateral force required to move the molecule on Au(111) surface and that on graphene nanoribbons reveals a superlubricity effect where the friction force is about two orders less than the friction force measured on Au(111). The measured friction force of the molecule on Au(111) is further confirmed by non-contact atomic force microscopy experiments, which provide a good agreement with the scanning tunneling microscope results. 5 for Xuanyi, now and always and my family 6 ACKNOWLEDGMENTS First and foremost, I would like heartily thank my advisor Prof. Saw-Wai Hla for the years of support. His mentoring, patience, understanding, and motivation have guided me towards exciting experimental studies in nanoscience, and have afforded me unimagined research opportunities at Center for Nanoscale Materials (CNM) of Argonne National Laboratory. This work would not have been possible without great collaborators: Dr. Anh Tuan Ngo and Prof. Sergio Ulloa for the DFT calculations, theoretical support and valuable discussions. I want to thank the members of my doctoral committee for their time, help and valuable suggestions to my research. I appreciate the help of students from the surface science laboratory at Ohio University: Dr. Andrew R. DiLullo, Dr. Uduwanage Gayni Perera, Dr. Yuan Zhang, Dr. Sajida Khan and Kyaw-Zin Latt. I would also like thank all professors in the department of Physics and Astronomy for offering me a wonderful studying experience. I also want to thank staff in the department of Physics and Astronomy, specially: Candy, Tracy, Wayne, Todd, Doug, Chris for their prompt help whenever I needed. I would like thank Mr. Brandon Fisher at CNM for his great support during my research at CNM. I also want to appreciate the great research experience at CNM with Dr. Volker Rose, Dr. Nozomi Shirato, Dr. Marvin Cummings, Mr. Daniel Rosenmann. I would also like to appreciate the great groups of researchers I worked with and studied along at CNM. 7 Finally, I would like to thank my family, and particularly Xuanyi, whose support, patience, and caring throughout my studies and life have motivated me to pass through difficulties and challenges. 8 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Acknowledgments ............................................................................................................... 6 List of figures .................................................................................................................... 10 Chapter 1: Introduction ..................................................................................................... 15 Chapter 2: Introduction to instruments ............................................................................. 18 2.1 Scanning tunneling microscope (STM) .................................................................. 18 2.1.1 Principle of STM .............................................................................................. 18 2.1.2 Tunneling spectroscopy ................................................................................... 20 2.1.3 Tip induced atom/molecule manipulation ........................................................ 21 2.2 Atomic force microscope (AFM) and qPlus AFM ................................................. 23 2.2.1 Atomic force microscope ................................................................................. 23 2.2.2 Constant frequency mode and constant height mode ....................................... 24 2.2.3 QPlus AFM ...................................................................................................... 24 2.2.2 Force spectroscopy ........................................................................................... 26 2.3 Ultra-high vacuum and cryogenic system .............................................................. 27 2.3.1 Ultra-high vacuum ........................................................................................... 27 2.3.2 Cryogenic system ............................................................................................. 28 2.4 Sample preparation ................................................................................................. 29 2.5 Tip preparation ....................................................................................................... 30 2.6 Molecular deposition system .................................................................................. 31 Chapter 3: Introduction to substrate and molecules .......................................................... 32 3.1 Au(111) substrate ................................................................................................... 32 3.1.1 Surface state of Au(111) .................................................................................. 33 3.2 TBrPP-Co molecules .............................................................................................. 34 3.3 Graphene nanoribbons (GNR) ................................................................................ 35 3.3.1 Synthesis of AGNR .......................................................................................... 36 Chapter 4: Kondo effect on Au(111) ................................................................................ 42 4.1 Introduction ............................................................................................................ 42 9 4.2 Sample preparation ................................................................................................. 44 4.3 Properties of TBrPP-Co on Au(111) ...................................................................... 44 4.4 Kondo resonance of TBrPP-Co on Au(111) .......................................................... 51 4.5 Switch Kondo resonance of TBrPP-Co on Au(111) by STM manipulation .......... 55 4.6 Discussion and summary ........................................................................................ 57 Chapter 5: Anomalous Kondo effect in TBrPP-Co/AGNR/Au(111) heterostructures ..... 59 5.1 Introduction ............................................................................................................ 59 5.2 Synthesis of TBrPP-Co/AGNR/Au(111) heterostructures ..................................... 60 5.3 Properties of TBrPP-Co/AGNR/Au(111) heterostructures .................................... 65 5.4 Discussion .............................................................................................................. 77 Chapter 6: Molecular superlubricity of TBrPP-Co on AGNRs ........................................ 79 6.1 Introduction ............................................................................................................ 79 6.2 Superlubricity ......................................................................................................... 79 6.3 Sample preparation ................................................................................................. 82 6.4 Force measurement ................................................................................................