Single Molecule Spintronics and Friction a Dissertation Presented to the Faculty of the College of Arts and Sciences of Ohio

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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

This dissertation titled

Single Molecule Spintronics and Friction

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.

for Xuanyi, now and always

and my family

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.

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 ...... 83 6.4.1 Force measurement using STM lateral manipulation ...... 83 6.4.2 Force measurement using qPlus AFM ...... 86 6.5 Friction force of TBrPP-Co on Au(111) and TBrPP-Co on AGNRs ...... 87 6.6 Discussion ...... 94 Chapter 7: Summary and outlook ...... 95 References ...... 98 Appendix A: Theoretical calculation of Kondo resonance ...... 105 Appendix B: Tip-molecule total force ...... 107

LIST OF FIGURES

Page

Figure 2.1: (A) A diagram shows the principle of constant current mode of STM. The tip is scanning from the left to the right across the substrate. The red curve shows the line profile to sample topography recorded by computer. The tip- sample bias voltage is V. (B) A diagram shows the quantum tunneling that happens between a STM tip and a sample. The electrons from the sample and the tip have energy E; a bias voltage V is added between tip and sample...... 19 Figure 2.2: STM topographic images of a manipulation sequence on Ag(111), VT = - 0.100 V, IT = 100 pA. The scale bar is for 10nm. A) Before manipulation. B) During manipulation. (C) Finished CNM logo. (D) A rendered topography of CNM atomic logo ...... 22 Figure 2.3: Diagrams of lateral manipulation. The red line shows the tip movement, which is recorded by computer. The manipulation direction is from left to right. (A) As the tip moving along the profile of the particle, the lateral force Fx is gradually increasing. (B) As the Fx is large enough, the particle hops forward, and the tip retracted to maintain constant tunneling current. (C) (A)-(B) process are repeated during the manipulation, and a zigzag shape manipulation curve is recorded...... 23 Figure 2.4: A diagram shows the structure of a qPlus AFM tip. Golden color represent the electrodes...... 26 Figure 2.5: (A) A top view diagram shows the structure of a UHV system. The dark gray parts are gate valves. The light gray parts are sample transfer systems. The chambers are consist of load-lock chamber, preparation chamber, evaporator chamber and STM chamber. (B) A picture of the system used for this thesis...... 27 Figure 2.6: A cross-sectional diagram of a bath cryostat...... 29 Figure 2.7: Diagrams of tip etching systems. The blue color represents the NaOH solution. (A) Coarse etching system. (B) Fine etching system...... 30 Figure 2.8: A diagram of homebuilt Knudsen cell. The red curve represents the filament, and the blue dot represent the thermalcouple...... 31 Figure 3.1. STM images of Au(111) substrate. (A) A large scale image (VT= 1.0 V, It=100 pA) shows FCC, HCP regions and herringbone reconstructions. The arrow indicates one close-packed direction. (B) An atomic resolution image (VT=8.28 mV, IT=150 pA)...... 33 Figure 3.2. Au(111) reconstruction. Blue dots indicate the FCC and HCP regions, and light blue dots indicate the transition regions, which are at bridge adsorption sites and higher than other regions...... 33 Figure 3.3. Au(111) surface state measured by tunneling spectroscopy. The surface state appears like a step at -490 mV...... 34 11

Figure 3.4. TBrPP-Co molecule. A) A ball-and-stick model of TBrPP-Co. B) A STM image of TBrPP-Co (VT = 1.5 V, IT =100 pA)...... 35 Figure 3.5: Ball-and-stick models of AGNR. (A) A DBBA monomer. (B) Upper: a DBBA Polymer. Lower: A side view of DBBA polymer shows that each segment of the polymer is twisted against its neighbors. (C) AGNR...... 37 Figure 3.6: STM image of AGNR on Au(111), and the arrows indicate three close- packed directions [110], [101], and [011] separately on Au(111). (A) Low coverage AGNRs (VT = 2.0 V, IT = 100 pA). (B) High coverage AGNRs (VT =1.0 V, IT=100 pA). The bright strips are the second layers of DBBA polymers...... 38 Figure 3.7: AGNR images. (A) A STM image shows the structures of AGNRs and the alignment of two AGNRs (VT = 0.04 V, IT = 1.0 nA). (B) A profile taken on (A) shows that the center-center distance between two neighboring AGNRs. (C) Ball-and-stick models of 7-, 14-,21-,28-AGNRs...... 39 Figure 3.8: Electronic structure of AGNRs. (A) STM image of AGNRs with different widths (VT = 1.0 V, IT = 100 pA). (B) Long range tunneling spectra taken on 7-, 14-, 28-AGNRs (from top to bottom). Arrows indicate the locations of HOMO and LUMO...... 41 Figure 4.1: A diagram shows the Kondo resonance. (A) When there is only free electrons, the spin of free electrons are randomly arranged. (B) When there is a net spin from magnetic impurities, the nearby free electrons (electrons in the red circle) interact with this net spin, which created a new resonance. . 43 Figure 4.2: A STM image (VT = -0.1 V, IT = 100 pA) shows an overview of TBrPP-Co deposited on an Au(111) substrate...... 45 Figure 4.3. Self-assembled TBrPP-Co cluster. (A) A STM image (VT = -0.1 V, IT = 50 pA) of TBrPP-Co cluster. (B) A model shows the structure of the cluster...... 46 Figure 4.4. STM images (VT = 1.0 V, IT = 100 pA) and corresponding ball-and-stick models showing TBrPP-Co adsorption sites and orientations. The white bars indicate 1nm. (A, B) Hollow site. (C, D)Bridge site. (E, F) Top site...... 47 Figure 4.5. Electronic structure of single TBrPP-Co on Au(111). (A) A tunneling spectrum showing HOMO and LUMO of TBrPP-Co. (B) Tunneling spectra show different LUMO locations for TBrPP-Co at different adsorption sites. .48 2 Figure 4.6: STM images (2.4 x 2.4 nm , IT = 200 pA) of TBrPP-Co on Au(111) recorded at (A, B, C) 2.0 V, (D, E, F) 1.0 V, and (G, H, I) -1.0 V, for the top, bridge and hollow adsorption sites, which show four-lobe, one-lobe and two-lobe structures respectively...... 50 Figure 4.7: Energy resolved dI/dV maps of the TBrPP-Co on Au(111) substrate. The white bars represent 1nm. (A)STM topography and (B) dI/dV map taken simultaneously at 1.7 V, IT = 300 pA. (C)STM topography and (D) dI/dV map taken simultaneously at -1.0 V, IT = 300 pA...... 51 12

Figure 4.8: A tunneling spectrum taken at the center (indicated by the red dot) of a single TBrPP-Co showing a sharp asymmetric peak at Fermi level...... 52 Figure 4.9: (A)Tunneling spectra showing Kondo resonance with different resonance widths for molecules on different adsorption sites. (B) Kondo temperature of TBrPP-Co at different adsorption sites...... 53 Figure 4.10: Tunneling spectra along a line showing 1/r relationship between the Kondo amplitude and distances from Co center. (A) A STM image of a single TBrPP-Co molecule. Dots indicates locations where tunneling spectra are taken. (B) Tunneling spectra taken at locations in (A). (C) A plot of Kondo amplitude as a function of distance...... 54 Figure 4.11: STM images and corresponding tunneling spectra showing Kondo resonance can be turned on and off by manipulating TBrPP-Co molecules on and off the elbow of a herringbone reconstruction. VT = -0.1 V, IT = 100 pA. (A) No Kondo resonance when the molecule on elbow. (B) Kondo resonance appears when the molecule is manipulated off the elbow. (C) Kondo resonance disappeared when the same molecule is moved back to the elbow. (D) Kondo resonance reappeared once moved off the elbow...... 56 Figure 4.12: A STM image (VT = -0.1 V, IT = 100 pA) overlaid by a model of Au(111) herringbone reconstruction. Light blue dots represent relocated atoms, and dark blue dots represents HCP or FCC regions. The orange dot indicates the Co center of TBrPP-Co molecule...... 57 Figure 5.1: TBrPP-Co and AGNRs at low coverage. (A) An STM image showing self- assembled TBrPP-Co molecules are gown between AGNRs on Au(111) (VT = -0.1 V, IT = 30 pA). (B) An STM image of a single TBrPP-Co molecule adsorbed on Au(111) beside an AGNR (VT = -0.2 V, IT = 30 pA)...... 61 Figure 5.2: Manipulating TBrPP-Co toward a AGNR using lateral manipulation. (A) An STM image taken before manipulation (VT = 1.0 V, IT = 100 pA). (B) An STM image taken after manipulation (VT = 1.0 V, IT = 100 pA), where the TBrPP-Co molecule pushed the whole AGNR away. The original positions of the TBrPP-Co molecule and AGNR are indicated by the blue and red dashed contours separately. The manipulation direction of the TBrPP-Co molecule and the moving direction of AGNR are indicated by blue arrow and red arrow separately. The manipulation parameters are VLM = 0.1 V, 2 RLM = 1.0 MΩ. The 2 x 2 nm grid lines are drawn for eye guidance...... 62 Figure 5.3: STM images (VT = 2.0 V, IT = 50 pA) shows a same AGNR with different lengths of (A) 12.4 nm; (B) 4.3 nm; (C) 18.9 nm; (D) 11.0 nm, which indicated very weak interactions between AGNRs and Au(111) substrate. 63 Figure 5.4: TBrPP-Co on AGNRs with different adsorption configurations. (A) An STM image of TBrPP-Co molecular self-assembly on top of AGNR (VT = 0.4 V, IT = 10 pA). The overlaid model show the structure of TBrPP-Co molecules in the cluster. (B) An STM image of a single TBrPP-Co molecule adsorbed on the edge of a AGNR (VT = 0.5 V, IT = 10 pA). (C) An STM image of 13

TBrPP-Co molecules adsorbed on the DBBA polymers (VT = 2.0 V, IT = 50 pA)...... 64 Figure 5.5: STM images (VT = 1.0 V, IT = 10 pA) overlaid by models of AGNR and TBrPP-Co models. The upper area, indicated by the blue rectangle, is a high resolution STM image (VT = 0.01 V, IT = 200 pA) of AGNRs...... 66 Figure 5.6: Measurement of distance between preferred adsorption sites by manipulating single TBrPP-Co molecules on AGNRs. (A) An STM image of a single TBrPP-Co molecule on a 7-AGNR before manipulation (VT = 1.0 V, IT = 1.0 pA). The blue arrow indicates the manipulation direction. (B) An STM image of the molecule after manipulation (VT = 1.0 V, IT = 1.0 pA). (C) Lateral manipulation curve, which shows the distance between preferred adsorption sites is ~4.15 Å. (D) Models of AGNR and TBrPP-Co...... 68 Figure 5.7: Tunneling spectra of TBrPP-Co molecules on AGNR (blue), and on Au(111) (Red). The yellow bar and red bar indicate zero value of each dI/dV tunneling spectra. The HOMO and LUMO of TBrPP-Co on AGNR appear sharper and the spectrum within the gap between the HOMO and LUMO are flatter comparing to the case of TBrPP-Co on Au(111)...... 70 Figure 5.8: Kondo resonance of TBrPP-Co/AGNR/Au(111) heterostructure. (A) An STM image (VT = 1.0 V, IT = 3.0 pA) of a heterostructure formed by three TBrPP-Co molecules. The blue dot indicates the location where the tunneling spectrum was taken. (B) A tunneling spectrum showing the Kondo resonance at the Fermi level...... 71 Figure 5.9: (A) Tunneling spectra of TBrPP-Co on AGNRs showing different Kondo temperatures. (B) Kondo temperatures of TBrPP-Co on AGNRs is slightly smaller than the case of TBrPP-Co on Au(111), and showing the same trend at different adsorption sites...... 72 Figure 5.10: 1/r measurements of Kondo amplitude of a TBrPP-Co molecule on AGNRs. (A) An STM image (VT = 0.2 V, IT = 10 pA) of a TBrPP-Co on AGNRs. The dots indicate the locations where the tunneling spectra are taken. (B) Tunneling spectra taken at different distance from the Co atom of TBrPP- Co. (C) The normalized Kondo amplitudes decay with 1/r trend...... 74 Figure 5.11: Distribution of Kondo resonance. (A) An STM image (VT = 0.4 V, IT = 10 pA) of a TBrPP-Co molecule on AGNRs. (B) A tunneling spectroscopy map of the same area of (A) taken at 7 mV showing the Kondo resonance is located at the center (white circle) of TBrPP-Co molecule. The white bars represents 0.5 nm...... 75 Figure 5.12: Disappearance of Kondo resonance. (A) An STM image (VT = 0.5 V, IT = 1.0 pA) showing a TBrPP-Co molecule adsorbed between two 7-AGNRs. (C) An STM image (VT = 0.5 V, IT = 10 pA) showing a TBrPP-Co molecule adsorbed on the edge of a 7-AGNR. (B, D) Tunneling spectra taken at the blue dot in (C, A) shows no Kondo resonance respectively...... 76 Figure 6.1: (A) An STM image taken at a small tunneling current (VT = 1.0 V, IT = 10 pA) showing a single TBrPP-Co molecule sliding along a 7-AGNR during scanning. (B) An STM image (VT = 2.0 V, IT = 1.0 pA) of a single TBrPP- 14

Co on a 7-AGNR showing the molecule is slightly rotated by the scanning tip...... 82 Figure 6.2: A diagram shows the geometries of forces (F, FX, f), and threshold angle �!...... 83 Figure 6.3: A plot shows the relationship between tunneling resistances and tip-sample distances. The abrupt decrease in tunneling resistance at 0 distance indicates that the tip is directly in contact with the sample...... 85 Figure 6.4: Later manipulation of a single TBrPP-Co molecule on Au(111). (A) An STM image (VT = 1.0 V, IT = 100 pA) before manipulation. The red arrow shows the direction of manipulation. (B) An STM image ((VT = 1.0 V, IT = 100 pA) after manipulation...... 88 Figure 6.5: Later manipulation of a single TBrPP-Co molecule on 7-AGNRs. (A) An STM image (VT = 2.0 V, IT = 1.0 pA) before manipulation. The white arrow shows the direction of manipulation. (B) An STM image (VT = 2.0 V, IT = 1.0 pA) after manipulation...... 89 Figure 6.6: Plots of threshold angles as a function of tip-sample distances for single TBrPP-Co molecules on AGNR and Au(111) separately...... 90 Figure 6.7: A diagram shows frequency shift measurement using a qPlus AFM. (A) The AFM tip scans across the sample at constant heights, which is repeated at different tip heights. (B) Resonant frequency shift are recorded as functions of lateral distances. Curve 1 shows no frequency shift; curve 2 shows a constant frequency shift due to long-range interactions; the dip at the center of curve 3 shows the frequency shift due to short-range interactions ...... 91 Figure 6.8: A frequency spectroscopy shows an abrupt jump (red arrow) due to the movement of the TBrPP-Co molecule...... 92 Figure 6.9: (A) Force gradient of tip-molecule junction. (B) Vertical component of tip- molecule interaction force. (C) Tip-molecule potential energy. (D) Lateral force between the tip and the molecule. Jumping points are indicated by red arrows...... 93 Figure A: Kondo mechanism. (A) (1)Calculated geometries of isolated TBrPP-Co, (2) TBrPP-Co adsorbed on Au(111), (3) the heterostructure of TBrPP-Co adsorbed on AGNR/Au(111), and (4) TBrPP-Co located at 7.5 Å above Au(111) surface after removing the AGNR from the heterostructure. (B) Spin-polarized Co d!! density of states of the TBrPP-Co corresponding to (A). (C) Atomic SP-DOS Co d!! and N p-orbitals for TBrPP-Co adsorbed on top site of Au(111). (D) Atomic SP-DOS of Co d!!, N p, and C PZ orbitals in AGNR for the TBrPP-Co/AGNR/Au(111) heterostructure. Positive and negative DOS correspond to spin-up, and spin-down components, respectively. Copyright 2017 Nature Communications...... 106 Figure B: (A) Tip-molecule potential calculated by DFT as a function of tip-molecule distance. (B) Tip-molecule total force derived from (a) [73]...... 107

CHAPTER 1: INTRODUCTION

One important goal of nanotechnology is to develop nanodevices with desired performance using single atoms and molecules, as initially proposed by Richard Feynman in his famous talk ‘There’s Plenty of Room at the Bottom’ [1]. The utilization of the single atoms and molecules as devices requires investigating their electronic, magnetic and mechanical properties. This thesis is focused on the electronic, spintronic and mechanical properties of single molecules on metallic substrates, and vertically stacked heterostructures formed by molecule-semiconductor-metallic substrate.

One of the most useful techniques to characterize nanomaterials is the Scanning

Probe Microscope (SPM). In this thesis, Scanning Tunneling Microscope (STM) and

Atomic Force Microscope (AFM) are used. By scanning an atomically sharp tip on the sample, they both offer direct observation of sample topography with atomic resolution.

In addition, STM can measure the electronic structure of the sample using tunneling spectroscopy, and can measure friction force using lateral manipulation. AFM can measure the force field and potential energy of the system using force spectroscopy.

These techniques are explained in detail in Chapter 2. Properties of nanomaterials for experiments, including the TBrPP-Co molecules, armchair graphene nanoribbons

(AGNR), and the Au(111) substrate are introduced in Chapter 3. Different from graphene, AGNR is a semiconductor with large band gaps, which make it promising in electronic applications. The recipe to synthesize AGNR on a Au(111) substrate is also discussed in Chapter 3.

In Chapter 4, the adsorption site dependence of Kondo resonance of individual

TBrPP-Co molecules on Au(111) is investigated. Spin exchange interactions between a magnetic impurity and free electrons from the substrate can generate a Kondo resonance, which is detected by means of tunneling spectroscopy. Using lateral manipulation,

TBrPP-Co molecules are moved to different adsorption sites. Kondo temperature, which is related to the spin-electron interaction strength, is determined for each adsorption configuration. It is found that the average Kondo temperature of the molecule adsorbed on top surface site is higher than the bridge site, which is followed by the hollow site.

There is no trace of Kondo resonance when the molecule is adsorbed on the elbow of herringbone Au(111) surface reconstructions.

Since the Kondo resonance of TBrPP-Co can be greatly affected by the substrate structure, a natural question will be what will happen if the TBrPP-Co molecules are separated from the surface. In Chapter 5, vertically stacked heterostructures of TBrPP-

Co/AGNR/Au(111) are fabricated for the first time to answer this question. Although the

TBrPP-Co is electronically separated from the Au(111) substrate by semiconducting

AGNRs, surprisingly, a robust Kondo resonance is found in the tunneling spectra. The

Kondo temperature here are almost the same as ones observed for the TBrPP-Co directly adsorbed on the Au(111) substrate. Based on the support of spin polarized density functional theory calculation, it is proposed that the AGNRs mediate the spin coupling between TBrPP-Co and free electrons from the Au(111) substrate.

Chapter 6 is focused on measuring the friction force of TBrPP-Co molecules on

Au(111) surface and on AGNRs using scanning tunneling microscope manipulation schemes. By using density functional theory calculated potential energy for the distance dependent tip-molecule interactions, the experimental data are quantified to find the friction force values. The magnitude of friction force of molecules on AGNR is found to be 2 orders lower than molecules adsorbed on the Au(111) surface, thereby revealing superlubricity behavior of the molecules on AGNR for the first time. Moreover, the ultra low friction force of molecules on the Au(111) surface is confirmed by a non-contact atomic force microscopy.

Finally, Chapter 7 summarizes the thesis and future research directions emerging from the current work.

CHAPTER 2: INTRODUCTION TO INSTRUMENTS

2.1 Scanning tunneling microscope (STM)

Scanning Tunneling Microscope (STM), invented in early 1980s [2, 3], has greatly improved the understanding of physical properties of nanoscale materials. STM made it possible to directly observe and even to manipulate single atoms or molecules in real space [4]. Since the invention of STM, several Scanning Prove Microscopes (SPM), using physical probes to investigate different properties of materials with atomic precision [5-8], have been developed. In the following text, quantum tunneling is introduced to explain the principle of STM. A feedback mechanism and a high precision position control system are also introduced as important techniques to operate STM. This is followed by an overview of the capabilities of STM, including tunneling spectroscopy and tip induced atomic/molecular manipulation.

2.1.1 Principle of STM

STM operates by scanning a conductive and atomically sharp tip (ideally with one atom at the tip apex) across a conductive sample, while using quantum tunneling current as the feedback signal to maintain the tip-sample distance. As shown in figure 2.1(A), the tip positions, including lateral positions and vertical heights, are recorded to generate a 3- dimensional topography of the sample.

Figure 2.1. (A) A diagram shows the principle of constant current mode of STM. The tip is scanning from the left to the right across the substrate. The red curve shows the line profile to sample topography recorded by computer. The tip-sample bias voltage is V. (B) A diagram shows the quantum tunneling that happens between a STM tip and a sample. The electrons from the sample and the tip have energy E; a bias voltage V is added between tip and sample.

Quantum tunneling allows electrons to tunnel between the sample and the STM tip through vacuum, which cannot be done classically. Considering the tip-sample junction as a 1-D potential barrier, the tunneling current can be described as [9]:

!!!" � ∝ V�!(0, �!)� (2.1) where the ‘V’ is the bias voltage applied between the tip and the sample, ‘�!’ is the

Fermi energy. ‘�!(0, �!)’ is the local density of states at Fermi level, ‘�’ is the decay constant, and ‘z’ is the tip-sample distance.

Because the tunneling current is exponentially proportional to the tip-sample distance, a small change in the distance can cause a significant change in the tunneling current. In a typical STM experiment, tunneling current is used as the feedback signal to maintain the tip-sample distance. 20

If the Fermi energy of the sample surface is the same as the Fermi energy of the the tip material, the electrons will tunnel from the sample to the tip at the same rate as tunneling from the tip to the sample. As a result, there will be no net tunneling current. In order to generate net tunneling current, a bias V is applied between the tip and the sample

(figure 2.1 (B)) [9].

STM has two operation modes: the constant current and the constant height mode.

Constant current mode is used in this thesis. In this mode, the tip-sample distance is adjusted to maintain the tunneling current, and is recorded to generate a topographic map.

The tip height adjustment makes the constant current mode slower than the constant height mode. In the constant height mode, the tip scans across the sample at a constant height, while the varying tunneling current is recorded. However, in the constant height mode, tips are more likely to crash into the sample, and the exponentially sensitive current can be out of the detectable range of the system.

In order to achieve high spatial resolution (<1 Å), piezoelectric motors are used to precisely control the tip movement with sub-angstrom precision.

2.1.2 Tunneling spectroscopy

In addition to topographic imaging, tunneling spectroscopy is another important application of STM to investigate electronic properties of materials. With the assumption of constant density of states (DOS) of the tip, DOS of the sample is proportional to the tunneling conductance [9]:

!" ∝ � � − �� (2.2) !" ! ! 21 where ‘dI/dV’ is the differential conductance at voltage ‘V’, ‘�!’ is the function of sample’s density of state, ‘�!’ is the Fermi energy, and ‘e’ is the elementary charge and

‘V’ is the bias voltage between the tip and the sample. So we can use experimentally measured differential conductance dI/dV as a direct measurement of local sample density of states, which is an important physical quantity of samples.

Once the STM tip is brought to the desired position on top of the sample, the feedback loop is terminated and the tip-sample distance is fixed, and differential conductance as a function of bias is usually measured by lock-in technique. During measurement, a high frequency sinusoidal voltage modulation with small amplitude is added to the tip-sample bias. The output of the lock-in amplifier gives the differential conductance dI/dV.

In addition to point spectroscopy, tunneling spectroscopic mapping is an effective way to visualize the spatial distribution of sample density of states.

2.1.3 Tip induced atom/molecule manipulation

Because of the tip-sample interactions can be controlled, STM is capable to manipulate atoms and molecules on surfaces [10]. STM manipulation can be used to create and engineer novel nano-structures which don’t exist in nature. Figure 2.2 shows an atomic logo for Center for Nanoscale Materials (CNM) made of 29 silver atoms on

Ag(111) substrate.

Figure 2.2. STM topographic images of a manipulation sequence on Ag(111), VT = - 0.100 V, IT = 100 pA. The scale bar is for 10nm. (A) Before manipulation. (B) During manipulation. (C) Finished CNM logo. (D) A rendered topography of CNM atomic logo.

There are two types of manipulation schemes; lateral manipulation (LM) and vertical manipulation (VM). The LM procedure is mostly used in this thesis (figure 2.3).

During LM, the feedback loop is kept on and the tunneling current is set to a relatively larger value than normal scanning mode in order to form a stronger tip-sample interaction. With an optimized tip-sample interaction, as the tip moving along the profile of the particle (figure 2.3 (A)), the lateral component of the interaction will be strong enough to overcome the barrier between the adsorption sites, then the particle will hop to the next adsorption site toward the tip. At this moment, the tip will rapidly retract to maintain constant tunneling current, and one manipulation step is completed (figure

2.3(B)). By repeating this manipulation step, the particle is moved to the desired position. 23

Typically, a saw-tooth like shape manipulation curve is recorded as illustrated in figure

2.3(C).

A BC

Figure 2.3. Diagrams of lateral manipulation. The red line shows the tip movement, which is recorded by computer. The manipulation direction is from left to right. (A) As the tip moving along the profile of the particle, the lateral force Fx is gradually increasing. (B) As the Fx is large enough, the particle hops forward, and the tip retracted to maintain constant tunneling current. (C) (A)-(B) process are repeated during the manipulation, and a zigzag shape manipulation curve is recorded.

In addition to building novel nanostructures, tip induced manipulation also provide ways to characterize mechanical properties of nanomaterials [11, 12], which is discussed in detail in Chapter 6.

2.2 Atomic force microscope (AFM) and qPlus AFM

2.2.1 Atomic force microscope

In contrast to STM, where the feedback signal is the tunneling current, AFM uses tip-sample interactions as the feedback signal, which offers an advantage that the sample doesn’t need to be conductive. A non-contact AFM uses an oscillating cantilever to probe 24 the surface. Here, the shift in the oscillation amplitude for amplitude modulation (AM) mode or frequency for frequency modulation (FM) mode (2.6) due to the tip-sample interaction is used as feedback signals to control the tip height. FM employs a small oscillating amplitude leading to a higher resolution and it is easier to control. As a result,

FM is the preferred mode to operate a non-contact AFM and is used in this thesis.

2.2.2 Constant frequency mode and constant height mode

In constant frequency mode, the cantilever is kept oscillating at the resonant frequency by adjusting the tip-sample distance [13, 14, 15]. The frequency shift is described by the following equation,

∆� = � !!!"(!)! (2.3) ! !! where ‘∆�’ is the frequency shift; ‘�!’ and ‘�’ are the eigenfrequency and the spring constant of the cantilever and ‘< �!"(�) >’ is the force gradient between tip and sample.

In the constant height mode, the AFM tip scans across the surface at a constant height, and the frequency shift of the cantilever is recorded. Constant height mode is faster than the constant frequency mode because a feedback loop is not required.

2.2.3 QPlus AFM

QPlus AFM is the latest development of the non-contact AFM [16-19]. In order to achieve a high resolution, the AFM should detect the short-range forces between the tip and the sample underneath, and avoid the long-range forces. It has been proved that, when the oscillation amplitudes are small compared to the range of the short-range interaction, the detected signal is mostly contributed by the short-range forces [16]. Thus, 25 a cantilever with small oscillation amplitude is preferred. However, such a small oscillation amplitude will introduce more noise (2.7) [16]

�� ! ∝ ! (2.4) �� where ‘δfthermal’ is the frequency shift due to thermal noise; f0 is the resonant frequency of

! the cantilever; ‘��!"#/2’ is the energy stored in the cantilever; ‘Q’ is the quality factor.

According to formula 2.4, in order to eliminate the thermal noise, large k and Q are necessary.

Additionally, the high stiffness (k) enables the detection of larger forces before snapping to contact instabilities. The sensitivity is shown in the following formula [17]:

! ∝ �×� (2.5) !! where ‘V’ is the electrical potential difference between two forks of the cantilever that is due to the distortion by the force, ‘z’’ is the distance of cantilever distortion, ‘k’ is the stiffness of the cantilever, and ‘f’ is the resonant frequency of the cantilever. With larger stiffness and resonant frequency, the sensitivity of qPlus sensor is higher than other types of AFM cantilevers, which allows a qPlus sensor to vibrate with small amplitude and to effectively detect short-range forces.

Figure 2.4 shows the structure of qPlus AFM tip. The tuning fork is made from a silicon wafer with optimized crystal structures.

Tip

STM

AFM

Figure 2.4. A diagram shows the structure of a qPlus AFM tip. Golden color represent the electrodes.

One beam of the cantilever (lower one in figure 2.4) is tightly fixed. A sharp Pt/Ir tip is attached at the end of the other free beam (upper one in figure 2.4). There are three isolated electrodes on the sensor. One electrode for measuring tunneling current is directly connected to the Pt/Ir tip. Two other electrodes, each connects to one of the beams of the tuning fork, drive the tuning fork to vibrate at a resonant frequency.

2.2.2 Force spectroscopy

Force spectroscopy offers a method to directly measure extremely small forces

(<1 pN) [20], and to investigate mechanical properties of nanostructures. Force is derived from the measurement of ∆f, the frequency shift from resonant frequency, and ∆f is measured by scanning the tip across the sample at a constant height. The measurements start from large tip-sample distance, where ∆f is zero, and repeated as the tip-sample distance is progressively reducing until the adsorbate on the surface is moved. Detailed derivation will be discussed in Chapter 6. 27

2.3 Ultra-high vacuum and cryogenic system

2.3.1 Ultra-high vacuum

Ultra-high vacuum (UHV) environment is critical to maintain the cleanness of the sample, and to eliminate reactions with background gases during experiments. The time to form a monolayer is inversely proportional to the gas pressure. So by maintaining an

UHV environment, a sample can be kept clean for a long period, which allows more complex and long term experiments.

UHV environment is obtained in stainless steel chambers, which is shown in figure 2.5.

Figure 2.5. (A) A top view diagram shows the structure of a UHV system. The dark gray parts are gate valves. The light gray parts are sample transfer systems. The chambers consist of load-lock chamber, preparation chamber, evaporator chamber and STM chamber. (B) A picture of the system used for this thesis.

The designs of UHV chambers need to minimize the area of internal surfaces, to avoid trapping contaminations. In order to achieve the best pressure and to enable a quick sample transfer, the UHV system used in this thesis is designed to consist of multiple chambers with increasing vacuum levels, including a load-lock chamber (~10-8 Torr), a sample preparation chamber (~10-9 Torr) and a scanning tunneling microscope chamber

(~10-10 Torr). These chambers are separated from each other with gate valves. The load- lock chamber is used to introduce the sample from the atmospheric environment. The sample preparation chamber is used for cleaning of the samples and molecular deposition while the scanning probe microscope chamber is used for the measurements.

2.3.2 Cryogenic system

Cryogenic environment is necessary for several reasons. First, the thermal diffusion at low temperature can be minimized. Second, cryogenic temperature are necessary for a number of research problems [21-28]. For example, in Chapter 3, Kondo resonance can only be detected at low temperature. For the third reason, cryogenic environment can effectively reduce thermal noise.

Cryogenic temperature is achieved and maintained using cryostats. A low-noise bath cryostat is used in the experiments of this thesis. As shown in figure 2.6, the scanning probe microscope is attached on the heat exchanger located at the bottom of the inner cryostat. In order to reduce the blackbody radiation, the microscope is protected by an inner shield and an outer shield, which are attached to the inner cryostat and the outer cryostat separately. For all measurements in this thesis, the sample temperature is kept at

4.4 K. 29

LN2 LHe LN2 LHe Shield LN2 Shield Scanner

Vacuum

Figure 2.6. A cross-sectional diagram of a bath cryostat.

2.4 Sample preparation

In order to obtain atomically clean substrates, Au(111) surface is repeatedly sputtered and annealed for multiple cycles. Sputtering process is to use high energy inert gas ions (Ar+) to bombard the surface. Ar gas is ionized by high energy electrons generated from heated filament, and accelerated by high voltage (~1 kV). Then the high energy ion current is directed to the substrate to remove contaminations. After sputtering, annealing procedure is applied to recover and smoothen the bombarded surface. The substrate is annealed by radiation heat from a tungsten filament. The annealing temperature for Au(111) is as high as 800 °C. The temperature of the sample is precisely monitored by a type C thermalcouple, which is closely attached to the substrate. The sputtering and annealing processes are repeated until atomically flat and clean terraces on the substrates are formed. 30

2.5 Tip preparation

A high quality tip is critical to get high resolution topographic images, and reliable tunneling spectroscopy. In this thesis, tips are made by tungsten wires through a two-step electrochemical etching process.

Figure 2.7 demonstrates the experimental setup for tip etching. 2 M NaOH solution is used as the electrolyte, and a graphite rod and a tungsten wire are dipped into the solution as electrodes. First, the tungsten wire is roughly etched using high voltage (~

10 V) (figure 2.7(A)). Then the fine etching is done using a tungsten loop coated with a

NaOH solution film under an optical microscope (figure 2.7(B)). Finally, the finely etched tips are rinsed in deionized water, and inspected under high resolution optical microscopes or scanning electron microscopes.

Tip Graphite AC Voltage Supply

B Tip NaOH AC Voltage Supply

Figure 2.7. Diagrams of tip etching systems. The blue color represents the NaOH solution. (A) Coarse etching system. (B) Fine etching system.

Once a sharp tip is obtained by etching, it is important to condition the tip during experiments. The STM tip is brought to the tunneling region, and either apply a high voltage or gently crash into the substrate, which can generate local heat to reshape the tip and remove contaminations. This technique can be applied repeatedly, until the desired tip performance is achieved.

2.6 Molecular deposition system

Molecules are deposited using thermal evaporation. As shown in figure 2.8, a homebuilt Knudsen cell consists of crucible, filament, and thermalcouple. The crucible is made of tantalum to stand a high temperature, with a small opening at the top as the outlet of the molecular beam. A thermalcouple is attached on the external wall of the crucible to measure the temperature, and a tungsten filament is connected to a DC current supply to heat the crucible.

Figure 2.8. A diagram of homebuilt Knudsen cell. The red curve represents the filament, and the blue dot represent the thermalcouple.

CHAPTER 3: INTRODUCTION TO SUBSTRATE AND MOLECULES

3.1 Au(111) substrate

A single crystal Au(111) substrate is used for the experiments in this thesis.

Because of the termination of crystal structure, the surface atoms of Au(111) substrate experience imbalanced forces and rearrange from their equilibrium positions. Thus the top-layer becomes buckled [29]. This leads to periodic herringbone reconstruction patterns as shown in the STM image of an atomically clean Au(111) surfaces (figure 3.1).

The large area STM image on the left of figure 3.1 shows a typical herringbone reconstruction of a Au(111) surface. The straight step at the lower right corner (indicated with an arrow) is along a surface close-packed direction, i.e. along a [110] surface direction. An atomic resolution STM of Au(111) on the right shows a close-up image of herringbone reconstruction (figure 3.1). The atoms are closely packed in a hexagonal pattern with the measured nearest neighbors distances of 290 pm in regions between the herringbone reconstruction patterns.

Figure 3.1. STM images of Au(111) substrate. (A) A large scale image (VT= 1.0 V, It=100 pA) shows FCC, HCP regions and herringbone reconstructions. The arrow indicates one close-packed direction. (B) An atomic resolution image (VT=8.28 mV, IT=150 pA).

The herringbone surface reconstruction of Au(111) separates the Au(111) surface into HCP and FCC regions (figure 3.2) [29].

1st Layer 2nd Layer

Figure 3.2. Au(111) reconstruction. Blue dots indicate the FCC and HCP regions, and light blue dots indicate the transition regions, which are at bridge adsorption sites and higher than other regions.

3.1.1 Surface state of Au(111)

Surface states exist in the bulk band gaps of Au(111), which is caused by the termination of the periodic potential at the surface. The wave function decays both into 34 the vacuum and the bulk crystal, which creates a new state. This new state confines electrons in the direction perpendicular to the surface. The Shockley surface state of

Au(111) is well known [30] and it can be used to examine the quality of STM tip for the tunneling spectroscopy measurements. The surface state of the Au(111) substrate can be measured by means of dI/dV tunneling spectroscopy as shown in figure 3.3, where the surface state energy is found at ~ -490 mV.

dI/dV (a.u.)

−1000 −500 0 500 1000 Bias Voltage (mV)

Figure 3.3. Au(111) surface state measured by tunneling spectroscopy. The surface state appears as a step at -490 mV.

3.2 TBrPP-Co molecules

The magnetic molecule used for this thesis is TBrPP-Co (tetrakis bromo-cobalt- porphyrin). By non-covalent interactions, porphyrin molecules form self-assembled structures [23]. In TBrPP-Co, a cobalt atom is caged at the center of the porphyrin unit, and four bromo-phenyl groups are attached at the four corners of the porphyrin (figure

3.4). The central Co atom forms coordinated bonds with neighboring nitrogen atoms, and 35 it has a net spin ½ with the spin density localized at the 3d7 Co(II) state, which makes

TBrPP-Co magnetic and leads to interesting spintronic effects. The electronic structures of TBrPP-Co on other surfaces such as on Cu(111) have been previously studied, which shows the highest occupied molecular orbital (HOMO) at -0.7 eV and the lowest unoccupied molecular orbital (LUMO) at +0.6 eV [21, 23].

Br Br

Figure 3.4. TBrPP-Co molecule. (A) A ball-and-stick model of TBrPP-Co. (B) A STM image of TBrPP-Co on a Au(111) substrate (VT = 1.5 V, IT =100 pA).

3.3 Graphene nanoribbons (GNR)

GNRs are narrow and straight strips of graphene (figure 3.7(A)). In contrast to graphene, which exhibits semi-metallic behavior, GNR can be a semiconductor with large bandgaps because of the quantum confinement and edge effects [31, 32]. The band gaps of GNR is inversely proportional to the width of GNR and can be controlled during synthesis, which makes GNR a promising material in applications. For example, GNR heterostructures with varying widths and bandgaps have been made by fusing different 36

GNR monomers, therefore the band gap engineering at the atomic scale becomes a reality

[33].

There are two common types of GNRs. One is armchair GNR (AGNR) with armchair-shaped edges (figure 3.7(A)), and the other one is zig-zag GNR (ZGNR) with zig-zag shaped edges. AGNR is used for experiments in this thesis. AGNR is labeled by the number of carbons in the direction perpendicular to AGNR axis. 7-AGNR, 14-

AGNR, 21-AGNR, 28-AGNR are shown in figure 3.7(C) [34].

3.3.1 Synthesis of AGNR

AGNR are synthesized using a bottom-up process that consists a series of surface catalyzed reactions. In contrast to top-down synthesis, which starts from bulky material and using lithography and etching, bottom-up synthesis starts with monomer precursors, and build up large structures by linking monomers using self-assembling and chemical reactions. As a result, the structures made by bottom-up process can be atomically precise and defect-free. In order to make desired structures, it is critical to choose the right monomers. In this experiment, 10-10’-dibromo-9-9’-bianthryl (DBBA) (Figure 3.5(A)) is chosen to synthesize 7-AGNR.

DBBA molecules are deposited onto an atomically clean Au(111) substrate through thermal evaporation at ~200 °C. At this stage, bromine atoms are removed from

DBBA on Au(111) surface, and the DBBA molecules are left with two activated sites. In the next step, the sample is heated up and maintained at 250 °C for 30 minutes. At this temperature, the activated DBBA molecules with enough thermal energy diffuse on the substrate. They undergo radical addition reaction [35] once they meet each other, and 37 form linear DBBA polymers (Figure 3.5(B)). Because of the repulsive interactions between hydrogen on neighboring benzene rings, monomers in the polymers are alternately twisted (Figure 3.5(B)). Then the substrate is heated up to 400 °C, and a surface-assisted cyclodehydrogenation reactions occurs. This leads to forming a fully connected aromatic system and 7-AGNRs are synthesized (figure 3.5(C)) [35].

AB C

Figure 3.5. Ball-and-stick models of AGNR. (A) A DBBA monomer. (B) Upper: a DBBA Polymer. Lower: A side view of DBBA polymer shows that each segment of the polymer is twisted against its neighbors. (C) AGNR.

At a low coverage, the AGNRs remain separated from each other on the Au(111) surface (figure 3.6 (A)). Here, most of the AGNRs are aligned along surface close-packed directions of Au(111) (figure 3.6 (A)). At higher coverages, the AGNRs are tightly packed and position parallel to each other. The paralleled AGNRs are also mostly aligned 38 along the surface close-packed directions, i.e. [110] surface directions, of Au(111) (figure

3.6 (B)).

Figure 3.6. STM image of AGNR on Au(111), and the arrows indicate three close- packed directions [110], [101], and [011] separately on Au(111). (A) Low coverage AGNRs (VT = 2.0 V, IT = 100 pA). (B) High coverage AGNRs (VT =1.0 V, IT=100 pA). The bright strips are the second layers of DBBA polymers.

Within the parallel alignment, AGNRs position each other in such a way that the protrusive sites on one edge of a AGNR is located next to hollow sites on the edge of neighboring AGNR (figure 3.7 (A)). The distance between the neighboring 7-AGNRs is

0.861nm as shown in figure 3.7 (B). 39

Figure 3.7. AGNR images. (A) A STM image shows the structures of AGNRs and the alignment of two AGNRs (VT = 0.04 V, IT = 1.0 nA). (B) A profile taken on (A) shows that the center-center distance between two neighboring AGNRs. (C) Ball-and-stick models of 7-, 14-,21-,28-AGNRs.

The width of 7-AGNR is the same as the width of DBBA monomer. In order to obtain AGNR with different widths, one way is to select monomers with different widths 40

[33]. Another way is to anneal the sample at high temperature for a long time, where neighboring AGNRs link and form wider ribbons. The widths of AGNRs made by this way can only be an integer multiple of the the monomer width (figure 3.7(C)).

At high coverages, second layers of DBBA polymers are observed on top of

AGNRs (figure 3.6 (B)). Interestingly, the second layer of DBBA polymers cannot form

AGNRs even annealed at 400 °C for a long time. This is because cyclodehydrogenation reaction cannot happened without the Au(111) surface as the catalyst [35].

Electronic structures of 7-, 14-, 28- AGNRs are measured using long range (-

1200mV-2000mV) tunneling spectroscopy (figure 3.8). The HOMO of AGNR is measured at -0.96 V, which is in agreement with previous measurements [36, 37]. While the HOMO level remains the same for three GNRs with different widths, the LUMO level is reduced for increasing widths of AGNRs, which is expected. The tunneling spectroscopy shows a band gap of ~2.7 eV for 7-AGNR, ~2.3 eV for 14-AGNR, and ~1.6 eV for 28-AGNR, which are in good agreement with values in the literature [34, 36, 37].

A B dI/dV(a.u.)

-1000 0 1000 2000 BiasVoltage(mV)

Figure 3.8. Electronic structure of AGNRs. (A) STM image of AGNRs with different widths (VT = 1.0 V, IT = 100 pA). (B) Long range tunneling spectra taken on 7-, 14-, 28- AGNRs (from top to bottom). Arrows indicate the locations of HOMO and LUMO.

CHAPTER 4: KONDO EFFECT ON AU(111)

4.1 Introduction

As the temperature decreases, an increase in resistance for some metals containing small fraction of magnetic impurities was discovered in the 1930s and referred to as the

Kondo effect. In the 1960s, this anomalous phenomenon was explained using a scattering model, which takes into account the interaction of the spins of the conduction electrons from the host metal with the localized spin of the magnetic impurities [38, 39]. Thanks to the invention of the scanning tunneling microscope, the Kondo effect can be detected at the single atom/molecule scale. As a result, the Kondo effect has revived and become a subject of interest for many experimentalists and theorists.

Kondo effect of molecules with magnetic atoms adsorbed on metallic substrates are of great interests. Some molecular Kondo systems studied with an STM are TBrPP-

Co/Cu(111) [21, 23], FePc/Au(111) [50], and Co-(5,5’-Br2-Salophen)/Au(111) [25]. By slightly changing the conformation of molecules, the interactions between magnetic atom and metallic substrate, and therefore Kondo effect can be greatly affected [21, 23, 50]. In the following, Kondo effect of TBrPP-Co molecules on a Au(111) substrate is systematically studied. The reconstruction patterns on the Au(111) substrate provides an ideal environment to investigate the effect of substrate on Kondo effect.

When a magnetic molecule is adsorbed on a metal surface, spin exchange interactions between the magnetic molecule and free conduction electrons from the metal substrate lead to a resonance (figure 4.1), called the Kondo resonance, which can be detected by dI/dV tunneling spectroscopy [40]. 43

Figure 4.1. A diagram shows the Kondo resonance. (A) When there are only free electrons, the spins of free electrons are randomly arranged. (B) When there is a net spin from magnetic impurities, the nearby free electrons (electrons in the red circle) interact with this net spin.

At a sufficiently low temperature [41], the Kondo resonance becomes significant and electrons from the tip now have two interfering channels to tunnel into the sample: 1, directly tunneling into continuous conduction-band states of the metal; 2, tunneling into the discrete orbital of the magnetic impurity [38]. Depending on the ratio (q) between these two channels [42], the shape of the resulting dI/dV tunneling spectroscopy can appear as a peak or a dip. The observed Kondo resonance can be characterized by fitting with a Fano line-shape [43] using the relationship of [23]:

! !" = � ! !!"!!! + � (4.1) !" !!!!

� = !"!!! (4.2) !!!! where ‘C’ and ‘a’ are constants, ‘εK’ is the energy shift from Fermi level, ‘kB’ is the

Boltzmann constant, and ‘TK’ is the Kondo temperature. The Kondo temperature reflects 44 the strength of the coupling between conduction electrons from the substrate and the localized spin of the magnetic impurity [44]:

! ! !" �! ∝ � (4.3) where ρ is the substrate density of states at the Fermi Level, and J represents the coupling strength between magnetic impurities and the conduction electrons.

4.2 Sample preparation

For the experiment, TBrPP-Co molecules are deposited onto an atomically clean

Au(111) substrate by sublimation at ~350 °C under UHV environment. The sample is then transferred to the sample stage in the STM, and cooled down to ~4.4 K for the measurements.

4.3 Properties of TBrPP-Co on Au(111)

Figure 4.2 shows an overview of TBrPP-Co deposited on Au(111).

10nm

Figure 4.2. A STM image (VT = -0.1 V, IT = 100 pA) of TBrPP-Co deposited on an Au(111) substrate.

Here, most TBrPP-Co molecules form ordered self-assembled rectangular clusters with one of the edges of the cluster aligning with a surface close-packed direction on

Au(111) (figure 4.3). The distance between nearest neighbors in the molecular clusters is

~1.4 nm.

Figure 4.3. Self-assembled TBrPP-Co cluster. (A) A STM image (VT = -0.1 V, IT = 50 pA) of TBrPP-Co cluster. (B) A model shows the structure of the cluster.

At low coverages, single molecules are preferentially adsorbed on elbows of the

Au(111) herringbone reconstructions (figure 4.2).

Here, TBrPP-Co molecules are adsorbed with the porphyrin unit lying flat on the surface of Au(111), with three rotation orientations, which are aligned along the surface close-packed directions [110] on Au(111). The apparent rotated positions of the molecule is caused by the different adsorption sites where the Co center of the molecules is positioned either on top, bridge, or three-fold hollow sites of Au(111) (figure 4.4), while the Bromine atoms are adsorbed in three-fold hollow sites and anchor the molecules [45].

Figure 4.4. STM images (VT = 1.0 V, IT = 100 pA) and corresponding ball-and-stick models showing TBrPP-Co adsorption sites and orientations. The white bars indicate 1nm. (A, B) Hollow site. (C, D) Bridge site. (E, F) Top site.

Electronic properties of single TBrPP-Co molecules are characterized by using dI/dV tunneling spectroscopy taken at the Co center of the molecules [46]. During the spectroscopic measurements, STM tip is positioned static above the molecule and feedback loop is turned off. Then the bias is ramped from – 1.5 V to +1.5 V with 1 mV steps while the corresponding tunneling current and differential tunneling conductance 48

(dI/dV) are recorded directly or through a lock-in amplifier. A small AC bias modulation with a frequency of 700 Hz and an amplitude of 10 mV is added to the tunneling voltage for the lock-in detection. The recorded dI/dV spectroscopy data reveal that the highest occupied molecular orbital (HOMO) is located at ~0.9 eV below the Fermi level (0 V), and the lowest unoccupied molecular orbital (LUMO) is located at ~1.57 V above the

Fermi level, figure 4.5(A). Therefore the HOMO-LUMO gap of the molecule is ~2.47 eV.

Figure 4.5. Electronic structure of single TBrPP-Co on Au(111). (A) A tunneling spectrum showing HOMO and LUMO of TBrPP-Co. (B) Tunneling spectra show different LUMO locations for TBrPP-Co at different adsorption sites.

While HOMO levels for molecules with different adsorption sites are nearly the same at ~-0.9 V, the LUMO orbital energies vary from 1.38 V for the top site, to 1.41 V for the bridge site, and 1.57 V for the hollow site (figure 4.5(B)). As a result, the HOMO-

LUMO gaps for molecules on top sites are 2.28 V, followed by 2.31 V on bridge sites, and 2.47 V on hollow sites, respectively. The smaller the HOMO-LUMO gap, the 49 stronger the molecule-substrate interactions and therefore, the spin-electron interactions between the substrate and the magnetic moment of the molecule are also stronger [47].

STM topography with different biases are taken on TBrPP-Co molecules at different adsorption sites. When images are taken with biases within the HOMO-LUMO gap, a single protrusion appear at the center of the molecule (figure 4.6(D, E, F)). When the bias is at -1 V, which is lower than the HOMO energy, the molecule appears as a two- lobe shape for all adsorption sites (figure 4.6(G, H, I)). When the bias is at 2 V, which exceeds the LUMO energy, the molecule appears as a four-lobe shape for all adsorption sites (figure 4.6(A, B, C)). The same effect has been observed when TBrPP-Co molecules adsorbed on Cu(111) substrates [21] 50

2 Figure 4.6. STM images (2.4 x 2.4 nm , IT = 200 pA) of TBrPP-Co on Au(111) recorded at (A, B, C) 2.0 V, (D, E, F) 1.0 V, and (G, H, I) -1.0 V, for the top, bridge and hollow adsorption sites, which show four-lobe, one-lobe and two-lobe structures respectively.

Energy-resolved tunneling maps are taken at each orbital energy to obtain further insight into the different appearances of TBrPP-Co molecules. Spatial distributions of molecular orbitals are directly visualized using these maps (figure 4.7(B, D)). Figure

4.7(B) is a dI/dV map taken at 1.7 V, which shows the LUMO as a four-lobe structure, while figure 4.7(D) is a dI/dV map taken at -1.0 V, which shows the HOMO as a two- lobe structure. The corresponding STM topography images are shown in figure 4.7(A, C). 51

The dI/dV maps clearly show that the different appearances of TBrPP-Co molecules are caused by the molecular orbitals.

Figure 4.7. Energy resolved dI/dV maps of the TBrPP-Co on Au(111) substrate. The white bars represent 1nm. (A) STM topography and (B) dI/dV map taken simultaneously at 1.7 V, IT = 300 pA. (C) STM topography and (D) dI/dV map taken simultaneously at - 1.0 V, IT = 300 pA.

4.4 Kondo resonance of TBrPP-Co on Au(111)

Asymmetric sharp peaks at Fermi level are observed in small range (from -100 mV to 100 mV) for tunneling spectra taken at the Co center of TBrPP-Co. As expected, these asymmetric peaks indicate the Kondo resonance, which is observed on TBrPP-Co adsorbed on Cu(111) substrates [21]. 52

A tunneling spectrum of Kondo resonance is shown in figure 4.8. For the TBrPP-

Co adsorbed on Au(111), the Kondo resonance is always measured as a peak shape in the dI/dV signals. Several important characterizations of Kondo resonance, such as Kondo temperature TK, and q are extracted by fitting the dI/dV spectra using equations 4.1 and

4.2 (figure 4.8).

Figure 4.8. A tunneling spectrum taken at the center (indicated by the red dot) of a single TBrPP-Co showing a sharp asymmetric peak at Fermi level.

Kondo resonances are systematically measured for TBrPP-Co adsorbed at different sites on Au(111). Different resonance widths for each adsorption site (figure 4.9

(A)) indicate different Kondo temperatures [23]. By fitting the spectra, it is found that

Kondo temperature for TBrPP-Co is 162 K on top sites, 140 K on bridge sites, and 119 K on hollow sites (figure 4.9 (B)), and the q value is ~0.4. The relative high Kondo temperature is because of the enhanced spin exchange interactions caused by the porphyrin molecules [22, 48]. 53

Figure 4.9. (A) Tunneling spectra showing Kondo resonance with different resonance widths for molecules on different adsorption sites. (B) Kondo temperature of TBrPP-Co at different adsorption sites.

Because the Kondo resonance is originated from the spin exchange interactions between the magnetic moment of the molecule located at the Co center, the Kondo amplitude should decrease as the tip moves away from the Co center.

Multiple tunneling spectra are taken along a line (about 6.8 Å long)(figure 4.10(A,

B)) from the Co center of a TBrPP-Co molecule to the Au(111) substrate to determine the relationship between the Kondo amplitude and the distance. 54

Amplitudes of Kondo resonance are extracted from spectra and calculated using equation 4.1, 4.2, and 4.3 [25].

! � = �(�)(1 + �! ) (4.3)

Figure 4.10. Tunneling spectra along a line showing 1/r relationship between the Kondo amplitude and distances from Co center. (A) A STM image of a single TBrPP-Co molecule. Dots indicates locations where tunneling spectra are taken. (B) Tunneling spectra taken at locations in (A). (C) A plot of Kondo amplitude as a function of distance.

The Kondo resonance amplitudes were plotted as a function of distance r (figure

4.10(C)). It is found that the Kondo amplitude decreases as the tip moves away from the central Co atom, following a 1/r trend [25], with a Kondo temperature of 132 K. 55

4.5 Switch Kondo resonance of TBrPP-Co on Au(111) by STM manipulation

Since the Kondo temperatures are dependent on the top, bridge, and hollow adsorption sites of the molecule on Au(111) surface, a further question to ask is what will happen if the molecules are adsorbed on the elbow of herringbone reconstructions. To answer this question, tunneling spectra are taken on TBrPP-Co molecules adsorbed on elbow sites. Surprisingly, there is no Kondo peak in the spectra near the Fermi level

(4.11(a)) for the molecules on elbow sites. Then these molecules are moved off from the elbow sites by the STM lateral manipulation with a tunneling resistance of ~ 2.0 MΩ, and a bias voltage of 20 mV. After relocating the molecule to a different site on the surface, tunneling spectra are taken again on the same molecule. Surprisingly, a Kondo peak can now be observed in the dI/dV tunneling spectra (figure 4.11(B)). When the same molecule is moved back to the elbow site, the Kondo peak disappears again in the dI/dV spectra (figure 4.11(C)). In this way, the observed Kondo resonance can be switched on and off by manipulating molecules off and onto the elbow site of the herringbone reconstructions (figure 4.11(D)). Additionally, when the same molecule is adsorbed on the top site, the bridge site, or the hollow site, the measured Kondo temperatures match well with the previously measured Kondo temperatures for the three adsorption sites

(figure 4.9(B)). 56

Figure 4.11. STM images and corresponding tunneling spectra showing Kondo resonance can be turned on and off by manipulating TBrPP-Co molecules on and off the elbow of a herringbone reconstruction. VT = -0.1 V, IT = 100 pA. The white arrows indicate the directions of lateral manipulation, which lead to following images. (A) No Kondo resonance when the molecule on elbow. (B) Kondo resonance appears when the molecule is manipulated off the elbow. (C) Kondo resonance disappeared when the same molecule is moved back to the elbow. (D) Kondo resonance reappeared once moved off the elbow. 57

4.6 Discussion and summary

In order to explain the observed Kondo switching, the Au(111) substrate, herringbone reconstructions and a TBrPP-Co molecule are modeled to find out the adsorption configuration of the molecule on an elbow (figure 4.12). It is found that the elbow of the herringbone is a less symmetric location on the surface, and it is also the highest point on the same terrace. It has been reported that Kondo resonance strongly depends on the adsorption site of the molecule on a Au(111) surface [49, 50]. One possible reason of the disappearance of Kondo resonance is that the spin-electron interaction is reduced due to the asymmetry and protrusion of the elbow of herringbone reconstruction, thus the Kondo resonance cannot be observed by the tunneling spectroscopy.

Figure 4.12. A STM image (VT = -0.1 V, IT = 100 pA) overlaid by a model of Au(111) herringbone reconstruction. Light blue dots represent relocated atoms, and dark blue dots represents HCP or FCC regions. The orange dot indicates the Co center of TBrPP-Co molecule. 58

As a summary, the Kondo resonance of TBrPP-Co molecules on a Au(111) substrate is systematically investigated in this chapter. It is found that on Au(111) substrate, single TBrPP-Co molecules are located at three different adsorption sites: top site, bridge site, and hollow site. By measuring tunneling spectra, Kondo resonance is observed on all three adsorption sites. Statistical analysis shows that the Kondo temperatures are dependent on the adsorption sites. Surprisingly, no Kondo resonance is observed when the TBrPP-Co molecule is adsorbed on the elbow site of the Au(111) herringbone reconstruction. It is found that the Kondo resonance on the Au(111) surface can be repeatedly turned on and off by manipulating the molecule with the STM tip away from or onto the elbow site.

CHAPTER 5: ANOMALOUS KONDO EFFECT IN TBRPP-CO/AGNR/AU(111)

HETEROSTRUCTURES

5.1 Introduction

Recently, AGNRs adsorbed on different substrates, such as Au(111) [51], Cu(111)

[52] and Ag(111) [34] have been intensely studied [53-57]. In order to further understand the properties of AGNRs and to explore potential applications, it is necessary to investigate vertically stacked heterostructures made of AGNR, metal substrates and functional molecules. This chapter is focused on the structural, electronic and spintronic properties of heterostructures formed by magnetic TBrPP-Co molecules on AGNRs on the Au(111) surface.

TBrPP-Co molecules are selected to build heterostructures for several reasons.

First, we want to find out if such vertically stacked heterostructures can be synthesized.

Second, we want to investigate how TBrPP-Co molecules are adsorbed on AGNRs and to find out if they form self-assembled structures, as on Au(111) surface. Third, it is important to measure HOMO and LUMO of TBrPP-Co on semiconducting AGNRs, and compare with electronic structure of TBrPP-Co adsorbed on Au(111). Finally, we want to find out if there is Kondo resonance in such heterostructures. As discussed in the previous chapter, on a Au(111) substrate, the interactions between the net spin from

TBrPP-Co molecules and free conduction electrons from the substrate lead to the Kondo resonance. Since the spin-electron interaction is necessary for the Kondo effect, if the magnetic moment of the molecule is electronically decoupled from the substrate, then the

Kondo resonance should not be expected. It is known that there is no charge transfer 60 between the AGNR and Au(111) [37]. So there is no additional charges in AGNR, and therefore an effective electronic decoupling by AGNR can be expected.

5.2 Synthesis of TBrPP-Co/AGNR/Au(111) heterostructures

First, AGNRs were synthesized on a Au(111) substrate using a bottom-up process, where DBBA (10-10’-dibromo-9,9’-bianthryl) precursor molecules were used as building blocks. After the AGNRs are synthesized, the sample is transferred into STM to check the quality and coverage of AGNR. In order to fabricate heterostructures, defect-free

AGNRs with long lengths are necessary. The temperature, pressure, and annealing time are critical parameters of the synthesis process and are closely monitored. Parameters to synthesize high quality AGNRs are obtained after several trials.

Once AGNRs with desired quality and coverage are obtained, the next step is to deposit TBrPP-Co molecules onto AGNRs. TBrPP-Co molecules are deposited by thermal evaporation from a custom built Knudson cell onto the AGNR/Au(111) surface.

Then the sample is transferred back to STM and cooled down to ~4.4 K for the experiments.

Initially, TBrPP-Co molecules are deposited onto the substrate with low coverage

AGNRs. As shown in figure 5.1, it is found that TBrPP-Co molecules are preferentially adsorbed on Au(111) substrate rather than on top of the AGNRs, which indicates that the

TBrPP-Co-Au(111) interaction is stronger than TBrPP-Co-AGNR interaction. At regions with low coverage, single TBrPP-Co molecules are adsorbed on Au(111) substrate with three adsorption configurations, the same as discussed in Chapter 4. At regions with 61 higher coverage, the TBrPP-Co molecules form self-assembled structures on Au(111) substrate.

Figure 5.1. TBrPP-Co and AGNRs at low coverage. (A) An STM image showing self- assembled TBrPP-Co molecules are gown between AGNRs on Au(111) (VT = -0.1 V, IT = 30 pA). (B) An STM image of a single TBrPP-Co molecule adsorbed on Au(111) beside an AGNR (VT = -0.2 V, IT = 30 pA).

AGNRs are known to weakly adsorb on Au(111) surface and they have been shown to exhibit superlubricity on Au(111) [56, 58]. When trying to manipulate single

TBrPP-Co molecules onto the AGNRs (figure 5.2), instead of moving the molecule onto the top of AGNR, the TBrPP-Co molecule pushes the whole AGNR network away

(figure 5.2(B)).

Figure 5.2. Manipulating TBrPP-Co toward a AGNR using lateral manipulation. (A) An STM image taken before manipulation (VT = 1.0 V, IT = 100 pA). (B) An STM image taken after manipulation (VT = 1.0 V, IT = 100 pA), where the TBrPP-Co molecule pushed the whole AGNR away. The original positions of the TBrPP-Co molecule and AGNR are indicated by the blue and red dashed contours separately. The manipulation direction of the TBrPP-Co molecule and the moving direction of AGNR are indicated by blue arrow and red arrow separately. The manipulation parameters are VLM = 0.1 V, RLM = 1.0 MΩ. The 2 x 2 nm2 grid lines are drawn for eye guidance.

This manipulation experiment clearly highlights the weak interaction between the

AGNRs and the Au(111) surface. This weak interaction also leads to diffusion of the

AGNR even during scanning with the STM tip. Figure 5.3 presents a sequence of STM images where an AGNR has been moving along its long axis direction back and forth 63 thereby appearing as a long strip. In order to obtain stable images, a low tunneling current is necessary to keep the tip far from the AGNRs, and to reduce the tip-AGNRs interactions.

Figure 5.3. STM images (VT = 2.0 V, IT = 50 pA) shows a same AGNR with different lengths of (A) 12.4 nm; (B) 4.3 nm; (C) 18.9 nm; (D) 11.0 nm, which indicated very weak interactions between AGNRs and Au(111) substrate.

Since TBrPP-Co molecules are preferentially adsorbed on bare Au(111) surface areas, it is difficult to realize a vertically stacked heterostructures formed by TBrPP-Co on top of AGNRs. To circumvent this, we form a highly dense AGNR layer on Au(111), which denies the TBrPP-Co to access bare Au(111) surface areas. Deposition of TBrPP- 64

Co on such densely covered Au(111) surface by AGNR enables the formation of vertically stacked TBrPP-Co/AGNR/Au(111) heterostructures (figure 5.4).

Figure 5.4. TBrPP-Co on AGNRs with different adsorption configurations. (A) An STM image of TBrPP-Co molecular self-assembly on top of AGNR (VT = 0.4 V, IT = 10 pA). The overlaid model show the structure of TBrPP-Co molecules in the cluster. (B) An STM image of a single TBrPP-Co molecule adsorbed on the edge of a AGNR (VT = 0.5 V, IT = 10 pA). (C) An STM image of TBrPP-Co molecules adsorbed on the DBBA polymers (VT = 2.0 V, IT = 50 pA).

In the TBrPP-Co/AGNR/Au(111) heterostructures, most TBrPP-Co molecules form self-assembled clusters along the direction of AGNRs (figure 5.4(A)). Some

TBrPP-Co molecules are located next to the second layer of DBBA polymers (figure

5.4(C)). In addition, some TBrPP-Co molecules are found to adsorb half way on AGNRs while the other half of the molecule is on the Au(111) substrate thereby appearing as only half of the molecule in STM images (figure 5.4(B)).

5.3 Properties of TBrPP-Co/AGNR/Au(111) heterostructures

Like on Au(111), TBrPP-Co molecules are adsorbed on AGNRs with planar shapes. The vertical distance between the molecule and the AGNR is 4.1 Å, while the

AGNR is located at 3.4 Å above the Au(111) substrate. So the TBrPP-Co is located at 7.5

Å above the Au(111) substrate. In contrast to the molecules directly adsorbed on Au(111),

TBrPP-Co molecules here are weakly adsorbed on AGNRs. This is evident during STM imaging where the molecules can be easily relocated even at a low tunneling current of

~10 pA. In order to obtain a stable STM image of single TBrPP-Co molecule on AGNR, an extremely small tunneling current of ~1 pA is necessary. By taking advantage of this weak molecule-AGNR interaction, single TBrPP-Co molecules can be extracted from self-assembled molecular clusters on AGNRs using lateral manipulation to investigate their electronic and spintronic properties.

To determine the adsorption site of TBrPP-Co on AGNR, high resolution heterostructure images were taken to build models of AGNRs and TBrPP-Co on top. In order to get a high resolution image, it is necessary to bring the tip close to the sample.

However, because of the TBrPP-Co molecules are weakly bound to the AGNRs, it is 66 impossible to directly take high resolution images of a heterostructure. To solve this problem, high resolution images are taken on the AGNR next to the molecule. Then an atomic model is built on the AGNR, which is then extended over the molecule (figure

5.5). This scheme enables to identify the adsorption site of TBrPP-Co on AGNR and it is found that the TBrPP-Co prefers to position its Co center above the center of the honeycomb of AGNRs (figure 5.5), and the distance between neighboring adsorption sites is 4.17 Å.

Figure 5.5. STM images (VT = 1.0 V, IT = 10 pA) overlaid by models of AGNR and TBrPP-Co models. The upper area, indicated by the blue rectangle, is a high resolution STM image (VT = 0.01 V, IT = 200 pA) of AGNRs. 67

Because TBrPP-Co molecules are always adsorbed on their preferred sites, lateral manipulation can be used to determine the distance between the two preferred adsorption sites. For this purpose, a single TBrPP-Co molecule is manipulated along a 7-AGNR, with a large tunneling resistance of 5000 MΩ and a manipulation voltage of -1.0 V

(figure 5.6(A, B)). As shown in figure 5.6(C), each segment of the manipulation curve indicates the TBrPP-Co molecule hopped from one adsorption site to the next one. So the horizontal length of a segment corresponds to the distance between two preferred adsorption sites. Using this way, it is found that the distance between two adsorption sites is 4.15 Å, which is in a good agreement with the adsorption sites found in the model.

Figure 5.6. Measurement of distance between preferred adsorption sites by manipulating single TBrPP-Co molecules on AGNRs. (A) An STM image of a single TBrPP-Co molecule on a 7-AGNR before manipulation (VT = 1.0 V, IT = 1.0 pA). The blue arrow indicates the manipulation direction. (B) An STM image of the molecule after manipulation (VT = 1.0 V, IT = 1.0 pA). (C) Lateral manipulation curve, which shows the distance between preferred adsorption sites is ~4.15 Å. (D) Models of AGNR and TBrPP-Co.

Next, we use dI/dV tunneling spectroscopy to investigate the electrical properties of the heterostructures (figure 5.7). Because of the extremely small interactions between

TBrPP-Co molecules and AGNRs, tunneling spectra are taken on TBrPP-Co molecules in the self-assembled clusters. Long range tunneling spectra show that the HOMO is located at -1.2 V and the LUMO is located at 1.61 V, which gives an energy gap of 2.81 eV. 69

For molecules on AGNRs, when the bias voltage is within the energy gap, the intensity of dI/dV spectrum abruptly goes to almost zero, and appears completely flat; while on Au(111) the intensity of dI/dV spectrum within the band gap doesn’t reduce to zero. This indicates that the TBrPP-Co molecules are electronically decoupled from

AGNRs and the Au(111) substrate. Comparing to the similar dI/dV spectra of TBrPP-Co directly adsorbed onto Au(111), the energy gap of TBrPP-Co on AGNR is found to be slightly larger (figure 5.7), which also indicates weaker interactions between TBrPP-Co and AGNR [59]. From another point of view, since the AGNR is a semiconductor with a large band gap and is weakly adsorbed on the Au(111) substrate, and the TBrPP-Co molecules located on top of AGNRs are also weakly adsorbed on AGNRs, the electronic decoupling between the TBrPP-Co and the Au(111) substrate could be expected [60].

Like in the case of TBrPP-Co on Au(111), TBrPP-Co on AGNR taken at different biases exhibit a two-lobe structure when the energy of bias voltage is lower than the HOMO energy, a four-lobe structure for the tunneling biases higher than the LUMO energy, and single protrusion for the biases within the energy gap.

Figure 5.7. Tunneling spectra of TBrPP-Co molecules on AGNR (blue), and on Au(111) (Red). The yellow bar and red bar indicate zero value of each dI/dV tunneling spectra. The HOMO and LUMO of TBrPP-Co on AGNR appear sharper and the spectrum within the gap between the HOMO and LUMO are flatter comparing to the case of TBrPP-Co on Au(111).

Because the experimental results suggested a very weak electronic coupling between TBrPP-Co molecules and the Au(111) substrate because of the intermediate

AGNR layer, a much weaker Kondo resonance can be expected. However, surprisingly, a sharp peak at the Fermi level is clearly observed (figure 5.8(B)) in the short range (-100 mV-100 mV) dI/dV tunneling spectra. By fitting this spectrum, the Kondo temperature is measured as 144 K.

Figure 5.8. Kondo resonance of TBrPP-Co/AGNR/Au(111) heterostructure. (A) An STM image (VT = 1.0 V, IT = 3.0 pA) of a heterostructure formed by three TBrPP-Co molecules. The blue dot indicates the location where the tunneling spectrum was taken. (B) A tunneling spectrum showing the Kondo resonance at the Fermi level.

The Kondo resonances measured on multiple TBrPP-Co molecules on AGNR reveal that the Kondo temperatures fall into three levels. The measured Kondo temperatures of 145.6 K, 128.2 K, and 107.6 K (figure 5.9(A)) are very close (97%) to the three Kondo temperatures measured for TBrPP-Co molecules on Au(111) substrate

(figure 5.9(B)). The large q value (~250), and the almost symmetric shape of the Kondo resonance for the TBrPP-Co molecules on AGNR indicate that the electrons are mainly tunneling through the magnetic impurities, which is in good agreement with the picture of decoupling from TBrPP-Co and Au(111) substrate.

Figure 5.9. (A) Tunneling spectra of TBrPP-Co on AGNRs showing different Kondo temperatures. (B) Kondo temperatures of TBrPP-Co on AGNRs is slightly smaller than the case of TBrPP-Co on Au(111), and showing the same trend at different adsorption sites.

It is known that when a magnetic impurity is separated from a metal substrate by an atomic thick insulating layer, the Kondo resonance can still be detected with a much smaller Kondo temperature. For example, when Co atoms are adsorbed on a single layer of CuN, which is insulating and adsorbed on top of Cu(100) substrate, the Kondo temperature is found to be ~2.6 K, which is just ~3% of the Kondo temperature of ~88 K when the Co atoms are directly adsorbed on Cu(100) [61]. It is also known that for

TBrPP-Co molecules on a Cu(111) substrate, by slightly lifting up the Co atom, or by changing the density of the free electrons from the substrate, the Kondo temperature can 73 be largely influenced [21, 23]. Therefore, to observe almost the same Kondo temperatures of TBrPP-Co molecules on AGNR as the ones directly adsorbed on Au(111) substrate is a very surprising finding. According to equation 4.3, both the Kondo resonance and Kondo temperature are very sensitive to the density of states of free electron ρ and the exchange coupling to the magnetic impurity J. In order to induce such a strong Kondo resonance, the spin-interaction between the Co center of the molecule and the free electrons should also be large. The observed high Kondo temperature of TBrPP-

Co on AGNR indicates that the spin coupling between the TBrPP-Co molecules and surface conduction electrons through AGNR is almost the same as TBrPP-Co molecules directly adsorbed on Au(111) [62], although the TBrPP-Co is separated from the substrate by AGNR.

In order to obtain further insights into the Kondo resonance of TBrPP-Co molecules on AGNRs, a series of tunneling spectra are taken along a line from the Co center of a TBrPP-Co molecule on a AGNR (figure 5.10(A)), and the distance dependent

Kondo amplitudes are calculated and analyzed. As expected, the Kondo amplitude decreases as the tip position moves away from the Co center following a 1/r trend with

Kondo temperature of ~133 K, for all five curves shown in the figure 5.10(C).

Figure 5.10. 1/r measurements of Kondo amplitude of a TBrPP-Co molecule on AGNRs. (A) An STM image (VT = 0.2 V, IT = 10 pA) of a TBrPP-Co on AGNRs. The dots indicate the locations where the tunneling spectra are taken. (B) Tunneling spectra taken at different distance from the Co atom of TBrPP-Co. (C) The normalized Kondo amplitudes decay with 1/r trend.

To visualize the spatial distribution of the Kondo resonance, a 2-D grid of dI/dV tunneling spectra are taken on a TBrPP-Co molecule on an AGNR (figure 5.11(A)). A dI/dV map at 7 mV is extracted from the grid spectra (figure 5.11(B)), showing the

Kondo resonance is located at the center of TBrPP-Co molecule, where the Co atom and four N atoms are located.

Figure 5.11. Distribution of Kondo resonance. (A) An STM image (VT = 0.4 V, IT = 10 pA) of a TBrPP-Co molecule on AGNRs. (B) A tunneling spectroscopy map of the same area of (A) taken at 7 mV showing the Kondo resonance is located at the center (white circle) of TBrPP-Co molecule. The white bars represents 0.5 nm.

Kondo resonance is observed for all TBrPP-Co molecules located on top of the

AGNRs, and the Kondo temperatures are independent of the widths of AGNRs. To prove the important role of AGNR, a TBrPP-Co molecule is positioned between two neighboring AGNRs (figure 5.12(A)), with no AGNR between the molecule’s Co atom and Au(111) substrate. In this case, no Kondo resonance was observed in the tunneling spectrum (figure 5.12(B)). Tunneling spectra were also measured on the TBrPP-Co molecules adsorbed on the edge of AGNRs (figure 5.12(C)), where the Co atom of the molecule was above the hydrogen atom, and no Kondo resonance was observed (figure

5.12(D)). These findings are in agreement that the AGNRs play an important role in this

Kondo resonance. In other words, the AGNRs are spintronically transparent while electronically opaque.

Figure 5.12. Disappearance of Kondo resonance. (A) An STM image (VT = 0.5 V, IT = 1.0 pA) showing a TBrPP-Co molecule adsorbed between two 7-AGNRs. (C) An STM image (VT = 0.5 V, IT = 10 pA) showing a TBrPP-Co molecule adsorbed on the edge of a 7-AGNR. (B, D) Tunneling spectra taken at the blue dot in (C, A) shows no Kondo resonance respectively.

In Appendix A, the electronic structures calculated using spin polarized density functional theory (SP-DFT) confirm that the AGNRs mediate the spin exchange coupling between magnetic moment from TBrPP-Co and free electrons from the Au(111) substrate.

If the AGNRs are removed from the heterostructures, there will be no spin exchange between TBrPP-Co molecules and the substrate. 77

5.4 Discussion

In summary, vertically stacked heterostructures of TBrPP-Co molecules, AGNR, and Au(111) substrate are successfully synthesized for the first time using a bottom-up synthesis process. Structural, electronic and magnetic properties of the TBrPP-Co molecules adsorbed on AGNR are investigated using low temperature STM, lateral manipulation, tunneling spectroscopy and spin-polarized DFT. The obtained results have been compared with the results of the same molecules directly adsorbed on a Au(111) substrate. The tunneling spectra clearly show the electronic decoupling of the TBrPP-Co molecules from the Au(111) substrate by AGNR. However, the strong Kondo resonance observed by the tunneling spectroscopy indicates strong spintronic coupling between the magnetic moment and free surface electrons mediated by AGNRs. The Kondo amplitude decreases following 1/r relationship as the probe moves away from the magnetic center.

Three different Kondo temperatures due to adsorption sites on Au(111) are reproduced on

AGNRs. Kondo resonance is known to be very sensitive to the spin coupling, and a previous study has shown that the Kondo temperature was changed by ~30% [21], with a slight displacement (0.6 Å) of the Co atom from the substrate. However, very surprisingly, in this experiment, the Kondo temperature of TBrPP-Co on AGNR is almost identical to TBrPP-Co directly adsorbed on Au(111) substrate, although the TBrPP-Co molecules are located 7.5 Å above the substrate by the AGNRs. It has been confirmed that the AGNRs are playing important roles to mediate the spin coupling between TBrPP-

Co and free electrons from the Au(111) substrate, which is confirmed by SP-DFT 78 calculations. The findings of AGNRs mediating spin coupling might inspire new applications of AGNRs in spintronics and single molecular sensors.

CHAPTER 6: MOLECULAR SUPERLUBRICITY OF TBRPP-CO ON AGNRS

6.1 Introduction

Friction is one of the oldest physical phenomena known to humans, and it has been studied and applied for thousands of years. However, the origin and mechanism of friction is still unclear because friction is influenced by many factors in a complicated manner, and it has been difficult to separate these factors [63]. When two solid bodies contact and slide against each other, friction occurs. The friction force is the drag against sliding, appearing along the contacting surfaces, and the direction is opposite to the moving direction of the object, or the force applied on the object. Friction can cause additional energy, generate extra heat, which can affect an instrument’s performance, and even reduce its lifetime. As a result, friction needs to be eliminated in many applications.

Nowadays, with the development of nanoelectromechanical systems (NEMS) and nano- rotors, it becomes more and more important to understand the mechanism of friction at the atomic scale. STM and AFM offer advantageous ways to measure frictional forces.

The atomically clean and flat surface, UHV, and extremely low temperature provide an ideal environment to investigate the mechanism of friction at the molecular/atomic scale.

6.2 Superlubricity

The state where the friction between two solids is zero and the solids slide without resistance is called superlubricity [64], which is predicted first by theory in the 1990s

[63]. At an atomistic level, when two solids slide against each other, the surface atoms from one solid can feel the periodic potential of the other solid, and the frictional force is the sum of all forces received by individual atoms from neighboring atoms along the 80 sliding direction. If the total force is not zero, there will be friction force, and otherwise, there will be no friction. If the ratio of the lattice spacing of two solids is a rational number, the sum of forces will be non-zero and the friction will exist. When the lattice spacing ratio of two solids is an irrational number (incommensurable), the magnitude and direction of the forces received by the atoms will cancel each other and will lead to zero friction [65]. So superlubricity is highly dependent on the structure of two solids. For two solids with incommensurate structures, as the contact size between solids increases, the friction force per unit of contact area decreases, and finally goes to zero. Superlubricity has been predicted when sliding two graphene sheets, and the incommensurability is achieved by rotating the graphene sheet against each other [65].

Superlubricity has been observed in several systems [66-71]. A well designed experiment has shown superlubricity when sliding a piece of graphite flake against a graphite sheet [66]. The orientation of the sample can be rotated with respect to the graphite flake, and the frictional force was measured at each angle. The friction forces

(~203 pN) measured at 0° and 60° are over one order more than friction force (15.2 pN) measured at other angles, which shows a strong orientation dependence of the friction force [66]. The extremely small friction force indicated superlubricity that is caused by the incommensurability between the graphite flake and graphite layer.

Macroscale superlubricity [69, 71] has been observed in graphite, by combining graphene with nanodiamond particles and diamond-like carbon (DLC) [69]. Graphene wrapped around nanodiamond particles, which increased the incommensurability when sliding against the DLC surface, reduced the friction coefficient by one order of 81 magnitude from ~0.04 to ~0.004, and led to superlubricity. Recently, superlubricity of

AGNR on a Au(111) substrate has been discovered using qPlus AFM [58], where the friction force Fstat ~ 105 pN. This force is exceptionally low, considering that the force needed to move a single atom is in the range of ~210 pN [20], and the size of the AGNR is much larger than a single atom.

Since superlubricity is highly related with the structure and size of materials, it is important to explore the size limit of the superlubricity effect. In this chapter, friction and superlubricity of single molecules will be discussed. ref

Because AGNR shows superlubricity on Au(111), it is natural to use AGNR as the substrate for this project. TBrPP-Co molecules are selected to be studied, because their planar structures are incommensurate with AGNRs.

In addition, during scanning, it was noticed that even with extremely small tunneling current (~10 pA), the TBrPP-Co molecules can still be easily moved along

AGNRs by the scanning tip (Figure 6.1(A, B)), which indicates very small friction between TBrPP-Co and AGNRs.

Figure 6.1. (A) An STM image taken at a small tunneling current (VT = 1.0 V, IT = 10 pA) showing a single TBrPP-Co molecule (lower middle) sliding along a 7-AGNR during scanning. (B) An STM image (VT = 2.0 V, IT = 1.0 pA) of a single TBrPP-Co on a 7-AGNR showing the molecule is slightly rotated by the scanning tip.

6.3 Sample preparation

In order to measure the friction force between a single TBrPP-Co molecule and a

Au(111) substrate, TBrPP-Co molecules were thermally evaporated on the the Au(111) as described in Chapter 4. Because of the molecular interactions, most TBrPP-Co molecules formed self-assembled clusters. Then single TBrPP-Co molecules were extracted from the clusters, and moved to empty Au(111) substrate using lateral manipulation.

To obtain single TBrPP-Co molecules on AGNRs, first, vertically stacked structures of TBrPP-Co, AGNR and Au(111) were synthesized using the surface catalyzed reaction process described in the Chapter 5. Single TBrPP-Co molecules were carefully extracted from molecular clusters and manipulated onto 7-AGNR with an extremely small manipulation current of ~100 pA, and a large tunneling resistance of

~500 MΩ. 83

6.4 Force measurement

6.4.1 Force measurement using STM lateral manipulation

As described in Chapter 2, STM lateral manipulation can be used to position single atoms or molecules on a substrate. To move a molecule adsorbed on a surface, a lateral force large enough to overcome the friction force needs to be applied. During the manipulation, the feedback loop is always on.

As shown in figure 6.2, initially, the STM tip is brought close to the molecule until the tunneling current reaches the set point (ILM). The force between the tip and the molecule is F. Then the tip moves along the contour of the molecule with a constant ILM, and the angle � decreases. Because of the relationship shown in equation 2.3, the lateral force (FX) gradually increases. When FX overcomes the friction force (f) between the molecule and the substrate, the molecule hops toward the next preferred adsorption site, and f can be represented by FX, which is described in equation 6.1 [12].

f = F! = �× cos �! (6.1)

Figure 6.2. A diagram shows the geometries of forces (F, FX, f), and threshold angle �!. 84

The shape of a single atom or molecule adsorbed on a substrate can be approximately treated as a circular shape. As a result, the threshold angles can be found by fitting the segments of lateral manipulation curves using circles.

Tip-sample distance is important to find the total force between the tip and the sample. This distance can be found from the relationship between the tunneling resistance and the tip-sample distance (R-Z curve). R-Z curve is obtained by measuring the tunneling resistance and tip height while the tip approaches the sample. In this process, the tunneling resistance decreases as the tip-sample distance decreases, and is fitted by a linear function (figure 6.3). The fitting line is extended to cover a larger range of tip- sample distance. When the tip and the sample are in direct contact, the tunneling resistance drops, as shown by the arrow in figure 6.3, and the corresponding tip height is defined as 0 nm. Using this R-Z curve, the tip-sample distances corresponding to different tunneling resistances are found. 85

Figure 6.3. A plot shows the relationship between tunneling resistances and tip-sample distances. The abrupt decrease in tunneling resistance at 0 distance indicates that the tip is directly in contact with the sample.

As shown in Appendix B, the values of the force between the tip and the sample are calculated using density functional theory (DFT), as a function of the tip-sample distance. By using the tip-sample distance, and the critical angles obtained from experiments, the value of the friction force between the sample and the substrate can be obtained. 86

6.4.2 Force measurement using qPlus AFM

QPlus Atomic Force Microscope offers another way to measure the friction force between molecules and the substrate. During operation, the cantilever of the qPlus AFM is vibrating with a constant amplitude (~100 pm) and the resonant frequency is measured.

If any force gradient is applied on the cantilever, there will be a shift in the resonant frequency. When the vibration amplitude is small, the force gradient during the vibration can be treated as a constant, and the relationship between the frequency shift and the force gradient can be described as equation 2.3.

∆� = � !!!"(!)! (2.3) ! !!

To measure the force to move a molecule on a substrate, the qPlus sensor scans across the surface in a constant height mode, with feedback loop open, and the resonant frequency of the qPlus sensor is recorded during the manipulation, as a function of lateral and vertical positions. To avoid crashing the tip into the tilted sample, it is important to define a scan surface which is in parallel to the substrate before starting measurements.

To avoid the interactions due to the electric field, the bias voltage between the tip and the sample is set to 0 V. The measurements start from a large tip-sample distance, where there is no shift between resonant frequency and eigenfrequency f0. The measurements are repeated while progressively decreasing the tip-sample distance, until the molecule hops to the neighboring preferred adsorption site. When the molecule is moved, there is an abrupt change in the frequency shift. Using equation 2.3, the junction force gradient can be calculated as a function of vertical and lateral distances from the recorded frequency shift spectrum. 87

The vertical force FZ can be obtained by integrating the junction force gradient along the Z direction. As a result, the calculated FZ represents an average force on the cantilever during an oscillation cycle. The vertical force FZ is a combination of two components: one is the background force FB, and the other is the tip-sample interaction force FZ*, which is due to the interaction between the tip and the adsorbate [7]. The background force gradually increases as the tip approaches the substrate, but doesn’t depend on the lateral positions. FZ* increases quickly as the tip approaches the molecule, and depends on the lateral location of the scanning tip. Because the tip-sample interaction force is non-dissipative when the adsorbate doesn’t hop [7], the tip-sample interaction potential U can by calculated by integrating FZ along Z. Then the lateral force along tip scanning direction can be obtained by differentiating the tip-sample interaction potential along the manipulation direction.

6.5 Friction force of TBrPP-Co on Au(111) and TBrPP-Co on AGNRs

First, friction force between TBrPP-Co and Au(111) is measured using STM lateral manipulation. For this, single TBrPP-Co molecules are laterally manipulated along a close-packed surface direction, i.e. [110] surface direction, of Au(111)repeatedly, and the corresponding lateral manipulation curves are recorded (figure 6.4). Such measurements are repeated for different tip-sample distances. Then the lateral manipulation curves are fitted using circles to find the threshold angles, where the lateral component of total tip-sample force equals to the friction force. Because the friction force is a constant for the same molecule on Au(111), as the tip is getting closer to the molecule, the total force and the threshold angle increase, and the threshold lateral force 88 is always a constant, which equals the friction force. By analyzing the lateral manipulation curves at different tip-sample distances, it is found that the threshold angle increase linearly as the tip approaches the sample [12].

Figure 6.4. Later manipulation of a single TBrPP-Co molecule on Au(111). (A) An STM image (VT = 1.0 V, IT = 100 pA) before manipulation. The red arrow shows the direction of manipulation. (B) An STM image ((VT = 1.0 V, IT = 100 pA) after manipulation.

Then the friction forces for single TBrPP-Co molecules on AGNRs are measured using STM lateral manipulation. Single TBrPP-Co molecules are repeatedly manipulated along AGNRs, as the preferred sliding direction and lateral manipulation curves are recorded (figure 6.5). Such manipulations are repeated at different tip-molecule distances.

Again, the threshold angles for different tip-sample distances are extracted by fitting the 89 manipulation curves using circles. It is found that the threshold angles increase linearly as the tip-sample distances get smaller (figure 6.6).

Figure 6.5. Later manipulation of a single TBrPP-Co molecule on 7-AGNRs. (A) An STM image (VT = 2.0 V, IT = 1.0 pA) before manipulation. The white arrow shows the direction of manipulation. (B) An STM image (VT = 2.0 V, IT = 1.0 pA) after manipulation.

In comparison to the threshold angles for TBrPP-Co on Au(111), within the range of the measured tip-molecule distance, the threshold angles for TBrPP-Co on AGNRs are always larger, which indicates the friction force for TBrPP-Co molecules on AGNR is smaller than on Au(111) (figure 6.6).

Figure 6.6. Plots of threshold angles as a function of tip-sample distances for single TBrPP-Co molecules on AGNR and Au(111) separately.

Using the calculated total tip-molecule force as a function of the tip-molecule distance, the force values can be extracted (Appendix B). Then the friction force can be calculated using equation 6.1. The friction force on AGNR is ~2.5 pN, which is about 2 orders less than the friction force ~250 pN on Au(111). Such a minute frictional force required to move the molecule on AGNR constitutes superlubricity behavior.

In order to verify the friction force measured using the STM lateral manipulation, qPlus AFM is used to independently measure the friction force for TBrPP-Co on Au(111) separately. The eigenfrequency of the cantilever used is 30.4 kHz, and the stiffness is

~1800 N/m. Then starting from a far tip-sample distance, where no tip-sample force can 91 be detected by the cantilever (figure 6.7(A, B)(1)), the qPlus AFM tip scans across the

TBrPP-Co molecule along one of the close-packed directions, with frequency shift recorded as a function of lateral positions. Such measurements are repeated, as the tip approaches the molecule. Initially, only long range tip-substrate force can be detected, which shows a constant frequency shift(figure 6.7(A, B)(2)). Then short range tip- molecule interaction force starts to be detected as a dip in frequency shift (figure 6.7(A,

B)(3)). As the tip moves towards the molecule, the frequency shift increases, and as the tip moves away, the frequency shift decreases.

Figure 6.7. A diagram shows frequency shift measurement using a qPlus AFM. (A) The AFM tip scans across the sample at constant heights, which is repeated at different tip heights. (B) Resonant frequency shift are recorded as functions of lateral distances. Curve 1 shows no frequency shift; curve 2 shows a constant frequency shift due to long-range interactions; the dip at the center of curve 3 shows the frequency shift due to short-range interactions.

As the molecule is manipulated to the next adsorption site, there is an abrupt decrease in the frequency shift (figure 6.8) located 2.5 Å away from the center of the 92 molecule. This is the threshold tip location, where the lateral component of tip-sample force is equal to the frictional force.

Figure 6.8. A frequency spectroscopy shows an jump (black dashed line on the right) due to the movement of the TBrPP-Co molecule. The dashed blue line is the lateral manipulation curve before the molecule is moved; and the solid blue line is the lateral manipulation curve where the molecule is moved. The dashed black line on the left indicates the location of molecule, and the dashed black line on the right indicates the jumping point.

The force gradient of the tip-molecule junction is calculated using equation 2.3

(figure 6.9(A)). By integrating the force gradient along the vertical direction, the total tip- sample force (figure 6.10(B)) for different tip-sample distances can be obtained, and by integrating total tip-sample force along the vertical direction, the potential energy (figure

6.9(C)). Then the lateral force is calculated by differentiating the potential curve along 93 the horizontal direction. The threshold lateral force is found using the threshold tip location.

Using qPlus AFM, the measured friction force of TBrPP-Co on Au(111) is about

271.4 pN (figure 6.9(D)), which is in a good agreement with the friction force measured by STM lateral manipulation ~250 pN.

Figure 6.9. (A) Force gradient of tip-molecule junction. (B) Vertical component of tip- molecule interaction force. (C) Tip-molecule potential energy. (D) Lateral force between the tip and the molecule. Jumping points are indicated by red arrows.

6.6 Discussion

In this chapter, friction forces are measured for single TBrPP-Co molecules on

Au(111) and on AGNRs. STM lateral manipulation and DFT calculation are used to measure the friction forces, which are confirmed by qPlus AFM measurements.

Superlubricity is found for TBrPP-Co on AGNRs with an extremely small friction force

(~2.5 pN), which is about 2 orders less than the friction force (~250 pN) of TBrPP-Co on

Au(111). This results show that superlubricity still exists at the single molecule level. In contrast to TBrPP-Co molecule adsorbed on the Au(111) substrate, the TBrPP-Co molecules are weakly adsorbed on AGNR. This superlubricity effect is probably due to both the structural incommensurability and extremely weak interactions between single

TBrPP-Co molecules and AGNRs.

CHAPTER 7: SUMMARY AND OUTLOOK

In this thesis, STM and qPlus AFM techniques are used to investigate electronic, spintronic, and mechanical properties of TBrPP-Co molecules adsorbed on a Au(111) substrate and AGNRs, which lead to several inspiring and important discoveries.

In Chapter 4, Kondo resonance of TBrPP-Co molecules on Au(111) is systematically investigated. When a TBrPP-Co molecule is adsorbed on FCC or HCP region on Au(111) substrate, because of the spin exchange interactions, an adsorption-site dependent Kondo resonance is formed near the Fermi level, and is detected by tunneling spectroscopy. However, when TBrPP-Co molecules are adsorbed on elbows of herringbone reconstructions on Au(111) surface, a Kondo resonance cannot be observed.

The Kondo resonance reappears, once TBrPP-Co molecules are moved to HCP or FCC regions from the elbows. Because the strengths of the Kondo resonance is sensitive to the adsorption configuration, and the Co atom of TBrPP-Co molecules are adsorbed above protruded Au atoms on elbows, it is proposed that the disappearance of the Kondo resonance is due to the asymmetric and protrusive structure of herringbone elbow. In the future, it will be necessary to use theoretical calculations to clarify the mechanism of the disappearance of Kondo resonance on elbows.

In Chapter 5, a synthesis process has been developed to create vertically stacked heterostructures of TBrPP-Co/AGNR/Au(111). An anomalous Kondo resonance is discovered in such heterostructures. Although using tunneling spectroscopy, TBrPP-Co molecules on AGNR are found to be electrically decoupled from the substrate, the Kondo resonance is unexpectedly observed on TBrPP-Co molecules on AGNR. Furthermore, it 96 is found that the strengths of Kondo resonances are almost the same as the TBrPP-Co molecules directly adsorbed on the Au(111) substrate. The dI/dV tunneling spectra measured on TBrPP-Co molecules between two AGNRs and on edges of AGNRs reveals the important role of AGNRs in this Kondo resonance. SP-DFT calculations confirm that the AGNRs mediate the spin-coupling between the net spin from TBrPP-Co and free electrons from the Au(111) substrate, which leads to the Kondo resonance. This project shows that although electrically opaque, AGNR is a good medium for spin coupling.

In Chapter 6, the superlubricity effect of a single TBrPP-Co molecule on AGNR is observed and measured, where the magnitude of frictional force decreases by about 2 orders compared to TBrPP-Co molecules directly adsorbed on Au(111). Frictional force of TBrPP-Co molecules on AGNR and on Au(111) are measured using STM lateral manipulation and DFT calculations, and qPlus AFM. The friction force on Au(111) is measured as ~250 pN, while on AGNR is only ~2.5 pN. It is proposed that the superlubricity is due to the incommensurability and extremely weak interactions between the TBrPP-Co molecules and AGNRs. In the future, theoretical calculation is necessary to find out the origin of the superlubricity. This project shows that superlubricity still exists at the single molecule level.

Future potential research based on these presented results in this thesis are numerous and exciting. Theoretical calculation will be important to understand the mechanism of this location-dependent Kondo resonance. Nanoscale electrical circuits might be designed and formed on a modified Au(111) substrates using TBrPP-Co molecules and AGNRs. The spintronic transparency of AGNR may inspire spintronics 97 research on AGNR and lead to spintronic applications. It will be interesting to explore the limit of superlubricity on molecules with different structures and even smaller size than

TBrPP-Co. Superlubricity can help reduce friction and wear, and save energy in applications. Further research is necessary to make use of the single molecule superlubricity in NEMS and molecular devices.

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APPENDIX A: THEORETICAL CALCULATION OF KONDO RESONANCE

In [47], spin-polarized density functional theory is used to study the mechanism of the Kondo resonance of the vertically stacked heterostructure of TBrPP-

Co/AGNR/Au(111). As shown in figure A, density of state of spin up and spin down states of d!! orbital of Co atom from TBrPP-Co molecules are calculated for four different models: 1, free TBrPP-Co; 2, TBrPP-Co adsorbed on Au(111) substrate; 3,

TBrPP-Co/AGNR/Au(111); 4, the same structure as 3, but with the AGNR removed

(figure A(A)). It is found that, in TBrPP-Co/AGNR/Au(111) heterostructures (figure A(B)

(3)), the spin-up and spin-down states are broadened due to the spin interactions between the Co atom from TBrPP-Co and surface free electrons, which is the same as the TBrPP-

Co directly adsorbed on Au(111) substrate (figure A(B) (2)); while once the AGNR is removed from the heterostructure, the spin-up and spin-down states become narrow peaks with large amplitudes (figure A(B) (4)), which are the same as the free TBrPP-Co model

(figure A(B)(1)). When adsorbed on AGNR, in contrast to on Au(111) (figure A(C)) , the density of state of Co d!! orbital has a larger amplitude (figure A(D)), which leads to strong spin exchange interactions and high Kondo temperature, in spite of the large distance between TBrPP-Co and the substrate. Kondo temperatures are calculated for

TBrPP-Co on AGNRs and on Au(111) using a two-orbital model [72], which are in good agreement with experimental results [47].

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Figure A. Kondo mechanism. (A) (1)Calculated geometries of isolated TBrPP-Co, (2) TBrPP-Co adsorbed on Au(111), (3) the heterostructure of TBrPP-Co adsorbed on AGNR/Au(111), and (4) TBrPP-Co located at 7.5 Å above Au(111) surface after removing the AGNR from the heterostructure. (B) Spin-polarized Co d!! density of states of the TBrPP-Co corresponding to (a). (C) Atomic SP-DOS Co d!! and N p- orbitals for TBrPP-Co adsorbed on top site of Au(111). (D) Atomic SP-DOS of Co d!!, N p, and C PZ orbitals in AGNR for the TBrPP-Co/AGNR/Au(111) heterostructure. Positive and negative DOS correspond to spin-up, and spin-down components, respectively. Copyright 2017 Nature Communications.

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APPENDIX B: TIP-MOLECULE TOTAL FORCE

In [73], tip-molecule potential is calculated using density functional theory (DFT) as a function of tip-molecule distance (figure B (a)), from where the total tip-molecule force is derived (figure B (b)). Once tip-molecule distances are obtained using R-Z curve

(figure 6.3), tip-molecule total forces corresponding to different tip-molecule distances can be found.

Figure B . (a) Tip-molecule potential calculated by DFT as a function of tip-molecule distance. (b) Tip-molecule total force derived from (a) [73]. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

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