Probing Atomic-Scale Properties of Magnetic and Optoelectronic Nanostructures by Xiaowei Zhang A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Michael F. Crommie, Chair Professor Steven G. Louie Professor Rachel A. Segalman Fall 2012 Probing Atomic-Scale Properties of Magnetic and Optoelectronic Nanostructures c 2012 by Xiaowei Zhang 1 Abstract Probing Atomic-Scale Properties of Magnetic and Optoelectronic Nanostructures by Xiaowei Zhang Doctor of Philosophy in Physics University of California, Berkeley Professor Michael F. Crommie, Chair This dissertation presents scanning tunneling microscopy and spectroscopy studies of individual molecules and graphene nanoribbons (GNRs) bound to a substrate. Under- standing the local electronic properties of these systems is importance from a fundamental physics viewpoint and for advancing potential technological applications in nanoelectronics. Two molecular systems, tetracyanoethylene (TCNE), and bithiophene naphthalene diimide (BND), were investigated. The basic questions addressed are (1) how do molecules respond to a condensed matter environment (i.e. a metal or semiconducting surface), (2) how do spins behave in molecule-scale structures, and (3) how do the intrinsic electronic properties of molecules affect their self-assembly behavior. We find that TCNE molecules display vari- able surface coupling and enable tunable magnetic exchange coupling between covalently bonded spin centers in Vx(TCNE)y complexes. We also were able to determine the TCNE adsorption site within a molecular monolayer on Ag(100) through a combination of inelastic electron tunneling spectroscopy and density functional theory calculations. We find that BND molecules exhibit type-II heterojunction energy level alignment. The interplay be- tween the bipolar electronic nature of the molecule and the substrate results in different self-assembly patterns on a Au(111) surface. In GNRs we have demonstrated the presence of magnetic edge states for chiral nanoribbons with atomically smooth edges. We have fur- ther controlled GNR edges via hydrogen plasma etching, and have determined their exact edge termination. i Contents List of Figures iv List of Abbreviations vi Acknowledgments vii I Introduction 1 1 Why Nanoelectronics? 2 1.1 MolecularDevices................................ 2 1.1.1 Magnetic Molecules for Spintronics Applications . ........ 3 1.1.2 Bipolar Molecules for Photovoltaic Applications . ........ 4 1.2 GrapheneNanoribbonDevices. .. 4 1.3 OutlineofDissertation . ... 5 2 Scanning Tunneling Microscopy Principles 6 2.1 TheoryofOperation ............................... 6 2.2 STMTopography................................. 7 2.3 ElasticSpectroscopy ............................. .. 9 2.4 dI/dV Maps.................................... 9 2.5 Inelastic Electron Tunneling Spectroscopy . ......... 10 2.6 STMManipulation ................................ 12 3 Instrumentation 13 3.1 The Scanning Tunneling Microscope Instrument . ........ 13 3.1.1 UHVChambersandPumpingSystems . 13 CONTENTS ii 3.1.2 CryogenicSystem............................. 15 3.1.3 VibrationIsolation ............................ 15 3.1.4 STMScanner ............................... 15 3.1.5 STMElectronicsandSoftware. 16 3.1.6 CoarseApproachandFineMotion . 17 3.2 SamplePreparation............................... 18 3.2.1 SputteringandAnnealing . 18 3.2.2 ThinInsulatingLayers . 18 3.3 DepositionTechniques . .. 19 3.3.1 KnudsenCellEvaporator. 19 3.3.2 ElectronBeamEvaporator. 21 3.3.3 KGetterEvaporator ........................... 21 3.3.4 LeakValveEvaporator . 21 3.3.5 SpinCoating ............................... 25 II Magnetic Nanostructures 26 4 Spin Coupling Mediated by TCNE 27 4.1 TCNEonAg(100) ................................ 28 4.2 Vx(TCNE)y Complexes.............................. 29 4.3 V(TCNE)onNaCl ................................ 36 4.4 Conclusions .................................... 36 5 TCNEMonolayerAdsorptionSiteDetermination 39 5.1 TCNEMonolayeronAg(100) .......................... 40 5.2 STSandIETSofTCNEMonolayer . 41 5.3 DFT Calculations on TCNE Monolayer Vibrational Modes . ...... 43 5.4 Conclusions .................................... 45 III Photovoltaic Molecules 47 6 EnergyLevelAlignmentinaBipolarMolecule 48 6.1 IntroductiontoOrganicSolarCells . ...... 48 6.1.1 Photovoltaic Principles of Organic Solar Cells . ........ 48 6.1.2 OrganicPhotovoltaicMaterials . ... 49 6.1.3 DeviceArchitectures ........................... 50 6.2 BipolarMolecules................................ 51 6.3 BNDMoleculesonNaCl/Ag(100) . 52 6.4 DFTCalculationsonBNDMolecule . 56 6.5 Conclusions .................................... 58 CONTENTS iii 7 Self-AssemblyofBipolarMoleculesonAu(111) 59 7.1 MoleculesonAu(111)Terraces. ... 60 7.2 MoleculesnearStep-edges . ... 61 7.3 Conclusions .................................... 61 IV Graphene Nanoribbons 63 8 EdgeStatesofChiralGrapheneNanoribbons 64 8.1 GrapheneNanoribbonsonAu(111) . 64 8.2 EdgeStateofGNRs ............................... 68 8.3 TheoreticalCalculationsonGNRs. .... 71 8.4 Conclusions .................................... 75 9 ControllingtheEdgeTerminationofGNRs 76 9.1 HydrogenPlasmaTreatedGNRsonAu(111). 76 9.2 Thermodynamic Calculations of Edge Terminations . ......... 78 9.3 Conclusions .................................... 84 Bibliography 85 iv List of Figures 1.1 Screeningaroundalocalspinonthesurface. ....... 4 2.1 Tunnelingdiagram................................ 7 2.2 STMschematicdiagram............................. 8 2.3 AtomicresolutionofAg(100)surface. ..... 9 2.4 Simulated I V and dI/dV curves ...................... 10 − 2.5 Inelasticelectrontunnelingdiagram . ....... 11 2.6 STMmanipulationdiagram. 12 3.1 Overviewofthehome-builtUHVLTSTM . 14 3.2 STMstageandscanner ............................. 16 3.3 NaClislandsonAg(100) ............................ 19 3.4 CuNislandsonCu(100)............................. 20 3.5 Knudsencellevaporator. .. 20 3.6 Electronbeamevaporator. .. 22 3.7 Room temperature leak valve evaporator setup . ....... 23 3.8 Low temperature leak valve evaporator setup . ....... 24 4.1 TCNEmolecularstructure . 27 4.2 TCNEonAg(100)................................ 29 4.3 DFTcalculationsofTCNEHOMO . 30 4.4 Construction of Vx(TCNE)y onAg(100).................... 30 4.5 VariousVx(TCNE)y structures......................... 31 4.6 Vx(TCNE)y structuremodels.......................... 32 4.7 Vx(TCNE)y highbiasspectra.......................... 33 4.8 Vx(TCNE)y lowbiasspectra .......................... 34 4.9 V2(TCNE)spectra................................ 35 LIST OF FIGURES v 4.10 V(TCNE) and V2(TCNE)SP-DFTcalculations . 35 4.11 TCNEonNaCltopographs........................... 37 5.1 TopographofTCNEmonolayeronAg(100) . 41 5.2 STS and IETS of TCNE molecules within a monolayer. .... 42 5.3 DFTcalculationsofTCNEmonolayer . .. 44 6.1 Photovoltaic principles of organic solar cells . ........... 49 6.2 Organicsemiconductorexamples . .... 50 6.3 Organicsolarcellarichitectures. ....... 51 6.4 Bithiophene naphthalene diimide molecule structure . ........... 52 6.5 TopographofBNDmoleculesonNaClislands . .. 53 6.6 dI/dV spectroscopyofaBNDmoleculeonNaCl . 54 6.7 Other dI/dV spectroscopicbehaviorofBNDmolecules . 55 6.8 dI/dV mapsofaBNDmoleculeonNaCl . 55 6.9 Calculations of quasi-particle LDOS of BND molecule . ........ 57 7.1 DirectlybondedBNDmoleculestructure . .... 59 7.2 Self-assembly of direct BND molecules on terraces . ........ 60 7.3 Self-assembly of direct BND molecules near step-edges . .......... 61 7.4 Schematic and STM images of parallel molecular chains . ......... 62 8.1 GraphenenanoribbonsonAu(111). .. 65 8.2 Partially unzipped carbon nanotubes on Au(111) . ....... 66 8.3 FoldedGNRsonAu(111)............................ 67 8.4 DeterminationofchiralityofGNRs . ... 69 8.5 EdgestatesofGNRs .............................. 70 8.6 Position- and width-dependent edge-state properties . ............ 72 8.7 Theoretical band structure and DOS of a 20-nm-wide (8, 1)GNR...... 74 9.1 HydrogenplasmatreatedGNRsonAu(111) . 77 9.2 ExperimentandsimulatedimagesofGNRedges . .... 79 9.3 H-terminated GNR edges thermodynamic stability calculation ....... 80 9.4 SimulatedSTMimagesfor(2,1)chiraledge . ..... 82 9.5 Comparison of line profiles of experimental and simulationimages . 83 vi List of Abbreviations DFT Density Functional Theory dI/dV Differential Conductance DOS Electronic Density of States ec Absolute value of the Electron Charge EF Fermi energy FE FieldEmission FWHM Full Width at Half Maximum HOMO Highest Occupied Molecular Orbital IETS Inelastic Tunneling Spectroscopy LDOS Local Density of States LT Low Temperature (around 77K or lower) LUMO Lowest Unoccupied Molecular Orbital ML Monolayer rms RootMeanSquare RT RoomTemperature STM Scanning Tunneling Microscopy or Scanning Tunneling Microscope STS Scanning Tunneling Spectroscopy UHV Ultra High Vacuum vii Acknowledgments There are many people to thank for their support throughout the time this work was done. First and foremost, I would like to express my sincere thanks to my advisor, Professor Michael Crommie, for his guidance and support during the past five years. His insight into physics provides novel research ideas and his enthusiasm has always been an inspiration. He cares about his students and always makes time for them to discuss any questions. I feel very grateful to have him as my advisor. I would also