Atomically defined tips in scanning probe microscopy - McGill Physics

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32 CHAPTER 3. EXPERIMENTAL METHODSto characterize supported nanostructures and overcomes the difficulty of attachingthe electrodes to nanoscale structures [70]. The essential part of the e-EFM methodis an insulating film that separates the nanostructures under investigation from aback electrode and acts as a tunnel barrier, which is shown schematically usingan example of an Au nanoparticle supported on an NaCl film in Figure 3.2a. Toinduce tunneling between the nanoparticle and the back electrode, the AFM tip isplaced at a height within several nanometers above the nanoparticle. A DC biasvoltage, V B , is applied across the oscillating cantilever and the back electrode. Typically,the tip-sample separation (∼ 5 nm) is large enough to make the tunnelingbetween the tip and the nanoparticle negligible due to the large barrier height andwidth of the vacuum gap.As shown in Figure 3.2b the potential drop between the nanoparticle andthe back electrode, i.e. across the tunnel barrier, is a fraction of the applied biasvoltage, αV B (α < 1). The α(x, y, z) parameter, which similarly to the case of asingle-electron box described in Section 2.2.1 is called the lever-arm, depends onthe lateral and vertical position of the tip. When the tip oscillates, α is modulated,giving rise to an effective modulation of the voltage across the barrier. Figure 3.2bshows that tunneling of an electron between the back electrode and the Au nanoparticleonly takes place if the Coulomb blockade is lifted by selecting a propervalue of V B and tuning eαV B to one of the electrochemical potential levels of thenanoparticle. Under this condition, the oscillation of α leads to an oscillation ofthe number of electrons on the nanoparticle, N, between two neighboring groundstates characterized by N = n and N = n + 1 electrons. The oscillating chargeon the nanoparticle causes both a resonance frequency shift, ∆f, and damping ofthe cantilever, ∆γ [70, 71]. Although tunneling of electrons is a stochastic event,the average charge, 〈N(t)〉, follows the oscillating motion of the tip. As a result,the oscillating electrostatic force has not only an in-phase, but also a 90 ◦ out-ofphasecomponent that leads to energy dissipation (which is a similar effect to theQ-factor degradation in the Q-control system used in amplitude modulation AFM[72]).The rate of tunneling through the ultra-thin insulating film largely decidesthe sensitivity of the method by affecting the relative strength of the cantileverdissipation and frequency shift signals [73] which are measured in e-EFM. As will
3.1 ATOMIC FORCE MICROSCOPY IN ULTRA-HIGH VACUUM 33Figure 3.2: (a) The most essential part of the e-EFM method is an ultra-thin insulatingfilm that separates studied nanostructures from a back electrode and acts as a tunnel barrier.An oscillating AFM tip is used as a local gate by applying an electric potential, V B ,between the back electrode and the tip. (b) Energy diagram of the system for positivesample bias allowing for sequential unloading of electrons. (c) In an electron additionspectrum an increase in γ (and ∆f) appears near voltage values matching eαV B with oneof the electrochemical potential levels of the nanoparticle. The shape of the peaks are givenby Equation 8.3, from which α can be extracted.be discussed in Section 8.1, the dissipation of the cantilever per unit time, ∆γ, ismaximal when the tunneling rate is matched with the oscillation frequency of thecantilever, while the frequency shift, ∆f, increases with increasing tunneling rate.This means that the signal could be optimized by a proper choice of the cantilever,however this is limited by the range of resonance frequencies (10-1,000 kHz)of commercially available AFM probes. Alternatively, the e-EFM technique canbe made sensitive at room temperature by achieving full control over the tunnelbarrier thickness and adjusting the tunneling rate to the cantilever resonance frequency.The change in ∆f and γ is maximal only during tunneling of an electron.This condition can be identified by placing the tip above a nanoparticle and sweepingV B , simultaneously observing ∆f and γ. Figure 3.2c schematically shows anelectron addition spectrum, where an increase in γ appears near voltage valuesmatching eαV B exactly with one of the electrochemical potential levels of the nanoparticle,which lifts the Coulomb blockade. If the modulation of eαV B caused bythe cantilever oscillation is smaller than the thermal energy, a condition which iseasily met at room temperature, the changes in ∆f and ∆γ are described by a linearresponse [70, 74, 75]. In this case the shape of the Coulomb blockade peak is
Page 1: Ultra-high vacuum fabricationof nan
Page 5 and 6: RésuméDans ce travail, nous décr
Page 8: viACKNOWLEDGEMENTSDr. Margaret Magd
Page 11 and 12: Contributions of Co-authorsThis the
Page 13 and 14: xiRTSEMSETSPMSTMTEMTMRUHVVCOVLSXRDR
Page 15 and 16: ContentsAbstractRésuméAcknowledge
Page 17 and 18: CONTENTSxv7.3.2 Structure of 2 ML t
Page 19: 1MotivationLow-dimensional systems
Page 23: 5simulations can be used to charact
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Page 59 and 60: 4Description of instrumentationThis
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Page 73 and 74: 5Preparation of Fe(001) substratesT
Page 75 and 76: 5.1 GROWTH OF FE WHISKERS 57aHClWhi
Page 77 and 78: 5.1 GROWTH OF FE WHISKERS 59way. If
Page 79 and 80: 5.2 THE FE(001) SURFACE 61FeCl 2 is
Page 81 and 82: 5.3 THE FE(001)-P(1×1)O SURFACE 63
Page 83 and 84: 6Reactive growth of MgO films on Fe
Page 85 and 86: 6.1 INTRODUCTION 67either case MgO
Page 87 and 88: 6.2 EXPERIMENTAL DETAILS 69directly
Page 89 and 90: 6.3 RESULTS AND DISCUSSION 71aClean
Page 91 and 92: 6.3 RESULTS AND DISCUSSION 734r/p =
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84 CHAPTER 7. GROWTH OF NACL FILMS
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102 CHAPTER 8. ROOM-TEMPERATURE SIN
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126 CHAPTER 9. CONCLUSIONS AND OUTL
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AEffect of self-capacitance on sing
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138 APPENDIX A. EFFECT OF SELF-CAPA
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140 APPENDIX B. REPAIR OF LEED OPTI
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142 APPENDIX C. FIELD INDUCED DEPOS
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144 APPENDIX C. FIELD INDUCED DEPOS
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146 BIBLIOGRAPHY[9] Ray, V.; Subram
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148 BIBLIOGRAPHY[30] Parkin, S. S.
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150 BIBLIOGRAPHY[50] Simic-Milosevi
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152 BIBLIOGRAPHY[70] Cockins, L.; M
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154 BIBLIOGRAPHY[89] Gardner, R. Co
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156 BIBLIOGRAPHY[109] Müller, M.;
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158 BIBLIOGRAPHY[130] Blonski, P.;
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160 BIBLIOGRAPHY[150] Ohgi, T.; Fuj
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Atomically defined tips in scanning probe microscopy - McGill Physics

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