Probing Single Molecules with

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Probing Single Molecules with Scanning Probe
Microscopy
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Outline
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An Ideal Experiment for Probing Molecular
Conduction
M. C. Hersam, et al., MRS Bulletin, 29, 385
(2004).
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Real Experimental Strategies for Probing
Molecular Conduction
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The Origin of Scanning Probe Microscopy
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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The Scanning Tunneling Microscope (STM)

• STM invented by Gerd Binnig and Heinrich Rohrer
  in 1982
• Led to Nobel Prize in Physics, 1986

C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Si(111)-77 Stairway to Heaven
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Scanning Tunneling Microscope Schematic
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Tungsten STM Tip

• Electrochemically etched using NaOH
• Ideally, the tip is atomically sharp

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One-Dimensional Tunnel Junction
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Tunneling Current Approach 1
Assume metal-vacuum-metal junction, solve
Schrödinger Equation
I tunneling current ?s local density of
states of sample V tip-sample voltage W width
of barrier
Typically, f 4 eV ? k 1 Å-1 ? Current decays
by e2 7.4 times per Å
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Bardeen Tunneling Theory
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Tunneling Current Approach 2
Consider overlap of wavefunctions from either
side of barrier Using Fermis Golden Rule
(assuming kT ltlt energy resolution of the
measurement),
For a free electron metal tip, ?t is constant
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Atomic Force Microscopy (AFM)

• Invented at Stanford by Binnig and Quate in 1986
• Bring tip-mounted micromachined cantilever into
  contact
• or close proximity of the surface
• Atomic forces deflect cantilever and is
  detected with laser
• deflection into a position sensitive photodiode
• Cantilever deflection is control signal for the
  feedback loop
• AFM can be done on any surface (i.e.,
  conductive, insulating,
• semiconducting, biological, etc.) in any
  environment (i.e., air,
• vacuum, liquid, etc.)

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Force Detection with Optical Beam Deflection
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Atomic Force Microscope Cantilevers
L 100 µm w 20 µm t 0.5 µm

• Fabricated using conventional microfabrication
  procedures
• Backside coated with an optically reflective
  material (e.g., Au)

https//www.veecoprobes.com/
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Atomic Force Microscope Tips

• Typical radius of curvature is 10 nm
• Tips are often coated with conductive materials,
  magnetic materials, low wear materials, or
  organic/biological molecules.

https//www.veecoprobes.com/
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Fluid Cell for Atomic Force Microscopy
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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AFM Photographs
Custom Liquid Cell
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AFM Images of Hematite

• Measured atomic step height of 2.2 Å
• Vertical spatial resolution of 0.1 Å in air.

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Nano Oxidation of Silicon with Conductive AFM

• Oxide nanopatterns achieved via local
  anodization of Si(111)H
• Spatial resolution limited to 10 nm.

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Lateral Force Microscopy

• Chemical contrast monitored via frictional
  force.
• Care must be taken to extract quantitative LFM
  data.

M. W. Such, D. E. Kramer M. C. Hersam,
Ultramicroscopy, 99, 189 (2004).
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Selective Polymerization Chemistry
ROMP Ring Opening Metathesis Polymerization
Collaboration with SonBinh Nguyen
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Atomic Force Electroluminescence Microscopy (AFEM)
L. S. C. Pingree, M. C. Hersam, M. M. Kern, B. J.
Scott T. J. Marks, Appl. Phys. Lett., 85, 344
(2004).
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AFEM on Organic LED Arrays
Light Emission Map
Current Map
Quantum Efficiency Map
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Single Molecule Imaging with Ambient AFM
Typical ambient AFM resolution is 10 nm as
opposed to atomic resolution for STM ? STM is
typically the technique of choice for
intramolecular spatial resolution imaging
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Example Ultra-high Vacuum (UHV) STM Design

• Homebuilt STM in the Hersam lab at Northwestern
  University
• STM is a modified Lyding scanner

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Scanner Construction Piezotubes
Outer tube 0.650 OD 0.570 ID 0.750 Long
Inner tube 0.375 OD 0.315 ID 0.750 Long
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Scanner Construction Base Plug
Front View
Rear View
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Scanner Construction Piezotubes Soldered into
Base Plug
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Scanner Construction Course Translation Platform
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Scanner Construction Course Translation
Platform Soldered onto Outer Piezotube
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Scanner Construction End Cap Positioned onto
Inner Piezotube
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Scanner Construction Tip Contact Assembly
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Scanner Construction Full Tip Assembly
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Scanner Complete
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Cryogenic Variable Temperature UHV STM
E. T. Foley, et al., Rev. Sci. Instrum., 75, 5280
(2004).
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Vibration Isolation
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Detail of STM Stage
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Thermal Shields with Back Panel Removed
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Stage Locking Screw for Cooldown
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Front Doors Open for STM Access
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Sample and Probe Mounted for Scanning
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STM Suspended for Scanning
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UHV Chamber and Liquid Helium Dewar
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Si(100) Dosed with Cyclopentene at 80 K
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dI/dV Imaging of Cyclopentene on Si(100) at 80 K
150 Å x 150 Å, -2.15 V, 0.1 nA
dI/dV Map at -2.8 V
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Scanning Tunneling Microscopy Nanofabrication

• Many nanofabrication schemes have been developed
  with STM
• (spatial resolution down to the single atom
  level)
• Initially demonstrated by Eigler in 1989
• (IBM written with atoms at cryogenic
  temperatures)
• (2) Room temperature atom removal from Si(111) by
  Avouris
• (3) Field evaporation of gold
• (4) Electron stimulated desorption of hydrogen
  from Si(100)

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Tunable Bond Formation with STM
G. Timp, Nanotechnology, Chapter 11
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Sliding Adatoms with STM
G. Timp, Nanotechnology, Chapter 11
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The First Atom Moved with STM
Xenon on platinum ? requires a defect to prevent
tip-induced motion under normal scanning
conditions
G. Timp, Nanotechnology, Chapter 11
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STM Manipulation of Xenon on Nickel
G. Timp, Nanotechnology, Chapter 11
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Nanograffiti
Kanji for atom
Xenon atoms on Nickel (110)
Fe atoms on Cu(111)
Don Eigler, IBM Alamden, http//www.almaden.ibm.co
m/vis/stm/atomo.html
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Quantum Corrals
Fe atoms on Cu(111)
Don Eigler, IBM Alamden, http//www.almaden.ibm.co
m/vis/stm/atomo.html
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Quantum Mirage (Kondo Resonance)
Topography
Co atoms on Cu(111)
dI/dV
Don Eigler, IBM Alamden, http//www.almaden.ibm.co
m/vis/stm/atomo.html
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Room Temperature Manipulation of Si(111)
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Field Evaporation of Gold
C. Julian Chen, Introduction to Scanning
Tunneling Microscopy
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Hydrogen Passivated Si(100)
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STM Nanolithography on Si(100)-21H

• Selective chemistry can be accomplished on
  patterned areas.

J. W. Lyding, et al., Appl. Phys. Lett., 64, 2010
(1994).
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Selective Molecular Adsorption of Norbornadiene
on Silicon
G. C. Abeln, et al., J. Vac. Sci. Technol. B, 16,
3874 (1998).
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Feedback Controlled Lithography
M. C. Hersam, et al., Nanotechnology, 11, 70
(2000).
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Patterning Individual Molecules with FCL
M. C. Hersam, et al., J. Vac. Sci. Technol. A,
18, 1349 (2000).
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FCL Patterned TEMPO Molecules
(a)
2 nm
Nitroxyl free radical binding chemistry allows
for single molecule patterning with FCL
(b)
FCL pattern
(c)
M. E. Greene, N. P. Guisinger, R. Basu, A. S.
Baluch M. C. Hersam, Surface Science, 559, 16
(2004).
TEMPO molecules
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Self-Directed Growth of Styrene Chains from
Individual Dangling Bonds
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Patterning Styrene Chains with FCL
R. Basu, N. P. Guisinger, M. E. Greene M. C.
Hersam, Appl. Phys. Lett., 85, 2619 (2004).
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Heteromolecular Nanostructure Fabrication
R. Basu, N. P. Guisinger, M. E. Greene M. C.
Hersam, Appl. Phys. Lett., 85, 2619 (2004).
Multi-step FCL
Pre-patterned TEMPO molecules constrain styrene
chain growth