Scanning Probe Microscopy

1
Scanning Probe Microscopy
Paul S. Weiss Department of Chemistry, The
Pennsylvania State University http//stm1.chem.psu
.edu/
Imaging Mechanism and requirements. Variations
and extensions. Limitations and
pitfalls. Spectroscopy Variations vis-à-vis
chemical/electronic properties. Interactions and
manipulation Building and measuring
nanometer-scale structures. Measuring
interactions.
2
Scanning Probe Microscopy
Three essential parts Scanning Piezoelectric
transducers Feedback Tunneling current in
scanning tunneling microscope Force in atomic
force microscope Can be attractive (difficult)
or Can be repulsive (easy but
damaging) Recording Measure tip-sample
controlling piezo position Measure tunneling
current (at constant separation) Measure
cantilever deflection (for force) Measure
optical signal (at constant separation) Can
record additional channels simultaneously as well!
3
Imaging Single Atoms Xe on Ni110
Feature size does not correspond directly to
atomic size STM images record a convolution
electronic and geometric structure.
40Å x 40Å x 1.53Å T4K VTip20 mV, I1 nA
Eigler, Schweizer, Weiss, and Lang, Physical
Review Letters 66, 1189 (1991)
4
Theoretical and Experimental Cross Sections for
STM Images of Adsorbed Xe
Physical Review Letters 66, 1189
(1991) Calculation by Norton Lang, IBM Yorktown
Heights
5
Electronic States Far Away in Energy Contribute
to the Tunneling Current
Adsorbed Xe electrons at position of tip electron
overlap
6s
EFermi
5p
?6
?4
2
0
?2
4
N. D. Lang, IBM Yorktown
6
Differentiating Surface Atoms in STM
GaAs(110) Charge transfer puts filled states on
As and empty states on Ga. Two images are
superimposed. Joe Stroscio Randy Feenstra, IBM
7
Scanning Tunneling Microscopy
?
?
EF
Elastic Tunneling
E
Tip
EF - eV
Sample
TipDensity of States
SampleDensity of States
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Transient Mobility

• Measure single atoms and molecules motion, 2D
  scattering.

Stepped
Surface
Incoming Atoms/Molecules
Perfect Terrace
Incident atoms impinge on Pt111 at 4K
Terrace with Line Defects
Incident molecules impinge on Ni(110) with
H-induced added rows, at 4K
Xe
Benzene
H-induced Line Defect
9
Xe Transient Mobility on Pt111
100Å x 100Å T4K VTip at 5mV I10pA
Xe atoms travel hundred of Ångstroms across
terraces before running into step edges and
stopping. Weiss Eigler, Physical Review Letters
69, 2240 (1992)
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Momentum Transfer in 2D
Erhard Schweizer, IBM Almaden Xe atoms on
Ni110 Repulsive tip
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Benzene on Cu111
At 4K, molecules are frozen in place.
250Å x 250Å Vtip0.1V, I100pA Coverage 0.002
monolayers
Stranick, Kamna, Weiss, Surface Science 338, 41
(1995)
12
Understanding STM Images

• How do we understand and predict STM images?

Ultrastable low temperature studies provide the
critical data used to understand images.
Benzene at three different adsorption sites on
Pt111 at 4K 15Å x 15Å images Weiss
Eigler PRL 71, 3139 (1993) Calculations by Ph.
Sautet, ENS Lyon Israel J. Chem. 36, 63 (1996)
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Local Chemical Effects Due to Adsorbates

• What are the chemical consequences?
• How can we exploit these atomic-scale effects?

Benzene at two types of three-fold hollow sites
on Pt111 at T4K 15Å x 15Å Vbias50mV I100pA
15Å x 15Å Vbias10mV I100pA
0.6Å
0Å
Topographic Height
0.6Å
Weiss Eigler PRL 71, 3139 (1993)
0Å
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Benzene on Cu111
The electronic structural perturbations of
neighboring molecules interfere, thus setting up
new adsorption sites.
30Å x 30Å Vtip0.1V I100pA T77K
Kamna, Stranick, Weiss, Science 274, 118 (1996)
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Identifying Adsorbates in STM
H added row
S adatom
80Å x 80Å Vtip50mV, I1 nA Ni110 at 4K
Correlate Auger spectroscopy, LEED, and STM
images to identify a limited set of adsorbates.

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Scanning Tunneling Spectroscopy
Surface states in the band gap appear at
different energies at different sites on Si(111)
Wolkow and Avouris, Phys. Rev. Lett. 60, 1049
(1988)
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Identifying Adsorbates via Spectroscopy
250Å x 250Å VTip100mV, I0.13nA Fe on Pt111
at 4K
Three adatoms identified as Fe in STM via an
electronic resonance.
Crommie, Lutz, and Eigler, PRB 48, 2851 (1993)
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Inelastic Electron Tunneling Spectroscopy
E
?
I
?
EF
Elastic
Tip
V
Inelastic
EF - eV
Sample
dI/dV
V
A molecular vibration can be stimulated when V ?
/e
d2I/dV2
V
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Single Molecule Vibrational Spectroscopy
H added row
Inelastic electrontunneling spectroscopy
Vibrational Imaging
S adatom
C2H2C2D2Difference
A Topography B d2I/dV2 _at_ V 358 mV C
d2I/dV2 _at_ V 266 mV D d2I/dV2 _at_ V 311 mV

Stipe, Rezaei, and Ho, Science 280, 1735 (1998)
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STM Probe Tips Also Determine Contrast
Oxygen on Ni110 37Å x 47Å
W Tip
O Tip
F. Besenbacher and coworkers, Physical Review
Letters 70, 4079 (1993)
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Ni on MoS2
Ni promoter adatoms remain mobile at 77K,
andremain very easy to move with STM tip at 4K.
95Å x 30Å Vtip1.5V, I100pA T4K
Kushmerick Weiss, J. Phys. Chem. B 102, 10094
(1998)
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Ni3 Electronic Structure on MoS2
Ni3 cluster enhances empty states at 2V above the
Fermi level, and depletes filled states 2V below.
2V, 0.1nA
1.4V, 0.1nA
2V, 0.2nA
Ni3 on MoS2 60Å x 60Å T4K
Ni3 cluster optimizes binding of nucleophilic
molecules.
Kushmerick Weiss, J. Phys. Chem. B 102, 10094
(1998)
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Adsorbates Perturb Electronic Structure
Interference pattern from a circle of Fe atoms on
Cu111 at 4K
Eigler coworkers, Science 262, 218 (1993)
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Surface States are Dispersive
Crommie, Lutz, and Eigler, Nature 363, 524 (1993)

• Upper Terrace

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Anisotropic Effects of Vacancies on Graphite
Nominal W Tip 75Å x 75Å T77K VTip at
50mV I500pA
Pt/Rh Tip 40Å x 40Å Room temp, air VTip
100mVI2nA
C60 Tip 40Å x 40Å Room temp, air VTip at
100mV I1nA
J. G. Kushmerick, H.-P. Rust P. S. Weiss,
PSU K. F. Kelly Naomi Halas, Rice University
ECE Journal of Physical Chemistry B, 103, 1619
(1999)
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Ullmann Coupling Reactions
Aromatic rings can be coupled in solution or in
vacuum with 100 selectivity using Cu catalysts.
175K
300-400K
390K
950K
The energetics and structures of the prototypical
reaction of C6H5I on Cu111 were worked out by
Brian Bent.
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Phenyl on Cu111
100Å x 100Å Tip0.2V, I80pA T77K
50Å x 50Å Tip0.2V, I80pA T77K
Kamna, Graham, Weiss, submitted to JACS.
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Phenyl Intermediates Pair and Unpair on Cu111
24Å x 24Å, phenyl on Cu111, imaged over 6
hours VTip0.2V, I80pA, T77K
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Measuring Molecular Conductance

• Determine conductance differences of similar
  molecules in identical configuration and
  conditions

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Measuring Molecular Conductance

• Mix two molecules -- decanethiol and
  dodecanethiol.
• They have the identical surface structures on
  Au111.
• Measure them under identical conditions.

250Å x 250Å VTip1V, I10pA 95 CH3(CH2)9SH and
5 CH3(CH2)11SH self-assembled on Au111
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Control of Molecular Placement

• Use self-assembly, intermolecular interactions,
  deposition, and processing to select film
  structure.

Mixed monolayer
Separated monolayer
Note physically perfect boundary but chemically
distinct domains due to lateral epitaxy.
250Å x 250Å VTip1V, I10pA
250Å x 250Å VTip1V, I5pA
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Measuring Molecular Conductance

• Measure electron transport for single molecules
  (ß)

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S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R.
E. Smalley, L. J. Geerligs, and C. Dekker,
Nature 386, 474 (1997)
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Photon Emission Scanning Tunneling Microscope
Photomultiplier Tube
Photon Counting
Reflected Photons
Sample
STM Tip
Ellipsoidal Mirror
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Photon Emission from a Gold Nanoparticle
Photon emission from an asperity on a gold
nanoparticle bound to Au111 measured with
nanometer resolution.
180Åx180Å VTip2.0V I5nA
Topography
Photon Map
McCarty Weiss, Chemical Reviews 99, 1983 (1999)
36
AC Scanning Tunneling Microscopy
Replace dc voltage supply with microwave
source. Replace electrometer with network or
spectrum analyzer. Take care with transmission
lines, junctions, and cavities. No resonant
cavity allows ACSTM to be tunable. cf. Kochanski,
Dransfeld, Michel.
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Tunable Microwave Frequency ACSTM

• Developed to image insulator surfaces such as
  this lead silicate glass.

Imaged at 7.73 GHz 4800Å x 4800Å
Stranick Weiss J. Phys. Chem. 98, 1762 (1994)
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Measurements of Surface Charge and Ferroelectric
Effects

• Pole by charging surface and/or orienting film.
• Record with Coulombic force microscopy,
• Local piezoelectric response, or
• microwave AC STM/AFM -- stay tuned.

Surface potential measured by Coulombic force
microscopy 80 nm PZT on YBaCuO.
Surface piezoelectric effect measured by
conducting AFM 20 nm pVDF/TrFE on graphite.
Collaboration with K. Matsushige, H. Yamada, X.
Chen Kyoto University, Electronic Science
Engineering
39
Lateral Resolution Depends Upon Substrate
Interaction

• Poling parallel to the substrate-induced poled
  layer yields higher apparent resolution.

Parallel
Anti-parallel
Surface piezoelectric effect measured by
conducting AFM 23 ? 5 nm pVDF/TrFE on graphite.
Chen, Terai, Horiuchi, Yamada, Matsushige,
Weiss Thin Solid Films 353, 259 (1999)
40
Lateral Resolution Depends Upon Substrate
Interaction

• While the surface is patterned in terms of
  orienting the polymer, there is not change in the
  measured surface morphology.

Surface piezoelectric effect measured by
conducting AFM 23 ? 5 nm pVDF/TrFE on
graphite, 2µ between posts.
Chen, Terai, Horiuchi, Yamada, Matsushige,
Weiss Thin Solid Films 353, 259 (1999)
41
Transconductance Depends upon Molecular Backbone
Electrochemical measurement of ? vs.
conjugation/saturation of molecular backbone.
S. Sachs, C. E. D. Chidsey et al., JACS 119,
10563 (1997)
42
Nanometer-scale Phase Separation
Self-assembled monolayers phase separate (but
remain out of equilibrium). Showed that molecules
remain mobile after adsorption.
500Å x 390ÅVTip1VI1nA
75 CH3O2C(CH2)15S- 25 CH3(CH2)15S-on Au111
Stranick, Parikh, Tao, Allara, WeissJournal of
Physical Chemistry 98, 7636 (1994)
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Iodine Atoms Promote Aryl Coupling
40Å x 40Å Tip0.2V, I80pA T77K
Iodine atoms appear as protrusions surrounded by
depressions. I atoms are typically associated
with phenyl.
Kamna, Graham, Weiss, submitted to JACS.
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Basic Viewgraph
Still need AFM