Directing Molecules into Precise Positions

1
Directing Molecules into Precise Positions
Paul S. Weiss Departments of Chemistry Physics,
The Pennsylvania State University http//stm1.chem
.psu.edu/ stm_at_psu.edu
Use intermolecular interactions to drive precise
assembly of supermolecular structuresstructured
monolayers coupled reaction intermediates
Measure and understand the interactions
directly and via transient and stabilized
molecular positions Use scanning probe
microscopy and spectroscopy to measure structure
and properties (electronic, etc.) with
sub-Ångstrom resolution.
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1. Using Substrate-Mediates and Direct
Interactions
Use intermolecular interactions to drive precise
assembly of supermolecular structurescoupled
reaction intermediates Measure and understand
the interactions directly and via transient and
stabilized molecular positions mobile molecules
probe perturbed structures Use scanning probe
microscopy and spectroscopy to measure geometric
and electronic structures with sub-Ångstrom
resolution.
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1. 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 T 4 K 15 Å x 15 Å Vbias 50
mV I 100 pA 15 Å x 15 Å Vbias 10 mV I 100
pA
0.6Å
0Å
Topographic Height
0.6Å
Weiss Eigler PRL 71, 3139 (1993)
0Å
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1. Adsorbates Perturb Electronic Structure
Interference pattern from a circle of Fe atoms on
Cu111 at 4 K
Eigler coworkers, Science 262, 218 (1993)
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1. Electronic Perturbations and the
Adsorbate-Substrate Interaction Potential
Observing the residence times of mobile molecules
allows us to rank the relative strengths of the
electronic perturbations and their effects on the
lateral corrugation of the adsorbate-substrate
interactions.
Strongest Charge transfer across the step edge
Intermediate Local effects around molecular
adsorbates
Weakest Surface state scattering from the step
edge
Kamna, Stranick Weiss, Science 274, 118 (1996)
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1. Surface States are Dispersive
Dispersive states enable mapping the energies of
intermolecular and molecule-substrate interactions
Crommie, Lutz, and Eigler, Nature 363, 524 (1993)

• Upper Terrace

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1. Benzene on Cu111
The electronic structural perturbations of
neighboring molecules interfere, thus setting up
new adsorption sites.
30 Å x 30 Å Vsample 0.1 V I100 pA T77 K
Kamna, Stranick, Weiss, Science 274, 118 (1996)
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1. Benzene on Cu111
Fill new adsorption sites by sweeping molecules
in with the STM tip.
40 Å x 40 Å Vsample 0.1 V I100 pA T77 K
Kamna, Stranick, Weiss, Science 266, 99 (1994)
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1. Benzene on Ag(110) Adsorbs Only at 001 Steps

Significant density of states at 110 steps
allows perturbation and thus adsorption.Gap at
001 steps does not ? no adsorption.
200 Å x 200 Å Tip0.05 VI4 nA
210 Å x 210 Å Tip0.5 VI0.1 nA
Dosed at 66 K, imaged at 4 K
Pascual, Jackiw, Kelly, Conrad, Rust
WeissPhysical Review B 62, 12632 (2000)
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1. Adsorbed Aromatics to Date
Use substrate-mediated interactions to enhance
catalysis Phenyl and Phenylene on
Cu111 Species and reaction progress vs. T,
structures. Can freeze in phenyl and phenylene
radical intermediates at or below 300K. PSW,
Kamna, Graham, Stranick, Langmuir 14, 1284
(1998). Kamna, Graham, PSW, submitted to
Journal of the American Chemical Society.
McCarty PSW, submitted to Journal of the
American Chemical Society. Use
substrate-mediated interactions to grow
atomically precise films TCNQ on Cu111 Strong
anisotropic perturber of electronic
structure. Kamna, Graham, Love, PSW, Surface
Science 419, 12 (1998).
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1. Chemical Example 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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1. Phenyl Pairing on Cu111
Reactive intermediates form and break observe
in STM with gt8 orders of dynamic range. Determine
catalytic mechanism, promotion, and possible
enhancements.
100 Å x 100 Å Tip 0.2 V, I 80 pA T 77 K
50 Å x 50 Å Tip 0.2 V, I 80 pA T 77 K
Kamna, Graham, Weiss, submitted to JACS
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1. Iodine Atoms Promote Aryl Coupling
40 Å x 40 Å VTip 0.2 V I 80 pA T 77 K
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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1. Phenyl Intermediates Pair and Unpair on Cu111
Follow formation breakage of reactive
intermediate pairs.
24 Å x 24 Å, phenyl on Cu111, imaged over 6
hours VSample 0.2 V, I 80 pA, T 77 K
Kamna, Graham, Weiss, submitted to JACS
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1. Self-assemble Polyphenylene on Cu111

• Ullman coupling of para-diiodobenzene to form
  polyphenylene on Cu111
• Strong surface interactions hold polymer flat.
• Measure electronic properties for fully
  conjugated wire.

I
I
Polyphenylene
I
I
175 K
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1. Self-assembled Polyphenylene Protopolymer on
Cu111

• Phenylene forms an unprecedented protopolymer
  interacting intermediates align in extended
  strings, but are not yet covalently bound.
• Produced via adsorption of para-diiodobenzene on
  Cu111.

270 Å x 270 ÅVSample 1 V, I 200 pA T 77 K
100 Å x 100ÅVSample 1 V, I 200 pA T 77 K
McCarty Weiss, submitted to JACS
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1. Polyphenylene Protopolymer on Cu111

• Deliberate motion of a monomer moves a
  protopolymer chain.

120 Å x 120 ÅVSample 0.2 V, I 100 pA T 77
K
McCarty Weiss, submitted to JACS
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1. Substrate-Mediated Interactions
Use intermolecular interactions to drive precise
assembly of supermolecular structurescoupled
reaction intermediates Understand
intermolecular interactions to enhance/guide
chemical reactions.
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2. Self- and Directed Assembly in Monolayers
Use defects to advantage to control motion,
exchange, and accessInsertion, removal, and
diffusion are all defect-mediated. Use
intermolecular interactions to control motion,
stability, and even directionality in
film Control type and density of defectsFilm
and substrate defects can be introduced and
removed independently. Stay away from
equilibriumDo not allow sufficient mobility for
nanostructures created to dissolve Use scanning
probe microscopy and spectroscopy to measure
structure and properties with sub-Ångstrom
resolution.
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2. Self-Assembled Monolayers

• Learn to control positions of molecules by
  controlling chemistry, interactions, motion, and
  defects.

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2. Nanometer-scale Phase Separation
Self-assembled monolayers phase separate (but
remain out of equilibrium). Showed that molecules
remain mobile after adsorption. Key observations
that changed the field of self-assembly.
500Å x 390ÅVSample 1VI1nA
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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2. Control of Molecular Placement

• Use self-assembly, intermolecular interactions,
  deposition, and processing to select film
  structure.
• Films of identical composition can be made to be
  mixed or completely separated using these
  strategies.

Mixed monolayer
Separated monolayer
Note physically perfect boundary but chemically
distinct domains due to lateral epitaxy.
250Å x 250Å VTip1 V, I10 pA
250Å x 250Å VTip1 V, I5 pA
Bumm, Arnold, Charles, Dunbar, Allara
Weiss, JACS 121, 8017 (1999)
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2. Nanometer-scale Phase Separation
Self-assembled monolayers phase separate due to
different interaction strengths between chains
The hydrogen bond in the amide chain doubles the
interaction strength between molecules
250Å x 250ÅVTip1 V, I1 pA
1000Å x 1000ÅVTip1 V, I1 pA
50 CH3(CH2)8NHCO(CH2)2SH 50 CH3(CH2)9SH
self-assembled on Au111
Smith, Reed, Monnell, Lewis, Clegg, Kelly, Bumm,
Hutchison PSW, JPCB 105, 1119 (2001)
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2. Vapor Deposition
Molecules can be inserted selectively into
defects from the gas phase
Au111
CH3(CH2)9SH in EtOH
CH3(CH2)11SH 80 ºC
1000Å x 1000ÅVTip1 V, I1 pA
50 CH3(CH2)8NHCO(CH2)2SH 50 CH3(CH2)9SH
self-assembled on Au111
Donhauser, Price, Tour, PSW, JACS (2003), in
press
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2. Some of Current Strategies for Structural
Control of Films
Structural Control Defects mediate adsorption,
exchange, and insertion. ? Control defect
densities and types by processing films. Vary
chain length, chain functionality, terminal
functional group, head group. Structures --
multi-component films Codeposit to create a mixed
film. Sequentially deposit to insert a small
amount of a second component at defect
sites. Desorb part of a film and replace with a
second component. Applications to date Probe
diffusion of marker molecules. Exploit phase
behavior. Probe single inserted molecules and
bundles of molecules. Create and maintain
nano-scale structures. Elaborate and expand
these strategies.
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Directing Molecules into Precise Positions
Paul S. Weiss Departments of Chemistry Physics,
The Pennsylvania State University http//stm1.chem
.psu.edu/
Develop tools and strategies to drive precise
assembly of molecules in small groups and in
structured monolayers. Chemical reaction
systems Monolayers and functional assemblies
Measure and understand the interactions using
scanning probe measurements with sub-Ångstrom
resolution. Applications in Molecular devices
operation, testing, assembly Chemical
reactions Precision synthesis of novel materials