Self-Assembled Monolayers of Thiolates on Metals as - Whitesides ...

1132 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
that the monolayer is not well packed along the outer
edge.
6.3.1. Spectroscopic Evidence for SAM Structure on
Nanoparticles
Spectroscopy (IR and NMR) provides information
about the conformation and packing <strong>of</strong> the alkyl
chains on the nanoparticles. 166,168,170,171,467,476,477 A
solid-state IR study <strong>of</strong> the structure <strong>of</strong> SAMs <strong>of</strong>
n-alkanethiolates on 1-2 nm gold clusters showed
that the major difference between planar SAMs and
SAMs on nanoparticles is that the SAMs on nanoparticles
exhibit a higher number (10-25%) <strong>of</strong> chain
end-gauche defects (for all chain lengths) than SAMs
on planar substrates. 171 The same study found that
SAMs on nanoparticles have a number <strong>of</strong> nearsurface
and internal kink defects similar to that <strong>of</strong>
planar SAMs formed from alkanethiols <strong>of</strong> similar
lengths. IR spectra <strong>of</strong> the same nanoparticles in
carbon tetrachloride show a degree <strong>of</strong> disorder comparable
to that <strong>of</strong> liquid n-alkanes. 470 One interpretation
<strong>of</strong> the difference in IR spectra between solution
and the solid phase is that the packing <strong>of</strong> the
nanoparticles in the solid state induces some degree
<strong>of</strong> order on the alkanethiolates. Alternatively, the
solvation <strong>of</strong> the alkyl chains by carbon tetrachloride
could account for the observed disorder.
As the size <strong>of</strong> the particle increases, the properties
<strong>of</strong> the SAM become more similar to a SAM on a
planar surface: particles with a core diameter greater
than 4.4 nm, coated with a SAM <strong>of</strong> dodecanethiolates,
have spectroscopic and physical properties approximating
that <strong>of</strong> a planar SAM. 166 For 4.4 nm particles
the majority <strong>of</strong> the surface comprises flat {111}
terraces rather than edges and corners; this geometry
leads to “bundles” <strong>of</strong> ordered alkanethiolates with
gaps (areas with a disordered organic layer) at the
corners and vertexes (Figure 9b). 170,472,473 These
“bundles” have been hypothesized to play an important
role in the solid-state packing <strong>of</strong> nanoparticles
into lattices (see section 6.4).
6.3.2. Evidence for the Structure <strong>of</strong> SAMs on
Nanoparticles based on Chemical Reactivity
The chemical reactivities, both <strong>of</strong> the metal core
and the alkanethiolate ligands, have been used to
evaluate the structure <strong>of</strong> SAMs on nanoparticles. For
example, Murray and co-workers studied the kinetics
and thermodynamics <strong>of</strong> the displacement <strong>of</strong> one
alkanethiolate for another on the surfaces <strong>of</strong> gold
nanoparticles (2 nm diameter) as a function <strong>of</strong> chain
length. 446,459,478 They find that the alkanethiolates
bound to the vertexes and edges have a higher rate
<strong>of</strong> exchange than those in the dense, well-packed
planar faces. The rate <strong>of</strong> exchange decreases as the
chain length and/or steric bulk <strong>of</strong> the initial SAM
increases.
The susceptibility <strong>of</strong> differently protected gold cores
to a cyanide etchant gives an indication <strong>of</strong> the density
<strong>of</strong> packing <strong>of</strong> the alkanethiolates in the SAM (section
8.1). 470,476,479 For nanoparticles, the rate <strong>of</strong> dissolution
(etching) decreases with increasing chain length; the
rate remains constant when the chain length is
greater than 10 carbons, however. 470 This result
complements the spectroscopic evidence, which in-
dicates an ordered inner core with increased fluidity
<strong>of</strong> the carbon chains in the outer shell. 170
<strong>Monolayers</strong> on nanoparticles composed <strong>of</strong> branched
alkane chains <strong>of</strong>fer a higher degree <strong>of</strong> protection to
chemical etching (sodium cyanide) than do straight
chain alkanes. 470,476 Murray and co-workers found
that SAMs formed from 2-butanethiol decreased the
rate <strong>of</strong> etching by NaCN to the same degree that
hexanethiol did. 470 In a related study, Rotello and coworkers
formed SAMs from alkanethiols functionalized
with a variety <strong>of</strong> amides and esters with branched
end groups and evaluated the stability <strong>of</strong> the monolayer-protected
clusters using cyanide etching and IR
spectroscopy. 476 They hypothesized that “cone-shaped”,
branched molecules would more effectively occupy the
volume available at the outer edge <strong>of</strong> the monolayer
than simple n-alkanethiolates, which have a linear
geometry when extended in an all-trans conformation
(Figure 9).
The chemical reactivities <strong>of</strong> terminal functional
groups displayed on SAMs formed on the surfaces <strong>of</strong>
nanoparticles are different than those on SAMs on
planar surfaces. For example, Murray and co-workers
demonstrated that SN2 reactions occur more readily
on the surfaces <strong>of</strong> nanoparticles than on planar
surfaces. 470 The headgroups <strong>of</strong> ω-bromoalkanethiolates
are less densely packed on curved surfaces
than they are on planar surfaces; this lower density
allows backside attack <strong>of</strong> the incoming nucleophile
(amine) to occur. The rate is a function <strong>of</strong> the steric
bulk <strong>of</strong> the incoming amine as well as <strong>of</strong> the relative
chain lengths <strong>of</strong> the bromoalkanethiolates and the
surrounding alkanethiolates. The measured rates are
similar to solution-phase rates for SN2 substitutions,
in agreement with the spectroscopic data (section
6.3.1) regarding the fluidity <strong>of</strong> the SAMs on nanoparticles.
6.4. SAMs and the Packing <strong>of</strong> Nanocrystals into
Superlattices
The same surfactants that are used to control the
size and shape <strong>of</strong> nanocrystals also influence the
organization <strong>of</strong> the particles into superlattices and
colloidal crystals. 480-483 In colloidal crystals the nanoparticles
are sometimes referred to as “molecules”
and the van der Waals contact <strong>of</strong> surfactant layers
on neighboring particles as intermolecular “bonds”. 54
When spherical nanocrystals, protected by a layer <strong>of</strong>
alkanethiolates, are allowed to self-assemble on a
TEM grid via slow evaporation <strong>of</strong> solvent, they form
hexagonal close-packed 2-D arrays (Figure 10). 483-486
Preparations with shorter alkyl chains (hexanethiol)
assemble in solution to form ordered 3-D, colloidal
crystals. 486 The separation <strong>of</strong> the close-packed spheres
is linearly dependent on the length <strong>of</strong> the alkyl chains
(Figure 10b). 486,487 The increase in particle spacing
per additional carbon (∼1.2 Å) is about one-half <strong>of</strong>
the expected value; this observation suggests that the
alkyl chains might interdigitate with the chains on
neighboring particles. 81,482,486,488 There are also examples
<strong>of</strong> hydrogen-bonding control over interparticle
spacing for gold nanoparticles with carboxylic-acidterminated
SAMs. 447,489
Nanocrystals with other morphologies assemble,
promoted by the hydrophobic tails <strong>of</strong> the capping