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

1122 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
adsorbates used to prepare them. These are the
defects that are intrinsic to the dynamic nature <strong>of</strong>
the SAM itself. 149,307 In this regard, one must consider
both the intrinsic structural (i.e., phase) dynamics
<strong>of</strong> the SAM and the thermodynamically imposed
constraints to its stability. The latter issue is one that
is easily understood. SAMs form via a thermodynamically
driven assembly <strong>of</strong> an adsorbate at a
surface/interface. Where the adsorbate-substrate
interaction is sufficiently strong (as for the case <strong>of</strong>
the prototypical layers formed by alkanethiols on
gold), the SAM may be safely removed from the
solution used to prepare it and studied or used
further. Although these SAMs may be kinetically
stable in the absence <strong>of</strong> a flux <strong>of</strong> adsorbate, the high
coverage <strong>of</strong> the adsorbate present in the SAM is, in
fact, thermodynamically unstable. Only in a case
where the rate <strong>of</strong> desorption is rigorously zero would
the SAM be expected to exist for an indeterminate
period outside the solution used to prepare it.
The unique aspects <strong>of</strong> the systems that have
attracted wide attention in studies <strong>of</strong> SAMs are the
essential abilities <strong>of</strong> the best adsorbate-substrate
pairings to resist the competitive binding <strong>of</strong> impurities
at the interface and their substantial stabilities
with respect to thermal desorption or displacement
by other chemical species. This stability is, however,
one that is limited by the finite strength <strong>of</strong> the M-S
bond and by the susceptibility <strong>of</strong> the simplest thiolate
systems toward decomposition (whether via oxidative
degradation or other dissociative pathways) reactions
that are sensitive to the ambients in which the SAMs
are used. Still, the main concern for stability remains
desorption. For simple SAMs <strong>of</strong> thiolates on gold, the
limit <strong>of</strong> thermal stability due to desorption is modest
but quite useful (especially at room temperature). 308
One also encounters classes <strong>of</strong> defects that are
related to intrinsic dynamics <strong>of</strong> the organic component
<strong>of</strong> the SAM. The chain dynamics <strong>of</strong> alkanethiolate
SAMs on gold provide an instructive example.
First, because the chains <strong>of</strong> these SAMs are canted
(reflecting the gold-sulfur spacings), the chains are
subject to a variety <strong>of</strong> complex phase transitionss
thermally driven population <strong>of</strong> gauche conformers
and tilt-order phase transitions are among some <strong>of</strong>
the phase dynamics that have been investigated and
used to rationalize aspects <strong>of</strong> their interfacial properties.
149,307 Order-order phase transitionsssuch as
those involving a posited thermal coexistence <strong>of</strong> the
c(4 × 2) and (�3×�3)R30° phasessconstitute another
example. In yet another example, Grunze
interpreted the relative protein-binding affinities <strong>of</strong>
oligo(ethylene glycol) (OEG)-modified SAMs on gold
as arising from a coverage-dependent rod-helix
ordering transition <strong>of</strong> the OEG chain end segments
(see section 8.4.1). 309,310 This last example illustrates
the subtle interplay <strong>of</strong> physical features that might
serve to modulate the properties <strong>of</strong> SAMs in a specific
application.
4. Removing SAMs from Surfaces
There are a number <strong>of</strong> different techniques for
removing SAMs from gold, silver, and other substrates.
Thermal desorption 311 or ion sputtering 312 are
convenient techniques for removing SAMs from
single-crystal substrates in UHV environments. SAMs
are mechanically fragile surfaces, and thus, techniques
for polishing or roughening surfaces <strong>of</strong> metals
can remove the SAM and expose a clean surface on
bulk metal substrates. 313 Chemical oxidants or reductants
such as concentrated acids or bases or
“piranha” solutions (H2O2:H2SO4) 227 also are effective
for cleaning substrates. Another method for removing
SAMs from metal substrates is plasma oxidation. 314
Some substrates such as patterned thin films or
suspensions <strong>of</strong> nanoparticles (colloids, rods, other
structures) can be damaged by harsh mechanical or
chemical treatments. We discuss three mild chemical
methods that are used to remove or exchange SAMs
on surfaces; these methods <strong>of</strong>fer mild conditions and
chemical selectivity.
4.1. Electrochemical Desorption <strong>of</strong> SAMs
Thiols undergo reductive desorption when a negative
potential is applied to the supporting metallic
film. 197,227,315 For electrochemical desorption SAMs
typically are immersed in an aqueous or ethanolic
solution with an electrolyte at a neutral or basic
pH. 316 The electrochemical half-reaction for alkanethiolates
adsorbed on metals is
RS-M + e - f RS - + M 0
(1)
Both the thiolate and the bare metal surface become
solvated, and the thiolate diffuses away from the
surface. The process is reversible: removing the
applied negative potential can result in readsorption
<strong>of</strong> the thiolates onto the metal surface. 317
Studies <strong>of</strong> the mechanism <strong>of</strong> this process suggest
that desorption occurs first at defect sites and grain
boundaries in the SAM and then at apparently
random nucleation sites within the well-organized,
crystalline regions <strong>of</strong> the SAM. 318 The electric fieldinduced
rate <strong>of</strong> desorption seems to be highest for
the adsorbate molecules at edges and defects. The
potential at which the desorption <strong>of</strong> alkanethiolates
occurs depends on a number <strong>of</strong> factors, including the
chain length, degree <strong>of</strong> ordering and number <strong>of</strong>
intermolecular interactions (hydrogen bonding) within
the organic film, and crystallinity <strong>of</strong> the substrate. 319
A typical desorption potential for n-alkanethiolates
is -1.0 V with respect to a Ag/AgCl (satd KCl)
reference electrode, but the value can vary for different
structures by (0.25 V. This range makes it
possible to desorb one component <strong>of</strong> a mixed SAM
selectively by controlling the applied potential. 320
4.2. Displacement <strong>of</strong> SAMs by Exchange
The molecules comprising a SAM exchange gradually
(minutes to hours) when exposed to solutions
containing other thiols or disulfides. This replacement
reaction generally does not yield a homogeneous
or uniform SAM nor does it provide a bare
substrate, but it does <strong>of</strong>fer a route to generate a new
organic surface on a substrate already supporting a
SAM. The mechanism <strong>of</strong> this reaction has been
studied on SAMs supported on thin films by a
number <strong>of</strong> techniques, including contact angle goni-