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

1116 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
Figure 3. Schematic diagram depicting the arrangement
<strong>of</strong> decanethiolates on Au(111) lattice when maximum
coverage <strong>of</strong> the thiolates is attained. (a) Structural model
<strong>of</strong> the commensurate adlayer formed by thiols on the gold
lattice. The arrangement shown is a (�3×�3)R30° structure
where the sulfur atoms (dark gray circles) are positioned
in the 3-fold hollows <strong>of</strong> the gold lattice (white circles,
a ) 2.88 Å). The light gray circles with the dashed lines
indicate the approximate projected surface area occupied
by each alkane chain; the dark wedges indicate the
projection <strong>of</strong> the CCC plane <strong>of</strong> the alkane chain onto the
surface. Note the alternating orientation <strong>of</strong> the alkane
chains defines a c(4 × 2) superlattice structure. The formal
c(4 × 2) unit cell is marked (long dashes); an equivalent
2�3 × 3 unit cell is marked by lines with short dashes.
The alkane chains tilt in the direction <strong>of</strong> their next-nearest
neighbors. (b) Cross-section <strong>of</strong> the SAM formed from
decanethiol. Note the alternating rotations <strong>of</strong> the carbon
chains in this view. The chains are labeled with twist
values (�) to indicate the relative orientations <strong>of</strong> the
neighboring chains.
<strong>of</strong> the chains corresponding to a c(4 × 2) superlattice.
270,271 Figure 3a shows this structure schematically.
The SAMs formed by n-alkanethiols were
originally described as thiolate overlayers (chemisorbed
structures formed by the activation <strong>of</strong> the
S-H bond at the gold surface). 22,258 This structure
had been called into question based on diffraction
experiments and STM imaging that seemed to suggest
a structure involving some degree <strong>of</strong> S-S bonding
between pairs <strong>of</strong> adjacent adsorbates on the
surface <strong>of</strong> the gold (a disulfide model). 161,272,273 Alternative
interpretations have been presented, and this
quasi-disulfide model has now been largely abandoned
in favor <strong>of</strong> the original thiolate model. 259,266,274
The latter is a simple adlayer model <strong>of</strong> the Au-S
bonding interactions. Within this model there has
been considerable discussion <strong>of</strong> the surface sites
involved in this bonding. 273,275 Bonding <strong>of</strong> the thiolates
at both 3-fold hollows and bridge sites has been
suggested on the basis <strong>of</strong> both experiment and
theory. 276,277 This aspect <strong>of</strong> the structure appears to
be unresolved as <strong>of</strong> this writing.
Most studies <strong>of</strong> SAMs on gold have employed
substrates presenting a strong (111) texture to support
the monolayer. Other studies have been directed
at different crystallographic textures, although the
structural literature available in these cases is far
more limited. The SAMs formed on Au(100) appear
to provide an enlightening example <strong>of</strong> the interplay
between surface-directed and organic-directed assembly
in the SAM. The adsorption <strong>of</strong> n-alkanethiols
on Au(100) appears to give thiolate structures organized
as a c(2 × 2) overlayer. 27 The packing density
<strong>of</strong> chains in a structure <strong>of</strong> this sort could not support
a structure canted to the same degree as that found
on Au(111). Such inferences are supported by the
results <strong>of</strong> direct experimental studies <strong>of</strong> the chain
tilts adopted on the two metal surfaces (section 3.2).
3.1.3. Surface Structure <strong>of</strong> <strong>Thiolates</strong> on Palladium
The structure <strong>of</strong> SAMs formed by the assembly <strong>of</strong>
n-alkanethiols on palladium surfaces has been described.
30 These monolayers form on top <strong>of</strong> a complex
surface phase-a palladium sulfide interphase. XPS
data reveal that the monolayer is bound in the form
<strong>of</strong> a thiolate that terminates the metal-sulfide interlayer.
For preparations carried out at room temperature,
at least two sulfur atoms are present in the
interface in the form <strong>of</strong> a palladium sulfide for every
one bound as the thiolate that anchors the SAM. The
sulfur atoms needed to construct this adlayer clearly
derive from a relatively clean excision <strong>of</strong> heteroatoms
from excess adsorbate molecules in solution. The
thiolates form a dense and highly oriented presentation
<strong>of</strong> chainssones well described by a model based
on all-trans chains oriented at a 14-18° angle with
respect to the direction <strong>of</strong> the surface normal. This
packing density correlates well with the expectations
fora(�3×�3)R30° hexagonal adlayer, a structure
that has been observed in UHV studies <strong>of</strong> sulfur
adlayers on Pd(111). 257 This point is, in fact, quite
interesting given the presumption that the (�3�3)-
R30° structure <strong>of</strong> the elemental sulfur phase is a
simple adlayer, that is, the coverage <strong>of</strong> sulfur is onethird
that <strong>of</strong> the atomic density <strong>of</strong> the Pd(111)
surface. The SAMs on Pd, however, clearly show that
nearly two additional sulfur atoms are present in the
form <strong>of</strong> a metal sulfide for every one bonded as a
thiolate; this result suggests that the (�3×�3)R30°
structure may be substitutional in nature.
3.1.4. Surface Structure <strong>of</strong> <strong>Thiolates</strong> on Silver
Most thiol-derived SAMs supported on silver and
copper show qualitative resemblances to properties
seen in assembly on either gold or palladium. 26,29,215,278
The case <strong>of</strong> SAMs on silver is instructive: the
available data again indicate the bonding <strong>of</strong> the
monolayer in the form <strong>of</strong> a thiolate. 26 This observation
is particularly important given that, under the
methods <strong>of</strong> preparation typically used, the silver
surface is generally present in the form <strong>of</strong> a complex
oxide-one that can carry fairly significant amounts
<strong>of</strong> environmentally sorbed contaminants. The interactions
<strong>of</strong> thiols with Ag samples immersed promptly