A R T I C L E S
Barrena et al.
The lowest density striped phase consists of rows of fully
stretched molecules parallel to the surface with a distance
between rows which is approximately twice the length of the
molecule. The stripes run along the next-nearest-neighbor
(NNN) direction of Au(111) with a periodicity of 5 Å along
the row. Increasing coverage leads to the formation of denser
striped phases with a lower distance between rows.20,21,25 The
striped phases have been generally denoted by p×x3, where
the corresponding x3 and p indicate the periodicities along and
between stripes, respectively, in terms of the Au(111) nearest-
neighbor distance (NN). Some of these phases have been also
obtained by annealing from the denser (x3×x3)R30° mono-
layer.14,15,17,22,26 Although most characterization has been ob-
tained from gas-phase deposition studies,12,14-17,21,23,25 identical
striped structures have been also obtained at low coverage
deposited from solution.20,29
Whereas the structures of the lowest density striped phases
and the full-coverage phase are well characterized, the picture
at intermediate coverage is rather complex and has been mostly
studied for short molecules (n e 10).12,13,8,21,30-34 In particular,
Poirier and co-workers have extensively studied the structural
phases of decanethiol molecules (C10) on Au(111)16,25,27
prepared under UHV conditions from vapor deposition. A phase
diagram of C10 on Au(111), in temperature and coverage space,
has been obtained by GIXD23 and STM.35
layer and ascribed to upright configurations.12,28,32,36-39 It is
likely that, at intermediate coverage, a broad spectrum of
different nonequilibrium structures, and coexistence between
them, would be achieved depending on preparation conditions
and chain length.1,40 However, many structural studies do not
provide direct information about the molecular density nor about
film height, making it difficult to determine the number of
molecules per unit cell and the true molecular configuration
(flat, tilted, or upright molecules).
Until now, most evidence for intermediate structures has been
obtained from scanning tunneling microscopy (STM), which
does not provide information for chains longer than 12 carbons
due to the large gap resistance of these films. However, because
GIXD studies on monolayers with 10 e n e 20 have revealed
some deviations of tilt and azimuthal angles at n < 14,41 we
believe that also for intermediate coverage phases, consisting
of nearly standing upright molecules, structural differences
would exist between short and long molecules. These differences
can be rather important due to the balance between competing
interactions.
A recent AFM study of low coverage films of octadecanethiol
(C18) prepared on Au(111) from solution has shown the
existence of a commensurate (2×x3) rectangular structure.42
The area per molecule is ∼29 Å2 (in the c(4×2), this area is
∼21.6 Å2 ), and the molecules are tilted ∼50° from the surface
normal. This structure as well as other tilted configurations arise
from the optimization of the van der Waals energy of the
molecular packing (interlocking of the chains) while keeping
the criteria of commensurability to optimize the energy of the
interface. This rectangular phase was shown to be metastable
because after perturbation by increasing the load of the scanning
AFM tip, the molecules rearrange in the lowest energy
(x3×x3)R30° structure.
Many other structures at intermediate coverage have been
reported for alkanethiols on Au(111),12,13,32-34,36-38 leading to
lack of agreement sometimes in the literature. Some of these
phases have been either grown (from solution or in UHV) or
obtained by annealing from the denser (x3×x3)R30° mono-
(12) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. J. Chem. Phys. 1993, 98,
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(13) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-
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G. Langmuir 1996, 12, 2737-2746.
On the basis of this observation, we have investigated whether
similar tilted phases exist for other alkanethiol molecules (C22,
C18, and C10) and how their stability depends on chain length.
The study has been performed using contact-mode AFM in
ambient conditions. By carefully controlling the applied force,
we could obtain an accurate measurement of the molecular
lattice periodicity as well as the film height without inducing
damage. Control of the applied force also permitted intentional
damage of the structures to test their stability. In addition to
the AFM experiments, some GIXD measurements have been
used to support structural determination of the observed phases.
The present study shows for the first time the formation of
ordered 50° tilted configurations at intermediate coverage
(between that of the striped phases and the close-packed
monolayer) and for molecules of different chain lengths.
Important differences are found for the shortest chain, in terms
of both structure and stability.
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