Sulfur-Substrate Interactions in Spontaneously Formed
Sulfur Adlayers on Au(111)
C. Vericat, M. E. Vela, G. Andreasen, and R. C. Salvarezza
Instituto de Investigaciones Fisicoquı ´micas Teo ´ ricas y Aplicadas (INIFTA),
(CIC-CONICET-UNLP), Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina
L. Va ´zquez and J. A. Martı ´n-Gago*
Instituto de Ciencia de Materiales de Madrid (CSIC), 28049-Madrid, Spain
Received December 30, 2000. In Final Form: April 3, 2001
The electroadsorption of S on Au(111) from 0.1 M NaOH + 3 × 10
-3
M Na2S solutions has been studied
by in situ scanning tunneling microscopy (STM), electrochemical methods, and ex situ X-ray photoemission
spectrocopy (XPS). By analyzing STM images, we have observed that S adsorbs on Au(111) forming a
3×3R30° superstructure. Under potential control this lattice slowly and continuously transforms into
S octomers (S8) in the range -0.7/-0.5 V (i.e., at typical potentials observed under open circuit conditions).
In this potential range, mixtures of both structures are present on the Au(111) surface. An XPS study of
the S 2p peak from the adlayers reveals the presence of three components that can be assigned to S forming
a 3×3R30° structure, S8, and bulk S at surface defects. The most important component is that
corresponding to S8, in good agreement with the STM images. Furthermore, XPS spectra recorded for
3×3R30° thiol adlayers on Au(111), characterized by STM and atomic force microscopy, lead to similar
S 2p XPS spectra. A comparison between these cases allows us to conclude that S in spontaneously formed
S8 on Au(111) exhibits the same binding energy of the core electronic levels (i.e., same chemical state) as
S in 3×3R30° spontaneously formed thiol lattices, although the adsorption sites are different.
Introduction
Self-assembled monolayers (SAMs) of alkanethiols on
metals have attracted considerable scientific interest.
1
These fascinating two-dimensional structures have po-
tential applications to modify wetting and wear properties
of solid surfaces and to anchor different functional groups
to be used in chemical and biochemical sensors. Also, they
can be used to protect metal surfaces against corrosion
and to be used as masks for the fabrication of nanodevices
for electronics and magnetic storage media.
2
One of the
main problems in understanding self-assembly of SAMs
on metals arises from the fact that alkanethiol-metal
and alkanethiol-alkanethiol interactions are not fully
understood. These interactions determine the stability of
SAMs, a crucial point for their use in many technological
applications.
In aqueous solutions, the most important environment
for technological applications, reductive electrodesorption
has been used to explore SAM stability.
3
In fact, SAMs on
Ag(111)
4
and Au(111)
5
are desorbed in sharp voltammetric
peaks whose peak potentials (E
p
) shift in the negative
direction as the length (n) of the alkanethiol hydrocarbon
chains, given in C units, increases. Based on the shift in
E
p
, stabilizing forces acting in 3×3R30° and related
superlattices of alkanethiols adsorbed on Au(111) and
Ag(111) in contact with aqueous solutions have been
estimated in ≈3-4 kJ/mol C units.
4,5
This energy involves
van der Waals and hydrophobic forces, both stabilizing
SAMs. Conversely to this progress in the understanding
of the lateral interactions prevailing in SAMs, the nature
of the S-Au bond is not fully understood. In fact, although
the chemical state of S atoms at the alkanethiol/Au
interface has been extensively studied by XPS, the
interpretation of experimental data still remains contro-
versial.
6-10
Recently, the behavior of electroreductive desorption
curves for 3×3R30° alkanethiol adlayers (and its
c(4×2) superlattice), recorded in aqueous 0.1 M NaOH,
has been compared to that found for similar S adlayers.
11
It has been found that the E
p
vs n plot leads to a value
for n ) 0, i.e., alkanethiols adsorbed on Au in the absence
of chain-chain interactions, 0.2 V positively shifted with
respect to the E
p
value for S electroreductive desorption.
This is a strong indication that the S-Au bond in
3×3R30° alkanethiol adlayers differs from the S-Au
bond in 3×3R30° S lattices.
In this paper we investigate the nature of the S-Au
bond for both spontaneously formed S and alkanethiol
adlayers using in situ scanning tunneling microscopy
(STM), ex situ atomic force microscopy (AFM), and
photoemission experiments. We have found that the
spontaneously formed S adlayer on Au consists mainly of
S
8
coexisting with 3×3R30° domains and bulk S. We
* Corresponding author.
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10.1021/la0018179 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/10/2001