Self-organized calix[4]arenes on Au„110…-„1 Ã 2…: A combined low-energy electron diffraction
and scanning tunneling microscopy experimental study with molecular mechanics calculations
Véronique Abad Langlais* and Yves Gauthier
Laboratoire de Cristallographie, CNRS-BP166, 38042 Grenoble Cedex 9, France
Hafid Belkhir
Département de Physique, Université d’Annaba, 23000 Annaba, Algeria
Olivier Maresca
Unitat de Química Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
Received 17 February 2005; revised manuscript received 12 May 2005; published 18 August 2005
We report on self-assembly of calix4arenes deposited on Au110-1 2. The molecules consist of a
macrocycle with four phenol rings linked by methylene groups in the ortho-position. The scanning tunneling
microscopy images together with the low-energy electron diffraction patterns allow us to identify the molecular
conformer and the unit cell. The intermolecular interactions are rather weak and are governed by stacking
and van der Waals–type interactions as deduced from molecular mechanics calculations. The two-dimensional
molecular self-ordering is due to a strong interaction with the gold substrate resulting in the replacement of the
initial 1 2 missing row reconstruction by a 1 3 structure upon molecular adsorption.
DOI: 10.1103/PhysRevB.72.085444 PACS numbers: 61.10.Nz, 05.65.b, 07.79.Cz, 02.70.c
I. INTRODUCTION
Organic molecules offer a large variety of complex mo-
lecular architectures. When deposited on a metallic substrate,
they provide functionality and high selectivity for applica-
tions such as selective catalysis, sensors, lithography, and
optoelectronics to name a few.
1–8
Moreover, organic mol-
ecules have the capability to self-organize at surfaces in
beautiful superstructures presenting long-range order.
9
In the
last few years, concomitantly with the development of sur-
face science techniques, they have attracted an increasing
interest due to the possibilities they offer to perform selective
two-dimensional 2D surface chemistry. The state of the art
is fully described in a recent review that deals with large
organic molecules.
10
To the large class of complex molecules
considered therein, one has to add the whole family of
calixnarenes where n =3–14 is the number of phenol
groups that form the macrocycle size. The name, which de-
rives from the Greek word for vase, was chosen because of
the conformational vaselike shape. Usually, the synthesis of
the calixarenes is based on phenol and formaldehyde con-
densation under various conditions.
11
Moreover, additional
complexity can be added since the aromatic cycles can be
doted with different substituents, which permit us to design
them for specific purposes. During the last decade, the calix-
arenes have been extensively studied for their properties as
receptors of neutral and ion guests.
12,13
Indeed, they play an
important role in molecular recognition as hosts for smaller
organic molecules, for ions, clusters or neutral guests. The
main applications of calixarenes reported up to date are their
use as sensors.
14,15
Functionalized calix4arene derivatives
exhibit, for example, sodium ion selectivity while
calix6arene has shown excellent sensitivity to cesium ions.
Moreover, calixarenes embedded into polymeric membranes
have been successfully used as ion-selective electrodes in
electrolyte-insulator-semiconductor EIS and ion-selective-
field-effect-transistors ISFETs type sensors. Often, this
kind of applications requires the elaboration of thin films.
Such films could be obtained 1 by self-assembly monolay-
ers SAMs when the calixarenes are functionalized by a -SH
group,
16,17
2 by sublimation of thick films as in the case of
the p-tert-butyl-calix8arenes used as recognition agents in
EIS and ISFETs sensors,
18–20
and 3 by electrochemical
deposition from solution, the most used deposition mode.
Moreover, recent fundamental studies have revealed a coher-
ent proton tunneling phenomenon in the cyclic network of
four hydrogen bonds in calix4arenes.
21,22
In addition, previous works on calix4arenes and deriva-
tives have revealed various adsorption modes on the dense
Au111 phase: the molecules may bind to the substrate via a
lateral phenol group or via the lower rim of the four phenol
groups.
23–25
The ability of these molecules to accept guest
species and/or to act as a sensor will highly depend on their
relative orientation and density and on their orientation rela-
tive to the substrate. In this respect, the crystallographic ori-
entation of the substrate is expected to play a crucial role.
While Au111 is nearly flat, the open Au110 surface is
highly corrugated with every other top row missing. Using
this ridge-and-valley structure, we aim at producing molecu-
lar arrangements at variance with those observed on Au111.
Indeed, the top rows, which extend over hundreds if not
thousands of Å, will possibly force the molecules to align, at
least in one direction, and the inter-ridge distance, of the
same order of magnitude as the long side of the molecules,
may help obtaining higher molecular density.
In the following, we show that the stablest conformer with
“crown shape” yields a compact stacking and that the mol-
ecules interacts so strongly with the substrate that the origi-
nal 1 2 missing row structure transforms into a 1 3
structure. This dense molecular layer is obtained at the price
PHYSICAL REVIEW B 72, 085444 2005
1098-0121/2005/728/0854447/$23.00 ©2005 The American Physical Society 085444-1