Surface Science 559 (2004) L187–L193
www.elsevier.com/locate/susc
Surface Science Letters
Sexiphenyl on a Ni(1 1 0)(2 · 1)-O surface: A single-molecule
STM study
G. Koller *, S. Surnev, M.G. Ramsey, F.P. Netzer
Institut f€ur Experimentalphysik, Karl-Franzens-Universit€at Graz, Universitaetsplatz 5, A-8010 Graz, Austria
Received 24 November 2003; accepted for publication 15 April 2004
Available online 10 May 2004
Abstract
The adsorption of p-sexiphenyl on the oxygen passivated Ni(1 1 0)(2 · 1)-O reconstructed surface has been investigated by STM. The sub-molecular resolution obtained allowed the individual phenyl rings to be observed along with
their orientation and registry on the surface. The molecules adsorb predominantly in a planar geometry within the
surface reconstruction in a manner which maximises the number of four-fold hollow sites for the individual rings on the
underlying Ni(1 1 0). Local compression of the Ni–O rows leads to the formation of adsorption channels which result in
the preferential arrangement of the molecules in quasi-periodic 1-dimensional molecular strings.
2004 Elsevier B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Self-assembly; Nickel; Aromatics; Low index single crystal surfaces
Organic molecules with conjugated functionalities are attracting increasingly interest as active
components in the field of organic electronics, and
devices such as light emitting diodes or field effect
transistors with promising characteristics have
been successfully fabricated recently [1,2]. The
growth of ordered thin films of the organic molecules on metal surfaces, which can be used both as
a growth template and contact materials, is an
important further step in the development of organic device optimisation procedures: for instance
in optoelectronic applications the preferential
*
Corresponding author. Tel.: +43-316-380-5219; fax: +43316-380-9816.
E-mail address: [email protected] (G. Koller).
alignment of the active molecules would allow the
tailoring of the emission in a desired way and
direction.
Since most complex molecules are semiconducting or insulating in their pristine state, these
organic materials can also be used as dielectric
layers instead of as active elements in hybrid organic–inorganic devices. In this case easy processability and cheap fabrication routes for large-area
applications can provide technological advantages
over all-inorganic systems. An interesting potential
impact of organic thin films has been proposed recently as dielectric layers in organo-metallic spintronic devices [3]. Here, the combination of an
organic dielectric layer sandwiched between two
ferromagnetic metals may be considered as a hybrid
alternative of all-metallic spin valves or metal-oxide
0039-6028/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.04.019
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based magnetic tunnelling junctions [4]. However,
the interface between an organic layer and ferromagnetic metals is a problematic issue, because the
ferromagnetic transition metals are typically very
reactive towards organic molecules. The strong
interactions between the metal and the molecules at
the interface may lead to decomposition and degradation of the organic layer and to undesired
interfacial properties in terms of charge transport
and stability. The passivation of the metal surface
by atomic adsorbates such as oxygen or sulphur
prior to the deposition of the organic layer has been
shown to be a successful route to reduce the reactivity of metal surfaces [5,6].
Certain complex molecules may however not
only be viewed as the starting points for bulk
electronic materials, but are considered good
candidates as the active components within electronic circuitry in molecular electronic devices [7].
Molecules have an inherent attractiveness because
of their size, representing the ultimate in terms of
atomic control over physical properties. The basic
science of a single-molecule electronics technology
is now just unfolding, and it involves the exact
knowledge of how molecules transport charge and
interact with the macroscopic world via surfaces
and interfaces. The bonding and configuration
of individual molecules at a molecule–electrode
interface is a critically important component of a
molecular junction: it determines the charge flow
and the electrical performance of the junction.
With this background in mind we have undertaken a single-molecule study of the coupling of the
linear p-sexiphenyl molecule (C36 H26 ) on the surface of a ferromagnetic metal, namely the Ni(1 1 0)
surface. We have used high-resolution scanning
tunnelling microscopy (STM) to obtain insights
into the fundamental processes related to the
interaction of sexiphenyl with the Ni surface and to
reveal the molecular conformation and the
adsorption site. p-Sexiphenyl (6P) is a prototypical
molecule in the field of molecular optoelectronics,
which is used as a dye and active component in
organic LEDs [8]. Since the interaction of aromatic
molecules with Ni surfaces is in general strong
[5,6,9], the reactivity of the Ni(1 1 0) surface has
been reduced by oxygen pretreatment, which leads
to a (2 · 1)-O surface reconstruction [10]. We find
that the Ni(1 1 0)(2 · 1)-O template surface does
not behave statically upon interaction with sexiphenyl, but that the –Ni–O– added rows of the
reconstruction rearrange in a ‘‘zipping action’’ to
accommodate the molecular species. We are able to
distinguish the individual phenyl moieties of the 6P
molecules together with the substrate atoms on the
surface in high-resolution STM images, and thus
can determine the exact details of the molecular
conformation and of the adsorption site. While the
majority of the 6P molecules adsorb with their
phenyl rings close to parallel to the Ni surface, 6P
species with twisted rings can also be observed.
The experiments have been performed in a
custom-designed ultrahigh vacuum system, operating at a base pressure of <1 · 1010 mbar. The
system is equipped with STM, LEED, Auger
electron spectroscopy, and the usual facilities for
crystal cleaning, surface preparation and physical
vapour deposition. The STM is a variable-temperature instrument, which has been operated
during the present study at room temperature; it is
described in detail elsewhere [11]. The STM images
were recorded in a constant current mode with
electrochemically etched W tips, cleaned in situ by
electron bombardment.
The nickel crystal was cleaned by repeated Arþ
ion sputtering and annealing (1000 K) cycles, and
surface cleanliness and order were checked by
LEED and Auger electron spectroscopy. The
Ni(1 1 0)(2 · 1)-O reconstruction has been prepared
by exposing the clean Ni(1 1 0) surface to 2 L O2 (1
Langmuir (L) ¼ 1 · 106 Torr s) at 400 K. This
resulted in a well ordered (2 · 1) reconstruction as
evidenced by LEED. During all stages of substrate
preparation STM images with atomic resolution
could be obtained and were used for calibration of
the scanner. Sexiphenyl (6P) molecules were dosed
in situ by organic molecular vapour deposition
using a home designed Knudsen-type evaporator
per minute)
(evaporation rates of the order of 1 A
with the substrate surface kept at room temperature. The system pressure during evaporation was
below 5 · 1010 mbar, with hydrogen constituting
the major component of the residual gas. The 6P
coverage has been determined by quantitative
evaluation of the STM images; it is quoted in
monolayers (ML), where 1 ML is defined as the
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coverage where the whole surface is covered by a
single layer of flat lying 6P molecules, using its van
· 27.2 A
182 A
2 [12]).
der Waals area (6.7 A
Oxygen chemisorbs dissociatively on the
Ni(1 1 0) surface and induces at RT various ordered
phases in the sequence: a (3 · 1), a (2 · 1), and again
a (3 · 1) reconstruction with increasing oxygen
coverage [13]. These reconstructions grow via an
added row growth mechanism of –Ni–O– rows,
which run along the [0 0 1] azimuth direction. The
overall morphology of the Ni(1 1 0)(2 · 1)-O surface is shown in the large scale STM image of Fig.
1(a). Although the surface is atomically flat over
one also observes
distances of more than 1000 A,
several large elongated areas of lower terraces,
where a single atomic layer of nickel has been re-
moved to form the added row (2 · 1)-O reconstruction. As seen in the high-resolution image of
Fig. 1(b), the (2 · 1)-O phase is well ordered and
displays parallel lines of atomic maxima running
diagonally across the image, i.e. along the [0 0 1]
azimuth. As reported in the literature [13] the
maxima in the STM image at the employed tunnelling conditions correspond to the Ni atoms of
the reconstruction.
Fig. 2 shows an STM image of the Ni(1 1 0)(2 · 1)-O surface covered with 0.05 ML of 6P at
RT. The molecules appear as rod-like features
with internal structure, in which the six phenyl
rings can be discerned. The molecules appear to be
preferentially attached to defects of the (2 · 1)
reconstruction, e.g. at the antiphase domain
boundary, which runs diagonally through the image. The direction of the molecular axes of 6P with
respect to the –Ni–O– rows of the substrate displays four distinct orientations: parallel (molecular
axis along [0 0 1]), and perpendicular (molecular
axis along [1 1 0]) to the –Ni–O– rows, and rotated
by ±35 relative to the –Ni–O– rows (diagonally to
the 2D unit cell, i.e. parallel to the [1 1 2] azimuth).
A statistical analysis of the molecular species in
Fig. 2 reveals that for this low coverage all orientations are of equal probability. Presumably the
2 ) of the
Fig. 1. (a) Large scale STM image (2300 · 2300 A
Ni(1 1 0)-p(2 · 1)-O surface at RT. (b) Atomically resolved im2 ) displaying the –Ni–O– rows. The principal
age (70 · 40 A
surface azimuths are indicated.
2 ; 2 V, 0.1 nA) of 0.05 ML
Fig. 2. STM image (500 · 380 A
sexiphenyl on the Ni(2 · 1)-O substrate. Ellipses highlight
curved species and arrows indicate molecules folded over –Ni–
O– rows.
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availability of the different defect sites over the
surface is similar. The apparent heights of the 6P
molecules in the STM image of Fig. 2 measure
(ii) 0.5 ± 0.1 A,
three different levels: (i) 0.3 ± 0.1 A,
and (iii) 1.9 ± 0.2 A above the –Ni–O– rows. These
may be interpreted in terms of molecules being
incorporated into the –Ni–O– rows (i), (ii) and
molecules lying on top of the rows (iii). The latter
higher, i.e. close to the 1.25 A
are 1.5 ± 0.3 A
height difference expected from the Ni interplanar
discrepancy is presumably
distance. The 0.3 A
due to electronic effects. The incorporated molecules along the closely packed rows of the second
Ni layer, i.e. those perpendicular to the –Ni–O–
rows of the reconstructed first layer, appear with
lower height (i) than the molecules lying across the
closely packed rows of the underlying Ni layer
(height ii). In several cases molecules are observed,
where the intramolecular contrast and the appar along the
ent height changes from 0.5 to 1.9 A
molecular axis (see arrows in the figure). This
is close to the nickel
height difference of 1.4 ± 0.2 A
and suggests that
interplanar distance of 1.25 A
these molecules are ‘‘anchored’’ at Ni atoms below
the reconstructed top layer with one end and are
located on top of the Ni–O rows with the other
end. This indicates that the interring C–C bond is
rather flexible, and this can even lead to curved
species (see the encircled molecules in Fig. 2).
Fig. 3(a) shows a high magnification STM image
of individual sexiphenyl molecules, which allows us
to detect the molecular details: chains of six bright
rings are clearly distinguished. On the basis of the
molecular dimensions these rings are assigned
as individual phenyl rings. The appearance of
the rings suggests that the molecular planes of the
phenyl rings are oriented closely parallel to the
surface plane. Since the atomic resolution can be
achieved for both the substrate and the molecule
under the same tunnelling conditions, the substrate
can be used as a ‘ruler’ to determine the registry
and to measure the apparent molecular dimensions
with high accuracy. A linescan taken along the
short molecular axis across a ring, shown in Fig.
3(b), displays two maxima separated by 2.4 ± 0.2 A.
The apparent length of the molecule measured
along the long molecular axis, in Fig. 3(c) is
(measured at full width at half maxi26.5 ± 0.5 A
2 ; 1.5 V, 0.1 nA) of
Fig. 3. (a) Atomic scale image (80 · 80 A
individual sexiphenyl molecules. A (1 · 1) grid, where the corners match the underlying Ni(1 1 0) four-fold hollow positions,
has been superimposed on the molecule in the top left corner.
The phenyl rings of the twisted molecule in the top right corner
are marked 1 to 6 according to the text. Line scans across and
along the planar molecule are shown in (b) and (c).
mum) with an interring distance (minimum–mini These values are consistent
mum) of 4.4 ± 0.3 A.
with the geometry parameters derived from the
known bond lengths and from X-ray diffraction
measurements [12]. The STM images indicate that
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most molecules have locally removed the Ni–O
reconstruction and that they are attached to the Ni
atoms of the second layer (see below). While the
majority of the phenyl rings is oriented parallel to
the Ni surface, tilted phenyl rings are also observed
occasionally––see the molecule labelled 1–6 (upper
right of Fig. 3(a)), where the phenyl rings 1, 5, and
6 appear to be planar, whereas the phenyl rings 2,
3, and 4 seem to be tilted away from the surface.
Although the reason for this tilting is not directly
apparent, it indicates the rotational flexibility of
the molecule along the interring C–C bonds.
The registry of the 6P molecules parallel to the –
Ni–O– rows will be discussed below. To determine
the precise adsorption site of the molecules, which
are oriented at ±35 with respect to the –Ni–O–
rows, a (1 · 1) substrate mesh has been placed over
the molecule in the upper left hand corner of Fig.
3(a). This shows that the centres of all the aromatic rings are in registry with the diagonal
direction of the unit cell on the Ni(1 1 0) surface
and that all rings are located above the four-fold
hollow Ni sites. Note that the four-fold hollow site
has also been established as the preferred adsorption site for benzene on Ni(1 1 0) [14].
An interesting question concerns the origin of the
observed molecular image contrast in the STM.
Sautet [15] has shown that the STM images of
conjugated molecules are often dominated by
quantum interference effects, which influence the
actual molecular shape. Not only orbitals close to
the Fermi level, but also deeper lying orbitals contribute significantly to the image contrast. The
images thus originate from a superposition of individual molecular orbital and substrate contributions. The STM image contrast of sexiphenyl on
Ni(1 1 0)(2 · 1)-O was fairly robust, and atomic-type
resolution could be obtained over a wide range of
bias voltages, from +2 to )2 V. The appearance of
the molecules remained very similar with bias, in
agreement with the theoretical calculations that the
imaging process is not determined by a single
orbital. The shape of the molecules as seen actually
seem to reflect the carbon backbone of the molecule.
On increasing the sexiphenyl coverage to 0.2
ML the formation of molecular chains running
along the [0 0 1] substrate direction can be seen in
the STM image of Fig. 4(a). Here, 50% of the
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molecules are arranged in such strings. The
appearance of these strings is always accompanied
by a local change of the (2 · 1) reconstruction on
either side of the molecular chain, i.e. the –Ni–O–
rows with (2 · 1) periodicity are compressed to
–Ni–O– double rows with a local (1 · 1) periodicity, as shown in the schematic diagram of Fig. 4(b).
It should be noted that such local (1 · 1) double
rows also exist in the (3 · 1)-O reconstruction of
Ni(1 1 0) [13]. We propose the following mechanism for the formation of the molecular chain
2 ; 2.3 V, 0.03 nA) showing
Fig. 4. (a) STM image (400 · 240 A
strings of sexiphenyl molecules. The arrows mark single atoms
between molecules. (b) Model displaying the observed sexiphenyl adsorption sites. Dark large circles are nickel atoms of
the (2 · 1) reconstruction, small circles are oxygen atoms and
the light large circles indicate the position of Ni in the underlying substrate. The bottom part of (b) illustrates the compression of the –Ni–O– substrate rows and the registry of
individual sexiphenyl molecules within the strings. The fourfold ring sites are indicated.
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structures. Since the spacing between two adjacent
is not enough to accommo–Ni–O– rows (5 A)
date a flat lying sexiphenyl molecule (van der
each –Ni–O– row is pushed
Waals width is 7 A),
aside by one nickel spacing. This movement of the
Ni–O rows can be imagined like the opening up of
an atomic zipper and it appears to be a non-local
effect and continues until stopped by a defect of
the reconstruction (note that the zipping sometimes extends to beyond the 6P chains). This
wide channel of bare Ni
process creates a 10 A
atoms with a high affinity for the adsorption of 6P
molecules as indicated in the model of Fig. 4(b),
and leads to the formation of the observed strings
of molecules. A local compression of –Ni–O– rows
has also been observed by Stensgaard et al. [14]
upon benzene adsorption on an oxygen precovered
Ni(1 1 0) surface, where the Ni(1 1 0)(3 · 1)-O
reconstruction has been compressed locally into a
(2 · 1) structure.
The arrangement of 6P in the molecular strings
is only quasi-periodic, and sometimes Ni atoms are
seen to be embedded in between the molecules (see
arrows Fig. 4(a)). Close inspection reveals that the
molecules within a string either start or end on a
four-fold hollow site leading to minimum ring-to or 6.8 ± 0.2 A
ring distances of either 6.0 ± 0.2 A
between neighbouring molecules (see Fig. 4(b)).
These separations are fairly small and within the
estimated closest van der Waals distance of the
molecules. The intra-molecular ring periodicity of
the 6P molecule is incommensurate with the Ni
[0 0 1] azimuthal direction, but the distance between the centres of the first and the fifth phenyl
ring corresponds roughly to 5 Ni–Ni distances
along [0 0 1], allowing rings 1 and 5 to adopt fourfold hollow sites. Thus, the number of four-fold
hollow sites is maximised. This determines the
registry of the 6P molecules in the channels opened
up in the (2 · 1)-O reconstruction (Fig. 4(b)), and
explains the two observed molecular head-to-tail
distances along the chains: it depends on whether
the head or the tail phenyl ring is placed above the
four-fold hollow site. The displacement of the
(2 · 1)-O reconstruction by the molecular chains
indicates that the substrate surface does not behave
as a rigid template, but rearranges in a dynamic
way to accommodate the molecular species in the
most favourable configuration. As illustrated in
Fig. 4(b), the most favoured configuration is that
which maximises the possible number of four-fold
hollow sites––six for diagonal, two for both [0 0 1]
and [1 1 0] oriented molecules. The latter are observed at the initial adsorption stages at defects and
when the (2 · 1)-O reconstruction is degraded by
hydrogen, which results in patches of (1 n)
reconstructed areas with bare Ni rows along [1 1 0],
which align the molecules (see Fig. 3(a)).
It has been mentioned in the introduction that
clean Ni surfaces are very reactive towards aromatic molecules, and that surface passivation by
e.g. preadsorbed oxygen is a useful concept to enable the formation of intact and well-defined
molecular layers. It is therefore interesting to see
that the 6P molecules bond preferentially to the Ni
sites of the second layer after restructuring the Ni–
O top surface layer. However, it appears that the
overall electronic structure of the Ni(1 1 0)(2 · 1)-O
surface is sufficiently modified to preclude the
break-up of 6P at the surface to a major extent.
This is supported by the fact that the majority of 6P
molecules can be desorbed from the surface in intact molecular form. Fig. 5 illustrates this point by
examining the 6P covered Ni(1 1 0)(2 · 1)-O surface
after heating to 400 K: the STM image shows the
2 ) of the surface after heating to
Fig. 5. STM image (155 · 155 A
450 K. Residues of phenyl rings have been indicated by arrows.
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adsorption induced trenches in the (2 · 1) reconstruction, which remain after most 6P molecules
have desorbed from the surface. The trenches seem
to be pinned by the star-like carbonaceous residues
of some phenyl rings (see arrows on the image),
which have been left at the surface after the fragmentation of a minority of 6P molecules.
In summary, we have obtained STM images of
sexiphenyl molecules on the Ni(1 1 0)(2 · 1)-O
reconstructed surface with sub-molecular resolution, which allow us to detect the molecular details
in terms of the individual phenyl rings as well as
their orientation and registry at the surface. Most
molecules are adsorbed in a planar configuration in
four well-defined directions, but some species with
twisted phenyl rings have also been observed. The
6P molecules assemble into molecular strings along
the [0 0 1] azimuth, after compressing the –Ni–O–
rows in the first layer of the (2 · 1) reconstruction
into a local (1 · 1) arrangement; the molecular
species are accommodated in the thus created
channels and bond to the second layer Ni atoms.
Acknowledgements
This work has been supported by the Austrian
Science Funds.
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