Self-assembled monolayers of cobalt(II)-(4-tert-butylphenyl)-porphyrins: The influence of the electronic dipole on scanning tunneling microscopy images

Article abstract of DOI:10.1021/ja0470970

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert- butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70° with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.

Full text of DOI:10.1021/ja0470970

Published on Web 12/04/2004
Self-Assembled Monolayers of Cobalt(II)-
(4-tert-Butylphenyl)-Porphyrins: The Influence of the
Electronic Dipole on Scanning Tunneling Microscopy Images
Valentina Arima,* Eduardo Fabiano, Robert I. R. Blyth, Fabio Della Sala,
Francesca Matino, Julie Thompson, Roberto Cingolani, and Ross Rinaldi
Contribution from the National Nanotechnology Laboratory of INFM, Distretto Tecnologico
ISUFI, UniVersita` di Lecce, Via Arnesano, 73100 Lecce, Italy
Received May 18, 2004; E-mail: valentina.arima@unile.it
Abstract: Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin,
a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared
on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques.
Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent
topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images
and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins
on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin
ring was tilted by about 70° with respect to the surface plane. The above effects are ascribed to the presence
of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison
with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have
been reproduced by a simple monodimensional tunneling model.
Raman scattering (SERS),8 electrochemical techniques,8 and
UV/Vis spectroscopy.9 We have focused our attention on these
Introduction
Ordered assemblies of metal tetraphenyl-porphyrins (MTPP)
have recently attracted great interest for the formation of
nanoscale molecular devices by virtue of their optical, photo-
electrochemical, and chemical properties. For this reason, many
ultrahigh vacuum scanning tunneling microscopy (UHV-STM)
and spectroscopy (STS),1-5 photoemission5,6 and near edge
X-ray absorption fine structure (NEXAFS)7 studies have been
performed on evaporated metal porphyrins. STM measurements
have always led to the identification of single porphyrin rings,
clearly indicating that the tunneling probability is sensitive to
electronic density of the molecular orbitals (essentially to the
highest occupied and lowest unoccupied molecular orbitals,
HOMO and LUMO, respectively). Moreover, photoemission and
NEXAFS measurements have been used to investigate the
occupied and unoccupied electronic states and to deduce the
molecular orientation of the porphyrin ring on the surface.7
Although monolayers of physisorbed MTPPs have been well-
studied, chemisorbed self-assembled monolayers (SAMs) of
metal-porphyrins have been studied only by surface-enhanced
SAMs because the preparation of the monolayer by wet-
processing is likely to be preferred to the relatively expensive
and time-consuming evaporation method, in real device imple-
mentation.
In this work, we have studied a self-assembled monolayer
of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin
(CoTBPP) on gold(111), using STM and STS in UHV,
photoelectron spectroscopy and NEXAFS. The aim of our work
is to investigate, by comparing experimental and theoretical
studies, how the molecules can be arranged and oriented on
the surface and how their chemisorption can affect the electronic
properties of the gold electrode.
To immobilize the porphyrin rings on the surface, we prepared
a 4-aminothiophenol (4-ATP) self-assembled monolayer on
gold(111) followed by axial legation in-situ of cobalt(II)
5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (CoTBPP).
STM is a well-established surface analysis technique.10,11
Although basic tunneling theory is well understood, the mech-
anism by which thin organic adsorbates (including physisorbed,
chemisorbed, and Langmuir-Blodgett films) on conducting
substrates are imaged is not yet unequivocally clarified. Studies
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A R T I C L E S
Arima et al.
of long-chain alkanes,12-15 thiols,16-21 liquid crystals,22-24 and
dialkyl-substituted benzenes6,22,25-28 on different conductive
substrates, have demonstrated that image contrast can be
controlled and modulated by changing the magnitude and
polarity of the tip-sample bias.
also for physisorbed molecules.35,36 Therefore, the intrinsic
dipole and polarizability of molecules are important factors in
determining the adsorbate’s effect on the surface work function
and hence the STM image contrast.
When an STM image is acquired, one or more of these
mechanisms may operate depending both on the coupling
between the energy levels of the adsorbate and the surface Fermi
levels, and on the spatial overlap between the STM probe and
the molecules oriented on the substrate. A crucial role in the
enhanced contrast and in the measured topographic height is
played by the orientation of the molecules with the respect to
the surface. A molecule, or ordered arrays of molecules, given
conformational freedom, can result in unexpected STM images
because they can assume different orientations on the surface.
Although the intrinsic electronic structure of the molecular layer
is only slightly modified, the geometry of the tip-molecule
spatial overlap changes completely.
Analysis of data from ex-situ prepared samples provides
certain challenges for the interpretation of surface sensitive
spectroscopies, in that the presence of impurities, both from
the sample preparation process and from adventitous adsorbates,
cannot be avoided. Analysis of the Au/4-ATP system using both
conventional XPS20 and synchrotron radiation photoemission37
indicates the presence of both free and oxidized 4-ATP, in
addition to 4-ATP bonded to Au. Detailed curve fitting of results
from our Au/4-ATP samples indicates that only 30% of the
molecules present are actually 4-ATP bonded to Au37. Therefore,
interpretation of NEXAFS data in particular will not be as
straightforward as is the case for samples of a single species
adsorbed on a clean surface under ultrahigh vacuum.
Bias-dependent images, reported in such studies, indicate that
there is a threshold bias at which the features of the monolayer
can be discerned, thereby facilitating the observation of an
adsorbate’s electronic structure independent of the underlying
substrate. When the threshold bias is achieved, the tunneling
process through adsorbates can take place and contrast heights
appear in the STM images. Two mechanisms have essentially
been proposed to explain the origin of the STM image
contrast: tunneling from molecular orbitals of the adsorbates
(or resonant tunneling),22,29 and tunneling from a conductive
substrate.15 In the first case, the frontier orbitals of the species
participate in the tunneling process because the shape of the
adsorbate, as imaged with the STM, approximately reflects the
calculated electron density of the molecular orbitals.22,29 The
second mechanism is often invoked to determine the nature of
image contrast for electrically insulating adsorbates, e.g., simple
and functionalized alkanes. Thibaudau and co-workers15 sug-
gested that the contrast in STM images of n-alkanes on HOPG
was dominated by elastic deformation of the substrate. Cyr et
al.30 related the contrast of several functional groups (NH2, CH3,
OH, SH, Cl, Br, and I) in primary-substituted hydrocarbons to
the local modulation of the substrate work function due to
adsorbate polarizability. In their model, the presence of the
polarizable layer induces a modification of the tunneling barrier
and makes the physisorbed molecules “visible”.
Furthermore, photoemission studies have shown the work
function of the substrate can be modified by the addition of
physisorbed or chemisorbed layers which introduce an oriented
dipole.31-34 This is true not only for chemisorbed layers, but
Experimental Section
Chemicals and Solvents. To allow wet processing, we have
synthesized a more soluble version of cobalt(II) 5,10,15,20-tetrakis-
phenyl-porphyrin (CoTPP), cobalt(II) 5,10,15,20-tetrakis(4-tert-butyl-
phenyl)-porphyrin (CoTBPP), by following the standard procedure38
for the metal-free porphyrin and using cobalt(II) acetate to insert the
cobalt into the ring. The resulting material was purified by column
chromatography, giving a final purity of ∼99% (characterized by liquid
chromatography-mass spectroscopy and UV-visible absorption spec-
troscopy). 4-aminothiophenol (4-ATP), 90%, was purchased from
Aldrich and used as received. Ethanol and chloroform used were
analytical grade. Chloroform was purified by filtration through a column
of basic alumina.
Formation of the CoTBPP SAM on Gold(111) (Au/4-ATP/
CoTBPP). We anchored CoTBPP to the Au(111) surface via axial
ligation to a self-assembled monolayer of 4-aminothiophenol (4-ATP),
by immersing an Au/4-ATP SAM into a chloroform solution of
CoTBPP (10-4 M) for 18 h. Further details of the immobilization
procedure are described in ref 39. The structures of the molecules are
shown in Figure 1.
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STM and STS Measurements. Soon after deposition, the sample
was inserted in the ultrahigh vacuum chamber to minimize possible
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SAMs of Cobalt (II)−(4-tert-Butylphenyl)-Porphyrins
A R T I C L E S
using a photon energy of 510 eV. A cylindrical mirror analyzer was
used to acquire the photoemission spectra, which had an overall
resolution (beamline plus analyzer) of 0.8 eV. Nitrogen K-edge
NEXAFS spectra were recorded using Auger yield, with a resolution
of 0.4 eV. Attempts were also made to obtain spectra of the Co 2p
levels, however the relatively low flux at photon energies sufficient to
excite the Co 2p levels,40,41 combined with the high background due to
Au photoemission and Auger features, made meaningful data acquisition
impossible.
Theoretical Calculations
Figure 1. Molecular structures of (a) 4-aminothiophenol (4-ATP) and (b)
Cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (CoTBPP).
We calculated the molecular geometry, binding energies and the
electronic properties (dipole moment and polarizability) of the Au/4-
ATP/CoTBPP assembly using a first-principles density-functional theory
(DFT) approach. Calculations were carried out using the parallel
quantum chemistry package TURBOMOLE.42 In the calculations, the
4-tert-butylphenyl substituent groups have been removed to reduce the
computational cost, as they have a negligible effect on the electronic
properties. The calculated system will be, hence, indicated as Au/4-
ATP/CoP. Pseudopotentials43 were used to describe the gold core
electrons. The resulting number of electrons is odd; therefore, an
unrestricted DFT treatment was required.
For geometry optimization and energies, the gold substrate was
simply modeled using a single gold atom,44 using the TZVP45-48 basis
set and the B3LYP49 exchange-correlation (XC) energy functional.
For electronic properties, the Becke-Perdew50 XC energy functional
was employed together with the resolution-identity technique.51 The
Au(111) substrate was modeled as a cluster of 21 gold atoms (in
crystalline geometry) and oriented to form an angle of 70° with the
porphyrin plane (as obtained from NEXAFS measurements, see later).
We checked that the number of gold atoms was sufficient to correctly
describe the electronic properties. We note moreover that the choice
of the XC functional influences the electronic dipole moment by less
than 20%. The sulfur atom was positioned in the hcp hollow site, as
recently described in ref 52.
Figure 2. Experimental geometry for the photoemission and NEXAFS
measurements.
The electronic polarizability was computed using a finite-field of
10-4 a.u., and the dipole moment derived from Mulliken charges only
on the molecule, i.e., without considering gold atoms. We note that
the calculation of the electronic polarizability of a molecule attached
to a metal cluster is completely dominated by the very large polariz-
ability of the cluster, which moreover increases linearly with the number
of metal atoms.53 This effect is not present in an open and semi-infinite
gold substrate, like the one we have considered for the description of
tunneling current.
contamination, which, however, cannot be avoided. The STM and STS
measurements were carried out in UHV (10-10 mbar) at room
temperature, using Pt-Ir tips. Tip quality was tested by acquiring
atomically resolved images of graphite. STM images were acquired in
constant current mode using tip-biases ranging from (0.2 to (1.5 V
and a set point of 0.5-1 nA.
During spectroscopy measurements, the feedback loop was switched
off and the set-point current, which sets the tip-sample distance, was
fixed during the bias scan. The spectroscopy measurements (I-V) were
performed by collecting grids of points spaced by 6.25 Å over scan
areas of 50 nm × 50 nm. The STS spectra shown in this work are
averaged over a set of several I-V curves in the grid over the scanning
area.
Experimental Results
Bias-Dependent STM Images. Morphological characteriza-
tions of the clean gold(111) surface and of the 4-ATP mono-
layer, reported in ref 39, were performed before studying the
CoTBPP monolayer. After deposition of the 4-ATP layer, the
In addition, high rsolution contact mode atomic force microscopy
(AFM) was also performed, in air, using a Thermomicroscopes CP
Research Instrument AFM.
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Photoemission and NEXAFS Measurements. Photoemission and
NEXAFS spectra, at room temperature, were taken using the BEAR
beamline40,41 at the ELETTRA synchrotron, Trieste, Italy. Samples were
introduced into the vacuum system via a load lock. The sample
mounting geometry used is shown in Figure 2. The analyzer was
mounted vertically over the sample, which was mounted at a slight
angle from the vertical. This gives high surface sensitivity, while the
sample was rotated about the vertical axis to change the polarization
of the incident light. Nitrogen 1s photoemission spectra were recorded
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for hydrogen.
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Naletto, G.; Pelizzo, M. G.; Tondello, G. AIP Conf. Proc. 2004, 705, 450.
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Tondello, G. ELETTRA News, 2003, 47, September 30th.
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A R T I C L E S
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Figure 4. Typical unfiltered STM image of Au/4-ATP/CoTBPP at (a) 1.5
V, 0.2 nA; (b) 1.5 V, 0.5 nA; (c) line scan along the direction perpendicular
to the Au(111) lines of Figure 4a (dotted line) and of Figure 4b (solid line).
Figure 3. (a) Typical unfiltered STM images of Au/4-ATP/CoTBPP (0.2
V, 1 nA); (b) high-resolution STM image of Au/4-ATP/CoTBPP (-1 V,
0.5 nA); and (c) High-resolution AFM image of Au/4-ATP/CoTBPP.
gold terraces did not exhibit the characteristic pattern of Au-
(111)-(22 × x3) herringbone surface reconstruction, but rather
exhibited a “web” pattern consisting of single and pairwise lines
often linked by “U” connections. After the axial ligation of
metalloporphyrins to the 4-ATP monolayer, the long range order
of the Au(111) reconstruction, lost after 4-ATP adsorption, was
recovered. In our previous work,39 we suggested that the
observed phenomena were due to thiol adsorption which induced
gold atom transport and modified the Au(111) surface morphol-
ogy.
Figure 3a shows an STM image of the gold(111) surface after
the CoTBPP deposition, recorded after positioning the tip close
to the metal surface, i.e., by setting a relatively high tunneling
current and low tip voltage. On the terraces, well-defined
herringbone turns and pairwise-arranged parallel lines are clearly
visible. The periodicity of the pairwise lines, whose nominal
height is 0.15 Å, is 6.3 nm, with 120° bending on the
herringbone turns, typical of the Au(111)-(22 × x3) her-
ringbone surface reconstruction.54
Figure 5. Typical unfiltered STM images of Au/4-ATP/CoTBPP at (a)
-0.2 V, 0.5 nA; (b) -0.5 V, 0.5 nA; (c) -0.8 V, 0.5 nA; (d) -1 V, 0.5
nA.
In an attempt to directly image the molecules, we gradually
increased the tip bias to 0.8 V; however, we did not observe
any features which could be directly identified as molecules.
High-resolution AFM, Figure 3c, does not indicate a pristine
gold surface, but neither shows the well-resolved long range
order seen in the STM images. However, the image shows the
presence of structures which could be compatible with an
ordered array of molecules, with a certain degree on confor-
mational freedom, resulting in the loss of resolution as seen in
Figure 3c.
4c) it can be seen that the apparent height of one of the lines is
0.09 Å, whereas that of the other is 0.02 Å lower. By further
reducing the tip-sample distance, i.e., by increasing the current
set point, the typical dimensions of the Au(111)-(22 × x3)
herringbone surface reconstruction were restored, as shown in
Figure 4b (solid line in Figure 4c). The highest positive bias at
which we collected data was +1.5 V, since higher voltages were
considered likely to cause molecular damage. Attempts to reduce
the tip-surface distance, without modifying the applied voltage,
led to loss of resolution.
We also observed a modulation of the Au(111) image by
changing the tip-bias from -0.2 to -1 V, keeping the current
set point fixed at 0.5nA, as shown in Figure 5. The relatively
broad raised lines seen in Figure 5a, gradually become resolved
on increasing the negative voltage of the tip, into the charac-
When we applied a bias in the range +1 to +1.5 V to the
tip, we observed a strong and asymmetrical decrease of the
apparent height of the pairwise-arranged lines, as shown in
Figure 4a. From the line profile in Figure 4 (dotted line in Figure
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SAMs of Cobalt (II)−(4-tert-Butylphenyl)-Porphyrins
A R T I C L E S
Figure 6. Line scan along the direction perpendicular to the Au(111) lines
of Figure 5a-d.
Figure 7. Scanning tunneling current-voltage curves (averaged over an
area of 50 × 50 nm) recorded on different samples: clean gold (dashed
line -0.5 V, 0.5 nA); Au/4-ATP/CoTBPP (solid line -1V, 0.5 nA). Inset:
asymmetry grade (k ) |I(V)| - |I(- V)|/(|I(V)| + |I(- V)|)/2) of: clean
gold (dashed line -0.5, 0.5 nA) and Au/4-ATP/CoTBPP (solid line -1 V,
0.5 nA) STS curves.
teristic paired discommensuration lines of the Au(111)-(22 ×
x3) reconstruction. As shown in Figure 5a, the profile of these
discommensuration lines is strongly modified at low negative
bias (-0.2 V), while it assumes the typical Au(111) profile at
higher biases (max. values -1 V), as shown in Figure 5d. While
the negative voltage is increasing, the line’s apparent height,
shown in Figure 6 (line profiles of figures 5a-d), gradually
increases (from 0.1 to 0.17 Å for one of the two lines) and the
asymmetrical nature of the two lines becomes more visible
(ranging from 0.01 to 0.03 Å). The central line becomes
apparently deeper, ranging from 0.014 to 0.11 Å (measured from
the top of the higher line). Attempts to image the surface at
higher negative bias again resulted in loss of resolution.
High-resolution images of the Au/4-ATP/CoTBPP surface,
such as that shown in Figure 3b, exhibit the atomically resolved
Au(111)-22 × x3 surface reconstruction. Note, however, that
the asymmetry of the two lines still persists.
No bias-dependence was observed in images of the clean Au-
(111) substrate. Bias dependent measurements of the Au/4-ATP
system37 showed only an increase in the apparent height of the
discommensuration lines at higher voltages.
STS Measurements. STS spectra of clean gold and Au/4-
ATP/CoTBPP are shown in Figure 7. The clean gold samples
show a symmetric I(V) curve, whereas the Au/4-ATP/CoTBPP
surfaces show asymmetric I(V) curves with a higher current for
negative sample bias. A quantitative estimate of the asymmetry
of such curves (see the inset in Figure 7), the asymmetry grade,
k, can be simply obtained from
|I(V)| - |I(- V)|
k )
(|I(V)| + |I(- V)|)/2
The inset to Figure 7 shows the asymmetry grades as a function
of voltage derived from the data of Figure 7. From this it can
be seen that for Au/4-ATP/CoTBPP there is a significant
asymmetry, while for clean Au there is not.
Photoemission and NEXAFS Measurements. Flux normal-
ized N 1s photoemission spectra of Au/4-ATP and Au/4-ATP/
CoTBPP are shown in Figure 8a. It is clear that for Au/4-ATP/
CoTBPP there is considerably more nitrogen present on the
surfacescomparing the areas of the peaks suggests a ratio of
about 4:1. In principle, the coverage could be estimated using
core level photoemission (i.e., comparing the N1s/Au4f ratio),
but this measurement suffers from a high degree of uncertainty
due to the inevitable presence of impuritiessthe measurement
Figure 8. (a) Flux normalized N 1s photoemission spectra of Au/4-ATP
and Au/4-ATP/CoTBPP; (b) Nitrogen K edge NEXAFS spectra of Au/4-
ATP/CoTBPP at grazing (80°) and normal (0°) incidence.
is not the usual calculation of the coverage of a surface species
on an otherwise clean surface, but effectively that of a buried
interface. We can perform a second order estimate by assuming
that the coverage of 4-ATP is a monolayer, and comparing the
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A R T I C L E S
Arima et al.
flux-normalized intensities of the N 1s peak, as we have done
in Figure 8a. This resulting 4:1 ratio suggests approximately
monolayer coverage of the porphyrin.
The presence of only one N 1s peak in the Au/4-ATP/
CoTBPP in the data of Figure 8a confirms the presence of a
metalloporphyrin, since metal-free porphyrin shows two peaks,
due to protonated and unprotonated nitrogen.55 The binding
energy of the porphyrin N1s peak is 400.6 eV, compared to
399.6 for that of 4-ATP. The value for 4-ATP is consistent with
that of Rosinck et al.20 The value for CoTBPP is slightly higher
than that of a neat CoTPP film,56 probably due to the reduced
screening of the hole. A similar result was obtained by Da Cruz
et al.56 when adsorbing CoTPP on silica via ligation to a silicon-
substituted pyridine.
Nitrogen K edge NEXAFS spectra of Au/4-ATP/CoTBPP
are shown in Figure 8b. Reference spectra of metalloporphyrin
evaporated films can be found in ref 7. The low energy region
of the NEXAFS spectra of metalloporphyrins consists of two
peaks due to π* resonances,7 labeled A and B (following the
notation of ref 7) in Figure 8b. A weak shoulder, due to a further
π* resonance was also noted in ref 7sthe approximate energy
of this peak is indicated by the label “C” in Figure 8b. The
broader σ peaks commence at around 407 eV, indicated by “D”
in Figure 8b. The peaks are somewhat broadened compared to
those of the evaporated Co porphyrin films,7 consistent with
the results of Okajima et al.7 who noted a broadening in the
spectra of Co-porphyrin when exposed to NO, a molecule
which would also be expected to bond to the porphyrin via the
metal.
The π* resonances A and B in Figure 8b show significant
angular variation. Measuring the intensity ratio of the π*
resonances in going from grazing to normal incidence, we obtain
values of 1.3:1 and 1.6:1 for peaks A and B, respectively. There
is a significant contribution to the N NEXAFS signal due to
the presence of the underlying 4-ATP layer. NEXAFS data from
the Au/4-ATP system37 show little angular variation, as ap-
proximately 70% of this system is randomly oriented free and/
or oxidized 4-ATP.37 Therefore, the 4-ATP contrubutes a finite
background at all angles to the data of Figure 8b, which we
can estimate as around 20% of the total signal from a
comparison of the N 1 s photoemission data of Figure 8a. We
should expect that a small contribution to the variation of peak
B is due to the intensity variation in this energy range from the
underlying 4-ATP spectrum;37 therefore, it is not unexpected
that the observed ratio is slightly higher for peak B compared
to peak A. Taking the finite background into account, from the
intensity plots of Stohr57 the observed intensity variations in
the π* orbitals A and B correspond to angles of 65° and 75°
relative to the surface. The intensity ratio of π* resonance
NEXAFS is quite sensitive to small angular variations in this
range;56 therefore, the uncertainty is not particularly high.
Figure 9. Optimized molecular geometry of Au/4-ATP/CoP. The arrow
indicates the dipole moment.
of 122°. This is due to the tetrahedral hybridization assumed
by the nitrogen atom. The computed Co-N bond-length is 2.44
Å. The binding-energy of the CoP to the Au/4-ATP has been
computed as the total energy difference between the 4-ATP/
CoP optimized structure, and the system with the porphyrin and
aniline well separated. The resulting energy is 49.50 kcal/mol
(2.15 eV), which shows that the Co-N bond is quite strong.
The calculated HOMO-LUMO gap (using the B3LYP XC
functional) of the 4-ATP/CoP is 3.01 eV. Both the HOMO and
LUMO are π orbitals localized in the porphyrin ring. Note that
in this case HOMO and LUMO refer to the system 4-ATP/
CoP (i.e., without gold atoms).
The dipole moment of the Au/4-ATP/CoP assembly is 7.82
D and is indicated by the arrow in Figure 9. This dipole moment
is mainly due to the 4-ATP molecule.
The static electronic polarization normal to the gold substrate
is 267 au. This implies that the molecular electronic properties
can be significantly modified by an external field, such as that
of an STM tip.
Discussion
Although the presence of the CoTBPP SAM is confirmed
by photoemission and NEXAFS measurements, it is completely
inVisible to the STM and appears electrically insulating in the
limited voltage range that we have used (to avoid molecular
damage). Therefore, to explain the experimental results in Figure
5 we cannot invoke the resonant tunneling mechanisms often
used to describe STM images of π-electron rich layers. CoTBPP
and similar molecules have been directly imaged with STM in
conditions where the ring lies parallel to the surface.1-5
However, the conformational freedom induced by ligation via
the 4-ATP, can result in a less conductive situation by permitting
an adsorption geometry in which the spatial overlap of the tip
wave functions with those of the molecular π orbitals is much
reduced. This would appear to be the situation in our case. From
the results of the NEXAFS measurements, DFT calculations
and molecular modeling we suggest that CoTBPP molecules
are likely to be attached to the substrate as shown in Figure 10.
This model takes into account the 70° inclination of the
porphyrin plane with respect to that of the substrate deduced
from the NEXAFS spectra, and the 120° calculated angle
between the 4-ATP and the porphyrin, and also the presence of
Theoretical Results
The optimized molecular geometry of Au/4-ATP/CoP is
shown in Figure 9. Interestingly, we found that the 4-ATP and
porphyrin molecular planes are not orthogonal, but form an angle
(55) Polzonetti, G.; Ferri, A.; Russo, M. V.; Iucci, G.; Licoccia, S.; Paolesse,
R. J. Vac. Sci. Technol. A 1999, 17, 832-839.
(56) Da Cruz, F.; Driaf, K.; Berthier, C.; Lamelle, J.-M.; Armand, F. Thin Solid
Films 1999, 349, 155-161.
(57) Sto¨hr, J. NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992.
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16956 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

SAMs of Cobalt (II)−(4-tert-Butylphenyl)-Porphyrins
A R T I C L E S
(Wentzel-Kramer-Brillouin) approximation
T(E) ) exp - z dx
8m
2 (U(x) - E)
(2)
∫
0
[
]
x
p
where z is the substrate-tip distance, U(x) is the potential profile
and m is the electron effective mass, for which we have assumed
m ) me. The WKB method is known to strongly underestimate
the transmission probability,61 nevertheless, it can be used to
obtain a qualitative description in absence of resonances. The
employed potential profile follows the molecular structure and
is shown in Figure 10, where Φ represents the substrate and
tip work functions and eV represents the applied voltage. For
simplicity, as in ref 60, the tip work function was assumed to
be equal to that of the substrate (5 eV): small differences in
barrier height do not have significant effects on transmission
probability, as is pointed out in refs 58 and 62. The permanent
dipole of the molecule is approximated by the field of two
parallel plane distributions (at distance d ) 6 Å, i.e., the length
of the aniline) of opposite charge density σ. Due to the high
polarizability of the system the value of σ actually depends on
the applied voltage
Figure 10. Model of the SAM. At right: schematic view of the potential
used for the calculation of the trasmission coefficient between the tip and
the substrate. Φ is the work-function of the substrate, V is the applied bias
and σ is the surface charge density.
tert-butyl groups which have been found to determine how the
porphyrin ring is rotated with respect to the Co-N axis. With
the orientation shown in Figure 10, the tert-butyls do not overlap
with the substrate nor with the adjacent molecules, allowing a
very close packing. In this configuration, the frontier orbital of
the 4-ATP/CoTBPP system, which from DFT calculations we
found located in the porphyrin ring, cannot spatially overlap
with the STM probe molecular orbitals, due to the presence of
tert-butyls, nor to the gold substrate, due to presence of 4-ATP.
Therefore, no tunneling current through the molecular orbitals
of the porphyrin ring is observed.
However, insulating monolayers can create voltage-dependent
STM images if they introduce a dipole and are polarizable by
the tip field: in this way, the substrate work function and the
tunneling barrier can be modulated by the applied voltage. The
DFT calculations show that the Au/4-ATP/CoTBPP system has
a large dipole moment and polarizability. From Figure 9, it can
be seen that the molecular dipole projection onto the substrate-
normal is almost complete. Moreover, experimental evidence
of the presence of a molecular dipole is provided by the
asymmetry of the STS measurements of Figure 7.58
V
z
σ ) σ0 + R ) σ0 + R˜V
(3)
where z is the tip-substrate distance, which in our case can be
considered almost constant (2 nm) due to the very small tip
movements (see Figure 6).
Equation 1 gives the I-V characteristics in the presence of
the molecular dipole. Equations 1 and 2 were computed by
numerical integration and in order to obtain the tip-substrate
distance, and thus to model STM line profiles, they can be
solved for z, with fixed I and V.
We have determined the values of σ0, R˜ in order to reproduce
the experimental STM line profiles of Figure 6 and thus to
determine in which region molecules are present. We divided
the Au herringbone reconstruction into its three known distinct
regions,54 A (FCC), B (Bridge) and C (HCP), as shown in Figure
11. STM contrast images (see Figure 6) suggest that CoTBPP
molecules induce effects along the gold(111) discommensuration
lines. In the model, values of σ0 and R˜ have been determined
in each of the three regions, with region B closer to the tip than
the others in order to follow the Au herringbone reconstruction.
It turns out that the experimental STM profile can be very well
reproduced assuming that no molecules are present in the C
region, which then serves as a reference point. With σ0 ) 0.006
e/Å2 and R˜ ) 0.005 e/(Å2V) in region A and reduced by a factor
of 2 in the B region, the obtained STM profile is shown in Figure
12, which is in excellent agreement with Figure 6 as far as the
voltage effects are concerned.
The obtained values of σ0 and R˜ can be directly related to
the molecular dipole and polarizability of the 4-ATP/CoTBPP
system, given an estimate of the molecular area. CoTBPP is 15
Å wide, thus the molecular area is 15L Å2, with L given in
Figure 10, which actually depends on the molecular coverage.
With close packing, L is about 1 nm, thus the fitted values of
σ0, R˜ correspond to a dipole of 25.4 D and a polarizability of
In the energy range far from resonance with any molecular
orbitals, the current is dominated by electron tunneling to the
substrate through the molecule, which acts as a potential
barrier.58,59 The current can be then expressed as
2e
h
I(V) )
eV Ft(E - eV)Fs(E)T(E)dE
(1)
∫
0
where Ft and Fs are tip and substrate-molecule local densities
of states (LDOS), V is the applied bias and T(E) is the
transmission probability through the barrier. The substrate-
molecule LDOS shows in general peaks at molecular orbital
energies but is flat within a small domain around the Fermi
energy.60 For low applied bias the substrate-molecule LDOS
can thus be assumed constant, and consequently the current
depends only on the transmission probability T(E).
In a simple monodimensional model, which we use here, the
transmission probability can be evaluated using the WKB
(58) (a) Rosink, J. J. W. M.; Blauw, M. A.; Geerlings, L. J.; Van der Drift, E.;
Radelaar, S. Phys. ReV. B 2000, 62, 10459-10466; (b) Rosink, J. J. W.
M.; Blauw, M. A.; Geerligs, L. J.; Van der Drift, E.; Rousseeuw, B. A. C.;
Radelaar, S. Mater. Sci. Eng. C 1999, 8-9, 267-272.
(59) Samanta, M. P.; Tian, W.; Datta, S.; Henderson, J. I.; Kubiak, C. P. Phys.
ReV. B 1996, 53, 7626-7629.
(60) Onipko, A. I.; Berggren, K.-F.; Klymenko, Y. O.; Malysheva, L. I.; Rosink,
J. J. W. M.; Geerlings, L. J.; Van der Drift, E.; Radelaar, S. Phys. ReV. B
2000, 61, 11118-11124.
(61) Cruz, E. L.; Helman, J. S. Phys ReV B 1974, 10, 1751-1754.
(62) Dhirani, A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, R. J. Chem.
Phys. 1997, 106, 5249-5253.
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A R T I C L E S
Arima et al.
mental profile, can be assigned to a reduced molecular coverage,
probably due to the different organization of the gold surface
near the discommensuration lines.54
These results can be used to discuss the STM profile of Figure
6. For negative tip bias, the electric field between the tip and
the sample, and the dipole projection in the field direction, have
opposite orientations. Moreover, the dipole projection in the field
direction changes according to the applied voltage due to the
high molecular polarizability. When the absolute value of the
tip voltage is decreased, the effect of the dipole projection in
the field direction is maximized. Therefore, when the negative
tip voltage is decreased, the tunneling current decreases and
the tip itself moves toward the surface in order to achieve the
set point current.
The current decrease near bridge positions is slightly different
if compared to the current decrease near the FCC sites because
the molecular coverage and packing probably change near the
discommensuration. For the above reasons, the traced profile
is dependent on the voltage applied to the tip and has the shape
shown in Figure 12. At low negative voltages (-0.2 V), the
dipole effect is stronger and the traced profile is significantly
different from the bare gold(111) line profile. When the voltage
absolute value increases, the dipole effect slowly disappears and
the resulting STM image reflects the structure of the gold
surface. The asymmetrical nature of the two lines, just visible
in the bias-dependent STM images (Figure 5) and particularly
evident in the current-dependent STM images (Figure 4), can
be ascribed to slightly different molecular packing of the 4-ATP/
CoTBPP system on the gold discommensuration lines.
Figure 11. Schematic diagram of the parallel lines seen in Figure 3b,
showing the geometric structure of the Au(111)-(22 × x3) surface
reconstruction, lateral and top view.
Conclusions
The CoTBPP SAM has been studied by means of UHV STM
and STS, photoemission spectroscopy, NEXAFS, and DFT
calculations. We have demonstrated, by a comparison between
experimental and theoretical results, that the chemisorbed
molecules induced an electronic pertubation on the gold surface.
The molecules, oriented as determined by NEXAFS, induced,
in atomic resolved STM images of the Au(111) herringbone
structure reconstruction, a contrast which varied as a function
of tip-sample bias. STS measurements showed strong asym-
metric characteristics which indicated the presence of an oriented
dipole layer between the metal and the organic material.
Theoretical studies on the molecular geometry and electronic
properties were performed in order to elaborate a theoretical
model of the STM image line profile, which was found to be
in good agreement with the experimental one.
Figure 12. Calculated STM profile at different voltages along regions
A,B,C.
Acknowledgment. We thank the MIUR FIRB project “Mo-
lecular Devices” for funding and the BEAR staff, particularly
Bryan Doyle, for assistance at ELETTRA. We thank R. Ahlrichs
for providing the TURBOMOLE program package and G.
Aloisio for his support. The calculations were carried on a 16cpu
Compaq Alpha SC Server at CACT/ISUFI (Lecce). We thank
the group of Prof. G. Vasapollo (NNL) for assistance with the
synthesis of the CoTBPPs.
308.9 a.u., which compare well with the values reported in the
theoretical results section. Considering the employed WKB and
monodimensional approximations in the tunneling model, the
obtained value can be considered in excellent agreement with
DFT results, suggesting that the obtained STM profile is due
to the intrinsic molecular properties of the 4-ATP/CoTBPP
system. Rows of closely packed 4-ATP/CoTBPP molecules are
thus present in region A. The reduced values of σ0 and R˜ in the
B region, which is indeed important to reproduce the experi-
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16958 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004