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Hydrogen-bonded networks
DOI: 10.1002/smll.200600407
Cyanuric Acid and Melamine on AuACHTUNGRE(111):
Structure and Energetics of HydrogenBonded Networks**
Wei Xu, Mingdong Dong, Henkjan Gersen,
Eva Rauls, Socorro Vzquez-Campos,
Mercedes Crego-Calama, David N. Reinhoudt,
Ivan Stensgaard, Erik Laegsgaard,
Trolle R. Linderoth,* and Flemming Besenbacher*
Supramolecular chemistry based on noncovalent interactions is a powerful synthetic tool for the preparation of complex molecular architectures.[1, 2] In particular, hydrogen
bonds are considered to be useful for controlling molecular
self-assembly due to the reversibility, specificity, directionality, and cooperative strength of this class of interactions.[3, 4]
In recent years, a number of studies have characterized hydrogen-bonded structures formed by molecules adsorbed on
solid surfaces under vacuum conditions.[5–7] Besides fundamental interest, these studies are driven by the technological
relevance of molecular surface structures from perspectives
such as surface coatings, biochemical sensors, organic electronics, and heterogeneous catalysis. Most studies so far
have involved only homomolecular interactions, and very
few structures based on heteromolecular interactions have
been investigated.[8–10] To enable the synthesis of thermally
stable self-assembled multicomponent nanostructures on
surfaces, suitable protocols of stoichiometry, deposition
order, and thermal treatment have to be established and
[*] W. Xu, M. Dong, Dr. H. Gersen,+ Dr. E. Rauls, Prof. I. Stensgaard,
Prof. E. Laegsgaard, Prof. T. R. Linderoth, Prof. F. Besenbacher
Interdisciplinary Nanoscience Center (iNANO)
and Department of Physics and Astronomy
University of Aarhus, 8000 Aarhus C (Denmark)
Fax: (+ 45) 89-423-690
E-mail: [email protected]
[email protected]
Dr. S. V?zquez-Campos, Prof. M. Crego-Calama,
Prof. D. N. Reinhoudt
Laboratory of Supramolecular Chemistry and Technology
MESA + Institute for Nanotechnology, University of Twente
P.O. Box 217 7500 AE Enschede (The Netherlands)
[+] Current address:
Nanophysics and Soft Matter Group
Dept. of Physics, University of Bristol, BS8 1TL (UK)
[**] This research has been supported financially by the EU through
a Marie-Curie Intra-European Fellowship (H.G. contract number
MEIF-CT-2004-010038). E.R. acknowledges the AvH foundation
for support in the form of a Feodorlynen fellowship. Partial support through the SONS Eurocores program FUN-SMARTS as well
as EU programs AMMIST and PICO-INSIDE is acknowledged.
Supporting information for this article is available on the WWW
under http://www.small-journal.com or from the author.
854
systems with sufficiently high intermolecular binding
strengths have to be identified.[11, 12]
An extensively studied heteromolecular H-bonding
motif results from the interaction between diaminopyridine
and diimide moieties, exhibiting three complementary
NH···O and NH···N hydrogen bonds.[3, 13–15] This classic
H-bonding interaction has been exploited in the solution
phase,[14, 15] in the solid state,[3] and more recently at interACHTUNGREfaces.[8, 9, 16–18] A prototypical molecular system exhibiting this
complementary interaction is the cyanuric acid/melamine
(CA/M) system. The basic structure formed from these compounds both in solution, in bulk, and on surfaces is a symmetric two-dimensional (2D) array consisting of cyclic hexamers of 3 M and 3 CA molecules (see Figure 1 a), which
Figure 1. a) Schematic representation of the cyanuric acid–melamineACHTUNGRE(CA1M1) lattice stabilized by O···H and N···H hydrogen bonds.
b–d) Homo- and heteromolecular interactions between CA and M
molecules (red: O, blue: N, white: H).
was predicted in early studies[13] and later characterized by
bulk X-ray diffraction.[19, 20] Despite the fundamental nature
of this model system, its self-assembly at a solid surface was
p p
only very recently studied at a Ag-SiACHTUNGRE(111) 3 : 3R308 surface with modest resolution.[18]
In this work we investigate the adsorption and coadsorption of M and CA molecules on a AuACHTUNGRE(111) surface using
scanning tunneling microscopy (STM). From the submolecularly resolved images, the M and CA molecules are clearly
distinguishable. The presence of the expected 1:1 stoichiometric phase after simultaneous deposition of CA and M is
verified. In addition, a novel nonstoichiometric phase
(CA1M3) is identified, which forms upon sequential deposition of M followed by CA. The energetics of the homo-and
heteromolecular interactions of M and CA has been evaluated from self-consistent charge density-functional-based
tight-binding (SCC-DFTB) calculations to explain the observed structures.
The experiments were performed in an ultrahigh
vacuum (UHV) chamber equipped with standard facilities
for sample preparation and characterization. Prior to deposition, the single-crystal AuACHTUNGRE(111) sample was cleaned by several cycles of 1.5 keV Ar + sputtering followed by annealing
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molecule1. A CA homodimer with slightly higher binding
to 770 K for 15 min, resulting in a well-ordered herringbone
[21]
reconstruction.
energy is discussed in the Supporting Information. Finally,
Powders of M and CA ( 99 % and
the heterodimer formed through complementary coupling
98 % purity, respectively; Acros) were thoroughly deof one M and one CA molecule involves one NH···N and
gassed prior to deposition. Both CA and M were deposited
two NH···O hydrogen bonds and has a binding energy of
by thermal sublimation from molecular evaporators held at
0.24 eV molecule1.
356 K. Simultaneous deposition of CA and M was
ACHTUNGREachieved by heating both evaporators and overlapping their
Adsorption structures formed upon deposition of CA or
output beam on the surface. Molecules were deposited onto
M individually on AuACHTUNGRE(111) are shown in Figure 2. In both
a sample held at room temperature ( 300 K). A typical
cases, large well-ordered islands are observed. The herringdeposition time of 5 min resulted in coverages below half
bone reconstruction does not appear to change upon molecsaturation of the first monolayer. After deposition, the
ular adsorption or affect the self-assembly patterns observed
sample was transferred in situ to a variable-temperature
in this study. When the molecular overlayer is commensuAarhus scanning tunneling microscope.[22, 23] Unless otherrate with the underlying lattice substantial variations of Hbond lengths may occur,[26] however, in our case, the meawise indicated, the STM measurements were obtained in a
temperature range of 100–160 K to thermally stabilize the
sured H-bond lengths are in good agreement with values
molecular structures.
obtained by bulk X-ray diffraction experiments.[19, 20] This
Theoretical modeling was performed within the frameconfirms that the molecule–substrate interaction here plays
work of the SCC-DFTB method.[24] This is a density-funca minor role for the molecular ordering. STM images of CA
and M, as shown in Figure 2 a and c, respectively, show that
tional tight-binding approach where the repulsive energy
individual M molecules are resolved with a characteristic
term is obtained for each pair of elements by fitting to
three-spoke shape, attributed to the position of the amino
ab initio results for a large set of molecules with various
bonding situations. The method has previously been used
successfully, for example, in
describing hydrogen bonding
between DNA bases.[25] The
modeling was carried out to
elucidate the intermolecular
interactions behind the observed structures. The molecule–surface interaction with
the relatively inert AuACHTUNGRE(111)
substrate is assumed to primarily confine the molecules
in two dimensions while playing a minor role for the ordering. The gold substrate was
therefore not included in the
simulations. In the calculations, periodic boundary conditions were used and all involved atoms were fully relaxed without constraints.
Homo- and heteromolecular dimers formed from M
and CA are depicted in Figure 1 b–d. The M dimer in
Figure 1 b
involves
two
H bonds between the NH
donor and N acceptor groups
and the calculated binding
energy (Eb) for the optimized
structure
is
0.17 eV molecule1 (corresponding to an
energy gain of 0.34 eV for formation of the dimer from the
isolated
molecules).
The
dimer formed from CA molecules shown in Figure 1 c has Figure 2. a, c) STM images (102 ) * 104 )) of a) CA (It = 0.72 nA, Vt = 1250 mV) and c) M (It = 0.57 nA,
a binding energy of 0.11 eV Vt = 1486 mV) on AuACHTUNGRE(111). b, d) Optimized models for the H-bonded network of CA and M, respectively.
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groups, which is distinctly different from the appearance of
the CA molecules with near-circular symmetry.
The overlayer formed from pure CA (Figure 2 a) is hexagonally close-packed and can be modeled by the structure
shown in Figure 2 b, in which each molecule associates
through six H bonds to symmetrically distributed neighbors.
In contrast, M forms a more complicated structure in which
the spokes of neighboring molecules interdigitate, leading to
the formation of six-membered rings surrounding open
pores in the network. From the corresponding optimized
model for M (Figure 2 d) the structure is seen to involve
double H-bonding interactions to three neighbors surrounding each molecule. The lattice parameters for the calculated
networks of M and CA correspond well to those obtained
from the STM images, as can be seen in Table 1.
ger binding energy of the CA networks reflects that the
NH···O hydrogen bonding interactions are stronger than the
NH···N bonds, as expected.[4]
Simultaneous deposition of CA and M leads to a self-assembled network as shown in Figure 3 a. The characteristic
Table 1. Measured and calculated lattice parameters for the unit cells
displayed in Figures 2–4. A good agreement is found for the M and
CA networks. Thermal drift during the measurements on the CA–M
networks hampers an accurate determination of the lattice parameters in these cases.
Network
a
[)]
M
CA
CA1M1
CA1M3
Measured
b
[)]
10.0 1.0
7.3 0.7
9.6 1.9
25.8 5.1
11.4 1.1
7.0 0.7
7.4 1.5
21.5 4.3
a
[)]
Calculated
b
[)]
10.6
6.8
9.8
20.3
10.8
6.9
9.8
20.3
The binding energies for the optimized periodic networks shown in Figure 2 b and d are 0.47 eV molecule1 (M)
and 0.65 eV molecule1 (CA), respectively. The networks
are built up of homodimers of the type depicted in Figure 1.
The corresponding binding energies obtained by appropriately summing the pairwise interaction energies for these
dimers are 0.51 eV for M and 0.66 eV for CA (see also
Table 2), that is, they are close to the calculated values for
the extended networks. This is in contrast to, for example,
networks of guanine molecules,[7] where pronounced cooperativity effects lead to a superlinear scaling of binding energy
with the number of hydrogen bonds. In both the M and CA
networks, each molecule forms six H bonds, and the stron-
Figure 3. a) STM image (102 ) * 104 )) of a self-assembled network
resulting from simultaneous deposition of M and CA on AuACHTUNGRE(111). The
characteristic ball- and three-spoke shapes for CA and M are clearly
resolved. The structure is a mixed phase corresponding to the CA1M1
network presented in Figure 1 (It = 0.56 nA, Vt = 1250 mV). b) Optimized model for CA1M1 network. The hexagon marked “2” in (a) and
(b) shows corresponding areas as a guide to the eye.
Table 2. Calculated binding energy (Eb) by the SCC-DFTB method for dimers and extended networks (see
1st and 5th column). These numbers can be rationalized by counting the molecules present in the unit
cells (see 2nd column) as well as the interactions between pairs of molecules (see 3rd column) in each
of the networks in Figures 2–4. Note that interactions with molecules in an adjacent unit cell only count
for half. This allows for the estimation of the binding energy purely based on Eb of the respective
dimers, as shown in the 4th column.
Calculated Eb per Number of molecules Pair interactions Network Eb per moldimer [eV]
per unit cell
per unit cell
ecule [eV][a]
M
CA
CA1M1
CA1M3
0.34
0.22
0.48
–
2*M
1 * CA
1 * M, 1 * CA
6 * M, 2 * CA
3 * M–M
3 * CA–CA
3 * CA–M
6 * M–M,
6 * M–CA
30:34
2
30:22
1
30:48
2
ð60:34Þþð60:48Þ
8
¼ 0:51
¼ 0:66
¼ 0:72
¼ 0:62
Calculated Eb per
molecule [eV][b]
0.47
0.65
0.82
0.68
[a] Based on Eb of the dimer. [b] Calculated for each network.
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circular and three-spoke
shape for CA and M is resolved (most clearly seen at
the top-left side of the
image), showing that the
structure is a binary mixed
phase involving heteromolecular H-bonding. Domains of
pure CA islands are found to
coexist with this mixed
phase, most likely due to a
slight excess of CA molecules on the surface. Comparison of the area indicated
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with the hexagon marked “1” in Figure 3 a to the model for
the CA1M1 structure shown in Figure 1 a reveals that the observed structure corresponds to a motif based on the complementary CA···M coupling of Figure 1 d. A binding energy
of 0.82 eV molecule1 is found for this stoichiometric CA1M1
network, which is thus energetically preferred over the
structures formed from the individual constituents. The corresponding binding energy calculated from the pairwise
M···CA dimer interaction energies is 0.72 eV molecule1
(see Table 2), showing a slight tendency to additional stabilization in the extended network, which may result from each
amino group on the M molecules engaging in two H-bonds
to oxygen atoms on adjacent CA molecules. The calculated
H-bond lengths of 1.86 G (O···H) and 1.96 G (N···H) for the
CA1M1 network are in good agreement to the values of 1.93
and 2.01 obtained by bulk X-ray diffraction experiments.[19, 20] The strength of the interaction between M and
CA has been addressed in bulk chemistry,[27, 28] but a direct
comparison to the values obtained here is hampered by the
complicated nature of the dissociation process for large rosette assemblies and also by solvent effects.
To investigate the influence of the deposition conditions
on the formation of the CA–M network we performed experiments in which the compounds were deposited sequentially onto samples held at room temperature. The thermal
stability of the pure CA and M phases was assessed by
imaging the surface with the STM held at 300 K, showing
that both the CA and M networks are stable and can be
imaged at room temperature. In the sequential deposition
experiments, molecules of the second component deposited
may thus land both on top of and adjacent to islands of the
first component deposited. If M is deposited after CA, we
observe primarily a separation into individual islands of CA
and M. However, if CA is deposited after M we find extended regions of a binary mixture of CA and M, of which an
STM image is shown in Figure 4 a. Simultaneously, areas of
pure M and CA can also be seen, the occurrence of which
depends on the relative ratio between the two compounds.
The characteristic hexagons observable in these networks,
shown in the inset, shows that the ratio of M to CA in this
network is 3:1. We conclude that sequential deposition leads
to a novel CA1M3 network not present in the originally suggested structure. This novel network is only observed as a
locally ordered structure as visible in Figure 4 a.
From the structural model in Figure 4 b, the intermolecular interaction in this novel phase is seen to be identical to
those of the pure M and the M–CA structures. In particular
the characteristic six-membered ring of M molecules is
found in the center of the indicated hexagons. A binding
energy of 0.68 eV molecule1 has been calculated for the
CA1M3 network in the optimized structure shown in Figure 4 b. To rationalize this value, one may evaluate the
number of M–M and M–CA interactions in each unit cell of
the rather complicated network structure (see Table 2). If
each of these interactions is assumed to contribute the same
binding energy as in the M and the CA dimers, respectively,
a binding energy for the CA1M3 network of 0.62 eV is
found, which is close to the calculated value for the optimized structure. Hence, the binding energies of all the exsmall 2007, 3, No. 5, 854 – 858
Figure 4. a) STM image (105 ) * 128 )) of a self-assembled network
on AuACHTUNGRE(111) resulting from deposition of CA after M. The inset
(38 ) * 41 )) shows the typical hexagon showing the shape of the
individual molecules. Dividing such an hexagon in triangles as
depicted schematically shows that in this network each CA molecule
bonds to three M molecules (It = 0.52 nA, Vt = 1250 mV). b) Optimized model for this novel CA1M3 network.
tended networks are well accounted for by the pairwise interaction energies of the molecular building blocks involved.
The binding energy for the CA1M3 phase is lower than
the 0.82 eV molecule1 found for the CA1M1 phase while it
is of comparable magnitude to the 0.65 eV molecule1 of the
CA network. Incorporating CA into an existing M network
to form a CA1M3 structure is thus energetically favorable.
The formation of a CA1M3 network may be explained as an
intermediate step to convert to a thermodynamically stable
CA1M1 binary mixture phase. One may speculate whether
the added CA molecules are embedded in the pores of the
existing M islands upon direct impingement from the gas
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phase or primarily interdiffuse from the edges of the islands
after deposition on the free terraces. Experiments performed at low CA dose indicate that the latter mechanism
is dominant, since intermixing is primarily observed at the
perimeters of the M islands while the M structure is unperturbed in the central areas of the islands. This would be consistent with the observation that nearly no binary mixture
phase is observed when depositing M on top of existing CA
networks due to the higher binding energies of the CA networks and associated problems with incorporating a molecule into it.
In summary, we have investigated adsorption and coACHTUNGREadsorption structures formed from CA and M molecules at
a AuACHTUNGRE(111) surface. In addition to the CA1M1 structure found
upon simultaneous deposition, our STM measurements
show the existence of a novel CA1M3 network upon sequential deposition of CA after M. Theoretical modeling by the
SCC-DFTB method explains the stability of the different
networks observed and gives a detailed insight into the intermolecular interactions underlying their formation. By
quantifying the hierarchy of homo- and heteromolecular interaction strengths, we have gained insight that can guide
future efforts towards using this important complementary
hydrogen-bonding motif in the synthesis of self-assembled
surface nanostructures.
Keywords:
hydrogen bonding · molecular recognition ·
nanotechnology · scanning probe microscopy ·
supramolecular chemistry
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Received: August 9, 2006
Revised: December 20, 2006
Published online on March 29, 2007
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