J. Phys. Chem. C 2009, 113, 8407–8411
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Template-Directed Molecular Nanostructures on the Ag/Pt(111) Dislocation Network Kamel Aı¨t-Mansour,* Matthias Treier, Pascal Ruffieux, Marco Bieri, Rached Jaafar, Pierangelo Gro¨ning, Roman Fasel, and Oliver Gro¨ning Empa, Swiss Federal Laboratories for Materials Testing and Research, nanotech@surfaces Laboratory, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland ReceiVed: February 15, 2009; ReVised Manuscript ReceiVed: March 30, 2009
We report on the high effectiveness of the Ag/Pt(111) dislocation network template for guiding the formation of well-ordered perylenetetracarboxylic diimide (PTCDI) molecular nanostructures due to its laterally strong inhomogeneous properties. Numerous PTCDI nanostructures show chiral arrangements driven by specific substrate-molecule and molecule-molecule interactions, with a higher strength coming from the substrate, which, per molecule, is quantified by simulations to be at least in the same order as a hydrogen bond. The fabrication of complex supramolecular structures by selfassembly of specifically designed molecular functional building blocks and their control at the single-molecule level remains a challenge of particular interest in materials science due to their potential use for applications in molecular nanoelectronics and other cutting edge technologies.1,2 Single crystal surfaces represent ideal supports for the formation of flexible molecular architectures and their characterization down to the submolecular scale.2 On these solid surfaces there are basically two different strategies pursued for controlling the self-assembly of molecular species. The first and the most prominent one is based on the exploitation of different types of molecule-molecule interactions such as aromatic interactions,3 hydrogen bonding,4,5 metal coordination,6 and even covalent bonding.7 The second, less frequently employed strategy uses site-specific molecule-substrate interactions to immobilize molecules at regularly distributed positions on template surfaces. Ideally such template surfaces should have periodicities similar to the dimensions of small molecules, which means below 10 nm, with a positional precision approaching 1 nm. Such requirements are not accessible to conventional lithographic techniques or to the recently demonstrated “lithographically controlled wetting” technique.8 In this context, a real alternative consists of using nanotemplate surfaces arising from two-dimensional (2D) porous molecular networks4-6 or self-organized surface structures.9-15 For instance, the triangular dislocation network formed in the system of two monolayers (ML) of Ag on Pt(111)16 known for templating the self-organized growth of Ag 2D nanostructures at 110 K17 has been found in our previous works to strongly influence the self-assembly of C60 molecules.9,10 In this paper, using scanning tunneling microscopy (STM), we show that the 2 ML Ag/Pt(111) dislocation network pattern (Figure 1a) is very effective for fabricating well-ordered molecular nanostructures directed by the template surface. The organic molecule chosen in this work is the planar 3,4,9,10perylenetetracarboxylic diimide (PTCDI) (Figure 1b) which in previous studies was associated with other molecular species in order to provide 2D networks4,5 as well as ribbons.14 We also demonstrate that small, regular PTCDI clusters formed on specific regions of the dislocation network surface show chiral * To whom correspondence
[email protected].
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arrangements whose structure is determined by a balance of specific substrate-molecule and molecule-molecule interactions. Experiments were carried out in ultrahigh vacuum using a low-temperature STM9-12 (Omicron) operated here at 77 K. STM images were measured in the constant-current mode; the stated voltage refers to the electric potential of the sample with respect to the tip (which has been mechanically cut from a Pt/ Ir wire). The STM images have been processed with the WSxM software.18 The template surface was prepared by annealing 2 ML Ag on Pt(111) to 800 K as previously described.9-11 PTCDI molecules (99+%, Fluka Chemie GmbH) were deposited on the template surface kept at 370 K via sublimation from a quartz crucible resistively heated in a Kentax evaporator (TCE-BSC) at about 660 K, where thickness calibration was monitored by a quartz crystal microbalance. Several PTCDI deposition sequences were employed on the same template surface.
Figure 1. (a) STM image of a 2 ML Ag/Pt(111) dislocation network surface showing long-range order (-1 V, 2 nA) and where three types of domains (fcc, hcp1, and hcp2) can be seen in the superstructure unit cell. (b) Chemical structure of PTCDI.
The periodic dislocation network surface (see STM image of Figure 1a) used for templating the growth of the PTCDI molecules (chemical structure shown in Figure 1b) contains in the unit cell three types of domains, one hexagonal fcc16 and two differently sized triangular hcp16 (hcp1 and hcp2) domains
10.1021/jp901378v CCC: $40.75 2009 American Chemical Society Published on Web 04/20/2009
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Figure 2. STM images showing the initial growth stages of PTCDI on the 2 ML Ag/Pt(111) dislocation network surface at 370 K and the preferential adsorption of the molecules on the hcp1 regions of the template surface: (a) few percent of ML PTCDI (-0.5 V, 50 pA) and (b) 0.1 ML PTCDI (-0.5 V, 5 pA).
which are separated by dark lines appearing in the STM image at lower topography, in particular the triangles enclosing the hcp1 regions whose corners reach the maximum depression of about 0.4 Å.9 These dark lines are attributed to Shockley partial dislocations16 very probably buried in the deeper layers11 as recently reported elsewhere for a semiconductor system.19 For a few 0.01 ML PTCDI on the dislocation network surface, the molecules decorate exclusively hcp1 regions of the bare terraces together with some step-edge sites (Figure 2a). For higher coverages of about 0.1 ML (Figure 2b), PTCDI chains connect the molecular clusters formed on the hcp1 regions via the hcp2 edges, whereas the step-edge sites are not fully occupied by the molecules. This observation shows that the adsorption energy of the PTCDI molecules on the hcp1 regions
Aı¨t-Mansour et al. is comparable to the one at step edges, which is unusual on noble metal surfaces.14,20 For PTCDI coverages even higher, of about 0.3 ML, as can be seen in the STM images of Figure 3, the molecules continue to grow in this manner, and now some of the molecular chains become larger and form close-packed ribbons or linear islands linking the molecular clusters of the hcp1 regions. Figure 3 clearly shows the strong influence of the template surface on the molecular assembly. In contrast to passive surfaces which do not show long-range templating features, like Ag/Si(111) where PTCDI molecules diffuse freely on the bare terraces and form large islands growing from step edges, in which the order is governed by intermolecular interactions,20 in the present case, the molecules form an ordered pattern of nanostructures that is strongly directed by the dislocation network. The very apparent preferential adsorption of the PTCDI molecules on the hcp1 regions can be explained by two reasons related to each other: (i) the hcp1 regions have the lowest work function on the bare terraces of the template surface (the work function difference between the fcc region where the work function is maximum and the hcp1 region is about 0.35 eV)12 and (ii) the PTCDI molecule is known to be an electron acceptor.21 From (ii) one can reasonably expect an electronic charge transfer from the substrate to the molecules. In fact, on Ag(111) which is the surface closest to our template but unreconstructed, the substrate to molecule charge transfer has been found in recent first-principle calculations to be of 0.9 electrons per PTCDI.21 The preferential adsorption of PTCDI in the hcp1 regions is attributed to the partially ionic character of the molecule-substrate interaction, which in the case of a molecule with a high electronegativity like PTCDI is expected to result in a higher binding energy in regions with a low work function, similarly to the recently reported case of the C60 molecule on the same template surface.10 Panels b and c of Figure 3 show that numerous PTCDI clusters adsorbed on the hcp1 regions exhibit a regular arrangement of six molecules, three at the hcp1 corners and three in the middle of the hcp1 edges. Interestingly, these six-molecule clusters show chiral structures with the two enantiomorphs highlighted by white and black circles in Figure 3c. Close-up STM images of both structures are shown in Figure 4a,b. We have observed that, like the apparent height of the PTCDI molecules that varies from 0.5 Å at -2 V (Figure 3c) to 1.7 Å at +2 V (Figure 3b), their apparent shape strongly depends on the tunneling bias, similarly to the case of the 3,4,9,10perylenetetracarboxylic dianhydride (PTCDA) molecule whose chemical structure is close to that of PTCDI (only each one of the two NH groups of PTCDI (see Figure 1b) is replaced by an O atom in the case of PTCDA) and whose apparent shape observed in STM has recently been studied experimentally and theoretically in detail.22 If one compares the STM images of Figure 3b,c measured at opposite voltages, one can see that the apparent large and small sides for each molecule are roughly inverted from one image to the other. As in the case of PTCDA,22 we note that in the intramolecular resolution STM images of Figure 4a,b the two principal maxima for each PTCDI correspond to the aromatic parts of the molecule and not to its carboxylic groups. Therefore, for the molecular arrangements of Figure 4a,b, we propose the structural models of Figure 4c. In the following we investigate using AMBER force-field calculations23 the energetics of the six-PTCDI clusters stabilized on the hcp1 regions. The starting point for these investigations consists in three different configurations of pairs of PTCDI molecules labeled R, β, and γ shown in Figure 4b,c. As we
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Figure 3. STM images of 0.3 ML PTCDI adsorbed at 370 K on 2 ML Ag/Pt(111). Overview image showing the formation of an ordered pattern of molecular nanostructures guided by the dislocation network (+2 V, 5 pA). (b, c) Close-up images of the upper right region of panel a measured with the same tunneling current (5 pA) but with opposite voltages, +2 and -2 V, respectively. Chiral six-PTCDI clusters formed on the hcp1 regions of the template surface are highlighted by white and black circles in panel c. (d) Region with a locally higher PTCDI coverage than in panels b and c and where an extended close-packed molecular island can be seen (+2 V, 5 pA).
assume a planar adsorption and ignore the influence of the surface atomic lattice, the free parameters of the energy optimization calculations are the relative position (x, y) of the centers of both molecules (intermolecular distance is d ) (x2 + y2)1/2) and the relative in-plane rotation angle φ between the long axes of the two molecules, where the long axis of one molecule is fixed to be parallel to the x-axis. These parameters are shown in the β-configuration in Figure 4c. Configuration R represents the energetically most favorable pair configuration with d ) 14.5 Å (x ) 14.3 Å, y ) 2.5 Å) and φ ) 0(1)°, and with an interaction energy of ER ) 0.33 eV (attractive) which is comparable to the value reported elsewhere.14 In this configuration the molecule can form two hydrogen bonds
between the cabroxylic and imide group. This bonding motive, characterized by the lateral offset of the molecules, is found in the single molecule chains connecting the clusters of the hcp1 regions (Figures 2b and 3b,c) as well as in the close-packed PTCDI islands (Figure 3d), where an intermolecular distance of d ) 14.4 Å is measured. Within the six-molecule clusters, however, this bonding motive is not found. Rather we observe the β and γ bonding motives. For the β-configuration we find by calculations an optimized geometry of d ) 12.2 Å (x ) 11.80 Å, y ) 3.1 Å) and φ ) 60(5)° with an interaction energy of Eβ ) 0.14 eV. Compared to R, the lower interaction energy can be readily understood from the less favorable hydrogen bond formation here. The six-molecule cluster can be understood as
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Aı¨t-Mansour et al. within extended compact PTCDI islands (Figure 3d), i.e., when the molecular coverage locally approches the ML, the molecules of the hcp1 regions are forced to change their orientation, more precisely the β-configuration disappears in favor of the standard R configuration characteristic of the large islands, and therefore the cluster chirality no longer exits. In conclusion, we show that the 2 ML Ag/Pt(111) dislocation network surface is very effective for templating the growth of PTCDI molecular assemblies due to its laterally strongly inhomogeneous adsorption properties. Clearly the template surface guides the formation of well-ordered PTCDI nanostructures, specifically the formation of regular, chiral PTCDI clusters containing six molecules. By comparison of the interaction energies, obtained by force-field simulations, of this specific local arrangement of molecules with the ideal molecular configuration, we can deduce that the template-related sitespecific contribution to the adsorption energy per molecule is at least comparable to that of a hydrogen bond. This opens the perspective of assembling molecules with different properties and/or complementary end-group functionalities into wellcontrolled complex nanostructures which can be relevant in technological applications. Acknowledgment. Financial support by the European Commission (RADSAS, NMP3-CT-2004-001561) is gratefully acknowledged. Kamel Aït-Mansour thanks the nanotech@surfaces Laboratory of Empa for its great hospitality.
Figure 4. (a, b) Close-up STM images showing the chiral six-PTCDI clusters (white and black circles) stabilized on the hcp1 regions as well as a small PTCDI chain (-0.5 V, 50 pA). (c) Calculated models for different molecular configurations R, β, and γ seen in panel b. As shown in panel c, the chiral six-PTCDI clusters consist of three β-units and three γ-units.
consisting of three β-units linked by three γ-type bonds. From the experimental images we can deduce the γ-configuration parameters which obviously do not correspond to the more ideal R-unit of similar configuration. For γ we find d ) 17 Å (x ) 17 Å, y < 0.5 Å) and φ ) 0(3)°, and, due to the comparatively large intermolecular distance, the interaction energy is rather small, i.e. of Eγ ) 0.006 eV. This sustains the picture that, regarding hydrogen bonding interactions, the six-PTCDI clusters of the hcp1 regions are mainly driven by the β-units. It is worth noticing here that these clusters do not adapt to form R-type bonds between the β-units, by which up to 0.97 [3(0.33 0.006)] eV in binding energy could be gained, depending on how much the β-unit bond would be weakened. This already gives the order of the template substrate related, configurational contribution to the adsorption energy of the six-PTCDI clusters. To quantify this contribution, further, we compare the molecular interaction energy of the cluster EC ) 3(Eβ + Eγ) ) 0.44 eV in the configuration on the hcp1 region of 2 ML Ag/Pt(111) with the ideal, free-floating six-PTCDI cluster characterized by the highest interaction energy which is found to be EI ) 1.74 eV. Such a floating six-PTCDI cluster would adopt a very similar configuration as inside the large PTCDI island seen in Figure 3d, except the orientation of the island which is strongly influenced by the template surface. This comparison shows that the site specific contribution to the adsorption energy of the six-PTCDI cluster is about (EI - EC) ) 1.30 eV, which gives 0.22 eV per molecule. We note that this site specific interaction energy (0.22 eV) is even higher than a hydrogen bond (0.17 eV) and therefore ranks at least in the same order as the guiding force to the chiral molecular network formation. We note that
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J. Phys. Chem. C, Vol. 113, No. 19, 2009 8411 (18) Horcas, I.; Ferna´ndez, R.; Go´mez-Rodrı´guez, J. M.; Colchero, J.; Go´mez-Herrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 78, 013705. (19) Liu, Y.; Cai, Y.; Zhang, L.; Xie, M. H.; Wang, N.; Zhang, S. B.; Wu, H. S. Appl. Phys. Lett. 2008, 92, 231907. (20) Swarbrick, J. C.; Ma, J.; Theobald, J. A.; Oxtoby, N. S.; O’Shea, J. N.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2005, 109, 12167. (21) Sassi, M.; Oison, V.; Debierre, J. M. Surf. Sci. 2008, 602, 2856. (22) Kraft, A.; Temirov, R.; Henze, S. K. M.; Soubatch, S.; Rohlfing, M.; Tautz, F. S. Phys. ReV. B 2006, 74, 041402. Rohlfing, M.; Temirov, R.; Tautz, F. S. Phys. ReV. B 2007, 76, 115421. Tautz, F. S. Prog. Surf. Sci. 2007, 82, 479. (23) Calculations for homo-molecular pairs have been performed with use of the AMBER force-field (Cornell, W. D.; Cieplak, P.; Bayly, C. I.; Gould, I. R.; Merz, K. M.; Ferguson, D. M.; Spellmeyer, D. C.; Fox, T.; Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 5179. ) as integrated in HyperChem 7.5 (Hypercube Inc., 1115 NW 4th Street, Gainesville, FL 32601). Molecular structures have been optimized at the AM1 level of theory and were kept fixed for the force-field calculations. All calculations have been performed for gas phase interactions with the effect of the surface being accounted for by forcing molecules to lie within the same plane.
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