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Exploring the F16CoPc/Ag(110) Interface Using Scanning Tunneling Microscopy and Spectroscopy. Part 1: Template-Guided Adlayer Structure Formation Marius Toader,* Thiruvancheril G. Gopakumar,† Mahmoud Abdel-Hafiez, and Michael Hietschold* Chemnitz UniVersity of Technology, Institute of Physics, Solid Surfaces Analysis Group, D-09107 Chemnitz, Germany ReceiVed: August 12, 2009; ReVised Manuscript ReceiVed: January 6, 2010
Adsorption of cobalt(II) hexadecafluoro-phthalocyanine (F16CoPc) onto a silver (110) surface, prepared by organic molecular beam epitaxy (OMBE) is investigated using ultra high vacuum scanning tunneling microscopy (UHV STM) operating at 40 K. The asymmetry of the metal substrate is found to act as a template for the organic adlayer that grows preferentially along the [11j0] crystallographic axis leading to the formation of commensurate structures with well-defined pinning centers. A line-to-line epitaxy is observed where the difference in interpinning line spacing superimposes the development of two different structures: oblique and square. Structural models are proposed and discussed for both adsorbate lattices where the first one is strongly sustained by the molecular self-assembly at the defect proximity and especially at the domain boundaries where a mirror structure is induced. Introduction The entire field of organic materials is expanding due to its immense interest in basic research and its applications in modern technology based on organic-inorganic structures. Metal phthalocyanines (MePc’s) are one family from the huge area of organics that have already proved themselves to be suitable candidates for molecular opto-electronic devices1,2 mainly because of their semiconductive behavior with a transport gap around 2.2 eV, as well as their much lower cost compared with the inorganic counterparts. Moreover, MePc’s offer a huge variety of electronic properties by simple substitution of the central metal ion as well as the modification of its periphery by suitable functional/non-functional groups, leading to new complex and less-studied molecules. We have investigated cobalt(II) hexadeca-fluoro-phthalocyanine (F16CoPc), one of the few organic n-type semiconductors that possess a high stability to electron beam irradiation allowing further bulk investigations. Understanding the interface formation at organic-metal interfaces is essential with regard to the fabrication and reliability of top and bottom contacts in thin film devices based on organic molecules. For purposes of systematic studies, this is indirectly achieved via organic thin film growth on well-defined crystalline metallic substrates. This is considered in the current investigation by depositing F16CoPc on a noble metal surface, namely a silver single crystal. Scanning tunneling microscopy (STM) under ultra high vacuum (UHV) conditions has already been proved to be one of the most powerful tools for investigating interfaces by monitoring the early stage of the thin film formation. This can be performed by simple imaging of the adsorbed adlayer starting with the first monolayer directly in contact with the substrate whose morphology plays a crucial role for further film growth. * Corresponding author. (M.T.) E-mail:
[email protected]. (M.H.) Fax: +49 371 531 21639; e-mail:
[email protected]. † Present address: Christian-Albrechts-Universita¨t zu Kiel, Institute of Experimental and Applied Physics, D-24118 Kiel, Germany.
Intense studies have been done on non-substituted phthalocyanines and its regular adsorption structures on surfaces by means of STM. So far there are many investigations using STM concerning the adsorption of MePc on different well-defined crystalline substrates, at solid-liquid interfaces,3 in air,4 or under UHV conditions.5,6 On weakly interacting substrates such as graphite, whose electronic surface is mainly dominated by π electrons, the organic ultrathin layers are reported to condense forming large areas of nearly defect-free square crystalline structures.5,6 The weak adsorbate-substrate interaction allows a nearly unrestricted molecular diffusion, and the final symmetry of the organic layer is determined mostly as a result of intermolecular interaction consisting of weak van der Waals forces and steric repulsions. As a result, the reported square symmetry of the self-assembled structure of MePc is induced by the shape of these molecules mostly having a planar structure characteristic to the D4h symmetry group, except for out-ofplane MePc’s such as SnPc’s,7 which belong to C4V. When investigating the molecular self-assembly process of MePc’s at metal surfaces, the situation changes considerably. Because of the nearly free electrons, which dominate the metal surface states, a stronger adsorbate-substrate interaction is expected, a charge transfer can occur at the interface, and the final molecular adlayer structure is strongly influenced by the metal surface symmetry, as has been shown in several reports on MePc crystalline structure formation on highly symmetric metal single crystal surfaces (mainly with 6-fold symmetry).8,9 Despite this growth scenario, which is dominated by the molecule-substrate interaction, still the 4-fold symmetry specific to MePc layers tends to also be conserved in the film formation, as a result of the weak van der Waals intermolecular interaction, either directly in the early deposition stages8,10–12 or induced by annealing processes.13 For fully halogenated MePc layers adsorbed on metal substrates, the literature offers a remarkable lower number of reports using STM investigation. Two recent works on the same molecular system F16CuPc adsorbed on equivalently symmetric metal single crystal surfaces Cu (111)14 and Ag (111)15 have investigated the molecular self-assembly
10.1021/jp9078019 2010 American Chemical Society Published on Web 02/10/2010
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process using low-temperature STM (LT-STM). Despite the same 4-fold symmetry of the molecule conserved after the substitution, the organic crystalline structure differs even from a pseudo-square symmetry. The monolayer formation is governed by the point-on-line epitaxy leading to a two-dimensional (2D) assembly with a rhombic or oblique unit cell, respectively. Still, halogenated molecular complexes have only recently become interesting as a result of the possibility of forming new structures. However, there are not very many reports attributing substituted MePc’s, especially using STM characterization. Especially there is a lack in literature concerning MePc adsorption on non-densely packed metal surfaces where the reduced surface symmetry is expected to act as a template for the molecular film growth. This is what motivated us to choose a silver single crystal substrate offering a face-centered cubic (fcc) (110) surface and fluorine-substituted CoPc’s in the present paper. On the other hand, the molecule of interest here, F16CoPc, has been reported previously by Hipps et al.16 to form a welldefined structure while mixing with nickel tetra-phenyl porphyrine (NiTPP) on Au (111), even at submonolayer coverage. When F16CoPc alone was deposited at the same coverage, although it seemed to aggregate, no ordered structure could be observed and no single molecule could be imaged with submolecular resolution. This drastic contrast compared with the usual protonated CoPc adsorbed on Au (111), which forms a 2D crystalline structure of high order,8 has been explained as a result of fluorine substitution with a great impact on the molecular electron affinity as well as molecular thermal motion induced at the surface. Experimental Details The experiments including both sample preparation and STM investigation were performed in a UHV chamber with a base pressure of 2-3 × 10-10 mbar. The metal crystal was cleaned in situ by repeated sputtering cycles using Ar ions at a pressure not exceeding 5 × 10-6 mbar, followed by annealing up to 625-675 K. The metal surface crystallinity and morphology was checked using low-energy electron diffraction (LEED; see inset of Figure 1c). Prior to deposition, thermal gradient sublimation was employed for purification of the organic molecules provided by Aldrich with a purity larger than 97%. The organic film was deposited onto the freshly cleaned crystalline substrate, using organic molecular beam epitaxy (OMBE) from a Knudsen cell fitted with multiquartz crucibles. The substrate was exposed to a molecular beam for half a minute at a deposition rate of 0.6 nm/min carefully controlled by a calibrated quartz-microbalance monitor from Tectra. Tungsten tips were prepared from polycrystalline W wire by electrochemical etching and cleaned in UHV by Ar+ sputtering and subsequent annealing. STM imaging was performed using a variable-temperature STM (VT-STM) from Omicron in which the samples can be cooled with a liquid helium flow cryostat. Therefore, all the results reported in this work were achieved at a coresponding working temperature of about 40 K. The data processing was performed using WSxM software from Nanotec.17 Results and Discussion The optimized geometrical structure of F16CoPc using density functional theory (DFT) performed by the Gaussian 03 software package18 is presented in Figure 1a. It is a planar 4-fold symmetrical aromatic macrocyclic organic molecule. In particular, the peripherical hydrogen atoms of the benzene rings
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Figure 1. (a) Optimized molecular structure of F16CoPc. (b) 3D hard sphere model of the Ag (110) metal surface; four major planes are visible. (c) High-resolution STM constant current topography (25 × 15 nm2) of F16CoPc adsorbed on Ag (110) (Vsp) 2 V and Isp) 200 pA) together with an LEED image of Ag (110) as the inset.
are substituted with fluorine atoms inducing a higher electron affinity to the entire molecule, consequently leading to a much more reactive system. In this work, F16CoPc is deposited on a lower-symmetry Ag (110) single crystal substrate with a coverage corresponding to a saturated monolayer. Belonging to the fcc crystalline structure, the silver cut along the (110) plane exhibits a surface with a rectangular unit cell described by a lattice constant of a ) 4.0862 Å along [001] and b ) 2.892 Å along [11j0].19 Figure 1b shows a three-dimensional (3D) hard sphere model of Ag (110) surface including the first four top layers, using a DSViewerPro software package from Accelrys. It is clearly visible that this relatively low-symmetry of the surface has much higher corrugation along [001] compared to [11j0], leading to the formation of real potential notches propagating along [11j0], which may act as pinning lines for the adsorbate molecules, restricting them to a highly anisotropic motion. Figure 1c shows a high-resolution STM image of F16CoPc adsorbed onto Ag (110) recorded in constant current mode where the inset presents a LEED image of the metal substrate surface. As expected, the adsorbate-substrate interaction plays an important role in the adlayer formation. The organic molecules grow preferentially along the [11j0] crystallographic axis of the metal surface. Despite the high negatively charged periphery, which involves a strong intermolecular repulsion, the molecules aggregate forming a well-defined relatively close-packed structure. The observed oblique structure b A×b B, denoted by black arrows, is described as a unit cell of 1.68 × 1.86 nm2 with an angle in between (θ) of 73° and a corresponding azimuthal angle (δ) of 43°. According to Oison et al.,20 half-halogenated F8ZnPc deposited on Ag (111) shows a close-packed square structure with a lattice constant of 1.5 nm as a result of the strong electrostatic H-F bonds formation. Therefore, we may conclude that the intermolecular spacing found here is in the right order of magnitude, considering the strong intermolecular repulsion as well as the geometrical length of 1.54 nm along one of the molecular axis. Each single molecule is identified as a four-lobe structure mainly due to the delocalized π orbitals extended over the benzene rings imposing a flat lying adsorption as a result of the electronic
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Figure 2. Rectangular model used to explain the organic adlayer structure (a); constant current topography (30 × 18 nm2) of F16CoPc adsorbed on Ag (110) recorded at Vsp ) 2 V and Isp ) 200pA (b); the corresponding height profiles labeled in panel b as 1 (c), 2 and 2′ (d), and 3 and 3′ (e).
wave function overlapping with the metal surface states. Due to a face-to-face arrangement of the molecular lobes along [11j0], a strong repulsion is induced resulting in a larger intermolecular spacing even if their orientation is along the atomically closest packed crystallographic axis. This arrangement is sustained by the presence of symmetrical voids along [11j0] compared with their lack along b A where molecules are brought closer by interdigitation (refer also to Figure 2a). For a better understanding of the epitaxial growth process the influence of the substrate symmetry on the adlayer structure is checked by measuring the intermolecular rows spacing along the [001] direction. In particular, two molecular central cavities are fairly well aligned along [001] over four molecular rows (follow the broken white line between the two white arrows in Figure 1c), leading to well-defined corrugations in the height profiles, which allows a better understanding (refer also to Figure 2b,c). The depicted length L has been measured, and the average ratio L/a has been determined to be very close to the integer number 16. In conclusion, the molecules arrange in well-defined molecular rows along [11j0] at 4a lattice constants along [001] separated from each other. At this moment we may describe the crystalline structure using a rectangular model, which is quite easy to be understood. It is important to mention that the perfect rectangularity might deviate with (2° due to the direction determination errors as well as the molecular central position determination since the central cavity extends over a certain area. Nevertheless, the proposed model in Figure 2a is important for reproducibility of the line profile shape particularities, which helped for a very precise intermolecular row spacing determination. The model is based on a rectangular unit cell with four molecules at the corners and three molecules located with their center along the diagonal, at 4a intermolecular row spacing. Figure 2c shows an experimental cross-sectional profile over four molecular rows along [001] connecting two molecular central cavities. The line was recorded from the STM image in Figure 2b and corresponds to the broken white arrow marked as 1. The color code was introduced for a better identification
of the line particularities and direct comparison between the model and the real profile line. Because of the four-lobe structure, the molecule is identified in the STM image, we marked the two equivalent pairs of lobes as LH and LV corresponding to horizontal and vertical axis in the plane of the image. The blue area is attributed to the LH, and the red one is attributed to the LV, while the yellow corresponds to the hollow area (H) characteristic to the intermolecular voids, and the green represents the central cavity C. The central cavity corresponds to a small depression, indicating that the cobalt metal ion does not contribute to the tunneling current when the substrate is positively biased. This one is followed by a peak as a result of line crossing a molecule and further by a double peak corresponding to the LH and LV and ending into a deep depression H. The second half of the profile line is totally symmetric, revealing a very good correlation between the model and the real image. Figure 2b shows a high resolution STM image where molecular defects (single and agglomerated vacancies) are present. The largest empty area appears at the boundary of two different domains labeled as D-I and D-II. The boundary is created by the mismatch between the molecular rows oriented along [11j0]. Therefore, the relaxation processes at the boundary, denoted by a white dotted line, induce larger intermolecular spacing, which deviates from the close-packed structure. The relaxation direction and its magnitude are strong indications for determining the favorable molecular adsorption sites. Figure 2d presents the height profiles 2 and 2′ corresponding to the green and red interrupted arrow, respectively. Each single molecule can be identified in the profile as a double-shouldered peak with a small dip in between corresponding to the central metal ion. The presence of the intermolecular dark area sustains a relaxation along [11j0] where the molecular dimer adsorbs at ∼2.32 nm away, which corresponds to an excess of ∼0.46 nm from the length of the unit cell vector b B. Because the height profile is not recorded exactly along [11j0] as a result of the domains mismatching, it should be corrected with a factor that
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Figure 3. the proposed commensurate adsorption geometry model of F16CoPc/Ag (110).
will lower the value slightly, and therefore the correction has not been performed. The relaxation magnitude is found to match very well with a value of (3/2)b ) 0.43 nm. Moreover, the edge molecule of D-II (the intersection of profiles 2 and 3′) exhibits a similar relaxation with a magnitude very close to (1/2)b ) 0.14 nm. It is not surprising to find this relaxations along [11j0] with magnitudes corresponding to integer numbers of half units of lattice constant b, since the length of the unit cell vector b B (∼1.86 nm) has a negligible mismatch comparing with (13/2)b ) 1.87 nm. Song et al.21 have reported using DFT slab calculations that, in the case of CuPc adsorption on the same substrate Ag (110), the most favorable adsorption site (the adsorption site refers to the position of the central Cu ion) is the hallow site (H: the top site of the Ag atom located in the second layer) followed energetically by the long bridge site (LB: the boundary site of two Ag atoms located in the second layer). Nevertheless, the relaxation processes corresponding to welldefined integers of half units of b are indications that one adsorption site is more favorable. On the other hand, the height profiles 3 and 3′ shown in the Figure 2e correspond to the blue and black interrupted arrows and are used to determine the mismatch between the domains. Starting from the same molecular row belonging to the D-II, we will end up on molecules belonging to different domains marked with white small arrows. The difference between these two lengths, corrected by sin θ, is equal to ∼0.80 nm, very close to 2a ) 0.81 nm, sustaining a pinning effect of the molecular rows along [11j0]. Moreover, the same molecule that shows a relaxation along [11j0] exhibits no deviation from the closed packed position along b A, and is thus implicit along [001] (see height profile 3′ from Figure 2e). Defects like single-molecule missing, marked using white dotted circles, impose no similar deviations (see Figure 2e, height profile 3). As for the molecule embedded in between the small dark arrows, we may not speak about a relaxation along [001] since it belongs to neither D-I nor D-II and adsorbs at the boundary site at equal distances from both domains.
The epitaxial structure can be completely understood, and a proposed commensurate model is shown in Figure 3. The molecules arrange exclusively on the potential notches along [11j0] adopting alternative adsorption sites, H and LB, while the neighboring molecule in the other direction adsorbs at four lattice constants a, preserving the same alternative selectivity for the adsoption site. Because two molecules relatively aligned with the central cavities along [001] possess different adsorption sites, the model deviates from a perfect rectangularity by an angle of +1.15°, which is within the previous model’s predicted error bar. The model unit cell, described as 1.69 × 1.87 nm2 and the angle in between of 75°, is in very good agreement with the measured parameters. The small difference in angle value might be due to the error in determination as well as small drift effects. The entire arrangement is not accidental, since in this configuration the main electron acceptors of the molecule, such as the pyrole nitrogen and especially the fluorine atoms, adsorb close to the first silver layer, enabling an efficient coupling between their electronic clouds and the surface states. Two neighbor molecules along b A exhibit an arrangement of the lateral fluorine atoms on opposite equivalent sites, imposing a symmetrical interaction along [001], suppressed by the pinning force responsible for the molecular anisotropic relaxation described previously. The formation of crystalline structure is well balanced by the molecule-substrate and molecule-molecule interactions. While the growth is superimposed through the pinning of each single organic molecule along [11j0], allowing a motion with well-defined pinning centers, the final 2D crystalline structure is adjusted by the intermolecular interaction. The epitaxial relation between the adlayer lattice and the substrate can be described as follows:
[ ] [ ][ ] A′ 4 0 ) 2B 0 13
a b
where A′ denotes the projection of Ab on [001] .
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Figure 4. STM images (30 × 25 nm2) (a) and (25 × 20 nm2) (b) of the self-assembled monolayer of F16CoPc at the defects proximity recorded at Vsp) 2 V and Isp) 300 pA and 200 pA, respectively. (a′,b′) Corresponding height profiles; (a′′,b′′) corresponding models for the packing mechanism .
The epitaxial process proposed previously is strongly sustained by the self-assembly mechanism in the proximity of defects. Figure 4a,b shows two STM images where molecular clusters on top of the monolayer are clearly observed as bright disordered areas. An equal number of steps (follow the blue line in Figure 4a) is not sufficient to enclose a molecular island, and, consequently, an additional vector, the so-called Burgers vector (denoted by the green arrow), is needed to close the loop. This is an indication that a 2D edge dislocation in the monolayer might act as a seed for the island growth, inducing a propagation of this dislocation in the crystalline structure, and finally leading to the formation of a new phase. In consequence, two domains are formed with a certain boundary, embedded in between two interrupted white lines, which belongs to none of the domains. In our case, the domain boundary consists of a single molecular row, which, together with the two adjacent rows, induces a new phase (between the dotted black lines) with a mirrored structure. Due to the soft pinning centers along [11j0] and to the 2-fold symmetry of the substrate, each adsorption site has an equivalent position mirrored with respect to [001], leading to the same packing mechanism responsible for the perfect fitting of the boundary to the domains. As a result, the mirrored structure is just a crossing phase between two identical close-packed structures (follow the interrupted arrow). Figure 4a′ shows the height profile recorded along the interrupted black arrow. No relaxation effects are induced over the intermolecular spacing as a result of the self-assembly process, which obeys to the same line-on-line pinning selectivity. The measured length between b|. An the small black arrows ∼3.39 nm corresponds to 2|A additional island encountered during the boundary phase propagation has no repercussions on the phase structure. Just a slight shifting with a magnitude of one molecular row was observed. The model for the boundary phase formation (see Figure 4a′′) is based on the same principles, and, in particular, it is just an extension of the proposed commensurate one where one molecular row along [11j0] glides adsorbing in equivalent adsorption sites symmetrically with respect to [001]. The unit cell corresponding to the mirrored structure (identified as a black parallelogram) matches perfectly to that describing the closepacked structure of the domains (depicted as white paral-
Figure 5. High-resolution tunneling voltage polarity-dependent STM image (20 × 12 nm2) of F16CoPc on Ag (110) revealing the effect of the contrast reversion when switching from +2 to -2 V at Isp ) 200 pA.
lelograms). A similar situation is encountered in Figure 4b. A perfect assembling is observed between two molecular domains independently growing or induced by the island present in the proximity, like in the previous case. Although no physical boundary (depicted by the interrupted white line) can be identified, still the same mirrored structure is induced by the two-edged molecular rows with a similar interspacing characteristic to the close-packed structure (see Figure 4b′). The selfassembly mechanism responsible for the boundary formations obeys the same adsorption site selectivity previously proposed, allowing the development of the accurate model denoted in Figure 4b′′. Being a 3d transition metal, the Co (II) d7 system exhibits after sharing the unpaired electrons with the pyrole nitrogen atoms, a half-filled 3dz2 orbital. The strong “z” character of this orbital, which points perpendicular to the molecular plane, imposes a better coupling of its electronic wave functions with the metal surface states. Figure 5 shows a high-resolution tunneling voltage polarity-dependent STM image of F16CoPc monolayer adsorbed on Ag (110), where the line separating scans with positive and negative sample polarity is indicated by two black arrows. When crossing from a positive to a
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Figure 6. (a) Topographic STM image (25 × 20 nm2) recorded at Vsp)- 2 V and Isp) 200 pA, denoting a pseudo-square molecular lattice. (b) Proposed commensurate model.
negative voltage (the sign of the voltage denotes the sample polarity) the molecular central cavity appearance changes from a shallow depression toward a bright protrusion. This orbital tunneling imaging denotes a tunneling process mediated by the dz2 orbital of the coordinated cobalt ion as a result of its electronic interaction with the underlying substrate. The bright contrast at the central cavity when negatively biased, indicates that after the interface formation the half-filled dz2 orbital of the cobalt ion exhibits a higher density of filled states most likely as a result of a charge transfer from the metal into the molecule. Hipps et al8,22 have explained the difference in tunneling contrast for MePc as a consequence of different occupation of the d orbital which leads to differences in the electronic configuration of transition metal ions. The strong adsorbate-substrate interaction mainly mediated by the dz2 orbital of the cobalt ion explains the adsorption sites selectivity, since these refer to the position of the central metal ion. A further explanation leads in the early stage of the STM history when Chen23 showed that localized metallic pz or dz2 tip states are required for atomic resolution in STM imaging. As a result of the molecular flat-lying adsorption, the dz2 orbital of the coordinated cobalt ion points perpendicular to the substrate. Its coupling with the metal surface states acts as a driving force for the adsorption sites selectivity, explained by the strong sensitivity to the atomic corrugation imposed by the “z” character of the mediated orbital. A second observed phase, presented in Figure 6a with a pseudo-square structure is characterized by a unit cell described b×b B) with a 90° angle in between. The as 2.05 × 1.99 nm2 (A unit cell vectors are very close in length to five atomic surface lattice constants a, 5a ) 2.04 nm, and to seven atomic surface lattice constants b, 7b ) 2.02 nm, respectively, leading to a perfect commensurable adlayer structure where each single molecule adopts an equivalent adsorption site. The epitaxial relation between the substrate and the adlayer lattice can be described using a matrix transformation between the unit cell vectors of the adlayer and the substrate as follows:
[ ] [ ][ ] 5 0 A ) 0 7 B
a b
A structural model explaining the adsorption geometry of F16CoPc on Ag (110) is presented in Figure 6b. Compared with the oblique structure where molecules adopt alternative adsorption sites H and LB, here all molecules possess the same adsorption site, indicating that one of them is energetically more favorable, in good agreement with the observed relaxation mechanism discussed previously for the oblique structure. Nevertheless, the question whether this site is H or LB, remains open since there are no indications to elucidate which one is
Toader et al. preferred. Even if the molecular growth obeys the same lineto-line pinning principles, a larger spacing, between the preferentially aligned molecular rows along [11j0], induces a totally different structure. The higher azimuthal angle reflects a relaxation from the superimposed face-to-face arrangement due to the intermolecular steric repulsion. It would have been expected that this relaxation to be a more attractive one since there is a higher molecular interdigitation. Still, a repulsive relaxation is observed as a result of the repulsive intermolecular interaction toward a crystalline structure formation governed by a tendency of the organic molecules to conserve their 4-fold symmetry, well accepted by the 2-fold symmetry of the substrate. This type of structure is very well-known and reported to be specific to MePc’s, which form square molecular crystals by symmetry conservation, with a lattice constant in the range of 14-16 Å.5,7,8 In this work a lattice constant of ∼20 Å sustains the stronger intermolecular repulsion achieved after the peripherial halogenation comparing with the non-substituted MePc, where the intermolecular interaction is governed by weak van der Waals forces. The inter-row spacing-dependent molecular structure formation can be explained based on packing density variations from one structure to another. Since the oblique phase shows a packing density of 0.33 molecules/nm2, the pseudosquare phase exhibits a structure described by a packing density of 0.24 molecules/nm2. Conclusion The growth process of an ultrathin monolayer of F16CoPc adsorbed on Ag (110) has been investigated using VT-STM. The epitaxial relation with the substrate denotes the formation of commensurate adlayer structures strongly superimposed by the surface symmetry. The strong difference in atomic corrugation exhibited by the metal surface leads to the formation of real potential notches along the [11j0] crystallographic axis which act as pinning centers for the monolayer growth. The well aligned molecular rows along these notches adsorb at an integer number of lattice constants along [001], 4a or 5a, leading to the formation of an oblique close-packed structure or a pseudosquare more loosely packed structure, respectively. The highly anisotropic molecular growth imposed by the line-on-line pinning effect allows soft relaxation processes along [11j0] with well-defined pinning centers. The selectivity for the adsorption sites is strongly correlated with the dz2 orbital of the coordinated cobalt ion responsible for the high sensitivity to the atomic corrugation. The coupling of this orbital with the metallic surface states was pointed by the different contrast appearance at the central cavity in the voltage polarity-dependent STM image as a result of the different occupation of the dz2 orbital with direct influences on the tunneling process. The self-assembly process of the oblique molecular adlayer at the defects proximity induces a mirrored structure accurately described by the same proposed packing mechanism. In conclusion, an inter-row spacing dependence of organic monolayer formation, toward two different well-defined crystalline structures, was observed, being strongly imposed by the adsorbate-substrate interaction, which acts as a driving force for the growth, carefully balanced by the intermolecular interaction with repercussions over the final 2D structure. Acknowledgment. M.T. thanks the Deutsche Forschunggemeinschaft (DFG) for funding this work in terms of Graduate College -International Research and Training Group (GRK 1215) - “Materials and Concepts for AdVanced Interconnects”.
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