Stabilizing CuPc Coordination Networks on Ag(100) by Ag Atoms

Article pubs.acs.org/JPCC

Stabilizing CuPc Coordination Networks on Ag(100) by Ag Atoms Grazẏ na Antczak,*,† Wojciech Kamiński,† and Karina Morgenstern‡ †

Institute of Experimental Physics, University of Wrocław, Wrocław, Poland Chair for Physical Chemistry I, Ruhr-Universität Bochum, Bochum, Germany

‡

ABSTRACT: We demonstrate that Ag adatoms are capable of stabilizing negatively charged copper-phthalocyanine (CuPc) molecules in a Ag−CuPc network at room temperature. For this aim, the structure of the Ag−CuPc coordination network at different molecule-adatom densities is investigated experimentally by scanning tunneling microscopy and theoretically by firstprinciples calculations. The islands formed at saturation adatom density, close to the source of adatoms, consist of a closed-packed layer without voids. The islands formed at lower adatom density consist of an irregular arrangement of larger entities, named subunits, mainly (CuPc)4Ag and (CuPc)6Ag2, which are interconnected in the same fashion as the CuPc molecules in the closed-packed layer. Silver adatoms in the subunits and between them differ by the number of molecules they link. The Ag−CuPc networks are stabilized, because the adsorption energy of CuPc molecules increases due to the presence of adatoms.

I. INTRODUCTION Phthalocyanines (Pc) represents a family of molecules with a wide range of applications due to promising semiconductor and optoelectronic properties.1−6 The molecules are studied with respect to the development of solar cells,7,8 sensors,9 and other organic electronic devices.10,11 The submonolayers (islands) of copper-phthalocyanine (CuPc) cannot be stabilized at room temperature (RT),12 and islands of CuPc were observed only at low temperatures (below 100 K) while after room temperature treatment only full monolayers are stable.12−14 For other molecules metal linkers were successfully employed to stabilize nanostructures (e.g., 9,10-anthracenedicarbonitrile,15 zinc5,10,15,20-tetra(4-pyridyl) porphyrin,16 1,3,5-benzoic tricarboxylic acid,17 4-[trans-2-pyrid-4-yl-vinyl)] benzoic acid18). The molecules form different island structures if different additional agents (adatoms) are present, and such molecular arrangements might provide novel functional materials.19−21 The source of metal adatoms can be fluctuating surface step-edges or coadsorption. For example, Pawin et al. investigated the creation of a coordination network of 9,10-anthracenedicarbonitrile on Cu(111) with copper adatoms released from the stepedges by annealing.15 Shi and Lin investigated the changes in the arrangement of zinc-5,10,15,20-tetra(4-pyridyl) porphyrin adsorbed on Au(111) with Cu adatoms from an external source.16 Apart from linkage, the metal atoms can metalate freebase phalocyanine as shown for instance for iron adatoms on the surface.22 Selective capturing of Fe adatom by tetrapyridylporphyrin molecules on a copper (111) surface was reported by Auwärter et al.23 The aim of this work is to stabilize CuPc coordination networks at RT with native Ag adatoms. We show that at a temperature of 200 K the molecular structure is not stabilized owing to insufficient Ag adatom density.24 At RT a twodimensional (2D) atomic Ag gas is present on the surface. A © 2014 American Chemical Society

close-packed molecular layer is formed close to step-edges where Ag atoms are abundant. A layer with voids is developed on terraces, because there is a deficiency of Ag adatoms on them. The voids are sometimes filled by additional Ag atoms. We thus outline a possibility to stabilize nanostructures consisting of CuPc on the Ag(100) surface at temperatures relevant for applications.

II. METHODS The Ag(100) surface was cleaned prior to deposition by performing twice a combination of Ar+ sputtering with an energy of 1.3 eV, a sputtering current of 8 μA and a pressure of 3 × 10−5 mbar, and annealing at around 500 °C. After this procedure, the cleanliness of the sample is checked by STM images the sample at room temperature and/or at 120 K. The CuPc source is kept at ∼350 °C for at least 72 h prior to deposition. Mass spectrometry ensures the purity of the deposit. Before deposition the temperature of the Knudsen cell is raised to 423 °C, then the mass spectrometer is turned off to avoid a possibility of decomposing the molecules and around 0.01 ML of CuPc is deposited onto the sample. The time of deposition is 10 min and the deposition rate was estimated as 2 × 10−5 ML/s. The Ag(100) sample is held during deposition either around 200 K or at RT. After deposition at RT the sample is cooled down to 100−150 K by liquid nitrogen, at which the measurements are performed with a commercial fast Aarhus STM that operates at 90−400 K. The STM images are analyzed with WSxM.25 The Ag linkers needed for formation of Ag−CuPc networks are released from surface step-edges. The adatom formation Received: October 15, 2014 Revised: December 17, 2014 Published: December 19, 2014 1442

DOI: 10.1021/jp5103803 J. Phys. Chem. C 2015, 119, 1442−1450

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The Journal of Physical Chemistry C

Figure 1. Submonolayer coverage of CuPc adsorbed at 200 K. (a) STM image of Ag(100) steps decorated by CuPc molecule. Scanning conditions: V = 313 mV, I = 60 pA, T = 120 K. (b) Zoom into square as indicated in (a), arrow points to single CuPc molecule attached to the step. (c) Schematics of CuPc molecule.

Figure 2. Ag−CuPc islands on Ag(100) assembled at RT, imaged at 120 K. (a) Island on the terrace. Scanning conditions V = 323 mV, I = 60 pA. (b) Island grown from lower step-edge. Scanning condition: V = 372 mV, I = 60 pA. (c) Island grown from upper step-edge. Scanning conditions: V = 331 mV, I = 50 pA. Images (d−f) magnifications into islands shown in (a−c). The image of the CuPc molecules is not completely quadratic due to a combination of thermal drift and creep. The rhombic unit cell is marked in (f).

Vienna ab initio simulation package (VASP, version 5.2).28,29 The projector-augmented wave (PAW) method 29 was employed using the gradient-corrected Perdew−Burke−Ernzerhof (PBE-GGA) exchange-correlation functional.30 A cutoff energy for plane-wave basis was set at 500 eV and the 4 × 4 × 1 Γ-centered Monkhorst−Pack mesh for k-points was used throughout the computations. The van der Waals (vdW) corrections were included using the scheme of Grimme.31 A standard supercell approach was applied with the substrate represented by a slab of five Ag atomic layers from which the two topmost layers where allowed to relax, whereas the remaining atoms were fixed in their bulk positions. We used the

energy on the Ag(100) surface from step-edges onto the terrace is as low as 0.42 eV.26 At 200 K the equilibrium concentration of Ag atoms on the terrace amounts to 10−11 while at RT concentration increases to 10−7 per surface atom. Adatoms released are mobile on the terrace because the activation energy for adatom diffusion on the terrace amounts to 0.4 eV.24,26,27 Thus, during RT growth Ag adatoms are present on the terrace and both CuPc molecules and Ag adatoms are mobile so islands can collect Ag adatoms from a bigger area than the area of the island. Our measurements were supported by spin-polarized density functional theory (DFT) calculations performed with the 1443


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the step-edges is responsible for the smaller separation of stepedge islands. (We occasionally observe black dots in the proximity of both types of islands. Such black dots might indicate a partial dehydrogenation of the CuPc molecules.) The structural arrangement of the molecules differs in islands on the terraces as compared to the ones in islands attached to the step-edges. A close-up view of both types of islands reveals that they are composed of two objects (Figure 2d−f): four-lobe structures and circular protrusions. We identify CuPc molecules by their characteristic four-lobe structure14 and Ag adatoms as circular protrusions. The arrangement within the islands reveals that the Ag adatoms play a role of linkers by enabling agglomeration, linking free molecules from the 2D gas existing on the surface at lower temperature. We checked for a residual number of not agglomerated molecules by measuring the structure at several bias voltages, Figure 3. While the images of

calculated Ag equilibrium bulk lattice parameter of 4.15 Å, which is in good agreement with other theoretical investigations,33,34 and slightly bigger than the experimental value of 4.08 Å.32 The convergence criteria for the structural optimization were forces smaller than 0.02 eV/Å and changes in total energy lower than 10−4 eV. Charge redistribution was calculated using a Bader charge analysis.35 The inclusion of vdW interactions was crucial to describe the adsorption process correctly: the use of PAW−PBE without vdW correction resulted in negative adsorption energy values, which means that there is no molecular adsorption. The molecular layers were investigated using three different unit cells each containing one CuPc molecule as suggested by experiment: the (7 × 7) unit cell, which represents quasiisolated CuPc molecules, the −51 15 unit cell with and without a Ag adatom, which represents the molecular square structure, and the 15 16 unit cell with and without two Ag adatoms, which represent the molecular rhombic structure.

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III. FORMATION OF AG−CUPC COORDINATION NETWORKS Adsorption of 0.01 ML of CuPc on Ag(100) at around 200 K results in the immobilization of molecules at the surface stepedges. The typical STM image scanned at 120 K is shown in Figure 1a,b; similar images were observed after each of eight independent preparations. The molecules at the step-edge show the usual four-lobe structure14 reminiscent of the molecular shape (Figure 1c). The molecules are stabilized by being adsorbed with their metal center above a step-edge atom. However, there is no aggregation of molecules on the terraces visible. Neither islands on the terraces nor islands growing from the step-edges are observed. The only indication for the presence of molecules on the terrace is a uniform diffusive noise, which vanishes close to step-edges or to molecules adsorbed at the step-edges. The presence of such noise suggests that molecules on the terraces are in a 2D gas state, moving much faster than the tip is scanning.36 The presence of such a 2D gas can result from a low activation energy for diffusion (around 0.2 eV)37 and/or the existence of repulsive interactions between the CuPc molecules adsorbed on Ag(111), suggested previously by Sadler et al.,12 due to charge transfer from the substrate to the molecule. Our DFT calculations show that a quasi-isolated CuPc molecule is negatively charged by −0.90 |e| resulting from a transfer of electrons from the substrate to the molecule, which supports the existence of repulsive molecule− molecule interactions. Nonetheless, the central Cu atom remains positively charged by 0.86 |e|, which is similar to the value of the molecule in the gas phase (0.89 |e|), meaning the charge transfer upon adsorption from the surface is almost entirely to the ligands of the molecule. Such a negative charge at the contour of molecule prevents molecular agglomeration of the CuPc molecules in submonolayer coverage. A similar charge transfer between the CuPc molecule and the Ag(100) surface was reported previously in ref 38. In contrast to 200 K adsorption, the adsorption of 0.01 ML of CuPc at RT leads to island nucleation on the terraces as well as at the upper and lower step-edges (Figure 2, imaged at 120 K). The experiment was repeated three times. The islands on the terraces are usually separated from each other by more than 200 nm. For the step-edge islands, the separation is much smaller (Figure 2c). The heterogeneous nucleation ability of

Figure 3. STM images of island border at opposite polarities: (−) V = −372 mV, (+) 372 mV; images taken at T = 120 K, I = 100 pA.

islands themselves are independent of polarity, the images of terraces close to islands are more noisy at negative polarity than at positive polarity. At positive polarity the molecules are moving too fast to be observed. Only occasional stripes are visible. At negative polarity, molecules are trapped under the tip causing that the terrace looks brighter (cf. Figure 3a,b). The trapped molecules are lost at the island edges. The effect was previously observed by Böhringer et al. for nitronaphthalene molecule on Au(111) surface.39 The noise observed at negative polarity is an indication of the presence of unlinked molecules on the surface. The coexistence of a molecular gas is another proof that the Ag adatoms are responsible for the hightemperature island formation. Only in the presence of Ag atom molecules are immobile.

IV. STRUCTURE OF AG−CUPC COORDINATION NETWORKS In order to characterize the island structure on the terraces. we analyzed in detail islands with different shapes and sizes ranging from around 35 to 120 nm2, as shown in Figure 4. The islands consist of subunits with a square unit cell (called square structure). Such subunits can also be named “organometallic supermolecules” and are mostly (CuPc)4Ag and (CuPc)6Ag2 and much less frequently (CuPc)8Ag3 and (CuPc)9Ag4, as shown in Figure 5a. In each subunit, one silver adatom connects four CuPc molecules. The arrangement of subunits (CuPc)4Ag and (CuPc)6Ag2 are marked in Figure 6a. The distribution of subunits sizes within all islands analyzed on the terraces is shown in Figure 6b. Most common are subunits containing four (41%) or six (25%) CuPc molecules. Isolated molecules are neglected in the pie plot. They can be considered 1444


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Figure 4. STM images of islands on terraces with regular arrangement of voids. All analyzed islands are presented. Temperature of imaging T = 120 K. Scanning conditions: (a) V = 372 mV, I = 80 pA; (b) V = 323 mV, I = 60 pA; (c) V = 313 mV, I = 60 pA; (d) island grown on the terrace up to a step-edge V = 372 mV, I = 60 pA; (e,f) V = 442 mV, I = 100 pA.

Figure 5. STM images of island on the terrace: (a) with the subunits marked by red lines; (b) with unit cells for square and rhombic structures marked by red lines.

as island defects, because they are sometimes located on the border of the islands and are also visible in less well-ordered regions of islands, which looks like an area of coalescence of two islands. Subunits with three and two molecules are usually located at the border of the island. Subunits with three molecules can thus be considered as an incomplete subunit of four, that is, the same arrangement with one molecule missing. This increases the relative abundance of subunits with four molecules to 57%. The subunits with two molecules amount to 14% of observed subunits. Subunits consisting of eight and nine molecules are much less frequantly observed, with only 4%. They have the same ordered structure with square unit cell as subunits of four and six. The distribution of the number of molecules attached to one Ag adatom is shown for islands on the terraces and for islands grown from the step-edges in Figure 6c. Almost all Ag atoms are 4-fold coordinated adatoms, but they differ by the number of molecules they interconnect. The vast majority of Ag atoms links three molecules independent of island position, though they are linked to four ligands (benzene rings) of the CuPc molecule. Only islands on the terraces contain Ag adatoms that link four CuPc molecules. For (CuPc)4Ag supermolecules the Ag adatom is exactly in the center of the supermolecule, as shown in Figure 7a. This leads to a lower adatom density observed for islands on the terrace.

Figure 6. (a) STM image with subunits consisting of 4, 6, and 8 CuPc molecules as marked. (b) Averaged distribution of subunits in islands on terraces; island border and not well ordered regions are neglected in the plot. (c) The distribution of the number of molecules attached to one Ag adatom. On the vertical axis the average number of observations is normalized to the total number of molecules.

V. STRUCTURE OF ISLANDS ON THE TERRACES The structural arrangement of molecules in the islands on the terraces differs from the arrangement observed in islands attached to step-edges. For islands on the terraces the arrangement is commensurate with the underlying lattice. It 1445


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(counted from the model) in the rhombic structure is 0.80 atoms per nm2 almost twice as large as the one in the square one being 0.45 atoms per nm2. Interestingly, similar square and rhombic structures were observed recently by Phan et al. for adsorption of 3-pyridyloxy-appended ZnPc on iodine precovered Cu(100)40 observed in electrolyte and anchored in place by molecule ligands not by the presence of adatoms. In order to gain insight into details of the structure and energetics of the two experimentally observed molecular arrangements, we use first-principles simulations to investigate the square and the rhombic structure with and without Ag adatoms present on the surface. The models derived for both structures are shown in Figure 7b,d next to STM images in Figure 7a,b. The comparison of the adsorption energies, the structural parameters, and the charge redistribution are presented in Table 1. In order to understand the formation mechanism of the experimentally observed molecular arrangements, we calculated the adsorption energy of CuPc molecules (Eads) using the following formula

Figure 7. Determination of Ag−CuPc superstructure. (a−b) Square ⎛b ⎞ a structure described with ⎜b1 ⎟ = −51 15 a12 vectors. (c−d) ⎝ 2⎠ ⎛ b′ ⎞ a Rhombic structure described with ⎜b′1 ⎟ = 15 16 a12 vectors. ⎝ 2⎠ (a,c) STM images corrected for drift and creep to achieve quadratic shape of CuPc molecule. Numbers: 2, 3, and 4 indicate the number of molecules attached to the Ag adatom. (b,d) Models of Ag−CuPc structure on surface structure. The unit cell is marked in red, a1 and a2 are vectors along [1−10] and [110] directions of the Ag(100) surface.

(

)( ) ( )( )

Eads = (ECuPc + EAgn /Ag(100)) − ECuPcAgn /Ag(100); n = 0, 1, 2 (1)

where ECuPc, EAgn/Ag(100), and ECuPcAgn/Ag(100) denote the energies of the CuPc molecule, the Ag(100) substrate, and the whole molecule−substrate system, respectively, while n is the number of Ag adatoms in the unit cell. Adsorption of the quasi-isolated CuPc molecule (i.e., in the (7 × 7) unit cell) without Ag adatoms (n = 0) is more favorable by 0.24 eV for the hollow position of the central Cu atom, compared to its bridge position. Consequently, the adsorption at the hollow site of the surface is considered as the most stable position of CuPc molecules on the Ag(100) surface, which is in agreement with the previous findings14 for the quasi-isolated molecule. In the presence of Ag adatoms the adsorption energy of CuPc molecules increases by 0.04 and 0.13 eV for the square and rhombic structure, respectively. This result indicates that the presence of Ag adatoms plays an important role in the formation of Ag−CuPc coordination networks on the Ag(100) surface. Further information about the stability of the three structures is obtained by a charge analysis. The positive charge of the central Cu atom is neither influenced by the periodicity of the studied networks nor by the presence of Ag adatoms, and remains the same (i.e., ∼0.86 |e|) as for the quasi-isolated molecule. However, the CuPc molecule is 0.29 |e| more negatively charged in the square structure than as quasi-isolated molecule. This negative charge (−1.19 |e|) is virtually the same

is a combination of subunits with a square structure described ⎛b ⎞ a by ⎜b1 ⎟ = −51 15 a12 (shown in Figures 5b and 7a,b) and ⎝ 2⎠ the subunit-border with rhombic structure described by ⎛ b′1 ⎞ a1 ⎜ ⎟ = 5 1 1 6 a 2 (shown in Figures 5b and 7c,d), where ⎝b′2 ⎠ a1 and a2 are the lattice vectors of Ag(100) surface. These subunits differ in five respects. First, the square structure consists of Ag adatoms linking four molecules. The rhombic structure contains only Ag adatoms linked to three molecules. Next, in the rhombic structure the angle between vectors b′1 and b′2 amounts to 69.23°, while it is 90° in the square structure. Then, there are two Ag adatoms present in the rhombic unit cell and only one Ag adatom in the square one. Furthermore, the area of the rhombic unit cell Sr is slightly bigger than the one of the square structure Ss with Sr/Ss ≈ 1.12. This does, however, not compensate for the larger number of adatoms in the structure. Finally, the density of adatoms

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Table 1. Comparison of the Adsorption Energies Eads, the Structural Parameters (zCuPc, Distance of Molecular Plane from Surface Layer; ΔzCu‑CuPc, Distance of Cu Atom from Molecular Plane; zAg, Distance of Ag Atom from Surface Layer, α Angle of Rotation of CuPc with Respect to the [110] Direction on the Surface), and the Charge Redistribution (qCuPc, Charge of CuPc Molecule; qCu, Charge of Cu Atom) in the calculated CuPc and Ag−CuPc Networksa CuPc square Ag−CuPc square CuPc rhombic Ag−CuPc rhombic CuPc quasi-isolated

Eads (eV)

zCuPc (Å)

ΔzCu‑CuPc (Å)

5.81 5.85 5.78 5.91 6.57

2.65 2.65 2.71 2.75 2.75

−0.32 −0.32 −0.23 −0.18 −0.12

zAg (Å) 2.03 1.76 1.52*

α (deg)

qCuPc (|e|)

qCu (|e|)

26.26 26.81 27.27 29.73 28.93

−1.19 −1.18 −1.04 −0.95 −0.90

+0.86 +0.85 +0.86 +0.87 +0.86

a

All results are for CuPc molecules adsorbed in the hollow position of the central Cu atom. Asterisk indicates result obtained for the Ag/Ag(100)− (7 × 7) system without CuPc molecules. 1446


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The Journal of Physical Chemistry C for the square structure with and without Ag adatoms. The situation changes for the rhombic structure, where the charge transfer to the CuPc molecule is influenced by the presence of Ag adatoms being −0.95 |e| for the Ag−CuPc network and −1.04 |e| for the adatom-free molecular structure. The adsorption height of a single Ag adatom in the (7 × 7) unit cell is 1.52 Å, the distance to nearest neighbor surface atom amounts to 2.77 Å and the binding energy to 2.82 eV. The values are in good agreement with previous findings for this system.33 According to our theoretical study, in the presence of CuPc molecules, the Ag adatom height increases from 1.52 to 1.76 and 2.03 Å for rhombic and square Ag−CuPc networks, respectively. Of course, in the experiment both structures are always interconnected, forcing the same level of adsorption of the Ag adatoms as visible in the measurements of the apparent height in the STM images. The apparent height of Ag adatoms is additionally influenced by the electronic effects described above. Nonetheless, according to theory, the formation of Ag− CuPc network enlarges the separation of Ag adatoms from the surface, as compared to a single Ag adatom. The adsorbed CuPc molecule accommodates more negative charge (from −0.90 to −1.19 |e|) and lowers the molecular adsorption height (from 2.75 to 2.65 Å) with decreasing molecule−molecule distance (i.e., from quasi-isolated molecule via rhombic to square structure). Simultaneously, the central Cu atom moves closer toward the Ag substrate increasing its separation to the molecular plane of the CuPc molecule from 0.12 to 0.32 Å. The final geometries obtained for adsorbed CuPc molecules in the square and the rhombic structures, in both CuPc and Ag−CuPc networks, are shown in Figure 8. In all networks, adsorbed CuPc molecules are planar (except of the Cu atom, which is closer to the surface), while the topmost Ag layer is slightly deformed laterally as can be seen in Figure 8. A rotation of the CuPc molecule as a whole by 2.5° is mediated by the presence

of the Ag adatoms only for the Ag−CuPc rhombic network. The effect of molecular rotation is not observed experimentally as both rhombic and square Ag−CuPc structures are mutually interconnected. Theory thus confirms that the metal atoms stabilize negatively charged molecules forming the CuPc coordination networks.

VI. VOIDS ARRAY IN ISLANDS ON TERRACES The specific arrangements of the subunits leads to the formation of voids for the islands on the terraces. Such voids have raised an increasing interest because the guest frame can host other adsorbates as metal adatoms or molecules for further investigation.17,20,41 Here, in islands on the terraces, subunits of equivalent size are arranged into rows misaligned by 11° from the [110] direction of Ag(100) surface, shown in Figure 6a. As a consequence of the rowlike arrangements of the subunits a long-range ordered array of 2D voids develops in the system, shown in Figure 9a. The voids have a crosslike shape (Figure 2d). They are too small to support an additional CuPc molecule, but their arms are wide enough to be occupied by adatoms. The possibility of the voids to be filled with adatoms is demonstrated here by the native Ag adatoms. Up to four of them can occupy the arms of voids, as shown in Figure 9b. The occupation of the center of the void is never observed. This further support the importance of adatom−molecule interactions in the system. The filling possibilities are shown in Figure 9b. The voids of type 0 do not contain any Ag adatoms. Type I has one adatom in one of the arms of the void. The adatom can occupy four equally probable positions in the arms of the cross. Type II has two adatoms present and two opposite arms are usually filled, while adatoms in the adjacent arms are observed less frequently. Type III contains three adatoms and three arms are occupied. Type IV has four adatoms and every arm of the cross contains an adatom. To estimate which kind of molecules could be hosted by our organometallic framework, we measure the size of the central part of the void between opposite molecules in the way shown in Figure 9c to allow for a comparison of different void types. This size is, in reality, bigger because the size of the tip used in experiment influences this measurement. The diameter of the central part of the voids ranges from 0.95−1.36 nm with a mean value of (1.12 ± 0.03) nm (193 pores measured). The variation of the void’s diameter results from a nonuniform distribution of subunits of different size, which also leads to a variety of voids distances. Figure 9d shows the distribution of the void distances. The most common distance between the voids is associated with the side length of the four molecule subunit of network. The voids are separated on average by a distance of (3.3 ± 0.1) nm (503 distances measured). The distance between voids might force specific interguest molecules separation if the network was used for confinement. VII. ISLANDS AT THE STEP-EDGES AND COMPARISON WITH ISLANDS ON THE TERRACES Figure 2b,c shows islands grown from the lower and upper step-edges, respectively. In contrast to islands on the terraces, those islands have a close-packed layer arrangenment without any voids (Figure 2e,f). Only occasionally an adatom is lacking or there is another defect in the structure. The molecules are arranged in the same rhombic structure as observed between subunits in the islands on the terraces. Thus, one Ag adatom

Figure 8. Top view of lowest energy structures for (a,b) square and (c,d) rhombic CuPc network; (b,d) with and (a,c) without Ag adatoms. Only the CuPc molecules and the topmost Ag layer are shown. The Ag adatoms are colored in yellow for clarity. 1447


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Figure 9. Voids in Ag−CuPc networks. (a) STM image showing arrangement of voids in island on the terrace. (b) Five types of voids; 0 is the largest void without adatoms, I−IV have voids with one, two, three or four adatoms filling “arms”, respectively. (c) The measurements procedure of central part of void. (d) The distribution of voids distances d in islands on terrace. Vertical axis shows the average number of observations normalized to total number of observations.

VIII. SUMMARY A submonolayer coverage (0.01 ML) of CuPc molecules adsorbed on the Ag(100) surface at 200 K is unable to nucleate islands due to high molecular diffusion mobility combined with weak repulsive electrostatic intermolecular interactions. The situation changes when not only CuPc molecules, but also Ag adatoms as released from step-edges are present during nucleation. The Ag adatoms play the role of linkers causing creation of two kinds of islands at different sites of the surface. Islands on the terraces consist of a combination of structures with square and rhombic unit cells and islands attached to the surface step-edges consist of the rhombic structure only. Both types of islands are Ag−CuPc coordination networks with Ag adatoms as a coordination agent. In the islands attached to the steps, we observe a close-packed arrangement of molecules with 4-fold coordinated Ag adatoms interconnecting three CuPc molecules. The angle between molecular rows and the stepedge amounts to 58° for islands grown from both upper and lower step-edges. The close-packed structure is associated with a higher number of the Ag adatoms available due to the short distance to the step-edge, which plays the role of adatom source. In the islands on the terraces, additionally, Ag adatoms interconnecting four CuPc molecules are observed. Our DFT calculations confirm the crucial role of Ag adatoms in the formation of the Ag−CuPc networks. The adsorption energy of CuPc molecules is by 0.04 and 0.13 eV more favorable for the square and rhombic structure, respectively, when Ag adatoms are present on the surface. The structure of islands on the terraces depends on the number of Ag adatoms available. Most frequently, CuPc molecules are arranged into subunits with square unit cell containing four or six molecules linked by 4-fold coordinated Ag adatoms. Subunits with a different number of molecules are also present. The subunits are interconnected by the subunitborder rhombic structure. The arrangement of subunits enforces an existence of voids separated from each other by a

usually links three CuPc molecules (see Figure 6c). However, the direction of the molecular rows depends on the step orientation. For all islands, the angle between the rows of molecules and the step-edge is around 58°. This angle is associated with adsorption of single CuPc molecule on the stepedges (cf. Figure 1b), which serve as a nucleation centers to this islands independent of the presence of linkers. The presence of the Ag adatoms makes island growth out of already anchored molecules possible. The orientation of the nucleating molecules on the step-edge is fixed by the step orientation and provides the starting point for the island growth. That is why the step orientation, not the lattice, dictates the molecular arrangement of molecular rows. The adatom-to-molecule ratio is larger in these step-edge islands than in islands on the terrace due to the rhombic structure of islands attached to step-edges, which has two Ag adatoms per unit cell (Figure 7c,d). In the terrace island shown in Figure 2a there are around 0.7 adatoms/nm2, while in the step-edge island shown in Figure 2b there are around 0.9 adatoms/nm2. As the islands on terraces are composed of a combination of square and rhombic structures and there are additional (not as counted for in the model) Ag adatoms in the voids, the adatom density should be bigger than 0.45 adatoms/ nm2 and the adatom-to-molecule ratio should be between one and two. In the ideal step-edge islands, the adatom density should be 0.85 adatoms/nm2 and the adatom-to-molecule ratio should be two. The adatom-to-molecule ratio measured from our STM images is ∼1.7 for islands on the terrace, and ∼1.9 for the islands grown from the step-edges. The value is slightly lower than two for step-edge islands due to the presence of defects in the structure. Thus, the density of adatoms measured from STM images are comparable with the one predicted by the model. We conclude that the different adatom density, higher close to the step edges and slightly lower at the terrace is responsible for the different molecular arrangements inside the islands. 1448


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mean value of 3.3 nm. The voids have primarily cross like shape, however their final shape depends on Ag adatom occupancy in arms of cross. Ag adatoms are always trapped in the voids “arms” what suggests an attractive interaction between the Ag adatom and CuPc molecule. The existence of such voids opens the possibility to confine other guest molecules of specific sizes (up to at least 1.1 nm) into a network with a spacing equal to 3.3 nm. The network is thus an example of 2D porous arrangement. We propose that the formation of Ag−CuPc networks cause a larger adsorption height of Ag adatoms as compared to the adsorption level of quasi-isolated Ag adatom on the surface. According to theory, the Ag adatom−surface distance increases with the number of molecules attached to the Ag adatom. Therefore, the Ag adatoms within the CuPc coordination network have to be considered rather as a part of Ag−CuPc network rather than as a separate entity. In conclusion, we demonstrate the formation of Ag−CuPc networks on the terraces and close to the step-edges of the surface and thus the capability to stabilize submonolayer covereges of CuPc molecules on Ag(100) by Ag adatoms at room temperature.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was supported by the Humboldt Stiftung as part of G.A. Humboldt Fellowship stay at the Leibniz University Hanover. W.K. thanks the project number 1010/S/IFD for support. We thank Professor Gert Ehrlich, University of Illinois, for frequent discussions about the project. Numerical calculations were performed at the Interdisciplinary Center for Mathematical and Computational Modelling of the University of Warsaw within the Grant G44−10.

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REFERENCES

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Stabilizing CuPc Coordination Networks on Ag(100) by Ag Atoms

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