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Structural Properties of Iron Phtalocyanines on Ag(111): From the Submonolayer to Monolayer Range S. C. Bobaru, E. Salomon,* J.-M. Layet, and T. Angot Physique des Interactions Ioniques et Moleculaires, UMR 6633 CNRS - Universite de Provence, Campus de Saint-Jer^ome, case 241, avenue de l'escadrille Normandie Niemen, 13397 Marseille Cedex 20, France ABSTRACT: An experimental investigation of the epitaxial growth of iron-phthalocyanines on Ag(111) has been conducted in the monolayer range by low-energy electron diffraction and scanning tunneling microscopy. During the growth, the molecular overlayer undergoes few structural phase transitions, and various superlattices with oblique or rectangular unit cells have been identified. We show that the morphologies of the most stable molecular superstructures formed for different molecular coverage are adopted by various phthalocyanines with different central metal atom and phthalocyanine derivatives.
’ INTRODUCTION Phthalocyanines (Pc) are macrocyclic compounds that have been extensively studied over the years due to their possible technological applications (e.g., solar cells, catalysis, molecular electronics, cancer therapy).1-4 The functionality of such applications strongly depends on the electronic and the structural properties of the molecular systems. It has been shown that the intermolecular interaction, the molecule-substrate interaction, the arrangement of the molecules within the thin films, as well as the molecular orientation with respect to the substrate have a strong influence on the physical and chemical properties of the thin films.5-7 Yet, a deeper understanding of the correlation between the geometrical structure of an organic molecular film adsorbed on a surface and the charge carrier dynamics at the molecular interface is crucial for the design of potential organic devices.8,9 The growth and the morphology of the metal-phthalocyanine (MPc, where M stands for a metal atom) thin films have been studied on various substrates, and at different molecular coverage, by employing numerous techniques of investigation.6,10-15 In a pioneering work using low-energy electron diffraction (LEED), J. C. Buchholz and G. A. Somorjai suggested that the chemical bonding of the metal-phthalocyanine monolayer is sensitive to the crystal face of the substrate.16 Scanning tunneling microscopy (STM) studies have shown that in the case of the adsorption of FePc on Cu(111) there is a strong interaction between the molecules and the substrate. At high coverage the deformation of the Pc skeleton, due to the repulsive (in-plane) molecule-molecule interactions, is observed, often accompanied by an attractive out-of-plane molecule-molecule interaction.17 Other STM investigations on the adsorption of FePc on HOPG revealed that the molecules adsorb with their 4-fold symmetry axis perpendicular to the surface. The STM r 2011 American Chemical Society
images of the adsorbed monolayer are dominated by the central Fe atom, which via the 3d orbital gives a major contribution to the electronic states close to both HOMO and LUMO.6,18,19 Recently, Manandhar et al. showed that on Ag(111) FePc molecules form commensurate ordered films at submonolayer coverage. The authors suggested that the commensurate lattice structure of FePc/Ag(111) results not only from the bonding interaction between π-electrons of phthalocyanine rings and Ag(111) but also from the strong interaction between the metalrich molecular orbitals centered on Fe2þ and the Ag 4dxz,yz.20 The adsorption of the nonplanar molecule SnPc onto Ag(111) showed the coexistence of the ordered and disordered phase. For the ordered phase the unit cell is almost quadratic, with one molecule orientated up and the other down.21 Another study of the SnPc evaporated onto Ag(111) using normal incidence X-ray standing wave and Auger spectroscopy revealed the formation of an incommensurate overlayer.22 All these studies dealing with the adsorption of MPc on Ag(111) have been done for various molecular coverage, but yet, no clear link has been done between them and more particularly on the evolution of the film order as a function of the coverage. In this paper, we therefore present a careful investigation of the growth’s behavior of FePc on Ag(111) up to one monolayer coverage using low-energy electron diffraction during the deposition process. Most of the significant phases have been then analyzed by means of scanning tunneling microscopy. By comparing our results with the literature we demonstrate that there is a common adsorption behavior of various Pc's and their derivatives on Ag(111), at least up to the monolayer range. Received: December 9, 2010 Revised: February 17, 2011 Published: March 07, 2011 5875
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Figure 1. Intensity profile of the (10) silver substrate spot during the continuous evaporation of FePc's together with the most significant LEED patterns corresponding to three phases of the molecular layer gaseous (G), commensurate (C2), and incommensurate (I). All LEED patterns have been recorded at the same primary beam energy of 11.3 eV.
’ EXPERIMENTAL SECTION Experiments were performed in a series of interconnected ultrahigh vacuum chambers equipped with standard surface preparation techniques, LEED, STM, and high-resolution electron energy loss spectroscopy (HREELS) analysis. The silver single crystal surface has been prepared by several cycles of ions sputtering in the Argon partial pressure of 1 10-6 for 45 min at 800 eV followed by annealing up to 750 K for 30 min until a sharp LEED pattern with a low elastic background, reflecting the 6-fold symmetry of the (111) plane of silver, was obtained. Cleanness and order of the substrate have been further checked by HREELS and STM. All measurements have been done under UHV conditions at room temperature (RT). The LEED optics used was a SpectraLeed Omicron system. The STM images have been taken with a commercial Omicron VT-STM. Home made STM tips were produced from electrochemically etched tungsten wires. STM images have been recorded in constant-current mode and processed using the WSxM software.23 Linear electronic drift correction was systematically used to compensate from possible thermal and mechanical drift of the probe. For the sake of the discussion, let us mention that STM images have not been recorded on the phases observed during the so-called “live-LEED” experiments. Images have been recorded on phases grown independently using similar evaporation conditions. Let us also point out that the stated angles and lattice parameters result from the matrix described in the manuscript, which have been extracted combining STM and LEED measurements. FePc molecules were synthesized using conventional methods and further purified by outgassing in UHV at 600 K.24,25 They have been thermally evaporated from a homemade crucible onto the silver substrate kept at RT. ’ RESULTS AND DISCUSSION Figure 1 displays the analysis of substrate (10) spot intensity together with three of the most significant LEED patterns recorded during deposition of FePc onto Ag(111). For this “live-LEED” experiment, LEED patterns were recorded every 4 s during the film growth using a CCD camera video acquisition
Figure 2. 35 nm 35 nm (a) and 11 nm 11 nm (b) STM images of the commensurate phase labeled C2 (tunneling parameters Vt = 1.2 V, It = 0.03 nA) and its corresponding LEED pattern (c) recorded for a primary energy beam of 11.3 eV. (d) Corresponds to the LEED pattern of the clean Ag(111) substrate recorded at a primary energy beam of 45 eV.
system, and the evaporation rate was stable and estimated at 0.22 ( 0.02 ML 3 min-1. From 0 to 180 s after the beginning of the evaporation, no molecular superstructure has been observed. However, one can notice a clear decrease of the substrate (10) spot intensity accompanied by a gradual increase of the elastic background in its vicinity. Then, after roughly 180 s a second regime was established, as indicated by a slow down of the growth process, during which a ring superstructure is formed as shown by the LEED pattern labeled G. The diameter of the diffraction ring, as well as its intensity, is increasing with further deposition 5876
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Figure 3. (a) 9 nm 9 nm STM image of the incommensurate phase labeled I (tunneling parameters Vt = -1.2 V, It = 0.01 nA) and its corresponding LEED pattern (b) recorded for a primary energy beam of 11.3 eV.
of the phthalocyanines. This G LEED pattern, which appears at low molecular coverage, results from the formation of a twodimensional gas phase, i.e., a disordered phase due to the high mobility of the molecules. At 240 s, a new pattern is observed and attributed to a commensurate FePc overlayer labeled C2. After 330 s of exposure we observed an incommensurate phase labeled I. As the coverage further increased, the phase became denser. These observations are strictly comparable to those done by C. Stadler and co-authors who recently published a similar phase diagram for SnPc and CuPc molecules adsorbed on Ag(111).26,27 The electronic structure of the Sn causes the metal atom to stand out of the SnPc macrocycle plane, while the Fe and Cu atoms are contained within the macrocycle. Yet, we can already conclude that Pc's present a common adsorption behavior independent of the central atom. Such an observation has been already pointed out for Pc's adsorbed onto semiconductor surfaces.28-30 In the case of the ring-shape phase (G), we did not manage to image molecules, probably because of the high mobility of the molecules at low coverage and RT. At higher coverage, STM images illustrating the molecular structure corresponding to the commensurate phase (C2) are displayed in Figure 2. From the large-scale image it is clearly visible that the molecules form longrange ordered chains and arrange themselves in two domains as illustrated by the arrows. These two domains are, respectively, rotated by an angle of about 120° bearing a relation to the symmetry of the underlying substrate. Within a unit cell, the molecules arrange themselves in a rectangular superlattice, which is commensurate with the silver substrate. The corresponding crystallographic matrix is ! ! ! A 5 0 i ¼ B 3 6 j where i and j are the basis vectors of Ag(111), while A and B represent the lattice basis vectors of the molecular overlayer. Within this unit cell, the latter vectors do not differ very much in length, A = 14.5 Å and B = 15.0 Å, and the angle between them is 90°. After 330 s of continuous deposition, the molecular adlayer undergoes a third structural transition resulting in an incommensurate phase labeled I. The morphology of the incommensurate structure is depicted in the STM image from Figure 3. Similarly to the commensurate phase C2 the molecules arrange themselves in two domains. The magnified view reveals a submolecular resolution of the FePc molecules with a bright
Figure 4. 48 nm 48 nm (a) and 12 nm 12 nm (b) STM images of the commensurate phase labeled C1 (tunneling parameters Vt = -1 V, It = 0.02 nA) and its corresponding LEED pattern (c) recorded at a primary energy beam of 11.3 eV.
spot surrounded by four fainter lobes. The center corresponds to the Fe atom, and we ascribe the four lobes to the four isoindole units of the molecules. On the basis of thorough analysis of the LEED and the STM data, we established that the superstructure I can be described by the following matrix ! ! ! A 4:80 -0:40 i ¼ B 2:58 5:68 j As compared to C2, the molecular superlattice basis vectors are shorter (respectively, 14.2 and 14.5 Å), and the angle between them opened up to 97°, describing a denser phase. As the coverage further increased, the incommensurate phase continuously densified up to the full monolayer completion. In Figure 4, we present two representative STM images of the FePc submonolayer on Ag(111) and the corresponding LEED pattern, obtained using a much higher evaporation rate, estimated at 3.1 ( 0.1 ML 3 min-1. In the large-scale STM image it appears that the molecules arrange themselves in long chains, parallel to the step edges of the silver substrate. Around the step edges one can observe strikes or spikes oriented in the scanning directions. These fluctuations might be due to the substrate since it is known that the Ag(111) surface exhibits one-dimensional mass transport around the step edges31 or could result from the rapid diffusion of adsorbed molecules during the scanning procedure, as a consequence of a relatively weak moleculesurface interaction. Finally, one can notice that the Ag terraces are not fully covered with molecules demonstrating that this phase, labeled as C1, exists for an intermediate coverage as compared to the G and C2 phases. Before going any further, let us recall that this phase was not observed during the continuous pattern recording during deposition of FePc onto Ag(111) but was observed in another set of experiments for which a higher deposition rate was used. We cannot exclude that the fact that we 5877
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Figure 5. Models of the molecular arragement for the C1, C2, and I phases. Gray, black, blue, red, and green balls correspond, respectively, to the Ag, C, N, H, and Fe atoms.
have not seen the C1 phase using a low evaporation rate necessarily means that it cannot be accessed under such conditions. However, it is well-known that growth mechanisms result from the competition between thermodynamics and kinetics and that different evaporation rates may stabilize different phases.32 Now, using the information obtained from both the LEED and STM measurements we have determined the transformation matrix between the substrate lattice and the overlayer as the following ! ! ! 5 -1 i A ¼ 5 6 j B The length of the molecular superlattice vectors are equal and close to 16.1 Å, while the angle between them is about 78°. It appears that the vector lengths of the molecular superlattice are longer than those discussed earlier in the text, while the angle between them is smaller. This confirms the conclusion, based on the STM images, that the C1 phase exists at a lower coverage than the C2 one. Let us notice that similar STM images, with similar parameters, of the FePc overlayer on Ag(111) have been also obtained by Manandhar et al.20 In their paper, solely based on STM measurements, the authors have proposed a structural matrix that actually does not match neither the length of the superlattice vectors nor the angle between them. Therefore, according to their STM measurements we believe that the authors have probably observed the so-called C1 phase as described here. We estimate the C1 phase to be metastable, existing over a narrow thermodynamic range. In their experiment, Manandhar et al. produced the C1 phase after annealing a FePc film, a procedure which may have induced molecular desorption resulting in a coverage similar to the one we have obtained for the C1 phase. In this case, we speculate that the presence of impurities has modified the subtle balance between substrate-molecule and molecule-molecule interactions stabilizing this metastable structure. Figure 5 sums the structural models of the lattice unit cells proposed for both the commensurate and incommensurate phases observed in the case of FePc films adsorbed onto Ag(111) in the submonolayer to monolayer range. If we compare the structural models proposed for the three reported phases, one can notice that from phase to phase, as the coverage increases, the length of the superlattice vectors shrinks, while the angle between them opens up. Therefore, we can conclude that at low coverage, i.e., the disordered/gas phase, the molecules maximize the distance to their closest neighbors because of repulsive interactions. As the coverage increases, the molecular superlattice
Figure 6. (a) 13 nm 13 nm STM image of a submonolayer coverage of ZnPcCl8 adsorbed on Ag(111) (tunneling parameters Vt = 0.9 V, It = 0.19 nA). (b) 14 nm 14 nm STM image of a monolayer coverage of ZnPcCl8 adsorbed on Ag(111) (tunneling parameters Vt = 1.2 V, It = 0.18 nA).
shrinks, and the angle between the two basis vectors opens up, resulting in an increase of the molecular density. This phenomenon clearly demonstrates that as the molecular coverage increases the chemical potential is modified and gradually overcomes the repulsive interactions. This brings molecules closer one to each other causing denser phases. Moreover, the two commensurate phases evidence the tendency of FePcs to adsorb in registry with substrate high-symmetry sites leading to a deformation of the film similar to the orientational epitaxy phenomena observed for the rare gas physisorption and first described by the Novaco-McTague theory.33-36 Finally, topographical images displayed in Figure 6 show the structural arrangements of the chlorinated phthalocyanines (ZnPcCl8) evaporated onto Ag(111). In the submonolayer range (Figure 6a), the adsorbed molecules organize themselves into a compact 2D, long-range ordered structure, which is also commensurate with the substrate. From a comparison between this picture and the one of the C1 phase (Figure 4), one can clearly notice that in both cases the molecules arrange and orient themselves in the same manner within a unit cell. Now, in the monolayer range, the configuration adopted by the ZnPcCl8 superstructure is illustrated in Figure 6b. This structure, which has already been carefully studied and discussed elsewhere, presents defect lines, every second or third molecular row, resulting from the competition between the intermolecular interaction forces and the molecule-substrate interaction.37,38 Ignoring these lines, we can clearly see that the arrangement of the molecules, in a unit cell, is similar to the structure adopted by FePc molecules in the I phase (Figure 3). This comparison clearly demonstrates that, at least on Ag(111), metallophthalocyanine 5878
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’ CONCLUSIONS The comparison between the STM results, LEED patterns, and the real space models yielded to the following information on the adsorption behavior of phthalocyanines on Ag(111). For both, submonolayer and monolayer coverage highly ordered superstructures have been found, supporting the fact that the repulsive interactions between molecules are finally overcome by an increase of the chemical potential as the molecular coverage increases. Two commensurate and one incommensurate phases were identified. The two commensurate phases evidence the tendency of FePc's to adsorb in registry with the substrate leading to a deformation of the film as in the case of rare gas physisorption. Finally, we emphasize that common structural properties are actually adopted by most phthalocyanines (FePc, SnPc, ZnPc) and some of their derivatives (ZnPcCl8) on Ag(111), the substrate acting more as a template. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: (33) 491 282 802. Fax: (33) 491 288 357.
’ ACKNOWLEDGMENT Financial support from the French National Research Agency (Grant PNANO 06-0251) is gratefully acknowledged. ’ REFERENCES (1) Petrisch, K.; Dittmer, J. J.; Marseglia, E. A.; Friend, R. H.; Lux, A.; Rozenberg, G. G.; Moratti, S. C.; Holmes, A. B Solar Energy Mater. Solar Cells 2000, 61, 63. (2) Forrest, S. R. Chem. Rev. 1997, 97, 1793. (3) Alvaro, M.; Carbonell, E.; Espla, M.; Garcia, H. Appl. Catal., B 2005, 57, 37. (4) Spikes, J. D. Photochem. Photobiol. 1986, 43, 691. (5) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Jiang, N.; Liu, Q.; Shi, D. X.; Du, S. X.; Guo, H. M.; Gao, H.-J. J. Phys. Chem. C 2007, 111, 9240. (6) Åhlund, J.; Schnadt, J.; Nilson, K.; G€othelid, E.; Schiessling, J.; Besenbacher, F.; Martensson, N.; Puglia, C. Surf. Sci. 2007, 601, 3661. (7) Barrena, E.; de Oteyza, D.; Dosch, H.; Wakayama, Y. Chem. Phys. Chem. 2007, 8, 1915. (8) Dholakia, G. R.; Meyyappan, M.; Facchetti, A.; Marks, T. J. Nano Lett. 2006, 6, 2447. (9) Koch, N.; Salzmann, I.; Johnson, R. L.; Pflaum, J.; Friedlein, R.; Rabe, J. P. Org. Electron. 2006, 7, 537. (10) Yim, S.; Jones, T. S. Surf. Sci. 2002, 521, 151. (11) Lozzi, L.; Santicci, S.; La Rosa, S. Appl. Phys. Lett. 2006, 88, 133505. (12) Nilson, K.; Palmgren, P.; Åhlund, J.; Schiessling, J.; G€othelid, E.; Martensson, N.; Puglia, C.; G€othelid, M. Surf. Sci. 2008, 602, 452. (13) H€aming, M.; Scheuermann, C.; Sch€oll, A.; Reinert, F.; Umbach, E. J. Electron Spectrosc. Relat. Phenom. 2009, 174, 59. (14) Day, P. N.; Zhiqiang Wang; Pachter, R. J. Mol. Struct. (Theochem) 1998, 455, 33. (15) Salomon, E.; Papageorgiou, N.; Angot, T.; Verdini, A.; Cossaro, A.; Floreano, L.; Morgante, A.; Giovanelli, L.; Le Lay, G. J. Phys. Chem. C 2007, 111, 12467. (16) Buchholz, J. C.; Somorjai, G. A. J. Chem. Phys. 1977, 66, 573. (17) Scarfato, A.; Chang, S.; Kuck, S.; Brede, J.; Hoffmann, G.; Wiesendanger, R. Surf. Sci. 2008, 602, 677.
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