Influence of Chlorine Substitution on the Self-Assembly of Zinc

Laboratoire de Matériaux et Microélectronique de Provence (L2MP), UMR CNRS ... Xavier Bouju , Cristina Mattioli , Grégory Franc , Adeline Pujol , a...
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10058

J. Phys. Chem. B 2006, 110, 10058-10062

Influence of Chlorine Substitution on the Self-Assembly of Zinc Phthalocyanine Mathieu Koudia,* Mathieu Abel,* Christian Maurel, Ariane Bliek, Daniel Catalin, Mireille Mossoyan, Jean-Charles Mossoyan, and Louis Porte Laboratoire de Mate´ riaux et Microe´ lectronique de ProVence (L2MP), UMR CNRS 6137, UniVersite´ s Paul Ce´ zanne, ProVence et Sud Toulon Var, Case 151, 13397 Marseille Cedex 20, France ReceiVed: December 9, 2005; In Final Form: March 27, 2006

The adsorption and ordering of zinc phthalocyanine (ZnPc) and octachloro zinc phthalocyanine (ZnPcCl8) on an Ag(111) surface is studied in situ by scanning tunneling microscopy under ultrahigh vacuum. Two-dimensional self-assembled supramolecular domains are observed for these two molecules. We show how substituting chlorine atoms for half of the peripheral hydrogen atoms on ZnPc influences the self-assembly mechanisms. While intermolecular interactions are dominated by van der Waals forces in ZnPc molecular networks, ZnPcCl8 molecular packing undergoes a sequential phase evolution driven by the creation of C-Cl‚‚‚H-C hydrogen bonds between adjacent molecules. At the end of this evolution, the final molecular assembly involves all possible hydrogen bonds. Our study also reveals the influence of molecule-substrate interactions through the presence of fault lines generating a stripe structure in the molecular film.

1. Introduction The controlled formation of noncovalent bonds between molecular building blocks opens numerous interesting routes to achieve supramolecular architectures at surfaces.1 One of the main strategies in the field of supramolecular chemistry is molecular self-assembly, as it can efficiently form twodimensional nanostructures thanks to hydrogen bonds,2-4 van der Waals,5-7 or metal-ligand8,9 interactions. The adsorption of large and flat metal phthalocyanine (MPc) molecules is welldocumented for both metallic and semiconductor surfaces.10,11 The tunability of their electronic and structural properties makes this class of molecules ideal for both fundamental science and technological applications, such as optoelectronic devices,12 sensors,13 or solar cells.14 From a fundamental point of view, the influence of the complex competition between moleculemolecule and molecule-substrate interactions on the molecular self-assembly is far from being well-understood. Investigations in this field are requested in order to gain better control on the building of supramolecular architectures at surfaces. Studies of several MPcs (M ) Fe, Ni, Co, or Cu) deposited on Au(111)15,16 indicate that the self-assembled structures obtained are governed by van der Waals interactions, whatever the central metal atom. This means that the van der Waals intermolecular interactions driving the assemblage are hardly affected by the metal substitution. In contrast, the substitution of peripheral hydrogen atoms directly influences molecule-molecule as well as molecule-substrate interactions, providing an interesting means of controlling the molecular self-assembly. Herein, we present a detailed scanning tunneling microscopy (STM) investigation of the self-assembly of zinc phthalocyanine (ZnPc) and 2,3,9,10,16,17,23,24-octachloro zinc phthalocyanine17 (ZnPcCl8) deposited on Ag(111). In the ZnPcCl8 molecule, represented in Figure 1, 8 chlorine atoms have been substituted for 8 out of the 16 hydrogen atoms in ZnPc in order to investigate how the self-assembly process and the molecular organization are influenced by such a modification at the * [email protected], [email protected].

Figure 1. Molecular structure of ZnPc and ZnPcCl8, with X ) H and X ) Cl, respectively. M1 and M2 are the symmetry mirrors of the molecule.

periphery of the phthalocyanine molecule. The choice of ZnPc, with its nonbonding Zn 3d band, and of Ag(111), a surface of low energy, should in principle allow a quite low moleculesubstrate interaction. In the following, we present evidence that the self-assembly obtained after the deposit of ZnPcCl8 molecules on the Ag(111) substrate at room temperature follows the principle of energy minimization, in a process of maturation of the molecular deposit driven by the sequential formation of hydrogen bonds between adjacent molecules.4 Furthermore, the fact that the influence of the intermolecular interactions increases while that of the molecule-substrate interactions remains fairly constant leads to regularly spaced faults in the final structure attributed to stress relaxation in the molecular film. 2. Experimental Section All experiments were performed in a multicharacterization chamber equipped with an ultrahigh vacuum (UHV) system allowing different in situ preparation and characterization methods. The base pressure was in the 10-10 mbar range. The Ag(111) monocrystal was prepared through repeated cycles of Ar+-ion sputtering and annealing up to 800 K. Prior to molecular deposition, the Ag(111) substrate was checked by X-ray

10.1021/jp0571980 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/04/2006

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Figure 3. Sequential occurrence of ZnPcCl8 phases (noted P1, P2, and P3) monitored by STM at 300 K. The transition zones are represented by dashed lines, because there is an uncertainty on experimental areas measured on STM images, due to the influence of the scanning tip. Note that the simultaneous occurrence of the three phases was never observed experimentally.

Figure 2. STM images of ZnPc molecules deposited onto Ag(111) at room temperature. (a) Large image (50 nm × 50 nm) showing different structural domains: labels I and II correspond to two chiral structures, while subscripts a and b correspond to two variants (out of three) induced by the substrate symmetry. (b,c) Close-up view of the two chiral structures Ia and IIa (10 nm × 10 nm). (d,e) Proposed model deduced from (b,c) for chiral structures I and II; the symmetry mirror is indicated by the dotted line along the [110] direction. Lattice parameters: A ) A′) 13.6 Å and B ) B′ ) 14.5 Å.

photoelectron spectroscopy, low-energy electron diffraction, and then scanning tunneling microscopy (VT STM OMICRON). ZnPc and ZnPcCl8 were synthesized using classical methods.18,19 They were purified by sublimation and outgassed in the UHV system until the pressure reached the 10-9 mbar range. Then, they were evaporated from a molybdenum crucible heated at 600 and 690 K for ZnPc and ZnPcCl8, respectively. This results in a deposition rate of about 0.2 monolayer (ML) per minute (1 ML corresponds to a full layer of the dense molecular phase). Scanning tunneling microscopy (STM) experiments were recorded in constant-current mode with It around 0.2 nA and Vt ranging between -0.8 and -1.6 V without noticeable spectroscopic effects. Distances measured by STM were averaged from several images. Images were plane-corrected and analyzed using the WSxM software.20 3. Results 3.1. Two-Dimensional Structure of a ZnPc Deposit on Ag(111). Figure 2a shows a typical STM image recorded after depositing ZnPc molecules on Ag(111). The substrate terraces are completely covered with molecules that self-organize within domains. Several domains labeled Ia, IIa, Ib, and IIb are observed. Labels I and II refer to the chiral symmetry, while subscripts a and b refer to the orientation symmetry. Indeed, a close-up view of domains Ia and IIa represented in Figure 2b,c respectively, shows individual molecules imaged as a symmetric

crosslike structure. The apparent height of the molecule is about 1 Å, a value in accordance with those of other MPcs recorded in a flat-lying configuration.15,16 Network Ia is related to network IIa through mirror symmetry. The unit cells corresponding to these two networks and the dashed line transecting the mirror plane are drawn in Figure 2d,e, respectively. From the STM images, one can obtain the average lattice parameters A ) A′ ) 13.6 Å, B ) B′ ) 14.5 Å, and an angle R ) 95° between basis vectors. One can notice that the 2D organization of the nonchiral ZnPc molecules on the Ag(111) surface leads to chiral networks. Moreover, Figure 2a reveals domains oriented at an angle of 60° with respect to networks Ia and IIa (or a multiple of 60°). This rotational symmetry reveals the influence of the Ag(111) substrate symmetry on the growth process. Due to the Ag(111) substrate symmetry, three orientations labeled by the subscript letters a, b, and c are found for each of networks I and II. In agreement with the threefold symmetry of the surface, three rotational networks exist for each of the two chiral structures. Only four out of the six possible networks are visible in Figure 2a (variant c not present in the figure). For all the networks observed using STM, the symmetry mirror M1 of the MPc molecule, represented in Figure 1, is aligned along one close-packed direction of the substrate. The submolecular resolution shows the orientation of the molecules within the ad-lattice and hence reveals an angle of 35° between the basis vector B and the mirror M1. All these features are rationalized by the model proposed in Figure 2d,e, where a montage of the Ag(111) atoms and ZnPc molecules is shown. This structure is very similar to the ones observed for MPc deposits on metallic surfaces, for example, CuPc and CoPc on Au(111)15,16 or CuPc on Ag(111).10 It reveals that in all these cases the self-assembly is mediated by van der Waals interactions. 3.2. Structural Evolution of ZnPcCl8 Deposit on Ag(111). The ZnPcCl8 molecule, represented in Figure 1, was obtained by substituting 8 chlorine atoms for 8 peripheral hydrogen atoms on ZnPc. After depositing around 0.7 ML of ZnPcCl8 molecule, sequential STM imaging was performed during 3 days, the sample being kept at room temperature. During this period of time, 3 self-assembled configurations were observed that developed in 3 steps. These configurations are depicted on Figure 3 where the full line corresponds to the existence of a unique phase and the dotted line represents the coexistence of two phases during the phase transformation. It is worth noticing that, due to the tip influence, the evaluation of the percentage of each phase on the surface cannot be well-established. Therefore, the first phase to be formed at the early beginning of the growth is phase P1. Then, 30 h later, the second phase P2 nucleates and prevails on the P1 phase, which slowly disappears.

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Figure 4. Characteristic STM images obtained at room temperature for the three two-dimensional arrangements of ZnPcCl8 molecules deposited onto Ag(111): (a) immediately after the deposit, phase P1; (b) about 40 h after deposit, phase P2; (c) about 70 h after deposit, phase P3; (d-f) zoom on phase P1 (lattice parameters A1 ) B1 ) 18 Å), intermediate phase P2 (A2 ) 15 Å, B2 ) 18 Å), and final phase P3 (A3 ) 15 Å, B3 ) 15 Å), respectively. Dimensions of the upper and lower images: 30 nm × 30 nm and 7.5 nm × 7.5 nm, respectively.

After about 50 h, a third phase P3 nucleates and progressively takes the place of the second one, which vanishes completely after about 75 h. Figure 4 reports typical STM images recorded for each of these phases. Figure 4a depicts the unique structure observable in the early stage of the growth. The unit cell of the structure of P1, represented in Figure 4d, is characterized by the lattice parameters A1 ) B1 ) 18 Å and an angle R1 ) 110° between basis vectors. This gives a packing density of 0.33 molecule/nm2. The orientation of the molecular lattice with respect to the lattice substrate is determined by imaging the Ag(111) surface in uncovered areas. A comparison of a number of images indicates the coexistence of three rotational networks related to the three equivalent principal directions of the Ag(111) surface. This reveals that the two-dimensional molecular assembly is influenced not only by intermolecular interactions but also significantly by molecule-substrate interactions. The second phase is represented in Figure 4b,e. The new lattice parameters are A2 ) 15 Å, B2 ) 18 Å, and R2 ) 105°, giving a packing density of 0.38 molecule/nm2 for P2. This corresponds to a 15% increase in the packing density. From P1 to P2, the molecules move so that the hydrogen atoms of one molecule get closer to the chlorine atoms of the neighboring molecule, leading to the formation of H‚‚‚Cl contacts. This increases compactness and molecule-molecule interactions in the A2 direction. Seventy hours after the deposit, the molecular structure becomes even more compact with the formation of P3, presented in Figure 4c. The molecules move so that hydrogen atoms and chlorine atoms of neighboring molecules interact finally in the B3 direction. The new lattice parameters, represented in Figure 4f, are A3 ) 15 Å, B3 ) 15 Å, and R3 ) 90°, giving a packing density of 0.44 molecule/nm2 for P3. The packing density is now 35% higher than that of P1. One can notice that P3 is not fully compact but presents stacking faults every three rows in the direction perpendicular to the stripes (direction B3 on Figure 4f). Occasionally, due to local defects, the stacking fault could arise after two rows. It may also be worth noticing that the local molecular organization around the fault is similar to the organization observed for P2. To gain more insight into the kinetics of the phase transformation, we now focus on real-time STM measurements

Figure 5. 300 nm × 300 nm STM images acquired: (a) 52 h after the depositsthe sharp lines delineate frontier domains between P2, P3, and gas phases; (b) 55 h after the depositsthe thicker line delineates the new frontier domains, while frontiers are reported from (a) to show evolutions.

performed during the P2-P3 transformation. Figure 5a,b shows two images of the same area recorded at t ) 52 h and t ) 55 h, respectively. Figure 6 gives a close-up view of the same area acquired by STM at t ) 60 h. It allows us to specify the positions of P2 and P3, which are surrounded by a gas phase,

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Figure 6. 50 nm × 60 nm STM image acquired 60 h after deposit of ZnPcCl8 on Ag(111). It shows P2 and P3 molecular structures linked by a grain boundary zone and surrounded by a gas phase (GP) composed of mobile ZnPcCl8 molecules. Note the matching of P2 and P3 structures at lines A and B.

denoted GP. The image of the gas phase appears blurred, due to the mobility of some isolated phthalocyanine molecules on the Ag(111) surface. For the sake of clarity, domain boundaries have been outlined in Figure 5. The sharp line delineating domain boundaries in Figure 5a has been redrawn in Figure 5b. It emphasizes the expansion of P3 over P2 and the major influence of the gas phase on the P2-P3 phase transformation. Indeed, the expansion of P3 occurs mainly at the P3 gas-phase boundary (arrows 2, Figure 6) while the P2-P3 boundary is hardly modified (arrow 1). Figure 6 shows that the P3 and P2 structures can be commensurate via the pinning of the defect lines of P3, as illustrated by lines A and B that penetrate P2. The displacement of the molecules in such a situation would be extremely difficult, because the creation of a molecular vacancy is needed first. The energy cost of this process is certainly higher than the energy cost of one molecule detaching itself from P2 to go to the gas phase (at least by a factor 2). Therefore, the latter process appears to be the favorable route for molecular species to leave the P2 phase for the gas phase, after which they can attach to the P3 phase. 4. Discussion Substituting chlorine atoms for peripheral hydrogen atoms in the ZnPc molecule strongly modifies the molecular selfassembly. This self-assembly is discussed in terms of modification of the intermolecular interactions due to the formation of chlorine-hydrogen bonds. Let us focus first on the influence of chlorine substitution on the molecule-substrate interactions. The substitution of electronegative atoms for the peripheral hydrogen atoms on an aromatic molecule directly influences the largely extended π-system. Eremtchenko et al. have shown that the addition of electronegative oxygen atoms in the periphery of a molecule of perylene largely enhances moleculesubstrate interactions.21 Similarly, it has been reported by Liu et al. that cobalt phthalocyanine (CoPc) molecules deposited on Au(111) self-assemble,15 whereas the fully fluorinated CoPcF16 molecules do not because of overly strong moleculesubstrate interactions and an increased repulsion between fluorine atoms.22 In the case of ZnPcCl8, where chlorine atoms are less electronegative than oxygen and fluorine atoms, the mobility of ZnPcCl8 molecules is not drastically reduced in comparison to that of ZnPc molecules, and despite moleculesubstrate interactions which should be substantially reinforced, the self-assembly process is still possible.

Figure 7. Proposed model deduced from STM experiments for P1, P2, and P3 molecular structures on an Ag(111) surface (a, b, and c, respectively). For the sake of clarity, H‚‚‚Cl bonds between molecules have been underlined by full lines for the molecule stacking in the [1-10] direction in P2 and P3, and dotted lines for the molecule stacking in the [110] direction in P3 only. The dH‚‚‚Cl bond lengths amount to 2.9 and 2.4 Å for full lines and dashed lines, respectively. There are 0, 4, and 8 H-bonds per unit cell for P1, P2, and P3 structures, respectively. Lattice parameters: phase P1 (A1 ) B1 ) 17.6 Å), intermediate phase P2 (A2 ) 15.0 Å, B2 ) 18.0 Å), and final phase P3 (A3 ) 15.0 Å, B3 ) 14.5 Å).

While ZnPc molecules self-assemble when deposited onto an Ag(111) surface under the influence of van der Waals forces, ZnPcCl8 molecules exhibit a structural evolution driven by the sequential formation of chlorine-hydrogen interactions.4 The observed supramolecular domains, P1, P2, and P3, are rationalized by the structural models proposed in Figure 7, which shows a montage of the unrelaxed ZnPcCl8 molecules on the Ag(111)

10062 J. Phys. Chem. B, Vol. 110, No. 20, 2006 surface. Intermolecular distances measured by STM are compatible with a molecular network commensurable with the substrate network. Thus, we conclude that all the molecules are located on equivalent adsorption sites on the Ag(111) surface. The representation of P1 in Figure 7a reveals a structural arrangement quite similar to the one observed for CuPcCl16 deposited on HOPG.23 One can deduce a distance dCl-Cl ) 4 Å between the nearest-neighbor chlorine atoms of two adjacent molecules. This reveals a self-assembly mediated by van der Waals interactions.4 After about 30 h at room temperature (300 K), it takes 15 h for P1 to slowly disappear, while the second phase P2 extends. During the formation of P2, the displacement of the molecules results in the creation of H‚‚‚Cl bonds between adjacent molecules. One finds four H‚‚‚Cl bonds per unit cell (or molecule), each with a length of 2.9 Å (full line in Figure 7b). The effectiveness of the H-bond interactions between chlorine and hydrogen atoms was proven by density functional theory (DFT) calculations.4 During the P2-P3 transformation (50 h after deposition), molecules still approach one another, thereby creating four additional H‚‚‚Cl bonds, each with a length of 2.4 Å (dashed line in Figure 7c). P3 is now stabilized by eight H‚‚‚Cl bonds per unit cell (or molecule). The evolution of the packing pattern with time represents an evolution toward as dense a packing as possible, due to the sequential formation of hydrogen bonds. Note that in the P3 structure one chlorine atom in two is engaged in two hydrogen bonds, that is to say four chlorine atoms and eight hydrogen atoms per phthalocyanine molecule are engaged in hydrogen bonds. Thus, all eight hydrogen atoms from the ZnPcCl8 molecule are engaged in H-bonds, and the P3 structure corresponds to the final term of this evolution driven by hydrogen bonding. However, it appears experimentally impossible to eliminate the fault lines of the P3 structure. Subsequent annealing of this film does not produce any alteration of this structure until molecules evaporate from the substrate. It just accelerates the evolution and produces molecular desorption above 380 K. Furthermore, real-time STM analyses reveal that the stripes start to form as soon as P3 start to grow, as represented in Figure 6. During the formation of P3, when three molecular rows stack perpendicular to the direction of the stripe, the fourth one does not stack but produces a stacking fault. We conclude that the P3 structure is thermodynamically stable, and the faults allow the stabilization of the molecular film. All these observations lead to the conclusion that substrate symmetry induces anisotropic stress within the molecular film. This stress originates in the competition between intermolecular interactions and molecule-substrate interactions. Substituting electronegative Cl atoms for H atoms in ZnPc to form ZnPcCl8 increases the molecule-substrate interactions and forces a site-specific adsorption of the molecules. Due to the surface symmetry, the lengths of the two molecular lattice vectors (A3, B3) are not equal: 14.5 and 15 Å, respectively. This leads to different Cl‚‚‚H bond lengths along and perpendicular to the stripe direction of 2.9 and 2.4 Å, respectively. The latter distance is therefore shortened by 17% compared to the former one. Theoretical calculations also give an equilibrium distance of 2.9 Å for the C-Cl‚‚‚H-C bond.4 While the longer hydrogen bond reaches its nominal equilibrium distance, the shorter one appears significantly stressed. The resulting stress energy accumulates each time short H-bonds form, that is, each time one molecular row is added. This explains why P3 closely packed organization is quasi infinite in one direction and develops regular fault lines to relax stress energy in the perpendicular direction.

Koudia et al. 5. Conclusion The self-assembly of zinc phthalocyanine and of octachloro zinc phthalocyanine on an Ag(111) surface was investigated by STM at room temperature. In the case of ZnPc deposition, six domain orientations were observed as a result of the combination of the threefold rotational substrate symmetry and the 2D chirality of the supramolecular lattice. Van der Waals forces are responsible for the self-assembly of the ZnPc molecules. To modify the intermolecular interactions, we studied ZnPcCl8 deposits. ZnPcCl8 is obtained from ZnPc by substitution of eight chlorine atoms for eight peripheral hydrogen atoms. The molecular organization observed after deposition of almost 1 ML of ZnPcCl8 on the Ag(111) surface revealed a surprising maturation process at room temperature. This maturation process is a structural evolution of the molecular assembly driven by the sequential formation of hydrogen bonds between adjacent molecules, from no H-bond in the first structure to four and finally eight H-bonds per molecule in the final structure. Furthermore, in the final structure, anisotropic stress develops within the molecular film as a consequence of moleculesubstrate interactions, leading to the formation of regularly spaced stripes to relax the structure. In short, the comparative study of ZnPc and ZnPcCl8 reveals how substituted chlorine atoms can create efficient hydrogen bonds to control the selfassembly process and produce original molecular nanostructures. Acknowledgment. The authors would like to thank V. Oison for fruitful discussions and theoretical calculations. References and Notes (1) Lehn, J. M. Science 1993, 260, 1762-1763. (2) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature (London) 2003, 424, 1029-1031. (3) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907-6912. (4) Abel, M.; Oison, V.; Koudia, M.; Maurel, C.; Katan, C.; Porte, L. ChemPhysChem 2006, 7, 82-85. (5) Stohr, M.; Wagner, T.; Gabriel, M.; Weyers, B.; Moller, R. AdV. Funct. Mater. 2001, 11, 175-178. (6) Suzuki, H.; Miki, H.; Yokoyama, S.; Mashiko, S. Thin Solid Films 2003, 438-439, 97-100. (7) Berner, S.; De Wild, M.; Ramoino, L.; Ivan, S.; Baratoff, A.; Gu¨ntherodt, H.-J.; Suzuki, H.; Schlettwein, D.; Jung, T. A. Phys. ReV. B 2003, 68, 115410. (8) Lingenfelder, M. A.; Spillmann, H.; Dmitriev, A.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. Chem.sEur. J. 2004, 10, 1913-1919. (9) Barth, J. V.; Weckesser, J.; Lin, N.; Dimitriev, A.; Kern, K. Appl. Phys. A 2003, 76, 645-652. (10) Grand, J.-Y.; Kunstmann, T.; Hoffmann, D.; Haas, A.; Dietsche, M.; Seifritz, J.; Moller, R. Surf. Sci. 1996, 366, 403-414. (11) Yim, S.; Jones, T. S.; Chen, Q.; Richardson, N. V. Phys. ReV. B 2004, 69, 235402. (12) Forrest, S. R. Chem. ReV. 1997, 97, 1793-1896. (13) Collins, R. A.; Mohammed, K. A. Thin Solid Films 1986, 145, 133-145. (14) Murata, K.; Ito, S.; Takahashi, K.; Hoffman, B. M. Appl. Phys. Lett. 1996, 68, 427-429. (15) Lu, X.; Hipps, K. W.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197-7202. (16) Lu, X.; Hipps, K. W. J. Am. Chem. Soc. 1997, 101, 5391-5396. (17) Phthalocyanines, properties and applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers, Inc.: New York, 1989; Vol. 1, p 6. (18) Linstead, R. P. J. Chem. Soc. 1934, 1016. (19) Barrett, P. A.; Dent, C. E.; Linstead, R. P. J. Chem. Soc. 1936, 1719. (20) WSxM; Nanotech Electronics; http://www.nanotec.es. (21) Eremtchenko, M.; Schaefer, J. A.; Tautz, F. S. Nature (London) 2003, 425, 602-605. (22) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903-2909. (23) Irie, S.; Hoshino, A.; Kuwamoto, K.; Isoda, S.; Miles, M. J.; Kobayashi, T. Appl. Surf. Sci. 1997, 114, 310-315.