pubs.acs.org/Langmuir © 2010 American Chemical Society
Coverage-Dependent Structures of Cobalt-Phthalocyanine Molecules Adsorbed on Cu(001) Surface Qinmin Guo,†,‡ Zhihui Qin,† Kan Zang,†,‡ Cunding Liu,†,‡ Yinghui Yu,† and Gengyu Cao*,† †
State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China Received January 8, 2010. Revised Manuscript Received June 2, 2010
The morphologies, self-assembly structures, and stability of cobalt-phthalocyanines (CoPc) molecules adsorbed on Cu(001) with coverage ranging from 0.2 monolayer (ML) to 1.6 ML are investigated by ultrahigh-vacuum lowtemperature scanning tunneling microscopy (UHV LT-STM) at liquid nitrogen temperature. Upon increasing √ √the deposition of CoPc molecules various structures, such as isolated adsorption, quasi-hexagonal structure, 29 29 structure, are well characterized by the corresponding high-resolution STM images. The CoPc-CoPc intermolecular interaction and CoPc-substrate interfacial interaction dominate the structural evolutions. For the√coverage √ higher than 1 ML, CoPc molecules preferentially locate on top of the molecules underneath √ √ and organize into 58 58 structure. As more and more√CoPc molecules adsorb on the first layer, in some 58 58 regions molecular insertion leads to the √ formation of the 29 29 domain to effectively decrease the energy of the whole system.
Introduction As tunable single-molecule magnets, metal phthalocyanines (MePcs) have attracted remarkable interests in fabricating organic molecular devices, especially in the field of storage device, gas, and radiation sensors.1-5 Among these applications, understanding the interplay of molecules and environment as well as the related electronic properties is the important prerequisites for the development of the devices. Recently, the growth of MePcs and the characterization of their orientations or/and structures on various substrates, such as Au(111), Au(100), Pb(111)/Si(111), Cu(111), and Cu(100), have been intensively studied in both *Corresponding author: tel þ86 27 8719 7737; fax þ86 27 8719 8576; e-mail
[email protected]. (1) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R. W.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521. (2) Forrest, S. R. Nature 2004, 428, 911. (3) Wang, S.; Liu, Y. Q.; Huang, X. B.; Yu, G.; Zhu, D. B. J. Phys. Chem. B 2003, 107, 12639. (4) Lei, S. B.; Deng, K.; Yang, D. L.; Zeng, Q. D.; Wang, C. J. Phys. Chem. B 2006, 110, 1256. (5) Jarosz, G. J. Non-Cryst. Solids 2006, 352, 4264. (6) Cheng, Z. H.; Gao, L.; Deng, Z. T.; Liu, Q.; Jiang, N.; Lin, X.; He, X. B.; Du, S. X.; Gao, H. J. J. Phys. Chem. C 2007, 111, 2656. (7) Takada, M.; Tada, H. Chem. Phys. Lett. 2004, 392, 265. (8) Ge, X.; Manzano, C.; Berndt, R.; Anger, L. T.; K€ohler, F.; Herges, R. J. Am. Chem. Soc. 2009, 131, 6096. (9) Chen, X.; Fu, Y. S.; Ji, S. H.; Zhang, T.; Cheng, P.; Ma, X. C.; Zou, X. L.; Duan, W. H.; Jia, J. F.; Xue, Q. K. Phys. Rev. Lett. 2008, 101, 197208. (10) Zhao, A. D.; Li, Q. X.; Chen, L.; Xiang, H. J.; Wang, W. H.; Pan, S.; Wang, B.; Xiao, X. D.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. Science 2005, 309, 1542. (11) Zhao, A. D.; Hu, Z. P.; Wang, B.; Xiao, X. D.; Yang, J. L.; Hou, J. G. J. Chem. Phys. 2008, 128, 234705. (12) Hipps, K. W.; Barlow, D. E.; Mazur, U. J. Phys. Chem. B 2000, 104, 2444. (13) Mazur, U.; Leonetti, M.; English, W. A.; Hipps, K. W. J. Phys. Chem. B 2004, 108, 17003. (14) Barlow, D. E.; Scudiero, L.; Hipps, K. W. Langmuir 2004, 20, 4413. (15) Ogunrinde, A.; Hipps, K. W.; Scudiero, L. Langmuir 2006, 22, 5697–5701. (16) Gopakumar, T. G.; Lackinger, M.; Hackert, M.; M€uller, F.; Hietschold, M. J. Phys. Chem. B 2004, 108, 7839. (17) Walzer, K.; Hietschold, M. Surf. Sci. 2001, 471, 1. (18) Molodtsova, O. V.; Knupfer, M.; Ossipyan, Y. A.; Aristov, V. Y. J. Appl. Phys. 2008, 104, 083704. (19) Suzuki, T.; Kurahashi, M.; Yamauchi, Y. J. Phys. Chem. B 2002, 106, 7643. (20) Lippel, P. H.; Wilson, R. J.; Miller, M. D.; W€oll, C.; Chiang, S. Phys. Rev. Lett. 1989, 62, 171.
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experimental and theoretical aspects.6-22 For example, Gao’s group found the FePc molecules of the second layer were tilted by ∼40° to the substrate Au(111) surface plane.6 On the other hand, CoPc molecules adsorbed on CoPc monolayer supported on Au(111) substrate were almost parallel to the metal surface with no skew angle with respect to the underlying molecules.7 While on Cu(111) and Pb(111)/Si(111), the CoPc molecules of the second layer rotated in the molecular plane.8,9 All the above results demonstrate that depending on the central metal ions and the substrate underneath MePcs take various adsorption structures and orientations. Additionally, the electronic properties of MePcs can thus be greatly affected as well. One of the most promising discovery is that the Kondo effect and the corresponding Kondo temperature of MePcs were strongly influenced by the molecule-substrate interfacial interactions, which has been proved in the case CoPc/Au(111) previously.10,11,23 It is worth noting that the molecule-molecule interactions (MMI) and molecule-substrate interactions (MSI) dominate the adsorption sites and thereby the self-assembly manner of the MePcs. Since the MMI and MSI vary with the molecular concentration, it is necessary to systemically investigate the coverage-dependent adsorption structures. However, the reports on the MMI and MSI dominated MePcs molecular adsorption structures and their evolutions are rare to date. In this study, on a selected substrate, Cu(001),24 we studied the molecular orientations and structures of CoPc molecules from disorder distribution, self-assembly monolayer (SAM), to periodic structures in the second layer. The coverage-dependent structures (21) Takada, M.; Tada, H. Ultramicroscopy 2005, 105, 22. (22) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (23) Aguiar-Hualde, J. M.; Chiappe, G.; Louis, E.; Anda, E. V.; Simonin, J. Phys. Rev. B 2009, 79, 155415. (24) Because of the symmetry matching between the fcc (001) metal surfaces and the MePc molecules, it can be expected that well-defined self-assembly MePc layers could be found on these surfaces. So far, there are seldom researches focusing on MePc/Cu(001) systems, except that P. H. Lippel et al. and M. Takada et al. discussed the adsorption and electronic structures of isolated CuPc and CoPc molecules on Cu(001) surface, respectively.20,21
Published on Web 06/18/2010
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and their evolutions dominated by MMI and MSI were discussed in detail.
Experimental Details The experiments were performed with an ultrahigh-vacuum scanning tunneling microscopy (UHV STM) (Unisoku, Japan) operated at liquid nitrogen temperature (∼78 K) with a base pressure better than 1 10-8 Pa. A single crystal copper(001) (Mateck, Germany), cleaned by repeated cycles of Neþ sputtering (∼30 min) and annealing at 750 K for 20 min, was used as substrate for CoPc molecule growth. After more than 48 h degassing for purification, CoPc (95% for purity, Aldrich) molecules were sublimated from a silica crucible onto the substrate kept at room temperature (RT). We keep the deposition rate at ∼0.03 ML/min, which is calibrated by the large-scale STM image of the sample with submonolayer molecules adsorption, and control the deposition time to obtain different molecular coverages. During deposition, the pressure was kept at ∼6.5 10-8 Pa. The STM images presented here were obtained with electrochemically etched polycrystalline tungsten tip in constant-current mode. Before STM scanning, the tip was subjected to e--beam heating to get rid of the contaminations and oxide layers. The bias voltage was applied to the sample in this work; therefore, positive biases probe the unoccupied states of the sample.
Results and Discussion The STM image (Figure 1a) obtained after room temperature (RT) deposition of ∼0.2 ML of CoPc on Cu(001) demonstrates that the molecules separately distribute on the terrace. The step edges indicated by circles are fully decorated by the molecules, which have a slight deformation with lobes on both sides of the steps in contrast to the ones on the terrace and assemble into single-molecular chains along the step edges. It seems that due to the relative high adsorption coefficient CoPc molecules have a strong tendency to locate at the step sites of Cu(001) at RT. Additionally, from high-resolution STM images (Figure 1b,c) of
the region indicated by a solid square, well-defined four-leafclover molecules adsorbed on the terrace are observed. Four lobes outside are referred to the phenyls of CoPc molecules and center Co2þ ions display as protrusions under negative bias voltage (Figure 1b) and as depressions under positive one (Figure 1c). This bias-dependent feature can be ascribed to the different Co2þ dz2 orbital contribution to tunneling current at different bias polarity.25 In Figure 1d, a CoPc molecule and the underlying substrate lattice are well-resolved simultaneously, from which we can determine the Co2þ ion in the cavity of molecule lies directly on top site of Cu(001), and the phenyl lobes locate at the 4-fold hollow sites where the mirror symmetry exists along [110] or [1-10] directions. Furthermore, the molecular lobes have an angle of (22.5° with respect to [1-10] direction. Herein, these two kinds of adsorption configurations are sketched in Figure 1e,f. The molecules showing both configurations have the same chemical environments and adsorption energy and thereby the equivalent adsorption ratio. No other adsorption configurations can be found even though the coverage is increased to monolayer, which indicates the CoPc molecules are stable with both adsorption configurations at RT. Another two metastable configurations (0° and 45° with respect to [1-10] direction of Cu(001)) of CoPc molecules at lower deposition temperature around 29 K were reported previously, and they could be transformed into these stable ones by annealing to RT.26 In our case, the molecules with both configurations (Figure 1e,f) cannot change their adsorption sites and orientations even if they suffer þ4 V scanning bias voltage, indicating they are the stable configurations. To further figure out the stability of CoPc molecules adsorbed on Cu(001), we anneal the sample covered by ∼0.7 ML CoPc molecules to different temperatures. Comparing with Figure 2a, the molecular concentration does not change upon annealing at ∼260 °C for 10 min as shown in Figure 2b. However, further annealing at 380 °C treatment results in random desorption of
Figure 1. (a) Typical STM image (30 nm 30 nm, V = 2 V, I = 59 pA) of CoPc molecules (∼0.2 ML) on Cu(001) surface. The molecules
preferentially decorate the steps of Cu(001). (b, c) Zoom-in STM images (5 nm 5 nm, I = 44 pA) of the region indicated by a solid square showing isolated CoPc molecules with different appearances. (b) V = -0.5 V, (c) V = 1.5 V. (d) STM image with a CoPc molecule and the substrate lattice resolved simultaneously. The central Co2þ ion resides on top site of the Cu(001), and the molecular lobes form ∼23° with respect to [110] direction. (e, f) Two equivalent adsorption schematic models of CoPc/Cu(001) derived from (b) and (c), respectively.
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Figure 2. STM images (45 nm 25 nm) of ∼0.7 ML CoPc on Cu(001) surface subjected to different temperatures annealing: (a) as prepared (0.8 V, 390 pA); (b) annealing at 260 °C for 10 min (-1.4 V, 387 pA), and (c) annealing at 380 °C for 10 min (-1.4 V, 457 pA).
almost half of the CoPc molecules away from the surface as shown in Figure 2c. That CoPc can stably exist on Cu(001) surface at least up to 260 °C suggests the substrate copper has relatively strong interfacial interaction with CoPc molecules. Upon increasing the coverage to 0.83 ML, as shown in Figure 3a, the molecules arrange in a quasi-hexagonal lattice manner, which can be proved by corresponding fast Fourier transform (FFT) as shown in the inset of Figure 3a. The intermolecular distances along these molecular rows √ are √ 1.58 ( 0.05, 1.42 ( 0.05, and 1.77 ( 0.05 nm, close to 41, 34, and 7 times of the lattice constant of Cu(001) (0.255 nm), respectively. So the unit cell vectors are along [1-90], [4-10], and [110] directions of Cu(001). The molecules randomly distribute in one domain with two kinds of adsorption configurations (Figure 1e,f) as clearly indicated by depicting molecular contours. According to the intermolecular spacing, the molecular adsorption structure is proposed as shown in Figure 3b, in which the blue solid circles represent the positions of CoPc molecules neglecting the orientation information and the yellow ones represent copper atoms underneath. In this adsorption structure, the molecules are in registry with Cu(001) with their center Co2þ ions residing on top sites of Cu(001) as the isolated molecules do. The CoPc molecules rarely adsorb on top of themselves until monolayer coverage is reached. Figure 4a shows a typical STM image of CoPc molecules with the coverage of ∼1.05 ML. As monolayer coverage is reached, the arrangement lattice of the first layer is not quasi-hexagonal any more, but assembles into quadrate lattice as shown in Figure 4c. The molecular rows form ∼22° with respect to Cu[110] direction, and the intermolecular distance in the molecular row is 1.36 ( 0.05 nm, which√is consistent with the lattice constant along Cu[7/23/20] direction ( 29 0.255 nm). √ On basis of this fact, a possible molecular reconstruction ( 29 ; √ 29)R21.8°;structure is proposed, as shown in Figure 4d. Note that molecules insertion is possible in quasi-hexagonal arrangement (Figure 3). With more CoPc molecules coming to the surface, molecules insertion induces the formation of quadrate reconstruction, a more compact arrangement. By comparing both 0.83 and 1.05 ML cases, in the latter case the molecules have (25) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207. (26) Chang, S. H.; Kuck, S.; Brede, J.; Lichtenstein, L.; Hoffmann, G.; Wiesendanger, R. Phys. Rev. B 2008, 78, 233409.
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Figure 3. (a) STM image (15 nm 15 nm, 1 V, 50 pA) of ∼0.83 ML of CoPc on Cu(001) surface. The inset is the corresponding FFT image suggesting molecular quasi-hexagonal arrangement. (b) Proposed reconstruction structure. The arrows indicate direction of the vectors. Yellow and blue spots represent the substrate copper atoms and the Co2þ ions of CoPc molecules, respectively.
Figure 4. (a) Molecularly resolved STM image (50 nm 50 nm, 0.5 V, 50 pA) of CoPc with 1.05 ML coverage. (b) Isolated CoPc molecule adsorbed on the first molecular layer with a compact arrangement structure shown in (c). Scan size: (b) 4 nm 4 nm and (c) 5 nm 5 nm. (d) The proposed structural model for a CoPc monolayer on Cu(001).
consistent orientation induced by intermolecular interactions, in which the short-ranged intermolecular repulsive force dominates their assembly. As the molecular distance is short enough, the energy associated with the repulsive force can overcome molecular Langmuir 2010, 26(14), 11804–11808
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Figure 6. (a) STM image (93 nm 47 nm, 2.05 V, 71 pA) of orien-
Figure 5. (a) Typical STM image (155 nm 70 nm) of the surface with CoPc coverage of ∼1.4 ML. (b, c) High-resolution STM images (10 nm 10 nm) of the second molecular layer with positive (1.5 V) and negative (-1.5 V) biases, respectively. (d, e) STM images (26 nm 26 nm, 2 V, 30 pA) of the same region before and after scanning upper left part with þ4 V bias voltage.
rotation potential barrier, and the molecules rotate and form the quadrate lattice. The total energy of the system is lowered down as a result of the consistent orientation. On the other hand, for the 0.83 ML case the CoPc molecules have a little longer intermolecular distance, and the repulsive force attenuates rapidly, resulting in inconsistent orientation in quasi-hexagonal arrangement. Additionally, in Figure 4a the bright spots correspond to the isolated CoPc molecules adsorbed on the first layer. A zoom-in STM image (Figure 4b) reveals the stacking relationship between the two layers. It is interesting that the CoPc molecules are not located at hollow sites in between four first layer CoPc units but lie vertically above the first layer CoPc, which demonstrates that these hollow sites is not the stable adsorption position for second layer CoPc molecules. The in-plane axes of the molecules in the second layer rotate 45° with respect to the ones in the first layer. The interlayer interaction dominates this stacking process via π-π interactions.27 Forming 45° can release the overlapping of the π-π orbitals and thereby low down the systemic energy. With increasing the molecular deposition to ∼1.4 ML, as shown in Figure 5a, it is clear that the molecules aggregate into a quadrate lattice but with a different lattice distance from that of the first monolayer. In terms of the careful measurements of the nearest molecular distance (1.93 √ √ ( 0.05 nm), it is concluded that the molecules form 58 58 reconstruction. Figure 5c,d dis√ √ plays the high-resolution STM images of 58 58 superlattice in positive and negative bias voltage conditions showing no bias dependence, in contrast to the molecule directly adsorbed on Cu(001). The CoPc molecule in the second layer forms dimer with the underlying one through a weak σ bond. The dz2-dz2 σ-bonding orbital is occupied by one electron and causes the bright spot in the center of the CoPc structure in the STM image (27) Huang, H.; Chen, W.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 14913.
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tation domains in second molecular layer. (9.25 nm √ (b)√An area√ √ 9.25 nm, 2.05 V, 72 pA) having both 58 58 and 29 29 structures and (c) is its schematic √ structure. √ (d) The √ proposed √ structural transition process from 58 58 to 29 29 structure.
obtained at negative bias. The antibonding combination (σ*) remains empty and gives rise to the bright spot at positive bias.8 Close inspection reveals that every phenyl lobe of CoPc adsorbed on top of the first CoPc layer changes into two fragments, relating to the free CoPc molecules orbital, and the central Co2þ ions are more pronounced, which is different from the molecules directly adsorbed on Cu(001) in appearance. As we know, orbital of molecules adsorbed on the metal substrate would hybridize with the d-band electronic states of the metal. Then the electronic states, contributing to the STM topography, involve electronic states of both the molecules and the underlying metal. An insulating layer between the molecules and the metal substrate can effectively decouple the interaction between the molecules and the metal. Alkali halides ultrathin films and oxidation films were good candidates as decoupling layers.28-31 Similarly, the CoPc molecular layer can also quench the electronic states of the metal substrate and serve as a decoupling layer. Hence, the molecular orbital of the second layer can expand farther, and the CoPc molecules show a little bigger contours than those directly adsorbed on Cu(001) substrate. In addition, √ when√the upper left part of Figure 5d is scanned at þ4 V, 58 58 structure is destroyed as shown in Figure 5e. In contrast, according to the above results the structure of the first layer keeps unchanged in such a scanning condition. It is concluded that the CoPc molecules of the second layer have smaller diffusion and rotation barriers than those of the first layer, indicating the interlayer interaction is weaker than that between CoPc molecules and Cu(001). Upon further increasing the deposition of CoPc to ∼1.6 ML, on the surface there are a few domains in which the molecules arrange more compactly such as one domain indicated by the arrow in Figure 6a. From the zoom-in STM image (Figure 6b) of (28) (29) (30) (31)
Repp, J.; Meyer, G. Phys. Rev. Lett. 2005, 94, 026803. Liljeroth, P.; Repp, J.; Meyer, G. Science 2007, 317, 1203. Repp, J.; Meyer, G.; Olsson, F. E.; Persson, M. Science 2004, 305, 493. Ogawa, N.; Mikaelian, G.; Ho, W. Phys. Rev. Lett. 2007, 98, 166103.
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this region, it can be clearly seen that there are two kinds of molecular √ √lattices (1.93 √ ( 0.05 √ and 1.36 ( 0.05 nm, i.e., forming 58 58 and 29 29 superlattice, respectively). The corresponding diagram √ is schematically √ √drawn√in Figure 6c. All the molecules in both 58 58 and 29 29 constructions locate on top sites of the first layer. However, the molecular orientation with respect to the √ underneath √ molecule is different. The molecules rotate 45° in 58 58 structure while no √ √ rotation is found in 29 29 structure. At the initial period of molecules adsorbing on the first layer, the molecules rotate 45° as mentioned in the 1.05 ML case. As more and more molecules first the √ adsorb √ on the first layer, √ √ molecules arrange into the 58 58 superlattice, i.e., 2 2 lattice with respect to the first layer √ (1.4 √ML case). Owing to the large intermolecular √ distance in 58 58 structure, a CoPc can insert into a 58 √ 58 unit cell as the molecular coverage is√high enough and √ consequently lead to the formation of 29 29 reconstruction, i.e., 1 1 lattice with respect to the first layer (1.6 ML case). Figure 6d shows the possible structural evolution process of √ √ √ √ 58 58 structure to √ 29 √ 29 structure. The insertion of a CoPc molecule into a 58 58 unit cell induces this molecule and its neighboring ones rotating and forming an angle of 0° with respect to the ones underneath in order to effectively decrease the √ √ energy of the whole system, and then the 29 29-R21.8° structure forms. Previous researches reveal that the CoPc molecules adsorbed on monolayer CoPc covered Au(111)7 and Pb(111)/Si(111)9 form 1 1 structure with respect to the underlying monolayer molecular lattice, while they arrange with different in-plane orientations. On Au(111) the molecules in the second layer form 0° with respect to the underlying molecules, while on the Pb(111)/Si(111), they form an angle of 45°. In our
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case, both molecular rotations are observed on Cu(001) supported √ CoPc √ saturated monolayer, i.e., 0° in 1 1 lattice and 45° in 2 2 lattice with respective to the molecular lattice underneath. The different molecules-metal substrate interfacial interactions and intermolecular interactions are supposed to play a key role for these orientation differences.
Conclusions We have used STM to systemically investigate the morphologies and structures of CoPc adsorbed on Cu(001) with different deposition coverages. At the initial adsorption, CoPc locates at the top sites of Cu(001) with their lobes rotating (22.5° with respect to the Cu[110] direction. As increasing the coverage, CoPc molecules form quasi-hexagonal structure with disordered in-plane orientation. Once √ √ monolayer coverage is reached, the molecules form 29 29 structure possessing the same in-plane orientation. Further √increasing √ the coverage to 1.4 ML, the CoPc molecules form 58 58 structure on top of the first layer with respect to Cu(001). As more and more molecules adsorb on the first molecule √ layer, √ in a few regions √ √ insertion process transforms 58 58 structure into 29 29 structure when the coverage reach 1.6 ML. The competition of MMI and MSI plays a key role in this assembly process. On the basis of the decoupling effect of the first CoPc layer, the discrepancy in aspects of the topography between the two molecular layers is also explained. Acknowledgment. This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 10804121).
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