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Ni(II)- and Vanadyloctaethylporphyrin Self-Assembled Layers Formed on Bare and 5-(Octadecyloxy)isophthalic Acid Covered Graphite Nuri Oncel and Steven L. Bernasek* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received March 9, 2009. Revised Manuscript Received July 8, 2009 Self-assembled monolayers of nickel- and vanadyloctaethylporphyrin molecules (NiOEP and VO-OEP, respectively) formed on bare and on 5-(octadecyloxy)isophthalic acid (5-OIA) covered highly ordered pyrolytic graphite (HOPG) substrates were studied with scanning tunneling microscopy (STM) at the solid-liquid interface under ambient conditions. A detailed comparison of the monolayer structures and lattice parameters of Ni-OEP and VO-OEP overlayers, along with previous information about the structure of Pt-OEP monolayers, suggests that coupling between the central metal atom and the substrate strongly affect the observed structures. In addition, the concentration of the solution and the nature of the solvent also affect the structure of these thin porphyrin films. These conditions can be used to guide the nanoscaled structures that form.
Introduction Thin films of organic molecules have seen use in various applications such as electroluminescent devices, sensors, lightemitting diodes, and photovoltaic cells.1-4 The performance of these devices crucially depends on the quality of the thin film. Self-assembly of organic molecules is a promising method to manufacture high-quality films. However, the mechanism that controls self-assembly of organic molecules is not trivial because organic molecules exhibit a wide spectrum of physical and chemical properties. In applications of organic thin films, porphyrin molecules have special importance. In general, porphyrin molecules have a nearly square core conformation, with a two-dimensional delocalized conjugated π-electron system. Metal porphyrins have been extensively studied as they have unique properties that make them suitable for device applications in areas such as light-emitting diodes, organic displays, and thin film transistors.5,6 In addition to this, metal porphyrins form the active centers of biologically important molecules such as heme and chlorophyll.7,8 There are extensive studies of thin films of these molecules in UHV and at the solid-liquid interface.9-13 However, it is not fully understood how the central metal atom affects the physical and chemical properties of the molecule and eventually the properties of the thin films formed from these molecules. Metal octaethylporphyrins may be considered an ideal match for studying the influence of *To whom correspondence should be addressed. (1) Malinski, T.; Tara, T. Nature 1992, 358, 676. (2) Maree, C. H. M.; Roosendaal, S. J.; Savenije, T. J.; Schropp, R. E. I.; Schaafsma, T. J.; Habraken, F. H. P. M. J. Appl. Phys. 1996, 80, 3381. (3) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332. (4) Harima, Y.; Okazaki, H.; Kunugi, Y.; Yamashita, K.; Ishii, H.; Seki, K. Appl. Phys. Lett. 1996, 69, 1059. (5) Tsuboi, T.; Wasai, Y.; Nabatova-Gabain, N. Thin Solid Films 2006, 496, 674. (6) Bansal, A.; Holzer, W.; Penzkofer, A.; Tsuboi, T. Chem. Phys. 2006, 330, 118. (7) Everse, J.; Vandegriff, K. D.; Wislow, R. M. Hemoglobins; Academic Press: San Diego, 1994. (8) Johnson, E. F.; Waterman, M. R. Cytochrome P450; Academic Press: San Diego, 1996. (9) Huber, V.; Lysetska, M.; W€urthner, F. Small 2007, 3, 1007. (10) Miyake, Y.; Tanaka, H.; Ogawa, T. Colloids Surf., A 2008, 313-314, 230. (11) Scudiero, L.; Hipps, K. W. J. Phys. Chem. C 2007, 111, 17516. (12) Ogunrinde, A.; Hipps, K. W.; Scudiero, L. Langmuir 2006, 22, 5697. (13) Mazur, U.; Hipps, K. W.; Riechers, S. L. J. Phys. Chem C 2008, 112, 20347.
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the metal center in the adsorption process since the influence of ethyl groups surrounding the porphyrin ring might be considered limited in affecting the adsorption of the porphyrin molecule. In contrast to this assumption, in this study it is shown that the ethyl groups can also have a significant influence on the lattice properties of the formed monolayer. In the present work, the properties of both Ni-OEP and VOOEP thin films are studied on bare and 5-OIA covered HOPG surfaces at the solid-liquid interface and under ambient conditions. These results are compared with previous studies of monolayers formed from Pt-OEP.14 Crystalline Ni-OEP exhibits both triclinic and tetragonal crystal forms.14 Previously studied PtOEP also exhibits the triclinic form.16 In crystalline form, the central Ni atom of the Ni-OEP porphyrin molecule, as with the Pt atom of the Pt-OEP molecule, is in the same plane as the surrounding porphyrin ring. Assuming that the porphyrin molecules will keep the same planar geometry in solution, it is possible to draw some conclusions about the influence of the size of the central atom on the lattice properties of the film by comparing the lattice properties of Pt- and Ni-OEP layers. On the other hand, the crystal form of VO-OEP is monoclinic and the V atom of the VO-OEP molecule sits out of the plane of the porphyrin ring since the oxygen atom bound to V moves the V atom off the center of the molecule.17 Therefore, even though V and Ni atoms have comparable sizes, in crystalline form the molecular structures of these porphyrins are different, and the expected moleculemolecule interactions in a formed monolayer are likely to differ. Similarly, assuming that VO-OEP will have the same geometry in solution, a comparison of the lattice properties of Ni and VOOEP molecules in the monolayer can reveal the influence of the molecular structure of the individual porphyrin molecule on the lattice parameters of the thin film. In addition, studying the lattice parameters of Ni- and VOOEP films formed on a monolayer of 5-OIA molecules and comparing these results with the Ni and VO-OEP films formed (14) Oncel, N.; Bernasek, S. L. Appl. Phys. Lett. 2008, 92, 133305. (15) Cullen, D. L.; Meyer, E. F.Jr. J. Am. Chem. Soc. 1974, 96, 2095. (16) Milgrom, L. R.; Sheppard, R. N.; Slawin, A. M. Z.; Williams, D. J. Polyhedron 1988, 7, 57. (17) Molinaro, F. S.; Ibers, J. A. Inorg. Chem. 1976, 15, 2278.
Published on Web 07/27/2009
Langmuir 2009, 25(16), 9290–9295
Oncel and Bernasek
Article
Figure 1. (a) A 6 nm 6 nm STM image of Ni-OEP monolayer formed on bare HOPG surface. Unit cell vectors are a = 1.48 ( 0.05 nm and b = 1.28 ( 0.1 nm, and the angle between them is about 70°. (b) 6.7 nm 6.7 nm STM image of VO-OEP monolayer formed on bare HOPG surface. The unit cell vectors are a = 1.45 - 0.1 nm and b = 1.34 - 0.1 nm, and the angle between them is about 60°. The tip bias and the tunneling current in both images are the same (0.6 V and 1 nA). (c, d) are schematic diagrams illustrating the Pt (Ni)-OEP/VO-OEP molecules. (e) is a schematic diagram of the 5-OIA molecule.
on the bare HOPG surface and with the lattice parameters of the Pt-OEP monolayer formed on bare and 5-OIA covered HOPG surfaces14 reveal how the size and position of the central atom in the porphyrin molecule influences the interaction of the molecules with the substrate.
Experimental Section HOPG substrates were purchased from Nanoscience Instruments. 1-Phenyloctane (98%), Ni-OEP (98%), VO-OEP (95%), and 5-OIA (98%) were purchased from Aldrich. 1-Phenyloctane was used as a solvent. Ni-OEP, VO-OEP, and 5-OIA were used without further purification. STM tips (Pt0.8Ir0.2, 0.25 mm diameter) were purchased from Nanoscience Instruments. Experiments were performed at the solid-liquid interface and under ambient conditions using a nano Surf EasyScan tabletop STM. Bias voltage and tunneling current conditions ranged from 0.5 to 0.8 V and 0.8 to 1.0 nA. Conditions are indicated in each image caption. Images were reproducibly obtained using different tips and multiple solution drops on the prepared substrates. Langmuir 2009, 25(16), 9290–9295
Results and Discussion Figure 1a is a high-resolution STM image of Ni-OEP molecules adsorbed from 1-phenyloctane solution (∼5 10-4 M) on the freshly cleaved HOPG surface. The concentration of the solution was chosen to be approximately the same as the concentration of the previously studied Pt-OEP solution14 in order to avoid any artifact caused by a difference in concentration. The unit cell vectors measured for the Ni-OEP monolayer are a = 1.48 ( 0.05 nm and b = 1.28 ( 0.1 nm, and the angle between them is about 70°. These values are significantly different than the lattice spacings (a = b = 1.2 nm) and the internal angle (60°) seen for the Pt-OEP monolayer.14 The unit cell of Ni-OEP is slightly larger than the unit cell of Pt-OEP. The 3-D crystal structure data for Pt- and Ni-OEP molecules show that Ni-OEP has a longer lattice spacing in the plane, but the volume of the unit cell of Pt-OEP is larger than the unit cell volume of Ni-OEP (0.75 nm3 vs 0.743 nm3).15,16 This is due to the larger separation between planes in the crystal caused by the extension of the Pt d-orbital of DOI: 10.1021/la900846s
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Figure 2. (a) is an STM image of Ni-OEP monolayer formed on top of a 5-OIA layer. The white lines reaching out to the edges of the image are domain boundaries. (b) Top: line scan of line 1. Bottom: line scan of line 2. (c) A high-resolution STM image of Ni-OEP monolayer formed on top of a 5-OIA layer. Arrows labeled as a and b show unit cell vectors. The unit cell vectors are a = 1.28 - 0.1 nm and b = 1.05 - 0.1 nm, and the angle between them is about 70°. The tunneling current and the tip bias for both STM images are 1 nA and 0.5 V, respectively.
Pt-OEP along the z-direction. This crystal structure data support what is observed with STM at the solid-liquid interface. Ogunrinde et al.12 studied the adsorption of Ni-OEP molecules on the HOPG surface from benzene and chloroform solutions (∼5 10-5 M). The unit cell vectors and the internal angle between them reported in this study are a = 1.47 ( 0.06 nm, b = 1.55 ( 0.03 nm, and 70°. The lattice spacing along the a direction and the internal angle is about the same as that seen for deposition from phenyloctane reported here. The contraction of the unit cell along the b direction can be ascribed to the difference in the concentration of the solution used in the experiments and/or the effect of solvent on the adsorption geometry. In order to test the influence of the concentration on the lattice properties, a monolayer deposited from a solution of Ni-OEP molecules dissolved in 1-phenyloctane at lower concentration (∼5 10-5 M) was examined. The unit cell vectors observed in this case are a = 1.66 ( 0.1 nm and b = 1.73 ( 0.1 nm, and the internal angle is about 70°. The unit cell of this monolayer is significantly larger than that of the monolayer formed from the solution with higher concentration. This suggests that the concentration has a significant impact on the film properties. Although the concentration of this new solution is the same as the concentration of the solution in benzene or chloroform used by Ogunrinde et al.,12 the 2-D unit cells of the two monolayers are quite different. The difference can be attributed to the solvent since benzene and chloroform have high vapor pressures and therefore evaporate more quickly. However, 1-phenyloctane remains on the surface and forms a thin liquid film in which the STM tip is immersed for scanning, and 9292 DOI: 10.1021/la900846s
we find no trace of evidence in the STM images for the coadsorption of 1-phenyloctane molecules together with OEP molecules. Therefore, the presence of solvent may give enough flexibility to reposition ethyl groups and therefore reduce the packing density. In Figure 1b, a high-resolution STM image of VO-OEP molecules adsorbed from 1-phenyloctane solution (∼5 10-4 M) on the freshly cleaved HOPG surface is presented. The concentration of this solution was chosen to be as close as possible to the Ni-OEP solution concentration in order to make a valid comparison. The unit cell vectors measured for the VO-OEP monolayer are a = 1.45 - 0.1 nm and b = 1.34 - 0.1 nm, and the angle between them is about 60°. A former study by Miyake et al.10 reports the lattice properties of VO-OEP self-assembled monolayer formed from a 1-tetradecene solution (
