Langmuir 2006, 22, 5697-5701
5697
A Scanning Tunneling Microscopy Study of Self-Assembled Nickel(II) Octaethylporphyrin Deposited from Solutions on HOPG Ayowale Ogunrinde, K. W. Hipps, and L. Scudiero* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed January 24, 2006. In Final Form: April 14, 2006 The adsorption of nickel(II) octaethylporphyrin (NiOEP) from benzene and chloroform solutions on highly ordered pyrolytic graphite (HOPG) was investigated with a scanning tunneling microscope (STM) operated in ambient conditions. STM images show that NiOEP self-assembles on the graphite surface and that the molecules lie flat and form 2D lattices with spacings of 1.58 ( 0.03 nm by 1.46 ( 0.06 nm with a lattice angle of 69° ( 4° averaged over both solvents. We were unable to eliminate the possibility that one unit cell distance is twice the above-reported distance. The corresponding molecular packing density, 4.5 ( 0.3 × 1013 molecules/cm2, was essentially the same for benzene and chloroform solution deposition. These results differ somewhat from the structure revealed by high-resolution STM images of NiOEP on Au (111). The lack of apparent height (image intensity) in the constant current STM image of the alkane region of alkane-substituted metal porphyrins is attributed to a combination of changes in alkane configuration relative to the ring and associated changes in electronic coupling with HOMO and LUMO.
Introduction Because of their important physical, electronic, and optical properties, porphyrins have been intensively studied. Their tendency to form ordered self-organized structures on a variety of substrates and the existence of an extended π-electron systems make the study of these compounds important from both fundamental and technological viewpoints. For instance, porphyrin thin films have been used as the basis for gas sensors,1 photovoltaic cells,2 organic light-emitting diodes,3 and an electrooptical data storage device,4 and when fused to rigid coplanar aromatic bridges, porphyrin structures have been proposed as molecular wires.5 Because of their very low roomtemperature vapor pressure (10-14 Torr), they have been sublimed in ultrahigh vacuum (UHV) and thus prepared and characterized as single-component thin films or multilayered films.6-14 However, for many applications, it is desirable to prepare monolayers of organic compounds on solid surfaces by solution phase deposition. These organic compounds can be dissolved in specific solvents and, when placed on metal and graphite surfaces, form self-organized structures. Organic monolayers on solid substrates are commonly prepared at a liquid-air interface (e.g., * Corresponding author. E-mail:
[email protected]. (1) Malinski, T.; Tara, Z. 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) Harima, Y.; Okazaki, H.; Kunugi, Y.; Yamashita, K.; Ishii, H.; Seki, K. Appl. Phys. Lett. 1996, 69, 1059. (4) Liu, C. Y.; Pan, H. I.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (5) Reimers, J. R.; Lu, T. X.; Crossley, M. J.; Hush, N. S. Nanotechnology 1996, 7, 424. (6) Duong, B.; Arechabaleta, R.; Tao, N. J. J. Electroanal. Chem. 1998, 447, 63. (7) Kuniatke, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (8) Ogaki, K.; Batina, N.; Kuniake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (9) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607. (10) Han, W.; Durantini, E. N.; Moore, T. A.; Moore, A. L.; Gust, D.; Rez, P.; Letherman, G.; Seely, G.; Tao, N.; Lindsay, S. M. J. Phys. Chem. B 1997, 101, 10719. (11) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066. (12) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (13) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 11899. (14) Scudiero, L.; Dan E. Barlow, D. E.; Mazur, U.; Hipps, K. W. J. Am. Chem. Soc. 2001, 123, 4073.
Figure 1. Stick molecular model of NiOEP as it occurs in the single crystal. Four ethyl substituents are oriented up and four are oriented down.
Langmuir-Blodgett techniques) or solid-liquid interface (e.g., self-assembly). The need to better understand the interaction between molecule and substrate at the liquid-solid interface has generated studies on self-organized porphyrins, substituted porphyrins adsorbed on Au (111) in the electrochemical environment,15-20 and graphite from solution.21-25 In all these studies, the end of the tip is immersed in a solution. (15) Yoshinoto, S.; Tda, A.; Suto, K.; Narita, R.; Itaya, K. Langmuir 2003, 19, 672. (16) Suto, K.; Yoshinoto, S.; Itaya, K. J. Am. Chem. Soc. 2003, 125, 14976. (17) He, Y.; Ye, T.; Borguet, E. J. Am. Chem. Soc. 2002, 124, 11964. The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) on Au (111) electrodes was investigated. The adlayer structure was found to depend on the electrode potential. (18) Yoshimoto, S.; Tada, A.; Itaya, K. J. Phys. Chem. B 2004, 108, 5171. (19) Yoshimoto, S.; Tsutsumi, E.; D’Souza, F.; Ito, O.; Itaya, K. Langmuir 2004, 20, 11046. (20) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540. (21) Wang, H.; Wang, C.; Zeng, Q.; Xu, S.; Yin, S.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 266. (22) Lei, S. B.; Wang, C.; Yin, S. X.; Wang, H. N.; Xi, F.; Liu, H. W.; Xu, B.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2001, 105, 10838. (23) Thomas, P. J.; Berovic, N.; Laitenberger, P.; Palmer, R. E.; Bampos, N.; Sanders, J. K. M. Chem. Phys. Lett. 1998, 294, 229. (24) Lei, S.; Deng, K.; Yang, D.; Feng, Q.; Wang, C.; J. Phys. Chem. B 2006, 110, 1256.
10.1021/la060233p CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006
5698 Langmuir, Vol. 22, No. 13, 2006
Ogunrinde et al.
Figure 2. Typical STM image of NiOEP adsorbed on HOPG from benzene or chloroform solution. The image was acquired at bias ) -0.5 V and setpoint ) 0.3 nA with a Pt/Ir tip and shows two uncovered graphite areas (A and B) and less than a monolayer NiOEP film (C) with two small defect regions in the film where molecules are missing. The height was calibrated using the atomic step of graphite (∼3.35 Å), as shown in the profile D. As shown in profile E, the molecule corrugation is about 0.8 Å and the overall molecular height is 2.5 Å.
Porphyrins modified by appending long alkane chains have been investigated on graphite by STM operated under ambient conditions. For these systems, the annealed droplet method was used. This method consists of depositing a drop of dissolved porphyrin either in acetone26 or toluene27 on a graphite substrate maintained at 40 °C. The samples were then allowed to cool slowly to room temperature. In these studies, self-assembled monolayer films were observed. As expected based upon simple alkane adsorption on graphite,28 the lamellar assembly of the alkane chains assists in the ordering of the porphyrin ring. These observations showed that molecular alkylation is an effective method for obtaining stable low-dimensional molecular assemblies on graphite in air. In the present work, our interest centered on the nature of small porphyrin species on graphite in air. Rather than use long alkane chains that form lamellar structures on graphite, or an electrochemical environment where the applied potential can play a significant role in the molecular ordering, we investigated the adsorption of nickel octaethyl porphyrin (NiOEP, Figure 1).29 STM images were acquired of monolayers formed on highly oriented pyrolytic graphite (HOPG) from benzene and from chloroform solutions at room temperature under ambient conditions. Self-organized monolayer of NiOEP formed 2D lattices on the clean graphite, and the unit cell dimensions will be compared to those observed for vapor-deposited films on Au (111).30
2p peak from a film where the porphyrin N 1s peak is clearly observed ensures that chloroform has been removed from the surface. A similar test could not be performed for benzene, but we assumed it was also removed by the extended vacuum drying. The self-organized thin films of NiOEP that formed on the graphite surface from both solutions were very stable and allowed images to be acquired from the same samples for days and even weeks. Both mechanically formed Pt0.8Ir0.2 and electrochemically etched W and Pt0.8Ir0.2 tips were used in this study. A PicoPlus STM (manufactured by Molecular Imaging) was utilized to acquire the images on graphite. A commercially available scanning probe image processor software (SPIP) was used to analyze the data. Vacuum Deposition and UHV Measurement. Epitaxial Au (111) films with well-defined terraces and single atomic steps were prepared on mica by previously described methods.33,34 The gold films were transferred via airlock into the UHV STM chamber (working pressure ∼8 × 10-10 Torr), where a monolayer of the porphyrin was thermally deposited and then studied without exposure to air. The STM head used was produced by McAllister Technical Services (Coeur d’Alene, ID), while a Digital Instruments Nanoscope III controller was used to acquire and process the reported data. Constant current images are reported, and any filtering is indicated in the appropriate figure caption. Both etched W and cut Pt0.8Ir0.2 tips were used. SPIP was also used for final data presentation.
Results and Discussion
Experimental Section
Figure 2 shows a typical STM constant current image of NiOEP self-organized on freshly cleaved graphite from benzene solution. This image was acquired under ambient conditions and shows
Solution Phase Deposition and in Air Measurement. The substrate used was highly ordered pyrolytic graphite (SPI-2) from SPI Supplies, and the organic compound, NiOEP, was purchased from Aldrich Chemical Co. and used as received. Liquid-phase deposition was used to deposit NiOEP onto the highly ordered pyrolytic graphite surface (HOPG). NiOEP was first dissolved in either benzene or chloroform at a concentration of 5 × 10-5 M. A 10 × 10 mm freshly cleaved HOPG piece was immersed in the solution at room temperature for 15-20 min, rinsed with the pure solvent for a few seconds, and then placed in a vacuum desiccator to dry for several hours to several days. This resulted in the formation of a thin film that covered most of the surface (some graphite surface was still apparent) with a single layer of molecules. The presence of residual chloroform in the films studied was tested with X-ray photoelectron spectroscopy. The complete absence of the chlorine
(25) Zou, Z.; Wei, L.; Chen, F.; Liu, Z.; Thamyongki, P.; Loewe, R. S.; Lindsey, Jonathan S.; Mohideen, U.; Bocian, D. F. J. Porphyrins Phthalocyanines 2005, 9, 387. (26) Zhou, Y.; Wang, B.; Zhu, M.; Hou, J. G. Chem. Phys. Lett. 2005, 403 140. (27) Wang, H.; Wang, C.; Zeng, Q.; Xu, S.; Yin, S.; Xu, B.; Bai, C. Surf. Interface Anal. 2001, 32, 266. (28) Giancarlo, L. C.; Fang, H.; Avila, L.; Fine, L. W.; Flynn, George, W. J. Chem. Educ. 2000, 77, 6671. (29) Cullen, D.; Meyer, E. F. J. Am. Chem. Soc. 1974, 96, 2095. (30) Scudiero, L.; Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2002, 106, 996. (31) Atamny, F.; Spillecke, O.; Schloe, R. J. Chem. Chem. Phys. 1999, 1, 4113. (32) Hipps, K. W.; Scudiero, L. J. Chem. Educ. 2005, 82, 704-711. (33) Lu, X.; Hipps, K. W. J. Phys. Chem. B 1997, 101, 5391. (34) Lu, X.; Hipps, K. W.; Wang. X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197.
Ni(II) Octaethylporphyrin Deposited on HOPG
three characteristic regions. Regions A and B are uncovered graphite with an atomic step at the top right of the image that was used for calibration The step height is about 3.4 Å, as shown in the top cross-section plot (in good agreement with the value of 3.35 Å of Atamny et al.31). The large area C is a dense island of organized NiOEP molecules. In addition, defects in the molecular film appear as two small regions in which molecules are missing. The lower cross-section plot taken over 10 molecules, and a small portion of graphite reveals a molecular corrugation of about 1 Å and a total height of 2.5 Å relative to graphite (the apparent height of the molecule). Using CPK model radii, the average height of NiOEP with the 8 ethyl group oriented up (or down) is about 5 Å, while the central porphyrin ring is about 3 Å in height. The smaller value revealed by this STM data reinforces the fact that STM images not only display the geometric structure of the surface, but also depend on the electronic density of states of the sample.32 High-resolution STM images of NiOEP adsorbed from both benzene and chloroform solutions on graphite are shown in Figure 3A and B, respectively. The images display individual porphyrin rings and a depression at the center of each molecule associated with the filled dz2 orbital of Ni. The contrast produced by the d orbital occupancy of metal ions has been explained elsewhere.13,33-35 In the case of samples made from benzene solution, cross-section plots (not shown) reveal that the intermolecular distances are a ) 1.55 ( 0.03 nm and b ) 1.47 ( 0.06 nm and that the angle between them is 70° ( 5°. The corresponding molecular packing density is 4.6 ( 0.3 × 1013 molecules/cm2. A similar value of 4.5 ( 0.3 × 1013 molecules/ cm2 was obtained from chloroform solution with lattice constants a ) 1.60 ( 0.03 nm, b ) 1.50 ( 0.08 nm, and γ ) 67° ( 5°. These intermolecular distances are somewhat different from the 1.65 and 1.4 nm measured on Au (111) where the internal angle is 60°.30 A UHV STM image of thermally deposited NiOEP on Au (111) is shown in Figure 3C. FeOEP adsorbed on the Au (111) surface in a 0.1 M HClO4 electrochemical environment also displays intermolecular distance of 1.65 and 1.45 nm.18 On Au (111), therefore, it appears that the packing density of octaethylporphyrins is about 5.0 × 1013, slightly greater or the same as we observe on HOPG. The difference in packing density and lattice shape could be related to: (1) the difference in sizes of the C (0001) and Au (111) lattices; both are hexagonal, but the carbon lattice is smaller (2.44 versus 2.88 nm); and (2) the difference in molecule-surface interactions between the graphite and Au surfaces. We are not able to assign the surface structure of NiOEP relative to the underlying HOPG lattice because we could not simultaneously image the NiOEP layer and the graphite substrate. We note that our inability to resolve the individual ethyl groups in air on graphite makes it difficult to ascertain the actual size of the unit cell. While the ethyl groups and, therefore the detailed ring orientation, are easily observed for the UHV sample on Au (111), Figure 3C, we were unable to resolve them in air on graphite. This inability to resolve the ethyl groups may be a property of the alkylated porphyrin on HOPG since, even for porphyrins ordered on HOPG by long alkane side chains, the alkane chain appears with much lower contrast than the porphyrin ring.21 The unit cell parameters given here assume that each NiOEP on HOPG is oriented the same, and there is only one molecule per unit cell. On Au (111), both NiOEP30 and FeOEP,18 the hexagonal unit cell has two molecules per unit because of the alternating twist of 15° between molecular rows. It is possible (35) Barlow, D.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 2444.
Langmuir, Vol. 22, No. 13, 2006 5699
Figure 3. High-resolution 8 × 8 nm STM images of NiOEP absorbed (A) on graphite from benzene solution, (B) on graphite from chloroform solution, and (C) vapor deposited on Au (111) and observed in UHV. The images were acquired at (I, V) ) (0.3 nA, -0.5 V), (I, V) ) (0.3 nA, -0.6 V), and (I, V) ) (0.3 nA, -0.6 V), respectively. Color scales are 0.4, 0.3, and 0.5 nm, respectively.
that a variant of the NiOEP-Au (111) surface is present on HOPG, and there are actually two molecules per unit cell, with one of the unit cell vectors being twice the molecular spacing. In any case, it is clear that the packing density is similar on both surfaces. The reason for the absence of the ethyl groups in the STM images is not clear, but suggests that the interaction between ethyl groups, HOPG is different than the one acting in the case
5700 Langmuir, Vol. 22, No. 13, 2006
Ogunrinde et al.
Figure 4. High-resolution 10 × 10 nm UHV STM images of NiOEP on Au (111) acquired at two different sample bias voltage settings (+1.0 V, right image, and -1.2 V, left image).
Figure 5. Bias voltage dependence STM images of NiOEP molecules deposited on graphite from chloroform solution. Constant current 10 × 10 nm images. The left image was acquired at negative -0.8 V and the right image was acquired at positive +0.8 V bias.
of Au substrate (in either UHV or electrochemical environment). This difference may be either physical, electronic, or both. On Au (111), the ethyl groups curl upward from the surface, forming a crown-like structure. On HOPG, they may lay flat as they do for the long chain alkane samples. On the electronic front, the difference could arise from the mixing of molecular orbitals with the HOPG surface states, or it might be due, in part, to the difference in the work function of graphite (∼5 eV) and of Au (111) (5.3 eV). To see how substrate work function changes may affect the STM image, one must recall that when the bias voltage plus the surface potential is near the energy of a molecular orbital, the STM image is heavily influence by that orbital.32 The very nice image of the entire NiOEP molecule (Figure 3C) can only be seen, even in UHV vapor deposited films on Au (111) in a fairly narrow bias region near 0 V. Images obtained from a vapordeposited sample in UHV, but at larger positive and negative bias are presented in Figure 4. Note that, while the porphyrin ring appears very differently, thereby reflecting the HOMO and LUMO electron densities, the ethyl groups are not observed.
Finally, Figure 5 shows the bias voltage dependence of STM images acquired from a chloroform-deposited film of NiOEP on HOPG in air. This strong dependency provides important information on the electron transport pathway at the submolecular scale. At negative bias voltage, the STM images reveal a dark spot at the center of the molecule and a bright porphyrin ring. At this voltage, most of the tunneling current flows through the outer periphery of the porphyrin ring (left image in Figure 5). On the other hand, when images are acquired at a positive bias voltage, a large bright region is seen that encompasses both the center and the inner region of the ring (right image in Figure 5). The corrugation height of the bright spots at positive voltage is about 3 Å, which is about twice the corrugation height of the ring at negative voltage (∼1.5 Å). To rule out any changes in oxidation state of the Ni (II) ion, we performed XPS measurements on the samples used in the present study and in a previous study of NiOEP adsorbed on Au.30 There is no evidence of oxidation or reduction of the Ni ion in our XPS data. Changes in oxidation state of the metal ion would have produced a shift of about 1.8 eV in the Ni 2p3/2 peak.
Ni(II) Octaethylporphyrin Deposited on HOPG
Langmuir, Vol. 22, No. 13, 2006 5701
Note that, at positive bias, reduction processes are stabilized32 so that oxidation of the Ni2+ under positive bias will not occur. Transient reductions, on the other hand, do occur. If one compares Figures 4 and 5, the similarities are striking. In both cases, orbital-mediated electron tunneling through the porphyrin dominates the images. Given the 0.3 V smaller work function of HOPG, the +0.8 V bias image should be very similar to that obtained on Au(111) at +1.1, always assuming that the adsorption is physisorptive in nature and induces no significant shifts or broadening of the NiOEP electronic levels. The image at -0.8 V bias, given the same assumptions, should be similar to that at -0.5 V bias on Au (111), and it is not. The absence of the ethyl groups in the NiOEP-HOPG image must involve more than simple work function changes. We speculate that a change in configuration of the ethyl groups relative to the ring further decouples the ring electron density from the alkane groups, making them appear very “low” in constant current images taken in the molecular band gap.
resolution images reveal that, for both solutions, NiOEP molecules pack into similar two-dimensional arrays where the molecules lay flat on the surface. The lattice constants are very similar for NiOEP adsorbed from either solvent on graphite but are somewhat different than the values obtained for vapor-deposited NiOEP on Au (111). Finally, we have shown that, independent of the method of deposition and on both Au (111) and HOPG, the bias dependence of the STM image reflects orbital-mediated tunneling through the HOMO (at negative bias) and LUMO (at positive bias). The polarity of the bias voltage determines where in the molecular structure the electrons will transfer between tip and substrate. The lack of apparent height (image intensity) in the alkane region of alkane-substituted metal porphyrins is attributed to a combination of changes in alkane configuration relative to the ring and associated changes in electronic coupling with HOMO and LUMO.
Conclusions
Acknowledgment. We thank the National Science foundation for support in the form of grants CHE 9709273 and CHE 9819318. Acknowledgment to Research Corporation for partial support of this research.
This STM study shows that, by simply immersing a freshly cleaved HOPG substrate into benzene and chloroform solutions containing NiOEP, self-organized islands of up to a monolayer of NiOEP can easily be formed on the HOPG surface. High-
LA060233P
