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XPS and SERS Study of Silicon Phthalocyanine Monolayers: Umbrella vs Octopus Design Strategies for Formation of Oriented SAMs Zhiyong Li and Marya Lieberman* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556
Wieland Hill Lambda Physik AG, Hans-Boeckler Strasse 12, 37079 Goettingen, Germany Received February 7, 2001 Two strategies are compared for the formation of self-assembled monolayers (SAMs) of silicon phthalocyanines on gold. Silicon phthalocyanines were synthesized with thiol anchoring groups in either eight peripheral side chains (the “octopus”) or with one short thiol in an axial position (the “umbrella”). Both approaches gave phthalocyanines capable of forming SAMs on gold surfaces. The orientation and coverage of the phthalocyanines were compared using ellipsometry, X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman scattering (SERS). The octopus silicon phthalocyanines form poorly organized SAMs in which the phthalocyanine (Pc) rings are strongly tilted with respect to the gold surface. On average, between 3 and 4 of the thiol “arms” fail to bind to the gold surface, even when limiting coverage is achieved after 7 days of soaking. The film thickness is 22 ( 5 Å. In contrast, the umbrella silicon phthalocyanine produces close-packed SAMs within 1 h in which the Pc rings lie parallel to the gold surface. The average thickness of the later SAMs is 11 ( 3 Å, and each phthalocyanine ring occupies an average area of 284 Å2.
Introduction Thin films of phthalocyanines are of interest for applications in photovoltaic, electronic, and sensing devices.1 Various techniques have been used to fabricate phthalocyanine (Pc) thin films, including LangmuirBlodgett deposition,2 spin-coating,3 and vapor deposition.4 The arrangement of phthalocyanines in these films is dominated by π-stacking interactions between neighboring phthalocyanines. When compared with these techniques, the self-assembled monolayer (SAM) technique developed in the groups of Whitesides, Ulman, and Nuzzo5 offers great advantages in that the monolayer films are chemically bound to substrates so that reproducible, stable films can be formed. By careful placement of thiol groups in the adsorbate, the SAM technique can constrain a molecule to adopt a packing preference on a surface that it would not otherwise adopt. This capability would be very useful for orienting aniosotropic molecules for device applications. Two main strategies have been applied to orienting large macrocycles in SAMs. We wanted to establish which of these attachment methods would be the most effective for a given molecule in terms of speed of attachment, control (1) Leznoff, C. C., Lever, A. B. P., Eds. Phthalocyanines: Properties and Applications; VCH: New York, 1989. (2) (a) Bourgion, J. P.; Doublet, F.; Palacin, S.; Vandevyver, M. Langmuir 1996, 12, 6473. (b) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357. (3) (a) Cook, M. J.; Mayes, D. A.; Poynter, R. H. J. Mater. Chem. 1995, 5, 2233. (b) Fujiki, M.; Tabei, H.; Kurihara, T. Langmuir 1988, 4, 1123-1128. (4) (a) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197-7202. (b) Vukusic, P. S.; Sambles, J. R. Thin Solid Films 1992, 221, 311. (5) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (b) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (c) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335.
of molecular orientation, and SAM stability. Several porphyrins were successfully anchored to gold surfaces by Porter6 and Murray7 using one or more thiol linkers at the porphyrin ring periphery. Cook and Russell et al. used this approach to make monolayer films of phthalocyanines on gold and SiO2 by reacting the surfaces with phthalocyanines that contain pendant thiols and trichlorosilanes.8 In the cases where single peripheral tethers were used, the macrocycle rings were found to be tilted at various angles to the substrate surface; in some cases, flat orientation is achieved, but more often, the ring-ring π-stacking interaction favors various degrees of tilt. Tethering a central metal ion to a surface offers another way to orient macrocycles. Recently, coordination of amines in a prefabricated SAM to axial sites in metallophthalocyanines or porphyrins was utilized by Palacin9 and Liu10 to bind the macrocycle ring to the surface with flat orientation. However, the quality of phthalocyanine or porphyrin SAMs formed by this method depends largely on the quality of the primer SAM delivering the coordinating ligands (amine or pyridine). This problem, however, could be avoided by derivatizing the axial site of a macrocycle with a thiol-linking group, as demonstrated by Boeckl et al.11 with phosphorus porphyrins and by this group12 with silicon phthalocyanines. (6) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772-2774. (7) Hutchison, J. E.; Postlethwaite, T. A.; Murray, T. W. Langmuir 1993, 9, 3277-3283. (8) (a) Simpson, T. R. E.; Revell, D. J.; Cook, M. J.; Russell, D. A. Langmuir 1997, 13, 460-464. (b) Cook, M. J.; Hersans, R.; McMurdo, J.; Russell, D. A. J. Mater. Chem. 1996, 6, 149-154. (c) Revell, D. J.; Chanbrier, I.; Cook, M. J.; Russell, D. A. J. Mater. Chem. 2000, 10, 31-37. (9) Huc, W.; Saveyroux, M.; Bourgoin, J. P.; Valin, F.; Zalczer, G.; Albouy, P. A.; Palacin, S. Langmuir 2000, 16, 1770-1776. (10) Zhang, Z.; Hou, S.; Zhu, Z.; Liu, Z. Langmuir 2000, 16, 537-540.
10.1021/la010203g CCC: $20.00 © 2001 American Chemical Society Published on Web 07/17/2001
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In this report, two strategies to force the same macrocycle ring to lie parallel to the surface are compared. For the “octopus” strategy, eight short thiol tethers were arranged around the periphery of a phthalocyanine ring, and for the “umbrella” strategy, a short tether was covalently attached to the central atom of the phthalocyanine. These design strategies were applied to a family of soluble silicon phthalocyanines XYSiPc(OR)8. These molecules contain two axial alkyl or alkoxide groups (X, Y) and eight alkoxy groups around the periphery of the ring. This paper describes the synthesis of such molecules, their self-assembly into monolayer films on gold surfaces, and the comparison of the coverage and the orientation of the molecules in the SAMs. Experimental Section General. Syntheses were carried out under dry argon using standard Schlenk methods. All procedures were carried out in the dark as much as possible (flasks and NMR tubes were wrapped in aluminum foil). AIBN and concentrated HCl (Aldrich) were used without purification. Quinoline (Aldrich) contained a yellow impurity and was vacuum distilled from barium oxide before use; thioacetic acid and mercaptoethanol were vacuum distilled from 3 Å molecular sieves and stored under Ar; methanol and 2-propanol were dried by storage over activated 3 Å molecular sieves; toluene and d6-benzene were dried over sodium, distilled under vacuum, and stored in a drybox under Ar. The yields for the phthalocyanines (based on diiminoisoindoline) range from 30 to 50% for most diiminoisoindolines and alkyltrichlorosilanes that we have tried. Solid products can be stored in a vacuum desiccator or drybox for months. 1H NMR spectra were taken on a Varian Unity+ 300 MHz instrument and were referenced to the solvent residual. IR spectra for bulk samples were obtained on a Perkin-Elmer Paragon 1000 FT-IR spectrometer; some spectra were taken as KBr pellets or Nujol mulls, while others were taken by the evaporation of a drop of CDCl3 solution to form a thin film on a KBr plate. UVVis spectra were obtained with a Perkin-Elmer Lambda 11 UVVis spectrometer. Elemental analyses were performed by M-H-W Laboratories, Phoenix, AZ. Mass spectra were obtained on a JEOL JUS-AX505 HA mass spectrometer using a JEOL 102517 electrospray source. A Nanoscope II STM (Digital Instruments) was used to image gold substrates. Before scanning each sample, the same tip was used to observe highly ordered pyrolytic graphite for which atomic resolution is obtained if the tip is good (not twinned). Molecular modeling was performed in SPARTAN 4.0.4 X11, Wavefunction Inc., Irvine, CA. Synthesis of 1: Compound 1 Was Synthesized by a Literature Method.12a Octopus and Umbrella Silicon Phthalocyanines 1, 2a, and 2b. (i) (OCH3)2SiPc(OCH2CH2CH2CH2CH2SCOCH3)8 (2a). (OCH3)2SiPc(OCH2CH2CH2CHdCH2)8 (250 mg)13 and AIBN (100 mg) was dissolved in 10 mL of dry toluene under Ar, then dry thioacetic acid (2 mL) was added, and the mixture was heated at 70 °C for 5 days. Excess thioacetic acid and solvent were removed under vacuum, and the dark green solid was washed with methanol and hexane and dried under vacuum overnight to give analytically pure product. 1H NMR (C D , 50 °C): δ 9.27 (s, 8H), 4.03 (t, b, 16H), 2.91 (t, 6 6 16H, -CH2SAc), 1.98 (s, 24H, -SCOCH3), 1.76 (m, 16H), 1.60 (m, 16H), 1.52 (m, 16H), -0.86 (s, 6H, axial methoxide). IR (thin film): 1689 (vs, CdO), 1607 (w), 1466 (s), 1430 (s), 1394 (s), 1363 (m), 1283 (s), 1208 (m), 1104 (s), 750 (m), 668 (m), 626 (m) cm-1. MS (electrospray) (m/e): calcd for C90H118O18N8SiS8, 1884.5; found, 1883.8 (M+), 1869.3 (M+ - CH3), 1853.0 (M+ - OCH3), 1776.2 (M+ - OCH3 - CH3COSH). Elemental analysis calcd: C, (11) Boeckl, M. S.; Bramblett, A. L.; Hauch, K. D.; Sasaki, T.; Ratner, B. D.; Rogers, J. W., Jr. Langmuir 2000, 16, 5644-5653. (12) (a) Li, Z.; Lieberman, M. Inorg. Chem. 2001, 40, 932-939. (b) Li, Z.; Lieberman, M. In Fundamental and Applied Aspects of Chemically Modified Surfaces; Blitz, J. P., Little, C. B., Eds.; Royal Society of Chemistry: Letchworth, U.K., 1999; pp 24-35. (c) Li, Z.; Lieberman, M. 1st International Conference on Porphyrins and Phthalocyanines, Dijon, 2000. (13) Li, Z.; Lieberman, M. Supramol. Sci. 1998, 5, 485-489.
Li et al. 57.36; H, 6.31; N, 5.94; S, 13.61. Found: C, 57.12; H, 6.07; N, 5.65; S, 13.73. UV-vis λmax, nm (): 691 (1.7 × 105 M-1 cm-1). (ii) (OCH3)2SiPc(OCH2CH2CH2CH2CH2SH)8 (2b). Twentyfour milligrams of 2a and 10 mL of degassed 50% HCl/MeOH were heated at 70 °C for 1 week under Ar; the dark green product was isolated by filtration, washed with methanol, and dried under vacuum. However, this product had very low solubility in most organic solvents, so further purification and characterization were limited. IR (Nujol): 1604 (m), 1482 (sh), 1463 (vs), 1432 (sh), 1378 (s), 1283 (s), 1209 (m), 1098 (s), 1055 (m), 1005 (w), 899 (w), 858 (w), 820 (w), 750 (m) cm-1. MS (electrospray) (m/e): (M+ - OCH3) calcd for C74H102O10N8SiS8, 1548.3; found, 1516.3. Preparation of Atomically Flat Gold Substrates. A total of 500 Å of gold was thermally evaporated onto clean mica sheets. After being deposited, the gold surfaces were annealed by passing a 4 cm long hydrogen flame across the gold about 50 times. Before being annealed, the gold was a mosaic of ∼60 nm crystallites with a range of crystal orientation; after being annealed, the gold surfaces showed atomically flat terraces >10 000 nm2 in area in the scanning tunneling microscope, and low angle XRD showed a strong [111] orientation. Preparation of Rough Gold Substrates. Surface-enhanced Raman scattering (SERS) substrates were prepared by spincoating clean glass slides with 150 µL of a 5% aqueous suspension of 0.3 µm agglomerate-free alumina particles according to a method described by Bello et al.14 A chromium layer with a thickness of 2 nm was evaporated in a vacuum onto the substrates to improve the adhesion between the glass and the gold. A gold layer with a thickness of 80 nm was evaporated at a rate of 0.1 nm/s immediately after the chromium was deposited. Procedure for Self-Assembly on Gold Substrates. Gold substrates (either flat or rough) were immersed in approximately 0.5 mM solutions of the desired phthalocyanine in toluene for 1-7 days, rinsed with fresh toluene or methylene chloride, and blown dry in a stream of argon. Because 2b had very poor solubility in toluene, a mixture of methanol/dichloromethane (1:1) was used for the deposition and the rinse. Ellipsometry. A Rudolph Research/AutoEL III ellipsometer was used to measure the thickness of SAMs on gold surfaces. The measurement was performed using a 632.8 nm He/Ne laser beam incident upon the sample at 70°. The optical parameters (ns, ks) of each batch of freshly annealed gold substrates were measured before the formation of SAMs. At least 10 different locations on each sample were measured to obtain average values. A refractive index of 1.50 was assumed for the phthalocyanine SAMs. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectra were obtained using a Kratos Analytical ESCA system with monochromatic Mg KR radiation at 1253.6 eV. The takeoff angle was fixed at 90°. Monolayer and powder samples were mounted on sample stubs with conductive carbon tape. Significant charging effects (up to 10 eV shifts in BE) were observed, so the binding energies for each peak were referenced to the Au 4f 7/2 peak at 84.0 eV. In addition to a survey scan, each element of interest was studied in a 200 s high-resolution region scan. A linear baseline was applied, but the spectra were not smoothed. The area of each peak was measured by fitting the individual region scans with one or more 60% Gaussian 40%Lorentzian peaks using the Kratos Vision II software. For elements that display spin multiplets (e.g., Si, S, and Au), the areas and peak-to-peak separation of the fit multiplets were constrained according to standard values.15 Each S 2p peak was fit as two linked components, 2p3/2 and 2p1/2. The peak area of the later component was set to be half of the former one, and the peak separation of these two components was set to be 1.2 eV.15 When the peak position of a sulfur 2p doublet is reported in this paper, it is the position of the 2p3/2 component. SERS and Raman Spectroscopy. Spectra were recorded with a FT-Raman spectrometer (Perkin-Elmer, System 2000R) in backscattering geometry. The instrument consists of the (14) Bello, J. M.; Stokes, D. L.; Vo-Dinh, T. Appl. Spectrosc. 1989, 43, 1325. (15) Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer: Eden Prairie, MN, 1992. (16) Reference deleted at the galley stage.
Umbrella vs Octopus Design Strategies for SAMs following main components: a diode-pumped Nd:YAG laser with an emission wavelength of 1.064 µm, a filter for cleaning up the laser emission, a sample compartment with focusing and collecting optics, a set of three holographic notch filters for the removal of elastically scattered laser light, an optical module with gold-coated optics, an aperture stop, a Jacquinot stop, a quartz beam splitter, a Michelson interferometer, and an InGaAs detector. In the present study, the laser power at the sample was about 25 mW. Spectra were recorded accumulating 500 scans in the Stokes-shifted region from 100 to 3500 cm-1 with a resolution of 8 cm-1. The detector was operated at room temperature. SERS substrates were placed behind the 2.5 mm diameter opening of the sample holder for solids of the instrument accessory kit. Raman reference spectra were taken from 5 mg of powder of each substance pressed into a thin layer on the bottom of a 4 mm diameter metal powder holder. Heating of the samples was minimized by the use of low laser powers and the thermal contact of the thin powder layers with the metal.
Results and Discussion Synthesis and SAM Deposition. The synthesis of (CH3)(HSCH2CH2O)SiPc(OC5H11)8 (1) was achieved by axial ligand exchange between methylisopropoxySiPc(OC5H11)8 and mercaptoethanol; the sole product is the O-bound mercaptoethanol adduct.12a Octasubstituted silicon phthalocyanine 2a was made by radical thioacetylation of octapentenyloxySiPc(OCH3)2.13 This precursor was synthesized via template cyclization of dipentenyloxydiiminoisoindoline after the method of Nolte et al.17 The AIBN-catalyzed addition of thioacetic acid to the eight vinyl groups was followed to completion by mass spectroscopy, NMR, and IR. Octopus 2a was soluble in most organic solvents and could be purified by column chromatography on silica gel. Deprotection of the eight thioacetate groups in 2a was carried out in a degassed HCl/MeOH solution under Ar. Deprotection was monitored by the disappearance of the thioacetate IR absorbance at 1690 cm-1. The dark green product, 2b, had very poor solubility in most organic solvents. Only a mixture of dichloromethane and methanol could dissolve it with sufficient concentration for the formation of SAMs. This low solubility of 2b might be due to the high polarity of the eight thiol groups on the phthalocyanine ring. Because the thioacetate group appears to be fully capable of SAM formation on gold21 and because 2a had much better solubility than 2b, 2a was used as the primary octopus phthalocyanine in this study. Ellipsometric Thickness. The gold substrates gave significantly different refractive indexes (ns) and extinction coeffecients (ks) before and after hydrogen flame annealing. The values changed from ns ) 0.108 and ks ) 3.54 immediately after annealing to ns ) 0.103 and ks ) 3.45 after several hours of storage in ambient conditions. This could be interpreted as the gold adsorbing contaminants from the laboratory atmosphere. A similar effect was found by Troughton et al., who observed a contact angle change from 60 to 90° after storing gold substrates in air for only a few hours.18 Hence, for all our SAM fabrications, freshly (17) Nostrum, C. F. V.; Picken, S. J.; Schouten, A. J.; Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 9957-9965. (18) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365-385. (19) Ulman, A.; Elman, J. F. In Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heinemann: Boston, 1995; p 223. (20) Collins, R. W.; Allara, D. L; Kim, Y.-T.; Lu, Y.; Shi, J. In Characterization of Organic Thin Films; Ulman, A., Ed.; ButterworthHeinemann: Boston, 1995; Chapter 3. (21) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529-9534.
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annealed gold substrates were immediately immersed in phthalocyanine solutions for monolayer assembly. For ellipsometric measurements of SAMs on such gold substrates, the parameters for freshly annealed gold (ns ) 0.108 and ks ) 3.54) and a refractive index of 1.5 for the organic film were used in a single-layer fitting model. On the basis of these parameters, the thickness of dodecanethiol and octadecanethiol SAMs on gold were calculated to be 17 ( 2 and 24 ( 3 Å, respectively. The theoretical thickness of these standard SAMs, according to Ulman et al.,19 should be 13.2 Å for dodecanethiol and 19.8 Å for octadecanethiol SAMs on gold. We attribute the 4 Å discrepancy between observed and theoretical thicknesses to the S/Au interface, which is poorly modeled by a single-layer fitting program. The ellipsometric thicknesses measured by Allara et al. for a series of thiols with different chain lengths on gold extrapolated to give a 2.6 Å interfacial thickness,20 which is not so different from what we observe. Thus, we have corrected the thicknesses measured by our ellipsometer by subtracting a 4 Å interfacial thickness. The thickness of SAMs of 2a and 1 were measured to be 26 ( 5 and 15 ( 3 Å, respectively. On the basis of the above calibration, the actual thickness of the SAMs of 2a and 1 are 22 ( 5 and 11 ( 3 Å, respectively. XPS. X-ray photoelectron spectroscopy was used to detect the appearance on the gold surface of elements found in the thiol-modified phthalocyanines and also to investigate how the thiols interacted with the gold surface. Carbon, nitrogen, oxygen, silicon, sulfur, and gold were observed in all the XPS spectra for umbrella and octopus powder and SAM samples. Because oxygen and carbon are almost always found on putatively bare surfaces due to contaminants (e.g., “adventitious carbon”), the appearance of silicon and sulfur in the SAMs gives the clearest evidence for the attachment of phthalocyanine molecules on gold surfaces. Silicon 2p peaks with peak widths of 2.5-3.5 eV were observed at binding energies of 102.2102.9 eV in SAMs of the octopus and the umbrella phthalocyanines; this range is similar to the binding energies observed for silicon in other silicon(IV) compounds, such as SiO2. Silicon 2s peaks, also unusually broad, were also seen at binding energies appropriate for silicon(IV) at 153-154 eV. The integrated intensities for the silicon signals were larger than expected when compared to the sulfur signals in both powder samples and SAMs. When the relative sensitivity factors (0.371 for Si 2p, 0.324 for Si 2s, and 0.723 for S 2p) were taken into account, the apparent silicon:sulfur atomic ratio was 3.4 for a powder sample of the umbrella Pc (theoretical ) 1.0), 1.9 for the umbrella SAM (theoretical ) 1.0), and 0.5 for the octopus SAM (theoretical ) 0.125). However, the low signal strength of the silicon peaks makes them relatively unreliable quantitative reporters, and the main conclusion we draw is that the silicon(IV) phthalocyanines 1 and 2a are deposited on the gold surface. Figure 1 shows the S 2p region of the octopus and the umbrella silicon phthalocyanines 2a and 1 on flat gold substrates after soaking times of 1 day and 1 week. As shown in Figure 1, SAMs of the octopus phthalocyanine 2a gave two sets of S 2p peaks, one centered at 161.7 eV and another at 163.4 eV. The peak at 161.7 eV is assigned as sulfur bound to gold, while the peak at 163.4 eV could indicate unbound RSH, RSSR, or RSAc groups on the surface.6 Tour et al. have reported that thiols protected as thioacetate form SAMs on gold just as disulfides and thiols themselves do.21 In the case of the octothioacetate 2a, the thioacetates might be sterically blocked from goldinduced deprotection. This suspicion led us to carry out
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Figure 1. XPS of sulfur 2p region of SAMs of octopus (2a) and umbrella (1) on gold after 1 (top panels) and 7 (bottom panels) days of soaking. The black line shows raw (unsmoothed) data; the colored lines show fits to sulfur doublets; and the magenta line shows total curve fit. Table 1. Peak Positions (and Raw Peak Areas) of S 2p and Au 4f in SAMs of 2a, 2b, and 1 S 2p and Au 4f Multiplet Peak BE (eV) and Raw Peak Area samples
1 day
1 week
2a
S 2p: 161.5(1.3 × 103), 162.7(6.7 × 102) 163.4(1.1 × 103), 164.6(5.7 × 102) Au 4f: 84.0(1.8 × 105), 87.7(1.5 × 105) S 2p: 161.7(9.3 × 102), 162.9(4.6 × 102) 163.4(7.4 × 102), 164.6(3.7 × 102) Au 4f: 84.0(1.5 × 105), 87.6(1.2 × 105) S 2p: 162.1(1.4 × 103), 163.3(7.2 × 102) Au 4f: 84.0(2.0 × 105), 87.6(1.6 × 105)
161.7(1.3 × 103), 162.9(6.4 × 102) 163.5(1.3 × 103), 164.7(6.6 × 102) 84.0(1.5 × 105), 87.7(1.2 × 105) 161.7(8.3 × 102), 162.9(4.2 × 102) 163.5(1.4 × 103), 164.7(6.8 × 102) 84.0(1.3 × 105), 87.7(1.0 × 105) 162.2(1.1 × 103), 163.4(5.4 × 102) 84.0(1.4 × 105), 87.7(1.1 × 105)
2b 1
a solution deprotection reaction to form octathiol phthalocyanine 2b. However, the XPS spectra from SAMs of 2b on gold were nearly identical to SAMs of 2a (see Table 1 and the following discussion). These unbound sulfur “arms” appear to be a consequence of the octopus strategy rather than a lack of reactivity of the thioacetate on the individual arms of the octopus molecule. In contrast, SAMs derived from the umbrella phthalocyanine 1 gave only one set of S 2p doublets at 162.2 eV, which indicates that all the sulfur atoms are bound to the gold surface. No oxidation of sulfur (SdO should give a peak at 168 eV)22 was observed in any of our phthalocyanine SAMs, which suggests that the molecules studied are stable during formation and characterization.23 The surface coverage of each phthalocyanine was estimated by comparing the limiting values for the sulfur to gold XPS raw peak area ratios with those seen in a SAM with known coverage. The S 2p(3/2 + 1/2)/Au 4f(7/2 + 5/2) XPS raw peak area ratio was found to be 0.011 for the SAMs of 2a, 0.0089 for 2b, and 0.0061 for (22) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. K.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700-2709. (23) We did see some oxidation of sulfur when SAM samples were exposed to the ambient atmosphere for several weeks before they were subjected to XPS measurements.
1 after 1 day of soaking. These ratios climbed asymptotically to 0.014 for 2a, 0.014 for 2b, and 0.0065 for 1 after 1 week of soaking. Hutt and Leggett24 studied a closepacked monolayer of butanethiol on gold using LEED and XPS and found the raw peak ratio of S/Au was 0.084. The surface coverage of the (x3 × x3)R30° butanethiol on gold is 7.6 × 10-10 mol/cm2,25 which corresponds to one sulfur atom per three gold atoms or one sulfur per 22 Å2. Because the length of butanethiol is about the same as the mercaptoethanol tether on the umbrella SiPc molecule 1, the XPS takeoff angles are similar, and 1 has only one S atom bound to the gold surface, the raw peak ratios can be compared to estimate the surface coverage of 1. Our observed S/Au ratio of 0.0065 corresponds to a sulfur surface coverage of 7.7% of the butanethiol SAM or one sulfur per 284 Å2. Both a CPK model of 1 and SPARTAN predict a molecular diameter of 21 Å, as shown in Scheme 1. The projected area of 1 (modeled as a circle with radius 10.5 Å) is 346 Å2, which is consistent with the XPS data if one considers the possibility of interweaving the alkyl chains between adjacent Pc rings on the gold surface. Thus, (24) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 3055-3058. (25) (a) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-58. (b) Chidsey, C. E. D.; Liu, G. Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421-4423.
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Scheme 1. Space-Filling Models and Structures of Representative Umbrella and Octopus Silicon Phthalocyanines 1 and 2a
Scheme 2. Possible Orientation of Umbrella 1 (a) and Octopus 2a and 2b (b) Silicon Phthalocyanines on Gold Surfaces
the surface atomic concentrations in a SAM of 1 suggest that the ring is oriented parallel to the gold surface as illustrated in Scheme 2a. The calculated height of the umbrella molecule 1 (from the sulfur to the axial methyl group) is 7.2 Å. Ellipsometric measurements of the SAMs of 1 gave a thickness of 11 ( 3 Å, which is consistent with a parallel ring orientation considering that the eight side chains might project up away from the surface. The surface atomic concentrations are more difficult to interpret in the case of the octopus phthalocyanines 2a and 2b. In the first place, the total S/Au ratios are unexpectedly low, considering that each Pc ring contains eight sulfur atoms instead of the single S atom in molecule 1. The limiting coverage of 2a and 2b (after 1 week of soaking) thus appears to be submonolayer. Since ellipsometric measurements indicated the thickness of the 2a film was 22 ( 5 Å, a correction of the XPS peak intensity was done to account for the absorption of the XPS electrons from the Au interface region by the overlying phthalocyanine film. The signal intensity from sulfur atoms at the gold interface was corrected based on26
Id ) e-d/λ Io
(1)
where Id is the photoelectron intensity from the layer of interest covered by an overlying film, d is the thickness of the film, Io is the intensity that would be observed with no overlying film, and λ is the escape depth of electrons of a given energy through the overlying film. According to Penn and co-workers, the escape depth of 1100 eV electrons (corresponding roughly to sulfur 2p electrons with 160 eV of binding energy) through a variety of organic compounds is ∼30 Å.27 So, for the bound sulfurs that are buried about 22 Å deep inside the film, the observed intensity will experience a 50% drop based on eq 1. For the octopus 2a SAMs formed on gold after 1 week of soaking, based on the fact that the measured intensity for bound sulfurs is roughly the same as that for free sulfurs, the S/Au intensity ratio of 0.014 should be corrected to be 0.021 according to the above discussion. This corresponds to a sulfur surface coverage of 25% of that of the butanethiol SAM mentioned before. Because there are eight sulfur atoms per Pc molecule, this corresponds to 704 Å2 per molecule. From molecular modeling (Scheme 1), the diameter of an octopus molecule is 28 Å. This would give a projected area of 615 Å2 with all the peripheral substituents fully extended (modeled as a circle with radius of 14 Å). The coverage of octopus phthalocyanine molecules on gold is thus 0.9 of that expected for a flat monolayer.28 (26) Ghosh, P. K. Introduction to Photoelectron Spectroscopy; John Wiley & Sons: New York, 1983. (27) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165-176.
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Figure 2. SERS spectra of umbrella 1 (a) and its normal Raman (b) in the region of 200-1800 cm-1.
For every SAM derived from octopus phthalocyanines 2a and 2b, both bound and unbound sulfur atoms coexist on the gold surface. Either the Pc rings in the SAMs of 2a and 2b are not actually parallel to the gold surface or the Pc rings are parallel to the gold but several of the sulfur arms are somehow constrained to point away from the gold surface (Scheme 2). Ellipsometric measurements of the thickness of these films gave values of 22 ( 5 Å, consistent with the former explanation but not the latter. Phthalocyanines with long alkyl side chains (C8-C18) are known to form columnar liquid crystalline arrays,29 and it is possible that a ring-stacking interaction could favor edge-on binding in these SAMs. However, edge-on binding would be expected to give much higher surface coverages of the phthalocyanines than observed. SERS. SERS was carried out to get an independent estimation of the orientation of Pc rings in the monolayers. Both Raman spectra of powder samples and SERS of the octopus SiPc 2a and umbrella SiPc 1 on rough gold substrates were measured. Attempts to measure SERS and Raman spectra of the phthalocyanines under investigation with visible excitation showed strong fluorescence. Apparently, the excitation wavelengths of 702 and 785 nm were sufficient to excite the fluorescence within the long wavelength tail of the Q-band of the phthalocyanines. In solution, a strong fluorescence at 715 nm was observed at an excitation wavelength of 660 nm for octopus 2a in dichloromethane. Fourier transform measurements with excitation at 1064 nm gave high quality Raman and surface-enhanced Raman spectra with little fluorescence background, as shown in Figures 2-4. Raman and SERS band positions are listed in Table 2. The main Raman bands have been tentatively assigned to the given vibration modes by comparison with literature data for silicon30 and other metal31-34 phthalocyanines. The Raman spectra of both phthalocyanines are dominated by the strong in-plane stretching and breathing (28) The limiting coverage for randomly dropping disks onto a plane is 53%, but if the disks can distort to allow 10% overlap at their periphery, it is possible to achieve packing densities of up to 96%. Meineke, M.; Gezelter, J. D. J. Phys. Chem. B, in press. (29) Cook, M. J.; Daniel, M. F.; Harrison, K. J.; Mckewon, N. B.; Thomson, A. J. J. Chem. Soc., Chem. Commun. 1987, 1086-1088. (30) Wijekoon, W. M. K. P.; Karna, S. P. J. Raman Spectrosc. 1994, 25, 949-952.
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Figure 3. SERS spectra of octopus 2a (a) and its normal Raman (b) in the region of 200-1800 cm-1.
Figure 4. SERS spectra of umbrella 1 (a) and octopus 2a (b) in the region of 2700-3200 cm-1.
modes of the macrocycle, characteristic bands for phthalocyanine Raman spectra.35 The intense band at 1536 cm-1 in 1 and 1555 cm-1 in 2a is assigned to the symmetric macrocycle in-plane stretching mode ν3. The band at 1332 cm-1 for both 1 and 2a is assigned to the isoindole stretching mode ν4. The strong band at 686 cm-1 in 1 and 689 cm-1 in 2a is assigned to an in-plane macrocycle breathing mode ν7, although for simple alkylthiols, this could also be assigned as a C-S stretch.36 The peak positions and intensities of the Raman spectra of 1 and 2a are very similar, as expected for two compounds with such similar structures. (31) (a) Jennings, C. A.; Aroca, R.; Kovacs, G. J.; Hsaio, C. J. Raman Spectrosc. 1996, 27, 867-872. (b) Jennings, C.; Aroca, R.; Hor, A.; Loutfy, R. O. J. Raman Spectrosc. 1984, 15, 34-37. (c) Aroca, R.; Dilella, D. P.; Loutfy, R. O. J. Phys. Chem. Solids 1982, 43, 707-711. (32) (a) Bovill, A. J.; McConnell, A. A.; Nimmo, J. A.; Smith, W. E. J. Phys. Chem. 1986, 90, 569-575. (b) McConnell, A. A.; Nimmo, J. A.; Smith, W. E. J. Raman Spectrosc. 1989, 20, 375-380. (33) Souto, J.; Gorbunova, Y.; Rodriguez-Mendez, M. L.; Kudrevich, S.; van Lier, J. E.; de Saja, J. A. J. Raman Spectrosc. 1996, 27, 649-655. (34) Abe, M.; Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978, 69, 4526-4534. (35) Menendez, J. R.; Martin, F. J. Raman Spectrosc. 1995, 26, 381385. (36) (a) Ambello, A.; Genet, F.; Nigretto, J. M.; Lucazeau, G. Surf. Sci. 1989, 15, 158. (b) Lee, H.; Kim, M. S.; Suh, S. W. J. Raman Spectrosc. 1991, 22, 91-96.
Umbrella vs Octopus Design Strategies for SAMs Table 2. Peak Positions (cm-1) in the Raman and SERS Spectra of the Investigated Phthalocyaninesa SiPc 2a (octopus) Raman
SERS
268 w 584 w 689 s 763 s 999 1150 1147 1173 1174 w 1279 1282 1332 s 1335 s 1397 1393 1443 1443 1489 1486 1555 vs 1552 vs 582 w 689 s 761 s
SiPc 1 (umbrella) Raman
SERS
686 s 757 s
288 w 579 690 753 1001 1153
1146 s 1178 w 1282 1332 s 1393 1432 1489 1536 vs
2919 3046 w a
1283 1351 1400 1456
assignment Au-S stretch ν7 macrocycle breathing ν15 macrocycle stretch pyrrole ring breathing ν4 pyrrole in-plane stretch ν28 isoindole stretch
1549
ν3 macrocycle in-plane stretch 2919 CH2 stretch 3046 vw aromatic ring C-H stretch
Key: w, weak; s, strong; vs, very strong. Table 3. Band Intensity Ratios of Phthalocyanine In-Plane Modes to Au-S Stretch in 1 and 2a umbrella 1 I1549/I288 ) 0.8 I1351/I288 ) 0.9 I753/I288 ) 1.5 I690/I288 ) 1.5
octopus 2a I1551/I268 ) 6.5 I1332/I268 ) 2.5 I763/I268 ) 2.9 I689/I268 ) 3.1
The surface-enhanced Raman spectra for SAMs of 1 and 2a show patterns of changes in peak intensities, which show that these two structurally similar molecules adopt very different molecular orientations on the gold surface. According to the surface selection rules of SERS,37,38 only those vibrations that have scattering tensor components perpendicular to the surface will be strongly enhanced. Creighton, Moskovits, and others have used the presence of the aromatic ring C-H stretching band at around 3048 cm-1 as an indication of a vertical or at least a tilted orientation of a benzyl ring in SAMs on gold surfaces.39 In Figure 4, the C-H vibration regions (2700-3200 cm-1) of SAMs of the umbrella 1 and the octopus 2a are compared. The band at 3046 cm-1 is the aromatic C-H stretch of the phthalocyanine ring, while the band at 2919 cm-1 is the C-H stretch in methylene groups of the peripheral substituents. The ratio of band intensities at 3046 vs 2919 cm-1 is found to be 0.6 and 0.3 in 2a and 1, respectively (a linear baseline was used in this region for calculating the ratios). Considering that there are the same number of methylene units in both phthalocyanines, the band at 3046 cm-1 is significantly more intense in 2a. This is indicative of a more edge-on or tilted orientation of octopus phthalocyanine ring 2a on the gold surface, as suggested in Scheme 2b. This conclusion is further supported by the enhancement of characteristic in-plane vibration modes of the Pc ring in SAMs of 2a. To make this comparison, one needs a band that is out-of-plane and common to both samples. The sulfur-gold stretching vibration, which has a large component perpendicular to the surface, can serve as an out-of-plane reference band. Carron40 and Garrell41 ob(37) Creighton, J. A. In Advances in Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley & Sons: New York, 1988; Vol. 16, p 37. (38) Kudelski, A.; Hill, W. Langmuir 1999, 15, 3162-3168. (39) (a) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 1999, 103, 10831-10837. (b) Moskovits, M.; Suh, J. S. J. Am. Chem. Soc. 1986, 108, 4711. (c) Creighton, J. A. Surf. Sci. 1983, 124, 209. (40) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 99799984.
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served this vibration as a broad peak at around 275 cm-1 in thiolphenol SAMs. In our spectra, the broad bands at 268 cm-1 for 2a and 288 cm-1 for 1 were assigned as Au-S stretches. The ratios of bands from in-plane Pc stretches at 1551, 1332, 763, and 689 cm-1 vs these out-of-plane Au-S stretches are listed in Table 3. These ratios were calculated directly from the peak heights using a linear baseline correction. For a phthalocyanine ring oriented face-on to the gold surface, the in-plane vibrations should not be enhanced. For a vertical or tilted orientation, the in-plane vibrations should be greatly enhanced. The ratio of the in-plane Pc stretch at 1549 cm-1 to the Au-S stretch is only 0.8 for 1, whereas in 2a, the same band ratio is eight times larger. The same trend is observed for the in-plane vibrations at 1335, 763, and 689 cm-1; the octopus phthalocyanine always shows enhancement of the in-plane vibrations when compared to the umbrella. These ratios were not corrected for the fact that there are roughly four times as many Au-S bonds per phthalocyanine unit for 2a as there are for 1. Hence, the difference between the ratios calculated in 2a and 1 could be even larger. This gives strong evidence for the proposed tilted orientation of 2a and the flat orientation of 1 in SAMs on gold (111). Conclusions Two strategies to control the phthalocyanine ring orientation in SAMs on gold surfaces have been tested in this study. The octopus silicon phthalocyanines 2a or 2b, which have eight thiol linkers in the periphery of the rings, do not bind all of their arms to the gold surface at the same time. Such coexistence of bound and unbound sulfur has been reported by several other groups for SAMs of molecules that contain multiple thiol linkers.42 In theory, it could result if the thiol groups attached to the molecule were rigidly held in an arrangement that was incommensurate with the underlying gold lattice or if there were just too many thiol groups constrained to bind in too small an area. Neither of these explanations applies to octopus phthalocyanine 2a. The thiols are at the end of a six atom long flexible chain, and the actual number of thiols bound to the gold surface in a monolayer of 2a is only about one-quarter of the number that can bind in a close-packed hydrocarbon SAM. The area accessible to the eight thiol arms is at least 300 Å2 due to the flexibility of the hydrocarbon chains, which corresponds to more than a dozen potential binding sites. Despite the apparent match between the multiple thiol-anchoring groups and the multiple surface binding sites, XPS, ellipsometry, and SERS suggest a strongly tilted orientation of the phthalocyanine rings on the gold surface. The umbrella silicon phthalocyanine 1, which has a short thiol tether in the axial position, can form wellordered and persistent monolayer films on gold surfaces. Both SERS and XPS results suggest that the rings are parallel to the surface. Further efforts to use this attachment method to make SAMs of silicon phthalocyanine dimers and larger oligomers are underway. Acknowledgment. We thank Dr. Yandong Li for assistance with the XPS experiments, Prof. E. E. Wolf for lending us his STM, and Prof. Olaf Wiest for assistance with molecular modeling. Prof. Dan Gezelter (ND) and (41) Szafranski, C. A.; Tanner, W.; Laibinis, P. E.; Garrell, R. L. Langmuir 1998, 14, 3570-3579. (42) (a) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277-3283. (b) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14, 816-820.
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Matthew Meineke worked out the “sticky disk” simulation. Z.L. is grateful for a Reilly Fellowship from the Department of Chemistry and Biochemistry at University of Notre Dame. This work was supported by the National Science Foundation through DMR 9875788 and by the
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DARPA Moletronics Program through Navy Grant ONR-00014-99-1-0472. LA010203G
