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Zirconium-Phosphonate Monolayers with Embedded Disulfide Bonds Xuejun Wang and Marya Lieberman* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received February 3, 2003. In Final Form: April 30, 2003 Disulfides are chemically and electrochemically reactive functional groups that could be useful for ultrahigh-resolution patterning of self-assembled monolayer films (SAMs) by atomic force microscopic (AFM) anodization. Zirconium-organophosphonate monolayers that contain embedded disulfide bonds were prepared by sequential deposition of Zr4+ and H2PO3-S-S-R components. Bis(10-phosphodecyl) disulfide and 10-phosphodecyldodecyl disulfide form monolayers on zirconated Si(100) and gold surfaces. X-ray photoelectron spectroscopy (XPS), AFM, contact angles, and ellipsometry were used to characterize these self-assembled films. The growth process is sluggish, and contact angle measurements suggest that the hydrocarbon chains in the film are disordered. The origin of these effects appears to be the preferred C-S-S-C dihedral angle of about 90°. Silicon substrates that have a thin native silicon oxide give a plasmon band in the XPS spectra, which can be mistaken for oxidized sulfur; we discuss a strategy for eliminating this interference. The disulfide groups embedded in these monolayers retain their chemical reactivity. On silicon substrates, dithiothreitol (DTT) readily cleaves the embedded disulfide bonds, leaving a thiol-terminated surface. However, on gold substrates, DTT also cleaves the Au-S bond and strips the monolayer from the gold. AFM anodization gave high-resolution patterning of the well-ordered methylterminated C12 monophosphonate, but the methyl-terminated 10-phosphonodecyl dodecyl disulfide SAMs on native silicon oxide were too disordered and the underlying silicon was oxidized too.
Introduction Organic thin films have received great interest during the past decade as they have potential applications in a range of fields including wetting,1 adhesion,2 electrochemistry,3 and optoelectronic and molecular electronic devices.4 Molecular self-assembly is a simple method for preparing organic films with controlled structure and composition and thus has become the preferred route for the preparation of organized molecular assemblies. The goal of fundamental studies of these organic thin films is to understand and control structures and chemical reactivity at the molecular level. Zirconium-phosphonate mono- and multilayers on solid substrates are one of the most widely studied class of selfassembled organic thin films.5-8 They have attracted wide interest because of their structural order and stability and because of their relative ease of preparation. Zirconium phosphonates can be adsorbed as strongly bound monolayer films on different substrates by a simple twostep procedure, consisting of dipping an appropriately functionalized substrate alternately into solutions of zirconium ion and of a long-chain alkyldiphosphoric acid. The self-limiting layer-by-layer growth mode makes it fairly easy to build different functional groups into Zr-P organic films, provided that relevent organic phosphonates * Corresponding author: e-mail
[email protected]; fax (574) 6316652; phone (574) 631-4665. (1) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (2) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 1230. (3) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986, 2, 239. (4) Ulman, A. Adv. Mater. 1990, 2, 573. (5) Lee, H.; Kepley L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (6) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (7) Katz, H. E. Chem. Mater. 1994, 6, 2227. (8) Katz, H. E.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe, S. B. J. Am. Chem. Soc. 1994, 116, 6631.
with specific functional groups are used. It is possible to use these films for different applications, such as nonlinear optical materials,7 molecular recognition,9 current rectification,10 and light-induced electron-hole pair separation.11 In this work, we incorporate disulfide bonds into zirconium-phosphonate films by sequential deposition of Zr4+ and H2PO3-S-S-R components. Our goal is to see if the embedded disulfide bonds have any effect on the structure of films and whether they react normally in the monolayer. Other researchers have also created embedded disulfide bonds on silicon surfaces using a thiol-disulfide exchange reaction with a thiol-terminated primer layer formed from (3-mercaptopropyl)triethoxysilane (MPTS).12 However, the yield of the thiol exchange reaction is not 100% and the MPTS is liable to form multilayers on the surface, so it is difficult to form a uniform monolayer with embedded disulfide bonds. It would be preferable to use orthogonal chemical steps to assemble the disulfide and to form the self-assembled monolayer (SAM). Sequential deposition of zirconium-phosphonate monolayers allow us to preform the disulfide and then assemble it into a SAM, so there are no free thiol groups on the surface and multilayer formation is not a problem. We are interested in using the embedded disulfide bonds as negative resists for ultra-high-resolution lithography. Disulfide bonds are relatively weak (bond energy ≈ 55 kcal/mol), and can be cleaved with dithiothreitol (DTT) in (9) Cao, G.; Garcia, M. E.; Alcala, M.; Burgess, L. F.; Mallouk, T. E. J. Am. Chem. Soc. 1992, 114, 7574. (10) Rong, D.; Hong, H.-G.; Kim, Y. I.; Kruger, J. E.; Mayer, J. E.; Mallouk, T. E. Coord. Chem. Rev. 1990, 97, 237. (11) Vermuelen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767. (12) (a) Yee, J. K.; Parry, D. B.; Caldwell, K. D.; Harris, J. M. Langmuir 1991, 7, 307-313. (b) Ledung, G.; Bergkvist, M.; Quist, A. P.; Gelius, U.; Carlsson, J.; Oscarsson, S. Langmuir 2001, 17, 6056.
10.1021/la030040r CCC: $25.00 © 2003 American Chemical Society Published on Web 08/08/2003
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Synthesis and Characterization of Mono- and Diphosphonate Precursors with Disulfide Bonds. Organophosphonates are generally synthesized as phosphonate esters via the Michaelis-Arbuzov reaction. The general synthetic approach for mono- and diphosphonates with embedded disulfide bonds is shown in Scheme 2. Bis(10-phosphonodecyl) disulfide (III) and 10-phosphonodecyl dodecyl disulfide (VI) were synthesized from the corresponding protected intermediates, bis[10-(diethoxyphosphinyl)decyl] disulfide (II) and 10-(diethoxyphosphinyl)decyl dodecyl disulfide (V). The phosphonate groups are deprotected with high yield (∼90%) by bromotrimethylsilane, which selectively cleaves P-O-R groups without affecting disulfide bonds. The protected intermediate II was synthesized by heating diethyl 10bromodecylphosphonate (I) and thiourea and then treating with basic water in air to form the disulfide. The yield of II was 41% from I. The protected intermediate V was synthesized by titrating diethyl (10-mercaptodecyl)phos-
phonate (IV) and excess dodecanethiol with I2. The yield of V was relatively low (about 10-20%) due to unselective disulfide formation. Both bis(10-phosphonodecyl) disulfide (III) and 10-phosphonodecyl dodecyl disulfide (VI) are soluble in methanol, ethanol, and basic water but not in acidic water. Methanol or ethanol were used as the solvents for monolayer growth. Priming of Substrates. Gold substrates were primed with (10-mercaptododecyl)phosphonate, which was formed through dissociation of bis(10-phosphonodecyl) disulfide (III). The measured ellipsometric thickness of the primer layer was 17.8 ( 1.5 Å, close to the expected 18 Å. The water contact angles changed from 75° on bare gold to below 20° on the phosphonate-primed SAM. The observed RMS roughness was 1 nm, about the same as the original gold surface. A high-resolution X-ray photoelectron spectroscopic (XPS) scan of the S2p region shows an asymmetric peak that was fitted as a spin doublet at 162.0 and 163.2 eV, corresponding to S2p3/2 and S2p1/2. The binding energy of these peaks indicates that the sulfur is bound to gold.16 The peak area ratio of S/Au is 0.066 ( 0.004; for comparison, the S/Au ratio of a well-ordered x3 × x3 (R30°) SAM of butanethiol on gold was 0.084.17 The low coverage (approximately 5.9 × 10-10 mol/cm2) is probably a consequence of the bulky, charged phosphonate groups. Silicon wafers were primed with an -NHPO(OH)2terminated monolayer by treating the clean SiO2 with (3-aminopropyl)methyldiethoxysilane and then POCl3. The ellipsometric thickness for this primer layer was 7.3 ( 1.3 Å and the measured water contact angle was 60°. The RMS roughness after priming was 1-2 Å, almost the same as that of the bare silicon wafer. Zirconium-Phosphonate Monolayers. Monolayer formation was carried out by sequential absorption of zirconium and bis(10-phosphodecyl) disulfide (III) onto the primed gold or silicon wafer substrates to form monolayers 3a, 3b, and 3c. For comparison purposes, a C10 diphosphonate (VIII), a methyl-terminated C12 monophosphonate (VII), and the methyl-terminated 10phosphonodecyl dodecyl disulfide (VI) were also deposited on the primed gold substrates to form monolayers 8a, 7a, and 6a. Although monolayer formation was reasonably fast for the simple alkyl mono- or diphosphonates, it took almost a week for the disulfide-containing mono- and diphosphonates to attain constant ellipsometric thickness and contact angle. The zirconium-diphosphonodecane monolayer 8a gave the expected values for ellipsometric thickness18 and advancing water contact angle.19 A high-resolution scan of the S2p region (Figure 1b) shows a broad peak that was fitted as a spin doublet at 162.1 and 163.3 eV, corresponding to S2p3/2 and S2p1/2. These signals originate from the sulfur atom in the priming layer. The measured S/Au ratio is similar to that found in the primer layer, showing that the primer is intact. The stoichiometric Zr/P ratio is 0.33, but the XPS ratio is expected to be about 0.26 because one of the phosphonate groups is at the surface of the monolayer and its signal will not be attenuated as much as that of the zirconium buried at the primer layer.20 Thus, the observed Zr/P ratio of 0.28 ( 0.02 suggests that the organophosphonate monolayer has monolayer coverage. Zirconium-dodecylphosphonate SAM 7a forms a wellstructured hydrophobic monolayer on gold, even though the primer layer is poorly packed. The ellipsometric
(13) Ichinose, N.; Sugimura, J.; Uchida, T.; Shimo, N. Chem. Lett. 1993, 11, 1961. (14) Mrksich, M. Cell. Mol. Life Sci. 1998, 54, 653. (15) Geyer, W.; Stadler, V.; Eck, W.; Go¨lzha¨user, A.; Sauer, M.; Weimann, Th.; Hinze, P. J. Vac. Sci. Technol. B 2001, 19, 6.
(16) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (17) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 3055. (18) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (19) Hoekstra, K. J.; Bein, T. Chem. Mater. 1996, 8, 1865.
Scheme 1. Formation of Zirconium-Phosphonate Monolayers That Contain Embedded Disulfide Bonds on Gold (top) and Thin or Thick Silicon Oxide (bottom)a
a (i) III/MeOH; (ii) Zr(acac)2/EtOH followed by III, VI, VII, or VIII; (iii) APS followed by POCl3/collidine; (iv) Zr(acac)2/ EtOH followed by III; (v) cleavage of disulfide with dithiothreitol/basic water.
solution or on a surface to form thiol groups.13 If the embedded disulfide bonds in the patterned region were oxidized by atomic force microscope (AFM) anodization, DTT could later be used to cleave the unoxidized disulfide bonds in the remaining region to form a thiol-terminated complementary pattern. The patterns of free thiol could then be selectively derivatized with chemical cross-linkers and hence with dyes, biomolecules, etc.,14 which would be very useful for molecular electronics, biosensors, and tissue engineering.15 Results and Discussion
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Scheme 2. Synthesis of Bis(10-phosphonodecyl) Disulfide (III) and 10-Phosphonodecyl Dodecyl Disulfide (VI)
thickness is 16.6 ( 0.9 Å, and the advancing and receding water contact angles are 110° and 96°, respectively. A high-resolution scan of the S2p region (Figure 1c) shows the expected signals from the sulfur in the primer layer: S2p3/2 and S2p1/2 are observed at 162.1 and 163.3 eV. The S/Au ratio is the same as it is in the primer layer. The Zr/P ratio of 0.44 ( 0.03 matches the stoichiometric Zr/P ratio of 0.50, as expected since the Zr and P atoms are at the same depth and their signals will be similarly attenuated by the organic overlayer. Introduction of a disulfide group in the diphosphonate causes several structural differences between the resulting zirconium-organophosphonate SAMs (3a and 6a) and
those that contain only simple hydrocarbon pillaring groups (7a and 8a). The ellipsometric thicknesses of the disulfide-containing monolayers are only 8-9 Å thicker than those formed by the C12 phosphonate, although both disulfides have at least a C10 chain above the disulfide site and a C12 chain below the disulfide. A thickness increase of 15 Å over the C12 phosphonate would be expected if the disulfide-containing chains acted like normal hydrocarbon chains. The ellipsometric thickness results indicate either that the disulfide monolayers have submonolayer coverage (enabling the chains to tilt more than normal) or that they contain more gauche-type kinks than a normal hydrocarbon chain. The Zr/P ratio for the 10-phosphonodecyl dodecyl disulfide monolayer 6a is identical to that found for the C12 phosphonate SAM 7a, which indicates that the coverage of the methyl-terminated 10-phosphonodecyl dodecyl disulfide monolayer is about the same as that of the methyl-terminated C12 phosphonate monolayer. In the case of the phosphonate-terminated zirconium-10-phosphonodecyl dodecyl disulfide monolayer 3a, the stoichiometric ratio of Zr/P is 0.33, but the zirconium and two of the phosphonates are buried deeply enough that their signals will be attenuated by a factor of 59%.21 The theoretical Zr/P ratio for a full monolayer is therefore 0.22; since the observed Zr/P ratio is 0.20, the coverage of the phosphonate disulfide overlayer is quite close to one monolayer. The advancing contact angle is about the same as that of the phosphonate-terminated diphosphonodecane monolayer 8a. To understand if the embedded disulfide bonds in the zirconium-phosphonate monolayers are oxidized during deposition, which would introduce kinks or bumps in the chain, zirconium-bis(10-phosphonodecyl) disulfide monolayers 3a were characterized with high-resolution XPS in the S2p region. As shown in Figure 1a, two types of S2p peaks are resolved. One is a spin doublet at 161.9 and 163.2 eV, which is consistent with the literature values for S atoms bound to gold in alkanethiol SAMs.16 The other one is a coupled doublet at 163.8 and 165.0 eV, corresponding to unbound sulfur. No oxidized sulfur peak (C-SOxH, BE 168 eV) was detected for this monolayer.
(20) (a) Signal intensity falls off as e-d/λ, where d is the film thickness and λ is the escape depth for electrons from the interface through the film material: Ghosh, P. K. Introduction to Photoelectron Spectroscopy; John Wiley and Sons: New York, 1983. (b) The escape depth for 1100 eV electrons through polyethylene is about 30 Å: Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165-176. For an 18 Å overlayer, about 55% of the electrons originating at the primer layer would penetrate.
(21) (a) Signal intensity falls off as e-d/λ, where d is the film thickness and λ is the escape depth for electrons from the interface through the film material: Ghosh, P. K. Introduction to Photoelectron Spectroscopy; John Wiley and Sons: New York, 1983. (b) The escape depth for 1100 eV electrons through polyethylene is about 30 Å: Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1993, 21, 165-176. For electrons originating from the primer layer, about 40% of the electrons would penetrate the 27 Å overlayer.
Figure 1. XPS spectra of three different zirconium-phosphonate monolayers on gold in the S2p region. The primer layer for all monolayers was Au-S(CH2)10PO(OH)2. Dotted lines show linear baseline and sum of fitted peaks. Data are unsmoothed. (a) Zirconium-bis(10-phosphonodecyl) disulfide monolayer 3a; (b) zirconium-PO3(CH2)10PO(OH)2 monolayer 8a; (c) zirconiumPO3(CH2)11CH3 monolayer 7a.
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Figure 2. High-resolution XPS spectra in the S2p region: (a) blank native silicon oxide, 10-12 Å thick, showing silicon plasmon band at 168 eV; (b) zirconium-bis(10-phosphonodecyl) disulfide overlayer on native silicon oxide; (c) 500 nm silicon oxide free of plasmon band; (d) zirconium-bis(10-phosphonodecyl) disulfide overlayer on thick silicon oxide.
The ratio of unbound sulfur to bound sulfur is 2.8:1, which is slightly higher than the stoichiometric 2:1 ratio for disulfide atoms to gold-bound thiolate, but this could be due to the greater attenuation of the gold-bound thiolate in the primer layer. The structures of the Zr-bis(10-phosphonodecyl) disulfide monolayers formed on silicon oxide substrates (3b, 3c) are quite similar to the monolayers formed on gold substrates (3a): the ellipsometric thickness is 28.0 ( 0.7 Å on silicon and the advancing contact angle is 70° ( 3°. The surface topography gives an RMS roughness of 3.0 Å (measured over 1 square micrometer). However, the Zr/P ratio of 0.30 works out to a submonolayer coverage of 65%.22 Silicon Bulk Plasmon Band Interference. Initial studies of the disulfide SAMs on silicon substrates were carried out on silicon wafers that had a thin native oxide; these surfaces are extremely smooth (RMS roughness
