8764
Langmuir 2006, 22, 8764-8769
Wetting Behavior of Porous Silicon Surfaces Functionalized with a Fulleropyrrolidine Davide Dattilo,† Lidia Armelao,‡ Michele Maggini,*,† Giovanni Fois,§ and Giampaolo Mistura*,§ Dipartimento di Scienze Chimiche and ISTM-CNR, UniVersita` di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy and Dipartimento di Fisica, UniVersita` di PadoVa, Via Marzolo 8, 35131 PadoVa, Italy ReceiVed March 28, 2006. In Final Form: July 26, 2006 We report the immobilization of a fulleropyrrolidine, bearing a dec-9-ynyl functionality, on silicon surfaces through a thermal hydrosilylation protocol. Contact angle measurements on porous silicon (PS) surfaces reveal an unusual dependence of the angle with the PS roughness that apparently contradicts Wenzel’s formula. This result has been explained by an extension of Wenzel’s model in which the critical angle, which discriminates between the hydrophilic/ hydrophobic character of a solid material, is substantially reduced below 90° by surface roughness.
Introduction Silicon surface chemistry is of fundamental importance because of the very relevant role of silicon in modern technology.1-3 Over the past decade, several methods have been developed to place layers of organic compounds on both crystalline and porous, hydrogen-terminated, silicon through formation of Si-C or Si-O bonds.4-6 Thermal hydrosilylation,7-9 UV irradiation,10,11 electrochemistry,12-14 Lewis acid- and metal-mediated hydrosilylation,15-18 and chemomechanical scribing19-21 have been * To whom correspondence should be addressed. M.M.: phone, +39049-8275662; fax, +39-049-8275792; e-mail:
[email protected]. G.M.: phone, +39-049-8277020; fax, +39-049-8277003; e-mail,
[email protected]. † Dipartimento di Scienze Chimiche. ‡ ISTM-CNR, Dipartimento di Scienze Chimiche. § Dipartimento di Fisica. (1) Waltenburg, H.; Yates, J. T. Chem. ReV. 1995, 95, 1589. (2) Turton, R. The Quantum Dot; Oxford University Press: New York, 1995. (3) Linford, M. R.; Fenter, P. J. Am. Chem. Soc. 1995, 117, 3145. (4) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 2, 23. (5) Buriak, J. M. Chem. ReV. 2002, 102, 1271. (6) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudholter, E. J. R. AdV. Mater. 2000, 12, 1457. (7) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1998, 14, 1759. (8) Sieval, A. B.; van den Hout, B.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2001, 17, 2172. (9) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2000, 16, 10359. (10) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (11) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. Engl. 1998, 37, 2462. (12) De Villeneuve, C. H.; Pinson, J.; Bernard, M. C.; Allongue, P. J. J. Phys. Chem. B 1997, 101, 2415. (13) Gurtner, C. W.; Wun, A. W.; Sailor, M. J. Angew. Chem., Int. Ed. 1999, 38, 1966. (14) Robins, E. G.; Stewart, M. P.; Buriak, J. M. Chem. Commun. 1999, 2479. (15) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 5. (16) Buriak, J. M.; Allen, M. J. J. Am. Chem. Soc. 1998, 120, 1339. (17) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491. (18) Holland, J. M.; Stewart, M. P.; Allen, M. J.; Buriak, J. M. J. Solid State Chem. 1999, 147, 251. (19) Niederhauser, T. L.; Jiang, G.; Lua, Y.-Y.; Dorff, M. J.; Woolley, A. T.; Asplund, M. C.; Bergers, D. A.; Linford, M. R. Langmuir 2001, 17, 5889. (20) Niederhauser, T. L.; Lua, Y.-Y.; Jiang, G.; Davis, S. D.; Matheson, R.; Hess, D. A.; Mowat, I. A.; Linford, M. R. Angew. Chem., Int. Ed. 2002, 41, 2353. (21) Niederhauser, T. L.; Lua, Y.-Y.; Sun, Y.; Jiang, G.; Strossman, G. S.; Pianetta, P.; Linford, M. R. Chem. Mater. 2002, 14, 27.
used to covalently link organic molecular structures to silicon surfaces with the long-term objectives of practical applications as diverse as chemical sensors,22,23 optoelectronic devices,24,25 photodetectors,26 and matrixes for photopumped tunable lasers.27 Furthermore, the convergence between surface science and organic chemistry represents a unique opportunity to enhance our general understanding of silicon surface reactivity and behavior toward the development of nanometer-scale materials and devices that are expected to shape future technologies. Some accounts, published in the past few years, contain excellent sections devoted to the organic modification of silicon surfaces.22,28-31 Porous silicon (PS) is a material that can be produced by anisotropic electrochemical32 or chemical33 corrosion of crystalline silicon by hydrofluoric acid. The resulting large surface area, populated with silicon hydrides (Si-Hx, x ) 1-3), Si-Si bonds, and defects,34 flourished as an attractive testing ground for organic functionalization because analysis and characterization could be relatively straightforward with conventional Fourier transform infrared or diffuse reflectance infrared spectroscopies (FTIR, DRIFT). In 1990 Canham and co-workers made the important discovery that nanocrystalline PS emits under exposure to UV light.35 As a result, modification and characterization of PS surfaces received further impulse since a decrease in emission intensity could be related to the presence of adsorbed or covalently linked species.23 This is particularly important for chemical and biological sensing, where PS could function as a direct transducer, (22) Lin, V. S. Y.; Motesharei, K.; Dancil, K. P. S.; Sailor, M. J.; Ghadiri, M. R. Science 1997, 278, 840. (23) Mulloni, V.; Pavesi, L. Appl. Phys. Lett. 2000, 76, 2523. (24) Hamilton, B. Semicond. Sci. Technol. 1995, 10, 1187. (25) Ghulinyan, M.; Oton, C. J.; Bonetti, G.; Gaburro, Z.; Pavesi, L. J. Appl. Phys. 2003, 93, 9724. (26) Sailor, M. J.; Heinrich, J. L.; Lauerhaas, J. M. Semiconductor Nanoclusters; Elsevier Science: New York, 1996; Vol. 103, p 209. (27) Canham, L. T. Appl. Phys. Lett. 1993, 63, 337. (28) Wei, J.; Buriak, J. M.; Siuzdak, G. Nature 1999, 399, 243. (29) Shinoda, H.; Nakajima, T.; Ueno, K.; Koshida, N. Nature 1999, 400, 853. (30) Li, Y. Y.; Cunin, F.; Link, J. R.; Gao, T.; Betts, R. E.; Reiver, S. H.; Chin, V.; Bhatia, S. N.; Sailor, M. J. Science 2003, 299, 2045. (31) Zhao, L.; Yosef, M.; Steinhart, M.; Goring, P.; Hofmeister, H.; Gosele, U.; Schlecht, S. Angew. Chem., Int. Ed. 2006, 45, 311. (32) Halimaoui, A. Properties of Porous Silicon; INSPEC: London, 1997. (33) Kelly, M. T.; Chun, J. K. M.; Bocarsly, A. B. Appl. Phys. Lett. 1994, 64, 1693. (34) Cullis, A. G.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909. (35) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046.
10.1021/la060833o CCC: $33.50 © 2006 American Chemical Society Published on Web 09/15/2006
Fulleropyrrolidine on Porous Silicon Surfaces
Langmuir, Vol. 22, No. 21, 2006 8765
Scheme 1. Synthesis of Fulleropyrrolidine 1
allowing in-situ and real-time detection of supramolecular or enzyme activity.36 A current area of activity is the integration of nanoscale systems because many interesting physicochemical properties may result from their interactions. Related to this work is the investigation of the wetting properties of solid surfaces that have been functionalized with organic molecular structures. Wettability is important in many industrial and biological processes, ranging from self-cleaning surfaces37 to microfluidics38 and biology.39 This paper presents our results on the preparation and characterization of PS surfaces containing a grafted [60]fullerene derivative and evaluation of their wetting properties, studied through water contact angle measurements. It has been found that the dependence of the water contact angle on the roughness of PS coated with fullerene apparently contradicts Wenzel’s formula,40,41 which is commonly used to model the wettability properties of rough hydrophobic solids.
Figure 1. Transmission FTIR spectrum of a freshly etched PS sample showing a broad ν(Si-Hx) stretch in the 2000-2200 cm-1 range. The spectrum was recorded against a Si(100) wafer background.
Results and Discussion Direct incorporation of pristine [60]fullerene on both flat and PS surfaces has been reported previously.42,43 In these studies [60]fullerene was covalently linked to H-terminated Si(100) surfaces directly without any intermediate functional hydrocarbon chain. In this work we used a fulleropyrrolidine functionalized at position 2 of the pyrrolidine ring with a dec-9-ynyl group for the known reactivity of the C-C triple bond with Si-H surface groups.4,5,9,44 Synthesis of Fulleropyrrolidine 1. Synthesis of derivative 1 (Scheme 1) is based on the well-established 1,3-dipolar cycloaddition reaction of azomethine ylides to [60]fullerene.45 To this end, N-methylglycine and 10-undecynal46 were reacted in refluxing toluene for 3 h in the presence of [60]fullerene to afford fulleropyrrolidine 1 in 28% yield. HPLC analysis was employed to assess the purity of derivative 1 (Figure S1, Supporting Information) whereas NMR spectroscopy and mass determination to validate its proposed molecular structure (Figures S2-S4, Supporting Information). Formation of PS Samples. Luminescent PS samples were formed by chemical etching of single-side polished Si(100) slabs in a HF/H2SO4/NaNO2 aqueous solution as previously reported (see Experimental Section).33 Typical red bright luminescence was observed, confirming porous layer formation after etching. (36) Letant, S. E.; Hart, B. R.; Kane, S. R.; Hadi, M. Z.; Shields, S. J.; Reynolds, J. G. AdV. Mater. 2004, 16, 689. (37) Blossey, R. Nat. Mater. 2003, 2, 301. (38) Nguyen, N. T.; Wereley, S. T. Fundamentals and applications of Microfluidics; Artech House: Boston, 2002. (39) Israelachvili, J. Intermolecular and surface forces; 2nd ed.; Academic Press: San Diego, 1992. (40) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (41) de Gennes, P. G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and Wetting Phenomena; Springer: New York, 2003. (42) Feng, W.; Miller, B. Electrochem. Solid State Lett. 1998, 1, 172. (43) Feng, W.; Miller, B. Langmuir 1999, 15, 3152. (44) Bateman, J. E.; Eagling, R. D.; Worral, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683. (45) Maggini, M.; Prato, M. Acc. Chem. Res. 1998, 31, 519. (46) Wang, H.; Wulff, W. D. J. Am. Chem. Soc. 1998, 120, 10573.
Figure 2. Transmission FTIR spectra of fulleropyrrolidine 1 (a) and a PS sample after thermal hydrosilylation with 1 (1-PS) (b). The IR profile of pristine, H-terminated, PS has been subtracted as background. The up-pointing arrows indicate new bonds on the surface (see the text).
Samples were stored in air for several days without any loss of luminescence. It is worth noting that the luminescence intensity of freshly etched PS samples was in all cases comparable. A scanning electron micrograph (SEM) confirmed the etching of the silicon surface (Figure S5, Supporting Information). Immobilization of Fulleropyrrolidine 1 on PS (1-PS). Grafting of compound 1 on PS surfaces was carried out following a reported thermal hydrosilylation procedure.44 A PS slab was poured into a solution of pyrrolidine 1 (3 mL of a 0.5 M solution in dry toluene) previously degassed under nitrogen for 1 h. Then toluene was brought to reflux temperature for 3 h under a gentle flow of nitrogen. Removal of noncovalently bound fullerene was achieved by washing the sample extensively with toluene with the aid of ultrasounds (SweepZone, 150 W, about 10 s for each cycle), ethanol, and dichloromethane and finally dried under nitrogen. Transmission FTIR Spectroscopy Characterization. FTIR spectra of PS samples before and after reaction with fulleropyrrolidine 1 are shown in Figures 1 and 2. The FTIR spectrum reported in Figure 1 shows the typical features reported in the literature47 for a freshly etched silicon sample. The broad band in the 2000-2200 cm-1 region is associated to the stretching modes of Si-H at 2089 cm-1, Si-H2 at 2116 cm-1, and Si-H3 at 2140 cm-1. The band at around 1100 (47) Liu, S.; Palsule, C.; Gangopadhyay, S. Phys. ReV. B 1994, 49, 10318.
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cm-1 indicates the presence of silicon oxides (SiOx), most likely of interstitial nature. To confirm this assumption, an oxidized PS sample was treated with a 48% aqueous HF/EtOH mixture (1:1 v/v), which is known to remove silicon oxides from the surface. After treatment, a strong decrease of the ν(Si-O-C) band at 1160 cm-1 and ν(Si-O-Si) band at 1060 cm-1 was recorded, whereas the 1100 cm-1 band remained substantially unaffected, thus suggesting the interstitial origin mentioned above (SiOx, Figure S6) whose presence is commonly observed after chemical etching.47 The peak at 912 cm-1 (Figure 1) is associated to a δ(Si-H2) scissor mode absorption. The low-energy part of the spectrum displays a strong absorption at around 617 cm-1 with a smaller shoulder at 665 cm-1: the former is commonly assigned to the ν(Si-Si) stretching mode and the latter to a ω(Si-H) wagging mode. Furthermore, some additional peaks were recorded: a weak absorption at around 715 cm-1, possibly due to O-Si-O bending mode,48 and a peak at 865 cm-1 that can be attributed to a ω(Si-H2) wagging mode.49 No changes in the IR spectra were observed upon air exposure for a few hours, whereas an extensive oxidation was apparent from the spectra after 1 week. Figure 2 shows the FTIR spectra of fulleropyrrolidine 1 (Figure 2a) and that of a PS sample after thermal hydrosilylation in the presence of 1 (Figure 2b). The fingerprint part of the spectrum in Figure 2a appears quite complex. However, a number of major features could be recognized and univocally attributed: the alkynyl hydrogen stretching at 3300 cm-1; the antisymmetric and symmetric methylene stretching vibrations appearing, respectively, at 2923 and 2854 cm-1; the N-methyl stretching at 2780 cm-1; and absorption of the [60]fullerene cage at 526 cm-1. Other features could be assigned as well: C-H deformation modes around 1460 and 1380 cm-1 and other [60]fullerene resonances at about 1430, 1180, and 630 cm-1. Figure 2b shows the difference spectrum of a PS sample before and after (1-PS) thermal hydrosilylation with 1. The negative peaks refer to loss of a particular functional group, whereas the positive ones refer to formation of a new bond on the surface. The lack of signals at 3300 cm-1 indicates that the CtC triple bond in 1 has reacted. Furthermore, the absence of any CdC double-bond vibrations (expected at around 1600 cm-1) confirmed that the alkyne in 1 reacted with the H-terminated silicon surface giving a sp3-hybridized C-Si bond. The antisymmetric and symmetric stretching of methylenes and N-methyl group in 1-PS appear at 2923, 2850, and 2780 cm-1, respectively, in line with the above-mentioned discussion presented for derivative 1. It is worth noting that the band due to ν(Si-Hx) still retains up to one-third of its original intensity after prolonged thermal treatment of the PS surface with 1. The broad band around 1460 cm-1 is due to partial overlapping of the 1465 and 1430 cm-1 peaks (assigned to the alkyl chain of 1). The important [60]fullerene vibration is evident in 1-PS at 526 cm-1. Some silicon oxides are present on the surface (at 1100 and 1060 cm-1, attributed, respectively, to SiOx and SiO-Si vibrations) in line with the known reactivity of H-terminated silicon surfaces44,50-52 (vide infra). The negative peak at around (48) Tsai, C.; Li, K. H.; Kinosky, D. S.; Qian, R. Z.; Hsu, T. C.; Irby, J. T.; Banerjee, S. K.; Tasch, A. F.; Campbell, J. C.; Hance, B. K.; White, J. M. Appl. Phys. Lett. 1992, 60, 1700. (49) Fuchs, H. D.; Stutzmann, N.; Brandt, M. S.; Rosenbauer, M.; Weber, J.; Breitschwerdt, A.; Deak, P.; Cardona, M. Phys. ReV. B 1993, 48, 8172. (50) Sailor, M. J.; Lee, E. J. AdV. Mater. 1997, 9, 783. (51) Bateman, J. E.; Eagling, R. D.; Horrocks, B. R.; Houlton, A.; Worral, D. R. Chem. Commun. 1997, 2275. (52) Lucovsky, G. Solid State Commun. 1979, 29, 571.
Dattilo et al.
660 cm-1 is related to a ω(Si-H) wagging mode and that at 620 cm-1 to a ν(Si-Si) stretching mode. X-ray photoelectron spectroscopy (XPS) characterization was carried out on H-terminated and functionalized PS to complement the FTIR analysis. Detailed scan XPS data, from a freshly etched H-terminated PS surface, exhibited an intense Si2p peak at a binding energy (BE) of 100.3 eV along with a lower intensity shoulder at 102.5 eV. The registered BE of the basic Si2p peak can be associated to the H-terminated silicon surface.53 The higher BE peak component is connected with the presence of silicon oxides, of the SiOx type,54 due to some surface oxidation. No signals attributed to SiO2 were detected in the high BE region (Figure S7). The relative amounts of different silicon species, determined by deconvolution of the XPS signal, were about 80% (Si-H-type silicon) and 20% (Si-O-type silicon). The Si2p band in the functionalized PS samples, on the other hand, is broader than that discussed above, owing to the presence of several species (Figure S8). The peak fitting of the experimental Si2p envelope revealed three components centered at 100.1 ( 70%). The higher BE component is the fingerprint for SiO2,55 whereas the two lower BE components could be associated to Si coordinated by oxide anions and alkyl organic groups (≈102 eV)56 and Si-C bonds (≈100 eV),54 in line with the FTIR characterization. These data suggest that only a partial coverage of the H-terminated silicon surface has been successfully obtained through formation of direct Si-C bonds, as reflected by the large amount of SiO2 on the surface. However, it has been reported that bulky molecular structures have an important impact on the surface coverage: the bulkier the terminus, the lower the surface packing.57 In our case, it is reasonable to assume that the immobilized bulky [60]fullerene protects unreacted Si-H groups toward further reactions during thermal treatment. This is generally believed to be the reason, in this kind of low densely packed monolayers, silicon oxide formation is always observed.57 The stability of 1-PS in air was also investigated. The infrared absorption profile of 1-PS, shown in Figure 2b, remains substantially unchanged upon air exposure for a few days. However, after more than 1 week, the surfaces showed extensive oxidation with a concomitant growth of IR absorptions at 1160 [ν(Si-O-C)], 1060 [ν(Si-O-Si)], and 2240 [ν(O3Si-H)] (Figure S9). The stability of 1-PS was also tested against a mixture of 48% aqueous HF/EtOH (1:1 v/v). A 5-min immersion of the silicon slab into the mixture left the fullerene derivative attached to the surface, as evidenced by FTIR analysis, thus indicating formation of Si-C bonds and not Si-OR at the PS surface. The accessible oxidized regions reacted with the HF/EtOH mixture, leading to disappearance of the band at 1068 cm-1 (Si-O-Si). However, the band at 1100 cm-1, indicative of some interstitial oxides, was resistant to acid treatment (Figure S6). Further evidence of the robust nature of the fullerene layer in 1-PS was its resistance toward prolonged sonication in toluene, with no change in the IR profile (Figure S10). The covalent linking of fulleropyrrolidine 1 to PS produces a substantial quenching of the original emission of the material. (53) Webb, L. J.; Nemanick, E. J.; Biteen, J. S.; Knapp, D. W.; Michalak, D. J.; Traub, M. C.; Chan, A. S. Y.; Brunschwig, B. S.; Lewis, N. S. J. Phys. Chem. B 2005, 109, 3930. (54) Lopez, E.; Chiussi, S.; Kosch, U.; Gonzalez, P.; Serra, J.; Serra, C.; Leon, B. Appl. Surf. Sci. 2005, 248, 113. (55) Moulder, J. F.; Stickle, W. F.; Sobol, P. W.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (56) Wagner, C. D. J. Vac. Sci. Technol. 1978, 15, 518. (57) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 2001, 17, 7554.
Fulleropyrrolidine on Porous Silicon Surfaces
Langmuir, Vol. 22, No. 21, 2006 8767 Table 1. Equilibrium Contact Angles of Milli-Q Water Drops on Flat Si and PS Surfaces with Different Functionalizationsa
a
Figure 3. (a) Emission spectra of a freshly etched PS sample before and after thermal hydrosilylation with derivative 1 for 3 h in toluene at reflux temperature under inert atmosphere. (b) Emission spectra of a freshly etched PS sample before and after treatment with toluene at reflux temperature for 3 h under inert atmosphere (λexc ) 350 nm).
Figure 3a shows the emission properties of the same PS sample before and after thermal hydrosilylation in the presence of fulleropyrrolidine 1. Figure 3b shows, on the other hand, the emission characteristics of a PS sample before and after treatment in refluxing toluene for 3 h under an inert atmosphere but without 1. Only about one-half of the original luminescence of PS was lost, possibly for the formation of silicon oxides. It is therefore reasonable to assume that the nearly quantitative quenching of the emission of PS (Figure 3a) can be reasonably ascribed to functionalization with 1 that acts as nonradiative recombination center,5,26 although other quenching mechanisms, involving fullerene excited states, cannot be ruled out. Hydrophobicity of 1-PS. The hydrophobic behavior of 1-PS was studied through measurements of water contact angles. Table 1 shows the equilibrium contact angles, Θ, measured on flat Si(111) wafers and on PS with different surface functionalizations. The spread in contact angles on bare Si wafers refers to data measured after the samples have been kept in air for a few days, Θ ≈ 54°, and right after a passage in UVO cleaner that effectively removes organic contaminants, Θ ≈ 10°. The data on bare PS have been acquired for five different samples. We also measured the contact angle for two samples after different etching time periods, ranging from 5 to 20 min in step of 5 min. Overall, we have found no precise correlation between Θ and the etching time, and the tabulated spread is then an indication of the reproducibility of our etching procedure. The contact angle of three flat Si wafers coated with fulleropyrrolidine 1 is very close to the standard value of 85°
The values in brackets refer to literature data.
measured on a clean graphite surface60 and to that of 80° on a [60]fullerene monolayer.58 Interestingly, chemisorption of 1 on five PS samples increases Θ to ∼103°, in apparent contradiction with Wenzel’s theory,41 which assumes that the liquid fills up the grooves on the rough surface. From energy considerations, it can be easily derived that the apparent contact angle Θ* of a rough material characterized by an equilibrium contact angle Θf when measured on a flat surface is given by
cos Θ* ) r cos Θf
(1)
where r is the ratio between the area of the rough surface and that of the geometric projection. This formula implies that the solid becomes more hydrophobic/hydrophilic with increasing roughness according to the value of Θf. If a flat solid shows an equilibrium contact angle Θf < 90°, roughening its surface reinforces its hydrophilic character. Vice versa, if the material has Θf > 90° a rough surface enhances its hydrophobicity. In other words, there exists a critical angle Θc ) 90° that separates the two regimes. To make sure that the results on 1-PS were not an artifact of our method, we measured the equilibrium contact angles of PS functionalized with three model molecular structures: a hydrophobic alkane made of 12 carbon atoms (C12), a hydrophilic methyl ester (methyl undecanoate, CH3-10), and a 2,2,2trifluorethyl undecanoate (CF3-10). The latter has been chosen because its wetting properties are similar to those displayed by a [60]fullerene monolayer on flat Si(111) surface.58 For more details, see the Supporting Information (Figures S11-S13) on the grafting of C12, CH3-10, and CF3-10 on PS. We checked that the surface chemistry was correct by functionalizing first flat silicon wafers with the above-mentioned (58) Ostrovskaya, L.; Podesta`, A.; Milani, P.; Ralchenko, V. Europhys. Lett. 2003, 63, 401. (59) Sun, Q.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Thune, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514. (60) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: San Diego, 2000; Vol. 3.
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compounds and then transferring the experimental protocol to porous silicon substrates. The contact angles agree well, within experimental uncertainties, with the values reported in the literature (see numbers within brackets in Table 1). The moderate discrepancy (five degrees maximum for CF3-10) between our results and those reported in the literature could be related to the known reactivity of the Si(111)-H surface that has to be used immediately after the etching treatment,61 although other reasons, such as a different morphology, cannot be ruled out. It is important to mention that the PS data in Table 1 refer to the same surface morphology since all samples for the analysis have been prepared from the same freshly etched silicon batch. The wetting behavior observed for C12 and CH3-10 is consistent with Wenzel’s predictions,41 namely, roughening a silicon substrate bearing a molecular structure such as C12 (CH310) increases its hydrophobic (hydrophilic) character. More interestingly, the data on CF3-10 show the same trend found for 1-PS. This suggests that the origin of this effect is related to the PS morphology rather than its surface functionalization. In other words, the PS critical angle Θc is not 90°, as implicitly assumed by Wenzel’s formula, but lies somewhere between 71° and 80°. A similar decrease in Θc below 65° has been recently found on cluster-assembled carbon films.58 Our observation is also consistent with theoretical studies of wetting on rough surfaces, according to which a reduction of the critical angle well below 90° is found at relatively high values of the surface roughness exponent.62 More generally, it has been suggested that a certain class of self-affine profiles of surface roughness may render any substrate with a nonzero contact angle non wet, that is Θ* > 90°.63
Conclusion In summary, we have shown that a fulleropyrrolidine, bearing a dec-9-ynyl functionality, can be grafted to silicon surfaces through a thermal hydrosilylation protocol. It has been found that the hydrophobic behavior of fullerene-linked silicon surfaces is enhanced by roughness. This result can be explained by an extension of Wenzel’s formula in which the critical angle, which discriminates between the hydrophilic/hydrophobic character of a solid, is substantially reduced below 90° in the case of very disordered surfaces. Since the roughness of PS can be controlled through the etching process, our findings suggest that it is possible to enhance the contact angle of water (i.e., the hydrophobicity) of silicon surfaces bearing molecular structures that display contact angles comprised between about 70° and 90° when deposited on flat substrates. Experimental Section General Information. Dodecylmagnesium bromide (1.0 M solution in diethyl ether), undecylenic acid, and methyl undec-10enoate [CH2dCH-(CH2)8-CO2CH3] were purchased from Aldrich. All solvents and methyl undec-10-enoate were distilled prior to use. [60]Fullerene was purchased from Bucky USA (99%). 10-Undecynal46 and 2,2,2-trifluoroethyl undec-10-enoate [CH2dCH(CH2)8-CO2CH2CF3]7 were prepared as reported in the literature. Homogeneous-phase reactions were monitored by thin-layer chromatography using Merck precoated silica gel 60-F254 (0.25 mm thickness). Flash column chromatography was performed employing 230-400 mesh silica gel (ICN Biomedicals). The silicon substrates were pieces (1 × 1 cm) of single side polished Si(100) (p-type, 500 µm thickness), or Si(111) (n-type, 500 µm thickness), purchased (61) Namyong, Y. N.; Laibinis, P. E. J. Am. Chem. Soc. 1998, 120, 4516. (62) Palasantzas, G.; De Hossn, J. T. M. Acta Mater. 2001, 49, 3533. (63) Herminghaus, S. Europhys. Lett. 2000, 52.
Dattilo et al. from Aldrich. NMR spectra were recorded in CDCl3 on a Bruker AC 250 (5.9 T, 250.1 MHz for 1H) at room temperature. Chemical shifts are given in parts per million (δ) relative to tetramethylsilane. UV-vis spectra were recorded on a Perkin-Elmer Lambda 45 spectrophotometer. Fluorescence spectra of the surfaces were recorded on a Perkin-Elmer LS55 spectrophotometer using a frontsurface accessory. FT-IR spectra were collected in transmission mode (against a clean silicon wafer as background) on a Nicolet 5700 FT-IR spectrophotometer with a DTGS-KBr detector and a nitrogenpurged sample chamber with 64 scans at 4 cm-1 resolution. All liquids and oils were measured between KBr windows; powders were measured making KBr pellets. ESI-MS spectra were performed with a MSD SL Trap mass spectrometer (Agilent Technologies, Palo Alto, CA) operating in positive-ion mode from m/z 100 to 2000. A 20 µL amount of a 1.4 × 10-6 M solution in methanol, containing 0.1% HCOOH, was injected in a 50 µL/min flow of the same solvent performed with a 1100 Series binary pump (G1312A). The purity of fulleropyrrolidine 1 was checked by HPLC using a Thermo Separation Spectra System P2000, with a Spectra system UV6000LP detector at 340 nm. HPLC analysis was performed with a Phenomenex Luna column (250 × 4.6 mm, SiO2, 5 µm) using toluene/ethyl acethate 8:2 as eluent, at 1 mL/min. In the mentioned HPLC conditions, compound 1 is eluted at 3 min. SEM images were recorded using a XL 30 ESEM Philips instrument at 20 KV and 4.7 spot; the sample was gold-sputtered prior to taking the image. Fulleropyrrolidine 1. [60]Fullerene (130 mg, 0.18 mmol), N-methylglycine (64 mg, 0.72 mmol), and 10-undecynal (60 mg, 0.36 mmol) were dissolved in toluene (100 mL) and brought to reflux. After 3 h (TLC, toluene/ethyl acetate 9:1, Rf (1) ) 0.28) the solution was concentrated on the rotary evaporator, poured on top of a SiO2 flash column chromatography, and eluted first with toluene, to remove unreacted [60]fullerene, and then with toluene/ethyl acetate 8:2. The fractions containing the product were concentrated under reduced pressure and transferred into a centrifuge tube. The product was precipitated from CH3CN, washed twice with the same solvent, and dried in vacuo. A 46 mg (28%) amount of 1 was isolated as a brownish powder. 1H NMR (250 MHz, CDCl3): δ 4.80 (d, 1H, J ) 9.85 Hz), 4.15 (d, 1H, J ) 9.50 Hz), 3.88 (t, 1H, J ) 5.12 Hz), 4.27 (s, 3H), 2.44 (m, 2H), 2.15 (td, 2H, J1 ) 6.95 Hz, J2 ) 2.57), 1.90 (t, 1H, J ) 2.55 Hz), 1.35 (m, 12H). 13C NMR (62.9 MHz, CDCl3): δ 154.48, 147.46, 146.81, 146.69, 146.51, 146.46, 146.38, 146.31, 146.26, 146.18, 146.00, 145.76, 145.63, 145.51, 145.44, 145.38, 144.94, 144.78, 144.62, 144.56, 143.39, 143.25, 142.84, 142.37, 142.30, 142.10, 142.03, 141.90, 140.48, 140.40, 139.98, 139.83, 137.37, 136.44, 136.02, 135.71, 84.94, 78.32, 70.24, 68.35, 40.15, 31.19, 30.37, 29.47, 29.20, 28.87, 27.67, 26.66, 18.60 ppm. IR (KBr) : ν 3300, 2928, 2850, 2780, 1460, 1425, 1331, 1180, 627, 527 cm-1. UV-vis (CH2Cl2): λmax() 255 (45691), 318 (14 570), 430 (2090), 460 (1050), 540 (580), 700 (200 M-1 cm-1) nm. ESIMS (C73H23N): m/z 915 [M+H]. Anal. Calcd for C73H23N: C, 95.93; H, 2.54; N, 1.53. Found: C, 95.72; H, 2.55; N, 1.51. HPLC (toluene/ ethyl acetate, 8:2): RT ) 3.12 min, purity >99% (Figure S1). X-ray Photoelectron Spectroscopy. XPS analyses were performed with a Perkin-Elmer Φ 5600-ci spectrometer using nonmonochromatized (15 kV, 400 W) Al KR radiation (1486.6 eV). The samples were mounted on a steel sample holder and introduced directly, by a fast-entry lock system, into the XPS analytical chamber. The sample analysis area was 800 µm in diameter, and the working pressure was
