Grafting Cavitands on the Si(100) Surface

In Final Form: August 16, 2006. Cavitand ... self-assembly properties of cavitands4 have been transferred from ... in fact, the surface of election fo...
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11126

Langmuir 2006, 22, 11126-11133

Grafting Cavitands on the Si(100) Surface Guglielmo G. Condorelli,† Alessandro Motta,† Maria Favazza,† Ignazio L. Fragala`,*,† Marco Busi,‡ Edoardo Menozzi,‡ Enrico Dalcanale,‡ and Luigi Cristofolini§ Dipartimento di Scienze Chimiche, UniVersita` degli Studi di Catania, and INSTM UdR di Catania, V.le A. Doria 6, 95100 Catania, Italy, Dipartimento di Chimica Organica e Industriale, UniVersita` di Parma, and INSTM UdR di Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy, and Dipartimento di Fisica and INFM, Centro di ricerca e sViluppo SOFT, UniVersita` di Parma, Parco Area delle Scienze 7/A, 43100 Parma, Italy ReceiVed March 13, 2006. In Final Form: August 16, 2006 Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.

Introduction The synthesis of hybrid organic/inorganic systems in which organic molecules are anchored by covalent/noncovalent bonding on inorganic surfaces represents a key point for the development of molecular devices. The most promising architecture for these systems is represented by a dense array of molecular devices hosted on a silicon-based microelectronic circuit.1 In this context, the development of hybrid nanoelectronic systems requires the functionalization of the silicon surface.2 In the past few years, the peculiar molecular recognition3 and self-assembly properties of cavitands4 have been transferred from solutions to surfaces, leading to highly selective chemical sensors5 and to the controlled assembly of nanosize molecular containers on gold.6 A recent paper has shown the possibility of using cavitands as selective sorbents for benzene detection at trace level by grafting them on silica gel.7 No reports have, however, appeared to date on the possibility of anchoring cavitands on a surface of technological importance such as Si(100), which is, in fact, the surface of election for the development of commercial microelectronic devices. * Corresponding author. Fax: +39-095-580138. E-mail: [email protected]. † Universita ` degli Studi di Catania and INSTM Udr di Catania. ‡ Universita ` di Parma and INSTM UdR di Parma. § Universita ` di Parma and INFM. (1) Cerofolini, G. F.; Ferla, G. J. Nanopart. Res. 2002, 4, 185-191. (2) Buriak J. M. Chem. ReV. 2002, 102, 1271-1308. (3) (a) Cram, D. J.; Cram, J. M. Container Molecules and Their Guests; Royal Society of Chemistry: Cambridge, 1994. (b) Trembleau, L.; Rebek, J., Jr. Science 2003, 301, 1219-1220. (4) (a) Heinz, T.; Rudkevich, D. M.; Rebek, J., Jr. Nature 1998, 394, 764766. (b) Fochi, F.; Jacopozzi, P.; Wegelius, E.; Rissanen, K.; Cozzini, P.; Marastoni, E.; Fisicaro, E.; Manini, P.; Fokkens, R.; Dalcanale, E. J. Am. Chem. Soc. 2001, 123, 7539-7552. (5) Pinalli, R.; Suman, M.; Dalcanale, E. Eur. J. Org. Chem. 2004, 451-462. (6) (a) Levi, S.; Guatteri, P.; van Veggel, F. C. J. M.; Vancso, G. J.; Dalcanale, E.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2001, 40, 1892-1896. (b) Menozzi, E.; Pinalli, R.; Speets, E. A.; Ravoo, B. J.; Dalcanale, E.; Reinhoudt, D. N. Chem.sEur. J. 2004, 10, 2199-2206. (7) Bianchi, F.; Pinalli, R.; Ugozzoli, F.; Spera, S.; Careri, M.; Dalcanale, E. New J. Chem. 2003, 27, 502-509.

Several recent studies on grafting of organic molecules on silicon surfaces, focused on the Si(111) plane because of the simple chemical etching procedure available to obtain atomically flat and chemically well-defined surfaces.8 Chemical etching of the Si(100) surface tends to roughen the surface by exposing Si(111) facets.9 Under some conditions it is, however, possible to produce relatively flat surfaces, predominantly SiH2-terminated, even though SiH and SiH3 groups are present.10 Among the various covalent bonding modes of organic molecules on silicon surfaces, hydrosilylation of molecules with multiple bonds on hydrogen-terminated surfaces appears the best suited for the largest potential applications.11 In the present paper, we report the direct anchoring of cavitands having ω-decylenic feet and different bridging groups at the upper rim (Scheme 1) on H-terminated Si(100) surfaces via photochemical hydrosilylation of the double bonds. It is well-documented11b,c,12 that the hydrosilylation of double bonds lead to the formation of robust Si-C bonds between the surface and the organic molecules. (8) (a) Wallart, X.; de Villeneuve, C. H.; Allongue, P. J. Am. Chem. Soc. 2005, 127, 7871-7878. (b) Liu, Y.-J.; Navasero, N. M.; Yu, H.-Z. Langmuir 2004, 20, 4039-4050. (c) Jin, H.; Kinser, C. R.; Bertin, P. A.; Kramer, D. E.; Libera, J. A.; Hersam, M. C.; Nguyen, S. T.; Bedzyk, M. J. Langmuir 2004, 20, 62526258. (d) Bansal, A.; Li, X.; Yi, S. I.; Weinberg, W. H.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 10266-10277. (e) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205-1209. (f) Linford, M. R.; Fenter, P.; Eisenberg, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (9) Cullis, A. G.; Cahnam, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909-965. (10) (a) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K.; Burrows, V. A. J. Vac. Sci. Technol., A 1989, 7, 2104-2109. (b) Cerofolini, G. F.; Galati, C.; Giorgi, G.; Motta, A.; Reina, S.; Renna, L.; Terrasi, A. Appl. Phys. A: Mater. Sci. Process. 2005, A81, 745-751. (11) (a) Sieval, A. B.; Demirel, A. L.; Nissink, J. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (b) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Condorelli, G. G.; Fragala`, I. L.; Giorgi, G.; Sgamellotti, A.; Re, N. Appl. Surf. Sci. 2005, 246, 52-67. (c) Sun, Q.-Y.; 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, 2514-2523. (12) (a) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. (b) Sung, M. M.; Kluth, G. J.; Yauw, O. K.; Maboudian, R. Langmuir 1997, 13, 6164-6168.

10.1021/la060682p CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006

Grafting CaVitands on Si(100)

Langmuir, Vol. 22, No. 26, 2006 11127 Scheme 1. Cavitands Used in the Grafting Process

Among the investigated molecules, methyl cavitands MeCavA with decenyl feet and MeCavB with butenyl feet have been studied as prototypes of cavitand molecules. The grafting processes of Br substituted methyl cavitand BrMeCavA and quinoxaline derivate cavitand QxCav have been also studied. The latter molecule has been proved an efficient sensor for aromatic molecules.7 The resulting cavitand-decorated surfaces have been characterized via angle resolved X-ray photoelectron spectroscopy (AR-XPS) and atomic force microscopy (AFM). Mixed monolayers of cavitands and 1-octene as spectator spacer were also prepared, and the quality of surface coverage compared with that of pure cavitand layers. Experimental Section Synthesis of MeCavA, QxCav, and BrMeCavA. MeCavA, QxCav, and BrMeCavA were synthesized following a published procedure.13,14 MeCavA. Anal. Calcd (%) for C72H96O8 (1089.53): C, 79.37; H, 8.88. Found: C, 79.01; H, 8.64. QxCav. Anal. Calcd (%) for C100H104O8N8‚CH2Cl2 (1630.91): C, 74.38; H, 6.55; N: 6.87. Found: C, 74.45; H, 6.79; N, 6.54. BrMeCavA. Anal. Calcd (%) for C72H92Br4O8 (1405.11): C, 61.54; H, 6.60; Br, 22.75%. Found: C, 61.89; H, 6.78; Br, 22.87. Synthesis of Resorcinarene 1. To a solution of resorcinol (2.0 g, 18.2 mmol) and 1-pentenal (1.53 g, 18.2 mmol) in ethanol (16 mL), 12 N hydrochloric acid (4 mL) was added at 0 °C in 1 h. The solution was stirred at 60 °C overnight. Addition of water (50 mL) led to the precipitation of 1 as an orange solid. The precipitate was filtered off and washed to neutrality with hot water (60 °C) and then dried under vacuum to give pure 1 as yellow powder (2.91 g, 91%).

M ) C44H48O8 (704.86 amu). 1H NMR (acetone-d6, 300 MHz, δ): 2.05 (m, 8H), 2.39 (m, 8H), 4.34 (t, 4H, 3J ) 7.8 Hz), 4.97 (m, 8H), 5.88 (m, 4H), 6.24 (s, 4H), 7.63 (s, 4H), 8.48 (s, 8H). Synthesis of MeCavB. K2CO3 (1.09 g, 7.95 mmol) and CH2Br2 (0.496 mL, 7.07 mmol) were added, under nitrogen, to a solution (0.623 g, 8.84 × 10-1 mmol) of resorcinarene 1 in 10 mL of dry DMSO. The mixture was stirred in a sealed tube at 50 °C for 5 h. The reaction was quenched by the addition of 10% HCl(aq); the precipitate was filtered and dried under vacuum. The crude was purified by column chromatography (SiO2, CH2Cl2/ethyl acetate 9:1) to give MeCavB as pale yellow solid in 18% yield (0.12 g). Rf ) 0.7. MALDI-TOF MS [(m/z) (%)] ) found 754.0 [(M + H)+ (100)]: calcd 753.9, in which M ) C48H48O8 (752.9 amu). 1H NMR (CDCl3, 300 MHz, δ): 2.13 (m, 8H), 2.33 (m, 8H), 4.42 (d, 4H, 2J ) 7.2 Hz), 4.77 (t, 4H, 3J ) 8.1 Hz), 5.02 (m, 8H), 5.74 (d, 4H, 2J ) 7.2 Hz), 5.91 (m, 4H), 6.49 (s, 4H), 7.12 (s, 4H). Anal. Calcd (%) for C48H48O8 (752.9): C, 76.57; H, 6.43. Found: C, 76.34; H, 6.22. Monolayer Preparation. Various cavitand/1-octene mixtures with the following cavitand mole fractions (1.0, 0.5 and 0.2) were dissolved in mesitylene (solution concentration ) 0.05 M), for grafting of pure and mixed monolayers. A total of 2.0 mL of cavitand solution was placed in a quartz cell and deoxygenated by stirring in a drybox for at least 1 h. Subsequently, a Si(100) substrate was etched in 2.5% hydrofluoric acid for 2 min and immediately placed in the solution. The cell remained under UV irradiation (254 nm) for 2 h. The sample was then removed from the solution and sonicated in dichloromethane for 10 min. Monolayer Characterization. The XPS spectra were run with a PHI multi-technique ESCA-Auger spectrometer equipped with a monochromated Al KR X-ray source. The analyses were carried out at various photoelectron angles (relative to the sample surface) in the 10°-80° range with an acceptance angle of ( 7°. XPS binding energy (BE) scale was calibrated by centering the C 1s peak due to the hydrocarbon moieties at 285.0 eV.15,16AFM images were obtained in high amplitude mode (tapping mode) by a NT-MTD instrument. The noise level before and after each measurement was 0.01 nm.

Result and Discussion

MALDI-TOF MS [(m/z) (%)] ) found 727.3 [(M + Na)+ (100)]: calcd 727.8; found 743.3 [(M + K)+ (40)], calcd 744.0, in which (13) van Velzen, T.E.U.; Engbersen, J. F. J.; Reinhoudt, D. N. Synthesys 1995, 8, 989-997. (14) Eggo, U.; van Velzen, T.E.U.; Engbersen, J. F. J.; De Lange, P. J.; Mhay, J. W. G.; Reihoudt, D. J. Am. Chem. Soc. 1995, 117, 6853-6862.

X-ray photoelectron spectroscopy (XPS) proved to be an ideal tool for the characterization of the nanometric layer as well as to probe the elemental depth distribution and their bonding states. (15) (a) Swift, I. L. Surf. Interface Anal. 1982, 4, 47-51. (b) Briggs, D.; Beamson, G. Anal. Chem. 1992, 64, 1729-1736. (16) (a) Cerofolini, G. F.; Galati, C.; Lorenti, S.; Renna, L.; Viscuso, O.; Bongiorno, C.; Raineri, V.; Spinella, C.; Condorelli, G. G.; Fragala`, I. L.; Terrasi, A. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 403-409; (b) Haber, J. A.; Lewis, N. S. J. Phys. Chem. B 2002, 106, 3639-3656.

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Table 1. Elemental Compositions of (a) HF Freshly Etched Silicon Surface, (b) Sample Treated in Pure Mesitylene under 2 h UV Irradiation, (c-f) Samples Decorated with MeCavA, BrMeCavA, MeCavB, and QxCav, Respectively, by UV Irradiation in Mesitylene Solution atomic fraction

Si C O N Br

HF etched (a)

Mes (b)

MeCavA (c)

BrMeCavA (d)

MeCavB (e)

QxCav (f)

87.3 9.4 3.3

57.1 23.2 19.7

43.5 46.6 8.9

36.3 46.7 14.6

47.1 38.9 13.4

40.1 48.5 10.1 1.2

1.3

The efficiency of the present grafting process of various cavitands has been therefore evaluated by XPS measurements. Table 1 compares elemental compositions of (a) HF freshly etched silicon surface, (b) sample treated in pure mesitylene under 2 h UV irradiation, (c-f) samples decorated with MeCavA, BrMeCavA, MeCavB, and QxCav, respectively, by UV irradiation in mesitylene solution. Data show that C 1s peaks are observed for both reference samples, HF freshly etched silicon and mesitylene-treated surface. These C 1s signals arise from adventitious contaminant species which cannot be completely eliminated during etching and handling steps.8d,16 However, cavitand decorated surfaces show an evident enhancement of C 1s related signals as compared to both freshly etched silicon surface and mesitylene-treated samples. This enhancement together with the observation of N for QxCav and Br XPS features for BrMeCavA represents an indication of the presence of cavitand molecules on the silicon surface. Changes of elemental carbon concentration moving from HF etched silicon to samples irradiated in mesitylene will be discussed in the following sections. Useful information on the nature of the grafted layers was obtained from high-resolution spectra of relevant photoemission bands. The Si 2p spectrum (takeoff angle 45°) of freshly etched silicon surface shows a well-defined 2p3/2-1/2 doublet (at 99.2 and 99.8 eV, respectively) associated to the elemental silicon (Si0). No evidence of fully oxidized silicon at 103 eV is observed. Immediately after the grafting process of pure cavitands, the Si 2p region is similar to that of the freshly etched silicon surface. Nevertheless, upon 1 week aging in air, the Si 2p region shows two distinct features (Figure 1): the 2p3/2-1/2 doublet associated to the elemental silicon (Si0) and a broad band at 103 eV due to fully oxidized silicon (Si4+).17 The presence of some surface oxidation indicates that the layers made from pure cavitands are not densely packed. This is not an unexpected behavior since the steric hindrance of the cavitand headgroups (whose cross-sectional area is about 140 Å2)14 precludes a close packing. The four legs of the basket, namely, the alkyl chains, do not completely fill the space underneath the cavitand headgroup because the sum of their cross-sectional areas (ca. 4 × 20 Å2) is considerably less than the cross-sectional area of the headgroup itself. Therefore, the alkyl chains underneath the headgroups of different molecules are not close enough to form a well-packed hydrocarbon layer. Figure 2 shows the high-resolution C 1s photoelectron spectral region obtained at a takeoff angle of 45°. In both, freshly etched silicon surface (Figure 2a) and samples irradiated in pure mesitylene (Figure 2b), the carbon region consists of two (17) (a) Cerofolini, G. F.; Bongiorno, C.; Camalleri, M.; Condorelli, G. G.; Fragala`, I. L.; Galati, C.; Lorenti, S.; Renna, L.; Spinella, C.; Viscuso, O. Appl. Phys. A: Mater. Sci. Process. 2002, 75, 585-590; (b) Cerofolini, G. F.; Galati, C.; Renna, L.; Re, N. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 515-521.

components. The first component is centered at 285.0 eV (referred to as C0). It represents aliphatic and aromatic hydrocarbons.18-20 The second component is centered at 286.5 eV (referred to as C+1). This feature can be attributed to carbon bonded to one oxygen.18-20 The latter component of this carbonaceous layer can be attributed to the formation of Si-O-C groups due to the reaction between Si-H and Si-OH groups on the silicon surface with oxidized carbon species. The formation of Si-O-C bonds on both Si-H and Si-OH termination has been well-documented in the literature.8,21,22 Si-OH terminations can be even formed in the reference samples due to trace of adventitious water in solution or in air. In this context, it is worth noting that the relative intensity of C 1s signal in the sample irradiated in mesitylene is higher compared to that of freshly HF etched surface (Table 1), since the increased Si oxidation of the sample irradiated in mesitylene results in a greater number of SiOH terminations available for the Si-O-C formation.18 The C 1s spectra (takeoff angles 45° and 10°) of the grafted cavitands (Figures 3 and 4) show a richer structure compared to reference samples, clearly due to the presence of further bonding states. They consist of three main components: (i) a C0 component at 285.0 eV (also observed in the previous samples) due to the aliphatic and aromatic hydrocarbon backbone; (ii) a C+1 component at 286.3 eV due to oxidized carbon centers both in the cavitand phenyl ring bonded to one oxygen atom19,20 (Scheme 1) as well as in adventitious Si-O-C frameworks.18 Note that this assignment also accounts for the increased relative intensity of the C+1 component compared to reference samples. A third component at 288.4 eV (referred to as C+2), only observed in cavitand decorated surfaces, can be used as fingerprint of the presence of cavitand molecules. It represents the methylene groups bridging the oxygen atoms at the upper rim (Scheme 1). In this case, the BE shift relative to hydrocarbon C 1s (3.4 eV) is slightly higher than the shift values (3.0 eV) expected for two C-O bonds.19 This difference is due to a secondary effect of phenyl groups (Ph), which induces in each C-O-Ph array a further shift of about 0.2 eV per Ph group.20 As expected the C 1s band of MeCavA and B are constituted by almost identical components. The C+2 component of the BrMeCavA (Figure 4a), by contrast, is broader compared to the correspondent component of the remainders MeCavs likely due to the presence of oxidized carbon contaminants. The BE of the C+2 component measured in the spectra of QxCav (Figure 4b) shifts toward lower values (287.6 eV). This component is due to quinoxalinic carbons that bond both oxygen and nitrogen atom. Finally, an accurate fitting of MeCavs spectra requires a further feature at 283.5 eV (referred to as C-1) assigned to the Si-C bond that forms upon hydrosilylation reaction.18 This component is, however, somewhat hidden due to two factors: (i) this bond is buried under the grafted molecules; therefore, it is scarcely detectable for the large QxCav molecules; (ii) the presence of proximal SiOx and SiOH terminations shifts the C-1 component (18) (a) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Viscuso, O.; Condorelli, G. G.; Fragala`, I. L. Mater. Sci. Eng., C 2003, 23, 989-994. (b) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Mater. Sci. Eng., C 2003, 23, 253-257. (c) Condorelli, G. G.; Motta, A.; Fragala`, I. L.; Giannazzo, F.; Raineri, V.; Caneschi, A.; Gatteschi, D. Angew. Chem., Int. Ed. 2004, 43, 4081-4084. (19) Briggs, D. In Practical Surfaces Analysis, 2nd ed.; Briggs, D., Seah, M. P., Eds.; Wiley-VCH: Weinheim, Germany, 1995; Vol. 1, p 444. (20) Beamson, G.; Briggs, D. High-Resolution XPS of Organic Polymers. The Scienta ESCA300 Database; Wiley & Sons: New York, 1992. (21) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429-7434. (22) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001-4003.

Grafting CaVitands on Si(100)

Langmuir, Vol. 22, No. 26, 2006 11129

Figure 1. High-resolution Si 2p spectra at a takeoff angle of 45° from (a) freshly HF etched silicon, (b) freshly cavitand decorated surface, and (c) 1 week aged cavitand decorated surface.

Figure 2. High-resolution C 1s XPS spectra (takeoff angle 45°) of HF etched Si(100) substrate (a) and of reference samples prepared by immersion of a freshly etched surface in pure mesitylene followed by the same treatments (UV exposure and sonication) adopted for cavitand decorated surfaces (b). The intensities are normalized to the total Si 2p intensity.

Figure 3. High-resolution C 1s XPS spectra of Si(100) substrates after grafting of the MeCavA (left side) and MeCavB (right side): (a) 45° and (b) 10°.

to slight higher BE values; hence, the component overlaps with the more intense C0 band.18b Therefore, the more oxidized are the samples, the less evident is the C-1 component.18b Nevertheless, its presence in freshly MeCavA and B decorated silicon wafers represents a direct evidence of the Si-C bond formation and, in turn, that cavitands are covalently bonded to the silicon surface.

Further experiments were made to rule out either any possible physisorption or alternative grafting to the surface through a Si-O-C bond. Thus, silicon surfaces were exposed to cavitand (MeCavA) solutions in the absence of UV irradiation. XPS C 1s spectrum (Figure 5) does not show significant evidences of cavitand molecules since the band consists of C0 and C+1 components due to Si-O-C adventitious carbon bonded to

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Figure 4. High-resolution C 1s XPS spectra (takeoff angle 45°) of Si(100) substrates after grafting of (a) BrMeCavA and (b) QxCav. Similar spectra have been obtained at a takeoff angle of 10°. Scheme 2. Arrangement of Pure Cavitands on the Si Surface: (a) as Prepared and (B) after Aging

Figure 5. High-resolution C 1s XPS spectrum of Si(100) surface after exposure to cavitand (MeCavA) solution in absence of UV irradiation (takeoff angle 45°).

silicon. No sizable contributions due to the C+2 component are apparent. This provides evidence that grafting of cavitand occurs only under UV irradiation which promotes the hydrosilylation of the olefin group and the formation of a Si-C bond. Further indication of the structure of the grafted layer has been obtained by comparing XPS spectra collected at different takeoff angles (45° and 10°) since they provide information on the in-depth distribution of the carbon bonding states. Figure 3 shows that for both MeCavs, the C+2 component of the oxygenbridging methylene groups slightly increases at the lower takeoff angle (i.e., for shorter sampling depth). By contrast, the C-1 component associated with the Si-C bond practically disappears at 10° takeoff angle. These results are consistent both with Si-C bond grafting and with the upper rim atop the layer. A possible arrangement of pure cavitands on the Si surface is sketched in the Scheme 2. For BrMeCavA and QxCav, no evident angular dependence was observed. In the case of QxCav this observation appears reasonable since the C+2 component is due to the quinoxalinic carbon bonded to the nitrogen and to the oxygen of the resorcin[4]arene ring, which is located in the middle of the molecule (Scheme 1). Instead, the behavior of BrMeCavA is unexpected since the carbon bonding two oxygen atoms (responsible for the C+2 component) is located at the upper rim of the molecule. This observation is consistent with a disordered layer in which the resorcin[4]arene headgroup is not always atop the layer. Figure 6 shows the high-resolution spectral region of the N 1s and Br 3d feature of QxCav and BrMeCavA. In the first case, the band centered at 399.9 eV (Nqx) clearly represents the nitrogens of quinoxaline rings.20 A much less intense side component (Nadv) at 401.1 eV is due to side reaction byproducts (Figure 6a). In the BrMeCavA the Br 3d signal consists of two components (Figure 6b). The first, at 70.5 eV, represents the Br-C moieties

of the cavitand while the other, at 69.0 eV, is evidence of the Br-Si bond (Figure 6b).8c The presence of these two components indicates that both the olefinic and bromoaryl moieties are reactive at H-Si(100) surface under UV irradiations. Previous data on the photochemical Si-grafting of 1-bromoalkenes8c indicate that the UV irradiation promotes the homolitic dissociation of almost all Br-C bonds with the formation of bromo and alkenyl radicals. They lead, respectively, to Si-Br terminations and to the upsidedown grafting of the alkenyl chain with the intact double bonds atop the layer. These results indicate that UV irradiation induces dissociation of some Br-aryl moieties which, analogous to Bralkyl moieties, leads to the formation of Br-Si termination and to the upside-down grafting (Scheme 3). In the present case, however, the Br-C component (70.5 eV) due to Br-aryl bonds is a clear indication that dissociation does not involve all Br-aryl moieties. This is due to the lower reactivity of Br-aryl groups with respect to the corresponding Br-alkyl ones. In addition the side formation of phenyl radical favors side reaction paths leading, in the presence of O2 or H2O traces, to oxidized carbon. These side reactions can explain the greater oxidized carbon contamination observed in the C 1s spectrum of the BrMeCavA as compared to the other MeCavs. The O 1s band of all cavitands consists of two main components centered at 533.5 eV (Ocav) and at 532.1 eV (OSiOx). They represent the Ph-O-C arrangement20 and the SiOx due to Si surface partial oxidation,17b,19 respectively (Figure 7). As expected for a carbonaceous overlayer on the silicon substrate, the OSiOx component decreases at the low takeoff angles. The aging process (1 week of air exposure) increases the relative intensity of OSiOx component and shifts the position to an higher value (532.8 eV) similar to that of the bulk SiO2.17b Both these two observations

Grafting CaVitands on Si(100)

Langmuir, Vol. 22, No. 26, 2006 11131

Figure 6. (a) High-resolution N 1s photoelectron region of the QxCav decorated sample. (b) High-resolution Br 3d photoelectron region of the BrMeCavA decorated surface (takeoff angle 45°).

Figure 7. O 1s region of fresh MeCavA cavitand decorated surfaces at takeoff angles of (a) 45°, (b) 10°, and (c) MeCavA after 1 week aging (takeoff angle 45 °C). Scheme 3. Species Present on the Silicon Surface after the BrMeCavA Grafting Process

Table 2. Relative (%) Contributions of Silicon and C2+ Components to Total Intensity for Pure Cavitand, Cavitand/ 1-Octene Mixtures, and Pure 1-Octene after Aginga sample

fSi 0

Si f+4

C f+2

Nqx/C

MeCavA RMeCavA/octene ) 1/4 QxCav RQxCav/octene ) 1/4 1-octene

89.9 100.0 84.4 100.0 100.0

10.1 0.0 15.6 0.0 0.0

4.8 3.0 7.9 5.1 0.0

5.9 4.6 0.0

a

Scheme 4. MeCav/Octane Monolayer Representation

where ISi is the total spectral intensity and ISi x denotes the intensity of the particular x component. fSi+4 is diagnostic of surface oxidation (SiOx). Present data show that in mixed monolayers the SiOx component due to the aging process decreases. This observation suggests that the layer packing improves for mixed monolayer with low cavitand/octene ratios as compared to pure cavitand layer. The dilution effect on the cavitand concentration over the C surface can be monitored in terms of intensity of the C+2 ( f+2 ) component (%) associated to the cavitand molecule relative to C is defined the total C 1s signal. In analogously with eq 1, f+2 as follow:

The Nqx/C ratio in the QxCav is also reported.

are consistent with the formation of significant amounts of fully oxidized silicon after aging. To improve the surface molecular packing, mixed monolayers were prepared by Si-grafting of mixtures of cavitands and 1-octene (1:4) in mesitylene solutions. Table 2 shows the relative intensities of the Si 2p components for pure cavitands, cavitand/1-octene mixture, and pure 1-octene after aging. The relative (%) contribution fSi x of each component to the total intensity of the Si 2p feature is given by the following equation: Si Si fSi x ) Ix /I × 100

(1)

C C f+2 ) I+2 /IC × 100

(2)

C where IC is the total C 1s spectral intensity and I+2 denotes the intensity of the C+2 component. In addition, in the case of QxCav, the elemental concentration ratio between the quinoxalinic nitrogen (Nqx) and the total carbon signal is also reported in Table 2 since this ratio can be also used as indicator of the surface concentration of the cavitand. Data show that both the C+2 component and the Nqx/C atomic ratio decrease, as expected, upon decreasing the cavitand mole fraction. There is, therefore, evidence that the grafting of cavitand/1octene mixtures leads to mixed monolayers in which the octyl

11132 Langmuir, Vol. 22, No. 26, 2006

Condorelli et al.

Figure 8. AFM images of (a) fresh HF etched Si(100) surface and (b) QxCav 20%. Cross section images on the left and z profiles on the right.

chains fill the voids left under cavitands, thus precluding uncovering of substrate regions as shown in Scheme 4. The roughness and homogeneity of the grafted layers were evaluated by AFM analysis. Figure 8 compares the AFM images of the surface and the cross section before and after grafting of QxCav/1-octene mixtures. QxCav, the largest cavitand used in this study, should protrude from the octane layer about 1.3 nm according to crystal structure measurements.23 The surface of a (23) (a) Dalcanale, E.; Soncini, P.; Bacchilega, G.; Ugozzoli, F. J. Chem. Soc., Chem. Commun. 1989, 500-501. (b) Soncini, P.; Bonsignore, S.; Dalcanale, E.; Ugozzoli, F. J. Org. Chem. 1992, 57, 4608-4612.

etched Si(100) substrate (Figure 8a) appears slightly rough. The average peak to peak Rmean value determined from various images in several areas (600 nm × 600 nm) is equal to 0.4 nm with a 0.12 nm roughness (RMS). After grafting with a 1:4 mixture of QxCav/1-octene (Figure 8b), the surface topography changes completely. Several peaks protruding from the surface are present with typical peak-to-valley height of 1.25 nm in line with the expected X-ray values. The large lateral dimensions of many of the peaks can be attributed either to QxCav cluster formation during grafting or to physisorption of organic molecules. The latter possibility was ruled out by reiterate rinsing of the silicon

Grafting CaVitands on Si(100)

wafer with different organic solvents followed by sonication in dichloromethane. The surface topography did not change after sonication, excluding physisorption as the origin of lateral clustering (see Supporting Information). It is important to notice that the relatively high inherent roughness of Si(100) limits the possibility to obtain molecular resolution topography of vicinal objects.11c,24

Conclusions This work reports the first example of cavitand molecules grafted on Si(100), hence on surfaces suited for integration into well-established silicon technologies. The XPS analysis has been used to provide an extensive study of the grafting process for four different cavitands. Cavitand grafting takes place through the hydrosilylation reaction of the terminal alkyl chain double bonds, which leads to the formation of strong Si-C bonds, with the cavities anchored in the atop layer. However, in the case of BrMeCavA the presence of Br-aryl moieties leads to side reactions that can determine the formation of Br-Si terminations of the substrate surface as well as the upside-down grafting (24) Cattaruzza, F.; Criceti, A.; Flamini, A.; Girasole, M.; Longo, G.; Mezzi, A.; Prosperi, T. J. Mater. Chem. 2004, 14, 1461-1468.

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of the cavitand. Pure cavitands are not densely packed due to the steric hindrance among their headgroups and substrate oxidation occurs, after aging, in the substrate surface left uncovered by the organic layer. By contrast, the use of cavitand/1-octene mixtures allows the anchoring of a denser layer in which the octyl chains cover the voids left under the cavitand heads and between cavitands, thus preventing silicon oxidation. A structured surface topography is observed upon grafting of QxCav/octane mixed monolayers, with peak-to-valley heights of the protruding objects in line with the expected values. Finally, the present study shows that complex functional molecules can be covalently bonded to a silicon surface, making them of relevance for the development of silicon-based molecular devices. Acknowledgment. The authors thank the MIUR (FIRB 2003 and PRIN 2005 research programs) for financial support. Supporting Information Available: AFM images of QxCav 20% grafted on Si(100) surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA060682P