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Comparison of Si-O-C Interfacial Bonding of Alcohols and Aldehydes on Si(111) Formed from Dilute Solution with Ultraviolet Irradiation Christina A. Hacker,*,† Kelly A. Anderson,‡ Lee J. Richter,§ and Curt A. Richter† National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899 Received May 10, 2004. In Final Form: October 8, 2004 Aliphatic alcohols and aldehydes were reacted with the Si(111)-H surface to form Si-O-C interfacial bonds from dilute solution by using ultraviolet light. The resulting monolayers were characterized by using transmission infrared spectroscopy, spectroscopic ellipsometry, and contact angle measurements. The effect of different solvents on monolayer quality is presented. The best monolayers were formed from CH2Cl2. The optimized monolayers were thoroughly characterized to determine the film structure and monolayer stability. The UV-promoted, alcohol-functionalized, and aldehyde-functionalized monolayers are of comparable quality to those previously prepared by other means. Although both molecules are tethered through a Si-O-C bond, the film reactivity is distinctly different with the aldehyde films being more chemically resistant. The differences in chemical reactivity, vibrational spectra, hydrophobicity, and ellipsometric thickness between the alcohol and aldehyde monolayers are attributed to a difference in molecular coverage and monolayer formation.
Introduction The attachment of organic molecules directly to silicon surfaces has been of great interest for a variety of applications including molecular electronics, passivating agents, immobilization of biological molecules, and sensors.1-9 As researchers look to more diverse and revolutionary uses of silicon, methods of functionalizing the surface become a key component to the success of these applications. Moreover, since each monolayer fabrication method has inherent advantages and disadvantages associated with it, it is important for researchers to have at hand a variety of well-understood surface modification procedures.10,11 As has recently been pointed out, a variety of fabrication approaches are necessary before the technically relevant breakthrough of silicon-molecule hybrids becomes available.12 Monolayers on silicon are particularly attractive for molecular electronics, as molecules are tethered through a strong covalent bond making them amenable to further * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Semiconductor Electronics Division, Electronics and Electrical Engineering Laboratory. ‡ NSF-Summer Undergraduate Research Fellow, currently at University of Maryland. § Surface and Microanalysis Science Division, Chemical Science and Technology Lab. (1) Stewart, M. P.; Maya, F.; Kosynkin, D. V.; Dirk, S. M.; Stapleton, J. J.; McGuiness, C. L.; Allara, D. L.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 370-378. (2) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2004, 302, 1543-1545. (3) Vuillaume, D.; Boulas, C.; Collet, C.; Davidovits, J. V.; Rondelez, F. Appl. Phys. Lett. 1996, 69, 1646-1648. (4) Vuillaume, D.; Boulas, C.; Collet, C.; Allan, G.; Deleru, C. Phys. Rev. B 1998, 58, 16491-16498. (5) Lee, H. N.; Cho, J. H.; Kim, H. J. Jpn. J. Appl. Phys. 2003, 42, 6678-6682. (6) Liu, Y. J.; Yu, H. Z. J. Phys. Chem. B 2003, 107, 7803-7811. (7) Headrick, J. J.; Sepaniak, M. J.; Lavrik, N. V.; Datskos, P. G. Ultramicroscopy 2003, 97, 417-424. (8) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289-1292. (9) Bergveld, P. Sens. Actuators, A 1996, 56, 65-73. (10) Buriak, J. M. Chem. Rev. 2002, 102, 1271-1308.
10.1021/la048841x
processing and less likely to desorb or degrade with time. The added advantage of the processing technology and infrastructure from the silicon microelectronics industry makes it easy to fabricate molecular electronic test structures and makes the emergence of molecular electronics a less-disruptive technology. For molecular electronics applications, siloxane monolayers on the silicon oxide surface have shown promise as insulators.3,4 However, to fully utilize the electronic properties of the semiconductor, it is desirable to attach monolayers directly to the silicon surface. In addition, monolayers bound directly to silicon are expected to have less interfacial capacitance,13,14 be more amenable to further processing, and be resistant to degradation due to the nature of the strong covalent bond.10 Much research has been performed to create organic monolayers bound to silicon directly through Si-O-C bonds by using radical initiators, heat, and surface pretreatment.15-24 Each of these procedures has certain disadvantages, in terms of available materials, surface (11) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 23-34. (12) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (13) Cahen, D.; Kahn, A. Adv. Mater. 2003, 15, 271-277. (14) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384-1389. (15) Zhu, X. Y.; Boiadjiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Langmuir 2000, 16, 6766-6772. (16) Chazalviel, J. N. J. Electroanal. Chem. 1987, 233, 37-48. (17) Zharnikov, M.; Kuller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Langmuir 2003, 19, 4682-4687. (18) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001-4003. (19) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Langmuir 2000, 16, 7429-7434. (20) Roth, K. M.; Yasseri, A. A.; Liu, Z. M.; Dabke, R. B.; Malinovskii, V.; Schweikart, K. H.; Yu, L. H.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505-517. (21) Barrelet, C. J.; Robinson, D. B.; Cheng, J.; Hunt, T. P.; Quate, C. F.; Chidsey, C. E. D. Langmuir 2001, 17, 3460-3465. (22) Bateman, J. E.; Eagling, R. D.; Horrocks, B. R.; Houlton, A. J. Phys. Chem. B 2000, 104, 5557-5565. (23) Haber, J. A.; Lauermann, I.; Michalak, D.; Vaid, T. P.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 9947-9950. (24) Haber, J. A.; Lewis, N. S. J. Phys. Chem. B 2002, 106, 36393656.
This article not subject to U.S. Copyright. Published 2005 by the American Chemical Society Published on Web 12/22/2004
Comparison of Si-O-C Interfacial Bonding on Si(111)
contamination, and surface patterning. Additionally, most of these techniques require the use of neat liquids leading to unnecessary use of costly or rare materials. Furthermore, larger molecules of interest for molecular electronic applications are nearly exclusively solids, necessitating the use of a solvent or heat. Previous researchers have focused primarily on thermal attachment of neat alcohols to the hydrogen-terminated silicon(111) surface via nucleophilic attack of the Si-H bond by the -OH of the alcohol.15,17,19,21,22 Previous work has achieved high-quality films obtained by heating dilute solutions (2.5-25% volume fraction) of alkenes, allowing the production of molecular films from smaller quantities of material.25 Ultraviolet irradiation has been utilized extensively to attach unsaturated molecules, such as alkenes and aldehydes, to the hydrogen-terminated Si(111) surface from neat liquid.10,11,26-29 This facilitates direct patterning of the substrate.26 Despite the widespread use of irradiation for attachment of organic molecules to Si(111), UV-promoted attachment of saturated molecules, such as alcohols, has not been investigated. Moreover, the optimal choice of solvent for monolayer formation has not been investigated. We report here investigations of the photochemical preparation of Si-O-C monolayers from dilute solutions of aliphatic alcohols and aldehydes at the H-terminated Si(111) surface. Transmission Fourier transform infrared spectroscopy (FTIR), contact angle (CA), and spectroscopic ellipsometry(SE) are used to characterize the molecular films formed by UV (254 nm) irradiation of solutions of primary alcohols and aldehydes varying in length from decanol to octadecanol and decyl aldehyde to tridecyl aldehyde. Heptane, isooctane, and dichloromethane were investigated as solvents. The structure and reactivity of optimal alcohol and aldehyde monolayers are presented and possible reaction mechanisms discussed. Experimental Methods
Langmuir, Vol. 21, No. 3, 2005 883 Table 1. Comparison of Octadecanol-Si Monolayers Formed from Different Solvents solvent
SE thicknessa (nm)
CAb (deg)
ν CH2(a) intensityc (abs)
heptane isooctane dichloromethane
2.0 1.9 2.2
100 93 112
10.6 × 10-4 9.0 × 10-4 14.5 × 10-4
a Fit using n ) 1.52. Values have an uncertainty of (0.1 nm. Values have an uncertainty of (2°. c Absorbance intensity has an uncertainty of 5 × 10-5.
b
were routinely illuminated for 2 h on each side with care taken to maintain a constant solvent level. After illumination, the samples were removed from solution, rinsed with CH2Cl2, sonicated twice in CH2Cl2 (5-10 min each), and dried with streaming nitrogen. Characterization of the organic-functionalized silicon samples occurred directly after preparation to minimize film degradation and substrate oxidation. Monolayer Characterization. CA measurements are an effective and relatively effortless means to discern macroscopic surface properties of molecular films.30,31 In comparison with similar molecular monolayers, CA data yield a quick indication of the hydrophobicity, which is often tied to molecular packing and surface coverage. Sessile water-drop CA measurements were obtained by using a video camera-based commercial apparatus. The reported value is the average of three measurements of deionized water (18.2 MΩ cm) on each organic monolayer. Transmission infrared spectroscopy was obtained by using a commercial Fourier transform infrared spectrometer with a wire grid polarizer and a sample holding accessory to attain nominally Brewster’s angle incidence. The resolution was 8 cm-1; typical spectrum acquisition time was 5 min. The spectral intensities are reported in terms of absorbance units (-log (T/To) where T is the transmitted power of the infrared beam from the sample and To is the transmitted power from the reference) using either a double-side polished Si(111) native oxide or a freshly prepared Si(111)-H sample as a reference. Reported intensities and characteristic frequencies were obtained from nonlinear leastsquares fits of the absorbance spectra to the sum of six Lorentzian lines and are the average over independently prepared samples. Spectroscopic ellipsometry measurements were performed over a wavelength range of 190-1000 nm by using a commercial instrument at a nominal angle of incidence and reflectance of 70° from the surface normal. The optical thickness of the organic monolayer was obtained by fitting the SE data to a three-phase model (Si|organic|air) assuming an index of refraction of n ) 1.52 for the alcohol and aldehyde monolayers.32 The value of 1.52 has been used extensively as the refractive index of alkanethiol self-assembled monolayers on gold and provides a comparison with that well-characterized aliphatic surface.33
Monolayer Formation. All chemicals were purchased ACS grade or better (decanol 99%, tridecanol 97%, tetradecanol 97%, pentadecanol 99%, hexadecanol 99%, heptadecanol 98%, octadecanol 99%, decyl aldehyde 95%, undecyl aldehyde 97%, dodecyl aldehyde 92%, tridecyl aldehyde 90%) and used as received. Double-side polished Si(111) wafers (p-type, 2-15 Ω cm) were diced into approximately 1 cm × 1 cm squares and cleaned by sonication in acetone and methanol then dried with streaming nitrogen. The hydrogen-terminated surface was created by immersion (3-5 min) into cleanroom-grade 6:1 buffered oxide etch (NH4F:HF). The Si(111)-H samples were rinsed with 18.2 MΩ cm water, dried with nitrogen, and placed in a previously cleaned reaction vessel inside a nitrogen glovebox. Alcohol and aldehyde solutions were made using heptane, isooctane (2,2,4trimethylpentane), and dichloromethane solvents with a typical concentration of ∼10 mM. A thin layer of alcohol or aldehyde solution was added to the reaction vessel by using a pipet, and care was taken to keep the Si(111)-H sample immersed while maintaining a thin solution layer to maximize UV intensity at the surface. Irradiation was carried out with a 6 W ultraviolet lamp (254 nm) with an estimated intensity of approximately ∼1500 µW cm-2 at the sample, based on manufacturer’s specifications. Illuminating the sample longer than 2 h showed no measurable changes in monolayer quality; therefore, samples
Solvent Effect. Dodecanol (C12H25OH) has a reported melting point range of 24-27 °C.34 With this as a reference, alcohol molecules shorter than 12 carbon atoms exist as liquids, while longer alcohols exist as solids under typical laboratory conditions. To attach solid alcohols to the silicon surface with ultraviolet radiation, dilute solutions were made by using solvents of heptane, isooctane, and dichloromethane. The physical and chemical properties of octadecanol (C18H37OH)-functionalized Si(111) samples formed from these solvents are presented in Table 1. Samples formed from dichloromethane solutions showed
(25) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudholter, E. J. R. Langmuir 1999, 15, 8288-8291. (26) Effenberger, F.; Gotz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem. Int. Ed. 1998, 37, 2462-2464. (27) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695. (28) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (29) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155.
(30) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974-2981. (31) Chang, S. C.; Chao, I.; Tao, Y. T. J. Am. Chem. Soc. 1994, 116, 6792-6805. (32) The thickness uncertainity is (0.1 nm based on one standard deviation. (33) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90-94. (34) Handbook of Chemistry and Physics; CRC Press: Boca Raton, 2004.
Results
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Table 2. Comparison of Tridecyl Aldehyde-Si Monolayers Formed from Different Reaction Conditions reaction conditions
SE thicknessa (nm)
CAb (deg)
ν CH2(a) intensityc (abs)
neat isooctane dichloromethane evaporated
1.8 1.5 1.9 2.7
98 79 98 65
4.0 × 10-4 2.6 × 10-4 6.2 × 10-4 14.0 × 10-4
a Fit using n ) 1.52. Values have an uncertainty of ( 0.1 nm. Values have an uncertainty of ( 2°. c Absorbance intensity has an uncertainty of 5 × 10-5.
b
higher CA, higher film thicknesses, and stronger vibrational intensity indicating a higher density of molecules on the surface than samples formed from heptane and isooctane solutions. The intermolecular forces between heptane and the aliphatic alcohol molecules may be large enough for heptane to become integrated into the alcohol monolayer, which may inhibit complete monolayer formation. Previous observations of alkanethiols on gold grown from heptane solutions have demonstrated increased pitting or voids in alkanethiol-gold monolayers attributed to solvent interactions.35 Although reported thermal reactions of alcohols from isooctane solutions produced high-quality monolayers,15 the photochemical monolayers formed from isooctane solutions in this study are of lesser quality despite the added steric hindrance of the branched aliphatic solvent. Since both heptane and isooctane are fully saturated molecules, they are unlikely to attach to the Si-H surface and are probably removed during the rinse, resulting in a less-dense monolayer chemically bonded to the surface. Dichloromethane, a smaller, more-polar solvent, formed monolayers of highest quality as indicated by the results shown in Table 1. However, the high vapor pressure of dichloromethane sometimes led to evaporation of the solvent, leaving behind a thin film of solid alcohol covering the silicon sample. Characterization of monolayers formed under these solvent evaporation conditions showed a lessdense monolayer was formed, suggesting the mobility provided by the solution was needed to form dense alcohol monolayers. Tridecyl aldehyde (CH3(CH2)11HCdO) has a reported melting point of 14 °C;34 however, under standard laboratory conditions, this molecule exists as a thick waxy substance. The data obtained from monolayers of tridecyl aldehyde-functionalized Si(111) samples prepared under differing reaction conditions are presented in Table 2. (For reference, optimal tridecanol monolayers exhibited a CA ) 108°, SE ) 1.65 nm, FTIR CH2(a) ) 5.8 × 10-4, as discussed in more detail later.) Since the UV lamp heats the silicon sample to roughly 27 °C, as measured from a thermocouple attached to the back of the sample, we initially tried to react neat tridecyl aldehyde. Although the tridecyl aldehyde remained a waxy substance throughout 24 h of ultraviolet illumination, FTIR analysis of these samples indicated the presence of organic molecules on the surface. Upon closer inspection, the FTIR spectra contained peaks consistent with physisorbed aldehyde. Most notably, a substantial CdO peak at 1736 cm-1 and a C-H stretch at 2820 cm-1 were evident in the spectra indicative of the presence of unreacted aldehyde molecules. Exposure to a 1 × 10-2 mol L-1 solution of tridecyl aldehyde in isooctane showed no evidence of physisorbed aldehydes; however, SE, CA, and FTIR analysis indicated incomplete monolayer formation. In an attempt to form complete monolayers, samples were prepared from dichloromethane solutions both by the normal procedure and by allowing the solvent to evaporate. Characterization of samples
Table 3. Summary of Contact Angle and Spectroscopic Ellipsometry Data Obtained from CH3(CH2)n-1O-Si Monolayers on Silicon CAa(deg)
SE thicknessb (nm)
n
alcohol
aldehyde
alcohol
aldehyde
10 11 12 13 14 15 16 17 18
105
96 100 97 98
1.60
1.53 1.76 1.72 1.93
108 104 104 103 106 112
1.65 1.67 1.83 1.84 2.13 2.20
a Values have an uncertainty of ( 2°. b Fit using n ) 1.52. Values have an uncertainty of ( 0.1 nm.
Figure 1. Monolayer thickness determined from SE data (n ) 1.52) of aldehyde (triangles)- and alcohol (squares)-functionalized monolayers. The nominal molecular length (gray line) shown for reference was measured from the oxygen atom to terminal carbon atom.
prepared with evaporated solvent (listed as “evaporated” in Table 2) showed substantial evidence of physisorbed material and multilayers. Samples remaining in dichloromethane solution throughout the length of UV illumination formed optimal monolayers with no evidence of physisorbed molecules. Thus, the optimal solvent for forming Si-O-C monolayers from both aldehydes and alcohols was determined to be dichloromethane. Properties of Optimal Monolayers. Samples produced by using the optimal dichloromethane solutions were subjected to detailed characterization to understand the chain-length dependence of the film structure. The data obtained from CA analysis of alcohol- and aldehydefunctionalized silicon samples are summarized in Table 3. The CA data consistently measured between 103° and 112° for alcohol samples, while the aldehyde samples exhibited a slightly lower CA, between 96° and 100°. Although there is a slight difference in the CA of the alcohol and aldehyde surfaces, both types of molecules form a hydrophobic surface, as indicated by the consistent CA value greater than 90°. The film thickness values obtained from spectroscopic ellipsometry characterization of alcohol- and aldehydefunctionalized monolayers are reported in Table 3. Figure 1 presents the data graphically with the ideal chain length measured from the oxygen atom to the terminal carbon atom, as a reference.36 As shown in Figure 1, the thicknesses of the alcohol monolayers agree reasonably well with the expected molecular length, with the exception of decanol, which is optically thicker than expected. The aldehyde-functionalized surfaces are slightly thicker than (35) Yamada, R.; Sakai, H.; Uosaki, K. Chem. Lett. 1999, 667-668. (36) Length determined from molecular structure obtained after MM2 energy minimization.
Comparison of Si-O-C Interfacial Bonding on Si(111)
Langmuir, Vol. 21, No. 3, 2005 885
Figure 2. Transmission FTIR spectra of the aliphatic stretch region for alcohol-functionalized monolayers and aldehydefunctionalized monolayers. Table 4. Summary of Transmission FTIR Dataa Obtained from CH3(CH2)n-1O-Si Monolayers on Silicon alcohol n 10 11 12 13 14 15 16 17 18
aldehyde
ν CH2(a) (cm-1)
ν CH2(a) (abs)
ν CH3(a) (abs)
ν CH2(a) (cm-1)
ν CH2(a) (abs)
2928
2.3 × 10-4
4.2 × 10-4
2922 2924 2925 2924 2924 2923
5.8 × 10-4 8.8 × 10-4 10.1 × 10-4 12.6 × 10-4 14.0 × 10-4 14.5 × 10-4
4.5 × 10-4 3.0 × 10-4 4.4 × 10-4 3.4 × 10-4 4.5 × 10-4 2.6 × 10-4
2920 2919 2921 2920
3.7 × 10-4 2.9 × 10-4 4.0 × 10-4 6.2 × 10-4
a Values have an uncertainty of (1 cm-1 and ( 5 × 10-5 absorbance units.
expected for all of the chain lengths but generally increase with increasing number of methylene groups. The monotonic increase in SE data for the alcohol-functionalized surface indicates the extent of attachment, and the molecular packing at the surface is similar for all of the alcohols. The thickness of alcohol-functionalized monolayers is comparable to reported SE thicknesses of Si-alkene29 and Au-alkanethiol33 monolayers, within 0.2 nm. X-ray reflectivity studies of thermally prepared octadecanol-Si(111) monolayers indicated a monolayer thickness on the order of 1.8-2.0 nm.15 These findings suggest UV-preparation of alcohol monolayers are of equivalent thickness to UV-prepared alkene monolayers, as well as thermally prepared alcohol monolayers. Representative vibrational spectra for alcohol and aldehyde monolayers are presented in Figure 2. The peak intensity of CH2 and CH3 asymmetric stretches and the frequency of each peak are listed in Table 4 for both the alcohol- and aldehyde-functionalized samples. The intensity of the CH2 asymmetric stretch (νd-, near 2925 cm-1) increases monotonically with increasing number of CH2 groups for both alcohol and aldehyde samples, indicating similar coverage for all of the different chain lengths (Figure 3A), consistent with the SE data. However, the frequency of the asymmetric CH2 stretch, νd-, is distinctly different for the alcohol- vs aldehyde-formed monolayers (see Table 4). In the alcohol-formed films, νd- appears closer in value to one obtained from an isotropic liquid spectrum (∼2925 cm-1), while the aldehyde films are closer in value to that of an all-trans ordered crystalline phase (∼2918 cm-1), indicating a greater amount of gauche defects within the alcohol monolayers.37-39 The peak intensity of the CH3 asymmetric stretch of the alcoholfunctionalized monolayer is grouped into two distinct absorbance values correlating with even and odd numbers of methylene repeat units, as shown in Figure 3B. The
Figure 3. Graphs showing chain length dependence of (A) intensity of the methylene asymmetric stretch and (B) intensity of methyl asymmetric stretch.
uniqueness of this feature suggests differing orientation of the methyl group for the two different classes of chain lengths. The asymmetric methyl stretch from the aldehyde data does not show this trend. The spectra from decanol were consistently distinct from the longer alcohols, with very broad features and blue-shifted νd- (2927 cm-1), suggestive of significant disorder within the film. The alcohol OH and aldehyde HCdO vibrational stretches can provide information on how the molecule is bonded to the silicon surface. The OH stretch, expected to occur near 3300 cm-1, and the OH bend, expected in the 1600 cm-1 region, were absent in the vibrational spectra of alcohol-functionalized samples. The optimal aldehydefunctionalized surfaces exhibited no features characteristic of an aldehyde at 1736 and 2820 cm-1, with the exception of the few samples containing physisorbed aldehyde molecules. The SiH vibrational mode at 2082 cm-1, indicative of a monohydride surface termination, is not observed after exposure to alcohols and aldehydes (see Supplemental Information). Although, it is physically impossible for 100% coverage due to geometric constraints (the area per surface Si atom is 0.128 nm2 and of a crystalline alkyl chain is 0.186 nm2 40), previous studies of high-quality monolayers also saw the disappearance of the Si-H stretch attributed to broadening.19 The absence of the Si-H, OH, and HCdO vibrational stretches in the infrared spectra indicates these groups are involved in monolayer formation. (37) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (38) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (39) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623-5630. (40) Craievich, A. F.; Denicolo, I.; Doucet, J. Phys. Rev. B 1984, 30, 4782-4787.
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Hacker et al. Table 5. Summary of Data Obtained from Controls of Octadecanol and Tridecyl Aldehyde Monolayers Formed from CH2Cl2 Solutions SE thicknessa (nm)
Figure 4. Transmission IR spectra obtained using Si-H as a reference of native oxide, dodecyl aldehyde-functionalized, octadecanol-functionalized, and octadecene-functionalized (magnified 3×) Si(111) substrates.
To achieve sufficient surface sensitivity, most researchers have used multiple internal reflection (MIR) geometry to acquire vibrational data from monolayers on silicon. However, phonon absorption of bulk silicon prohibits study of the spectral region below ∼1400 cm-1. Our transmission IR geometry allows us to examine the changes in the IR spectral region below 1400 cm-1 and observe the Si-O vibrational stretching region. The Si-O vibrational region is very sensitive to the orientation of Si-O bonds and sample preparation.41,42 Figure 4 shows a series of FTIR spectra in this region. Presented are native oxide, octadecanol, dodecyl aldehyde, and octadecene spectra referenced to that of a Si-H sample. The spectrum obtained from the cleaned native oxide surface contains peaks at 1050 and 1220 cm-1 attributed to Si-O LO and TO phonons and a peak at 1105 cm-1.41 Comparison of the molecular-functionalized spectra with the native oxide shows the oxide related peaks are not evident indicating the absence of a significant oxide layer at the interface. It is anticipated that a vertically oriented Si-O-C bond should lie at the 1105 cm-1 based on the spectra of model silane molecules.22,43-45 Unfortunately, a strong artifact can appear, peaked at 1105 cm-1, due to incomplete cancellation of the absorption of bulk interstitial oxygen contaminants.46 The magnitude of this artifact (based on studies of identically processed H terminated substrates) is comparable to the feature in Figure 4 and thus the origin of the 1105 cm-1 peak cannot be determined with certainty. The absence of vibrational modes attributed to the alcohol and aldehyde functional groups and the absence of the Si-H modes indicate an interfacial SiO-C bond is formed by ultraviolet irradiation of alcohols and aldehydes. The organic films are tethered by SiO-C bonds and are monolayer thick, as evidenced by the SE data. Nonspecific Adsorption. To further understand the role of ultraviolet light, control experiments were performed, with the results summarized in Table 5. Exposing a SiO2 surface to ultraviolet radiation in the presence of octadecanol and tridecyl aldehyde produced no attachment of either molecule to the oxide surface. Characterization (41) Weldon, M. K.; Queeney, K. T.; Chabal, Y. J.; Stefanov, B. B.; Raghavachari, K. J. Vac. Sci. Technol. B 1999, 17, 1795-1802. (42) Demkov, A. A.; Liu, R.; Zhang, X.; Loechelt, H. J. Vac. Sci. Technol. B 2000, 18, 2388-2394. (43) Bateman, J. E.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1997, 93, 2427-2431. (44) Kim, N. Y.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 22972298. (45) Deshmukh, S. C.; Aydil, E. S. J. Vac. Sci. Technol. A 1995, 13, 2355-2367. (46) Oates, A. S.; Lin, W. J. Cryst. Growth 1988, 89, 117-123.
CAb (deg)
ν CH2(a) intensityc (abs)
SiO2 UV Si-H UV Si-H ambient Si-H dark
Si-Octadecanol 2.0 66 2.2 112 1.7 92 1.3 84
none 14.5 × 10-4 3.4 × 10-4 1.4 × 10-4
SiO2 UV Si-H UV Si-H ambient Si-H dark
Si-Tridecyl Aldehyde 2.2 65 1.9 98 2.2 95 1.2 79
none 6.2 × 10-4 4.0 × 10-4 2.6 × 10-4
a Fit using n ) 1.52. Values have an uncertainty of ( 0.1 nm. Values have an uncertainty of ( 2°. c Absorbance intensity has an uncertainty of 5 × 10-5.
b
of SiH surfaces exposed to ambient light for 24 h in the presence of octadecanol and tridecyl aldehyde indicated an alcohol coverage only 20% that of the UV prepared sample and an aldehyde coverage about 60% that of the UV sample based on FTIR analysis. While it is apparent that UV light promotes the formation of denser films, there is evidence of organic attachment with extended exposure to ambient room lighting. SiH surfaces kept in the dark for 24 h in the presence of octadecanol and tridecyl aldehyde produced an alcohol coverage only 10% that of the UV prepared sample and an aldehyde coverage about 40% that of the UV sample based on FTIR analysis. Previous studies had observed the slow direct attachment of short-chain alcohols without light or heat,16,18 with coverages in agreement with those seen here. Close examination of the alcohol dark control FTIR spectra in the Si-O-C stretching region indicated the formation of an oxide layer. Thus, the SE thicknesses of the dark samples are greater than expected due to the formation of an SiOx oxide layer. Similar examination of the aldehyde dark control FTIR spectrum showed there were no peaks indicative of the parent aldehyde, suggesting the reaction of aldehydes with the Si-H surface in the dark is more favorable than the reaction of alcohols in the dark. Monolayer Stability. Monolayer quality on silicon is often associated with the ability to protect the silicon interface from oxidation. To understand whether oxidative aging alters chemically bonded alcohol and aldehyde monolayers, stored samples were further characterized. Octadecanol samples stored for 7 days in ambient conditions showed an immeasurable change in the ellipsometric thickness; however, the CA was 5-10° lower, indicating a change in the hydrophobicity possibly due to an increase in the disorder of the surface or contamination. These octadecanol-Si samples exhibited a slight decrease in the FTIR CH2 intensity, which changed from 12.5 × 10-4 to 10.6 × 10-4, and evolution of a weak silicon oxide feature suggesting oxide formation and loss of organic molecules. After these stored samples were re-sonicated in dichloromethane to remove any organic contamination, analysis of the CA and spectroscopic ellipsometry thickness indicated an additional decrease to ∼98° in the CA and a thickness decrease of 0.2 nm, suggesting further removal of organic molecules. In contrast, alcohol-functionalized samples stored for 7 days in an inert nitrogen atmosphere exhibited no measurable change in the SE, CA, or FTIR values. To understand whether fresh samples were susceptible to oxide formation, octadecanol-functionalized silicon samples were placed in 60 °C deionized water for 30 min and the CA and SE data were acquired again. The resulting
Comparison of Si-O-C Interfacial Bonding on Si(111)
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samples showed the CA dramatically changed from >100° to between 30° and 40°, while the ellipsometric thickness measured approximately 2.0 nm, suggesting significant loss of organic molecules and substantial oxide formation. In fact, IR spectra of a fresh octadecanol sample showed an asymmetric CH2 peak intensity of 1.4 × 10-3, while after immersion in a 60 °C water bath, the intensity was reduced to 6.2 × 10-4. These findings closely match previous research examining the chemical stability of thermally formed alcohol monolayers.19 Characterization of different-chain-length alcohols yielded similar SE, CA, and IR data, indicating that the extent of oxide formation was independent of the alkyl chain length. The chemical instability of the alcohol-modified substrates is consistent with previous electrochemical results postulating the polarity of the Si-O bond relative to the Si-C bond makes alcohol-functionalized silicon surfaces more susceptible to oxidation than alkene-functionalized surfaces.16,23 Aldehyde-functionalized samples allowed to age overnight and sonicated in dichloromethane showed only small decreases in the FTIR methylene stretches and immeasurable changes in the SE and CA values. Tridecyl aldehyde samples placed in 60 °C water for 30 min showed no significant changes in the FTIR, SE, or CA data. Because aldehyde samples exhibited resistance to oxide formation in water, the samples were also exposed to 0.24 mol L-1 HCl solutions for 10 min. Characterization of these samples showed minimal changes in the values obtained from SE and CA, while FTIR analysis of four samples ranging in length from decyl aldehyde to dodecyl aldehyde showed a 20% decrease in absorbance intensity. Thus, aldehyde monolayers appear to be more resistant to oxidation than alcohol-functionalized surfaces, despite the shorter chain lengths investigated here. Discussion The structure of the alcohol- and aldehyde-formed monolayers is qualitatively similar. Both films are hydrophobic, have optical thickness comparable to the molecular length, and are chemisorbed via a Si-O-C linkage. In all regards, the UV-promoted films are similar to previous reports of thermally prepared films.15,17,19,21,22 Given the strong similarities between the alcohol and aldehyde formed films, it is remarkable that their chemical stabilities are significantly different. A similar variation in the stability was observed for thermally formed decanol and decyl aldehyde monolayers.19 The variation in stability is attributed to greater areal density of the aldehyde films, consistent with both the ellipsometry and FTIR. In general, one cannot uniquely determine both the index of refraction and the film thickness from ellipsometric data on ultrathin films. This can be seen if one expands the expression for the ellipsometric ratio, F, to first order in the film thickness, t
F≡ r0p r0s
[
rp ≡ tan Ψei∆ ∼ rs
1-I
]
(n2 - 1) (n2 - ) 4π t + ... cos θi λ ( - 1) cot2 θi - 1 n2
where λ is the incident wavelength, θi is the angle of incident light, t is the film thickness, n the film index of refraction, and is the substrate dielectric function. On high-dielectric-constant substrates (such as metals or semiconductors), the ellipsometric response is determined by t(n2 - 1)/n2. The index of refraction of the film can be
modeled by the Clausius-Mossotti relationship
n2 - 1 4π Nσ R ) 3 t n2 + 2 where R is the polarizability of a methylene (1.83 × 10-24 cm3),47 σ is the areal density of chains, t is the film thickness, and N the number of carbons and oxygens per unit cell (neglecting the polarizability difference between the terminal methyl, Si-O-C, and the interior methylene groups). The methylene density in the film is essentially Nσ/t. The assumed value of 1.52 corresponds to a density of 0.92 g cm-3, comparable to that of high-density polyethylene (HDPE, n ) 1.54).48 The modeled film thickness for the aldehyde monolayers is slightly thicker than that of the alcohol monolayers and the ideal molecular length. The presence of multilayers and physisorbed material can be ruled out since there were no detectable peaks attributed to aldehyde groups seen in the FTIR. The greater optical thickness of the aldehyde layers suggests a greater density. If we assume the film thickness is the ideal molecular length (1.64 nm), then the ellipsometric data for tridecyl aldehyde indicates that the film index is 1.73, which corresponds to σ ) 6.1 × 1014 cm-2. Similar arguments for the tridecanol films indicate a film index of 1.53 and an areal density of 4.7 × 1014 cm-2. Since the details of the Si-O-C interface were neglected, these values most likely represent upper limits of the true areal density. Recent theoretical work modeled the monolayers formed by Si-O-C linkages and determined that the optimal areal density of an octadecyl aldehyde monolayer is 5.2 × 1014 cm-2.49 The vibrational spectra provide further insight into the contrast of the aldehyde and alcohol film structures. The νd- vibration, near 2920 cm-1, for the aldehyde films was consistently 3 cm-1 lower than that observed for the alcohol monolayers (see Table 4), indicating less gauche disorder in the aldehyde molecular chains, consistent with a higher areal density and greater steric constraints on chain flexibility. Similarly, the half width of the methylene asymmetric stretch was 2 cm-1 narrower for the aldehyde films with respect to the alcohol films (8.3 cm-1 vs 10.5 cm-1) again consistent with less disorder within the aldehyde films.50 Qualitative insight into the chain tilt can be obtained from the CH3 asymmetric vibrational data of Table 4, Figure 2, and Figure 3. For alcohol monolayers, the intensity alternates between two unique values, correlated with even and odd number of methylene units, as shown in Figure 3. This intensity alternation is not evident in the aldehyde spectra. The observance of this alternation suggests the average orientation of the alkane chain within the alcohol monolayer is tilted away from the surface normal, resulting in two distinct orientations for the terminal CH3 group depending on the number of carbon atoms. The lack of intensity alternation in the aldehyde data indicates the ensemble distribution of the terminal CH3 group does not alternate as a function of chain length, suggesting the molecular orientation of aldehydes may be more vertical than alcohols, consistent with a higher areal density. A more-vertical orientation is also supported (47) Derived from the Clausius-Mossotti relationship using highdensity polyethylene n ) 1.54 and s ) 0.95 obtained from ref 46. (48) Polymer Handbook; John Wiley and Sons: New York, 2003. (49) Pei, Y.; Ma, J.; Jiang, Y. S. Langmuir 2003, 19, 7652-7661. (50) The null hypothesis is rejected at the 95% confidence level in a two-sample, unequal variance t-test with 12 degrees of freedom and 22 observations. In other words, at the 95% confidence level, the two numbers are significantly different.
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by the absence of a detectable splitting of the CH3 symmetric stretch of the aldehyde films. For the neargrazing angle of incidence employed, the FTIR spectrum is most sensitive to vibrations with a transition dipole moment projection along the surface normal. For an upright chain, the out-of-plane asymmetric stretch (expected to lie near 2955 cm-1) will be significantly weaker than the in-plane asymmetric stretch (2965 cm-1). The increased line widths of the alcohol films prohibit characterization of the out-of-plane asymmetric stretch. Because the alcohol molecular chains contain gauche defects, we cannot unequivocally state whether this unique even-odd effect is caused by the Si-O-C interfacial bond angle or molecular packing that maximizes intermolecular interactions. Theoretical modeling of Si-O-C interfaces observed gauche defects are mainly concentrated near the substrate, allowing the chain length to maximize van der Waals interactions.49 Moreover, previous research on the influence of surface roughness concluded van der Waals interactions could play a role both in ordering the alkyl chains and in governing the growth mechanism itself.51 With these previous works in mind and the observance of substantial gauche defects within the film, it is more likely intermolecular interactions are giving rise to the orientation of the terminal methyl group rather than the Si-O-C bond. The CH3 intensity alternation and the SE data are consistent with the aldehyde monolayer consisting of a greater molecular density and a more-upright chain orientation than the alcohol monolayer. Although the differences in the chemical reactivity, SE, and FTIR data for the alcohol and aldehyde films can be explained by a slightly denser, better-ordered, moreupright aldehyde film, the CA data to a first approximation cannot. The CA values obtained from alcohol- and aldehyde-functionalized substrates indicate both formed hydrophobic surfaces while aldehyde-functionalized surfaces measured consistently lower, or more hydrophilic. The water CA of ultrasmooth octadecyltrichlorosilane (OTS) SAMs on SiO2 is reported as 112o 52 and the value of many of the alcohol-functionalized monolayers is nearly equivalent to this value. The CA is essentially a measure of the surface free energy for homogeneous surfaces and contains contributions from the polarity and ionization of functional groups at the surface as well as local structural details, such as roughness, at the solid-liquid interface.53 Polarity differences for alkane chains would be between the methyl and methylene groups and vary as a function of the tilt of the monolayer. The water CA of a CH2 group has been observed to be less than that of a CH3 group, and lessdense alkanethiol SAMs have exhibited lower CA than well-packed oriented SAMs.54,55 Thus, a tilted alkane chain might result in a lower CA. From the IR, SE, and chemical reactivity, it is likely the aldehyde monolayer is more dense and upright than the alcohol despite the lower CA. The second contribution to the CA is the local structural details. The roughness of a molecular monolayer has been associated with an increase in apparent hydrophobicity;51 for OTS on SiO2, the water CA on rough (multilayer) films increased ∼5°.52 AFM studies of thermally formed decanol and decyl aldehyde monolayers found the alcohol film surfaces to be rougher, attributed to water contamination (51) More, S. D.; Graaf, H.; Baune, M.; Wang, C. S.; Urisu, T. Jpn. J. Appl. Phys. 1 2002, 41, 4390-4394. (52) Wang, Y. L.; Lieberman, M. Langmuir 2003, 19, 1159-1167. (53) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (54) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921-937. (55) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167.
Hacker et al.
Figure 5. Possible mechanisms for the reaction of alcohols and aldehydes with Si(111)-H
of the liquid.19 A likely explanation of the CA disparity between alcohols and aldehydes is a difference in the surface roughness between the two films. In the absence of AFM data for the films formed in this report, a microscopic interpretation of the origin of the CA differences cannot be definitive. Comparison with Other Methods of Forming Si-Organic Bonds. Formation of Si-C bonds by reacting alkenes with Si-H surfaces has been widely studied experimentally and theoretically.10,11 Studies performed in our lab indicate attachment of alcohols and aldehydes produce films more densely packed than alkenes under similar reaction conditions.56 This is consistent with previous experimental observations of photochemically formed aldehyde and alkene monolayers.26 Moreover, theoretical modeling of Si-O-C monolayers reported a higher molecular density than Si-C bonded monolayers due to the differing interfacial chemistry leading to dramatically differing molecular packing.49 Although Si-O-C monolayers are denser than Si-C monolayers, Si-O-C monolayers have the disadvantage of being more reactive than Si-C monolayers due to the increased polarity of the Si-O bond. Ultraviolet attachment of alcohols and aldehydes from dilute solution is a good way to produce relatively dense, chemically tethered monolayers as long as precautions are taken to avoid subsequent substrate oxidation or monolayer degradation. Possible Mechanism of Attachment. The difference in reactivity and structure suggest that the mechanism of monolayer formation differs for alcohol- and aldehydefunctionalized films. In comparison with the dark control samples, it is clear that UV light is critical to monolayer formation. Additionally, these two molecules are distinct in that alcohols are saturated molecules and aldehdyes are unsaturated molecules. It is believed that alcohol insertion proceeds via a mechanism that is initiated by the interaction of a lone pair of electrons on the oxygen with the states in the conduction band of silicon (electron injection) followed by loss of molecular hydrogen, as depicted in Figure 5.11,19,21-24 There are at least two possible mechanisms for unsaturated molecules in these reaction conditions shown in Figure 6. First, a hydrosilylation reaction mechanism in which a surface silicon radical reacts with an unsaturated organic molecule to form an aliphatic radical. The aliphatic radical then abstracts a hydrogen from a nearby Si-H bond, the solvent, or other solute molecules.11,22,28,29 The second is nucleophilic charge donation in which light generates electrons and holes in the substrate and a reacting (56) Hacker, C. A.; Anderson, K. A. unpublished work.
Comparison of Si-O-C Interfacial Bonding on Si(111)
Figure 6. Si(111)-H.
Mechanism of reaction of aldehydes with
molecule inserts into the Si-H bond, forming an aliphatic radical, then abstracts a hydrogen, forming a neutral molecule.57-59 In the hydrosilylation reaction, a crucial step is the formation of the silicon surface radicals. The minimal energy needed for Si-H bond homolysis in solution appears to be 3.5 eV (338 kJ mol-1), requiring wavelengths shorter than 350 nm.27 Previous researchers have reported generation of silicon radicals by mulitphoton absorption using a wavelength of 254 nm, the same used in our experiments.27 Molecular silane radicals have rate constants for reaction with tristrimethylsilane (TTMS, ((CH3)3Si)3Si‚) on the order of 106 (M-1 s-1) for alkenes and 105 (M-1 s-1) for aldehdyes. Reactions with alcohols tended to fragment the molecule.60 While chemistry of the semiconductor surface is bound to differ from that of small molecules, a radical-based reaction should contain the same trend. Our results indicate that molecular density (presumably determined by the kinetics of the reaction rates) is largest for aldehdyde films, followed by alcohols, and least for alkenes, contrary to the reported trend. Consistent with our results, researchers have reported the reaction of oxygen in air contains a threshold near 350 nm and is more reactive than octadecene using 254 nm.27 While it is not possible to rule out the occurrence of this reaction mechanism, the observed trends are not (57) Schmeltzer, J. M.; Porter, L. A., Jr.; Stewart, M. P.; Buriak, J. M. Langmuir 2002, 18, 2971-2974. (58) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 78217830. (59) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudholter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 13521355. (60) Chatgilialoglu, C. Acc. Chem. Res. 1992, 25, 188-194.
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consistent with a homolytic cleavage of the Si-H bond and radical propogation of attachment. Within the second mechanism, a crucial step is nucleophilic attack of the silicon surface. Thus, in this reaction mechanism, the functional groups are largely responsible for the surface coverage. The oxygen of an aldehyde group is more electronegative than the oxygen of an alcohol group, making the attack of aldehdyes more favorable. Additionally, the poor coverage of the alkene samples can be understood in terms of the poor nucleophilicty of alkenes. Photogeneration of near-surface holes on n-type Si has been proposed to promote nucleophilic attack in the photostimulated reactions of I- and Brterminated surfaces.23,61 Although we are using lightly doped p-type wafers, this mechanism may account for the stimulation of alcohol attachment in both the ambient lighting and ultraviolet conditions, as significant populations of carriers are generated for all wavelengths above the 3.45 eV E1 transition, corresponding to wavelengths shorter than 365 nm.62 Additionally, while it is possible to understand how UV light promotes the nucleophilic attack of alcohols, it is difficult to reconcile a radical-based hydrosilylation reaction for an unsaturated molecule. Although it is difficult to distinguish between these reaction mechanisms and they may be operating concurrently, we believe the electron injection mechanism is responsible for the disparity between the aldehyde- and alcohol-functionalized monolayers. While it is clear that ultraviolet light enhances the attachment of alcohols and aldehydes to the silicon surface, more-detailed studies need to be performed to unambiguously determine the reaction mechanism. Conclusion A myriad of approaches to form monolayers chemically tethered to silicon is useful for differing applications. The use of dilute solutions is advantageous for costly, lowquantity, or insoluble molecules, and photoexcitation holds the promise for patterning surfaces and avoids thermal input, which could damage delicate features on the silicon wafer. We have demonstrated chemical attachment of aliphatic alcohols and aldehydes to the hydrogenterminated silicon surface induced by ultraviolet irradiation from dilute solution. The attachment proceeds through a Si-O-C bond and produces monolayers of comparable quality to those prepared by previous researchers by using alternate means. Differences in chemical reactivity, SE, and FTIR data for the two different types of molecules suggest differences in molecular orientation and coverage between the alcohol and aldehyde monolayers. Acknowledgment. C.A.H. gratefully thanks the National Research Council (Postdoctoral Fellowship, 2002-2004) and K.A.A. gratefully thanks the National Science Foundation (Summer Undergraduate Research Fellowship, 2003) for funding. Supporting Information Available: FTIR data showing the hydrogen-terminated reference used for the vibrational analysis. This material is available free of charge via the Internet at http://pubs.acs.org. LA048841X (61) Cai, W.; Lin, Z.; Strother, T.; Smith, L. M.; Hamers, R. J. J. Phys. Chem. B 2002, 106, 2656-2664. (62) Yu, P. Y.; Cardona, M. Fundamentals of Semiconductors; Springer: Berlin, 2001.
