Langmuir 1994,10, 984-987
984
Packing of Hydrocarbon and Perfluorocarbon Chains Planted on Oxidized Surface of Silicon As Studied by Ellipsometry and Atomic Force Microscopy M. Fujii,' S. Sugisawa, K. Fukada, T. Kato, T. Shirakawa, and T. Seimiya Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Minamioosawa, Hachiohji 192-03, Japan Received December 6, 1993. In Final Form: February 7,1994" The oxidized surface of a single crystal of silicon was silylized with octadecyltrichlorosilane (OTS), perfluorooctylmethyldichlorosilane (FOMDS), or perfluorodecyltrichlorosilane (FDTS)to give a hydrophobic surface that shows a contact angle larger than l l O o to water. The optimal condition of silylation was established to give a well-qualified hydrophobic surface. Both the thickness and the refractive index of the oxide and the alkylated layers grown on silicon surface were measured by ellipsometry. Each octadecyl chain was found to occupy 0.6 f 0.1 nm2 of silicon surface and to have an orientation slightly slanted to the surface normal. The hydrophobic surfaces obtained were also examined in water by atomic force microscopy. The atomic force micrograph shows that the alkyl chains of OTS,FOMDS, or FDTS are oriented at the surface to give an occupied area of 0.43 f 0.07,0.52 f 0.07, or 0.50 f 0.08nm2,respectively. The occupied area of fluorocarbon chain was found 15% larger than that of hydrocarbon chain.
Introduction The silylation technique1p2is one of the popular methods of modifying metal, metal oxide, or polymer surfaces as well as silica to prepare any functional surfaces. The major advantage of the silylation technique is that the groups planted at the surface are bound covalently, which necessarily limits the modified layer to one molecule thick.The silylation processes, however, accompany The problem one serious difficulties to o~ercome.B*~ encounters at first may be the polymerization of silylating agent induced by water as supplied from ambient atmosphere and also from the solvent used. The second problem should lie at the conditioning of the silica substrate to make it reactive to the coupling agent which is alkylsilane halide in the present case. The stabilization of the modified layer by blocking unreacted group of the agent left on the surface may be another problem. In the present studies, the thermally oxidized surface of silicon(ll1) wafer was silylized and the surface modified was examined by means of ellipsometer and atomic force microscope (AFMP to correlate the degree of packing of implanted hydrocarbon or perfluorocarbon chains implanted with the hydrophobicity of surface which was estimated by the contact angle of water drop placed on it.
Experimental Section Reagent. Octadecyltrichlorosilane (OTS), lH,lH,W,Wpertluorooctylmethyldichlorosilane(FOMDS),and lH,lH,W,Wperfluorodecyltrichlorosilane (FDTS)were the reagents used for silylation. They were purchased from Shin-Etau Chemical Co., Ltd., Japan, H a s American Inc., USA, and from PCR, Inc., USA. The mixture of n-hexadecane, carbon tetrachloride, and chlo-
* Abstractpublished in Advance ACS Abstracts, March 15,1994.
(1)Leyden,D.E.,Collinus, W., Eds.; Silylated Surfaces;Gordon and Breach, Science Publishers, Inc.: New York, 1980. (2)Leyden, D. E., Ed. Silanes Surfaces and Interfaces; Gordon and Breach, Science Publishers, Inc.: New York, 1986. (3)Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983,100, 67. (4)Guyot-Sionnest,P.; Superfine,R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (5)Ogawa, K.;Mino, N.; Tamura,H.; Hatada, M. Jpn. J . Appl. Phys. 1989,28, L1854. (6)Nishiyama, N.; Shick, R.; Ishida, H. J . Colloid Interface Sci. 1991, 143, 146. (7)Trau, M.; Murray, B. S.; Grant, K.; Grieser, F. J. Colloid Interface Sci. 1992, 148, 182. (8)Binnig, G.; Quate, C.; Gerber, Ch. Phys. Reu. Lett. 1986,56, 930.
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roform of reagent grade was used as a solvent for silylation. Each solvent was thoroughly dried with molecular sieves (Wako Pure Chemical Industries, Ltd., Japan) prior to use. Other chemicals used, aqueous ammonia (29%), hydrochloric acid (35%), and hydrogen peroxide (30%)for cleaning mixture, were electronic grade of Kanto Chemical Co., Inc., Japan. The water used in the present studies was all purified by a Milli-Q system. Pretreatment of Silicon Wafer. Silicon wafer ( p = loo0 fhcm) as kindly supplied by Shin-Etau Handoutai Co., Ltd., was cut along the (111)plane and has a thickness of 600 pm. The wafer was cleaned by immersing it in RCA-1 cleaning mixture composed of aqueous NHs, H202, and Ha0 (1:1:5 by volume) for 15min at 75 O C then transferred to the second cleaningsolution called RCA-2 after a thorough rinse with water. RCA-2 is a mixture of HCl, H202, and HzO in a volume ratio of 1:1:6 and was used under the same condition as that for RCA-1. The surface of the silicon wafer was etched by aqueous HF to remove the oxidized layer spontaneously grown on the surface. The clean surface of silicon was oxidized at loo0 OC for 3 h under a stream of dry oxygen to form a well-qualifiedoxide layer. This procedure led to the growth of a dense oxide layer ca. 200 nm thick on the silicon surface. Planting of Hydrocarbon Chains. The surface of the oxidized plate was cleaned again with RCA-1 and RCA-2 to remove minute fatty contaminationfromthe ambientatmosphere and also to introduce silanol groups on the surface by breaking siloxane bonds. The substrate was heated to 300 O C for 30 min under vacuum to remove adsorbed water and was successively transferred to a silylation bath containing 100 mmol/dms of coupling agent in a mixed solvent of 80% n-hexadecane + 12% CC4 + 8% CHCls by volume.s After 30 min of silylation, the substrate was withdrawn and rinsed with CHCla repeatedly to remove excess silylatingreagent followed by the rinse with water. The plate was heated again in vacuo at 300 O C for 1h to make sure the reaction was complete. Finally it was rinsed with tetrahydrofuran to remove the ash of unreacted reagent if any. Characterization of Surface. The hydrophobicity of the surface after silylation was evaluated by measuring the contanct angle of tiny 5-fiLwater drops with a goniometer in the field of a microscope. The homogeneity of the surface was confirmed by measuring the contact angle for 10drops of water placed at various places on the surface. The overall thickness and the refactive index of the modified layer were estimatedfrom the ellipsometric analyses of the reflected laser beam (A = 633 nm) for the surface of the area about 1 mm X 3mm before and after surfacetreatments of oxidation and silylization. The refractive index and the thickness of the silylated layer were separately calculated as well (9)Sagiv, J. J. Am. Chem. SOC.1980, 102, 92.
0 1994 American Chemical Society
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Figure 1. AFM micrographs of Si02 surface treated with OTS. The scan area is (a) 10 nm X 10 nm and (b) 5 nm X 5 nm. Images were taken under constant force mode in water. The elastic modulus of the cantilever used was 0.06 N/m. as that of oxide layer by assuming the additivity rule to hold. All the ellipsometric measurements were made for the substrate immersed in water to avoid any organic contaminations from ambient atmosphere during the measurement. The detail of the ellipsometricdevices used is seen in our previous paper.1° The calculation of the data was performed by following the computational technique of McCrackinll which is based on the Drude equation (1889).12 The observation of the surfaceby AFM (NanoScope-11,Digital Instruments, USA) was also made in water. Microfabricated Si3Nd cantilever having an elastic modulus 0.06 N/m was used. All AF'M images were taken in constant force mode. In order to make the measurements free from the effects of thermal drift, we compared the pair of images which were taken under the opposite scanning direction, up and down, successively. If pairs of images had different size of separations, we reject the image to add statistical population.
Results and Discussion The silylation of Si02 surface by OTS, FOMDS, and FDTS was made under carefully controlled conditions, by (10) Watanabe, N.; Shirakawa, T.; Iwahashi, M.; Ohbu, K.; Seimiya, T. Colloid Polym. Sci. 1986, 264, 903; 1987,266, 254. (11)McCrackin, F. L. NBS Technical Note 479, A Fortran Program for Analysis of Ellipsometer Measurements; US.Government Printing Office: Washington, DC, 1969. (12) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; Elsevier Science Publishers B.V.: Amsterdam, 1977.
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Figure 2. AFM micrographs of Si02 surface treated with (a) FOMDS and (b) FDTS. The scan area was 5 nm X 5 nm and was measured in water under constant force mode. The elastic modulus of the cantilever was 0.06 N/m. which highly hydrophobic uniform surfaces were reproducibly obtained. The contact angle of water drops was found to be 110 f 5" for all modified surfaces with the reagents given above. The surface heterogeneity was regarded to be within the experimental error of the angle measurements. The contact angle observed for the modified surfaces is either equal or larger than that of typical hydrophobic materials, namely PTFE (108") and . paraffin (1O6O),l3and is good compared with the previously reported values for the surfaces treated with OTS.3*7 The refractive index, N, and the thickness, d, of the silylized layer with OTS were estimated to be N = 1.393 f 0.002 and d = 1.8f 0.2 nm, respectively. The refractive index of liquid OTS is reported to be 1.46,14 and the molecular length of OTS is about 2.5 nm that is calculated by MM2 structure simulation. The refractive index and the thickness of the silylated layer estimated in the present experiment are both smaller than those expected for liquid OTS, indicating that the OTS layer a t the surface assumes a density lower than that of liquid. The molecular volume of OTS a t the surface may be calculated from the data of the refractive index and the thickness of the adsorbed (13)Chemical Society of Japan Kagaku Benran (Handbook of Chemistry), 3rd ed.; Maruzen Inc.: Tokyo, 1984; Vol. 11, p 90 (in Japanese). (14) Catalog; Shin-Etsu Chemical Co. Ltd.; 1993.
986 Langmuir, Vol. 10, No. 4, 1994
Figure 3. (a) Autocorrelation image derived from Figure lb. The image area was 5 nm X 5 nm. (b) The schematic drawing of (a). The bright spot at the center of (a) is expressed in this figure as an original domain 0. Other domains denoted A,, (n = 1-4) are indicated nearest neighbor spots to appear at this distance from the origin. Spacings of the nearest (A1,s) and nextnearest (A2,4) neighbor spots are estimated to be 0.58 and 0.71 nm, respectively.
layer, provided that the refractive index is given as a function of the density of OTS layer. Sincethe dependence of the density of a substance on its refractive index is scarcely influenced by the chemical structure for simple organic compounds, we have adopted a value of d C / W = 7.4 X lo3 g/dm3 for the analysis,15 where C is the concentration of organic substance and N is the refractive index of the solution. The free molecular volume of a substance in the layer was thus estimated to be 1.1f 0.2 nm3 from which molecular area of OTS at the surface is expected to be 0.6 f 0.1 nm2 by assuming the monomolecular OTS layer to have the thickness of 1.8 nm. Unfortunately, the calculations of the optical constants for the surfaces modified by FOMDS and FDTS were not successful because of their lower refractive indices than that of the underlying oxidized silicon layer. The AFM images of the surface modified by OTS are shown in parts a and b of Figure 1each observed in water at two different scan sizes of 10 nm X 10 nm and 5 nm x 5 nm, respectively. The appearance of the surface images is affected neither by scan speed nor by probe pressure to (15)The value was measured for dodecyltrimethylammoniumbromide with an Abbe refractometer (A = 633 nm, 25 "C).
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Figure 4. Autocorrelation images of (a) FOMDS (Figure 2a) and (b) FDTS (Figure 2b). Both images cover the area of 3.5 nm X 3.5 nm. Spacings of nearest and next-nearest neighbor spots are 0.75 and 0.75 nm for (a) and 0.69 and 0.71 nm for (b), respectively.
the sample surface. The magnification of the observed periodicity proportional to their scan size as seen in Figure 1implicates that the images observed here reflect actual surface morphologies. The AFM images of the surfaces modified by FOMDS and FDTS are also shown in Figure 2. A large number of spots appear in these four micrographs, distributed randomly over the entire areas observed. The average diameter of the spots in Figure 1is about 0.4 nm, and the distance between nearest neighbors is estimated about 0.6 nm from which we may reasonably assume that all the spots are likely to correspond to the terminal methyl group of the individual alkyl chain planted. For the sake of statistical analysis of the packing of alkyl chains a t the surface, the images were transformed by employing an autocorrelation device equipped in NanoScope-I1software. From the autocorrelation images, we can estimate the periodicity and the symmetry of the terminal methyl groups at the modified surface. Figure 3a shows the autocorrelation image displayed in four symmetric quadrants as derived from Figure lb. The bright spots in this micrograph form a regular array along the diagonal lines of the figure as schematically shown in Figure 3b. The regularity is more dominant around the center spot, implying that the short range order is likely
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to exist in this silylized layer. The bright spot a t the center of Figure 3a is indicated by domain 0 in Figure 3b and other domains marked An (n = 1-4) are representing the distribution of the spacing along A n 4 directions. The long range order is not observed for this silylized Si02 surface, while the long range order is clearly seen in the autocorrelated AFM micrograph for the cleaved surface of stearic acid crystal.l6 Therefore we can estimate an average area occupied by a terminal methyl group in the silylized layer (Figure lb) by measuring the separation of the neighboring spot from the origin in Figure 3a. The separations of the nearest and next-nearest neighbored spota from the origin are found to be 0.58 and 0.71 nm, respectively. Therefore, the average occupied area of the spot in Figure l b is calculated to be 0.40 nm2. With similar analysis for about 60 different micrographs of the OTS treated plates, the average occupied area of the spot is calculated to be'0.43 f 0.07 nm2. This value is good compared with that deduced from the ellipsometric data, namely 0.6 f 0.1 nm2. As for the perfluorocarbon chains, the images of FOMDS (Figure 2a) and FDTS (Figure 2b) are also transformed to give the autocorrelation images (Figure 4). Similar analysis to that of OTS was made for the perfluoroalkylated surfaces which revealed the average (16)Fujii, M.; Sugisawa, S.; Fukada, K.;Kato, T.; Shirakawa, T.; Seimiya,T. Hyoumenn Kagaku (J.Surf.Sci. SOC.Jpn.) 1993,14,366 (in Japanese).
Langmuir, Vol. 10, No. 4, 1994 987 occupied areas of the terminal CF3 group for each FOMDS and FDTS to be 0.52 f 0.07 and 0.50 f 0.08 nm2, respectively. The cross-sectional area of perfluorocarbon chains is larger than that of hydrocarbon chains of OTS by 15%, which is the tendency consistent with the fact that the van der Waals diameter of the perfluoromethyl group is larger than that of hydrocarbon methyl by the similar amount. We may safely conclude that the silylation of the Si02 surface by OTS according to our procedure covers the surface with octadecyl chains, reproducibly, whose crosssectional area is estimated to be 0.4 nm2 and rendered the surface highly hydrophobic. Despite the half numbers of carbon atom in the alkyl chain, the FOMDS- and FDTStreated surfaces also have high hydrophobicity; the occupied area of the fluorocarbon chains was about 0.5 nm2.
Acknowledgment. The silicon wafers were kindly supplied by Shin-Etsu Handoutai Co., Ltd., Japan. This study was partly supported by the Fund for Special Research Project at Tokyo Metropolitan University. Registry No. Supplied by Author. OTS, 112-04-9; FOMDS, 73609-36-6; FDTS, 78560-44-8; Si02,7631-86-9; Si, 7440-21-3; CHs(CH2)&Hs, 544-76-3; CCb, 56-23-5; CHC13, 67-66-3; THF, 109-99-9; HC1, 7647-01-0; NH3, 1336-21-6; H202, 7722-84-1.
