Langmuir 1994,10, 525-529
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Atomic Force Microscope Images of Monolayers from Alkyltrichlorosilane on Mica Surfaces and Studies on an Anchoring Mechanism of Alkyltrichlorosilane Molecules to the Surface Tohru Nakagawa' and Kazufumi Ogawa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd., 3-4, Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-02, Japan
Toshimitsu Kurumizawa Matsushita Technoresearch, Inc., 3-1 -1 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan Received April 16, 199P Topographies of ultrathin organic films from octadecyltrichlorosilane (OTS)on mica surfaces pretreated with sodium ethoxide and an anchoring mechanism of the OTS molecules to the mica surface were investigated in ethanol by using an atomic force microscope (AFM). The film on the mica surface could be imaged repeatedly without being damaged when an applied force on the AFM tip was set at about 1 nN. The AFM image showed that the OTS molecules were reacted and arranged perpendicular to the surface to form a monolayer containing some pinholes, whose diameters range from several nanometers to about 100 nm. The monolayer on the mica surface was peeled off from the surface by the AFM tip at an applied force of 20nN. The driving force to remove one molecule from the mica surface was calculated from the repulsive force exerted on the monolayer and was much smaller than that for breaking a typical covalent bond. These results indicate that almost all of the molecules in the monolayer are anchored to the mica surface not by covalent bonds but by physical adsorption.
Introduction Organic monolayers such as Langmuir-Blodgett films' and self-amembled f i h from alcohols: amines: carboxylic acids,3-8 thiols: or chlorosilaness are interesting for their technological and fundamental applications. Above all, chlorosilanes seem very attractive because they react and form closed packed monolayers on various materials, which are most durable to chemical and physical stress. After the pioneering work on chlorosilane monolayers by Sagiv and co-workers,bl0 the properties of the monolayers were widely ~tudied.11-l~ In those studies, the substrates for adsorbing chlorosilanes were restricted to oxide substrates having many hydroxyl groups on their publiiedinAdvance ACSAbstracts, January15,1994. (1)Blodgett, K.B.; Langmuir, I. phy8. Rev. 1937,51,964. (2)Troughton, E.B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4,365. (3)A h a , D.L.; Nuzzo, R. G. Langmuir 1985,1,45. (4)Allara, D.L.; Nuzzo,R. G. Langmuir 1985,1,52. (5)Ogawa, H.;Chihera, T.; Taya, K. J. Am. Chem. SOC. 1985,107, 1365. (6)Chen, S.H.;Frank, C. F. Langmuir 1989,5,978. (7) Porter, M.D.; Bright, T. B.; A h a , D. L.; Chidaey, C. E. D. J. Am. Chem. Soe. 1987,109,3569. (8) Sagiv, J. J. Am. Chem. Soc. 1980,102,92. (9)Sagiv, J. Zsr. J. Chem. 1979,18,339. (10)Sagiv, J. Xsr. J. Chem. 1979,18,346. (11)Pomerantz, M.;Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Film 1985,132,153. (12)Tillman, N.; Ulman, A.; Schildkraut, J. S.;Penner, T. L. J. Am. C k m . SOC. 1988,110,6136. (13)Wasaerman,S.R.;Whitesides,G.M.;Tidswell,I.M.;Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. SOC. 1989,111,5852. (14)Waseerman, S.R.;Tao,Yu-Tai;Whitesides, G. M. Langmuir 1989, 5,1074. (16)Angst, D.L.; Simmons, G. W. Langmuir 1991,7,2236. (16)Silbenan, P.; Leger, L.; Awerre, D.; Benettar, J. J. Langmuir 1991,7,1647. (17)Ogawa, K.;Mino, N.; Nakajima, K.; Azuma, Y.; Ohmura, T. Langmuir 1991,7, 1473. (18)Ohtake, T.;Mino, N.; Ogawa, K. Langmuir 1992,8,2081. (19)Bnoeka, J. B.; Shahidzadeh, N.;Rondelez, F. Nature 1992,360, 719. 9 Abstract
surfaces (glass, silicon oxide, metal oxide, etc.), where the chlorosilyl groups (-SiCl) of the chlorosilanes react with the hydroxyl groups and then covalent bonds are formed between the reacted chlorosilanes and the solid surfaces. Recently, chlorosilanes were adsorbed on other subThe gold and mica strates, such as gold and covered with monolayers were very useful for investigating electrochemical processes through the thin films20 and measuring interactions between two surfaces,2l respectively. It is however difficult to adsorb the chlorosilanes on these substrates because these surfaces are inert and have few hydroxyl groups to react with chlorosilyl groups. Thus, some pretreatments of the surfaces have been performed for adsorbing the chlorosilanes. John and co-workers induced silanol groups (-SiOH) on the mica surface by exposing it to water vapor plasma and could adsorb (tridecafluoro-l,1,2,2-tetrahydrooctyl)-l-dimethylchlorosilane on the surface.21 Finkles and co-workers controlled quantity of HzO on the gold to adsorb octadecyltrichlorosilane ( C H ~ ( C H ~ ) ~ T S ~ C ~on ~ :the O Tsurface.20 S) Carson and co-workers could also adsorb OTS on the mica surface after treating with argon plasma or In the last three pretreatments, the H20 adsorbed on the surfaces may react with OTS and thus seemed to play an important role in the OTS adsorption although it is not so important to control the water on the surfaces in the adsorption of the chlorosilanes to oxide substrates. Thus the assembling mechanism of the OTS molecules to gold surface and mica surfaces may be different from that to oxide substrates even if the monolayers on both substrates (20)Finklea,H.0.; Robinson, L. R.; Blackburn,A.; Richer, B.;Allara, D.; Bright, T. Langmuir 1986,2, 239. (21)Parker, J.L.;Cho, D. L.; Claesson, P. M. J. Phys. Chem. 1989,93, 6121. (22)Carson, G.A,; Granick, S. J. Appl. Polym. Sci. 1989,37,2767. (23)Carson, G.A,; Granick, S. J . Mater. Res. 1990,5,1745.
Q143-1463/94/241Q-Q525$Q4.5QJQ0 1994 American Chemical Society
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have almost the same chemical properties. It had been speculated that the OTS molecules reacted on the gold form a two-dimensionally networked monolayer which is not covalently bound to the surface,2owhile both physical adsorption and covalent bonding to the mica surface were proposed by Carson and co-w0rkers.~3 It has been found that the chlorosilanes could be adsorbed on mica surfaces which were treated with sodium ethoxide before adsorption. Sodium ethoxide is generally used to introduce ethoxyl groups (C2H50-) which are easily hydrolyzed to ethanol molecules and hydroxyl groups. Thus by treating the mica with sodium ethoxide, hydroxyl groups which react with chlorosilyl groups of the chlorosilanes might be introduced to the mica surfaces;however detailed information on the mica surface and the anchoring mechanism of the chlorosilane to the mica surfacesremains unclear. On the other hand, an atomic force microscope (AFM) was recognized as a powerful tool to modify the surface as well as to characterize the topography of the surface at atomic resolution, especially for insulators.2P26 Modification by the AFM seemsto be very useful for investigating the anchoring mechanism of the chlorosilanes to the surfaces. The main purpose of this paper is to clarify the anchoring mechanism of the chlorosilanes to the mica surface by using the AFM. In this paper, the topographies of the monolayers from OTS on the mica surface pretreated with sodium ethoxide have been imaged using the AFM, and the anchoring mechanism of the OTS molecules to the mica surface has been investigated by peeling off the adsorbed OTS film using the AFM tip.
Experimental Section Muscovite micas were cleaved on a (111) surface and then immersedin 10%sodium ethoxide solution of ethanol (by weight) at 50 "C for 1min, followed by washing with ethanol and with ultrapure water for 30 min. These mica substrates were finally dried in nitrogen atmosphere overnight to remove excess water from the mica surface. The OTS molecules were adsorbed on the pretreated mica by immersing the substrate in a solution composed of 1%OTS, 119% carbon tetrachloride, 159% chloroform, and 73% hexadecane (by weight) for 2 h in a nitrogen atmosphere followed by washing with chloroform and with water. As a reference, the OTS molecules were also adsorbed on the silicon substrate which was cleaned with HzO2 and NH3 boiling before the adsorption. The mica and the silicon substrate reacted with OTS molecules became very hydrophobic, indicating that the molecules had been adsorbed. AFM images of the samples were obtained by using a Nan0 Scope I1 (DigitalInstruments). Microfabricated cantileverswith spring constants of 0.16 N/m were purchased from Olympus Opt. Co., Ltd. At the end of the cantilever, a silicon nitride tip smaller than 20 nm in radius was mounted. For the AFM observations, the sample was mounted on the XYZ piezoelectric scanner and was scannedunder the tip. In our experiments,the sample surface was kept touching the tip. The force being applied to the tip caused cantilever deflection, which was measured by detecting a laser beam reflected from the back of the cantilever. During the scan, the applied force on the tip was kept constant by moving the sample up and down, and the AFM images were drawn from the XYZ movements of the piezoelectric scanner. The X and Y scalesof the AFM imageswere calibrated beforehand by assigning the observed a and b axis of the muscovite mica lattice to the (24) Binnig, G.; Quate, C.; Gerber, Ch. Phys. Reu. Lett. 1986,56,930. (25) Quate, C. F. Scanning Tunneling Microscopy and Related Methods; Behm, R. J., et al., Eds.; Kluwer Academic Publishers: The Netherlands, 1990; pp 281-297. (26) Nakagawa,T.;Ogawa,K.;Kurumizawa,T.;Ozaki,S.Jpn. J. Appl. Phys. 1993, 32, L294.
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Figure 1. Top view image of the adsorbed OTS film on the mica surface observed at about 1nN. Vertical scale of the image is shown on the right side of the image, where low areas are dark color and high areas are light color (vertical unit is nm). data obtained from X-ray anal~sis.~'The height scale of the AFM images was also calibrated within 0.15 nm precision by comparing the step height measured on the cleaved mica in ethanol with the c axis data from X-ray analysis. All of the observations on the samples were performed in ethanol to suppress the van der Waals' force and the adhesion between the tip and the surface.28 The coefficient of static friction of silicon nitride against the adsorbed OTS film on the solid surface was obtained as follows. The angle of the incline of the silicon nitride substrate was altered from the value 0" to that at which the OTS modified silicon substrate staying on the silicon nitride substrate just begins to slide, and the coefficientof the static friction was evaluated from the tangent of this critical angle.
Results and Discussions AFM images of a 1000 X 1000 nm2area of the adsorbed OTS film on the mica surface were obtained repeatedly without damaging the samples when the applied force on the tip was set at about 1nN, as shown in Figure 1. This AFM image of the OTS film was very different from that of a 1000X 1000nm2area of sodium ethoxide treated mica which showed an atomically smooth surface. In the adsorbed OTS film image, many pinholes from several nanometers to about 100 nm in diameter appeared. After the 1000 X 1000 nm2 area of the adsorbed OTS film was observed at an applied force of about 1nN, as shown in Figure 1,the 200 X 200 nm2 area in the center of the film was scanned under the tip at an applied force of 20 nN, and then the 1000X 1000nm2area of the film was observed again a t an applied force of about 1nN, as shown in Figure 2. In the AFM image, a large square hole 200 X 200 nm2 appeared near the center of the image, indicating that the film was disrupted by the tip. The inside of the AFM induced hole was further observed a t atomic resolution at an applied force of 20 nN, as shown in Figure 3. The periodicity of the image was consistent with that of the muscovite mica's a-b lattice, indicating that in the hole, the majority of the adsorbed OTS moleculeswere removed and the mica surface appeared. The removed OTS molecules should adhere to the AFM tip or dissolve into ethanol because any removed molecules were not observed around the AFM induced hole in the film. (27) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Interscience Publishers: USA, 1960; Vol. 4. (28) Weisenhorn, A. L.; Hansma, P. K. Appl. Phys. Lett. 1989, 54, 2651.
AFM Images of OTS Monolayers
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I
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1
O
A #
I
100
200
300
Hole Diameter (nm)
Figure 2. Top view image of the adsorbed OTS film on the mica surfaceobserved at about 1nN at almost the same position shown in Figure 1after the film was scanned under the AFM tip at 20 nN in the area of 200 nm2 squares. Vertical scale of the image is shown on the right side of the image, where low areas are dark color and high areas are light color (vertical unit is nm).
Figure 3. Top view image observed at 20 nN in the hole induced by the AFM tip at 20 nN. Vertical scale of the image is shown on the right side of the image, where low areas are dark color and high areas are light color (vertical unit is nm).
The depth D and diameter 4 of the pinholes and AFMinduced holes were estimated from AFM cross-sectional views of the surfaces, and D was plotted as a function of 4, as shown in Figure 4, where the solid circles and open circles indicate the pinholes and the AFM induced holes, respectively. Twice the value of the tip radius (2R) was also indicated in the figure by an arrow. In Figure 4, D values of the pinholes and the AFM-induced holes are on the same curve and increase with increasing 4 below 2R and level off a t 2.6 nm for 4 over 2R, indicating that the top of the tip can contact the bottoms of the holes for 4 over 2R and cannot contact below 2R. Thus, the film thickness can be evaluated from the hole depth for 4 over 2R by considering deformation of the film by the tip. When soft samples such as adsorbed OTS films are pressed by an AFM tip, the sample surfaces are deformed. The sample deformation normal to the surface can be roughly estimated by considering Hertzian contact29 ~
(29) Timeshenko, S. P.; Goodier, J. N. Theory of Elasticity; McGraw-Hill: London, 1970.
Figure 4. Hole depth estimated from cross sectional views of the AFM images of adsorbed OTS monolayer as a function of the hole diameter. The solid circles and the open circles indicate data obtained at the pinholes and the AFM induced holes, respectively. Twice the value of the tip radius is indicated by an arrow.
between a spherical tip and an elastic surface. Given a repulsive force P between the sample surface and the tip, and a tip radius R, the sample deformation h normal to the surface is calculated by eq 1
h = ((9/16)p(1/R)p)1/3 (1) where Y = (1 - ul2)/E1+ (1- ~ 2 ~ ) / and E 2 El, 01 and E2, u2 are Young’s modulus and Poisson’s ratio of the tip and the sample, respectively. A repulsive force P i s in general larger than an applied force on the tip (which is evaluated from the cantilever deflection) because the peripheral regions of the tip are attracted to the sample surface by the van der Waals’ force or the adhesion.28 The attractive force in ethanol between the tip and the adsorbed OTS film was estimated to be 0.1 nN by force curve measurements. As the applied force on the tip for the AFM measurements is about 1nN, which is much larger than the attractive force of 0.1 nN, the repulsive force P can be approximated by the applied force on the tip. Young’s modulus E1 and Poisson’s ratio u1 of the silicon nitride are 280 X lo9N/m2 3O and 0.2,3l respectively, and the tip radius is 20 nm. Thus by using the elastic constants obtained on a Langmuir-Blodgett film (E2 = 9.3 X lo9 N/m2,30 u2 = 0.431) as those on the adsorbed OTS film, the deformation of the adsorbed OTS film a t an applied force of 1nN is calculated to be 0.06 nm. On the other hand, the measured hole depth of the adsorbed OTS film for 4 over 2R is 2.6 nm, as shown in Figure 4, and much smaller than the deformation. Thus the film deformation by the tip a t an applied force of about 1nN can be neglected in measuring the hole depth, indicating that the thickness of the adsorbed OTS film can be approximated by 2.6 nm. The estimated film thickness was in good agreement with the molecular length of the OTS moleculeswhich are assumed to have fully extended trans zigzag hydrocarbon chains, if the molecules aligned normal to the surface.8 Thus the adsorbed OTS molecules on the mica surface should form a monolayer. The adsorbed OTS molecules on the mica surface are pulled parallel to the surface by the AFM tip as well as pushed normal to the surface when the sample was scanned below the tip. Because the molecules may be pulled out from the surface by the tip, the lateral force pulling the (30) Weihs, T. P.; Nawaz, Z.; Jarvis, S. P.; Pethica, J. B. Appl. Phys. Lett. 1991,59, 3536. (31) Tables of Physical and Chemistry Constants and Some Material Functions, 13th ed.; originally complied by G. W. C. Kaye; Longmana Green and Co., Ltd.: London, 1966.
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528 Langmuir, Vol. 10, No. 2, 1994 Alkyl Chain
+ Mica
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-Si-0dH
si-0
dH
-si-o
6
-si-o-si-odH
dH
(c)
Figure 5. A possible chemisorption mechanism of the alkyltrichlorosilanes on mica surfaces.
molecules should be the driving force to remove the molecules from the mica surface and is roughly calculated by eq 2
F = uP/(Sn) (2) where F, u, P, S,and n are the lateral force exerted on one molecule, the coefficient of the kinetic friction between the tip and the surface, the repulsive force between the film and the tip, the contacting area of the tip with the surface, and number of the adsorbed OTS molecules per unit area, respectively. The coefficient of the kinetic friction u is approximated by that of the static friction of 0.26. The repulsive force P can be approximated by the applied force on the tip. The contact area S is roughly calculated by considering Hertzian c0ntact~930and is 29 nm2 when the applied force on the tip is 20 nN. On the other hand, from the AFM image of the adsorbed OTS monolayer on the mica surface, we can speculate that the adsorbed OTS molecules are closely packed and the occupied area of one molecule is 0.25 nm2,8 thus n is calculated to be 4 nm-2. By use of these values, the lateral force F a t an applied force of 20 nN is calculated to be 0.045 nN. As the force constant of a covalent bond is larger than 200 nN/nm in general,31 the stretch of the covalent bond by the force is too small to break one bond, although the force is enough to remove the adsorbed OTS molecule from the mica surface. Thus the average bond strength of the adsorbed OTS moleculesto the mica surface is much smaller than that of typical covalent bond. The above speculation of the bond strength was also supported experimentally for OTS molecules adsorbed on silicon substrates. The OTS molecules are assumed to be covalently bound to the silicon surface and were not removed from the surfaces with lateral frictional forces of 0.045 nN. The anchoring mechanism of the OTS molecules to the mica surface may be summarized as follows. The adsorption process of the OTS molecules on the mica surface may be almost the same as on oxide substrates: as shown in Figure 5. First, the molecules are attracted to the mica surface via the trichlorosilyl groups, because the mica
surface is polar and is adsorbing H20, as shown in Figure 5a. And then, as shown in Figure 5b, the chlorosilylgroups are hydroxylated with water to form silanol groups (-SiOH). The silanol groups of neighbor molecules react with each other to form lateral siloxane bonds. The other silanol groups react with hydroxyl groups on the mica surface to make covalent bonds or physically adsorb to the surface, as shown in Figure 5c. The ratio of the molecules covalently bound to the surface to those physically adsorbed should depend on the density of the hydroxyl groups on the mica surface. The estimated average bond strength of the OTS molecules to the mica surface is much smaller than that of the pure covalent bonds. This indicates that the density of the hydroxyl groups on the mica surface is low even though the mica was pretreated with sodium ethoxide. Thus almost all the adsorbed OTS molecules should be anchored not by covalent bonds but by physical adsorption. The covalent bonds and the two-dimensional network of siloxane bonds may mainly connect the monolayer to the mica surface.20 Barrat and co-workers have recently imaged pinholes in the adsorbed OTS monolayers on the silica substrates by using the AFM and concluded that the pinholes are due to substrate ~ o n t a m i n a t i o n .However, ~~ the origin of the pinholes observed in our monolayers on the mica surfaces cannot be explained only by contamination because the monolayers have large pinholes a t reduced coverage, indicating that the interaction between the OTS molecules on the surfaces plays some role in forming the pinholes. Thus the pinholes in the monolayer on the mica surfaces may be caused from imperfections of the network between adsorbed OTS molecules, that is, the physically adsorbed OTS molecules which are not covalently bound to neighbor molecules or to the mica surfaces may be easily washed away by ethanol or water, and the pinholes may be formed.
Conclusions Monolayer formation from OTS on mica surfaces pretreated with sodium ethoxide has been investigated by using AFM. The monolayers included many pinholes from several nanometers to about 100 nm in diameter. Those pinholes may be caused from imperfections of the lateral siloxane bonds. The average bond strength of the OTS molecules to the mica surface was estimated by disrupting the monolayer from the surface with the AFM tip and was much smaller than the typical covalent bond strength. These results indicate that almost all the adsorbed OTS molecules are anchored to the mica surface not by covalent bonds but by physical adsorption. Although an accurate bond strength of the individual OTS molecules to the mica surface could not be measured in our experiment, some further information obtained by using a lateral force microscope will be reported else~here.~3-~~ In this research by using the AFM, although we naturally tried to obtain molecular images of the adsorbed OTS molecules on the mica surface, no clear images could be obtained so far. The reason why we could not obtain the (32) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992,20, 633. (33) Meyer, G.; h e r , N. M. Appl. Phys. Lett. 1990,57,2089. (34) Mate, C. M.; McClelland, G. M.; Erlandsson,R.; Chiang, S. Phys. Rev. Lett. 1987,59, 1942. (35) Erlandsson, R.; Hadziioannou,G.; Mate, C. M.; McClelland, G. M.; Chiang, S. J . Chem. Phys. 1988,89,5190. (36) Overney, R. M.; Meyer, E.; Frommer,J.; Brodbeck, D.; Luthi, R.; Howald,L.; Guntherodt,H.-J.;Fujihira,M.;Takano,H.; Gotoh, Y.Nature 1992,359,133.
AFM Images of OTS Monolayers molecular images may be that the adsorbed OTS molecules on the mica surface are not closely packed as a crysw, thus hydrocarbon chains of the molecules are vibrating in the monolayer at room temperature. If the hydrocarbon chains were frozen, i.e., the AFM were operated a t cryogenic temperature, perhaps the OTS molecules could be observed clearly.
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Acknowledgment. The authors wish to thank Senior Researcher H. Higashino for fruitful discussions of the data and also thank Director Dr. T. Nitta of Mataushita Electric Industrial Co., Ltd., Central Research Laboratories, and Director Dr. K. Kanai of Matsushita Electric Industrial Co., Ltd., Component and Material Basic Research Laboratories in Central Research Laboratories.
