Langmuir 1992,8, 733-735
733
Notes Surface Roughness of Plasma-Treated Mica
Tim J. Senden' and William A. Ducker Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, GPO Box 4, Canberra, ACT 2601, Australia Received April 2, 1991. In Final Form: November 12, 1991
For many years muscovite mica has been a common substrate in surface chemical techniques, particularly those employing the surface forces apparatus (SFA) of Isbut also in streaming potential raelachvili and determinations3p4and force micro~copy.~ The utility of mica lies in the fact that sheets of the material can be obtained which are extremely smooth (-0.2 nm) over macroscopic dimensions ( cm). Because this property is virtually unique, it is useful to be able to modify the surface of mica to obtain different surface groups while maintaining a smooth surface. However, because mica is very unreactive, covalent attachment to the native surface is difficult. To overcome this problem, Parker et aln6Jhave developed a technique whereby the surface of mica can be made reactive by exposure to an oxygen or water plasma, but without modification of its bulk properties. Examination of plasma-modified mica by electron spectroscopy for chemical analysis (ESCA)G and surface force measurements7 suggests that the surface contains silanol groups, which are not present on the mica surface. Silanol groups can be reacted with silanating agents to produce a wide variety of surface groups.8 However, the advantage of surface modification is contingent on the preservation of a smooth surface. Indirect evidence for molecular smoothness of plasma-modified mica has been obtained by measurement of solvent-structure forcess which to date have only been measured when at least one of the surfaces is molecularly s m ~ o t h . ~ In a typical experiment using the surface forces apparatus the area of interaction is about 30 pm2 and the interferometer used in this technique allows the experimenter to examine surface roughness on a scale greater than 1pm. In this note we present a more direct surfaceroughness estimate on a smaller scale obtained by atomic
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(1)Israelachvilli, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1986; Chapters 10, 12. (2) Parker, J. L.; Christenson,H. K.; Ninham, B. W.Reu.Sci. Instrum. 1989,60,3135. (3) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: New York, 1987; Vol. 1, p 555. (4) Scales, P. J.; Griezer, F.; Healy, T. H. Langmuir 1990, 6, 582. (5) Drake, B.;Prater, C. B.;Weisenhorn, A. L.; Could, S. A. C.; Al-
brecht, T. R.; Quate, C. F.; Cannell, D. S.; Hansma, H. G.; Hansma, P. K. Science 1989,243, 1586. (6) Parker, J. L.; Cho, D. L.; Claesson, P. M. J.Phys. Chem. 1989,93, 6121.
(7) Parker, J. L.; Claesson, P. M.; Cho, D. L.; Ahlberg, A.; Tidblad, J.; Blomberg, E. J. Coll. Interface Sci. 1990, 134,449. (8) Plueddemann, E. P. Silane Coupling Agents; Plenum: New York, 1982. (9) Parker, J. L.; Christenson, H. K. J . Chem. Phys. 1988, 88, 12.
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force microscopy.1° Three types of mica surface were imaged under double-distilled water-one native sample and two water-plasma treated samples. One of the plasmatreated samples was rendered hydrophobic by exposure to trimethylchlorosilane vapor" for 3 min. This reaction causes a large increase in the contact angle of water on the surface. Native mica and plasma-modified mica both have an advancing contact angle of less than lo", whereas after silanation the angle increases to about 85'. Images were taken after the substrates had been underwater for about 10 min. We also examined for possible contamination of surface force experiments with epoxy resin. A thin layer of resin (epikote 1004)was spread on the surface of mica, &id the force between the tip and resin was measured at a number of points on the mica. This force was found to be longrange and attractive, presumably due to hydrophobic forces. In this roughness study, the substrates were glued in the same manner as in SFA experiments, allowing us to check for contamination. The attractive forcewas never observed in our measurements on mica, suggesting that neither this work nor that of the SFA is contaminated by the resin. Images of mica are shown in Figure 1. The native mica in Figure la is clearly crystalline; the variation in height within a unit cell is about 0.13 nm, and the image is similar to those previouslyr e p ~ r t e d A . ~similar variation in height is observed over our maximum scanning range 670 nm2. After plasma modification (Figure lb), the mica surface is predominantly amorphous although small regions of crystallinity are apparent. In water very flat circular features about 25 nm in diameter are also imaged. These are believed to be a result of localized regions of higher silanol density, and hence surface charge. A similar degree of roughness was observed with areas of up to 670 nm2. Modifications to the AFM, with subsequent losses in lateral resolution, allowed areas of 140 pm2to be studied. These images too were flat, as above, and featureless. However, it is important to add that to obtain large areas of stepfree surface the mica must be cleaved as a thin flexible sheet. For the methylated mica the surface is amorphous and the surface roughness remains about 0.13 nm (Figure IC)despite the fact that the 85' contact angle suggests incomplete methylation. However, if care is not taken to exclude water vapor during methylation, a surface film is observed. This film is probably a mixture of trimethylsilanol and hexamethylsiloxane produced as a result of condensation of the silanating agent with water. Holes in this film can be made with the cantilever tip by imaging at high force (- 10 nN) and reveal that the film is about 1-3 nm thick. For comparison, we have also studied a material containing surface silanol groups but which was made smooth by polishing. We imaged the surface of a polished silicon wafer12 which had an oxide surface 30 nm thick. The oxidation was performed at 920 'C in clean oxygen, (10)Binnig, G.; Quate, C.; Gerber, G. Phys. Reu. Lett. 1986,56, 930. Our force microscope ie a commercial Nanoscope 11, manufactured by Digital Instruments, Santa Barbara, CA. (11)Trimethylchlorosilane was obtained from Petrarch Systems, Inc., PA. (12) The wafer was obtained from Okemtic, P.O. Box 44, F1-02631 Espoo, Finland.
0 1992 American Chemical Society
734 Langmuir, Vol. 8, No. 2,1992
Notes
a
b
C
Figure 1. Surface of mica and modified mica under double-distilled water. The images are unfiltered, and were obtained with a 0.58 N m-l V-shaped silicon nitride cantilever in constant force mode. (a) Freshly cleaved native mica. Height differences are displayed as variation in brightness, with a 0.13-nm range between black (lowest)and white (highest). No further variation in height is recorded over areas up to 670 nm2. (b) Water plasma modified mica. In the plasma modification process, the mica was sealed in a chamber containing 0.03-Torr partial pressure of argon, and 0.02-Torr partial pressure of water vapor. A 25-W, 18-MHz rf source was then used to create an environment of water plasma, to which the mica was exposed for 3 min. In this image the height range is 0.2 nm and the scan area is 6.7 nm2. (c) Water plasma modified mica rendered hydrophobic by reaction with trimethylchlorosilane vapor for 3 min. The scan area is 6.7 nm2, and the height range is 0.2 nm. a
--
%,*-
Figure 2. Force microscope image of oxidized silicon wafer/water interface. The wafers were washed in ethanol immediately before use. These unfiltered images were obtained with a 0.58 N m-l V-shaped silicon nitride cantilever in constant force mode. (a) A 330 mm2 image with a height range of 0.67 nm. (b) A 6.7 nm2 image of the same surface with a height range of 0.33 nm.
Notes
and then the wafer was annealed and cooled in argon. Figure 2 reveals that unlike mica, the roughness of the oxidized wafer depends on the size of the area examined. In scans of area 330nm2,the highest asperity was typically 0.7 nm above the mean height while the standard deviation from the mean was 0.2 nm. In 6.7 nm2 scans the highest asperity was 0.33 nm above the mean height and the surface deviation was 0.10 nm. Thus, force microscopy indicates that plasma-modified mica provides extremely smooth surfaces which are suitable for surface force measurements but can also be reacted with silanating agents to yield a variety of surfaces. Because of the combination of extreme smoothness and
Langmuir, Vol. 8, No. 2, 1992 735
chemical reactivity of the surface layer, plasma-modified mica should be useful for AFM investigations of biological materials. Currently, immobilization of organic or biological molecules on mica is difficult because of nonspecific binding. Plasma-modified (and silanated) mica offers the possibility of hydrogen and covalent bonding and thus a way to immobilize molecules on this very smooth substrate.
Acknowledgment. The plasma reactor was designed and built by John Parker whom we thank for ita use. We also thank Tommy Nylander for providing the silicon wafers, and Per Claesson for helpful discussions. Registry No. Si, 7440-21-3.
