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Particles on Melt-Cut Mica Sheets Are Platinum Mika M. Kohonen,*,† Fiona C. Meldrum,‡ and Hugo K. Christenson*,§ Department of Applied Physics, University of Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany; Department of Chemistry, Queen Mary University of London, Mile End Road, London E1 4NS, U.K.; and Department of Physics and Astronomy, The University of Leeds, Leeds LS2 9JT, U.K. Received September 6, 2002. In Final Form: November 14, 2002
It has recently been shown that mica sheets prepared by melt-cutting freshly cleaved mica for use in the surface force apparatus (SFA) have numerous particles on the surface.1 These particles are too small (typically 20-25 nm in diameter and 2-3 nm high according to atomic force microscope (AFM) images presented in ref 1) and too sparsely distributed to be analyzed spectroscopically. At the time, one of us speculated that they consist of droplets of solidified mica, scattered across the surface as the white-hot platinum wire used for cutting melts the mica. Recent interest in the influence of these particles on measurements with the SFA of solvation forces and friction has now prompted us to look anew at this artifact of surface preparation.2,3 We have confirmed the basic observations of ref 1, i.e., that particles are invariably found on melt-cut mica sheets (Figure 1a), although the number density varies considerably from sheet to sheet (in the range 0.1-10 µm-2), and is much larger on thicker sheets. According to the AFM images obtained in this work,4 the particles are typically 2-10 nm high and 30-150 nm in diameter, slightly larger than previously reported. The ratio of height to diameter of the particles appears to be approximately 0.08, though the effects of tip convolution5 preclude a detailed analysis of their morphology. Further observations suggest that these particles consist of platinum, and not of mica, as previously thought. Particles of a similar size and number density are found on a mica sheet (Figure 1b) that has merely been held close (∼ 5 mm) to a white-hot platinum wire for 5-10 ssthe typical time it might take to cut out a mica sheet for use in the SFA. The number density of particles on an exposed surface increases with proximity to the wire and increases with the length of time that the surface is exposed to the wire (Figure 1c). Similar particles are also observed on a silicon wafer surface exposed to the wire * Corresponding authors. E-mail: (M.M.K.) mika.kohonen@ physik.uni-ulm.de; (H.K.C.)
[email protected]. † University of Ulm. ‡ Queen Mary University of London. § The University of Leeds. (1) Ohnishi, S.; Hato, M.; Tamada, T.; Christenson, H. K. Langmuir 1999, 15, 3312. (2) Ohnishi, S.; Stewart, A. M. Langmuir 2002, 18, 6140. Becker, T.; Herminghaus, S.; Mugele, F. Preprint. (3) Granick, S. Personal communications. (4) AFM images were obtained in air using a commercial instrument (Bioscope, Digital Instruments, Santa Barbara, CA) operated in tapping mode. We used Si cantilevers of type NCHR obtained from Nanosensors, Neuchatel, Switzerland (with a nominal tip radius of 10 nm). (5) See, e.g.: Markiewicz, P.; Goh, M. C. Langmuir 1994, 10, 5. Grabar, K. C.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S.-L.; Natan, M. J. Anal. Chem. 1997, 69, 471. Hulteen, J. C.; Treichel, D. A.; Smith, M. T.; Duval, M. L.; Jensen, T. R.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 3854.
Figure 1. AFM images (2.5 µm × 2.5 µm): (a) a mica surface (∼1 cm × 2 cm; thickness ∼5-10 µm) cut with a white-hot platinum wire; (b) a mica surface held parallel to a white-hot platinum wire at a distance of ∼5 mm for 5-10 s; (c) a mica surface held parallel to a white-hot platinum wire at a distance of ∼5 mm for ∼30 s; (d), a silicon wafer surface held parallel to a white-hot platinum wire at a distance of ∼5 mm for ∼30 s. The horizontal streaks in part d suggest that some of the particles are moved by the scanning AFM tip.
Figure 2. Photographs of mica surfaces (3-4 cm in width) exposed to a glowing platinum wire for (a) ∼10 and (b) ∼30 min. Both surfaces are coated with a reflective film in the region which was in closest proximity to the wire. In part a, the film is silvery-white. Curiously, in part b, the reflective film is silverywhite at the edges but colored toward the middle.
(Figure 1d). After exposure to a glowing wire for about a minute, a dark “stain” is apparent on the surface, and after more than 10 min, a mica surface takes on a metallic appearance with a silvery-white, reflective coating (Figure 2). This suggests that the glowing wire is coating the mica (or silicon) surface with platinum, and this was confirmed with EDX analysis of a mica sample with a reflective coating. Comparison of the elements present on freshly cleaved mica (Figure 3a) with those on mica after prolonged
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Figure 3. EDX spectra of (a) a freshly cleaved mica surface and (b) a mica surface with a reflective coating produced by exposure to a glowing platinum wire for ∼30 min. Samples were prepared for EDX analysis by mounting mica samples on Al stubs using conducting carbon tape and sputter-coating with a thin layer of carbon prior to analysis. All samples were examined in a JEOL JSM-6300 scanning electron microscope operating at 20 kV, using an Oxford Instruments, Inca Energy EDX analysis system.
exposure to the hot platinum wire (Figure 3b) clearly demonstrates that platinum is deposited on the mica surface due to proximity to the wire. Evaporation of platinum has been shown to occur at temperatures above 1300 °C,6 and since mica melts at about 1320 °C,7 a platinum wire hot enough to cut mica will necessarily evaporate and deposit platinum onto any surface in the vicinity. It is thus likely that the particles on melt-cut mica surfaces consist of platinum (although we cannot exclude the possibility that platinum oxides (6) Smentkowski, V. S.; Yates, J. T., Jr. J. Vac. Sci. Technol. A 1994, 12, 224. (7) Value quoted by mica suppliers Boud Marketing Limited (Longend Lane, Green Lane, Marden, Kent, TN12 9SE, U.K.) and Micronized Group (PO Box 91575, 2006 Auckland Park, Johnannesburg, South Africa).
Notes
may also be produced by the wire6). This explains the increase in number density of particles with proximity to the wire. The earlier found correlation with mica thickness is also easy to explain. Thicker mica requires a hotter wire and slower cutting, i.e., increased exposure time. Given that the evaporation occurs under atmospheric pressure it may be that the particles are formed prior to contact with the surface,8 although dewetting of a thin platinum film on the surface is also a possibility9 (our AFM images were not of sufficient resolution to allow us to comment on the possible presence of monolayer or submonolayer films of Pt on the mica). We note that the draught in a laminar flow cabinet is very efficient at carrying away the metal atoms. No discernible film is formed after 10 min if the air flow is allowed to pass freely between the wire and the mica surface. Slight variations in the airflow, temperature of the wire, and the rate of cutting would be expected to have an important effect on the size and number density of the deposited particles. The adhesion of the particles to the surface may vary considerably, and they are at times displaced even by imaging in airsmore easily so on silicon than on mica. In ref 1, it was found that the particles usually disappeared on repeated scanning under water. Experiments on the capillary condensation of water between melt-cut mica surfaces suggest that some fraction of the particles are highly mobile in water and may be removed by rinsing the surfaces.10 We have observed, however, that the majority of particles on mica surfaces are not removed by simply rinsing the surfaces with water. To avoid completely particles on mica sheets for SFA measurements it would be necessary to either employ the second-cleavage method of Frantz and Salmeron,11 or to use a cutting wire of some less volatile metal, although oxide formation at elevated temperatures may preclude the use of most such metals. With the former method, one ends up with a nonsymmetrical interferometer, as the mica sheets are of different thickness, and a consequent loss of resolution. Note, however, that with use of thin mica and quick and efficient cutting at as low a wire temperature as possible, one may minimize particle deposition even if a platinum wire is employed. Particle deposition could also be reduced by maintaining a high rate of laminar air flow and by cutting large pieces of mica which can subsequently be cut into smaller pieces using a scalpel.3,10 Acknowledgment. M.M.K. acknowledges the Alexander von Humboldt Foundation for financial support. H.K.C. and F.C.M. acknowledge support from the EPSRC. LA026520K (8) Granqvist, C. G.; Buhrman, R. A. J. Appl. Phys. 1976, 47, 2200. (9) Woodward. J. T.; Zasadzinski, J. A. J. Microsc. 1996, 184, 157. (10) Kohonen, M. M.; Christenson, H. K. Langmuir 2000, 16, 7285. (11) Frantz, P.; Salmeron, M. Tribol. Lett. 1998, 5, 151.
