This same study also reported the force required to rupture bonds in oligomers of inosine (I) averaging 160 bases in length (22). Cytosine-functionalized silica spheres on a surface and tip provided tethering sites for the oligomers and considerable force was needed to pull apart the tip and the surface, presumably because of the snapping of the oligomer chain. In some cases multiple snapping points could be seen in the force curve. These multiple snapping points are believed to be caused by the variation of the chain length of the poly (I) and multiple connections between the tip and surface. Future applications An unproven but intriguing application of the force microscope has been proposed by Sidles and co-workers (23, 28), who suggest using the cantilever as a detector for making magnetic resonance measurements. A strong magnetic field gradient is created by a magnetic particle on the end of a cantilever. When the sample is brought close to this particle and oscillated from side to side, it can induce an oscillation in the cantilever because of the coupling of the magnetic moments of the surface atoms and die magnet on the cantilever. Because the distance between the sample and cantilever is small, high resolution should be possible. The forces that are generated will be extremely small (~ 10"19 N), so advances in cantilever and force detection technology are needed for this experiment to be viable. A wide variety of forces have been detected and imaged with many variations of the force microscope, and the measurement of other molecular properties, such as individual electron and nuclear spins, may be possible. The increasing availability of force microscopes as well as their many potential applications should result in additional variations of force microscopy in the near future.
(3) Sand, D. Scanning Force Microscopy, 1st éd.; Oxford University Press: New York, 1991. (4) Albrecht, T. S.; Akamine, S.; Carver, T. E.; Quate, C. F.J. Vac. Sci. Technol. A 1990, 8, 3386. (5) Griffith, J. E.; Grigg, D. A. J. Appl. Phys. 1993, 74, R83. (6) Atkins, P. W. In Physical Chemistry, 3rd éd.; Oxford University Press: Oxford, U.K., 1986. (7) Burnham, Ν. Α.; Colton, R. J. In Scanning Tunneling Microscopy and Spectroscopy Theory, Techniques, and Applications; Bun nell, D. A., Ed.; VCH: New York, 1993. Ch.7. (8) Hutter, J. L; Bechhoefer, ].]. Appl Phys. 1993, 73,4123. (9) Ohnesorge, F.; Binnig, G. Science 1993, 260,1451. (10) Martin, Y.; Williams, C. C; Wickramasinghe, H. K.J. Appl. Phys. 1987, 61, 4723. (11) Hansma, P. K.; Cleveland, J. P.; Radmacher, M.; et al. Appl. Phys. Lett. 1994, 64, 1738. (12) Stem, J. E.; Terris, Β. D.; Mamin, H. J.; Rugar, D. Appl. Phys. Lett. 1988,53, 2717. (13) Scandella, L.; Schumacher, A; Kruse, N.; et al. Thin Solid Films 1994,240,101. (14) Frisbie, C. D.; Rozsnyai, L. F.; Noy, Α.; Wrighton, M. S.; Lieber, C. M. Science 1994,265,2071. (15) Delawski, E.; Parkinson, B. A . / Am.
Chem. Soc. 1992,114,1161. (16) Ueno, Κ.; Koma, Α.; Ohuchi, F. S.; Parkin son, Β. Α. Appl. Phys. Lett. 1991, 58,472. (17) Chen, C-H.; Gewirth, A. A / . Am. Chem. Soc. 1992,114,5439. (18) Majumdar, A; Carrejo, J. P.; Jai, J. Appl. Phys. Lett. 1993, 62, 2501. (19) Gimzewski, J. K.; Gerber, C; Meyer, E.; Schlitter, R. Chem. Phys. Lett. 1994,217, 589. (20) Lee, G. U.; Kidwell, D. Α.; Colton, R. J. Langmuir 1994,10, 354. (21) Lee, G. U.; Chrisey, L. A; Colton, R J. Sci ence 1994,266,771. (22) Deng, G.; Wu, R Methods Enzymol. 1983, 100,96. (23) Sidles, J. A; Garbini, J. L.; Drobny, G. P. Rev. Set. Instrum. 1992, 63,3881. (24) Sidles, J. Α.; Rugar, D. Phys. Rev. Lett. 1993, 70, 3506.
Darrell R. Louder, a doctoral candidate at Colorado State University, is studying dop ing and defects in layered dichalcogenides by STM and SFM. Bruce A. Parkinson, pro fessor of chemistry at CSU, conducts re search in photoelectrochemistry, scanning probe microscopies, and surface and materi als chemistry. Address correspondence to Parkinson at the Dept. of Chemistry, CSU, Fort Collins, CO 80523.
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We thank Jane Frommer and Andrew Gewirth for providing some of the images used in this article and the U.S. Department of Energy, Division of Chemical Sciences, for support.
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References (1) Binnig, G.; Roher, H.; Gerber, C; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (2) Binnig, G.; Quate, C. R; Gerber, C. Phys. Rev. Lett. 1986, 56,930.
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