Adsorption of Water on Solid Surfaces Studied by Scanning Force Microscopy A. Gil, J. Colchero,* M. Luna, J. Go ´mez-Herrero, and A. M. Baro ´ Departamento de Fı ´sica de la Materia Condensada C-III, Universidad Auto ´ noma de Madrid, Campus de Cantoblanco, E-28049, Madrid, Spain Received October 8, 1999. In Final Form: February 25, 2000 Tip-sample interaction of an oscillating tip near a surface is determined. The experimental results show that the presence of the surface can be detected without mechanically touching the surface. By adjusting the appropriate operating conditions of a scanning force microscope setup, tip-sample contact can be avoided during imaging at atmospheric pressure. This allows study of even the softest samples. In the present work, we demonstrate that molecularly thin water films can be imaged with nanometer resolution on different substrates such as mica, gold, and highly oriented pyrolitic graphite. Correspondingly, scanning force microscopy can be used to investigate wetting properties of liquids with very high spatial resolution. Introduction The physics and chemistry of surfaces are of funda- mental as well as technological interest, since many processes such as melting and chemical reactions are known to initiate at surfaces. Also, many biological processes occur on membranes and, thus, at the solid- liquid interface. Correspondingly, surface science is an important and well-established field. In particular, one topic that has drawn much attention is the adsorption of materials on surfaces. For a detailed understanding of the corresponding processes, investigations on an atomic and molecular scale are needed. Techniques such as low- energy electron diffraction, He scattering, or X-ray dif- fraction have been the main tool for this kind of inves- tigation. Since these techniques require ultrahigh vacuum (UHV) conditions for operation, investigation of adsorption on surfaces under ambient conditions on a nanometer scale has been rather limited. The development of the scanning force microscope 1 (SFM) has opened new expectations and possibilities in surface science due to its ability to work not only in UHV but also under ambient conditions as well as in liquids. SFM is one member, probably the most extended and versatile one, of the by now quite numerous family of scanning probe microscopes. The general principle on which these types of microscopes relies is the scanning of a tip in very close proximity to the sample that is to be studied. In the case of SFM, the force exerted by the tip- sample interaction is detected through a cantilever acting as a force sensor. The tip is attached to the free end of this microfabricated cantilever. Typically, two different kinds of experiments can be performed with a SFM. On one hand, a so-called force vs distance curve 2 can be acquired at a fixed position on the sample. In this case, the tip- sample distance is varied to explore the interaction potential between the sample and the probe tip. On the other hand, the interaction can be kept constant by means of an appropriate feedback system while the tip (or the sample) is scanned laterally to acquire topographical images. In addition to these topographical images, further sample properties can be measured and displayed as complementary images. For many processes in air, water on surfaces is fundamental. This is also true for SFM studies. In fact, effects of water have been observed in adhesion 3 as well as lateral force 4 measurements. However, direct observa- tion of liquid films by SFM techniques has been difficult, mainly due to the fact that most SFM modes work in the contact regime; that is, tip and sample are in mechanical contact. Therefore, structures that are weakly adhered to the surface are usually destroyed or moved away. However, recently approaches based on SFM techniques have been developed allowing reproducible and stable imaging in the noncontact regime. Scanning force polarization mi- croscopy, 5 which is based on the electrostatic interaction between tip and sample, is one of them, and a special version of the so-called “intermittent contact” mode (IC- SFM) is another one. With these techniques, water adsorption on mica 6 and alkali halides 7 has been studied, and the formation and shape of liquid drops 8,9 have been investigated. In addition, also the wetting behavior of liquid crystals and metal films has been determined. 10 In the present work, we report on our experiments to determine tip-sample interaction of a vibrating tip near a surface and show that the presence of the surface can be detected without mechanically touching the surface. By adjustment of the appropriate operating conditions of the SFM setup, tip-sample contact can be avoided during imaging and the adsorption of water can be studied on different surfaces. Tip-Sample Interaction Several operation modes have been developed, either to measure different surface properties or to minimize (1) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930. (2) Weisenhorn, A. L.; Hansma, P. K.; Albrecht, T. R.; Quate, C. F. Appl. Phys. Lett. 1989, 54, 2651. (3) De Pablo, P. J.; Colchero, J.; Go ´ mez-Herrero, J.; Baro ´ , A. M. Appl. Phys. Lett. 1998, 73, 3300. (4) Piner, R. D.; Mirkin, C. A. Langmuir 1997, 13, 6864-6868. (5) Hu, J.; Xiao, X.-D.; Salmero ´n, M. Appl. Phys. Lett. 1995, 67, 476. (6) Hu, J.; Xiao, X.-D.; Ogletree, D. F.; Salmero ´n, M. Science 1995, 267, 268. (7) Luna, M.; Rieutord, F.; Melman, N. A.; Dai, Q.; Salmero ´n, M. J. Phys. Chem. A 1998, 102, 6793. (8) Herminghaus, S.; Fery, A.; Reim, D. Ultramicroscopy 1997, 69, 211. 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