SFM Studies of the Surface Morphology of ICE
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21 Mat. Res. Soc. Symp. Proc. Vol. 355 @1995 Materials Research Society
EXPERIMENTAL In our experiments we have used a commercial atomic force microscope NanoScope III (Digital Instruments, Inc.). The principles of AFM operation are described elsewhere14 . Briefly, in the AFM a very sharp tip, mounted on a flexible cantilever, is scanned over the surface of the sample. The deflections of the cantilever are detected by a laser beam reflected from the tip. The horizontal resolution is determined by the radius of the tip (R = 50 nm) and the vertical resolution is about 0.1 nm. We used commercial gold coated silicon nitride cantilevers (200 gm long) with the spring constant k = 0.38 N/m. NanoScope III allows operation in both contact and non-contact modes. In most of our experiments the contact mode was used. One of the major advantages of SF microscopy is that it can be used for the investigation of dielectrics and materials with low conductivity, such as ice. Measurements of the force interaction between the tip and the surface, as a function of relative tip-sample position, yield a force curve. From a force curve a wealth of information about the properties of the surface can be extracted. For example, one can determine the type of tipsurface interaction, the adhesion force and the thickness of a liquid film present on the surface1 5- 17 . Recently it has been shown that force curves allow even to measure the strength of an individual hydrogen bond between the tip and the surface 18. This makes the force curve technique extremely appropriate for studying the surface of ice. In addition, in non-contact mode SFM is capable of measuring electric fields produced by the surface. Thus, at least theoretically, it would allow to measure the polarization charge on the ice surface, if any. The SFM head was placed in a cold room, with temperature maintained in the range from 00 C to -45°C with precision _+0.2°C. The head was oriented in such a way, as to allow an efficient air circulation through the cantilever housing. This was necessary in order to provide a cooling of the cantilever by the air flow. The SFM controller and the computer were kept outside the cold room. The ice specimen was prepared from a deionized, degassed water. Special precautions were taken to prevent possible surface contamination which has a crucial effect on the properties of the surface layer. In the course of this study we found that ice under cantilever sometimes melts. Melting manifests itself in the gradual formation of a "hole" in the sample surface in the center of the scanning field. This can be attributed to the heating of the cantilever by the laser beam or to the heating from the electronic circuits located in the base of the head. The second factor, however, according to our estimations appears to be insignificant. To prevent the heating of the cantilever by the laser we reduced the power of the laser beam by a factor of approximately f= 0.1 using optical filters. The initial power of the laser used was Wo = 1 mW. The absorption coefficien
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