How dry are dried samples? Water adsorption measured by STM

When operating scanning probe microscopes, like STM or AFM, under ambient conditions, the presence of water on the sample and the tip always plays an important role. The water not only influences the structure of the sample itself, but also the imaging process; in the case of the STM using a wet etched w-tip, by interfering with the electron transfer process, and in the case of the AFM, due to the capillary forces in the micro Newton range that dominate the tip surface interaction forces. In this paper, the distribution and the amount of adsorbed water on different surfaces is investigated with the help of the STM, which can provide information by imaging and by current/distance spectroscopy. Hydrophilic and hydrophobic surfaces like titanium, gold, and graphite were studied at a relative humidity between 10 and 90%. Under very dry conditions with relative humidity below 15%, the presence of water was only detectable by the longer decay length of the measured current with distance compared to samples prepared in UHV completely free of water. At less dry conditions on gold surfaces, water was found as droplets. With increasing humidity, the quantity and the size of these droplets increased until the whole surface became covered with water. Above 55% humidity, the thickness of the water film increased with increasing humidity up to several 10 nm. On titanium and graphite, water was always present in the form of closed layers growing in thickness with increasing humidity.