How dry are dried samples? Water adsorption measured by STM

When operating scanning probe microscopes, like STM or AFM, under ambient c onditions, 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 i tself, but also the imaging process; in the case of the STM using a wet etc hed w-tip, by interfering with the electron transfer process, and in the ca se of the AFM, due to the capillary forces in the micro Newton range that d ominate the tip surface interaction forces. In this paper, the distribution and the amount of adsorbed water on different surfaces is investigated wit h the help of the STM, which can provide information by imaging and by curr ent/distance spectroscopy. Hydrophilic and hydrophobic surfaces like titani um, gold, and graphite were studied at a relative humidity between 10 and 9 0%. Under very dry conditions with relative humidity below 15%, the presenc e of water was only detectable by the longer decay length of the measured c urrent with distance compared to samples prepared in UHV completely free of water. At less dry conditions on gold surfaces, water was found as droplet s. With increasing humidity, the quantity and the size of these droplets in creased until the whole surface became covered with water. Above 55% humidi ty, the thickness of the water film increased with increasing humidity up t o several 10 nm. On titanium and graphite, water was always present in the form of closed layers growing in thickness with increasing humidity. (C) 19 99 Wiley-Liss, Inc.