Research On Substrate-scanning Tip Interactions Under Complex Conditions

At the nanoscale, some surface interactions increase dramatically and they are much sensitive to the environment due to the increasing influence of size effect. In air, the adsorption of gas molecules on the surfaces has strong effects on the nanoscale physical and mechanical properties of materials and makes them different from that in vacuum. Under the nanoscale confinement, the physical properties of fluid can be changed significantly. Applying an external field such as electrical field, heat etc. may enhance the changes of the above properties of materials; even induce some chemical property changes. The changes in material properties can influence the stability of application performance of materials and related devices, on the other hand, these changes can also lead to the emergence of new principles. So it is necessary to study the surface interactions of materials under the nanoscale and different complex conditions. The scanning probe microscope is an important tool for studying surface forces at the nanoscale. Measurements using scanning probe microscope can be conducted in multiple conditions such as in air, vacuum and applying the external field etc., and a nanoscale confined condition is easily formed between the tip and the substrate, which provide helpful conditions for studying the surface interactions under the complex environment and lead to a lot of studies of physics mechanics based on the tips. In this thesis, we studied the adhesion and friction behaviors using the scanning probe microscope between different tips and freshly cleaved mica surface in air, in vacuum and under bias electric field, physical and mechanical properties of capillary water condensate under the electric field, and the interactions between the conductive tips and highly oriented pyrolytic graphite (HOPG) under the applied electric field. The following progresses are achieved:(1) The effect of external loads on the friction forces between the Si3N4 tip and the mica surface has been investigated both in air and in vacuum of 10-3 Pa. It is found that at the loads lower than 20 nN, the friction force increases linearly with the external load, which suggests that the contact between the tip and the mica surface is multi-asperity contact; the friction force in air is lower than that in vacuum due to the lubricant effect of water absorbed on the mica surface. The effect of scanning velocity below 2μm/s on the friction force was studied in air using the Si3N4 tip and diamond-coated tip at different loads and under the applied electric fields. We found that the friction force increases nonlinearly with the scanning velocity; combining with the thermally activated mode that is usually used to interpret the origin of nanoscale friction, we theoretically analyzed the relationship between the friction and the scanning velocity and suggested that the logarithmic dependence of the friction force on the velocity, i.e. FL～lnV is universally applicable for the different friction systems and environments. When a negative or positive bias is applied to the substrate, the friction force between the Si3N4 tip and the mica surface measured in air increases with the absolute value of the bias voltage; but the friction force at the negative bias is higher than that at the positive bias. This indicates that the negative bias has stronger influences on the friction force between the Si3N4 tip and the mica surface.(2) Under the applied electric field, the friction and adhesive behaviors between the hydrophilic Pt-coated tip and the mica surface have been studied at room temperature both in air and in vacuum. It was found that in air when the negative bias was applied to the substrate, the friction curve and force-displacement curve showed surprising shapes different from the traditional ones, in which the friction curve tilted totally, an abrupt flex occurred in the retraction of force-displacement curve and the adhesive force decreased evidently; while the shape of the curves kept unchanged under no bias or the positive bias. In vacuum the bias has no effect on the curves, which indicates that in air the capillary water condensate between the tip and the substrate causes the unusual changes in the curves under the effect of the negative bias. Mechanically, the Young's modulus of the capillary condensate is similar to that of the multilayer ice, which suggests that the capillary water is induced to freeze into the icelike structure at the room temperature under electric fields. With the hydrophobic diamond-coated tip, the analogous experiments have been done. It showed that the change in the friction curve is similar to that measured by the Pt-coated tip and the mechanical property of the capillary condensate is close to that of icelike solid, but the measured friction curves by the diamond-coated tip show a slow tilting process and the tilting of the curve gradually develops with increasing the negative bias or its remaining time. The effect of the bias on the force-displacement curve measured by the diamond-coated tip is very different from the former. Here the curve shows the noticeable hysteresis behaviors both after jump-to-contact and before jump-off-contact in air and under the positive bias and the adhesive force decreases, which suggests that the capillary water behaves like the bulk water. While the force-displacement curve shows no changes under other conditions. The comparative study using the two tips showed that the effect of the applied bias on the friction and adhesive behaviors between the tip and the mica surface is closely related to the properties of tip materials.(3) Convex and concave nano structures were created on HOPG surface by applying a positive voltage pulse to the the sample surface. With increasing voltage or its duration over a threshold, the convex structures fractured and converted into concave ones. The depth of the concave structure increases rapidly with increasing amplitude and duration of the applied voltage. The voltage threshold of transition decreases with increasing duration and keeps constant at about +5 V. The functional relation between the voltage threshold and the duration has been obtained. Analysis of the experimental data shows that the minimum voltage threshold ranges from 4 to 5 V. Deeper mechanism investagtion shows that under the experimental conditions, the formation of convex intermediates is mainly ascribed to the dissociative adsorption of water and oxygen in atoms induced by intensive hole concentration under the external electric field, and the following defect-induced oxidation of graphite facilitates the formation of concave.