Nanomechanics and Dynamics of Tip-Substrate Interactions

The understanding of the atomic processes occurring at the interface of two materials being brought together, separated or moved with respect to each other is central to many technological problems, including adhesion, contact formation, friction, wear, lubrication, nano-indentation and fracture. A single fine tip as used as specimen in the field ion microscope (FIM) may allow the inspection of the true contact area, lattice defects and material transfer with atomic resolution. The atom probe may provide the chemical composition position sensitive on atomic level. A single fine tip as well is used in the scanning tunnelling microscope (STM) to scan metal surfaces to provide topographic information of the surface with sub-nanometer resolution. Comprehensive studies with the STM have been performed to study induced defects and material transfer on the flat substrate surface. Very little experimental work has been carried out, in this context, to study the consequences on the tip itself. The main objective of this project was the study of the consequences of tip-surface interactions on an atomic level on both, the flat substrate sample in the STM and the modifications on the tip in the FIM. Another essential feature of the experiment was the chemical analysis of the tip in the case of material transfer in the 3D atom probe. The unique aspect of the project was the combining use of all three experimental techniques in one single UHV system. In the first stage of the project the experimental set-up has been established. Unfortunately only a fraction of the instrument requested has been granted. Therefore only a refurbished STM instrument could be purchased without the essential electronic control system. To make the system running a homemade electronic control system has been arranged based on a PC, I/O interface cards, low signal and high voltage amplifiers. After this rather time consuming stage the essential experiments with the tip in the STM could be performed. The experiments on tip-substrate interaction had been carried out in several steps: Tip approach close to the surface (tunnelling regime), jump to contact position, mechanical contact and indentation of the tip into the substrate surface. As model system a hard tip and soft substrate metal have been used. Already before mechanical contact occurs, a certain transfer of species from the substrate surface to the tip can be observed. The quantities detected on the tip are far below one monolayer. After mechanical contact a material transfer exceeding several monolayers can be measured. The chemical nature of the deposit on the tip has been determined by time-of-flight mass spectrometry. Hereby substrate material can clearly be identified beside residual gas adsorbates. After indentation of the tip of several nanometers plastic deformation on the tip can be observed too. The experimental results have been compared with already documented theoretical model calculations (molecular dynamics). There is a considerably good agreement with the experiment for the tip-substrate metal combination which has been studied. The most attractive final result is that within the project a new almost unique instrument, combining imaging and analytical methods on atomic level, has been realised in one single UHV system. The scanning tunnelling microscope (STM) with in situ tool for formation, characterisation and controlled modification of the tip (FIM - atom probe) is a great challenge for new investigations on more complex surfaces where a well defined shape of the tip is a key issue.