Multidimensional force spectroscopy for nanoscale characterization of electrostatic and dispersion interactions

Abstract

Multidimensional force spectroscopy for nanoscale characterization of electrostatic and dispersion interactions The interaction between the tip and the sample is the fundamental physical principle on which the operation of the AFM is based. It is through the analysis of this interaction that the AFM can access the different properties of the samples in which it is used. There are many, and of a very different nature, contributions to the total interaction between the tip and the sample. Elastic, capillary, dispersion, electrostatic or magnetic forces are some of the forces that are usually present in our experiments and contribute to the total effective interaction between the tip and the sample. All of them act together and, therefore, it is necessary and fundamental to develop experimental techniques that are capable of discerning between the different contributions. This is precisely the aim of this thesis: to develop an experimental technique that, quantitatively, is capable of characterizing two of the most interesting contributions to the total interaction between the tip and the sample: electrostatic and dispersion interactions. Such a technique, capable of characterizing surfaces through chemical properties such as contact potential difference (CPD), Hamaker constant or dielectric constant, would give AFM a novel ability to characterize and chemically identify the samples in which it is used. It would, however, be impossible to achieve this objective without first addressing other considerations of a more technical or methodological nature. Having an accurate vision of the physical phenomena that occur between the tip and the sample will be fundamental, both when proposing and developing experiments, and when analyzing quantitatively the results they produce. Part of the effort devoted to this work will consist of identifying physical phenomena such as the formation of liquid necks and understanding how these affect the interaction and the data collected in the AFM measurements, giving us the ability to operate the AFM in a more controlled manner and also avoiding the non-linearities associated with dissipative interactions. We introduce what we call the true non-contact mode. A measuring mode, generally operated using frequency shift as a control signal, characterized by small amplitudes of oscillation at a sufficient distance from the sample to prevent the formation of a capillary neck between the tip and the sample. Only in this case, the only contributions to the total interaction of the point-sample system are electrostatics and dispersion. For the use of this technique of characterization of electrostatic and dispersion interactions it has been essential to use a previous technique called interaction images. In addition, it is necessary to develop a program that is in charge of the particularities of the analysis of the data obtained through this technique and its subsequent processing and representation to obtain quantitative results. Making use of this new technique of characterization of the interaction between the tip and the sample, in this work we are able to reconstruct in a precise way the electrostatic interaction and the dispersion one and to obtain from them characteristic properties of the materials as the CPD. By comparing the two interactions, we were able to avoid dependence on the geometry of the tip and calculate the Hamaker constant and the actual position of the surface with respect to the tip. The development of this technique has meant, in parallel, an advance in the understanding of the interaction between the tip and the sample in AFM, which has led to a reclassification of the measurement modes that we believe will be important to take into account in order to advance in obtaining qualitative results with AFM techniques. We apply this technique in two different experiments. In the first we apply the technique described above to different conductive samples and calculate both the CPD that characterizes them and the Hamaker constant, finding reproducible values and differences between the different materials significant enough to distinguish between them. In particular, we found that a joint representation of both properties provides a novel ability so far to classify and identify materials. From a second work it is concluded that not only the contamination in the tip, but sometimes also in the sample, is difficult to visualize and to detect, but it is, nevertheless, very important and determines significantly the results of AFM and its interpretation, demonstrating that it is important to have well defined surfaces in air and that the contamination is a factor to take into account when interpreting the results. In addition, this method allows a simple, effective and in situ characterization of the point-sample system without the need to remove either the tip or the sample from the experimental set-up.