Non-contact friction studied with pendulum AFM

Understanding nanoscale friction and dissipation is central to nanotechnology. The recent detection of the electronic friction drop caused by the onset of superconductivity in Nb by means of an ultrasensitive non-contact pendulum atomic force microscope (AFM) raised hopes that a wider variety of mechanical-dissipation mechanisms become accessible. In this thesis several mechanisms leading to the existence of non-contact friction at the nanoscale are studied and discussed. Chapters 2 and 3 concern the tool of choice, the pendulum AFM and the relevant forces and why it is a wonderful tool to investigate non-contact friction, chapter 4. The fundamental concepts of the experimental setup are discussed in chapter 5. Since the pendulum AFM is not a common tool in the research field and it is hardly discussed in literature, a detailed introduction of the working principal and sensing mechanism is given for the first time in chapter 6. Furthermore, the basic concepts are applied on a widely studied reference system, copper with sodium chloride, see chapter 7, where the non-contact friction behavior of a metal-insulator substrate is investigated. The result of this chapter show a distinguishable difference in the smooth rise of the non-contact friction due to the existence of an dielectric media on top of a metal, causing Joule dissipation. Moving on to more challenging system in the chapter 8, with a rather special electronic phase transition, the Charge Density Wave (CDW), observed in niobiumdiselenide NbSe2 below T = 34K. Here, we state for the first time the observation of discrete non-contact friction maxima multiplets. In collaboration with group of Prof. Dr. Erio Tosatti it was possible to find the theoretical explanation to the newly discovered non-contact friction mechanism. We observed hysteresis of the local 2 pi phase slips in the CDW phase order parameter, under the local influence of the tip potential. Furthermore, in the last chapter of the thesis, the preliminary results of niobium doped strontium titanate Nb : SrTiO3 (STO) are presented. We observed noncontact friction in the vicinity of the structural phase transition, which was theoretically proposed by Benassi et al., due to the softening of phononic modes at the structural anomaly, when the system becomes slow and soft. In addition, STO offers a second phase transition from the paraelectric state to the ferroelectric, and the random oriented electric dipoles become ordered and form domains in the XY plane. In this phase, we observed a remarkable train of non-contact friction maxima, which still lack a quantitative theoretical explanation. However, a possible candidate for the mechanism is briefly discussed. In summary, this thesis presents an overview over yet unknown mechanism of noncontact friction of matter, giving an adequate contribution to the understanding of dissipation processes of matter on the nanoscale.