Combined Scanning Tunneling and Atomic Force Microscopy and Spectroscopy on Molecular Nanostructures

Abstract

Several key aspects in the exciting field of surface science on the nanoscale were addressed during this thesis. The common denominator of this work is the microscope used for the experiments; the combined scanning tunneling and atomic force microscope (STM and AFM). These scanning probe methods allow the study of electronic, magnetic and mechanical properties on surfaces down to the level of an individual atom.In the scope of this thesis and a previous diploma thesis, I designed and built a combined STM/AFM head for an existing low temperature ultra-high vacuum system with a 14 Tesla magnet. The development of such novel instruments in this competitive field of research is a precondition to be capable of addressing the increasingly complex questions and to conquer ambitious experiments. In Chapter 3 I will summarize its design and highlight key features and important technological advancements, especially concerning the quartz tuning fork based force sensor. The versatility and reliability of the instrument was proven by a series of experiments at the atomic level.The interplay between the individual atoms or molecules under investigation and the supporting substrate can have a significant influence on the properties derived. Therefore, also the investigation and thorough characterization of the versatile surfaces available is an important requirement. With the combined STM and AFM capabilities of the instrument I investigated the mechanical properties of the insulating layer of hexagonal boron nitride on a rhodium crystal (h-BN/Rh(111)). I will present a thorough data analysis that reveals surprisingly low stiffnesses of the strongly corrugated layer with unprecedented resolution. By this study also the experimental performance of the instrument and its potential to detect forces with sub-pico Newton resolution is demonstrated.Also magnetism at the individual atomic and molecular level is accessible by scanning probe methods with excellent spatial and energy resolution. Spin-flip processes and the Kondo effect that arises due to the interaction between a localized magnetic impurity and the surrounding electrons, are the physical framework and a “fingerprint of magnetism" of the experiments on cobalt atoms I will discuss in Chapter 5. The perturbative approach to describe the spin-spin interaction can be derived from Kondo's original approach and will be introduced. It describes the data with convincing agreement. Already the topographic investigation of the cobalt atoms and cobalt-hydrogen complexes that formed on the h-BN/Rh(111) will reveal the richness of the underlying physics. I will further present the broad variety of spectroscopic signatures obtained during the investigation by scanning tunneling spectroscopy (STS). Here, the huge spread in magnetocrystalline anisotropies derived is worth highlighting. I will conclude with some elucidation and also some puzzling observations.The ability of the AFM to measure very small forces with high spatial resolution is the key to investigating the fundamental force related to the exchange coupling, which governs magnetism. In Chapter 6 I will present our first experimental findings and a theoretical discussion of the ambitious experiment to measure the force related to the transition of two spins being parallelly aligned in an external magnetic field to the antiparallel alignment, when they are brought into closer proximity.The first experimental observations related to the Kondo effect date back to the 1930s, while the first theoretical descriptions only emerged in the 1960s. These theories require discussion of the Kondo effect in different regimes, including strong and weak coupling and ferromagnetic as well as antiferromagnetic coupling. Most Kondo studies are discussed in terms of the strong coupling regime. The experiments done on a purely organic radical molecule that was directly deposited on a gold surface led to the intensive discussion of the different regimes and the underlying physics and models, which I will outline. The convincing agreement between the perturbation theory model and the experimental result on this true spin-1/2 system in the antiferromagnetic weak coupling regime will be summarized in the last chapter.Single-molecule magnets (SMM) are envisioned for a broad range of possible application in information technology, but are at the same time of tremendous scientific interest, as they can be considered the link between quantum mechanical and classical magnetic systems. While previous studies (on Mn12) had shown to be very successful, the STM/STS investigation of the SMM Cr7Ni on a gold surface and on the h-BN layer did not result in conclusive findings. The work is discussed in Appendix A. I will conclude this thesis with a summary and a selection of suggestions for interesting, as well as challenging further experiments the microscope should be capable of addressing.