Low-temperature scanning probe microscopy

Abstract

This chapter is dedicated to scanning probe microscopy (SPM ) operated at cryogenic temperatures, where the more fundamental aspects of phenomena important in the fields of nanoscience and nanotechnology can be investigated with high sensitivity under well-defined conditions. In general, scanning probe techniques allow the measurement of physical properties down to the nanometer scale. Some techniques, such as scanning tunneling microscopy (STM ) and scanning force microscopy (SFM ), even go down to the atomic scale. Various properties are accessible. Most importantly, one can image the arrangement of atoms on conducting surfaces by STM and on insulating samples by SFM. However, electronic states (scanning tunneling spectroscopy), force interaction between different atoms (scanning force spectroscopy), magnetic domains (magnetic force microscopy), magnetic exchange interactions (magnetic exchange force microscopy and spectroscopy), local capacitance (scanning capacitance microscopy), local contact potential differences (Kelvin probe force microscopy), local temperature (scanning thermal microscopy), and local light-induced excitations (scanning near-field microscopy) can also be measured with high spatial resolution, among others. In addition, some modern techniques even allow the controlled manipulation of individual atoms/molecules and the visualization of the internal structure of individual molecules. Moreover, combined STM/SFM experiments are now possible, mainly thanks to the advent of tuning forks as sensing elements in low-temperature (LT) SPM systems. Probably the most important advantage associated with the low-temperature operation of scanning probes is that it leads to a significantly better signal-to-noise ratio than measuring at room temperature. This is why many researchers work below 100 K. However, there are also physical reasons to use low-temperature equipment. For example, visualizing the internal structure of molecules with SFM or the utilization of scanning tunneling spectroscopy with high energy resolution can only be realized at low temperatures. Moreover, some physical effects such as superconductivity or the Kondo effect are restricted to low temperatures. Here, we describe the advantages of low-temperature scanning probe operation and the basics of related instrumentation. Additionally, some of the important results achieved by low-temperature scanning probe microscopy are summarized. We first focus on the STM, giving examples of atomic manipulation and the analysis of electronic properties in different material arrangements, among others. Afterwards, we describe results obtained by SFM, reporting on atomic-scale and submolecular imaging, as well as three-dimensional (3-D ) force spectroscopy. Results obtained with the method of Kelvin probe force microscopy (KPFM ) that is used to study variations in local contact potential difference (LCPD ) are briefly discussed. Magnetic force microscopy (MFM ), magnetic exchange force microscopy (MExFM ), and magnetic resonance force microscopy (MRFM ) are also introduced. Although the presented selection of results is far from complete, we feel that it gives an adequate impression of the fascinating possibilities of low-temperature scanning probe instruments. In this chapter low temperatures are defined as lower than about 100 K and are normally achieved by cooling with liquid nitrogen or liquid helium. Applications in which SPMs are operated close to 0∘C are not covered in this chapter.