Alkali Halides
Surfaces of alkali halide crystals have properties which make them interesting model surfaces for the surface science of insulators: they perfectly terminate ionic crystals, they are chemically inert, and they have no free electrons. Structural and point
- PDF / 2,293,244 Bytes
- 15 Pages / 439 x 666 pts Page_size
- 53 Downloads / 192 Views
Alkali Halides
Roland Bennewitz, Martin Bammerlin, and Ernst Meyer
5.1
Introduction
Surfaces of alkali halide crystals have properties which make them interesting model surfaces for the surface science of insulators: they perfectly terminate ionic crystals, they are chemically inert, and they have no free electrons. Structural and point defects are well described. Some model systems for film growth like Au on NaCI have been extensively studied for decades. With the invention of force microscopy, experimental studies on the atomic scale became feasible. For the above reasons, surfaces of alkali halide crystals have become important test samples in the development of force microscopy. They were the first non-layered insulators whose atomic periodicity could be resolved [1,2J. However, only after the invention of non-contact dynamic force microscopy did true atomic resolution of single point defects and steps become possible. In this chapter we will describe some general aspects and typical results of dynamic non-contact force microscopy on alkali halide surfaces. To begin with, we briefly introduce the experimental techniques and the tip~sample forces to be considered. Imaging of single crystals and of thin films grown on metal substrates will be discussed, with emphasis on atomic-scale contrast formation. As an application we describe topographical effects of electron irradiation on surfaces of CaF 2 and KBr. Finally, we discuss results for the damping of tip oscillation in our measurements. 5.1.1
Experimental Techniques
The dynamic force microscopy experiments described in this section were performed with a tip oscillation of constant amplitude A. The amplitude is controlled by measuring the root mean square of the oscillation signal and varying the amplitude Aexc of the voltage applied to the piezo-actuator. Typically, oscillation amplitudes between 1.5 nm and 30 nm are used. The actual eigenfrequency f of the cantilever is determined by a digital phase-locked loop circuit. A numerical oscillator is locked to the oscillation signal with fixed phase [3J. The sinusoidal signal of the numerical oscillator is used to excite the cantilever with the amplitude Aexc mentioned in the preceding paragraph. S. Morita et al. (eds.), Noncontact Atomic Force Microscopy © Springer-Verlag Berlin Heidelberg 2002
94
Roland Bennewitz et al.
In order to record the topography of a surface, the frequency shift of the tip oscillation is kept constant by varying the tip-sample distance. The system consists of three controllers: one for the oscillation amplitude, one for adjusting the frequency in the phase-locked loop to that of the cantilever, and one for the tip-sample distance. The speed of the distance controller is limited by the need to avoid mechanical eigenfrequencies of the scanner and to make sure that the other two controllers are always working properly. The typical scanning velocity while recording high-resolution images is thus 10 nm/s.
5.1.2
Relevant Forces
The relevant forces in force microscopy of alkali hali
Data Loading...
