Preparation and interpretation of Kelvin probe force microscopy experiments on bulk insulators
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Abstract
Molecular electronics is a promising field in nanotechnology. It is based in the possibility of building up miniaturized electronic devices using single molecules or molecular structures with specific functions, such as switches, transistors or resistors. However, before we can build up these devices, we need to understand all different aspects that influence the formation of different molecular structures on surfaces.
Molecular self-assembly is the field of study of the different structures that molecules can form by interacting with each other. For molecular electronics, it is necessary to have these molecular structures arranged on surfaces. Furthermore, we need to build up molecular structures which are electronically decoupled from the substrate. Because of this, we work on insulating substrates. For studying these molecular structures, we use atomic force microscopy (AFM). This technique allows to perform local studies with nanometre resolution and does not need conducting substrates, as scanning tunnelling microscopy (STM) does.
One of the interactions playing a crucial role in molecular self-assembly is the electrostatic interaction. Investigating the electrostatic interaction implies studying the charge distribution. Kelvin probe force microscopy (KPFM) is a variation of AFM that allows mapping the charge distribution on a sample surface. Many studies have been performed using this technique on semiconductors, molecular structures on bulk substrates or single molecules. Nevertheless, there was not a unifying theory that explains the KPFM signal for all types of samples. Recently, new theory for explaining KPFM was developed. The theory predicts that if point charges are located between the metallic tip and metallic sample or sample holder, the KPFM signal depends on the tip-sample distance. It also states that all point charges located between metallic tip and sample holder contribute to the KPFM signal. The direct implication is that, if there are point charges localized on the tip apex, these charges will also contribute to the KPFM signal. Thus, it is necessary to develop a procedure to obtain AFM tips free of localized point charges.
Considering this new theory, in this thesis a systematic study of KPFM experiments on insulators in ultra-high vacuum (UHV) is presented. It is demonstrated that all point charges present between tip and sample holder contribute to the KPFM signal. The procedures to clean the metal-coated AFM tips and to test their contamination state are described in detail. Finally, experiments performed on calcite (10.4) and on 2,5-dihydroxybenzoic acid (2,5-DHBA) deposited on calcite using these prepared metal-coated AFM tips are presented.
In summary, this thesis provides experimental evidence of the contribution of all point charges present between tip apex and metallic counter electrode to the KPFM signal. It presents a procedure to clean the metal-coated AFM tips for KPFM experiments and to test the tips before and after experiments on molecules deposited on bulk insulators. It demonstrates that obtaining reliable KPFM data sets in experiments on bulk insulators using these clean metal-coated AFM tips is possible.