In this review article, we clarified what we can image on semiconductor surfaces using noncontact atomic force microscopy (NC-AFM): 1. We can image covalent bonding force on Si(111)7×7 surface. 2. We can discriminate Si adatoms from Sb adatoms on Si(111)5√3×5√3-Sb surface with a Si tip and a Sb adsorbed tip. 3. We can image tilted dangling bond on Si(100)2×1 surface. 4. We can image individual hydrogen atom on Si(100)2×1:H monohydride surface. 5. We can measure atomic strain around missing dimer. 6. We can image a potential map on Si(111)√3×√3-Ag surface by measuring NC-AFM image as a function of tip-sample distance. 7. We can selectively control atomic force by placing an adequate atom on the tip apex. 8. We can pull up lower Si atoms constituting buckled Si dimer by increasing attractive force between tip and sample surface.
Our recent studies on metal oxide surfaces by noncontact atomic force microscopy (NC-AFM) are reviewed. We have demonstrated that atom-resolved images of a TiO2(110) surface can be obtained by NC-AFM. We have also revealed that isolated species of formate ions and hydrogen adatoms on TiO2(110) can be imaged by NC-AFM. As an example of observation of dynamic behavior of adsorbed molecules by NC-AFM, images of fluctuating molecules of acetate ions at domain boudaries of (2×1)-acetate overlayer on TiO2(110) are presented. CeO2−x has been believed to play a crucial role in automotive exhaust catalysis with its special oxygen storage capacity. Surface oxygen atoms at an oxygen-terminated CeO2(111) surface, oxygen point vacancies, and multiple oxygen vacancies such as triangular defects and line defects were visualized by NC-AFM. Unexpected high mobility of surface oxygen atoms at room temperature was observed on slightly reduced CeO2(111) with multiple defects. This observation is probably relevant to facile oxidation-reduction cycles of CeO2−x.
In order to elucidate the difference of the mechanisms of imaging between scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM), the atomic structure and local physical properties of SrTiO3(100) surface were studied by using STM and NC-AFM. We theoretically simulated a model cluster with first-principles total energy calculation. Calculated density of states (DOS), work function, images for STM and NC-AFM were in good agreement with experimental data.
Many kinds of scanning probe microscopes have been proposed for imaging surface magnetic structures. Theoretical calculations predict that the detection of the short-range magnetic interaction such as exchange interaction reveals the magnetic structures on an atomic scale. The non-contact atomic force microscopy (NC-AFM) detecting the short-range magnetic interaction is called exchange force microscopy (EFM). In order to detect the exchange interaction between a magnetic tip and a sample, we performed NC-AFM imaging of an antiferromagnet NiO(001) surface using a ferromagnetic Fe-coated tip. To discuss the interaction atomically resolved images obtained are analyzed by the superposition method. The results of the analysis demonstrates that the spin configuration revealed from the superimposed images coincides with the expected one of the NiO(001) surface. To evaluate the corrugation amplitude of the atoms, we introduced the topographic asymmetry, which indicates that the short-range magnetic interaction between the Fe-coated tip and sample can be detected by NC-AFM.
Applications of non-contact atomic force microscopy (NC-AFM) to the investigations of organic molecular films are described. Molecular arrangements in model organic films such as fullerene (C60) thin films on a Si(111)-7×7 surface and self-assembled monolayers (SAMs) of alkanethiol molecules deposited on a Au(111) substrate were successfully imaged. Since both samples have been intensively studied by STM, they are suitable for the comprehensive understanding of the contrast mechanisms in NC-AFM. Some technical issues in NC-AFM imaging of molecules are also discussed. Furthermore, Kelvin probe force microscopy (KFM) was used to map the local surface potential of molecular films of oligothiophenes as well as fullerenes. Since surface potential of molecular films is caused by either polarization of the film originating from the dipole moment of molecules or the charge transfer at a molecule-substrate interface, the local potential variation is directly related to the electronic structures in the films on the substrate.
Non-contact atomic force microscopy (NC-AFM) has been employed to observe double-stranded DNA. Cu(111) surface is found to be useful to realize high-resolution imaging of DNA molecules without the effect of water-layer and to improve the tip shape during the scan for imaging. NC-AFM images reveal the double-helix structure whose averaged repeat distance agrees with Watson-Crick model. However, the observed height of DNA molecules is only 1 nm that is the half of natural diameter of double-stranded DNA suggesting the strong deformation due to the surface adsorption. The simultaneous measurements of frequency-shift and tunneling current indicate that DNA molecules show tunneling conductivity across the strand with the attenuation factor β = 1.1.
Noncontact atomic force microscopy (NC-AFM) images of the mixed monolayers of carboxylates prepared on a TiO2(110)-(1×1) surface are presented. The order of the height of formate, acetate, pivalate, and propiolate molecules is reproduced in the NC-AFM images, while the difference in the height is smaller than that expected from their true heights. The shrinkage of the height difference is ascribed to the contribution of several molecules to the tip-molecule force. On the other hand, trifluoroacetate is imaged as a spot shorter than the acetate though the two ions have an equal true height. This indicates that the image contrast reflects the permanent dipole moment of the molecules. The experimental results show that the organic molecules can be individually identified by the NC-AFM and the experimentally obtained topography is interpreted referring to physical and chemical characteristics of the molecules.