Multiparameter Imaging and Understanding the Role of the Tip - Atomic Resolution Images of Rutile TiO 2 (110)
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Multiparameter Imaging and Understanding the Role of the Tip - Atomic Resolution Images of Rutile TiO2 (110) S. J. O’Brien, H. Ozgur Ozer*, G. L.W. Cross and J. B. Pethica Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and School of Physics, Trinity College, Dublin 2, Ireland ABSTRACT A major challenge for scanned probe microscopy is to identify structures and chemical species on a surface, which have not already been inferred from other analytical techniques. Progress is impeded by the fact that in general the structure and composition of the tip atom is not known. To illustrate some of the issues involved, we report simultaneous scanning tunneling microscopy/atomic force microscopy (STM/AFM) of the TiO2 (110) surface. The use of small amplitudes enabled the simultaneous acquisition of force gradient and barrier height images during standard STM imaging. Surprisingly, we find most STM images exhibit a corrugation contrast inverse to that usually reported in the literature. However, regardless of the contrast in STM, force gradient images always showed greater attraction over O rows. Barrier height images also show this consistency, always being greater over O rows. This supports the theoretical model of the electronic structure of the surface, but shows that the tip structure and interaction cannot be ignored in modeling STM images. We conclude that there is a ¿ne balance between topography and local density of states (LDOS) in STM imaging of this surface; which of them dominates the STM image is determined by the tip. Simultaneous multi-parameter imaging is useful in interpreting images reliably, particularly on multi-component surfaces. INTRODUCTION Atom resolved scanning probe microscopy (SPM) of multi-element surfaces has the ability, in principle, to identify individual atomic species in their local chemical environment. However, its practicality is limited because the microscopic structure of the imaging tip, and its interaction with the surface in image formation, is in general not known. Elucidating the electronic and geometric structure of the imaging tip from experimental data can require extensive and laborious theoretical simulations using numerous tip models [1]. One possible way to improve matters is to perform both STM and AFM on the same surface. This gives information on different but still local properties, unlike the spatial averages obtained by other surface analytical techniques. The combination can constrain the range of possible tip and surface structures which fit any given image. Truly simultaneous operation of STM and AFM also eliminates ambiguities which arise from possible changes in tip and surface structure and in operating conditions between separate STM and AFM measurements. We have previously shown that simultaneous STM/AFM is possible with subangstrom oscillation amplitudes [2]. The use of small amplitudes is important not only for a direct linear measurement of force gradients in non-contact AFM, but also for the
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proper acquisition of tunnel current
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