Theories of scanning probe microscopes at the atomic scale

Abstract

Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods-scanning tunneling microscopy and scanning force microscopy-in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface-the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors' own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation. CONTENTS

Imaging nanoclusters in theconstant height modeof the dynamic SFM

“…Dynamic SFM measures the change in frequency (detuning f ) of an oscillating cantilever due to the interaction of a tip at the end of the cantilever with the surface [5][6][7]. Before taking measurements, the fast scanning direction was changed during scanning in the constant f mode by an adjustment of the scanning angle until the fast scanning direction was parallel to the surface with an error of a few degrees.…”

“…For an in situ characterization and for substrates of insulating materials such as MgO or Al 2 O 3 , only the scanning force microscope can be used. Especially in its dynamic mode, the SFM offers in principle the ability to image surfaces with high resolution-in the best case with atomic resolution [5][6][7].…”

Chemical Resolution at Ionic Crystal Surfaces Using Dynamic Atomic Force Microscopy with Metallic Tips

“…On binary and more complex ionic surfaces, the polarity of the tip apex determines whether anions or cations are imaged as protrusions [3]. Therefore, it is impossible to assign image features to particular surface atoms without knowing the tip polarity.…”

“…The ability to characterize insulating surfaces and control surface processes down to the atomic scale is extremely important for numerous applications in chemistry, catalysis, nanoscience, and nanotechnology and can be achieved only using AFM. The exact chemical nature and structure of the AFM tip apex as well as the identity of the foremost tip atom are ultimately responsible for the formation of atomic-scale contrast [3], but are notoriously difficult to control. AFM tips are typically fabricated ex situ from silicon or metals and hence possess an oxide layer, either due to the manufacturing process itself or by oxidation in air.…”

Site-specific force-distance characteristics on NaCl(001): Measurements versus atomistic simulations

“…25 This tip model has been widely used to simulate SFM imaging and provided good qualitative agreement with experiments on other ionic surfaces. 26 For this work, calculations were performed using both MgO and NaCl tips, each consisting of a cubic cluster of 32 cations and 32 anions with stable ͕100͖ facets oriented such that ͗111͘ direction is perpendicular to the sample surface. Ions in the top half of the cluster were kept fixed.…”