Simultaneous Measurement of Multiple Independent Atomic-Scale Interactions Using Scanning Probe Microscopy: Data Interpretation and the Effect of Cross-Talk

Abstract

In high-resolution scanning probe microscopy, it is becoming increasingly common to simultaneously record multiple channels representing different tip−sample interactions to collect complementary information about the sample surface. A popular choice involves simultaneous scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) measurements, which are thought to reflect the chemical and electronic properties of the sample surface. With surface-oxidized Cu(100) as an example, we investigate whether atomic-scale information on chemical interactions can be reliably extracted from frequency shift maps obtained while using the tunneling current as the feedback parameter. Ab initio calculations of interaction forces between specific tip apexes and the surface are utilized to compare experiments with theoretical expectations. The examination reveals that constant-current operation may induce a noticeable influence of topography-feedback-induced cross-talk on the frequency shift data, resulting in misleading interpretations of local chemical interactions on the surface. Consequently, the need to apply methods such as 3D-AFM is emphasized when accurate conclusions about both the local charge density near the Fermi level, as provided by the STM channel, and the sitespecific strength of tip−sample interactions (NC-AFM channel) are desired. We conclude by generalizing to the case where multiple atomic-scale interactions are being probed while only one of them is kept constant. ■ INTRODUCTIONWith the accelerating trend toward miniaturization in functional electromechanical systems as well as the advent of twodimensional materials with exceptional physical properties such as graphene 1 and silica bilayers, 2 measuring and understanding chemical surface forces with atomic specificity has become increasingly important for fields as diverse as molecular electronics, catalysis, and tribology. 3 To image and characterize surfaces with atomic resolution, scanning tunneling microscopy (STM) 4 and noncontact atomic force microscopy (NC-AFM) 5 are most frequently used. While STM relies on the tunneling current between an atomically sharp probe tip and a (semi-) conducting sample to deliver atomic scale information about the electronic properties of surfaces, NC-AFM maps how much the resonance frequency f of an oscillating cantilever has changed from its value away from the surface (i.e., from its eigenf requency f 0 ) due to the chemical interaction forces acting between the tip and the sample (corresponding to the f requency shif t, Δf = f − f 0 ). Consequently, the concurrent recording of both the STM and the NC-AFM channel has in principle the potential to yield complementary information on both atomic scale variations in surface interactions and the underlying electronic structure responsible for them. However, using a combination of atomic-scale topography data acquired with tunneling current-based feedback and simultaneously recorded frequency shift images on surface oxidized Cu(100), we sho...

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