Effect of the tip state during qPlus noncontact atomic force microscopy of Si(100) at 5 K: Probing the probe

Abstract

Summary Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range of surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the tip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and theoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip apex in the observed contrast. Results: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum (UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a range of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we eliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of silicon dimers, which is a key issue in scanning probe studies of Si(100). Conclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by variations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians working on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the tip termination critically affects both the short-range force and dissipated energy.

Structural development and energy dissipation in simulated silicon apices

“…From the calculated geometries we can see that the tip structures in (d) and (f) differ, thus modifying the tip–surface interaction, which in turn leads to the observed hysteresis. This theoretical result is very similar to experimental observations on the Si(100) surface that recorded a dissipation of up to 0.5 eV/cycle [40] for a tip that demonstrated a “dimer-tip”-type atomic resolution [44]. It has also been shown [34] that very large simulated tip clusters demonstrate the same behaviour, which is attributed to more permanent structural changes that are likely to occur within the much larger experimental tip.…”

Atomic Structures of Silicene Layers Grown on Ag(111): Scanning Tunneling Microscopy and Noncontact Atomic Force Microscopy Observations

“…1 (a) to the atoms of the surface, which then gets a strong, timely support from also our nc-AFM data. Indeed, for perfect symmetric tips and well-chosen scanning parameters nc-AFM on silicon surfaces has been found to fairly well represent the true positions of surface atoms as often demonstrated on, e.g., the Si(111)(7 × 7)2526 or Si(001) surface27.…”

“…From our experiences with the nc-AFM imaging of the silicene film we can state that it is generally quite difficult to obtain the atomic scale contrast on the silicene film. Furthermore, if an atomic contrast is obtained, the contrast is relatively weak, particularly in comparison to the strong atomic contrast that can be regularly measured on the Si(111)(7 × 7)25 or Si(001)27 surface. We believe that this might be a consequence of the sp 2 /sp 3 character of the Si atoms within the silicene layer, which makes it different from the Si(111) surface with its pure sp 3 dangling bonds protruding from the Si atoms.…”

“…Furthermore, the atomic scale contrast strongly depends on many parameters like the tip chemistry and structure2627, or on the tip-surface distance2611, which is generally not precisely known as it was the case for each of the two measurements shown in Fig. 1.…”

Quantification of Atomic-Scale Elasticity on Ge(001)-c(4×2)Surfaces via Noncontact Atomic Force Microscopy with a Tungsten-Coated Tip

“…However, this technique cannot be used to detect the intrinsic bonding states of the surface atoms because the detected interaction contains other contributions from the back-bonding state of the tip apex atom and the interaction between the tip apex atom and the surface atom. In addition, any structural changes to the tip apex or invertible atomic structure formations at the tip-surface gap allow dissipative interactions to easily occur when the tip is sufficiently close to the surface such that the repulsive interaction force becomes dominant [11]. Therefore, the precise experimental extraction of intrinsic physical quantities, such as the binding energy and the individual atomic bonding state on a surface, is difficult.…”