Chemical and Crystallographic Characterization of the Tip Apex in Scanning Probe Microscopy

Abstract

The apex atom of a W scanning probe tip reveals a nonspherical charge distribution as probed by a CO molecule bonded to a Cu(111) surface [Welker et al., Science 336, 444 (2012). Three high-symmetry images were observed and related to three low-index crystallographic directions of the W bcc crystal. Open questions remained, such as the detectability of a contamination of W tips by sample material (here Cu), and the applicability of the method to distinguish other atomic species. In this work, we investigate bulk Cu and Fe tips. In both cases, we can associate our data with the fcc (Cu) and bcc (Fe) crystal structures using a simple electrostatic model that is based on the partial filling of d orbitals.

Force Field Analysis Suggests a Lowering of Diffusion Barriers in Atomic Manipulation Due to Presence of STM Tip

“…The FðzÞ curve of the RCT [see Fig. S2(d), [35]] shows quantitative agreement with the W(111) tips reported in [33]. Possible manipulation paths pointing into the h110i directions of the Cu(111) surface are indicated in both COFI images.…”

“…The force versus distance curve FðzÞ of that tip yields a minimum of −120 pN [see Fig. S2(b), [35]], identifying it as a Cu(100) tip [33]. The FðzÞ curve of the RCT [see Fig.…”

“…To characterize the force field of the metal tip, threedimensional frequency-shift data (Δf) over an adsorbed CO molecule [21] were acquired and deconvoluted into forces [32]. The COFI images obtained can be classified into two categories [21,33]: attractive center tips (ACTs) and repulsive center tips (RCTs). Recent results show, that ACTs are monoatomic tips, whereas RCTs are multiatomic tips, with the number of attractive dips corresponding to the size of the cluster that forms the tip apex [34].…”

Unique Determination of “Subatomic” Contrast by Imaging Covalent Backbonding

“…al. 32 a circular symmetry was observed for both Cu(111) and Cu(100) tips, and a twofold symmetry for a Cu(110) tip. Consequently we note that an n-fold symmetry in the DFM data does not simply result from n backbonding neighbours.…”

Attempts to test an alternative electrodynamic theory of superconductors by low-temperature scanning tunneling and atomic force microscopy

“…A subsequent brief sputtering removed this reconstruction, leaving a surface with nanometer-scale asperities. Additional measurements involved a Cu(111) and a Cu(110) sample (MaTecK GmbH, purity 99.9999%), prepared by repeated sputtering and annealing up to 785 K. We used an etched tungsten tip, prepared by field evaporation in UHV and in situ poking into a clean copper sample, likely resulting in a copper-coated tip apex [15]. The tip is mounted on a qPlus sensor [16] operated in frequency-modulation mode [17], with a quality factor at low temperature ranging from 250 000 to 540 000 [18].…”