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

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 $\mathit{d}$ orbitals.

Figure 1

Cu tips yield three high-symmetry COFI images (c, h and m), which can be assigned to Cu tip clusters pointing into the $⟨100⟩$ (a), $⟨111⟩$ (f) and $⟨110⟩$ directions (k). To justify the assignment, we develop a model for the tip atom and calculate its electrostatic interaction with the dipole of the CO molecule. The model tip atom consists of a positive charge in the center, surrounded by unequally occupied $3d$ orbitals. Their occupation (b, g and l) is determined by their angular overlap with the nearest neighbors. The bold labels refer to the three orbitals with the highest occupation. The central charge is adjusted to optimize the fit between model and experimental results for the force versus distance behavior (e, j and o) and the contrast in the model images, which are calculated at a core-core distance of 200 (d), 145 (i) and 160 pm (n). The black arrows in the force versus distance graphs indicate the distances of the COFI images. The experimental distance is determined from the conductance of the tunneling gap [18]. In the model, the $z$-value reflects the core-core distance between the Cu tip atom and the oxygen [18].

Figure 2

Comparison of W (a), Fe (b) and Cu tips (c and d), which show a circular symmetry in the COFI image [c and d are the same as in Figs. 1 and 1]. e) Force versus distance curves of $\mathrm{W}⟨100⟩$ (squares), $\mathrm{Fe}⟨100⟩$ tips (triangles), $\mathrm{Cu}⟨100⟩$ (circle) and $\mathrm{Cu}⟨111⟩$ tips (stars). The solid (black) line represents a W tip, which is contaminated with Cu.

Figure 3

Comparison of W, Fe and Cu tips revealing two- or threefold symmetry. a–e) COFI images of the corresponding W (a, b), Fe (c, d) and Cu tips (e) [a uses the same raw data as Fig. 1H in [2], e is the same as in Fig. 1]. f) Force versus distance curves of several W, Fe and Cu tips.