Chemical tip fingerprinting in scanning probe microscopy of an oxidized Cu(110) surface

We present a joint experimental/theoretical study of the oxygen-terminated Cu(110) surface. Noncontact atomic force microscopy (NC-AFM) identifies patches of $p(2\times 1)$ and $c(6\times 2)$ structures. For the latter reconstruction, we observe distinctly different NC-AFM images depending on the chemical tip identity, which were identified as either Cu, O, or Si terminated. Since knowledge of the chemical structure of the tip asperity in NC-AFM is crucial for controlled imaging and manipulation of atoms and/or molecules on surfaces, this surface is proposed to be used for the tip termination “fingerprinting,” i.e., identifying the chemical species at its apex. Practical ways of enforcing the chemical structure of the tip asperity both before as well as at any stage of the imaging or manipulation experiment when numerous tip changes may take place, are also discussed.

Figure 1

(a) Ball model of the $p(2\times 1)$ phase (top and side views), where light red, grey and light grey balls depict low O, added Cu, and bulk Cu atoms, respectively. (b) Ball model of the $c(6\times 2)$ phase, where additionally dark grey and red balls depict the super Cu and high O atoms.

Figure 2

NC-AFM measurements of (a) the pure $c(6\times 2)$ surface with the Si tip before contamination with the surface species, and (b) a large area of the oxidized Cu(110) surface containing a Cu-O cluster (indicated by an arrow) used to modify the tip apex. The experimental procedure is depicted by a cartoon (c), illustrating the Si tip crashing into a Cu-O cluster on the surface, thereby resulting in tips of either types I or II. NC-AFM topography images of the pure $c(6\times 2)$ surface performed with tip type I (d) and tip type II (e) have the schematic of the top layer of the $c(6\times 2)$ surface overlaid in each case. At the bottom of images (a), (d), and (e), the corresponding schematic of the $c(6\times 2)$ surface is given to highlight the imaged atoms.

Figure 3

NC-AFM topography images of the same area of the oxidized Cu(110) surface performed using tip type I (a) and II (b). An elongated island of (most likely) Cu and O atoms on top of the Cu(110) $c(6\times 2)$ surface is clearly visible in the bottom right corner of the images.

Figure 4

NC-AFM topography images of the same area of the oxidized Cu(110) surface performed using tip types I (a) and II (b). The circled area in both images shows initiation of the reconstruction of the Cu-O (001) rows from missing row to the double row/missing row structure. A reduced distance between the Cu-O (001) rows can also be seen at the edges of the $c(6\times 2)$ island (indicated by white arrows). Black arrows indicate the registry of $p(2\times 1)$ and $c(6\times 2)$ features.

Figure 5

(a) NC-AFM topography image, where type I contrast changes into type II. (b) NC-AFM topography scan of a $c(6\times 2)$ island on the $p(2\times 1)$ surface in which a Cu atom is deposited from the tip onto the $p(2\times 1)$ surface (indicated by arrows). In both images, schematics of some of the super Cu atoms surrounded by high O atoms are overlaid for clarity and the slow scan direction is from the top to the bottom.

Figure 6

Experimental (black solid curves) and theoretical (red dashed curves) scan lines of the Cu(110) $c(6\times 2)$ surface. (a) Scan lines with experimental tip type I and the theoretical O terminated tip; (b) the corresponding tip model and (c) the experimental topography image. (d) Scan lines with experimental tip type II and the theoretical Cu terminated tip; (e) the corresponding tip model and (f) the experimental topography image. All scan lines go across three missing rows, over the super Cu, high O, and hollow sites as shown in the schematic beneath (d). The same color code is used as in Fig. 1. Experimental results for type I and II tips were obtained using $\Delta f$ values of $-$19.5 Hz and and $-$24.4 Hz, respectively. Theoretical results for O and Cu terminated tips were obtained using $\Delta f$ values of $-$22 Hz and 24 Hz, respectively.

Figure 7

(a) Experimental (left panels) and (b) theoretical (right panels) spectroscopies of the Cu(110) $c(6\times 2)$ surface over super Cu atom (red lines), high O atom (blue lines), and hollow site (cyan lines) for type I apex. The theoretical tip model is shown in (c). (d) The corresponding experimental image with spectroscopic sites indicated by crosses. Note that bright spots around super Cu atoms have oval-like shapes. (e)–(h) The same information for type II apex.

Figure 8

Schematic of the cell model used in our theoretical calculations. Lattice vectors ${\mathbf{a}}_{\mathbf{1}}$ $=$ 8.53 Å and ${\mathbf{a}}_{\mathbf{2}}$ $=$ 14.55 Å and the angle between them $\alpha$ $=$ 64.76${}^{\circ }$. Plan view of the surface is on the left and side view on the right. Light grey, grey, and dark grey balls correspond to bulk, added, and super Cu atoms. Light red and red balls correspond to low and high O atoms as in Fig. 1.

Figure 9

Schematic of the tip model used to calculate theoretical scan lines/spectroscopy curves Fig. 4/Fig. 5. It consists of a macroscopic sphere terminated with a microscopic (atomistic) model. The portion of the microscopic tip that sits inside the macroscopic sphere is defined by the offset.