Atom-specific forces and defect identification on surface-oxidized Cu(100) with combined 3D-AFM and STM measurements

The influence of defects on the local structural, electronic, and chemical properties of a surface oxide on Cu(100) were investigated using atomic resolution three-dimensional force mapping combined with tunneling current measurements and ab initio density functional theory. Results reveal that the maximum attractive force between tip and sample occurs above the oxygen atoms; theory indicates that the tip, in this case, terminates in a Cu atom. Meanwhile, simultaneously acquired tunneling current images emphasize the positions of Cu atoms, thereby, providing species-selective contrast in the two complementary data channels. One immediate outcome is that defects due to the displacement of surface copper are exposed in the current maps, even though force maps only reflect a well-ordered oxygen sublattice. The exact nature of the defects is confirmed by the simulations, which also reveal that the arrangement of the oxygen atoms is not disrupted by the copper displacement. In addition, the experimental force maps uncover a position-dependent modulation of the attractive forces between the surface oxygen and the copper-terminated tips, which is found to reflect the surface's inhomogeneous chemical and structural environment. As a consequence, the demonstrated method has the potential to directly probe how defects affect surface chemical interactions.

Figure 1

Missing-row model of the surface oxide layer on the Cu(100) surface. A rectangular unit cell, the positions of Cu1 and Cu2 atoms and the missing rows, as well as the alternating separation of O atoms are highlighted.

Figure 2

The 2×4 unit-cell segment of the optimized Cu(100)-O surface oxide structure with Cu atoms shown in gold, O atoms shown in red, and subsurface Cu slabs shown in dimmed gray. (a) The vertical displacement of surface atoms around the missing-row feature seen in the $x$-$z$ plane (side view). (b) The difference in O and Cu2 atom lateral displacements towards the missing row, perspective view.

Figure 3

Three-dimensional representation of chemical interaction forces over an area of 2.89×2.89 nm. The color scale ranges from −12 pN (dark blue, least attractive) to $+$10 pN (dark red, most attractive) with the average force at each height subtracted.[18, 19] Note that the $z$ axis has been arbitrarily set to $z$ $=$ 0 pm at the distance of closest approach, i.e., the displayed values do not coincide with the definition of the distance $d$ in the simulations.

Figure 4

Simultaneous 3D-AFM and tunneling current measurements on Cu(100)-O (2.89×2.89 nm). (a) Horizontal force map cut from the data set shown in Fig. 3 at the distance of closest approach. Circular maxima reveal the oxygen atoms’ alternating spacing of 3.5/3.7 Å. Total force contrast is 23 pN; dashed lines circle the locations of defects identified from the current map in Fig. 2b. (b) Tunneling current data displaying a ladder-type contrast as well as defects in the form of apparent nonmissing rows. The gray scale covers the range of 0–7 pA (dark to bright). (c) Structural model of the imaged area where defects formed through lateral movement of Cu2 atoms are highlighted. Comparing (a) and (b) reveals that the current and force images are offset by ≈140 pm.

Figure 5

Probe tips considered in the calculations (perspective views with copper atoms in gold and oxygen atoms in red). (a) Cu-terminated tip; (b) O-terminated tip; (c) dangling bond O tip; (d) O-adatom tip (“CuO tip”).

Figure 6

Force-distance curves computed for (a) Cu and (b) CuO tips (insets: zoom of the forces in the 400–500-pm distance range). (c) and (d) display, for Cu and CuO tips, the force difference ($\mathit{dF}$) between individual lattice sites and the missing-row bridge site (mr:bridge) located between two Cu atoms on both sides of a missing row. A comparison with experimental force contrast reveals that the closest tip-sample distance during the experiments is 425–450 pm.

Figure 7

Tip and defect identification. (a) Clean Cu-terminated tip (side view). (b) Cu-terminated tip stabilized by an oxygen atom located at the side (CuO tip; side view). (c) Origin of the offset between force and current channels for the CuO tip: The oxygen atom close to the apex causes the current maximum to shift towards the direction of the oxygen, whereas, the strongest force interaction remains with the apex atom. (d) Structural model for the defect-free surface; insets of tip models define the notation used to describe tip orientation. (e) Corresponding simulated STM signature, revealing a ladder-type contrast. The upper panel has been computed for a symmetric Cu tip, and the bottom panel has been computed for a CuO tip with θ $=$ 45°. The offsets resulting from tip asymmetry are highlighted. (f) and (g) Model and computed STM signatures for a missing row partially filled with four Cu atoms (filled-row defect, represented by green spheres in blue enclosure). Simulations for a CuO tip with θ $=$ 45° predict a dark row along the defect in strong contrast to experimental results. (h) Energetically favorable defect model created by lateral displacement of Cu2 atoms (purple spheres, highlighted by blue enclosure). For this “displacement defect,” distortions in the positions of the O atoms are minimal both in lateral and in vertical directions. (i) Corresponding STM image calculated for a CuO tip with θ $=$ 45°. The appearance of an apparent nonmissing row is clearly observed along the length of the defect. All current maps are calculated at a constant height of 450 pm and at a bias voltage of −0.4 V.

Figure 8

The 2×6 unit-cell segment of the optimized Cu(100)-O surface oxide structure containing defect atoms in the missing row with Cu atoms shown in gold, O atoms shown in red, and subsurface Cu slabs shown in dimmed gray. Filled-row and displacement Cu defects are colored green and purple, respectively. (a) and (c) The vertical displacement of surface atoms around the defect-filled missing row seen in the $x$-$z$ plane (side view), demonstrating the additional displacement of O atoms around the filled-row defects in comparison to the case of displacement defects. (b) and (d) The difference in O- and Cu2-atom lateral displacements towards the defect-filled missing row, perspective view.