CO Tip Functionalization Inverts Atomic Force Microscopy Contrast via Short-Range Electrostatic Forces

We investigate insulating ${\mathrm{Cu}}_2\mathrm{N}$ islands grown on Cu(100) by means of combined scanning tunneling microscopy and atomic force microscopy with two vastly different tips: a bare metal tip and a CO-terminated tip. We use scanning tunneling microscopy data as proposed by Choi, Ruggiero, and Gupta to unambiguously identify atomic positions. Atomic force microscopy images taken with the two different tips show an inverted contrast over ${\mathrm{Cu}}_2\mathrm{N}$. The observed force contrast can be explained with an electrostatic model, where the two tips have dipole moments of opposite directions. This highlights the importance of short-range electrostatic forces in the formation of atomic contrast on polar surfaces in noncontact atomic force microscopy.

Figure 1

(a) Charge redistribution in a metallic tip due to the Smoluchowski effect, leading to an electric dipole moment pointing towards the sample. (b) A CO molecule adsorbed on a copper tip, carrying an electric dipole moment pointing away from the sample. (c) Model of the ${\mathrm{Cu}}_2\mathrm{N}$ network on Cu(100) with the numbers being the net charge of Cu and N atoms in units of elementary charge $e$ [17]. (d) Cross section of ${\mathrm{Cu}}_2\mathrm{N}$ along a N-Cu-N row and (e) Cu-hollow site-Cu row showing the first two atomic layers of the reconstructed surface and the net charges of the atoms, respectively. In (c), (d), and (e), larger circles indicate Cu and smaller circles indicate N.

Figure 2

Data acquired with a Cu-terminated tip [(a)–(c)] and with a CO-terminated tip [(d)–(f)]. Top row: STM topographs with the structural model of ${\mathrm{Cu}}_2\mathrm{N}$ overlaid. Lighter circles indicate Cu, darker circles indicate N: (a) is recorded at constant current at 500 pA and 10 mV applied to the tip and Laplace filtered to enhance contrast; (b) 100 pA and 10 mV, raw data. Center row: Force between tip and sample at closest approach. Bottom row: Force contrast vs distance curves at two high-symmetry locations.

Figure 3

Top row: Central area of Figs. 2 and 2 (unit-cell averaged and $3\times 3$ Gauss filtered). Center row: Calculated electrostatic force between the tip dipole and the point charges of a ${\mathrm{Cu}}_2\mathrm{N}$ structure, including the second-layer Cu atoms (see also Fig. 1). Simulation for a Cu-terminated tip with a dipole moment of 0.874 D (b) and a CO-terminated tip with 0.035 D (e). In the first two rows, the force at the Cu site was set to zero to emphasize the force difference. Bottom row: Line profiles from (a), (b), (d), and (e) along N-Cu-N and Cu-hollow site-Cu directions, respectively.