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

Abstract

We investigate insulating Cu 2 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 Cu 2 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. DOI: 10.1103/PhysRevLett.112.166102 PACS numbers: 68.37.Ps, 61.46.−w, 68.37.Ef The combination of scanning tunneling microscopy (STM) with noncontact atomic force microscopy (NC-AFM) in a single probe enables a wide range of atomic-scale studies on surfaces. Whereas contrast mechanisms in STM for different tip-sample systems are widely understood, the interpretation of NC-AFM data remains challenging. In NC-AFM the sum over all tip-sample interactions is measured, and the source of atomic resolution is often hard to identify. On semiconductors [1]-as well as on metals [2]-imaged with reactive tips (e.g., Si) atomic contrast is dominated by the formation of covalent bonds that often reach magnitudes of nanonewtons. For nonreactive CO-functionalized tips, Pauli repulsion was attributed to the observed intramolecular resolution [3,4]. Lantz et al. [5] showed that the dangling bonds of Si(111)-(7 × 7) can induce a dipole moment in (nonreactive) oxidized Si tips resulting in a short-range electrostatic interaction, which contributes to atomic resolution. Electrostatic interaction and an induced tip dipole moment was also used to explain atomic contrast on ionic crystals [6]. A similar model describes the interaction with charged adatoms on thin insulating layers [7,8]. Moreover, it was found that clean metallic tips carry an intrinsic dipole moment [9,10], which is caused by the Smoluchowski effect [11]. All of these examples underline the importance of atomic-scale electrostatic interactions in NC-AFM.Electrostatic forces become even more meaningful as polar thin insulating layers (e.g., NaCl, MgO, Cu 2 N) are used to decouple adsorbates in STM and AFM experiments [3,7,[12][13][14][15][16]. In this study we explore the influence of electrostatic forces in NC-AFM on Cu 2 N islands on Cu(100). N and Cu atoms on Cu 2 N form a periodic charge arrangement, as calculated by density functional theory (DFT) [17] [Figs. 1(c)-1(e)]. Compared to alkali halides, the Cu 2 N's cð2 × 2Þ unit cell structure has a lower symmetry; thus, its atomic positions are easier to designate. STM experiments led to two criteria to locate N atoms within the islands [18]: first, N adsorbs on the hollow sites of Cu(100) [19][20][21] and should therefore appear fourfold symmetric; second, island boundaries and sharp edg...

Simulating contrast inversion in atomic force microscopy imaging with real-space pseudopotentials

“…Here the hollow sites show the darkest contrast whereas the brightest spots occur above the nitrogen atoms. These images match previous experimental findings on Cu 2 N with Cu-and Co-terminated tips 19 . To examine the contrast mapping more closely, we plotted the vertical force on each tip as a function of tip height over the different sites (Fig.…”

“…A previous study has reported that long-range electrostatic forces can also influence contrast inversion 19 . We find that at very large tip distances for the CO tip (z = 6.88Å for Cu 2 N and z = 5.21Å for graphene), AFM images are inverted compared to those at intermediate distances (z ∼ 3.39Å).…”

Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory

“…1(e)]. The metallic tip apex atom has a dipole moment induced by the Smoluchowski effect [25], whose direction is the same as that of the CO molecule adsorbed on the surface [26]. Thus in the distance regime where the electrostatic interaction between the tip and the molecule dominates, the force between them is attractive.…”

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

“…Here z is the tip-sample distance, and p is the dipole of the tip, estimated to be 0.5 D [23] or 0.9 D [24] for copper tips [25]. The corresponding frequency shift…”

“…2(a), we need to set the zero of the z axis; that is, we need to estimate the position of the surface. To this end, we employed a commonly used approximation assuming z = 0 at the "point contact", where the tunneling conductance would reach G 0 = 2e 2 /h ≈ 77.5 μS with a nonoscillating sensor [24,27].…”