Metal clusters really close-up Atomic force microscopy (AFM) can be used to reveal subatomic structures. By this means, Emmrich et al. found that individual copper and iron atoms formed toroidal structures on a copper surface. These shapes arise from the electrostatic attractions in the center of the atoms and Pauli repulsions at their edges. Individual atoms within clusters have underlying surface symmetry and can bind to different surface sites as clusters form. Science , this issue p. 308

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...

We study the physics of atomic manipulation of CO on a Cu(111) surface by combined scanning tunneling microscopy and atomic force microscopy at liquid helium temperatures. In atomic manipulation, an adsorbed atom or molecule is arranged on the surface using the interaction of the adsorbate with substrate and tip. While previous experiments are consistent with a linear superposition model of tip and substrate forces, we find that the force threshold depends on the force field of the tip. Here, we use carbon monoxide front atom identification (COFI) to characterize the tip's force field. Tips that show COFI profiles with an attractive center can manipulate CO in any direction while tips with a repulsive center can only manipulate in certain directions. The force thresholds are independent of bias voltage in a range from 1 to 10 mV and independent of temperature in a range of 4.5 to 7.5 K.

SummaryMagnetic force microscopy (MFM) allows one to image the domain structure of ferromagnetic samples by probing the dipole forces between a magnetic probe tip and a magnetic sample. The magnetic domain structure of the sample depends on the alignment of the individual atomic magnetic moments. It is desirable to be able to image both individual atoms and domain structures with a single probe. However, the force gradients of the interactions responsible for atomic contrast and those causing domain contrast are orders of magnitude apart, ranging from up to 100 Nm−1 for atomic interactions down to 0.0001 Nm−1 for magnetic dipole interactions. Here, we show that this gap can be bridged with a qPlus sensor, with a stiffness of 1800 Nm−1 (optimized for atomic interaction), which is sensitive enough to measure millihertz frequency contrast caused by magnetic dipole–dipole interactions. Thus we have succeeded in establishing a sensing technique that performs scanning tunneling microscopy, atomic force microscopy and MFM with a single probe.