Chemical Resolution at Ionic Crystal Surfaces Using Dynamic Atomic Force Microscopy with Metallic Tips

Teobaldi, Gilberto; Lämmle, Knud; Trevethan, Thomas; Watkins, Matthew; Schwarz, Alexander; Wiesendanger, R.; Shluger, Alexander L.

doi:10.1103/physrevlett.106.216102

Cited by 56 publications

(76 citation statements)

References 21 publications

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“…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.…”

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confidence: 82%

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

et al. 2014

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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...

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confidence: 82%

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

et al. 2014

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“…Localized charges of the sample stem from the ions and the vacancy. The localized charges at the tip, in particular, a tip dipole, arise from the tip shape because of the Smoluchowski effect [29,43] and, additionally, from the tip functionalization. Moreover, also image charges have to be taken into account.…”

Section: Theory and Discussionmentioning

confidence: 99%

“…A direct way to identify the lattice sites experimentally, without a priori knowledge of the tip termination and its imaging contrast, is the application of markers with known adsorption sites [29]. We used Cl vacancies in the top layer of NaCl(100), which have previously been studied using scanning tunneling microscopy (STM) by Repp et al [30], to provide unambiguous lattice site identification.…”

Section: Introductionmentioning

confidence: 99%

Investigating atomic contrast in atomic force microscopy and Kelvin probe force microscopy on ionic systems using functionalized tips

et al. 2014

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Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) have become important tools for nanotechnology; however, their contrast mechanisms on the atomic scale are not entirely understood. Here we used chlorine vacancies in NaCl bilayers on Cu(111) as a model system to investigate atomic contrast as a function of applied voltage, tip height, and tip functionalization. We demonstrate that the AFM contrast on the atomic scale decisively depends on both the tip termination and the sample voltage. On the contrary, the local contact potential difference (LCPD) acquired with KPFM showed the same qualitative contrast for all tip terminations investigated, which resembled the contrast of the electric field of the sample. We find that the AFM contrast stems mainly from electrostatic interactions but its tip dependence cannot be explained by the tip dipole alone. With the aid of a simple electrostatic model and by density functional theory we investigate the underlying contrast mechanisms.

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“…15 Force spectroscopy (FS) experiments, in which tip-sample forces are determined as a function of distance for specific surface sites 16 or over a two-dimensional (2D) area (3D mapping), 17 impose an even stronger constrain on the tip structure and the nature of the interactions. FS has been used to discriminate between the two ionic sublattices on several insulator surfaces, [18][19][20][21][22] to achieve single-atom chemical identification on semiconductors, 23 and to understand the nc-AFM contrast on carbon nanostructures. 24,25 In this work we combine site-specific force measurements and extensive first-principles calculations on TiO 2 (110)-1 × 1, aiming to clarify the origin of the observed nc-AFM contrast and to characterize the tip structures responsible for the protrusion and hole imaging modes.…”

Section: Introductionmentioning

confidence: 99%

Understanding image contrast formation in TiO2with force spectroscopy

et al. 2012

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Site-specific force measurements on a rutile TiO 2 (110) surface are combined with first-principles calculations in order to clarify the origin of the force contrast and to characterize the tip structures responsible for the two most common imaging modes. Our force data, collected over a broad range of distances, are only consistent with a tip apex contaminated with clusters of surface material. A flexible model tip terminated with an oxygen explains the protrusion mode. For the hole mode we rule out previously proposed Ti-terminated tips, pointing instead to a chemically inert, OH-terminated apex. These two tips, just differing in the terminal H, provide a natural explanation for the frequent contrast changes found in the experiments. As tip-sample contact is difficult to avoid while imaging oxide surfaces, we expect our tip models to be relevant to interpret scanning probe studies of defects and adsorbates on TiO 2 and other technologically relevant metal oxides.

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Chemical Resolution at Ionic Crystal Surfaces Using Dynamic Atomic Force Microscopy with Metallic Tips

Cited by 56 publications

References 21 publications

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

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

Investigating atomic contrast in atomic force microscopy and Kelvin probe force microscopy on ionic systems using functionalized tips

Understanding image contrast formation in TiO2with force spectroscopy

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