Bond-Level Imaging of the 3D Conformation of Adsorbed Organic Molecules Using Atomic Force Microscopy with Simultaneous Tunneling Feedback

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Bond-Level Imaging of the 3D Conformation of Adsorbed Organic Molecules Using Atomic Force Microscopy with Simultaneous Tunneling Feedback

Daniel Martin-Jimenez, Sebastian Ahles, Doreen Mollenhauer, Hermann A. Wegner, Andre Schirmeisen, and Daniel Ebeling

Phys. Rev. Lett. 122, 196101 – Published 13 May 2019

See Viewpoint: Atomic Force Microscope Images Molecules in 3D

Abstract

The chemical structure and orientation of molecules on surfaces can be visualized using low temperature atomic force microscopy with CO-functionalized tips. Conventionally, this is done in constant-height mode by measuring the frequency shift of the oscillating force sensor. However, this method is unsuitable for analyzing 3D objects. We are using the tunneling current to track the topography while simultaneously obtaining submolecular resolution from the frequency shift signal. Thereby, the conformation of 3D molecules and the adsorption sites on the atomic lattice can be reliably determined.

Received 13 February 2019

DOI:https://doi.org/10.1103/PhysRevLett.122.196101

Physics Subject Headings (PhySH)

Atomic force microscopy Noncontact atomic force microscopy Scanning techniques Scanning tunneling microscopy

Condensed Matter, Materials & Applied Physics

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Atomic Force Microscope Images Molecules in 3D

Published 13 May 2019

A new trick simplifies the atomic force microscope imaging of the 3D structure of nonflat molecules.

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Authors & Affiliations

Daniel Martin-Jimenez^1,4, Sebastian Ahles^2,4, Doreen Mollenhauer^3,4, Hermann A. Wegner^2,4, Andre Schirmeisen^1,4, and Daniel Ebeling^1,4,*

¹Institute of Applied Physics (IAP), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany
²Institute of Organic Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany
³Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany
⁴Center for Materials Research (LaMa), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany

^*Corresponding author. daniel.ebeling@ap.physik.uni-giessen.de

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Issue

Vol. 122, Iss. 19 — 17 May 2019

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Images

Figure 1
(a) Scheme of the conventional bond imaging technique. A CO-functionalized tip that is attached to an oscillating tuning fork sensor is scanned in constant-height mode above the surface. The frequency shift signal reveals the chemical structure of the imaged molecule. (b) Scheme of the proposed constant-current mode, where the tunneling current is used to track the sample topography while the frequency shift signal is simultaneously captured for providing submolecular contrast.
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Figure 2
(a),(d) Topography and (b),(e) frequency shift images of ITP in constant-current mode for two tunneling currents ( $I = 8.7$ and 25.9 pA, $V_{gap} = 4 mV$ , oscillation $amplitude = 52 pm$ ). (c),(f) Constant-height frequency shift images for two imaging distances ( $z = 130$ and 66 pm). (g),(h) Topography and frequency shift profiles along red and blue dashed lines in Fig. 2 for $I = 8.7$ , 11.6, 15.5, 20.4, and 25.9 pA (constant-current mode) and $z = 130$ , 114, 98, 82, and 66 pm (constant-height mode). Red and blue arrows indicate increasing currents ( $I$ ) and decreasing distances ( $z$ ), respectively. (i) $Δ f (z)$ curves on ITP (red color) and Ag(111) (blue color) for $V_{gap} = 4$ and $- 0.57 mV$ , respectively.
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Figure 3
(a) Sketch of ITP on Ag(111). (b) Topography image of ITP at $I = 11.6 pA$ and $V_{gap} = 4 mV$ (oscillation $amplitude = 52 pm$ ). (c),(d) Topography images at constant $Δ f = - 1 Hz$ for constant-current and constant-height scanning modes. Image pixels that lie outside the proper frequency range for calculating the topography are transparent. The gray planes are set at $z = 66 pm$ . (e) Topography profiles at three constant $Δ f$ values ( $- 2$ , 0, 2 Hz) along the red and blue lines in Figs. 3 and topography profiles at constant current ( $I = 8.7$ , 11.6, 15.5, 20.4, and 25.9 pA) along the orange line in Fig. 3.
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Figure 4
(a) Topography and (b),(c) frequency shift images of TP radical in constant-current mode ( $I = 9.2 pA$ , $V_{gap} = 4 mV$ , oscillation $amplitude = 52 pm$ ) and constant-height mode ( $z = 102 pm$ ), respectively. (d),(e) Topography images of TP radical at constant $Δ f = - 1 Hz$ for constant-current and constant-height scanning modes. The gray planes are set at $z = - 60 pm$ . (f) Topography profiles at three constant $Δ f$ values ( $- 2$ , 0, 2 Hz) along the lines in Figs. 4 and topography profiles at constant current ( $I = 12.8$ , 17.8, 24.3, and 32.5 pA) along the orange line in Fig. 4. The black dashed lines represent linear fits to the topography profiles for determining the tilt of the different regions of the molecule.
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Figure 5
(a) Frequency shift image of single and self-assembled ITP molecules on Ag(111) in constant-current mode ( $I = 25 pA$ , $V_{gap} = 7 mV$ and oscillation $amplitude = 68 pm$ ). (b),(c) Frequency shift images of TP radical on Ag(111) in constant-current mode [ $I = 600 pA$ in (b) for imaging Ag(111) surface atoms, $I = 15 pA$ in (c) for submolecular resolution on ITP molecule]. Other parameters: $V_{gap} = 2 mV$ , oscillation $amplitude = 43 pm$ . (d) Overlay of fitted Ag(111) lattice (yellow circles) and molecular structure of TP (orange). The adsorption site is indicated by a red circle.
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