Force Field Analysis Suggests a Lowering of Diffusion Barriers in Atomic Manipulation Due to Presence of STM Tip

Matthias Emmrich, Maximilian Schneiderbauer, Ferdinand Huber, Alfred J. Weymouth, Norio Okabayashi, and Franz J. Giessibl

Phys. Rev. Lett. 114, 146101 – Published 6 April 2015

Abstract

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.

Received 18 November 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.146101

Authors & Affiliations

Matthias Emmrich^1,*, Maximilian Schneiderbauer¹, Ferdinand Huber¹, Alfred J. Weymouth¹, Norio Okabayashi², and Franz J. Giessibl¹

¹Institute of Experimental and Applied Physics, University of Regensburg, D-93053 Regensburg, Germany
²Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan

^*matthias.emmrich@ur.de

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Vol. 114, Iss. 14 — 10 April 2015

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Images

Figure 1
(a) An attractive-center tip is scanned along one of the six nearest neighbor directions over a CO molecule in constant height. Before retrace, the tip is lifted by 50pm and lowered by 55 pm before commencing the next trace (red arrows) such that each scan cycle lowers the tip by 5 pm. (b) Vertical force as a function of the lateral tip position $x$ at different tip heights. Darker lines refer to closer distances. A tilt of the tip gives rise to an asymmetric profile at small $z$ separations. The red dashed line indicates the attractive minimum. (c) Conductance in units of the conductance quantum $G_{0} = 2 e^{2} / h$ . Because of the tip’s tilt, the dip in conductance becomes distorted at close distances and the position of the minimum shifts to the right as marked by the green bar. Steplike features indicate a stick-slip motion of the CO. (d) Lateral forces acting on the tip are obtained by differentiation of the energy in the $x$ direction. For an ACT, lateral forces develop a maximum followed by a minimum where a positive sign indicates that the CO pulls the tip to the right and vice versa. Because of the slightly tilted tip, the profile of $F_{lat}$ is asymmetric with extrema of different magnitudes. $F_{lat}$ needs to exceed the manipulation threshold $F_{thr}$ , indicated by black dashed lines in (d). At the distance shown here, manipulation to the right is feasible. To be able to move CO to the left, the tip would need to be lowered further such that $F_{lat} > F_{thr}$ for negative $x$ values, too.
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Figure 2
Directionality of atomic manipulation for (a) a tilted attractive- and (b) a repulsive-center tip. (c) For the ACT, two asymmetric directions $x_{A}$ and $x_{B}$ , and one almost symmetric direction $x_{C}$ are selected (profiles taken at $z_{man} + 5 pm$ ). (d) For the RCT, manipulation with the sickle shaped part (direction $x_{A}$ ) is compared to the case where the beamlike feature comes first (direction $x_{B}$ ). The difference in the profiles results from the CO bending to the side. Evidence of the CO motion is found in the average normalized conductance $G / G_{0}$ at $z_{man}$ : (e) For the ACT manipulation is observed in directions $x_{A}$ and $x_{C}$ . The shape of the curve in direction $x_{B}$ indicates that the CO has moved to the left. (f) Manipulation with a RCT is only possible in direction $x_{A}$ . When the beam hits the CO first, it is pushed to the side. (g) The lateral force curves acquired with the ACT reflect its symmetry properties (see insets). In direction $x_{B}$ the threshold is reached by positive forces explaining the motion of the CO to the left. (h) Repulsive lateral forces point away from the molecule. Regardless of the slight asymmetry the CO is always moved when the tip is left of the CO. (i) During sliding, the CO is forced under the tip by lateral forces of different sign but equal strength. (j) Tilted ACTs that favor forces pointing in the direction of the tip motion pull the CO.
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Figure 3
(a) Manipulation threshold at $z_{man} + 5 pm$ as a function of the maximal attractive normal force determined from $F (z)$ for several tips. Tip terminations are assigned according to [33]. Apparently, the manipulation threshold increases with the maximal attraction a tip can generate. (b) The manipulation threshold $F_{thr}$ does not depend on the normal forces. (c) $F_{thr}$ as a function of bias voltage, showing no apparent correlation for voltages between 1 and 10 mV. (d) The manipulation threshold has a weak temperature dependence. At 7.5 K it is 5 pN lower than at 4.5 K.
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