"Investigation of inelastic electron tunneling process by combining STM and AFM"

Who: Norio Okabayashi, Kanazawa University

Place: Donostia International Physics Center

Date: Wednesday, 7 November 2018, 12:00

The vibration of a molecule on a surface contains critical information on the bond of the molecule with the surface and within the molecule, which is crucial for understanding surface phenomena and for technologically important processes such as catalysis and epitaxial growth. Combination of scanning tunneling microscopy (STM) with inelastic electron tunneling spectroscopy (IETS) enable us to investigate vibrational energies at the atomic scale precision [1]. However, it has been known that the intensity of IETS strongly changes between different tips [2] and the vibrational energies acquired by IETS are strongly influenced by the distance between the tip and molecule [3,4], i.e., the force from the tip to the molecule. In order to clear these problems, we have incorporated atomic force microscopy (AFM) into STM-IETS by collaboration with Franz Giessibl laboratory in Regensburg University, by which we can get the information on the geometrical structure of a tip apex [5,6] and directly measure the force exerting on the molecule from the tip.
By combining AFM and STM-IETS, we have firstly found that a metallic tip whose apex consists of a single atom provides the stronger IETS signal to a CO molecule on a Cu(111) surface, whereas a metallic tip whose apex consists of multiple atoms provides the smaller IETS signal [7]. This difference of the IETS intensity by the geometrical structure of a tip apex becomes negligible when the vertical position of a CO molecule is elevated by inserting the Cu adatom between the CO molecule and Cu(111) substrate. These results suggest that (i) the IETS intensity is strongly governed by the current passing through the CO molecule and (ii) the efficiency of the inelastic process is almost constant regardless of the geometrical structure of a metallic tip apex, which is also confirmed by the theoretical calculations [7].
Secondary, we have found that the a tip which exerts the stronger force causes a larger energy shift of lateral vibrational modes of a CO molecule on a Cu(111) surface [8]. In addition, these energy shifts can be precisely reproduced by the classical model considering both of the additive perturbation from the tip to the molecule and the bond weakening effect by the vertical force [8]. However, there remained one problem: in our report [8], the vibrational energy shift could not be investigated for the very short tip molecule distances owing to the method of the current measurement, where considerable decreases and drastic changes of vibrational energies were reported by using STM-IETS [3][4]. 
In order to clear this problem, we have improved our current measurement method, by which we have extended the simultaneous measurements of forces and vibrational energies to very small tip molecule distances for the system of a CO molecule on a Cu(111) surface. We have found that the improved method provides the data consistent with the previous experiments [3,4] and the previous conclusion [8]: a tip that exerts the stronger force causes larger energy shift.

[1] B. C. Stipe, M. A. Rezaei, and W. Ho, Science 280, 1732 (1998). 
[2] L. J. Lauhon and W. Ho, PRB 60, R8525 (1999)
[3] L. Vitali et al., Nano Lett. 10, 657 (2010).
[4] F. Mahmood, https://purl.stanford.edu/cy977mf7313. 
[5] J. Welker and F. J. Giessibl, Science 336, 444 (2012)
[6] M. Emmrich et. al., Science 348, 308 (2015).
[7] N. Okabayashi et. al., PRB 93, 165415 (2016).
[8] N. Okabayashi et. al., PNAS 115, 4571 (2018).

Host: Thomas Frederiksen

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