Investigation of inelastic electron tunneling process by combining STM and AFM
DIPC Seminars
- Speaker
-
Norio Okabayashi, Kanazawa University
- When
-
2018/11/07
13:00 - Place
- Donostia International Physics Center
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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