Lateral scattering potential of the PTCDA/Ag(111) interface state

A. Sabitova, R. Temirov, and F. S. Tautz

Phys. Rev. B 98, 205429 – Published 29 November 2018

Abstract

A modified scheme of scanning tunneling spectroscopy data analysis has been used to reconstruct the band structure of a strongly dispersive two-dimensional unoccupied electronic state located at the commensurate interface between a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and Ag(111) surface up to the fourth band in the reduced Brillouin zone of the molecular monolayer. The two-dimensional scattering potential, evaluated with the help of the measured band structure data, is highly corrugated. This shows that the accepted picture, describing the interface state of PTCDA/Ag(111) as an upshifted Shockley state of Ag(111), is incomplete. Our findings indicate that the hybridization of unoccupied molecular orbitals of PTCDA on one of the two sublattices with the metal states of Ag(111) substantially influences the interface state properties.

Received 23 March 2018

DOI:https://doi.org/10.1103/PhysRevB.98.205429

Physics Subject Headings (PhySH)

Local density of states Surface & interfacial phenomena Surface states

Monolayer films

Scanning tunneling microscopy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

A. Sabitova^1,2,3, R. Temirov^1,2,*, and F. S. Tautz^1,2,3

¹Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany
²Jülich Aachen Research Alliance, Fundamentals of Future Information Technology, 52425 Jülich, Germany
³Experimental Physics IV A, RWTH Aachen University, 52056 Aachen, Germany

^*Corresponding author: r.temirov@fz-juelich.de

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Vol. 98, Iss. 20 — 15 November 2018

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Figure 1
Structural and electronic properties of PTCDA/Ag(111). (a) STS spectra (smoothed), measured above the centers of type-A (red) and type-B (blue) PTCDA molecules on Ag(111). The inset shows a constant-current STM image of PTCDA/Ag(111), measured at $V_{b} = - 340$ mV. The unit cell with $| {\vec{a}}_{1} | = 18.96$ Å and $| {\vec{a}}_{2} | = 12.61$ Å is indicated with a black dashed line. Type-A molecules, occupying the corners of the drawn unit cell, are aligned with the $[1 \bar{1} 0]$ direction of the substrate [26]. (b) Numerical derivative $d^{2} I / d V^{2}$ , obtained from the smoothed STS spectrum in (a), as used for the detection of $d I / d V$ peaks (type-B molecule). Dashed lines indicate two examples of detected peaks, labeled P1 and P2. $E_{P 1}$ and $E_{P 2}$ denote the detected peak positions; $W_{P 1}$ and $W_{P 2}$ are the corresponding weights.
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Figure 2
PTCDA/Ag(111) interface state dispersion as determined by FT STS. (a) Constant-current $d I / d V$ image measured at $V_{b} = 930$ mV. (b) FFT of the image shown in (a). White arrows: reciprocal lattice vectors of the PTCDA/Ag(111) interface. When the ringlike FFT pattern reaches the red circles labeled ${\bar{X}}_{1}^{'}$ , ${\bar{X}}_{2}^{'}$ , and ${\bar{M}}^{'}$ , $\vec{k}$ hits the 1BZ boundaries at the corresponding ${\bar{X}}_{1}$ , ${\bar{X}}_{2}$ , and $\bar{M}$ points. Note that ${\bar{X}}_{1}^{'}$ , ${\bar{X}}_{2}^{'}$ , and ${\bar{M}}^{'}$ are not located at the 1BZ boundary (see text). (c) Parabolic dispersion of the interface state extracted from FT STS. The horizontal $\vec{k}$ scale is calibrated using FFT spots of the molecular lattice which correspond to the reciprocal vectors $| {\vec{b}}_{1} | = 2 π / | {\vec{a}}_{1} |$ and $| {\vec{b}}_{2} | = 2 π / | {\vec{a}}_{2} |$ shown in (b). (d) First Brillouin zone of PTCDA/Ag(111). The truncated corners in the top right and bottom left result from the slightly skewed (approximately $1^{\circ}$ ) real-space unit cell of PTCDA/Ag(111) [24]. For simplicity, this tiny deviation from a rectangular 1BZ is neglected.
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Figure 3
Band structure of the PTCDA/Ag(111) interface state as derived from FD STS and a NFE model calculation fitted to FD STS data. (a)–(g) Left column: Experimental feature distribution maps (FDMs) extracted from specific features of the measured energy distribution histogram (EDH) displayed on the left-hand side of (h). Right column: $| ψ {(\vec{r})}_{i, \vec{k}} |^{2}$ of the calculated NFE band structure for (a) and (b) $\vec{k} = {\bar{X}}_{1}$ , (c) and (d) ${\bar{X}}_{2}$ , (e) and (f) $\bar{M}$ , and (g) $\bar{Γ}$ points of PTCDA/Ag(111), based on the approximated rectangular 1BZ. The real-space unit cell defined in the inset of Fig. 1 is indicated by white dashed rectangles. (h) Experimental EDH (left) and 2D NFE model band structure (right), the latter calculated with $U_{\pm {\vec{b}}_{1}} = 38 mV$ , $U_{\pm {\vec{b}}_{2}} = 57 mV$ , $U_{\pm ({\vec{b}}_{1} + {\vec{b}}_{2})} = U_{\pm ({\vec{b}}_{1} - {\vec{b}}_{2})} = - 61 mV$ , as obtained by fitting the NFE model to the EDH (see text). Only the four lowest bands of the calculated band structure are shown. The black parabola shows the dispersion of a free-electron state with $m^{*} = 0.48 m_{e}$ and an onset energy of 700 meV. Horizontal dashed lines indicate the correspondence between the features of the model band structure and EDH features. The inset shows the calculated $| ψ {(\vec{r})}_{2, \vec{M}} |^{2}$ , which has no counterpart in the experimental EDH and FDM (see text).
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Figure 4
Map of 2D scattering potential $U (\vec{r})$ of the PTCDA/Ag(111) interface state, overlaid with the structure of the PTCDA monolayer. The corners of the PTCDA/Ag(111) unit cell are occupied by type-A molecules; the centers are occupied by type-B molecules [see the inset of Fig. 1]. Values of $U (\vec{r})$ at high-symmetry points of the unit cell are $U (A) = - 420$ meV, $U (B) = - 50$ meV, $U (C) = 190$ meV, $U (D) = 260$ meV.
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