^{1,2,a)}, Benjamin Stadtmüller

^{1,2}, Christian Wagner

^{1,2}, Christian Weiss

^{1,2}, Ruslan Temirov

^{1,2}, F. Stefan Tautz

^{1,2}and Christian Kumpf

^{1,2,b)}

### Abstract

The understanding and control of epitaxial growth of organic thin films is of crucial importance in order to optimize the performance of future electronic devices. In particular, the start of the *submonolayer* growth plays an important role since it often determines the structure of the first layer and subsequently of the entire molecular film. We have investigated the structure formation of 3,4,9,10-perylene-tetracarboxylic dianhydride and copper-phthalocyanine molecules on Au(111) using pair-potential calculations based on van der Waals and electrostaticintermolecular interactions. The results are compared with the fundamental lateral structures known from experiment and an excellent agreement was found for these weakly interacting systems. Furthermore, the calculations are even suitable for chemisorptive adsorption as demonstrated for copper-phthalocyanine/Cu(111), if the influence of charge transfer between substrate and molecules is known and the corresponding charge redistribution in the molecules can be estimated. The calculations are of general applicability for molecular adsorbate systems which are dominated by electrostatic and van der Waals interaction.

We thank Christoph Kleimann for stimulating discussions. This work was supported by the Deutsche Forschungsgemeinschaft, project number KU 15312-1.

I. INTRODUCTION

II. PAIR POTENTIAL CALCULATIONS

III. PTCDA/AU(111)

IV. CUPC/AU(111)

V. CUPC/CU(111)

VI. SUMMARY

### Key Topics

- Molecule surface interactions
- 22.0
- Intermolecular forces
- 19.0
- Charge transfer
- 11.0
- Monolayers
- 11.0
- Electrostatics
- 10.0

## Figures

Molecular structure of CuPc and PTCDA. The labels refer to the atomic partial charges in Table I.

Molecular structure of CuPc and PTCDA. The labels refer to the atomic partial charges in Table I.

Potential curves resulting from Eq. (3) for symmetric (a) and unsymmetric pairs (b). The vertical dotted lines indicate the corresponding sum of van der Waals radii.

Potential curves resulting from Eq. (3) for symmetric (a) and unsymmetric pairs (b). The vertical dotted lines indicate the corresponding sum of van der Waals radii.

Scanning tunneling hydrogen microscopy image of the PTCDA/ Au(111) herringbone structure. The lattice spanned by the unit cell vectors and is indicated by the dotted lines. The two molecules (M1, M2) within the unit cell are rotated by 101° with respect to each other. The six neighbors of one PTCDA molecule are indicated by red and blue arrows.

Scanning tunneling hydrogen microscopy image of the PTCDA/ Au(111) herringbone structure. The lattice spanned by the unit cell vectors and is indicated by the dotted lines. The two molecules (M1, M2) within the unit cell are rotated by 101° with respect to each other. The six neighbors of one PTCDA molecule are indicated by red and blue arrows.

(a)–(e) Pair potential maps for five different rotational orientations of two PTCDA molecules between Θ_{ Z } = 0° and Θ_{ Z } = 101°. Δ*Z*, Θ_{ X }, and Θ_{ Y } are zero in all cases. Gray areas indicate repulsion between the molecules, red areas indicate attraction. Red and blue arrows in (a) and (e) denote the distance vectors between neighboring molecules as they were determined experimentally from the STHM image (Fig. 3). The insets in (a) and (e) illustrate the corresponding geometries. Panel (f) shows the minimum value of Φ extracted from the (Δ*X*, Δ*Y*)-maps for each rotation angle Θ_{ Z }, plotted versus Θ_{ Z }. For details see text.

(a)–(e) Pair potential maps for five different rotational orientations of two PTCDA molecules between Θ_{ Z } = 0° and Θ_{ Z } = 101°. Δ*Z*, Θ_{ X }, and Θ_{ Y } are zero in all cases. Gray areas indicate repulsion between the molecules, red areas indicate attraction. Red and blue arrows in (a) and (e) denote the distance vectors between neighboring molecules as they were determined experimentally from the STHM image (Fig. 3). The insets in (a) and (e) illustrate the corresponding geometries. Panel (f) shows the minimum value of Φ extracted from the (Δ*X*, Δ*Y*)-maps for each rotation angle Θ_{ Z }, plotted versus Θ_{ Z }. For details see text.

Pair potential maps for the interaction of one molecule with its six nearest neighbors in the unit cell for PTCDA/Au(111). In (a), the result for the variation of the distance vector is shown. The allowed range lies only between 5.5 Å and 7.0 Å for Δ*X* and 8.5 Å and 10.5 Å for Δ*Y*. The minimum is located precisely in the center of the unit cell. The central molecule and two of its symmetrically equivalent neighbors (at the left and right) are not drawn since their position is varied in this plot. In (b), the azimuthal orientations of both molecules were varied. Angles are measured between the long molecular axis and the long vector of the unit cell. The minimum is found for angles of 40° and 140° for the corner- and center-molecule, respectively.

Pair potential maps for the interaction of one molecule with its six nearest neighbors in the unit cell for PTCDA/Au(111). In (a), the result for the variation of the distance vector is shown. The allowed range lies only between 5.5 Å and 7.0 Å for Δ*X* and 8.5 Å and 10.5 Å for Δ*Y*. The minimum is located precisely in the center of the unit cell. The central molecule and two of its symmetrically equivalent neighbors (at the left and right) are not drawn since their position is varied in this plot. In (b), the azimuthal orientations of both molecules were varied. Angles are measured between the long molecular axis and the long vector of the unit cell. The minimum is found for angles of 40° and 140° for the corner- and center-molecule, respectively.

Δ*X*-Δ*Z* pair potential map for PTCDA molecules arranged in π-stacking direction (Δ*Y* = 0). In the right, a cut in Δ*Z* direction through the minimum at Δ*X* = 2.8 Å and Δ*Y* = 0 is shown. The minimum corresponds nicely to the molecular stacking geometry of a bulk crystal shown in the inset.

Δ*X*-Δ*Z* pair potential map for PTCDA molecules arranged in π-stacking direction (Δ*Y* = 0). In the right, a cut in Δ*Z* direction through the minimum at Δ*X* = 2.8 Å and Δ*Y* = 0 is shown. The minimum corresponds nicely to the molecular stacking geometry of a bulk crystal shown in the inset.

(a) Structural phase diagram of CuPc/Au(111), taken from Ref. 10. (b) Calculated pair potential (black) and its gradient (red) plotted versus the radial displacement along the direction (distance of the two molecules) and versus coverage (nonlinear scale at the top).

(a) Structural phase diagram of CuPc/Au(111), taken from Ref. 10. (b) Calculated pair potential (black) and its gradient (red) plotted versus the radial displacement along the direction (distance of the two molecules) and versus coverage (nonlinear scale at the top).

(a) Pair potential map of equally oriented CuPc molecules. The lattice vectors of the LT-phase structure ( Å, Å) lie close to the minima of the potential map, if the superimposed experimental unit cell and lattice of the reconstructed Au(111) surface are rotated by 26° relative to the molecular axis. (b) Vicinity of the minima of the potential map. (c) Corresponding real space model.

(a) Pair potential map of equally oriented CuPc molecules. The lattice vectors of the LT-phase structure ( Å, Å) lie close to the minima of the potential map, if the superimposed experimental unit cell and lattice of the reconstructed Au(111) surface are rotated by 26° relative to the molecular axis. (b) Vicinity of the minima of the potential map. (c) Corresponding real space model.

(a) Illustration of the geometry of the two CuPc LUMO states which are degenerate in the gas phase. Their different registry with the substrate causes differences in the charge transfer with the surface and, therefore, lifts the degeneracy. (b) The favorable adsorption geometry (right) obtained from pair potential calculations agrees well with the experimental data. The empty-states STM image (left) is reproduced from Ref. 42. Dotted lines represent the molecular orientation, solid lines the direction of rows of copper atoms. (c) Pair potential map for the interaction of two equally oriented CuPc molecules including the modeled charge redistribution based on the donation/back-donation effect. (d) Pair potential map for the interaction between a third CuPc molecule that is attaching to a cluster of two molecules which are oriented according to the best configuration.

(a) Illustration of the geometry of the two CuPc LUMO states which are degenerate in the gas phase. Their different registry with the substrate causes differences in the charge transfer with the surface and, therefore, lifts the degeneracy. (b) The favorable adsorption geometry (right) obtained from pair potential calculations agrees well with the experimental data. The empty-states STM image (left) is reproduced from Ref. 42. Dotted lines represent the molecular orientation, solid lines the direction of rows of copper atoms. (c) Pair potential map for the interaction of two equally oriented CuPc molecules including the modeled charge redistribution based on the donation/back-donation effect. (d) Pair potential map for the interaction between a third CuPc molecule that is attaching to a cluster of two molecules which are oriented according to the best configuration.

## Tables

Atomic partial charges of PTCDA and CuPc derived from NBO population analysis.^{27} The indices refer to atomic labels in Fig. 1.

Atomic partial charges of PTCDA and CuPc derived from NBO population analysis.^{27} The indices refer to atomic labels in Fig. 1.

Coefficients of the van der Waals interaction and the Pauli repulsion as well as van der Waals radii used for the pair potential calculations according to Eq. (3). *b* _{ n } are taken from Ref. 30, *c* _{ n } mostly from Refs. 33 and 34, and from Ref. 35.

Coefficients of the van der Waals interaction and the Pauli repulsion as well as van der Waals radii used for the pair potential calculations according to Eq. (3). *b* _{ n } are taken from Ref. 30, *c* _{ n } mostly from Refs. 33 and 34, and from Ref. 35.

Article metrics loading...

Full text loading...

Commenting has been disabled for this content