Naphthalocyanine on Au Surfaces - Adsorption and Structural Formation

  • Fig. 1: Schematic model of the H2Nc molecule including mirror planes m1 and m2 and its van der Waals diameter (a). Reconstructed Au(100) and Au(111) surfaces shown as linear lines (b) and herringbone structure, respectively (c).
  •  Fig. 2: After H2Nc deposition a striped phase is found on Au(100) (a) and Au(111) (c) surface. On Au(100) two different directions are observed, on Au(111) three directions. With increasing coverage the molecules‘ form a densely packed phase (b), (d). Differences in island size and orientation are apparent comparing both surfaces.
  • Fig. 3: The surface map shows three different island orientations on Au(100) (a), two of them are favored. An angle between the molecular rows and the surface reconstruction lines is α = β = 60° (b). Up to six different island orientations are observed on Au(111) (c). The surface reconstruction is visible beneath the H2Nc molecules, on Au(100) (d).
  • Fig. 4: The 10 x 10 nm2 STM images reveal the molecules orientation within the densely packed domains. They are aligned with their axes along the surface reconstruction lines on (a), (b). The unit cell of both structures is similar. The structure model is proposed on the basis of the data (c). Vectors , and , denote the gold lattice directions and the unit-cell directions, respectively.

The formation of metal-free naphthalocyanine (H2Nc) self-assembled monolayers (SAMs) on reconstructed Au(100) and Au(111) surfaces is investigated by scanning tunneling microscopy (STM) at room temperature. STM images reveal the formation of a striped phase and a densely packed H2Nc structure on both surfaces depending on the molecular coverage. The H2Nc formation is mainly driven by molecule-molecule interactions rather than molecule-substrate interactions.


Phthalocyanine and the Derivative Naphthalocyanine

The self-assembly of phthalocyanines (Pc) on a variety of surfaces has received large attention within the recent years [1, 2]. Their unique properties and the possibility of inserting various different central metal ions provides excellent conditions for many applications like solar cells [3], light emitting diodes [4], and thin film transistors [5]. Therefore, the adsorption and structural formation was investigated from initial low coverage to the multilayer regime [6, 7]. In the past, some studies determined the properties of the Pc derivative naphthalocyanine (Nc) on graphite and insulator surfaces [8]. However, detailed information on metal surfaces is rare in literature.

Sample Preparation and Instrumentation


Au(100) and Au(111) single crystals were used as substrates. Both surfaces were cleaned by several sputter and annealing cycles of 500 eV and 500°C, respectively. Napthalocyanine (95%) was purchased from Sigma-Aldrich and degassed at 500 K for 24 h. The molecules were deposited on the Au surfaces with a modified electron beam evaporator at a rate of 0.5 layers per minute. The STM images were recorded with an Omicron AFM/STM system with a base pressure well below 2 x 10-10 mbar. All measurements were performed in constant current mode with 0.1-0.2 nA and 0.7-1.2 V for tunnel current and gap voltage, respectively.

Results

A schematic model of the H2Nc molecule and STM images of the reconstructed Au(111) and Au(100) surfaces are presented in figure 1. The H2Nc molecule consists of a phthalocyanine skeleton with additional benzene rings symmetrically attached.

The molecule contains two mirror planes, as denoted by m1 and m2 in figure 1a. It is of planar geometry, and the van der Waals diameter is approximately 2 nm. Both Au surfaces show the typical reconstruction lines depending on the crystal orientation shown in the STM images in figure 1b and 1c.

Figure 2 presents STM images after H2Nc deposition on both surfaces. The molecules form a striped phase at low coverage on Au(100) and Au(111) as shown in figure 2a and 2c. These stripes are orientated along the high symmetry surface directions of Au(100) and Au(111). Thereby, two orientations on Au(100) and three orientations on Au(111) are apparent. With increasing coverage densely packed H2Nc islands are formed on both surfaces, as shown in figure 2b and 2d. The formation of these domains indicates a net attractive interaction between the molecules. The H2Nc formation is mainly driven by molecule-molecule interactions rather than molecule-substrate interactions. A closer look at these islands reveals differences in size and orientation depending on the substrate.

Figure 3 presents a surface map of the STM images shown in figure 2b and 2d and the densely packed phase in a higher resolution. Three island orientations (I), (II), and (III) on Au(100) can be identified in figure 3a. Clearly two of them, orientation (I) and (II) are favored. Orientation (III) is rather rare on the surface. The average island size is approximately 185 nm2 for orientation (I) and 190 nm2 for orientation (II). The higher resolution STM image provides access to the molecule orientation within the domains. It is noticeable that the substrate reconstruction is visible in the island structure. The molecules are orientated with their axes parallel to these linear lines in both island orientations. The angle between the molecule rows and the reconstruction lines is α = β = 60° which is in agreement with the 3-fold symmetry of the reconstructed Au(100)hex surface. The surface map of H2Nc on Au(111) shows a higher number of different island orientations. In the map four large islands with four different orientations are identified. Three of them follow the high symmetry surface directions of Au(111), and the fourth is rotated by 15°. No clear differences in size are apparent. In all, up to six different orientations are found on the Au(111) surface. The STM image with higher resolution identifies the herringbone reconstruction of the surface beneath the island structure. The molecules are orientated with their axes along the reconstruction lines, as observed on Au(100).

In figure 4 the densely packed H2Nc structure is presented together with a structure model. The 10 x 10 nm2 images show that H2Nc molecules are absorbed with the conjugated ring parallel to the surface. They are imaged as crosses with a depression in the center. Their diameter is in good agreement with the gas-phase van der Waals diameter of approximately 2 nm. The molecules' axes are aligned along the Au(100) and Au(111) reconstruction lines. The unit-cell is indicated by a black rhombus on both surfaces. When comparing both unit-cells no clear differences are found. They are of rhombic shape and comprise a single molecule. In figure 4c a structure model is presented summarizing the experimental findings. The green lines denote the surface reconstruction lines of Au(100) with a distance of 1.44 nm. Gold lattice directions and the unit-cell directions are denoted by vectors a, b and A, B respectively. The unit-cell is aligned along the high-symmetry direction along vectors a, and the angle between the directions of vektor A, and vektor B is γ = 95°. From the experimental data, we propose a rhombic unit-cell with vectors magnitude | A| = | B| ≈ 1.7 nm. The orientation of the molecules‘ with respect to the surface symmetry is denoted by the azimuth angle θ = 25°.

Conclusion

The formation of H2Nc SAMs was investigated on Au(100) and Au(111) by STM measurements at room temperature under UHV conditions. The molecules assemble in a striped structure on both surfaces at low coverage. With increasing coverage densely packed domains are formed on Au(100) and Au(111). The molecules are aligned with their axes along the reconstruction lines of both surfaces and the unit-cell is similar. The H2Nc SAMs formation is mainly driven by molecule-molecule interactions rather than molecule-substrate interactions.

Acknowledgements
This work was financially supported by the Land Nordrhein-Westfalen.

References
[1] Bottari G. et al.: Chem. Rev. 110, 6768-6816 (2010)
[2] Mack J. et al.: Chem. Rev. 111, 281-321 (2011)
[3] Koeppe R. et al.: Appl. Phys. Lett.87, 244102 (2005)
[4] Forrest S. R.: Nature 428, 911 (2004)
[5] Singh V. K. et al.: AIP Adv. 1, 042123 (2011)
[6] Hipps K. W. et al.: J. Phys. Chem. B 104, 2444-2447 (2000)
[7] Ogunrinde A. et al.: Langmuir 22, 5697-5701 (2006)
[8] Gopakumar T. G. et al.: J. Phys. Chem. B 110, 6051-6059 (2006)



Authors
Patrick Mehring
(corresponding author)
Prof. Dr. Carsten Westphal


Experimentelle Physik I
Technische Universität Dortmund
Dortmund, Germany
http://e1.physik.uni-dortmund.de/e1b/

Contact

TU Dortmund
Otto-Hahn-Str. 4
44227 Dortmund
Germany

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