Article pubs.acs.org/JPCC
Metal-Free Naphthalocyanine Structures on Au(100) at Submonolayer Coverage Patrick Mehring,* Axel Beimborn, Tobias Lühr, and Carsten Westphal Experimentelle Physik I, Technische Universität Dortmund, 44221 Dortmund, Germany ABSTRACT: The formation of metal-free naphthalocyanine (H2Nc) self-assembled monolayers (SAMs) on reconstructed Au(100)hex is investigated by scanning tunneling microscopy (STM) at submonolayer coverage. The STM images show the aggregation of clusters, short stripes, and densely packed islands at the surface. These islands are orientated along two favored surface directions. The molecules’ orientation within these islands is determined. It is shown that the molecules’ axes are aligned parallel to the reconstruction lines along the [011]̅ surface direction in both island orientations. When comparing the molecules’ position, the island types are identified as mirror domains. Subsequent thermal annealing induces a slight change in the densely packed island structure. The molecules’ axes are rotated by 5° compared to the reconstruction lines. The offset induces dislocations in the H2Nc structure increasing the unit-cell. A structure model is presented for the densely packed structures before and after annealing. The results are discussed within the context of phthalocyanine (Pc) findings in order to show the influence of the extended side-group system of the H2Nc molecule on the structural formation.
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INTRODUCTION The class of phthalocyanine (Pc) and its derivates has received considerable attention within the recent years.1−4 Their properties in electronic and optic fields provide promising results for their integration in organic devices like light-emitting diodes (oLEDs),5,6 thin film transistors,7−9 solar cells,10 and as single molecule electronic devices.11,12 The performance of these devices is particularly dependent on the quality of the self-assembled monolayer (SAM) and the metal−organic interface. Therefore, a detailed knowledge about the adsorption and the structural formation of the molecules on different substrates is required. While several studies determined the adsorption,13−15 electronic properties,16−18 and the structural formation19−22 of both, metal-free Pc and Pc-containing central metal ions on various substrates, detailed information about its derivative H2Nc on metal surfaces is rare in literature. These molecules are characterized mainly on graphite and on insulator surfaces.23−25 The Pc studies addressed a molecular coverage in the range from single molecules up to several layers. It was shown that the presence of a central metal ion affects the planar geometry of the molecules and the assembly process especially in the multilayer regime. In those cases, the unit-cell of the formed molecular structure was shifted compared to the unit-cell of the first layer. This effect was observed for CoPc, FePc, and DyPc.26,27 The formation of the first layer is mediated by an interplay of molecule−molecule and molecule−substrate interactions. It was shown for the adsorption of Pc that the molecule−substrate interaction becomes more important when the Pc coverage is low. Both, metal-free Pc and FePc, prefer © 2012 American Chemical Society
adsorbing dispersedly as isolated molecules on hcp and fcc regions of the Au(111) surface. Komeda et al. showed that the orientation of single H2Pc molecules is not evenly distributed among the high-symmetry surface directions.28 This effect was contributed to the uniaxial contraction of the surface atoms and clearly indicated the dominant molecule−substrate interaction. Cheng et al. proposed a direct dependence of the molecule− molecule interaction on the coverage of FePc on Au(111).29 Starting from initial state low-coverage to the monolayer regime linear aggregations of FePc were found as the result of increasing molecule−molecule interaction. Furthermore, dimer, trimer, and hexamer FePc structures were formed before the monolayer regime was reached. Contrary to those results, STM studies on Au(100) and Cu(100) revealed the absence of densely packed structures of CuPc and FePc even at monolayer coverage.30−32 The molecules assembled with their planes parallel to the surface in two orientations, according to the surface symmetry without any long-range order. In all, the results discussed in the previous paragraphs show a dominant molecule−substrate interaction of metal Pc and metal-free Pc rather than a lateral molecule−molecule interaction at submonolayer coverage on Au surfaces. However, these results on Pc offer the possibility for investigating the effect of the extended side-group system of H2Nc on the structural formation on Au(100). Received: December 22, 2011 Revised: May 15, 2012 Published: May 25, 2012 12819
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In this article, we report on the adsorption and structural formation of H2Nc SAMs scanning tunneling microscopy by (STM) measurements at a coverage of half a monolayer. STM images reveal clusters, striped aggregations, and densely packed H2Nc islands on the Au(100)hex surface. The islands show a favored orientation along two high-symmetry surface directions. Both island types are identified as mirror domains, and the molecules are orientated with their axes parallel to the surface reconstruction lines. Thermal annealing induces a change in the island structure. The molecules are rotated by 5° compared to the previous position. This leads to the formation of 1Ddislocation lines, which are repeated every three molecular rows. These results demonstrate a very different absorption and structural formation of H2Nc molecules in comparison to the very similar but smaller Pc molecules.
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EXPERIMENTAL DETAILS The experiments were performed in a UHV chamber equipped with a room temperature STM system at a base pressure better than 1 × 10−10 mbar. An Au(100) single-crystal with 10 mm in diameter and 2 mm thickness was used as a substrate; its orientation was better than 0.1° as checked by Laue diffraction. The sample was prepared by repeated sputter and annealing cycles of 500 eV and 500 °C, respectively. STM pictures and LEED diffraction were used identifying the Au(100) hex reconstruction, which clearly indicates a clean surface. Naphthalocyanine (95%) was purchased from Sigma Aldrich and degassed at 500 K for 24 h. H2Nc was evaporated from a modified electron beam evaporator with a rate of ∼0.5 layers per minute. All STM 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. The STM tips were prepared by electrochemical etching from a polycrystalline tungsten wire and degassed in UHV.
Figure 1. Schematic model of the H2Nc molecule including mirror planes m1 and m2 and its van der Waals diameter (a). The 150 × 150 nm2 STM image of H2Nc on Au(100) was recorded with 0.01 nA and 1 V as current and bias voltage, respectively (b). The molecules were found in 2D-islands (A), in short stripe-like aggregations (B), and clusters (B) at the surface. An analysis of the favored island orientation (A) shows an alignment along the high-symmetry surface directions (c). The 50 × 50 nm2 STM image was recorded with 0.02 nA and 1.1 V as current and bias voltage, respectively. The surface map shows a preferred alignment of the islands in orientation (I) or (II) (d).
high number of defects in their structure is noticeable in the form of missing molecules. This effect can be explained by the low coverage at the surface and the preparation parameters. Furthermore, the formation of H2Nc islands indicates a net attractive interaction between the molecules. The uncovered surface areas show the typical Au(100)hex reconstruction lines. They arise from a reconstruction of the topmost atom layer very similar to Au(111). The clean Au(100) surface tends to reconstruct with the topmost atom layer rearranged in a hexagonal lattice, which grows incommensurable to the bulk layers. For the reconstructed surface, the atoms located at bride- or 4-fold coordinated positions appear usually darker in the STM images than the atoms located at on-top positions of the bulk layer. Because of the reconstruction, straight lines along the [011]̅ surface direction are formed, which provide an easy access to the adlayer position even without atomic resolution. In the past, different superlattices for the Au(100)hex reconstruction with different unit-cells, for example, (5 × 1) or (5 × 20), were reported in literature.33 The island orientation with respect to the surface reconstruction lines is shown in Figure 1c. The close-up view displays three distinct orientations consistent with the 3-fold symmetry of the hexagonal surface. Each of them is indicated by a blue line. The corresponding island orientations are denoted by (I) and (II) for an angle of α = β = 60° and (III) for an orientation parallel to the [011]̅ direction. Figure 1d provides a schematic map of the surface area presented in Figure 1b. Each individual island is indicated by (I), (II), or (III) corresponding to its orientation at the surface. Clearly,
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RESULTS AND DISCUSSION A schematic model of the H2Nc molecule and STM images recorded after deposition on the Au(100) surface are presented in Figure 1. The H2Nc molecule consists of a phthalocyanine skeleton with additional benzene rings attached symmetrically along both molecular axes. Therefore, it shows a strong similarity to Pc with an extended side-group system. 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. Figure 1b shows a 150 × 150 nm2 STM image after H2Nc deposition on the Au(100) surface at room temperature. The coverage corresponds to half a monolayer. In the image, three large terraces of the surface are displayed. The H2Nc molecules can clearly be identified by their higher contrast to the substrate. Although the 2-fold symmetry as depicted in Figure 1a of the Nc molecules is not resolved due to the large image size of 150 × 150 nm2, some first information on the assembly can be seen directly. At first, the formation of 2D-islands (A) can be identified at the surface. These structures are accompanied by short stripes and clusters of H2Nc molecules (B). Their size ranges from a few nm2 up to 2000 nm2. While there are large areas of terraces without molecules, the areas near the step edges show a high density of molecules. Overall, the formation of clusters, short stripes, 2Dislands, and the increased density of these structures at step edges directly indicate a high mobility of H2Nc on Au(100) at room temperature. When looking closely to the 2D-islands, a 12820
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orientations (I) and (II) with an angle of α = β = 60° to the surface reconstruction lines are more prominent than orientation (III). Approximately one-third of the molecules is arranged in islands of orientation (I), one-third in orientation (II), and the remaining in clusters, stripes, or in orientation (III). Overall, orientation (III) is rather rare at the surface as proved by several STM images. An analysis of the typical island size shows a range of a few nm2 up to 2000 nm2 for orientation (I) as well as (II). The average size is determined from several STM images to 180 nm2 and 190 nm2 for orientation (I) and (II), respectively. The unequal distribution of orientations (I), (II), and (III) can be explained by a favored molecule orientation within these structures and will be discussed in Figure 2.
substrate interaction leads to a molecular adsorption orientation following the surface symmetry rather than forming densely packed 2D-islands. Even for an increased coverage, only striped phases or dimer/trimer aggregations are formed on Au(111).28 Close packed structures on Au(100) are completely missing at all coverages.30−32 In all, the Pc adsorption is mainly driven by a molecule−substrate interaction rather than a molecule−molecule interaction. Here, for H2Nc, the STM images clearly show a different adsorption mechanism. The high mobility at the surface and a net attractive interaction between the molecules leads to the formation of large 2Dislands. Overall, a possible explanation for these experimental findings could be that the adlayer formation is mainly driven by molecule−molecule interactions rather than molecule−substrate interactions as shown in literature for Pc molecules. Figure 2 presents a closer look at the H2Nc islands in higher resolution for further characterization. The two favorite island orientations (I) and (II) are shown with molecular resolution in Figure 2a. The molecules are imaged as crosses with a depression in their center. These crosses represent the molecules’ position and orientation within the islands. At first, it can be seen that H2Nc is adsorbed with the π-conjugated ring parallel to the surface. The diameter is in good agreement with the van der Waals radius of approximately 2 nm. The molecular adsorption of H2Nc on Au(100) preserves the surface reconstruction contrary to results reported for the molecule CuPc on Au(100).31 Clearly, the ridge of the reconstruction along the [011̅] direction is present in both the molecules’ domain structure and in the molecule free areas. Therefore, the island orientation can clearly be determined from the STM images. The corresponding angles of orientation (I) and (II) are α = β = 60° to the surface reconstruction lines. The experimental data show that the molecules are aligned along the [012] and [021] surface directions. Therefore, they follow the 3-fold symmetry of the hexagonal surface. The molecules’ orientation within the islands can be determined from the STM images shown in Figure 2b,c. It is noticeable that, for both island orientations (I) and (II), the molecules’ orientation is the same. They are aligned with their axes parallel to the [011̅] surface direction. No other orientation was found in all experimental data. While the molecular orientation is identical for both island orientations, a different molecule sequence along the [011]̅ direction is observed, as displayed by dashed zigzag lines in Figure 2b,c. At the left side of a zigzag line, a molecule is located on-top of the ridge with the axis parallel to the reconstruction line, while the following one is shifted by half a stripe-to-stripe distance. The island orientation is directly depending on what zigzag line is realized at the surface. It is not possible to transfer island orientation (I) into orientation (II) by a rotation within the surface plane. Thus, island orientations (I) and (II) are mirror domains. The number and average size of H2Nc islands is similar for both orientations, as shown before. Obviously, both domains show the same unit-cell as indicated by a black rhombus in Figure 2b,c. Each unit-cell contains one H2Nc molecule. The density of the adlayer can be calculated to approximately 0.33 molecules per nm2. Figure 2d proposes a structure model from the STM images. Only a model of island orientation (I) is shown here due to the identical but mirrored structural formation of orientation (II). The green lines denote surface reconstruction lines with a distance of 1.44 nm. The exact molecular adsorption site on the substrate can not be identified by STM since this technique
Figure 2. Two orientations of the densely packed domains in molecular resolution (a). The 50 × 50 nm2 image was recorded with 0.02 nA and 0.8 V, with the Au(100)hex reconstruction lines clearly visible beneath the H2Nc layer. Images with higher resolution and 8.5 × 8.5 or 12 × 12 nm2 in size, respectively, recorded with 0.01 nA and 0.7 V, show the orientation of single molecules within the mirror domains (b,c). The molecules are aligned with their axes parallel to the surface reconstruction lines. Structure model for orientation (I) proposed on the basis of the data (d). Vectors a,⃗ b⃗ and A⃗ , B⃗ denote the gold lattice directions and the unit-cell directions, respectively.
However, these experimental findings on H2Nc on Au(100) clearly provide some interesting information on the adsorption and structural formation at room temperature. The strong chemical similarity of H2Nc and Pc offers an opportunity for discussing and comparing these results with Pc findings reported in literature. Both molecules are imaged as crosses where H2Nc is consisting of the Pc skeleton with additional benzene rings. Their different adsorption may be attributed to the benzene side-rings. Pc molecules prefer adsorbing as isolated molecules on Au(111) and Au(100) at initial stages of molecular coverage.28−32 Recent studies showed that the repulsive interaction of the Pc molecules may arise from charge transfer from the molecules to the surface. A possible interaction mediated by surface electrons was discarded.34,35 The dominant molecule− 12821
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molecules’ mobility prevents a closer look into the structure of type (B); therefore, that structure remains unresolved. Finally, areas without molecules (C) are found, too. These areas are indicated by the Au(100)hex reconstruction lines, and they clearly identify the orientatation of the adlayer. When comparing areas of the densely packed island type (A) before and after annealing, the absence of mirror domains and the decreased number of different island orientations is noticeable. Before annealing, two preferred orientations are found at the surface, and only one orientation remains after annealing. In addition, 1D-dislocation lines are found within the islands. The dislocation lines are following the direction of the molecular row, and they are repeated every three rows. A careful analysis of the average island size after annealing shows an increased value of 1100 nm2. Figure 3b presents a close-up view of structure type (A) in Figure 3a. In this 18 × 18 nm2 image, the slight structural change of the densely packed island structure is presented. First, the surface reconstruction lines are clearly present in the island structure and also at molecule free areas at the lower part of the image, as seen before in Figure 2b,c. The green dashed line indicates the corresponding [011]̅ direction. Apparently, the orientation of molecular axes is rotated by 5° with respect to this direction as indicated by the solid green lines in Figure 3b. These lines represent the direction of the molecules’ axes within a 3-molecule block, resulting from the 5° rotation. Clearly, a shift is found, which is repeated every three rows of molecules. This displacement leads to dislocation lines in the island structure. The direction of the dislocation lines is indicated by solid blue lines along the [012] direction with an offset of 5°, which is consistent with the direction of the molecular rows. Overall, the dislocation lines are repeated every 5.5 nm, and clearly, an increased unit-cell results for the annealed system compared to the densely packed structure without annealing presented in Figure 2. The black indicated unit-cell is of rhombic shape and comprises three H2Nc molecules. Single molecules in a different orientation are found at domain boundaries. In Figure 3b, two of these molecules are schematically drawn, their axis is rotated by 40° with respect to the molecules in structure type (A). In all, this orientation is rarely found at the surface. The experimental findings are summarized in the structure model presented in Figure 3c. Surface reconstruction lines of the Au(100)hex reconstruction are indicated by green solid lines. The surface lattice and adlayer lattice vectors are denoted by a,⃗ b⃗ and A⃗ , B⃗ , respectively. Further, the H2Nc molecules are rotated by 5° with respect to the surface reconstruction lines. Thus, dislocation lines with an offset of 5° to the [012] direction are induced as indicated in blue in the adlayer structure. These lines are repeated every 5.5 nm; thus, an increased unit-cell is obtained as a result from the annealing step. A rhombic-shaped unit-cell is aligned along the direction indicated by A⃗ parallel to the dislocation lines. The vector B⃗ of the rhombic unit-cell has an angle of γ = 95° with respect to A⃗ . From the experimental data, we determined the length of unit vectors to |A⃗ | ≈ 1.7 nm and |B⃗ | ≈ 5.5 nm; thus, the unit-cell comprises three molecules. The azimuth angle indicates the density of this adlayer is θ = 25°, which is in good agreement with the azimuth angle before annealing. Therefore, a similar density of molecules of approximately 0.33 molecules per nm2 is found. A possible explanation of the observed structural change of the H2Nc islands could be a rearrangement of metastable
records electron densities of the uppermost surface layer. However, in this case, the images clearly provide the molecules’ structural arrangement at the surface, and therefore, a detailed model may be proposed. In Figure 2d, gold lattice directions and the unit-cell directions are denoted by vectors a,⃗ b⃗ and A⃗ , B⃗ , respectively. The unit-cell is aligned along the high-symmetry direction along vector a,⃗ and the angle between the directions of A⃗ and B⃗ is γ = 95°. From the experimental data, we propose a rhombic unit-cell with vectors |A⃗ | = |B⃗ | ≈ 1.7 nm. The orientation of the molecules with respect to the adlayer’s unitcell is denoted by the azimuth angle θ = 25°. A large azimuth angle indicates a high molecular density, which is approximately 0.33 molecules per nm2 for this H2Nc adlayer. In a next step, the influence of an increased surface mobility of the molecules on the H2Nc adlayer’s unit-cell is studied. Therefore, the sample was heated at 130 °C for 20 min. Figure 3 presents STM images of the surface after the annealing step,
Figure 3. H2Nc adlayer after annealing at 130 °C for 20 min. The 50 × 50 nm2 STM image was recorded with 0.01 nA and 1.2 V and shows three different surface areas (a). A densely packed phase (A), areas without long-range order (B), and molecule free areas (C) are displayed. After heating, the island size is increased, and 1D-dislocation lines are found at the surface, as indicated by blue lines. A close-up view (b) of 18 × 18 nm2 in size was recorded with 0.01 nA and 1 V. A slight rotation of the molecules is indicated by the direction of the solid green lines with respect to the surface reconstruction in [011]̅ direction, indicated by the green dashed line. The proposed structure model (c) displays the molecules and the unit-cell vectors A⃗ and B⃗ . Surface reconstruction lines and adlayer dislocation lines are denoted by wide green and solid blue lines, respectively.
showing a slight change of the H2Nc adlayer structure. First, we discuss the observed change in an overview image of the surface, as shown in Figure 3a. In this 100 × 100 nm2 image, three different surface areas are displayed. Clearly, large 2Dislands of H2Nc molecules (A) are surrounded by loose-packed structures (B). As a consequence, only a low number of H2Nc molecules is found in the striped arrangement as observed previously, and most of the molecules form larger aggregations at the surface. Unfortunately, at room temperature, the 12822
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structures. This can be seen by comparing the island size before and after annealing at the surface. After preparation, small islands with an average size below 200 nm2 are formed. These islands exist in three orientations with respect to the highsymmetry surface directions and are accompanied by short stripes of molecules and clusters. Thermal annealing leads to a significant increase of the island size and reduces the possible orientations. Thus, only one island orientation remains and the average island size is increased to approximately 1100 nm2. Also, the number of molecular stripes and clusters is reduced at the surface.
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CONCLUSIONS The formation of H2Nc SAMs on an Au(100)hex surface was investigated by STM measurements. The H2Nc molecules are adsorbed with their molecular plane parallel to the surface. Clusters, stripe aggregations, and densely packed islands are found at a coverage of half a monolayer. The islands show a preferred alignment along two high-symmetry directions of the reconstructed hexagonal Au(100) surface. Within the islands, the molecules’ axes orientation is parallel to the surface reconstruction lines for both island types (I) and (II). It was shown that the island orientations are mirror domains. The adlayer’s unit-cell was determined to be of rhombic shape with an angle of 95° between the lattice vectors. The azimuth angle of θ = 25° indicates a densely packed structure of the H2Nc islands. Thermal annealing at 130 °C for 20 min slightly changes the H2Nc island structure. After the heating, an increased island size without mirror domains was observed at the surface, which can be explained by a rearrangement of metastable structures. Furthermore, the molecules’ orientation within the islands was slightly changed with the H2Nc axis being rotated by 5° off the direction of the reconstruction lines. This leads to the formation of dislocation lines along the [012] direction with an offset of 5°. The dislocation line periodicity is 5.5 nm. The increased unit-cell of this adlayer comprises three molecules. After heating, the rhombic shape is preserved with an angle of 95° between the lattice vectors. Summarizing, we report a rather different adsorption and structure formation of H2Nc compared to Pc.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Land NordrheinWestfalen.
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REFERENCES
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