Article pubs.acs.org/Langmuir
Ordered Superstructures of a Molecular Electron Donor on Au(111) A. Mehler,†,§ T. Kirchhuebel,‡,§ N. Néel,† F. Sojka,‡ R. Forker,‡ T. Fritz,‡ and J. Kröger*,† †
Institut für Physik, Technische Universität Ilmenau, D-98693 Ilmenau, Germany Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, D-07743 Jena, Germany
‡
S Supporting Information *
ABSTRACT: The molecular donor tetraphenyldibenzoperiflanthene (DBP) forms coverage-dependent superstructures on Au(111). At submonolayer coverage, the molecules align parallel to each other. They arrange in row-like structures, which exhibit a nearly rectangular primitive unit cell. By contrast, the molecular monolayer is characterized by a herringbone-type DBP arrangement spanned by an almost square unit cell containing two molecules. Both superstructures occur simultaneously in a narrow coverage range close to completion of the molecular monolayer. The adsorbate−substrate interaction is similar to other physisorbed molecular films on Au(111), but differs for the two adsorption phases as inferred from the different modification of the Au(111) surface reconstruction. Structural properties were consistently probed in real and reciprocal space by scanning tunneling microscopy and low-energy electron diffraction, respectively.
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INTRODUCTION Tetraphenyldibenzoperiflanthene (C64H36, DBP, Figure 1) is currently attracting attention as an organic electron donor in
diffraction (LEED) analysis of DBP self-assembly in the monolayer range on Ag(111) has been reported.9 In particular, herringbone-type superstructures with two DBP molecules per unit cell were demonstrated to occur as the only type of ordered molecular arrangement on that surface.9 This type of molecular superstructure has been reported to be driven by an intramolecular quadrupole moment. Such a quadrupole moment has been shown to determine diverse molecular superstructures, e.g., 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films and crystals.10−12 Similarly, 1,4,5,8naphthalene-tetracarboxylic dianhydride on Ag(111)13 as well as 3,4,9,10-perylene tertracarboxylic diimide on graphenecovered SiC(0001)14 have recently been reported to adopt a herringbone-type superstructure. Here we present the findings of a combination of STM and LEED experiments performed on DBP-covered Au(111). Deviating from previous results obtained for DBP on Ag(111),9 coverage-dependent molecular arrangements are found for DBP on Au(111). At submonolayer coverages DBP molecules form a superstructure that is characterized by a nearly rectangular unit cell containing a single molecule. Adjacent molecules are aligned parallel to each other. At coverages close to the full monolayer DBP molecules adopt a herringbone-type superstructure, where the nearly square molecular unit cell contains two DBP molecules. Each molecular arrangement modifies the Au(111) surface reconstruction. Slightly below the closed monolayer, in a narrow
Figure 1. Ball-and-stick-model of the relaxed vacuum structure of 5,10,15,20-tetraphenylbisbenz[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene (DBP). The optimized structure was obtained by density functional calculations at the B3LYP/6-311G++(d, p) level. C (H) atoms appear red (white).
photovoltaic applications1−4 and as an assistant dopant in organic light-emitting diodes.5 While vapor-deposited thin polycrystalline films of DBP have thoroughly been studied,6−8 structural aspects at the single-layer and single-molecule level have scarcely been reported to date. Recently, a scanning tunneling microscopy (STM) and low-energy electron © 2017 American Chemical Society
Received: January 27, 2017 Revised: April 28, 2017 Published: June 12, 2017 6978
DOI: 10.1021/acs.langmuir.7b00306 Langmuir 2017, 33, 6978−6984
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Figure 2. (a) Overview STM image (1 V, 50 pA, 100 nm × 100 nm) showing A-phase DBP molecules on Au(111) assembled in islands as well as at substrate step edges. The surface coverage is 0.7 MLE. The soliton walls of the Au(111) reconstruction are visible on the pristine surface as well as beneath DBP molecules as protruding lines. They are aligned with ⟨112̅⟩ surface directions. A crystallographic direction, ⟨11̅0⟩, is likewise indicated. The color scale was chosen such as to see both the individual molecules and the Au(111) dislocation lines on all terraces. (b) Close-up STM image (−0.8 V, 80 pA, 5 nm × 5 nm) of the inner region of an A-phase molecular island. a1, a2 span the nearly rectangular unit cell of the molecular superstructure, as determined from the LEED pattern in panel d. (c) Close-up STM image (1 V, 50 pA, 7.1 nm × 7.1 nm) of a step edge region showing individual DBP molecules adsorbing with their backbone perpendicular (top) and parallel (bottom) to the Au(111) step edge. (d) LEED pattern of DBP adsorbed on Au(111) at a coverage of 0.7 MLE, which is clearly dominated by A-phase molecules. The kinetic energy of the incident electrons was 90.5 eV and the sample temperature was ≈20 K. The simulated diffraction spots together with the unit cell of the adsorbate structure are superimposed in green. Blue lines indicate the reciprocal lattice vectors of the unreconstructed substrate. Open orange circles depict the diffraction spots of the reconstructed Au(111) surface.
Table 1. Geometric Aspects of DBP Adsorption Phases A and B on Au(111)a phase
M
δ
x1 (nm)
x2 (nm)
∠(x1,x2) (deg)
∠(x1,s1) (deg)
ϱ (nm−2)
A
⎛ 4.87(1) 3.12(1)⎞ ⎜ ⎟ ⎝− 0.85(2) 6.85(1)⎠
0.005
1.23(2)
2.11(3)
86.5(2)
39.2(1)
0.385(2)
B
⎛ 8.14(4) 5.96(5)⎞ ⎜ ⎟ ⎝− 2.51(4) 6.72(6)⎠
0.005
2.10(3)
2.38(4)
90.2(4)
45.0(3)
0.399(6)
a x1, x2 (s1, s2) denote the primitive adsorbate (substrate) lattice vectors. M is the epitaxy matrix with (x1, x2)T = (1 ± δ) M(s1, s2)T and ∠(s1, s2) = 120°. δ is the scaling error. ϱ denotes the molecular surface density. The numbers in parentheses indicate the uncertainties of the last significant digits.
In the LEED experiments (10−8 Pa) the DBP coverages were determined via the occurrence of distinct diffraction patterns.9,17 Deposition rates in the range of ≈0.07 MLE min−1 to ≈0.1 MLE min−1 were used. Electron diffraction patterns of DBP-covered Au(111) surfaces were recorded with LEED optics equipped with two microchannel plates (OCI Vacuum Microengineering). Some LEED patterns were acquired at a sample temperature of ≈20 K. Apart from an enhancement of the diffraction spot intensities, the low temperature left the LEED pattern invariant. Lattice parameters of the substrate and molecular superstructures were inferred by comparing calculated diffraction patterns of simulated structures with experimental LEED data. 18,19 The analysis was performed using LEEDLab,20 which takes all possible rotational and mirror domains of the adsorbate into account for the numerical lattice fit. Prior to the analysis, the LEED patterns were thoroughly corrected for distortions with LEEDCal.20 For contrast enhancement, diffraction intensities are plotted on a logarithmic scale with inverted grayscale (diffraction spots appear black) to facilitate the comparison with simulated diffraction patterns.
coverage range, both superstructures are observed simultaneously.
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EXPERIMENTAL METHODS
STM and LEED experiments were performed in separate ultrahighvacuum recipients, i.e., in Ilmenau and Jena, respectively. In both chambers, single-crystalline Au(111) surfaces were prepared by Ar+ bombardment (Ar purity: 99.995%, Ar pressure: 10−4 Pa) and annealing (≈ 800 K). DBP molecules (obtained from Lumtec with a nominal purity of >99%, further purified by two cycles of temperature gradient sublimation)9 were sublimated from a heated (≈ 600 K) crucible and deposited onto the clean surface at room temperature. Surface coverages are defined as fractions of a monolayer equivalent (MLE), where 1 MLE corresponds to a single closed molecular layer with a surface density of DBP molecules of ≈0.4 nm−2. The deposition rate in the STM experiments was determined as the ratio of surface coverages inferred from STM images and the exposure time. A deposition rate of ≈3 MLE min−1 was used in these experiments. STM experiments were performed in ultrahigh vacuum (10−9 Pa) and at low temperature (77 K) with a custom-built STM. All STM images were acquired at constant current with the bias voltage applied to the sample. Topographic data are plotted using Gwyddion15 and WSXM.16
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RESULTS AND DISCUSSION Figure 2a shows a representative STM image of DBP on Au(111) at a coverage of 0.7 MLE. The molecules form an ordered superstructure within compact molecular islands on 6979
DOI: 10.1021/acs.langmuir.7b00306 Langmuir 2017, 33, 6978−6984
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Figure 3. (a) Overview STM image (1.1 V, 50 pA, 50 nm × 50 nm) of Au(111) covered with 1 MLE of DBP showing several adjacent terraces covered with a closed molecular layer. All molecules adopt the B-phase arrangement. The color scale was chosen such as to see both the individual molecules and the Au(111) dislocation lines on all terraces. (b) Close-up STM image (1.1 V, 50 pA, 6 nm × 6 nm) of the inner region of the closed DBP film. b1 and b2 denote vectors spanning the unit cell of the DBP assembly. These unit cell vectors were inferred from the average lattice as measured in LEED. (c) STM image (1.1 V, 50 pA, 15.6 nm × 13.8 nm) where a superimposed contrast due to Au(111) dislocation lines is visible. (d) LEED pattern of DBP-covered Au(111) (1 MLE), superimposed with the simulated diffraction spots and the reciprocal adorbate unit cell in red. Blue lines indicate the unreconstructed Au(111) reciprocal lattice vectors. The kinetic energy of impinging electrons was set to 33.5 eV.
unit cell as determined by LEED reveal that the reconstruction periodicity is extended toward a ((24 ± 1) × √3) superstructure. A point-on-line epitaxy with respect to this modified reconstruction is possible within the accuracy margin of the analysis (Figure S6 in the Supporting Information). In order to analyze the adsorption of DBP in islands and at step edges close-up STM images were acquired. Figure 2b shows the DBP arrangement within a molecular island. The space-filling model DBP was added to facilitate the identification of individual molecules. The central part of DBP was imaged with uniform contrast and without further submolecular structure in a wide range of bias voltages (−2 V ≤ V ≤ 2 V). In agreement with previous findings reported for DBP on Ag(111)9 and for DBP on Au(111) based on vibrational spectroscopy,17 the molecular backbone adsorbs parallel to the substrate surface. The four phenyl groups located at the sides of the molecular backbone appear as bright protrusions. From the STM images alone a possible inclination of the phenyl groups with respect to the surface normal is difficult to infer. Recent vibrational spectroscopy experiments indicated that the phenyl groups of the adsorbed DBP molecule are slightly tilted away from the surface normal.17 The analysis of several A-phase DBP islands revealed that the molecular unit cell prefers specific orientations, that is, a1 encloses an angle with ⟨11̅0⟩ crystallographic directions of ≈39° (Table 1). Figure 2a shows in addition that a changing direction of the dislocation lines leaves the orientation of the DBP molecules invariant, which hints at a rather low influence of the surface reconstruction on the DBP assembly. A similar behavior was previously reported for PTCDA on Au(111).26 At low coverage we did not observe the exclusive occupation of fcc stacking regions as previously reported for other molecules.27−31 This additionally corroborates the weak impact of the Au(111) reconstruction on the arrangement of DBP molecules. Occasionally, one-dimensional chains of DBP molecules tend to be localized on fcc sites (Figure S1 in the Supporting Information).
terraces (Figure 2b) and decorate step edges (Figure 2c, Figures S1 and S2 in the Supporting Information). Regions of pristine Au(111) are visible at this coverage. In particular, the dislocation lines of the (22 × √3) surface reconstruction are still discernible. These lines run along ⟨112̅⟩ directions and reflect the partitioning of the crystal surface into regions with alternating face-centered cubic (fcc) and hexagonal closepacked (hcp) stacking of atomic planes.21 Similar pairs of dislocation lines along ⟨11̅0⟩ directions underneath the DBP islands (Figure 2a) are observed. The widths of fcc and hcp stacking regions are close to the values reported for clean Au(111), 3.9 and 2.7 nm, respectively.22−24 In the following, this new DBP superstructure will be referred to as phase A. To determine the unit cell dimensions accurately, quantitative LEED experiments were performed. Figure 2d shows a LEED pattern of Au(111) covered with 0.7 MLE of DBP. At this coverage, A-phase molecules dominate the adsorption structure. The Au(111) diffraction spots are visible at the circumference of the pattern. The satellite spots to each substrate diffraction spot are either due to the Au(111) surface reconstruction or to the A-phase (cf. LEED simulation). Table 1 summarizes the results of the LEED analysis, containing the relevant adsorbate lattice parameters together with the corresponding epitaxy matrix. For phase A, the sums of the matrix elements of each row are integers within the experimental accuracy, which indicates a point-on-line epitaxy of DBP in phase A with respect to the unreconstructed Au(111) lattice.25 The lowest indexed coincidence in reciprocal space occurs for the (ha = 8, ka = 6) lattice points of the adsorbate and the (hs = 1, ks = 1) lattice points of the substrate. The Au(111) surface reconstruction has not been considered in this analysis since irregularities of the reconstruction underneath A-phase domains hamper the determination of an epitaxy relation with sufficient precision. Indeed, our data indicate slight modifications of the reconstruction supercell by the adsorbate. Fourier transforms of STM images (Figures S3 and S5 in the Supporting Information) scaled to the adsorbate 6980
DOI: 10.1021/acs.langmuir.7b00306 Langmuir 2017, 33, 6978−6984
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Similar to superstructure A the Au(111) surface reconstruction is influenced by the presence of B-phase molecules. Figure 3c shows STM data of DBP-covered Au(111) where both the adsorbed B-phase molecules and the dislocation lines of the reconstruction are visible. The dislocation lines appear to be separated nearly equidistantly by 3.2 ± 0.2 nm, which corresponds to about half of the periodicity of the undistorted surface reconstruction. Consequently, at 1 MLE adsorption phase B modifies the widths of hcp and fcc stacking regions of the Au(111) surface reconstruction. This type of modification where hcp zones as well as fcc stacking regions host a single DBP row each was frequently observed for B-phase domains. Within the accuracy of the measurement the long molecular axis of every second DBP is parallel to the dislocation lines. A precise determination of the dislocation line spacing is impeded due to a moiré lattice contrast modulation, which results from the superposition of the reconstruction pattern with the molecular array. Analogous to phase A the modification of the Au(111) surface reconstruction was investigated by means of Fourier-transformed STM images (Figure S5 in the Supporting Information). A ((26 ± 2) × √3) supercell was inferred for the modified substrate surface with respect to the bulk Au(111) lattice. Compared to the (22 × √3) reconstructed Au(111) surface, this corresponds to an 18% larger periodicity in ⟨11̅0⟩ directions of the Au bulk lattice. Our data evidence that the modified reconstruction is not regular on length scales of several 10 nm, and different modifications of the reconstruction are observed. While some adsorbates on Au(111) left the reconstruction invariant42−44 other examples reveal adsorbate-induced variations. Monolayers of C60,45 hexa-peri-hexabenzocoronene,46 PTCDA,26 α-sexithiophene,47 and Azure A48 were reported to cause uniaxial expansions of the reconstruction supercell to different extents. In these cases, the modified reconstruction was described as (n × √3) with n > 22, where n atoms of the topmost Au layer occupy n − 1 bulk lattice sites along ⟨112̅⟩. Tetrathiotetracene was demonstrated to even eliminate the reconstruction upon adsorption of a molecular monolayer.49 In the reports listed above, the degree of the reconstruction modification was unanimously considered to be a measure for the molecule−metal interaction. In summary, our findings for DBP films on Au(111) exhibit similarities with previous reports on the modification of the (22 × √3) reconstruction. Both adsorption phases A and B cause a uniaxial expansion of the reconstruction. Therefore, the DBP− Au interaction is average in comparison to other physisorbed aromatic molecules on Au(111). This is surprising since the aromatic backbone is not in direct contact with the metal surface due to the phenyl spacer group of DBP. Moreover, this interaction is stronger for phase B than for phase A, which may be inferred from the stronger modification of the reconstruction periodicity and of the widths of hcp and fcc stacking regions by B-phase domains. A reason for this observation is unclear at present. Possibly, the rather flexible lattice of phase B facilitates a stronger interaction by the occupation of those Au lattice sites where the interaction is strongest. At the same time, however, both A and B phase retain their orientation when the Au(111) dislocation lines change their direction. The STM and LEED experimental findings reported here consistently reveal that the A-phase DBP adsorption structure is characteristic for a coverage below ≈0.7 MLE while adsorption phase B determines the closed-layer structure starting from ≈0.9 MLE. In the narrow coverage range 0.7−0.9 MLE, both
At very low coverage step edges of Au(111) were decorated by DBP molecules prior to the occupation of terrace sites and the formation of islands. Figure 2c shows a close-up STM image of a Au(111) step edge, which runs from the top to the bottom of the STM image, with attached DBP molecules. The majority of the DBP molecules are adsorbed with their backbone nearly perpendicular (top sketch) to the step edge and, as indicated by the STM image, bridge two adjacent terraces. Similar results were previously reported for cobaltphthalocyanine on a vicinal Au surface.30 Some DBP molecules are aligned with their long molecular axis nearly parallel to the step edges (bottom sketch). As shown in the Supporting Information (Figure S1) these molecules prefer adsorption sites in the hcp stacking region at the edge of the upper Au terrace. The closed DBP monolayer is characterized by a different ordered superstructure, which will be referred to as B in the following. Figure 3a shows an overview STM image with superstructure B covering several adjacent terraces. The unit cell geometry and (local) modifications of the Au(111) surface reconstruction are depicted in Figure 3b and 3c, respectively. In all analyzed STM images, the periodicity of the molecular arrangement was retained across step edges. A representative LEED pattern of Au(111) covered with 1 MLE of DBP is depicted in Figure 3d. Adsorption phase B contains two glide planes parallel to each of the B-phase lattice vectors. They give rise to an extinction of the (h,0) and (0,k) diffraction orders for odd values of h and k.32 This effect has been considered in the LEED analysis and, thus, no simulated diffraction spots occur at these positions for phase B. In contrast to structure A, the epitaxy matrix does not reveal any particular type of lattice epitaxy for phase B with respect to the Au(111) bulk lattice. Indeed, B-phase arrays do not exhibit perfect translational symmetry. Small variations of the crossshaped space between the interdigitated phenyl groups of four adjacent DBP molecules are visible (Figure 3b). The molecular arrangement within the unit cell as well as the unit cell geometry are subject to disorder. In addition, the Au(111) surface reconstruction is modified in an irregular manner by B-phase molecules (Figure 3c, Figures S4 and S5 in the Supporting Information). The modifications are stronger than observed for adsorption phase A. A detailed analysis of the various epitaxial relations for adsorption phase B is hampered due to the low contrast in LEED patterns, which most likely results from the local adaptations between the adsorbate and the substrate lattice. The arrangement of B-phase DBP molecules is akin to the herringbone assembly of PTCDA reported for several surfaces.12,33−40 Figure 3b shows a close-up view of the Bphase assembly with indicated unit cell vectors b1 and b2 that are inferred from the average lattice as measured in LEED. In analogy to the PTCDA adsorption phase, we suggest that the B-phase arrangement of DBP is energetically favored due to a (partial) compensation of the intrinsic quadrupole moment of individual DBP molecules.9 Indeed, it was shown previously that DBP on Ag(111)9 exhibits a distribution of quadrupole charges comparable to PTCDA.41 In particular, negative charges are concentrated at the molecular backbone while positive charges are located at the outer phenyl groups.9 On Ag(111), herringbone-type arrangements of DBP were reported as the only ordered assemblies.9 Before suggesting possible origins for the presence of two distinct superstructures on Au(111) rather than a single DBP assembly, further characteristics of the A and B phase will be elaborated. 6981
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Figure 4. (a) Overview STM image (1 V, 50 pA, 90 nm × 90 nm) of Au(111) covered with 0.85 MLE of DBP. Both adsorption phases A and B are present. The color scale was chosen such as to see both the individual molecules and the Au(111) dislocation lines on all terraces. (b) Close-up STM image (20 nm × 20 nm) measured in the region that is marked by a white box in the right part of (a). Au(111) dislocation lines are visible. (c) LEED pattern of Au(111) covered with 0.75 MLE DBP (kinetic energy of incident electrons: 33.5 eV, sample temperature: ≈ 20 K). Simulated diffraction spots together with one of the corresponding reciprocal unit cells of A-phase and B-phase superstructures are superimposed in green and red, respectively. The direction of the unreconstructed Au(111) lattice vectors is indicated by blue lines.
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CONCLUSIONS Adsorption of DBP on Au(111) unexpectedly exhibits a submonolayer superstructure that is not typical for molecules with an internal quadrupole moment. Only near and at the closed monolayer the characteristic herringbone-type adsorption phase occurs. STM experiments reveal modifications of the Au(111) surface reconstruction by the presence of DBP molecules in both adsorption phases. Therefore, DBP films exhibit a coupling to the metal surface that is seemingly typical for physisorbed aromatic molecules, albeit DBP being a lander molecule. The uniaxial expansion of the reconstruction supercell as well as the alterations of the hcp and fcc stacking regions is more pronounced for adsorption phase B than for A, which may entail a stronger molecule−substrate coupling for the closed molecular monolayer. The observation of the exceptional low-coverage adsorption phase together with the coverage-dependent DBP−Au(111) interactions will hopefully spark theoretical investigations.
adsorption phases were simultaneously observed. Figure 4a,b show STM data at a DBP coverage of 0.85 MLE and LEED data (Figure 4c) recorded at 0.75 MLE. Both phases obviously coexist. The dislocation lines of the Au(111) surface reconstruction are not significantly altered at the transition between A-phase and B-phase DBP islands (Figure 4a,b). The occurrence of adsorption phase A is remarkable. On Ag(111), exclusively the herringbone-type arrangement B was reported for all coverages and traced to the intramolecular quadrupole moment.9 Therefore, additional mechanisms are likely to be operative for the A-phase structure observed on Au(111). It is tempting to relate the occurrence of the A phase to the (22 × √3) Au(111) reconstruction. However, a possible template effect of the (22 × √3) surface reconstruction for the adsorption of DBP molecules appears to play a minor role due to a rather low DBP−Au(111) interaction. The different packing motif may enable the molecules to adopt other arrangements that effectively reduce the electric field of adjacent quadrupole moments. Figure 2b shows that adjacent molecules are laterally displaced along the long molecular axis, which additionally leads to a close stacking of the phenyl groups. This mutual lateral shift may reduce the total electric field of the molecular quadrupole moments and favor an intermolecular coupling via the phenyl groups. In addition, a possible influence of substrate step edges on the adsorption structure may be noteworthy. STM images reveal that A-phase islands do not reach step edges, while B-phase islands do (Figure 4a, Figures S1 and S2 in the Supporting Information). Since adsorption at step edges is characterized by DBP molecules with backbones aligned parallel and perpendicular to the orientation of the step edge (Figure 2c), the formation of the herringbone-type assembly on terraces may be facilitated. Apart from these observations, spectra of the differential conductance (Figure S7 in the Supporting Information) acquired on A-phase and B-phase molecules are similar. DBP frontier orbitals appear with essentially identical energies and line shapes indicating comparable DBP−Au interactions in the different adsorption phases.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b00306. Au(111) step edges: decoration by single molecules and influence on A- and B-phase domains; interaction between the DBP adsorption phases and the Au(111) surface reconstruction; epitaxy of the DBP adsorption phases; DBP orbital electronic structure on Au(111) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
R. Forker: 0000-0003-0969-9180 J. Kröger: 0000-0002-6452-5864 Author Contributions §
These authors contributed equally to this work.
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DOI: 10.1021/acs.langmuir.7b00306 Langmuir 2017, 33, 6978−6984
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Langmuir Notes
(17) Rouillé, G.; Kirchhuebel, T.; Rink, M.; Gruenewald, M.; Kröger, J.; Forker, R.; Fritz, T. Identification of vibrational excitations and optical transitions of the organic electron donor tetraphenyldibenzoperiflanthene (DBP). Phys. Chem. Chem. Phys. 2015, 17, 30404− 30416. (18) Sojka, F.; Meissner, M.; Zwick, C.; Forker, R.; Fritz, T. Determination and correction of distortions and systematic errors in low-energy electron diffraction. Rev. Sci. Instrum. 2013, 84, 015111. (19) Sojka, F.; Meissner, M.; Zwick, C.; Forker, R.; Vyshnepolsky, M.; Klein, C.; Horn-von Hoegen, M.; Fritz, T. To tilt or not to tilt: Correction of the distortion caused by inclined sample surfaces in lowenergy electron diffraction. Ultramicroscopy 2013, 133, 35−40. (20) LEEDLab and LEEDCal are commercially available at http:// www.scientaomicron.com/en/products/350/1155 and http://www. scientaomicron.com/en/products/350/1154, respectively. (21) Narasimhan, S.; Vanderbilt, D. Elastic stress domains and the herringbone reconstruction on Au (111). Phys. Rev. Lett. 1992, 69, 1564−1567. (22) Wöll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Determination of atom positions at stacking-fault dislocations on Au(111) by scanning tunneling microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 39, 7988−7991. (23) Barth, J. V.; Brune, H.; Ertl, G.; Behm, R. J. Scanning tunneling microscopy observations on the reconstructed Au(111) surface: Atomic structure, long-range superstructure, rotational domains, and surface defects. Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 42, 9307−9318. (24) Hanke, F.; Björk, J. Structure and local reactivity of the Au(111) surface reconstruction. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 235422. (25) Forker, R.; Meissner, M.; Fritz, T. Classification of epitaxy in reciprocal and real space: rigid versus flexible lattices. Soft Matter 2017, 13, 1748−1758. (26) Kilian, L.; Umbach, E.; Sokolowski, M. A refined structural analysis of the PTCDA monolayer on the reconstructed Au(111) surface − “Rigid or distorted carpet?”. Surf. Sci. 2006, 600, 2633−2643. (27) Böhringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Two-dimensional self-assembly of supramolecular clusters and chains. Phys. Rev. Lett. 1999, 83, 324−327. (28) Néel, N.; Kröger, J.; Berndt, R. Highly periodic fullerene nanomesh. Adv. Mater. 2006, 18, 174−177. (29) Altman, E. I.; Colton, R. J. Nucleation, growth, and structure of fullerene films on Au(111). Surf. Sci. 1992, 279, 49−67. (30) Kröger, J.; Jensen, H.; Néel, N.; Berndt, R. Self-organization of cobalt-phthalocyanine on a vicinal gold surface revealed by scanning tunnelling microscopy. Surf. Sci. 2007, 601, 4180−4184. (31) Vladimirova, M.; Stengel, M.; De Vita, A.; Baldereschi, A.; Bö hringer, M.; Morgenstern, K.; Berndt, R.; Schneider, W.-D. Supramolecular self-assembly and selective step decoration on the Au(111) surface. EPL (Europhysics Letters) 2001, 56, 254−260. (32) Holland, B. W.; Woodruff, D. P. Missing spots in low energy electron diffraction. Surf. Sci. 1973, 36, 488−493. (33) Lovinger, A. J.; Forrest, S. R.; Kaplan, M. L.; Schmidt, P. H.; Venkatesan, T. Structural and morphological investigation of the development of electrical conductivity in ion-irradiated thin films of an organic material. J. Appl. Phys. 1984, 55, 476−482. (34) Möbus, M.; Karl, N.; Kobayashi, T. Structure of perylenetetracarboxylic-dianhydride thin films on alkali halide crystal substrates. J. Cryst. Growth 1992, 116, 495−504. (35) Ludwig, C.; Gompf, B.; Glatz, W.; Petersen, J.; Eisenmenger, W.; Möbus, M.; Zimmermann, U.; Karl, N. Video-STM, LEED and Xray diffraction investigations of PTCDA on graphite. Z. Phys. B: Condens. Matter 1992, 86, 397−404. (36) Hoshino, A.; Isoda, S.; Kurata, H.; Kobayashi, T. Scanning tunneling microscope contrast of perylene-3, 4, 9, 10-tetracarboxylicdianhydride on graphite and its application to the study of epitaxy. J. Appl. Phys. 1994, 76, 4113−4120. (37) Schuerlein, T. J.; Armstrong, N. R. Formation and characterization of epitaxial phthalocyanine and perylene monolayers and
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.K., F.S., R.F., and T.F. acknowledge funding by the Deutsche Forschungsgemeinschaft through Grant No. FO 770/2-1 and FR 875/16-1. T.K. thanks for financial support by the Evonik Stiftung through a Ph.D. scholarship. A.M., N.N., J.K. acknowledge funding by the Deutsche Forschungsgemeinschaft through Grant No. KR 2912/7-1 and KR 2912/12-1.
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