Structural Investigation of 1,1′-Biphenyl-4-thiol Self-Assembled

Article pubs.acs.org/Langmuir

Structural Investigation of 1,1′-Biphenyl-4-thiol Self-Assembled Monolayers on Au(111) by Scanning Tunneling Microscopy and LowEnergy Electron Diffraction D. G. Matei, H. Muzik, A. Gölzhaü ser, and A. Turchanin* Physics of Supramolecular Systems and Surfaces, University of Bielefeld, Germany ABSTRACT: Self-assembled monolayers (SAMs) of 1,1′-biphenyl-4-thiol (H− (C6H4)2−SH) on Au(111) were prepared from solution or via vapor deposition in ultrahigh vacuum and characterized by scanning tunneling microscopy (STM), lowenergy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). In contrast to the typically observed for densely packed alkane-thiol SAMs on Au(111) (√3 × √3)R30° structure, the densely packed aromatic biphenylthiol SAMs prepared by both methods exhibit an unusual hexagonal (2 × 2) structure. Upon annealing at 100 °C, this structure evolves into the (2 × 7√3) structure resulting in the formation of highly ordered pinstripes oriented along the ⟨1 −1 0⟩ directions. Lower density SAMs, prepared by vapor deposition in vacuum, show mixed structures comprising the hexagonal (2 × 2) structure and two rectangular arrangements with the unit cells of (3√3 × 9) and (2√3 × 8). An extinction of the (3√3 × 9) structure in the favor of the (2√3 × 8) structure is observed upon annealing at temperatures of ∼100 °C.

■

INTRODUCTION In the 3 decades following the preparation of the first selfassembled monolayers (SAMs),1,2 molecular self-assembly on solid surfaces has attracted a significant amount of research.3−9 Thereby a prominent role has been played by thiols on gold surfaces, as thiols allow a rather simple SAM preparation from organic solvents. In particular alkanethiol SAMs10,11 have been intensively studied and are extensively used as organic surfaces in a broad variety of applications, such as lubrication, corrosion inhibition, electrochemistry, and biotechnology.6 Conversely, SAMs from aromatic thiols 12−16 were less frequently investigated, which is quite surprising as these systems also constitute commonly used materials for nanotechnology applications.17 A prominent example is the interaction of aromatic SAMs with low energy electrons18 or extreme ultraviolet (EUV)19 light, which results in a molecular crosslinking20 within the self-assembled molecular layer that strongly enhances its thermal and mechanical stability21,22 and opens a path for the creation of carbon nanomembranes (CNMs) and graphene.23−32 In spite of these developments, the fundamental molecular processes associated with the adsorption of aromatic molecules on gold surfaces are much less understood than for their aliphatic counterparts. Some reports investigate the structure of 4-methyl-4′-mercaptobiphenyl (CH3−(C6H4)2− SH, MMB)13,33,34 and similar molecules that are characterized by the insertion of an alkyl chain between the thiol and the aromatic biphenyl groups.33,35−39 A comprehensive scanning tunneling microscopy (STM) characterization of SAMs built with MMB and its derivatives exists,34−39 but no detailed STM data are available for 1,1′-biphenyl-4-thiol (H−(C6H4)2−SH, BPT). However, BPT is a frequently used precursor for the fabrication of CNMs 17 and the understanding of the fundamentals of BPT SAM formation should provide insight into the mechanisms of CNM formation. In this study, we © 2012 American Chemical Society

investigate the structure of BPT self-assembled monolayers adsorbed on gold surfaces using scanning tunneling microscopy and low energy electron diffraction (LEED). Complementarily, we analyze the chemical composition of the monolayers by Xray photoelectron spectroscopy (XPS).

■

EXPERIMENTAL SECTION

1,1′-Biphenyl-4-thiol (H−(C6H4)2−SH) was purchased from Platte Valley Scientific and purified by sublimation. BPT SAMs were prepared on clean Au(111) surfaces either from Au/mica substrates (G. Albert PVD-Coatings) or from an Au(111) single crystal (MaTecK). Au/mica substrates were cleaned in ozone atmosphere for 6 min and then rinsed in ethanol. The Au(111) single crystal was cleaned in vacuum by a combination of Ar+ sputtering and annealing. Two SAM preparation procedures were used. The first procedure involved the immersion of the substrates into a 0.2 μM solution of BPT in dimethylformamide (DMF) at room temperature for an immersion time of 3 days, followed by rinsing in DMF and ethanol. The second method was vapor deposition of BPT from a quartz crucible heated at 60 °C in an ultrahigh vacuum (UHV) chamber at pressures of ∼10−7 mbar, with the substrate held at room temperature. The evaporation time was set between 1 and 2 h. A multichamber UHV-system (Omicron) consisting of an analysis chamber equipped with STM (Multiscan VT), LEED, and XPS; a preparation chamber equipped with a Knudsen-type organic evaporator (TCE-BSC, Kentax); a heatable sample manipulator; and an interconnecting transfer system was used for all experiments. STM images were obtained using electro-chemically etched tungsten tips with tunneling currents of 30−80 pA and bias voltages of 300 mV. LEED patterns were recorded using a BDL600IR-MCP (OCI Vacuum Microengineering) system with a multichannel plate detector. Experimental LEED patterns were simulated using the LEEDsim Received: July 12, 2012 Revised: August 23, 2012 Published: September 6, 2012 13905

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

Figure 1. STM images of a BPT SAM obtained from solution: (a) overview image and (b) detailed image showing the (2 × 2) hexagonal arrangement of the molecules. The inset in part (a) shows the profile across the vertical dashed line in part (b); the insets in part (b) shows the profile across the oblique dashed line. The ⟨1 −1 0⟩ directions are indicated in the lower-right part of part (b) and the Fourier transform of the image, respectively. software. X-ray photoelectron spectra were recorded using a monochromatic X-ray source (Al Kα) and a Sphera electron analyzer (Omicron) with a resolution of 0.9 eV. The effective thickness of the monolayers was estimated from the exponential attenuation of the substrate Au 4f signal in comparison to the signal of a clean Au(111) reference, using an attenuation length of 36 Å. Following the short exposures to electron or X-ray beams from LEED and XPS instruments, respectively, no changes were detected in the SAMs. Annealing of the samples was performed at about 100 °C using a resistive heater placed behind the sample holder for 15 h. The samples were then cooled down to room temperature before further investigations were done.

■

RESULTS AND DISCUSSION 1. BPT SAMs on Au(111) Directly after Preparation from the Solution or Vapor Phase. 1.a. Solution Preparation. Figure 1a shows an overview STM image of a BPT SAM prepared from solution. The topography is determined by the terraces and single atomic steps of the underlying Au(111) surface. The terraces are populated with two-dimensional islands with irregular shapes and lateral sizes below 20 nm. The islands appear bright in Figure 1a. They have a density of a few thousands per square micrometer and are characterized by a height of 2.5 ± 0.2 Å, equal to that of monatomic steps of the gold substrate, indicating that the topography of the molecular layer at this level is determined by the arrangement of the Au atoms of the substrate. It has been proposed that the nucleation of such islands is connected to the formation of thiolate-Au complexes.9 The STM images reveal molecules grouped tightly in patches and separated by gaps which appear as dark areas with measured depths in the range 1.5−2 Å. These molecular patches cover about 80% of the total surface area. Figure 1b shows a molecularly resolved image that reveals a compact structure of surface features arranged in a hexagonal lattice. The lattice spacing obtained from the line scans and from the Fourier transforms is 5.8 ± 0.5 Å, which corroborates with the crystalline orientation allowing one to

Figure 2. XP spectra of a BPT SAM obtained from solution: (a) S2p, (b) C1s, and (c) O1s.

assign the structure as a (2 × 2) overstructure. The star in the bottom right corner of the image indicates the ⟨1 −1 0⟩ 13906

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

covered with molecules whose long axis is almost parallel to the substrate surface, as compared with the other close packed molecules with the molecular axis almost normal to the surface. This observation is confirmed by a reduced value of the effective layer thickness, as determined by XPS, of ∼5 Å. A detailed image obtained from a part of the surface with flat lying molecules is shown in Figure 3d, it was obtained from the bottom-left part of Figure 3c. It is noteworthy that the coexistence of differently orientated molecules after vapor phase deposition, was also observed for alkanethiols.41 Our STM data indicate that BPT shows a similar behavior at low coverage. It has been suggested that the lying-down arrangement of the molecules provides a kinetic barrier against the formation of a densely packed SAM at the earlier stages of growth.41 To identify the molecular superstructures of low-coverage BPT SAMs obtained by vapor deposition, we further investigated the LEED patterns from these samples. Figure 3e reveals that the gold surface is covered with two different molecular arrangements: (i) a rectangular lattice characterized by the matrix

directions. This is a surprising result, as the typical unit cell of densely packed alkanethiols, MMBs, as well of sulfur atoms on Au(111)40 is found to be either (√3 × √3)R30° or related structures.6,9 Figure 2 shows XP spectra from solution prepared BPT SAMs that reveal the presence of carbon, sulfur, gold, and a small amount of oxygen. The S2p signal (Figure 2a) consists of a spin−orbit-split doublet, with a S2p3/2 peak at 162.0 eV that is assigned to thiolate and indicates the chemical binding of the sulfur atoms to the substrate.18 The carbon spectrum shows a C1s peak with the main peak at 284.4 eV whose shape and intensity is characteristic for carbon in the phenyl rings.18,22 The carbon to sulfur ratio obtained from XPS is close to 12:1 and thus indicates that the BPT molecules are adsorbed intact. A small amount of oxygen is seen in the graph of Figure 2c (black trace) indicating that only few contaminants, most probably water, adsorbed on the surface while manipulating the sample in air. Upon annealing the sample, the oxygen peak reduces; see the red trace in Figure 2c. The effective thickness of SAMs prepared from solution, calculated from XPS data, amounts to about 8 Å. Considering the theoretical length of the BPT molecule of 10 Å12 and taking into account an angle of about 30° between the molecular axis and the normal to the surface,20 this value further supports the compact structure observed in STM. 1.b. Vapor Deposition. BPT SAMs were also prepared by vapor deposition. The BPT molecules were evaporated from a quartz crucible, heated at 60 °C for 2 h, and subsequently imaged by STM. Figure 3a shows an STM image of a vapor deposited BPT SAM, indicating a local ordering of the molecules described by the (2 × 2) superstructure also observed for the BPT SAMs prepared from solution. The hexagonal structure of the substrate can be best seen from the two-dimensional Fourier transform of the inset of the figure. A LEED pattern obtained at 10 eV is shown in Figure 3b, which further confirms the presence of the (2 × 2) superstructure. The (0,0) spot is seen in the upper part of the screen; it is accompanied by two broad spots originating from the molecular structure in the lower half of the screen. The inplane orientation of the hexagonal superstructure is the same as the orientation of the Au(111) structure (see the inset), and the first-order reflections of the Au(111) structure appear at electron energies approximately 4 times higher than those at which the first-order reflections of the molecular structure appear. Considering the inverse square root dependence of the distances on the LEED screen as a function of the beam energy, this indicates a ratio of 2 between the corresponding lattice spacing. From XPS data, the SAM thickness is estimated to be ∼7 Å, which is slightly lower than the thickness of SAMs prepared from solution but still indicative of a compact arrangement of the molecules. The C1s and S2p spectra of the vapor deposited SAMs (not shown) have similar characteristics as for the SAMs prepared from solution. However, the O1s signal was not observed in these monolayers directly after the preparation, which demonstrates the absence of contamination. Figure 3c shows STM data of a BPT SAM obtained after an evaporation time of 1 h. The molecules are tightly packed along step edges and near steps. On the terraces, areas with different molecular arrangements can be distinguished, coexisting with domains where no structure is visible in the STM images. The height difference between various structures on the same terrace, as determined by STM (see line profile in inset), is about 1 Å. This clearly indicates that we find regions that are

⎛6 3⎞ ⎜ ⎟ ⎝0 9⎠ having dimensions of (14.96, 25.92) Å or (3√3 × 9) in terms of the lattice spacing of Au (111) and (ii) a rectangular lattice with a smaller unit cell, characterized by the matrix ⎛4 2⎞ ⎜ ⎟ ⎝0 8⎠

with dimensions (9.98, 23.04) Å or (2√3 × 8), following the conventions of Merz and Ernst42 for unit cell selection and matrix notation. Figure 3f shows a simulated LEED pattern of the two structures that very well matches the observed pattern, taking into account the symmetry of the substrate as well as the presence of rotational domains. We note for both structures the extinction of the LEED spots of type (m, 0) and (0, n), with m and n odd, suggesting the presence of additional symmetry elements in the unit cell.42 It is thus necessary to impose a p2gg symmetry of the unit cell for both structures to have a full correspondence between the simulated and the real patterns. The rectangular (3√3 × 9) unit cell (blue rectangle) and a possible arrangement of the molecules on the surface are overlaid in Figure 3d. The unit cell (2√3 × 8) will be discussed in detail together with the results after annealing below. 2. Changes of the Structure of BPT SAMs on Au(111) after Annealing in Vacuum. 2.a. Solution Preparation. After the BPT SAMs prepared from solution were annealed at 100 °C for about 15 h, a significant surface reorganization is observed, cf. Figure 4a. The long annealing time was employed to ensure a reproducible thermal equilibration across the sample. After annealing, the step edges and the edges of the islands become faceted along the ⟨1 −1 0⟩ directions, which induces a zigzag direction in the steps and a triangular shape of the islands, like the island in the right side of Figure 4b. Upon annealing at higher temperatures, the overall surface coverage decreased and at 120 °C almost all BPT molecules are desorbed from the surface, which is in agreement with the earlier electron spectroscopy results (see ref 20.). A further consequence of surface annealing is the rearrangement of the initial randomly scattered molecular patches into long (>50 nm) stripes with a periodicity of 3.8 ± 0.4 nm, running along 13907

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

Figure 3. (a) STM image of a BPT SAM obtained by vapor deposition for 2 h; the inset shows the 2D Fourier transform. (b) LEED pattern of the same sample obtained at 10 eV, with the sample tilted so that the (0,0) spot is visible; the inset shows the (1,1) reflections of the Au substrate. (c) STM image of a BPT SAM obtained by vapor-phase deposition for 1 h; the inset shows the profile along the dashed line; (d) Detail showing a rectangular unit cell having dimensions of (3√3 × 9) (blue rectangle with a possible molecular arrangement). (e) LEED pattern obtained at 15 eV from the same sample; (f) Simulated LEED pattern from the coexistence of two rectangular structures with size (3√3 × 9) (square spots) and (2√3 × 8) (circular spots); both structures belong to the p2gg symmetry group.

⎛2 0 ⎞ ⎜ ⎟ ⎝ 7 14 ⎠

the ⟨1 −1 0⟩ directions. This is clearly observed in ordered areas of the surface where no islands are present and the step density is low, as in the example shown in Figure 4b. Molecularly resolved STM images, cf. the upper-left inset of Figure 4b, show that the (2 × 2) hexagonal packing is preserved inside the stripes. These areas cover about 60% of the total surface area. LEED patterns like the one shown in Figure 4c, where the reorganization in stripes was dominant, allow an accurate determination of the structure parameters. We also have simulated the LEED pattern of a structure with a rectangular (2 × 7√3) unit cell defined by the matrix (upperright inset in Figure 4b).

The simulation is shown in Figure 4d; it very well reproduces the experimentally observed LEED pattern shown in Figure 4c. Considering the original (2 × 2) structure, the formation of a new structure after annealing could be explained by the removal of every seventh row of molecules. An additional shift with one unit distance of Au (111) of every second stripe, along the stripe direction, is necessary to obtain the required unit cell, resulting in a stripe periodicity of 3.49 nm. The high-resolution image from the upper-left inset in Figure 4b supports this assumption, a similar structure denoted “pinstripe pattern” was 13908

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

Figure 4. (a) STM image of a BPT SAM from solution, annealed in UHV at 100 °C, showing a faceting of steps along ⟨1 −1 0⟩. (b) STM of an area where molecules arranged into stripes with 3.49 nm periodicity; the unit cell is marked with a rectangle; the upper-left inset is a STM image recorded from the area within the square, the scale bar is 5 Å; a model of the molecular arrangement is drawn on the upper-right inset, the red dots are the positions of the molecules, and the blue rectangle is the unit cell of the molecular structure. (c) LEED pattern recorded at 18 eV from a sample where the striped phase was dominant; the sample was tilted with respect to the electron beam, so that the spots around the (0,0) reflection are visible; note the different crystalline orientation, compared to images in parts a and b. (d) simulation of the pattern obtained in part c using a rectangular unit cell, with dimensions (2 × 7√3) and oriented like the image in part b.

packing retain their local (2 × 2) structure, as shown in the insets of Figure 5a. In the areas covered by flat lying molecules, as the one shown in Figure 5b, LEED patterns show that the rectangular (3√3 × 9) structure becomes extinct in favor of the rectangular (2√3 × 8) arrangement. A LEED pattern obtained at an energy of 26 eV with the sample tilted with respect to the electron beam is shown in Figure 5c. A simulation that considers only the latter structure with p2gg symmetry is shown in Figure 5d, which qualitatively matches the experimental pattern in Figure 5c. Note that a rectangular unit cell with the same size of (2√3 × 8), obtained by the absorption of MMB on Au(111), together with the extinction of certain diffraction spots, this time obtained by grazingincidence X-ray diffraction, has been reported by Ulman and co-workers.13 Looking at the image from Figure 5b, we observe a rectangular lattice with the unit cell having the dimensions (2√3 × 8), with a rotational symmetry of the second order, marked by the white rectangle. However, glide symmetry is not readily visible in the STM images. Here it should be noted that the question of what is actually seen in STM images of SAMs of organic molecules is still under discussion,9 leaving uncertainties in establishing the true molecular arrangement. On the other hand, the diffraction of electrons in LEED experiments may be sensitive to other structural features than the tunneling of electrons in an STM measurement. For instance, the LEED

also observed for short-chain alkanethiols.43 It was noted as (p × √3), being derived from the (√3 × √3) structure and the periodicity p being variable. In the case of annealed BPT, p shows only a small deviation from the value of 7. The corresponding narrow distribution of stripe widths is most likely an effect of the long-time annealing. The progressive decrease in the amount of material with increasing annealing time or temperature is also supported by XPS measurements, which show a decrease in the effective thickness down to about 7 Å for a sample annealed at 100 °C for 15 h, demonstrating a consistency with the STM data within the accuracy of measurements. Moreover, relative intensities and shapes of the XPS signals show that annealing temperatures up to 100 °C do not lead to the decomposition of a BPT SAM, which is in agreement with the results of an earlier study.20 2.b. Vapor Deposition. After annealing, the samples prepared by vapor deposition for 2 h (see Figure 3a), which are the samples with the highest coverage, behave similar to the samples prepared from solution and demonstrate the same structures as presented in Figure 4. Therefore, we concentrate in this section on the temperature-induced structural evolution of the vapor deposited samples with a lower coverage (1 h, Figure 3c). Figure 5a shows a low magnification STM image of such a sample after annealing 15 h at 100 °C in ultrahigh vacuum. The scattered molecular patches with hexagonal 13909

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

Figure 5. (a) STM image of a BPT SAM from the vapor phase (1 h, see Figure 3c), annealed in vacuum at 100 °C; the insets show a hexagonal (2 × 2) structure of upright molecules. (b) Detail obtained from an area covered by flat lying molecules; the unit cell is drawn with a rectangle; the inset shows a possible molecular arrangement, where the red dots represent sulfur atoms arranged in p2gg symmetry; the positions of the phenyl rings do not follow this symmetry; (c) LEED pattern obtained at 26 eV; the sample was tilted so that the (0,0) spot appears at the top of the image, and a spot corresponding to the Au crystal is visible in the bottom-left part of the image. (d) Simulated LEED pattern for a molecular structure with a rectangular unit cell of size (2√3 × 8) and symmetry p2gg adsorbed on a Au(111) surface.

than in the densely packed (2 × 2) regions. Both arrangements are rectangular, with unit cells that slightly differ in size: (3√3 × 9) and (2√3 × 8). After annealing, only the (2√3 × 8) phase is preserved. The symmetry of the unit cell of this structure, as judged from the STM images, is p2, which differs from the p2gg symmetry observed by LEED, which we attribute to the fact that STM and LEED may probe different parts of the SAM, i.e., the aromatic ring and the sulfur−gold bond, respectively. We relate the slightly increased molecular distances of the densely packed BPT SAMs on Au(111) exhibiting the (2 × 2) structure in comparison to the typical (√3 × √3) structure of the densely packed alkane-thiol SAMs on Au(111) to the increased van der Waals radii of the studied molecules and their intrinsic rigidity.

patterns can originate from the arrangements of the sulfur atoms on the gold surface, while the topography of the STM images could be influenced by the two phenyl rings which are arranged in a way that they do not preserve all the symmetries of the structure formed with sulfur atoms. This is depicted schematically in the inset overlaid on Figure 5b, where a possible arrangement of BPT molecules with 4 molecules per unit cell is shown. The p2gg symmetry is lost, remaining just a unit cell belonging to the p2 group.

■

CONCLUSIONS Densely packed BPT SAMs on Au(111) obtained from a solution in DMF and by vapor deposition in ultrahigh vacuum exhibit a hexagonal (2 × 2) molecular superstructure of the nearly upright standing BPT molecules on Au(111). This molecular arrangement is unambiguously proven by both STM and LEED measurements. Upon annealing at 100 °C, the (2 × 2) structure evolves into the highly ordered (2 × 7√3) structure forming pinstripes along the ⟨1 −1 0⟩ directions. Inside these stripes, the hexagonal structure is still preserved. The XPS data of all studied samples demonstrate chemical integrity of the BPT molecules before and after annealing. Lower-density BPT SAMs obtained by vapor deposition in ultrahigh vacuum show a mixed structure comprising the already-mentioned (2 × 2) hexagonal structure and two arrangements formed by molecules that are more flat-lying

■

AUTHOR INFORMATION

Corresponding Author

*Andrey Turchanin: phone, +49 521 106 5376; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (Grants SFB613, TU149/2-1, and TU149/3-1) and the 13910

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911

Langmuir

Article

(31) Rhinow, D.; Büenfeld, M.; Weber, N. E.; Beyer, A.; Gölzhäuser, A.; Kühlbrandt, W.; Hampp, N.; Turchanin, A. Ultramicroscopy 2011, 111 (5), 342−349. (32) Rhinow, D.; Weber, N.-E.; Turchanin, A. J. Phys. Chem. C 2012, 116, 12295−12303. (33) Frey, S.; Rong, H. T.; Heister, K.; Yang, Y. J.; Buck, M.; Zharnikov, M. Langmuir 2002, 18 (8), 3142−3150. (34) Azzam, W.; Fuxen, C.; Birkner, A.; Rong, H. T.; Buck, M.; Wöll, C. Langmuir 2003, 19 (12), 4958−4968. (35) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wöll, C. Langmuir 2003, 19 (20), 8262−8270. (36) Cyganik, P.; Buck, M. J. Am. Chem. Soc. 2004, 126 (19), 5960− 5961. (37) Cyganik, P.; Buck, M.; Azzam, W.; Wöll, C. J. Phys. Chem. B 2004, 108 (16), 4989−4996. (38) Cyganik, P.; Buck, M.; Wilton-Ely, J. D. E. T.; Wöll, C. J. Phys. Chem. B 2005, 109 (21), 10902−10908. (39) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; WiltonEly, J.; Zharnikov, M.; Wöll, C. J. Am. Chem. Soc. 2006, 128 (42), 13868−13878. (40) Min, B. K.; Alemozafar, A. R.; Biener, M. M.; Biener, J.; Friend, C. M. Top. Catal. 2005, 36 (1−4), 77−90. (41) Poirier, G. E.; Pylant, E. D. Science 1996, 272 (5265), 1145− 1148. (42) Van Hove, M. A.; Weinberg, W. H.; Chan, C.-M. Low-Energy Electron Diffraction: Experiment, Theory and Surface Structure Determination; Springer-Verlag: Berlin, Germany, 1986. (43) Hagenström, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15 (7), 2435−2443.

German Bundesministerium für Bildung and Forschung (BMBF) for financial support.

■

REFERENCES

(1) Sagiv, J. J. Am. Chem. Soc. 1980, 102 (1), 92−98. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105 (13), 4481−4483. (3) Ulman, A., An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly. Academic Press: San Diego, CA, 1991. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65 (5−8), 151−256. (5) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13 (49), 11333−11365. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105 (4), 1103−1169. (7) Woodruff, D. P. Phys. Chem. Chem. Phys. 2008, 10 (48), 7211− 7221. (8) Kind, M.; Wöll, C. Prog. Surf. Sci. 2009, 84 (7−8), 230−278. (9) Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Chem. Soc. Rev. 2010, 39 (5), 1805−1834. (10) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111 (1), 321−335. (11) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113 (19), 7152− 7167. (12) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9 (11), 2974−2981. (13) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458 (1−3), 34−52. (14) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17 (8), 2408− 2415. (15) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105 (19), 4058−4061. (16) Korolkov, V. V.; Allen, S.; Roberts, C. J.; Tendler, S. J. B. J. Phys. Chem. C 2011, 115 (30), 14899−14906. (17) Turchanin, A.; Gölzhäuser, A. Prog. Surf. Sci. 2012, 87, 108−162. (18) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75 (16), 2401−2403. (19) Turchanin, A.; Schnietz, M.; El-Desawy, M.; Solak, H. H.; David, C.; Gölzhäuser, A. Small 2007, 3 (12), 2114−2119. (20) Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Langmuir 2009, 25 (13), 7342−7352. (21) Eck, W.; Küller, A.; Grunze, M.; Völkel, B.; Gölzhäuser, A. Adv. Mater. 2005, 17 (21), 2583−2587. (22) Turchanin, A.; El-Desawy, M.; Gölzhäuser, A. Appl. Phys. Lett. 2007, 90 (5), 053102. (23) Schnietz, M.; Turchanin, A.; Nottbohm, C. T.; Beyer, A.; Solak, H. H.; Hinze, P.; Weimann, T.; Gölzhäuser, A. Small 2009, 5 (23), 2651−2655. (24) Turchanin, A.; Beyer, A.; Nottbohm, C. T.; Zhang, X. H.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Gölzhäuser, A. Adv. Mater. 2009, 21 (12), 1233−1237. (25) Beyer, A.; Turchanin, A.; Nottbohm, C. T.; Mellech, N.; Schnietz, M.; Gölzhäuser, A. J. Vac. Sci. Technol. B 2010, 28 (6), C6D5−C6D10. (26) Nottbohm, C. T.; Sopher, R.; Heilemann, M.; Sauer, M.; Gölzhäuser, A. J. Biotechnol. 2010, 149 (4), 267−271. (27) Nottbohm, C. T.; Wiegmann, S.; Beyer, A.; Gölzhäuser, A. Phys. Chem. Chem. Phys. 2010, 12 (17), 4324−4328. (28) Zheng, Z.; Nottbohm, C. T.; Turchanin, A.; Muzik, H.; Beyer, A.; Heilemann, M.; Sauer, M.; Gölzhäuser, A. Angew. Chem., Int. Ed. 2010, 49 (45), 8493−8497. (29) Amin, I.; Steenackers, M.; Zhang, N.; Schubel, R.; Beyer, A.; Gölzhäuser, A.; Jordan, R. Small 2011, 7 (5), 683−687. (30) Nottbohm, C. T.; Turchanin, A.; Beyer, A.; Stosch, R.; Gölzhäuser, A. Small 2011, 7 (7), 874−883. 13911

dx.doi.org/10.1021/la302821w | Langmuir 2012, 28, 13905−13911