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Interaction of Alkanethiols with Nanoporous Cluster-Assembled Au Films F. Bisio,*,† M. Prato,‡ E. Barborini,§ and M. Canepa|| †
CNR-SPIN, C. so Perrone 24, I-16152 Genova, Italy Istituto Nazionale di Fisica Nucleare, Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy § Tethis S. p. A., Via Franco Russoli 3, 20143 Milan, Italy CNISM, Sede Consorziata di Genova and Dipartimento di Fisica, Universita di Genova, Via Dodecaneso 33, I-16146 Genova, Italy
)
‡
ABSTRACT: This article presents a study of the interaction of octadecanethiol molecules (C18) with nanoporous cluster-assembled gold films under a liquid environment based on a combined spectroscopic ellipsometry and X-ray photoelectron spectroscopy investigation. By comparing the optical response, following the deposition of C18, of cluster-assembled films with varying degrees of porosity with that of flat surfaces and by resolving the corresponding features of the moleculeAu bond, we have been able to define the conditions that either favor molecular in-depth diffusion into the pores or promote the formation of a molecular selfassembled monolayer (SAM) restricted to the film surface. In the presence of abundant open pores, C18 molecules strongly diffuse within the film interior and bind to the pore walls, whereas in the presence of porous films with less abundant open pores we have observed that the molecules tend to remain confined to the surface region, adopting a SAM-like configuration.
’ INTRODUCTION Within the broad field of self-assembled monolayers (SAMs), the thiol/metal interface combination (particularly alkanethiol SAMs on gold) is one of the most relevant and investigated systems.14 The interplay between the covalent SAu bond and the interchain van der Waals forces ensures the stabilization of high-quality, close-packed layers with controlled thickness (by varying the alkyl chain length) and variable outer functionality (by varying the end group). These molecular layers can be exploited as linkers for other materials, notably organic and biological molecules.511 Functionalized thiols are well-established ingredients of supramolecular assemblies for the bottomup design of devices for sensing, recognition, and molecular electronics, to name a few.4 Over time, the vast amount of research on thiolate SAMs on gold has evolved along two major directions. On one hand, many studies have addressed the interaction of increasingly complex molecules with ideal or quasi-ideal substrates.1218 On the other hand, the adsorption of simple thiols on more complex substrates has attracted increasing interest. Nanoporous systems, characterized by high surface-to-volume ratios and a large density of “defects”, stand out as remarkably interesting substrates for which the thiolgold binding energy can be even stronger than at terraces.19,20 Nanoporous gold films can be prepared by various methods ranging from electrochemical selective dealloying19 or etching21 to the aggregation (chemically or physically r 2011 American Chemical Society
driven) of preformed 3D clusters. The morphological parameters of the nanoporous matrix and the molecular absorption geometries clearly depend on the fabrication methods, thus providing a large flexibility in designing thiol/metal structures. The so-called “black-gold”, consisting of columnar-like overlayers a few hundred of nanometers thick with a large density of pores, claims improved resistance to SAM degradation and enhanced stability in the electrochemical environment.21,22 A different class of nanoporous gold systems is represented by intrinsically granular samples such as those formed by 3D nanoparticle assemblies. This kind of film can be obtained chemically or physically by the layer-by-layer growth of suitably functionalized clusters23 or by the molecular-beam-assisted deposition of preformed clusters. The latter is the case for the so-called cluster-assembled films whose granular structure and nanoscale porosity provide an enhancement of the specific area, useful for sensing purposes,24 and whose surface morphology well mimicks natural systems such as the extracellular matrix structure.25 In a recent study, we applied spectroscopic ellipsometry (SE) to the study of cluster-assembled gold films. We were able to obtain insights into the interplay of the single-cluster functionalities and their collective behavior in determining their optical Received: February 1, 2011 Revised: May 10, 2011 Published: May 31, 2011 8371
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properties26 and into the in-depth penetration of simple liquids such as ethanol inside a nanoporous matrix.27 In this article, by combining SE performed under a liquid environment and X-ray photoelectron spectroscopy (XPS) on dried samples, we exploit the interaction of alkanethiol molecules (octadecanethiol, C18) with the uncapped nanoparticles of cluster-assembled gold films in order to estimate the penetration depth of molecules within the porous structure of the film, thus expanding and corroborating the results of recent papers.27 By comparing the results obtained for cluster-assembled films with different degrees of porosity with data gathered on flat surfaces and by resolving the corresponding features of the moleculeAu bond, we have been able to define the conditions that favor either molecular in-depth diffusion across the pores or the formation of a molecular selfassembled monolayer (SAM) typically restricted to the system’s surface.
’ MATERIALS AND METHODS Fabrication and Characterization of Cluster-Assembled Films. Cluster-assembled Au films ∼40 nm thick were deposited by
Tethis Spa onto 15 15 mm2 silicon substrates by means of a supersonic cluster beam deposition apparatus equipped with a pulsed microplasma cluster source (PMCS). The operating principle of a PMCS and the deposition of Au/Si cluster-assembled films have been thoroughly described in previous publications.26,28 The films have been characterized by spectroscopic ellipsometry (SE) measurements performed on a J. A. Woollam M-2000 rotating-compensator ellipsometer operating in the 245725 nm spectral range at an angle of incidence of 65°. The output of the standard SE measurements is the ratio F of the p-polarized and s-polarized complex Fresnel reflection coefficients rpp and rss, routinely expressed in terms of angles Ψ and Δ as2931 F¼
rpp ¼ tan ΨeiΔ rss
ð1Þ
Information on the optical and morphological properties of the samples can be obtained by the SE measurements with simulations performed using the WVASE32 code provided by the instrument supplier. In the simulations, the films are modeled as a stack of layers, characterized by their complex dielectric function ε = (ε1,ε2) and their effective thickness d to be optimized by means of best-fit procedures performed with the aid of the WVASE32 program. The optical model of the nanoporous Au samples, returning the film thickness, porosity, and mean cluster size, has been presented and discussed in detail in previous work.26,27 Deposition of Alkanethiols. 1-Octadecanethiol molecules (C18, Aldrich, purity 98%) were deposited from an ethanol (EtOH, Fluka, purity >99.8%) solution (0.1 mM). Dry nitrogen gas was bubbled in the solution in order to favor the removal of dissolved oxygen. The deposition was performed in a homemade Teflon cell designed to allow high-precision in situ SE measurements.32 The samples were first characterized by SE in air and then inserted into the empty deposition cell. Pure ethanol was subsequently introduced into the cell, and the corresponding SE spectrum was recorded. C18 molecules, previously dissolved in 5 mL of ethanol, were then quickly admitted to the cell, eventually reaching a 0.1 mM concentration. The SE spectrum was monitored as a function of the incubation time (100 min. typically). The effect of the molecular interaction with the substrate was considered by looking at the so-called SE difference spectra (in brief, the δ spectra)3336 (i.e., the difference between the Ψ(Λ) and Δ(Λ) curves measured in ethanol after and before the deposition of the C18 molecules, defined by eq 2) δΔ ¼ ΔðΛÞAuþC18 ΔðΛÞAu
ð2Þ
Figure 1. Schematics of the morphology and degree of interaction of differently aged cluster-assembled Au films with ethanol solvent, as found in refs 26 and 27. Left panel: a freshly deposited Au film. Right panel: an aged film. For each film, the thickness and density (F) of the layers are shown. The degree of pore filling in the liquid EtOH environment is expressed by the size of the shaded areas.27 The percentage of EtOH in the film pores is shown. The Au cluster mean sizes are about 67 and 810 nm for fresh and aged films, respectively.26 δΨ ¼ ΨðΛÞAuþC18 ΨðΛÞAu
ð3Þ
δ spectra have long been employed as a method for assessing surface coverage in the monolayer and submonolayer regimes.3337 Our spectra have been analyzed following an original approach described in detail in ref 36 and successfully applied to several thiolate films on gold.36,3840 After the C18 deposition and in situ measurements, the Au films were extracted from the deposition cell, thoroughly rinsed in pure solvent, dried under a flow of dry nitrogen, and then quickly placed under ultrahigh vacuum conditions to perform XPS measurements (PHI ESCA 5600 system with a monochromatized Al source). The irradiation time was optimized to minimize the damage while maintaining a good enough resolution and signal-to-noise ratio to allow a reliable deconvolution of the spectral subcomponents. The exposure time to X-rays has been kept identical for all of the samples under scrutiny in order to avoid any systematic difference due to beam damage.
’ RESULTS AND DISCUSSION Morphology of Nanoporous Au Films. To discriminate the effect of the surface roughness from the sample porosity on the interaction with C18 molecules, we will consider two sets of nanoporous Au films characterized by different nanoscale morphologies and nanopore accessibilities. The sample classes are either cluster-assembled films in a freshly deposited state (a few tens of hours after supersonic cluster beam deposition) or identically prepared systems that have undergone approximately 80 days of aging at room temperature prior to experiments. Samples were aged under a dry atmosphere in a protected environment and did not undergo any cleaning procedure prior to experiments. No relevant increase in the contamination level of the samples during the aging process could be detected. The morphology of the two sample classes (film thickness and degree of porosity) and their interaction with the EtOH solvent have been thoroughly described in previous works.26,27 Representative AFM images can be found as Supporting Information to ref 27. Their main features are pictorially represented in Figure 1 and synthetically described below. The first set of Au films, henceforth referred to as “fresh” or FNP, can be approximated (Figure 1, left) as a stack of two nanostructured layers, each one a few tens of nanometers thick. 8372
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Figure 2. Representative in situ δΔ and δΨ spectra recorded after the deposition of C18 on aged (red symbols) and fresh cluster-assembled films (gray symbols), compared with analogous data obtained for an optically thick (200 nm), morphologically flat Au sample (Arrandee, blue lines). The angle of incidence was 65° for all samples.
The two layers are composed of stacked Au clusters with a similar mean size of 67 nm, but they exhibit different degrees of porosity. The outer layer has a porosity of ∼80%, and the inner layer has a porosity of ∼20%, as we found in a previous study.26 This two-layer morphology represents an effective simplification compared to more complex, likely graded structures. The second set of Au films, referred to as “aged” or ANP, exhibits (Figure 1, right) significant differences with respect to the first set, as thoroughly described in ref 27. The storage at room temperature indeed causes a partial annealing of the samples, which consequently exhibit a thinner and more compact outer layer, a slightly denser inner layer, and a larger mean Au cluster size that is estimated to fall in the range of 810 nm. The different morphology of the two sets of samples plays a major role in the interaction with liquids.27 When immersed in liquid EtOH, fresh samples allow the solvent to fill the topmost-layer voids (up to 85%) almost completely and access a significant fraction of inner voids, as pictorially shown in Figure 1 by the shaded areas. In contrast, the topmost layer of aged samples apparently acts as an impermeable barrier against the liquid. As a consequence, the inner-layer pores remain dry. Interaction with Alkanethiols. When C18 molecules are admitted in ethanol, the Ψ and Δ spectra of all of the Au samples exhibit small yet reproducible variations that are best appreciated by looking at the corresponding δ spectra. Representative in situ δΔ and δΨ spectra, recorded after the deposition of C18 on ANP (red symbols) and FNP films (gray symbols), are compared in Figure 2 with analogous data obtained for an optically thick (200 nm), morphologically flat Au sample (Arrandee, blue lines). A thorough discussion of the data collected on flat films is reported in ref 36. Some defined trends can be observed when moving from the flat surface case toward the ANP and FNP samples. The overall shape of the ANP δΔ curve resembles that found for flat films, apart from two interesting details: an almost consistent upward shift of ∼0.5° and a red shift of the relative maximum beyond 500 nm. Note that the spectrum always remains negatively valued. For FNP samples, the δΔ curve displays a more pronounced maximum that appears approximately at the same position as in ANP films. In addition, for a large portion of the δΔ curve the δ value is positive. The
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latter spectral feature, reproducibly observed on a number of FNP films, is particularly relevant because in experiments on many different thiolate compounds on gold flat films36,3841 negative values of δΔ were systematically observed. Analogously, the δΨ spectra for the flat and ANP samples present some overall similarities. A close look reveals that their zero crossing indeed occurs at larger values of Λ, in agreement with the red shift of the δΔ maximum. The fresh-case spectrum clearly stands out by featuring larger positive and negative values. In the case of flat surfaces, the difference spectra bear, both in shape and magnitude, the optical fingerprint of the formation of a compact, 23-nm-thick alkanethiol SAM on the gold surface, which is further characterized by the formation of an optically absorbing AuS interface layer, as has been thoroughly discussed in previous work.36 In this respect, the overall resemblance to the ANP data suggests that the two sets of systems share a rather common molecular configuration. The surfaces of ANP films, despite presenting a nanoscale roughness, are likely compact enough to prevent significant molecular penetration into the bulk porous structure, therefore apparently supporting SAM-like C18 adsorption geometry. These findings are fully consistent with the results found in the case of the interaction of NP films with pure solvent.27 Judging from the difference spectra, this situation radically changes for FNP samples and clearly deserves a more detailed analysis. We therefore attempted to reproduce the experimental FNP δ spectra by means of simulations, assuming as a starting point that the optical model is capable of describing the interaction between the nanoporous film and ethanol.27 The model is schematically described in Figure 1. For the purpose of model simplification, the bulk C18 film (i.e., the alkyl chains) has been treated as a transparent medium with a nondispersive refractive index of n = 1.475.35,36 From the common starting point, different physical schemes of the molecule/film interaction can be envisaged, each giving rise to different δΔ and δΨ spectra. The simulations are compared to the experimental data in Figure 3. The simplest possible model, where C18 molecules were naively thought to form a compact SAM on top of the substrate, without penetration, provided simulated curves (not shown) with a very poor fit to the experimental data and was consequently discarded. In a more realistic scheme, C18 molecules were allowed to diffuse within the pores. In the model, the void fractions within both Au layers were kept fixed and a variable fraction of ethanol was allowed to be “replaced” by C18. Such a situation was modeled by modifying the dielectric function of the void/ethanol mixture to mimick the presence of C18 (model b in Figure 3). This was achieved by employing a “filler” effective medium whose optical dielectric function was a linear combination of those of EtOH and C18. In an initial version, the model did not account for optical effects related to the formation of thiolate interfaces. The best fit to the experimental δΔ and δΨ spectra (Figure 3a, blue lines) corresponds to the case where slightly more than 50% of the ethanol, in the topmost layer only, was replaced by C18. The comparison between simulations and experiments indeed shows rather poor agreement. The agreement drastically improved by explicitly taking into account the optical effect of the formation of a strong SAu chemical bond.35,36,42,43 On flat films, the SAu bond was modeled by introducing an interface layer bearing a Drude-like absorption band of growing magnitude moving from the visible toward the NIR spectral region.36 Although a first principles model for this 8373
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Figure 3. (a) Gray symbols: experimental difference spectra δΔ and δΨ in ethanol following the deposition of C18 on the FNP film. Simulated δΔ and δΨ spectra calculated according to model b (blue lines) and model c (red lines). (b, c) Schematic of optical-layer models for the interaction of C18 molecules with the FNP film. (d) Imaginary part of the best-fit dielectric function of Au clusters in the FNP film before (blue line) and after the deposition of C18 molecules. The ε2 of bulk Au is reported for comparison (black symbols). See the text for details.
behavior has not yet been developed, it seemed fair to consider the interface absorption properties in relation to a modification of the nearly free electron behavior (in particular, a decrease in the scattering length) related to nanoscale morphological modifications.36,44 In the case of films made of nanoparticles, a decrease in the scattering length could be implemented in our model by increasing the so-called Drude tail (i.e., the freeelectron scattering contribution to the dielectric constant of the cluster Au εNP, pictorially represented in Figure 3c by the red cluster edges).26 Under these assumptions, the best agreement between experiments and simulation (red lines in Figure 3a) is found by assuming that C18 roughly replaces 80% of EtOH in the topmost Au layer, leaving the inner layer unaffected, and that the Drude contribution to the Au cluster dielectric function has increased by ∼25% upon C18 deposition (Figure 3d). The corresponding calculated δΔ and δΨ spectra reproduce important experimental features. In particular, the model is able to predict the position and the positive value of the δΔ maximum and the correct order of magnitude of δΨ, including its zerocrossing wavelength. The good overall agreement between experimental data and simulations suggests that this model, despite its relative simplicity in relation to the system complexity, might provide a fair representation of the molecule/nanopore interaction. The results obtained for the FNP may also help to refine the understanding of ANP spectra. It indeed seems fair to ascribe the above-mentioned upward δΔ shift and the blue shift of the δΔ maximum to a limited molecular penetration into the film and interaction with pores. AuS Bond in Cluster-Assembled Films. To investigate more deeply the C18/ Au interaction in nanoporous systems, the details of the SAu bond can be selectively addressed by photoelectron spectroscopy. Figure 4 shows typical XPS spectra of the S 2p region for FNP (top) and ANP films (bottom) following the deposition of C18. The data, shown after the
Figure 4. (Top) S 2p XPS spectra after C18 deposition on the FNP films. (Bottom) S 2p XPS spectra after C18 deposition on the ANP films. The data are shown after subtracting a Shirley-type background.
subtraction of a Shirley-type background, have been treated according to procedures developed in previous papers on related systems.45,46 The spectra were fitted using one Voigt-shaped doublet for each chemically shifted contribution, having a fixed 1.2 eV spinorbit 8374
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Langmuir splitting and a 2:1 branching ratio between S 2p3/2 and 2p1/2 components. The peak width and shape have been kept identical for all contributions in order to minimize the number of free parameters of the fit. The peaks have been labeled according to the binding energy (BE) of the S 2p3/2 component (with a (0.1 eV estimated uncertainty), calibrated by reference to the Au 4f7/2 peak at 84.0 eV. In the ANP case (Figure 4, bottom), the best fit to the experimental data required two doublets. The spectrum was dominated by an intense peak (Sth, ∼80% of the total S 2p intensity) at 162 eV BE, with a minor contribution (Sun, ∼15% of the total S 2p intensity) at 163.3 eV. A very small feature can indeed be glimpsed in the 167170 eV region that could eventually be fitted with another doublet, Sox, at ∼168.5 eV. In the FNP case, the best fit to the experimental data required four S 2p doublets. Starting from the lowest binding energy, the Sth peak (161.9 eV BE, about 55% of the total S 2p intensity) was followed by a smaller Sun contribution (163.3 eV, ∼1520% of the total S 2p intensity). In this case, at higher BE, a relatively intense feature (∼2530% of the total S 2p intensity) could be observed. The feature is broad, and its fitting required two Sox doublets at approximately 168 and 169 eV whose energy was found to exhibit slight sample-to-sample variations on the order of fractions of electronvolts (typically 0.4 eV). Models for the Molecule/Substrate Interaction. The SE results suggest a possible scheme for the interaction of alkanethiols with the cluster-assembled films. In the ANP case, the film morphology has evolved to form a substantially impenetrable surface barrier for molecules, as shown also by the response to liquid EtOH.27 The aged films are therefore short on accessible pores and have a relatively shallow surface roughness. The C18 molecules remain mostly confined to the film surface, thereby adopting a structure that, from an optical point of view, resembles that of a SAM on flat Au surfaces.36 In the FNP case, where abundant open pores (i.e., pores accessible from the outside environment) are available, alkanethiol molecules diffuse within the whole outer layer, almost completely displacing the solvent and binding to the gold cluster surface throughout the layer depth. The XPS spectra complement such a molecule/surface interaction scheme. The presence, in both FNP and ANP spectra, of the Sth doublet at 161.9 eV demonstrates the formation of a thiolate-bound species.42,45,4749 This species, the only one encountered upon the deposition of well-organized alkanethiolfunctionalized SAMs on flat Au surfaces,50 is by far dominant in the ANP case whereas it accounts only for roughly half of the total S 2p signal in the FNP case. The different weights of the Sth moiety in the fresh and aged spectra are counterbalanced by the different relative intensities of the Sox peaks in the ∼167170 eV range. In FNP films, such features account for almost 30% of the total signal but less than 10% in the aged case. Although there is no doubt that such highBE peaks belong to oxidized S moieties,50,51 it is useful to discuss their possible origin. First, the fact that the sulfonate signal observed in the FNP case roughly compensates for the corresponding missing fraction of the thiolate moiety suggests the catalytic action of the Au clusters in the oxidation of the thiol end group, implying that the oxidation agent must be close to the cluster surface. Two mechanisms can be envisaged that may account for the different intensities of the sulfonate peak in fresh and aged samples, according to whether the oxidation agent (likely oxygen
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in the present case) comes from the atmosphere or from the inner porous structure itself. The first mechanism entails oxygen reaching the cluster surface when samples are removed from ethanol and prior to their introduction into the XPS chamber. On the ANP film surface, similar to what was observed on flat surfaces and in the case of black gold,22 this process is counteracted by the effective barrier created by the compactness of the SAM. The larger degree of oxidation of FNP films would therefore have to be ascribed to the less compact structure of the C18 layer coating the Au clusters, which would allow for the penetration of oxygen through the SAM and even a better detection of oxidized species. The second mechanism is more intriguing and involves the oxidation of sulfur by the residual atmospheric gas residing in the film pores, especially small inner pores, and trapped there after immersion in solution and the subsequent deposition of C18. In this second case, the larger degree of oxidation in the FNP films must be ascribed to a large density of accessible pores present in the film interior that could release the gas, which then diffuses toward the C18 layer. In this respect, the Au clusters can be thought of as catalysts for the oxidation of the SH group that takes place provided that atmospheric oxygen (i.e., the oxidizing agent) is available near the Au surface following the dissociation of the thiol group. In the case of aged samples, the same morphology that prevents the diffusion of solvent and molecules toward the film interior acts reversely as a barrier, preventing trapped gas from reaching the C18 layer. We noted that on FNP two oxidized species were necessary to fit the data. However, both the low signal-to-noise ratio and the sample-to-sample spread advise against the interpretation of finer spectral details. The thiolate-induced peak is accompanied in both the FNP and ANP cases by the Sun peak at ∼163.2 eV BE, although more substantially in the FNP case. This peak is commonly ascribed to “unbound” sulfur due to, for example, unreacted SH groups or even disulfide species produced by radiation damage or chemical reaction.22,47,50,52 On flat surfaces this peak, usually less prominent, is associated with a somewhat disordered absorption geometry that is encountered in several situations, for example, with poor solvent or insufficient rinsing.47,50 The latter factor could play a role in our samples, whose morphology on the one hand enhances the probability of “intact” molecules being trapped in the pores and on the other hand makes the rinsing off more difficult.
’ CONCLUSIONS We have performed a combined spectroellipsometric and XPS investigation of the interaction of 1-octadecanethiol molecules with nanoporous cluster-assembled Au films in a liquid environment (ethanol). We observed that the surface morphology of the nanoporous films is crucial in determining the degree of interaction of the films with solvent molcules in the liquid phase. Such sensitivity is reflected in the different configurations that C18 molecules adopt when deposited on morphologically different substrates. Freshly deposited Au films exhibit an abundance of pores that are easily filled by the solvent molecules, indicating a relatively high permeability of the system. Under these conditions, C18 molecules diffuse deeply within the film, subsequently binding to the cluster surface. Cluster-assembled films, for which a partial cluster coalescence was allowed by letting the systems age for 80 days at room 8375
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Langmuir temperature, showed very different responses to the presence of foreign molecules. C18 molecules adsorbed at the corrugated film surface adopt a configuration strongly resembling that of a SAM on a flat surface. Apparently, this surface SAM, which is relatively compact, acts as an impermeable barrier against the diffusion of solvent molecules toward the film’s interior. The different adsorption properties of C18 on fresh and aged samples were also found to affect the stability against oxidizing agents. Fresh films indeed show a sizable XPS signal coming from oxidixed sulfur species. Two possible mechanisms have been proposed to account for this behavior. In the first one, the loose compactness of the C18 layer coating the surface Au clusters would allow for the penetration of oxygen through the SAM from the atmosphere. In the second mechanism, the large density of accessible pores present in the film’s interior could lead to the release of trapped oxidation agents, which then diffuse in ethanol toward the C18 layer.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank M. Palombo for assistance during measurements, O. Cavalleri for useful discussions, R. Parodi and INFN-GE for providing access to XPS facilities, and F. Parodi for a critical reading of this article. Financial support from the University of Genova and Fondazione Carige is gratefully acknowledged. ’ REFERENCES (1) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (3) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (5) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (6) Liedberg, B.; Tengvall, P. Langmuir 1995, 11, 3821. (7) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (8) Pflaum, J.; Bracco, G.; Schreiber, F.; Colorado, R.; Shmakova, O. E.; Lee, T. R.; Scoles, G.; Kahn, A. Surf. Sci. 2002, 498, 89. (9) Vericat, C.; Vela, M. E.; Salvarezza, R. C. Phys. Chem. Chem. Phys. 2005, 7, 3258. (10) Liang, J.; Rosa, L. G.; Scoles, G. J. Phys. Chem. C 2007, 111, 17275. (11) Ekblad, T.; Liedberg, B. Curr. Opin. Colloid Interface Sci. 2010, 15, 499. (12) Maartensson, J.; Arwin, H. Langmuir 1995, 11, 963. (13) Gupta, P.; Loos, K.; Korniakov, A.; Spagnoli, C.; Cowman, M.; Ulman, A. Angew. Chem., Int. Ed. 2004, 43, 520. (14) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. Macromolecules 1999, 32, 4343. (15) Tinazli, A.; Tang, J.; Valiokas, R.; Picuric, S.; Lata, S.; Piehler, J.; Liedberg, B.; Tampe, R. Chem.—Eur. J. 2005, 11, 5249. (16) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Floreano, L.; Morgante, A.; Canepa, M.; Rolandi, R. Phys. Chem. Chem. Phys. 2004, 6, 4042. (17) Richter, L. J.; Yang, C. S.-C.; Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547. (18) Pasquali, L.; Terzi, F.; Zanardi, C.; Seeber, R.; Paolicelli, G.; Mahne, N.; Nannarone, S. J. Phys.: Condens. Matter 2007, 19, 305020. (19) Huang, J. F.; Sun, I. W. Adv. Funct. Mater. 2005, 15, 989.
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