Triphenylene Silanes for Direct Surface Anchoring in Binary Mixed

Article pubs.acs.org/Langmuir

Triphenylene Silanes for Direct Surface Anchoring in Binary Mixed Self-Assembled Monolayers Markus Mansueto,†,‡ Sven Sauer,† Martin Butschies,† Martin Kaller,† Angelika Baro,† Rebecca Woerner,§ Nis Hauke Hansen,§ Guenter Tovar,*,‡,∥ Jens Pflaum,*,§ and Sabine Laschat*,† †

Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany Institut für Grenzflächenverfahrenstechnik IGVT, Universität Stuttgart, Nobelstr. 12, D-70569 Stuttgart, Germany § Lehrstuhl für Experimentalphysik VI, Universität Würzburg und Bayerisches Zentrum für Angewandte Energieforschung e.V. (ZAE), Am Hubland, D-97074 Würzburg, Germany ∥ Fraunhofer-Institut für Grenzflächen- und Bioverfahrenstechnik IGB, Nobelstr. 12, D-70569 Stuttgart, Germany ‡

S Supporting Information *

ABSTRACT: New triphenylene-based silanes 2-(ω-(chlorodimethylsilyl)-n-alkyl)-3,6,7,10,11-penta-m-alkoxytriphenylene 4 (Tm-Cn) with n = 8 or 9 and m = 7, 8, 9, 10, or 11 were synthesized, and their self-assembly behavior in the liquid state and at glass and silicon oxide surfaces was investigated. The mesomorphic properties of triphenylene silanes 4 (Tm-Cn) and their precursors 3 (Tm-Cn) were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. From the smallangle X-ray scattering (SAXS) regime, a preferential discotic lamellar mesophase can be deduced, and wide-angle X-ray scattering (WAXS) highlights the liquid-like characteristics of the alkyl side chains. To transfer these bulk structural properties to thin films, self-assembled monolayers (SAMs) were obtained by adsorption from solution and characterized by water contact angle measurements, null ellipsometry, and atomic force microscopy (AFM). Employing the concentration as an additional degree of freedom, binary SAMs of 2-(ω-(chlorodimethylsilyl)-undecyl)3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11) were coassembled with chlorodecyldimethylsilane or chlorodimethyloctadecylsilane, and their capability as model systems for organic templating was evaluated. The structure of the resulting binary mixed SAMs was analyzed by water contact angle measurements, null ellipsometry, and X-ray reflectivity (XRR) in combination with theoretical modeling by a multidimensional Parratt algorithm and AFM. The composition dependence of film thickness and roughness can be explained by a microscopic model including the steric hindrance of the respective molecular constituents.

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INTRODUCTION For the successful application of discotic liquid crystals in optoelectronic devices such as high-resolution laser printers, gas sensors, organic light emitting diodes (OLEDs), organic solar cells, organic field effect transistors (OFETs), it is of outmost importance to control their alignment in the liquid crystalline (LC) phase with special emphasis on substrate effects.1 For example, organic field effect transistors require a planar (i.e., edge on) alignment with the columnar directors oriented parallel to the substrate, whereas a homeotropic (i.e., face on) alignment with the columnar directors oriented perpendicular to the substrate is required for photovoltaic devices and light emitting diodes.2 Therefore, a lot of effort has been put into working on the alignment control of discotic liquid crystals by Langmuir− Blodgett techniques,3 mechanical alignment-employing, friction-transferred poly(tetrafluoroethylene) (PTFE) layers,4 zone-casting of solutions on untreated glass,5 surface-assisted photoalignment control,6 thermal annealing,7 electrical,8 or magnetic alignment.9 © 2012 American Chemical Society

In order to obtain stable systems which are ordered laterally up to the micrometer scale, i.e., the length scale relevant for, e.g., charge carrier transport in thin film devices, self-assembled monolayers (SAMs) constitute an attractive approach: utilizing chemisorption of the monolayer to a substrate such as a metal or an oxide layer surface.10−12 An extensively studied system is the grafting of thiols on gold.13 It has been recently shown by Piot et al.14 that the flexibility and length of the spacer determines the density of packing and stability of the SAMs on Au(111) for hexa-peri-hexabenzocoronenes which are tethered to thioesters or disulfides.14 SAMs derived from siloxanes on glass differ not only by the type of binding from thiolate SAMs on gold, but also with respect to layer formation, packing motifs, and mobilities of the adsorbed molecules on the respective substrates.13,15 Received: February 24, 2012 Revised: April 27, 2012 Published: April 28, 2012 8399

dx.doi.org/10.1021/la300775n | Langmuir 2012, 28, 8399−8407

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be discussed and evaluated in the context of their utilization as templates in thin film application.

Organosilane-SAMs on Si/SiO2 have very interesting properties in relation to their use in molecular electronics, because they can be chemically grafted to the surface and offer exceptionally high chemical and physical stability and are compatible with Si-based technology.16,17 The possibility to tune specific properties of the surface-grafted molecules, for example, the electrical conduction of aromatic conjugated molecules, makes these substances interesting compounds for an application in hybrid Si-technology.18−20 Furthermore, it has been demonstrated that the use of self-assembled monolayers as dielectrics enables the realization of efficient organic thin-film devices.21 With operational voltages of approximately 1 V and power losses in the nW range, these innovative device architectures pave the way toward portable, all-organic electronics. The packing density of the silane-SAMs can be adjusted in a controlled manner by the use of alkyltrichlorosilanes or alkylchlorodimethylsilanes,13,22 branched tethers, or binary systems.23,24 Alkyltrichlorosilanes can build dense films of organized, cross-linked alkyl chains, whereas monochlorosilanes cannot cross-link via siloxane bonds and suffer from steric hindrance; thus, the resulting film is less dense, but is homogeneous in composition.25−27 In a mixture of aromatic and alkyl silanes in binary SAMs, it is possible to adjust favored surface properties by controlling the relative amount of individual species and thereby minimize the level of defects.17,25 Unfortunately, there are only a few examples of these systems in the literature.17,25,28,29 The resulting films can differ in their surface composition, i.e., a uniformly blended film or phase segregation of the molecules to nanoislands could be observed. The literature stated that an alkyl linker between the surface and the aromatic core improved the packing density and surface coverage of the film and the mobility of the molecules,25,28 because it was shown by Desbief and coauthors17 that grafting to the surface is a two-step process. The first step is the chemisorption of the headgroup and the second is a densification step, in which the reaction rate depends on the interactions between the aromatic rings and is faster for short alkyl linkers. Furthermore, the surface composition and water contact angle of the film showed a strong dependence on the aromatic silanes.25 Here, we address a new class of triphenylene-based molecules which are designed to combine liquid crystalline properties with surface self-assembly behavior on oxide surfaces. Therefore, a series of alkyl ether triphenylenes was synthesized where one alkyl ether side chain was substituted by a alkyl chlorodimethylsilane tether with variation of its alkyl chain length. The triphenylenes were chosen as model systems due to their tendency to self-assemble into columns leading to a pronounced spatially anisotropic hole carrier mobility;30 the possibility of self-healing of defects in the LC phase and the chemical grafting to the SiO2 surface should improve and stabilize planar alignment, thus making them ideally suited for silicon-based molecular thin film devices. The self-assembly behavior of these compounds was investigated, both in the liquid state and when adsorbed at solid substrates. A triphenylene derivative with appropriate self-assembling behavior was chosen for coassembly at silicon oxide surfaces with long-chain alkyl silanes, and the structure of the resulting binary mixed SAMs was analyzed. Such binary mixed SAMs may provide an additional degree of freedom to control the growth dynamics and the film structure of liquid crystalline triphenylenes. The resulting morphological characteristics will

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EXPERIMENTAL SECTION

General. Contact angles were obtained on a DataPhysics OCA 40 with water four times on three different spots in static measurements and were evaluated with the SCA 20 software. Ellipsometric data were determined by null ellipsometry on a Sentech SE 800 with a measurement range from 280 to 850 nm at three different spots and analyzed with the Spectra Ray software. The optical constants were the following: nSAM = 1.7, nSiO2 = 1.46, nSi = 3.87, and kSi = 0.02. The extinction of the SAM and the SiO2 is insignificant because of the large band gap of more than 8 eV. X-ray reflectivitiy (XRR) measurements were performed on a GE Inspection Technologies XRD3003 employing Cu Kα radiation (wavelength as above). Tapping-mode atomic force microscopy (AFM) measurements were realized on a Veeco Dimension Icon under ambient conditions using an n-doped Si tip operating at a resonance frequency of 287 kHz. Si wafers with a native silicon oxide surface were obtained from the Institut für Mikroelektronik Stuttgart. Commercial reagents were used without further purification. All reactions were carried out using standard Schlenk techniques unless otherwise noted. Hydroxypentaalkyloxytriphenylenes 1a−e and ω-bromoalkenes 2a−b were prepared according to the literature procedure;31,32 the synthesis and structural data of all other compounds can be found in the Supporting Information. Preparation of SAMs. The silicon wafers were cleaned by sonication in acetone (10 min) and isopropanol (10 min) and dried under a flow of N2 [note: commonly, the literature reports more complex cleaning processes of silicon wafers;13,33 however, our simple cleaning procedure yields comparable water contact angles (104°) and thicknesses (ellipsometry: 2.24 nm; XRR: 2.3 nm) for octadecyltrichlorosilane-SAMs (OTS-SAMs) as indicators of similar surface quality]. The substrates were transferred in Schlenk tubes using N2 as inert gas which were filled with a solution of trialkylchlorosilanetethered triphenylene 4 (Tm-Cn) (4.5 μmol) in CH2Cl2 (3 mL) and kept in solution at room temperature overnight. Afterward, the wafers were cleaned through sonication in CH2Cl2 (10 min) to eliminate physisorbed silanes. For binary SAMs, the silanization solution consisted of a mixture of trialkylchlorosilane-tethered triphenylene 4 (Tm-Cn) and chlorodecyldimethylsilane 5 (C10) or chlorodimethyloctadecylsilane 6 (C18) (Table 1) in different mole fractions.

Table 1. Declaration of the Nomenclature Used in the Publication

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RESULTS AND DISCUSSION For a better approach to the molecules used in the discussion, the nomenclature elucidated in Table 1 is used. Synthesis of Triphenylenes 3 (Tm-Cm) and 4 (Tm-Cm). The synthesis of dialkylchlorosilane-tethered triphenylenes 4 (Tm-Cm) started with the reaction of the known hydrox8400


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ypentaalkyloxytriphenylenes 1a−e31 with 11-bromoundecene 2a32 or 10-bromodecene 2b32 in the presence of K2CO3 in refluxing acetonitrile (Scheme 1). The resulting alkene-tethered

and 54 °C and for 3 (T10-C10) between 54 and 62 °C during second heating, whereas 4 (T9-C11) and 4 (T10-C10) displayed mesomorphic properties (Table 2).

Scheme 1. Synthesis of Triphenylenes 3 (Tm-Cn) and 4 (Tm-Cn)

Table 2. Phase Transition Temperatures (and Phase Transition Enthalpies ΔH) of the Triphenylenes 3 (T9C11), 3 (T10-C10) and the Triphenylene Silanes 4 (T9C11) and 4 (T10 C10)a triphenylene 3 3 3 3 4 4 4 4

(T9-C11) (T9-C11) (T10-C10) (T10-C10) (T9-C11) (T9-C11) (T10-C10) (T10-C10)

transition temperatures [°C] (ΔH [kJ mol−1]) Cr 30 (49.0) P Cr 41 (−37.9) P 54 (−0.6) I Cr 43 (83.8) P Cr 54 (−64.7) P 62 (−7.3) I Cr 28 (23.4) DL 45 (3.4) I Cr 26 (−26.5) DL 52 (−3.9) I Cr 32 (28.4) DL 50 (8.1) I Cr1 28 (−3.0) Cr2 40 (−24.2) DL 57 (−6.7) I

1st cooling 2nd heating 1st cooling 2nd heating 1st cooling 2nd heating 1st cooling 2nd heating

a

Phase transitions were determined by DSC (heating/cooling rate 5 K min−1). The following phases were observed: Cr, crystalline; P, plastic crystalline; DL, discotic lamellar; I, isotropic. P and DL phases were assigned by X-ray diffraction analysis.

Recently, Cammidge and Gopee35 and Stackhouse and Hird36 have studied symmetrical and nonsymmetrical mixed alkenyl-alkoxy- and alkinyl-alkoxy-triphenylenes in comparison with the parent hexaalkoxytriphenylenes and found a subtle balance of different factors which contribute to mesophase stability. They also reported that the rigidity has a significant influence on the mesomorphic properties. In our case, it seems that the increased rigidity of 3 (T9-C11) and 3 (T10-C10) as compared to the parent symmetrically substituted hexanonyloxy- and hexadecyloxytriphenylene, respectively, resulted in decreased phase width of mesophases and increased molecular order; i.e., the hexagonal columnar phase is lost and the plastic crystalline phase is observed instead. Upon first cooling, derivatives 4 (T9-C11) and 4 (T10-C10) displayed enantiotropic mesophases between 28 and 45 °C for 4 (T9-C11) and 32 and 50 °C for 4 (T10-C10). During second heating, the melting transitions were observed at 26 and 40 °C, respectively, and the clearing transitions at 52 and 57 °C (Table 2). Typical DSC curves for 3 (T10-C10) and 4 (T10-C10) are shown in the Supporting Information (Figure S1 and S2). Upon cooling from the isotropic phase, fan-shaped textures were observed by POM as shown in the Supporting Information (Figure S3) for 4 (T9-C11) and 4 (T10-C10). However, a detailed structural assignment of the observed mesophase was not possible on the basis of the POM textures, and thus, the samples were examined by X-ray diffraction (SAXS and WAXS). In the XRD small angle scattering regime, compounds 3 (T9C11) and 3 (T10-C10) displayed diffraction characteristics of plastic crystals, while compounds 4 (T9-C11) and 4 (T10C10) (see Supporting Information, Figure S4) showed a scattering pattern typical for discotic lamellar mesophases (DL). This is unexpected, because columnar mesophases are more common for triphenylenes.1 In the WAXS section of 4 (T9C11) and 4 (T10-C10), broad halos were observed, which originate from the liquid-like alkyl side chains. From the XRD data, layer distances d001 of 2.23 and 2.31 nm were calculated for 4 (T9-C11) and 4 (T10-C10), respectively, which are shorter by about 30% compared to the molecular lengths of 4 (T9-C11) (3.55 nm) and 4 (T10-C10) (3.52 nm),

triphenylenes 3 (Tm-Cm), which were isolated in 62−88% yield, were subsequently treated with chlorodimethylsilane in the presence of catalytic amounts of H2PtCl6 in refluxing CH2Cl2 for 18 h following a procedure by Coqueret and Wagner34 to afford the target triphenylene-silanes 4 (Tm-Cm) (Scheme 1). After evaporation of the solvent, compounds 4 (Tm-Cm) were used without further purification directly for the preparation of SAMs. Mesomorphic Properties of Triphenylenes 3 (Tm-Cn) and 4 (Tm-Cn). The mesomorphic properties of compounds 3 (Tm-Cn) and 4 (Tm-Cn) were studied by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Triphenylenes 3 (T7-C11), 3 (T8-C11), 3 (T10-C11), and 3 (T11-C11) and the corresponding derivatives 4 (T7-C11), 4 (T8-C11), 4 (T10-C11), and 4 (T11-C11) were nonmesomorphic and revealed only isotropic melting. Plastic crystalline phases were obtained for 3 (T9-C11) between 41 8401


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respectively. The latter have been calculated by molecular modeling with Chem3D Pro (version 12.0) taking into account the atomic van der Waals radii. The results suggest that the presence of the silane moiety suppressed the formation of hexagonal columnar mesophases and that discotic lamellar phases are energetically favored instead. In this configuration, the layers are strongly interdigitated with the silane moieties buried within the alkyl chains (Figure 1). The observation that

Table 3. Water Contact Angles and Film Thicknesses by Null Ellipsometry of Films Derived from the Triphenylene Silanes 4 (Tm-Cn) entry 1 2 3 4 5

compounds 4 4 4 4 4

(T7-C11) (T8-C11) (T9-C11) (T10-C11) (T11-C11)

contact angle [deg]

film thickness [nm]a

± ± ± ± ±

1.0 3.3 1.2 1.9 1.1

58 89 54 86 59

2 4 2 3 1

a As the statistical variation of the film thicknesses was too small, we assumed a more realistic absolute error of 0.1 nm.

surface roughness in the case of 4 (T8-C11) and 4 (T10-C11) (see Supporting Information, Figure S6). Disregarding defectinduced agglomeration, the film thickness of triphenylenes 4 (T7-C11), 4 (T9-C11), and 4 (T11-C11) extracted from the respective height profiles reveals a range between 2 and 3 nm (see Supporting Information, Figure S5b), proving the local coverage of, at most, 1 monolayer. In this case, water contact angles are not only determined by the inherent SAM properties, but also influenced by surface corrugation and interaction of the underlying SiO2, both lowering the contact angles.37 Therefore, higher surface corrugation caused by partial monolayer coverage explains not only the difference in film thickness via an effective medium approach (EMA), but also the smaller water contact angles in the case of the odd chain compounds 4 (T7-C11), 4 (T9-C11), and 4 (T11-C11).13 Structural Properties of Binary Mixed SAMs from 2(ω-(Chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11) with Chlorodecyldimethylsilane 5 (C10) and Chlorodimethyloctadecylsilane 6 (C18). For the utilization as self-organized templates in allorganic electronic devices, the composition of binary SAM mixtures offers an additional degree of freedom to control the morphological and electronic characteristics. Therefore, we studied the concentration-dependent contact angle, film thickness, and roughness of prototypical mixtures of 2-(ω(chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11). The choice of 4 (T10-C11) was motivated by its most dense and homogeneous packing as indicated by the AFM results in the previous section. To address the influence of steric constraints on the lateral order and thereby on the resulting surface properties of the SAMs, we have employed chlorodecyldimethylsilane 5 (C10) and chlorodimethyloctadecylsilane 6 (C18) as binary partner in the blends. Whereas the nominal chain length of compound 5 (C10) matches that of the 4 (T10-C11) undecyl chain, the longer chain length of 6 (C18) is expected to impose lateral tension in the mixed 4 (T10-C11) SAMs. Films from binary mixed solutions of the triphenylene silane 4 (T10-C11) with chlorodecyldimethylsilane 5 (C10) were obtained over the entire X4 (T10‑C11) mole fraction regime. At any mixing ratio, films from 4 (T10-C11) with 5 (C10) result in water contact angles between 78° and 91° and thus exhibited a general hydrophobic character; therefore, the values are nearly independent from the surface composition. In contrast to the water contact angles of the pure triphenylene silanes 4 (Table 3), no significant deviations from the mean value can be observed, indicating more dense packing and vice versa a reduced influence of the bare SiO2 substrate areas. Ellipsometric film thicknesses varied more strongly over the mole fraction regime. The thickness of 0.3 nm obtained for

Figure 1. Proposed packing model of the DL phase of 4 (T10-C10), in which the experimentally obtained layer distance d001 (2.31 nm) is determined by the regular interdigitation of the alkyl chains. The calculated layer distance dcalc (3.41 nm) for fully extended alkyl side chains was obtained by molecular modeling with Chem3D Pro (version 12.0).

mesophases (or plastic crystalline phases) were found only for derivatives 3 (T9-C11) and 4 (T10-C10) with nonyloxy side chains and undecyl spacer, and for derivatives 3 (T10-C10) and 4 (T10-C10) with decyloxy side chains and decyl spacer, might be due to the fact that the alkene or the silane moiety can be accommodated best with optimal space filling and minimal steric constraints as compared to other combinations of side chains and spacer lengths. X-ray diffraction patterns of the 4 (T10-C10) DL phase from SAXS and WAXS measurements and their corresponding intensity profiles can be found in the Supporting Information in Figure S4. SAM Preparations from Trialkylchlorosilane-Tethered Triphenylenes 4 (Tm-Cn): Water Contact Angle Measurements, Film Thicknesses by Null Ellipsometry, and Surface Structure by AFM. All triphenylene-silanes 4 (TmCn) were adsorbed at silicon oxide surfaces and rendered hydrophobic films. The contact angles, however, differed significantly for the films composed of different molecular moieties showing rather two different types of hydrophobicity (Table 3). Whereas layers formed from 4 (T7-C11), 4 (T9C11), and 4 (T11-C11) displayed comparable water contact angles (Table 3, Entries 1, 3, 5), layers from 4 (T8-C11) and 4 (T10-C11) revealed significantly larger ones (Entries 2, 4). The ellipsometric film thicknesses for 4 (T7-C11), 4 (T9-C11), and 4 (T11-C11) were smaller than those of films from 4 (T8C11) and 4 (T10-C11) (Table 3). These results suggest that the trialkylchlorosilane-tethered triphenylenes with odd alkoxy chain lengths rendered less densely packed monolayers than for even chain lengths. Additional AFM studies corroborate these findings by a distinct surface corrugation for odd chain lengths (see Supporting Information, Figure S5a) versus a statistical 8402


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films from the pure alkyl silane 5 (C10) can be attributed to a coverage of less than 1 ML, which results in predominately flatlying alkyl chains as also indicated by AFM height profiles (see Supporting Information, Figure S7). In contrast, already a concentration of X4 (T10‑C11) = 0.2 rendered a SAM with a film thickness of 1.24 nm, indicating a tilted alignment of the molecular entities with respect to the surface normal. Upon increasing triphenylene silane content, the effective layer thickness increased continuously until saturation was reached at a film thickness of 1.7 nm above X4 (T10‑C11) = 0.4. Remarkably, the thickness at saturation coincides with that of the pure triphenylene silane 4 (T10-C11) layer (Figure 2).

Figure 2. Ellipsometric thickness data as a function of triphenylene derivative X4 (T10‑C11) mole fraction. Dotted lines are guides to the eye.

Figure 3. SAM thickness (a) and roughness (b) estimated by XRR as a function of X4 (T10‑C11) mole fraction. Dotted lines represent guides to the eye. The inset in (a) depicts the good agreement between experimental data and modeling by the Parratt fit algorithm.

A substantially different behavior was found for films from binary mixed solutions of triphenylene silane 4 (T10-C11) with silane 6 (C18) (Figure 2). Whereas water contact angles between 75° and 87° resembled the general hydrophobic character of the films, the corresponding thicknesses exhibited a pronounced maximum at lower mole fractions in the range of 0.2 < X4 (T10‑C11) < 0.4 followed by saturation at 0.6 < X4 (T10‑C11). Again, the pure silane 6 (C18) SAM showed submonolayer coverage as indicated by AFM height profiles (see Supporting Information, Figure S8). For complementary quantitative information on the SAM alignment, its density, and interface roughness, XRR investigations in combination with theoretical modeling by a multidimensional Parratt algorithm have been applied. In analogy to the optical transfer matrix formalism, the Parratt model is based on multiple-reflection and transmission at the layer boundaries in a multilayered stack.38 The real and imaginary parts of the respective reflection coefficient depend on the electronic density of each layer. Furthermore, the interface roughness is considered by a Gaussian distribution of the height fluctuations within an individual layer. As a result, from oscillations at low scattering angles (so-called Kiessig fringes) the thickness of each individual layer in the stack can be deduced and the damping of the X-ray intensity oscillations provides information on the respective interface roughness. Exemplarily, a reflectivity scan together with a fit according to the Parratt formalism is illustrated in the inset of Figure 3a). The XRR film thicknesses and related roughnesses of binary mixed monolayers formed upon adsorption from mixed solutions from 4 (T10-C11) and 5 (C10) or 6 (C18) are shown in Figure 3.

The estimated layer thicknesses of SAMs from 5 (C10) or 6 (C18) revealed a tilt angle of 50° and 62°, respectively. These angles have been derived from the corresponding molecular lengths, namely, 1.50 and 2.49 nm. The tilt angles turned out to be much larger than those reported, e.g., for densely packed alkanethiols on Au(111) or OTS-SAMs on Si/SiO2.13,33 Several factors might lead to the observed deviations. Supported by the much larger space-filling of the individual silanes together with a significantly enhanced number of degrees of freedom, the amorphous SiO2 substrate seems to suppress the formation of a long-range-ordered anchoring of the SAM head-groups. In addition, the large binding energy of the chemisorbed headgroups and the high energetic surface corrugation strongly reduce the surface mobility of the silanes. Also, the monochlorosilane grafting head, instead of a trichlorosilane, seems to have a role in the increase of the tilt angle. In contrast, for alkanethiols on Au(111) the flat corrugation of the surface potential allows for site changes of the chemisorbed thiols groups on the metal surface and therefore supports molecular ordering on large length scales.39 For the silane films under study, immediate immobilization upon binding to the surface leads to randomly distributed SAM patches, though homogeneous in thickness. In the case of binary solutions, an increase of the film thicknesses at mole fractions above 0.2 is confirmed by the XRR data. Above this concentration, all films revealed an almost constant height of 1.95 nm. Taking into account the electron density along the molecular scaffold, we attribute this height to the distance between the silane anchoring group at 8403


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Figure 4. AFM reference measurements of the 6 (C18):4 (T10-C11) mixture at X4 (T10‑C11) = 0 (a), 0.3 (b), and 1 (c). The corresponding height distributions are shown in (d).

experimental data. Accordingly, the height fluctuations of the SAM film become smallest for the pure films constituted from 5 (C10), 6 (C18), or 4 (T10-C11) corresponding to mole fractions of 0 or 1. By analyzing the concentration-dependent roughness for mixed films with the alkyl-silane 6 (C18), a qualitatively similar behavior can be found, however, with the roughness maximum shifted toward lower mole fractions of 0.2 instead of 0.6. As there is no obvious reason to relate this shift to an intrinsic property of the SAM, we suggest that this effect originates from a change in packing density accompanied by a change of the tilting angle. Complementarily, Figure 3b shows that the maximum roughness occurs at a mole fraction where the SAM films adopt a constant height of 1.95 nm and, for higher mole fractions, i.e., constant layer thicknesses, the roughness decreases to its initial value of about 0.5 nm. From these results, the following conclusion can be drawn. As the alkyl silane 6 (C18) and compound 4 (T10-C11) are of comparable length but due to steric hindrances cannot pack in a dense space-filling phase, an increasing thickness correlates with enlargement of the effective layer roughness. Comparing the roughness peaks of both binary systems, the combination 4 (T10-C11) with C10 alkyl-silane 5 (C10) offers a smaller maximum roughness, as can be seen in Figure 3b. This difference can be explained by the ability of the molecule 5 (C10) to create a densely packed layer together with the C11

the substrate and the densely packed triphenylene unit. Taking into account the total length of about 2.5 nm for the derivative 4 (T10-C11), a tilting angle of 39° is required to achieve the observed effective height of 1.95 nm. The almost constant height over a broad concentration range indicates that derivative 4 (T10-C11) dominates the film morphology even at low mole fractions. These findings are in qualitative agreement with the ellipsometry data discussed above except for the maximum of the 6 (C18):4 (T10-C11) mixture at X4 (T10‑C11) ≈ 0.3, which will be discussed in the context of AFM measurements later on. Quantitative deviations between ellipsometry and XRR thickness data might originate from variations of the effective medium density as a function of composition. Furthermore, as XRR allows for a precise estimation of the interface and surface roughness we were able to estimate these parameters as a function of stoichiometry. As a key result, the various binary mixtures showed a defined roughness behavior characterized by a maximum at a defined composition. For mixed films of the alkyl-silane 5 (C10) and the triphenylenesilane 4 (T10-C11), this maximum occurs at a mole fraction ratio of around 40:60 (Figure 3b), thereby close to the archetypical case of a binary molecular layer whose constituents of fixed height difference are randomly distributed at a statistical weight of 50%. In this case, due to the enhanced destructive interference, the amplitude of the oscillations in the XRR spectra should be strongly reduced as confirmed by in the 8404


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coherent fraction of the reflected intensity. As this loss of coherence translates into a damping of the Kiessig oscillations, the roughness maximum in XRR (Figure 3) occurs at the same concentration (X4 (T10‑C11) = 0.3) as the thickness maximum in ellipsometry (Figure 2) and the roughness maximum in AFM (Figure 5). Accordingly, thickness deviations observed in ellipsometry and XRR can be attributed to the same effect of height fluctuations on short length scales, and the differences observed in the AFM versus XRR roughness provide complementary information on surface topography at various length scales (local versus integral roughness). This discussion illustrates the importance of a concerted approach by various techniques to aim for the full structural information of the blended binary SAM films. The structural results discussed above are summarized schematically in Figure 6. Driven by

alkyl spacer of the 4 (T10-C11) compound without disturbing the packing of the triphenylene group on top. Additionally, AFM measurements have been conducted to verify the trends in roughness of the binary SAMs deduced from the XRR data. Reference surface images, together with their respective height distribution, are shown in Figure 4. The related rms-roughness, derived from the width of the height distribution by means of the software package WSxM 5.0,40 is displayed for various X4 (T10‑C11) mole fractions in Figure 5.

Figure 5. Rms-roughness evolution estimated by AFM as a function of the X4 (T10‑C11) mole fraction. The dotted lines define guides to the eye.

Again, for each mixture 5 (C10):4 (T10-C11) and 6 (C18):4 (T10-C11) a defined roughness peak evolves. However, with respect to the XRR data the absolute roughness values obtained by AFM are somewhat enhanced, presumably due to local agglomerates induced by structural or chemical inhomogeneities in the underlying SAM. As the surface topography of the 5 (C10):4 (T10-C11) blended layer at X4 (T10‑C11) = 0.3 exhibits an unusual amount of agglomerates, this mole fraction has been disregarded in the further analysis. While these AFM data confirm the general trend of a singular roughness maximum for each mixture and of a higher roughness peak for the 6 (C18):4 (T10-C11) alloy compared to the 5 (C10):4 (T10-C11) blend, the latter is shifted toward a higher X4 (T10‑C11) mole fraction of 0.8 with respect to the XRR results (Figure 3). Comparing the ellipsometry thickness data for the 6 (C18):4 (T10-C11) mixture (Figure 2) with that obtained by XRR (Figure 3a) poses a question on the absence of the maximum at 0.2 < X4 (T10‑C11) < 0.4 in the case of the latter. A possible explanation might be rendered by the complementary AFM data shown in Figure 5. Obviously, in the concentration range of interest there exists a correlation between AFM rmsroughness and thickness determined by ellipsometry. Bearing in mind that both techniques are sensitive to height variations on short length scalesellipsometry by its effective medium approach and AFM by its superior lateral resolutionthe presence of laterally restricted height fluctuations, caused for instance by agglomerates, might lead to the observed correlation between these two properties of the binary SAMs. The AFM sensitivity on such localized elevations is corroborated by the asymmetric height distribution accompanied by a tail toward larger height values for X4 (T10‑C11) = 0.3 in Figure 4d. In the case of structural analysis by XRR, information on the SAM film thickness originates from constructive interference of X-rays. Therefore, localized variations in height will not yield an increase of the effective film thickness but rather a loss in the

Figure 6. Schematic illustration of the morphologies in binary SAM mixtures at different compositions. The silanes 5 (C10) and 6 (C18) have been chosen to form blends with triphenylene silane 4 (T10C11), as they were expected to generate precedents according to the steric hindrance imposed by their respective alkyl chain length. Pure chlorodecyldimethylsilane 5 (C10) (a), a mixture consisting of 5 (C10) and the triphenylene derivative 4 (T10-C11) exhibiting dense packing (b), pure 4 (T10-C11) (c), chlorodimethyloctadecylsilane 6 (d), and a mixture consisting of 6 (C18) and 4 (T10-C11) displaying a rather sparse packing (e).

these space-filling effects, it becomes obvious that self-organized binary systems such as those presented in this work provide additional degrees of freedom to control the effective thickness in combination with the surface morphology. This might be of great importance for organic thin film transistor applications, either for the implementation as monolayer thick gate dielectrics with defined (permanent) electronic polarization or as templates with tailored surface properties to control the growth dynamics of active transport channel on top.

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CONCLUSION We presented novel nonsymmetrically substituted triphenylene-based silane derivatives 4 (Tm-Cn). The structural analyses revealed the triphenylene silanes 4 (T9-C11) and 4 (T10-C10) to form discotic lamellar mesophases, whereas the 8405


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(8) Cristadoro, A.; Lieser, G.; Räder, H. J.; Müllen, K. Field-Force Alignment of Disc-Type π Systems. ChemPhysChem 2007, 8, 586− 591. (9) Kim, H.-S.; Choi, S.-M.; Lee, J.-H.; Busch, P.; Koza, S. J.; Verploegen, E. A.; Pate, B. D. Uniaxially Oriented, Highly Ordered, Large Area Columnar Superstructures of Discotic Supramolecules using Magnetic Field and Surface Interactions. Adv. Mater. 2008, 20, 1105−1109. (10) Kind, M.; Wöll, C. Organic surfaces exposed by self-assembled organothiol monolayers: Preparation, characterization, and application. Prog. Surf. Sci. 2009, 84, 230−278. (11) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications. Adv. Mater. 2009, 21, 1407−1433. (12) Boden, N.; Bushby, R. J.; Martin, P. S.; Evans, S. D.; Owens, R. W.; Smith, D. A. Triphenylene-Based Discotic Liquid Crystals as SelfAssembled Monolayers. Langmuir 1999, 15, 3790−3797 and references cited therein.. (13) Schreiber, F. Self-assembled monolayers: from ’simple’ model systems to biofunctionalized interfaces. J. Phys.: Condens. Matter 2004, 16, R881−R900 and references cited therein.. (14) Piot, L.; Marie, C.; Dou, X.; Feng, X.; Müllen, K.; Fichou, D. Growth of Long, Highly Stable, and Densely Packed Worm-Like Nanocolumns of Hexa-peri-Hexabenzocoronenes via Chemisorption on Au(111). J. Am. Chem. Soc. 2009, 131, 1378−1379. (15) Schwartz, D. K. Mechanisms and kinetics of self-assembled monolayer formation. Annu. Rev. Phys. Chem. 2001, 107−137. (16) Maoz, R.; Sagiv, J. On the formation and structure of selfassembling monolayers. I. A comparative atr-wettability study of LangmuirBlodgett and adsorbed films on flat substrates and glass microbeads. J. Colloid Interface Sci. 1984, 100, 465−496. (17) Patrone, L.; Gadenne, V.; Desbief, S. Single and Binary SelfAssembled Monolayers of Phenyl- and Pentafluorophenyl-Based Silane Species, and Their Phase Separation with Octadecyltrichlorosilane. Langmuir 2010, 26, 17111−17118. (18) Aswal, D. K.; Lenfant, S.; Guerin, D.; Yakhmi, J. V.; Vuillaume, D. Self assembled monolayers on silicon for molecular electronics. Anal. Chim. Acta 2006, 568, 84−108. (19) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.; Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Low-voltage organic transistors with an amorphous molecular gate dielectric. Nature 2004, 431, 963−966. (20) Vuillaume, D. Molecular-scale electronics. C. R. Phys. 2008, 9, 78−94. (21) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Ultralowpower organic complementary circuits. Nature 2007, 445, 745−748. (22) Fadeev, A. Y.; McCarthy, T. J. Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon. Langmuir 2000, 16, 7268−7274 and references cited therein.. (23) Fadeev, A. Y.; McCarthy, T. J. Binary Monolayer Mixtures: Modification of Nanopores in Silicon-Supported Tris(trimethylsiloxy)silyl Monolayers. Langmuir 1999, 15, 7238−7243. (24) Mathauer, K.; Frank, C. W. Binary Self-Assembled Monolayers As Prepared by Successive Adsorption of Alkyltrichlorosilanes. Langmuir 1993, 9, 3446−3451. (25) Smith, M. B.; Efimenko, K.; Fischer, D. A.; Lappi, S. E.; Kilpatrick, P. K.; Genzer, J. Study of the Packing Density and Molecular Orientation of Bimolecular Self-Assembled Monolayers of Aromatic and Aliphatic Organosilanes on Silica. Langmuir 2007, 23, 673−683. (26) Granier, M.; Lanneau, G. F.; Moineau, J.; Girard, P.; Ramonda, M. Improved Strain Relief of Self-Assembled Monolayers from Organohydridochlorosilanes Grafted onto Oxidized (1,0,0) Silicon Wafers. Langmuir 2003, 19, 2691−2695.

corresponding alkene-substituted triphenylenes 3 (Tm-Cn) displayed plastic crystalline phases. Furthermore, the morphology of coassembled SAM blends of 4 (T10-C11) and chlorodecyldimethylsilane 5 (C10) or chlorodimethyloctadecylsilane 6 (C18) is strongly affected by the packing of the triphenylene silane. The bilayer with 5 (C10) indicated a more densely packed SAM in comparison with 6 (C18) due to a better space matching with the undecyl-spaced triphenylene silane 4 (T10-C11) in the binary SAM. In contrast, the much longer derivative 6 (C18) has a higher tilt angle (62°) than 5 (C10) (50°), and therefore provides an effective height comparable to that of 4 (T10-C11) (39°). However, due to steric hindrances, compounds 4 (T10-C11) and 6 (C18) cannot form a densely packed layer even at low mole fractions of 4 (T10-C11). These results demonstrate the capability but also the structural complexity of binary SAM systems dedicated as, e.g., ultrathin dielectrics or organic templates for silicon/ organic hybrid devices.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Generous financial support by the Deutsche Forschungsgemeinschaft, the Ministerium für Wissenschaft, Forschung und Kunst des Landes Baden-Württemberg, the Bundesministerium für Bildung und Forschung (shared instrumentation grant) and the Fonds der Chemischen Industrie is gratefully acknowledged. We are further grateful for support by the BMBF within the GREKOS project.

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