Mixing of Nonsubstituted and Partly Fluorinated Alkanethiols in a

Feb 5, 2009 - The binary NSAT-PFAT films were found to exhibit interesting effects in photoemission related to the superposition of contra-directed di...
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J. Phys. Chem. C 2009, 113, 3697–3706

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Mixing of Nonsubstituted and Partly Fluorinated Alkanethiols in a Binary Self-Assembled Monolayer Nirmalya Ballav,†,§ Andreas Terfort,‡ and Michael Zharnikov*,† Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, 69120 Heidelberg, Germany, and Institut fu¨r Anorganische and Analytische Chemie, Goethe-UniVersita¨t Frankfurt, Max-Von-Laue-Straβe 7, 60438 Frankfurt, Germany ReceiVed: September 18, 2008; ReVised Manuscript ReceiVed: January 2, 2009

An applicability of irradiation-promoted exchange reaction (IPER) to the fabrication of binary self-assembled monolayers comprised of nonsubstituted and partly fluorinated alkanethiols (NSAT and PFAT, respectively) is demonstrated. This particular combination is interesting because of the intrinsic differences between these two species in terms of molecular volume and dipole moment. Within the IPER framework, the composition of the binary NSAT-PFAT SAMs could be precisely controlled by the irradiation dose up to 40% PFAT portion. This threshold of the intermixing is presumably related to a limited ability of the primary NSAT matrix to accumulate the bulky PFAT moieties and to the onset of extensive cross-linking in this matrix, which sets the upper limit of the applied promoting dose (at ca. 1-2 mC/cm2). The binary SAMs fabricated by IPER were compared with the analogous films prepared by coadsorption. A detailed analysis of the experimental data suggests that the differently prepared SAMs differ to some extent. The binary NSATPFAT films were found to exhibit interesting effects in photoemission related to the superposition of contradirected dipole moments of the NSAT and PFAT species. The effects could only be explained within an electrostatic framework, which shows that the standard chemical shift description is not always sufficient or even applicable to describe photoemission from a SAM-based system. 1. Introduction Control over the surface properties is an important challenge that has a broad relevance for different areas ranging from simple protection issues to the fabrication of medical implants and biochips. A perspective approach in this regard is functionalization of the target surface by a chemisorbed monomolecular film, a self-assembled monolayer (SAM),1-3 which gives the surface or interface a new chemical identity. A SAM is usually comprised by rod-like constituents that, in their turn, consist of three major building blocks, namely, a headgroup that makes the anchoring to the substrate, a tail group that is exposed to ambient, and a spacer that separates the head and tail groups. Within this general architecture, the properties of the functionalized surface are predominantly defined by the chemical identity of the tail group as far as the SAM remains intact. To get a specific property one has to design a molecule with a suitable tail group, which, depending on a specific application, can be quite complex. This approach can involve significant synthetic efforts since one needs a new molecule for any desired surface property. But these efforts can increase even further if not a discrete sampling but a smooth variation of a surface property is required; one will then need a series of molecules with variable tail group as far as such variation will be not completely impossible. The solution to this dilemma is the fabrication of mixed SAMs comprised of two (or three) individual molecules. Changing the * To whom correspondence should be addressed: E-mail: michael.zharnikov@ urz.uni-heidelberg.de. † Universita¨t Heidelberg. ‡ Goethe-Universita¨t Frankfurt. § Present address: Laboratory for Micro and Nanotechnology, Paul Scherrer Institut, 5232 Villigen, Switzerland.

portions of both constituents in the blend film one can vary the desired surface property continuously in a broad range, with comparably small synthetic efforts. However, regretfully, mixed SAMs can not always be easily fabricated even if the molecules are available. The standard approach to the SAM fabrication, that is, immersion of the substrate in the solution containing both molecular constituents of the target mixed SAM (i.e., coadsorption) works only in limited cases. In particular, test studies on SAMs comprised of binary mixtures of different molecules revealed that the solution composition of the chemical species and the resulting surface composition can be quite different.4-9 In many cases, preferential adsorption of one species over the other and phase segregation of different molecules in the SAM take place. To overcome the respective limitations, a merge of both functional moieties comprising a binary SAM in one molecule can be performed (e.g., dialkyldisulfides or dialkylsulfides with different chain parts), which, however, can involve significant synthetic efforts and allows only the 50-50% composition of these constituents.10-15 But even more importantly, the quality of the resulting blend SAMs is quite low in most cases. In particular, poorly ordered and liquid-like films, large deviations from the expected 1:1 composition, and phase segregation were observed quite frequently, although certain notable exceptions have been reported.11,12 A further approach involves a preparation of the primary one-component film with subsequent implanting of the second SAM constituent into the primary matrix (see e.g., ref 16). However, for successful implanting, the quality of the primary matrix should be quite low, which frequently result in an overall poor quality of the resulting mixing film. In the case of the high quality primary matrix (i.e, well-ordered and dense molecular packing), the implanting of a second component molecule can only occur by the exchange

10.1021/jp808303z CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

3698 J. Phys. Chem. C, Vol. 113, No. 9, 2009 of a first component moiety. Such an exchange reaction is a well-known phenomenon.3,4,17-27 Regretfully, it works efficiently only for certain combinations of molecules (e.g., short-chain species are efficiently exchanged for long-chain ones) and has frequently a very slow kinetics (days and even weeks).3 Recently, we have shown that the efficiency of the exchange reaction can be significantly enhanced and its kinetics noticeably accelerated if the reaction is promoted by electron irradiation or UV light exposure.28-31 This approach, irradiation-promoted exchange reaction (IPER), is based on the well-known fact that SAMs with less order and more defects undergo exchangereaction more easily than well-ordered and defect-free ones.3 So, the idea was to take a primary high-quality SAM and introduce defects intentionally, by electron irradiation or UV light exposure.28,31 Significantly, the best primary SAMs for IPER are those of commercially available alkanethiols (ATs), which is due to the dominance of the damage over cross-linking at the earlier stages of the irradiation in these particular systems.32,33 A further advantage is a progressive character of the effect, a higher irradiation dose up to the offset of the extensive cross-linking (at ca. 1 mC/cm2) results in a higher defect density which, in its turn, results in a larger extent of the exchange reaction.29 Thus, the composition of the resulting binary SAMs in the irradiated areas can be precisely adjusted by selection of a proper irradiation dose. Combining IPER with lithography one can then easily fabricate gradient-like chemical patterns on a length scale ranging from nm to cm.28,34,35 So far, we have demonstrated the versatility of IPER by using different binary systems, including nonsubstituted ATs (NSATs) as the primary matrix and different ω-substituted ATs,28,29,34,35 nonsubstituted biphenylthiols (NS-BPTs),29 and 4′-substituted BPTs29 as substituents. Some of the substituents carried quite bulky tail groups such as, for example, ferrocene29 or oligoethyleneglycol,35 which, however, were protruding over the densely packed matrix of the binary films comprised by the primary NSATs and the aliphatic or aromatic “foots” of the substituents. In this study, we investigated the mixing of “slim” and “bulky” elements within the SAM matrix by IPER and compared the results with the standard coadsorption approach. As the primary films, we used SAMs of NSATs on Au(111); the van der Waals diameter of these molecules is 4.2 Å.36 As the substituent, we took a partly fluorinated AT (PFAT). This molecule combines hydrocarbon and fluorocarbon segments, which generally have different molecular volumes with van der Waals diameters of 4.2 and 5.6 Å, respectively.36 The larger volume of fluorocarbon chain is essentially caused by its helical conformation, which is characteristic of respective bulk materials such as poly(tetrafluoroethylene), whereas the hydrocarbon chain typically has a planar zigzag conformation as, for example, in poly(ethylene). This conformation persists upon packing the PFAT molecules in SAMs.37-39 As a result, the intermolecular spacing in PFAT SAMs (5.8 Å)37,38,40-42 is usually larger than that in AT ones (ca. 5 Å for Au(111)).1 Apart from the molecular volume difference between NSATs and PFATs, these molecules have distinctly different dipole moments associated primarily with the CH3 (NSAT) and CF3 (PFAT) tail groups, which can result in interesting effects in the mixed films as has been correctly pointed recently in ref 43. In particular, ab initio Hartree-Fock calculations of the molecular dipole moments of CH3(CH2)9SH and CF3(CF2)7(CH2)2SH give values of 2.24 and -1.69 D, respectively (the direction from the head to tail group is taken as positive).44 The assembly of the above molecules on Ag

Ballav et al. substrate changes the surface potential with respect to pristine Ag by -0.70 and 0.85 eV, respectively.44 To our knowledge, so far there were only a few attempts to mix NSATs and PFATs in a SAM.43,45,46 In one case, the fluorocarbon part of PFAT contained only the CF3 moiety, that is, represented a ω-substituted AT with the CF3 tailgroup.43,45 It was found that such molecules can be easily mixed with NSATs by coadsorption, with the relative composition of the blend films being somewhat different from the solution composition.43 Note, however, that even though the CF3 moiety is bulkier than the CH3 tailgroup of a NSAT, there is no additional steric strain in the interior of the respective binary SAMs but only at the SAM-ambient interface (as far as the chain lengths of both constituents are equal). Therefore, the substitution of the CF3-terminated AT into a NSAT SAM does not distort its packing and crystallinity.45 In the second case, a NSAT (hexadecanethiol: HDT) and a more highly fluorinated PFAT compound were mixed in a SAM by subsequent adsorption as described above.46 The primary poor-quality NSAT matrix was prepared in a gradient-like fashion using a specially designed poly(dimethylsiloxane) stamp, and the subsequent adsorption of the PFAT molecules resulted in the formation of NSAT-PFAT gradient (linear and radial) film.46 The fraction of PFAT in this film was varied from 0 to 100%. The films were characterized by X-ray photoelectron spectroscopy (XPS) and contact angle goniometry. In this study, we prepared and characterized mixed SAMs of dodecanethiol (CH3(CH2)11SH: DDT) and a PFAT compound (CF3(CF2)9(CH2)2SH: F10H2) on Au(111) substrates. The characterization has been performed by XPS and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. In the following, we provide a brief description of the experimental procedure and setup. Thereafter, results are presented and discussed in detail in section 3. Finally, the results are summarized in section 4. 2. Experimental Section 2.1. Substrates and Compounds. The gold substrates were prepared by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. Such evaporated films are polycrystalline in nature with a grain size of 20-50 nm as observed by atomic force microscopy.47 They are considered to be standard substrates for thiol-derived SAMs. The grains predominantly exhibit a (111) orientation, which is, in particular, supported by the observation of the corresponding forward scattering maxima in the angular distributions of the Au 4f photoelectrons and by the characteristic binding energy (BE) shift of the Au 4f surface component.47,48 DDT and all solvents were purchased from Sigma-Aldrich Chemie GmbH (Germany) and used without a further purification. F10H2 was synthesized according to the protocol described in ref 39. 2.2. Fabrication of the Mixed SAMs by IPER. The primary DDT SAMs were formed by immersion of freshly prepared gold substrates into 1 mmol solution (in ethanol) of the target compound for 24 h at room temperature. After immersion, the samples were carefully rinsed with pure ethanol and blown dry with argon. The advancing and receding water contact angles of the SAMs were 111 and 102°, respectively. The fabricated DDT films were irradiated with 10 eV electrons, which are especially effective for a gentle modification of SAMs and AT SAMs in particular.49,50 The doses were estimated by multiplication of the exposure time with the current

Alkanethiols in a Binary Self-Assembled Monolayer density (≈2.5 µA/cm2). The electron gun was mounted at a distance of ≈11.5 cm from the sample to ensure uniform illumination. The base pressure in the chamber during the irradiation was 1 × 10-8 mbar. The exchange reactions were performed by immersion of the pristine or irradiated DDT SAMs into a solution of F10H2 in dichlormethane (DCM) for 2 h at room temperature. According to our previous results, this immersion time corresponds to the saturation of IPER.29 After immersion, the samples were carefully rinsed with DCM followed by pure ethanol, blown dry with argon. The samples were either characterized immediately or, in the case of synchrotron-based experiments, kept for several days in argon-filled containers until the characterization. No evidence for impurities or oxidative degradation products was found by XPS (see below). 2.3. Fabrication of the Mixed SAMs by Coadsorption. The SAMs were formed by immersion of freshly prepared gold substrates into a DCM solution containing both DDT and F10H2 molecules in a definite molar ratio. The immersion occurred for 24 h at room temperature. After immersion, the samples were carefully rinsed with DCM followed by pure ethanol, blown dry with argon, and used either immediately or, in the case of synchrotron-based experiments (see below), kept for several days in argon-filled containers until the characterization. No evidence for impurities or oxidative degradation products was found by XPS (see below). Also, for the reference purpose, single-component SAMs of F10H2 were prepared. The advancing and receding water contact angles of these SAMs were 117 and 108°, respectively. 2.4. Characterization. The fabricated films were characterized by XPS and angle resolved NEXAFS spectroscopy. All experiments were performed at room temperature. The measurements were carried out under UHV conditions at a base pressure better than 1 × 10-9 mbar. The spectra acquisition time was selected in such a way that no noticeable damage by the primary X-rays occurred during the experiments.51-53 Note that this measure is especially important in the case of PFAT-containing films because of their enormous sensitivity to the ionizing radiation.54,55 The XPS measurements were performed in a custom-made UHV chamber using a Mg KR X-ray source and a LHS 11 analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ≈0.9 eV. The X-ray source was operated at a power of 260 W and positioned ≈1.5 cm away from the samples. The energy scale was referenced to the Au 4f7/2 peak of AT-coated gold at a binding energy (BE) of 84.0 eV.56 For each sample, a wide scan spectrum and the C 1s, O 1s, S 2p, and Au 4f narrow scan spectra were measured. XPS spectra were fitted by symmetric Voigt functions using a Shirley-type background. To fit the S 2p3/2,1/2 doublet we used two such peaks with the same full width at half-maximum (fwhm), a standard56 spin-orbit splitting of ≈1.18 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently; the same fit parameters were used for identical spectral regions. During the analysis of the spectra of the PFAT-DDT films to derive the film composition, we used the intensity values of the characteristic emissions without any correction for the attenuation of the photoelectron signal. This procedure is justified in our opinion because of the additivity of the XPS signals for the individual molecules. Note, however, that the attenuation lengths for photoelectrons in the hydrocarbon and fluorocarbon films are reported to be indistinguishable.57

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Figure 1. C 1s XPS spectra of single-component F10H2/Au (top curve), pristine DDT/Au (bottom curve), irradiated (1 mC/cm2) DDT/ Au, and irradiated (1 and 2 mC/cm2) DDT/Au immersed into the F10H2/DCM solution for 2 h (IPER approach). Also, the spectra of mixed DDT-F10H2/Au prepared by coadsorption (from 50-50% DDTF10H2 solution) are shown for comparison. The positions of the characteristic emissions are indicated by vertical dotted lines. The observed BE shifts will be discussed in detail in section 3.4.

NEXAFS measurements were carried out at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The spectra acquisition was carried out at the C and F K-edges in the partial electron yield mode with a retarding voltage of -150 and -450 V, respectively. Linear polarized synchrotron light with a polarization factor of 0.82 was used and the energy resolution was ≈0.40 eV. To monitor the orientational order in the SAMs, the incidence angle of the light was varied from 90 (E-vector in surface plane) to 20° (E-vector near surface normal) in steps of 10-20°. This approach is based on the dependence of the cross-section of the resonant photoexcitation process on the orientation of the electric field vector of the synchrotron light with respect to the molecular orbital of interest (so-called linear dichroism in X-ray absorption).58 This effect results in a characteristic dependence of an adsorption resonance intensity on the incidence angle of X-rays as far as there is an orientational order in the probed system. The photon energy (PE) scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite at 285.38 eV.59 The relative PE shift at the F K-edge was calibrated using the Au 4f photoemission from a bare Au sample. Raw NEXAFS spectra were corrected for the photon energy dependence of the incident X-ray flux by division through a spectrum of a clean, freshly sputtered gold sample. Further, the C K-edge spectra were reduced to the standard form by subtracting linear pre-edge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40-50 eV above the respective absorption edges). In the case of the F K-edge, the spectra were normalized to the pre-edge intensity, after which linear pre-edge background was subtracted. As the result, the edge jump became representative of the packing density of the F10H2 molecules in the films. 3. Results and Discussion 3.1. XPS. The selected C 1s, F 1s, and S 2p XPS spectra of F10H2 SAMs, pristine and irradiated DDT SAMs, and mixed DDT-F10H2 films fabricated by IPER and coadsorption are given in Figures 1-3, respectively. The C 1s spectrum of the pristine DDT film exhibit a characteristic emission at a BE of about 284.9 eV,48,60 whereas the analogous spectrum of the

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Figure 2. F 1s XPS spectra of single-component F10H2/Au (top curve), pristine DDT/Au immersed into F10H2/DCM solution for 2 h (bottom curve), and mixed DDT-F10H2/Au. The mixed films were fabricated by either IPER (the irradiation dosages are given in the mC/ cm2 units at the respective curves) or the coadsorption (only the spectrum for the 50-50% solution composition is shown). The positions of the characteristic emission are indicated by vertical dotted lines. The observed BE shifts will be discussed in detail in section 3.4.

Figure 3. S 2p XPS spectra of single-component F10H2/Au (top curve), pristine DDT/Au (bottom curve), irradiated (1 mC/cm2) DDT/ Au, and mixed DDT-F10H2/Au fabricated by IPER (only the spectra for doses of 0.5 and 1 mC/cm2 are shown). The experimental spectra (open circles) are decomposed into the individual contributions related to the thiolate and irradiation-induced sulfur species. The solid lines represent the respective fits.

F10H2 SAM shows three peaks at BEs of about 284.7, 290.6, and 293.1 eV representative for the hydrocarbon stem, fluorocarbon stem, and terminal CF3 group, respectively.39,54,61,62 The F1s spectrum of F10H2/Au (Figure 2) exhibits a characteristic emission at a BE of about 688.0 eV, which is representative for the entire fluorocarbon stem, including the CF3 group.39,54,61,62 The S 2p spectra of both DDT/Au and F10H2/Au exhibit a single S 2p3/2,1/2 doublet at a BE of about 162.0 eV (S 2p3/2) related to the Au-thiolate bond (Figure 3, bottom and top curves).48,63,64 In agreement with expectations,32,33 the irradiation of the pristine DDT film results in its partial disordering and damage of the thiolate-Au interface, as follows from the broadening of the C 1s emission in the respective spectrum in Figure 1, decrease of the intensity of the S 2p doublet characteristic of thiolate in the spectrum in Figure 3, and appearance of an additional S 2p doublet at 163.4 eV (S 2p3/2) in the same spectrum. This doublet is characteristic of weakly bound sulfur and dialkylsulfide species evolving after the cleavage of the

Ballav et al. primary thiolate-gold bond.53 Subsequent immersion of the irradiated DDT/Au into F10H2/DCM solution resulted in the appearance of the emissions characteristic of the F10H2 molecule in both C 1s (Figure 1) and F 1s (Figure 2) spectra, suggesting an extensive exchange of the DDT moieties in the SAM for F10H2 ones, that is, formation of a mixed DDT-F10H2 film. The intensity of the respective features increases progressively with increasing irradiation dose, whereas that of the emission characteristic of DDT decreases. Note that almost no exchange occurred for the nonirradiated DDT/Au; this is evidenced by the respective F 1s spectrum in Figure 2. As for the S 2p spectra (Figure 3), the immersion of the irradiated DDT/ Au samples into the F10H2/DCM solution caused a partial intensity recover of the S 2p doublet related to thiolate and complete (0.5 mC/cm2) or partial (1.0 mC/cm2) disappearance of the S 2p doublet related to the damaged DDT species. This suggests that the F10H2 substituents preferably exchanged the latter species (as can be expected) and adsorbed onto the respective sites. An important finding is the fact that not all the DDT species with the cleaved S-Au bond are substituted by the F10H2 molecules, which occurs at irradiation doses of 1-2 mC/cm2. According to Figure 3, the portion of the damaged DDT molecules in the mixed DDT-F10H2 SAMs prepared at such irradiation doses amounts to about 20%. Similar to the C 1s and F 1s spectra of the mixed DDTF10H2 SAMs fabricated by IPER, the analogous spectra of the films prepared by the coadsorption represent a superposition of the respective spectra of the single-component DDT and F10H2 SAMs (see Figures 1 and 2). The relative intensity of the F10H2 component increases with increasing portion of the F10H2 compound in the primary solution (not shown). These findings will be discussed in detail below in sections 3.3 and 3.4. 3.2. NEXAFS Spectroscopy. NEXAFS measurements provide information about the chemical identity of the target film and average orientation of its constituents, on the basis of the linear dichroism effect (see section 2.4). An efficient way to monitor the linear dichroism is to plot the difference of the NEXAFS spectra acquired at normal (90°) and grazing (20°) angles of X-ray incidence. In contrast, a spectrum acquired at the so-called magic angle of X-ray incidence (55°) is not affected by any effects related to molecular orientation and gives only information on the chemical identity of investigated samples. Note that a deviation from the ideal 100% linear polarization results in a change of the magic angle;58 in particular, it is about 51° for our polarization factor (0.82). C K-edge NEXAFS spectra acquired at an X-ray incidence angle of 51° (a) and 90-20° difference curves (b) for pristine DDT/Au, pristine DDT/Au immersed into F10H2/DCM solution for 2 h, and mixed DDT-F10H2/Au fabricated by either IPER or coadsorption are shown in Figure 4. For comparison, the spectrum and difference curve for single-component F10H2/ Au are also presented. The spectrum of pristine DDT/Au exhibits a pronounced absorption resonance at ≈287.7 eV and two broader resonances at ≈293.4 and ≈301.6 eV, respectively (the resonance at ≈301.6 eV is not clearly seen in Figure 4a but can be distinguished in the respective difference spectrum in Figure 4b; see below).32 The two latter resonances are commonly related to valence, antibonding C-C σ* and C-C′ σ* orbitals,58,65 while the resonance at 287.7 eV is alternatively attributed to the excitations into pure valence orbitals,58,66 predominantly Rydberg states,67,68 and mixed valence/Rydberg states.69 This resonance consists of several individual resonances, which are merged together.66-68 We will denote it as a R* resonance but take into account a

Alkanethiols in a Binary Self-Assembled Monolayer

Figure 4. C K-edge NEXAFS spectra acquired at an X-ray incidence angle of 51° (a) and 90-20° difference curves (b) for single-component F10H2/Au (top curves), pristine DDT/Au (bottom curves), pristine DDT/Au immersed into F10H2/DCM solution for 2 h (bottom but one curves), and mixed DDT-F10H2/Au. The mixed films were fabricated by either IPER (the irradiation dosages are given in the mC/cm2 units at the respective curves) or the coadsorption (only the spectrum for the 50-50% solution composition is shown). The positions of the characteristic absorption resonances (see the assignments) are indicated by vertical dotted lines.

possible admixture of antibonding C-H* orbitals. The transition dipole moments (TDMs) of the molecular orbitals related to the R* and C-C/C-C′ σ* resonances are believed to be oriented perpendicular to and along the axis of the alkyl chain, respectively.65,70,71 In accordance with this orientation and expected upright orientation of the DDT molecules in the respective film, the R* and C-C/C-C′ σ* resonances exhibit positive and negative anisotropy peaks in the 90°-20° curve for pristine DDT/Au. The spectra of single-component F10H2/Au contain two absorption edges at ≈287.8 and ≈294.0 eV related to the C1sf continuum excitations for carbon atoms bonded to hydrogen and fluorine, respectively. The spectra are dominated by the pronounced resonances at ≈292.4, ≈295.4, and ≈298.9 eV related to the transitions from the C1s state to the C-F σ*, C-C σ*,andC-F′σ*orbitalsofthefluorocarbonpart,respectively.39,61,72-75 The corresponding TDMs are believed to be oriented almost perpendicular (C1s f C-F σ* or C-F′ σ*) or along the chain axis (C1s f C-C σ*), respectively.61,70,72,73 As to the hydrocarbon part, only a weak feature at ≈288 eV, related to the R* resonance (see above) is exhibited in the spectra. The characteristic C-C σ* and C-C′ σ* resonances of the hydrocarbon part at ≈293 eV and ≈302 eV (see above) overlap with the strong features related to the fluorocarbon part and are, therefore, indistinguishable. The absorption resonances related to the fluorocarbon part of the F10H2 molecules exhibit a pronounced linear dichroism. This behavior implies an expected39 high orientational order in the F10H2 films. The resonances with TDMs oriented along the fluorocarbon chain (C-C σ*) show the intensity increase

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Figure 5. F K-edge NEXAFS spectra acquired at an X-ray incidence angle of 51° (a) and 90-20° difference curves (b) for single-component F10H2/Au (top curves), pristine DDT/Au immersed into F10H2/DCM solution for 2 h (bottom curve in the upper panel), and mixed DDTF10H2/Au. The mixed films were fabricated by either IPER (the irradiation dosages are given in the mC/cm2 units at the respective curves) or the coadsorption (only the spectrum for the 50-50% solution composition is shown). The positions of the characteristic absorption resonances (see text for the assignment) are indicated by vertical dotted lines.

with increasing angle of X-ray incidence, whereas those with TDMs oriented perpendicular to the fluorocarbon chain axis (C-F σ* and C-F′ σ*) reveal an opposite behavior. Considering that the E vector is parallel to the substrate surface at normal incidence and becomes almost perpendicular to it with decreasing angle of X-ray incidence, such a behavior implies the expected,39 almost perpendicular orientation of the fluorocarbon moieties in the F10H2 SAMs, even though with a certain inclination. The C K-edge spectra of the mixed films fabricated by both IPER and coadsorption represent a superposition of the spectra of the single-component DDT and F10H2 films. For the IPER case, the relative intensity of the F10H2 component increases with increasing irradiation dose, with no visible F10H2 features for the nonirradiated (0 mC/cm2) SAM exposed to the F10H2/ DCM solution. For the coadsorption case, the relative intensity of the F10H2 component increases with increasing portion of the F10H2 compound in the primary solution (not shown). The above findings suggest, in full agreement with the XPS data, the formation of mixed DDT-F10H2 SAMs by both IPER and coadsorption. Significantly, the difference curves for the mixed films in Figure 4b exhibit a clear linear dichroism for the resonances related both to the DDT and F10H2 molecules, which suggest that the mixed films possess orientational order. The C K-edge data are supported by the F K-edge ones, which are presented in Figure 5 in a similar fashion. The F K-edge spectra of F10H2 in Figure 5a exhibit several overlapping resonances. These resonance are related to the transitions from the F1s state to the different σ* orbitals of the fluorocarbon part

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Figure 6. Schematic illustration of the IPER procedure in the case of DDT-F10H2/Au. All damaged DDT species are exchanged for the F10H2 moieties after electron irradiation with a low dose (0-1 mC/cm2). Such an exchange does not occur to the full extent in the films prepared with high irradiation doses (1-2 mC/cm2); these films contain some amount (ca. 20%) of the damaged DDT species.

of the F10H2 molecules.73,76 Whereas an exact identification and assignment of all resonances is difficult due to their overlap, the dominant feature at ≈689.1 eV can be definitely ascribed to the C-F σ* orbital.73,76 The respective difference peak in the 90-20° spectra in Figure 5b is positive, suggesting, in view of the TDM orientation for the C1s f C-F σ* transition (see above), an expected, upright orientation of the F10H2 moieties in the respective SAM. The F K-edge spectra of the mixed films fabricated by both IPER and coadsorption exhibit all the characteristic features of the F10H2 molecules. For the IPER case, the relative intensity of the NEXAFS signal increases with increasing irradiation dose. For the coadsorption case, the relative intensity of the NEXAFS signal increases with increasing portion of the F10H2 compound in the primary solution (not shown). The above findings suggest, in full agreement with the XPS and C K-edge NEXAFS data, formation of mixed DDT-F10H2 SAMs by both IPER and coadsorption. Significantly, the difference curves for the mixed films in Figure 5b exhibit a clear dichroism, suggesting, along with the C K-edge data, an orientational order in the mixed films.

3.3. Composition of the Mixed Films. The IPER is obviously working, even though to a limited extent, in the case of the DDT-F10H2 combination. A schematic drawing of this process is shown in Figure 6: both defects in the SAM matrix and at the SAM-ambient interface promote the exchange, with the damaged DDT species being predominantly exchanged. The portion of F10H2 in the mixed DDT-F10H2 SAMs prepared by IPER is given in Figure 7 as a function of the irradiation dose. The respective values were determined on the basis of the XPS and NEXAFS data taken the pristine DDT and F10H2 SAMs as references. According to the resulting curve, which was calculated as an average between the XPS- and NEXAFSderived values, the portion of the F10H2 molecules in the mixed films can be varied in a controlled way from 0 to ≈40% by the selection of a proper irradiation dose in a range of 0-2 mC/ cm2. The upper limit of the dose range is already higher than the onset of extensive cross-linking (at ca. 1 mC/cm2), so that a further dose increase is not reasonable. Also, as shown by the S 2p spectra in Figure 3, not all damaged DDT molecules are substituted by the F10H2 species at large dose, which results in a lower quality of the resulting binary DDT-F10H2 SAMs.

Alkanethiols in a Binary Self-Assembled Monolayer

Figure 7. Relative content of F10H2 in the mixed DDT-F10H2 films prepared by IPER as a function of irradiation dose. The values were derived from the F1s XPS spectra (b), C 1s XPS spectra ([), and F K-edge NEXAFS spectra (O). The average of the XPS- and NEXAFSderived values gave the resulting curve (gray solid line).

Interestingly, the upper limit of the F10H2 admixture to the primary DDT SAM by IPER is close to that for the biphenylthiol species (35%),30 whereas it is significantly higher (70-75%)29 if the substituent differs from the primary SAM molecule by the ω substitution only. Of course, it is difficult to make a general conclusion on the basis of a limited data set, but it looks so that admixture of noticeably different (with respect to the primary) species by IPER is more difficult than admixture of similar species. The DDT-F10H2 combination is especially interesting since both molecules are not only distinctly different, but differ by the molecular volume. The insertion of a bulky F10H2 species into the DDT matrix should involve a rearrangement or/and reorientation of the neighboring DDT molecules. Of course, the respective changes can be accumulated to some extent by elimination of structural defects, but, starting from some point, they should affect the “true” 2D lattice of the primary molecules. The respective rearrangement can presumably occur to a limited extent only, which gives the upper limit of the substituent fraction in the case of IPER. The presence of the damaged DDT molecules in the mixed DDT-F10H2 SAMs prepared by IPER with a high irradiation dose (1-2 mC/cm2) is a clear drawback of these films. According to Figure 7, this dose range corresponds to the F10H2 concentration range of 30-40%. However, at low irradiation doses, the amount of the damaged DDT molecules in the mixed DDT-F10H2 SAMs is negligible (see the spectrum for 0.5 mC/ cm2 in Figure 3), so that the respective films do not suffer from the above drawback. According to Figure 7, this dose range corresponds to the F10H2 concentration range of 0-30%. Interestingly, no residual damaged species of the primary matrix were observed in all other mixed SAMs prepared by IPER.28-30 The reasons that these species are present in the DDT-F10H2 SAMs are probably the comparably large diameter of the F10H2 species and relatively high doses. Usually, we keep the dose below 1 mC/cm2 in the case of IPER because of the offset of extensive cross-linking between the individual molecular species at higher doses.28,29 However, in the case of the DDT-F10H2 SAMs, we had to go above the 1 mC/cm2 limit to increase the portion of F10H2 species. It is no wonder then that the exchange does not occur to the full extent under these circumstances. The portion of F10H2 in the mixed DDT-F10H2 films prepared by coadsorption is given in Figure 8 as a function of the relative content of F10H2 (molar fraction) in the primary solution. The respective values were obtained on the basis of the XPS data; NEXAFS spectra were only measured for several films. As follows from Figure 8 the composition of the binary DDT-F10H2 films can be varied in the entire relative content

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3703

Figure 8. Relative content of F10H2 in the mixed DDT-F10H2 films prepared by coadsorption as a function of the relative content of F10H2 in the primary solution.

range, even though the dependence of the F10H2 portion in the SAMs on its molar fraction in the solution is distinctly nonlinear, with preferable adsorption of the F10H2 molecules. Considering that the lengths of the DDT and F10H2 species are similar and the density of the F10H2 films is smaller by about 30-40% because of the larger van der Waals diameter (see section 1), this behavior can only be explained by a higher energy gain related to the intermolecular interaction of the F10H2 species as compared to DDT ones. Note that quite a different situation occurs in the case that the fluorinated part of a PFAT species is only represented by the tailgroup. In this case, the molar fraction of the PFAT species in the mixed PFATNSAT film is lower than the fraction of these molecules in solution.45 3.4. Detailed Analysis of the C 1s and F 1s XPS Spectra. An attentive look at the C 1s and F 1s XPS data in Figures 1 and 2, respectively, reveals that the BE positions of all the characteristic emissions in the spectra of the mixed DDT-F10H2 films change with the film composition and differ from those for the one-component DDT and F10H2 SAMs. Whereas this effect can be clearly monitored in the case of the F 1s spectra (Figure 2), it is more difficult for the more complex C 1s spectra. Therefore, we have presented separately the representative regions of these spectra for several selected DDT-F10H2 films in Figures 9 (C-H region) and 10 (C-F region). Note that the emission in the C-H region is mostly representative for the DDT molecules, except for the cases of single-component F10H2/Au and, to some extent, the film prepared from the 50-50% DDT-F10H2 solution. The emission in the C-F region is exclusively representative for the F10H2 molecules. The BE positions of the C 1s (C-H) and C 1s (C-F) emissions are given in Figures 11a and b, respectively, as functions of the F10H2 portion in the mixed SAMs. The analogous plot for the F 1s emission is presented in Figure 12. The general tendency observed both for the C 1s and F 1s emissions is a progressive downward BE shift with increasing portion of F10H2 in the mixed SAMs. The extent of this shift is very similar for the C 1s (C-F) and F 1s emissions. In particular, for the films prepared by the coadsorption, BE of the both emissions decreases by about 0.9 eV upon a F10H2 portion increase from 5 to 80%. The analogous decrease of the C 1s (C-H) BE is only 0.4 eV. Whereas an exact description of the above effects is presumably quite complex and will require theoretical modeling of the entire molecular ensemble, a qualitative explanation can be given. As was shown recently, in photoemission experiments the alignment of the Fermi and vacuum levels of a SAM with the analogous levels of the substrate and spectrometer occurs

3704 J. Phys. Chem. C, Vol. 113, No. 9, 2009

Figure 9. Selected region (C-H) of the C 1s XPS spectra of F10H2/ Au (100%; black) and mixed DDT-F10H2/Au. The mixed films were fabricated by either coadsorption or IPER. In the case of coadsorption, relative content of F10H2 in the primary solution is given at the respective curve: 50% (red), 16.7% (green), 10% (blue), 6.25% (cyan). In the case of IPER, irradiation dose is given at the respective curve: 0.25 (magenta), 0.5 (dark yellow), 1 (olive), 2 (orange) mC/cm2.

Figure 10. Selected region (C-F) of the C 1s XPS spectra of F10H2/ Au (100%; black) and mixed DDT-F10H2/Au. The mixed films were fabricated by either coadsorption or IPER. In the case of coadsorption, relative content of F10H2 in the primary solution is given at the respective curve: 50% (red), 16.7% (green), 10% (blue), 6.25% (cyan). In the case of IPER, irradiation dose is given at the respective curve: 0.25 (magenta), 0.5 (dark yellow), 1 (olive), 2 (orange) mC/cm2.

in a very special way, which is distinctly different from the standard Fermi level pinning of a conductive sample.77 In contrast, the pinning of a SAM and the substrate occurs over the vacuum level, which results in electrostatic effects, that is, in the dependence of the C 1s BE on the work functions (WF) of the SAM and substrate.77 A presumable reason for this behavior is a comparably slow (with respect to the lifetime of the excited-state in a photoemission experiment) charge compensation by tunneling through the molecular backbone.78 In most cases, a standard description of a photoemission experiment for a SAM in terms of the chemical shift is fully adequate and sufficient, but, in some selected cases, as, for example, for an embedded dipole layer,77,79 electrostatic pinning of the associated energy states comes to the foreground. As was mentioned in section 1, NSAT and PFAT molecules have distinctly different dipole moments, directing into opposite directions. In the case of NSATs, it is directed from the head

Ballav et al.

Figure 11. Binding energy positions of the C 1s (C-H) (a) and C 1s (C-F) (b) emissions in the XPS spectra of DDT/Au (only for C-H), F10H2/Au (100%), and mixed DDT-F10H2/Au as functions of the F10H2 content in the mixed films. The latter films were fabricated by either coadsorption (b) or IPER (O).

Figure 12. Binding energy position of the F1s emission in the XPS spectra of F10H2/Au (100%) and mixed DDT-F10H2/Au as a function of the F10H2 in the mixed films. The latter films were fabricated by either coadsorption (b) or IPER (O).

to the tail group; in the case of PFATs from the tail to the headgroup (the convention used here defines dipole moments as acting from - to +).44 As a result, upon assembling these molecules in a SAM, the WF of the substrate decreases in the case of ATs and increases in the case of PFATs.44,80-82 Note, however, that the respective changes in the WF are not only associated with the dipole moments of the primary molecules but also contain minor contributions from the charge transfer between the adsorbed molecule and the substrate, including that related to the formation of the headgroup-substrate bond (some authors consider this contribution as negligible).48,80,82-91 Also, the WF of a SAM is usually affected by mutual interaction between the individual molecular dipoles, which results in a decrease of the respective dipole moments as compared to the values for the insolated molecule.92 In any case, one can consider DDT and F10H2 SAMs as molecular assemblies comprised of dipoles directed in opposite directions: upward in the case of DDT and downward in the case of F10H2. A binary DDTF10H2 film contains thus a mixture of the counter-directed dipoles with the net dipole moment and the respective WF change (as related to the substrate) depending on the ratio of

Alkanethiols in a Binary Self-Assembled Monolayer both constituents. According to the previous results, the introduction of a positive dipole layer at the SAM-ambient interface or within the SAM results in the upward shift of the C 1s XPS peak.77,79 Also in our case, the BE of the C 1s and F 1s peaks increases with increasing portion of the positive dipoles in the binary DDT-F10H2 films. The different extent of the respective shift for the peaks associated with the DDT molecules (C 1s C-H) and F10H2 species (C 1s C-F and F 1s) allows us to speculate that the pinning of the Fermi and vacuum level occurs individually for both constituents of the mixed SAMs, due, probably, of the different dynamics of the charge transfer along the molecular backbone (filling of the photoemission hole). Significantly, the exact BE positions and FWHMs of the C 1s and F 1s emissions for the DDT-F10H2 films prepared by IPER and coadsorption differ to some extent. The same is true for the fwhm of these peaks (the data are not shown); in particular, the fwhm of the C 1s (C-F) emission for the DDTF10H2 film containing ca. 35% of F10H2 is about 1.2 eV for IPER and about 1.6 eV for the coadsorption. These findings indicate, even though indirect, a difference between the DDTF10H2 films prepared by the different methods. On one hand, these differences can be explained by the presence of the damaged DDT species in the mixed films fabricated by IPER, even though not to a full extent, since the differences are also observed in the films fabricated by low dose (small content of F10H2), where all damaged DDT species are exchanged by the IPER moieties (see the spectrum for 0.5 mC/cm2 in Figure 3). On the other hand, the differences observed by XPS can be related to the possible phase separation, which, in our opinion, can occur only in the case of coadsorption; this effect has been observed for some AT systems previously.11,15,93 Note that we do not expect a phase separation in the case of IPER because of a stochastical character of the irradiation-induced defects promoting the exchange reaction. A scanning-tunneling microscopy (STM) or atomic-force microscopy (AFM) characterization of the binary DDT-F10H2 films would be very desirable to verify the above hypothesis. An additional information, namely, the degree of crystallinity and molecular conformation in such films, can be obtained by surface-sensitive vibrational spectroscopy.45 4. Conclusions A mixing of NSAT and PFAT molecules in a binary SAM is of interest because of the intrinsic differences between these two species in terms of molecular volume and the dipole moment. Using the model system of DDT SAMs and F10H2 substituents, we demonstrated that such a mixing can be performed by the IPER approach. Varying the irradiation dose, the portion of F10H2 species in the binary DDT-F10H2 film could be varied up to about 40%. This upper limit of the intermixing is presumably related to a limited ability of the DDT matrix to accumulate the relatively bulky F10H2 moieties and to the onset of extensive cross-linking in this matrix which sets a limit of the applied dose (at about 1-2 mC/cm2). Another consequence of the latter effects is incomplete exchange of the damaged DDT species for the F10H2 moieties in the case of high irradiation doses (above 1 mC/cm2); the respective DDTF10H2 films contain up to 20% of the damaged DDT species. The mixed SAMs prepared by IPER were compared to the analogous films fabricated by the coadsorption. Analysis of the XPS data indicated that the films prepared by the different methods are different to some extent. We speculate that this difference is the phase separation, which can occur in the films fabricated by the coadsorption but is not expected in the case of IPER.

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3705 For the mixed DDT-PFAT SAMs, a composition-dependent shift of the characteristic photoemission peaks was observed. This shift correlates qualitatiVely with the composition-dependent change of the work function. The above effect was explained within the electrostatic framework assuming a complex alignment of the Fermi and vacuum levels between the SAM, substrate, and spectrometer in a photoemission experiment. The results of this study can be important from the viewpoint of applications. In particular, SAMs are considered to be suitable to manipulate the Schottky barrier between a metal electrode and the organic electronic material (see, e.g., refs 44 and 94). A continuous variation of the work function by molecular intermixing opens a new route within this approach. Acknowledgment. We thank M. Grunze for the support of this work, Ch. Wo¨ll and A. Nefedov for the technical cooperation at BESSY II, and the BESSY II staff for the assistance during the synchrotron-based experiments. This work has been supported by DFG (ZH 63/9-2, ZH 63/10-1, and TE 247/6-2). References and Notes (1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Thin films: self-assembled monolayers of thiols; Ulman, A., Ed.; Academic Press: San Diego, CA, 1998. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (4) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (5) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (6) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (7) Bertisson, L.; Liedberg, B. Langmuir 1993, 9, 141. (8) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1994, 98, 563. (9) Salaita, K.; Amarnath, A.; Maspoch, D.; Higgins, T.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 11283. (10) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (11) Shon, Y.-S.; Lee, S.; Colorado, R., Jr; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (12) Shon, Y.-S.; Lee, S.; Colorado, R., Jr; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (13) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440. (14) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H. -J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898. (15) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. (16) Shaporenko, A.; Ro¨βler, K.; Lang, H.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 24621. (17) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (18) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167. (19) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192. (20) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (21) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (22) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 1 1997, 36, 2379. (23) Kajikawa, K.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys., Part 2 1997, 36, 1116. (24) Chung, C.; Lee, M. J. Electroanal. Chem. 1999, 468, 91. (25) Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192. (26) Felgenhauer, T.; Rong, H. T.; Buck, M. J. Electroanal. Chem. 2003, 550-551, 309. (27) Yang, G.; Amro, N. A.; Starkewolfe, Z. B.; Liu, G.-y. Langmuir 2004, 20, 3995. (28) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. AdV. Mater. 2007, 19, 998. (29) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772. (30) Ballav, N.; Weidner, T.; Ro¨βler, K.; Lang, H.; Zharnikov, M. ChemPhysChem 2007, 8, 819.

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