Orientation and Order of Self-Assembled p-Benzenedimethanethiol

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J. Phys. Chem. C 2007, 111, 6357-6364

6357

Orientation and Order of Self-Assembled p-Benzenedimethanethiol Films on Pt(111) Obtained by Direct Adsorption and via Alkanethiol Displacement Christophe Silien,* Laurent Dreesen, Francesca Cecchet, Paul A. Thiry, and Andre´ Peremans Laboratoire de Spectroscopie Mole´ culaire de Surface, UniVersity of Namur, B-5000 Namur, Belgium ReceiVed: October 31, 2006; In Final Form: January 9, 2007

Self-assembly of p-benzenedimethanethiol (p-BDMT) on Pt(111) has been investigated by high-resolution electron energy loss spectroscopy and sum-frequency generation spectroscopy. The molecules adsorb on Pt(111) like on gold, attached through one single thiol, leaving the second thiol moiety unreacted and exposed to the outer surface. Yet, the order of the p-BDMT self-assembled film is poor in comparison to alkanethiol SAMs. Displacement in solution of a preadsorbed pentanethiol (PTT) SAM by p-BDMT molecules proved to be a successful approach to achieve films of higher quality, where the size of the dithiol molecular domains tends toward the size measured for PTT. Nearly complete exchange is accomplished after 42 h. However, no significant exchange is observed when dodecanethiol replaces PTT. Displacing the alkanethiols by benzenethiol leads to the same conclusions, suggesting that the exchange processes are lessened as compared to gold.

Introduction Self-assembly of R,ω-dithiols is pertinent for achieving a dependable integration of functionalized molecular wires between two electrodes. Technological relevance requires however the self-assembly process to produce organized layers, where the individual molecules are bonded through one thiol on a substrate, acting as the first electrode, and the second unreacted thiol moiety exposed to the outer surface, therefore available for bonding to the second electrode. In fact, numerous reports indicate that, upon self-assembly on gold, R,ω-dithiols lose a single thiol proton and align with their molecular axis close to the surface normal, so that the film is terminated by the unreacted thiol.1-8 On the other hand, on silver, the bonding of R,ω-dithiols generally proceeds through both thiol moieties.2,3,9-12 Variations from these trends are seldom observed. However, although 1,8-octanedithiol adsorbs on gold as a monothiolate,4 1,6-hexanedithiol is reported to lie flat on the same surface when vapor deposited or self-assembled from a solution.13 Yet, this molecule stands up when the surface potential is electrochemically controlled.14 In addition, self-assembling p-benzenedimethanethiol (p-BDMT) on gold particles leads to either monothiolate5 or dithiolate adsorption.12 On silver, 4,4′-biphenyldithiol is reported standing up on the surface.15 When the adsorption process results in the presence of one unbound thiol at the outer surface of the film, the second electrode can be connected. This can be achieved by various processes such as direct deposition of gold clusters or colloidal particles,16-18 nanotransfer printing,19 electrochemistry,20 and metallic vapor deposition.19,21 However, in the last two cases, not all authors agree that the metal does not penetrate the film and nucleates only at the outer surface by bonding to the exposed thiol moiety. A key aspect appears to be the quality of the molecular packing.19,21 Therefore, it seems important to produce compact and ordered R,ω-dithiols films. In comparison to gold, platinum has attracted less attention as a potential substrate. Use of platinum, and in particular of * To whom correspondence should be addressed. Current address: School of Chemistry, University of St Andrews, North Haugh, St. Andrews, KY16 9ST, U.K. E-mail: [email protected].

Pt(111) single crystal, presents several advantages. Indeed, unlike Ag, for example, ordered and large terraces can be easily obtained on Pt(111) by soaking in a corrosive solution and subsequent flame annealing. Furthermore, akin to gold, highquality alkanethiol SAMs can be also realized on Pt(111)22 with a tilt angle expected to be smaller than 15° for long molecules while an angle slightly less than 30° is normally obtained on Au(111).23 The alkanethiol surface density is also reported to be higher on Pt(111) than on Au(111),24 which is expected to result from the smaller lattice parameter of Pt. Therefore, platinum surfaces appear well suited for fabrication of dense and well-packed R,ω-dithiols films. It is only recently that the electrodeposition of dithiols, namely, 1,6-hexanedithiol, 1,9-nonanedithiol, 1,2-benzenedithiol, and 1,3-benzenedithiol, was investigated on crystalline platinum electrodes.25 Depending on the deposition conditions, formation of (x3×x3)R30° and p(2 × 2) overlayers was reported. The persistent observation of a same arrangement for all molecules prompted these authors to suggest that all these molecules adopt an upward orientation with the establishment of only one Pt-S bond per dithiol. Yet, no spectroscopic characterization is reported. The self-assembly process of dithiols on platinum has also been used to investigate the molecular electrical transport.26,27 In this study, p-BDMT is used to coat one Pt electrode, which is separated from a second one by a gap of ∼100 nm. After self-assembly, application of a potential to the junction in an aqueous solution containing Pt ions triggers growth of a platinum film only on the second electrode, eventually closing the gap. The authors however report an asymmetric current behavior, which indicates establishment of only a weak link between the electrochemically grown Pt electrode and the selfassembled film of p-BDMT. Because the true nature of the interface is unfortunately unknown, self-assembly of dithiols on platinum requires further characterization. Moreover, other authors have reported that direct selfassembly of dithiols on gold usually leads to a poorly ordered layer. However, they observed that using a mixture of monothiol and dithiol substantially improves the film quality and that, provided proper relative concentrations are used, coassembly

10.1021/jp067153e CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007

6358 J. Phys. Chem. C, Vol. 111, No. 17, 2007 results in an almost pure dithiol film.19 Similar results are achieved by replacing a self-assembled film by immersion into a solution of the dithiol or another molecule of interest. The early works using this methodology targeted introduction of individual linear conjugated molecules inside alkanethiol SAM by limiting the exchange time.28 It was observed that the alkanethiol layer remains compact around the inserted molecules. However, complete replacement of the SAM, which acts then as a template layer, can be achieved. In particular, we note the exchange of octadecanethiol SAM on gold by biphenyldithiol and p-BDMT. In this case, complete replacement is achieved in 15 h.16 The sequence of events occurring upon exchange in solution for SAM on gold has been repeatedly reported.29-34 It is observed that replacement takes place over the first hours at the domain boundaries of the original SAM and then proceeds much more slowly toward the inside of the domains. In this work, we report the investigation of the growth of p-BDMT films on Pt(111) by either direct self-assembly or displacement in solution of a preadsorbed alkanethiol SAM, aiming for formation of ordered and compact monolayers where one thiol moiety of the dithiol molecules is left unreacted and exposed at the outer surface. We used high-resolution electron energy loss spectroscopy (HREELS) to deduce the molecular orientation and estimate the order of the films.35 In addition, we employed sum-frequency generation spectroscopy (SFG)36 to perform a systematic investigation of the exchange process of SAMs on Pt(111). We compared the replacement of two alkanethiols of different length, namely, pentanethiol (PTT) and dodecanethiol (DDT), by p-BDMT and benzenethiol (BT). Experimental Section High-Resolution Electron Energy Loss Spectroscopy. The electron energy loss measurements were performed in an ultrahigh vacuum (UHV) system equipped with a Delta 0.5 from VSI GmbH. The spectral resolution (full width at half-maximum of the elastic peak) achieved in this study is comprised between 15 and 20 cm-1. The primary energy was kept at 1.5 eV in order to prevent the molecular films from being damaged by long exposure to the electron beam. The angle of incidence was set at 75° with respect to the surface normal, which favors electron scattering through the dipolar interaction mechanism.35 In order to single out the energy losses arising from dipolar scattering (i.e., with a dynamic dipole moment component normal to the surface), we recorded the data both in specular and off-specular geometries. The angular positions of the analyzer were 75° and 35°, respectively. All data shown in this paper have been normalized to the same integral of the elastic peak intensity. In off-specular geometry the intensity was corrected to take into account the angular inhomogeneity of impact diffusion. For the HREELS experiments Pt(111), Au(111), and Ag(111) single crystals were prepared under vacuum by repeating sequences of Ar+ ion sputtering and annealing with an electron beam irradiating the back of the sample for platinum and gold while by radiative heating for silver. Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) were systematically used to verify the cleanness and order of the metal surfaces. The UHV-cleaned crystals were exposed to ambient atmosphere for less than 1 min before chemical functionalization. All HREELS measurements have been performed at room temperature. Sum-Frequency Generation Spectroscopy. For SFG spectroscopy, we used two optical parametric oscillators built around LiNbO3 and BBO crystals and synchronously pumped by a

Silien et al. single Nd:YAG laser.37,38 We used the co-propagative configuration, with the IR and visible beams at 65° and 55° with respect to the sample surface, both beams being p polarized. The SFG photons were detected by a photomultiplier positioned behind a monochromator, which limited measurement to the p polarization and was set at the sum of the frequencies of the two incident beams. For the present investigation we fixed the visible wavelength at 532 nm while the IR frequency was scanned over the spectral range of the CH stretch vibrations, between 2750 and 3150 cm-1. The SFG measurements were performed at room temperature under ambient atmosphere with a spectral resolution better than 2 cm-1. Noteworthy, provided the laser beams are out of focus, we did not observe any evolution of the SAMs spectra. The sample lifetime in air appears long enough for our analysis. The absolute intensity of the SFG spectra could not be normalized in this work because of slight daily changes in the IR and visible powers. Henceforth, the spectra are stretched so as to maximize the occupation of the figures. The mechanically polished Pt(111) single crystals were first cleaned in freshly prepared hot piranha solution (98% H2SO4/30% H2O2 7:3 v/v) and subsequently rinsed with 18 MΩ cm water. The substrates were then thoroughly annealed with an oxygen-enriched butane flame. Although we did not systematically assess the quality of the surface, we repeatedly verified that the procedure produces large and clean terraces by scanning tunneling microscopy in air and cyclic voltammetry in H2SO4 0.1 M (aqueous media). After annealing, the samples were immersed into the appropriate solution for chemical functionalization. Materials and Sample Preparation. DDT, PTT, p-BDMT, and BT were purchased from Aldrich and used without further purification. The alkanethiol molecules were dissolved (5 mM) in absolute ethanol (better than 99.9%), while p-BDMT and BT were dissolved (5 mM) in dichloromethane (∼99.8%). Both the SFG and HREELS samples were prepared by dipping the single crystals into the appropriate solutions, kept at room temperature. The displacement experiments were achieved by successively immersing the clean platinum surface overnight into PTT or DDT and then into p-BDMT or BT solutions for periods varying from 1 h to several days. Before any measurement, all samples were thoroughly rinsed with ethanol or dichloromethane and blown dry with high-purity nitrogen. Because of the long sample manipulation and acquisition times in HREELS, fresh samples were prepared before every replacement step. More systematic measurements were obtained by SFG because in this case the same sample is used during the whole replacement process. Results and Discussion p-BDMT Orientation on Pt(111). In the first part of this paper we present our HREELS investigation of the self-assembly of p-BDMT on Pt(111) single crystal. Because the orientation of p-BDMT on silver2,10,12 and gold2,5,7,12,14,39 is well documented in the literature on the basis of SERS and infrared spectroscopy, we also recorded HREELS spectra on Ag(111) and Au(111). These analyses provide us with reference samples for clarifying the unknown adsorption behavior on Pt(111). In the left panel of Figure 1 we show the HREELS spectra of p-BDMT self-assembled on Pt(111), Au(111), and Ag(111). The spectra have been recorded in the specular (solid lines) and off-specular geometries (dashed lines). The difference highlights the dipolar activity of the vibrational modes. Given the relevance of having or not having free thiol moieties after bonding to the surface, we emphasize the wavenumber range between 2400

Self-Assembled p-Benzenedimethanethiol Films

Figure 1. (i) HREELS spectra of p-BDMT films self-assembled on Pt(111) (a), Au(111) (b), and Ag(111) (c). The data have been recorded in specular (solid line) [75°-75°] and off-specular geometry (dashed line) [75°-35°]. On curves b and c dipole-enhanced modes are indicated by the letter d. (ii) Off-specular HREELS data recorded in the wavenumber range of the SH stretch and for the same samples as shown on the first panel (a-c). The upper curve (d) has been measured for p-BDMT on Pt(111), obtained by replacement of PTT during 42 h.

and 2800 cm-1 in the right panel of Figure 1. These spectra were recorded on the same samples as those shown in panel i, in off-specular geometry, and with a longer integration time for Ag(111) and Pt(111). For p-BDMT on Ag(111), very few peaks exhibit a clear contrast (labeled d in the figure) between the specular and offspecular spectra, which indicates that the molecules adsorb with a preferred orientation and form ordered films. The mode of prevalent dipolar strength is detected at 725 cm-1. Some dipolar contribution is also detected for the mode at 1460 cm-1. Infrared measurements show that, upon adsorption on silver powder, BT exhibits one CH out-of-plane deformation mode (γCH) at 733 cm-1.40 Given that for SERS measurements for p-BDMT on polycrystalline Ag one vibration at 743 cm-1 is also assigned to one γCH mode,2 we attribute the dipole active mode at 725 cm-1 to a γCH vibration that involves the benzene moiety of the molecule. The attribution of one CH2 deformation at 1419 cm-1 in the SERS spectra recorded on Ag particles in solution3 and at 1430 cm-1 in ordinary Raman spectra2,3 gives a possible assignment for the HREELS peak at 1460 cm-1. The observation by HREELS of the CH2 scissor mode between 1455 and 1460 cm-1 for alkanethiol self-assembled monolayers on Au(111) clearly supports our attribution.41 Finally, the CH2 symmetric and asymmetric stretches (νCH2), located around 2865 and 2925 cm-1, are also clearly visible in specular HREELS, even though only a weak dipolar contribution is observed at 2925 cm-1. Our

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6359 assignment is obviously supported by the literature on pBDMT.2,3,9,10 Let us note that no clear contribution of the aromatic CH stretching (νCH) is observed and that no peak stemming from the SH stretching (νSH) is visible (see right panel of Figure 1). The HREELS spectra of p-BDMT self-assembled on Ag(111) thus exhibit only a significant dipolar activity for modes involving the γCH out-of-plane ring mode and to a lesser extent one CH2 bending. Our HREELS investigation is therefore consistent with an adsorption geometry where the benzene moiety is parallel to the silver surface. Furthermore, given that we do not observe any νSH vibration, we conclude that both thiol protons are lost during adsorption and that p-BDMT is indeed bonded as a dithiolate on Ag(111), in complete agreement with the available literature.2,10,12 The HREELS spectra of p-BDMT SAM films on Au(111), shown in Figure 1(i)b, displays a much richer spectrum than on Ag(111). However, in line with the IR spectra of the electrodeposited monolayer of p-BDMT on Au(111) reported by Rifai et al.,14 all modes are of disputable or weak dipolar activity. They are measured at 465, 516, 560, 679 (νCS), 770 (δCSH), 835 (γCH), 945 (γCH), 1015, 1118, 1184 (βCH) (in-plane deformation), 1225 (CH2 wagging), 1410 (CH2 scissor), 1504 (νCC), 2583 (νSH) (see right panel), 2919 (νCH2), and 3041 cm-1 (νCH). The assignment, given in parentheses, is based on the SERS investigation of polycrystalline gold presented by Murty et al.2 The modes at 1015 and 1118 cm-1 correspond well to the IR active modes observed by Venkataramanan et al.9 at 1020 and 1106 cm-1 and assigned to in-plane ring modes. Thus far, the three modes at 465, 516, and 560 cm-1 are left unassigned. The SERS spectra recorded on Ag particles in solution by Lee et al.10 exhibit Raman active modes at 464, 515, and 551 cm-1, which match the frequencies detected here by HREELS on Au(111). They have been assigned to in-plane, out-of-plane, and out-of-plane modes involving the benzene moiety. Murty et al. also report observation of SERS active modes on polycrystalline gold and silver at 515 and 500 cm-1, respectively.2 Although we do not claim that our assignments of the energy losses on Au(111) are definitive, it is clear that all peaks can be attributed to intrinsic vibrations of p-BDMT. Nonetheless, the three energy losses at 465, 516, and 560 cm-1 could also account for the presence of disulfide14, 41-43 and therefore indicate formation of a p-BDMT multilayer. In fact, several authors reported formation of p-BDMT multilayer on gold in specific conditions. Pugmire et al. showed by XPS that p-BDMT films self-assembled in the dark for 1 day in dry dichloromethane actually consisted of 2.2 monolayers.7 Joo et al. studied by ellipsometry the assembly of p-BDMT multilayers in various solvents.5 For exposure times exceeding 50 h, less than one monolayer was achieved in ethanol, while two to three monolayer thick films were grown in hexane. A similar behavior was reported for alkanedithiol as well by Kohli et al.3 They measured by ellipsometry multilayer formation when immersion in ethanol was longer than 1 night. Noteworthy, they also observed that a short exposure of the dithiol multilayer to ultraviolet radiation reduces the film thickness to a single monolayer. In our experiment, because the duration of the deposition is only about 15 h, a film thickness less than two p-BDMT monolayers is expected. In addition, since the solution is left exposed to ambient light during self-assembly as well as during the rinsing process, we believe that if a multilayer is partially formed only a marginal fraction of it remains on the surface. Therefore, we infer that our p-BDMT film is indeed representative of a single monolayer.

6360 J. Phys. Chem. C, Vol. 111, No. 17, 2007 The comparison between specular and off-specular HREELS shows that on Au(111) more modes possess a dipolar character (labeled d in Figure 1), although for none of them it is as evident as on Ag(111). Obviously, the molecular orientation is very different on both surfaces. Because the assignment proposed above indicates that both in-plane and out-of-plane modes are enhanced in the specular spectrum, our data indicate that now the plane of the benzene moiety is neither parallel nor perpendicular to the surface. In addition, observation of one clear shoulder at 294 cm-1, imputable to the Au-S bond,41 and δCSH at 770 cm-1 and νSH at 2583 cm-1 indicates that only one thiol proton is lost upon adsorption on Au(111). The second thiol group is left unreacted and, given the orientation of the benzene moiety, exposed to the outer surface of the film. The seamless agreement between our HREELS spectra and the SERS and IR data recorded by other authors on Au(111) and Ag(111) allows us to make a definitive determination of the orientation of p-BDMT on Pt(111) whose HREELS spectra are shown in Figure 1(i)a. Clearly, the spectra acquired on Pt(111) and Au(111) display strong similarities, while the one on Ag(111) appears markedly different. We observe peaks at 470, 510, 568, 703, 774, and 832 cm-1 along with two broad features around 1200 and 1445 cm-1, two smaller ones at 1608 and 1696 cm-1, and finally at 2919 and 3033 cm-1. A similar assignment to the one made for gold can thus be proposed here. In addition, long signal integration between 2400 and 2800 cm-1 reveals one mode at 2567 cm-1, which is assigned to νSH of one free thiol moiety. Therefore, the HREELS spectra recorded on Pt(111), supported by those measured on Ag(111) and Au(111), clearly demonstrate that p-BDMT adsorbs on Pt with the same configuration as on Au(111), i.e., as a monothiolate, with a tilted benzene ring and, consequently, one thiol pointing outward. Displacement of PTT by p-BDMT. Given that the direct self-assembly of dithiol leads to films of relatively poor quality, as known from the literature and demonstrated by our HREELS investigation (as we will discuss below), we studied the possibility of producing p-BDMT films by displacing alkanethiol SAMs. The objective is to use the alkanethiol matrix as a template that is progressively replaced by p-BDMT molecules so that, after completion of the process, the dithiol domains tend to match the alkanethiol ones. We first present the HREELS results obtained for PTT. The technique is indeed sensitive to the size of the molecular domains and therefore capable of determining the extent to which the dithiol film quality can be improved. In the left panel of Figure 2 we show the HREELS spectrum of a PTT SAM on Pt(111) (curve a) along with spectra recorded after dipping a fresh PTT layer in the p-BDMT solution for 21 and 42 h (curves b and c). The upper curve (d) corresponds to a p-BDMT layer directly self-assembled on Pt(111), as already shown and discussed above. For PTT, the most intense energy losses in specular geometry are measured at 743, 1100 (with a shoulder at 1048), 1376, 1455, and 2918 cm-1 (with shoulders at 2858 and 2953 cm-1). In keeping with a previous HREELS analysis of a large panel of alkanethiol SAM grown on Au(111),41 those peaks are assigned to the CH2 rocking, C-C stretching, CH3 symmetric and asymmetric bending (with contribution from the CH2 scissor), and the unresolved CH stretching modes. One mode appearing here at 1630 cm-1 is occasionally visible on Au(111) although not discussed.41 Other weak energy losses are also detected here. In particular, we note the shoulder around 360 cm-1, which is attributed to excitation of the S-Pt bond.44 At this stage it is appropriate to discuss a little further the implication of the HREELS spectra of PTT. Indeed, if the

Silien et al.

Figure 2. (i) HREELS spectra of a pentanethiol SAM (a), pentanethiol SAM immersed into p-BDMT for 21 (b) and 42 h (c), and p-BDMT film directly self-assembled on a clean platinum surface (d). The data were recorded in specular [75°-75°] (solid line) and off-specular [75°35°] (dashed line) geometry with a primary energy of 1.5 eV. On curve a, dipole-enhanced modes are indicated by the letter d. (ii) Angular dependence of the intensity of the elastic beam recorded on the samples shown in the left panel (a-d) and on the clean Pt(111) surface (e). The electron energy was 1.5 eV, and the angle of incidence is 55° with respect to the surface normal. The angular fwhm of each profile is also indicated.

orientation of alkanethiol SAM like DDT on Pt(111) has been discussed in the past, similar investigations are lacking for PTT. Besides, if it is generally admitted that longer alkanethiols only exhibit a moderate tilt angle on Pt(111) (smaller than 15° as estimated by Li et al.22), other experiments suggest that a short alkanethiol like butanethiol lies flat on the surface.45 In comparison to what is observed for alkanethiol on Au(111),23 specular and off-specular HREELS of PTT on Pt(111) are less contrasted. However, since the angular dispersion for our PTT SAM (3.6°) [Figure 2(ii)a] is only slightly larger than the value measured on clean Pt(111) (3.4°) [Figure 2(ii)e], the PTT film appears sufficiently ordered to unravel dipole active modes (labeled d in Figure 2). The HREELS data indicate thus that both CH2 rocking (743 cm-1) and CH3 symmetric deformation (1376 cm-1) modes possess a weak oscillating dipole perpendicular to the surface. In keeping with the observations on Au(111),41 the dipolar activity of the CH3 symmetric deformation suggests that the alkanethiol molecules are oriented with the CH3 end group pointing outward. Supporting this, enhancement of the CH2 rocking is indeed not compatible with the PTT lying flat on Pt(111). Although, it is not possible to determine accurately the tilt angle on the sole basis of HREELS, let us note that for an odd number of carbon atoms, HREELS shows

Self-Assembled p-Benzenedimethanethiol Films that, on Au(111), the CH3 symmetric stretching does not possess a measurable dipolar activity. Hence, it is tempting to propose that the tilt angle of PTT on Pt(111) is at least as small as on Au(111).41 The HREELS spectra measured after dipping the PTT SAM in the dithiol solution for 21 and 42 h show that PTT is progressively replaced by p-BDMT. Indeed, after 42 h, the fingerprint of p-BDMT can be clearly recognized, including νSH at ∼2600 cm-1 observed after longer averaging. The excellent match between the HREELS spectra recorded after replacement of PTT and direct growth strongly indicate that the molecules adsorb with the same geometry for both methods and replacement is very effective. Yet, the remaining differences suggest that the exchange is only nearly, but not completely, achieved. A good assessment of the order can be achieved by measuring the HREELS elastic angular dispersion after reflection onto the sample surface. Such a measurement is reminiscent of the angular profile of the [00] Bragg beam in LEED and achieved by recording the intensity of the electron beam at zero energy loss as a function of the detector angular position. Sharp Bragg diffraction is achieved on ordered surfaces, while diffuse intensity is observed on rough surfaces. The normalized data shown in the right panels of Figure 2 were recorded with an incidence angle of 55° and energy of 1.5 eV. The fwhm of the dispersion profile includes several contributions that cannot be separated here: angular dispersion of the incident beam, angular acceptance of the detector, and surface properties. On one hand, the fwhm value of 8.0°, obtained after direct deposition of p-BDMT [Figure 2(ii)d] indicates poor order of the film. On the other hand, the fwhm value of 3.6°, measured for PTT [Figure 2(ii)a], is quite close to the clean Pt(111) dispersion of 3.4° [Figure 2(ii)e]. After 42 h of dipping in p-BDMT, an angular dispersion of 4.2° is observed [Figure 2(ii)c], almost twice as low as compared to direct growth. This indicates that the size of the molecular domains of p-BDMT is roughly doubled and tends toward the PTT ones.46 A series of SFG spectra illustrating the progressive replacement of PTT is presented in Figure 3. A typical SFG spectrum of a PTT SAM on Pt(111) is shown in Figure 3a. On the basis of the SFG literature of alkanethiols on Pt(111),47,48 the three peaks observed at 2877, 2937, and 2968 cm-1 are assigned to the CH3 symmetric stretching (CH3-SS), Fermi resonance (CH3FR), and CH3 degenerated asymmetric stretching CH3 (CH3DS), respectively. The two shoulders at 2853 and 2912 cm-1 are assigned to the CH2 symmetric (CH2-SS) and asymmetric (CH2-AS) stretches, respectively. The upper spectrum (Figure 3e) corresponds to a p-BDMT film obtained by direct selfassembly on Pt(111). The SFG signal is characterized by an intense nonresonant background49 which is most likely of molecular nature given that it is not observed on clean platinum and systematically observed when aromatic moieties are present,50 see also ref 51. The vibrational bands of p-BDMT appear as dips located at 2885, 2945, 2974, and 3064 cm-1. In this wavelength range the Raman spectrum of p-BDMT exhibits a strong mode at 2840 cm-1, a weaker one at 2928 cm-1, and a strong one at 3056 cm-1 that have been assigned to the CH2SS, CH2-AS, and aromatic CH stretching, respectively.2,52 In the solid state the infrared spectrum shows one weak mode at 2856 cm-1, one strong at 2930 cm-1, and three of moderate intensity at 2965, 3028, and 3057 cm-1.9 Given the magnitude of the nonresonant SFG background for p-BDMT, a complex pattern of interferences48 is expected and the dip position may overestimate the exact wavenumber of the vibration. Hence, we assign the SFG dips at 2885, 2945, and 3064 cm-1 to CH2-SS,

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Figure 3. SFG spectra of PTT SAM (a), PTT replaced by p-BDMT (1 (b), 21 (c), and 42 h (d) immersion into the dithiol solution), and p-BDMT monolayer directly self-assembled (e). All layers are adsorbed on Pt(111). The zero SFG intensity is marked by horizontal dashed lines.

CH2-AS, and aromatic CH stretching, respectively. The dip at 2974 cm-1 is obviously related to the infrared active mode at 2965 cm-1, not clearly assigned in the literature but without doubt involving the aliphatic moieties of p-BDMT. We now turn to spectra b, c, and d of Figure 3 that were measured after dipping a fresh PTT SAM into the p-BDMT solution for 1, 21, and 42 h, respectively. No further changes were observed when keeping the sample up to 113 h in the solution. Obviously the p-BDMT fingerprint progressively takes over from the PTT signature, since curve d recorded after 42 h of dipping is reminiscent of the dithiol (e). Indeed, the average value of the nonresonant background rises and the PTT peaks at 2853, 2877, 2912, 2937, and 2968 cm-1 are replaced by the p-BDMT dips at 2886, 2945, 2973, and 3063 cm-1. A closer comparison between spectra d and e shows that the intensity of νCH at 3063 cm-1 (with respect to the SFG background) is similar. However, differences are perceptible in the width and relative intensity of the CH2-related modes. Indeed, in spectrum d obtained after PTT replacement the dip at 2886 cm-1 appears more pronounced and the width of the one at 2945 cm-1 is clearly reduced. These spectral differences may be indicative of incomplete replacement of the alkanethiol after 42 h, as already suggested by the HREELS measurements. The SFG and HREELS measurements demonstrate thus that a PTT monolayer can be efficiently replaced by p-BDMT within a time scale of 42 h. Yet, given the remaining differences between the spectra recorded after 42 h of replacement and direct self-assembly on the clean Pt(111), it is likely that replacement

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Figure 4. Evolution of the SFG spectra of alkanethiol SAMs on Pt(111) upon immersion time into solutions of aromatic thiol molecules. (i) PTT SAM dipped into BT. (ii) DDT into BT. (iii) DDT into p-BDMT.

of PTT is not complete. It is however noticeable that the molecular orientation of p-BDMT is unchanged and the domain size is increased and tends to match the one of the PTT SAM. Displacement of PTT and DDT by BT and of DDT by p-BDMT. Given that the alkanethiol SAM quality is improved for longer chains,22 we studied the replacement of dodecanethiol (DDT), hoping that a better organized matrix would further improve the order of the dithiol film. Longer chains than DDT were not tried here because extremely low replacement rates are expected. Indeed, it has been shown that, after 50 h, mercaptoundecanoic acid replaces more than 50% of octanethiol but only less than 10% of octadecanethiol53 as a result of an increased intermolecular interaction for longer molecules. Because both HREELS and SFG led to similar conclusions when studying the exchange of PTT with p-BDMT, we focus here only on SFG measurements. Since this technique does not require a UHV environment, more systematic measurements are indeed possible, and consequently, the same SAM is studied all along the displacement process. Before discussing replacement of DDT by p-BDMT, we shall first analyze displacement

of PTT and DDT by BT. Indeed, the SFG spectra for BT are simpler in the wavenumber range studied in this work since limited to one νCH stretching vibration. Given that the frequencies νCH2 and νCH3 of alkanethiol appear at much lower frequencies, we have two well-separated fingerprints that allow us to single out any perturbation of the alkanethiol matrix as well as the extent of the replacement. All these results are shown in Figure 4. The left panel (i) displays the evolution of a PTT SAM for various dipping times in a solution of BT. The longer time presented here is 61 h (upper curve). At this stage, PTT is mostly, although not completely, replaced by BT, as testified by the very intense nonresonant SFG background, the large intensity of the aromatic CH vibration (dip at 3071 cm-1), and the strongly weakened signal stemming from the alkanethiol νCH2 and νCH3. The spectra recorded for shorter dipping times (1, 18, and 37 h) show a progressive reduction of the intensity of PTT νCH3 and rise of BT νCH. Increasing the dipping time to more than 61 h did not cause further change in the SFG spectrum, suggesting that only a nearly complete displacement is feasible. Replacement of PTT

Self-Assembled p-Benzenedimethanethiol Films

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6363

Figure 5. Illustrations of p-BDMT and BT incorporated into compact alkanethiol monolayers on Pt(111). (i) p-BDMT into PTT SAM, (ii) BT into PTT, (iii) p-BDMT into DDT, and (iv) BT into DDT. The large difference in molecular length between BT and p-BDMT with DDT may lead to a bending of the alkane chain and the subsequent appearance of gauche defects (iii and iv). Note that the precise tilt angle of p-BDMT and PTT on Pt(111) is unknown. The representation is however compatible with our HREELS observations (see text for details).

by BT appears thus to closely follow the observations made for p-BDMT before. Noteworthy, before 37 h of immersion, we do not observe any particular effect on the alkanethiol SAM except for the progressive disappearance of its SFG fingerprint. Let us note a faint resurgence of νCH2 after 37 h (marked by arrows in the figure), which possibly suggests a proportionally larger amount of defects in the remaining alkanethiol film. The second panel (ii) of Figure 4 shows SFG spectra of displacement of a DDT SAM by BT. The lower curve corresponds to the spectrum of DDT. Five peaks are clearly observed at 2852, 2879, 2919, 2940, and 2968 cm-1 and assigned to the CH2 and CH3 stretching modes in accordance with our earlier discussion of the SFG spectra of PTT SAM. In clear contrast to what is observed for PTT, only a moderate substitution of DDT is observed after 60 h of dipping in the BT solution. This assertion is demonstrated by the modest SFG intensity, relative to the nonresonant background, of the aromatic νCH of BT (dip at 3075 cm-1) in comparison to the one measured after substitution of PTT (61 h), for which we showed that nearly complete replacement occurs. On gold, faster replacements are normally observed since complete replacement of octadecanethiol by biphenyldithiol and p-BDMT are achieved in only 15 h.16 This difference between gold and platinum can be explained by a greater stability of alkanethiol SAMs on Pt(111). This results from the larger molecular density on platinum,24 which favors the intermolecular interactions, and from the larger bond strength of sulfur with Pt, which is roughly twice as large compared to Au.54 Although only a moderate change of the PTT SFG spectral features was seen after 37 h of immersion, an important evolution of the DDT spectral features is observed after only 1 h. The relative intensity of the three νCH3 is indeed strongly reduced compared to the first νCH2 [see arrow in Figure 4(ii)],

suggesting an important change in the DDT structure. Compared to PTT, the stronger perturbation of DDT can be attributed to the larger difference in molecular length and consequently to the role played by the molecular interactions in both SAMs. BT is indeed considerably shorter than DDT, so that when the aromatic molecule is incorporated in the monolayer, the surrounding aliphatic chains are likely to bend, leading to more gauche defects in the film. In SFG, gauche defects in alkanethiol SAMs result indeed in the appearance of strong νCH2 modes.47,55-59 The process is illustrated in Figure 5. Noteworthy, in line with the literature,29-34 the absence of a visible aromatic νCH after 1 h indicates that the amount of exchange is still marginal. Yet, it induces a large change in the SFG spectra, suggesting that the perturbation extends further than one neighboring molecule. We finally turn to the SFG spectra of the displacement of DDT by p-BDMT that are displayed in Figure 4(iii). The data were recorded before and after 1, 17, 38, and 60 h of dipping in the p-BDMT solution. The evolution of the SFG signature is similar to the one discussed here above for the substitution of DDT by BT. Although the SFG spectral signature of the dithiol is hard to distinguish from the distorted DDT one, it never matches the one presented in Figure 3 for directly self-assembled p-BDMT. Poor replacement is thus again taking place after 60 h. In addition, a large decrease in intensity of the three νCH3 is seen after 1 h of dipping, in line with what is seen for BT. We conclude thus again that a strong perturbation is induced and that displacement is extremely poor compared to the one observed for PTT. Conclusions We investigated the growth of p-BDMT monolayers on Pt(111) single crystal by self-assembly and displacement of

6364 J. Phys. Chem. C, Vol. 111, No. 17, 2007 preadsorbed alkanethiol SAM. On the basis of HREELS measurement we infer that for both methods the molecule is bonded through one single thiol, leaving the second thiol unreacted and ready for further chemistry. The ordering of the directly self-assembled film is poor as seen from the angular profile of the HREELS elastic beam. However, we showed that displacement in solution of a PTT SAM, which is almost complete after 42 h, is an effective way to improve the ordering of dithiol monolayer with an increase of the size of the molecular domain toward the one measured for a pure PTT SAM. Despite the success achieved when precovering the surface with PTT, very poor exchange is observed with DDT. A similar conclusion is obtained when BT is used for replacement. Yet, the drastic difference in behavior as a function of the chain length as well as the noticeable improvement of the p-BDMT SAM order are interesting for realization of nanostructured, compact, highquality films on Pt(111). Acknowledgment. F.C. and A.P are, respectively, Postdoctoral Researcher and Senior Research Associate of the FNRS (Belgian National Fund for Scientific Research). L.D. acknowledges the Walloon Region for financial support. At the time of this research, C.S. was a Scientific Research Worker of the FNRS. The authors also acknowledge financial support from the Interuniversity Attraction Pole (IUAP-05) on “Quantum Size Effect in Nanostructured Materials”, initiated by BELSPO (Belgian Science Policy). References and Notes (1) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Withesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529-9534. (2) Murty, K. V. G. K.; Venkataramanan, M.; Pradeep, T. Langmuir 1998, 14, 5446-5456. (3) Kohli, P.; Taylor, K. K.; Harris, J. J; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962-11968. (4) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147-5133. (5) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 1999, 103, 10831-10837. (6) de Boer, B.; Meng, H.; Perepichka, D. F.; Zheng, J.; Frank, M. M.; Chabal, Y. J.; Bao, Z. Langmuir 2003, 19, 4272-4284. (7) Pugmire, D. L.; Tarlov, M. J.; van Zee, R. D. Langmuir 2003, 19, 3720-3726. (8) Tai, Y.; Shaporenko, A.; Rong, H.-T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806-16810. (9) Kwon, C. K.; Kim, K.; Kim, M. S. J. Mol. Struct. 1989, 197, 171180. (10) Lee, T. G.; Kim, K.; Kim, M. S. J. Phys. Chem. 1991, 95, 99509955. (11) Cho, S. H.; Han, H. S.; Jang, D.-J.; Kim, K.; Kim, M. S. J. Phys. Chem. 1995, 99, 10594-10599. (12) Venkataramanan, M.; Ma, S.; Pradeep, T. J. Colloid Interface Sci. 1999, 216, 134-142. (13) Leung, T. Y. B.; Gernstenberg, M. C.; Lavrich, D. J.; Scoles, G.; Schreiber, F.; Poirier, G. E. Langmuir 2000, 16, 549-561. (14) Rifai, S.; Lopinski, G. P.; Ward, T.; Wayner, D. D. M.; Morin, M. Langmuir 2003, 19, 8916-8921. (15) Weckenmann, S. U.; Mittler, S.; Naumann, K.; Fischer, R. A. Langmuir 2002, 18, 5479-5486. (16) Henderson, J. I.; Feng, S.; Ferrence, G. M.; Beins, T.; Kubiak, C. P. Inorg. Chim. Acta 1996, 242, 115-124. (17) Dorogi, M.; Gomez, J. M.; Osifchin, R. J.; Andres, R. P.; Reifenberger, R. P. Phys. ReV. B 1995, 52, 9071-9077. (18) Dadosh, T.; Gordin, Y.; Krahne, R.; Khivrich, I.; Mahalu, D.; Frydman, V.; Sperling, J.; Yacoby, A.; Bar-Joseph, I. Nature 2005, 436, 677-680. (19) Jiang, W.; Zhitenev, N.; Bao, Z.; Meng, H.; Abusch-Magder, D.; Tennant, D.; Garfunkel, E. Langmuir 2005, 21, 8751-8757.

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