Self-Assembly of 1,4-Benzenedimethanethiol Self-Assembled

pubs.acs.org/Langmuir © 2010 American Chemical Society

)

Self-Assembly of 1,4-Benzenedimethanethiol Self-Assembled Monolayers on Gold Hicham Hamoudi,†,‡ Mirko Prato,§, C
eline Dablemont,†,‡ Ornella Cavalleri,§ Maurizio Canepa,§ and Vladimir A. Esaulov*,†,‡ Universit
e-Paris Sud, and ‡CNRS, UMR 8625, Laboratoire des Collisions Atomiques et Mol
eculaires, LCAM, B^ atiment 351, UPS-11, 91405 Orsay, France, §CNISM and Dipartimento di Fisica, Universit a di Genova, Via Dodecaneso 33, 16146 Genova, Italy, and Istituto Nazionale di Fisica Nucleare, Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy )

†

Received November 16, 2009. Revised Manuscript Received February 10, 2010 A study of the self-assembly of 1,4-benzenedimethanethiol (BDMT; HS-CH2-(C6H4)-CH2-SH) monolayers on gold is presented. Self-assembled monolayers (SAMs) are characterized by reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE) measurements. The ensemble of measurements consistently shows that well-organized BDMT SAMs, with “standing-up” molecules, can be obtained on high quality gold films with incubation in n-hexane provided that N2-degassed solutions are used and all preparation steps are performed at 60 °C in the absence of ambient light. SE data indicate that the optical interface properties of the BDMT-Au system are different from those of simple alkanethiol SAMs. A possible mechanism for the formation of the “standing-up” phase from the lying-down phase via a hydrogen exchange reaction involving chemisorbed lying-down and free dithiol molecules is discussed.

Introduction Self-assembly of organic molecules at surfaces has been the object of a very large number of investigations in relation to a variety of applications in the fields of corrosion protection, sensors, and molecular electronics, to name a few. Regarding specifically metal surfaces, many studies have focused on molecules bearing a thiol termination because of the ease of S-metal binding. In particular, the assembly of many different kinds of sulfur-based molecules, for example, alkane thiols and dithiols, thiophenes, and thiophenols on different substrates, such as Au, Ag, and Cu, Pt have been investigated.1-5 Dithiol molecules have attracted particular attention because of the possibility of using the two thiol terminations to bind to different metallic entities.6-12 A dithiol self-assembled monolayer (SAM) with free pendent SH groups allows grafting of metallic atoms or nanoparticles,11,12 that can in turn become a useful platform for further growth of metallic contacts and more complex heterostructures.9 *To whom correspondence should be addressed. E-mail: vladimir.esaulov@ u-psud.fr. (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (3) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440–5443. (4) Guo, Z.; Zheng, W.; Hamoudi, H.; Dablemont, C.; Esaulov, V. A.; Bourguignon, B. Surf. Sci. 2008, 602, 3551–3559. (5) Subramanian, S.; Sampath, S. J. Indian Inst. Sci. 2009, 89, 1–7. (6) Liang, J.; Rosa, L. G.; Scoles, G. J. Phys. Chem. C 2007, 111, 17275–17284. (7) K€ashammer, J.; Wohlfart, P.; Weiβ, J.; Winter, C.; Fisher, R.; MittlerNeher, S. Opt. Mater. 1998, 9, 406–410. (8) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147–5153. (9) Sarathy, K. V.; Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1999, 103, 399–401. (10) Aliganda, A. K. A.; Lieberwirth, I.; Glasser, G.; Duwez, A.-S.; Sun, Y.; Mittler, S. Org. Electron. 2007, 8, 161–174. (11) Sakotsubo, Y.; Ohgi, T.; Fujita, D.; Ootuka, Y. Phys. E 2005, 29, 601–605. (12) Pethkar, S.; Aslam, M.; Mulla, I. S.; Ganeshan, P.; Vijayamohanan, K. J. Mater. Chem. 2001, 11, 1710–1714.

7242 DOI: 10.1021/la904317b

We recently investigated assembly of butane-, hexane-, and nonanedithiols (C9DT) using a combination of reflectionabsorption infrared spectroscopy (RAIRS), sum frequency generation (SFG), spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and electrochemistry methods.13,14 A protocol allowing formation of well-ordered hexanedithiol (HDT) and nonanedithiol (NDT) SAMs from degassed n-hexane solutions in the absence of light was established. Measurements showed that phases consisting of upright molecules and also some lying-down molecules coexist. The transition to the upright phase is favored for the longer chain dithiols. Here we extend this study and address the case of the 1,4-benzenedimethanethiol (BDMT; HS-CH2-(C6H4)-CH2-SH) SAMs. Dithiols with phenyl units allow conduction at the molecular scale and pave the way to create molecular contacts between nanosized metallic electrodes.6,15-26 In fact, the first electrical (13) Hamoudi, H.; Guo, Z. A.; Prato, M.; Dablemont, C.; Zheng, W. Q.; Bourguignon, B.; Canepa, M.; Esaulov, V. A. Phys. Chem. Chem. Phys. 2008, 10, 6836–6841. (14) Daza Milone, M. A.; Hamoudi, H.; Rodrı´ guez, L. M.; Rubert, A.; Benitez, G. A.; Vela, M. E.; Salvarezza, R. C.; Gayone, J. E.; S
anchez, E. A.; Grizzi, O.; Dablemont, C.; Esaulov, V. A. Langmuir 2009, 25, 12945–12953. (15) Weckenmann, U.; Mittler, S.; Naumann, K.; Fischer, R. A. Langmuir 2002, 18, 5479–5486. (16) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; W€oll, C. Langmuir 2002, 18, 7766–7769. (17) Tai, Y.; Shaporenko, A.; Rong, H.-T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806–16810. (18) Yang, Y.-C.; Lee, Y.-L.; Ou Yang, L.-Y.; Yau, S.-L. Langmuir 2006, 22, 5189–5195. (19) Pugmire, D. L.; Tarlov, M. J.; Van Zee, R. D. Langmuir 2003, 19, 3720–3726. (20) Pasquali, L.; Terzi, F.; Zanardi, C.; Pigani, L.; Seeber, R.; Paolicelli, G.; Suturin, S. M.; Mahne, N.; Nannarone, S. Surf. Sci. 2007, 601, 1419–1427. (21) Pasquali, L.; Terzi, F.; Seeber, R.; Doyle, B. P.; Nannarone, S. J. Chem. Phys. 2008, 128, 134711. (22) Joo, S. W.; Han, S. W.; Kim, K. J. Phys. Chem. B 1999, 103, 10831–10837. (23) Rifai, S.; Morin, M. J. Electroanal. Chem. 2003, 550-551, 277–289. (24) Qu, D.; Uosaki, K. J. Phys. Chem. B 2006, 110, 17570–17577. (25) Silien, C.; Dreesen, L.; Cecchet, F.; Thiry, P. A.; Peremans, A. J. Phys. Chem. C 2007, 111, 6357–6364. (26) Venkataramanan, M.; Ma, S.; Pradeep, T. J. Colloid Interface Sci. 1999, 216, 134–142.

Published on Web 03/03/2010

Langmuir 2010, 26(10), 7242–7247

Hamoudi et al.

Article

measurements on molecules involved measuring the resistance of a 1,4-benzenedimethanethiol (BDMT) molecule.27 This was performed using a scanning tunneling microscopy (STM) tip held above a gold nanoparticle deposited on top of a BDMT SAM on a gold surface. In this and other experiments, one often assumes that the BDMT SAM is in fact formed with one sulfur attached to gold and the other sulfur pointing up. SAMs based on BDMT grown on noble metals from solution can be considered as a prototype system for the study of an aromatic dithiol assembly. In this case, the assembly should also be governed by lateral interactions between the aromatic rings of adjacent molecules. The presence of the methylene groups between the sulfur head terminals and the aromatic ring may also favor the packing of the molecules on Au.28-31 It would appear that in this case better assembly could be obtained with respect to cases where the SH group is directly linked to the aromatic ring. The system has been studied by several authors in some detail.19-26 Despite this effort, the conditions for the reproducible formation of highly ordered compact SAMs, with free -SH end groups at the outer interface, depending subtly on factors such as the quality of the substrate and reagents, solvent strength, photochemical action, and so on, are still the subject of debate. In some cases, existence of multilayered films involving disulfide bridge formation has been reported.20-22 Here we have used a combination of RAIRS, SE, and XPS to study BDMT self-assembly on high quality Au films grown on a mica substrate from an n-hexane solution previously used for alkanedithiol assembly. RAIRS, in particular, through the detection of sharp peaks related to the methylene group stretching modes allowed us to obtain information on the assembly quality. The results of this paper contribute to define a protocol allowing formation of highly ordered BDMT SAMs.

Experimental Section SAM Preparation. In order to get well-organized SAMs, we used gold on mica substrates (bought from PHASIS, which have a 200 nm thick final gold layer). For this work, the substrates were annealed at 600 °C for 30 s in an oven, followed by cooling performed under N2 flow. Final rinsing was done with hexane before drying under N2. The annealed substrates were checked by atomic force microscopy (AFM), showing the existence of large (a few micrometers) and flat Au(111) terraces. They were also checked by SE just prior to incubation in the solution. The dielectric function derived from SE on annealed samples gave results in excellent agreement with benchmark literature.32,33 Use of these samples has allowed us (see the Supporting Information) to obtain excellent results for an alkane SAM assembly similar to some of the best results available in the literature.5 BDMT (98% purity) was obtained from Aldrich and used without further purification. Some measurements were also performed as a reference for dodecanethiol (C12SH). C12SH (98% purity) was purchased from Aldrich. (27) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323–1325. (28) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y. J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359–3362. (29) Rong, H.-T.; Frey, S.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Langmuir 2001, 17, 1582–1593. (30) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; W€oll, C. Langmuir 2003, 19, 8262–8270. (31) Shaporenko, A.; Brunnbauer, M.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 14462–14469. (32) Prato, M.; Moroni, R.; Bisio, F.; Rolandi, R.; Mattera, L.; Cavalleri, O.; Canepa, M. J. Phys. Chem. C 2008, 112, 3899–3906. (33) Aspnes, D. E.; Kinsbron, E.; Bacon, D. D. Phys. Rev. B 1980, 21, 3290– 3299.

Langmuir 2010, 26(10), 7242–7247

The SAMs were prepared by immersing the gold support into a freshly prepared 1 mM solution of n-hexane for about 30 min at 60 °C. We used solutions freshly degassed by N2 bubbling. The samples were then rinsed with the same (fresh) solvent of the solution and dried with N2. To minimize photooxidation effects, all these procedures were carried out in the absence of ambient light. Experiments involving incubation in n-hexane at room temperature led to a quite large dispersion of results. This was also the case of experiments involving ethanol solutions, where we also observed anomalously large intensities in infrared spectra (see below). Spectroscopic Ellipsometry (SE). Principles of SE are described at length in books34,35 and reviews.36 Specific application to SAMs is addressed in detail in ref 32. SE measurements were performed in Genova on a recently upgraded rotating compensator spectroscopic ellipsometer (M-2000, J.A. Woollam Co. Inc.), which allows simultaneous measurements at 674 different wavelengths in the range 245-1700 nm. The experimental protocol adopted for SE measurements on ultrathin layers has been thoroughly described in a recent article on thiolate SAMs on gold.32 The tiny spectral variations induced by the formation of nanometer thick layers are best appreciated by calculating the difference between the data for pristine and SAM covered substrates (δΨ = Ψfilm - ΨAu, δΔ = Δfilm - ΔAu), for several zones of each sample. In this work spectra have been collected at 65° and 70° angles of incidence. The spot size on the sample was of the order of a few square millimeters. Measurements were performed ex situ, in laboratory atmosphere, immediately after extraction from solution and adequate rinsing. The instrument was also used to perform a transmission measurement in liquid to check the absorption characteristics of the BDMT solution in the wavelength range investigated. Infrared Spectroscopy. The Fourier transform infrared (FTIR) spectrometer used for analysis is a Bruker Vertex 70. It can be equipped either with a demountable gas cell with ZnSe windows for transmission spectra or with a homemade reflection attachment for RAIRS measurements. The incident angle was 80° to the surface normal. A deuterated triglycine sulfate (DTGS) detector was used to detect the reflected light. For the SAM measurements, the spectral resolution was set to 4 cm-1. The spectrometer and sample are flushed first with dry air and during measurements by N2 flow. XPS Measurements. XPS analysis was carried out in Genova with a PHI ESCA 5600 MultiTechnique apparatus. The system consists of an X-ray monochromatized Al source (hν = 1486.6 eV) and a spherical electron energy analyzer, used at a constant pass energy of 5.85 eV. Measurements at different takeoff angles (20° and 70°) have been performed. The binding energy (BE) scale was referenced to the Au 4f7/2 level at 84.0 eV. The data treatment follows a procedure outlined previously37,38 as discussed below.

Results and Discussion SE measurements performed on BDMT SAMs grown in nhexane at 60 °C are shown in Figure 1. The data are shown in the 245-1000 nm range for a 65° angle of incidence (blue continuous curve). The data have been obtained after averaging over typically four zones of five samples. (34) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland: Amsterdam, 1987. (35) Tompkins, H.; Irene, E. Handbook of Ellipsometry; Noyes Data Corporation/ Noyes Publications: Park Ridge, NJ, 2005. (36) Woollam, J. A.; Johs, B.; Herzinger, C. M.; Hilfiker, J. N.; Synowicki, R.; Bungay, C. Proc. SPIE 1999, CR72, 1. (37) Cavalleri, O.; Gonella, G.; Terreni, S.; Vignolo, M.; Floreano, L.; Morgente, A.; Canepa, M.; Rolandi, R. Phys. Chem. Chem. Phys. 2004, 6, 4042–4046. (38) Gonella, G.; Terreni, S.; Cvetko, D.; Cossaro, A.; Mattera, L.; Cavalleri, O.; Rolandi, R.; Morgente, A.; Floreano, L.; Canepa, M. J. Phys. Chem. B 2005, 109, 18003–18009.

DOI: 10.1021/la904317b

7243

Article

Figure 1. SE measurements performed on BDMT SAMs grown in n-hexane at 60 °C (blue continuous line). Data for C12SH SAMs are shown for comparison (red dotted line). Dashed lines (green and black) represent the results of simulations for transparent layers (n = 1.475 and 1.67) with thicknesses of 1.0 and 1.5 nm (see text). The vertical bars indicate the data dispersion related to sample-to-sample variations. A schematic drawing of the BDMT molecule derived from DFT calculations obtained with the GAUSSIAN package40 is also shown.

The shape of BDMT δΨ and δΔ spectra show an overall resemblance with the data obtained on alkanethiols and C9DT SAMs, that have been extensively discussed in previous works.13,32 In the figure, the BDMT data are directly compared with those obtained for C12SH SAMs deposited under the same conditions of substrate preparation and incubation. Note that the C12SH data present some subtle variations with respect to those presented in ref 32 which were obtained for RT deposition with a different solvent (ethanol) and on a different type of gold film, deposited on glass substrate via a Cr primer layer. In the present data, in particular, the small δΨ dip at about 330 nm, likely related to interface effects,32 appears better defined. It is well-known that SE measurements cannot give an independent evaluation of the thickness and optical properties of ultrathin layers. In order to suggest a reliable order of magnitude of thickness, besides the comparison with C12 SAMs, a few simulations (dashed green lines) are also reported. The simulations are representative of a three-phase model (ambient (air)/ transparent layer/substrate) with layers of variable thickness.32 In these simulations, the interfaces were considered sharp and the refraction index of the isotropous transparent layer was fixed first at 1.475, a reasonable value for packed alkyl chains39 and then at 1.67 as used for molecules with phenyl rings.39 Looking at the comparison of simulations with the δΔ data (lower panel), which bear most easily accessible information,32 a thickness of the order of 1.5 nm could be inferred. Three important aspects should be pointed out here. First, the same comparison based on δΨ suggests a thickness of 1 nm, which may mean that the model used is oversimplified. Second, the estimated thickness includes also the interface region, which may extend up to several a˚ngstroms.32 Finally, the refractive index of a very compact BDMT layer may be even larger than 1.67 as also speculated in ref 39, and hence, the actual thickness could be lower. The BDMT δΨ curve (upper panel) exhibits a sharp transition from (39) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90–94. Richter, L. J.; Yang, C. S.-C.; Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547–12559.

7244 DOI: 10.1021/la904317b

Hamoudi et al.

positive to negative values at about 500 nm wavelength which is typical of thiolate SAMs on gold.32,41 Some differences with respect to the C12SH pattern should be noted. The first difference regards the well-defined intensity decrease, which is observed on the δΨ BDMT curve for wavelengths less than 300 nm. This spectral feature cannot be related to a transparent layer (e.g., dashed curves of Figure 1). It occurs in the energy region of interest for UV absorption of aromatic rings. It is worth mentioning that we have observed rather narrow δΨ and δΔ dips also for other molecules displaying absorption in the UV-vis region.42 Thus, we proceeded to check the BDMT transmission in solution. As shown in the inset, a strong attenuation of transmission can be observed, in reasonably good agreement with the δΨ feature. Therefore, we assigned the dip to BDMT absorption, most probably related to aromatic rings, eventually influenced by intermolecular interactions in the SAM. The effect of optical absorption is visible, even if less sharp, also on the BDMT δΔ pattern, as it can be appreciated by the different curve slope below 300 nm with respect to the simulations of transparent films. A second relatively strong difference with respect to the C12SH pattern concerns the negative values above 500 nm in the δΨ spectrum. The BDMT values appear definitely larger (e.g., about 40% more at 700 nm) than those observed on C12SH and many other thiolate interfaces,13,32 and even display a rather broad minimum in the 600 nm region. As shown in Figure 1, a sharply interfaced transparent layer (e.g., dashed curves) is not able to reproduce the δΨ negative values. As it was broadly discussed in a previous work,32 such δΨ negative values must be related to what, in ellipsometry, is called the transition layer from the optical properties of bulk substrate to those of the SAM. The optical properties and the spatial extent of the transition layer are expected to depend on many inter-related factors consequent to the formation of the molecule-surface bond such as charge redistribution43 and chemisorption-induced structural and morphological changes,44-46 in turn strongly affected by packing density. Relying on results of ref 32, the measurements of Figure 1 indicate that the interface properties of the BDMT-Au SAM are different from the case of a simple alkanethiol SAMs, such as C12SH. On qualitative grounds, the larger negative BDMT δΨ (40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; , Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford CT, 2004. (41) Bordi, F.; Prato, M.; Cavalleri, O.; Cametti, C.; Canepa, M.; Gliozzi, A. J. Phys. Chem. B 2004, 108, 20263–20272. (42) Prato, M.; Alloisio, M.; Jadhav, S. A.; Chincarini, A.; Svaldo-Lanero, T.; Bisio, F.; Cavalleri, O.; Canepa, M. J. Phys. Chem. C 2009, 113, 20683–20688. (43) Heimel, G.; Ambrosch-Draxl, C.; Zojer, E. Adv. Funct. Mater. 2008, 18, 3999–4006. (44) Maksymovych, P.; Sorescu, D. C.; Yates, J. T., Jr. Phys. Rev. Lett. 2006, 97, 146103. (45) Yu, M.; Bovet, N.; Satterley, C. J.; Bengio, S.; Lovelock, K. R. J.; Milligan, P. K.; Jones, R. G.; Woodruff, D. P.; Dhanak, V. Phys. Rev. Lett. 2006, 97, 166102. (46) Mazzarello, R.; Cossaro, A.; Verdini, A.; Rousseau, R.; Casalis, L.; Danisman, M. F.; Floreano, L.; Scandolo, S.; Morgante, A.; Scoles, G. Phys. Rev. Lett. 2007, 98, 016102.

Langmuir 2010, 26(10), 7242–7247

Hamoudi et al.

Figure 2. XPS spectra of BDMT SAMs taken at two different takeoff angles (measured with respect to the surface).

values in the red-NIR spectral region could suggest stronger interface effects and/or a better organization. Regarding the former aspect, one should consider the different molecular structure of BDMT and simple alkanethiols. In this respect, recent density functional theory (DFT) calculations47 performed for several thiolate SAMs on gold claimed that the surface-to-molecule charge transfer (CT) depends significantly on the molecular groups which follow the thiol termination. In particular, CT was found to always include a contribution of approximately 0.20e to the S headgroup. If the S atom is followed by a saturated alkane chain, only a very small additional charge transfer, of the order of 10-2e, was calculated. Phenyl units instead were found to involve a much larger increase of the CT and a charge spreading along the molecule. It should be mentioned that this point is still controversial, as other works on thiolate SAMs, although reporting on a substantial charge redistribution at the interface, partly involving the phenyl unit, questioned the occurrence of a net surface-to-molecule charge transfer.48 It seems anyway quite possible that in BDMT the single CH2 unit could not prevent that some charge redistribution would involve the phenyl unit, thus affecting the electronic properties as well as the “spatial extent” of the interface region. XPS Measurements. In our experimental approach, we initially performed relatively low resolution survey scans. A typical survey scan on BDMT SAMs has been reported in the Supporting Information, where one can observe the C 1s level peak at about 284.1 eV BE, in good agreement with literature on related systems31 and relatively intense S 2p and S 1s levels. SAMs thickness values in the 0.9-1.2 nm range were derived from the adlayer-induced Au 4f intensity attenuation, for “reasonable” values of the so-called electron attenuation length of the order of 2.5 nm, obtained for alkanethiolate SAMs.50 We then concentrated on narrow scan measurements of the S 2p level. The irradiation time was optimized in order to minimize the damage, while maintaining good enough resolution and signal-to-noise ratio, to allow a reliable deconvolution of spectral subcomponents. XPS S 2p spectra are shown in Figure 2. Note the absence of high binding energy components related to oxidized sulfur species (47) Sun, Q.; Selloni, A.; Scoles, G. J. Phys. Chem. B 2006, 110, 3493–3498. (48) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. Acc. Chem. Res. 2008, 41, 721–729. (49) Lindberg, B. J.; Hamrin, K.; Johansson, G.; Gelius, U.; Fahlman, A.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 286–298. (50) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037–2042.

Langmuir 2010, 26(10), 7242–7247

Article

(at about 168 eV BE). For the deconvolution of S 2p peak subcomponents, we adopted the same procedure as in previous works on thiolate SAMs.37,38 In brief, after subtraction of a Shirley-like background, Gaussian functions were fitted to the photoemission peaks. For each S 2p doublet, a 1.2 eV spin orbit splitting and a 2:1 branching ratio between the 3/2 and 1/2 components were adopted.10 The peaks were assumed to have the same width. Each doublet was identified in terms of the position of the 3/2 peak. The spectral deconvolution shown required two doublets at 161.9 ( 0.2 eV (Sth) and 163.1 ( 0.2 eV (Sun) BE with a fwhm of 0.95 eV, where the error bars account for both the experimental and fitting uncertainties. The energy position of the peaks are in excellent agreement with high resolution synchrotron-based measurements on BDMT SAMs grown from solution with several solvents20 and from the vapor phase.21 The Sth/Sun intensity ratio changed appreciably as a function of the emission angle. In particular, Sth appears considerably attenuated at grazing emission, indicating that electrons belonging to this state had to travel through a larger portion of the adlayer. In fact, the binding energy location of Sth supports an assignment to the S-Au bond. The energy position of Sun is compatible with what is sometimes claimed as “unbound” sulfur, that is, sulfur which is not directly engaged in a thiolate molecule-surface bond, as, for example, free thiol (SH) groups or S-S bonds49 We note that the intensity of Sun is much less affected by the angle of emission. This means that this state is localized in the outer part of the adlayer. We assigned this state to pending SH groups of first-layer standing-up molecules. This assignment is also in reasonable agreement with simple arguments based on the so-called attenuation length of photoelectrons. In fact, if we assume that Sth and Sun electrons come from the bottom and the top of the layer, respectively, then we can fairly state that the Sth/Sun intensity ratio should be about exp(-d/λ sin j), where d is the layer thickness, λ is the mean free path of electrons in the SAM, and j is the takeoff angle. Assuming for λ a reasonable value of about 2.5 nm50 and taking into account the fit uncertainty on peak intensity, we obtain d values in the 1.1-1.5 nm range, which appear consistent with the value obtained from Au 4f attenuation and with the estimate based on SE data. RAIRS. Representative RAIRS spectra on BDMT SAMs are shown in Figure 3 (lower panel). We were able to observe sharp structures (line A) which we assign to the symmetric υs(CH2) stretch at about 2830 cm-1 and asymmetric stretch υas(CH2) at wavenumbers in the 2917 and 2921 cm-1 ranges. We also observe structures due to CH vibrations of the benzene ring in the 3100 cm-1 range. All these features are rather weak when compared to the ones observed, for example, for hexanethiol in similar conditions (see the Supporting Information). We also performed some measurements on a solid BDMT film obtained from a dried drop of BDMT. The resulting spectrum is shown in Figure 3 (top panel). The spectral features are much broader and shifted with respect to the positions of those of the SAM. We would assign these differences to a more disordered phase in this dried sample. Here we find that the most intense peak, which we attribute to υas(CH2), is located at 2930 cm-1. The quite broad peak centered at about 2838 cm-1 should correspond to the symmetric stretch υs(CH2). This may not be contradictory to the 2856 cm-1 assignment for disordered alkanethiols, because here we have a benzene ring. Thus, in some previous studies of other systems with benzene rings, for example, for liquid and solid 2-benzylphenol,51 (51) Katsyuba, S.; Chernova, A.; Schmutzler, R. Org. Biomol. Chem. 2003, 1, 714–719.

DOI: 10.1021/la904317b

7245

Article

Figure 3. RAIRS spectra of BDMT SAM on gold: (A) ordered SAM and (B) less ordered SAM and a solidified sample of BDMT (for details, see text).

υs(CH2) is located at 2841 cm-1. Also for some other molecules such as ethylhexene,52 ethylamine,53 or 2-(1-cyclohexenyl)ethylamine,54 υs(CH2) is located in the 2836-2840 cm-1 range. In the 3000 cm-1 region, we observe several features related to CH vibrations in the ring.17,23,26,29,31,55,56 In an earlier work on BDMT on gold nanoparticles, the main structures reported were at 3018 and 3045 cm-1. We did not find in existing literature assignments of all these peaks for 1,4-BDMT (see, e.g., 57). For 1,3-BDMT, peaks at 3055 and 2029 cm-1 have been reported in SERS spectra.58 In earlier works29,31 for ω-(40 -methylbiphenyl-4yl)alkanethiol (CH3-(C6H4)2-(CH2)n-SH (n = 0-6)) and 4,40 terphenyl-substituted alkanethiol (C6H5-(C6H4)2-(CH2)n-SH (n = 1-6)) SAMs, RAIRS spectra have shown peaks at 3029 cm-1 and at 3035 and 3060 cm-1. For 1,2-benzenedithiol liquid, several structures in infrared and Raman spectra have been observed and some assignments provided.59,60 We will not delve into these features here. Returning to the lower lying features, we recall here that, in extensively studied alkanethiol SAMs, better ordering corresponds to lowering of the value of υas(CH2) toward what is generally considered as an “optimum” value of 2917 cm-1, whereas for less ordered SAMs this value can be of the order of 2920 cm-1.1,5 In disordered SAMs, just as in the vapor phase, this is usually found around 2928 cm-1. Similarly, in well-ordered alkanethiol SAMs, υs(CH2) is shifted down to about 2848 cm-1, whereas it is at 2855 cm-1 in disordered SAMs as in the vapor phase. (52) Zeroka, D.; Jensen, J. O.; Samuels, A. C. THEOCHEM 1999, 465, 119–139. (53) Sent€urk, S.; Parlak, C.; Aytekin, M. T.; Senyel, M. Z. Naturforsch. 2005, A60, 532–536. € Parlak, C.; Aytekin, M. T.; Senyel, M. Spectrochim. (54) Izgi, T.; Alver, O.; Acta,Part A 2007, 68, 55–62. (55) Duan, L. L.; Garrett, S. J. Langmuir 2001, 17, 2986–2994. (56) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parkh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529–9534. (57) Varsanyi, G.; Szoke, S. Vibrational spectra of benzene derivatives; Academic Press: New York, 1969. (58) Lim, J. K.; Kim, Y.; Kwon, O.; Joo, S. W. ChemPhysChem 2008, 9, 1781– 1787. (59) Griffith, W. P.; Koh, T. Y. Spectrochim. Acta 1995, 51A, 253–267. (60) Varsanyi, G. Assignment for Vibrational Spectra of Seven Hundred Benzene Derivatives; Academic Kaido: Budapest, 1974.

7246 DOI: 10.1021/la904317b

Hamoudi et al.

In earlier works, infrared spectra of BDMT on Au nanoparticles26 showed a feature assigned to υs(CH2) at 2920 cm-1. In SERS spectra on gold nanoparticles,22 υas(CH2) and υs(CH2) were reported at 2917 and 2845 cm-1, respectively. Our assignments of the peaks at 2917 and 2830 cm-1 to υas(CH2) and υs(CH2), respectively, in an ordered BDMT SAM is done by analogy with the downward wavenumber shift of these modes from 2928 to 2917 cm-1 and 2855 to 2848 cm-1 for alkanethiol SAMs. The fact that υas(CH2) is found at the same value as that for an alkanethiol SAM could be coincidental. We have observed some degree of variability between different samples. We thus present another spectrum (line B) measured for a different sample, with a less ordered SAM. Here, the spectrum with the peak at 2921 cm-1 assigned to υas(CH2) is broader, with a shoulder extending to lower wavenumbers. An interesting feature is the change in the positions and intensities of the peaks related to CH vibrations in the ring. One can observe a shift of these peaks and changes in relative intensities as the υas(CH2) moves to 2917 from 2921 cm-1, that is, when we believe a better order is reached. This is difficult to interpret at present, without supporting calculations. One of the reasons might be due to differences in BDMT orientation with increasing order, possibly due to a transition from a more inclined to a more vertical molecular orientation and a compact parallel packing of the phenyl units increasing the lateral interactions. The former effect would be to some extent akin to earlier observations on orientational changes in an azobenzenealkanethiol SAM.61 Finally, we want to add a brief comment about measurements performed in ethanol. In this case, the υas(CH2) peak was found at 2926 cm-1, indicating a disordered layer. In a number of cases, the intensity of this peak was much larger than that for the n-hexane prepared film (see also the Supporting Information), suggesting that there were additional molecules bonded to the SAM, possibly through disulfide bonds. The possibility of forming BDMT SAMs with more than a monolayer equivalent of molecules from ethanol has been mentioned in earlier works.20

Concluding Remarks We have presented a study of BDMT SAMs on gold. The combined analysis of RAIRS, SE, and XPS data show that wellorganized BDMT SAMs can be obtained in degassed n-hexane solutions at 60 °C, with all the preparation procedures performed in the absence of ambient light. It has been noted in the past by us and other authors62 that, for example, for alkane SAMs, annealing the SAM at 60 °C leads to better order with larger domains and distribution of pits along domain boundaries. As mentioned, better order and also transition from a more inclined to a more vertical molecular orientation and a compact parallel packing of the phenyl units at higher temperatures have also been reported.62,63 Our method of assembly at the higher 60 °C temperature follows these observations, and we believe that the better assembly results are due to larger mobility. Overall, this appears to lead to better order and packing of the SAM and much more reproducible results. On average, values of film thickness, including both the interface and molecular layer, estimated in this paper by SE and XPS, are in the 1-1.5 nm range, and they appear slightly larger than those expected when taking into account the molecular size alone. (61) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071–6082. (62) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998. (63) Xu, Q.; Ma, H.; Yip, H.; Jen, A. K.-Y. Nanotechnology 2008, 19, 135605.

Langmuir 2010, 26(10), 7242–7247

Hamoudi et al.

In cases when order, as seen in RAIRS, was not optimal, this finding does not allow us to entirely rule out existence of a limited amount of disulfide oligomers incorporated into the SAM, as suggested by our results here at room temperature (and in ethanol) and also observed earlier in some conditions for alkanedithiols,13,14 which could lead to an increase of the effective thickness. SE and RAIRS results showed some interesting features. Thus, in SE, a more negative value in the 500-1000 nm region of the δΨ curves with respect to the C12 SAMs is observed and could be related to different characteristics of the S-Au interface. This may be due to different degrees of charge transfer and/or a different packing of the SAM. In RAIRS, spectra show that, with increasing order in the BDMT SAM, when υas(CH2) moves to 2917 cm-1, there is a concurrent change in position and relative intensities of the features related to CH vibrations in the ring. A possible reason might be a change in BDMT molecular orientation tending to a vertical position, with increasing order and a compact parallel packing of the phenyl units increasing the lateral interactions. For benzenethiol SAMs, both such a parallel configuration of molecules and a herringbone type assembly have 64 For√this system, recent STM been considered theoretically. √ 65 measurements proposed ( 13  2 5)R46°. More input from theory is necessary to make more definitive statements about our observations for BDMT. Many works on dithiols have mentioned the existence of lyingdown phases, with both S atoms attached to gold. This has also been reported for BDMT in the case of, for example, evaporative absorption in vacuum.21 An interesting question regards the (64) Nara, J.; Higai, S.; Morikawa, Y.; Ohno, T. J. Chem. Phys. 2004, 120, 6705. (65) Kang, H.; Lee, H.; Kang, Y.; Harab, M.; Noh, J. Chem. Commun. 2008, 5197–5199.

Langmuir 2010, 26(10), 7242–7247

Article

reasons behind the transition of the dithiol from the lying-down phase to the standing-up phase. In this case, one needs to generate a free SH group from the lying-down molecule, a process that should be active both in solution and in vacuum adsorption. Free SH groups on nonanedithiol SAMs have been reported by us.13 A possible explanation could be a hydrogen substitution reaction involving interaction of a free BDMT molecule (HSRSH) with a lying-down chemisorbed molecule on gold (AuSRSAu), with a surface mediated exchange of an H atom between the two: HSRSH þ AuSRSAu f 2AuSRSH This would lead to “liberation” of one end of the chemisorbed molecule as the -SH end and adsorption of the initially free molecule, with one S attached to gold. We hope this work will stimulate theoretical investigations into some of these fascinating aspects. The procedure of BDMT SAM preparation we outline could be useful in applications involving growth of metallic films and nanoparticles, as in molecular electronics. Acknowledgment. M.C. thanks the University of Genova for funding and Fondazione Carige and INFN which supported the upgrading of the spectroscopic ellipsometer. M.C. and M.P. thank A. Chincarini for discussion about XPS measurements and the Genova section of INFN for the availability of the XPS apparatus. Supporting Information Available: RAIRS spectra of hexanethiol and BDMT; survey X-ray photoemission scan of BDMT on Au/mica. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la904317b

7247