Substitutional Self-Assembly of Alkanethiol and Selenol SAMs from a

ARTICLE pubs.acs.org/JPCC

Substitutional Self-Assembly of Alkanethiol and Selenol SAMs from a Lying-Down Doubly Tethered Butanedithiol SAM on Gold Vijay Chaudhari,†,‡ Harish Makri Nimbegondi Kotresh,§ Sampath Srinivasan,§ and Vladimir A. Esaulov*,†,‡ †

Institut des Sciences Mol
eculaires d’Orsay, Universit
e-Paris Sud, 91405 Orsay, France CNRS, UMR 8214, Institut des Sciences Mol
eculaires d’Orsay, Orsay ISMO, B^atiment 351, Universit
e Paris Sud, 91405 Orsay, France § Department of Physical & Inorganic Chemistry, Indian Institute of Science, Bangalore, India ‡

ABSTRACT: Substitutional self-assembly of thiol and selenol SAMs from a lying-down phase of butanedithiol (C4DT) (SAM) were investigated using thiols, disulfide, and diselenide molecules. The intent was to address the question if formation of a lying-down dithiol phase is an impediment to formation of standing-up dithiol phases as it has been assumed. It is demonstrated that this is not the case, and the C4DT SAM, where both the sulfur atoms are chemisorbed on gold, is removed and replaced in all cases. Differences in substitution kinetics are observed.

1. INTRODUCTION Self-assembled monolayers of organic molecules have been investigated intensively over recent years because of a variety of possible applications ranging from molecular electronics, biomimetic devices, corrosion protection, etc. A vast body of work is now available on prototype systems like alkanethiol1
15 SAMs assembled on Au(111). Another interesting type of molecules are alkanedithiols which have attracted much attention because of interest in using such molecules with two thiol groups to attach to different metallic entities, such as two nanoparticles, or bind nanoparticles onto dithiol SAMs on metal substrates for molecular electronic measurements, or else for creation of complex heterostructures, etc.16,17 In this case a problem could be the formation of a lyingdown phase in which both thiol ends are chemisorbed on the surface. While formation of ordered alkanethiol SAMs is an established fact, the formation of ordered dithiol SAMs has been a highly controversial subject,18
32 and in many cases disordered phases of dithiol films have been obtained. Recent work, in particular by some of us,27
31 shows that actually quite well ordered SAMs of standing-up molecules can be produced for e.g. nonanedithiol molecules,27,28 although it was also noted that some amount of lying-down phase persisted and the fraction of lying-down phase increased with decreasing chain length, suggesting that increasing van der Waals forces helped in attaining the standing-up phase.27 Thus, for butanedithiol only a lyingdown phase was found with both S atoms chemisorbed on Au.27 An interesting aspect in dithiol self-assembly is that one usually considers that assembly proceeds as for alkanethiols via an intermediate formation of a striped lying-down phase. This means that both thiol ends are initially chemisorbed following loss of hydrogen atoms. It has been suggested that formation of such a lying-down phase with both sulfur ends tethered to gold is an impediment to formation of a standing-up phase (see e.g. r 2011 American Chemical Society

refs 18 and 26) because it would not be possible to remove the chemisorbed sulfur ends. However, standing-up phases have been observed. Assuming that a lying-down phase is indeed initially formed (though in solution assembly there is no direct evidence of such an intermediate for dithiols), a question of much heuristic interest then concerns the mechanisms behind the transition of the dithiol from the lying-down phase to the standing-up phase. Indeed in this case, one needs to detach one chemisorbed S end from the substrate and generate a free
SH group from the lying-down molecule, a process that should be active both in solution and in vacuum adsorption. Generation of such SAMs with free
SH groups has been reported by us.26
31 A possible mechanism as suggested by us could28 be a hydrogen substitution reaction involving interaction of a free dithiol 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 with a
SH end and adsorption of the initially free molecule, with one S attached to gold. To our knowledge this question in dithiol self-assembly has not been investigated in any detail. One can also wonder in what other cases such a stripping of a doubly tethered dithiols could occur, i.e., if this would only occur for the above mechanism. To investigate this further, we studied substitutional assembly of alkanethiol SAMs from butanedithiol SAMs on gold using alkane disulfides (CH3(CH2)nS
S(CH2)nCH3). In this case we do not Received: May 9, 2011 Revised: July 18, 2011 Published: July 28, 2011 16518

dx.doi.org/10.1021/jp2042922 | J. Phys. Chem. C 2011, 115, 16518–16523

The Journal of Physical Chemistry C have an
SH group and an exchangeable H atom, but an intermediate
S
S
group. Note that self-assembled monolayers of alkanethiols formed from alkane disulfides have been investigated previously. We chose butanedithiol as our recent studies27 show that in this case we deal with only a lying-down phase with both sulfur atoms tethered to gold. This thus provides an unambiguous case to investigate this exchange process, where we know that we start with a lying-down phase with both S atoms attached to Au. We first investigated the case of substitution using a dodecanethiol CH3(CH2)11SH (or C12T) in order to confirm our previous conclusions. We then extended the study to didodecyl disulfide: CH3(CH2)11S
S(CH2)11CH3 (or DC12DS). Another related point we investigate regards selenide SAMs. These have recently attracted attention33
40 since they are viewed as alternatives to thiol-derived SAMs, in e.g. molecular electronics applications, since selenium is claimed to offer a better electronic match for the metal surface than the usually used sulfur,33
40 and contact resistance of the anchor group is of crucial importance for the electronic transport in molecular junctions. These SAMs (Se(CH2)nCH3) are usually formed35,37 starting from diselenides (CH3(CH2)nSe
Se(CH2)nCH3). Here we chose to investigate if we could form a selenide SAM by substitutional assembly from the lying-down butanedithiol SAM. For substitution we used didecyl diselenide: CH3(CH2)9Se
Se(CH2)9CH3 (DC10DSe) The results of this investigation, which clearly demonstrate that substitution does in fact proceed in all cases and reveals some interesting features regarding substitution reactions, are described below. In the context of dithiol self-assembly, these results clearly demonstrate that the existence of a lying- down dithiol SAM, doubly tethered by sulfur ends, is not an impediment for producing a standing-up dithiol SAM.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Butanedithiol and dodecanethiol were purchased from Alfa Aesar and Sigma. The DC12DS was purchased from Assemblon. The DC10DSe was synthesized as described below. We used ethanol and n-hexane solutions. n-Hexane was purchased from Riedel-de Ha€en. All chemicals were used without further purification. 2.2. Diselenide Synthesis. Didecyl diselenide was synthesized in Bangalore by a reported method.41 Step 1: Preparation of Sodium Diselenide (Ethanolic). Absolute ethanol (150 mL) was added in stirring mixture of 3.0 g (38 mmol) of selenium and 1.0 g (27 mmol) of ice-cooled sodium borohydride. After the initial reaction had subsided, the mixture was stirred, heated, and refluxed in a nitrogen environment in order to dissolve the selenium and expel H2Se (caution). The brownish-red ethanolic solution of sodium diselenide (Na2Se2) was then ready for further use. Step 2: Synthesis of Didecyl Diselenide. Decyl bromide (3 g, 25 mmol) was added to the ethanolic solution of Na2Se2 and refluxed for 4 h. Room temperature cooled reaction mixture was acidified with glacial acetic acid. N2 was passed for 15 min to remove any H2Se present. The reaction mixture was concentrated and treated with water. The orange oily product obtained was extracted with CH2Cl2 and dried over anhydrous MgSO4. The CH2Cl2 extract was evaporated, and the crude product was purified by column chromatography (n-hexane, silica gel 60
120

ARTICLE

mesh). The final yield is 83%. NMR cross-checks did not reveal presence of triselenides, which can sometimes be present. 2.3. SAMs Preparation. SAMs were prepared on gold-covered (200 nm) mica substrates purchased from Phasis. The gold area is a 6 mm wide strip on a 10 mm mica disk. The substrates were annealed in a furnace to 600 °C. The quality of these substrates has been checked previously29 by AFM and ellipsometry, showing appearance of large (111) terraces, and the dielectric constant derived from spectroscopic ellipsometry gave results in excellent agreement with literature. The assembly was carried out, following the procedure used previously for dithiols.27
29 For butanedithiol we follow the procedure described in27 to ensure the lying-down phase. C4DT SAMs were prepared by immersing the gold support into a freshly prepared 1 mM solution of C4DT n-hexane purged with N2 for 60 min at room temperature in the absence of light. Final rinsing was done with hexane. The SAM was then transferred into millimolar hexane solutions of the thiols, disulfides, and diselenides for variable times. Upon extraction the resulting samples were rinsed in hexane and dried under N2. Note that in this work we limited ourselves to preparation in n-hexane as outlined above because in our previous studies with dithiols we had observed a number of problems when assembly was performed with ethanol and in particular in order to avoid potential problems of oxidation, etc., with the starting C4DT. 2.4. Electrochemical Measurements. A three-electrode electrochemical cell and potentiostat were used. We used a saturated calomel electrode (SCE) and a platinum wire as reference and counter electrode, respectively. All potentials in the text are referred to the SCE electrode. The base electrolyte, 0.1 M NaOH aqueous solution, was prepared with pure water and solid analytical grade NaOH. The electrolyte was degassed with nitrogen prior to the experiments. Reductive electrodesorption from the Au substrates was performed at typically 0.05 V s
1 at room temperature (actual scan voltage step was of 10 mV). For electrochemical measurements a small portion (ca. 2 mm of the 10 mm strip) of the gold on mica sample is immersed into the solution. The actual area in the electrodesorption measurements is referred to the Au peak. 2.5. Infrared Spectroscopy. For RAIRS (reflection
absorption infrared spectroscopy) measurements we use a Bruker Vertex 70 FT-IR spectrometer, equipped with a grazing incidence reflection attachment with an incident angle of 80° to the surface normal. A deuterated triglycine sulfate detector was used to detect the reflected light. The spectral resolution was set to 4 cm
1. The spectrometer and sample compartment are first flushed with dry air and then, during measurements, by a N2 flow. Prior to measurements on the SAM-covered sample, reference measurements were made on the cleaned and annealed sample. PMIRRAS has not been performed and IR light is not polarized. Some complementary RAIRS measurements for the DC10DSe SAM were also performed in Bangalore using a Nicolet FTIR spectrometer and a mercury cadmium telluride detector.

3. RESULTS In the following we present results of our study of the possibility of substitutional assembly of alkanethiol and selenide SAMs by replacing a formed lying-down phase of C4DT SAM. As mentioned above, in an earlier work we found that C4DT forms a lying-down phase with both thiol ends bound to the gold substrate. This was inferred from XPS measurements 16519

dx.doi.org/10.1021/jp2042922 |J. Phys. Chem. C 2011, 115, 16518–16523

The Journal of Physical Chemistry C

ARTICLE

Figure 2. RAIRS spectrum of SAMs from C4DT and C12T and after substitution (C4DT/C12T).

Figure 1. (a) CV recorded for C4DT SAM, C12T SAM, and C4DT SAM exchanged with C12T for 5 and 24 h incubation time. (b) CV recorded for C4DT SAM exchanged with C12T SAM at different concentrations for 24 h.

which clearly showed absence of “free”, nonchemisorbed sulfur atoms.27 In studying substitution, we performed two types of measurements, which can delineate the change from the C4DT SAM to the final SAM. We perform RAIRS measurements, where we look for a strong change in the RAIRS spectrum finger printing the formation of the longer chain alkane SAM and bearing a
CH3 group. Also, we perform cyclic voltammetry (CV). Here we base ourselves on an earlier observation25,27 that the position of the molecular desorption peak shifts to lower (more negative) voltages for longer alkane chains, and this shift is almost linearly proportional to chain length. 3.1. Substitution by C12T. We first consider substitution by C12T. A C4DT SAM was formed as described above. The sample was then rinsed and immersed into a C12T solution for variable times and for two different concentrations of the C12T molecules: 2 and 10 mM solutions. This was done in the initial experiments since it is known that in general efficiency of substitutional assembly depends on various factors including solution concentrations and immersion times (see e.g. ref 31). The results of cyclic voltammetry are shown in Figure 1a,b. The black line (Figure 1) corresponds to the reductive desorption of the C4DT SAM, with a peak appearing at
0.915 V/SCE and a charge density of 76 ( 3 μC cm
2, corresponding closely to a complete coverage, as described in our previous report on dithiol self-assembly.27 The red line corresponds to the reductive desorption curve for a pure C12T SAM. The desorption peak appears at a significantly lower voltage:
1.1 V/SCE with a slightly higher charge density 83 ( 5 μC cm
2.

The blue lines in Figure 1a and the data in Figure 1b report data after immersion of a C4DT SAM into the C12T solution for different times and concentrations. One can observe in all cases a shift of the reductive desorption peak to more negative voltage values, with respect to the C4DT peak, indicating that substitution occurs. In panel a, we compare two different immersion times in a 10 mM solution. We see that after 5 h the peak shifts toward the C12T position but is still far, showing clearly that substitution does occur, but is not complete. After 24 h in solution we observe a rather broad structure closer to the C12T position signifying more complete substitution. Note that in this work our objective was to demonstrate that substitution in fact occurs, but we were not focused on attaining full substitution. Longer immersion times were therefore not attempted. We noted that at lower concentrations substitution was less efficient for the same immersion times. Thus, in panel b, we show the results of substitution by immersion for 24 h into 2 and 10 mM solutions. Here we see that immersion in the higher concentration solution results in a larger degree of substitution based on the position of the reductive desorption peak. The presence of the peaks with shoulders in the voltammogram below (more negative) the C4DT position and also the structure at around 0.6 V, for both concentrations and different immersion times, suggests the existence of differently arranged phases of C12T SAM, whose relative proportions vary. This may be related to the not entirely full exchange within the selected substitution time leading to differences in order and packing and reflects itself in the lack of ordering in the IR spectra. The peak position in CV reflects interaction between the alkane chains, and in the disordered layer some less interacting species may be present in the layer. To analyze these results further, we report RAIRS measurement is Figure 2 for the 10 mM concentration and incubation for 24 h. The black line is the RAIRS spectrum for C4DT which does not show any marked structures because of the lying-down configuration of the short chain alkane molelcule. The red line shows a reference spectrum for the C12T SAM, prepared directly with no substitution, where we clearly observe the usual peaks corresponding to CH2 and CH3 vibrations. As described in previous works,1
4,14 in general in best ordered alkanethiol SAMs one observes well-defined quite narrow peaks at 2917 cm
1 corresponding to υas(CH2) and 2850 cm
1 for υs(CH2). Furthermore, one observes structures at 2964 cm
1 due to νas(CH3) vibration and at 2877 cm
1 due to 16520

dx.doi.org/10.1021/jp2042922 |J. Phys. Chem. C 2011, 115, 16518–16523

The Journal of Physical Chemistry C

ARTICLE

Figure 3. (a) CV and (b) RAIRS spectra of SAMs from C4DT and DC12DS and after substitution (C4DT/DC12DS).

Figure 4. (a) CV and (b) RAIRS spectrum of SAMs of C4DT and DC10DSe and after substitution (C4DT/DC10DSe).

νs(CH3). The relative intensities of the latter two peaks changes for even
odd chain length molecules because of final group orientation change (see e.g. ref 14). In the gas phase the CH2 related structures appear at 2926 cm
1 corresponding to υas(CH2) and 2854 cm
1 for υs(CH2), and this is also the case for disordered SAMs, where a shift from the 2917 and 2850 cm
1 positions toward the gas phase positions occurs. In some cases one observes intermediate shifts and broadening of these structures related to appearance of more or less well-ordered domains. The C12T SAM spectrum is similar to what one finds in the literature and is reasonably well ordered with υas(CH2) at about 2919 cm
1. The blue line in Figure 2 shows the RAIRS spectrum after substitution for 24 h in the 10 mM solution. We can see that we clearly observe structures similar to the case of the C12T SAM. In particular, we also see structures related to CH3 vibrations. This spectrum thus confirms the occurrence of substitution by C12T. The different relative intensities of the CH3 are presumably related to different orientation of the final group in the less well ordered SAM. The position of the υas(CH2) peak at 2923 cm
1 is indicative of poor order and it is somewhat broad, suggesting that the SAM has differently ordered domains. This would seem compatible with the broad peak in the reductive desorption. Summarizing these results clearly demonstrates that the C4DT SAM with both thiol ends chemisorbed on Au could be replaced by a C12T SAM. This could occur by the mechanism suggested by us previously and mentioned above, involving a hydrogen substitution reaction. 3.2. Substitution by DC12DS. We now consider the case of DC12DS, i.e., a molecule not having an SH termination, but an S
S group.

Results for cyclic voltammetry are shown in Figure 3a. The black line corresponds to C4DT and the red line to a DC12DSrelated SAM with a peak for the reductive desorption at 1.14 V/ SCE (charge density 71 ( 5 μC/cm2). The blue line corresponds to reductive desorption from a sample of C4DT SAM after immersion into a 1 mM DC12DS solution for 24 h. Here we used a 1 mM solution in order to have the same number of sulfur atoms as in the 2 mM C12T case. We see that the reductive desorption peak for substituted SAM occurs at
1.10 V (68 ( 5 μC/cm2) has shifted to almost the position of the peak for the pure DC12DS SAM, indicating that almost complete substitution of the C4DT SAM clearly took place. Also, the narrower desorption peak obtained here as opposed to the C12T case shows that the substitution appears more effective than in the case of the C12T SAM. We observe a sharp anodic peak for the DC12DS SAM possibly due to variation in oxidation process, which suggests a different adsorption mechanism for DC12DS SAM as compare to thiol ones. Figure 3b shows the RAIRS spectra for the DC12DS SAM and the substituted C4DT SAM. As in the case of substitution by C12T, we see that the RAIRS spectrum changes from the one for C4DT to a spectrum resembling the one obtained by direct assembly from a DC12DS solution. This clearly supports our CV data indicating that efficient substitution of the C4DT SAM does also occur for the case of a molecule having no
SH end group but an
S
S
group. Interestingly, substitution is much more efficient for similar S atom numbers in solution for the same immersion times when compared to C12T. 3.3. Substitution by DC10DSe. Figure 4a,b shows results of the investigation of C4DT substitution by DC10DSe. The black 16521

dx.doi.org/10.1021/jp2042922 |J. Phys. Chem. C 2011, 115, 16518–16523

The Journal of Physical Chemistry C and red lines in Figure 4a correspond to reductive desorption from the C4DT SAM and the SAM formed from DC10DSe. The desorption peak lies at
1.14 V (charge density is 60 μC/cm2) for DC10DSe SAM. The desorption peak position appears compatible with the data for alkanethiol SAMs and the result for C12T given above. The somewhat smaller charge would indicate a somewhat smaller packing, which could be related to differences in assembly characteristics and intermediate phases noted previously in the literature for Se SAMs.36 The blue line shows the CV substitution data after immersion of the C4DT SAM into a 1 mM solution of DC10DSe for 24 h. We clearly see a strong displacement of the reductive desorption peak to the position characteristic of the DC10DSe SAM, indicating an essentially complete substitution of the C4DT. The desorption peak lies at
1.17 V/SCE (charge density is 63 ( 5 μC/cm2) for the substituted DC10DSe SAM. The comparable charge density for both the DC10DSe and substituted SAM indicates the reasonable substitution. This is further tested by performing RAIRS measurements shown in Figure 4b. We see that the RAIRS spectrum of the substituted C4DT SAM is now characteristic of the DC10DSe SAM. Again the ordering of the SAM is not very good. Summarizing, we see again that the lying-down phase of the C4DT SAM could be substituted by the diselenide. For similar immersion times the substitution seems as efficient as for DC12DS and more effective than for C12T.

4. CONCLUSIONS We have presented an investigation carried out in the context of dithiol self-assembly problems, on the possibility to replace a lying-down phase of a dithiol SAM, in which both sulfur atoms are bound to gold by thiols, disulfides, and diselenides. We were able to show that the C4DT dithiol SAM could be efficiently replaced by C12T, DC10DS, and DC10DSe molecules. In earlier studies we proposed that in dithiol self-assembly formation of a standing-up phase from the lying-down one could be mediated by a hydrogen exchange reaction involving one of the
SH ends of the “incoming” free dithiol. The results with disulfide and diselenide molecules show that the S/Au surface bond can also be broken in case of presence of the
S
S
and
Se
Se
groups, leading to the C4DT removal and replacement of the detached end by one of the
S(Se)
R chains. At present without calculations it is difficult to give a simple model, since different scenarios could occur including possibly formation of some intermediate species e.g. between the detached dithiol and one of the “remaining”
S
R entities of the (S
R)2 molecule (or (Se
R)2)). This investigation clearly shows that the formation of a lyingdown dithiol phase is not an impediment to formation of a standing-up phase in dithiol assembly. Concerning the efficiency of these processes we note that higher concentrations of C12T in solution led to more rapid substitution, suggesting a “flux” type effect. Finally, an interesting feature was the observation that a much more efficient replacement occurs in case of a disulfide and diselenide molecule than in case of the thiol for similar incubation times and similar S atom numbers. This maybe related to a more efficient reactive exchange process and also again to a “flux” like effect in which immediate “availability” of a second RS (RSe) entity helps in substitution.

ARTICLE

It would be very interesting to investigate theoretically the reaction pathways and barriers for these substitution processes, and we hope these results stimulate such studies.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +33169157680. Fax: +33169157671.

’ ACKNOWLEDGMENT This work was supported by the ARCUS program of cooperation between French and Indian Universities. We thank Botao Ji and C. Dablemont for some assistance. Vijay Chaudhuri acknowledges a postdoctoral position from the Universit
e Paris Sud. ’ REFERENCES (1) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (2) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358–2368. (3) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569. (4) 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. (5) Hooper, A.; Fisher, G. L.; Konstadinidis, K.; Jung, D.; Nguyen, H.; Opila, R.; Collins, R. W.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 1999, 121, 8052–8064. (6) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440–5443. (7) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (8) Noh, J.; Kato, H. S.; Kawai, M.; Hara, M. J. Phys. Chem. B 2006, 110, 2793–2797. (9) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (10) Camillone, N.; Chidsey, C. E. D.; Liu, G. Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503–3511. (11) Danisman, M. F.; Casalis, L.; Bracco, G.; Scoles, G. J. Phys. Chem. B 2002, 106, 11771–11777. (12) Bu, D.; Mullen, T. J.; Liu, G. ACS Nano 2010, 4, 6863–6873. (13) Sarkar, S.; Sampath, S. Langmuir 2006, 22, 3388–3395. (14) Guo, Z.; Zheng, W.; Hamoudi, H.; Dablemont, C.; Esaulov, V. A.; Bourguignon, B. Surf. Sci. 2008, 602, 3551–3559. (15) Prato, M.; Moroni, R.; Bisio, F.; Rolandi, R.; Mattera, L.; Cavalleri, O.; Canepa, M. J. Phys. Chem. C 2008, 112, 3899–3906. (16) Sarathy, K. V.; Thomas, P. J.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1999, 103, 399–401. (17) Ohgi, T.; Sheng, H. Y.; Nejoh, H. Appl. Surf. Sci. 1998, 130
132, 919–924. (18) Liang, J.; Rosa, L. G.; Scoles, G. J. Phys. Chem. C 2007, 111, 17275–17284. (19) Tai, Y.; Shaporenko, A.; Rong, H. T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806–16810. (20) Niklewski, A.; Azzam, W.; Strunskus, T.; Fischer, R. A.; W€oll, C. Langmuir 2004, 20, 8620–8624. (21) Yang, Y. C.; Lee, Y. L.; Yang, L. Y. O.; Yau, S. L. Langmuir 2006, 22, 5189–5195. (22) 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. (23) Pasquali, L.; Terzi, F.; Zanardi, C.; Seeber, R.; Paolicelli, G.; Mahne, N.; Nannarone, S. J. Phys.: Condens. Matter 2007, 19, 305020. 16522

dx.doi.org/10.1021/jp2042922 |J. Phys. Chem. C 2011, 115, 16518–16523

The Journal of Physical Chemistry C

ARTICLE

(24) Kohli, P.; Taylor, K. K.; Harris, J. J.; Blanchard, G. J. J. Am. Chem. Soc. 1998, 120, 11962–11968. (25) Qu, D.; Kim, B. C.; Lee, C. W. J.; Uosaki, K. Bull. Korean Chem. Soc. 2009, 30, 2549–2554. (26) Pasquali, L.; Terzi, F.; Seeber, R.; Doyle, B. P.; Nannarone, S. J. Chem. Phys. 2008, 128, 134711–134721. (27) Millone, M. A. D.; Hamoudi, H.; Rodríguez, 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. (28) Hamoudi, H.; Prato, M.; Dablemont, C.; Cavalleri, O.; Canepa, M.; Esaulov, V. A. Langmuir 2010, 26, 7242–7247. (29) 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. (30) Alarcon, L. S.; Chen, L.; Esaulov, V. A.; Gayone, J. E.; Snchez, E. A.; Grizzi, O. J. Phys. Chem. C 2010, 114, 19993–19999. (31) Hamoudi, H.; Dablemont, C.; Esaulov, V. A. Surf. Sci. 2011, 605, 116–120. (32) Pasquali, L.; Terzi, F.; Seeber, R.; Nannarone, S.; Datta, D.; Dablemont, C.; Hamoudi, H.; Canepa, M.; Esaulov, V. A. Langmuir 2011, 27, 4713–4720. (33) Dischner, M.; Hemminger, J. C.; Feher, F. J. Langmuir 1997, 13, 4788–4790. (34) Yaliraki, S. N.; Kemp, M.; Ratner, M. A. J. Am. Chem. Soc. 1999, 121, 3428–3434. (35) Weidner, T.; Shaporenko, A.; M€uller, J.; Hotlig, M.; Terefort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 11627–11635. (36) Monnell, J. D.; Stapleton, J. J.; Jackiw, J. J.; Dunbar, T.; Reinerth, W. A.; Dirk, S. M.; Tour, J. M.; Allara, D. L.; Wiess, P. S. J. Phys. Chem. B 2004, 108, 9834–9841. (37) Subramanian, S.; Sampath, S. J. Colloid Interface Sci. 2007, 312, 413–424. (38) K€afer, D.; Bashir, A.; Witte, G. J. Phys. Chem. C 2007, 111, 10546–10551. (39) Fonder, G.; Cecchet, F.; Peremans, A.; Thiry, P. A.; Delhalle, J.; Mekhalif, Z. Surf. Sci. 2009, 603, 2276–2282. (40) Aslam, M.; Bandyopadhyay, K.; Vijayamohanan, K.; Lakshminarayanan, V. J. Colloid Interface Sci. 2001, 234, 410–417. (41) Klayman, D. L.; Griffin, T. S. J. Am. Chem. Soc. 1973, 95, 197–199.

16523

dx.doi.org/10.1021/jp2042922 |J. Phys. Chem. C 2011, 115, 16518–16523