Thermal study and structural characterization of self-assembled monolayers generated from diadamantane disulfide on Au(1 1 1)

Applied Surface Science 257 (2011) 3739–3747

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Thermal study and structural characterization of self-assembled monolayers generated from diadamantane disulfide on Au(1 1 1) Waleed Azzam a,∗ , Asif Bashir b , Osama Shekhah c a b c

Tafila Technical University, Department of Chemistry, P.O. Box 179, 66110 Tafila, Jordan Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

a r t i c l e

i n f o

Article history: Received 20 June 2010 Received in revised form 2 November 2010 Accepted 20 November 2010 Available online 27 November 2010 Keywords: SAM Adamantanethiol STM FTIR XPS Thermal

a b s t r a c t Self-assembled monolayers (SAMs) formed from diadamantane disulfide (DADS) on Au(1 1 1) have been characterized using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and infrared reflection absorption spectroscopy (IRRAS). SAMs of DADS prepared at room temperature (RT) were found to form at least four different orientational ordered domains having densely packed and well˚ The spectroscopic techniques used in this ordered structure with nearest neighbour distance of 7.0 ± 0.3 A. work reveal the formation of highly oriented SAMs with molecular axis orientated perpendicular to the surface. Annealing the SAMs prepared at RT to temperatures below 353 K results in no structural changes from that observed at RT. The samples that annealed at temperatures higher than 353 K exhibited different surface morphologies and structural changes were observed for these SAMs. In addition, the number of the different rotational domains is reduced to three after this annealing. After annealing at 353–393 K, the domain boundaries that were missed in SAMs prepared at RT or after annealing at temperature lower than 353 K appear in a shape similar to those observed for n-alkanethiols on Au(1 1 1). Moreover, the SAMs show a highly ordered hexagonal close-packed molecular lattice with a measured nearest neighbour √ √ ˚ corresponding to a ( 7 × 7)R19.12◦ unit cell. distance of 7.6 ± 0.1 A, © 2010 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the formation of stable and well-defined molecular adlayers using self-assembly has been the subject of enormous scientific interest due to their potential applications in thin-film sensor devices [1,2], wetting [3,4], lubrication [5,6], corrosion inhibition [7–9] and surface patterning [10–12]. It is essential to achieve a thorough understanding of the forces acting at the molecular level in self-assembled monolayers (SAMs), since they play a major role in establishing the monolayer properties [13,14]. The structure of the SAMs and their stability are primarily determined by the competation between two main forces, these are the interaction between the adsorbates and the substrate and the interchain interactions. SAMs composed of alkanethiols on Au(1 1 1) are the most frequently studied systems. In this case a variation of interchain forces can easily be achieved by altering the alkyl chain length. In addition it is possible to change the terminal functional group of the component of the molecules [15–23]. Besides the strong sulphur-gold interactions, van der Waals interactions that exist between the adjacent alkyl chains in the monolayer play a

∗ Corresponding author. Tel.: +962 799027858; fax: +962 32250002. E-mail address: [email protected] (W. Azzam). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.129

crucial role in determining the structure of the SAMs made from alkanethiols √Au(1 1 1). At the S/Au interface the sulphur atoms √ on form a ( 3 × 3)R30◦ overlayer with S–S distance of about 5 A˚ [22,24]. In contrast, SAMs of aromatic oligophenyl thiols stabilized by ␲–␲ interactions between the neighbouring adsorbates as well as the weak chemisorption of the thiolate on the gold substrate [14,25–33]. In some cases the sulphur atoms of the√aromaticthi√ ols are arranged in hexagonal structures based on ( 3 × 3)R30◦ two-dimensional lattices [25,29]. A deviation from this simple hexagonal arrangement has been recently reported for some aromaticthiols [13,29]. Previous works have demonstrated that the annealing as well as the immersion times play a significant role in determining the orientation and the ordering in the SAMs [18,19,21,25]. It was observed that low density packed phases of n-alkanethiols and aromaticthiols corresponding to striped phases can be achieved either by dosing from the gas phase by short immersion times [25] or by annealing [18,19,21]. Typically, the symmetry of alkanethiolate or aromaticthiolate self-assembled monolayers is lower than the high three-fold symmetry of the Au(1 1 1) substrate. As a result, different rotational domains and correspondingly domain boundaries exist. If the molecular overlayer exhibits at three-fold symmetry, then only three domains rotated with respect to each other by 120◦

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are present [34]. The ordered domains are separated by domain boundaries, which can be free of molecules or contain molecules adsorbed at irregular adsorption sites. Typically, these domain boundaries have to be considered as film defects. The ability to control and reduce the thin-organic film defects has important implications in technological applications. For example, pin-holefree and domain-boundaries-free monolayers would provide ideal matrices for device fabrication [35,36] and patterned interfaces [10–12]. The presence of domain boundaries to some extent limits the applicability of n-alkanethiolate and aromaticthiolate SAMs for nano device fabrication, although in some cases fairly large regions (several 100 nm) of perfectly ordered, domain boundary free regions have been observed [37]. In order to achieve high quality, long range ordered and defect free monolayers, it would be favourable to use thiolates that adsorb with their molecular axis orientated perpendicular to the surface. In such cases a higher, possibly three-fold, local symmetry of the molecular adlayer can be expected leading to the disappearance of rotational domain boundaries. One of the candidate molecules that have the potential to be orientated perpendicular to the surface is adamantanethiol. This bulky spherical compound has no free rotational joint in its molecular backbone, the sulphur-binding atom is directly attached to a spherical hydrocarbon cage. Adamantane (tricyclo[3.3.1.13,7]decane), C10 H16 , is a highly symmetric globular molecule and thermodynamically stable cage compound with the same structure as a diamond lattice [38–40]. The molecular structure of adamantane consists of three CH groups at the corners of a tetrahedron and six CH2 groups interconnecting the remaining C atoms. The structure consists of three cyclic rings each with six carbon atoms, made up of three alternating CH and CH2 groups. The unusual structure of adamantane conveys many constructive chemical and physical properties, such as high thermal and oxidative stability, extreme lipophilicity, low surface energy, and high density [41–43]. Moreover, the van der Waals interaction between the adjacent molecules is expected to be much weaker than that between the alkyl chains in n-alkanethiolate films with similar molecular length or the ␲–␲ interactions in the aromaticthiolates. Previous studies report that the adamantanethiol and its derivatives form highly ordered SAMs. SAMs formed from disulfide containing pair of adamantane moieties, namely, bis(tricyclo 3,3,1,1 decylmethyl) disulfide have been assembled on Au(1 1 1) and studied using STM [44]. The results showed the formation of four different ordered domains having hexago˚ Recently, nal arrangement with lattice constants of about 6.67 A. SAMs generated from 1-adamantanethiol revealed the formation of a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbour distance of 6.9 ± 0.4 A˚ [45]. Moreover, the SAMs showed five different rotational domains. ◦ Several unit cells were√proposed √ √ namely, (7 × 7), (7 × 7)R21.8 , √ ( 91 × 91)R27◦ , and ( 91 × 91)R5.2◦ . More recently, trithiols containing CH2 SH groups at the three bridgehead positions of the adamantine framework and a halogen-containing group Br, p-BrC6 H4 , and p-IC6 H4 at the fourth bridgehead were studied on Au(1 1 1) [46]. It has been reported that the molecules adsorbed on the gold surface via the three sulphur atoms. STM studies on Brterminated one exhibited a hexagonal arrangement of the adsorbed ˚ molecule with a lattice constant of 8.7 A. In an extension of the previous works on SAMs fabricated from adamantine thiols, in this study we report the results of a rather extensive, multi-technique study on SAMs fabricated from diadamantane disulfide (DADS) on Au(1 1 1) using FTIR, XPS, and STM. In the present study, the high resolution STM images enables us to determine the structure of DADS SAMs more precisely than that reported in literature [44–46]. In addition to the structural characterization, we have examined the temperature stability of these SAMs by annealing the samples in vacuum at different

temperatures. Previous results showed that thiols and disulfides form similar films, and there is strong evidence that the final chemisorbed state of the molecules is the same. A general agreement has been reached in the literature such that disulphides adsorb on the gold substrate by a completely cleaving of the S–S bonds thus forming an adamantane thiolate adlayer on the gold surface. 2. Experimental IRRAS spectra were recorded using a Biorad Excalibur Fourier transform infrared spectrometer (FTS 3000) equipped with a grazing incidence reflection unit (Biorad Uniflex) and a narrow band MCT detector. All spectra were measured with 2 cm−1 resolution at an angle of incidence of 80◦ relative to the surface normal and further processed by using boxcar apodization. Acquisition of STM data was carried out in air by using two different instruments, a commercial Nanoscope IIIa microscope Multimode (Digital Instruments, Santa Barbara, CA) equipped with a type E scanner and a JOEL STM 4210 instrument. The tips were mechanically prepared by cutting a 0.25 mm Pt0.8 Ir0.2 (Goodfellow). The data were collected in constant current mode using tunneling currents between 200 pA and 300 pA and a sample bias of between 650 mV and 700 mV. No tip-induced changes were observed. 2.1. Sample preparation Chemicals. Diadamantanedisulfide was synthesized using a previously described procedure [44] and ethanol (A.R. grade, 99.8%, Baker) was used as received. Gold substrates. For IR, the Au substrates were prepared by first evaporating 5 nm of titanium and subsequently 140 nm of gold onto Si(1 0 0) wafers in a recipient with a residual gas pressure of 10−7 mbar. STM measurements have been carried out on substrates that have been prepared by evaporating 140 nm of Au onto freshly cleaved mica, which had been heated to about 600 K for 3 days in the evaporation chamber. After evaporation of the metal, the substrates were cooled and the chamber was backfilled with nitrogen. The substrates were stored under argon and flame-annealed in a butane/oxygen flame immediately before the adsorption experiments were carried out. This procedure yields Au substrates with large terraces (several hundreds of nanometers, as evidenced by STM) exhibiting a (1 1 1) surface. Monolayers. The adsorption of the organothiolate monolayers was carried out by immersing the substrates into 0.1 mM of ethanolic solutions of the (DADS) for 24 h at RT. To study the effect of temperature on the SAMs, the samples prepared at RT was annealed in vacuum at the desired temperature for 30 min. 3. Results 3.1. STM 3.1.1. Common features for SAMs before and after annealing Immersion of Au(1 1 1) substrates into an ethanolic solution of DADS at room temperature generates a stable SAM which is conspicuously comparable to the SAMs fabricated from alkanethiols or dialkyl disulfides [18,47]. STM images in Fig. 1(a) show surface structures of DADS SAMs on Au(1 1 1) formed after 1-day immersion of gold substrates in a 0.1 mM ethanolic solution at RT. The typical large scale STM image demonstrate signs of well-defined step edges and vacancy islands or “etch-pits” within the top gold layer [13,25,48]. The depth of these features corresponds to the interlayer distance along the Au(1 1 1) direction, i.e. they corre-

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Fig. 1. Summary of STM topographs of DADS SAMs on Au(1 1 1): (a) a 300 × 300 nm2 scan of a freshly prepared SAM at room temperature, (b) a freshly prepared SAM annealed at 343 K for 30 min and imaged at RT, (c) a freshly prepared SAM annealed at 353 K for 30 min and imaged at RT, and (d) one after annealing at 373 K for 30 min. Parts (e)–(h) are high resolution images of parts (a)–(d), respectively.

spond to vacancies within the top gold layer [13]. In previous works it has been reported that these vacancy islands can be frequently healed by annealing the monolayer at temperature higher than RT. The size, shape and density of the vacancy islands of DADS SAMs are similar to those observed for n-alkanethiols. In Fig. 1(b), (c), and (d) we display DADS SAMs for samples prepared at room temperature and after annealing to 343 K, 353 K, and 373 K, respectively. The STM image displayed in (b) shows no significant changes in the surface morphology from that in (a). Only a slight increase in the size of the depressions at the expense of their density was observed. In Fig. 1(c) and (d) the size and density of the vacancy islands are dramatically changed from that observed for samples prepared at RT (a) and that annealed at 343 K (b). It is clear from panels (a)–(d) of Fig. 1 that the total number of vacancy islands decreases and their size increases with annealing. When the sample was annealed at 353 K, the size of the depressions increased from 4 to 5 nm at RT and 6–8 nm (for samples annealed at 343 K) to about 15–20 nm. The STM image displayed in (d), demonstrates that after annealing to 373 K the vacancy islands are largely absent. Another interesting point is the size of the ordered domains which follows the same trend as that found for the vacancy islands upon annealing. The sizes of the ordered domains become larger when the DADS SAMs are subjected to the thermal annealing at higher temperatures. From the panels (e)–(h) which shows STM images of freshly prepared SAMs at RT, freshly prepared SAMs annealed at 343 K, 353 K, and 373 K, respectively, the ordered domain are 4–10 nm, 8–20 nm, 15–35 nm, and 30–60 nm in size, respectively. Moreover, for samples annealed at 373 K the size of the ordered domains is limited by the dimension of the Au(1 1 1) terraces (see (h)). More interestingly, the well-known domain boundaries which are usually observed in the SAMs of n-alkanethiols and aromaticthiols are absent in the STM images Fig. 1(e) and (f) of DADS monolayers. Only after annealing the freshly prepared samples to temperature above 353 the usual domain boundaries could be observed (see (g)). The translational domain boundaries sep-

arated by disordered regions are the most frequently appeared in the STM images (see the region enclosed in Fig. 1(g)). Previously [13,14,19,29], it has been reported that the density of domain boundaries could be substantially reduced in the SAMs of n-alkanethiols and aromaticthiols via thermal annealing at high temperatures. The reasons for the different behaviour of DADS SAMs namely, an occurrence of these domain boundaries upon annealing, will be explained later in this paper. 3.1.2. DADS SAMs at RT In Fig. 2(a) and (b), we present STM topographs showing molecular resolution recorded for DADS SAMs on Au(1 1 1) prepared at RT for 24 h. Individual adamantane AD species (DADS dissociate on surface to yield AD) appear as hexagonally ordered spots in fairly large domains extending over the flat terraces of the Au(1 1 1) substrate. These images and the one displayed in Fig. 1(e), contain several important features worth to be discussed in detail. Firstly, two different kinds of depressions (or vacancy islands) can be easily distinguished. The previously described vacancy islands which ˚ as mentioned above and are in depth of atomic gold layer (2.4 A) appearing in the STM images as darker regions with a uniform distribution on the gold surface. These depressions are covered with the AD having the same packing as those in the vicinity of the vacancy islands. A representative example is the region enclosed by the yellow circles in Fig. 2(a) and Fig. 1(e). The second type of the depressions is distributing randomly on the surface and appearing in the STM images as dark regions and differ substantially from the vacancy islands. They have smaller size and lower depth; they are shallower than 0.5 A˚ related to the ordered arrays (see the regions surrounded by white circles in Fig. 2(a)). This result implies that these defects are not due to the removal of gold atoms in the first layer, actually, individual molecules were also observed in some of the defect areas, as indicated by the black arrows. In general, such defects have not been previously observed neither for n-alkanethiols nor for aromaticthiols SAMs chemisorbed on gold. Thirdly, in high resolution images such as these depicted in Fig. 2(a) new structural aspect is resolved which cannot be

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Fig. 2. (a) and (b) Show molecular level constant-current STM images of DADS SAMs on Au(1 1 1) substrate, which have been immersed for 24 h into 0.1 mM solution of DADS in ethanol at RT. (c) Shows cross-sectional height profiles taken along the lines A, B, and C which are presented in the STM image (b).

explained by the commensurate structure between the adlayer and the Au(1 1 1) substrate. Instead of three, at least four equivalent domains in terms of rotational angles are present. Actually, determination the number of the different orientational domains turns out to be difficult due to fact that the rows of the AD species alter direction somewhat throughout the molecularly resolved images. For accuracy, several STM images have been acquired from two different types of STM instruments, DI and JOEL, and then analyzed. In many of them four different orientational domains have been frequently observed. A representative STM image showing the four different orientational domains is depicted in Fig. 2(a). The relative orientation angles (the smallest) between these domains are 28◦ ± 2◦ (A and B), 19◦ ± 1◦ (A and C), 14◦ ± 1◦ (A and D), 32◦ ± 2◦ (B and C), 30◦ ± 2◦ (B and D), and 7◦ ± 0.5◦ (C and D). Herein, we would like to point out that the precise rotation of the ordered domains with regard to the underlying Au(1 1 1) substrate cannot be settled on by imaging only the adlayer. The estimation of this rotation is only achievable from the straight step edges of the Au(1 1 1) substrate, which follow close-packed substrate direction. Within the ordered domains, the AD species form a slightly distorted hexagonal structure which makes the possibility to analyze the dimensions of the unit cell with a high precision more tricky. Therefore, different ordered domains in several STM images were analyzed very carefully to extract the accurate lattice parameters of the unit cell with small errors. In Fig. 2, we display STM image for the AD adlayer (Fig. 2(b)) together with cross-sectional line profiles along the line A, B, and C (Fig. 2(c)) labelled in the STM image. The data show that the periodicity along the line A and B (a nearest neighbour distance (NN)) amounts to 7.0 ± 0.3 A˚ while that

˚ along line C (next nearest neighbour distance (NNN)) is 12 ± 1 A. Based on the value of the lattice constants, the most likely unit cell assignments are displayed √ 1. For example, the two rota√ in Table tionally equivalent cells ( 57 × 57)R6.59◦ regarding the Au(1 1 1) substrate, each unit cell of them consists of nine molecules having lattice constant of 21.79 A˚ with NN distance of 7.25 A˚ and NNN dis˚ In all possible unit cells, only the molecules that tance of 12.56 A. are adsorbed on the corner of each unit cells are commensurate with gold substrate and adsorb on one adsorption site. The others are bounded to different adsorption sites with slight deviation from the equally spaced hexagonal molecular sites. The area occupied by a single AD species for the different proposed structures is provided in the seventh column in Table 1. The value is ranging from 39.21 to 45. 66 A˚ 2 . 3.1.3. Annealed DADS SAMs Fig. 3(a) shows high resolution STM for freshly prepared DADS SAM, which were prepared at room temperature and then annealed to 343 K for 30 min and finally imaged at RT. This image reveals some interesting features. Firstly, bright protrusions with height of 0.1 nm above the surrounding areas are observed within the ordered layers (see Figs. 1(f) and 3(a)). These protrusions have different lateral dimensions with sizes corresponding to between one and three of the individual AD adsorbates. Secondly, slightly deviation in the hexagonal arrangement formed by AD molecules is observed in the samples annealed at 343 K. Analyzing the lattice parameters formed by AD species at these conditions yield a hexagonal lattice with a nearest neighbour distance of 7.0 ± 0.3 A˚ and next ˚ i.e. the same parameter latnearest neighbour distance of 12 ± 1 A. tice formed by AD at RT. Thirdly, at least three equivalent domains

Table 1 Proposed unit cell assignments for DADS SAMs prepared at 298–352 K. Unit cell

Number of molecules per unit cell

Lattice ˚ constant (A)

Next Neighbour distance (NN) ˚ (A)

Next nearest neighbour distance (NNN) ˚ (A)

Number of incomensurate adsorbates per unit cell

Molecular area (A˚ 2 )

(7 × 7) (7 × 7)R + 21.79 ◦ (7 − 21.79 ◦ √× 7)R√ (√57 × √57)R + 6.59◦ (√57 × √57)R − 6.59◦ (√91 × √91)R + 5.21◦ (√91 × √91)R − 5.21◦ (√91 × √91)R + 27.00◦ (√91 × √91)R − 27.00◦ (√93 × √93)R + 21..05◦ (√93 × √93)R − 21..05◦ (√97 × √97)R + 15.30◦ ( 97 × 97)R + 15.30◦

9 9 9 9 9 16 16 16 16 16 16 16 16

20.20 20.20 20.20 21.79 21.79 27.53 27.53 27.53 27.53 27.83 27.83 28.42 28.42

6.72 6.72 6.72 7.25 7.25 6.87 6.87 6.87 6.87 6.94 6.94 7.09 7.09

11.68 11.68 11.68 12.56 12.56 11.90 11.90 11.90 11.90 12.03 12.03 12.29 12.29

8 8 8 8 8 15 15 15 15 15 15 15 15

39.21 39.21 39.21 45.66 45.66 40.96 40.96 40.96 40.96 41.86 41.86 41.86 41.86

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Fig. 3. Shows molecular level constant-current STM images of DADS SAMs on Au(1 1 1) substrate, which have been immersed for 24 h into 0.1 mM solution of DADS in ethanol at RT and then annealed (a) at 343 K and (b) 353 K for 30 min. (c) Shows cross-sectional height profiles taken along the lines A, B, C, and D which are presented in the STM image (b).

are present. The relative orientation angles between these domains are 10◦ ± 1◦ (C and B), 7◦ ± 1◦ (A and C), and 4◦ ± 0.5◦ (A and B). In Fig. 3(b), we displayed a high-resolution STM micrograph recorded for a DADS SAM, which was prepared at room temperature and then annealed at 353 K for 30 min before it was imaged again at room temperature. Surprisingly, the terrace is covered with three different domains that are barely visible at this scale; in Fig. 1(g), two orientational domains rotated by 120 ± 5◦ with respect to each other are obviously visible. The three domains of the AD species are a consequence of the 3-fold symmetry of Au(1 1 1) substrate. The dimensions of the unit cell were determined from the height profiles along the lines A, B, C, and D labelled in the high-resolution STM topograph in Fig. 3(b). The parameters equal 7.6 ± 1 A˚ for the nearest neighbour distance and 13 ± 1.5 A˚ for the next nearest neighbour distance (see Fig. 3(C)). The observed orientations showed no deviation from the hexagonal symmetry of the Au(1 1 1), which suggests that there is simple commensurate relationship between the lattice of SAM and Au(1 1 1). Based on the above results, the most likely unit √ cell√assignments are the two rotationally equivalent unit cells ( 7 × 7)R19.12◦ with respect ˚ to the√Au(1√1 1) substrate. Using the gold lattice spacing of 2.886 A, ˚ and has a 7.64 A the ( 7 × 7) unit cell has a lattice constant of √ √ single molecule per unit cell. As a result, the ( 7 × 7) unit cell ˚ The adsorprovides a next nearest neighbour distance of 13.2 A. bates are bonded to a single type of adsorption sites, which implies a commensurate structure with respect to the underlying substrate. The AD species area is calculated to be 50.41 A˚ 2 . STM images (not shown) for DADS SAMs which were prepared at room temperature and then annealed at 373 K for 30 min show no changes in the unit cell dimensions and in the rotational angles between the ordered domains from that √ observed √ for the samples annealed at 353 K. Therefore, the same ( 7 × 7) structure was proposed.

˚ The thickness of the decanethiol film was assumed to be 13.0 A, corresponding to a tilt angle of the molecules of 30◦ with respect ˚ The length to the surface normal and an Au–S bond distance of 2 A. ˚ and the S–Au distance, to of one DADS molecule amounts to 4.7 A, 2 A˚ [26]. The XPS results, consequently, point to the presence of a densely packed, well-defined monolayer with the molecular axes orientated perpendicular to the gold surface. In Fig. 4(c), we display a XP spectrum recorded for the S 2p region of the DADS film. The results reveal a single S 2p doublet with the S 2p3/2 component located at 161.6 eV assigned to thiolate species [26]. The last finding implies that the DADS molecules adsorb on the Au(1 1 1) as

3.2. XPS Fig. 4 shows XPS data recorded for DADS SAMs and decanethiolate SAMs films prepared at RT. The two samples were mounted on the same sample holder to yield the same geometric (i.e., distance of X-ray source, angle of incidence, and energy analyzer toward the sample) conditions during the measurements. The energy scales of all spectra were referenced to the Au 4f7/2 peak located at a binding energy of 84.0 eV [28,49]. From the relative intensities of the Au 4f7/2 and the C 1s peaks (see Fig. 4(b)), the film thickness was calculated by using decanethiol on Au as a reference system. Assuming the same photoelectron escape depths of the gold (Au = 4.5 nm at a kinetic photoelectron energy of 1402 eV) and carbon (C = 3.5 nm at 1202 eV) photoelectrons for of DADS and decanethiolate films, Eq. (1) was used for the calculation and yielded thicknesses of 6.64 A˚ for DADS film. 1 − exp{−dsample /C }exp{−dreference /Au } (IC /IAu ) (sample) = (IC /IAu ) (reference) exp{−dsample /Au }1 − exp{−dreference /C }

(1)

Fig. 4. XP spectra recorded for polycrystalline gold substrates showing (a) the gold 4f regions, (b) the carbon 1s regions of DADS and C10 layers prepared at RT on Au(1 1 1). (c) Sulphur 2p region of DADS SAM prepared at RT.

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Fig. 5. Comparison of the IR spectrum of bulk DADS (KBr pellet) with the spectrum of the SAM of DADS on Au(1 1 1) prepared at 298 K; (a) low-frequency region and (b) high-frequency region.

adamantanethiolate (AD species) via the cleavage of the S–S bond in the DADS molecules. The XP spectrum of the S 2p region did not indicate the presence of extra peaks, e.g. arising from oxidized sulphur at (166–170) eV [49]. 3.3. IR 3.3.1. Room-Temperature infrared Absorption and KBr spectra Fig. 5 shows the grazing angle IR spectrum recorded for a DADS SAM prepared at RT, and the IR spectra of a corresponding bulk sample (DADS/KBr pallet). Several peak positions in the spectrum of the DADS SAM are slightly different from that in the bulk spectrum. In the powder spectrum (b), the infrared peaks between 2800 and 3000 cm−1 are assigned to ␯CH (CH2 ) stretching modes. An assignment of the different peaks is provided in Table 2. Briefly, in the KBr spectrum we observe the ␯s (CH2 ) symmetric (d+) occurs at 2845 and ␯as (CH2 ) antisymmetric (d−) stretching mode at 2916 cm−126 . In the low frequency region, the characteristic modes between 1200 and 1400 cm−1 are assigned to the wagging and twisting vibrations of the methylene groups [50]. C–C–C skeletal vibrational modes are observed between 1039 and 1176 cm−1 , followed by a number of bands between 750 and 1000 cm−1 arising from CH2 rocking modes and CH bending modes [51]. In the SAM spectrum (Fig. 5) and within the region 2950 to 2800 cm−1 , two peaks are observed at 2852 (␯s (CH2 )) and at 2914 (␯as (CH2 )). As can be seen, a considerable change in the peak position from that of the KBr spectrum is observed. In the low-frequency region (1500–700 cm−1 ), several peaks that seen in the KBr spectrum at 1454 (CH2 def, scissor), 1312 (CH2 Wag), 1252 (CH bend/CH2 twist), 977 (CH2 rock), 930 (CH2 rock), and 766 cm−1 (CH2 rock) are disappeared in the SAM spectrum. Table 2 Mode assignment and frequencies of adamantanethiol in KBr and in SAM: nr = not resolved. Peak assignment

KBr

SAM

CH2 rock CH bending out of plane CH2 rock CH2 rock CH2 rock CC str/CH bend CC str/CH bend CC str/CH bend CH bend/CH2 twist CH2 twist CH2 Wag CH bend/CH2 Wag CH bend CH2 def, scissor ␯s (CH2 ), sym str

766 810 824 930 977 1039 1105 1176 1252 1298 1312 1340 1363 1454 2845

nr 806 827 nr nr 1031 1099 1178 nr 1294 nr 1340 1363 nr 2852

3.3.2. Infrared Data of Samples Annealed at high temperatures The high-frequency region between 2800 and 3000 cm−1 of DADS SAMs prepared at RT and annealed at 333, 343, 353, 363, and 373 K are shown in Fig. 6(a). With increasing the annealing temperature, neither shift in the absorption modes to higher or lower energy nor appearance or disappearance of new peaks are observed. The low-frequency region (700–1400 cm−1 ) is shown in the Fig. 6(b) and (c). Upon increasing the temperature of annealing, the progression modes (bending, rocking, twisting, and wagging) gain intensity. A significance increase in the intensity of the CH bending mode (CH2 rock) was observed for the samples that annealed at high temperatures especially at the 373 K.

4. Discussion 4.1. SAMs of DADS prepared at RT There is no doubt that the large body of the spectroscopic and microscopic data collected in this study support the presence of densely packed, highly orientated, and well-ordered SAMs of DADS molecules on Au(1 1 1). For simplicity, we will first discuss the data collected for the SAMs of DADS prepared at room temperature. The XPS data show that the thiolate-gold linkage is formed upon the adsorption of DADS molecules, via cleavage of the S–S bond. This conclusion was deduced from the XP spectrum collected for S 2p region, where only a single S 2p doublet assigned to thiolate species was observed. Therefore, the DADS (disulfide) molecules are chemically converted into AD species (thiolate) upon adsorption on Au(1 1 1). The observed adsorption-induced cleavage of the S–S bond in DADS agrees very well with the previous obtained results for systems having disulfide moieties [52–54]. In addition to the analysis of the elemental composition, the XPS results were used to obtain an estimate of the thickness of the molecular adlayer formed on top of the gold substrate. The length of AD is assumed to be 4.7 A˚ ˚ From the calculations, the and the S–Au distance is taken as 2 A. ˚ This result indicates thickness of the AD-film is found to be 6.64 A. that the AD species are orientated perpendicular to the surface. As a result of this molecular orientation, the bond angle in the Au–S–C segment is expected to be 180◦ since the AD species bound to the gold have no free rotational joint within its molecular structure. STM data collected for SAMs of DADS prepared and measured at room temperature exhibit the formation of a highly ordered hexagonally close-packed molecular lattice with a measured near˚ The distance between the est neighbour distance of 7.0 ± 0.3 A. neighbouring S–S species on Au(1 1 1) is significantly larger than 5 A˚ that is found for n-alkanethiols and for some aromaticthiols on Au(1 1 1) [13,25,29]. In n-alkanethiols, the molecules form a

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Fig. 6. (a) Low, (b) moderate, and (c) high-frequency regions of the IR spectra of the DADS SAMs prepared and annealed at different temperatures; (a) at RT, (b) at 333 K, (c) at 343 K, (d) at 353 K, (e) at 363 K, and (f) at 373 K.

√ √ commensurate ( 3 × 3) R30◦ structure, in which the molecular axes are tilted away from the surface normal to by about 30◦ to increase the adsorbate–adsorbate interaction. The n-alkanethiols adopt this orientation because the cross-sectional area of the alkyl chain ca. 18.7 A˚ 2 is smaller than the area per molecule of the unit cell (21.6 A˚ 2 ). In contrast to the n-alkanethiolates, the AD adsorbate consists of bulky adamantane moiety, the adamantine √ √ therefore, molecule is not compatible with the ( 3 × 3) structure observed for alkane thiolates. We, therefore, expect that the AD adsorbates form commensurate or incommensurate structures with a substantially larger unit cell, depending on dominancy between the substrate–sulphur interaction and molecule-molecule interaction. When the substrate–sulphur interaction is dominating, the lattices of SAMs are commensurate with underlying gold substrate, whereas, when the adsorbate–adsorbate interaction is governing, it is not essential that the lattice is commensurate with Au(1 1 1). From our STM results, the later situation is expected to be applicable for the SAMs of DADS. This is clearly evident by the presence of four equivalent domains in terms of rotational angles that cannot be explained by the commensurate structure between the adlayer and the Au(1 1 1) substrate. An interesting observation, which is in pronounced contrast to the behaviour of n-alkanethiols, and aromaticthiols SAMs is the disappearance of the domain boundaries within the DA thiolate films as can be clearly seen in the STM images. One reason behind this SAM property is might be the upright orientation of the AD species within the SAM. By taking all the above observations together with the nearest neighbour distance between the adsorbate–adsorbate, ˚ into our considerations, several proposed unit that is 7.0 ± 0.3 A, cell assignments are proposed and displayed in Table 1. Moreover, models demonstrating these probable unit cells are displayed in Fig. 7. In these models, the experimentally obtained nearest neighbour distance is satisfied. Moreover, in each proposed unit cell the four adsorbates located on the corner are assumed to be commensurate with Au(1 1 1) and are bounded with the underlying gold substrate having the same adsorption sites. The hollow site was chosen since it is the energetically preferred adsorption site. The adsorbates sandwiched between ones adsorbed on the corners are incommensurate with Au(1 1 1) via adsorption on different sites such as the bridge, the hollow and the on-top sites. These models explain clearly the slight deviation from hexagonally observed within the domains, moreover, they succeed to demon˚ depressions, seen in the strate the origin of the low-depth (∼0.5 A) STM images, that is the adsorption on different sites. In previous study, SAMs formed from organodisulfide with a pair of adamantane moieties adsorbed on Au(1 1 1) were studied

√ √ Fig. 7. The possible √ √ unit cells for DADS SAMs on Au(1 1 1) (a) a ( 91 × 91)R √ ± ◦ ◦ ◦ 5.21 and a ( 91 × 91)R √ ± 27.00 √ , (b) a (7 × 7) and a (7 × 7)R ± 21.79 , (c) a ( 57 × √ 57)R ± 6.59◦ , and (d) a ( 93 × 93)R ± 21.05◦ .

[44]. The molecules were found to form four orientationally dif˚ ferent hexagonals with nearest neighbour distance of 6.6 ± 0.17 A. Recently, SAMs of 1-adamantanethiol have been investigated on Au(1 1 1) [45]. The SAMs exhibited five rotational domains with ˚ nearest neighbour distance of 6.9 ± √ 0.4 A.√In both studies, a combi√ ◦ , ( 91 × 91)R27.00◦ , and ( 57 × nation of (7 × 7), (7 × 7)R21.79 √ 57)R − 6.59◦ unit cells were proposed. Moreover, in both cases the STM results of the respective SAMs showed the same morphology, regarding the missing of the domain boundaries, as found for DADS SAMs. Our results are in good agreement with these studies with respect to the appearance of several rotational domains, nearest neighbour distance (to some extent), and missing the domain boundaries within the SAMs. The nearest neighbour distance found ˚ is slightly larger that that found in the first in our work (7.0 ± 0.3 A) study. In our work, several samples were microscopically scanned using two types of microscopes (DI and JOEL) to determine exactly the unit cell. The analyses revealed that the nearest neighbour dis˚ This value agrees very well with the value tance equals 7.0 ± 0.3 A. obtained in the previous second study [45]. The IR results obtained for the DADS SAMs prepared and measured at RT showed the emergence of different absorption bands that found in the KBr spectrum of DADS. These peaks are the ␯CH stretching, wagging, twisting, rocking, bending, and C–C–C skeletal vibrational modes. Information on the molecular orientation can be

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√ √ Fig. 8. Top view of a ( 7 × 7)R19.12◦ model for DADS SAMs prepared at RT and annealed at 353 K for 30 min.

acquired by considering the transition dipole moments (TDMs) of the different IR modes and by applying so called surface selection rule. In the SAM spectrum, only the vibrational peaks assigned to the ␯s (CH2 ) symmetric (d+), ␯as (CH2 ) antisymmetric (d−) stretching modes, and CH bending at 1031 cm−1 are observed. The TDMs generated from these vibrational modes are orientated in-plane (ip) of their mean rotational axes of C atoms. As consequence, the appearance of these ip bands suggests the up right orientation of the AD species. 4.2. SAMs of DADS prepared at RT and annealed at different temperatures Like in the case of n-alkanethiols and aromaticthiols SAMs [19,29], temperature has a significant effect on the size and density of the vacancy islands that formed after adsorption of DADS on Au(1 1 1). Their size increases at the expense of their density with increasing the annealing temperature. After annealing to a temperature of 393 K, the vacancy islands typical for the morphology of the DADS SAM prepared at room temperature are almost completely absent. Effect of SAM annealing is also observed by analyzing the size of the ordered domains that has been found to increase linearly with the annealing temperature (see Fig. 1). An additional interesting obtained result that emerges as a consequence of annealing the SAMs of DADS at high temperature is the re-arrangement of the molecular overlayer into a new phase with lower packing density. Annealing at 353 K results in the formation of a new hexagonal phase with a nearest neighbour (NN) distance of ˚ The simplest commensurate overlayer consistent with 7.6 ± 0.1 A. √ √ this distance is a ( 7 × 7)R19.12◦ structure. The presence of a low order commensurate structure of the molecular overlayer is consistent with the fact that there are only three different rotational domains by 120◦ with respect to each other. Moreover, √ rotated √ the ( 7 × 7)R19.12◦ structure corresponds to a packing density reduced by about 25% compared to that observed at RT or at temperatures lower than 352 K. In Fig. 8, we provide a structural model √ √ for these ( 7 × 7)R19.12◦ structure which with all √ is consistent √ the available experimental data. The same ( 7 × 7)R19.12◦ structure was observed for SAMs annealed at higher temperatures than 353 K as can be seen from the STM images (see Fig. 3(b)). Therefore, annealing the DADS SAMs at temperatures up to 393 K results in no structural changes from that observed at 353 K, only an increment in the size of the ordered domains was observed. An interesting observation seen in the STM images of the DADSSAMs is the appearance of domain boundaries after annealing at

Fig. 9. Molecular structure of adamantanethiol.

353 K. From the STM images, it can be seen that the regions of domain boundaries consist of fewer disordered adsorbate DADS molecules than those in the vicinity regions. This indicates that upon thermal treatment some molecules are desorbed from the domain boundaries regions√ and from √ the ordered domains regions due to the formation of the ( 7 × 7)R19.12◦ structure, which have a lower density of packing. This expected scenario is evident by the presence of the protruding features that are swinging above the monolayer and appearing in the STM images with topographical height of 0.1 nm higher than their underneath molecules. These protrusions are attributed to AD species molecules that desorbed from the high densely packed phase (that having distance of √ NN √ ˚ to form the lower densely packed ( 7 × 7)R19.12◦ 7.0 ± 0.3 A) structure upon treating with temperature. The temperatureinduced changes for SAMs of DADS are different from those observed for corresponding SAMs made from alkanethiols and aromatic thiols [13,19]. In the latter two cases, thermal annealing of the most densely packed RT-phases results in the formation of striped phases [19] in which the molecular axes of the adsorbates are orientated parallel to the surface. Our STM data recorded for DADS SAMs clearly that no such striped phases are formed. √ demonstrate √ In the ( 7 × 7)R19.12◦ structure, the area occupied by a single molecule is (50.41 A˚ 2 ) larger than that found in the structure formed at RT and in those formed after annealing at temperatures lower than 353 K. As a consequence, the molecular axis of the AD species within the SAM formed after being annealed at 353 K is expected to tilt away from the surface normal. The portion in the molecular structure that is responsible for such orientation is expected to be the Au–S–C bond angle. A bond angle of smaller than 180◦ is expected to be figured. The change in orientation of AD species upon thermal annealing is confirmed by the IR results. Adamantine consists of six –CH2 groups labelled in Fig. 9 by C4 , C6 , C5 , C12 , C10 , and C8 . Three –CH groups indicated by C7 , C9 , and C11 and one C (C3 ) without bonding with H. From Fig. 6, annealing the DADS monolayer results in an increase in the intensity of the vibrational bands located at 806, 1031, 1178, 1252, and 1349 cm−1 . The assigned vibrational modes, CH2 twisting and CH2 wagging located at 1252 and 1340 cm−1 , respectively, having TDMs orientated outof-plane perpendicular to their mean rotational axes of C atoms. These orientations are directed parallel to the gold surface. Therefore, according to the surface selection rule the intensity of these vibrational modes should be vanish at room temperature where the molecules adopting upright orientation. As mentioned above, upon annealing the intensity of these vibrational modes increases. This behaviour indicates that the AD species are tilted away from

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the surface normal. The TDMs of the vibrational modes of –CH groups are in-plane for the bands located at 1031 and 1178 cm−1 and out-of-plane for the one located at 806 cm−1 . After annealing, the intensity of the band at 806 cm−1 increases. A slight increase in the intensity was observed for the 1031 and 1178 cm−1 peaks. The increase in the out-of-plane band is expected due to the inclination of the molecules away from surface normal. The slight increase in the in-plane bands upon annealing is unexpected. This behaviour could be due to the contribution of the overlapped C–C stretching which might affect the direction of the –CH vibrational modes upon annealing. 5. Conclusions Several complementary experimental techniques were used to characterize SAMs formed from DADS on Au(1 1 1). In addition, temperature-induced changes in the structure and in the morphology of the DADS SAMs were studied by annealing the Au substrates to different temperatures. Preparation at room temperature results in the formation of densely packed, well ordered DADS SAMs. The molecules are bound to the substrate via a thiolate-metal linkage, which is formed after the cleavage of the S–S bond in the DADS molecules. The results exhibited the formation of highly ordered monolayers with hexagonal arrangement of the adsorbates having ˚ A slight deviation from the hexagonally a NN distance of 7.0 ± 0.3 A. within the molecular rows was observed. The most likely models, which satisfy these observations, were proposed. To explain the deviation of the adsorbates from the hexagonal arrangement, the sulphur atoms that bind on the corners of the unit cell are assumed to adsorb on the same adsorption sites, the hcp hollow site. Whereas, those exist inside the unit cell are proposed to adsorb on different adsorption sits. The IR and XPS results are in full agreement with the microscopic data and indicated an upright orientation with the molecules are orientated completely perpendicular from the Au(1 1 1). Annealing of the DADS SAMs to temperatures above 350 K results in the formation of a new hexagonal phase, which also exhibit a different morphology. A structural change to lower density of molecular packing structure was observed. In this structure, the molecules obey totally the hexagonal arrangement with no deviation. The same structure was observed for samples those annealed at 363 K and 373 K. Therefore, SAMs of DADS were observed to be stable against annealing up to 373 K. References [1] [2] [3] [4]

S. Fink, F. van Veggel, D.N. Reinhoudt, Adv. Mater. 12 (2000) 1315. I. Willner, E. Katz, Chem. Int. Ed. 39 (2000) 1180. J.P. Folkers, P.E. Laibinis, G.M. Whitesides, Langmuir 8 (1992) 1330. J.P. Folkers, P.E. Laibinis, G.M. Whitesides, J. Deutch, J. Phys. Chem. 98 (1994) 563. [5] C.E.D. Chidsey, D.N. Loiacono, Langmuir 6 (1990) 682.

3747

[6] F. Hipler, S. Gil Girol, W. Azzam, R.A. Fischer, C. Wöll, Langmuir 19 (2003) 6072–6080. [7] G.M. Whitesides, P.E. Laibinis, Langmuir 6 (1990) 87. [8] O. Chailapakul, L. Sun, C. Xu, R.M. Crooks, J. Am. Chem. Soc. 115 (1993) 12459. [9] P.E. Laibinis, G.M. Whitesides, J. Am. Chem. Soc. 114 (1992) 9022. [10] Y.N. Xia, G.M. Whitesides, Annu. Rev. Mater. Sci. 28 (1998) 153. [11] Y.N. Xia, J.A. Rogers, K.E. Paul, G.M. Whitesides, Chem. Rev. 99 (1999) 1823. [12] G.Y. Liu, S. Xu, Y.L. Qian, Acc. Chem. Res. 33 (2000) 457. [13] W. Azzam, P. Cyganik, G. Witte, M. Buck, C. Wöll, Langmuir 19 (2003) 8262–8270. [14] P. Cyganik, M. Buck, W. Azzam, C. Wöll, J. Phys. Chem. B 108 (2004) 4989– 4996. [15] D.K. Schwartz, Annu. Rev. Phys. Chem. 52 (2001) 107–137. [16] F. Schreiber, Prog. Surf. Sci. 65 (2000) 151–256. [17] S.S. Wong, M.D. Porter, J. Electroanal. Chem. 485 (2000) 135–143. [18] G.E. Poirier, W.P. Fitts, J.M. White, Langmuir 17 (4) (2001) 1176–1183. [19] Y.L. Qian, G. Yang, J. Yu, T.A. Jung, G.Y. Liu, Langmuir 19 (2003) 6056–6065. [20] Y.-C. Yang, Y.-L. Lee, L.-Y. Yang, S.-L. Yau, Langmuir 22 (2006) 5189–5195. [21] W.P. Fitts, J.M. White, G.E. Poirier, Langmuir 18 (2002) 1561–1566. [22] J. Noh, H.S. Kato, M. Kawai, M. Hara, J. Phys. Chem. B 110 (2006) 2793–2797. [23] L. Müller-Meskamp, B. Lüssem, S. Karthäuser, R. Waser, J. Phys. Chem. B 109 (2005) 11424–11426. [24] H. Sellers, A. Ulman, Y. Shnidman, J.E. Eilers, J. Am. Chem. Soc. 115 (1993) 9389. [25] W. Azzam, C. Fuxen, A. Birkner, H.-T. Rong, M. Buck, C. Wöll, Langmuir 19 (2003) 4958–4968. [26] R. Arnold, W. Azzam, A. Terfort, C. Wöll, Langmuir 10 (2002) 3980–3992. [27] R. Arnold, A. Terfort, C. Wöll, Langmuir 17 (2001) 4980–4989. [28] W. Azzam, B.I. Wehner, R.A. Fischer, A. Terfort, C. Wöll, Langmuir 18 (21) (2002) 7766–7769. [29] W. Azzam, A. Bashir, A. Terfort, T. Strunskus, C. Wöll, Langmuir 22 (2006) 3647–3655. [30] N.A.F. Al-Rawashdeh, W. Azzam, C. Wöll, Z. Phys. Chem. 222 (2008) 965–978. [31] W. Azzam, Cent. Eur. J. Chem. 7 (4) (2009) 884–899. [32] W. Azzam, Appl. Surf. Sci. 256 (2010) 2299. [33] U. Siemeling, C. Bruhn, F. Brettauer, M. Borg, F. Träger, F. Vogel, W. Azzam, M. Badin, T. Strunskus, C. Wöll, Dalton Trans. (2009) 8593–8604. [34] A.A. Dhirani, R.W. Zehner, R.P. Hsung, P. GuyotSionnest, L.R. Sita, J. Am. Chem. Soc. 118 (13) (1996) 3319–3320. [35] J.M. Tour, Acc. Chem. Res. 33 (2000) 791. [36] C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541. [37] P. Cyganik, M. Buck, T. Strunskus, A. Shaporenko, G. Witte, M. Zharnikov, C. Wöll, J. Phys. Chem. C 111 (2007) 16909–16919. [38] W. Nowacki, Helv. Chim. Acta 28 (1945) 1233. [39] Y. Hu, S.B. Sinnot, Surf. Sci. 526 (2003) 230. [40] W. Nowacki, K.W. Hedberg, J. Am. Chem. Soc. 70 (1948) 1497. [41] P.V.R. Schleyer, J.E. Williams, K.R. Blanchard, J. Am. Chem. Soc. 92 (1970) 2377. [42] K.K. Khullar, J. Bauer, J. Org. Chem. 36 (1971) 3038. [43] M. Shen, H.F. Schaefer III, C. Liang, J.-H. Lii, N.L. Allinger, P.V.R. Schleyer, J. Am. Chem. Soc. 114 (1992) 497. [44] S. Fujii, U. Akiba, M. Fujihira, J. Am. Chem. Soc. 124 (2002) 13629–13635. [45] A.A. Dameron, L.F. Charles, P.S. Weiss, J. Am. Chem. Soc. 127 (2005) 8697–8704. [46] T. Kitagawa, Y. Idomoto, H. Matsubara, D. Hobara, T. Kakiuchi, T. Okazaki, K. Komatsu, J. Org. Chem. 71 (2006) 1362–1369. [47] N. Prathima, M. Harini, N. Rai, R.H. Chandrashekara, K.G. Ayappa, S. Sampath, S.K. Biswas, Langmuir 21 (2005) 2364–2374. [48] K. Edinger, M. Grunze, C. Wöll, Ber. Bunsenges. Phys. Chem. 101 (1997) 1811–1815. [49] C. Fuxen, W. Azzam, R. Arnold, A. Terfort, G. Witte, C. Wöll, Langmuir 17 (2001) 3689–3695. [50] J.-F. Bardeau, A.N. Parikh, J.D. Beers, B.I. Swanson, J. Phys. Chem. B 104 (2000) 627–635. [51] A.N. Parikh, S.D. Gillmor, J.D. Beers, K.M. Beardmore, R.W. Cutts, B.I. Swanson, J. Phys. Chem. B 103 (1999) 2850–2861. [52] L. Li, S. Cheng, S. Jiang, Langmuir 19 (2003) 3266. [53] A.V. Shevade, J. Zhou, M.T. Zin, S. Jiang, Langmuir 17 (2001) 7566. [54] H. Schönherr, H. Ringsdorf, Langmuir 12 (1996) 3891.