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Observation of stable mixed alkanethiolate–chloride adlayer on Ag(1 1 1): structural correlation with pure alkanethiolate monolayers
Surface Science 549 (2004) 237–245 www.elsevier.com/locate/susc
Observation of stable mixed alkanethiolate–chloride adlayer on Ag(1 1 1): structural correlation with pure alkanethiolate monolayers Mitsuo Kawasaki *, Hiromichi Nagayama Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishigyo-ku, Kyoto 615-8510, Japan Received 30 September 2003; accepted for publication 14 November 2003
Abstract The self-assembly of 1-alkanethiols, CH3 (CH2 )n1 SH (n ¼ 2–16), on Ag(1 1 1) film initially covered with a native oxide monolayer and that on a chloride-covered Ag(1 1 1) were comparatively studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy. The native oxide monolayer was readily substituted completely by thiop p lates irrespective of chain length, leading to a dense ð 7 7ÞR19.1 monolayer for n < 3, or a distorted p p ( 7 7ÞR19.1 structure for n > 3 accompanied by distinct island and fine domain structures previously reported by other groups. In contrast, the chloride-to-thiolate conversion was far from complete for long alkanethiols (n P 8), and at sufficiently high conversion temperature (>50 C), we found a highly stable mixed thiolate–chloride monolayer with a p p well-defined 2:1 S/Cl atomic ratio, suggesting the occurrence of a mixed ð 7 7ÞR19.1 adlayer ordering. The combined effects of substrate-molecule and intermolecular interactions behind these contrastive phenomena are discussed. 2003 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Scanning tunneling microscopy; Chemisorption; Self-assembly; Silver; Halides; Sulphides
1. Introduction Strong chemisorption and self-assembly of organosulfur compounds on metal surfaces offer a useful method to organize a variety of ordered monolayers to modify and/or functionalize the respective solid surfaces [1,2], and have been intensively studied by many groups in different disci-
*
Corresponding author. Tel./fax: +81-75-383-2574. E-mail address: [email protected] Kawasaki).
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plines. Among others, the self-assembled monolayers (SAMs) of 1-alkanethiols [CH3 (CH2 )n1 SH, Cn in short] on Au(1 1 1) are particularly simple, well structured, and best characterized model systems. It is known that at least long (e.g., n P 6) alkanethiols invariably form a densely packed crystalline-like assembly p pon Au(1 1 1) either with a commensurate ð 3 3ÞR30 structure [3], or p with a more complex 3 2 3 structure [c(4 · 2) p p superlattice of a ð 3 3ÞR30 structure] as first observed by He atom diffraction [4] and confirmed later by a number of high-impedance scanning tunneling microscopy (STM) studies [5–13]. The
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reported phase behaviors of short alkanethiols (n 6 4) on Au(1 1 1) are apparently more diverse, however; some studies revealing p either no molecular order [5] or low-density p 3 phase [14–17], while others as dense (e.g., p 3p· 4 phase) ordered monolayers as the ð 3 3ÞR30 structure [9,13,18–20]. The two major forces that determine the ultimate SAM structures are the substrate–sulfur chemisorption bonding and the interchain van der Waals dispersion forces. There can be also sulfur– sulfur interactions between the head groups and the alkyl–substrate interactions (particularly in the flat-lying adsorption case) that are by no means negligible. The observations of largely different molecular orders for short-chain SAMs may possibly be a reflection of delicate balance of these elementary forces that may even depend on specific SAM preparation conditions. To gain further insight into the competing (or synergic) roles of the substrate-molecule and intermolecular interactions in SAM formation, it is useful to compare the SAM structures on different substrates. The Ag(1 1 1) surface offers a particularly interesting comparison, because despite only 0.3% difference in the (1 1 1) nearest neighbor spacing, the alkanethiol SAMs on Ag(1 1 1) have been found to yield markedly different packing order and density from those on Au(1 1 1) [2]. In the case of sulfur (sulfide) and the extremely short alkanethiolate (Cp 1 ) on Ag(1 1 1), occurrence p of a commensurate ð 7 7ÞR19.1 structure was evidenced both by early diffraction studies [21–23] and by the more recent STM work of Heinz and Rabe [24]. This structure can be constructed by binding one-third of the sulfur atoms on top sites and the rest on alternate hpc and fcc hollow sites of the Ag(1 1 1) lattice. The resulting packing density, 5.93 · 1014 cm2 , with 4.41 A nearest neighbor spacing is 30% greater than that p p (4.6 · 1014 cm2 ) expected for the ð 3 3ÞR30 structure. This high packing density becomes an immediate problem for longer alkanethiolates, wherein much stronger alkyl chain packing constraint prevents the interchain spacing any smaller (i.e., the ideal hydrocarbon spacing in than 4.6 A bulk n-alkanes [2]). Indeed, the experimentally (by He and X-ray diffractions [25], STM [26], and
electrochemical measurements [27]) determined average spacing in Cn (n P 4) SAMs on Ag(1 1 1) range, which is invariably fell in the 4.6–4.8 A incommensurate with respect to the Ag lattice. The corresponding SAM structure p p may be best described as a distorted ð 7 7ÞR19.1 structure, in which the competing silver–sulfur and interchain interactions must be somehow reconciled. Importantly, the distortion from the p substantial p commensurate ð 7 7ÞR19.1 structure (as imposed by the interchain repulsive forces) likely introduces some significant stress in the Ag(1 1 1) surface lattice and its undesirable structural consequences. This has been most clearly demonstrated in the STM study of decanethiol SAMs reported by Dhirani and co-workers [26]. They showed that the SAM-covered Ag(1 1 1) surface produced a large number of elevated monatomic This silver islands ranging in size from 20 to 180 A. interesting feature represents another striking difference from Au(1 1 1) where the SAM formation is known to produce a number of monatomic depressions or pits [28]. Furthermore, the molecularly resolved STM images clearly uncovered fine with hexagonal domain (average size of 70 A pattern) structures separated by shallowly (by or less) depressed domain boundaries. 0.5 A The resulting complex vertical undulation of the image, together with the distinct island structure, gives us the impression that the long-chain SAMs on Ag(1 1 1) could be more defective pthan on p Au(1 1 1), even though the distorted ð 7 7ÞR19.1 structure yields a still significantly denser packing of alkanethiolates than does the p p ð 3 3ÞR30 structure. The purpose of this paper is to reilluminate these unique SAM features on Ag(1 1 1) and their origin in a little different perspective. We first present our own STM (along with X-ray photoelectron spectroscopy, XPS) characterization of a series of 1-alkanethiols (n ¼ 2, 3, 4, and 8) on Ag(1 1 1), which not only reproduces well the previously reported results, but also adds some complementary information regarding how the characteristic SAM features on Ag(1 1 1) for long alkanethiols evolve with chain length. We then introduce the formation of a novel SAM phase of long alkanethiols (n ¼ 8 and 16) co-adsorbed with
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chlorides in a simple 2:1 S/Cl atomic ratio. This interesting, markedly stable, new phase emerged typically as a result of substitutional chemisorption of long alkanethiols from ethanolic solution at relatively high temperatures (>50 C) onto an initially chloride-covered Ag(1 1 1). While short alkanethiols (n 6 4) completely displaced the initial chloride adlayer, the substitution by long alkanethiols ceased at the well-defined S/Cl atomic ratio, thereby p p suggesting the formation of a mixed ( 7 7ÞR19.1 adlayer structure. There, the characteristic island and domain structures associated with pure alkanethiol SAMs on Ag(1 1 1) were no longer observed. Overall these comparative observations for pure and mixed alkanethiolate–chloride SAMs provide more comprehensive understanding of the combined roles of substrate-molecule and intermolecular interactions on Ag(1 1 1).
2. Experimental The Ag(1 1 1) film was epitaxially grown on Au(1 1 1) predeposited on freshly cleaved natural mica. Both depositions were done by using the simple DC glow-discharge sputtering in an Ar atmosphere as described elsewhere [29,30]. On a freshly cleaved, 0.1 mm thick mica substrate typically 3 · 4 cm2 in area, an atomically flat high-quality Au(1 1 1) film, 0.2 lm thick, was grown at 300 C with 40–50 A/min deposition rate. The epitaxial growth of Ag(1 1 1) thereupon was allowed at a lower temperature of 180 C to the thickness of 0.05 lm. When Ag(1 1 1) is directly grown on mica at similar temperature or higher, the resulting film frequently gives whitish foggy appearance to the eye due to the formation of large grains [26,31]. The epitaxial growth of Ag(1 1 1) on Au(1 1 1) prevents this problem and yields an extensively terraced surface texture that could be even smoother than the predeposited Au(1 1 1). It should be noted that the as-deposited Ag(1 1 1) film, when taken out of the deposition chamber to the ambient air, is immediately covered by a native oxide monolayer. Thus even in our control experiment where the alkanethiol adsorp-
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tion (see below) is allowed onto the as-deposited substrate, the adsorption occurs substitutionally by replacing the native oxide monolayer. The preparation of a chloride-covered Ag(1 1 1) for the comparative experiment also uses substitutional oxide-to-chloride conversion in dilute alkali chloride solution. The structures of these halide monolayers are as analyzed in detail [31], p elsewhere p where we suggested a quasi- 3 3 structure average spacing. The oxide-to-chlowith 4.6 A ride conversion was allowed by bathing the asgrown substrate into 1 mM KCl (guaranteed reagent grade) solution in a mixed water (deionized and singly distilled) and ethanol (guaranteed reagent grade) solvent of 1:1 volume ratio for a few minutes at room temperature. The series of short alkanethiols [ethanethiol (C2 ), 1-propanethiol (C3 ), 1-butanethiol (C4 ), 1octanethiol (C8 ), and 1-hexadecanethiol (C16 )] were purchased from Wako Pure Chemical Industry Ltd. They were used without further purification in the form of typically 1 mM solution in ethanol. The as-grown (oxide-covered) or chloride-covered Au(1 1 1) films were bathed in each solution for desired periods at various temperatures (up to 70 C), thoroughly rinsed by running ethanol, and then blown dry with nitrogen. The XPS data were taken in an ESCA-750 spectrometer (Shimadzu Corporation) with MgKa radiation of 1253.6 eV for samples typically 6 · 6 mm2 in area. The photoemission angle was fixed at 90. All STM images were taken in the ambient atmosphere by using a Nanoscope I microscope (Digital Instruments Inc.). Pt/Ir tips obtained from Digital Instruments Inc. were mechanically cut before use so as to gain the optimum resolution. The microscope was operated under the constant current mode, with the sample bias of 400–600 mV (negative) and the tunneling current of 0.1 nA. The scan rates along the X and Y directions were adjusted to 8 and 0.033 Hz, respectively, irrespective of the scan width. The real-time changes in the piezo drive voltages during each scan were sampled at regular time intervals by using a multi-channel A-to-D converter and recorded as series of digital data in a personal computer.
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Height-mapped STM images were produced therein from 150 · 150 data points at maximum.
3. Results 3.1. SAM formation on As-deposited Ag(1 1 1) In the case of good planar Ag(1 1 1) substrates, the S2p and Ag3d XPS spectra and the corresponding S/Ag intensity ratio provide convenient information about the average molecular packing density of alkanethiolates on Ag(1 1 1). Since the S2p and the substrate Ag3d signals undergo virtually equal intensity attenuation by whatever kinds of overlayers, the S/Ag intensity ratio is controlled by the absolute surface thiolate density irrespective of the alkyl chain length. Fig. 1 presents a typical result obtained for C8 SAMs, showing the relationships between the S/ Ag intensity ratio and the immersion time (at room temperature) for three widely different thiol concentrations in solution. It can be seen that an identical saturation coverage was reached sooner (within 10 min) or later (taking a few hours) in the given range of thiol concentration from 1 mM down to 0.01 mM. The observed adsorption rate was noticeably smaller than on Au(1 1 1) studied
S/Ag intensity ratio
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Fig. 1. Chemisorption kinetics of C8 on as-deposited Ag(1 1 1) substrate from room-temperature ethanolic solution measured by XPS for widely different thiol concentrations. S2p /Ag3d intensity ratio, a convenient measure of the surface thiolate density, is plotted as a function of immersion time.
previously [12], which is attributed at least in part to the substitutional mode of thiol chemisorption on Ag(1 1 1) originally covered with a native oxide monolayer. The decrease in the O1s signal intensity indeed paralleled with the growth of S2p and we confirmed that the oxygen signal totally disappeared when the saturation thiolate coverage was reached. By assuming a perfectly smooth Ag(1 1 1) surface, the theoretical S/Ag intensity ratio can be readily calculated for an arbitrary thiolate packing density from the instrumental sensitivity factors for each XPS signal along with the density of Ag atoms and the mean electron escape depth [32] in the Ag substrate. The thus S/Ag intenp calculated p sity ratio was 0.0061 for ð 3 3ÞR30 structure p p and 0.0078 for ð 7 7ÞR19.1. Note that the experimental saturation S/Ag ratio in Fig. 1, 0.008, is close to the latter value. This of course does not necessarily point to any specific adlayer structure, but gives us one straightforward confirmation of the fact that the saturation thiolate packing density on Ag(1 1 1) is significantly higher than on Au(1 1 1). The kinetics of thiolate chemisorption studied by XPS for other kinds of alkanethiols was similar to that shown in Fig. 1, and no particularly significant effects of chain length were observed in the framework of XPS analysis. One apparent exception was the behavior of C2 SAM, whose adsorption time profile frequently exhibited further monotonic rise in the apparent sulfur coverage above the common saturation level upon extended treatment in thiol solution. This unusual behavior probably stems from the fact that a corrosive reaction beyond the simple chemisorption scheme may be possible between the Ag substrate and such short alkanethiols [33]. However, judging from the results of the following STM study, this reaction could be more like a local event that only set in at limited defect sites of the substrate. The images scanned over good atomic terraces (see below) had no indication of such corrosive reactions that would have roughened the surface more or less. Fig. 2 shows a series of wide-scan (150 nm) and molecular resolution STM images of the Ag(1 1 1) terraces covered with C2 –C8 SAMs (taken in the saturation coverage regime). The
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Fig. 2. Series of STM images taken for C2 –C8 SAMs on Ag(1 1 1) in the saturation coverage regime. Wide-scan (150 · 140 nm2 ) images show that monatomic islands begin to evolve at C3 . Molecular resolution images (10 · 9 nm2 ) show that characteristic domain structures become dominant also for chain lengths longer than C3 .
wide-scan images clearly show that emergence of elevated monatomic islands (as first observed by Dhirani and co-workers for C10 SAM [26]) is strongly dependent on chain length: No islands were seen for C2 SAM; a relatively small number of islands began to show up at C3 ; and an equally large number of islands formed for C4 and longer alkanethiol SAMs. The molecular resolution STM images in Fig. 2 also exhibit a systematic chain-length dependence in parallel to that of the island structure. The image of C2 SAM, with some atomic-size depressions, is analogous to that taken earlier by Heinz and Rabe [24] with the average spacing close and we attribute this image to a top 4.4 pA, ð 7 7ÞR19.1 structure not yet distorted any significantly. The C3 SAM again served as the intermediate case, where the characteristic domain structure as noted by Dhirani and co-workers [26] became clearly noticeable. This pattern then spread out over the entire terraces for C4 and longer alkanethiol SAMs. Each domain preserves a hexagonal pattern with a little enlarged spacing up p to p4.7 A characteristic of the distorted ð 7 7ÞR19.1 structure. Fig. 2 thus confirms the critical role of chain length in the evolution of monatomic islands as well as fine domain structures on SAM-covered Ag(1 1 1).
3.2. Substitutional SAM formation on chloridecovered Ag(1 1 1) The interactions between the tail alkyl chains may be largely modified when alkanethiolates could be effectively diluted by other co-adsorbates. Chlorine adatoms, which also make strong ionic bonds with Ag [31] as in the case of S–Ag bond [26,27,33–37], are good candidates that possibly mix well in the sulfur adlayer. The fact that their for Cl()1) and 1.84 A for S()2) ionic radii, 1.81 A [38], are close to each other is also advantageous. As a most convenient way to examine whether or not there could be any distinguishable mixed phase, we simply followed a substitutional thiolate chemisorption onto an initially chloride-covered Ag(1 1 1). Fig. 3 shows what typically occurred in the case of C8 . In Fig. 3, we plotted the sensitivity corrected, normalized intensities of the sulfur (S2p ) and chlorine (Cl2p ) XPS signals as a function of immersion time in 1 mM thiol solution. Here the unit intensity was chosen so as to approximately match p the number density of adatoms that make p up a ð 7 7ÞR19.1 structure. At all the processing temperatures up to 70 C, the observed time profiles invariably evidenced far from complete substitution. Of these, the room-temperature
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Fig. 3. XPS time profiles of substitutional chemisorption of C8 onto initially chloride-covered Ag(1 1 1) in 1 mM thiol solution at varied temperatures. Normalized intensities of S2p (j) and Cl2p () are plotted as functions of immersion time.
profile (at the top) may not be particularly interesting, because the incomplete substitution at this low temperature may simply stem from a kinetic barrier. The processing at much higher temperatures (50 and 70 C) indeed allowed a higher level of chloride-to-thiolate conversion. Note, however, that the net increase in the relative sulfur coverage at these elavated temperatures was not yet so significant. More importantly, the steady-state S/Cl ratio most likely converged to 2:1 (as indicated by two dotted lines in Fig. 3) in the high temperature limit. Fig. 4 shows the effect of chain length on the chloride-to-thiolate conversion at 70 C. It can be seen that short alkanethiols (C3 and C4 ) caused quite rapid, complete substitution of the chloride
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Fig. 4. Comparison of XPS time profiles of substitutional chemisorption of C3 , C4 , and C16 onto initially chloride-covered Ag(1 1 1) at 70 C.
monolayer without any plateau at any atomic ratio. In contrast, the formation of again 2:1 mixed alkanethiolate–chloride monolayer was evidenced in the case of C16 . The results thus indicate that the interchain interactions somehow play dominant roles for the formation of stable mixed monolayer. This phase continued to keep the 2:1 atomic ratio for hours in the high temperature thiol solution (see the long plateau in Fig. 4 for C16 ). This fact alone sufficiently testifies to the remarkable stability of the mixed phase. The simple 2:1 p atomic p ratio is reminiscent of the fact that in a ð 7 7ÞR19.1 structure of pure sulfur adlayer on Ag(1 1 1) one-third of the sulfur atoms are on top sites and the remaining twothirds on hollow sites of the Ag(1 1 1) lattice. Replacement of the top-site sulfur atoms by chlo-
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Fig. 5. Model lattice of circles for a 2:1 mixed thiolate–chloride p p adlayer conformed to ð 7 7ÞR19.1 structure, with chlorine atoms on top sites and sulfur atoms on hollow sites of p p Ag(1 1 1) lattice. A rhombic box shows 7 7 unit cell with edge length of 7.65 A.
p rine p atoms then would lead to a mixed ð 7 7ÞR19.1 adlayer with just the observed 2:1 S/Cl ratio, as illustrated by the model lattice of circles presented in Fig. 5. In order to examine further this coincidence, we have tried STM imaging of the 2:1 mixed monolayer with C8 thiolates. Fig. 6 shows typical examples. The extent of molecular order should be higher with C16 thiolates, but the corresponding alkyl chains are too long to be compatible with non-destructive STM imaging. In Fig. 6, we first note that the wide-scan image no longer exhibits any island structures that were so dominant in the case of pure C8 SAM on Ag(1 1 1) (see Fig. 2). The molecular resolution image appears to be a little noisy due probably to incomplete order of the tail C8 alkyl chains. Nev spacing ertheless, hexagonal patterns with 7.5pA p [i.e., the dimension of the unit cell of ð 7 7ÞR19.1 structure] can be clearly seen in the image (easier to notice on the left side of the image). Although there is no direct proof, the observed spacing can be understood if chlorine atoms in the model of Fig. 5 are selectively imaged brighter than sulfur atoms. It is at least unlikely that inhomogeneous mixing of sulfur and chlorine gives rise to this kind of periodic structure. Overall, both
Fig. 6. STM images of 2:1 mixed thiolate (C8 )–chloride adlayer: (a) 150 · 136 nm2 image showing disappearance of island structure; (b) 15 · 14 nm2 image partially but unambiguously spacing. uncovering a hexagonal pattern with 7.5 A
XPS and STM data support that long alkanethiolate and chloride can produce a highly stable mixed on Ag(1 1 1) likely conformed to pmonolayer p the ð 7 7ÞR19.1 registry.
4. Discussion As suggested by Dhirani and co-workers [26], the strongly ionic silver–sulfur bonds can significantly weaken the underlying Ag–Ag bonds, which in turn enhances the surface mobility of thiolates (as thiol–silver complexes). Their rearrangement aided by this increased mobility and driven by the interchain repulsive forces [against the substrate– sulfur interactions that favor the commensurate p p ð 7 7ÞR19.1 structure] seems to reasonably account for the strongly chain-length dependent SAM structures of alkanethiolates on Ag(1 1 1). In addition, as suggested by Rieley and Kendall in their X-ray studies [37], the strongly ionic nature of S–Ag bond makes the direction of S–C bond
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more flexible than on Au(1 1 1), which would also help the reorganization of alkyl chains into whatever necessary forms that match the ultimate SAM structure. Furthermore, the distortion p significant p from the commensurate ð 7 7ÞR19.1 structure would give rise to considerable stress in the surface Ag(1 1 1) lattice directly bonded to the thiolates. The distinct island and fine domain structures are thought to be natural ways to release this excess stress in the incommensurate registry. Overall, the relationship between the substrate– sulfur and interchain interactions in the formation of pure alkanethiolate SAMs on Ag(1 1 1) is believed to be strongly competitive, and the observed SAM structure may as well represent reconciliation between these counteracting forces. Importantly, the formation of stable 2:1 mixed thiolate–chloride monolayer also supports this interpretation indirectly. As discussed below, the appearance of this new phase is likely based on a drastic change of the above relationship into more like a synergic one. The simple, well-defined, 2:1 atomic ratio, together with the result of STM imaging, suggests that the mixed chlorine atoms are well dispersed in ap regular p pattern being incorporated in the ð 7 7ÞR19.1 adlayer lattice together with sulfur atoms. As noted already, both kinds of atoms form strongly ionic bonds with Ag and their ionic radii are very close to each other, so it may be of no particular surprise that they form a relatively stable mixed adlayer on Ag(1 1 1). Nevertheless, p this does not necessarily justify their ð 7 p 7ÞR19.1 arrangement with such a specific occupation of chlorine atoms as assumed in the model of Fig. 5. Moreover, in the case of short alkanethiols, the chloride monolayer was completely displaced by thiolates without any temporal plateau at the 2:1 S/Cl ratio (see Fig. 4). We therefore suggest that the interchain interactions of long alkanethiols play important roles not only in stabilizing the 2:1 mixed monolayer but also in producing the given positional order in the mixed sulfur–chlorine adlayer. Specifically, if the chloride-to-thiolate substitution occurred in such a way that discrete patches of pure alkanethiolate phase formed separately from
the residual chloride monolayer, the interchain repulsive forces immediately come up again even though locally, so this kind of substitution would be energetically disfavored. However, our results also indicate that randomly diluted thiolates with an arbitrary number of intervening chlorine atoms were never the stable product either. The specialty ofp the p2:1 mixed monolayer conformed to the ð 7 7ÞR19.1 structure likely stems from the fact that the interchain van der Waals forces are probably optimized with this particular atomic ratio and with the given positional order in the mixed sulfur and chlorine adlayer [the alkyl chain packing density in the 2:1 mixed p monolayer p amounts to 86% of that in the ð 3 3ÞR30 p structure]. In other words, the mixed ð 7 p 7ÞR19.1 adlayer structure may allow an ideal interchain spacing in the mixed phase and, conversely, the thus organized alkyl chains may further solidify the underlying mixed sulfur–chlorine adlayer structure. This is what we refer to as synergic substrate-adlayer and interchain interactions. That long alkyl chains in the 2:1 mixed phase are substantially (if not completely) ordered is reflected at least in two experimental results. First, the extent of molecular resolution in the STM imaging as exemplified in Fig. 6 seems to be hardly achieved without fair ordering of the overlying alkyl chains. Secondly, partial chloride-to-thiolate substitution, as high as 50% or more, easily occurred even at room temperature and for long alkanethiols (see Fig. 3). Nevertheless, as the S/Cl ratio approached 2:1 the conversion rate was drastically decreased so that completion of the 2:1 mixed phase required considerable thermal activation. This is difficult to rationalize unless some significant alkyl chain order begins to develop near the 2:1 atomic ratio.
5. Conclusions The substitutional self-assembly of 1-alkanethiols on as-deposited Ag(1 1 1) but covered with a native oxide monolayer was relatively rapid, completing well within 10 min in 1 mM thiol solution, thereby the native oxide monolayer was
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totally replaced by thiolates irrespective of the chain length. The resultant SAM structures exhibited clearly p chain-length dependent transition p from a dense ð 7 p 7ÞR19.1 monolayer for p n < 3 to a distorted ð 7 7ÞR19.1 phase for n > 3 accompanied by distinct island and fine domain structures. Our observations generally reproduce well the previously reported SAM structures on Ag(1 1 1), and can be understood in terms of competing interaction p substrate-molecule p [favoring the ð 7 7ÞR19.1 ordering] and interchain van der Waals dispersion forces (limiting the minimum interchain spacing). By contrast, chloride-to-thiolate conversion turned out to be far from complete for long alkanethiols (n P 8). In particular, a highly stable mixed thiolate–chloride monolayer with a welldefined 2:1 S/Cl ratio showed up at sufficiently high conversion temperature above 50 C. The simple 2:1patomic p ratio suggests the occurrence of a mixed ð 7 7ÞR19.1 ordering, which was at least partially supported by molecular resolution STM images. The large stability of the 2:1 mixed phase suggests synergetic roles of substrate-molecule and interchain interactions, under which the specific positional order in the sulfur–chlorine adlayer reinforces the overlying alkyl chain order, and vice versa.
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Observation of stable mixed alkanethiolate–chloride adlayer on Ag(1 1 1): structural correlation with pure alkanethiolate monolayers Mitsuo Kawasaki *, Hiromichi Nagayama Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishigyo-ku, Kyoto 615-8510, Japan Received 30 September 2003; accepted for publication 14 November 2003
Abstract The self-assembly of 1-alkanethiols, CH3 (CH2 )n1 SH (n ¼ 2–16), on Ag(1 1 1) film initially covered with a native oxide monolayer and that on a chloride-covered Ag(1 1 1) were comparatively studied by X-ray photoelectron spectroscopy and scanning tunneling microscopy. The native oxide monolayer was readily substituted completely by thiop p lates irrespective of chain length, leading to a dense ð 7 7ÞR19.1 monolayer for n < 3, or a distorted p p ( 7 7ÞR19.1 structure for n > 3 accompanied by distinct island and fine domain structures previously reported by other groups. In contrast, the chloride-to-thiolate conversion was far from complete for long alkanethiols (n P 8), and at sufficiently high conversion temperature (>50 C), we found a highly stable mixed thiolate–chloride monolayer with a p p well-defined 2:1 S/Cl atomic ratio, suggesting the occurrence of a mixed ð 7 7ÞR19.1 adlayer ordering. The combined effects of substrate-molecule and intermolecular interactions behind these contrastive phenomena are discussed. 2003 Elsevier B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Scanning tunneling microscopy; Chemisorption; Self-assembly; Silver; Halides; Sulphides
1. Introduction Strong chemisorption and self-assembly of organosulfur compounds on metal surfaces offer a useful method to organize a variety of ordered monolayers to modify and/or functionalize the respective solid surfaces [1,2], and have been intensively studied by many groups in different disci-
*
Corresponding author. Tel./fax: +81-75-383-2574. E-mail address: [email protected] Kawasaki).
(M.
plines. Among others, the self-assembled monolayers (SAMs) of 1-alkanethiols [CH3 (CH2 )n1 SH, Cn in short] on Au(1 1 1) are particularly simple, well structured, and best characterized model systems. It is known that at least long (e.g., n P 6) alkanethiols invariably form a densely packed crystalline-like assembly p pon Au(1 1 1) either with a commensurate ð 3 3ÞR30 structure [3], or p with a more complex 3 2 3 structure [c(4 · 2) p p superlattice of a ð 3 3ÞR30 structure] as first observed by He atom diffraction [4] and confirmed later by a number of high-impedance scanning tunneling microscopy (STM) studies [5–13]. The
0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2003.11.017
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reported phase behaviors of short alkanethiols (n 6 4) on Au(1 1 1) are apparently more diverse, however; some studies revealing p either no molecular order [5] or low-density p 3 phase [14–17], while others as dense (e.g., p 3p· 4 phase) ordered monolayers as the ð 3 3ÞR30 structure [9,13,18–20]. The two major forces that determine the ultimate SAM structures are the substrate–sulfur chemisorption bonding and the interchain van der Waals dispersion forces. There can be also sulfur– sulfur interactions between the head groups and the alkyl–substrate interactions (particularly in the flat-lying adsorption case) that are by no means negligible. The observations of largely different molecular orders for short-chain SAMs may possibly be a reflection of delicate balance of these elementary forces that may even depend on specific SAM preparation conditions. To gain further insight into the competing (or synergic) roles of the substrate-molecule and intermolecular interactions in SAM formation, it is useful to compare the SAM structures on different substrates. The Ag(1 1 1) surface offers a particularly interesting comparison, because despite only 0.3% difference in the (1 1 1) nearest neighbor spacing, the alkanethiol SAMs on Ag(1 1 1) have been found to yield markedly different packing order and density from those on Au(1 1 1) [2]. In the case of sulfur (sulfide) and the extremely short alkanethiolate (Cp 1 ) on Ag(1 1 1), occurrence p of a commensurate ð 7 7ÞR19.1 structure was evidenced both by early diffraction studies [21–23] and by the more recent STM work of Heinz and Rabe [24]. This structure can be constructed by binding one-third of the sulfur atoms on top sites and the rest on alternate hpc and fcc hollow sites of the Ag(1 1 1) lattice. The resulting packing density, 5.93 · 1014 cm2 , with 4.41 A nearest neighbor spacing is 30% greater than that p p (4.6 · 1014 cm2 ) expected for the ð 3 3ÞR30 structure. This high packing density becomes an immediate problem for longer alkanethiolates, wherein much stronger alkyl chain packing constraint prevents the interchain spacing any smaller (i.e., the ideal hydrocarbon spacing in than 4.6 A bulk n-alkanes [2]). Indeed, the experimentally (by He and X-ray diffractions [25], STM [26], and
electrochemical measurements [27]) determined average spacing in Cn (n P 4) SAMs on Ag(1 1 1) range, which is invariably fell in the 4.6–4.8 A incommensurate with respect to the Ag lattice. The corresponding SAM structure p p may be best described as a distorted ð 7 7ÞR19.1 structure, in which the competing silver–sulfur and interchain interactions must be somehow reconciled. Importantly, the distortion from the p substantial p commensurate ð 7 7ÞR19.1 structure (as imposed by the interchain repulsive forces) likely introduces some significant stress in the Ag(1 1 1) surface lattice and its undesirable structural consequences. This has been most clearly demonstrated in the STM study of decanethiol SAMs reported by Dhirani and co-workers [26]. They showed that the SAM-covered Ag(1 1 1) surface produced a large number of elevated monatomic This silver islands ranging in size from 20 to 180 A. interesting feature represents another striking difference from Au(1 1 1) where the SAM formation is known to produce a number of monatomic depressions or pits [28]. Furthermore, the molecularly resolved STM images clearly uncovered fine with hexagonal domain (average size of 70 A pattern) structures separated by shallowly (by or less) depressed domain boundaries. 0.5 A The resulting complex vertical undulation of the image, together with the distinct island structure, gives us the impression that the long-chain SAMs on Ag(1 1 1) could be more defective pthan on p Au(1 1 1), even though the distorted ð 7 7ÞR19.1 structure yields a still significantly denser packing of alkanethiolates than does the p p ð 3 3ÞR30 structure. The purpose of this paper is to reilluminate these unique SAM features on Ag(1 1 1) and their origin in a little different perspective. We first present our own STM (along with X-ray photoelectron spectroscopy, XPS) characterization of a series of 1-alkanethiols (n ¼ 2, 3, 4, and 8) on Ag(1 1 1), which not only reproduces well the previously reported results, but also adds some complementary information regarding how the characteristic SAM features on Ag(1 1 1) for long alkanethiols evolve with chain length. We then introduce the formation of a novel SAM phase of long alkanethiols (n ¼ 8 and 16) co-adsorbed with
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chlorides in a simple 2:1 S/Cl atomic ratio. This interesting, markedly stable, new phase emerged typically as a result of substitutional chemisorption of long alkanethiols from ethanolic solution at relatively high temperatures (>50 C) onto an initially chloride-covered Ag(1 1 1). While short alkanethiols (n 6 4) completely displaced the initial chloride adlayer, the substitution by long alkanethiols ceased at the well-defined S/Cl atomic ratio, thereby p p suggesting the formation of a mixed ( 7 7ÞR19.1 adlayer structure. There, the characteristic island and domain structures associated with pure alkanethiol SAMs on Ag(1 1 1) were no longer observed. Overall these comparative observations for pure and mixed alkanethiolate–chloride SAMs provide more comprehensive understanding of the combined roles of substrate-molecule and intermolecular interactions on Ag(1 1 1).
2. Experimental The Ag(1 1 1) film was epitaxially grown on Au(1 1 1) predeposited on freshly cleaved natural mica. Both depositions were done by using the simple DC glow-discharge sputtering in an Ar atmosphere as described elsewhere [29,30]. On a freshly cleaved, 0.1 mm thick mica substrate typically 3 · 4 cm2 in area, an atomically flat high-quality Au(1 1 1) film, 0.2 lm thick, was grown at 300 C with 40–50 A/min deposition rate. The epitaxial growth of Ag(1 1 1) thereupon was allowed at a lower temperature of 180 C to the thickness of 0.05 lm. When Ag(1 1 1) is directly grown on mica at similar temperature or higher, the resulting film frequently gives whitish foggy appearance to the eye due to the formation of large grains [26,31]. The epitaxial growth of Ag(1 1 1) on Au(1 1 1) prevents this problem and yields an extensively terraced surface texture that could be even smoother than the predeposited Au(1 1 1). It should be noted that the as-deposited Ag(1 1 1) film, when taken out of the deposition chamber to the ambient air, is immediately covered by a native oxide monolayer. Thus even in our control experiment where the alkanethiol adsorp-
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tion (see below) is allowed onto the as-deposited substrate, the adsorption occurs substitutionally by replacing the native oxide monolayer. The preparation of a chloride-covered Ag(1 1 1) for the comparative experiment also uses substitutional oxide-to-chloride conversion in dilute alkali chloride solution. The structures of these halide monolayers are as analyzed in detail [31], p elsewhere p where we suggested a quasi- 3 3 structure average spacing. The oxide-to-chlowith 4.6 A ride conversion was allowed by bathing the asgrown substrate into 1 mM KCl (guaranteed reagent grade) solution in a mixed water (deionized and singly distilled) and ethanol (guaranteed reagent grade) solvent of 1:1 volume ratio for a few minutes at room temperature. The series of short alkanethiols [ethanethiol (C2 ), 1-propanethiol (C3 ), 1-butanethiol (C4 ), 1octanethiol (C8 ), and 1-hexadecanethiol (C16 )] were purchased from Wako Pure Chemical Industry Ltd. They were used without further purification in the form of typically 1 mM solution in ethanol. The as-grown (oxide-covered) or chloride-covered Au(1 1 1) films were bathed in each solution for desired periods at various temperatures (up to 70 C), thoroughly rinsed by running ethanol, and then blown dry with nitrogen. The XPS data were taken in an ESCA-750 spectrometer (Shimadzu Corporation) with MgKa radiation of 1253.6 eV for samples typically 6 · 6 mm2 in area. The photoemission angle was fixed at 90. All STM images were taken in the ambient atmosphere by using a Nanoscope I microscope (Digital Instruments Inc.). Pt/Ir tips obtained from Digital Instruments Inc. were mechanically cut before use so as to gain the optimum resolution. The microscope was operated under the constant current mode, with the sample bias of 400–600 mV (negative) and the tunneling current of 0.1 nA. The scan rates along the X and Y directions were adjusted to 8 and 0.033 Hz, respectively, irrespective of the scan width. The real-time changes in the piezo drive voltages during each scan were sampled at regular time intervals by using a multi-channel A-to-D converter and recorded as series of digital data in a personal computer.
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Height-mapped STM images were produced therein from 150 · 150 data points at maximum.
3. Results 3.1. SAM formation on As-deposited Ag(1 1 1) In the case of good planar Ag(1 1 1) substrates, the S2p and Ag3d XPS spectra and the corresponding S/Ag intensity ratio provide convenient information about the average molecular packing density of alkanethiolates on Ag(1 1 1). Since the S2p and the substrate Ag3d signals undergo virtually equal intensity attenuation by whatever kinds of overlayers, the S/Ag intensity ratio is controlled by the absolute surface thiolate density irrespective of the alkyl chain length. Fig. 1 presents a typical result obtained for C8 SAMs, showing the relationships between the S/ Ag intensity ratio and the immersion time (at room temperature) for three widely different thiol concentrations in solution. It can be seen that an identical saturation coverage was reached sooner (within 10 min) or later (taking a few hours) in the given range of thiol concentration from 1 mM down to 0.01 mM. The observed adsorption rate was noticeably smaller than on Au(1 1 1) studied
S/Ag intensity ratio
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.006 1 mM 0.1 mM
.004
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.002 0
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Fig. 1. Chemisorption kinetics of C8 on as-deposited Ag(1 1 1) substrate from room-temperature ethanolic solution measured by XPS for widely different thiol concentrations. S2p /Ag3d intensity ratio, a convenient measure of the surface thiolate density, is plotted as a function of immersion time.
previously [12], which is attributed at least in part to the substitutional mode of thiol chemisorption on Ag(1 1 1) originally covered with a native oxide monolayer. The decrease in the O1s signal intensity indeed paralleled with the growth of S2p and we confirmed that the oxygen signal totally disappeared when the saturation thiolate coverage was reached. By assuming a perfectly smooth Ag(1 1 1) surface, the theoretical S/Ag intensity ratio can be readily calculated for an arbitrary thiolate packing density from the instrumental sensitivity factors for each XPS signal along with the density of Ag atoms and the mean electron escape depth [32] in the Ag substrate. The thus S/Ag intenp calculated p sity ratio was 0.0061 for ð 3 3ÞR30 structure p p and 0.0078 for ð 7 7ÞR19.1. Note that the experimental saturation S/Ag ratio in Fig. 1, 0.008, is close to the latter value. This of course does not necessarily point to any specific adlayer structure, but gives us one straightforward confirmation of the fact that the saturation thiolate packing density on Ag(1 1 1) is significantly higher than on Au(1 1 1). The kinetics of thiolate chemisorption studied by XPS for other kinds of alkanethiols was similar to that shown in Fig. 1, and no particularly significant effects of chain length were observed in the framework of XPS analysis. One apparent exception was the behavior of C2 SAM, whose adsorption time profile frequently exhibited further monotonic rise in the apparent sulfur coverage above the common saturation level upon extended treatment in thiol solution. This unusual behavior probably stems from the fact that a corrosive reaction beyond the simple chemisorption scheme may be possible between the Ag substrate and such short alkanethiols [33]. However, judging from the results of the following STM study, this reaction could be more like a local event that only set in at limited defect sites of the substrate. The images scanned over good atomic terraces (see below) had no indication of such corrosive reactions that would have roughened the surface more or less. Fig. 2 shows a series of wide-scan (150 nm) and molecular resolution STM images of the Ag(1 1 1) terraces covered with C2 –C8 SAMs (taken in the saturation coverage regime). The
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Fig. 2. Series of STM images taken for C2 –C8 SAMs on Ag(1 1 1) in the saturation coverage regime. Wide-scan (150 · 140 nm2 ) images show that monatomic islands begin to evolve at C3 . Molecular resolution images (10 · 9 nm2 ) show that characteristic domain structures become dominant also for chain lengths longer than C3 .
wide-scan images clearly show that emergence of elevated monatomic islands (as first observed by Dhirani and co-workers for C10 SAM [26]) is strongly dependent on chain length: No islands were seen for C2 SAM; a relatively small number of islands began to show up at C3 ; and an equally large number of islands formed for C4 and longer alkanethiol SAMs. The molecular resolution STM images in Fig. 2 also exhibit a systematic chain-length dependence in parallel to that of the island structure. The image of C2 SAM, with some atomic-size depressions, is analogous to that taken earlier by Heinz and Rabe [24] with the average spacing close and we attribute this image to a top 4.4 pA, ð 7 7ÞR19.1 structure not yet distorted any significantly. The C3 SAM again served as the intermediate case, where the characteristic domain structure as noted by Dhirani and co-workers [26] became clearly noticeable. This pattern then spread out over the entire terraces for C4 and longer alkanethiol SAMs. Each domain preserves a hexagonal pattern with a little enlarged spacing up p to p4.7 A characteristic of the distorted ð 7 7ÞR19.1 structure. Fig. 2 thus confirms the critical role of chain length in the evolution of monatomic islands as well as fine domain structures on SAM-covered Ag(1 1 1).
3.2. Substitutional SAM formation on chloridecovered Ag(1 1 1) The interactions between the tail alkyl chains may be largely modified when alkanethiolates could be effectively diluted by other co-adsorbates. Chlorine adatoms, which also make strong ionic bonds with Ag [31] as in the case of S–Ag bond [26,27,33–37], are good candidates that possibly mix well in the sulfur adlayer. The fact that their for Cl()1) and 1.84 A for S()2) ionic radii, 1.81 A [38], are close to each other is also advantageous. As a most convenient way to examine whether or not there could be any distinguishable mixed phase, we simply followed a substitutional thiolate chemisorption onto an initially chloride-covered Ag(1 1 1). Fig. 3 shows what typically occurred in the case of C8 . In Fig. 3, we plotted the sensitivity corrected, normalized intensities of the sulfur (S2p ) and chlorine (Cl2p ) XPS signals as a function of immersion time in 1 mM thiol solution. Here the unit intensity was chosen so as to approximately match p the number density of adatoms that make p up a ð 7 7ÞR19.1 structure. At all the processing temperatures up to 70 C, the observed time profiles invariably evidenced far from complete substitution. Of these, the room-temperature
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Fig. 3. XPS time profiles of substitutional chemisorption of C8 onto initially chloride-covered Ag(1 1 1) in 1 mM thiol solution at varied temperatures. Normalized intensities of S2p (j) and Cl2p () are plotted as functions of immersion time.
profile (at the top) may not be particularly interesting, because the incomplete substitution at this low temperature may simply stem from a kinetic barrier. The processing at much higher temperatures (50 and 70 C) indeed allowed a higher level of chloride-to-thiolate conversion. Note, however, that the net increase in the relative sulfur coverage at these elavated temperatures was not yet so significant. More importantly, the steady-state S/Cl ratio most likely converged to 2:1 (as indicated by two dotted lines in Fig. 3) in the high temperature limit. Fig. 4 shows the effect of chain length on the chloride-to-thiolate conversion at 70 C. It can be seen that short alkanethiols (C3 and C4 ) caused quite rapid, complete substitution of the chloride
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Fig. 4. Comparison of XPS time profiles of substitutional chemisorption of C3 , C4 , and C16 onto initially chloride-covered Ag(1 1 1) at 70 C.
monolayer without any plateau at any atomic ratio. In contrast, the formation of again 2:1 mixed alkanethiolate–chloride monolayer was evidenced in the case of C16 . The results thus indicate that the interchain interactions somehow play dominant roles for the formation of stable mixed monolayer. This phase continued to keep the 2:1 atomic ratio for hours in the high temperature thiol solution (see the long plateau in Fig. 4 for C16 ). This fact alone sufficiently testifies to the remarkable stability of the mixed phase. The simple 2:1 p atomic p ratio is reminiscent of the fact that in a ð 7 7ÞR19.1 structure of pure sulfur adlayer on Ag(1 1 1) one-third of the sulfur atoms are on top sites and the remaining twothirds on hollow sites of the Ag(1 1 1) lattice. Replacement of the top-site sulfur atoms by chlo-
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Fig. 5. Model lattice of circles for a 2:1 mixed thiolate–chloride p p adlayer conformed to ð 7 7ÞR19.1 structure, with chlorine atoms on top sites and sulfur atoms on hollow sites of p p Ag(1 1 1) lattice. A rhombic box shows 7 7 unit cell with edge length of 7.65 A.
p rine p atoms then would lead to a mixed ð 7 7ÞR19.1 adlayer with just the observed 2:1 S/Cl ratio, as illustrated by the model lattice of circles presented in Fig. 5. In order to examine further this coincidence, we have tried STM imaging of the 2:1 mixed monolayer with C8 thiolates. Fig. 6 shows typical examples. The extent of molecular order should be higher with C16 thiolates, but the corresponding alkyl chains are too long to be compatible with non-destructive STM imaging. In Fig. 6, we first note that the wide-scan image no longer exhibits any island structures that were so dominant in the case of pure C8 SAM on Ag(1 1 1) (see Fig. 2). The molecular resolution image appears to be a little noisy due probably to incomplete order of the tail C8 alkyl chains. Nev spacing ertheless, hexagonal patterns with 7.5pA p [i.e., the dimension of the unit cell of ð 7 7ÞR19.1 structure] can be clearly seen in the image (easier to notice on the left side of the image). Although there is no direct proof, the observed spacing can be understood if chlorine atoms in the model of Fig. 5 are selectively imaged brighter than sulfur atoms. It is at least unlikely that inhomogeneous mixing of sulfur and chlorine gives rise to this kind of periodic structure. Overall, both
Fig. 6. STM images of 2:1 mixed thiolate (C8 )–chloride adlayer: (a) 150 · 136 nm2 image showing disappearance of island structure; (b) 15 · 14 nm2 image partially but unambiguously spacing. uncovering a hexagonal pattern with 7.5 A
XPS and STM data support that long alkanethiolate and chloride can produce a highly stable mixed on Ag(1 1 1) likely conformed to pmonolayer p the ð 7 7ÞR19.1 registry.
4. Discussion As suggested by Dhirani and co-workers [26], the strongly ionic silver–sulfur bonds can significantly weaken the underlying Ag–Ag bonds, which in turn enhances the surface mobility of thiolates (as thiol–silver complexes). Their rearrangement aided by this increased mobility and driven by the interchain repulsive forces [against the substrate– sulfur interactions that favor the commensurate p p ð 7 7ÞR19.1 structure] seems to reasonably account for the strongly chain-length dependent SAM structures of alkanethiolates on Ag(1 1 1). In addition, as suggested by Rieley and Kendall in their X-ray studies [37], the strongly ionic nature of S–Ag bond makes the direction of S–C bond
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more flexible than on Au(1 1 1), which would also help the reorganization of alkyl chains into whatever necessary forms that match the ultimate SAM structure. Furthermore, the distortion p significant p from the commensurate ð 7 7ÞR19.1 structure would give rise to considerable stress in the surface Ag(1 1 1) lattice directly bonded to the thiolates. The distinct island and fine domain structures are thought to be natural ways to release this excess stress in the incommensurate registry. Overall, the relationship between the substrate– sulfur and interchain interactions in the formation of pure alkanethiolate SAMs on Ag(1 1 1) is believed to be strongly competitive, and the observed SAM structure may as well represent reconciliation between these counteracting forces. Importantly, the formation of stable 2:1 mixed thiolate–chloride monolayer also supports this interpretation indirectly. As discussed below, the appearance of this new phase is likely based on a drastic change of the above relationship into more like a synergic one. The simple, well-defined, 2:1 atomic ratio, together with the result of STM imaging, suggests that the mixed chlorine atoms are well dispersed in ap regular p pattern being incorporated in the ð 7 7ÞR19.1 adlayer lattice together with sulfur atoms. As noted already, both kinds of atoms form strongly ionic bonds with Ag and their ionic radii are very close to each other, so it may be of no particular surprise that they form a relatively stable mixed adlayer on Ag(1 1 1). Nevertheless, p this does not necessarily justify their ð 7 p 7ÞR19.1 arrangement with such a specific occupation of chlorine atoms as assumed in the model of Fig. 5. Moreover, in the case of short alkanethiols, the chloride monolayer was completely displaced by thiolates without any temporal plateau at the 2:1 S/Cl ratio (see Fig. 4). We therefore suggest that the interchain interactions of long alkanethiols play important roles not only in stabilizing the 2:1 mixed monolayer but also in producing the given positional order in the mixed sulfur–chlorine adlayer. Specifically, if the chloride-to-thiolate substitution occurred in such a way that discrete patches of pure alkanethiolate phase formed separately from
the residual chloride monolayer, the interchain repulsive forces immediately come up again even though locally, so this kind of substitution would be energetically disfavored. However, our results also indicate that randomly diluted thiolates with an arbitrary number of intervening chlorine atoms were never the stable product either. The specialty ofp the p2:1 mixed monolayer conformed to the ð 7 7ÞR19.1 structure likely stems from the fact that the interchain van der Waals forces are probably optimized with this particular atomic ratio and with the given positional order in the mixed sulfur and chlorine adlayer [the alkyl chain packing density in the 2:1 mixed p monolayer p amounts to 86% of that in the ð 3 3ÞR30 p structure]. In other words, the mixed ð 7 p 7ÞR19.1 adlayer structure may allow an ideal interchain spacing in the mixed phase and, conversely, the thus organized alkyl chains may further solidify the underlying mixed sulfur–chlorine adlayer structure. This is what we refer to as synergic substrate-adlayer and interchain interactions. That long alkyl chains in the 2:1 mixed phase are substantially (if not completely) ordered is reflected at least in two experimental results. First, the extent of molecular resolution in the STM imaging as exemplified in Fig. 6 seems to be hardly achieved without fair ordering of the overlying alkyl chains. Secondly, partial chloride-to-thiolate substitution, as high as 50% or more, easily occurred even at room temperature and for long alkanethiols (see Fig. 3). Nevertheless, as the S/Cl ratio approached 2:1 the conversion rate was drastically decreased so that completion of the 2:1 mixed phase required considerable thermal activation. This is difficult to rationalize unless some significant alkyl chain order begins to develop near the 2:1 atomic ratio.
5. Conclusions The substitutional self-assembly of 1-alkanethiols on as-deposited Ag(1 1 1) but covered with a native oxide monolayer was relatively rapid, completing well within 10 min in 1 mM thiol solution, thereby the native oxide monolayer was
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totally replaced by thiolates irrespective of the chain length. The resultant SAM structures exhibited clearly p chain-length dependent transition p from a dense ð 7 p 7ÞR19.1 monolayer for p n < 3 to a distorted ð 7 7ÞR19.1 phase for n > 3 accompanied by distinct island and fine domain structures. Our observations generally reproduce well the previously reported SAM structures on Ag(1 1 1), and can be understood in terms of competing interaction p substrate-molecule p [favoring the ð 7 7ÞR19.1 ordering] and interchain van der Waals dispersion forces (limiting the minimum interchain spacing). By contrast, chloride-to-thiolate conversion turned out to be far from complete for long alkanethiols (n P 8). In particular, a highly stable mixed thiolate–chloride monolayer with a welldefined 2:1 S/Cl ratio showed up at sufficiently high conversion temperature above 50 C. The simple 2:1patomic p ratio suggests the occurrence of a mixed ð 7 7ÞR19.1 ordering, which was at least partially supported by molecular resolution STM images. The large stability of the 2:1 mixed phase suggests synergetic roles of substrate-molecule and interchain interactions, under which the specific positional order in the sulfur–chlorine adlayer reinforces the overlying alkyl chain order, and vice versa.
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