- Email: [email protected]
Distinct self-assembly of dithiol monolayers on Au(1 1 1) in water and hexane
Chemical Physics 441 (2014) 77–82
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Distinct self-assembly of dithiol monolayers on Au(1 1 1) in water and hexane Aisyah M. Sharif a,b, Fathima R. Laffir b, D. Noel Buckley a,b, Christophe Silien a,b,⇑ a b
Department of Physics and Energy, University of Limerick, Ireland Materials and Surface Science Institute, University of Limerick, Ireland
a r t i c l e
i n f o
Article history: Received 17 February 2014 In final form 30 June 2014 Available online 10 July 2014 Keywords: Scanning tunneling microscopy (STM) X-ray photoemission spectroscopy (XPS) Benzenedimethanethiol Adsorption Solvent Self-assembled monolayer (SAM)
a b s t r a c t The self-assembly of 1,4-benzenedimethanethiol on Au(1 1 1), at low concentration in water and in hexane which are respectively polar and non-polar solvent, has been studied by scanning tunneling microscopy (STM). The data reveal that, on clean Au(1 1 1), a complete and ordered self-assembled monolayer (SAM) of lying-down dithiols can form within a few seconds in water. While in hexane the adsorption is initially impeded by the rapid growth of an ordered hexane film that is gradually replaced by disordered domains of dithiol until completion of a saturated monolayer. Complemented by X-ray photoelectron spectroscopy measurements, the STM images resolve the progression of the self-assembly in both these polar and non-polar solvent, and highlight how the self-assembly depends on the trio solvent, dithiol, and substrate. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thiol monolayers are today among the most widely investigated self-assembled monolayers (SAMs) on metals [1–4], and the ability of linear dithiols to bond to two metal electrodes makes these molecules pertinent to the development of molecular electronics [5]. Yet, although the formation of highly ordered saturated films of standing-up monothiols is well established [1–4], the involvement of two thiol moieties complicates the achievement of comparable organization in saturated dithiol monolayers [6–9], since undesirable intermolecular sulfur–sulfur bonding [10] and multilayer formation [11–16] may occur in SAMs of standing-up dithiols and since the attachment of both thiol moieties on the substrate may constrain the molecules to remain lying-down onto the substrate [17–19]. Procedures for dithiol SAM preparation have nonetheless been studied for a little more than a decade. For example, with respect to the preparation of dithiol monolayers in solution, earlier works have established that the quality of the dithiol SAMs is concomitant with purging the solutions with an inert gas, which is believed to prevent the oxidative formation of sulfur–sulfur bonds, thus limiting multilayer growth and intralayer cross-linking, two of
⇑ Corresponding author at: Department of Physics and Energy, University of Limerick, Ireland. Tel.: +353 61234177. E-mail address: [email protected] (C. Silien). http://dx.doi.org/10.1016/j.chemphys.2014.06.019 0301-0104/Ó 2014 Elsevier B.V. All rights reserved.
the main sources of defects [6,20–22]. Moreover variations in the amount of structural imperfections in dithiol monolayer have been identified depending on the solvent. Recent comparative studies have highlighted that the non-polar hexane leads to saturated monolayers of standing-up molecules which are of higher spectroscopic quality than those grown in polar solvents such as ethanol [20,23,24]. In addition to spectroscopy-based investigations with techniques including vibrational spectroscopies and electrochemical methods [6,20,23], scanning tunneling microscopy (STM) has been used at several occasions to image dithiol SAMs and to assess their structure. However, although ordered arrangements have often been resolved at low molecular coverage when the dithiols are lying-down on the surface [17,25–27], STM has in most cases proved unable to confirm lateral order at monolayer saturation coverage, when the molecules are standing-up [7,17,25,26]. It remains unclear whether the lack of resolution results from the layer being actually disordered or from an increased reactivity of the surface owing to the top-most unbound thiol moieties, as it is sometimes proposed [7], since very high quality STM images of standing-up octanedithiol SAMs have been recorded in ultra-high vacuum [28]. Moreover, in a particular study of the self-assembly of 1,4-benzenedimethanethiol (BDMT) on Au(1 1 1) in ethanol, STM performed in ambient air revealed small domains of two types of ordered patches that have been interpreted as corresponding to dithiols lying-down and standing-up, respectively [13]. The same study also reported the observation of regular features in saturated standing-up BDMT monolayers [13].
78
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
The preparation and generic properties of saturated SAMs of alkanedithiols and of other dithiols incorporating aromatic moieties such as BDMT are thus generally known. Yet, it has also appeared that the relationship between the solution used for the self-assembly and the growing interface is critical in determining the structural properties of the dithiol SAMs. Rationalizing that the relative contributions of the solvents at the interface are emphasized for low dithiol concentrations, and accounting for the capacity of STM to image surfaces with molecular resolution, we have studied by STM the self-assembly of BDMT from dilute water and hexane solutions, which are polar and non-polar solvent. By complementing the STM data with X-ray photoelectron spectroscopy (XPS) measurements, various steps of the growth are understood and the molecular level implications of water and hexane are resolved. 2. Materials and methods Dodecanethiol (DDT), BDMT, hexane and ethanol were purchased from ALDRICH. Thin epitaxial Au(1 1 1) films (300 nm thick) grown on mica (GEORG ALBERT PVD) were used as substrates and stored in vacuum upon reception. The BDMT samples were prepared by immersion of the freshly flame annealed Au(1 1 1) substrates in thoroughly degassed (N2) deionized water (18 MX) or hexane dithiol solutions kept in the dark, respectively at ca. 50 C and room temperature (RT), and with a dithiol nominal concentration of ca. 0.5 nM, unless stated otherwise. Aqueous solutions were prepared at ca. 80 C to ensure good dithiol dissolution. DDT SAMs were self-assembled for a period of 16 h at RT in ethanol solutions (1.0 mM). All the samples were then rinsed using the same solvent as used for self-assembly and dried with N2 gas. STM measurements were performed in ambient air with a 5500 AFM/STM platform (AGILENT) and tips prepared by mechanically cutting/breaking Pt:Ir (80:20) wire (GOODFELLOW). All STM images were acquired at constant current, specified along with the sample bias in the figure captions below. XPS measurements were carried out with an AXIS 165 apparatus (KRATOS), equipped with a monochromatized Al source at 1486.6 eV. All spectra were recorded with a take-off angle of 30° and the binding energies (BE) were corrected by adjusting the Au 4f7/2 core-level at 84.0 eV. 3. Results XPS spectra of the core-levels S 2p were recorded in order to verify the actual molecular coverage and get more insight in the bonding configuration of the dithiols. The core-levels S 2p for thiol or dithiol SAMs on Au(1 1 1) exhibit up to three components appearing as doublet due to spin–orbit coupling that are labeled A, B, and C in Fig. 1. The corresponding S 2p3/2 core-levels are measured respectively around 161.2, 162.2, and 163.2 eV, in good agreement with values reported by others for thiols [29–39] and dithiols on Au [6,7,9,12,22–24,36,40]. The component B, around 162.2 eV, marks the gold thiolate typically seen for all saturated SAMs with thiol moieties [29–39]. The component C, around 163.2 eV, is seen in dithiol SAMs when only a single moiety bonds to Au while the other remains either as a free thiol or possibly bonded to an adjacent sulfur atom [6,7,9,12,22–24,36,40]. Thus for dithiols, the observation of the component B alone typically indicates molecules that are lying down on the substrate, while the observation of both contributions B and C marks standing up molecules in a monolayer, possibly with additional sulfur–sulfur intermolecular bonds arising from a partial multilayer or intralayer cross-linking. The component A, around 161.2 eV, is typically weaker than B. It has been observed in low coverage thiol or dithiol films prepared by vapor phase
Fig. 1. S 2p XPS core level measurements of BDMT on Au(1 1 1). The XPS data were recorded on samples prepared during 16 h immersion of DDT in ethanol at RT (a), in 500 pM BDMT in water at 50 C (10 h) showing low molecular coverage (b), in 500 pM BDMT in water at 50 C (10 h) showing high molecular coverage (c), in 100 lM BDMT in hexane (30 min) at RT (d) and in 500 pM BDMT in hexane (10 h) at RT (e).
deposition, by self-assembly in solution and annealing, and occasionally for complete SAMs such as those made of cyclopentanethiol and biphenylthiol [9,12,31,35–37]. It is believed that the component highlights an alternative S–Au hybridization [35,36], which is reasonable since sulfur atoms adsorbed on Au(1 1 1) exhibit indeed S 2p3/2 components at ca. 160.8 and 161.6 eV upon adsorption on different sites [41]. The series of S 2p core level spectra recorded for DDT and BDMT SAMs are presented in Fig. 1. The core levels were fitted by a set of three doublets characterized by a spin–orbit coupling of 1.2 eV and a 2:1 branching ratio for the 2p3/2 and 2p1/2 components, and by a peak shape defined by a fixed mixture of Gaussian and Lorentzian functions with a full-width at half maximum of 0.8 eV, representative of the instrumental resolution. For DDT SAMs, the S 2p3/2 BE is recorded at 161.9 eV (Fig. 1a) in agreement with what is generally found for thiol SAMs on Au(1 1 1) [29–39]. The intensity ratio between the core levels S 2p and Au 4f7/2 is measured at 0.11, which matches with values expected for such saturated alkanethiol SAMs. A spectrum recorded on a BDMT sample self-assembled 10 h in water is presented in Fig. 1b and reveals two components A and B, around 161.1 and 162.0 eV, imputable both to Au–S bonds with different atomic configurations. The total S 2p peak area is 0.7 of that measured for DDT, suggesting a coverage less than for a saturated BDMT monolayer [9]. It is thus concluded that, despite the prolonged immersion in the BDMT solution, the layer is only partial and characterized by lying-down molecules. Noteworthy, as it will be discussed below, STM measurements on the same sample revealed periodically-arranged stripes which are independently also understood as features of low density BDMT SAMs. Yet, the very same sample preparation in water may also lead to samples showing S 2p spectra such as the one shown in Fig. 1c, where the cumulated S 2p thiolate components A and B, at 161.4 and 162.4 eV, exhibit a total intensity 1.4 of that measured for
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
DDT. Anticipating once again with the presentation of the STM results below, the same sample does not reveal periodicallyarranged stripes but rather features characteristic of high coverage dithiol SAMs [25,26,38,42]. These last samples clearly consist of higher molecular density films where, given the component C at 163.4 eV, most molecules only bond a single sulfur moiety to the Au substrate. The difference in coverage between the spectra in Fig. 1b and c is further addressed below and is understood as resulting from the formation, or not, of an ordered lying-down BDMT SAM (i.e., the periodically-arranged stripes) capable of sensibly lessening the film growth in water. S 2p spectra of BDMT films prepared in hexane at high (100 lM, 30 min) and low concentration (ca. 0.5 nM, 10 h) are presented in Fig. 1d and e. A concentration of 100 lM is typical for thiol or dithiol self-assembly, and in spite of the relatively short immersion time (30 min), the spectrum reveals a cumulated S 2p peak area for components A and B, at 161.2 and 162.3 eV, equals to 1.3 of that measured for DDT. The spectrum is similar to that presented in Ref. [6], also prepared in hexane, and exhibits a component C at 163.4 eV marking preferentially free thiol moieties at the surface [6,7,9,12,22–24,36,40]. The XPS data are thus suggesting the formation of a saturated BDMT SAM. When BDMT is assembled during 10 h in hexane with a very low concentration (ca. 0.5 nM), the S 2p thiolate intensity is 2.2 that measured in DDT SAMs (Fig. 1e), suggesting a relatively large BDMT coverage, beyond that of saturated thiol and dithiol SAMs. The relative intensity of the components C at 163.0 eV is however weak with respect to the thiolate ones indicating that the film quality is poor with only a fraction of the molecules arranged with thiol moieties not bonded with Au. Moreover, the relatively large intensity of the S 2p3/2 component A at 161.2 eV suggests that Au atomic rearrangement concomitant with thiol and dithiol selfassembly is crucially lessened in hexane when the BDMT concentration is low. Noteworthy, in agreement with earlier works [35,36], the S 2p3/2 components A at ca. 161 eV is not attributed to atomic sulfur species that could arise from a slow molecular decomposition in the solution. The immersion times used in the study remain indeed well within typical values and the magnitude of the component does not correlate with longer immersions at same concentration (see for example Fig. 1c and e). It is hypothesized that the low BDMT SAM quality thus seen follows differences in the Au atomic surface reconstruction and subsequent variations in Au–S–C bond. Noteworthy, unlike for the growth in water, XPS results do not show large sample-to-sample variations when these are prepared in hexane. XPS measurements highlight differences between BDMT SAMs prepared in water and hexane. In particular, in aqueous solution, when the conditions are met, the growth is impeded prior to the formation of a BDMT monolayer of density equivalent to alkanethiol or biphenylalkanethiol saturated SAMs. To further clarify the molecular arrangement and the nature of the species present at the substrate surface, STM data are now presented. Images of BDMT films grown in water for periods of 30 s and 10 h are shown in Fig. 2a and b, respectively. STM reveals that the self-assembly of BDMT in water follows a pattern alike to the one recently described for other dithiols, more specifically hexanedithiol and biphenyldimethanethiol [27]. The image in Fig. 2a shows that within several seconds of deposition the clean Au(1 1 1) surface was covered by an ordered monolayer characterized in large scale images by a periodicity of 5.6 nm along the 0i substrate axis. The molecular arrangement is further h1 1 resolved in the high resolution image presented in inset. Since the interfacial structure of the BDMT SAM here is the same as the one reported for hexanedithiol and biphenyldimethanethiol in Ref. [27], only a brief description is made. Rows of bright protrusions, labeled A, and rows of dark depressions, labeled C, are seen
79
Fig. 2. STM images of BDMT films on Au(1 1 1) prepared in water. The samples were prepared in 30 s in water (a), in 7 s in water (inset), in 10 h in water (b), sequentially 30 min in water and 30 min in hexane (c). The images were recorded with the sample bias and current set at 0.75 V, 0.03 nA (a); 0.50 V, 0.05 nA (inset); +1.0 V, 0.025 nA (b); and +0.75 V, 0.025 nA (c). Scale bars are 50 nm (a, b, c) and 5 nm for inset.
substrate axis. The rows A repeat every running along the h1 1 2i 3.2 and 2.4 nm in sequence with either one row C or one row B in between. In line with Ref. [27] where it is fully justified, it is proposed that four lying-down and asymmetrically bonded dithiols are accommodated within the total 5.6 nm sequence. For these ordered samples, only a slow evolution of the BDMT SAM is observed when the duration of immersion in the aqueous solution is increased. The STM image in Fig. 2b reveals that after 10 h the sample still exhibits the same 5.6 nm periodicity characteristic of the dithiol arrangement achieved within the first few seconds, albeit domains of increased roughness are now also observed within the original structure. However, the absence of vacancy islands, which form systematically during monothiol or dithiol deposition on Au(1 1 1) when the molecular coverage is such that the Au adatoms accommodated within the final interfacial structure are extracted from the substrate terraces [44,45], indicates here that the BDMT coverage remains low, in agreement with the corresponding XPS spectrum (Fig. 1b). Thus, when this 5.6 nm-periodic stripe phase is formed, the self-assembly in the aqueous solution is severely hindered. A STM image recorded on sample, sequentially prepared first in water (30 min) and then in hexane (30 min), is shown in Fig. 3c and it reveals a surface that is seemingly homogeneous at the scale of tens of nanometers, with features reminiscent of vacancy islands, and where small molecular patches are visible at the nanoscale with no apparent order. As it is shown below in the manuscript, the same arrangement is seen for samples prepared in a single step in hexane, for sufficient duration of self-assembly, and occasionally upon preparation in water albeit with then an XPS spectra alike the one shown in Fig. 1c, indicative of a large BDMT coverage. Thus, the STM image confirms the eventual formation of a saturated monolayer of BDMT when the lying-down ordered monolayer firstly made in water is further modified in hexane. Consequently, the hindrance of the transition from lyingdown (i.e., 5.6 nm-periodic stripe phase) to standing-up SAM seen
80
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
Fig. 3. STM images of BDMT films on Au(1 1 1) prepared in hexane. The samples were prepared in 10 s in hexane (a, b, c), prepared in 2 s in hexane and aged for 35 days in a sealed vial (d), prepared in 1 h in hexane (e). The images were recorded with the sample bias and current set at 0.75 V, 0.03 nA (a, b, c); +0.5 V, 0.05 nA (d); and +0.1 V, 0.05 nA (e). Height profiles P1–P6 marked in the images (a–c) and (e) are presented in (f). Scale bars are 10 (a), 5 (b), 5 (c), 50 (d), 50 nm (e) and 10 nm for inset.
in water upholds because of the solvent implication at the interface and not because of the bonding of BDMT through both thiol moieties. The self-assembly of BDMT in hexane strongly differs from the one seen in water, and differences are observed from the very first instants of the growth. Indeed, after only a few seconds in the hexane solution, STM reveals a surface as shown in Fig. 3a–c, which is completely unlike the ordered monolayer of lying-down BDMT formed in water (Fig. 2a). The large scale image in Fig. 3a shows the formation of two sorts of ordered domain (v, d), along with wide patches (e) and narrows bands (c) where no order is detected. A small fraction of the surface exhibits ordered domains of the first sort (v), one of which is reproduced in Fig. 3b. These domains are substrate characterized by bright stripes running along the h1 1 2i 0i direction, and are axis with a periodicity of 1.2 nm in the h1 1 embedded in disordered patches which extend over several tens of nanometers. These ordered domains are seldom observed, and it is thus believed that very peculiar conditions are necessary for their nucleation. Yet they are reminiscent of a structure reported in an earlier study of the self-assembly of BDMT in ethanol [13],
where the stripes were attributed to lying-down BDMT molecules. The same interpretation is proposed here, noting that the symmetric appearance of the dithiols contrasts with the asymmetry observed in the lying-down monolayers made in water. The differences are attributed to variations in the arrangement of the Au atoms and adatoms at the interface justified by the difference in growth and sample history. The surface is mostly covered by ordered domains of the second sort (d), inside which disordered bands (c) are seen following the axis. These bands are a few nanometers wide for substrate h1 1 2i the sample presented in Fig. 3a, and their average height is 0.3 Å more than that of the extended ordered domains. A small undulation of the surface reminiscent of the Au(1 1 1) herringbone reconstruction is also seen, indicating that unlike for brief exposure to BDMT in water, the original reconstruction of the substrate [43,45] is not lifted. The higher resolution image presented in Fig. 3c shows rows with a periodicity of 4.5 Å, where dots are resolved every 4.2 Å when the scanning conditions are favorable. Self-assembly of BDMT in ethanol [13] revealed occasionally a much alike pattern consisting of a nearly rectangular lattice of dimensions 3.9 and 3.7 Å. The structure was tentatively ascribed by the authors of that study to an ordered monolayer of BDMT [13]. However, despite the similarities, it is proposed here to associate the structure to a monolayer of hexane physisorbed on the reconstructed Au(1 1 1) substrate. This interpretation is substantiated by several experimental evidences. The density of BDMT on the sample at that stage is indeed necessarily too low to account for the vast area covered by the ordered structure. This is supported by the linear sweep voltammetry (LSV) data shown in Fig. S1 which reveals a desorption charge an order of magnitude less than for saturated BDMT SAMs, as well as by the STM image presented in Fig. 3d which shows that, when aged 35 days in a dark and air-sealed vial, a sample initially prepared during several seconds in hexane solution exhibits a very low density of typically 10 nm long stripes that are occasionally arranged in small domains of parallel rows of periodicity 1.2 nm and that are reminiscent of domains of low density dithiol SAMs [17,27]. Furthermore, since the chemisorption of thiol on Au(1 1 1) involves Au adatoms that are first provided by deconstruction of the herringbone pattern [27,43–46], the observation of the pattern in Fig. 3a is further evidence that the sample is for the most part covered by a physisorbed monolayer. Finally, the rapid formation of a hexane monolayer was verified in an independent experiment by immersion a fresh Au(1 1 1) substrate in pure hexane (see supporting information, Fig. S2). The sub-monolayer amount of BDMT revealed in the LSV (Fig. S1) and in the STM images of an aged sample (Fig. 3d) are then associated to the wide patches and narrows bands where no order is detected in Fig. 3a and to the few ordered domains such as the one reproduced in Fig. 3b. A STM image recorded on a sample immersed for 1 h in the hexane solution and presented in Fig. 3e reveals that, for longer immersion, the BDMT films are very much alike SAMs of other dithiols reported in the literature [7,17,25,26]. The STM image is also undistinguishable from the one recorded in air on samples prepared sequentially in water and in hexane and discussed earlier (see Fig. 2c). Moreover, since XPS also indicates a coverage equivalent or higher than saturated alkanethiol SAMS (Fig. 1d), it is concluded that a saturated monolayer of BDMT is achieved; presumably growing from the disordered bands which originally follow the herringbone reconstruction as well as from the occasional wide disordered patches (Fig. 3a). Thus, after a brief immersion of the gold substrate in the hexane solution, STM reveals the formation of an ordered hexane layer (Fig. 3a, d) that initially impede the growth of BDMT. The hexane layer is progressively replaced by a BDMT monolayer upon further immersion (Fig. 3e).
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
4. Discussion The experiments reveal that, in water, the dithiol can organize within seconds in a complete and ordered monolayer where the molecules are lying-down on the substrate, akin to other similar dithiols [27]. This ordered monolayer hardly evolves when further exposed to the aqueous solution. Yet, growth into a saturated dithiol SAM is readily achieved when the lying-down monolayer made in water is exposed afterward to a hexane solution of BDMT, or when the ordered periodic lying-down monolayer does not form over the entire surface. When the self-assembly is initiated directly in hexane, for the same nominal BDMT concentration, the completion within seconds of a monolayer of hexane is first observed. Although this layer initially impedes the adsorption of the dithiols, BDMT domains eventually develop and a saturated BDMT SAM is completed in less than 30 min. This BDMT layer does not reveal any order in STM, although small ordered dithiol domains of a lying-down phase are occasionally observed during the growth. Furthermore, the quality of a saturated SAM grown at low concentration is sensibly less than the layer grown at higher concentration, namely here 100 lM. Given that water and hexane are respectively polar and nonpolar solvent, variations in the self-assembly on Au(1 1 1) of molecules such as BDMT are not unexpected. For example, considering the duo solvent-BDMT, the weaker solubility of the dithiol in the polar solvent and the larger viscosity of the latter are a priori important factors to consider with respect to the growth kinetic of the SAMs. Yet, the data indicate that very low concentrations of BDMT in the solvent are sufficient to ensure the very rapid formation of a complete film of lying-down BDMT in water. Thus, the amount of dithiols in the vicinity of the substrate surface is not limiting the growth. On the other hand, the experiments highlight the necessity to consider the complete trio solvent, BDMT, and substrate, as well as their mutual interactions to satisfactorily account for the growth in water and in hexane. The initial formation of an ordered monolayer of hexane reveals the relative stability of this surface when in contact with the solution. However, its progressive replacement by BDMT confirms that the chemisorption of the dithiols remains more favorable energetically. Yet, a subtle balance is achieved here since an increase in the van der Waals forces within the physisorbed alkane layer may be sufficient to make the attachment of dithiol unfavorable, as seen in a separate experiment where hexadecane was used instead of hexane. Moreover, the growth of BDMT SAMs at low concentration leads to monolayers of poorer structural quality than those grown at higher concentration. The observation can perhaps be rationalized by the faster replacement of the hexane layer at higher concentration and by a subsequent increased BDMT freedom to selfassemble. In water, the observation of a complete and ordered monolayer of lying-down BDMT after a brief deposition highlights the absence of an overlayer capable of affecting significantly the growth kinetic. Conversely the experiments also highlight the stability of this lying-down BDMT monolayer in the aqueous solution since the film hardly evolves upon prolonged immersion. However, the experiments show that this ordered lying-down phase does not always form, in which case the growth proceeds towards a saturated BDMT layer. It is not entirely clear why the lying-down layer forms or not, however, since variations are seen for different batches of substrates of Au on mica, it is proposed that the quality or purity of the Au(1 1 1) surface is critical. Yet, regardless of that, a SAM of standing-up BDMT should always remain more favorable energetically, as confirmed by the lying-down to standing-up transition seen in vacuum deposition experiments [9], and presumably because a standing-up monolayer involves a larger density of
81
thiol–Au bonds and increased lateral interactions between molecules. Since the monolayer is formed here directly on a clean herringbone reconstructed Au(1 1 1) surface as it is done in vacuum deposition experiment, it is reasonable to assume that in both cases the lying-down BDMT SAM have similar properties and stability, and one may also speculate that both correspond to the same arrangement of Au adatoms and molecules. Moreover, the transition is also seen when monolayers of lying-down BDMT prepared in water are immersed in the hexane solution afterward. Thus, although the initial chemisorption of both thiol moieties (i.e., ordered lying-down SAM) to the substrate certainly increases the energy barrier for the transition in comparison to monothiols, it does not justify the halt of the growth that is seen upon completion of the lying-down monolayer. The latter must thus be discussed considering the interface as a whole, including the Au substrate, the BDMT SAM, and the aqueous solution. At the beginning of the self-assembly, when the Au(1 1 1) surface is mostly bared, the adsorption of lying-down BDMT is expected to be favorable as it is observed in the experiments. This is readily conceivable since, at the molecular level, the two-thiol adsorption scheme on Au reduces the interaction between BDMT and the polar water molecules due to the interface energy is reduced by maximizing the number of thiol–Au bonds. After the ordered lying-down monolayer is completed, the addition of BDMT requires disruption of the already adsorbed molecules, and this is not observed at the interface with water. Yet, that the energy balance is then in favor of the status quo can also be justified considering the polarity of water because the reorientation of one molecule into a standingup configuration leads to the unfavorable increase in the amount of water molecules in contact with the BDMT molecules, unless the SAM is saturated in which case the interaction between dithiols should make the standing-up configuration more stable. Yet, when the ordered striped lying-down monolayer does not form, a standing-up configuration is obtained also in water. Thus it is a fine balance of interactions and sample history that determines the final interfacial structure. Conversely, hexane as non-polar solvent is likely to be promoting the transition, as this is indeed observed in our experiments. The role of the solvent is thus complex: the polarity is crucial in the conversion of the ordered lying-down phase as it is not seen in water but occurs readily in hexane, but also the precursors are different in both water and in hexane with the ordered lying-down phase not forming in hexane where a monolayer of hexane in formed instead. 5. Conclusions The self-assembly of BDMT in water and hexane, respectively polar and non-polar solvent, has been studied by STM and XPS. The growth at low BDMT concentration allowed the observation of precursor BDMT arrangements and coadsorption events at molecular resolution by STM. The STM images stress the implications of the solvents at the interface. The data fully support the conclusion that the solvent in which the self-assembly is performed strongly influences the structural properties of the dithiol SAMs. More specifically, on clean Au(1 1 1), the poor solvation of BDMT in water is associated to the stabilization at the liquid-sample interface of a complete and ordered monolayer of lying-down BDMT. The latter rapidly evolves nonetheless into a saturated SAM of standing-up dithiols at the interface with hexane. The self-assembly in hexane on Au(1 1 1) is initially impeded by the formation of an ordered hexane monolayer that is resolved by STM. The variation in structural properties in SAMs for different solvent and concentration have thus roots in the very first instant of selfassembly and the polarity of the solvent has been considered to justify at the molecular level the growth in water and hexane.
82
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
Conflict of interest The authors declare no conflict of interest. Acknowledgments A.M.S. acknowledges a Grant from the Ministry of Higher Education (MOHE) in Malaysia and C.S. funding from the Integrated Nanoscience Platform for Ireland (INSPIRE), initiated by the Higher Education Authority in Ireland within the PRTLI4 framework. At the time of this research, A.M.S. was on leave from the Sultan Idris University of Education (Malaysia). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemphys.2014. 06.019. References [1] C. Vericat, M.E. Vela, G. Benitez, P. Carro, R.C. Salvarezza, Chem. Soc. Rev. 39 (2010) 1805. [2] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. [3] D.K. Schwartz, Annu. Rev. Phys. Chem. 52 (2001) 107. [4] F. Schreiber, Prog. Surf. Sci. 65 (2000) 151. [5] R.L. McCreery, A.J. Bergren, Adv. Mater. 21 (2009) 4303. [6] H. Hamoudi, M. Prato, C. Dablemont, O. Cavalleri, M. Canepa, V.A. Esaulov, Langmuir 26 (2010) 7242. [7] D. Qu, B.C. Kim, C.W.J. Lee, M. Ito, H. Noguchi, K. Uosaki, J. Phys. Chem. C 114 (2010) 497. [8] C. Silien, L. Dreesen, F. Cecchet, P.A. Thiry, A. Peremans, J. Phys. Chem. C 111 (2007) 6357. [9] L. Pasquali, F. Terzi, R. Seeber, B.P. Doyle, S. Nannarone, J. Chem. Phys. 128 (2008) 134711. [10] R. Koole, B. Luigjes, M. Tachiya, R. Pool, T.J.H. Vlugt, C.D.M. Donega, A. Meijerink, D. Vanmaekelbergh, J. Phys. Chem. C 111 (2007) 11208. [11] D. Garcia-Raya, R. Madueno, M. Blazquez, T. Pineda, J. Phys. Chem. C 114 (2010) 3568. [12] D.L. Pugmire, M.J. Tarlov, R.D. van Zee, J. Naciri, Langmuir 19 (2003) 3720. [13] H.M. Zareie, A.M. McDonagh, J. Edgar, M.J. Ford, M.B. Cortie, M.R. Phillips, Chem. Mater. 18 (2006) 2376. [14] S. Rifai, M. Laferriere, D. Qu, D.D.M. Wayner, C.P. Wilde, M. Morin, J. Electroanal. Chem. 531 (2002) 111.
[15] S. Rifai, G.P. Lopinski, T. Ward, D.D.M. Wayner, M. Morin, Langmuir 19 (2003) 8916. [16] S. Rifai, M. Morin, J. Electroanal. Chem. 550–551 (2003) 277. [17] T.Y.B. Leung, M.C. Gerstenberg, D.J. Lavrich, G. Scoles, F. Schreiber, G.E. Poirier, Langmuir 16 (2000) 549. [18] H.B. Akkerman, A.J. Kronemeijer, P.A. van Hal, D.M. de Leeuw, P.W.M. Blom, B. de Boer, Small 4 (2008) 100. [19] Y.H. Kim, C.B. Gorman, Langmuir 27 (2011) 6069. [20] H. Hamoudi, Z.A. Guo, M. Prato, C. Dablemont, W.Q. Zheng, B. Bourguignon, M. Canepa, V.A. Esaulov, Phys. Chem. Chem. Phys. 10 (2008) 6836. [21] S.W. Chah, J.H. Fendler, J.H. Yi, Chem. Commun. (2002) 2094. [22] L. Pasquali, F. Terzi, R. Seeber, S. Nannarone, D. Datta, C. Dablemont, H. Hamoudi, M. Canepa, V.A. Esaulov, Langmuir 27 (2011) 4713. [23] M.A.D. Millone, H. Hamoudi, L. Rodriguez, A. Rubert, G.A. Benitez, M.E. Vela, R.C. Salvarezza, J.E. Gayone, E.A. Sanchez, O. Grizzi, C. Dablemont, V. Esaulov, Langmuir 25 (2009) 12945. [24] L. Pasquali, F. Terzi, C. Zanardi, L. Pigani, R. Seeber, G. Paolicelli, S.M. Suturin, N. Mahne, S. Nannarone, Surf. Sci. 601 (2007) 1419. [25] M.J. Esplandiu, M.L. Carot, F.P. Cometto, V.A. Macagno, E.M. Patrito, Surf. Sci. 600 (2006) 155. [26] C. Li, I. Pobelov, T. Wandlowski, A. Bagrets, A. Arnold, F. Evers, J. Am. Chem. Soc. 130 (2008) 318. [27] A.M. Sharif, D.N. Buckley, M. Buck, C. Silien, J. Phys. Chem. C 115 (2011) 21800. [28] K. Seo, H. Lee, ACS Nano 3 (2009) 2469. [29] Y.W. Yang, L.J. Fan, Langmuir 18 (2002) 1157. [30] M. Zharnikov, M. Grunze, J. Phys. – Condens. Matter 13 (2001) 11333. [31] P. Cyganik, M. Buck, T. Strunskus, A. Shaporenko, J.D.E.T. Wilton-Ely, M. Zharnikov, C. Wöll, J. Am. Chem. Soc. 128 (2006) 13868. [32] A. Shaporenko, A. Terfort, M. Grunze, M. Zharnikov, J. Electron Spectrosc. Relat. Phenom. 151 (2006) 45. [33] E. Cortes, A.A. Rubert, G. Benitez, P. Carro, M.E. Vela, R.C. Salvarezza, Langmuir 25 (2009) 5661. [34] Y.M. Yang, L.J. Fan, Langmuir 18 (2002) 1157. [35] T. Ishida, N. Choi, W. Mizutani, H. Tokumoto, I. Kojima, H. Azehara, H. Hokari, U. Akiba, M. Fujihira, Langmuir 15 (1999) 6799. [36] T. Ishida, M. Hara, I. Kojima, S. Tsuneda, N. Nishida, H. Sasabe, W. Knoll, Langmuir 14 (1998) 2092. [37] H. Kang, Y. Kim, T. Park, J.B. Park, E. Ito, M. Hara, J. Noh, Bull. Korean Chem. Soc. 32 (2011) 1253. [38] W. Azzam, B.I. Wehner, R.A. Fischer, A. Terfort, C. Woll, Langmuir 18 (2002) 7766. [39] W. Chen, A.T.S. Wee, J. Electron Spectrosc. Relat. Phenom. 172 (2009) 54. [40] Y. Tai, A. Shaporenko, H.T. Rong, M. Buck, W. Eck, M. Grunze, M. Zharnikov, J. Phys. Chem. B 108 (2004) 16806. [41] J.A. Rodriguez, J. Dvorak, T. Jirsak, G. Liu, J. Hrbek, Y. Aray, C. Gonzalez, J. Am. Chem. Soc. 125 (2003) 276. [42] A. Niklewski, W. Azzam, T. Strunskus, R.A. Fischer, C. Woll, Langmuir 20 (2004) 8620. [43] J.V. Barth, H. Brune, G. Ertl, R.J. Behm, Phys. Rev. B 42 (1990) 9307. [44] P. Maksymovych, O. Voznyy, D.B. Dougherty, D.C. Sorescu, J.T. Yates, Prog. Surf. Sci. 85 (2010) 206. [45] G.E. Poirier, Langmuir 13 (1997) 2019. [46] P. Maksymovych, D.C. Sorescu, J.T. Yates, Phys. Rev. Lett. 97 (2006) 146103.
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Distinct self-assembly of dithiol monolayers on Au(1 1 1) in water and hexane Aisyah M. Sharif a,b, Fathima R. Laffir b, D. Noel Buckley a,b, Christophe Silien a,b,⇑ a b
Department of Physics and Energy, University of Limerick, Ireland Materials and Surface Science Institute, University of Limerick, Ireland
a r t i c l e
i n f o
Article history: Received 17 February 2014 In final form 30 June 2014 Available online 10 July 2014 Keywords: Scanning tunneling microscopy (STM) X-ray photoemission spectroscopy (XPS) Benzenedimethanethiol Adsorption Solvent Self-assembled monolayer (SAM)
a b s t r a c t The self-assembly of 1,4-benzenedimethanethiol on Au(1 1 1), at low concentration in water and in hexane which are respectively polar and non-polar solvent, has been studied by scanning tunneling microscopy (STM). The data reveal that, on clean Au(1 1 1), a complete and ordered self-assembled monolayer (SAM) of lying-down dithiols can form within a few seconds in water. While in hexane the adsorption is initially impeded by the rapid growth of an ordered hexane film that is gradually replaced by disordered domains of dithiol until completion of a saturated monolayer. Complemented by X-ray photoelectron spectroscopy measurements, the STM images resolve the progression of the self-assembly in both these polar and non-polar solvent, and highlight how the self-assembly depends on the trio solvent, dithiol, and substrate. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Thiol monolayers are today among the most widely investigated self-assembled monolayers (SAMs) on metals [1–4], and the ability of linear dithiols to bond to two metal electrodes makes these molecules pertinent to the development of molecular electronics [5]. Yet, although the formation of highly ordered saturated films of standing-up monothiols is well established [1–4], the involvement of two thiol moieties complicates the achievement of comparable organization in saturated dithiol monolayers [6–9], since undesirable intermolecular sulfur–sulfur bonding [10] and multilayer formation [11–16] may occur in SAMs of standing-up dithiols and since the attachment of both thiol moieties on the substrate may constrain the molecules to remain lying-down onto the substrate [17–19]. Procedures for dithiol SAM preparation have nonetheless been studied for a little more than a decade. For example, with respect to the preparation of dithiol monolayers in solution, earlier works have established that the quality of the dithiol SAMs is concomitant with purging the solutions with an inert gas, which is believed to prevent the oxidative formation of sulfur–sulfur bonds, thus limiting multilayer growth and intralayer cross-linking, two of
⇑ Corresponding author at: Department of Physics and Energy, University of Limerick, Ireland. Tel.: +353 61234177. E-mail address: [email protected] (C. Silien). http://dx.doi.org/10.1016/j.chemphys.2014.06.019 0301-0104/Ó 2014 Elsevier B.V. All rights reserved.
the main sources of defects [6,20–22]. Moreover variations in the amount of structural imperfections in dithiol monolayer have been identified depending on the solvent. Recent comparative studies have highlighted that the non-polar hexane leads to saturated monolayers of standing-up molecules which are of higher spectroscopic quality than those grown in polar solvents such as ethanol [20,23,24]. In addition to spectroscopy-based investigations with techniques including vibrational spectroscopies and electrochemical methods [6,20,23], scanning tunneling microscopy (STM) has been used at several occasions to image dithiol SAMs and to assess their structure. However, although ordered arrangements have often been resolved at low molecular coverage when the dithiols are lying-down on the surface [17,25–27], STM has in most cases proved unable to confirm lateral order at monolayer saturation coverage, when the molecules are standing-up [7,17,25,26]. It remains unclear whether the lack of resolution results from the layer being actually disordered or from an increased reactivity of the surface owing to the top-most unbound thiol moieties, as it is sometimes proposed [7], since very high quality STM images of standing-up octanedithiol SAMs have been recorded in ultra-high vacuum [28]. Moreover, in a particular study of the self-assembly of 1,4-benzenedimethanethiol (BDMT) on Au(1 1 1) in ethanol, STM performed in ambient air revealed small domains of two types of ordered patches that have been interpreted as corresponding to dithiols lying-down and standing-up, respectively [13]. The same study also reported the observation of regular features in saturated standing-up BDMT monolayers [13].
78
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
The preparation and generic properties of saturated SAMs of alkanedithiols and of other dithiols incorporating aromatic moieties such as BDMT are thus generally known. Yet, it has also appeared that the relationship between the solution used for the self-assembly and the growing interface is critical in determining the structural properties of the dithiol SAMs. Rationalizing that the relative contributions of the solvents at the interface are emphasized for low dithiol concentrations, and accounting for the capacity of STM to image surfaces with molecular resolution, we have studied by STM the self-assembly of BDMT from dilute water and hexane solutions, which are polar and non-polar solvent. By complementing the STM data with X-ray photoelectron spectroscopy (XPS) measurements, various steps of the growth are understood and the molecular level implications of water and hexane are resolved. 2. Materials and methods Dodecanethiol (DDT), BDMT, hexane and ethanol were purchased from ALDRICH. Thin epitaxial Au(1 1 1) films (300 nm thick) grown on mica (GEORG ALBERT PVD) were used as substrates and stored in vacuum upon reception. The BDMT samples were prepared by immersion of the freshly flame annealed Au(1 1 1) substrates in thoroughly degassed (N2) deionized water (18 MX) or hexane dithiol solutions kept in the dark, respectively at ca. 50 C and room temperature (RT), and with a dithiol nominal concentration of ca. 0.5 nM, unless stated otherwise. Aqueous solutions were prepared at ca. 80 C to ensure good dithiol dissolution. DDT SAMs were self-assembled for a period of 16 h at RT in ethanol solutions (1.0 mM). All the samples were then rinsed using the same solvent as used for self-assembly and dried with N2 gas. STM measurements were performed in ambient air with a 5500 AFM/STM platform (AGILENT) and tips prepared by mechanically cutting/breaking Pt:Ir (80:20) wire (GOODFELLOW). All STM images were acquired at constant current, specified along with the sample bias in the figure captions below. XPS measurements were carried out with an AXIS 165 apparatus (KRATOS), equipped with a monochromatized Al source at 1486.6 eV. All spectra were recorded with a take-off angle of 30° and the binding energies (BE) were corrected by adjusting the Au 4f7/2 core-level at 84.0 eV. 3. Results XPS spectra of the core-levels S 2p were recorded in order to verify the actual molecular coverage and get more insight in the bonding configuration of the dithiols. The core-levels S 2p for thiol or dithiol SAMs on Au(1 1 1) exhibit up to three components appearing as doublet due to spin–orbit coupling that are labeled A, B, and C in Fig. 1. The corresponding S 2p3/2 core-levels are measured respectively around 161.2, 162.2, and 163.2 eV, in good agreement with values reported by others for thiols [29–39] and dithiols on Au [6,7,9,12,22–24,36,40]. The component B, around 162.2 eV, marks the gold thiolate typically seen for all saturated SAMs with thiol moieties [29–39]. The component C, around 163.2 eV, is seen in dithiol SAMs when only a single moiety bonds to Au while the other remains either as a free thiol or possibly bonded to an adjacent sulfur atom [6,7,9,12,22–24,36,40]. Thus for dithiols, the observation of the component B alone typically indicates molecules that are lying down on the substrate, while the observation of both contributions B and C marks standing up molecules in a monolayer, possibly with additional sulfur–sulfur intermolecular bonds arising from a partial multilayer or intralayer cross-linking. The component A, around 161.2 eV, is typically weaker than B. It has been observed in low coverage thiol or dithiol films prepared by vapor phase
Fig. 1. S 2p XPS core level measurements of BDMT on Au(1 1 1). The XPS data were recorded on samples prepared during 16 h immersion of DDT in ethanol at RT (a), in 500 pM BDMT in water at 50 C (10 h) showing low molecular coverage (b), in 500 pM BDMT in water at 50 C (10 h) showing high molecular coverage (c), in 100 lM BDMT in hexane (30 min) at RT (d) and in 500 pM BDMT in hexane (10 h) at RT (e).
deposition, by self-assembly in solution and annealing, and occasionally for complete SAMs such as those made of cyclopentanethiol and biphenylthiol [9,12,31,35–37]. It is believed that the component highlights an alternative S–Au hybridization [35,36], which is reasonable since sulfur atoms adsorbed on Au(1 1 1) exhibit indeed S 2p3/2 components at ca. 160.8 and 161.6 eV upon adsorption on different sites [41]. The series of S 2p core level spectra recorded for DDT and BDMT SAMs are presented in Fig. 1. The core levels were fitted by a set of three doublets characterized by a spin–orbit coupling of 1.2 eV and a 2:1 branching ratio for the 2p3/2 and 2p1/2 components, and by a peak shape defined by a fixed mixture of Gaussian and Lorentzian functions with a full-width at half maximum of 0.8 eV, representative of the instrumental resolution. For DDT SAMs, the S 2p3/2 BE is recorded at 161.9 eV (Fig. 1a) in agreement with what is generally found for thiol SAMs on Au(1 1 1) [29–39]. The intensity ratio between the core levels S 2p and Au 4f7/2 is measured at 0.11, which matches with values expected for such saturated alkanethiol SAMs. A spectrum recorded on a BDMT sample self-assembled 10 h in water is presented in Fig. 1b and reveals two components A and B, around 161.1 and 162.0 eV, imputable both to Au–S bonds with different atomic configurations. The total S 2p peak area is 0.7 of that measured for DDT, suggesting a coverage less than for a saturated BDMT monolayer [9]. It is thus concluded that, despite the prolonged immersion in the BDMT solution, the layer is only partial and characterized by lying-down molecules. Noteworthy, as it will be discussed below, STM measurements on the same sample revealed periodically-arranged stripes which are independently also understood as features of low density BDMT SAMs. Yet, the very same sample preparation in water may also lead to samples showing S 2p spectra such as the one shown in Fig. 1c, where the cumulated S 2p thiolate components A and B, at 161.4 and 162.4 eV, exhibit a total intensity 1.4 of that measured for
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
DDT. Anticipating once again with the presentation of the STM results below, the same sample does not reveal periodicallyarranged stripes but rather features characteristic of high coverage dithiol SAMs [25,26,38,42]. These last samples clearly consist of higher molecular density films where, given the component C at 163.4 eV, most molecules only bond a single sulfur moiety to the Au substrate. The difference in coverage between the spectra in Fig. 1b and c is further addressed below and is understood as resulting from the formation, or not, of an ordered lying-down BDMT SAM (i.e., the periodically-arranged stripes) capable of sensibly lessening the film growth in water. S 2p spectra of BDMT films prepared in hexane at high (100 lM, 30 min) and low concentration (ca. 0.5 nM, 10 h) are presented in Fig. 1d and e. A concentration of 100 lM is typical for thiol or dithiol self-assembly, and in spite of the relatively short immersion time (30 min), the spectrum reveals a cumulated S 2p peak area for components A and B, at 161.2 and 162.3 eV, equals to 1.3 of that measured for DDT. The spectrum is similar to that presented in Ref. [6], also prepared in hexane, and exhibits a component C at 163.4 eV marking preferentially free thiol moieties at the surface [6,7,9,12,22–24,36,40]. The XPS data are thus suggesting the formation of a saturated BDMT SAM. When BDMT is assembled during 10 h in hexane with a very low concentration (ca. 0.5 nM), the S 2p thiolate intensity is 2.2 that measured in DDT SAMs (Fig. 1e), suggesting a relatively large BDMT coverage, beyond that of saturated thiol and dithiol SAMs. The relative intensity of the components C at 163.0 eV is however weak with respect to the thiolate ones indicating that the film quality is poor with only a fraction of the molecules arranged with thiol moieties not bonded with Au. Moreover, the relatively large intensity of the S 2p3/2 component A at 161.2 eV suggests that Au atomic rearrangement concomitant with thiol and dithiol selfassembly is crucially lessened in hexane when the BDMT concentration is low. Noteworthy, in agreement with earlier works [35,36], the S 2p3/2 components A at ca. 161 eV is not attributed to atomic sulfur species that could arise from a slow molecular decomposition in the solution. The immersion times used in the study remain indeed well within typical values and the magnitude of the component does not correlate with longer immersions at same concentration (see for example Fig. 1c and e). It is hypothesized that the low BDMT SAM quality thus seen follows differences in the Au atomic surface reconstruction and subsequent variations in Au–S–C bond. Noteworthy, unlike for the growth in water, XPS results do not show large sample-to-sample variations when these are prepared in hexane. XPS measurements highlight differences between BDMT SAMs prepared in water and hexane. In particular, in aqueous solution, when the conditions are met, the growth is impeded prior to the formation of a BDMT monolayer of density equivalent to alkanethiol or biphenylalkanethiol saturated SAMs. To further clarify the molecular arrangement and the nature of the species present at the substrate surface, STM data are now presented. Images of BDMT films grown in water for periods of 30 s and 10 h are shown in Fig. 2a and b, respectively. STM reveals that the self-assembly of BDMT in water follows a pattern alike to the one recently described for other dithiols, more specifically hexanedithiol and biphenyldimethanethiol [27]. The image in Fig. 2a shows that within several seconds of deposition the clean Au(1 1 1) surface was covered by an ordered monolayer characterized in large scale images by a periodicity of 5.6 nm along the 0i substrate axis. The molecular arrangement is further h1 1 resolved in the high resolution image presented in inset. Since the interfacial structure of the BDMT SAM here is the same as the one reported for hexanedithiol and biphenyldimethanethiol in Ref. [27], only a brief description is made. Rows of bright protrusions, labeled A, and rows of dark depressions, labeled C, are seen
79
Fig. 2. STM images of BDMT films on Au(1 1 1) prepared in water. The samples were prepared in 30 s in water (a), in 7 s in water (inset), in 10 h in water (b), sequentially 30 min in water and 30 min in hexane (c). The images were recorded with the sample bias and current set at 0.75 V, 0.03 nA (a); 0.50 V, 0.05 nA (inset); +1.0 V, 0.025 nA (b); and +0.75 V, 0.025 nA (c). Scale bars are 50 nm (a, b, c) and 5 nm for inset.
substrate axis. The rows A repeat every running along the h1 1 2i 3.2 and 2.4 nm in sequence with either one row C or one row B in between. In line with Ref. [27] where it is fully justified, it is proposed that four lying-down and asymmetrically bonded dithiols are accommodated within the total 5.6 nm sequence. For these ordered samples, only a slow evolution of the BDMT SAM is observed when the duration of immersion in the aqueous solution is increased. The STM image in Fig. 2b reveals that after 10 h the sample still exhibits the same 5.6 nm periodicity characteristic of the dithiol arrangement achieved within the first few seconds, albeit domains of increased roughness are now also observed within the original structure. However, the absence of vacancy islands, which form systematically during monothiol or dithiol deposition on Au(1 1 1) when the molecular coverage is such that the Au adatoms accommodated within the final interfacial structure are extracted from the substrate terraces [44,45], indicates here that the BDMT coverage remains low, in agreement with the corresponding XPS spectrum (Fig. 1b). Thus, when this 5.6 nm-periodic stripe phase is formed, the self-assembly in the aqueous solution is severely hindered. A STM image recorded on sample, sequentially prepared first in water (30 min) and then in hexane (30 min), is shown in Fig. 3c and it reveals a surface that is seemingly homogeneous at the scale of tens of nanometers, with features reminiscent of vacancy islands, and where small molecular patches are visible at the nanoscale with no apparent order. As it is shown below in the manuscript, the same arrangement is seen for samples prepared in a single step in hexane, for sufficient duration of self-assembly, and occasionally upon preparation in water albeit with then an XPS spectra alike the one shown in Fig. 1c, indicative of a large BDMT coverage. Thus, the STM image confirms the eventual formation of a saturated monolayer of BDMT when the lying-down ordered monolayer firstly made in water is further modified in hexane. Consequently, the hindrance of the transition from lyingdown (i.e., 5.6 nm-periodic stripe phase) to standing-up SAM seen
80
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
Fig. 3. STM images of BDMT films on Au(1 1 1) prepared in hexane. The samples were prepared in 10 s in hexane (a, b, c), prepared in 2 s in hexane and aged for 35 days in a sealed vial (d), prepared in 1 h in hexane (e). The images were recorded with the sample bias and current set at 0.75 V, 0.03 nA (a, b, c); +0.5 V, 0.05 nA (d); and +0.1 V, 0.05 nA (e). Height profiles P1–P6 marked in the images (a–c) and (e) are presented in (f). Scale bars are 10 (a), 5 (b), 5 (c), 50 (d), 50 nm (e) and 10 nm for inset.
in water upholds because of the solvent implication at the interface and not because of the bonding of BDMT through both thiol moieties. The self-assembly of BDMT in hexane strongly differs from the one seen in water, and differences are observed from the very first instants of the growth. Indeed, after only a few seconds in the hexane solution, STM reveals a surface as shown in Fig. 3a–c, which is completely unlike the ordered monolayer of lying-down BDMT formed in water (Fig. 2a). The large scale image in Fig. 3a shows the formation of two sorts of ordered domain (v, d), along with wide patches (e) and narrows bands (c) where no order is detected. A small fraction of the surface exhibits ordered domains of the first sort (v), one of which is reproduced in Fig. 3b. These domains are substrate characterized by bright stripes running along the h1 1 2i 0i direction, and are axis with a periodicity of 1.2 nm in the h1 1 embedded in disordered patches which extend over several tens of nanometers. These ordered domains are seldom observed, and it is thus believed that very peculiar conditions are necessary for their nucleation. Yet they are reminiscent of a structure reported in an earlier study of the self-assembly of BDMT in ethanol [13],
where the stripes were attributed to lying-down BDMT molecules. The same interpretation is proposed here, noting that the symmetric appearance of the dithiols contrasts with the asymmetry observed in the lying-down monolayers made in water. The differences are attributed to variations in the arrangement of the Au atoms and adatoms at the interface justified by the difference in growth and sample history. The surface is mostly covered by ordered domains of the second sort (d), inside which disordered bands (c) are seen following the axis. These bands are a few nanometers wide for substrate h1 1 2i the sample presented in Fig. 3a, and their average height is 0.3 Å more than that of the extended ordered domains. A small undulation of the surface reminiscent of the Au(1 1 1) herringbone reconstruction is also seen, indicating that unlike for brief exposure to BDMT in water, the original reconstruction of the substrate [43,45] is not lifted. The higher resolution image presented in Fig. 3c shows rows with a periodicity of 4.5 Å, where dots are resolved every 4.2 Å when the scanning conditions are favorable. Self-assembly of BDMT in ethanol [13] revealed occasionally a much alike pattern consisting of a nearly rectangular lattice of dimensions 3.9 and 3.7 Å. The structure was tentatively ascribed by the authors of that study to an ordered monolayer of BDMT [13]. However, despite the similarities, it is proposed here to associate the structure to a monolayer of hexane physisorbed on the reconstructed Au(1 1 1) substrate. This interpretation is substantiated by several experimental evidences. The density of BDMT on the sample at that stage is indeed necessarily too low to account for the vast area covered by the ordered structure. This is supported by the linear sweep voltammetry (LSV) data shown in Fig. S1 which reveals a desorption charge an order of magnitude less than for saturated BDMT SAMs, as well as by the STM image presented in Fig. 3d which shows that, when aged 35 days in a dark and air-sealed vial, a sample initially prepared during several seconds in hexane solution exhibits a very low density of typically 10 nm long stripes that are occasionally arranged in small domains of parallel rows of periodicity 1.2 nm and that are reminiscent of domains of low density dithiol SAMs [17,27]. Furthermore, since the chemisorption of thiol on Au(1 1 1) involves Au adatoms that are first provided by deconstruction of the herringbone pattern [27,43–46], the observation of the pattern in Fig. 3a is further evidence that the sample is for the most part covered by a physisorbed monolayer. Finally, the rapid formation of a hexane monolayer was verified in an independent experiment by immersion a fresh Au(1 1 1) substrate in pure hexane (see supporting information, Fig. S2). The sub-monolayer amount of BDMT revealed in the LSV (Fig. S1) and in the STM images of an aged sample (Fig. 3d) are then associated to the wide patches and narrows bands where no order is detected in Fig. 3a and to the few ordered domains such as the one reproduced in Fig. 3b. A STM image recorded on a sample immersed for 1 h in the hexane solution and presented in Fig. 3e reveals that, for longer immersion, the BDMT films are very much alike SAMs of other dithiols reported in the literature [7,17,25,26]. The STM image is also undistinguishable from the one recorded in air on samples prepared sequentially in water and in hexane and discussed earlier (see Fig. 2c). Moreover, since XPS also indicates a coverage equivalent or higher than saturated alkanethiol SAMS (Fig. 1d), it is concluded that a saturated monolayer of BDMT is achieved; presumably growing from the disordered bands which originally follow the herringbone reconstruction as well as from the occasional wide disordered patches (Fig. 3a). Thus, after a brief immersion of the gold substrate in the hexane solution, STM reveals the formation of an ordered hexane layer (Fig. 3a, d) that initially impede the growth of BDMT. The hexane layer is progressively replaced by a BDMT monolayer upon further immersion (Fig. 3e).
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
4. Discussion The experiments reveal that, in water, the dithiol can organize within seconds in a complete and ordered monolayer where the molecules are lying-down on the substrate, akin to other similar dithiols [27]. This ordered monolayer hardly evolves when further exposed to the aqueous solution. Yet, growth into a saturated dithiol SAM is readily achieved when the lying-down monolayer made in water is exposed afterward to a hexane solution of BDMT, or when the ordered periodic lying-down monolayer does not form over the entire surface. When the self-assembly is initiated directly in hexane, for the same nominal BDMT concentration, the completion within seconds of a monolayer of hexane is first observed. Although this layer initially impedes the adsorption of the dithiols, BDMT domains eventually develop and a saturated BDMT SAM is completed in less than 30 min. This BDMT layer does not reveal any order in STM, although small ordered dithiol domains of a lying-down phase are occasionally observed during the growth. Furthermore, the quality of a saturated SAM grown at low concentration is sensibly less than the layer grown at higher concentration, namely here 100 lM. Given that water and hexane are respectively polar and nonpolar solvent, variations in the self-assembly on Au(1 1 1) of molecules such as BDMT are not unexpected. For example, considering the duo solvent-BDMT, the weaker solubility of the dithiol in the polar solvent and the larger viscosity of the latter are a priori important factors to consider with respect to the growth kinetic of the SAMs. Yet, the data indicate that very low concentrations of BDMT in the solvent are sufficient to ensure the very rapid formation of a complete film of lying-down BDMT in water. Thus, the amount of dithiols in the vicinity of the substrate surface is not limiting the growth. On the other hand, the experiments highlight the necessity to consider the complete trio solvent, BDMT, and substrate, as well as their mutual interactions to satisfactorily account for the growth in water and in hexane. The initial formation of an ordered monolayer of hexane reveals the relative stability of this surface when in contact with the solution. However, its progressive replacement by BDMT confirms that the chemisorption of the dithiols remains more favorable energetically. Yet, a subtle balance is achieved here since an increase in the van der Waals forces within the physisorbed alkane layer may be sufficient to make the attachment of dithiol unfavorable, as seen in a separate experiment where hexadecane was used instead of hexane. Moreover, the growth of BDMT SAMs at low concentration leads to monolayers of poorer structural quality than those grown at higher concentration. The observation can perhaps be rationalized by the faster replacement of the hexane layer at higher concentration and by a subsequent increased BDMT freedom to selfassemble. In water, the observation of a complete and ordered monolayer of lying-down BDMT after a brief deposition highlights the absence of an overlayer capable of affecting significantly the growth kinetic. Conversely the experiments also highlight the stability of this lying-down BDMT monolayer in the aqueous solution since the film hardly evolves upon prolonged immersion. However, the experiments show that this ordered lying-down phase does not always form, in which case the growth proceeds towards a saturated BDMT layer. It is not entirely clear why the lying-down layer forms or not, however, since variations are seen for different batches of substrates of Au on mica, it is proposed that the quality or purity of the Au(1 1 1) surface is critical. Yet, regardless of that, a SAM of standing-up BDMT should always remain more favorable energetically, as confirmed by the lying-down to standing-up transition seen in vacuum deposition experiments [9], and presumably because a standing-up monolayer involves a larger density of
81
thiol–Au bonds and increased lateral interactions between molecules. Since the monolayer is formed here directly on a clean herringbone reconstructed Au(1 1 1) surface as it is done in vacuum deposition experiment, it is reasonable to assume that in both cases the lying-down BDMT SAM have similar properties and stability, and one may also speculate that both correspond to the same arrangement of Au adatoms and molecules. Moreover, the transition is also seen when monolayers of lying-down BDMT prepared in water are immersed in the hexane solution afterward. Thus, although the initial chemisorption of both thiol moieties (i.e., ordered lying-down SAM) to the substrate certainly increases the energy barrier for the transition in comparison to monothiols, it does not justify the halt of the growth that is seen upon completion of the lying-down monolayer. The latter must thus be discussed considering the interface as a whole, including the Au substrate, the BDMT SAM, and the aqueous solution. At the beginning of the self-assembly, when the Au(1 1 1) surface is mostly bared, the adsorption of lying-down BDMT is expected to be favorable as it is observed in the experiments. This is readily conceivable since, at the molecular level, the two-thiol adsorption scheme on Au reduces the interaction between BDMT and the polar water molecules due to the interface energy is reduced by maximizing the number of thiol–Au bonds. After the ordered lying-down monolayer is completed, the addition of BDMT requires disruption of the already adsorbed molecules, and this is not observed at the interface with water. Yet, that the energy balance is then in favor of the status quo can also be justified considering the polarity of water because the reorientation of one molecule into a standingup configuration leads to the unfavorable increase in the amount of water molecules in contact with the BDMT molecules, unless the SAM is saturated in which case the interaction between dithiols should make the standing-up configuration more stable. Yet, when the ordered striped lying-down monolayer does not form, a standing-up configuration is obtained also in water. Thus it is a fine balance of interactions and sample history that determines the final interfacial structure. Conversely, hexane as non-polar solvent is likely to be promoting the transition, as this is indeed observed in our experiments. The role of the solvent is thus complex: the polarity is crucial in the conversion of the ordered lying-down phase as it is not seen in water but occurs readily in hexane, but also the precursors are different in both water and in hexane with the ordered lying-down phase not forming in hexane where a monolayer of hexane in formed instead. 5. Conclusions The self-assembly of BDMT in water and hexane, respectively polar and non-polar solvent, has been studied by STM and XPS. The growth at low BDMT concentration allowed the observation of precursor BDMT arrangements and coadsorption events at molecular resolution by STM. The STM images stress the implications of the solvents at the interface. The data fully support the conclusion that the solvent in which the self-assembly is performed strongly influences the structural properties of the dithiol SAMs. More specifically, on clean Au(1 1 1), the poor solvation of BDMT in water is associated to the stabilization at the liquid-sample interface of a complete and ordered monolayer of lying-down BDMT. The latter rapidly evolves nonetheless into a saturated SAM of standing-up dithiols at the interface with hexane. The self-assembly in hexane on Au(1 1 1) is initially impeded by the formation of an ordered hexane monolayer that is resolved by STM. The variation in structural properties in SAMs for different solvent and concentration have thus roots in the very first instant of selfassembly and the polarity of the solvent has been considered to justify at the molecular level the growth in water and hexane.
82
A.M. Sharif et al. / Chemical Physics 441 (2014) 77–82
Conflict of interest The authors declare no conflict of interest. Acknowledgments A.M.S. acknowledges a Grant from the Ministry of Higher Education (MOHE) in Malaysia and C.S. funding from the Integrated Nanoscience Platform for Ireland (INSPIRE), initiated by the Higher Education Authority in Ireland within the PRTLI4 framework. At the time of this research, A.M.S. was on leave from the Sultan Idris University of Education (Malaysia). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemphys.2014. 06.019. References [1] C. Vericat, M.E. Vela, G. Benitez, P. Carro, R.C. Salvarezza, Chem. Soc. Rev. 39 (2010) 1805. [2] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. [3] D.K. Schwartz, Annu. Rev. Phys. Chem. 52 (2001) 107. [4] F. Schreiber, Prog. Surf. Sci. 65 (2000) 151. [5] R.L. McCreery, A.J. Bergren, Adv. Mater. 21 (2009) 4303. [6] H. Hamoudi, M. Prato, C. Dablemont, O. Cavalleri, M. Canepa, V.A. Esaulov, Langmuir 26 (2010) 7242. [7] D. Qu, B.C. Kim, C.W.J. Lee, M. Ito, H. Noguchi, K. Uosaki, J. Phys. Chem. C 114 (2010) 497. [8] C. Silien, L. Dreesen, F. Cecchet, P.A. Thiry, A. Peremans, J. Phys. Chem. C 111 (2007) 6357. [9] L. Pasquali, F. Terzi, R. Seeber, B.P. Doyle, S. Nannarone, J. Chem. Phys. 128 (2008) 134711. [10] R. Koole, B. Luigjes, M. Tachiya, R. Pool, T.J.H. Vlugt, C.D.M. Donega, A. Meijerink, D. Vanmaekelbergh, J. Phys. Chem. C 111 (2007) 11208. [11] D. Garcia-Raya, R. Madueno, M. Blazquez, T. Pineda, J. Phys. Chem. C 114 (2010) 3568. [12] D.L. Pugmire, M.J. Tarlov, R.D. van Zee, J. Naciri, Langmuir 19 (2003) 3720. [13] H.M. Zareie, A.M. McDonagh, J. Edgar, M.J. Ford, M.B. Cortie, M.R. Phillips, Chem. Mater. 18 (2006) 2376. [14] S. Rifai, M. Laferriere, D. Qu, D.D.M. Wayner, C.P. Wilde, M. Morin, J. Electroanal. Chem. 531 (2002) 111.
[15] S. Rifai, G.P. Lopinski, T. Ward, D.D.M. Wayner, M. Morin, Langmuir 19 (2003) 8916. [16] S. Rifai, M. Morin, J. Electroanal. Chem. 550–551 (2003) 277. [17] T.Y.B. Leung, M.C. Gerstenberg, D.J. Lavrich, G. Scoles, F. Schreiber, G.E. Poirier, Langmuir 16 (2000) 549. [18] H.B. Akkerman, A.J. Kronemeijer, P.A. van Hal, D.M. de Leeuw, P.W.M. Blom, B. de Boer, Small 4 (2008) 100. [19] Y.H. Kim, C.B. Gorman, Langmuir 27 (2011) 6069. [20] H. Hamoudi, Z.A. Guo, M. Prato, C. Dablemont, W.Q. Zheng, B. Bourguignon, M. Canepa, V.A. Esaulov, Phys. Chem. Chem. Phys. 10 (2008) 6836. [21] S.W. Chah, J.H. Fendler, J.H. Yi, Chem. Commun. (2002) 2094. [22] L. Pasquali, F. Terzi, R. Seeber, S. Nannarone, D. Datta, C. Dablemont, H. Hamoudi, M. Canepa, V.A. Esaulov, Langmuir 27 (2011) 4713. [23] M.A.D. Millone, H. Hamoudi, L. Rodriguez, A. Rubert, G.A. Benitez, M.E. Vela, R.C. Salvarezza, J.E. Gayone, E.A. Sanchez, O. Grizzi, C. Dablemont, V. Esaulov, Langmuir 25 (2009) 12945. [24] L. Pasquali, F. Terzi, C. Zanardi, L. Pigani, R. Seeber, G. Paolicelli, S.M. Suturin, N. Mahne, S. Nannarone, Surf. Sci. 601 (2007) 1419. [25] M.J. Esplandiu, M.L. Carot, F.P. Cometto, V.A. Macagno, E.M. Patrito, Surf. Sci. 600 (2006) 155. [26] C. Li, I. Pobelov, T. Wandlowski, A. Bagrets, A. Arnold, F. Evers, J. Am. Chem. Soc. 130 (2008) 318. [27] A.M. Sharif, D.N. Buckley, M. Buck, C. Silien, J. Phys. Chem. C 115 (2011) 21800. [28] K. Seo, H. Lee, ACS Nano 3 (2009) 2469. [29] Y.W. Yang, L.J. Fan, Langmuir 18 (2002) 1157. [30] M. Zharnikov, M. Grunze, J. Phys. – Condens. Matter 13 (2001) 11333. [31] P. Cyganik, M. Buck, T. Strunskus, A. Shaporenko, J.D.E.T. Wilton-Ely, M. Zharnikov, C. Wöll, J. Am. Chem. Soc. 128 (2006) 13868. [32] A. Shaporenko, A. Terfort, M. Grunze, M. Zharnikov, J. Electron Spectrosc. Relat. Phenom. 151 (2006) 45. [33] E. Cortes, A.A. Rubert, G. Benitez, P. Carro, M.E. Vela, R.C. Salvarezza, Langmuir 25 (2009) 5661. [34] Y.M. Yang, L.J. Fan, Langmuir 18 (2002) 1157. [35] T. Ishida, N. Choi, W. Mizutani, H. Tokumoto, I. Kojima, H. Azehara, H. Hokari, U. Akiba, M. Fujihira, Langmuir 15 (1999) 6799. [36] T. Ishida, M. Hara, I. Kojima, S. Tsuneda, N. Nishida, H. Sasabe, W. Knoll, Langmuir 14 (1998) 2092. [37] H. Kang, Y. Kim, T. Park, J.B. Park, E. Ito, M. Hara, J. Noh, Bull. Korean Chem. Soc. 32 (2011) 1253. [38] W. Azzam, B.I. Wehner, R.A. Fischer, A. Terfort, C. Woll, Langmuir 18 (2002) 7766. [39] W. Chen, A.T.S. Wee, J. Electron Spectrosc. Relat. Phenom. 172 (2009) 54. [40] Y. Tai, A. Shaporenko, H.T. Rong, M. Buck, W. Eck, M. Grunze, M. Zharnikov, J. Phys. Chem. B 108 (2004) 16806. [41] J.A. Rodriguez, J. Dvorak, T. Jirsak, G. Liu, J. Hrbek, Y. Aray, C. Gonzalez, J. Am. Chem. Soc. 125 (2003) 276. [42] A. Niklewski, W. Azzam, T. Strunskus, R.A. Fischer, C. Woll, Langmuir 20 (2004) 8620. [43] J.V. Barth, H. Brune, G. Ertl, R.J. Behm, Phys. Rev. B 42 (1990) 9307. [44] P. Maksymovych, O. Voznyy, D.B. Dougherty, D.C. Sorescu, J.T. Yates, Prog. Surf. Sci. 85 (2010) 206. [45] G.E. Poirier, Langmuir 13 (1997) 2019. [46] P. Maksymovych, D.C. Sorescu, J.T. Yates, Phys. Rev. Lett. 97 (2006) 146103.