Spectroscopic Ellipsometry and Electrochemical and X-ray

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Spectroscopic Ellipsometry, Electrochemical and XPS Investigation of the Influence of the Crystalline Plane on the Adsorption of #,#-Alkanedithiols: Mono vs. Bi-Coordinated Configurations Nicolas Arisnabarreta, Gustavo Ruano, Daniela K. Jacquelín, Eduardo Martin Patrito, and Fernando Pablo Cometto J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10641 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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The Journal of Physical Chemistry

Spectroscopic Ellipsometry, Electrochemical and XPS Investigation of the Influence of the Crystalline Plane on the Adsorption of α,ω−Alkanedithiols: Mono vs. Bi-coordinated Configurations

Nicolás Arisnabarreta,1* Gustavo D. Ruano,2 Daniela K. Jacquelín,1 E. Martín Patrito1* and Fernando P. Cometto,1*

1. Departamento de Fisicoquímica, Instituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC), Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Córdoba, Argentina. 2. Instituto de Física del Litoral (IFIS), Santa Fe, Argentina.

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ABSTRACT The effect of the Au surface crystalline plane on the configuration of Self-Assembled Monolayers (SAMs) of α,ω-Alkanedithiols (DTs) is studied via Reductive Desorption (RD), X-Ray photoelectron Spectroscopy (XPS) and Spectroscopic Ellipsometry (SE) experiments. A remarkable difference in the SAM configuration of short DTs, such as 1,4-butanedithiol (C4DT) on both Au (111) and Au (100) is observed; while standing up (SU) configured SAMs are formed on the (111) plane, lying down (LD) molecules form the structure on (100). The higher stability of the bi-coordinated structure on Au (100) is interpreted, indirectly, as higher bonding energies for the RS-Au(100) bond, in agreement with the RD and XPS results. Since hardly no SU phases are detected when adsorbing C4DT on Au (100), a longer DT is needed in order to achieve this configuration, namely 1,6-hexanedithiol (C6DT). In this case, although the RSC6DT-Au(100) bond is similar to RSC4DT-Au(100) in the bi-coordinated phase, the larger lateral interactions in the SU C6DT becomes the driving force for the DT lifting (i.e. the mono-coordinated phase formation). SE experiments reveal the thickness of the SU SAMs and detect the differences in the C4DT behavior on both surfaces. A discussion on the effect of the aging of the DT immersion solutions on the outcome of the SAM configuration is also presented.


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INTRODUCTION The modification of metallic surfaces using α,ω-alkanedithiols (DTs) was widely studied during the last 2 decades as it is an easy-to-handle approach to bestow specific functionalities upon them. The presence of thiols groups (-SH) at both ends of the molecule allows either mono or bi-coordination to the surface. While in the bicoordinated structure, also known as the Lying Down1–3 (LD) configuration, both S atoms are bonded to the surface, the Standing Up4–6 (SU) phase presents only one S attached and large lateral interactions between the hydrocarbon chains stabilize the structure. The SU phase is attractive due to the presence of a free thiol head group that envisages further functionalization. In this context, metal intercalated DT multilayers7 and DTmodificated-nanoparticles8,9 have been studied recently; as well as the potential application in highly popular fields such as molecular electronics.7,10–15 The effects of the DT chain length, immersion solution and immersion time on the formation of DT-SAM on Au (111) have been extensively studied.16–19,20 In this context, the formation of either SU or LD configuration strongly depends on the preparation method. It is known that when clean Au (111) samples are immersed during (at least) 24 h in DTs ethanolic solutions, SAMs present a SU configuration for long DTs and LD configuration for short ones.18 As we shall see, this is only a valid recipe if the preparation solutions are previously deoxygenated and freshly prepared. Otherwise, SU multilayer structures will be obtained, regardless the length of the spacer group.21 A post-treatment with mild reducing agents, may shave away multilayers, leaving short (or long) SU DTs monolayers. This situation would be desirable in the tailoring of functionalized surfaces, i.e. –SH terminal groups could be used to attach different species, such as molecules, nanoparticles, etc. In a previous work22 we have investigated the effect of the immersion time as well as the use of a disulfide reducing agent on the formation of DT-SAMs.


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Different preparation conditions surveyed for 24 h immersion in diluted ethanolic solutions gave rise to multilayer formation. Post-treatment with the disulfide reducing agent (Tris(2-carboxyethyl) phosphine hydrochloride (TCEP) produced a SAM with a SU configuration. However, immersion in a solution containing both the DTs and the reducing agent gave rise to a SAM with a LD configuration. In a further study, we have also focused on the role of the disulfide reducing agent on the DT lifting mechanism and its role on the formation of mono and bi-coordinated SAM.21 A substantial amount of work has been invested in the study of mono and bi-coordinated S containing molecules on the (111) plane of gold due to the greater stability and easier preparation respect to the (100) surface. The study of the interaction of thiols (and selenols) with the Au (100) surface has only begun recently,23,24,25 mostly driven by the need of stirring the coating of gold nanoparticles to enable customized functionalization.26 In that regard, we have studied the electrochemical stability of a range of aliphatic and aromatic thiol adlayers on Au (100) and Au (111) formed by the immersion in ethanolic solutions.25 We reported an overall increased electrochemical stability for the molecular arrangements on the (100) surface respect to the (111). The same trend was also observed studying the thermal stability of n-hexanethiol on both surfaces.27 To the extent of our knowledge, no studies of DTs adsorption on Au (100) have been reported yet. Hence, in this work we analyzed the effect of the surface crystalline plane i.e. Au (111) and Au (100), on the adsorption of α,ω-alkanedithiols (DTs) via cyclic voltammetry (CV), photoemission (XPS) and spectroscopic ellipsometry (SE) experiments. In this context, we followed the peak potential of the reductive desorption profiles to determine whether the DTs are in the LD or SU configuration. Since short immersion times (15 s) give rise to mono-(or sub-)monolayers in a LD configuration, its CV reduction potential value is a fingerprint of the bi-coordinated configuration.


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Furthermore, these results were supported by XPS results in which the presence/absence of the component at around 163.5 eV, attributed to the S atoms in either free terminal−SH groups or in S−S bonds, would determine the DT surface coordination. Finally, the thicknesses of the SU SAMs prepared on both surfaces as well as the differences on the C4DT behavior were determined by SE measurements and compared with those obtained by XPS.

EXPERIMENTAL SECTION Chemicals. Immersion solutions were prepared using 1,2-ethanedithiol (C2DT), 1,3propanedithiol (C3DT), 1,6-hexanedithiol (C6DT), 1,8-octanedithiol (C8DT), tris(2carboxyethyl)phosphine (TCEP) from Sigma-Aldrich and absolute ethanol from Cicarelli. Electrolytic NaOH (Baker) solution was prepared with Milli-Q water (Millipore Corp., Billerica, MA) Gold Substrates: For electrochemical measurements we used 100 and 111 monocrystals (4 mm in diameter, oriented better than 1º towards the face and polished down to 0.03 μm) from MaTeck, Jülich, Germany. Both crystals were annealed in a H2 flame for 2 minutes, cooled under constant N2 flux, and put in contact with water after 1 minute. Then, substrates are immersed for the assembly process. For the XPS and SE measurements we used Au films evaporated on borosilicate glass and for (100) substrate, we used a monocrystal (10 mm in diameter, MaTeck). SAM preparation. Self-Assembled Monolayers (SAMs) were formed following the standard immersion procedure. Previously annealed electrodes were immersed, for either 15 s or 24 h, in milimollar ethanolic solutions containing the DT precursors. Extensive rinse with both Mille-Q water and ethanol finished the procedure. Samples prepared during 24 h immersion, were washed with an aqueous/ethanolic (50%/50%) solution of


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TCEP (20 mM) during 10 min, as a post-treatment procedure to reduce the disulfide bonds S-S.19 Electrochemical Measurements. Cyclic Voltammograms (CV) were performed using an Autolab PGSTAT100 electrochemical interface using the NOVA 1.8 software package. A conventional three-electrode electrochemical cell (Reference Ag/AgCl (NaCl 3M) with a Pt counter electrode) was employed. Electrical contact with the working electrode was made by means of a meniscus on the surface of the, previously deoxygenated, NaOH 0.1 M solution. XPS Measurements. A commercial surface analysis SPECS system, equipped with a hemispherical energy analyzer, double-anode X-ray source was used for surveying the photoemission spectra. The base pressure measured in the main chamber was in the low 10−10 mbar range. The photoionization of the samples was induced by nonmonochromatized Mg Kα photons at 1253.6 eV and the resulting photoelectrons probed with a 150 mm hemispherical electron energy analyzer (SPECS Phoibos 150). The surface chemical purity was checked with survey XPS spectra in which only the characteristic signals of S, C and Au were detected. The high- resolution S 2p, C 1s and Au 4f core-level spectra were acquired using the fixed analyzer transmission (FAT) mode with an analyzer pass energy of 10 eV to characterize the adlayers. The binding energy (BE) scale for all samples was set to satisfy that their respective Au 4f7/2 BE substrate signal appear at 84.0 eV respective to the Fermi level. The detailed S 2p spectra were fitted considering a Shirley-type background and elemental components (2p3/2−2p1/2 doublet) being Voigt functions. The spin−orbit doublet separation of the S 2p signal was fixed to 1.18 eV and and the Lorentzian fwhm was fixed at 0.15 eV, as done in previous works.22,28


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Spectroscopic Ellipsometry: SE experiments were performed in the 250 - 750 nm range using a Horiba Jobin Yvon UV-vis spectroscopic ellipsometer. All measurements were performed with an angle of incidence of 65 degrees. For each experiment, the ellipsometric Δ and Ψ angles were first measured for the bare Au(111) or Au(100) substrates and after this the corresponding layer was formed. Special care was taken to make all measurements on the same spot on the surface. Data modeling was performed with the DeltaPsi2 software provided by Horiba.

RESULTS In order to evaluate the DTs adlayer configuration (i.e. whether LD or SU) on the Au surfaces we first performed reductive desorption (RD) experiments. Short (15 s) and long (24 h) immersion times of freshly annealed Au (111) and Au (100) were carried out using C4DT, C6DT and C8DT ethanolic solutions. When samples are immersed during short periods of time in freshly DT ethanolic solutions, adlayers (mono or submonolayers) in a LD configuration, are formed.29 On the other hand, to avoid multilayer formation, after longer immersion times, samples were immersed during 10 minutes in solutions containing a mild reducing agent (aqueous/ethanolic -50%/50%- 20 mM TCEP solution). Fig. 1 shows the RD profiles for short and long immersions, of the DTs on Au (111) (black line) and Au (100) (red line). The second sweep is shown as a thinner line only for SAMs prepared by long immersion times. The RD profile of the Au (111) sample prepared after 15 s immersed in C4DT, shows a sharp peak at around -0.934 V; attributed to the characteristic RD potential for a LD configuration. Similar results were obtained for the RD of C6DT and C8DT on Au (111), with a minor shift towards negative potentials due to greater vdWs interactions between the carbon chain and the gold surface (an increment of ca. 0.005 V per methyl group in the chain length is observed). According


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to our interpretation, C6DT SAM may also present patches with SU configuration since its RD profile exhibits a shoulder at around -0.990 V (characteristic for the desorption of C6DT species from SU configuration).19 RD of these DTs on Au (100) shows broader peaks at -1.129, 1.152 and -1.150 V, in increasing chain length order, suggesting more disordered SAMs in LD configuration. The RD profiles of C4DT, C6DT and C8DT species prepared after 24 h substrates immersion from Au (111) and Au (100) are also shown in Fig. 1. In the Au (111) substrate the profiles present large peaks at -1.000, -1.033 and 1.116 V, respectively, suggesting at first, mono-coordinated SU SAMs. This shift towards negative potentials is expected since lateral interactions between neighbor C chains increase for longer DTs in the SU configuration. In the case of C4DT, the RD peak does not present a significant variation, going from -0.934 to -1.000 V, as for C6DT and C8DT case. This result, along with the quite broad peak obtained, suggest that C4DT adlayer (formed by a 24 h immersion) may be composed by a mixed LD and SU structure. For longer DTs (C6DT and C8DT), it is clear from Fig. 1 that pure SU configuration is obtained, since RD peaks are quite narrower and shifted towards more negative values compared to that obtained for LD configurations (Table 1). Interestingly, the second sweeps (thinner lines) show peaks at similar RD potentials (Table 1) to those of short time DT immersion. This fact suggests that after RD process, DTs re-adsorb on the surface with a LD configuration. In a previous in-situ STM investigation, we effectively observed lying-down molecules during the oxidative readsorption of C8DT molecules.5 The second sweep for C8DT exhibits a peak at -0.943 V and a second peak at -1.100 V, suggesting a mixed (phase segregated) SU and LD configuration. Thus, as we shall illustrate by XPS measurements, RD technique shows the formation of C6DT and C8DT SU monolayers and mixed SU and LD C4DT


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adlayers, upon Au (111) surfaces immersion in freshly prepared deoxygenated ethanolic solutions during 24 h. A remarkable difference is obtained when these experiments were performed using Au (100). The RD for C4DT exhibits almost the same RD potential regardless the immersion time, namely -1.129 V and -1.145 V for 15 s and 24 h immersion, respectively, suggesting a LD configuration for both immersion times. The RD profile of C8DT on Au (100) exhibits a sharp peak at -1.228 V revealing pure SU configuration. In the second sweep, two peaks are obtained suggesting a re-adsorption with a mixed SU and LD configuration, analog to the adsorption on Au (111). As in the case of C4DT on Au (111), RD of C6DT on Au (100) produce a broad peak at -1.180 V that could be associated to the desorption from a mixed SU and LD structures. The second sweep, presents a minor peak at -1.146 V, as expected for LD configuration after the oxidative re-adsorption. Our results suggest a clear difference on the behavior of DTs on both Au (111) and Au (100): while C4DT is the shortest DT that may form SU SAMs on Au (111), a longer DT is needed to achieve this configuration on Au (100), namely C6DT. This issue implies that LD configuration is more stable on Au (100) than on Au (111): the same DT (C4DT) produces LD SAMs on Au (100) and SU SAMs on Au (111). Thus, considering that vdW interactions between the hydrocarbon chains and Au surface would be similar irrespective the surface crystalline plane, the higher stability of the LD configuration on Au (100) suggests, indirectly, that the binding energy S-Au(100) is higher than the S-Au(111). The fact that a larger DT (such as C6DT) would form SU SAMs on Au (100), indicates that lateral interactions between the hydrocarbon chains on the SU configuration are now higher than on the C4DT/SU,30 gaining stabilization and inducing the DT lifting.


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Figure 1. Reductive desorption profiles obtained for C4DT, C6DT, C8DT-layers on Au (111) (black line) and Au (100) (red line). The solid curves correspond to the first potential scan while the thin curves to the second potential scan. Immersion periods of 15 s and 24 h in ethanolic solutions. Electrolyte: NaOH 0.1M. Scan rate: 50 mV/s.


10

C6DT C8DT

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C4DT

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Crystalline plane Immersion Time 1st Sweep

Au (111)

Au (100)

15 s

24 h

15 s

24 h

-0.934 (LD)

-1.000 (LD+SU)

-1.129 (LD)

-1.145 (LD)

2nd Sweep

-

-0.942 (LD)

-

-1.145 (LD)

1st Sweep

-0.956 (LD)

-1.033 (SU)

-1.152 (LD)

-1.180 (LD+SU)

2nd Sweep

-

-0.935 (LD)

-

- 1.146 (LD)

1st Sweep

0.961 (LD)

-1.116 (SU)

-1.150 (LD)

-1.228 (SU)

2nd Sweep

-

-0.943 (LD)

-

-1.138 (LD)

Table 1. Reductive desorption peak potentials and SAM configuration for DTs on Au (111) and Au (100) prepared by immersion of 15 s and 24 h. Average error: ±5 mV.

In a recent work we reported the electrochemical stability of several thiols on Au (100) and Au (111) and we showed that the difference in the RD potential for a given alkanethiol on each substrate (Ep(100-111)) is in the order of 0.15 V. This is in very good agreement with the analog value we report this time for alkanedithiols (0.15 V for C4DT and C6DT and 0.11 V for C8DT), that can be readily calculated from Table I. According to our proposed model, Ep(100-111) for the studied SU dithiols and their analog thiols differs only slightly due to the mild polarity provided by the terminal SH group (which in turn has very limited effect on the potential of zero charge of the complex SAM-Au(hkl)). On the other hand, LD adlayers lacking of order and with poor lateral interactions resembles the case of aromatic thiols for which a higher Ep(100-111) is observed at around 0.26 V.25 The value for this last case is 0.21 eV that is reasonable within our proposed model. XPS measurements have been performed to confirm the transition from LD to SU configuration, obtained for both crystalline faces. Fig. 2 shows the results obtained on the S 2p region for different DTs adsorbed on Au (111) and on Au (100), for short and long immersion times. As in the EC experiments, an aqueous/ethanolic solution of TCEP is


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used as a post-treatment procedure to reduce the disulfide bonds S-S. This post-treatment eliminates any possible multilayer and guarantees a SAM with a SU configuration. In a previous work,25 we showed that for thiols with the C atom immediately bond to S containing an sp3 configuration, present a 0.2 eV core level shift on the Eb for the S1 signal (attributed to the S atoms in the thiol/Au interface) from Au (111) to Au (100), regardless the immersion time. It is noteworthy that since DTs present the same hybridization in this C atom, a 0.2 eV core level shift is also obtained in the S1 signals, as expected. XPS spectra for SAMs prepared during short immersion times (Fig. 2 (a)) on both Au (111) and Au (100) present almost a pure S1 signal. The absence (or negligible presence) of the S2 signal (attributed to the S atoms in either free terminal−SH groups or in S−S bonds) eliminates the possibility of SU configuration since both S atoms of the DT are bond to the Au surface (bi-coordinated). This suggests a SAM with a LD configuration, in agreement with the results obtained on the RD experiments for short immersion time. XPS results for longer immersion times (24 h) are presented in Fig. 2 (b). It is known that short DTs, such as C3DT, present only a LD configuration,18 even for SAMs formed by long immersion time. This is also suggested by our results of C3DT on Au (111). XPS spectrum for C4DT on Au (111) presents both S1 and S2 signals with similar areas. The S1 signal confirms the bonding to the surface while the S2 feature suggests the presence of SU species. Also, little contamination with chemisorbed S atoms is evidenced by the S3 signal. When this SAM is treated with the disulfide reductive phosphine (TCEP) the S2 signal decreases due to the S-S bridge cleavage (Figure S1). Since the S1 doublet remains more prominent, a mixed LD and SU configuration is present. As already discussed in the RD experiments, the effect of the surface plane is evidenced in C4DT/Au (100) since only S1 doublet is obtained for 24 h immersion. Almost pure LD configuration


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is present in the SAM formed by C4DT on Au (100) regardless the immersion time, in contrast with the results on Au (111). For larger DT such as C8DT (after treatment with TCEP) pure SU configuration is suggested on both Au (111) and Au (100) due to the presence of both S1 and S2 signals.

Figure 2. Photoemission spectra showing the S 2p region for SAMs prepared during a) short immersions (15 s) using C2DT, C4DT, C6DT, C8DT; and b) long immersions (24 h) using C3DT, C4DT, C8DT; formed on Au (111) and Au (100). TCEP was used as a post-treatment when samples were immersed during 24 h.


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In order to analyze the previous observations in a more quantitative framework we propose a simple model (detailed in the supporting information). The model considers a first layer formed by the coexistence of LD and SU phases up to a full SU monolayer and, for coverages over a SU monolayer, the development of a second SU layer commesurated with the first one will be assumed. Table 2 summarizes some quantitative information of the application of the model to the experimental results depicted in Fig. 2 b; the experimental ratio of the Au-S* intensities to that produced by the S*-S* or S*-H groups (𝐼𝐴𝑢 ― 𝑆 ∗ /𝐼𝑆 ∗ ― 𝑆 ∗ 𝑜𝑟 𝑆 ∗ ― 𝐻 ), the makeup of the adlayer through the fraction of single layer of dithiol in the SU configuration, the fraction of area covered by such an arrangement and finally the effective thickness of the adlayer as a non-local technique would measure. Additionally, theoretical information is included as a quick reference, for instance the ratio values expected for a full single (and double) SU layer as well as the thickness expected for a SU monolayer. The first general trend is the presence of a bilayer in development for C8DT (on both substrates) and monolayer SU+LD phases for the rest of the samples. According to our model the transition in between these situations is marked by the critical value of 𝑟 = exp (―

𝑑(𝑛) 𝑙 )

that corresponds to a full SU monolayer. As shown, none of the studied adlayers

reaches the onset of a full bilayer (𝑟 = exp ( ―

2𝑑(𝑛) 𝑙 ))

and thus, as discussed in previous

works,21,22 confirms that TCEP is an effective method to shave multilayers close to a single SU layer. Analyzing with more detail the effectiveness of this process, we noticed that it is dependent on the DT chain length, and in a lesser extent of the crystalline face of the gold substrate. For both (100) and (111) gold faces we observe that the longer the dithiol the bigger the proportion of a SU monolayer. Nevertheless, as we described above, from the comparison of DT molecules with the same chain length, it can be noticed that


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the fraction of SU phases is always bigger on Au(111). For example, the fraction of C4DT molecules on SU configuration on Au(111) is almost twice the SU molecules on Au(100) (0.64 vs 0.34).

Adlayer preparation

Exp.

Fraction of

Fraction of area

Effective thickness of

𝐼𝐴𝑢 ― 𝑆 ∗ / 𝐼𝑆 ∗ ― 𝑆 ∗ 𝑜𝑟 𝑆 ∗ ― 𝐻

molecules in a

covered by a

the adlayer (theor. SU

monolayer (SU

monolayer in a SU

monolayer) in Å

configuration)

configuration

and (theor. SU monolayer, SU bilayer)

Au (100) C8DT+TCEP(24h)

0.45 (0.95,0.29)

0.52

0.52

15.66 (12.25)

C6DT+TCEP(24h)

1.14 (0.89,0.25)

0.89

0.80

6.95 (9.81)

C4DT+TCEP(24h)

4.82 (0.92,0.27)

0.34

0.23

2.15 (7.23)

Au (111) C8DT+TCEP(24h)

0.38(0.95,0.29)

0.38

0.38

17.19 (12.25)

C4DT+TCEP(24h)

2.05 (0.92,0.27)

0.64

0.51

3.65 (7.23)

C3DT+TCEP(24h)

4.52 (0.93,0.28)

0.36

0.27

2.04 (5.97)

Table 2: Intensity ratio of the S-Au to S-S/H signals from Fig 2 b, estimation of molecular and area fraction of a SU monolayer and effective thickness of the adlayer. In parentheses, theoretical values are offered for quick reference. (See uncertainty of the model in the SI).

Another key aspect to consider in the formation of SU SAMs is the condition of the immersion solution. A notable difference on the DT configuration is observed when using either a deoxygenated and freshly prepared or an aged immersion solution, obtaining LD or SU phases for the same DT (and immersion time), respectively. Thus, we have to


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consider the fact that on the latter, two DTs may be oxidized in solution as expressed in equation (a): 𝑧

𝑧 𝐻𝑆𝐶𝑛𝑆𝐻𝑠𝑜𝑙 +𝑦 𝑂2 → 2 𝐻𝑆(𝐶𝑛𝑆 ― 𝑆𝐶𝑛)𝑆𝐻 +

𝑦 2

𝐻2𝑂

(eq. a)

Considering equation (a), the oxidized DT (𝐻𝑆(𝐶𝑛𝑆 ― 𝑆𝐶𝑛)𝑆𝐻, can now reach the surface as a polymeric longer DT. The main difference is that these species may be adsorbed directly in the SU configuration to maximize the lateral interactions in the monocoordinated phase. Thus, the possibility of producing SU SAMs with short DTs increases with the amount of disulfide bonds produced in the immersion solution, i. e. the freshness of the immersion solution. This is a quite relevant aspect since it may be used as another variable to control the surface configuration of α,ω-alkanedithiols on Au. Photoemission results showing the effect of the immersion solution freshness on short DTs SAMs, such as C2DT, are shown in Fig. 3. It can be seen that after 24 h immersion in a fresh deoxygenated solution, a LD C2DT structure is obtained. If the same solution is aged during 24 or 48 h, multilayers of SU C2DT adlayers are formed and then, these multilayers can be shaved with TCEP to obtain SU C2DT monolayers (Fig. 3).


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Figure 3. Photoemission spectra of the S 2p region showing the effect of aged C2DT solutions and TCEP as a post-treatment. The aging of the solution is indicated in parentheses.

In Table 3 we present a quantitative analysis using the same model described above. While the immersion of Au (111) substrates in fresh C2DT solutions (either for short or long periods) produce mostly LD phases, any aging of the solution generates more than a single layer upon 24 h immersion. In turn, a solution aged 24 h will generate a bilayer in 24 h immersion but it will generate more than double that coverage if the solution used


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is aged during 48 h. As seen before a post-treatment in a TCEP solution would shave the adlayer to produce almost as single SU layer (with less than 30% of excess bilayer).

Exp.

Fraction of molecules in a

Effective thickness of the

𝐼𝐴𝑢 ― 𝑆 ∗ / 𝐼𝑆 ∗ ― 𝑆 ∗ 𝑜𝑟 𝑆 ∗ ― 𝐻

monolayer (SU configuration)

adlayer (theo. SU

( area fraction of 1 SU

monolayer) in Å

and monolayer) (theor. SU monolayer, SU bilayer) Au (111) C2DT 24h (48h aged) +10

0.63 (0.95,0.29)

0.71

5.26 (4.72)

C2DT 24h (48h aged)

0.12 (0.95,0.29)

Multilayer (4-5 ML)

16 (4.72)

C2DT 24h (24h aged)

0.33 (0.95,0.29)

0.10

7.78 (4.72)

C2DT 24h (fresh)

3.21(0.95,0.29)

0.47

2.59 (4.72)

C2DT 15s (fresh)

4.59 (0.95,0.29)

0.35

1.82 (4.72)

min TCEP

Table 3: Intensity ratio of the S-Au to S-S/H signals from Fig 3, estimation of molecular and area fraction (in this case almost the same) of a SU monolayer and effective thickness of the adlayer. In parentheses, theoretical values are offered for quick reference. (See uncertainty of the model in the SI).

Spectroscopic ellipsometry is a powerful tool to investigate conformational transitions on ultrathin soft matter. In a recent paper, we investigated the pH-dependent switching behavior of a mixed thiolate monolayer measuring the ellipsometric thickness of different thin layers.31 The information on the layered system under investigation (layer thickness and complex refractive index) is extracted through the comparison of the experimentally measured Δ and Ψ angles with those calculated from the corresponding optical models based on the Fresnel relations for the layered structure.32 In the case of a transparent


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organic layer on a metal substrate, the simplest optical model has a three-phase metal/film/air structure assuming sharp metal/film and film/air interfaces. In this model, the increase in the thickness of the film is characterized by a decrease (increase) in Δ (Ψ) angles. It is well known that in the near infrared region, Δ angles change linearly with the film thickness.33 In the case of thiol and selenol molecules bond to gold surfaces, the optical model is more complex as the sharp metal/film interface cannot be assumed. The electron scattering properties in the interfacial region involving the S−Au bonding are different from those in the region comprising the organic chains. Therefore, the most appropriate model for these systems involves four phases: the metal, the interfacial region, the organic dielectric and air (Au/interface/dielectric/air).34,35 Fig. 4 a compares the Δ and Ψ spectra for the clean Au (111) substrate (thin line) with those obtained after 24 h immersion in an aged C2DT solution (thick line) and after immersion of the previous film in a solution of TCEP (dotted line) during 10 min. As expected, the growth of a film on the Au surface produces a decrease in Δ values in the whole spectral region. In order to evidence more clearly the small changes in the ellipsometric angles, it is customary to subtract the angles of the bare substrate and present δΔ and δΨ changes after the film formation, as shown in Figs. 4 b and 4 c, respectively. Fig. 4 b shows that δΔ values become less negative after treatment with TCEP which indicates a thinning of the C2DT layer as a consequence of the breakage of interlayer disulfide bonds produced by the reducing agent. At wavelengths lower than 500 nm, δΨ values are also sensitive to changes in the film thickness and consequently, they decrease as the film thickness decreases (Fig. 4 c). Therefore, ellipsometry clearly shows the shaving effect of TCEP. Fig. 4 c also shows that at wavelengths higher than 500 nm, δΨ values become negative and insensitive to the changes in the film thickness. A simulation with a three phase model


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for two dielectrics of different thicknesses also predicts that this spectral region is rather insensitive to the film thickness, but in all cases δΨ values are always positive and tend to zero as the wavelength increases. The transition from positive to negative δΨ values in Fig. 4 c cannot be explained with the three phase model and this is a consequence of the presence of the interface layer associated with thiolate-gold bonding.34,35 This layer has been consistently observed for compact thiolate SAMs in which the constituting molecules are in a standing-up configuration.7,33,34,36,37 We therefore used four phases (Au/interface/dielectric/air) to model the Δ and Ψ spectra. The dielectric function of bare gold was measured before the film formation. The dielectric function of the interface region was modeled with three Lorentz oscillators33 and the dielectric representing the alkyl chains was modeled with a non-dispersive Cauchy formula.33 The total thickness d of the adlayer was calculated as the addition of the thickness of the interface layer (dint) and the thickness of the dielectric (ddiel). From the fitting of Δ, Ψ spectra to this four phase model we obtained d = 0.85 ± 0.04 nm after 24 h substrate immersion in the aged C2DT solution which corresponds to multilayer. This thickness compares well with the obtained by XPS (0.78 nm). After shaving with TCEP during 10 min. the thickness decreased to 0.279 ± 0.012 nm which may correspond to a mixture of LD and SU. For the interface layer we obtained transitions centered at 1.19, 2.82 and 3.6 eV in accordance with previous studies of SAMs on Au (111).34,38,39


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Figure 4. a) SE results for C2DT on Au (111); SE difference spectra of b) Delta and c) Psi of Au (111) 24h/C2DT immersion previous (thick blue) and after (thin blue) TCEP shaving. Angle of incidence: 65º. The rise of the delta angle confirms the shaving of the SU multilayer formed from a C2DT solution. The solid lines in figures 4a and 4b correspond to the fitting to the four-phase model.


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Fig. 5 shows ex-situ SE measurements for C4DT, C6DT and C8DT layers prepared after different immersion times on Au (111) and Au (100). Fig. 5 a shows that δΔ values become more negative as the chain length increases for layers prepared at long immersion times and the corresponding δΨ values become more positive at wavelengths lower than 500 nm. The solid lines in Figs. 5a and 5b correspond to the fitting with the four-phase model. The δΔ, δΨ spectra for C4DT, C6DT and C8DT were fitted simultaneously to the model (“bound multi model” option in DeltaPsi2 software) sharing the same dielectric function and thickness for the interface layer and the same refractive index for the dielectric corresponding to the alkyl chains. Figs. 5 a and 5 b show that the model adequately fits the experimental data from which we obtained d values of 0.29 ± 0.02 nm, 0.45 ± 0.02 nm and 0.84 ± 0.05 nm for C4DT, C6DT and C8DT, respectively. Despite these thickness values are lower than those obtained by XPS data analysis, are in good agreement with previous works of thiolate adsorption on Au (111).34,38 We think that the presence of intralayer disulfide bonds formed after exposing the samples to air, produce these consistent differences in the adlayers. Thus, we attributed these thicknesses to SU monolayers of DTs on Au (111). In some experiments we obtained higher thicknesses, indicating that eventually multilayers may be formed. As mentioned above, the deoxygenation of the forming solution is critical to obtain monolayers. δΔ and δΨ spectra for C4DT/Au(111) obtained after a short immersion time of 15 s shows a completely different behaviour. δΔ values are positive (rather than negative) in the whole spectral range investigated, whereas δΨ values are slightly negative (rather than positive) at λ < 500 nm and then become more negative at λ > 500 nm. This behavior does not correspond to a dielectric film on the gold surface and cannot be modeled with the four-phase model.


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We associate it to a new interface layer structure corresponding to lying-down C4DT molecules as shown by the photoemission and electrochemical experiments. Figs. 5 c and 5 d show δΔ and δΨ spectra for C4DT, C6DT and C8DT layers on Au (100) prepared under exactly the same conditions as on the Au (111) surface. For long immersion times, we could only fit the spectra to the four phase model in the case of C6DT and C8DT adlayers (solid lines in Figs. 5 c and 5 d) indicating that they are in a standing-up configuration. As on the Au (111) surface, δΔ values are more negative for C8DT /Au (100) than for C6DT /Au (100) indicating higher thicknesses for the longest alkyl chain. From comparison of δΔ and δΨ spectra in Fig. 5 for C6DT and C8DT on both crystal faces, it can be observed that δΔ (δΨ) values are more negative (positive) for the Au (100) surface than for the Au (111) surface. This clearly indicates that ellipsometry is very sensitive to the crystal face. From the fitting procedure of the ellipsometric spectra for C6DT/Au (100) and C8DT/Au (100) we obtained thicknesses of 0.97 ± 0.04 nm and 1.37 ± 0.04, respectively. These values, are higher than those on Au (111) but, are in completely agreement with the theoretical thicknesses of SU monolayers (0.98 nm and 1.22 nm for C6DT and C8DT, respectively). We remark that under the same preparation conditions, we sometimes observed multilayers on Au (111) whereas we consistently observed multilayers on Au (100). This is a clear indication of the effect of the crystal face on the reactivity of terminal –SH groups which, upon oxidation by O2, may either give rise to intralayer disulfide bonds between adjacent alkyl chains or to interlayer disulfides giving rise to multilayers. Probably, the more open structure of the Au (100) surface gives additional degrees of freedom to the alkyl chains in the SAM thus inducing a higher reactivity; whereas the closer Au (111) structure favors the formation of intralayer S-S bonds. The ellipsometric spectra for C4DT/Au (100) show striking differences as nearly the same δΔ values are obtained for 15 s and 24 h of immersion time


23


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(Fig. 5 c). This validates the electrochemical and photoemission experiments that show the presence of bi-coordinated molecules for C4DT/Au(100) irrespective of the immersion time. XPS measurements indicate that both C2DT/Au(111) (15 s immersion time) and C4DT/Au(100) SAMs have bi-coordinated molecules. However, δΔ values are slightly positive in the first case (Fig. 5 a) whereas they are slightly negative in the second. The latter is compatible with the presence of a very thin dielectric layer and this could be attributed to bi-coordinated C4DT molecules forming U-loops, whereas this is impossible for short bi-coordinated C2DT molecules.


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Figure 5. SE difference spectra of ∆ (a and c) and Ψ (b and d) of 15 s immersion of Au(111) and Au(100) in C4DT (I) and 24 h immersion in C4DT(II), C6DT (III), and C8DT (IV). Thick lines correspond to the data modeling. Angle of incidence: 65º


25


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CONCLUSIONS In this work, the effect of the surface Au crystalline plane on the configuration of DTsSAMs has been investigated. The difference in the behavior of such DTs on both surfaces have been studied via reductive desorption, photoemission and spectroscopic ellipsometry studies. The fact that freshly annealed Au (111) and Au (100) immersion in C4DT solution build-up standing up and lying down SAMs, respectively, suggests a higher S-Au(100) binding energy. Thus, a longer DT, such as C6DT, is needed to form SU SAMs on Au (100). In this context, the vdW interactions in the larger DT are the driving force that induces the DT lifting. Spectroscopic ellipsometry clearly revealed the complex structure of either SU or LD layers and it was shown to be very sensitive to the crystalline face. The optical response of all SU layers had contributions from the interface layer region and the dielectric layer on top whose thickness increased with the number of C atoms in the alkyl chain. Whereas on Au (111) we mostly obtained monolayers for C6DT and C8DT, on Au (100) we obtained multilayers under the same conditions, indicating the influence of the crystalline face on the reactivity of the dithiol SAMs. A completely different optical response was obtained for the C2DT/Au(111) and C4DT/Au(100) SAMs for which photoemission spectra indicate the presence of bicoordinated molecules. The slightly positive δΔ values in the former indicate the presence of an interface layer with a different structure to that for SU dithiols, whereas the slightly negative δΔ for bi-coordinated C4DT molecules on Au(111) are interpreted as a U-loop adsortion structure. The fact that the same spectra are obtained for C4DT/Au(100) SAMs irrespective of the immersion time, indicate stronger S−Au bonding on Au (100) in agreement with the electrochemical and photoemission results.


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Supporting Information. Includes XPS spectra of C4DT before and after TCEP, and the explanation of the procedure to estimate the effective thickness of DT monolayers by XPS data.

ACKNOWLEDGMENTS This work was supported by CONICET (PIP grants 112-200801-000983 and 11220080100958). EMP acknowledges funding from Foncyt (PICT-2014-2199) and SecytUNC. FC and GR thank to Dr. G. Zampieri and Dr. J. Ferrón for fruitful discussions and XPS facilities at Bariloche and Santa Fe, respectively. AUTHOR INFORMATION Corresponding Authors * Prof. Dr. Fernando P. Cometto e-mail: [email protected]. * Lic. Nicolás Arisnabarreta e-mail: [email protected]. * Prof. Dr. E. Martín Patrito e-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. REFERENCES (1) (2) (3)

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(34) Prato, M.; Moroni, R.; Bisio, F.; Rolandi, R.; Mattera, L.; Cavalleri, O.; Canepa, M. Optical Characterization of Thiolate Self-Assembled Monolayers on Au(111). J. Phys. Chem. C 2008, 112 (10), 3899–3906. (35) Canepa, M.; Maidecchi, G.; Toccafondi, C.; Cavalleri, O.; Prato, M.; Chaudhari, V.; Esaulov, V. A. Spectroscopic Ellipsometry of Self Assembled Monolayers: Interface Effects. The Case of Phenyl Selenide SAMs on Gold. Phys. Chem. Chem. Phys. PCCP 2013, 15 (27), 11559–11565. (36) Prato, M.; Alloisio, M.; Jadhav, S. A.; Chincarini, A.; Svaldo-Lanero, T.; Bisio, F.; Cavalleri, O.; Canepa, M. Optical Properties of Disulfide-Functionalized Diacetylene Self-Assembled Monolayers on Gold: A Spectroscopic Ellipsometry Study. J. Phys. Chem. C 2009, 113 (48), 20683–20688. (37) Hamoudi, H.; Prato, M.; Dablemont, C.; Cavalleri, O.; Canepa, M.; Esaulov, V. A. Self-Assembly of 1,4-Benzenedimethanethiol Self-Assembled Monolayers on Gold. Langmuir 2010, 26 (10), 7242–7247. (38) Bordi, F.; Prato, M.; Cavalleri, O.; Cametti, C.; Canepa, M.; Gliozzi, A. Azurin Self-Assembled Monolayers Characterized by Coupling Electrical Impedance Spectroscopy and Spectroscopic Ellipsometry. J. Phys. Chem. B 2004, 108 (52), 20263–20272. (39) Neuman, O.; Naaman, R. New Optical Absorption Band Resulting from the Organization of Self-Assembled Monolayers of Organic Thiols on Gold. J. Phys. Chem. B 2006, 110 (11), 5163–5165.


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Spectroscopic Ellipsometry and Electrochemical and X-ray

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