Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the

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Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic and Sum-Frequency Generation Thiers Massami Uehara, Hilton B. de Aguiar, Kleber Bergamaski, and Paulo Barbeitas Miranda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5054919 • Publication Date (Web): 13 Aug 2014 Downloaded from http://pubs.acs.org on August 14, 2014

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The Journal of Physical Chemistry

Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic and Sum-Frequency Generation

T. M. Uehara,1 H. B. de Aguiar,1,* K. Bergamaski1,2 and P. B. Miranda1

1

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, São Carlos, São Paulo, Brazil, 13560-970 2

Departamento de Química, Universidade Federal de Sergipe, São Cristóvão, Sergipe, Brazil, 49100-000

* Current address: Aix-Marseille Université, Institut Fresnel, Domaine Universitaire de Saint Jérôme, F-13397 Marseille Cedex 20, France.

1 ACS Paragon Plus Environment


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ABSTRACT

Self-assembled monolayers (SAMs) of alkylthiols on gold have called considerable attention due to several applications in the modification and control of surface properties, such as wettability, tribology and biocompatibility. Therefore, understanding the adsorption and molecular structure of these SAMs is crucial to optimize their quality for a given application. Although many studies in this area have been performed, the effects of substrate morphology on the quality of long chain alkylthiols on gold have not been satisfactorily clarified. This study focuses on the adsorption and conformation of SAMs of 1-hexadecanethiol (C16SH) as a function of adsorption time, substrate roughness and morphology. The adsorption of C16SH on atomically flat Au surfaces was analyzed via cyclic voltammetry (CV), scanning tunneling microscopy (STM) and second-harmonic generation (SHG), while the molecular conformation and

orientation

were

characterized

via

sum-frequency

generation

(SFG)

vibrational

spectroscopy. We show that CV and STM are much more sensitive to defects in the monolayer than SFG spectroscopy, while SHG is useful to monitor the final stages of adsorption (defect healing). However, SFG spectroscopy suggests that the disordered regions observed in STM are not due to flat-lying molecules, but to conformationally disordered upright alkylthiols. Finally, the formation of a nearly perfect monolayer is only obtained at long adsorption times for very flat substrates, with the packing and organization of the alkyl chains depending strongly on the roughness of the gold surface. These findings may have important implications to the preparation of high quality SAMs for sensitive applications, and also highlight the advantages and drawbacks of each technique for assessing the quality of SAMs.

Keywords: Alkanethiol; Self-Assembled Monolayers; Gold; Surface Roughness; Cyclic Voltammetry; Sum-Frequency Generation; Second-Harmonic Generation; Scanning Tunneling Microscopy.


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1. INTRODUCTION

Modification of metal surfaces with self-assembled monolayers (SAMs) has emerged as a powerful approach to control their physical and chemical properties (for instance, substrate electronic structure and surface interactions), with applications in tribology, wettability, corrosion protection, catalysis, biocompatibility, to cite some.1,2 One of the most studied model system of SAMs are alkanethiols on Au. Alkanethiols are chemically “simple”, thus forming the basis for understanding the formation of SAMs.3 Gold is a relatively inert metal, straightforward to clean and has well-known surface properties, with sulphur-containing molecules strongly adsorbing on it.1,2 The structure and adsorption mechanism of alkanethiol SAMs has been widely investigated due to their technological relevance and relatively simple surface chemistry. It is in agreement that the adsorption proceeds in two major steps.4 For low concentration and low immersion time (adsorption from solution),5,6 or low exposure (adsorption in ultra-high vacuum),7 the molecules are physisorbed with chains parallel to substrate.8 Longer immersion times and concentrations lead to chemisorption with the alkanethiol bound through the sulphur end group. Generally, the molecular density is high enough to obtain an all-trans conformation of the alkyl chains. If the concentration is raised, the physisorption stage is too short, and the monolayer goes directly to the chemisorbed phase.6,9-11 These kinetics studies have reported different adsorption rate constants for different stages.12 Discrepancies may arise from different techniques used, substrate cleanness, solvent type, and the methodology used to prepare Au substrates (Au/mica, Au/glass, Au/Si, etc.). Roughness and crystallinity plays a role in quantitative data analysis for different probes.13-16 Currently, there are a lot of efforts in the literature to control substrate morphology and to investigate what consequences these have in SAMs integrity.17-27



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The majority of reports use an electrochemical approach to probe the integrity of SAMs. It relies in the exponential dependence of heterogeneous electron-transfer rate between donoracceptor on pair distance.28 For example, using one molecule that has a reversible redox couple, one can probe the molecule-substrate distance by the rate of electron transfer (current density) in a cyclic voltammogram. Thus, this methodology probes the extent of defects in a SAM. Two major defects could arise: pinholes or grain boundaries. Both pinholes and grain boundaries lead to molecules with gauche defects,29 which decreases the probe-substrate distance. So far, no comparative work has described which types of defects are the major sources of the current density in a cyclic voltammogram, and how substrate morphology affects the molecular structure of SAMs and the surface concentration of defects. Here we will investigate SAMs on Au substrates prepared with different methods that lead to significant changes in substrate morphology and roughness. We will then use scanning tunneling microscopy (STM) to characterize the Au substrates and follow the thiol adsorption on ultraflat Au surfaces. Cyclic voltammetry (CV) will be employed to quantitatively assess the quality of the SAMs, and sum-frequency generation (SFG) vibrational spectroscopy will be used to probe the conformation and coverage of alkylthiol molecules. For one hand, SFG spectroscopy is a surface-specific technique which is sensitive to molecular conformation and orientation in sub-monolayers,30 but no quantitative relation between the SFG spectra and average defect density in SAMs is available. On the other hand, many studies deal with the integrity of SAMs, mainly probed by electrochemical methods, giving quantitative results.31,32 In this work, we will combine and compare the different techniques to probe the nature of SAM defects, and make quantitative analysis of SFG spectra in comparison with electrochemical, STM and second-harmonic generation (SHG) results, to investigate both the adsorption of alkylthiols and the effect of substrate roughness on the quality of SAMs.


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2. SFG BACKGROUND

SFG is an optical technique with surface specificity and chemical selectivity capable of obtaining vibrational spectra of species at surfaces and interfaces.30,33-39 SFG is a second-order nonlinear optical process in which two input laser pulses, one in the visible (Vis) and another on the infrared (IR), overlap spatially and temporally at a surface to generate a coherent signal beam at the sum-frequency of the input beams. The source of the sum-frequency photons is the and

second-order polarization induced by IR and Vis electric fields, respectively:

,

where

(1)

is the second-order susceptibility tensor of the interface (the indices ijk are the

Cartesian coordinates x,y,z). Since

is enhanced when ωIR is near a vibrational resonance,

the intensity of the sum-frequency beam can be written as

,

(2)

where ωIR is the frequency of the tunable infrared beam, ωv is the frequency, Γv is linewidth and Bv is the SFG amplitude of the vibrational mode.

is a frequency-independent term which

results from the contributions of all other vibrational and electronic transitions. The CH stretch region of the SFG spectrum reveals the molecular conformation of the alkane molecules. For an alkyl chain with an all-trans conformation, adjacent CH2 groups point in opposite directions, leading to a cancellation of their SFG contribution. However, the CH3 groups in alkylthiols always generate SFG signal since on average they point in the same direction,



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away from the substrate. At a gauche defect, the local inversion symmetry of adjacent CH2 groups is broken, thus making their vibrations SFG allowed. At the same time, the average orientational order of CH3 groups is decreased, which leads to a reduction of their amplitude in the SFG spectrum. Therefore, the amplitude ratio between CH2 and CH3 modes is a good metric to infer the degree of conformational order for adsorbed alkyl chains. This methodology has been widely used by many groups to qualitatively assess the degree of conformational order of alkyl chains at interfaces.30,33-35,40,41

3. EXPERIMENTAL DETAILS

3.1. Substrate preparation. In this work, we used three strategies to prepare gold substrates with different surface roughness. Initially, the glass substrates were loaded inside a high vacuum chamber (Edwards model 306/FL 400) at pressure of 10-5 Torr. A 50 nm thick Cr layer was first evaporated (to improve adhesion) and subsequently followed by evaporation of a 150 nm thick Au layer. The deposition rate was 0.5 nm s-1. The second strategy is similar to the previous one, but with Au evaporated on mica substrates. Before the evaporation, the mica substrate was heated to 200 °C inside the vacuum chamber to remove water and other contaminants. After the evaporation, the gold film was annealed at 300 °C for approximately 2 hours inside the vacuum chamber to obtain larger grain sizes and lower surface roughness. For the third strategy, we followed the method of Lee et al.42 to produce atomically flat gold substrates. Mica sheets were cut in pieces of 8 x 14 mm and loaded inside a high vacuum chamber and followed by evaporation of a 150 nm thick Au layer. The deposition rate was 0.5 nm s-1. The mica/Au substrates were glued (UV Curing Lens Bond/Optical Cement/Type J-91 – U.S.A.) on glass slides using the Au-covered surface side (the back side of mica exposed to air). Another glass slide was placed over the mica back side, forming a “sandwich” that remained pressed for 20 hours to obtain a stronger adhesion of gold and glass slide. The “sandwich”


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assembly was then exposed to ultraviolet light for 1 hour to cure the adhesive glue. Afterwards, the glass slide (the one not glued) was removed, and the mica sheets were gently cleaved in order to keep the gold film adhered to the glass slide. The advantage of this method is that the Au surface has almost the same roughness as the mica surface, which is nearly atomically flat. This substrate will be referred hereafter as AuTS (template-stripped Au). Each Au substrate was then immersed in 10 mL beakers containing 1 mM 1hexadecanethiol (CH3(CH2)15SH, > 96 %, Alfa Aesar, hereafter referred as C16SH) solutions in ethanol (> 99%, J. T. Baker) for different adsorption times. After removal from the thiol solution, all samples were thoroughly rinsed with ethanol and blow dried with nitrogen. Although the purity of the hexadecanethiol used in this study is not the highest available, its interaction with gold is very strong, so it is unlikely that impurities would affect significantly our main conclusions. Indeed, the most likely impurities are alkanes, dialkyldisulfides and alkylthiols of different chain lengths. Alkanes have low adsorption energy and should be readily displaced by alkylthiols. Biebuyck et. al43 have compared SAMs assembled from alkylthiols and dialkyldisulfides and have shown that both the final monolayer and their adsorption kinetics are identical. Finally, if the SAMs contain a small fraction of alkylthiols of different chain lengths, this would not be noticeable in our studies, since we do not have atomic resolution in the STM images, SHG and CV would not be affected significantly by that, and SFG spectroscopy would not be sensitive enough to detect a few percent of a monolayer. Furthermore, many authors44-47,58 also use alkylthiols with similar purities, and therefore our results are directly relevant to those studies, if not of general applicability. 3.2. STM images. Scanning Tunneling Microscopy was performed either with a Discoverer 2010 (Topometrix) system using 0.25 mm diameter tungsten tips in a constant current mode (Figures 7a and 7b), or with a Nanoscope 3A (Veeco) in a constant height mode (Figure 1) or constant current mode (Figure 7c). The tip bias voltage was 200 mV with respect to the grounded Au substrate.



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3.3. Cyclic Voltammetry. We used a potenciostat/galvanostat EG&G PAR 283 for the cyclic voltammetry (CV) measurements. The electrochemical cell used was made of Teflon with a ~1.4 cm2 probed area. Pt wire was used as counter electrode and Ag/AgCl as a reference electrode. The electrolyte consisted of 1 mM K3Fe(CN)6 and 0.5 M KCl aqueous solution, and the scan rate was 50 mVs-1. 3.4. SFG setup. A thorough description of our SFG setup can be found elsewhere.39 Briefly, a commercial SFG setup (EKSPLA, Lithuania) generates VIS (532 nm, 600 µJ) and IR (120 µJ) laser pulses at a 20 Hz repetition rate. IR and VIS beams are temporally and spatially overlapped at the sample surface, with incidence angles of 55° and 60°, respectively. The spot sizes of VIS and IR beams on the sample are ~1 mm2 and 0.5 mm2, respectively. The polarization combination used in all measurements was PPP (P-SFG, P-VIS and P-IR). 3.5. SHG setup. For the SHG measurements we used a portion of the fundamental beam of the Nd3+:YAG laser in the SFG setup, producing pulses of 25 ps duration at a 20 Hz repetition rate and wavelength of 1064 nm. The pulse energy was 2 mJ and the beam incidence angle was 60°, illuminating an area of ≅ 2 mm2 at the sample. The polarization combination in the measurement was Sin and Pout. The AuTS substrates were placed inside a thoroughly cleaned Teflon cell covered by a glass window, which was then filled with the C16SH solution in ethanol.48 The cell was quickly aligned at the optical setup and data acquisition started after about one minute after filling the cell.

4. RESULTS AND DISCUSSION

We begin our discussion with the results on the nearly atomically flat AuTS substrates. We will then present a brief comparison among the different experimental techniques in assessing the quality of the SAMs. In the last section we will compare the results with SAMs prepared on


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different Au substrates and discuss the effect of surface roughness on the quality of the SAMs, as perceived by the different techniques. 4.1. STM. Figure 1 shows STM images, in constant height mode, of the bare Au surface and C16SH SAMs with 10 minutes, 1 hour, 2 hours, 3 hours, 6 hours, 12 hours and 24 hours immersion times. Figure 1a, shows a featureless image of the AuTS substrate, with high tunneling current and low roughness (for clarity, the current scale is not the same of the other images), demonstrating that our methodology leads to very flat substrates (2.1 Å rms roughness). Substrates with C16SH SAMs show a decreasing average tunneling current due to increasing surface coverage of regions with low tunneling current (which correspond to the bright areas in the images) as function of immersion time (between 10 minutes and 24 hours, Figures 1b to 1h). At first one might assign the bright areas to patches of upright, packed thiol molecules, and the dark regions (larger tunneling current) to the bare substrate or regions covered by flatlying C16SH molecules. However, atomic force microscopy (AFM) experiments have shown that at high thiol concentration (0.1 mM) a fully-packed SAM is attained within a few seconds of adsorption.49 Therefore, the contrast seen in Figures 1b to 1h is not due to gross topographic changes of the monolayer (such as covered/uncovered regions), but to a combination of electronic effects in the substrate and facilitated tunneling through patches of alkylthiols with defective conformations. In fact, small changes in monolayer height should occur in such conformationally disordered patches, but they are below the usual sensitivity of AFM imaging. Contrast in AFM images of mixed monolayers is seen only with appreciable differences in molecular lengths.50 In contrast, STM is known to be very sensitive to the presence of defects in SAMs.51 From the images in Figure 1 we have calculated the coverage (θ) of “fully-packed” SAM patches (bright regions, of low tunneling current) by the ratio between the bright area and the image size. The bright area was calculated from the current histograms of each image, as the area corresponding to the low current peak of the histogram. Since the low and high current peaks strongly overlap in the histograms, there is an ambiguity at defining exactly the threshold



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current below which it corresponds to bright regions. The uncertainty in coverage shown in Table 1 is an estimate due to this ambiguity. This quantitative measurement of the (mesoscopic) coverage of “defect-free” regions for different immersion times is shown on Table 1. As an aid to our discussion of the SAM formation, Table 1 also displays the average tunneling current for the dark areas (Id – corresponding to regions with conformational and packing defects) at different adsorption times. The current for the bright regions (Ib – corresponding to regions of fully-packed SAM) was quite low and did not vary appreciably, and therefore it is not shown in Table 1. Id and Ib were calculated from the current histograms of each image, as the average current of the high and low current peaks, respectively. Up to 1 hour of adsorption of C16SH on gold, the contrast (Id/Ib) between dark and bright regions is not very different. For the images with 2 and 3 hours of adsorption (Figures 1d and 1e), the contrast increases markedly, with the brighter regions being more uniform, and surrounded by much darker regions. At longer times (6 hours and beyond), the contrast in tunneling current is gradually reduced again, with the dark regions not as dark as before (reduced tunneling current).


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Figure 1. STM images (constant height mode) of Au

TS

as function of immersion times in the C16SH

solution: (a) bare gold; (b) 10 min; (c) 1 h; (d) 2 h; (e) 3 h; (f) 6 h; (g) 12 h and (h) 24 h. The images size is 0.5 µm x 0.5 µm.

Table 1. Average Tunneling Current Through Dark Areas of the Image (Id) and Surface Coverage (θ θ) of Fully-packed Regions of C16SH on Au Adsorption Time

TS

as a Function of Adsorption Times

10 min

1h

2h

3h

6h

12 h

24 h

Id (pA)

4.4 ± 1.2

5.2 ± 1.2

7.4 ± 1.2

8.5 ± 1.2

7.3 ± 2.0

6.7 ± 4.1

4.6 ± 2.5

θ (%)

50 ± 9

54 ± 7

64 ± 5

65 ± 4

79 ± 5

95 ± 2

95 ± 2

According to the images in Figure 1 and their contrast changes shown in Table 1, the adsorption of alkylthiols on AuTS appears to occur in three different regimes. At early times (10 min), the formation of fully-packed thiol islands on the gold surface is already quite obvious, but with a large fraction of the image with higher current (dark regions), although not at the same value as for the bare substrate. Therefore, these dark regions are probably covered by adsorbed molecules, but in a more disordered arrangement. At 1 h adsorption time (Figure 1c), the islands become smaller and several small dark spots can be seen within the dark regions, and as result the image appears “grainy”. This is surprising, since the conventional wisdom is that the monolayer becomes more ordered with increasing adsorption times.5 Between 1 and 3 h, the images change qualitatively. Larger and more homogeneous fully-packed thiol islands can be seen (areas with a complete coverage by a compact monolayer), separated by narrow dark regions with a higher contrast, which probably correspond to regions of almost bare gold. Beyond 3 h, the large patches of alkylthiols begin to coalesce and the dark regions become brighter, indicating that the “trenches” between islands start to fill in with thiol molecules. For the sample with immersion time of 12 hours (Figure 1g), an almost complete monolayer is already



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formed (coverage of 95%), and for longer immersion times (Figure 1h, for 24 h adsorption) there is only a slight reduction in the amount of defects in the monolayer (Id is further reduced). The decreased contrast between bright and dark regions at long adsorption times indicates that the defect regions are becoming increasingly ordered. 4.2. Cyclic Voltammetry. Electron transfer from a molecule in solution to a substrate through a closed packed SAM of alkyl chains occurs at a rate that depends exponentially on the monolayer thickness.52-54 As the structural integrity of the monolayer decreases, due to presence of pinholes, grain boundaries, loosely packed regions, etc., the electron transfer rate (the current across the monolayer) will be strongly increased. In certain cases the degree of integrity of the structure can be calculated with theoretical models for partially covered electrodes.55 The electrochemical probe Fe(CN)63- was selected because it represents a convenient and electrochemically reversible, one-electron, outer-sphere redox couple.56,57 Figure 2 shows the CV for the AuTS/SAM substrate in contact with the solution containing Fe(CN)63-, for SAMs prepared with different immersion times. It can be observed that the longer the immersion time, the smaller the redox current for the Fe(CN)63-/4- system. This indicates that the SAMs become more densely packed and with fewer defects as the immersion time increases. It is interesting to note that for short immersion times (~ 1 h), the peak current reduction for the CV correlates well with the surface coverage obtained by STM (Table 1), but for long times (more than 12 h), the CV changes are more appreciable than expected from the increase in surface coverage, reflecting the further reduction of Id for STM images, as shown in Table 1. This is another indication that for long adsorption times, the few remaining defect regions are being gradually filled by C16SH molecules and becoming more ordered, reducing the net charge transfer rate.


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Figure 2. CVs of modified Au substrates with SAMs for different immersion times in 1 mM C16SH ethanolic solution. (The probe concentration is 1 mM K3Fe(CN)6 in a 0.5 M KCl supporting electrolyte at a -1

scan rate of 50 mVs ).

4.3. SHG. The short-time (few minutes) adsorption kinetics of a series of n-alkylthiols on polycrystalline gold was investigated in detail by SHG in the work of Dannenberger et al.58 More recently, Gan et al.59 used SHG to study the adsorption kinetics of ethanethiol, 1,2benzenedithiol and 3-mercapto-propanesulfonate on Ag nanoparticles. Here we used the SHG technique to probe the long-time (hours) adsorption of C16SH on gold in situ. Figure 3 presents the SHG signal vs time (hours) during the adsorption of C16SH on the AuTS substrate. Each data point is the average of the SHG intensity during one minute (1200 laser shots).



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TS

Figure 3. Intensity of the SHG signal as a function of time for the Au

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substrate in contact with a 1mM

ethanolic solution of C16SH.

The time plot of Figure 3 shows a complex behavior up to 3 h, with an initial rise up to 35 min, followed by decay to a minimum at 95 min, and rise back to a maximum intensity at nearly 3 h of adsorption. After 3 h, the SHG intensity presents a monotonic decrease, stabilizing at around 12 h. These SHG data may be rationalized by comparing with the results obtained via CV and STM. Although we do not have a complete understanding of the initial behavior (up 3 h) of the SHG signal, this time span is exactly the same where the STM images show a drastic change in the morphology of the monolayer. If we recall that with 1064 nm excitation the SHG beam at 532 nm is resonant with the d-band of gold,60 it is likely that the non-monotonic changes in SHG intensity occur due to different modes of interaction of alkylthiol molecules with the substrate. This would affect the electronic structure at the gold/solution interface, with corresponding shifts of the d-band resonance, which in turn would change the intensity of the SHG output. Beyond 3 h the STM images show a similar morphology, with large thiol islands separated by narrow defects, which become increasingly filled by ordered thiol molecules upon increasing the adsorption time, with defects almost completely healed after 24 h. This “defect healing” stage translates in the gradual reduction of SHG intensity at long times. It is interesting to note that from 12 to 24 h, the changes in SHG and STM images are within the uncertainty of


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the measurements, but the CV still shows a significant decrease in the electron transfer rate through the SAM defects. 4.4. SFG spectroscopy. Prior to CV measurements, the same AuTS/SAM samples were characterized by SFG spectroscopy. The results are presented in Figure 4 for the CH stretch spectral region (2700 to 3100 cm-1). The spectra contain five resonances characteristic of alkyl chains and their assignment is summarized in Table 2. However, the modes due to CH2 groups along the chain (d+ and d-) are much weaker, indicating that the chains are nearly in an all trans conformation, where the CH2 groups point in opposite directions, leading to a cancelation of their SFG contribution,40 as briefly described in Section 2.

Figure 4. SFG spectra of C16SH SAMs on AuTS substrates for different immersion times in 1 mM ethanolic solution. The polarization combination is PPP. The spectra have been displaced for clarity, and the zero intensity for each curve is indicated by a horizontal mark with the same color. The solid lines are fits to Eq. (2).



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Table 2. Assignment of Observed CH Stretching Modes in the SFG Spectra Peak

-1

Assignment

Wavenumber (cm ) +

~ 2845

+

~ 2880

1

Symmetric stretching of CH2, d

2

Symmetric stretching of CH3, r

3

Antisymmetric stretching of CH2, d

4

-

~ 2915

Symmetric stretching of CH3 (Fermi + Resonance), r FR

5

Antisymmetric stretching of CH3, r

~ 2935 -

~ 2965

Table 3. Parameters Obtained from Curve Fitting of the SFG Spectra as a Function of Immersion Time 2 minutes 1 hours 14 hours 24 hours 1.56 1.45 1.5 1.5 θ (rad) 0.92 0.75 0.49 0.76 B1 Γ1 ω1 B2 Γ2 ω2 B3 Γ3 ω3 B4 Γ4 ω4 B5

0.1 7 2843 3.4 11 2881 0.75 7.2 2913 2.3 10.42 2935

0.25 6 2847 5.5 15 2882 0.67 5 2915 2.5 7.1 2937

0.1 6 2847 3.5 9 2881 0.8 5 2917 1.3 5.8 2937

0.18 6.3 2845 5.6 10.1 2880 0.5 3.7 2917 2.3 6.4 2937

1.1

1

0.5

0.9

Γ5 ω5

6.88 2963

7 2965

5.5 2965

3.8 2964

The molecular resonances interfere with the non-resonant contribution from the gold substrate,

, leading to dips in the spectra and a complex lineshape. In order to extract

amplitude (B), width (Γ) and resonant frequency (ω) of the vibrational modes, each spectrum was fitted to Eq. (2), where θ is the phase of the non-resonant susceptibility. This procedure is


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fundamental for quantitative comparisons since peak amplitudes, positions and widths are not as obvious as in conventional (linear) spectroscopy. The parameters obtained from the curve fitting are listed in Table 3. From the fitting parameters, we now try to interpret quantitatively the changes in SFG spectra. The amplitude ratio for the symmetric stretches of CH2 and CH3 groups in an alkyl chain (d+ / r+, or B1 / B2 in our fittings) is an indication of its relative conformational order, with smaller values for more ordered (nearly all trans) chains.40,41. This ratio as a function of the adsorption time is shown as red squares in the plot of Figure 5. Although the uncertainty in the fitting is quite large, the d+/r+ ratio is always very small, even for times as short as 2 min, indicating a nearly all trans conformation for the SAM in all cases. In addition, the molecular orientation of the terminal methyl group of the SAMs can be obtained quantitatively from the ratio of the amplitudes for the symmetric and asymmetric CH3 stretches (r+ / r-, or B2 / B5 in our fittings) in the spectrum with PPP polarization, as described in detail more recently by Jacob et al.41 Figure 5 also shows this amplitude ratio as a function of the adsorption time (black dots). Again, the uncertainty in the fitting is too large for a quantitative interpretation, but the large value for the r+ / r- ratio indicates that the average methyl orientation is close to the expected value of 27°,61 and seems to be nearly constant for all adsorption times. The slightly lower value for the sample prepared in 2 min is probably because the monolayer is less homogeneous in terms of chain orientation.41 Although the quantitative analysis of these SFG spectra is useful, one important qualitative feature can be readily seen in Figure 4, with striking consequences: the amplitude of the r+ mode at ~2880 cm-1 (see inset of Figure 5) is always quite large, even for 2 min adsorption time. In fact, the SFG spectra for all samples are similar, with only minor quantitative changes, as described in the previous paragraph. This is not what would be expected based on the STM images of Figure 1, if we interpret the bright regions as patches of ordered SAM, and the dark regions as monolayers with molecules lying flat on the substrate (or with a low surface density



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and highly disordered conformation). In this case, these dark regions should not yield any SFG signal, and the intensity of the SFG resonances should increase quadratically with the surface coverage of ordered patches. We see from Table 1 that the coverage of ordered regions nearly doubles from 10 min to 1 h, and from 1 h to 24 h adsorption. In contrast, the SFG amplitude for the r+ mode presents only a modest increase from 2 min to 1 h, but remains approximately constant afterwards. Therefore, the dark regions in the STM images must correspond to patches of upright alkylthiol molecules, perhaps with more disordered conformation, such as with kink defects. Such conformation would not affect the methyl group orientation and also would not contribute to the d+ mode amplitude, due to cancelation of the contributions from the two gauche defects along the chain (g+tg-, for example). In this case, such a conformational defect would not affect very much the SFG spectrum, but would facilitate electron transfer through the monolayer, affecting both STM and CV experiments.

+

-

+

+

Figure 5. Amplitude ratios r / r and d / r as a function of immersion time. The inset shows the r

+

amplitude as a function of the time.

4.5. Effects of substrate roughness. Figure 6 compares STM images, CV measurements and SFG spectra for three substrates with markedly different roughness: (a) Au/glass; (b) Au/mica annealed at 300 °C; (c) AuTS (stripped from mica template). STM images of the bare substrates were obtained in constant current mode (topography), and show a


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dramatic change in substrate morphology for the three preparation methods. Gold evaporated on glass presents small grains of a few tens of nm, resulting in an rms surface roughness of 13.3 Å. Upon annealing at 300 °C inside the vacuum chamber after evaporation, the small grains coalesce and large terraces with several tens to a few hundred nm can be seen. Although the rms roughness over the whole image increases to 47.5 Å for the Au/mica sample (compared to 13.3 Å for the Au/glass sample), this is due to large differences in height from terrace to terrace. However, the local roughness within the terraces is much smaller (we estimate it as ~4 Å). In contrast, the template-stripped Au surface (AuTS) is nearly atomically flat, with an rms roughness of only 2.1 Å. The CV measurements for SAMs prepared with long adsorption times (at least 24 h) on the three substrates (see Figure 6) show that the charge transfer rate across the SAM is very sensitive to surface roughness. The SAM on Au/glass led to only a minor reduction in electrochemical charge transfer with respect to that of a bare gold substrate, even after 2 weeks of adsorption time. For the SAM on the annealed Au/glass substrate, the charge transfer blocking was significantly improved, with the CV area after 48 h of adsorption being 20 times times smaller than that for bare Au. However, the best reduction in charge transfer rate was obtained for a SAM on the AuTS substrate, where the CV area was reduced by a factor of 45 after 24 h adsorption of C16SH. From these results, it is clear that substrate roughness leads to defects in the SAM that have a great influence on charge transfer across the organic film. This is consistent with the observations of Haiss et al.,62 who compared several STM-based methods for measuring single molecule conductance of alkyldithiol monolayers prepared on substrates with different step densities. With a statistical analysis of molecular conductances on various substrates, they found that the molecular conductance on step sites was always higher than on flat terraces. It has also been shown theoretically that the net tunneling conductance of metalinsulator-metal junctions is quite sensitive to moderate amounts of surface roughness, which lead to an apparent barrier that is thinner than the mean of the real barrier thickness



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distribution.63 In addition, it is interesting to note that substrate roughness not only affects the electric transport properties of organic monolayers, but also other important physico-chemical properties, such as the apparent pKa of acid-terminated SAMs.27

Figure 6. Comparison among results on different substrates. STM topographic images (constant current mode) of bare substrates (image size = 500 nm X 500 nm), CV measurements and SFG spectra, with the same conditions as previous Figures: (a) Au/Glass; (b) Au/mica annealed at 300 °C; (c) Au

TS

(stripped

from mica template). For the CV measurements and SFG spectra, the C16SH adsorption time is indicated in each panel.

Unlike the CV measurements, the SFG spectra of Figure 6 for SAMs on the different substrates are quite similar. All of them are dominated by vibrations from the terminal methyl group, which is an indication of nearly upright, all-trans conformation of the C16SH molecules.


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Only for the Au/glass substrate (Figure 6(a)) there is a weak but noticeable shoulder at 2845 cm, characteristic of the d+ mode, due to chains with gauche (disordered) conformations.

1

Therefore, as discussed before (in connection with the fitting parameters of Figure 5), the darker regions in the STM images are associated with grain boundaries of the Au substrate, where defects in the molecular conformation of the SAM must happen, with appreciable effects in electrochemical charge transfer. However, these defects do not show up significantly in the SFG spectra, either because of particular types of defects (kinks), or because the molecules are in valleys of the metallic substrate, where the local light field is reduced and the SFG sensitivity is lowered. In any case, it is clear that SFG spectroscopy is not as sensitive to defects in the SAM as CV measurements. According to these results, the presence of substrate roughness has a major effect on the quality of alkythiol SAMs, particularly when charge transfer across the monolayer is involved. Thermal evaporation of Au on glass is a very simple methodology for substrate preparation, but leads to high surface roughness and several defects in the monolayer, as a result of many grain boundaries. Evaporated gold films annealed at ~300 °C had a much better morphology, with much larger grains with flat terraces, but imperfections at grain boundaries still remain, with noticeable effects on the quality of the SAM. The AuTS substrate, stripped from the mica template, although still macroscopically polycrystalline, is nearly atomically flat and leads to almost perfect SAMs, with a much reduced amount of defects. 4.6. Comparison among the techniques. From the previous discussions, it is apparent that each of the techniques used in this work has its own advantages and disadvantages in respect to assessing the adsorption and quality of the SAMs. STM has been widely used to probe the adsorption and characterize alkylthiol SAMs5,6,8-11 and is indeed very sensitive to the local order of the monolayer. However, it probes relatively small regions (tens of µm) and is not straightforward to obtain good images with high resolution. Furthermore, it may be difficult to relate changes in tunneling current to the molecular structure of the organic film. In contrast,



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SFG spectroscopy is a powerful technique to characterize the orientation and conformation of thiol molecules on SAMs, thus providing information at the molecular level, either averaged over large regions (~ mm) or with micrometer resolution, in the case of SFG microscopy.41 Nevertheless, we showed that its sensitivity to defects in the monolayer is much smaller than in STM, and it is also a more demanding technique in terms of instrumentation and data analysis. The related SHG technique has the advantage of being of simple implementation and capable of probing molecular adsorption in situ and with good temporal resolution (a few seconds), but here we showed that it probes the electronic response of the substrate to molecular adsorption, making it difficult to relate to the SAM molecular structure. Finally, the technique of cyclic voltammetry is the simplest and most sensitive to defects in the monolayer, and is a good choice for a quick assessment of the quality of thiol SAMs. The drawbacks are a lack of information about the molecular structure, and probing relatively large areas (a few mm).

5. CONCLUSIONS

Here we initially investigated the adsorption of 1-hexadecanethiol on atomically flat gold substrates (AuTS) from 1 mM ethanolic solutions. From 10 min up to 3h of adsorption, the coverage of fully-packed thiol regions increases moderately from ~ 50 to 65%, but the morphology of the densely packed islands change significantly. Between 3 h and 24 h, CV, STM and SHG indicate a “defect healing stage”, where the disordered regions between islands become increasingly ordered by further thiol adsorption. We find that CV and STM are much more sensitive to defects in the monolayer than SFG spectroscopy, while SHG is useful to monitor the final stages of adsorption (defect healing). However, SFG spectroscopy suggests that the disordered regions observed in STM are not due to flat-lying molecules, but to conformationally disordered and upright alkylthiols.


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The comparison among different Au substrates with increasing surface roughness shows that the formation of a nearly perfect monolayer, with good electrochemical blocking, is obtained only at long adsorption times on atomically flat substrates. On rough substrates, the presence of conformational defects due to low packing and organization of the alkyl chains is unavoidable, leading to poor charge transfer blocking properties. These findings may have important implications to the preparation of high quality SAMs for sensitive applications.

ACKNOWLEDGMENTS

The authors would like to thank Dr. Marcelo de Assumpção Pereira da Silva for the STM images and Mr. José Roberto Bertho for assistance with gold film evaporation. TMU, HBA and KB also acknowledge scholarship from the Brazilian agencies CAPES and FAPESP (2010/00486-2 and 05/02284-0). This work was funded by FINEP, CNPq and FAPESP.

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Figure 1. STM images of AuTS as function of immersion times in the C16SH solution: (a) bare gold; (b) 10 min; (c) 1 h; (d) 2 h; (e) 3 h; (f) 6 h; (g) 12 h and (h) 24 h. The images size is 0.5 µm x 0.5 µm. 76x37mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2. CVs of modified Au substrates with SAMs for different immersion times in 1 mM C16SH ethanolic solution. (The probe concentration is 1 mM K3Fe(CN)6 in a 0.5 M KCl supporting electrolyte at a scan rate of 50 mVs-1). 62x48mm (600 x 600 DPI)


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Figure 3. Intensity of the SHG signal as a function of time for the AuTS substrate in contact with a 1mM ethanolic solution of C16SH. 61x47mm (600 x 600 DPI)



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Figure 4. SFG spectra of C16SH SAMs on AuTS substrates for different immersion times in 1 mM ethanolic solution. The polarization combination is PPP. The spectra have been displaced for clarity, and the zero intensity for each curve is indicated by a horizontal mark with the same color. The solid lines are fits to Eq. (2). 65x53mm (600 x 600 DPI)


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Figure 5. Amplitude ratios r+ / r- and d+ / r+ as a function of immersion time. The inset shows the r+ amplitude as a function of the time. 61x46mm (600 x 600 DPI)



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Figure 6. Comparison among results on different substrates (STM topographic images of bare substrates – size = 500 nm X 500 nm, CV measurements and SFG spectra, with the same conditions as previous Figures): (a) Au/Glass; (b) Au/mica annealed at 300°C; (c) AuTS (stripped from mica template). For the CV measurements and SFG spectra, the C16SH adsorption time is indicated in each panel. 222x177mm (150 x 150 DPI)


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230x180mm (72 x 72 DPI)


Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the

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