Optical Characterization of Thiolate Self-Assembled Monolayers on

J. Phys. Chem. C 2008, 112, 3899-3906

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Optical Characterization of Thiolate Self-Assembled Monolayers on Au(111) Mirko Prato, Riccardo Moroni, Francesco Bisio, Ranieri Rolandi, Lorenzo Mattera, Ornella Cavalleri, and Maurizio Canepa* CNISM and Department of Physics, UniVersity of GenoVa, Via Dodecaneso 33, 16146 GenoVa, Italy ReceiVed: NoVember 26, 2007; In Final Form: January 4, 2008

We have investigated the optical response of thiolate self-assembled monolayers (SAMs) deposited from the liquid phase on well characterized, (111)-textured gold films based on the use of in situ and ex situ optical spectroscopic ellipsometry. We considered SAMs formed by several molecules with thiol functionality, focusing on the octadecanethiol (C18) SAM model system. We were able to show the tiny spectroscopic variations induced by the monolayer thick films with great reproducibility and high signal-to-noise ratio. We identified spectral features related to the alkyl chain and to the S-Au interface, providing a reliable spectral “fingerprint” of the formation of densely packed thiolate layers. By comparing data with simulations based on several effective models developed within the framework of Fresnel approach, we identified the main optical features related to the thiolate interface and in particular an absorption band whose spectral weight increases regularly from 500 nm toward the IR limit. We also obtained reliable estimations of the SAM thickness. The interface absorption properties have been tentatively assigned to a modification of the nearly free electron behavior, related to nanoscale morphological modifications following the formation of Au-thiolate moieties.

1. Introduction Self-assembled monolayers (SAMs) of organosulphur compounds on metal surfaces are well-recognized components of nanoscience and nanotechnology.1,2 Many applications can be found in diverse fields such as the modeling of bio-surfaces,3 the design of bioadhesion-resistant surfaces,4,5 the study of wetting, adhesion, and friction phenomena,6,7 and the development of molecular electronics devices8,9 and nanolitography methods10-12 to name a few. To this regard, SAMs of n-alkanethiols (HS(CH2)n-1CH3, in brief Cn) on gold substrates represent a paradigmatic system1,2,13 Well-established protocols, notably for long-chain molecules such as C18, ensure the reproducible preparation of densely packed monolayers endowed with a high degree of molecular order.14 The structural and morphological properties of Cn SAMs, together with the main aspects of the assembly kinetics, are known in reasonable detail.14-19 Compared to morphological information, knowledge about the electronic structure of SAMs and the interface between the metal substrate and the organic layer in particular would appear to be less comprehensive.20,21 With specific regard to interface formation, optical methods represent a practicable means of investigating the in situ properties under wet conditions.19,22-24 Optical spectroscopic ellipsometry (SE) can be utilized to probe the electronic interface properties of ultrathin organic layers in extended portions of the UV-vis spectral region.25 Studies in the IR region (IRSE) can offer insight on molecular vibrations and provide specific information about adsorption configuration.26-30 Modern instruments that combine fast parallel detection and accuracy, enhanced sensitivity, and high signal-to-noise ratio can also be used to perform in situ monitoring of deposition processes.31 As long as the samples are not photosensitive, ellipsometry is absolutely not destructive. As a result, it can be used to integrate information derived from “dry’’ methods, for * Corresponding author. E-mail: [email protected].

example, photoelectron spectroscopy, which ensures “direct’’ access to the valence band structure of the interface at the cost of modification and ultimate destruction of soft matter samples. Despite these advantages, the application of UV-vis SE to the study of thiolate interfaces is rather limited.32-34 In this paper, we will discuss the optical properties of the S-Au interface concentrating on C18 SAMs as a benchmark system. Nevertheless, our analysis will be based upon a broad database obtained on shorter chain alkanethiols and other organosulphur compounds. By extending the results of previous works,32,35 we will provide a clear spectral identification of the formation of thiolate-mediated bond on gold. From a general point of view, it is important to understand the electronic properties of the thiolate Au layer to comprehend the transport properties across the interface.36,37 Such an understanding also provides the preliminary information needed to appreciate other interesting and recently disclosed phenomena, such as the magnetic behavior of several compounds on gold.38,39 Furthermore, our data could be useful to the large community of scientists who routinely utilize ellipsometry to effectively evaluate the thickness of bio-organic overlayers. 2. Methods and Materials 2.1. Spectroscopic Ellipsometry. Measurements were performed using a rotating compensator ellipsometer (M-2000S, J. A. Woollam Co., Inc.). The instrument, which can be used to perform simultaneous detection at 225 wavelengths in the 245-725 nm range, was tested in previous experiments.40 In situ measurements were carried out at room temperature in a homemade Teflon cell equipped with high quality fused silica windows that ensure precise definition of four incidence and reflection angles.41 The principles and applications of SE are described at length in books42,43 and reviews.44 In brief, the output of standard ellipsometry measurements is the complex reflection coefficient

10.1021/jp711194s CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008

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F, expressed in terms of two angles Ψ and ∆, according to the relation

F)

rpp ) tan Ψ exp(i∆) rss

(1)

where rpp and rss are the Fresnel reflection coefficients for pand s-polarization, respectively. The phase parameter ∆ ensures enhanced sensitivity to the presence of ultrathin films.42 Information on films can be derived by comparing the experimental data with simulations. Under the Fresnel scheme, the system is usually conceived as a stack of layers on a substrate. In the simplest approach, the layers are separated by sharp interfaces. Each layer is characterized by an optical complex function Nj (nj(λ), kj(λ)) and an effective thickness dj, that can serve as parameters to be adjusted until obtaining the best agreement between data and calculations.43 Simulations and fitting operations were carried out by the WVASE32 software supplied by the manufacturer. The spectral changes induced by the formation of nanometer thick layers are best appreciated by looking at difference spectra between the SAM-covered sample and its own substrate (δΨ ) ΨSAM - ΨAu and δ∆ ) ∆SAM - ∆Au). This procedure involves specific limitations and a certain degree of caution. First, the random variability of measurements performed over different substrates related to surface morphology and to adventitious contamination must be considered.45,46 Second, spot-to-spot variations on the same sample may also be significant and should be averaged accordingly, especially on ex situ measurements. Third, in situ sample positioning and alignment through windows may represent other factors of both systematic and casual errors. All these factors have been carefully considered to minimize measurement uncertainty. Two types of SE difference spectra will be presented: spectra taken as a function of the wavelength at a given time and dynamic scans at selected wavelengths, (δΨλ(t), δ∆λ(t)), useful to monitor the assembly process (deposition curves). In situ measurements will be shown in the high transparency range of the solvents. 2.2. Preparation of Substrates and SAMs. Thickness and roughness of gold films are well-recognized morphological parameters affecting the characteristics of alkanethiolate SAMs.47 We concentrated on optically thick (200 nm), flat samples, combining typical grain size on the scale of microns and homogeneity on the macroscopic scale. These two conditions were achieved using flame-annealed films deposited on glass slides coated with a Cr primer (Arrandee). Flame annealing (up to a dark red glow) to flatten the samples facilitates the desorption of loosely bound contaminants. The quality of each sample was assessed by means of SE through spot-checks with AFM measurements. The agreement of the dielectric function of annealed substrates derived by inversion of SE spectra43 with benchmark results45,48 and more recent studies49 proved satisfactory, as indicated in Figure 1. A more thorough discussion of the gold dielectric function is provided in the classical work by Aspnes and co-workers45 in addition to the more recent work in ref 50. With reference to these works, we observe that our substrates show low Drude tails with values of 2 close to 0.9 at about 1.6 eV; further, the threshold for d-band to Fermi level transitions is appreciable at about 1.8 eV. With reference to ref 46, we note that typical values of the ∆ parameter at 70° and for λ ) 632.2 nm on many freshly annealed substrates were closely distributed around 108°. This value can be compared with 109.4° calculated for

Figure 1. Real and imaginary part of the gold substrates dielectric function obtained from SE spectra. Shaded areas show the acceptable variability range of our samples. Reference data45 are shown for comparison (dotted lines).

an ideally flat surface and with 106° calculated for a RMS roughness of 5 nm.46 An RMS roughness of the order of 2 nm turned out consistent with atomic force microscopy (AFM) cross-checks. Substrates clearly differing from the average quality were rejected. To ensure high reproducibility of experimental results over a large set of samples, we followed a strict SAM preparation protocol consisting of the following: 1. Chemical etching, flame annealing, and immediate ex situ SE check of several zones of the Au substrates. The check involved a few minutes of exposure to atmospheric contamination. The typical decrease of ∆ observed after 300 s, about 0.03° at 632.2 nm, is equivalent to a layer of contaminants with an effective thickness of 30 pm. 2. SE check in liquid environment. 3. Inlet of the solution containing the molecules under study and incubation. The solution was injected using a syringe while the deposition process was monitored through SE dynamic scans. Volume and concentration of the admitted solution were chosen to ensure the desired equilibrium concentration in the cell, according to previous experiments.51 Typical incubation times ranged from 1 h to 1 day. 4. Ex situ SE check (again on several zones) after copious rinsing with the solvent and drying under nitrogen stream. 2.3. Reagents. 1-Hexanethiol (C6, Aldrich, purity 95%), 1-dodecanethiol (C12, Aldrich, purity 98.5%), 1-octadecanethiol (C18, Aldrich, purity 98%), L-cysteine (HSCH2CH(NH2)COOH, Sigma, purity g 98%), and L-methionine (CH3SCH2CH2CH(NH2)COOH, Sigma-Aldrich, purity g 98%) were used as received. Solutions were prepared in ethanol (Fluka, purity g 99.8%), methanol (Fluka, purity g 99.8%), and MilliQ water (Millipore, resistivity 18 MΩ cm). 3. Results 3.1 Experimental Data. Representative dynamic scans obtained monitoring the C18 deposition in ethanol are shown in Figure 2, using two wavelengths as an example. Regarding δ∆λ(t) data (upper panels), well-defined drops were observed at molecular admission, irrespective of the selected wavelength. The drops were followed by a slow drift toward more negative, steady values attained about 1 h after admission. The decrease of ∆ values is roughly proportional to the thickness of the ultrathin growing overlayer;25,42 the steplike variations amount to approximately 85% of steady values. Instead, the δΨλ(t) curves (lower panels) were dependent upon wavelength; for λ below 500 nm (panel B), sharp increments were observed, and after the jump the curve became practically stationary. On the opposite, Ψ drops were observed

Thiolate Self-Assembled Monolayers on Au(111)

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Figure 2. Dynamic scans (deposition curves) for C18 in an ultrapure ethanol solution. Time zero marks molecular admission. Molecular concentration at equilibrium 1 mM. Angle of incidence 70° . Panels A and B: λ1 ) 349.0 nm. Panels C and D: λ2 ) 632.2 nm. The insets show the time evolution of the signal close to admission.

Figure 4. Ex situ difference spectra δΨ(λ) and δ∆(λ) measured on C6, C12, and C18 SAMs (colors online). Angle of incidence 65°. Dotted curves (green): three-phases (ambient/film/substrate) simulations for 1 and 2 nm thick nonabsorbing films with refraction index n ) 1.475.

Figure 3. Representative difference spectra δΨ(λ), δ∆(λ) for C18 SAMs. Angle of incidence 70°. (A,B) In situ (ethanol) SAMs. Data are shown in the high transmission range of the solvent. Gray symbols, measurements just after the first jump of Figure 2; black symbols, layer saturation. (C,D) Ex situ measurements. Dotted curves (green): threephases (ambient/film/substrate) simulations for 2 and 3 nm thick nonabsorbing films with refraction index n ) 1.475.

above 500 nm (panel D), followed by a slow downward drift. Very similar trends were observed for deposition in methanol. Representative δΨ(λ) and δ∆(λ) difference spectra in ethanol are reported in Figure 3(A,B) for 70° angle of incidence. The δ∆ patterns present negative values, with a relative maximum at about 490-495 nm. The δΨ(λ) curves exhibit a well-defined transition from positive to negative values at the position of the δ∆ maximum. A more detailed look at δΨ data above 500 nm shows that the curve recorded just after the first adsorption step is significantly above the steady state one. The latter approaches a value of (-0.17/-0.18°) in the IR-limit. With regard to sample-to-sample variations, tiny offsets of the δ∆ patterns were typically observed, leading to a distribution comprised within (0.05° from the patterns reported in the figure. Such δ∆ offsets were coupled to minimal “deformations’’ of the δΨ profiles; typically, less negative δ∆ patterns (e.g., thinner films) were coupled with a tiny decrease of the positive δΨ values in the UV region and of negative δΨ values in the near IR. Spectra obtained in methanol (not shown) once corrected for the different refractive index of the ambient medium were comparable to those of Figure 3. Ex situ results proved substantially independent of the type of solvent employed.

Averaged ex situ δΨ(λ) and δ∆(λ) spectra are shown in panels Figure 3C,D; δ∆(λ) values are larger than in situ ones, and the relative maximum is shifted to 500 nm. The values of the ex situ δΨ(λ) curve (Figure 3 panel D) are definitely larger than in situ ones, except for the near-IR limit where the curve converges to about -0.13°. Most differences between in situ and ex situ values can be ascribed to the different refraction index of the ambient medium. In this respect, we added some simulations in Figure 3 that describe transparent films of variable thickness taking into account the ambient medium index of refraction. This approximation, reliable for the alkyl chain, is commonly adopted in single-wave ellipsometry experiments. A nondispersing value of 1.475 was tentatively assumed for the film refraction index to be consistent with the values presented in literature; such values are scattered mainly between 1.45 and 1.50.19,23,32 The simulations qualitatively reproduced the experimental δ∆ profiles, thus suggesting a monolayer thickness for both experimental conditions reported. A close look at the spacing between simulated δ∆ curves shows that the previously mentioned sample-to-sample variability corresponds to a thickness variability of (0.2 nm. The simulations could not account for negative δΨ values, likely due to the failure of the sharp interface assumption; a detailed analysis of the spectra is presented in the next section. To gain greater insight into the interface’s influence on δΨ values, we performed some measurements on SAMs formed by shorter chain alkanethiols (C6, C12). Ex situ data obtained on C6, C12, and C18 for an angle of incidence of 65° are shown in Figure 4. Data on C16 SAMs can be found in ref 33. In the figure, absolute δ∆ values, as well as the δΨ values in the near UV limit, increase with the length of the alkyl chain. More interestingly, irrespective of chain length,, all δΨ patterns converged to similar negative values (about -0.10°) in the nearIR limit. Large negative δΨ values were observed also in the case of L-cysteine, a small amino acid known to form compact thiolate SAMs.52 Representative ex situ patterns (incidence angle 70°)

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Figure 5. Representative ex situ SE patterns measured on L-cysteine and L-methionine SAMs on gold. Angle of incidence 70°.

are reported in Figure 5 to be compared with C18 ex situ data. Whereas δ∆ values amount to about one-fourth of the values obtained on C18, the δΨ spectrum in the near IR region converges to values similar to those found for alkanethiols SAMs. Large negative δΨ values were observed for other thiolate species as well.53 Much less negative values were obtained instead with regard to weakly bound molecules, such as Azurin (a metalloprotein)33 and L-methionine (another amino acid). Representative spectra for L-methionine are also shown in Figure 5 for comparison. 3.2. Analysis. An initial evaluation of the C18 SAMs thickness, dC18, was obtained by fitting the three-phase model used in Figure 3, for example, a nonabsorbing film sharply interfaced to the substrate, to ex situ data. Considering the well-known correlation between thickness and optical functions of ultrathin transparent films, we let the refraction index assume values in the 1.45-1.5 range,19,23,32 while neglecting dispersion. The model also neglected the conceivable anisotropy. Equivalent fits (dashed lines in Figure 6) were obtained for dC18 in the 2.3-2.1 nm range, respectively (mean-squared error,44 MSE ∼23.5). The substantial reproduction of δ∆ data confirms the transparency of the largest portion of the overlayer. A closer simulation of data, especially for δΨ, required the introduction of a four-phase optical system involving the substrate, the interface region, the transparent layer corresponding to alkyl chains, and the ambient medium.25,32 Application of four-phase effective models made it possible to find a close relation between negative δΨ values and interfacial optical properties. In the so-called Arwin model,25 the dielectric constant of the interface layer is calculated using the Bruggeman effective medium approximation (EMA) formula54

Au -  C18 -  + fC18 )0 fAu Au + 2 C18 + 2

(2)

where the parameters fAu and fC18 represent the so-called gold and film formal volume fractions, respectively (fAu + fC18 ) 1). For the sake of simplicity, after setting the alkyl chain layer refraction index to the value nC18 ) 1.475, best fit patterns were obtained with d′C18 ) 1.9 nm, d′int ) 0.3 nm, and fAu ) 0.24 ( 0.08 where d′C18 and d′int are the thickness of the alkyl chain and interface layers, respectively. The overall fit to the data

Figure 6. Comparison between experimental and calculated difference spectra for C18 ex situ SAMs. Symbols: data. Dashed lines (green): simulations obtained considering a sharp interface between a transparent layer and the substrate. Dotted lines (blue): simulations calculated with the EMA model. Continuous lines (red): simulations obtained with the SLR model. Insets: real and imaginary part of the interface layer effective dielectric constant obtained within the framework of EMA (dotted line) and SLR (continuous line) models, respectively. For details on models refer to the text.

(dotted lines in Figure 6) significantly improved (MSE ) 10.6) in particular with regard to the transition from positive to negative δΨ values. Real and imaginary parts of the resulting EMA are reported in the insets of Figure 6 (dotted curves). Allowing nC18 to assume values in the 1.45-1.50 range, equivalent fits were obtained with d′C18 between 2.0 and 1.8 nm and with negligible variations of both d′int and EMA. The fit to the data could be further improved by making EMA parameters (such as the volume fractions) λ dependent.25 The Arwin model, which helps to identify absorptions and provides an effective evaluation of the interface layer thickness, represented a solid starting point for implementing another approach. In this approach, which is consistent with the Kramers-Kronig relations, the effective interface dielectric function 2,eff was “directly’’ modeled as a superposition of Lorentzian resonances (hereafter the SLR model).32 Because of the correlation between thickness and optical parameters, in accordance to ref 32 an iterative approach was utilized to reproduce the data, aiming at a self-consistent adjustment of the thickness and resonance parameters. Consistent with the outcome of the EMA model, the simulation of the negative level in the high wavelength portion of the δΨ spectrum required a continuously increasing 2,eff in the 550-750 nm range. This behavior was achieved by introducing an intense oscillator centered in the near IR with a wide tail protruding into the visible range down to about 500 nm and a few, relatively broad oscillators in the red region. Furthermore, three weaker oscillators were necessary for the best reproduction of the data in the 250-350 nm range. Best results were obtained with the interface layer thickness d′′int in the 0.20-0.40 nm range. The continuous curves in Figure 6 were calculated with d′′int ) 0.35 nm and d′′C18 ) 1.80 nm for which d′′C18 is the alkyl

Thiolate Self-Assembled Monolayers on Au(111)

J. Phys. Chem. C, Vol. 112, No. 10, 2008 3903 TABLE 2: Summary of the Analysis of Measurements on ex situ C6 and C12 SAMs dchain (nm) n ) 1.50-1.45 C6a C6b C12a C12b

0.4 0.1 (1.1 ÷ 1.2) (0.7 ÷ 0.8)

dinterface (nm) 0.3 (0.3 ÷ 0.4)

total thickness (nm) 0.4 0.4 (1.1 ÷ 1.2) (1.0 ÷ 1.2)

a

Cauchy model. bSLR model using the interface optical functions determined for the C18 SAMs.

Figure 7. Comparison between in situ experimental data of Figure 3 and calculated spectra. (A,B) Data measured just after the first jumps of Figure 2; (C,D) layer saturation. Continuous lines (red): simulations obtained with the SLR model described in the text.

TABLE 1: Summary of the Analysis of SE Data on C18 SAMs.

Cauchyb EMAb SLRb SLRc SLRd

dchain (nm) n ) 1.50-1.45

dinterface (nm)a

total thickness (nm)

(2.1 ÷ 2.3) (1.8 ÷ 2.0) (1.7 ÷ 1.9) (2.2 ÷ 2.3) (2.2 ÷ 2.3)

0.3 (0.3 ÷ 0.4) (0.2 ÷ 0.3) (0.2 ÷ 0.4)

(2.1 ÷ 2.3) (2.1 ÷ 2.3) (2.0 ÷ 2.3) (2.4 ÷ 2.6) (2.4 ÷ 2.7)

aThe effective optical functions of the interface layer are shown in the insets of Figure 6. bEx situ data. cIn situ data: short incubation time (after the steps of Figure 2). dIn situ data: long incubation time (layer saturation).

chain layer thickness. The corresponding dielectric functions are plotted in the inset of Figure 6 (continuous curves). Variations of d′′int between 0.20 and 0.40 nm did not affect the overall shape of absorption bands, leading only to some intensity scaling of 2,eff. Not surprisingly, given the overall resemblance of experimental curves of Figure 3 the eff derived from ex situ data proved effective with minimal variations in reproducing in situ spectra (Figure 7). For spectra taken at short incubation times, the simulated curves (left panels), were obtained with din int ) 0.25 nm and din C18 ) 2.25 nm (with the obvious meaning of the super/subscripts). To reproduce the slightly larger negative δΨ values obtained at saturation (right panels), din int had to be slightly increased to 0.3 nm, which is closer to the value found for ex situ data. The results obtained on C18 SAMs after assuming that the refraction index lies in the 1.45-1.50 range are summarized in Table 1. After setting a definite value of nC18, the three models considered agree within less than 0.1 nm on the value of the total SAM ellipsometric thickness expressed as dC18, (d′C18 + d′int) and (d′′C18 + d′′int). This general agreement reflects the fact that information about film thickness is substantially supported by δ∆ values. The total thickness compares well with previous SE determinations32 and with the value of about 2.2 nm, measured by grazing incidence X-ray diffraction.55 An estimation of about 2.2 nm was also obtained by AFM on compact SAMS displaying a hexagonal (x3 × x3)R30° structure.16 In this “standing-up’’ phase, the alkyl chains are generally believed to be tilted ∼30-35° from the surface normal.14 Unlike IRSE,26,30 optical SE allows only an indirect evaluation of tilt angles. To this regard, we note that the thickness of the alkyl chain layer obtained in this work compares well to the height

of the 30° tilted chain calculated by the valence shell electron pair repulsion theory,56 that is, about 1.85 nm. The thickness of in situ SAMs also appears compatible with the formation of a single monolayer. We never observed the multilayer formation claimed in some reports.57 The slight excess thickness with respect to ex situ film is likely due to molecules weakly adsorbed on the first layer, then washed out after thorough rinsing.23,58 These findings demonstrate the overall consistency of our approach to the data analysis. The dielectric functions derived for the C18-Au interface were also used with perturbative variations to fit the data for SAMs of shorter chain alkanethiols. Thickness estimations obtained after assuming that the alkyl chain layer refraction index varies in the 1.45-1.50 range are summarized in Table 2. The reported values are compatible with previous SE determinations.32,59 As can be noted in Table 2, as far as short thiols such as C6 are considered, the distinction between interface and alkyl chain layers tends to lose its full meaning. For shorter molecules, the SAM tends to be seen by the light probe as a single “allinterface’’ layer. This fact came to full evidence in the analysis of ex situ L-cysteine SAMs. Simulations of SE spectra generated with the same interface properties found for C18 (not shown), led to a qualitative fit to the experimental results for which the L-cysteine SAM thickness values were about 0.4 nm, which is consistent with the formation of a single compact monolayer.52 While confirming the close association between negative δΨ values and interface-related absorptions, the outcome of the data analysis suggested that L-cysteine-Au SAM optical properties could be significantly different from the case involving alkanethiols. This is quite conceivable because L-cysteine layers assembly involves molecular networking typical of aminoacids,60-62 and we cannot exclude the role of molecular side groups in the substrate bonding process.60,63 4. Discussion The data analysis has demonstrated that “large’’ negative δΨ values in the 600-750 nm range are a direct consequence of the formation of the S-Au interface. The effective models that were utilized helped to identify the absorptions features related to this interface. To this regard, our results, thanks to the improved signalto-noise ratio, refine and extend early results obtained by Shi et al.32 In that work, which deals with the adsorption of alkanethiols on gold in a methanol solution, two oscillators were identified. One weak and broad oscillator, centered at 4-4.3 eV (e.g., about 300 nm), appears in substantial agreement with our higher energy absorption band. The second one, more intense and peaked at about 550 nm, agrees well with the onset of our broad absorption band in the low-energy region. In ref 32, the two oscillators appear to be separated by a region (about 380-470 nm) of very weak absorption, which is rather consistent with our results. Our observations also match the

3904 J. Phys. Chem. C, Vol. 112, No. 10, 2008 results of a recent reflection spectroscopy study on several alkanethiolate SAMs,35 where an absorption band centered in the near IR and protruding into the visible range down to 600650 nm was reported. This band was speculatively assigned to delocalized states involved in the formation of long range ordered SAMs. The Drude region is known to be strongly affected by microstructural factors modifying the scattering mechanism of s-p electrons45,64,65 In a recent study of gold colloidal nanoparticles,49 the limited particle size in relation to the electron mean free path was found to lead to size-dependent modification of the bulk dielectric function. The correction, ∆(λ,r) was expressed in terms of a difference between two Drude-like expressions.49 The enhanced electron scattering, decreasing the electron mean free path, involved a strong increase of the imaginary part in the 550-800 nm range,49 a condition similar in nature to our findings. On the other hand, intense absorptions in the 550-750 nm region were reported on unannealed ultrathin gold island films on mica66 and assigned to modifications of the surface plasmon band. In electrodeposited gold mesoparticles, absorption peak maxima were found to shift from 610 to 675 nm as the mean particle diameter increases from 42 to 74 nm.67 Similarly, a linear relationship was found between the absorption maximum of the longitudinal plasmon resonance in the 600-800 nm wavelength range and the mean aspect ratio of gold nanoparticles.68 Additionally, branched gold nanocrystals were found to exhibit a shape-dependent plasmon resonance that is redshifted by 130-180 nm from the spherical particle wavelength, for example, in the 680-830 range.69 A series of recent experiments on model short chain thiols (CH3S) report a new and interesting concept regarding the selfassembly process. In these papers, emphasis was placed on the organization of Au-thiolate complexes, rather than to the molecular assembly on an atomically flat substrate.18,70,71 More specifically, in ref 70 the herringbone structure was conceived as a source of reactive adatoms, which would be involved in initiating the thiolate self-assembly. Yu and coworkers18 pointed out a relation between self-organization and the movement of Au-S-R moieties between distinct local hollow sites on the surface. Very recently, Mazzarello et al.,71 combining structural data obtained with synchrotron radiation methods and first-principle molecular dynamics simulations, proposed a dynamic equilibrium between bridge site adsorption and a novel structure where two CH3S radicals are bound to an Au adatom lifted from the gold substrate. As a result, the interface is characterized by large atomic roughness. Although the extension of these findings and concepts to high coverage three-dimemsional alkanethiols self-assembled layers or to even more complex thiolate layers, such as those formed by L-cysteine, remains to be fully established, it is tempting to relate the low energy absorption band to morphological modifications at the nanoscale level driven by the formation of strong bonds and affecting the mean free path of free electrons and/or red-shifting the plasmonic mode in the near surface region. Concerning the interface absorption band at lower wavelengths, it should be recalled that in the formation of the thiolate species, the highest occupied molecular orbital (a π-orbital with sulfur 2p character) couples to the metal d-orbitals, giving rise to bonding and antibonding states. These states are expected to be strongly localized at the interface.63 Optical excitation from antibonding states, as well as from Au states modified by molecular adsorption, to empty states could provide an explanation for our observations. Antibonding

Prato et al. states are likely located at binding energies between the Fermi energy and the d-band threshold (about -2.5 eV). However, dedicated calculations are necessary to definitely assess this point. In view of the proposed relation between negative δΨ values and the formation of the thiolate interface, it is interesting to reconsider the C18 dynamic scans of Figure 2. δ∆ scans, providing information on the time evolution of the total SAM thickness, appear to be consistent with the accepted concept of the assembly kinetics of alkanethiol SAMs in terms of a fast initial process followed by a slower one.23,51,72,73 The amplitude of the first δ∆ (t) step (about 85% of steady values) would appear to be in substantial agreement with data of refs 72 and 73, whereas it may appear relatively large in comparison to results from other papers.19,23 However, it is well known that the relative weight of the fast versus slow adsorption process depends on a number of experimental factors including the morphology of the substrates, the amount of preadsorbed contaminants to be displaced by thiols, solvent effects, and the solution-phase fluid dynamics that eventually determines the effective molecular concentration at the substrate.19,74,75 Apparently conflicting reports can be often explained in terms of different experimental conditions. Here we note that a large contribution from the fast process is typical of relatively high concentrations (in the mM range) and flat, clean substrates. δΨ (t) curves in the red spectral region, provide direct access to the time evolution of the interface region during the deposition process. While the body of our data demonstrates that under the investigated conditions the layer displays the spectral features typical of the high coverage SAM already at very short incubation times, the downward drift that follows the initial steplike feature in panel D of Figure 2 demonstrates a slight increase of the interface layer thickness. This can be interpreted as a marker of the ongoing packing process, leading to the compact “standing-up’’ phase in consistence with the pictures emerging from scanning probe microscopy measurements.16,75 We finally note that the estimated interface layer thickness values reasonably compare with the Au-S distance (about 0.25 nm) determined for several alkanethiols by several type of measurements21,76,77 and by calculations.78-81 5. Conclusions We have presented an experimental study of the optical properties of thiolate SAMs deposited from the liquid phase on well-characterized, (111)-textured gold films investigated using spectroscopic ellipsometry in the 245-725 nm wavelength range. We focused on the C18 SAM model system while also considering a wider database of SAMs prepared with shorter chain alkanethiols and other organosulphur compounds. By detecting with great accuracy and reproducibility the tiny spectroscopic variations induced by films with thickness on the nm scale, we were able to unambiguously identify spectral features related to the alkyl chain and to the interface layer. δΨ(λ) difference spectra in particular showed a well-defined transition at about 500 nm from positive values to a negative level quasi-constant in the 600-750 nm range and poorly dependent on the angle of incidence. While such a negative level was almost unchanged for several alkanethiols (C18, C16, C12, C6) and even for more complex thiolate species (L-cysteine), it changed significantly to much less negative or even substantially null values in the case of loosely bound SAMs such as those formed by L-methionine or Azurin. Therefore, the δΨ(λ) negative level in the red region provided a spectral “fingerprint’’ of the dynamic formation of densely packed thiolate layers. Such

Thiolate Self-Assembled Monolayers on Au(111) a fingerprint could be useful for single wavelength ellipsometry users who generally utilize HeNe (632.8 nm) or diode lasers (670 nm). To quantitatively ascertain the “effective’’ dielectric properties of the Au-SAM interface, we compared our data with simulations derived from several effective models developed within the framework of the Fresnel approach. Simulations helped to identify the main optical features related to the thiolate interface, about 0.3 nm thick, and in particular an absorption band whose spectral weight increases regularly from about 500 nm toward the IR limit in close agreement with a recent work.35 We tentatively related the interface layer absorptions to nanoscale morphological modifications following the formation of Au thiolate moieties,71 which likely involve a reduction of the mean free path of Drude electrons and/or red shift the plasmonic mode in the near surface region. An in-depth theoretical examination of the optical properties of thiolate SAMs would be highly recommended. Acknowledgment. The authors extend their gratitude to G. Gonella for her assistance in the first stages of the experiment, A. Gussoni for his help in the deposition cell design, A. Morgante and L. Floreano for discussions, and S. Terreni and A. Gliozzi for their ongoing support. Funding by MIUR PRIN 2006020543_003 and by University of Genova (PRA_06) are also acknowledged. References and Notes (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (3) Castner, D. G.; Ratner, B. D. Surf. Sci. 2002, 500, 28. (4) Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 10714. (5) Balamurugan, S.; Ista, L. K.; Yan, J.; Lopez, G. P.; Fick, J.; Himmelhaus, M.; Grunze, M. J. Am. Chem. Soc. 2005, 127, 14548. (6) Houston, J. E.; Doelling, C. M.; Vanderlick, T. K.; Hu, Y.; Scoles, G.; Wenzl, I.; Lee., T. R. Langmuir 2005, 21, 3926. (7) D’Acunto, M. Nanotechnology 2006, 17, 2954. (8) Akkerman, H. B.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B. Nature 2006, 441, 69. (9) Maisch, S.; Buckel, F.; Effenberger, F. J. Am. Chem. Soc. 2005, 127, 17315. (10) Xu, S.; Laibinis, P. E.; Liu, G.-Y. J. Am. Chem. Soc. 1998, 120, 9356. (11) Barsotti, R. J.; O’Connell, M. S.; Stellacci, F. Langmuir 2004, 20, 4795. (12) Hu, Y.; Das, A.; Hecht, M. H.; Scoles, G. Langmuir 2005, 21, 9103. (13) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (14) Schreiber, F. J. Phys.: Condes. Matter 2004, 16, R881. (15) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (16) Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2001, 114, 4210. (17) Danisman, M. F.; Casalis, L.; Bracco, G.; Scoles, G. J. Phys. Chem. B 2002, 106, 11771. (18) Yu, M.; Bovet, N.; Satterley, C. J.; Bengio, S.; Lovelock, K. R. J.; Milligan, P. K.; Jones, R. G.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2006, 97, 166102. (19) Damos, F. S.; Luz, R. C. S.; Kubota, L. T. Langmuir 2005, 21, 602. (20) Duwez, A. S. J. Electron Spectrosc. Relat. Phenom. 2004, 134, 97. (21) Rousseau, R.; De Renzi, V.; Mazzarello, R.; Marchetto, D.; Biagi, R.; Scandolo, S.; del Pennino, U. J. Phys. Chem. B 2006, 110, 10862. (22) Tengvall, P.; Lundstro¨m, I.; Liedberg, B. Biomaterials 1998, 19, 407. (23) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (24) Isted, G. E.; Martin, D. S.; Smith, C. I.; LeParc, R.; Cole, R. J.; Weightman, P. Phys. Status Solidi C 2005, 12, 4012. (25) Mårtensson, J.; Arwin, H. Langmuir 1995, 11, 963. (26) Meuse, C. W. Langmuir 2000, 16, 9483.

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