Theoretical Calculations and Experimental Measurements of the

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Theoretical Calculations and Experimental Measurements of the Vibrational Response of p-NTP SAMs: An Orientational Analysis Francesca Cecchet,*,† Dan Lis,† Julien Guthmuller,‡,§ Benoıˆt Champagne,‡ Gregory Fonder,| Zineb Mekhalif,| Yves Caudano,† Alaa Addin Mani,†,⊥ Paul A. Thiry,† and Andre´ Peremans† Research Centre in Physics of Matter and Radiation (PMR), Laboratory of Theoretical Chemistry (LCT), and Laboratory of Chemistry and Electrochemistry of Surfaces (CES), FUNDP - UniVersity of Namur, Belgium ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: January 29, 2010

The vibrational response of p-nitrothiophenol (p-NTP) self-assembled monolayers (SAMs), on Pt and on Au, has been investigated by combining theoretical methods with vibrational spectroscopies. Experimentally, the vibrational signatures have been measured using infrared spectroscopy (IR), polarization-modulation reflection absorption infrared spectroscopy (RAIRS), and with sum frequency generation spectroscopy (SFG). Theoretically, density functional theory calculations (DFT) have predicted the molecular vibrations and have estimated their IR vectors and Raman tensors, necessary to simulate the spectra and therefore to interpret the vibrational signatures as well as to retrieve the molecular orientation. So, a tilt angle (ϑ) of 60° for the molecular 1,4-axis of p-NTP has been directly estimated for both Pt and Au SAMs by the polarizationdependent SFG data. Then, combining these results with RAIRS measurements has allowed to determine rotation angles (ξ) of 56° and 66° for p-NTP SAMs on Pt and Au, respectively. I. Introduction The vibrational response of an interface holds chemical and orientational information.1-7 Optical techniques probing the vibrational signatures of molecules adsorbed on surfaces are therefore efficient tools for quantifying the film orientation at the molecular scale. Among the vibrational spectroscopies, reflection absorption infrared spectroscopy (RAIRS) and sum frequency generation spectroscopy (SFG) have emerged in the characterization of thin organic layers adsorbed onto surfaces. The SFG being a more recent technique than RAIRS spectroscopy, its potentialities are still under expansion and need to be further explored.8-10 One main difference between those techniques is that infrared spectroscopy is based on a linear optical process, whereas SFG spectroscopy is based on a second-order nonlinear optical phenomenon. So, within the harmonic approximation, the RAIRS signal of each vibrational mode depends on the direction and amplitude of the dipole moment derivative with respect to the corresponding normal coordinate (IR vectors). Therefore, the spectral analysis shall in principle provide the molecular orientation at the surface. However, on metals the electric field parallel to the surface (s-polarized) is strongly screened in the mid-IR region. This confines the abilities of RAIRS spectroscopy to probe only the vibrational modes with a component of the IR vector perpendicular to the surface (Z-axis). On the other hand, in SFG spectroscopy the molecular orientation is determined from the second-order nonlinear optical susceptibility tensor, which is composed of the IR vectors and of the derivatives of the polarizability with respect to the normal * Corresponding author. Phone: +32 81 725487. Fax: +32 81 724718. E-mail: [email protected]. † Research Centre in Physics of Matter and Radiation (PMR). ‡ Laboratory of Theoretical Chemistry (LCT). § Current address: Institut fu¨r Physikalische Chemie, Friedrich Schiller Universita¨t Jena, Helmholtzweg 4, 07743, Jena, Germany. | Laboratory of Chemistry and Electrochemistry of Surfaces (CES). ⊥ Permanent address: AEC, Damascus P.O. Box 6091, Syria.

coordinates (Raman tensors). With the assumption of an achiral and azimuthally isotropic surface, only four components (ZZZ, ZXX, XZX, XXZ) of the susceptibility tensor remain independent, and these can be probed using four different polarization combinations of the incident and generated beams, namely, the ppp, pss, sps, and ssp sets [defined by the individual polarization of the sum frequency generated (SFG) beam, and of the incident visible (vis) and infrared (IR) beams, respectively]. This polarization-dependent SFG approach is already commonly employed to retrieve orientational data at insulating interfaces by combining sps and pss data sets probing the IR vector components parallel to the interface, with ppp and ssp data sets exploring the components of the IR vectors perpendicular to the surface. Instead, on metals, due to the screening of the IR field parallel to the surface (s-polarized), only the ppp and ssp polarization combinations (p-polarized IR beam) provide a detectable signal.5 This explains why a systematic application of polarization-dependent SFG measurements remains unusual on metals. In fact, a limited number of studies has used polarization-dependent SFG measurements at metal interfaces11-17 and few of them have led to extract orientational information.18-20 Up to now, SFG determination of the molecular orientation on metals was mainly carried out with a single ppp polarization configuration through the relative intensities of different vibrational modes. Unfortunately, this approach is not always applicable and therefore other strategies must be considered to provide accurate orientational information. To this aim, we show here how orientational data at metal interfaces can be inferred by analyzing the intensity ratio of a single vibration measured with two sets of polarizations, ppp and ssp. This allows overcoming the lack of information held in one polarization set. Moreover, a successful determination of the full molecular orientation is presented here by combining SFG with RAIRS measurements and theoretical calculations. To retrieve orientational information, the results of DFT calculations (vibrational frequencies, IR vectors, and Raman tensors) are used to simulate the linear and nonlinear vibrational responses of the interface

10.1021/jp911836k  2010 American Chemical Society Published on Web 02/17/2010

Vibrational Response of p-NTP SAMs

Figure 1. Representation of the p-NTP geometry and definition of the Euler angles (θ, ξ, φ) connecting the laboratory coordinate system (X, Y, Z) to the molecular coordinate system (x, y, z). The z axis corresponds to the molecular 1,4-axis.

as a function of the molecular orientation. The approach is applied to the molecule of p-nitrothiophenol (p-NTP; Figure 1), which is representative of more complex chemical architectures containing aromatic and nitro groups and which is of interest as model for nonlinear optical materials21 or biologically active materials.22 This represents also a technical challenge, because scanning the entire frequency window of molecular vibrations with SFG is still not regularly achieved due either to physical or technical limitations. Here, we have investigated vibrational modes down to 1290 cm-1, thus, measuring, with different polarization sets, vibrations that hold rather lower SFG activities than the most commonly investigated vibrations, such as the CH stretchings at 3000 cm-1 and the CO and the CN stretchings at 2000-2500 cm-1.23-27 This paper is organized as follows: section II describes the experimental part, the preparation of the sample and the SFG and RAIRS techniques, as well as the DFT simulation methods. Then, section III presents and discusses the results before conclusions are drawn in section IV. II. Experimental Section II.A. Sample Preparation. Self-assembled monolayers of p-nitrothiophenol were grown on Pt and on Au surfaces (Arrandee). The substrates are thin films of Au(111) deposited on borosilicate glass or Pt(111) deposited on Si and are specifically developed for scanning probe microscopy applications. AFM images of the substrates provide rms values of about 0.2 nm. The surfaces were cleaned in a UV ozone discharge for 15 min and rinsed in ethanol for 20 min before being employed. SAMs have been prepared by immersion of the substrates in a 1 mM ethanol solution of p-NTP overnight. The modified surfaces were abundantly rinsed in pure ethanol and dried under a stream of nitrogen before SFG and RAIRS investigations. p-NTP and absolute ethanol have been purchased from Aldrich and used as supplied. II.B. Sum-Frequency Generation Spectroscopy (SFG). The SFG setup is a homemade spectrometer with two optical parametric oscillators (OPOs) synchronously pumped by a picosecond Nd:YAG laser.28,29 The IR OPO is built around an AgGaS2 type-II crystal for generating IR frequencies from 2300 to 1000 cm-1, while the visible OPO is made around a BBO crystal. The measurements have been carried out in a nearly copropagative configuration, where the IR and vis incident beams make angles of 65 and 55° with respect to the normal of the sample surface. After spectral filtering by double grating monochromator, the SFG signal has been amplified by a

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4107 photomultiplier tube and monitored by an oscilloscope. The SFG spectra have been recorded for two sets of polarization combinations, namely, ppp and ssp sets. The polarization combination has been switched by rotating two half-wave plates placed on the paths of the incident visible and of the generated SFG beams in such a manner that all the other experimental conditions have been kept undisturbed (e.g., beam powers and focus, spatial overlapping of the IR and vis beams over the sample, signal detection). This has allowed attributing any intensity variation in the spectrum only to the polarization change. In our work the visible wavelength has been kept constant at 532 nm, while the IR frequency has been scanned over the spectral range from 1640 to 1290 cm-1. The SFG measurements have been performed at room temperature and under ambient atmosphere conditions, with a spectral resolution equal to 2 cm-1. II.C. Infrared Spectroscopy (Solid-State IR and RAIRS). Infrared spectra have been collected from a Bruker Equinox55 PMA37 equipped with a liquid nitrogen cooled MCT detector and a ZnSe photoelastic modulator. Transmission measurements of the solid state p-NTP molecule have been performed using KBr windows. For the investigation of the metal interface, the IR light, incident at an angle of 85°, has been modulated between s- and p-polarization at a frequency of 50 kHz. Signals generated from each polarization (Rs and Rp) have been synchronously detected by a lock-in amplifier and have been used to calculate the differential surface reflectivity (∆R/R) ) (Rp - Rs)/(Rp + Rs). The spectra have been taken by collecting 512 scans with a spectral resolution of about 2 cm-1. III. Theoretical and Modeling Section III.A. Theoretical Background. By assuming a Lorentzian response for the molecular vibrations and the harmonic approximation, the SFG spectra can be modeled according to eq 14

|

SFG (2)NR,eff Ippp/ssp ∝ χppp/ssp +

{∑ ν

ωSFG Ns c cos θSFG

1 ων(ων - ωIR - iΓν)

IR,k ∑ Fp/sSFG,iFp/svis,jFp/p

〈∑

i,j,k

lmn Tijk

l,m,n

∂Rlm(ωvis) ∂µn ∂Qν ∂Qν

〉}|

2

(1)

while the IR absorption on metallic surfaces can be represented by eq 2

ωIR Ns ∆RRAIRS ∝ R(p) c cos θIR

{[ Im

∑ ν

2 ∑ |FIR,k p | k

1 (ων - ωIR - iΓν)

〈(

∂µ

∑ Tnk ∂Qnν n

) 〉]} 2

(2)

χ(2)NR,eff (eq 1) represents the effective nonresonant susceptibility of the substrate-adsorbate system. ∂Rlm(ωvis)/∂Qν (eq 1), ∂µn/ ∂Qν (eqs 1 and 2) are the derivatives of the molecular polarizability and of the molecular dipole moment with respect to the Vth vibrational normal coordinate, ωV and ωIR are the vibrational frequency of mode V and of the infrared beam, respectively, and c is the speed of light. The Fresnel factors, SFG,i vis,j IR,k ,Fp/s (eq 1), and Fp/p (eqs 1 and 2), describe the coupling Fp/s strength between the interfacial nonlinear susceptibility and each of the three coherent beams. They represent the amplitude of

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Figure 2. Top: representation of atomic displacements (black arrows), IR vectors (red), and Raman tensors (green and red stemming for positive and negative eigenvalues, respectively) of the vibrational modes of p-NTP. Bottom: simulated and experimental IR spectrum of bulk p-NTP.

each component of the total electric field on the surface at a n given frequency (SFG, vis, IR). Tlmn ijk (eq 1) and Tk (eq 2) rotate the molecular tensors to the surface coordinate system. Γν (eqs 1 and 2) is the half width at half-maximum of the Lorentzian band shape. Ns (eqs 1 and 2) is the density of molecules on the surface. The brackets 〈〉 (eqs 1 and 2) represent an average over the orientation distribution of the ad-molecules, which entails the assumption of an azimuthally isotropic ad-layer.4,30 Here, the assumption of an azimuthally isotropic distribution is justified because the macroscopic probed area over the (111) surface integrates a large number of microscopic domains, each of them having a distinct azimuthal angle resulting from the self-assembled process. In the same theoretical frame and after averaging over all possible orientations of the molecule, the IR absorption intensity of the νth vibration is obtained from the expression

IIR,ν ∝

∑

n)x,y,z

( ) ∂µn ∂Qν

2

(3)

while the total Raman intensity is given by31

IRaman,ν ∝ 45(Rν)2 + 7(γν)2

(4)

where Rν and γν are the spherical part and the anisotropy of the ∂Rlm(ωVis)/∂Qν tensor, respectively. III.B. Spectra Simulation. The SFG, IR, and PM-RAIRS spectra of p-NTP have been analyzed and simulated according to eqs 1-4 with a homemade program employing quantum chemical calculations performed on an isolated molecule using the GAUSSIAN 03 program.32 Thus, to simulate the experimental spectral intensities and the spectral profiles, the input parameters encompass the frequencies, the Raman tensors ∂Rlm(ωvis)/∂Qν and IR vectors ∂µn/∂Qν for each vibrational mode. Figure 2 shows a 3D representation of the atomic displacements (black arrows) of the IR vectors (red) and of the Raman tensors (green and red stemming for positive and negative eigenvalues, respectively) of the vibrational modes of p-NTP, which occur in the frequency region between 1640 and 1290 cm-1. These calculations have been performed at the density functional theory (DFT) level of approximation with the B3LYP exchangecorrelation functional and the 6-311++G(d,p) basis set. The ground state geometry of the molecule was optimized under the condition that the residual forces are smaller than 10-5 a.u. Thereafter, harmonic vibrational frequencies and normal coordinates were analytically determined at the same level of theory. To correct for the lack of anharmonicity and the approximate treatment of electron correlation,33 the frequencies have been scaled by a factor of 0.97. The derivatives of the dipole moment vector ∂µn/∂Qν with respect to the normal coordinates Qν have

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TABLE 1: Components of the IR Vectors (10-3 a.u.) and Raman Tensors (a.u.) of p-NTP Vibrations in the Molecular Coordinate System (Figure 1)a

a

vibration/cm-1

δµx/δQ

δµz/δQ

δRxx/δQ

δRxz/δQ

δRyy/δQ

δRzz/δQ

(g) 1304 (f) 1321 (e) 1393 (d) 1463 (c) 1521 (b) 1574 (a) 1585

1.654 0.069 1.303 -0.081 11.038 0.519 -6.386

0.113 -17.231 0.287 -4.155 0.059 7.938 0.936

-0.0031 0.0695 0.0000 -0.0042 0.0000 0.0979 0.0065

0.0111 -0.0075 0.0442 0.0015 -0.0627 -0.0067 0.1318

-0.0015 -0.0110 -0.0003 -0.0097 -0.0005 0.0291 0.0023

-0.0698 1.3528 0.0035 -0.1146 0.0163 -0.7706 -0.0628

Due to symmetry reasons, the y components of the IR vectors as well as the xy and yz components of the Raman tensors are equal to zero.

TABLE 2: Square Modulus of the Fresnel Factors Computed for Pt and Au SFG,i vis,j IR,k 2 |Fp/s × Fp/s × Fp/p | [vis ) 532.0 nma (55°)d; IR ) 1350 cm-1 b (65°)d; SFG ) 496.4 nmc (55.6°)d]

tensor elements

Pt

Au

polarization set

ZZZ XXZ XZX ZXX YYZ ZYY YZY

8.5 × 100 7.2 × 10-2 1.2 × 10-3 1.1 × 10-3 1.4 × 10-2 9.0 × 10-5 1.0 × 10-4

4.7 × 100 4.9 × 10-1 1.1 × 10-3 7.9 × 10-4 1.2 × 10-1 6.0 × 10-5 1.1 × 10-4

ppp ppp ppp ppp ssp pss sps

a Pt: n ) 2.080 and k ) 3.630; Au: n ) 0.465 and k ) 2.400. b Pt: n ) 6.961 and k ) 27.700; Au: n ) 7.477 and k ) 43.600. c Pt: n ) 1.960 and k ) 3.420; Au: n ) 0.910 and k ) 1.850. d Beams direction with respect to the surface normal. nSAM ) 1.0. Optical constants n and k of metals have been interpolated using the values from Palik.53

been evaluated at the ground state geometry from the first-order nuclear coordinate derivatives of the Kohn-Sham orbitals. On the other hand, the Raman tensors ∂Rlm(ωvis)/∂Qν have been computed by using a two-point numerical differentiation procedure in which the dynamic polarizability tensor at 532 nm is calculated for the distorted structures resulting from the addition or subtraction of a finite displacement of the normal coordinate to the equilibrium geometry. The values of the Raman tensors and IR vectors components of each vibration occurring in the measured spectral region are summarized in Table 1. Similar procedures and approximations have already proved to be highly reliable for predicting and interpreting vibrational spectra.18,34-48 In particular, we have recently shown that this approach is efficient for predicting the molecular properties responsible for the SFG intensities.49 Such firstprinciples approaches constitute with molecular dynamics (MD) techniques the two categories of theoretical methods to simulate and interpret SFG spectra. MD is however better suited for studying systems with numerous structural configurations like water surfaces, whereas organic films with more rigid species are better treated with first-principles methods.50,51 In the simulation of the SFG and PM-RAIRS intensities, it is assumed that all the molecules adsorbed on the surface have the same orientation defined by the Euler angles ϑ and ξ (Figure 1). This can be justified because the substrates used in this work have a very low roughness (rms ) 0.2 nm), which favors the growth of directionally ordered monolayers. Moreover, the good correlation between the SFG and the PM-RAIRS measurements (as it will be shown later) implies narrow angular distributions.18 The Fresnel factors have been computed in the framework of the three-layer model52 by using the dielectric constant for Pt and for Au substrates at the wavelengths involved in the experiments,53 considering a refractive index of SAMs equal to n ) 1.054 and including the angles of the beams with respect to the surface normal (see Experimental Section). It is important to note that the magnitude of the refractive indices affects the simulated intensities and this influences the quantitative analysis of the experimental data. Here, we have calculated that an

increase by 20% of the refractive index of p-NTP SAMs results in a decrease of the tilt angle by about 6°. This is consistent with the uncertainty correlated to all techniques, which require the use of the refractive index for the calculation of structural parameters, as shown by Eisert et al. for second harmonic generation experiments (SHG).55 To give an order of magnitude of the Fresnel factors contributing to the SFG intensity (eq 2), Table 2 reports the square modulus of the Fresnel factor products as well as the parameters used for their computation. IV. Results and Discussion IV.A. DFT Calculations and IR Spectra of Bulk p-NTP. Figure 2 represents the calculated frequencies, normal modes, IR vectors, and Raman tensors of the seven vibrations of p-NTP located in the experimental frequency range. The theoretical frequencies are compared to the experimental ones in Table 3. The vibrations occurring at 1585 cm-1 and at 1574 cm-1 correspond to the superposition of CC stretchings with CH inplane bendings of the phenyl ring [rCC-δCH (a) and (b)]; the vibrations showing up at 1521 and 1321 cm-1 are assigned to the antisymmetric [r-NO2 (c)] and the symmetric [r+NO2 (f)] stretchings of the NO2 group, while those at 1463, 1393, and 1304 cm-1 are due to the CH bending modes of the phenyl ring [δCH (d), (e), and (g)]. The IR spectrum of p-NTP, which has been simulated from the DFT intensities by assuming a Lorentzian broadening with a fwhm equal to 12 cm-1, shows a good agreement with the experimental IR spectrum of bulk p-NTP, displayed in the bottom part of Figure 2. It can be noted that the experimental bands at 1420 and 1315 cm-1 are not visible in the simulated spectrum. Nevertheless, they were assigned to the modes (e) and (g) on the basis of their frequency positions. Overall, the experimental frequencies have been reproduced by the calculations with a mean deviation of 14 cm-1, which is the expected accuracy at this level of approximation. In particular, the overestimation of the frequency gap between the r-NO2 and r+NO2 vibrations calculated at 200 cm-1 can be attributed to the neglect of interactions with the

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TABLE 3: Theoretical and Experimental Data of p-NTP Vibrational Spectra mode

DFT IR int. a

DFT Raman int. a

rCC-δCH (a) rCC-δCH (b) r-NO2 (c) δCH (d) δCH (e) r+NO2 (f) δCH (g)

74 112 217 31 3 528 5

59 996 12 23 6 3181 9

DFT (cm-1)

b

1585 1574 1521 1463 1393 1321 1304

IR bulk (cm-1) b

RAIRS/Pt (cm-1) b

RAIRS/Au (cm-1) b

1595 1577 1508 1474 1420 1344 1315

1593 1573 1514

1593 1574 1522 1464

1346

1346

SFG/ Pt (cm-1) b

SFG/Au (cm-1) b

1572

1580

1344

1346

a IR (km/mol) and Raman (Å4/amu) intensities from DFT calculations. b Vibrational frequencies from DFT calculations, from experimental IR spectra of bulk p-NTP, from RAIRS spectra of p-NTP SAMs on Pt and on Au, and from SFG spectra of p-NTP SAMs on Pt and on Au.

Figure 3. SFG spectra of p-NTP SAMs on Pt (left) and on Au (right) at the metal air interface, recorded with ppp (top) and ssp (bottom) sets of polarization, respectively.

neighboring molecules and the surface in the DFT calculations. However, to simulate the effect of the environment, we have carried out additional calculations performed with the polarizable continuum model (PCM), which have shown that the r-NO2 and r+NO2 frequency gap decreases to 186 cm-1, which is in better agreement with the experimental value of 164 cm-1. IV.B. Polarization-Dependent SFG Measurements: Tilt Angle T. Figure 3a,b shows the SFG spectra of p-NTP SAMs on Pt and on Au, which have been recorded for the ppp (top panel) and ssp (bottom panel) sets of polarizations. On both metals, two resonant SFG signals have been detected. On Pt, the vibrations show up at 1572 cm-1 and at 1344 cm-1, while on Au they are centered at 1580 cm-1 and 1346 cm-1. DFT calculations have predicted two vibrations around 1580 cm-1 [the rCC-δCH (a) and rCC-δCH (b) modes in Figure 2]. In the spectra, considering the much larger Raman intensity predicted for mode (b), the peak at 1572/1580 cm-1 can be ascribed to the rCC-δCH (b) vibration. Note that a frequency shift may be observed from Au to Pt. Nevertheless, peak (b) appears to be particularly broad in the ppp configuration. Two reasons might be invoked to explain that: i) due to the mutual orientation of the Raman tensor and IR vector, and depending on the molecular orientation, the rCC-δCH (a) vibration can also give rise to a small SFG signal; ii) moisture water absorption occurs in this spectral region causing thereby fluctuations in the incident IR beam intensity. A fitting of the experimental profile with two components (or even three if a negative absorption for water is taken into account) in order to separate the different contributions is, however, very hazardous, as too many variable parameters would occur in the fitting procedure. The second remarkable SFG signature is assigned to the r+NO2 (f) vibration. In both sets of polarization, its intensity is

at least 1 order of magnitude larger than the one of rCC-δCH (b). This is ascribed to the larger IR vector and Raman tensor of r+NO2 (f) compared to that of rCC-δCH (b), as predicted by DFT calculations (Tables 1 and 3). As discussed in the Introduction, on metals the orientation of molecular films is commonly obtained from the SFG intensity ratio between two vibrations measured with ppp polarization. Unfortunately, this approach may not be useful with the SFGactive vibrations (b) and (f) of p-NTP SAMs. In fact, since the IR vector and Raman tensor components of rCC-δCH (b) and of r+NO2 (f) are mostly oriented along the same molecular 1,4axis (Figure 2), their peak intensity ratio remains nearly constant for any molecular orientation. It, therefore, follows that the molecular orientation of p-NTP cannot be inferred from the analysis of the relative SFG intensity of these vibrational modes. The measure of the SFG activity of p-NTP SAMs on metals with two polarization sets, namely, ppp and ssp, overcomes the above limitation. Indeed, switching from ppp to ssp results in a strong decrease in the total SFG intensity on both metals, and this behavior is related to the variation in the Fresnel factors to which depends the SFG intensity (eq 1).4 Table 2 lists the magnitude of the square modulus of the Fresnel factors calculated for ppp, ssp, pss, and sps sets of polarization on Pt and on Au. One can notice that the pss and sps polarization sets exhibit the lowest Fresnel factors because the electric field of the IR beam parallel to the surface is strongly damped. On the other hand, the ppp and ssp polarization sets provide much stronger signals, which arise from the fact that the IR electric field has a component normal to the surface. The same reason explains that the ZZZ and XXZ terms largely dominate over the XZX and ZXX terms in the ppp polarization set. Furthermore, in the case of ssp configuration, the Fresnel factors for the YYZ

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Figure 4. Theoretical dependence of the ppp to ssp intensity ratios on the tilt angle ϑ for the r+NO2 (f) and rCC-δCH (b) modes, while the rotation angle is kept equal to ξ ) 0° (-b-) and to ξ ) 90° (-2-) on Pt and on Au, respectively (top: entire curves; bottom: expanded views).

Figure 5. (a) Experimental (top) and simulated (bottom) RAIRS spectra of p-NTP SAMs on Pt (left) and on Au (right), respectively; (b) peak intensity ratio for r-NO2 (c)/r+NO2 (f), rCC-δCH (a)/ r+NO2 (f), r-NO2 (c)/rCC-δCH (b), and rCC-δCH (a)/rCC-δCH (b), computed for the tilt angle ϑ ) 60°, as a function of the rotation angle ξ.

component is between 1 and 2 orders of magnitude smaller than the one weighing the ZZZ component in the ppp polarization set (see Table 2). Because the ppp and ssp sets probe different components of the second-order nonlinear susceptibility, the ppp to ssp intensity ratio depends on the molecular orientation and allows assessing it. Figure 4 shows the ppp/ssp intensity ratio predicted by simulation for r+NO2 (f) and rCC-δCH (b) as a function of the tilt angle, ϑ, while the rotation angle is kept equal to ξ ) 0° or to ξ ) 90°. The ppp/ssp experimental ratio for r+NO2 (f) is equal to 45 on Pt and to 10 on Au; the ppp/ssp ratio recorded for rCC-δCH (b) is 7 on Au, while it has not been measured on Pt because no resonant signal has been detected with the ssp configuration (this signal being below our detection threshold). The comparison of the experimental with the theoretical data of the ppp/ssp ratio of r+NO2 (f) leads to estimate the tilt angle of p-NTP SAM to about ϑ ) 60° on both metals (Figure 4a). This result is further corroborated by the agreement of the orientation obtained from the rCC-δCH (b) ratio, which corresponds to ϑ ) 61° on Au (Figure 4b). Moreover, these results validate the assignment of the peak at 1580 cm-1 to vibration rCC-δCH (b). Indeed, calculations of the ppp/ssp ratios for mode rCC-δCH (a) and comparison with the experimental ratio would

provide a ϑ angle close to 10° for ξ ) 0°, while for ξ ) 90°, the (a) peak does not show up for any ϑ angle. The dependence of the ppp/ssp ratio on the rotation angle ξ is less pronounced. Indeed, for r+NO2 (f) on both metals, this ratio is relatively independent of the rotation angle for all ranges of ϑ values (Figure 4a). For rCC-δCH (b), the ratio shows a clear dependence on the rotation angle for ϑ comprised between 5 and 30°, while for tilt angles above 30° it becomes almost independent of ξ (Figure 4b). Therefore, because the tilt angle ϑ has been found with a value of 60°, in this specific conformation, the analysis of the ppp/ssp intensity ratio does not allow an unambiguous determination of ξ. The full molecular orientation can therefore be obtained by combining SFG with RAIRS measurements, as will be shown in section IV.C. IV.C. RAIRS Measurements: Rotation Angle ξ. Figure 5a shows the experimental RAIRS spectra of p-NTP SAMs on Pt (left-hand side) and on Au (right-hand side), recorded between 1650 and 1250 cm-1. On Pt, four main vibrations are observed: the rCC-δCH modes show up at 1593 cm-1 (a) and at 1573 cm-1 (b), while the r-NO2 (c) and r+NO2 (f) stretchings occur at 1514 and 1346 cm-1, respectively. On Au, the RAIRS spectrum exhibits five main vibrations, namely, the rCC-δCH vibrations at 1593 cm-1 (a) and at 1574 cm-1 (b), the r-NO2

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(c) and r+NO2 (f) modes at 1522 cm-1 and at 1346 cm-1, and the δCH (d) bending at 1464 cm-1. The weak intensity of the δCH (d) vibration mode is consistent with the low IR activity predicted by DFT calculations (Tables 1 and 3) and observed in the IR spectrum of bulk p-NTP (Figure 2, bottom) where it appears as a weak shoulder of the r-NO2 (c) vibration. Note that, owing to a very small intensity estimated by the calculations, RAIRS spectra do not exhibit the δCH bending signatures expected at 1393 cm-1 (e) and at 1304 cm-1 (g). Because the IR vectors of modes (b) and (f) are oriented parallel to the molecular 1,4-axis and the IR vectors of modes (a) and (c) are almost perpendicular to the molecular 1,4-axis, their relative RAIRS intensities depend on the tilt and rotation angles. Therefore, the analysis of these intensity ratios is expected to provide information on the molecular orientation. However, an efficient use of these data requires taking advantage of the already available structural information deduced from SFG, that is, ϑ ) 60°. Indeed, as simulated from DFT data, without this information, RAIRS analysis would provide many combinations of ϑ and ξ, from ϑ > 30° and ξ > 0°, which all reproduce the experimental intensities of RAIRS spectra. Figure 5b displays the calculated peak-intensity ratios for pairs of vibrational modes having perpendicular IR vectors, namely, r-NO2 (c)/r+NO2 (f), rCC-δCH (a)/r+NO2 (f), r-NO2 (c)/rCCδCH (b), and rCC-δCH (a)/rCC-δCH (b), when the tilt angle is fixed to ϑ ) 60° and the rotation angle ξ is scanned between 0 and 90°. For p-NTP SAM on Au, the experimental r-NO2 (c)/r+NO2 (f), rCC-δCH (a)/r+NO2 (f), r-NO2 (c)/rCC-δCH (b), and rCC-δCH (a)/rCC-δCH (b) intensity ratios are equal to 0.27, 0.12, 0.82, and 0.36, respectively. Fitting the theoretical data to the experimental values defines a value of 66° for the rotation angle of p-NTP SAMs on Au and of 56° for p-NTP SAMs on Pt (data not shown). These latter results call for additional considerations on the experimental and theoretical analysis carried out in this work: (i) the RAIRS data alone exclude an upright orientation of the molecular 1,4-axis (because the spectra can be reproduced only with ϑ > 30°), and this corroborates once more the assignment of the SFG peak at 1580 cm-1 to vibration rCC-δCH (b) and confirms the reliability of the close agreement in the ϑ angles, as obtained from the ppp/ssp ratios; (ii) the ξ angles obtained from the RAIRS peak ratios show a narrow distribution ((5°; Figure 5b), which substantiates the quantitative parameters obtained from the DFT calculations and supports our analysis and its results. V. Conclusions The molecular orientation of p-nitrothiophenol self-assembled monolayers at the metal interface has been fully determined through the combination of polarization-dependent SFG spectroscopy, polarization-modulation IR reflection absorption spectroscopy, and density functional theory calculations. Density functional theory calculations have been used to evaluate the vibrational frequencies and normal modes as well as the IR vectors and Raman tensors necessary to simulate SFG and RAIRS spectra. On the one hand, a tilt angle of 60°, on both Au and Pt, has been estimated from the SFG ppp to ssp intensity ratios of the r+NO2 (f) and rCC/δCH (b) modes. Then, a rotation angle of 56° (66°) for p-NTP on Pt (Au) has been obtained from using the RAIRS data and, in particular, from the peakintensity ratios of pairs of vibrational modes having perpendicular IR vectors. These absolute orientation of p-NTP SAM shows a good agreement with previous studies, indicating that short thioaromatic SAMs are strongly flat-lying to the surface.56-60

Cecchet et al. This validates the application of polarization-dependent SFG spectroscopy to the investigation of the molecular orientation on metals, despite the achievable polarization sets are limited to ppp and ssp and despite the Fresnel factors strongly weight the total SFG intensity. Moreover, this approach is further validated by the high correlation between SFG, RAIRS, and DFT data. Acknowledgment. F.C., Y.C., and A.P. are a postdoctoral researcher, research associate, and research director of the Fund for the Scientific Research F.R.S.-FNRS. D.L. thanks the FRIA for his Ph.D. student fellowship. A.M. thanks the F.R.S.-FNRS for his financial support under the convention No. 2.4.509.04 and the AECS. J.G. thanks the F.R.S.-FNRS for his postdoctoral grant under the conventions No. 2.4.509.04.F. The calculations were performed on the Interuniversity Scientific Computing Facility (ISCF, University of Namur, Belgium) for which the authors gratefully acknowledge the financial support of the F.R.S.-FRFC and the “Loterie Nationale” under Contract No. 2.4.617.07.F and of the FUNDP. References and Notes (1) Kudelski, A. Vib. Spectrosc. 2005, 39, 200. (2) Shen, Y. R. Pure Appl. Chem. 2001, 10, 1589. (3) Rao, Y.; Comstock, M.; Eisenthal, K. B. J. Phys. Chem. B 2006, 110, 1727. (4) Wang, H.-F.; Gan, W.; Lu, R.; Rao, Y.; Wu, B.-H. Int. ReV. Phys. Chem. 2005, 24, 191. (5) Hore, D. K.; Beaman, D. K.; Parks, D. H.; Richmond, G. L. J. Phys. Chem. B 2005, 109, 16846. (6) Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreey, R. L. Anal. Chem. 2006, 78, 3104. (7) Tanaka, M.; Young, R. J. J. Mater. Sci. 2006, 41, 963. (8) Shen, Y. R. Nature 1989, 337, 519. (9) Guyot-Sionnest, P. Surf. Sci. 2005, 585, 1. (10) Smith, J. P.; Hinson-Smith, V. Anal. Chem. 2004, 76, 287 A.. (11) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (12) Baldelli, S.; Markovic, N.; Ross, P.; Shen, Y.-R.; Somorjai, G. A. J. Phys. Chem. B 1999, 103, 8920. (13) Romero, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 11936. (14) Lu, X.; Shephard, N.; Han, J.; Xue, G.; Chen, Z. Macromolecules 2008, 41, 8770. (15) Beattie, D. A.; Fraenkel, R.; Winget, S. A.; Petersen, A.; Bain, C. D. J. Phys. Chem. B 2006, 110, 2278. (16) Lu, X.; Shephard, N.; Han, J.; Xue, G.; Chen, Z. Macromolecules 2008, 41, 8770. (17) Bordenyuk, A. N.; Weeraman, C.; Yatawara, A.; Jayathilake, H. D.; Stiopkin, I.; Liu, Y.; Banderskii, A. V. J. Phys. Chem. C 2007, 111, 8925. (18) Richter, L. J.; Yang, C. S. C.; Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Yao, Y.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547. (19) Lis, D.; Peremans, A.; Sartenaer, Y.; Caudano, Y.; Mani, A. A.; Dreesen, L.; Thiry, P. A.; Guthmuller, J.; Champagne, B.; Cecchet, F. J. Phys. Chem. C 2009, 113, 9857. (20) Cecchet, F. ; Lis, D. ; Guthmuller, J. ; Champagne, B. ; Caudano, Y. ; Silien, C. ; Mani, A. A; Thiry, P. A.; Peremans, A. ChemPhysChem 2010, published online Jan 27, 2010. (21) Perreault, F.; Champagne, B.; Soldera, A. Chem. Phys. Lett. 2007, 440, 116. (22) Orth, R. N.; Kameoka, J.; Zipfel, W. R.; Ilic, B.; Webb, W. W.; Clark, T. G.; Craighead, H. G Biophys. J. 2003, 85, 3066. (23) Masashi, N.; Wtanabe, K.; Matsumoto, Y. J. Phys. Chem. C 2009, 113, 11712. (24) Kweskin, S. J.; Rioux, R. M.; Habas, S. E.; Komvopoulos, K.; Yang, P.; Somorjai, G. A. J. Phys. Chem. B 2006, 110, 15920. (25) Wagner, S.; Leyssner, F.; Ko¨rdel, K.; Zarwell, S.; Schmidt, R.; Weinelt, M.; Ru¨ck-Braun, K.; Wolf, M.; Tegeder, P. Phys. Chem. Chem. Phys. 2009, 11, 6242. (26) Bozzini, B.; D’Urzo, L.; Mele, C.; Busson, B.; Humbert, C.; Tadjeddine, A. J. Phys. Chem. C 2008, 112, 11791. (27) Humbert, C.; Busson, B.; Six, C.; Gayral, A.; Gruselle, M.; Villain, F.; Tadjeddine, A. J. Electroanal. Chem. 2008, 621, 314. (28) Mani, A. A.; Dreesen, L.; Humbert, C.; Hollander, P.; Caudano, Y.; Thiry, P. A.; Peremans, A. Appl. Surf. Sci. 2002, 502-503, 261. (29) Mani, A. A.; Dreesen, L.; Hollander, P.; Humbert, C.; Caudano, Y.; Thiry, P. A.; Peremans, A. Appl. Phys. Lett. 2001, 79, 1945.

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