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Directly Probing Molecular Ordering at the Buried Polymer/Metal Interface 2: Using P‑Polarized Input Beams Xiaolin Lu,†,* Gi Xue,‡ Xinping Wang,† Jianglong Han,‡ Xiaofeng Han,§ Jeanne Hankett,§ Dawei Li,‡ and Zhan Chen§,* †

Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou, 310018, China ‡ Department of Polymer Science, Nanjing University, Nanjing, 210093, China § Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Previously, we developed several methods to use sum frequency generation (SFG) vibrational spectroscopy to probe buried polymer/metal interfaces in situ by depositing polymer films with different thicknesses on metal surfaces or sandwiching a polymer thin film between a metal surface and a fused silica window. In this study, we developed a new and easier method to directly probe the polymer/ metal interface by collecting ppp SFG spectra using a poly(ethyl methacrylate) (PEMA)/silver (Ag) interface as an example. We confirmed that for a thin polymer film on metal, the dominant SFG signals were contributed from the polymer surface in air and/ or the polymer metal interface, while the contribution from the polymer bulk could be ignored. Previously, we showed that the ssp spectra were contributed by both the polymer/air and polymer/metal interfaces. Here we demonstrated that the SFG ppp spectra were dominated by signals from the buried polymer/metal interface from which the structural information on the buried interface can be deduced. This method to probe the buried polymer/metal interface via SFG is relatively simple compared to our previous sample preparation techniques and/or data analysis methods. (PMMA)/silver (Ag) interface as an example.17 We also reported an approach to directly probe the buried polymer/ metal interface by sandwiching a thin polymer film between a metal surface and a fused silica window.18 In this paper, we present a simpler means of directly investigating the buried polymer/metal interface by collecting ppp SFG spectra using a poly(ethyl methacrylate)/Ag interface as an example. We also discuss several issues which were not discussed in detail in our previous SFG studies on polymer/metal interfaces. In particular, we (1) confirm that for a thin amorphous polymer film on metal, the dominating SFG signals come from the polymer surface and/or the polymer/metal interface rather than the polymer bulk and (2) discuss why ppp SFG spectral intensities detected from polymer films on metal in air do not have substantial film thickness dependence, different from those of ssp spectra. From these studies, we have developed a simpler method to directly probe the polymer/metal interface, without the need to vary the polymer film thickness or to use a fused silica cap.

1. INTRODUCTION Probing and understanding molecular structures at polymer/ metal interfaces can provide important information for optimizing desired properties of these interfaces in many applications such as microelectronics and anticorrosion coatings.1,2 Since a polymer/metal interface is buried, the best way to understand its molecular structure is to directly probe it in situ using nondestructive characterization techniques without breaking the interface into two surfaces. Over the past several decades, surface spectroscopic techniques have been rapidly developed to be able to study the molecular interfacial structures of organic materials. Examples include surface enhanced Raman spectroscopy (SERS),3−6 near edge X-ray fine structure spectroscopy (NEXFS),7,8 and sum frequency generation (SFG) vibrational spectroscopy.9−16 SERS has been extensively applied to examine many different materials deposited onto metal surfaces.3−6 The SERS signal deteriorates exponentially with respect to the distance from a metal surface and therefore obtains information on the polymer/metal interface. However, SERS does probe the entire film instead of the polymer/metal interface specifically. We desired to probe interfacial molecules only and therefore used SFG as our characterization technique. We previously developed a methodology to use SFG to probe polymer/metal interfaces by systematically varying the polymer film thicknesses using poly(methyl methacrylate) © 2012 American Chemical Society

Received: May 29, 2012 Revised: July 6, 2012 Published: July 24, 2012 6087

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direction for the sum frequency, visible, and infrared fields, respectively. It has been shown for a centro-symmetric medium this bulk electric-quadrupole contribution is smaller than or of the same order of the magnitude as the surface electric dipolar contribution.45−47 As for the case of the amorphous polymers like the PEMA thin films discussed in this paper, the random arrangement of the polymer chains in the bulk may further reduce the bulk electric-quadrupole contribution. For the SFG signal collected in the reflection geometry as used in this research, the bulk contribution is usually much smaller than the surface contribution. Nevertheless, it is important to evaluate the bulk signal contribution. The evaluation of the bulk contribution for polymer films has been completed by comparing the SFG spectral features and intensities detected in the reflection direction with those collected in the transmission direction. The bulk contribution was found negligible in the SFG reflection experimental geometry.46 This evaluation is based on the markedly different coherent length scales in the reflection and transmission directions: the much longer coherent length in the transmission direction leads to stronger bulk contributions in the SFG transmission spectra, while the much shorter coherent length in the reflection direction leads to negligible bulk contribution in SFG reflection spectra. Clearly, if the bulk contribution cannot be ignored, then different spectral features in the SFG transmission and reflection spectra would be observed. In fact in this approach the underlying assumption was adopted: the light fields across the polymer film within the coherent length were more or less constant. This assumption is generally valid for the polymer films prepared on glass or silica substrates because their refractive indices are similar to those of polymers. In the case of PEMA thin films on Ag substrates, the above approach may need to be discussed further because the light fields can vary rapidly inside the polymer films as well as change their propagation phases due to the multiple reflection effect. However, in this case the PEMA thin films can be considered isotropic in the bulk and a constant bulk nonlinear optical susceptibility per unit length scale in the z direction can be assumed over the entire polymer film (χ′B,ijk). So for a polymer thin film with a thickness of “d” the effective bulk contribution can be written in an integral form as shown in eq 2:

2. EXPERIMENTAL SECTION Poly(ethyl methacrylate) (PEMA, Mw ≈ 280 000) was purchased from Scientific Polymer Products, Inc. and used as received. The metal substrates were prepared by depositing 500-nm thick Ag layers on glass slides using an electron-beam evaporator (Cooke Evaporator, Cooke Vacuum Products). PEMA films of different thicknesses were prepared by spin-coating a PEMA toluene solution onto the substrates. Film thicknesses were controlled by adjusting the spin speed and the polymer solution concentration. Before the SFG experiments, all PEMA samples were annealed at 80 °C for 2 h. The PEMA film thicknesses were measured by an ellipsometer (EP3-SW imaging ellipsometer, Nanofilm Technologie, GmbH). SFG theory and experimental setups have already been extensively published so we will not delve into those details here.9−16,19−23 An EKSPLA SFG system pumped with a pico-second Nd:YAG laser was used in this study.17,18 SFG has been applied to investigate surface and interfacial structures of many different polymer materials,17,18,23−44 including different polymethacrylates.17,18,30−33,43,44 In our SFG experiment, a face-up experimental geometry was used as shown in Figure 1. In this experimental geometry, the SFG input visible and

Figure 1. Schematic of the SFG experimental geometry for a PEMA thin film with a thickness of “d” on a Ag substrate. infrared (IR) beams were directly overlapped spatially and temporally at the surfaces of PEMA thin films on the Ag substrates. The visible and IR input angles (before reaching the sample) were 60° and 54° vs the surface normal respectively with beam diameters of approximately 500 μm. In this study, SFG spectra were collected in the infrared frequency range between 2830 and 3020 cm−1 using ssp (s-polarized SFG signal, s-polarized input visible, and p-polarized input infrared beam) and ppp polarization combinations. Our further analysis and discussion is based on the collected ssp and ppp spectra in terms of the PEMA film thickness.

3. RESULTS AND DISCUSSION 3.1. Sources of the Resonant Signals in the SFG Spectra. As a second-order nonlinear optical technique, an SFG signal cannot be generated from a material with inversion symmetry but can be produced at a surface or an interface where the inversion symmetry is broken (under the electricdipole approximation).19 However, beyond this approximation, the electric-quadrupole contributions can also result in a nonvanishing bulk nonlinear susceptibility which mixes with the surface and/or interfacial contributions, as shown in eq 1.45−47 Q

χB , eff , ijk (d) =

d

Lsum , ii(z)Lvis , jj(z)Lir , kk (z)χ ′B, ijk dz

(2)

Here Lsum,ii(z), Lvis,jj(z), and Lir,kk(z) are the diagonal Fresnel coefficients responsible for the local field correction of the one output and two input light fields respectively at the z position. For our polymer thin films on the Ag substrates, the air/ polymer and polymer/Ag interfaces (or the “surface” and “interface” for simplification) are considered to be ordered such that the surface and interfacial effective nonlinear susceptibilities corresponding to the electric-dipole contributions and can be explicitly expressed as eqs 3 and 4:

Q

Q χB , ijk = χijkm ksm − χijkm1 k1m − χijkm2 k 2m

∫0

(1) 1 χQijkm

In terms of the induced polarization, accounts for the dipolar polarization induced by an electric dipolar coupling to the infrared and sum frequency fields and a quadrupolar Q2 coupling to the visible field. χijkm accounts for the dipolar polarization induced by an electric dipolar coupling to the visible and sum frequency fields and a quadrupolar coupling to the infrared field. χQijkm accounts for the quadrupolar polarization induced by the dipolar coupling to both the visible and infrared fields and a quadrupolar coupling to the sum frequency field. ksm, k1m, and k2m are the wave vector components in the m

χS , eff , ijk = Lsum , ii(z = 0)Lvis , jj(z = 0)Lir , kk (z = 0)χS , ijk

(3)

χI , eff , ijk = Lsum , ii(z = d)Lvis , jj(z = d)Lir , kk (z = d)χI , ijk

(4)

Here χS,ijk and χI,ijk are the second-order nonlinear susceptibility tensor components of the surface and interface, respectively. In addition a nonresonant background χNR is present due to the Ag surface optical nonlinearity.48 So the total 6088

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effective nonlinear susceptibility of a PEMA film on Ag should be expressed as: χtot , eff = χNR + χS , eff + χI , eff + χB , eff (5) And the SFG output signal intensity can be written as

Isum = |χtot , eff |2 IvisIir

(6)

where Ivis and Iir are the incident visible and infrared light intensities, respectively. Figure 2 shows the collected ssp and ppp spectra from the PEMA thin films with different thicknesses on the Ag Figure 3. Collected ssp and ppp spectra for the PEMA film on the silica substrate, contributed by the PEMA/air interface.

silica and the PEMA films on the Ag substrates were fitted using eqs 7 and 8, respectively. I(ω) ∝ χNR +

∑ q

iϕ

I(ω) ∝ χNR e +

2

Aq ω2 − ωq + i Γq

∑ q

(7) 2

Aq ω2 − ωq + i Γq

(8)

where χNR is the nonresonant background. Aq, ωq, and Γq are the strength, resonant frequency, and damping coefficient of the vibrational mode q, and φ is the phase term for the nonresonant background. The fitting results with the peak assignments33 for the PEMA film on the silica substrate are shown in Table 1. The fitting results for the PEMA films with Table 1. Fitting Results for the SFG Spectra of the PEMA Film on the Silica Substratea Ai

Figure 2. Collected ssp (A and B) and ppp (C and D) spectra for the PEMA thin films on the Ag substrates. The thicknesses of samples a, b, c, d, and e are 19, 27, 40, 75, and 130 nm, respectively. In part B, the ssp spectra were rescaled and offset. In part D, the ppp spectra were only offset for clarity. It can be seen clearly that ssp SFG spectral intensity strongly depends on the PEMA film thickness, which ppp spectral intensity is independent of the film thickness. In parts B and D, the dots are experimental data, and the lines are spectral fitting results.

ωi (cm−1)

ssp

ppp

Γi

assignment

2879

39

−

11

2904

23

−

11

2935

69

−

11

2957 2979

− −

28 39

11 11

methyl ss mode or Fermi resonance of −O− CH2−CH3 methylene ss mode of backbone −CH2− or −O−CH2−CH3 methyl ss mode or Fermi resonance of −O− CH2−CH3 methylene as mode of −O−CH2−CH3 methyl as mode of −O−CH2−CH3

a

Key: ss, symmetric stretching; as, anti-symmetric stretching.

different thicknesses on the Ag substrates are shown in Table 2. For the ssp spectra of the PEMA thin films on the Ag substrates, the most significant feature is the strong dependence of the SFG spectral intensity on the film thickness, similar to what was observed for PMMA films on Ag.17 We calculated the Fresnel coefficients for χS,eff and χI,ef f as a function of the film thickness.17,49−51 We found that the variation trends of the observed ssp SFG spectral intensity and the Fresnel coefficients have similar dependence on the polymer film thickness as shown in Figure 4. For example, the strongest SFG ssp signal was observed from the polymer film with a thickness of 75 nm. The calculation shows that the film with a thickness around 75 nm has the largest Fresnel coefficients. This indicates that the strong thickness-dependent spectral intensity for the PEMA

substrates. As references of the surface SFG spectra, the ssp and ppp spectra from a PEMA thin film on a silica substrate using the reflection experimental geometry were also collected, as shown in Figure 3. It was shown previously that typical SFG spectra from a polymer film on silica in air are dominated by the contributions from the polymer surface in air, not the buried polymer/silica interface.30,43 We believe that this is also true for PEMA.33 The SFG spectra of the PEMA film on the 6089

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Table 2. Fitting Results for the SFG Spectra of the PEMA Films with Different Thicknesses on the Ag Substrates ωi (cm−1) 2879

2904

2935

2953

2976

film thickness (nm)

polarization combination

Ai

Γi

Ai

Γi

Ai

Γi

Ai

Γi

Ai

Γi

χNR

φ

19

ssp ppp ssp ppp ssp ppp ssp ppp ssp ppp

51 132 57 117 110 124 231 129 71 122

11 11 11 11 11 11 11 11 11 11

24 − 25 − 42 − 68 − 37 −

11 − 11 − 11 − 11 − 11 −

80 128 111 119 203 117 506 124 168 115

11 11 11 11 11 11 11 11 11 11

−169 −325 −188 −310 −303 −331 −569 −313 −190 −311

11 11 11 11 11 11 11 11 11 11

−111 −111 −126 −122 −182 −132 −365 −116 −107 −113

11 11 11 11 11 11 11 11 11 11

56 200 64 195 86 191 159 198 74 197

−1.3 −1.2 −1.3 −1.2 −1.3 −1.2 −1.3 −1.2 −1.2 −1.1

27 40 75 130

The numerical integration was calculated for χB,ef f,yyz and χB,ef f,yyx as a function of the film thickness, and the result is shown in Figure 5. The change of the bulk contribution as a

Figure 4. Absolute Fresnel coefficients of the PEMA/air interface and the PEMA/Ag interface calculated in terms of the film thickness for ssp polarization combination. Clearly the Fresnel coefficients for ssp signal have strong film thickness dependence. Figure 5. Absolute value of the effective bulk contribution for ssp polarization combination; unit of y-axis is χ′B,eff,ijk.

thin films on the Ag substrates is due to the Fresnel coefficient’s variation in terms of the film thickness. For the ssp spectra of the PEMA thin films on the Ag substrates, regardless of the polymer thickness, part of the spectra which contain the three negative peaks at around 2879, 2904, and 2935 cm−1 replicate the feature of the PEMA (deposited on silica) surface spectrum. Here even though the above peaks still have positive counts, we define them as “negative” peaks because they have different phases relative to the nonresonant background so that they point down. In this study we define the positive peaks as those peaks which constructively interfere with the nonresonant background, or the peaks point up. The negative peak around 2935 cm−1 is distorted due to its interference with a nearby positive peak around 2953 cm−1 and the nonresonant background. The appearance of the two strong positive peaks around 2953 and 2976 cm−1 is another significant spectral feature different from the PEMA surface spectrum. In order to understand the origin of these two peaks, eq 9 was used to evaluate the possible bulk contribution. For the ssp polarization combination the effective bulk contribution can be written as:

function of the film thickness is much more significant than that of the surface and interface as shown in Figure 4. It is obvious that the intensity change of the two resonant peaks around 2953 and 2976 cm−1 does not follow the variation trend of the bulk contribution in terms of the film thickness. For example, Aq/Γqs of the two peaks from fitting the ssp spectrum of 40 nm thick film (spectrum c in panel A in 2) are ∼28 and 17 and Aq/ Γqs of the two peaks from fitting the ssp spectrum of 75 nm thick film (spectrum d in panel A in 2) are ∼52 and 33. Therefore, the signal strengths for these two thicknesses are about two times’ difference. If these two peaks are generated mainly from the polymer bulk (contributed by the sum of χB,ef f,yyz and χB,ef f,yyx), the difference of Aq/Γqs of the two peaks for 40 and 75 nm thick PEMA films on Ag should be about six times (from Figure 5: for 40 nm, χB,ef f,yyz + χB,ef f,yyx ∼20; for 75 nm, χB,eff,yyz + χB,ef f,yyx ∼120). So the two resonant peaks at around 2953 and 2976 cm−1 can only be generated from the PEMA surface and/or the PEMA/Ag interface (Figure 4), not the polymer bulk. Using the same method, we can infer that the experimentally observed intensity change of any resonant peak in ssp spectra is not consistent with the variation of the bulk contribution, but is compatible with the variation of the Fresnel coefficients for the surface and interface in terms of the film thickness. Therefore, it is clear for the ssp spectra of the PEMA thin films on Ag substrates that the two resonant peaks around 2953 and 2976 cm−1 are generated from the PEMA surface in air and/or the PEMA/Ag interface. A similar approach can be

χB , eff (ssp) = χB , eff , yyz + χB , eff , yyx d

=

∫0 Lsum,yy(z)Lvis,yy(z)Lir ,zz(z)χ′B,yyz dz d + ∫ Lsum , yy(z)Lvis , yy(z)Lir , xx(z)χ ′B , yyx dz 0 (9) 6090

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used to show that other peaks in the ssp spectra are not from the polymer film bulk contributions either. We then examined the ppp SFG spectra in more detail. All the ppp spectra of the PEMA thin films on the Ag substrates in Figure 2 show the same spectral features. The ppp spectra have similar spectral intensities as well, independent of the polymer film thickness. The independence of the spectral intensities in ppp spectra is different from those in the ssp spectra collected from samples with different thicknesses. In the ppp spectra, there are four discernible resonances including two negative peaks around 2879 and 2935 cm−1 and two positive peaks around 2953 and 2975 cm−1. The fitting results of these four peaks are listed in Table 2. They are contributed by the methyl and methylene groups on the side chains. They can be sequentially assigned to the methyl symmetric stretching and Fermi resonance, methylene asymmetric stretching, and methyl asymmetric stretching modes, respectively.33 For ppp polarization combination, the effective bulk contribution can be divided into eight terms, i.e., χB,ef f,xxx, χB,ef f,xxz, χB,eff,xzx, χB,ef f,xzz, χB,ef f,zxx, χB,ef f,zxz, χB,ef f,zzx, and χB,ef f,zzz. Similar to the analysis of the ssp spectra we evaluated the bulk contributions to the ppp spectra as a function of the polymer film thickness (see Supporting Information). We found that each of the eight bulk terms changes as a function of the polymer film thickness. This is against the observation that the ppp spectral intensity is independent of the film thickness. We therefore believe that ppp SFG signals also cannot come from the polymer bulk contribution. 3.2. Buried PEMA/Silver Interface. For the ssp spectra, simply by comparing the spectral features between the PEMA film on silica and the PEMA films on the Ag substrates, we can conclude the two positive peaks at 2953 and 2976 cm−1 for the PEMA films on the Ag substrates (Figure 2) are generated from the PEMA/Ag interface. The other three negative peaks at 2879 cm−1, 2904 and 2935 cm−1 are mainly from the PEMA surface in air with the interference from the PEMA/Ag interface. The effect of this interference has been discussed in detail in our previous paper and will not be repeated here.17 Now we focus on the ppp spectra. In the SFG experimental geometry shown in Figure 1, the ppp SFG signal probes four second-order nonlinear optical susceptibility components, namely χxxz, χxzx,, χzxx, and χzzz. We calculated the absolute values of the corresponding Fresnel coefficients for these four components at the surface and the buried interface as a function of the film thickness as shown in Figure 6. It can be seen that all the Fresnel coefficients for these four surface (PEMA surface in air) susceptibility components are strongly thickness-dependent while those for the interfacial susceptibility components (at PEMA/Ag interface) remain nearly the same regardless of the film thickness. Judging from the independence of the spectral feature on the film thickness for the ppp spectra, it can be concluded that the observed SFG ppp spectra are dominated by the contributions from the PEMA/Ag interface instead of the PEMA surface in air. In Figure 6 it can be found that for the buried interface the values of Fresnel coefficients of χI,xzx, and χI,zxx are much smaller than those of χI,xxz and χI,zzz, indicating that the contributions of the effective components χI,eff,xxz and χI,eff,zzz dominate the ppp spectra. As mentioned above, a metal has a complex refractive index with a small real part and a large imaginary part. Thus, a light beam cannot penetrate into the metal bulk but is mostly reflect back from the metal surface.

Figure 6. Absolute Fresnel coefficients of the PEMA/air interface (top) and the PEMA/Ag interface (bottom) in terms of the film thickness for ppp polarization combination. Clearly the Fresnel coefficients for the ppp SFG signals from the PEMA/air interface have strong dependence on the film thickness, but those from the PEMA/ Ag interface do not strongly depend on the film thickness.

In Figure 7, a light beam is incident upon and reflected at a polymer/metal interface. Most of the light is reflected and the

Figure 7. Schematic showing the simple reflection of a light beam at the polymer/metal interface when the imaginary part of the complex refractive index of the metal is much higher than the real part (normally in the infrared frequency range).

light field on the metal surface is simply the addition of the incident and the reflected light fields. Taking the PEMA/Ag interface as an example and applying the Fresnel equations for the s- and p- polarized beams in the infrared frequency range, we found the reflective coefficients of rs = −0.99−0.10i and rp = 0.98 + 0.14i. This can be understood by an out-of-phase change for the s-polarized light and in-phase change for the p-polarized light after reflection. This optical effect had been discussed (e.g., by Greenler and Allara et al.) when concerning the spectroscopy of the specular reflection mode on metal substrates.52−54 As stated, the light field above the metal surface is a superposition of the incident and reflective light fields in this case. A simple projection was completed for the coefficients of the local field correction in the x, y, z directions (Figure 1) with respect to the incident infrared light. From this 6091

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we found Lyy = 0.01−0.10i, Lxx = 0.02−0.14i, Lzz = 1.98 + 0.14i. This indicates that the local fields parallel to the metal surface (x, y directions) diminish while the local field perpendicular to the metal surface (z direction) strengthens due to this field superposition. In the visible range, this dependence of the local field correction on the field direction (x, y, z) still exists, but it is not as significant as that in the infrared range, due to the relatively smaller imaginary part of the refractive index in the visible range. The coefficients of the local field correction for the input visible and infrared beams are listed in Table 3. The output Table 3. Coefficients of the Local Field Correction for the Two Input and One Output Lights on the Ag Substrate Surface with Respect to the Incident Light Fields coefficient

Lxx

Lyy

Lzz

infrared light visible light

0.02−0.14i 0.56−0.88i

0.01−0.10i 0.26−0.65i

1.98 + 0.14i 1.44 + 0.88i

Figure 8. Collected SFG ppp spectra for the PMMA thin films on the Ag substrates. The thicknesses of sample a, b, c, d, and e are 20, 45, 80, 122, and 161 nm, respectively; the spectra in Graph B have been offset for clarity. Clearly the SFG ppp spectral intensity does not depend on the PMMA film thickness.

SFG intensity Iijk(ω) generated from the second-order nonlinear susceptibility tensor component χ(2) ijk can be written as Iijk(ω) ∝ |Lii(ω)|2 |χijk(2) |2 |Ljj(ω1)E(ω1)|2 |Lkk (ω2)E(ω2)|2

groups30 and combination mode17 respectively. Since the methyl ss mode of PEMA at the PEMA/Ag interface is also a negative peak, this indicates that both methyl groups of PMMA and PEMA at the Ag surface adopt the same absolute orientation away from the Ag side.17 Because we know the resonant signals in the ppp spectra of the PEMA thin films on the Ag substrates are dominated by the contributions from the PEMA/Ag interface, we can deduce the side methyl group orientation at the polymer/metal interface from the ratio of χppp,as over χppp,s. The symmetric C−H stretching, Fermi resonance, and asymmetric C−H stretching signals at 2879, 2935 and 2976 cm−1 were used to determine the orientation of the side chain methyl groups. Referring to the previous literatures concerning tilt angle calculation of methyl groups21,22,55 we have

(10)

Here E(ω1) and E(ω2) are the input fields of the visible and infrared lights and Lnn(ωn)n=i,j,k is the local field coefficient. In the surface or interfacial coordinate system, i, j, k has a direct corresponding relationship to x, y, z. For s-polarized infrared light, the y-directional local field on the metal surface is nearly diminished. For p-polarized infrared light, the x-directional local field is also nearly diminished while only the z-directional local field is strengthened. Since the molecular vibration is pumped by the infrared light in a SFG process, it indicates for the infrared light, the p-polarization over s-polarization should be used for the SFG process generated at the polymer/metal interface. Consequently, for the generally used ssp, sps, pss, and ppp polarization combinations, only ssp and ppp are suitable for the detection of molecular vibrations at the metal surface or interface. For the visible light, the coefficients of the local fields have the relationship of Lzz > Lxx > Lyy (Table 1). Since Lzz and Lxx are related to the signals contributed from the p-polarized light and Lyy is related to signals generated from the s-polarized light, the p-polarized input beam should lead to stronger signals than those generated from the s-polarized input beam. Therefore, the ppp SFG polarization combination is preferred over the ssp polarization combination for detecting SFG signals on the metal surfaces. Furthermore, for the four second-order nonlinear susceptibility tensor components detected using the ppp polarization combination, signals contributed from the χxzx and χzxx components could be difficult to detect due to the diminished local infrared fields compared to those of χxxz and χzzz. In order to generalize this observation we have studied several other polymer samples using ppp spectra. Similar ppp SFG spectra from polymer thin films on the Ag substrates were observed independent of the film thickness. As an example, Figure 8 shows ppp spectra for poly(methyl methacrylate) thin films on the Ag substrates with a variety of different thicknesses. Like the PEMA/Ag samples, the spectral features for different PMMA films are similar, dominated by a strong negative peak at 2955 cm−1 and a weak negative peak at 2844 cm−1, which are assigned to the symmetric stretching mode of the ester methyl

χppp , as χppp , ss

=

0.32(cos θ − cos3 θ) (0.11 + 0.38r )cos θ + (0.16 − 0.16r )cos3 θ ×

βcaa , as βccc , s

(11)

where r is the ratio of βaac/βccc for methyl symmetric stretching and θ is the tilt angle of the methyl group which is defined as the angle between the methyl c-axis and the surface normal. Because of the uncertainty of the r value, which is thought to be in the range from 1.6 to 4.2,21 we chose a middle value of 3.0 for plotting the curve of |χppp,as/χppp,ss| as a function of tilt angle θ. δ-distribution and Gaussian distribution (f(θ) = C exp[−(θ − θ0)2/2σ2]) were used for describing the possible tilt angle distribution. In Gaussian distribution, θ0 and σ are defined as average orientation angle and orientation angle distribution. The plotted curves and the experimental value are shown in Figure 9. From the intersection points we can deduce that the side methyl groups at the PEMA/Ag interface adopt a large tilt angle. This is very different from the orientation of the side methyl groups at the PEMA surface in air33 due to different interactions at different surfaces/interfaces. 6092

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ACKNOWLEDGMENTS

■

REFERENCES

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This work was supported by the National Science Foundation of China (Grant No. 51173169 and 21004054), the Zhejiang Provincial Natural Science Foundation of China (Grant No. Y4100390), the Qianjiang Talents Project of Department of Science and Technology in Zhejiang province (Grant No. 2011R10025) and SRC (2012-KJ-2282). The fund from the Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University (Grant No. ZYG2011005) is also acknowledged. We thank Chi Zhang for designing the TOC graphic.

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Figure 9. Theoretical curves of |χppp,as/χppp,ss| as a function of tilt angle θ0 and angle distribution σ using a middle r value of 3.0; the intersection points suggest the possible methyl tilt angles with angle distribution at the PEMA/Ag interface (the experimental value of 0.50 was an averaged one from fitting the ppp spectra of five samples with different thicknesses).

4. CONCLUSION We have demonstrated that for amorphous PEMA thin films in the thickness range of ∼100 nm on Ag substrates, the observed SFG resonant signals are generated from the PEMA surface in air and the PEMA/Ag interface with negligible bulk contribution. Both the PEMA surface and the PEMA/Ag interface contribute to the ssp SFG spectra. As we showed previously6 by analyzing SFG ssp spectra collected from polymer films with different thicknesses we can deconvolute SFG signals contributed from the buried polymer/metal interface from which structural information on the buried interface can be deduced. This research demonstrated that the polymer/metal interface solely contributes to the SFG ppp spectra collected from the PEMA films on metal because of the optical character of the metal surface. The orientation information on methyl functional groups at the PEMA/Ag buried interface was deduced directly from the observed methyl symmetric and asymmetric vibrational modes in the ppp spectra, without the need to collect SFG spectra from polymer films with different thicknesses. The methodology developed in this example study on the PEMA/Ag interface can be generalized for other amorphous polymer/metal interfaces. We believe that for amorphous polymer films on metal surfaces the bulk contribution to SFG signal could be ignored. Furthermore, we showed that the polymer/metal interface signals dominate SFG ppp spectra, thus these ppp spectra can be used to study molecular structures of the buried polymer/metal interfaces.

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ASSOCIATED CONTENT

S Supporting Information *

The estimated bulk contribution for ppp spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (X.L.) [email protected]; (Z.C.) [email protected]. Fax: 1-734-647-4685. Notes

The authors declare no competing financial interest. 6093

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