Abnormal Anti-Stokes Raman Emission and Infrared Dichroism

Article pubs.acs.org/JPCC

Abnormal Anti-Stokes Raman Emission and Infrared Dichroism Studies on Poly(p‑phenylenevinylene)/Single-Walled Carbon Nanotube Composites M. Baibarac,*,† I. Baltog,† J. Wery,‡ S. Lefrant,‡ and J. Y. Mevellec‡ †

Lab. Optical Processes in Nanostructured Materials, National Institute of Materials Physics, P.O. Box MG-7, Bucharest R077125, Romania ‡ Institut des Matériaux “Jean Rouxel”, 2 rue de la Houssinière, B.P. 32229, F-44322 Nantes, France ABSTRACT: Using surface-enhanced Raman scattering (SERS) and Fourier transform infrared spectroscopy (FTIR) in the grazing angle incident reflection geometry, new data concerning the abnormal anti-Stokes Raman emission (AASRE) and the molecular orientation of poly(p-phenylenevinylene) (PPV) deposited on metallic supports of Ag and Au are reported. The particular dependencies of the anti-Stokes Raman intensity on the Raman shift, the sample thickness, and the incident pump intensity are highlighted in this paper. In the spectral range of 800−1000 cm−1, the FTIR spectra of PPV films deposited on Ag and Au supports show a dichroism similar to that reported for the free-standing PPV film. Significant differences are reported in the spectral range of 1400−1700 cm−1 for the s- and p-polarized FTIR spectra of PPV films deposited on Au and Ag supports. The annealing treatment at 110 °C of the PPV precursor solution in the presence of single-walled carbon nanotubes (SWNTs) induces a noncovalent functionalization of carbon nanotubes with the polymer molecules, as evidenced by a decrease of the radial breathing mode intensity and a decrease in the strength of AASRE. The process of noncovalent functionalization of SWNTs with PPV induces a change of the orientation angle of the transition dipole moment vector for the absorption band at 835 cm−1. This fact is explained on the basis of the π−π* interaction between the phenyl group of PPV and the sidewall of the nanotubes.

1. INTRODUCTION Recent progress concerning the synthesis and applications of conjugated polymers/carbon nanotubes (CPs/CNTs) composites has resulted in a better understanding of the chemical and physical properties of these materials. Many works have been conducted on the vibrational characterization of the following four composites: (i) bilayers of CPs and CNTs, (ii) CPs doped with CNTs, (iii) CNTs covalently functionalized with CPs, and (iv) CPs noncovalently functionalized CNTs. In the past decade, special attention has been given to CNTs noncovalently functionalized with poly(p-phenylenevinylene) (PPV) because of the multiple applications reported in different fields as solar cells,1 organic photodiodes,2 organic thin-films transistors,3 photovoltaic devices,4 and photodetectors.5 To establish the type of the interactions between the two constituents that form the PPV/CNT composites, the experimental methods primarily used have been photoluminescence,6 electroluminescence,7 photoconductivity,8 resonant Stokes Raman scattering,9 and, more recently, antiStokes luminescence.10 The presence of the abnormal antiStokes Raman emission (AASRE) in a composite based on PPV and SWNTs with a concentration of 8 wt % was reported for the first time in 2011.11 In this paper, new results are reported concerning the dependence of the AASRE on the nanotube concentration in the PPV/SWNT composite mass when singlewalled carbon nanotubes (SWNTs) films that are noncovalently functionalized with PPV are deposited onto metallic supports active in surface-enhanced Raman scattering (SERS) © 2012 American Chemical Society

spectroscopy. Until now, AASRE has been observed only for composites based on SWNTs and conjugated polymers of the poly(2,2′-bithiophene) and poly(3,4-ethylenedioxythiophene) types.12,13 As shown in this paper, the layout of polymers and composites as thin films onto SERS active metallic supports, i.e., Au and Ag, creates new opportunities for studying the molecular orientation by FTIR spectroscopy in the grazing angle incident reflection geometry. An early study regarding the analysis of dichroic ratios of the main IR absorption bands of PPV free-standing films was performed in 1986.14 To our knowledge, no further paper has been published concerning the molecular orientation of PPV films and PPV/SWNT composites layered on metallic supports of Au and Ag. Indeed, only three articles are devoted to infrared dichroism studies on composites based on multiwalled carbon nanotubes and macromolecular compounds of polystyrene and styrene− butadiene copolymers.15−17 The dependence of the FTIR spectra recorded in the grazing angle incident reflection geometry of the PPV film as a function of the concentration of SWNTs in composites mass is shown as well. Received: July 15, 2012 Revised: October 25, 2012 Published: November 15, 2012 25537

dx.doi.org/10.1021/jp3070008 | J. Phys. Chem. C 2012, 116, 25537−25545

The Journal of Physical Chemistry C

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2. EXPERIMENTAL SECTION The solution of the PPV precursor was synthesized according to work published in 1989.18 The procedure described by H. Aarab and collaborators was used for the preparation of a solution based on the sulfonium polyelectrolyte precursor of PPV with different SWNT weights.9 Blends with different weight concentrations of SWNTs of 8, 16, 32 and 64% were ultrasonically homogenized for 5 min. The ultrasonic peak power maximum of the ultrasonic setup is of 800 W at standard sine-wave modulation. The deposition of a film of PPV precursor/SWNT composite on Ag and Au supports was carried out in an inert gas atmosphere at room temperature. Ag and Au supports were obtained by the vacuum evaporation technique under grazing incidence. To obtain films based on the PPV/SWNT composites, an annealing treatment of the PPV precursor/SWNT films was conducted under vacuum at a temperature of 110 °C. Figures 1a and 1b show SEM pictures

Raman and SERS spectra were recorded with a resolution of 2 cm−1 in a backscattering geometry under an excitation wavelength of 676.4 nm, using a HORIBA Jobin Yvon T64000 spectrophotometer. FTIR spectra were recorded using a Brüker Vertex 70 FTIR spectrometer, equipped with a Hyperion 2000 FTIR microscope endowed with a grazing angle objective. The resolution of FTIR spectra was of 4 cm−1. UV−vis−NIR spectra were recorded on films deposited onto quartz supports, using a PerkinElmer spectrometer, Lambda 950 model. The resolution of UV−vis−NIR spectra was 1 nm. SEM and HRTEM pictures were acquired with a JEOL JSM 6400F microscope and a Hitachi H9000 NAR, respectively, operated at 300 kV.

3. RESULTS AND DISCUSSION Figure 2 shows the Raman and SERS spectra of PPV films with a thickness of ∼100 nm, which were deposited on a quartz plate

Figure 1. SEM pictures of a PPV film and the PPV/SWNT composite are shown in (a) and (b), respectively. (c) and (d) show HRTEM pictures of SWNTs in the initial state and the PPV/SWNTs composite, respectively.

of a PPV film and PPV/SWNT composite with a concentration of SWNTs of 16%. In the latter case, the presence of bundles of carbon nanotubes in the polymer mass is clearly observed. Figures 1c and 1d show HRTEM pictures of SWNTs in the initial state and of the PPV/SWNT composite. An analysis of the HRTEM pictures indicates that SWNT bundles have diameters between 6 and 12 nm in the initial state while the bundles in the samples of composite in which SWNTs are functionalized with PPV molecules are thicker, with a diameter of ∼20 nm. The increase of the diameter of carbon nanotubes bundles is a result of coating of polymer molecules on the SWNT surface. According to data reported previously, the conversion of the PPV precursor solution to the polymer film, which was conducted at 110 °C, leads to highly oriented films and the fraction of oriented PPV molecules in samples was estimated to be 97%.19

Figure 2. Raman (a) and SERS spectra (b) of a PPV film, obtained by conversion at 110 °C on quartz and rough Au supports, respectively. All spectra were recorded at λexc = 676.4 nm.

and an Au support under an annealing conversion temperature of 110 °C. The film thickness was calculated by considering the concentration of the precursor solution, the density of PPV,20 and the volume of solution deposited on a known area. An additional verification of the procedure for determining the thickness of the PPV films was made by atomic force microscopy. According to Figure 1, in the spectral range of 1100−1700 cm−1, the main Raman lines of the PPV are situated at ca. 1172, 1323, 1419, 1547, 1589, and 1628 cm−1. They are assigned to the following vibration modes: C−C stretching + C−H bending of the phenyl ring, CC stretching + C−H bending of the vinyl group, symmetrical phenyl ring 25538



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spectral range from −1700 to −1000 cm−1 in Figure 2 reveals an AASRE process that may be evaluated by the ratio between the anti-Stokes intensity measured experimentally and the antiStokes intensity calculated by the Boltzmann law applied to the Stokes Raman spectra ((Iexp/Icalc)aStokes), which becomes larger than unity. Some supplementary explanations are necessary for a better understanding of the AASRE effect. The AASRE term defines an anti-Stokes Raman emission for which the anti-Stokes/Stokes intensity ratio (IaS/IS) is much greater than expected on the basis of the equilibrium population of the excited vibration states provided by the Boltzmann law, which is described by eq 1:

bending, CC stretching of the phenyl ring, C−C stretching of the phenyl ring, and CC stretching of the vinyl group, respectively.21 As expected, the use of the rough Au support leads to a great enhancement in the intensity of the Raman spectrum of PPV in comparison with the case when the film is deposited on a quartz support. This enhancement originates in a complex process on which the technique of surface-enhanced Raman scattering (SERS) is based. At present, it is well established that the enhancement of the Raman lines is due to two different mechanisms: electromagnetic and chemical. The electromagnetic mechanism is achieved by the resonant excitation of the surface plasmons, and the chemical mechanism is due to charge-transfer processes between the metallic support and adsorbed molecules. The main signature of the first mechanism is the increase in the intensity of the Raman lines while the second is identified by a shift of some Raman lines. In our opinion, the downshift of the Raman line assigned to the vibrational mode C−C stretching of the phenyl ring from 1593 to 1589 cm−1 when the Raman spectra recorded on a film deposited on quartz and Au, respectively, were due to a chargetransfer process that occurred at the PPV/Au support interface. An interesting result is presented in Figure 3, which shows that the anti-Stokes SERS spectra of PPV films with thicknesses between 100 and 200 nm are more intense than those provided by the Boltzmann law, which establishes the equilibrium population of the excited vibration states. The analysis of the

4 ⎛ hΩ ⎞−1 Ias ⎛ σ(αΩ)aS ⎞⎛ ω l + Ω ⎞ ⎟ =⎜ ⎟⎜ ⎟ exp⎜ ⎝ kT ⎠ Is ⎝ σ(αΩ)aS ⎠⎝ ω l − Ω ⎠

(1)

In eq 1, ωl and Ω, h, k, and T are the wavenumbers of the excitation light and the Raman line (cm−1), the Planck constant, the Boltzmann constant, and the temperature, respectively. In eq 1, the terms σ(αΩ)aS and σ(αΩ)S indicate the anti-Stokes and the Stokes cross sections associated with the Ω wavenumber. These terms depend on the polarizability of the material and are equal for the normal (nonresonant) spontaneous Raman process. The literature devoted to this subject indicates that AASRE is commonly observed in carbon nanotubes22−27 and in other materials under resonant optical excitation when the exciting light produces an electronic transition between two levels of the material under study.12,13,28 On the macroscopic scale, the response of a material to a resonant optical excitation is described by the polarization P = P0 + χ(1)E + χ(2)EE + χ(3)EEE + ... in which terms of higher order appear, such as χ(2)EE and χ(3)EEE, which is a key condition for generating a nonlinear optical process. However, AASRE is mostly observed when a SERS measurement configuration is used, which ensures the production of a resonant Raman process in the adsorbate−metallic surface complex that induces unequal cross sections for the Stokes and anti-Stokes Raman emission.12,13,28−33 In addition, the SERS configuration using the surface plasmons excitation allows a high density electromagnetic field at the interface metal/ dielectric that causes the Raman process to occur under intense excitation light, which is another requirement to generate a nonlinear optical process similar to a CARS (coherent antiStokes Raman scattering) effect.34 CARS is a nonlinear optical phenomenon resulting from a wave-mixing process. Pump, Stokes, and probe incident beams with frequencies of ωl, ωS, and ωp, respectively, interact with a sample and generate an anti-Stokes signal at ωaS = ωl − ωS + ωp. In most experiments, the pump and the probe fields derive from the same laser beam, ωl = ωp, such that ωaS = 2ωl − ωS. If ωl − ωS coincides with the frequency of a Raman active molecular vibration Ω, the antiStokes Raman signal is enhanced by several orders of magnitude in comparison with the Stokes Raman intensity that results from a spontaneous Raman process. In the planewave approximation and for a nonabsorbing medium, the timeaveraged CARS signal intensity is given by eq 2:

Figure 3. SERS spectra of PPV films with thicknesses of 100 nm (a1 and a2) and 200 nm (b1 and b2) in the anti-Stokes and Stokes ranges. The films were obtained by conversion at 110 °C on Au supports. All spectra were recorded at λexc = 676.4 nm with an exciting laser intensity power equal to 14 mW. The red curve shows the anti-Stokes spectra calculated with the Boltzmann formulas applied to the measured Stokes spectra.

IaS = ICARS = NA

ωaS2N 2 16ε0 2c 4

|χ (3) |2 Il 2ISL2

sin 2(ΔkL /2) (ΔkL /2)2

(2)

This equation reveals the particular dependences of the antiStokes Raman intensity on the anti-Stokes Raman shift ωaS, the interaction length between the mixing light beams L, i.e., the 25539



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sample slab thickness, the incident pump intensity Il, the thirdorder nonlinear dielectric susceptibility χ(3), and the numerical aperture (NA) of the collecting lens. The coherent nature of the CARS process is reflected by fulfilling the phase-matching condition |Δk|L ≪ π where Δk = kaS − (2kp − kS) and kaS, kS, and kp are the wave vectors of the anti-Stokes, Stokes, and pump light, respectively.35 A good phase matching is achieved for Δk = 0. For condensed media Δk ≈ 0 is fulfilled if the beams cross at an angle θ. For tightly focused beams, this requirement relaxes and the system is no longer sensitive to the Raman shift. Thus, a CARS spectrum can be observed at an angle θaS larger than the Stokes angle θ. The current major inconvenience of using this Raman spectroscopic technique is that it is typically performed with a multibeam-source technique, which requires a complex experimental set up able to implement the strict requirement of the spatial and temporal overlap of the excitation beams. Experimentally, the AASRE described by eq 2 is characterized by (i) an anti-Stokes Raman intensity that is increasing quadratically with the vibration wavenumber, (ii) a square dependence on the film thickness, (iii) a square dependence on the exciting laser intensity, and (iv) a linear dependence on the numerical aperture (NA) of the microscope objective used for the detection of the anti-Stokes emission.28 All of these suggest that AASRE behaves as a single beam CARS process. In this context, it is predictable that the AASRE effect can be observed on both materials that attain nonlinear optical properties when they are subjected to a resonant optical excitations and materials that are intrinsically optically nonlinear, which indicates that the Boltzmann law, i.e., eq 1, no longer operates because σ(αΩ)aS > σ(αΩ)S, which explains the increases of the IaS/IS ratio.36 An important consequence of this fact is that the AASRE may be considered a complementary experimental technique to normal Raman spectroscopy, and it is already used in the detection of surface interactions that occur in polymer/ carbon nanotubes composites.12,13 According to Figure 3, the observation of AASRE on PPV films is justified in two ways, by the resonant optical excitation which is amplified by the SERS measuring configuration, and by χ(3) ≠ 0, which characterizes this material.37 To demonstrate the intrinsic absorption properties of PPV, SWNTs, and the PPV/SWNT composite, a briefly presentation of the UV−vis− NIR spectra of these compounds is shown in Figure 4. The UV−vis−NIR spectrum of SWNTs shows three absorption bands with maximum situated at 0.65, 1.2, and 1.75 eV. The first two bands are associated with the S11 and S22 transitions in the semiconducting tubes, while the third band originates in the M11 transition of the metallic tubes.38 In Figure 4, the UV−vis spectrum of the PPV film shows an absorption edged at 3.15 eV, which is attributed to the π−π* electronic transition.21 Its position is often correlated with the effective conjugation length of the macromolecular chain.21 According with a paper published in 2005, a residual absorption sub-band gap was found, extending to λ ≥ 676.4 nm, which explains the resonance excitation effect.39 The UV−vis−NIR spectrum of the PPV/SWNT composite shows the absorption bands of the two constituents. In addition, an upshift of the gap transitions in semiconducting and metallic tubes of ca. 0.11 and 0.07 eV and in the absorption band of PPV of ca. 0.14 eV is also noted. These changes originate in the functionalization of SWNTs with PPV molecules. In summary, we conclude that the AASRE on PPV films originates both in the residual sub-band-gap

Figure 4. UV−vis−NIR spectra of the films of PPV, SWNTs, and the PPV/SWNT composite.

absorption, which is amplified by the SERS measuring configuration, and χ(3) ≠ 0, which characterizes this material. The strength of AASRE may be evaluated by the ratio (Iexp/ Icalc)aStokes, where Icalc is the theoretical value that results if the process is governed only by the Boltzmann law that describes the ratio between Stokes and anti-Stokes Raman intensity under conditions of normal thermal equilibrium and nonresonant excitation in accordance with eq 2. Figure 3a shows that the intensity of the anti-Stokes Raman lines becomes more intense with an increase of the anti-Stokes Raman shift for the PPV film with a thickness of ∼100 nm. The values of the ratio (Iexp/Icalc)aStokes for Raman lines with maxima at −1170 and −1585 cm−1 were calculated to be 8 and 17, respectively. As observed in Figure 3b, the values of this ratio increase to 53 and 144, respectively, when the film thickness is of ca. 200 nm. This result is due to two causes: one related to the resonant optical excitation that causes χ(3) ≠ 0 and another due to the increase in the film thickness, which are both already given above as conditions of an AASRE effect. A nonlinear dependence of the ratio IaS/IS on the exciting laser intensity in the case of Raman lines that peak at −1585 and −1170 cm−1 for the PPV films with a thickness of ca. 100 nm is highlighted in the insets of Figure 5. In accordance with the previsions of eq 2, (IaS/IS) ∼ Il2, this ratio should increase nonlinearly when the intensity of the exciting laser light increases from 1.4 to 14 mW. This variation reveals the difference between the two types of Raman emissions: the Stokes emission is a spontaneous optical process while anti-Stokes emission bears the signature of a nonlinear optical process. 25540



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Figure 5. Stokes and anti-Stokes SERS spectra of the PPV film deposited on a rough Au support, with a thickness of 100 nm, versus the intensity of the exciting light of the Raman signal. The inset shows the variation of the IaS/IS ratio for the anti-Stokes branch of the Raman lines with maxima situated at −1585 and −1170 cm−1. All spectra are recorded at λexc = 676.4 nm.

Figure 6. Stokes and anti-Stokes SERS spectra at λexc = 676.4 nm of SWNTs (a) and the PPV/SWNT composites with SWNT weight percentages equal to 64 (b), 32, (c) and 16 wt % (d). The red curves show the anti-Stokes spectra calculated with the Boltzmann formulas applied to the measured Stokes spectra. The exciting laser intensity power is equal to 14 mW.

The effect of adding SWNTs to the PPV precursor solution on the AASRE of the films with a thickness of ca. 100 nm is shown in Figure 6. All Raman spectra were recorded with an exciting laser intensity of 14.0 mW. Before giving comments on the AASRE effect observed on PPV/SWNT composites obtained by an annealing treatment at 110 °C, it is necessary to recall the features of the Raman spectrum of carbon nanotubes. According to Figure 6a, the Raman spectrum of SWNTs recorded at an excitation wavelength of 676.4 nm shows an intense Raman line with a maximum at 174 cm−1 attributed to the radial breathing vibration mode (RBM) and two other Raman lines with maxima at 1310 and 1540−1583 cm−1 assigned to the disorder state or defects induced in the carbon nanotube structure and a tangential vibration mode, respectively.40 As shown, the wide band situated in the spectral range of 1500−1650 cm−1 reveals a complex profile formed by two components labeled G+ and G−, which, in the case of the metallic SWNTs, are attributed to the transversal optical (TO)circumferential and longitudinal optic (LO)-axial vibration modes, respectively.41 According to Figures 6b−d, the addition of SWNTs to the PPV precursor solution induces two main changes in the Raman spectra: (i) a decrease of the (Iexp/Icalc)aStokes ratio associated with the Raman line, peaking at −1583 cm−1, compared to the value obtained for the SWNT film; (ii) a gradual decrease in the relative intensity of the Raman line at 1170 cm−1 as the SWNT concentration increases in the composite mass and an unexpected increase in the value of the (Iexp/Icalc)aStokes ratio in the anti-Stokes range for the same line;

and (iii) a decrease in the relative intensity of the Raman line associated with the RBM of the SWNTs and a simultaneous upshift from 174 to 180 cm−1 when the SWNT concentration in the composite mass decreases. The decrease in the relative intensity of the RBM band of the SWNTs accompanied by an upshift from 174 to 180 cm−1 is clear evidence of a modification in the characteristics of the two constituents of the composite material. In fact, this variation indicates the shielding effects induced by polymer molecules on carbon nanotubes as a result of the noncovalent functionalization of SWNTs with PPV. The behavior of the Raman line with a maximum at 1170 cm−1 must be correlated with the π−π* interaction between the phenyl groups of PPV and the sidewall of the nanotubes because steric hindrance effects are induced in the phenyl ring. A priori, a steric hindrance effect must be observed equally in the Stokes and anti-Stokes branches. Thus, the higher intensity in the anti-Stokes branch observed when the concentration of SWNTs in the composite mass increases may be explained as a result of an enhancement via plasmons generated at the interface of the metallic nanotubes and the polymer. For a better understanding of the functionalization process of SWNTs with PPV, new details concerning the changes induced in the molecular orientation of the PPV film by adding SWNTs to the precursor solution are shown below. They are found by using FTIR spectroscopy in the grazing angle incident 25541



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plane bending, phenyl ring C−C in-ring stretching, and CO stretching, respectively.14,42−45 The first two vibrational modes are perpendicular to the plane of the PPV chains, while the latter three modes are parallel to the plane of the PPV chains. The absorption FTIR band with the maximum at 1691 cm−1, which is assigned to the carbonyl stretching vibration mode is observed only in the case of the PPV film deposited on Au support. The absence of this band in the case of FTIR spectra of the PPV films deposited on Ag supports indicates that the annealing treatment does not lead to the creation of oxygenated defects in the macromolecular chain.46 This difference observed in the case of PPV films deposited onto Ag and Au supports can be explained by the affinity of oxygen for the Ag support when the formation of an Ag2O layer occurs. In the case of Au support, which is a metal that does not have an affinity with oxygen even at high temperature,47 the lack of a covering oxide layer permits direct interaction of oxygen sources with the macromolecular chain, generating new functional groups of the type CO. As observed in Figure 7, the intensities of the absorption bands peaked at 835 and 966 cm−1 strongly depend on the angle of polarization. This fact indicates a preferential orientation of the polymer chain in the plane of the film. In the case of s polarization, these bands vanish partially or totally when the PPV film is deposited on Ag and Au supports, respectively. Regardless of the type of metallic support, the two absorption bands for p polarization appear the strongest. For the FTIR absorption bands with maxima situated at 1421 and 1516 cm−1, which are associated with vibration modes of the CC in-plane bending and C−C in-ring stretching of the phenyl ring, respectively, a dependence on polarization is noted only in the case of Ag support. Other differences observed in Figure 7 concern the ratio between the relative intensities of the FTIR absorption bands with maxima at 964 and 1516 cm−1 when the FTIR spectra of PPV are recorded in the following geometries: (i) in the s polarization geometry, when the electric field vector E is parallel to the film plane, the values for the ratio I964/I1516 in the case of Ag and Au supports are 1:2.2 and 1:7.5, respectively; and (ii) in the p polarization geometry, when the electric field vector E is perpendicular to the film plane, the values of the ratio I964/I1516 in the case of Ag and Au supports are 6:1 and 1:1, respectively. These differences must be correlated with the absorption of PPV molecules onto the metallic surface. For a better assessment of the molecular orientation of the polymer versus the metallic surface, we show the calculated values of the dichroic ratio R in the case of the PPV film deposited on Ag and Au supports in Table 1. In comparison with the PPV free-standing film, the most important change when the PPV film is deposited on Ag and

reflection geometry. Given that our samples consist of thin films deposited onto rough Ag and Au supports, an evaluation of the influence of the metallic support on PPV without SWNTs is necessary. Figure 7 shows the FTIR spectra of PPV

Figure 7. FTIR spectra of the PPV film with a thickness of ca. 200 nm deposited on Ag and Au supports recorded in the grazing angle incident reflection geometry with p and s polarizations.

in polarized light as a function of the type of metallic support. According to previous FTIR studies, the main absorption FTIR bands are situated at ca. 835, 966, 1059−1109, 1267−1334, 1421 −1423, 1516, and 1601−1691 cm−1 and are associated with the following vibrational modes: phenyl ring C−H out-ofplane bending, trans-vinylene C−H out-of-plane bending, C−H in-plane bending, carboxyl stretching, phenyl ring CC in-

Table 1. Observed Dichroic Ratio (R) and the Angle θ Valuesa for the PPV Film Deposited on Ag and Au Support ν (cm−1)

θ (deg) for PPV free-standing film (ref 14)

θ (deg) for PPV free-standing film (ref 42)

R for PPV film deposited on Ag support (this work)

θ (deg) for PPV film deposited on Ag support (this work)

R for PPV film deposited on Au support (this work)

θ (deg) for PPV film deposited on Au support (this work)

835 965 1421 1517

83 84 64 9

85 85 60 10

8.836 5.899 1.659 0.417

79 76 62 43

12.639 7.109 1.287 1

82 77 58 55

a R = Ap/As = (sin2 θ + s)/(2 cos2 θ + s), where As and Ap are absorbances measured in the s and p polarization, respectively; θ is the angle between the transition moment for a vibration mode and the polymer chain axis; and s is the orientation parameter which has a value equal to 0.04 for the PPV films obtained by annealing treatment of the precursor solution in the temperature range between 60 and 110 °C.14

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Au supports is the change induced for the C−C in-ring stretching vibration mode of the phenyl ring, which corresponds to the absorption band with a maximum at 1516 cm−1. For the PPV films deposited onto Au and Ag supports, the transition dipole moment vector of the vibration mode of the absorption band with a maximum at 1516 cm−1 is oriented at angles of θ = 55° and 45°, respectively. Figure 8 shows the

Figure 9. FTIR spectra of the PPV film with thickness of ca. 200 nm deposited on an Au support, recorded in the grazing angle incident reflection geometry in p (a) and s polarization (b), prepared in the absence (a1, b1) and in the presence of different SWNT weight percentages equal to 8 (a2, b2), 16 (a3, b3), 32 (a4, b4), and 64 wt % (a5, b5).

Figure 8. FTIR spectra of the PPV film with a thickness of ca. 200 nm deposited on Ag and Au supports recorded in the grazing angle incident reflection geometry in the p polarization, obtained in the absence and presence of different SWNTs weight percentages equal to 64 and 32 wt %.

Table 2. Angle θ Values for the PPV Films Prepared in Absence and in the Presence of Different SWNTs Weight Percentages (x wt %) Deposited onto Au Support

influence of the amount of SWNT added to the PPV precursor solution on the FTIR spectra recorded in the grazing angle incident reflection geometry with Ag and Au supports. For FTIR spectra recorded in the p polarization, as the SWNT weight in the PPV precursor solution mass increases, a decrease in the relative intensity of the absorption bands with maxima at 839 and 965 cm−1 is observed in the case of Ag support, whereas for the Au support, an increase of the relative intensities of these bands is observed. According to Figure 9, an increase in the relative intensities of the absorption bands at 839 and 965 cm−1 is also observed in the FTIR spectra recorded in the s polarization. For a sample of PPV with a SWNT concentration of 64 wt % deposited on Au and Ag supports, the FTIR spectra recorded in the p polarization show a ratio between the relative intensities of the absorption bands with maxima at 965 and 1516 cm−1 of ∼3:1 (Figure 8). To achieve a better quantification of the influence of the SWNT concentration in the PPV mass, Table 2 shows the modifications to the orientation angle of the transition dipole moment vector for the absorption bands that peak at 835, 965, 1421, and 1516 cm−1 when the films deposited on Au supports are prepared by annealing at 110 °C using the PPV precursor solutions with different SWNT weight percentages.

ν (cm−1)

θ (deg) values for PPV

θ (deg) values for PPV + 8 wt % SWNTs

835 965 1421 1516

82 77 58 55

72 71 57 55




71 71 61 55

71 70 63 55

69 69 63 55

According to Table 2, the orientation angle of the transition dipole moment vector for the absorption band with a maximum at 1516 cm−1 does not vary, regardless of the SWNT weight percentages added to the PPV precursor solution. In our opinion, this fact indicates that the adsorption of SWNTs noncovalently functionalized with PPV occurs via the C−C stretching vibration mode of the phenyl ring. Significant differences are noted for the orientation angle of the transition dipole moment vector for the absorption band at 835 cm−1 even when a low SWNT weight percentage (8 wt %) is added to precursor solution due to the embedding of SWNTs in the PPV matrix when an interaction of the phenyl ring of the PPV with a side wall of an SWNT occurs. This result agrees with 25543



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reports concerning the interaction of the pyrenyl groups with the basal plane of a graphitic surface via π-stacking and the sidewalls of SWNTs.48−50 This hypothesis is well supported by the variations reported in Figure 6. A progressive decrease in the relative intensity of the RBM band accompanied by an upshift from 174 to 180 cm−1 is observed when the SWNT concentration decreases in the PPV/SWNT composite matrix, a fact that must be correlated with the noncovalent functionalization of SWNTs with PPV. A consequence of the interaction between the SWNTs and the phenyl rings of PPV is the modification of the angle of the transition dipole moment vector for the C−H out-of-plane vibrational mode of the phenyl ring.

(iv) Significant differences are reported for the orientation angle of the transition dipole moment vector for the absorption band at 835 cm−1 even when a low SWNT weight percentage (8 wt %) is added to the precursor solution. This difference results from the functionalization process of SWNTs with PPV, when a π−π* interaction between the sidewall of the carbon nanotubes with the phenyl rings of the PPV occurs. A result of this interaction is a modification of the angle of the transition dipole moment vector for the phenyl ring C−H in the out-of plane vibrational mode.

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4. CONCLUSIONS This paper reports new data concerning the abnormal antiStokes Raman emission (AASRE) of PPV and the molecular orientation of this polymer on metallic supports of Ag and Au. The influence of SWNTs added to the PPV precursor solution on the AASRE and the changes induced by SWNTs to the molecular orientation of PPV chains are also studied. The highlights are given below: (i) In the case of PPV, the AASRE has the features of a CARS effect. This statement is verified by the following relationships: (a) the quadratic increase of anti-Stokes Raman intensities of the main Raman lines of PPV with the vibration wavenumber, (b) the square dependence of the AASRE on the film thickness, and (c) the nonlinear dependence of the AASRE on the exciting laser intensity. (ii) Using the ratio (Iexp/Icalc)anti‑Stokes for Raman line with the maximum at 1593 cm−1 as a measurement of the AASRE strength, a decrease in this ratio is reported when SWNTs are added to the PPV precursor solution, which is a consequence of the noncovalent functionalization of SWNTs with PPV, a process evidenced by a progressive decrease in the relative intensity of the RBM band together with their upshift from 174 to 180 cm−1. The increase of the value of the (Iexp/Icalc)anti‑Stokes ratio associated with the Raman line at −1170 cm−1 is reported when the SWNT concentration in the composite mass increases due to an enhancement process via surface plasmons generated at the interface of the metallic nanotubes and the polymer. (iii) Similar to free-standing PPV films, the FTIR absorption bands situated at 835 and 966 cm−1 strongly depend on the angle of polarization for PPV films deposited on Ag and Au supports. This fact indicates a preferred orientation of the polymer chain in the plane of the film. The FTIR spectra of PPV films deposited on Ag and Au supports, when recorded in the grazing angle incident reflection geometry, reveal significant changes in the absorption band that peaks at 1516 cm−1, assigned to the C−C stretching vibration mode of the phenyl ring, as a function of s or p polarization. For the PPV film deposited on Au and Ag supports, we report that the transition dipole moment vector of the vibration mode of the band with a maximum at 1516 cm−1 is oriented at the angles θ = 55° and 45°, respectively. These angles are different than those reported for free-standing PPV films as a result of the absorption of PPV molecules on the metallic support.

*Tel + 40 21 3690170; Fax + 40 21 3690177; e-mail barac@ infim.ro. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was funded by the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, projects PN-II-ID-PCE-2011-3-0623 and PN-II-ID-PCE-2011-3-0619. This work was performed under the auspices of Scientific Cooperation between the Institute of Materials Jean Rouxel in Nantes and Laboratory of Optical Processes in Nanostructured Materials of the National Institute of Materials Physics, Bucharest.

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Abnormal Anti-Stokes Raman Emission and Infrared Dichroism

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