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1Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA. 2Abbott Laboratories, Abbott Park, Illinois 60064-3500, USA. 3e-mail: ...
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OPTICS LETTERS / Vol. 36, No. 12 / June 15, 2011

Examining surface and bulk structures using combined nonlinear vibrational spectroscopies Chi Zhang,1,* Jie Wang,2 Alexander Khmaladze,1 Yuwei Liu,1 Bei Ding,1 Joshua Jasensky,1 and Zhan Chen1,3 1

Department of Chemistry, University of Michigan, Ann Arbor, Michigan, 48109, USA 2 Abbott Laboratories, Abbott Park, Illinois 60064-3500, USA 3

e-mail: [email protected] *Corresponding author: [email protected] Received January 18, 2011; revised March 11, 2011; accepted April 28, 2011; posted May 11, 2011 (Doc. ID 141285); published June 10, 2011 We combined sum-frequency generation (SFG) vibrational spectroscopy with coherent anti-Stokes Raman scattering (CARS) spectroscopy in one system to examine both surface and bulk structures of materials with the same geometry and without the need to move the sample. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) thin films were tested before and after plasma treatment. The sensitivities of SFG and CARS were tested by varying polymer film thickness and using a lipid monolayer. © 2011 Optical Society of America OCIS codes: 240.4350, 300.6490, 300.6230.

In the last two decades, coherent nonlinear spectroscopies have been rapidly developed due to advances in ultrafast laser techniques. For example, sum-frequency generation (SFG) vibration spectroscopy has evolved into a powerful tool to study surfaces and buried interfaces [1–4]. SFG probes a material’s second-order nonlinear susceptibility, therefore, its selection rule provides intrinsic surface specificity [1]. SFG can probe vibrational modes of functional groups at interfaces, providing molecular insight into interfacial structures of complicated molecules [1–3]. However, SFG cannot provide structural information in a bulk phase with inversion symmetry. Surface structures may or may not be related to the structures of the bulk. To understand properties of the material, it is important to study both surface and bulk structures. Vibrational spectroscopic techniques such as Fourier transform infrared spectroscopy (FTIR) including attenuated total reflectance-FTIR (ATR-FTIR), conventional Raman spectroscopy, and coherent antiStokes Raman scattering (CARS) spectroscopy, which do not have the intrinsic surface selectivity, have been used to examine the structures of bulk or thin films [5,6]. On many occasions, it is important to study the material’s surface and bulk structures in the exact same environment; a slightly varied environment may lead to sample changes. Accordingly, it is important to develop vibrational spectroscopic techniques that can probe both surface and bulk structures without moving the sample or varying the sample environment. ATR-FTIR is a linear absorption spectroscopy that can be used to probe chemical structures throughout a thin film sample, e.g., with a thickness of tens to hundreds of nanometers. The combination of ATR-FTIR and SFG allows for measurement of both the bulk and surface structures of a thin film. However, the two measurement techniques require different experimental geometries. Therefore, different sample preparations for SFG and ATR-FTIR studies are involved, and they can be lengthy. Problems may be caused by alignment and sample mounting in different instruments or sample exposure to changing environments. Conventional Raman spectroscopy is also a linear optical spectroscopy. Its signal collection efficiency is low, and its experimental geometry is quite different from 0146-9592/11/122272-03$15.00/0

that of SFG. Therefore, using a combination of SFG and these linear optical spectroscopies to study both surface and bulk structures in an identical environment is challenging. CARS is a coherent Raman process that probes the third-order nonlinear susceptibility of the sample. CARS microscopy has been developed into a powerful tool to study many different systems [7,8]. CARS spectroscopy has also been used in the study of thin films and biomolecules [6,9], even in tandem with conventional Raman spectroscopy [6]. The signal generated from a CARS process has coherence similar to that of SFG, thus providing the possibility of achieving CARS measurement using the same experimental geometry as SFG. CARS signal collection of polymer thin films based on a commercial SFG system has been previously reported [6]. However, no studies utilizing both SFG and CARS have been developed to provide surface and bulk information concurrently, to our knowledge. Also, CARS spectra collected using SFG system required reconstruction and were only in one polarization combination, and thus cannot measure different third-order hyperpolarizability tensor components.

Fig. 1. Experimental setup. SHG, Second harmonic generation; SFG, sum-frequency generation; OPG/OPA, optical parametric generation/amplification; HWP1-4, half-wave plates; P1-4, polarizers; PMT, photomultiplier tube. Dotted line indicates CARS beam path. © 2011 Optical Society of America

June 15, 2011 / Vol. 36, No. 12 / OPTICS LETTERS

In this research, we collected SFG and CARS vibrational spectra from the same sample to probe its surface and bulk structures without sample relocation. Both SFG and CARS measurements were performed using a SFG spectrometer. It takes only 1 min to change between either technique with no spectral reconstruction required. The combined SFG and CARS studies on polymer thin films after plasma treatment successfully demonstrated the different surface and bulk changes. We also proved that our combined techniques have monolayer sensitivity under different polarization combinations. The experimental setup is shown in Fig. 1. The SFG spectrometer used here (EKSPLA, Vilnius, Lithuania) is composed of a picosecond Nd:YAG laser, a harmonic unit, an optical parametric generation (OPG)/amplification (OPA)/difference frequency generation (DFG) system, and a detection system. The visible beam (532 nm) is generated by frequency-doubling the fundamental output pulses of 20 ps pulsewidth from the Nd:YAG laser. OPG and OPA can generate a signal beam (420 to 680 nm) and an idler beam (740 to 2300 nm). The idler beam and the 1064 nm pump beam are used in DFG to generate a tunable mid-IR light (1000 cm−1 to 4300 cm−1 ). For SFG experiments, the input visible and IR pulse energies are both ∼100 μJ. The incident angles of the visible and IR input beams are 57° and 54° versus the surface normal, respectively. The SFG signal from the surface is collected by a photomultiplier tube (PMT) attached to a monochromator. To collect CARS spectra using the SFG spectrometer, we used the signal output from the OPG/OPA system as the Stokes beam, and the 532 nm visible beam as the pump/probe beam. The pulse energies for the pump/ probe and Stokes beams are ∼300 μJ and ∼100 μJ, respectively. An additional delay line is used to provide temporal overlap of the input beams on sample when shifting from SFG to CARS spectral collection. The CARS Stokes beam reaches the sample at the same angle as the SFG input IR beam. The CARS signal, which can be calculated according to the input Stokes and pump/probe beam

Fig. 2. (a),(b) SFG/CARS spectra of PMMA film before (top) and after (bottom) plasma treatment; (c), (d) SFG/CARS spectra of PS film before (top) and after (bottom) plasma treatment. The symbols are spectra collected in the experiments; curves are fitted results.

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directions, is generated at a different angle compared to the SFG signal. A He-Ne laser is used to track the calculated CARS signal, which is collected by the same monochromator and PMT as SFG. Flip mirrors are used to change between SFG and CARS spectral collections. In this study, SFG spectra were collected using different polarization combinations of ssp (s-polarized SFG, spolarized visible, p-polarized IR), and sps. CARS spectra were collected using ssss (s-polarized CARS, s-polarized probe, s-polarized Stokes, s-polarized pump) and spsp polarization combinations. Poly(methyl methacrylate) (PMMA) (Mw ¼ 75000) and polystyrene (PS) (Mw ¼ 19300) films (∼40 nm) were prepared by spin coating 1 wt% polymer solutions onto clean silica windows at 3000 rpm. We collected SFG and CARS spectra (Fig. 2) from polymer thin films before and after a 5 s air plasma treatment. For PMMA, the dominant peak at 2955 cm−1 in the ssp SFG spectrum is attributed to the C-H symmetric stretch of the ester methyl group, showing that the surface is dominated by the ester methyl groups [10]. After the plasma treatment, the SFG signal intensity decreases, indicating that the surface ester methyl groups undergo changes. Figure 2(b) shows ssss CARS spectra of PMMA films. The strong 2955 cm−1 and weak 3005 cm−1 peaks are due to the ester methyl group symmetric and asymmetric stretches [10]. Two peaks at 2845 and 2885 cm−1 are from the backbone methylene groups. The shoulder at 2935 cm−1 is from the alpha methyl groups. The CARS signals are contributed from the entire PMMA film, so that, compared with SFG, which only probes the surface (dominated by the ester methyl groups), different peaks can be detected. However, PMMA CARS spectra before and after plasma treatment are the same. This indicates that even though the surface structure changes substantially after the plasma treatment, the polymer bulk does not change noticeably. In the PS SFG spectra, various phenyl vibrational modes between 3023 − 3081 cm−1 [11] can be recognized by spectral fitting. After the 5 s plasma treatment, no SFG signal can be resolved. In CARS measurement, both signals from aromatic side groups (3023 − 3081 cm−1 ) and backbone methylene groups (2850 and 2905 cm−1 ) were observed. Similar to PMMA, plasma treatment creates significant changes for the PS surface [12] (i.e., it disorders originally ordered surface aromatic side groups), but not the PS bulk. We tested CARS sensitivity by varying the polymer film thicknesses and using a lipid monolayer. Polymer film

Fig. 3. Absolute value of the peak amplitude/width ratio of the PMMA or PS mode centered at 2935 or 3069 cm−1 as a function of the polymer film thickness.

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Fig. 4. (a) DPPG monolayer SFG spectra collected with ssp and sps polarization combinations; (b) DPPG monolayer CARS spectra collected with ssss and spsp polarization combinations.

thicknesses were varied by using different polymer solution concentrations in toluene (0:2 wt%, 0:5 wt%, 1 wt%, 2 wt%, and 3 wt%) and measured using a depth profilometer. CARS signal intensity can be expressed as I CARS ∝ jχ ð3Þ j2 I p I s I pr :

ð1Þ

I p , I s , and I pr are intensities of the pump, Stokes, and probe beams respectively, and χ ð3Þ has the nonresonant ð3Þ ð3Þ χ NR and resonant χ R contributions  2  ð3Þ X  Ai ð3Þ ð3Þ 2 ð3Þ 2   : jχ j ¼ jχ NR þ χ R j ¼ χ NR þ  Ω − ðω − ω Þ þ iΓ i p s i i ð2Þ The resonant contribution is modeled as the sum of Lorentzians with signal strength or amplitude Ai , frequency Ωi , and linewidth Γi . Resonant susceptibility ð3Þ χ R ∝ N, while N is the number of molecules probed. For thin films, the film thickness is proportional to N. Equation (2) was used to fit the CARS spectra, and the fitted amplitude/width ratio has a linear dependence on the film thickness (when the film thickness is thinner than the coherence length, which is ∼94 nm in our experiment; Fig. 3). The noise level in the CARS spectra is below 1 count; Fig. 3 indicates the feasibility of detecting CARS signal from films as thin as several nanometers. We further demonstrated this by probing a monolayer of lipid molecules. We used the Langmuir–Blodgett method to deposit a monolayer of 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac(1-glycerol)] (DPPG) on glass slides [13]. Figure 4(a) shows ssp and sps SFG spectra from a DPPG monolayer. The 2880 and 2945 cm−1 peaks are attributed to the symmetric stretch and Fermi resonance of the terminal methyl group. A peak at 2960 cm−1 is due to the methyl asymmetric stretch and can be extracted by spectral fitting. Only the asymmetric methyl stretch at 2960 cm−1

is resolvable in the sps spectrum. Figure 4(b) shows ssss and spsp CARS spectra. Two peaks at 2845 and 2885 cm−1 correspond to the methylene symmetric and asymmetric stretches, respectively [14]. In the spsp CARS spectrum, only the methylene asymmetric stretch at 2885 cm−1 was observed. This demonstrates that like SFG, CARS can be used to study monolayers. Since SFG and CARS measure different functional groups and structural parameters [15], the combined SFG and CARS studies on monolayer materials may lead to a more complete picture of its structure. In conclusion, we demonstrate that both second- and third-order nonlinear optical spectroscopic measurements can be performed using the same spectrometer to probe surface and bulk. The same experimental geometry can be adopted and spectral collection can be switched conveniently with flipping mirrors. In the combined spectroscopic studies, surface changes of polymer thin films were observed before and after plasma treatment, but the changes in the bulk were negligible. CARS spectra can be collected from monolayers under different polarization combinations. References 1. Z. Chen, Y. R. Shen, and G. A. Somorjai, Annu. Rev. Phys. Chem. 53, 437 (2002). 2. S. Ye, S. Morita, G. Li, H. Noda, M. Tanaka, K. Uosaki, and M. Osawa, Macromolecules 36, 5694 (2003). 3. X. Zhuang, P. B. Miranda, D. Kim, and Y. R. Shen, Phys. Rev. B 59, 12632 (1999). 4. S. Baldelli, Acc. Chem. Res. 41, 421 (2008). 5. M. W. Urban, Vibrational Spectroscopy of Molecules and Macromolecules on Surfaces (Wiley, 1993). 6. Z. D. Schultz, M. C. Gurau, and L. J. Richter, Appl. Spectrosc. 60, 1097 (2006). 7. A. Zumbusch, G. R. Holtom, and X. S. Xie, Phys. Rev. Lett. 82, 4142 (1999). 8. E. O. Potma and X. S. Xie, J. Raman Spectrosc. 34, 642 (2003). 9. J. Cheng, L. D. Book, and X. S. Xie, Opt. Lett. 26, 1341 (2001). 10. J. Wang, C. Chen, S. M. Buck, and Z. Chen, J. Phys. Chem. B 105, 12118 (2001). 11. K. S. Gautam, A. D. Schwab, A. Dhinojwala, D. Zhang, S. M. Dougal, and M. S. Yeganeh, Phys. Rev. Lett. 85, 3854 (2000). 12. D. Zhang, S. M. Dougal, and M. S. Yeganeh, Langmuir 16, 4528 (2000). 13. K. T. Nguyen, S. V. Le Clair, S. Ye, and Z. Chen, J. Phys. Chem. B 113, 12358 (2009). 14. R. G. Snyder, S. L. Hsu, and S. Krimm, Spectrochim. Acta 34A, 395 (1978). 15. J. Wang, Z. Paszti, M. L. Clarke, X. Chen, and Z. Chen, J. Phys. Chem. B 111, 6088 (2007).