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Preparation and Characterization of Plasma-Polymerized Benzonitrile Films With Ultrafast Optical Kerr Effect.

XIAO HU [*]

Plasma-polymerized benzonitrile (PPBN) thin films were successfully prepared by radio frequency glow discharge techniques. The results of Fourier transform infrared (FTIR), UV-Visible absorption spectra, X-ray photoelectron spectroscopy (XPS) and on-line MS revealed that extensive conjugation has been formed in the PPBN thin films and the polymerization of benzonitrile took place mainly through the opening of carbon-nitrogen triple bonds. The morphology characterized by field emission microscopy (FEM) indicated that the fine, homogenous and brown transparent films could be obtained at comparatively low discharge power. Furthermore, the subpicosecond time-resolved optical Kerr effect (OKE) was used to measure the third-order optical nonlinearity. For the first time, an ultrafast response and non-resonant optical Kerr effect of PPBN films was observed.

INTRODUCTION

Interest in nonlinear optical materials is being driven by the development of fiber optics, laser diodes optical information storage, optical signal processing and optical computing [1, 2]. Conjugated polymers, with strong delocalization of electrons contributing to the large and fast optical response, have provoked increasing interest in their great potential for high technology applications [3, 4]. Polynitrile is a typical conjugated polymer, and was found to exhibit unusually large third-order susceptibility and hyperpolarizability [5] as well as excellent and reproducible electrical bistability [6]. However, progress on the polynitriles has been hampered by the fact that the polymerization of nitrile to the corresponding carbon-nitrogen conjugated polymer is very difficult and cannot occur under conditions typical of polymerization of vinyl and acetylenic monomers [7, 8]. To our knowledge, no reports have yet been made concerning the study of nonlinear optical properties of conjugated polybenzonitrile fil ms.

Plasma polymerization is gaining recognition as an important technique for direct film deposition of entirely new kinds of polymeric materials that are hardly possible to obtain. Some of these polymers, when polymerized using conventional methods, often suffer from many disadvantages such as instability in air and poor solubility in common organic solvents. The films obtained by plasma polymerization are generally of high quality, homogeneous, adherent, and pinhole free [9]. Such films are found to have potential applications in electronic devices, optical wave-guides, thin film lenses, reverse osmosis membranes, photovoltaic membrane and sensors [10, 11].

In this work, the technique of plasma polymerization was used in the preparation of plasma-polymerized benzonitrile (PPBN) thin films, which were directly deposited onto calcium fluoride (Ca[F.sub.2]) plates and potassium bromide (KBr) pellets placed in the reaction chamber under glow discharge. The PPBN films were characterized by FT-IR, UV-vis, XPS, on-line MS and FEM. Furthermore, the third-order optical nonlinearity of plasma-polymerized conjugated PBN films was measured by subpicosecond time-resolved optical Kerr effect.

EXPERIEMENTAL DETAILS

Benzonitrile (minimum purity, 99%) was purchased from TCI, Japan. It is fully degassed under reduced pressure before experiments. Figure 1 shows the molecular structure of benzonitrile monomer and the desired structure of PPBN.

Plasma polymerization of benzonitrile was carried out using a radiofrequency (13.56 MHz) capacitive coupled glow discharge system (Fig. 2). A cylinder-shaped, stainless steel plasma polymerization reactor was fitted with parallel plate electrodes. After evacuation and purging with high pure nitrogen three times, the benzonitrile monomer vapor was introduced directly into the reaction chamber. When the system was adjusted to the pre-set powers, glow discharge was allowed to occur for a certain duration, typically 5-10 min. The pressure of the reaction chamber was maintained at about 5 X [10.sup.-2] torr during the glow discharge. The PPBN films deposited onto calcium fluoride (Ca[F.sub.2]) plates and potassium bromide (KBr) pellets were obtained respectively. The thickness of the PPBN films ranged from 200 nm to 450 nm.

The FT-IR spectra were measured on a Nicolet 750FT-IR spectrometer. The UV-VIS absorption spectra were recorded using a Shimadzu UV-3100 spectrophotometer. Mass spectrum was recorded on a ZLS-250 quadrupole mass spectrometer attached to the plasma chamber. A photoelectron spectrometer (VG ESCA-LAB-5] was used for XPS measurements with an Al K[alpha] X-ray source in a high vacuum of less than [10.sup.-6] Pa. The atomic compositions of the deposited plasma polymers were estimated from the relative peak area and the sensitivity factor of different core levels. An Amray-1910 FE field emission microscope (FEM) was used to examine the surface structure of the PPBN thin films obtained. The Optical Kerr Effect (OKE) measurements were performed using ultrashort pulses generated from a Satori Model 774 ultrafast dye laser. The full width at half maximum (FWHM) of laser pulse was about 165 fs. The detailed procedures have been described elsewhere (12).

RESULTS AND DISCUSSION

Characterization of PPBN Films

Figure 3 shows the FT-IR spectra of benzonitrile monomer and the plasma-polymerized thin films. Assignments for the main absorption bands are listed in Table 1. There appeared a broader and considerably stronger absorption band of polymer around 1600 [cm.sup.-1] attributed to the conjugated C=N stretching vibration as compared with its corresponding monomer. This suggests that extensively conjugated C=N double bonds were formed during the plasma polymerization of benzonitrile. In the meantime, the C[equivalent]N stretching vibration at 2228 [cm.sup.-1] and the C-C[equivalent]N deformation vibration at 548 [cm.sup.-1] were also observed in the polymer spectrum, indicating that a certain amount of C[equivalent]N triple bonds remained unreacted.

The UV-vis spectra of benzonitrile and its corresponding plasma polymerized thin films are shown in Fig. 4. The maximum absorption of benzonitrile at 222 nm shifts to about 246 nm in the polymer spectrum, which is much stronger and extends to the visible region. This also indicates that a larger conjugated [pi]-system has been formed.

Mass spectra recorded in-situ during plasma polymerization can give direct information about the polymerization mechanisms of benzonitrile. The relative intensity of fragment ion peaks at different discharge powers is listed in Table 2. It can be seen that the number of the ionic fragments remained unchanged at lower discharge powers. In fact the mass spectra obtained at low discharge powers were found to have no obvious differences compared with that obtained without discharge. It is clearly evident that the majority of the benzonitrile molecules were intact in the plasma zone reactor under lower discharge powers. This was further confirmed by XPS results. Budzikiewicz (13) pointed out that C-CN bonds are very stable, and few C-CN bonds are cleaved by electron attack during plasma polymerization of rutrile monomers. This also provides strong evidence that the plasma polymerization of benzonitrile took place mainly via the opening of C[equivalent]N triple bonds under lower discharge powers. In the case of higher discha rge powers, on the other hand, both the number of the ionic fragments and the relative intensity exhibited an obvious increase, indicating that more benzonitrile molecules were fragmented during higher power plasma polymerization. In other words, plasma polymerization of benzonitrile at higher discharge powers includes not only the formation of C=N conjugated polymer via the opening of C[equivalent]N triple bonds but also the formation of non-conjugated polymer by the combination of a variety of reactive species.

XPS spectra give important information on the chemical composition of the plasma-polymerized films. Figure 5 shows the XPS spectra of PPBN films. Besides carbon and nitrogen, oxygen was also detected in the XPS measurements. The presence of oxygen is normally expected in the plasma-polymerized films owing to the existence of trace oxygen absorbed on the wall of the reaction chamber system and also to the exposure to the atmosphere after deposition.

In order to get the detailed surface stoichiometry of PPBN films, high-resolution XPS spectra of C 1s and N 1s were evaluated and are shown in Fig. 6 and Fig. 7, respectively. Curve fitting performed on the C 1s spectrum of the PPBN film shows three peaks, located at 284.79, 286.19, and 287.95eV respectively. This suggests that there are three different species of carbon, and the corresponding area ratio of three peaks was approximately 1:5:1 (31.905:160.496:31.905). The observation is in good agreement with the expected three different chemical environments of carbon in PPBN structure. For N 1s spectrum, on the other hand, only one peak at 400.39eV is observed. This also agrees well with the fact that nitrogen has only one chemical environment in the molecular structure of PPBN. The above results further confirm that the plasma polymerization of benzonitrile took place mainly at the unsaturated C[equivalent]N triple bonds rather than at the aromatic ring. In fact, Yasuda (14) has observed that the rates of the plasma polymerization for monomers containing C[equivalent]N triple bond in their molecular structure are greater than those of saturated monomers. This suggests that the C[equivalent]N triple bonds in the monomers are very reactive in the plasma process and are much more subject to cleavage than those without cyano groups.

The morphology of the PPBN films deposited on Ca[F.sub.2] plates at different discharge powers was observed using FEM. The FEM micrographs in Fig. 8 show that high-quality PPBN films (Fig. 8a and 8b) could be obtained under the relative lower discharge powers. They are transparent, homogeneous and pinhole-free, suitable for the measurement of nonlinear optical properties. In fact, the films are so smooth that it was difficult to obtain good focus during the observation by FEM. On the other hand, in the case of higher discharge power such as 60W, some obvious folds were formed on the surface of PPBN films (Fig. 8c). To the best of our knowledge, there is no similar phenomenon as yet reported in the literature. This can be attributed to a combination of two factors. First, the higher discharge power is favorable for activating more monomer molecules to join polymerization. Second, the concentration of the free radical on the surface of substrate increases rapidly with increasing discharge power, which has been confirmed by our electron spin resonance (ESR) results. The above two factors will result in higher film forming rate on the surface of substrate.

Ultrafast Optical Kerr Effect of PPBN Films

The third-order optical nonlinearity of a PPBN film was measured by subpicosecond time-resolved optical Kerr effect at the wavelength of 647 rim. Because the absorption of PPBN films is negligible at the wavelength of 647 rim used in our laser (see Fig. 4), the nonresonant third-order nonlinearities of PPBN films could be measured from the time-resolved transient optical Kerr signals as shown in Fig. 9. It can be seen that the PPBN film has a strong OKE signal and the third-order nonlinear susceptibility calculated according to Kleinman's conjecture (15) is 3.0 X [10.sup.-12] esu, obviously larger than that of the PBN solution (12). This is due to the much larger molecule number density in the solid film than that in the solution, especially to the much larger delocalized [pi]-electrons conjugated system formed in the film. In addition, we also found that the signal profile was approximately symmetric with respect to the delay time, which indicates a primarily pulse-width-limited response. To estimate the re laxation time of Kerr medium, the experimental curve was fitted with an exponential function (16). It was observed that the time constant of PPBN film is less than the pulse duration. Thus, it can be deduced that the relaxation time of the PPBN film is shorter than the laser pulse width, i.e., less than 165 fs. The ultrafast optical response may be resulted from the [pi]-electroncloud distortion occurring upon the non-resonant excitation (17).

CONCLUSIONS

FT-IR, UV-vis, XPS and on-line MS studies revealed that a large [pi]-conjugated system has been formed in the PPBN thin films, and the polymerization of benzonitrile monomer took place mainly through the opening of carbon-nitrogen triple bonds by the free radical polymerization process.

Smooth and homogeneous plasma-polymerized poly-benzontrile films with strongly delocalized [pi]-electrons along the backbones could be obtained at the relative lower discharge powers. In the case of higher discharge powers, some obvious folds were formed on the surface of PPBN films owing to the higher film deposition rate.

The extremely fast and larger nonlinear response of PPBN film was observed, which shows that PPBN film is a promising material for applications as fast optical switches and modulators. Further studies on the effects of the quality of PBN films such as the thickness and morphology on the third-order optical nonlinearity as well as the relaxation time are being undertaken.

(*.) Corresponding author.

REFERENCES

(1.) P. N. Prasad, J. Swiatkiewicz, and J. Ptleger, Mol. Cryst. Liq. Cryst., 160, 53 (1988).

(2.) J. L. Bredas and R. R Chance, Conjugated Polymer Materials-Opportunities in Electronics, and Molecular Electronics, Kluwer Academic Publishers, Dordrecht (1991).

(3.) W. R. Salaneck, I. Lundstrom, and B. Ranby, Conjugated Polymer and Related Materials--The Interaction of Chemical and Electronic Structure, Oxford University Press, Oxford (1993).

(4.) P. N. Prasad and D. J. Williams, Introduction to Nonlinear Optical Effects in Molecules & Polymers, Wiley, New York (1991).

(5.) Q. H. Gong, D. Qing, and H. Y. Chen, Proceeding of International Conference on Laser, STSs Press, Mclean (1992).

(6.) H. Y. Chen, Y. K. He, and F. Geng, Chin. Chem, Lett., 5, 197 (1994).

(7.) V. A. Kabanov and V. P. Zubov, J. Polym. Sci., Part C, 4, 1009 (1964).

(8.) E. Oikawa and S. Kambara, Polym. Lett. 2, 649 (1964).

(9.) H. Yasuda, Plasma Polymerization, Academic Press, New York (1985).

(10.) R. K. Sadhir, W. J. James, and R. A. Auerbach, Thin Solid Films, 97, 17 (1982).

(11.) W. B. Liang, M. A. Masse, and F. E. Kasasz, Polymer, 35, 3101 (1992).

(12.) C. F. Wang, X. Y. Zhao, Z. J. Xia, and Y. H. Zou, Appl. Phys., B 64, 45 (1997).

(13.) H. Budzikiewicz, C. Djerassi, and D. H. Williams, Mass Spectrometry of Organic Compounds, Holden Day, San Francisco (1967).

(14.) H. Yasuda, J. Polym. Sci. Macromol. Rev., 16, 199 (1981).

(15.) D. A. Kelinman, Phys. Rev., 126, 1977 (1962].

(16.) P. P. Ho and R. R. Alfano, Phys. Rev., A20, 2170 (1979).

(17.) Y. R. Shen, The Principle of Nonlinear Optics, Wiley, New York (1984).
 Assignment of Main FT-IR Absorption
 Bands of PPBN Films.
Absorption Bands
 ([cm.sup.-1]) Assignment
 3037 aromatic C-H stretching vibration
 2228 C[equivalent]N stretching vibration
 1604 conjugated C=N stretching vibration
 1581
 1483 benzene ring backbone stretching vibration
 1447
 690 aromatic C-H nonplanar bending vibration
 760 (monosubstituted benzene rings)
 548 C-CN deformation vibration)
 The Relative Intensity of Benzonitrile
 Ionized Mass Peaks.
 Relative Intensity (%)
Reaction Conditions [C.sub.6][H.sub.5][CN.sup.+] [C.sub.6][[H.sub.5].sup.+]
Without discharge 100 38.8
Discharge at 40W 100 40.8
Discharge at 60W 100 63.4
Reaction Conditions [C.sub.4][[H.sub.3].sup.+] [C.sub.4][[H.sub.2].sup.+]
Without discharge 13.8 7.7
Discharge at 40W 10.8 9.2
Discharge at 60W 25.3 13.7
Reaction Conditions [CN.sup.+]
Without discharge --
Discharge at 40W --
Discharge at 60W 6.3
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Author:ZHAO, XIONGYAN; HU, XIAO; HE, YUANKANG; CHEN, HUIYING
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:Dec 1, 2000
Words:2485
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