- Email: [email protected]
Chemical modifications of polystyrene under swift Ar ion irradiation: A study of the energy loss effects
Nuclear Instruments and Methods in Physics Research B 169 (2000) 83±88
www.elsevier.nl/locate/nimb
Chemical modi®cations of polystyrene under swift Ar ion irradiation: A study of the energy loss eects Zhiyong Zhu *, Yunfan Jin, Changlong Liu, Youmei Sun, Mingdong Hou, Chonghong Zhang, Zhiguang Wang, Jie Liu, Xiaoxi Chen, Baoquan Li, Yanbin Wang Institute of Modern Physics, Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, People's Republic of China
Abstract Polystyrene (PS) ®lms of about 53 lm in thickness are stacked together and are irradiated with 1.37 GeV Ar ions at room temperature to ¯uences ranging from 1.1 ´ 1010 to 5.5 ´ 1012 cmÿ2 . The radiations induced chemical changes are studied by the Fourier transform infrared (FTIR) and ultraviolet/visible (UV/Vis) spectroscopies. It is found that the material undergoes serious degradation under irradiation and the chemical modi®cations depend strongly on the electronic energy loss. The main chains of PS as well as the phenyl ring are destroyed in the track core simultaneously with a damage cross-section of about 28.6 nm2 . Signi®cant reduction in the absorbance of bands at 1602, 2853 and 3059 cmÿ1 occurs above about 0.77 keV/nm at an energy deposition of about 6.4 MGy. The stronger reduction in intensity of the band at 706 cmÿ1 compared to the band at 1602 cmÿ1 is partly attributed to cross-links outside the track core. Alkynes are produced above a threshold of about 0.77 keV/nm. The progressive shift of the absorption edge from UV towards the visible is attributed to the carbonization of the material. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.41.+e; 78.66.Qn; 82.50.Gw; 82.80.Ch Keywords: Swift Ar ion irradiation; Polystyrene; Chemical modi®cation; Electronic energy loss
1. Introduction Ion irradiation of polymers induces irreversible changes of target properties and the study of these eects is important for technological applications such as ion beam lithography, improvement of polymer electrical [1] and mechanical properties [2], etc. Previous studies have shown that the changes induced by irradiation depend both on materials and on the energy deposition processes *
Corresponding author. Fax: +86-931-8881100. E-mail address: [email protected] (Z. Zhu).
[3,4]. However, there is still a lack of detailed knowledge about the correlation between structure changes and macroscopic properties, and the role of dierent energy deposition mechanisms in the modi®cation processes [4,5]. Energetic heavy ions in matter lose energy mainly through electronic excitation and ionization. Most of the primary ionizations and excitations occur close to the ion trajectory in a core of few nanometers in diameter. The dense energy deposition in the track core could be so high that actually all bonds inside the track can be destroyed. It is therefore expected that the damage
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 2 1 - 5
84
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
created by high-energy heavy ions would be different from that induced by low-energy light ions. In this work the energy loss eects on chemical modi®cations of polystyrene (PS) under swift Ar ion irradiation are studied through optical absorption measurements. 2. Experimental details PS samples, formed by stacking 32 PS ®lms of about 53 lm thickness together, are irradiated in vacuum and under normal incidence with swift Ar ions to ¯uences of 1:1 1010 ; 1:1 1011 ; 5:5 1011 ; 1:1 1012 and 5:5 1012 cmÿ2 , respectively. The irradiations are conducted at the radiation terminal of the Heavy Ion Research Facility in Lanzhou (HIRFL) by using 1.4 GeV Ar17 ions. During irradiation the beam ¯ux is controlled at about 5 107 cmÿ2 sÿ1 to avoid extra heating of the samples. The ¯uence is continuously monitored by measuring the emission of secondary electrons induced when the ions passed through an 8-lm thick stack of Al/Al/Al foils in front of the sample and is calibrated by a Faraday cup. The energy of Ar ions coming through the foils is estimated by the TRIM program [6] to be around 1.374 GeV. Accordingly the average energy losses of the ion in each of the PS ®lms are calculated. The irradiated PS ®lms are studied by Fourier transform infrared (FTIR) and ultraviolet/visible (UV/Vis) spectroscopies after being placed in air condition for several months. Only the PS ®lms passed by ions are selected for analysis in order to avoid any implantation eects. The FTIR spectra are collected by using a Perkin Elmer spectrophotometer of model 1640 working in specula re¯ection mode and are analysed using base line method. The UV/Vis spectra are collected in transmission mode by a Lambda 9 Perkin Elmer UV/Vis spectrometer in the wavelength ranging from 200 to 2500 nm. 3. Results and discussion 3.1. UV/Vis Fig. 1 shows the UV/Vis spectra for PS ®lms irradiated to 5.5 ´ 1012 cmÿ2 but at dierent elec-
Fig. 1. UV/Vis transmission spectra of 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
tronic stopping powers. With increase of electronic energy loss the ®lms become gradually opaque to the visible light and the absorption edge shifts from UV towards the visible. This is consistent with the observation that the material changes from transparence to brown and ®nally deep dark with increase of energy deposition. The shifting of absorption edge towards visible is generally considered to result from carbonization of the material under irradiation [7]. During irradiation PS loses gradually hydrogen atoms and the enrichment of carbon atoms leads to the formation of hydrogenated amorphous carbon with optical energy gaps depending on the H/C atom ratio [7]. In the present case it is believed that carbonization occurs mainly in the track core where the energy deposition could reach several keV/nm3 . In Fig. 2(a) and (b) the radiation-induced changes in absorbance at various wavelengths are shown as a function of the Ar ion ¯uence at a constant energy loss value of about 1.86 keV/nm and as a function of the electronic energy loss at a constant ¯uence of about 5:5 1012 cmÿ2 , respectively. Here the change in absorbance (A ) A0 ) is de®ned as the absorbance after irradiation minus the absorbance before irradiation. It can be seen that the A ) A0 is roughly linear with ¯uence but shows a power law dependence on the electronic energy loss with the power increasing from 2.5 to 3.4 with increasing wavelength from 400 to 600 nm. According to [8], the data in Fig. 2(a) can be
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
85
Fig. 2. Radiation-induced changes in absorbance at various wavelengths: (a) as a function of ¯uence at a constant energy loss value of about 1.86 keV/nm and (b) as a function of the electronic energy loss at a constant ¯uence of about 5.5 ´ 1012 cmÿ2 .
®tted by the relation A ÿ A0 kk nion rU, where nion is the number of chromophore that absorb at a certain k created per ion and per unit area, kk the absorption coecient of the chromophores at the wavelength k, r the production cross-section of the chromophores and U the ion ¯uence. Table 1 lists the ®tting results of kk nion r as a whole. It can be seen that kk nion r decreases with increase of wavelength. The decrease of kk nion r with increase of wavelength can be accounted if one considers that the chromophores which absorb at longer wavelength are those with high degree of conjugation. Since the production of the chromophores with high degree of conjugation needs higher energy deposition, they should be produced mainly in small regions close to the track centre. The power law dependence of A ) A0 on the electronic energy loss (Fig. 2(b)) means that kk nion r scales as (dE/ dX)n with n 2.5 for the wavelength of 400 nm. Similar dependence has been reported for PPS [8] and PC [9]. Table 1 Fitting results of the data in Fig. 2(a) by using the relation A ÿ A0 kk nion rU (see text for details) Wavelength (nm)
kk nion r (cm2 )
400 450 500 600
3:6 10ÿ13 1:4 10ÿ13 5:3 10ÿ14 9:6 10ÿ15
3.2. FTIR The FTIR spectra after irradiation show an overall reduction in intensity of the typical bands of virgin PS. Also some new bands appeared. Figs. 3 and 4 show the selected areas of FTIR spectra obtained for PS ®lms irradiated to 5:5 1012 cmÿ2 but at dierent electronic stopping powers. In the wavenumber range 4000±2500 cmÿ1 (Fig. 3), one can see the bands corresponding to C±H stretching of aromatic ring (band at 3059 cmÿ1 ) and C±H symmetric stretching of ±CH2 ± (band at 2853 cmÿ1 ), etc. With increase of electronic energy loss the intensity of these bands reduces. This is accompanied by the appearance of the O±H bonds, which give a broad band around 3465 cmÿ1 . Besides, alkyne end group (R±C¹C± H, band at 3296 cmÿ1 ) shows up at high electronic energy loss. In the wavenumber region from 1800 to 600 cmÿ1 (Fig. 4) typical vibration modes corresponding to bending of phenyl ring (1602 cmÿ1 ), out-plane bending mode of hydrogen attached to phenyl ring (706 cmÿ1 ), etc., can be seen. In Fig. 5(a) and (b) the normalized absorbance A/A0 (infrared absorbance after irradiation/infrared absorbance of the pristine sample) is shown for the 706, 1602 2853 and 3059 cmÿ1 bands as a function of the Ar ion ¯uence at a constant energy loss value of about 1.86 keV/nm and as a function of the electronic energy loss at a constant ¯uence of about 5:5 1012 cmÿ2 , respectively. It can be
86
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
Fig. 3. FTIR spectra in the wavenumber region of 4000±2500 cmÿ1 obtained for PS irradiated with 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
Fig. 4. FTIR spectra in the wavenumber region of 1800±600 cmÿ1 obtained for PS irradiated with 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
seen that the decay of the bands at 1602, 2853 and 3059 cmÿ1 is almost the same with the absorbance decreasing gradually with increase of ion ¯uence
and electronic energy loss. Signi®cant changes occur above 0.77 keV/nm at the ion ¯uence of 5:5 1012 cmÿ2 , corresponding to an energy deposition of about 6.4 MGy. The decay of the band at 706 cmÿ1 follows the same manner but is much stronger than the other vibration modes. It indicates that the vibration of the out-plane bending mode of hydrogen attached to phenyl ring are more restricted by the irradiation. The data points in Fig. 5(a) are ®tted by using the relation A=A0 1 ÿ c1 ÿ exp ÿrU, where r is the damage cross-section and c is a constant. Damage cross-sections around 28.6 nm2 are found for the bands of 1602, 2853 and 3059 cmÿ1 . For the band at 706 cmÿ1 a damage cross-section of about 111.2 nm2 is extracted. Phenyl ring in polycarbonate has been found to be the most stable functional group at low ¯uence under light ion irradiation [10]. Swift heavy ion deposits tremendous energy in the track core of a few nanometers in diameter within an extremely short time of about 10ÿ15 s. The energy density in the track core could go several keV/nm3 . Considering that the energy of carbon bonds is only a few eV, this high-energy deposition is sucient to break all bonds in the track core. Based on this consideration, we can conclude that the phenyl ring will be destroyed in the same way as the other functional groups in the track core. The larger reduction in intensity of the band at 706 cmÿ1 compared to that of the band of phenyl ring at 1602 cmÿ1 could be due to the lower stability of the corresponding bond under irradiation. Since a certain amount of energetic electrons emitted from the ionized target atoms will come to the region far from the track core, the bonds that are sensitive to radiolysis will change in the region which is referred to as track hallo. It has been known that for PS mainly cross-link occurs under light ion irradiation [11], we therefore suppose that the intensity change of the band at 706 cmÿ1 are partly related to the cross-link of macromolecular in PS in the track hallo. In Fig. 6 the absorbances of the bands at 3465 and 3296 cmÿ1 corresponding to O±H bonds and C±H stretching of alkynes, are shown as a function of the electronic energy loss. It can be seen that a threshold, which is around 0.77 keV/nm, is
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
87
Fig. 5. Radiation induced changes in absorbance: (a) as a function of ion ¯uence and (b) as a function of electronic energy loss for the 706, 1602, 2853 and 3059 cmÿ1 bands.
formation of triple bonds under irradiation is speci®c to high energy loss and insensitive to the chemical composition. Since the formation of alkynes requires a remarkable reorganization of molecular bonds, it can therefore be produced only in track cores, where the energy deposition is suciently high.
4. Summary and conclusion
Fig. 6. Absorbances of bands corresponding to O±H bonds (3465 cmÿ1 ) and alkyne end groups (3296 cmÿ1 ) as a function of electronic energy loss.
present for the band at 3296 cmÿ1 . Above the threshold the absorbance increases linearly with increase of electronic energy loss. The production of alkynes has been found in Sn (5.2 MeV/u) [12] irradiated polyethylene, PS, polyvinylidene ¯uoride and in Kr (8.6 MeV/u) as well as in Mo (5.6 MeV/u) irradiated polyethylene terephthalate [13]. However it has not been detected after electron and carbon irradiations [12]. The independence of alkyne production on polymer structure and threshold for the production indicates that the
PS ®lms are stacked together and are irradiated in normal incidence with 1.37 GeV Ar ions at room temperature. FTIR specula re¯ection and UV/Vis transmission spectra are measured to investigate the modi®ed material. It is found that the main chains of PS as well as the phenyl ring are destroyed in the track core simultaneously with damage cross-sections of about 28.6 nm2 . Signi®cant reduction in absorbances of the bands at 1602, 2853 and 3059 cmÿ1 occurs above 0.77 keV/nm at an energy deposition of about 6.4 MGy. The stronger reduction in intensity of the band at 706 cmÿ1 compared to the band at 1602 cmÿ1 is partly attributed to cross-links outside the track core. Alkynes are produced above a threshold of about 0.77 keV/nm. The progressive shift of the absorption edge from UV towards the visible is attributed to the carbonization of the material.
88
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
Acknowledgements This work is supported by the foundation of the Chinese Academy of Sciences (projected no. KJ952-S1-423), the foundation of XIBUZHIGUANG and the Natural Science foundation of Gansu province. References [1] T. Hioki, S. Noda, M. Sugiura, M. Kakeno, K. Yamada, J. Kawamoto, Appl. Phys. Lett. 43 (1983) 30. [2] G.R. Rao, E.H. Lee, L.K. Mansur, Wear 162±164 (1993) 739. [3] L. Calcagno, G. Compagnini, G. Foti, Nucl. Instr. and Meth. B 65 (1992) 413. [4] F. Abel, V. Quillet, M. Schott, Nucl. Instr. and Meth. B 105 (1995) 86.
[5] O. Puglisi, A. Licciardello, S. Pignataro, L. Calcagno, G. Foti, Radiat. E. 98 (1986) 161. [6] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. [7] G. Compagnini, G. Foti, R. Reitano, G. Mondio, Appl. Phys. Lett. 57 (1990) 2546. [8] L.S. Farenzena, R.M. Papaleo, A. Hallen, M.A. de Ara ujo, R.P. Livi, B.U.R. Sundqvist, Nucl. Instr. and Meth. B 105 (1995) 134. [9] Y. Wang, Y. Jin, Z. Zhu, C. Liu, Y. Sun, Z. Wang, M. Hou, X. Chen, C. Zhang, J. Liu, B. Li, Nucl. Instr. and Meth. B 164±165 (2000) 420. [10] C. Gagnadre, J.L. Decossas, J.C. Vareille, Nucl. Instr. and Meth. B 73 (1993) 48. [11] Ma.E. Martinez-Pardo, J. Cardoso, H. Vazquez, M. Aguilar, J. Rickards, E. Andrade, Nucl. Instr. and Meth. B 131 (1997) 219. [12] E. Balanzat, N. Betz, S. Bouard, Nucl. Instr. and Meth. B 105 (1995) 46. [13] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instr. and Meth. B 131 (1997) 159.
www.elsevier.nl/locate/nimb
Chemical modi®cations of polystyrene under swift Ar ion irradiation: A study of the energy loss eects Zhiyong Zhu *, Yunfan Jin, Changlong Liu, Youmei Sun, Mingdong Hou, Chonghong Zhang, Zhiguang Wang, Jie Liu, Xiaoxi Chen, Baoquan Li, Yanbin Wang Institute of Modern Physics, Chinese Academy of Sciences, P.O. Box 31, Lanzhou 730000, People's Republic of China
Abstract Polystyrene (PS) ®lms of about 53 lm in thickness are stacked together and are irradiated with 1.37 GeV Ar ions at room temperature to ¯uences ranging from 1.1 ´ 1010 to 5.5 ´ 1012 cmÿ2 . The radiations induced chemical changes are studied by the Fourier transform infrared (FTIR) and ultraviolet/visible (UV/Vis) spectroscopies. It is found that the material undergoes serious degradation under irradiation and the chemical modi®cations depend strongly on the electronic energy loss. The main chains of PS as well as the phenyl ring are destroyed in the track core simultaneously with a damage cross-section of about 28.6 nm2 . Signi®cant reduction in the absorbance of bands at 1602, 2853 and 3059 cmÿ1 occurs above about 0.77 keV/nm at an energy deposition of about 6.4 MGy. The stronger reduction in intensity of the band at 706 cmÿ1 compared to the band at 1602 cmÿ1 is partly attributed to cross-links outside the track core. Alkynes are produced above a threshold of about 0.77 keV/nm. The progressive shift of the absorption edge from UV towards the visible is attributed to the carbonization of the material. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 61.41.+e; 78.66.Qn; 82.50.Gw; 82.80.Ch Keywords: Swift Ar ion irradiation; Polystyrene; Chemical modi®cation; Electronic energy loss
1. Introduction Ion irradiation of polymers induces irreversible changes of target properties and the study of these eects is important for technological applications such as ion beam lithography, improvement of polymer electrical [1] and mechanical properties [2], etc. Previous studies have shown that the changes induced by irradiation depend both on materials and on the energy deposition processes *
Corresponding author. Fax: +86-931-8881100. E-mail address: [email protected] (Z. Zhu).
[3,4]. However, there is still a lack of detailed knowledge about the correlation between structure changes and macroscopic properties, and the role of dierent energy deposition mechanisms in the modi®cation processes [4,5]. Energetic heavy ions in matter lose energy mainly through electronic excitation and ionization. Most of the primary ionizations and excitations occur close to the ion trajectory in a core of few nanometers in diameter. The dense energy deposition in the track core could be so high that actually all bonds inside the track can be destroyed. It is therefore expected that the damage
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 2 1 - 5
84
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
created by high-energy heavy ions would be different from that induced by low-energy light ions. In this work the energy loss eects on chemical modi®cations of polystyrene (PS) under swift Ar ion irradiation are studied through optical absorption measurements. 2. Experimental details PS samples, formed by stacking 32 PS ®lms of about 53 lm thickness together, are irradiated in vacuum and under normal incidence with swift Ar ions to ¯uences of 1:1 1010 ; 1:1 1011 ; 5:5 1011 ; 1:1 1012 and 5:5 1012 cmÿ2 , respectively. The irradiations are conducted at the radiation terminal of the Heavy Ion Research Facility in Lanzhou (HIRFL) by using 1.4 GeV Ar17 ions. During irradiation the beam ¯ux is controlled at about 5 107 cmÿ2 sÿ1 to avoid extra heating of the samples. The ¯uence is continuously monitored by measuring the emission of secondary electrons induced when the ions passed through an 8-lm thick stack of Al/Al/Al foils in front of the sample and is calibrated by a Faraday cup. The energy of Ar ions coming through the foils is estimated by the TRIM program [6] to be around 1.374 GeV. Accordingly the average energy losses of the ion in each of the PS ®lms are calculated. The irradiated PS ®lms are studied by Fourier transform infrared (FTIR) and ultraviolet/visible (UV/Vis) spectroscopies after being placed in air condition for several months. Only the PS ®lms passed by ions are selected for analysis in order to avoid any implantation eects. The FTIR spectra are collected by using a Perkin Elmer spectrophotometer of model 1640 working in specula re¯ection mode and are analysed using base line method. The UV/Vis spectra are collected in transmission mode by a Lambda 9 Perkin Elmer UV/Vis spectrometer in the wavelength ranging from 200 to 2500 nm. 3. Results and discussion 3.1. UV/Vis Fig. 1 shows the UV/Vis spectra for PS ®lms irradiated to 5.5 ´ 1012 cmÿ2 but at dierent elec-
Fig. 1. UV/Vis transmission spectra of 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
tronic stopping powers. With increase of electronic energy loss the ®lms become gradually opaque to the visible light and the absorption edge shifts from UV towards the visible. This is consistent with the observation that the material changes from transparence to brown and ®nally deep dark with increase of energy deposition. The shifting of absorption edge towards visible is generally considered to result from carbonization of the material under irradiation [7]. During irradiation PS loses gradually hydrogen atoms and the enrichment of carbon atoms leads to the formation of hydrogenated amorphous carbon with optical energy gaps depending on the H/C atom ratio [7]. In the present case it is believed that carbonization occurs mainly in the track core where the energy deposition could reach several keV/nm3 . In Fig. 2(a) and (b) the radiation-induced changes in absorbance at various wavelengths are shown as a function of the Ar ion ¯uence at a constant energy loss value of about 1.86 keV/nm and as a function of the electronic energy loss at a constant ¯uence of about 5:5 1012 cmÿ2 , respectively. Here the change in absorbance (A ) A0 ) is de®ned as the absorbance after irradiation minus the absorbance before irradiation. It can be seen that the A ) A0 is roughly linear with ¯uence but shows a power law dependence on the electronic energy loss with the power increasing from 2.5 to 3.4 with increasing wavelength from 400 to 600 nm. According to [8], the data in Fig. 2(a) can be
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
85
Fig. 2. Radiation-induced changes in absorbance at various wavelengths: (a) as a function of ¯uence at a constant energy loss value of about 1.86 keV/nm and (b) as a function of the electronic energy loss at a constant ¯uence of about 5.5 ´ 1012 cmÿ2 .
®tted by the relation A ÿ A0 kk nion rU, where nion is the number of chromophore that absorb at a certain k created per ion and per unit area, kk the absorption coecient of the chromophores at the wavelength k, r the production cross-section of the chromophores and U the ion ¯uence. Table 1 lists the ®tting results of kk nion r as a whole. It can be seen that kk nion r decreases with increase of wavelength. The decrease of kk nion r with increase of wavelength can be accounted if one considers that the chromophores which absorb at longer wavelength are those with high degree of conjugation. Since the production of the chromophores with high degree of conjugation needs higher energy deposition, they should be produced mainly in small regions close to the track centre. The power law dependence of A ) A0 on the electronic energy loss (Fig. 2(b)) means that kk nion r scales as (dE/ dX)n with n 2.5 for the wavelength of 400 nm. Similar dependence has been reported for PPS [8] and PC [9]. Table 1 Fitting results of the data in Fig. 2(a) by using the relation A ÿ A0 kk nion rU (see text for details) Wavelength (nm)
kk nion r (cm2 )
400 450 500 600
3:6 10ÿ13 1:4 10ÿ13 5:3 10ÿ14 9:6 10ÿ15
3.2. FTIR The FTIR spectra after irradiation show an overall reduction in intensity of the typical bands of virgin PS. Also some new bands appeared. Figs. 3 and 4 show the selected areas of FTIR spectra obtained for PS ®lms irradiated to 5:5 1012 cmÿ2 but at dierent electronic stopping powers. In the wavenumber range 4000±2500 cmÿ1 (Fig. 3), one can see the bands corresponding to C±H stretching of aromatic ring (band at 3059 cmÿ1 ) and C±H symmetric stretching of ±CH2 ± (band at 2853 cmÿ1 ), etc. With increase of electronic energy loss the intensity of these bands reduces. This is accompanied by the appearance of the O±H bonds, which give a broad band around 3465 cmÿ1 . Besides, alkyne end group (R±C¹C± H, band at 3296 cmÿ1 ) shows up at high electronic energy loss. In the wavenumber region from 1800 to 600 cmÿ1 (Fig. 4) typical vibration modes corresponding to bending of phenyl ring (1602 cmÿ1 ), out-plane bending mode of hydrogen attached to phenyl ring (706 cmÿ1 ), etc., can be seen. In Fig. 5(a) and (b) the normalized absorbance A/A0 (infrared absorbance after irradiation/infrared absorbance of the pristine sample) is shown for the 706, 1602 2853 and 3059 cmÿ1 bands as a function of the Ar ion ¯uence at a constant energy loss value of about 1.86 keV/nm and as a function of the electronic energy loss at a constant ¯uence of about 5:5 1012 cmÿ2 , respectively. It can be
86
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
Fig. 3. FTIR spectra in the wavenumber region of 4000±2500 cmÿ1 obtained for PS irradiated with 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
Fig. 4. FTIR spectra in the wavenumber region of 1800±600 cmÿ1 obtained for PS irradiated with 1.37 GeV Ar ion irradiated polystyrene to 5.5 ´ 1012 cmÿ2 but at dierent electronic energy loss.
seen that the decay of the bands at 1602, 2853 and 3059 cmÿ1 is almost the same with the absorbance decreasing gradually with increase of ion ¯uence
and electronic energy loss. Signi®cant changes occur above 0.77 keV/nm at the ion ¯uence of 5:5 1012 cmÿ2 , corresponding to an energy deposition of about 6.4 MGy. The decay of the band at 706 cmÿ1 follows the same manner but is much stronger than the other vibration modes. It indicates that the vibration of the out-plane bending mode of hydrogen attached to phenyl ring are more restricted by the irradiation. The data points in Fig. 5(a) are ®tted by using the relation A=A0 1 ÿ c1 ÿ exp ÿrU, where r is the damage cross-section and c is a constant. Damage cross-sections around 28.6 nm2 are found for the bands of 1602, 2853 and 3059 cmÿ1 . For the band at 706 cmÿ1 a damage cross-section of about 111.2 nm2 is extracted. Phenyl ring in polycarbonate has been found to be the most stable functional group at low ¯uence under light ion irradiation [10]. Swift heavy ion deposits tremendous energy in the track core of a few nanometers in diameter within an extremely short time of about 10ÿ15 s. The energy density in the track core could go several keV/nm3 . Considering that the energy of carbon bonds is only a few eV, this high-energy deposition is sucient to break all bonds in the track core. Based on this consideration, we can conclude that the phenyl ring will be destroyed in the same way as the other functional groups in the track core. The larger reduction in intensity of the band at 706 cmÿ1 compared to that of the band of phenyl ring at 1602 cmÿ1 could be due to the lower stability of the corresponding bond under irradiation. Since a certain amount of energetic electrons emitted from the ionized target atoms will come to the region far from the track core, the bonds that are sensitive to radiolysis will change in the region which is referred to as track hallo. It has been known that for PS mainly cross-link occurs under light ion irradiation [11], we therefore suppose that the intensity change of the band at 706 cmÿ1 are partly related to the cross-link of macromolecular in PS in the track hallo. In Fig. 6 the absorbances of the bands at 3465 and 3296 cmÿ1 corresponding to O±H bonds and C±H stretching of alkynes, are shown as a function of the electronic energy loss. It can be seen that a threshold, which is around 0.77 keV/nm, is
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
87
Fig. 5. Radiation induced changes in absorbance: (a) as a function of ion ¯uence and (b) as a function of electronic energy loss for the 706, 1602, 2853 and 3059 cmÿ1 bands.
formation of triple bonds under irradiation is speci®c to high energy loss and insensitive to the chemical composition. Since the formation of alkynes requires a remarkable reorganization of molecular bonds, it can therefore be produced only in track cores, where the energy deposition is suciently high.
4. Summary and conclusion
Fig. 6. Absorbances of bands corresponding to O±H bonds (3465 cmÿ1 ) and alkyne end groups (3296 cmÿ1 ) as a function of electronic energy loss.
present for the band at 3296 cmÿ1 . Above the threshold the absorbance increases linearly with increase of electronic energy loss. The production of alkynes has been found in Sn (5.2 MeV/u) [12] irradiated polyethylene, PS, polyvinylidene ¯uoride and in Kr (8.6 MeV/u) as well as in Mo (5.6 MeV/u) irradiated polyethylene terephthalate [13]. However it has not been detected after electron and carbon irradiations [12]. The independence of alkyne production on polymer structure and threshold for the production indicates that the
PS ®lms are stacked together and are irradiated in normal incidence with 1.37 GeV Ar ions at room temperature. FTIR specula re¯ection and UV/Vis transmission spectra are measured to investigate the modi®ed material. It is found that the main chains of PS as well as the phenyl ring are destroyed in the track core simultaneously with damage cross-sections of about 28.6 nm2 . Signi®cant reduction in absorbances of the bands at 1602, 2853 and 3059 cmÿ1 occurs above 0.77 keV/nm at an energy deposition of about 6.4 MGy. The stronger reduction in intensity of the band at 706 cmÿ1 compared to the band at 1602 cmÿ1 is partly attributed to cross-links outside the track core. Alkynes are produced above a threshold of about 0.77 keV/nm. The progressive shift of the absorption edge from UV towards the visible is attributed to the carbonization of the material.
88
Z. Zhu et al. / Nucl. Instr. and Meth. in Phys. Res. B 169 (2000) 83±88
Acknowledgements This work is supported by the foundation of the Chinese Academy of Sciences (projected no. KJ952-S1-423), the foundation of XIBUZHIGUANG and the Natural Science foundation of Gansu province. References [1] T. Hioki, S. Noda, M. Sugiura, M. Kakeno, K. Yamada, J. Kawamoto, Appl. Phys. Lett. 43 (1983) 30. [2] G.R. Rao, E.H. Lee, L.K. Mansur, Wear 162±164 (1993) 739. [3] L. Calcagno, G. Compagnini, G. Foti, Nucl. Instr. and Meth. B 65 (1992) 413. [4] F. Abel, V. Quillet, M. Schott, Nucl. Instr. and Meth. B 105 (1995) 86.
[5] O. Puglisi, A. Licciardello, S. Pignataro, L. Calcagno, G. Foti, Radiat. E. 98 (1986) 161. [6] J.F. Ziegler, J.P. Biersack, U. Littmark, in: The Stopping and Ranges of Ions in Solids, Pergamon, New York, 1985. [7] G. Compagnini, G. Foti, R. Reitano, G. Mondio, Appl. Phys. Lett. 57 (1990) 2546. [8] L.S. Farenzena, R.M. Papaleo, A. Hallen, M.A. de Ara ujo, R.P. Livi, B.U.R. Sundqvist, Nucl. Instr. and Meth. B 105 (1995) 134. [9] Y. Wang, Y. Jin, Z. Zhu, C. Liu, Y. Sun, Z. Wang, M. Hou, X. Chen, C. Zhang, J. Liu, B. Li, Nucl. Instr. and Meth. B 164±165 (2000) 420. [10] C. Gagnadre, J.L. Decossas, J.C. Vareille, Nucl. Instr. and Meth. B 73 (1993) 48. [11] Ma.E. Martinez-Pardo, J. Cardoso, H. Vazquez, M. Aguilar, J. Rickards, E. Andrade, Nucl. Instr. and Meth. B 131 (1997) 219. [12] E. Balanzat, N. Betz, S. Bouard, Nucl. Instr. and Meth. B 105 (1995) 46. [13] T. Steckenreiter, E. Balanzat, H. Fuess, C. Trautmann, Nucl. Instr. and Meth. B 131 (1997) 159.