12110
J . Phys. Chem. 1993,97, 12110-12113
Transient Absorption Spectroscopic Study on Photothermal Process and Laser Ablation of Poly (N-vinylcarbazole) Film Hirosbi FUkumura,*J Kouji Hamano,* and Hiroshi Masubara'J Department of Polymer Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan Received: July 16, 1993; In Final Form: September 28, 1993'
Nanosecond laser photolysis of poly(N-vinylcarbazole) film did not give any appreciable transient absorption above an excitation intensity of 25 mJ/cm2. Only a tail of the ground-state absorption was observed, which was analyzed to estimate the temperature below, at, and above laser ablation threshold (60 mJ/cm2). The temperature rise time was less than 50 ns, and the film cooled in a few microseconds. The maximum temperature attained at the excitation intensity just below the threshold was close to the decomposition temperature of the film.
Introduction Poly(N-vinylcarbazole) (PVCz) is a representative organic photoconductor, and its photophysical and photochemical processes have been elucidated in detail on the basis of photoconductivity, emission, and absorption spectral measurements.' Absorption spectra of ground, excited singlet, triplet, cationic, and anionic states were measured by nanosecond and picosecond laser photolysis methods, and relations among interchromophoric interaction, spectrum, and relative geometrical structure of dicarbazolyl systems were well considered. These studies were carried out for its single polymer chain in dilute solution2 and for its thin film.3 Characteristic processes of PVCz excited by an intense laser pulse have received much attention: an efficient mutual interaction between excited states ( S I S Ia n n i h i l a t i ~ n )and ~ , ~transient polyelectrolyte f ~ r m a t i o n .These ~ processes should be related to laser-induced morphological changes of the film: laser annealing and ablation.6 We have measured total emission spectra as a function of laser fluence, time-resolved spectra, and their rise and decay curves upon laser ablation. Plasmalike emission and emission from decomposed species such as C2 and C N were detected far above the ablation threshold; however, only two kinds of PVCz excimer fluorescence wereobserved around the threshold. Analyzing the relative intensity of two excimer bands, we came to the conclusion that SI-SI annihilation in PVCz films has a key role in the primary processes of laser ablation. Since laser ablation is induced through bond breaking, as reported in the early 1980s and reviewed recently by Srinivasan and Braren,' neutral decomposed radicals should be formed. Furthermore, ion radicals may be involved since SISI annihilation produces the higher excited (S,) state and the latter has a higher energy than ionization potential. Simultaneous and successive multiphoton absorption might be responsible for the formation of S, states. All these processes are considered to result in decomposition of the film. It is therefore necessary to apply the laser photolysis method to identify these species and to examine laser ablation mechanisms. As far as we know, however, few reports have been given for transient absorption spectroscopic studies of neat polymer films just upon laser ablation.8,g In the present paper a laser photolysis study of PVCz film is reported, and photothermal processes and ablation mechanisms are considered. PVCz is one of the best polymers for measuring Present address: Departmentof Applied Physics, Osaka University, Suita 565, Japan. t Present address: Materials and Electronic Devices Laboratory, Mitsubishi Electric Co. Ltd., Tsukaguchi, Amagasaki 661, Japan. *Abstract published in Advance ACS Abstracts. November 15, 1993.
0022-3654/93/2091- 121 10%04.00/0
the transient species involved in laser ablation, since the spectroscopic information is available as described above. We used a very thin film, and therefore, an appreciable thickness was etched by one-shot irradiation. This is a somewhat unusual condition; however, in this case we can correlate transient absorption spectral information closely to laser ablation.
Experimental Section PVCz (Takasago International Co. Ltd.), prepared by radical polymerization, was three times reprecipitated from benzenemethanol mixed solvent. Anisole (Nacalai tesque, Special Grade) was purified by removing impurities with H2S04, dried, and distilled. Anisole containing 5 wt 7'% PVCz was coated on a quartz plate by a spinner (Mikasa IH-DZ), and the prepared film was dried in a vacuum chamber for 3 h. The film thickness was about 900 nm. No impurity such as anthracene or phenylcarbazole was detected in the fluorescence spectrum of the film. The nanosecond laser photolysis system was similar to that described elsewhere.8J0 Excitation pulses were generated by excimer lasers (Lambda Physik EMG 101 MSC, 351 nm; Lumonics Excimer 500, 308 nm). Absorption rise and decay curves were measured from wavelength to wavelength, using a storage scope (IWATSU TS-8123). Monitor optics were aligned to measure transient absorption of the laser-ablated area only. The angle between the excitation and monitoring beams was 45O. All the measurements were performed in air at room temperature. The data were obtained only for fresh surfaces and were not averaged over several measurements. The origin of the time axis was defined as the time of the laser pulse maximum. Etch depth was measured by a depth profiler (Sloan Dektak 3030) and plotted against laser intensity.
Results and Discussion Excitation of a PVCz film with a high-intensity laser pulse induced ablation, and an irradiated area was removed. Just upon laser ablation, transient absorption in the region 12 500-25 000 cm-1 (400-800 nm) was examined in the time range from nanoseconds to a few tens of microseconds. In this wavenumber range, the excited singlet, triplet, cationic, and anionic states of monomer and dimer of the carbazolyl chromophore should have an absorption, as confirmed for a typical excitation intensity of laser photolysis;2J however, nothing was detected. An absorption spectral change was observed only in the low wavenumber region, very close to the excitation wavelength. As shown in Figure lA, transient absorption just below the excitation photon energy was observed as a tail of the ground-state absorption, and it extended 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 12111
Letters
0.6 0 A
55 mJ/cm* o 38 mJ/cm’ 0 25 mJ/cm’
0)
d
2 P 3
QS mJ/cm‘
0.4
0.2
0 27 IO
27500
28000
28500
29000
Wavenumber (cm-’) 0 0.lps
05
/.Ls
0
40
20
Time (ps) Figure 2. Decay curves of absorption tails of poly(N-vinylcarbazole)
film. Excitation wavelength is 351 nm, excitation intensity was 35 mJ/ cm2, and observation wavelength is given in the figure.
4
o,l
15 mJ/cm2
27000 27500 28000 28500 29000
W a v e n u m b e r (cm-’ ) Figure 1. Transient absorption tails of poly(N4nylcarbazole) film. (A,
top) Excitation wavelength was 351 nm, gate time was 50-100 ns, and excitation intensity is given in the figure. (B, bottom) Excitation wavelength was 308 nm, delay time was 0.1 ps (0)and 5 p s (O), and excitation intensity is given in the figure. to low wavenumber (long wavelength) as excitation intensity increased. Since strong fluorescence interfered with the precise absorption measurement, the absorption tail before 50 ns was not measured. When excited at 308 nm, a negative absorption above 28 736 cm-’ (below 348 nm) was found to accompany the transient absorption as shown in Figure 1B. As described later, the film undergoes ablation around the excitation intensity of 60 mJ/ cm2. It is worth noting that the absorption behavior is observed similarly below and above the ablation threshold. No new absorption was observed when the ablation was induced, suggesting that ablation mechanism can be elucidated by analyzing the photophysical behavior below the threshold. This is very easy since repetitive examination is possible without any morphological change. Absorption decay was rather slow and observed up to a few tens of microseconds. In Figure 2, decay curves at an excitation intensity of 35 mJ/cm2 are given for a different observation wavenumber. As the wavenumber is lowered, the decay becomes faster. If a new transient species such as excited states or radicals is responsible for the absorption in the wavenumber range, decay curves observed at different wavenumber should give the same time profile. This is inconsistent with the results. The spectral characteristics and behavior can be interpreted by assigning the tail to a hot band of the ground-state absorption. The negative transient absorption in the high-wavenumber region (Figure 1B) means that the ground state is depleted to some extent, and an appreciable population in the higher vibrational level of the ground state is produced by 308-nm excitation. It is surprising that the transient absorbance at 28 249 cm-1 (354 nm) becomes 0.30-0.35 upon excitation at 60 mJ/cm2. This
means that the effective absorbance at the laser wavelength during excitation is not identical to that of the ground state. As the film was set at 45’ to the laser excitation, an effective path length is 4 2 X (film thickness). The absorbance of the present groundstate PVCz at 351 nm is 0.52, so that log(Zo/Z,) = 0.52 X d 2 = 0.735. Here IOand I, are an incident monitoring intensity and a transmitted one without laser-induced absorbance change, respectively. A transmitted monitoring intensity (Z) with laser excitation is not identical with Is as the transient absorbance is overlapped. We assumed that the absorbance change a t 351 nm is the same as that at 354 nm, which is supported by the spectra in Figure 1B. Experimentally, the absorption a t 28 249 cm-1 (354 nm),observedat 5&100nsafterexcitation, was proportional to the excitation intensity (@o, in units of mJ/cm2) of the 308-nm pulse, and its slope was 5 X lW3. The following relations are derived:
h 3 ( ~ o / z )= 10g(Zo/Zs) + log(Is/Z) = 0.735
+ 5 x 10-3@0
It is reasonable to consider that PVCz is etched to the depth where the incident laser intensity decreases to the ablation threshold (ath). If the absorbance a t the laser wavelength has nothing to do with transient absorption, the Lambert-Beer equation @th = 00exp(-dth), where a is absorption coefficient and Xth is the ablation depth, can be applied. Namely, the following is derived by replacing @th with 60 mJ/cm2 in the equation
= (l/a)ln(@o/60) (2) which will show a linear relation between the etch depth and the logarithm of @o. However, this expectation was denied. In Figure 3, the etch depth is given as a function of laser intensity, indicating a sublinear deviation. The obtained curve is well explained by replacing a by an alternative absorption coefficient a’ which depends on the excitation intensity. The value of a’is given by dividing log(lo/Z) of eq 1 by an effective path length (900 nm X 4 2 ) and by multiplying by 2.303 to get the natural logarithm. xth
a’ = 2.303 l0g(I0/1)/(90O nm X d 2 ) An etch depth is now estimated by the following equation:
x:h = (1/a)ln(@.,/6O)
+
= 5.47 X lO”(O.735 5 X ln(@.,/60) (3) As shown in Figure 3, the laser intensity dependence of the etch depth was well reproduced by eq 3. Therefore, absorbancechange
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12112 The Journal of Physical Chemistry, Vol. 97, No. 47, 1993 I
I
1
.
0 10
100
1000
Laser intensity (mJ/cmz) Figure 3. Etch depth of poly(Nvinylcarbazo1e)film as a function of 351-nm excitation intensity. Solid curves a and b are calculated by eqs 2 and 3, respectively (see text).
during the excitation pulse is crucial to determine the etch profile of laser ablation and is confirmed here on the basis of transient absorption spectroscopy. It is important to remember that a linear relation was observed for polyimide and poly(methy1 methacrylate) f i l m ~ . I * - ~The 3 PVCz film is concluded to be inefficient in etching because the transient absorbance is appreciable a t the laser wavelength. Analysis of the hot band can clarify the temperature of the laser-excited film and its dynamics. Polyatomic molecules in gas and liquid phases have been studied along this line, and one efficient approach is to observe the changes in the low wavenumber region of the ground-state absorption."16 As the temperature is raised, the tail is extended to low wavenumber, which is just the behavior we observed. The absorption coefficient is given as a sum of all possible transitions from thermally occupied vibrational levels in the electronic ground state to vibrational levels in the lowest excited singlet state. Here we analyze the data in the theoretical framework given by Scherer et a1.16 They simplified the treatment by assuming the following: (a) a few vibrational modes with energy larger than kTchange occupation number, (b) the harmonic approximation is applicable, and (c) low-frequency mode and modes with small Franck-Condon factors are lumped together. It was clearly shown that the change of absorption coefficient with temperature consists of three terms: an absorption change from the increase in thermal population, a line broadening with temperature, and a temperature-dependent shift of the absorption spectrum. Scherer et al. concluded that the first term dominates, and the result can be analyzed by the following equation
6 In Abs(v)/(l/kT) = E(uO)- E(Y)
(4) where Abs(v) is the absorbance at the monitored wavenumber Y, E(v0) and E(v) are the mean vibrational energy in thermal equilibrium and the energy of the absorption tail we monitor, respectively, k is Boltzmann's constant, and T is temperature. E(v0) is experimentally given as the energy of the absorption peak. The theoretical equation is rearranged to the following
+
+
In Abs(u) = -(E(vO) - E(v))/kT In Abs(vo) const (5) This means that absorption tail descends as an exponential function of the energy gap between the peak and monitored energy. This is already confirmed for some dyes in s01ution.I~ It is also wellknown that the Urbach rule in molecular crystals represents the same equation.'* Therefore, it is possible to estimate the temperature by fitting the absorption tail to the exponential function. The present data of the PVCz film can be fitted well by an exponential function, but the absolute temperature estimated by eq 5 was found not to be so reliable. Here we focused our attention on temperature change upon laser excitation. The absorption
T=20 'C -6 800
1200
1600
2000
AE Wavenumber (cm-') Figure 4. Calculated temperatureof poly(N-vinylcarbazole) film at the gate time of 50-100 ns upon 351-nm excitation. Excitation intensity
dependence. tails of the film at room temperature and with irradiation were fitted, and the temperature difference was calculated. In Figure 4, the present data of the PVCz film and the temperature calculated in this way are shown. The results show how the temperature of the film is raised upon laser excitation. The estimated temperature attained at 25 mJ/cm2 is 30 OC, while that at 55 mJ is 260 OC. Using eq 1, absorbed energy at excitation intensity of 25 and 55 mJ/cm2 is calculated to be 21.5 and 49 mJ/cm2, respectively. Therefore, if the temperature change is proportional to the absorbed energy, the present result is difficult to accept. As given in eq 1, the effective absorbance is a function of excitation intensity and the absorbed energy is larger than that expected with the Lambert-Beer equation. The gradient of adsorbed energy along the depth becomes sharper as the intensity is increased. Thus, the local temperature near the surface is higher than that estimated, as the excitation intensity is increased. This is one factor for the present unusual temperature jump, but still minor. Additional important information comes from a relation between fluorescence and excitation intensity in the range of a few tens of mJ/cm2. We have already reported that the fluorescence intensity is almost saturated with an increase of the excitation intensity and shows even a decrease above 100 mJ/cm2.6 By increasing the excitation intensity from 25 to 55 mJ/cm2, the fluorescence increased by only 15%. As no transient species was detected, the increased amount of energy should be used mostly for temperature elevation. More quantitative discussion is difficult as the absolute fluorescence quantum yield is not determined for PVCz film. Furthermore, thermal properties such as heat capacity and thermal conductivity are a function of temperature; hence, we consider that the present temperature change cannot be excluded as an extraordinary estimation. At 95 mJ/cm2, the temperature was calculated to be 750 O C at 50-100 ns after excitation. According to our recent transient absorption spectroscopy and nanosecond photography,19 fragmented particles were confirmed to be ejected around 50 ns after excitation and increased up to 1 1 s at a laser intensity just above the threshold. Also in the present PVCz film, the scattering of the monitoring light by fragmented particles may affect the transient absorbance. Hence, the temperature estimation at 95 mJ/cm2 may be less reliable compared to the others. The estimated temperature of 260 OC at 55 mJ/cm2 is just between 230 and 300 OC at which the molecular weight distribution of PVCz begins to change and random chain scission takes place, respectively.20 Since 55 mJ/cm2 is slightly below the ablation threshold, it is suggested that the ablation of PVCz is initiated by thermal decomposition under the present condition. As mentioned above, no transient absorption was detected in the wavenumber range 12 500-25 000 cm-l (400-800 nm) at 50100 ns after excitation. Ion radicals and chemical intermediates indicating directly photochemical ablation were not identified.
Letters
The Journal of Physical Chemistry, V O ~97, . NO. 47, 1993 12113
-li
a
Laser intensity = 5 5 m ~ / c m *
v -
-5'
800
1200
1800
2000
I
A E Wavenumber (cm-') Figure 5. Calculated temperature of poly(N4nylcarbazole) film upon 351-nm excitation of 55 mJ/cm2. Delay time dependence.
No neutral radical, expected in photothermal decomposition, was also observed, whose reasons are still beyond our knowledge. The temperature fall dynamics is elucidated at an excitation intensity of 55 mJ/cm2, as given in Figure 5 . Absorption tails at each delay time satisfied the exponential function of the energy gap; hence, the temperature was directly calculated and confirmed to be lowered in a few microseconds. Since the thermal energy in the film was conducted to the substrate, its time ( t ) can be estimated by I = dot,where 1is the film thickness and D is the thermal diffusivity of PVCz film. By introducing I = 900 nm and D = l t 3 cmz/s. t was calculated to be 8 ~ s which , is in good agreement with the measured value. Dynamic aspects of photothermal processes have a key role in understanding ablation mechanism of polymer films, and recently relevant works have been reported.*' More details are being revealed by transient absorption spectroscopy and will be published shortly.
Acknowledgment. The present work is partly supported by the Grant-in-Aid from the Ministry of Education, Science, and Culture of Japan (63430003). The authors are indebted to reviewers for their critical comments and important discussion. References and Notes (1) Mort, J.; Pfister, G. In Electronic Properties of Polymers; Mort, J., Pfister, G., Eds.; Wiley-Interscience: New York, 1982; Chapter 2. Stolzen-
burg, F.; Ries, B.; BBssler, H. Ber. Bunsen-Ges. Phys. Chem. 1987, 91,853. Masuhara, H.; Itaya, A. In Macromolecular Complexes Dynamic Interaction and Electronic Processes; Tsuchida, E., Ed.; VCH Publisher: New York, 1991; Chapter 4. Masuhara, H.; Itaya, A. In Dynamics and Mechanisms of Photoinduced Electron Transferand Related Phenomena;Matap,N.,Okada, T., Masuhara, H., Eds.; Elsevier: Amsterdam, 1992; pp 363-375. (2) Masuhara, H.; Tamai, N.; Mataga, N.; De Schryver, F. C.; Vandendriesche, J.; Boens, N . Chem. Phys. Lett. 1983,95,471. Masuhara, H.; Tamai, N.; Mataga, N.; De, Schryver, F. C.; Vandendriesche, J. J . Am. Chem. SOC.1983, 105, 7256. (3) Itaya, A.; Yamada, T.; Masuhara, H. Chem. Phys. Lett. 1990,174, 145. (4) Masuhara, H.; Ohwada, S.; Mataga, N.; Itaya, A.; Okamoto, K.; Kusabayashi, S. J . Phys. Chem. 1980,84, 2363. ( 5 ) Masuhara,H.;Tamai,N.;Ikeda,N.; Mataga,N.;Itaya, A.;Okamoto, K.; Kusabayashi, S. Chem. Phys. Lett. 1982, 91, 113. (6) Masuhara, H.; Eura, S.; Fukumura, H.; Itaya, A. Chem. Phys. Lett. 1989, 156, 446. (7) Srinivasan, S.; Braren, B. Chem. Reu. 1989, 89, 1303. (8) Hamano, K.; Fukumura, H.; Masuhara, H. Absrr. Ann. Meeting Chem. SOC.Jpn. 1989,21028. Fukumura, H.; Masuhara, H. J . Photopolym. Sci. Technol. 1992,5,223. Fukumura, H.; Hamano, K.; Masuhara,H. Chem. Lett. 1993, 245. (9) Arnold, B. A.; Scaiano, J. C. Macromolecules 1992, 25, 1582. (10) Koshioka, M.; Mizuma, H.; Imagi, K.; Ikcda, N.; Fukumura, H.; Masuhara, H.; Kryschi, C. Bull. Chem. SOC.Jpn. 1990, 63, 3495. (1 1) Srinivasan, R.; Braren, B.; Dreyfus, R. W. J. Appl. Phys. 1987,61, 372. (12) Srinivasan, R.; Braren, B.; Seeger, D. E.; Dreyfus, R. W. Macromolecules 1986, 19, 916. (13) Srinivasan, R.; Braren, B. J . Polym. Sci.: Polym. Chem. Ed. 1984, 22, 2601. (14) Nakashima, N.; Yoshihara, K. J. Chem. Phys. 1983, 79, 26. (15) Wondrazek, F.; Seilmeier, A.; Kaiser, W. Chem. Phys. Lett. 1984, 104,121. Seilmeier, A.; Scherer, P.0.J.; Kaiser, W. Chem. Phys. Lett. 1984, 105, 104. Hiibner, H.-J.; WBrner, M.; Kaiser, W. Chem. Phys. Lett. 1991, 182, 315. (16) Scherer, P. 0.J.;Seilmeier, A.; Kaiser, W. J . Chem. Phys. 1985,83, 3948. (17) Kinoshita, S.; Nishi, N.; Saitoh, A,; Kushida, T. J . Phys. SOC.Jpn. 1987, 56,4162. (18) Urbach, F. Phys. Rev. 1953, 92, 1324. (19) Takahashi, E.; Fukumura, H.; Masuhara, H. Polym. Prepr. Jpn. (Engl. Ed.) 1991, 40, E695. (20) Chu, J. Y. C.; Stolka, M. J . Polym. Sci., Chem. Ed. 1975,13,2867. (21) Wen, X . ; Tolbert, W. A.; Dlott, D. D. Chem. Phys. 1992, 192, 315. Chen, S.; Lee, I.-Y. S.; Tolbert, W. A.; Wen, X.;Dlott, D. D. J . Phys. Chem. 1992, 96, 7178.
