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In situ SHG investigation on the gelation process of organic doped silica film
1 November 1999
Physics Letters A 262 Ž1999. 206–211 www.elsevier.nlrlocaterphysleta
In situ SHG investigation on the gelation process of organic doped silica film Liying Liu, Lei Xu ) , Zhanjia Hou, Zhiling Xu, Jie Chen, Wencheng Wang, Fuming Li Laboratory of Laser Physics and Optics, Department of Physics and State Key Lab of Materials, Shanghai 200433, China Received 9 February 1999; received in revised form 28 June 1999; accepted 16 August 1999 Communicated by A. Lagendijk
Abstract The gelation process of organic doped silica film fabricated by the sol–gel technique was investigated by in-situ second harmonic generation ŽSHG. measurements. The SH signal came from the doped hemicyanine molecules which automatically oriented in the film at or near the film–substrate interface. We found that film shrinkage during gelation resulted in disordering of the doped hemicyanine molecules, so the SH intensity decreased during annealing. Thus, the SHG measurement can be used as a semi-quantitative probe of the gelation process in that particular region, which was hard to detect using conventional methods. We found that the time for gelation depends sensitively on the annealing temperatures. The activation energy of the gelling process was obtained from the SHG relaxation measurement. In addition, aggregated hemicyanine molecules dissociated with increasing annealing temperature. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In 1980, Sagiv w1x posed for the first time that if amphiphilic molecules, which are hydrophilic on one end and hydrophobic on the other end, were dispersed on a hydrophilic substrate, the molecules will automatically stand up on the substrate. If the molecules selected have second-order hyperpolarizability, the ordered alignment of the monolayer on the substrate will result in macro-second-order nonlinearity. However, this method can only be used to obtain a monolayer on substrate, because orientation cannot be maintained layer by layer without external
)
Corresponding author.
confinement. Several techniques can be used to achieve multilayer alignment, such as the LB technique w2x, self-assembly w3x, poled polymer w4x, etc. In 1997, we fabricated hemicyanine-doped Žwhich is an amphiphilic molecule with a second-order hyperpolarizability of 1.27 = 10y2 8 esu w6x. silica film w5x by a sol–gel technique. We found that the doped hemicyanine molecules showed some extent of self alignment. For a film of 50 nm thick, we obtained a second order nonlinearity of 6.6 pmrV. However, we found that only part of the film showed efficient nonlinearity. Later on, we also found proof that the oriented molecules only existed at and near the film–substrate interface. As a simple and efficient method, sol–gel technique has been widely used in fabricating bulk and
0375-9601r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 Ž 9 9 . 0 0 5 5 8 - 7
L. Liu et al.r Physics Letters A 262 (1999) 206–211
thin film glass materials w7x. In a sol–gel process, glass is formed from solution, via hydrolysis, condensation, gelation, drying and densification. The nature of this type of glass is its porosity which depends mainly on the annealing temperature. For thin-film formation, steps from gelation to densification are completed after the film has been coated and held at elevated temperature. Gelation is a process in which the silica network after hydrolysis grows to such an extent that the film becomes stiff. The gelation process is accompanied by evaporation of solvents and large reduction in film volume Žshrinkage. w8x. If films are kept at high temperature Žmore than 1008C., densification will result in further reduction of the film volume and porosity. Obviously, these steps are essential to the quality of the thin film. Various methods, such as in situ NMR, have been developed to reveal the dynamic process of gelation and densification. Recently it was also found that even for a thin film, optical properties Žsuch as the refractive index. could be depth-dependent w9x, implying that gelation process is depth-dependent. Thus, though macroscopic gelation process was clear long time ago, the microscopic process of gelation still need investigation. In this work, we report that by in-situ nonlinear optical measurements, we can monitor the gelation process easily and readily. The method relies on the fact of self-alignment of amphiphilic molecules in the glass film. Such a kind of alignment will be destroyed by gelation-induced film shrinkage. Consequently, oriented amphiphilic molecules can be considered as probe molecules for gelation. We will show that oriented molecules only exist at or near the film–substrate interface, so the dynamic information of gelation in that particular region can be obtained by in situ second harmonic generation ŽSHG. measurements.
207
kept for 2–3 days before use. Thin films were spin-coated on pyrex glass substrates. The fundamental light of a mode-locked laser irradiated on sample at an incident angle of 458. The sample was vertically mounted in an oven which can heat to temperature as high as 1508C. The SHG signal was detected by a photomultiplier and recorded on a X–Y recorder. UV–Visible absorption was measured on a Shimatsu UV-3101PC spectrophotometer.
3. Experimental results and discussion Fig. 1 shows the change of SHG intensity as a function of time at different temperatures. Obviously the evolution of the SH intensity varies a lot at different temperatures. At 308C, the SH intensity increases monotonously until it reaches its maximum. When the annealing temperature was increased to to 508C, the growth of SHG intensity becomes faster, but is still kept at its maximum value later on. On the contrary, when the temperature rises further to 728C, the SHG-signal goes up very fast, but decreases fast as well. We have found in our previous work w5x that the SH intensity originates from the ordered alignment of the hemicyanine molecules. Alignment was achieved by the evaporation of the solvent, which is completed after spin-coating. However, a large amount of doped molecules are in a state of protonation. As is well known, the protonated molecules have a much-reduced hyper-polarizability. After film formation, deprotonation takes place and lasts for
2. Experiment The detailed experimental steps for thin-film fabrication using the sol–gel technique and in situ SHG measurement have been described in our previous paper w5x. Here we just give a brief description. 0.025 mol hemicyanine was mixed with 1 mol TEOS together with water and ethanol. The solution was
Fig. 1. The evolution of SH intensity at different annealing temperatures.
208
L. Liu et al.r Physics Letters A 262 (1999) 206–211
Fig. 2. The evolution of SH intensity at different annealing temperatures. A SH intensity plateau can be clearly seen for the case of 608C annealing.
several hours at room temperature. After that, the SH intensity apparently is larger than that of the just-spun sample. For a sol–gel fabricated film, gelation, drying and densification occur after the film formation as well. For annealing below 1008C, the main process that occurs after film formation is gelation. The two steps in gelation are evaporation of the solvents and shrinkage of the film. Thus we can imagine that deprotonation and gelation are two processes that take place simultaneously but have opposite effect on the nonlinearity of the film. Deprotonation will result in enhancement of the nonlinearity without any influence on the molecular alignment. On the contrary, gelation will inevitably result in a shrinkage of the film; the initial alignment will be destroyed largely due to the deformation of the matrix. Fig. 1 tells us that the deprotonation effect occurred much faster than the gelation. Because a. at low temperature, the SH intensity rises monotonously, indicating that the deprotonation effect plays the main role, and b. even at higher temperature, we still see a rapid growth of the SH intensity, this implies that the gelation time at this temperature is still much slower than the deprotonation. In order to clearly distinguish the processes of deprotonation and gelation, an in-situ SHG measurement was carried out at temperatures of 608C, 678C and 808C respectively. The results are shown in Fig. 2. The SHG evolution at 608C separates the two processes apparently. Deprotonation completes 3–4 minutes after annealing, then the SHG intensity stays at a plateau for about 8 minutes before it begins to
decrease. The beginning of gelation-induced shrinking can be set at the point where the SHG intensity starts to fall. With the increase of temperature, the plateau at the maximum SHG intensity becomes shorter and shorter, which means that the gelation accelerates at higher temperatures. So the self-alignment of hemicyanine molecules become a sensitive probe for gelation. In our previous work, we found that self-alignment was maintained not through the whole film, but just for those molecules lying on side Žor sides. of the film. We did not examine further whether the alignment was achieved on both sides of the film or just one side. The following experiment was performed to show the details of the alignment. Prepared sol is further diluted with a different amount of ethanol, so solutions of different viscosity were obtained. Films spun from these solutions have different thickness. Obviously, evaporation and de-protonation process for heavy diluted film should be much faster than those of less diluted film. We have examined the SHG evolution of these films after spinning; it is plotted in Fig. 3. A film of high viscosity Žless diluted. shows a retarded growth of SHG intensity. The retardation time is sensitive to the viscosity of the solution. The retardation can be explained as the result of the fact that the evaporation and deprotonation processes starts from the top surface of the film. So the appearance of retarding strongly implies that alignment was only achieved at or near the film– substrate interface, as a result of the interaction between amphiphilic molecules with the hydrophilic substrate. So the SH evolution in Figs. 1 and 2
Fig. 3. SHG evolution of films fabricated from various ethanol diluted sols.
L. Liu et al.r Physics Letters A 262 (1999) 206–211
reflect the gelation process of the film–substrate interface. Fig. 4 shows the UV–Visible absorption spectra of samples annealed for 30 minutes at various temperatures. Clearly the absorption spectrum of the sample annealed at 308C consists of two peaks, one at about 500 nm corresponds to the monomer absorption of hemicyanine, while the absorption at around 410 nm corresponds to the absorption of hemicyanine H-aggregates. With an increase annealing temperature, the peak at 410 nm becoms less and less, and ultimately, the peak can be hardly found at 808C. Fig. 4 indicates clearly that a large amount of molecules are in aggregated state after the film formation. Annealing at elevated temperatures tends to separate the aggregated molecules due to heating. For films annealed at temperatures below 508C, the SH signal does not decrease in a period of 30 minutes. In order to examine whether gelation-induced disordering and aggregation–separation still exist, one sample is kept at 508C for 9 hours. The absorption spectrum after annealing is given in Fig. 5. The absorption spectrum of films annealed at the same temperature for 30 minutes is shown in the same Figure for comparison. We find that for films annealed for longer time, the absorption profile is closer to the one annealed at higher temperature. Consequently, we can conclude that no critical temperature exists for the gelation to take place, but the gelation process depends sensitively on temperature.
Fig. 4. Absorption spectra of films annealed at various temperatures for 30 minutes. The peak at around 500 nm corresponds to the absorption of hemicyanine. Absorption from a H-aggregate at 410 nm was clearly observed when annealing at 308C.
209
Fig. 5. Absorption spectra of films annealed at 508C for 9 hours and 30 minutes respectively.
SHG measurement also show that the film annealed for 9 hours has much less SHG intensity than the one annealed for only 30 minutes, which means that gelation induced-shrinkage exists at low temperatures as well. It must be noted that even for films annealed at a high temperature, a residual SH intensity is left after relaxation Žsee Figs. 1 and 2 for curves annealed at temperatures higher than 608C.. The residual intensity probably comes from ordered alignment of molecules at film–substrate interface. The ordering is maintained because the substrate is hydrophilic, the doped hemicyanine is an amphiphilic molecule, thus the orientation of the molecules at the interface is not possible to flip, even at high temperature. On the contrary, the orientation of molecules inside the film will loose gradually due to compression the matrix. The orientation does not recover when the annealing temperature goes down, which means that the damage of orientation is an irreversible process. The consequence of gelation is partly demonstrated in a measurement of the changes of SH intensity versus temperature, as shown in Fig. 6. In this figure, we have plotted two curves; one is the result of just-spun film, and the other is a film annealed formerly at 508C. For a previously annealed sample, a flat plateau is observed between 40–608C, this is in great contrast to the one of the just-spun sample. For just-spun film, the evolution of the SH intensity can be considered to result from two processes described above: deprotonation induced en-
L. Liu et al.r Physics Letters A 262 (1999) 206–211
210
Fig. 6. Plots of SH intensity versus temperature for a just-spun film and a film annealed formerly at 508C.
hancement Žbelow 508C. and gelation-induced drop Žabove 508C.. For a formerly annealed sample, the silica network inside the film becomes a porous solid after gelation, so the first drop of SH intensity at low temperature comes from vibration of molecules in the free volume of the network, rather than gelationinduced shrinkage. However, the vibration is confined by the pore sizes that are exactly determine by the annealing temperature, which is the reason of the formation of the plateau. When the temperature rises beyond the pre-annealing temperature, the silica network will collapse again, which resultes in further dropping of the SH intensity. The great difference between the first drop of SH intensity at low temperatures and the second drop at higher temperatures is: the first drop can completely recover when the temperature draws back Žsee Fig. 7., but for the second drop, it is an irreversible process. Fig. 7 demonstrates clearly that heating below the former annealing temperature does not result in any physical damage to the film structure.
will have good thermal stability. Ea is deduced from the Arrhenius formula w11x to be: log
k1 k2
s Ea
T1 y T2 2.302 RT1T2
Ž 1.
in which k 1 and k 2 are the reaction Žrelaxation or randomization. rates at temperatures T1 and T2 respectively. A modified Arrhenius formula was given by Ibar in 1979 w12x who obtained the equation by combining the Arrhenius formula and the WLF ŽWilliams, Landel and Ferry. equation w13x. For poled materials, k 1 and k 2 are the relaxation lifetimes of nonlinear optical intensity at elevated temperatures. In our case, relaxation of self-aligned molecules is caused by compression of matrix network. We have shown in Figs. 1 and 2 the relaxation
3.1. ActiÕation energy The activation energy Ea , which is a chemical concept, was introduced to treat nonlinear optical relaxation of poled materials w10x. Physically, Ea represents the energy barrier that an oriented molecule must overcome so as to transfer to a random state. A large Ea means that the poled guest in the matrix is difficult to randomize, so the material
Fig. 7. Changes of SH intensity for a heating circle between 30–508C.
L. Liu et al.r Physics Letters A 262 (1999) 206–211
211
critical temperature exists for gelation. Short-timehigh-temperature annealing has an equal effect as long-time-low-temperature. Since self-alignment of amphiphilic molecules is only achieved at or near the film–substrate interface, all the gelation information comes from that particular region which is hard to obtain using conventional methods.
Acknowledgements Fig. 8. Arrhenius-fit of the randomization rates. The dots are experimental results. The line is a linear fitting.
curves of SH intensity. The randomization rates of orderly self-aligned molecules are obtained by using single exponential decay fitting. The results are plotted in Fig. 8. Both the Arrhenius formula and the modified Ibar formula were used to fit the experimental points in Fig. 8. As a result, we obtain a activation energies of 40.8 kcalrmol ŽArrhenius. and 39.6 kcalrmol ŽIbar. respectively. The fitted activation energy by the two methods shows excellent coincidence. The values are apparently higher than that in the guest–host type polymer. We believe that the high value of Ea originates dominantly from the better rigidity of the silica network. It shows the possibility of obtaining much better nonlinear optical thermo-stability using the sol–gel technique.
4. Conclusion As we have shown above, self-alignment of amphiphilic molecules in the glass film fabricated using the sol–gel technique can be used as a sensitive probe for the gelation process. In order to achieve this goal, in-situ temperature dependent SHG measurements have been carried out. We find that the gelation speed dependeds much on the annealing temperature. In addition, de-aggregation is observed at elevated annealing temperatures. As expected, no
The financial support by the Climb Project from the Ministry of Science and Technology of China, National Natural Science Foundation of China ŽGrant No. 69708005, 69808001., Trans-century Training Program Foundation for the Talents by the Ministry of Education of China and The ShuGuang Project by the Education Commission of Shanghai ŽGrant No. 97SG02. are acknowledged.
References w1x J. Sagiv, J. Am. Chem. Soc. 102 Ž1980. 92. w2x G.L. Ashwell, P.D. Jackson, W.A. Crossland, Nature 368 Ž1994. 438. w3x D. Li, M.A. Ratner, T.J. Marks, C.H. Zhang, J. Yang, G.K. Wong, J. Am. Chem. Soc. 112 Ž1990. 7389. w4x M.A. Mortazavi, A. Knoesen, S.T. Kowel, B.G. Higgins, A. Dienes, J. Opt. Soc. Am. B 6 Ž1989. 733. w5x L. Xu, L. Liu, J. Yu, W. Wang, F. Li, J. Phys. D: Appl. Phys. 30 Ž1997. 19. w6x I.R. Girling, N.A. Cade, P.V. Kolinsky, R.J. Jones, I.R. Peterson, M.M. Ahmad, B. Neal, M.C. Petty, G.G. Roberts, W.J. Feast, J. Opt. Soc. Am. B 4 Ž1987. 950. w7x See for example, Sol–Gel Optics IV, Proceedings of SPIE 3136 Ž1997.. w8x D.R. Ulrich, J. Non-Cryst. Solids 100 Ž1988. 174. w9x M. Mosaddeq-ur-Rahman, G. Yu, K. Krishna, T. Soga, J. Watanabe, T. Jimbo, M. Umeno, Appl. Opt. 37 Ž1998. 691. w10x M. Stahelin, D.M. Burland, M. Ebert, R.D. Miller, B.A. Smith, R.J. Twieg, W. Volksen, C.A. Walsh, Appl. Phys. Lett. 61 Ž1992. 1626. w11x S. Arrhenius, Z. Phys. Chem. 4 Ž1889. 226. w12x J.P. Ibar, J. Macromol. Sci. Phys. B 16 Ž1979. 61. w13x M.L. Williams, R.F. Landel, J.D. Ferry, J. Am. Chem. Soc. 77 Ž1955. 3701.
Physics Letters A 262 Ž1999. 206–211 www.elsevier.nlrlocaterphysleta
In situ SHG investigation on the gelation process of organic doped silica film Liying Liu, Lei Xu ) , Zhanjia Hou, Zhiling Xu, Jie Chen, Wencheng Wang, Fuming Li Laboratory of Laser Physics and Optics, Department of Physics and State Key Lab of Materials, Shanghai 200433, China Received 9 February 1999; received in revised form 28 June 1999; accepted 16 August 1999 Communicated by A. Lagendijk
Abstract The gelation process of organic doped silica film fabricated by the sol–gel technique was investigated by in-situ second harmonic generation ŽSHG. measurements. The SH signal came from the doped hemicyanine molecules which automatically oriented in the film at or near the film–substrate interface. We found that film shrinkage during gelation resulted in disordering of the doped hemicyanine molecules, so the SH intensity decreased during annealing. Thus, the SHG measurement can be used as a semi-quantitative probe of the gelation process in that particular region, which was hard to detect using conventional methods. We found that the time for gelation depends sensitively on the annealing temperatures. The activation energy of the gelling process was obtained from the SHG relaxation measurement. In addition, aggregated hemicyanine molecules dissociated with increasing annealing temperature. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction In 1980, Sagiv w1x posed for the first time that if amphiphilic molecules, which are hydrophilic on one end and hydrophobic on the other end, were dispersed on a hydrophilic substrate, the molecules will automatically stand up on the substrate. If the molecules selected have second-order hyperpolarizability, the ordered alignment of the monolayer on the substrate will result in macro-second-order nonlinearity. However, this method can only be used to obtain a monolayer on substrate, because orientation cannot be maintained layer by layer without external
)
Corresponding author.
confinement. Several techniques can be used to achieve multilayer alignment, such as the LB technique w2x, self-assembly w3x, poled polymer w4x, etc. In 1997, we fabricated hemicyanine-doped Žwhich is an amphiphilic molecule with a second-order hyperpolarizability of 1.27 = 10y2 8 esu w6x. silica film w5x by a sol–gel technique. We found that the doped hemicyanine molecules showed some extent of self alignment. For a film of 50 nm thick, we obtained a second order nonlinearity of 6.6 pmrV. However, we found that only part of the film showed efficient nonlinearity. Later on, we also found proof that the oriented molecules only existed at and near the film–substrate interface. As a simple and efficient method, sol–gel technique has been widely used in fabricating bulk and
0375-9601r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 Ž 9 9 . 0 0 5 5 8 - 7
L. Liu et al.r Physics Letters A 262 (1999) 206–211
thin film glass materials w7x. In a sol–gel process, glass is formed from solution, via hydrolysis, condensation, gelation, drying and densification. The nature of this type of glass is its porosity which depends mainly on the annealing temperature. For thin-film formation, steps from gelation to densification are completed after the film has been coated and held at elevated temperature. Gelation is a process in which the silica network after hydrolysis grows to such an extent that the film becomes stiff. The gelation process is accompanied by evaporation of solvents and large reduction in film volume Žshrinkage. w8x. If films are kept at high temperature Žmore than 1008C., densification will result in further reduction of the film volume and porosity. Obviously, these steps are essential to the quality of the thin film. Various methods, such as in situ NMR, have been developed to reveal the dynamic process of gelation and densification. Recently it was also found that even for a thin film, optical properties Žsuch as the refractive index. could be depth-dependent w9x, implying that gelation process is depth-dependent. Thus, though macroscopic gelation process was clear long time ago, the microscopic process of gelation still need investigation. In this work, we report that by in-situ nonlinear optical measurements, we can monitor the gelation process easily and readily. The method relies on the fact of self-alignment of amphiphilic molecules in the glass film. Such a kind of alignment will be destroyed by gelation-induced film shrinkage. Consequently, oriented amphiphilic molecules can be considered as probe molecules for gelation. We will show that oriented molecules only exist at or near the film–substrate interface, so the dynamic information of gelation in that particular region can be obtained by in situ second harmonic generation ŽSHG. measurements.
207
kept for 2–3 days before use. Thin films were spin-coated on pyrex glass substrates. The fundamental light of a mode-locked laser irradiated on sample at an incident angle of 458. The sample was vertically mounted in an oven which can heat to temperature as high as 1508C. The SHG signal was detected by a photomultiplier and recorded on a X–Y recorder. UV–Visible absorption was measured on a Shimatsu UV-3101PC spectrophotometer.
3. Experimental results and discussion Fig. 1 shows the change of SHG intensity as a function of time at different temperatures. Obviously the evolution of the SH intensity varies a lot at different temperatures. At 308C, the SH intensity increases monotonously until it reaches its maximum. When the annealing temperature was increased to to 508C, the growth of SHG intensity becomes faster, but is still kept at its maximum value later on. On the contrary, when the temperature rises further to 728C, the SHG-signal goes up very fast, but decreases fast as well. We have found in our previous work w5x that the SH intensity originates from the ordered alignment of the hemicyanine molecules. Alignment was achieved by the evaporation of the solvent, which is completed after spin-coating. However, a large amount of doped molecules are in a state of protonation. As is well known, the protonated molecules have a much-reduced hyper-polarizability. After film formation, deprotonation takes place and lasts for
2. Experiment The detailed experimental steps for thin-film fabrication using the sol–gel technique and in situ SHG measurement have been described in our previous paper w5x. Here we just give a brief description. 0.025 mol hemicyanine was mixed with 1 mol TEOS together with water and ethanol. The solution was
Fig. 1. The evolution of SH intensity at different annealing temperatures.
208
L. Liu et al.r Physics Letters A 262 (1999) 206–211
Fig. 2. The evolution of SH intensity at different annealing temperatures. A SH intensity plateau can be clearly seen for the case of 608C annealing.
several hours at room temperature. After that, the SH intensity apparently is larger than that of the just-spun sample. For a sol–gel fabricated film, gelation, drying and densification occur after the film formation as well. For annealing below 1008C, the main process that occurs after film formation is gelation. The two steps in gelation are evaporation of the solvents and shrinkage of the film. Thus we can imagine that deprotonation and gelation are two processes that take place simultaneously but have opposite effect on the nonlinearity of the film. Deprotonation will result in enhancement of the nonlinearity without any influence on the molecular alignment. On the contrary, gelation will inevitably result in a shrinkage of the film; the initial alignment will be destroyed largely due to the deformation of the matrix. Fig. 1 tells us that the deprotonation effect occurred much faster than the gelation. Because a. at low temperature, the SH intensity rises monotonously, indicating that the deprotonation effect plays the main role, and b. even at higher temperature, we still see a rapid growth of the SH intensity, this implies that the gelation time at this temperature is still much slower than the deprotonation. In order to clearly distinguish the processes of deprotonation and gelation, an in-situ SHG measurement was carried out at temperatures of 608C, 678C and 808C respectively. The results are shown in Fig. 2. The SHG evolution at 608C separates the two processes apparently. Deprotonation completes 3–4 minutes after annealing, then the SHG intensity stays at a plateau for about 8 minutes before it begins to
decrease. The beginning of gelation-induced shrinking can be set at the point where the SHG intensity starts to fall. With the increase of temperature, the plateau at the maximum SHG intensity becomes shorter and shorter, which means that the gelation accelerates at higher temperatures. So the self-alignment of hemicyanine molecules become a sensitive probe for gelation. In our previous work, we found that self-alignment was maintained not through the whole film, but just for those molecules lying on side Žor sides. of the film. We did not examine further whether the alignment was achieved on both sides of the film or just one side. The following experiment was performed to show the details of the alignment. Prepared sol is further diluted with a different amount of ethanol, so solutions of different viscosity were obtained. Films spun from these solutions have different thickness. Obviously, evaporation and de-protonation process for heavy diluted film should be much faster than those of less diluted film. We have examined the SHG evolution of these films after spinning; it is plotted in Fig. 3. A film of high viscosity Žless diluted. shows a retarded growth of SHG intensity. The retardation time is sensitive to the viscosity of the solution. The retardation can be explained as the result of the fact that the evaporation and deprotonation processes starts from the top surface of the film. So the appearance of retarding strongly implies that alignment was only achieved at or near the film– substrate interface, as a result of the interaction between amphiphilic molecules with the hydrophilic substrate. So the SH evolution in Figs. 1 and 2
Fig. 3. SHG evolution of films fabricated from various ethanol diluted sols.
L. Liu et al.r Physics Letters A 262 (1999) 206–211
reflect the gelation process of the film–substrate interface. Fig. 4 shows the UV–Visible absorption spectra of samples annealed for 30 minutes at various temperatures. Clearly the absorption spectrum of the sample annealed at 308C consists of two peaks, one at about 500 nm corresponds to the monomer absorption of hemicyanine, while the absorption at around 410 nm corresponds to the absorption of hemicyanine H-aggregates. With an increase annealing temperature, the peak at 410 nm becoms less and less, and ultimately, the peak can be hardly found at 808C. Fig. 4 indicates clearly that a large amount of molecules are in aggregated state after the film formation. Annealing at elevated temperatures tends to separate the aggregated molecules due to heating. For films annealed at temperatures below 508C, the SH signal does not decrease in a period of 30 minutes. In order to examine whether gelation-induced disordering and aggregation–separation still exist, one sample is kept at 508C for 9 hours. The absorption spectrum after annealing is given in Fig. 5. The absorption spectrum of films annealed at the same temperature for 30 minutes is shown in the same Figure for comparison. We find that for films annealed for longer time, the absorption profile is closer to the one annealed at higher temperature. Consequently, we can conclude that no critical temperature exists for the gelation to take place, but the gelation process depends sensitively on temperature.
Fig. 4. Absorption spectra of films annealed at various temperatures for 30 minutes. The peak at around 500 nm corresponds to the absorption of hemicyanine. Absorption from a H-aggregate at 410 nm was clearly observed when annealing at 308C.
209
Fig. 5. Absorption spectra of films annealed at 508C for 9 hours and 30 minutes respectively.
SHG measurement also show that the film annealed for 9 hours has much less SHG intensity than the one annealed for only 30 minutes, which means that gelation induced-shrinkage exists at low temperatures as well. It must be noted that even for films annealed at a high temperature, a residual SH intensity is left after relaxation Žsee Figs. 1 and 2 for curves annealed at temperatures higher than 608C.. The residual intensity probably comes from ordered alignment of molecules at film–substrate interface. The ordering is maintained because the substrate is hydrophilic, the doped hemicyanine is an amphiphilic molecule, thus the orientation of the molecules at the interface is not possible to flip, even at high temperature. On the contrary, the orientation of molecules inside the film will loose gradually due to compression the matrix. The orientation does not recover when the annealing temperature goes down, which means that the damage of orientation is an irreversible process. The consequence of gelation is partly demonstrated in a measurement of the changes of SH intensity versus temperature, as shown in Fig. 6. In this figure, we have plotted two curves; one is the result of just-spun film, and the other is a film annealed formerly at 508C. For a previously annealed sample, a flat plateau is observed between 40–608C, this is in great contrast to the one of the just-spun sample. For just-spun film, the evolution of the SH intensity can be considered to result from two processes described above: deprotonation induced en-
L. Liu et al.r Physics Letters A 262 (1999) 206–211
210
Fig. 6. Plots of SH intensity versus temperature for a just-spun film and a film annealed formerly at 508C.
hancement Žbelow 508C. and gelation-induced drop Žabove 508C.. For a formerly annealed sample, the silica network inside the film becomes a porous solid after gelation, so the first drop of SH intensity at low temperature comes from vibration of molecules in the free volume of the network, rather than gelationinduced shrinkage. However, the vibration is confined by the pore sizes that are exactly determine by the annealing temperature, which is the reason of the formation of the plateau. When the temperature rises beyond the pre-annealing temperature, the silica network will collapse again, which resultes in further dropping of the SH intensity. The great difference between the first drop of SH intensity at low temperatures and the second drop at higher temperatures is: the first drop can completely recover when the temperature draws back Žsee Fig. 7., but for the second drop, it is an irreversible process. Fig. 7 demonstrates clearly that heating below the former annealing temperature does not result in any physical damage to the film structure.
will have good thermal stability. Ea is deduced from the Arrhenius formula w11x to be: log
k1 k2
s Ea
T1 y T2 2.302 RT1T2
Ž 1.
in which k 1 and k 2 are the reaction Žrelaxation or randomization. rates at temperatures T1 and T2 respectively. A modified Arrhenius formula was given by Ibar in 1979 w12x who obtained the equation by combining the Arrhenius formula and the WLF ŽWilliams, Landel and Ferry. equation w13x. For poled materials, k 1 and k 2 are the relaxation lifetimes of nonlinear optical intensity at elevated temperatures. In our case, relaxation of self-aligned molecules is caused by compression of matrix network. We have shown in Figs. 1 and 2 the relaxation
3.1. ActiÕation energy The activation energy Ea , which is a chemical concept, was introduced to treat nonlinear optical relaxation of poled materials w10x. Physically, Ea represents the energy barrier that an oriented molecule must overcome so as to transfer to a random state. A large Ea means that the poled guest in the matrix is difficult to randomize, so the material
Fig. 7. Changes of SH intensity for a heating circle between 30–508C.
L. Liu et al.r Physics Letters A 262 (1999) 206–211
211
critical temperature exists for gelation. Short-timehigh-temperature annealing has an equal effect as long-time-low-temperature. Since self-alignment of amphiphilic molecules is only achieved at or near the film–substrate interface, all the gelation information comes from that particular region which is hard to obtain using conventional methods.
Acknowledgements Fig. 8. Arrhenius-fit of the randomization rates. The dots are experimental results. The line is a linear fitting.
curves of SH intensity. The randomization rates of orderly self-aligned molecules are obtained by using single exponential decay fitting. The results are plotted in Fig. 8. Both the Arrhenius formula and the modified Ibar formula were used to fit the experimental points in Fig. 8. As a result, we obtain a activation energies of 40.8 kcalrmol ŽArrhenius. and 39.6 kcalrmol ŽIbar. respectively. The fitted activation energy by the two methods shows excellent coincidence. The values are apparently higher than that in the guest–host type polymer. We believe that the high value of Ea originates dominantly from the better rigidity of the silica network. It shows the possibility of obtaining much better nonlinear optical thermo-stability using the sol–gel technique.
4. Conclusion As we have shown above, self-alignment of amphiphilic molecules in the glass film fabricated using the sol–gel technique can be used as a sensitive probe for the gelation process. In order to achieve this goal, in-situ temperature dependent SHG measurements have been carried out. We find that the gelation speed dependeds much on the annealing temperature. In addition, de-aggregation is observed at elevated annealing temperatures. As expected, no
The financial support by the Climb Project from the Ministry of Science and Technology of China, National Natural Science Foundation of China ŽGrant No. 69708005, 69808001., Trans-century Training Program Foundation for the Talents by the Ministry of Education of China and The ShuGuang Project by the Education Commission of Shanghai ŽGrant No. 97SG02. are acknowledged.
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