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SYNTHESIS & CHARACTERIZATION OF SILICON HYBRID NANO COMPOSITES CONTAINING NONLINEAR OPTICAL DYES Y. Aghili1, 3, M. Mohseni1, E. Mohajerani2, H.R. Shirvani2, S. Moradian1, A.R. Khosravi1 1
Department of Polymer Eng., Amirkabir University of Technology, Hafez St., Tehran, Iran Phone: +98-21-64542441, e-mail:
[email protected] 2 Laser Research Institute, Shahid Beheshti University, Evin St., Tehran, Iran Phone: +98-21-29902598, e-mail:
[email protected] 3 Young Researchers Club (YRC), Shariati St., Tehran, Iran Phone: +98-912-289-5886, e-mail:
[email protected]
Abstract-A stable guest-host system was prepared to improve the nonlinear optical (NLO) effects as well as the mechanical and morphological properties of the polymeric host matrices. A hybrid organic-inorganic nano composite synthesized via the sol-gel process in our previous work and Disperse Red 1 (DR1) was utilized as the host and the guest, respectively. Thin films were deposited on glass slides using a dip-coater rig. Dichroism measurements were carried out to gain information on the aggregation of the dye molecules and also the orientation mechanisms. In addition, birefringence studies were performed to determine the desired variations of the refractive indices. The diagrams analyses were especially focused on the changing tendency of three parameters in different samples; the signal level, the rise time and the fall time. Keywords: Nonlinear Optics, Sol-gel, Dichroism, Birefringence, Guest-Host System.
INTRODUCTION Current trends in information technology (IT) suggest that light, rather than electricity has been labeled as one of the most important 21st century technology. However, the development of the photonic science largely relies on the progress achieved in fabrication of the new optical materials with better performance. In that respect, materials with a nonlinear optical (NLO) response are expected to play a major role in enabling optoelectronic and photonic technologies. On the other hand, organic-inorganic hybrid materials have increasingly become an important field of research during the recent years, due to their extraordinary properties attributed to the synergistic effects of their components. In addition, the interest of organic-inorganic hybrid materials in telecommunication devices has constantly grown, thanks to the ease with which functionalized hybrid materials can be processed into practical devices having low electric constant and high nonlinear optical coefficients [1-3]. In our previous work [4], we introduced a new hybrid organic-inorganic nano composites synthesized via the sol-gel route with lots of improved physical, mechanical and morphological properties. In the current study, we have actually applied the proven consequent matrix in the realm of nonlinear optics in order to examine its behaviors as a hybrid host component incorporated into the dye-doped guest-host NLO systems; focused on the photoinduced birefringence and dichroism analyses. The silica-based thin films were produced by sol-gel technique doped with Disperse Red 1 (DR1) molecules.
EXPERIMENTAL The organic monomer, 2-hydroxyethylmethacrylate (HEMA) obtained from Degussa and the organic-inorganic precursor, 3- (trimethoxysilyl) propyl methacrylate (MPTMS) purchased from Wacker, were used as received. Ethanol and acetone, as the solvents, were both gained from Merck. The thermal initiator, , ’-azoisobutyronitrile (AIBN), was supplied from Akzo-Nobel and purified by recrystalization from ethanol prior to use. Disperse 978-1-4244- /08/$25.00 ©2008 IEEE
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Red 1 (DR1), utilized as the NLO dye, and was obtained from Sigma-Aldrich. The organicinorganic host was prepared in two consecutive steps. The first step was included the free radical copolymerization of HEMA and MPTMS monomers and in the second step, a traditional sol-gel process was carried out which could, in turn, be divided into two stages: hydrolysis and condensation. The synthesis’s route and the details of the sol-gel preparation process were completely done according to our previous work, described in [4]. It should be mentioned that the 50 mol% content of the silane monomer (i.e. PHM50) was chosen as the synthesized composition ratio in order to prepare the hybrid network participating in the current study. This was due to the highest cross-linking density and the optimum value of the other enhanced properties which the mentioned composition had formerly achieved among the other applied ratios [4]. Furthermore, Disperse Red 1, with 3 wt% concentration, was being completely dissolved in acetone by the help of a magnetic stirrer prior to impregnating into the hybrid network in order to avoid aggregation of the dye molecules. The resulting solution was then added to the polymeric prepared precursor and again stirred to obtain a homogeneous true dispersion. Consequently, a series of thin films were deposited from the final colored solution onto precleaned glass-slide substrate by the help of a homemade dip-coater rig, using the withdrawal speed of 5 mm/sec (leading to the thickness of ∼2μm measured with a Dektak profilometer). Actually, the obtained films were highly transparent in the visible region of the spectrum which suggests the formation of compatible uniform sol-gel network having nano particle sizes well below the visible wavelengths. To mainly determine the influence of the samples curing process on the birefringence and dichroism measurements; the photoinduced studies were carried out, in which the prepared hybrid thin films were encountered a specific heating procedure, according to Table 1. Table 1 – Curing state of the studied samples in the heating procedure
Sample 1 2 3 4 5
Curing State Uncured Curing up to 140°C without exposing time Curing @ 140°C for 45 min Curing @ 140°C for 90 min Curing @ 140°C for 135min
RESULTS AND DISCUSSION All the investigations represent that the three concerning parameters, means the saturated signal level (or the maximum probe transmission), the rise time and the fall time of both photoinduced effects (according to the definition given by the IEEE society); obviously have a flourishing trend by increasing the heating time and completing the curing procedure. These behaviors may be attributed to the greater cross-linking density of the hybrid host structure according to the more complete hydrolysis and condensation reactions of the alkoxy groups, resulting from the continual process of curing. It implies that chromophore dipoles need some kind of free space (known as the free volume or cavity) for their molecular rotation. In thermoplastic polymeric systems (without any cross-linking reaction), since the host polymer chains around the guest chromophores can be re-oriented after the dye excitation, therefore, the mentioned rate constants depends on the number of cavities with sufficient free volumes to allow the back and forth reactions. In such organic host structures, chromophore molecule should just get over the molecules interaction energy with the cavity walls in order to change the orientation, it which accordingly results in fast orientation and re-orientation
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0.02 0.018 0.016 0.014 0.012 0.01 0.008 0.006 0.004 0.002 0
Saturated Signal Intensity (a.u.)
Saturated Signal Intensity (a.u.)
mechanisms during the Rise Time and Fall Time, respectively [5, 6]. In fact, the current thermoset nano composite structure approximately behaves as a thermoplastic polymer at its uncured state; means the state of just being deposited from a fresh synthesized solution which may only have a negligible cross-linking density. In this case, the chromophore molecules can extensively undergo thermal fluctuation in the existing free volumes. These easily thermal fluctuations cause more randomized alignments, leading to lower saturated signal intensity (or the probe transmission value) in both photoinduced birefringence and dichroism measurements. But, the obtained diagrams of the present work demonstrate that the concerning parameter increases by performing the curing procedure (see Fig. 1 and 2).
1
2
3
4
7 6.7 6.4 6.1 5.8 5.5 5.2 4.9 4.6 4.3 4 1
5
Sample Number (State of Curing)
Fig.1 - Variation of the saturated signal intensity as a function of the curing state, obtained in the birefringence measurements.
2
3
4
5
Sample Number (State of Curing)
Fig.2 – Variation of the saturated signal intensity as a function of the curing state, obtained in the dichroism measurements.
64100 60100 56100 52100 48100 44100 40100 36100 32100 28100 24100 20100 16100 12100 8100 4100 100
Fall Time Rise Time Time (msec)
Time (msec)
This is undoubtedly attributed to the improving tendency of the cross-linking reactions between the silanol groups achieved by evaporating more side product (i.e. water or alcohol) through an endothermic process. Reaching higher cross-linking density will give rise to a drastic reduction of the size and the amounts of pores (free volumes), increasing therefore the steric hindrance for randomized molecular orientation. Consequently, the pump power will be more efficient to subdue the fluctuation motions, in order to achieve the desired alignment with higher saturated signal intensity which actually results in receiving desired variations of the refractive indices. Moreover, this increasing trend is coincided with higher photoisomerization time parameters (i.e. the R.T. and F.T.). Fig. 3 and 4 show the changing trend of the photoinduced birefringence and dichroism, respectively. By decreasing the free volume of the polymer matrix, the thermal rotation (also known as the diffusion rotation) of both cis and trans molecules (more in the cis form) will hardly occur. Thus, the diffusion rotation procedure will happen with a slower rate, i.e. a higher Rise Time constant, compared to the uncured sample.
1
2
3
4
5
Sample Number (State of Curing)
Fig.3 – The changing tendency of R.T. and F.T. due to state of curing, observed for the birefringence effects.
2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0
Fall Time Rise Time
1
2
3
4
5
Sample Number (State of Curing)
Fig.4 – The changing tendency of R.T. and F.T. due to state of curing, observed for the dichroism effects.
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Qualitatively, the relaxation (i.e. the Fall Time duration) of the probe transmittance could also be analyzed by two steps: an initial fast decay related to the thermal cis-trans isomerization and a slow decay related to the angular redistribution of the oriented trans molecules [7]. It should be also mentioned that the obtained Fall Time duration always has a greater order of magnitude comparing with the relating Rise Time. This stems from the lower probability of the thermal rotation for the stable perpendicular trans molecules in order to get back to the randomized orientation.
CONCLUSION The almost high birefringence intensity of the sol-gel films and, therefore, the higher refractive index obtained in this study is comparable to some other conventional polymeric guest-host systems. So, in the prospect of future, there would be a promise achievement for such systems in optical applications, especially the waveguides and the switchers. Moreover, the more efficient dichroic dye-doped films obtained via the easy and inexpensive sol-gel process accompanied by lower chromophore aggregation and, of course, with no covalent bonding incorporation would be also a reliable field of future study for designing the new devices and appliances. In addition, it should be also mentioned that the new generation of the guest-host (G/H) system with a hybrid organic-inorganic structures do have the enough potential to be one of the active area in the nonlinear optics, especially using the coronapoling procedure. This is due to its several modifying behaviors and advantages accompanied with the existing benefits of the conventional G/H systems; i.e. the low cost production of NLO materials, the possibility of applying a wide variety of centrosymmetric NLO chromophores along with a wide range of operating frequencies, the ease of fabrication onto a large area of any dimension and also the facile processing into thin films by spray coating, dipping or spinning. In other words, applying the sol-gel hybrid matrices in the field of G/H systems with highly stable NLO properties can hopefully overcome the drawbacks of the conventional G/H systems involved the all-organic thermoplastic polymers as the host may have a low glass transition temperature (Tg) or the dye molecules can easily diffuse to the surface of the film at elevated temperatures; e.g. during the corona-poling procedure.
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