Nonlinear Refraction and Absorption Phenomena in a Novel

Aug 20, 2008 - Nonlinear optical behavior of a novel nanocomposite material based on chloroaluminum phthalocyanine (ClAlPc) embedded in silica and ...
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J. Phys. Chem. C 2008, 112, 14336–14342

Nonlinear Refraction and Absorption Phenomena in a Novel Nanocomposite Based on a Dye Stabilized in a Solid Hybrid Matrix K. Sathiyamoorthy and C. Vijayan* Department of Physics, Indian Institute of Technology Madras, Chennai 600 036 India ReceiVed: February 19, 2008; ReVised Manuscript ReceiVed: May 1, 2008

Nonlinear optical behavior of a novel nanocomposite material based on chloroaluminum phthalocyanine (ClAlPc) embedded in silica and incorporated in the structural nanopores of a Nafion membrane is investigated using the Z-scan technique. The experiments are performed at 532 nm using 8 ns pulses at a pulse repetition rate of 10 Hz from a frequency-doubled Nd:YAG laser. The nonlinearity in the hybrid membranes is characterized by obtaining the nonlinear refractive index and nonlinear absorption coefficient. Fluorescence lifetimes are measured using a time-correlated single photon counting technique. The effects of dynamic and static quenching on photostability are studied. Incorporating phthalocynanine molecules in the stable hybrid membrane renders them stable against photobleaching and provides a convenient solid matrix. 1. Introduction Nonlinear optical response of dyes stabilized in suitable solid matrices is of recent interest in view of their potential for photonic device applications.1–8 Development of efficient nonlinear optical devices is often hindered by the difficulty in obtaining materials with the proper combination of desirable properties such as large nonlinear response, short response times, and high damage thresholds. One way of overcoming some of these problems is designing composite materials of appropriate components with tailor-made properties.9–18 Current interest in this area is focused on designing hybrid organic-inorganic composites containing optical materials such as dyes which show high third-order nonlinear susceptibility. Such composites have recently been studied as laser gain media,9–14 an important class of optical materials. A proper choice of the organic polymer host and the optical material can be made to obtain stable media with controllable physical properties such as optical, electrical, and mechanical. This paper is on the nonlinear optical response of a new type of dye-doped silica-Nafion nanocomposite material. Silica sol-gel provides a transparent, rigid, and compact environment for encapsulating chromophore molecules. Nafion (a commercially available membrane from DuPont), a transparent membrane with very high thermal and mechanical stability, incorporates the chromophore in its structural nanopores. The linear optical absorption coefficient at the wavelength of study is an important parameter in understanding the mechanisms of nonlinearity operative in the system, and hence the optical properties of the sample are characterized using optical absorption spectroscopy. The optical properties of the composite are found to be different from those of the dye in the solution form. Chloroaluminum phthalocyanine (ClAlPc) is an organometallic compound which has been reported as a promising material for photonics applications. This compound is known to exhibit a substantially lower threshold for optical limiting application compared to most of the organic materials based on studies on this material in the solution form.19–32 Results on the nonlinear optical response of nanocomposites incorporating host-stabilized ClAlPc studied using the Z-scan technique at a wavelength of * Corresponding author e-mail: [email protected].

Figure 1. Flowchart representing the synthesis procedure of the Nafion-silica nanocomposite membrane.

532 nm is presented in this paper. The large nonlinear optical absorption observed in these nanocomposites renders them ideal candidates materials for optical limiting devices. 2. Membrane Preparation Nafion 117 from Dupont, is cut into pieces of 1 cm × 1 cm size, boiled in concd nitric acid at 70 °C for about 0.5 h and then washed with cold water. This cleaning process is repeated until the brown color of the film fades and it becomes transparent. Figure 1 represents a flowchart containing the details of the synthesis procedure of the Nafion-SiO2 nanocomposite

10.1021/jp801461x CCC: $40.75  2008 American Chemical Society Published on Web 08/20/2008

Novel Nanocomposite Based on a Dye membrane. The preparation of Nafion-SiO2 nanocomposite membrane for our present study as a solid matrix for dye entrapment is discussed below.33,34 The washed Nafion is soaked in 1 M NaOH solution and heated for 6 h to convert the membrane to the ionic form Na+. This process ensures good mechanical strength for the membrane for withstanding the subsequent processing steps. Then the membrane is rinsed in triply deionized water at 60 °C for 30 min. This causes purified Na+ ions to be formed in the membrane, which is then placed in a vacuum oven and heat treated at 100 °C for 12 h. The membrane was immersed in 10:1 ethanol/H2O solution for 1 h in order to maximize the absorption of the precursor by swelling the pores of the membrane. Dye of concentration 10-6 M is prepared in ethanol. This dye solution is used to prepare 0.5 M 70 wt% TEOS. Finally the membrane is immersed in 0.5 M 70 wt% TEOS solution for 6 h and then rinsed with acetone to remove surface deposition. Then the membrane is kept in vacuum oven and heated at 110 °C for 24 h to complete the condensation reaction. For comparative studies, one more 1 cm × 1 cm size Nafion membrane is taken and cleaned thoroughly as described above said procedure and is just immersed in 10-6 M of dye solution for 2 min and vaccuum-dried. Then the membrane is removed and washed with ethanol to remove excess dye molecules remains over the surface of the membrane.

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Figure 2. Experimental setup for (a) closed aperture Z-scan and (b) open aperture Z-scan.

3. Experimental Section Optical absorption of the material is characterized using a spectrophotometer (JASCO, Japan). The experiment is done at a scanning rate of 400 nm/min with a fixed bandwidth of 1 nm. Fluorescence spectra are recorded using a flourimeter from JASCO. The excitation bandwidth is kept at about 1 nm whereas emission bandwidth is kept at about 3 nm. The experiment is performed at a scanning rate of about 400 nm/min. The Z-scan technique is a convenient method to measure the laser induced nonlinear properties of the material.35 The basic idea of the closed aperture Z-scan technique is that the sample is moved along the axis of propagation of a focused Gaussian beam through its focal point, and the far field transmittance is monitored by energy meter with an aperture of finite size in front of the detector. A frequency doubled Nd:YAG laser is used as a source and is operated in Q-switched mode. It delivered 8 ns pulses at 532 nm with a repetition rate of 10 Hz. The laser beam is focused on the sample by a lens of focal length 10 cm. The beam waist ωo at the focus is ≈33 µm. The thickness of the sample is L ≈ 0.18 mm. The sample can be considered as an optically thin medium as the thickness lies well within the Rayleigh range Zo ) πω02/λ of our system. The closed aperture Z-scan studies are performed by measuring the far field sample transmittance by using an energy meter (Gentec, Canada), with an aperture in front of the detector. The experimental setup for closed aperture Z-scan is shown in Figure 2a. The closed aperture Z-scan curve tends to deviate from the expected theoretical curve representing the effects of pure nonlinear refraction in case nonlinear absorption also is present in the sample at the wavelength used. This is the case with most of the samples when the laser power is sufficiently high. Thus it is necessary to separately evaluate nonlinear absorption by performing the Z-scan experiment with the aperture removed. This is the open scan mode of the Z-scan experiment35 which can be used to probe nonlinear absorption in the material. Figure 2b represents the open aperture Z-scan apparatus. The data from the closed aperture Z-scan experiment are normalized with respect to the data from the open aperture Z-scan experiment in order to obtain the close scan trace representing purely

Figure 3. Optical absorption spectra of ClNf (denoted by circles) and ClSNf (denoted by triangles).

nonlinear refraction. Both open and close aperture Z-scans are performed at a pulse energy of 0.085 mJ. A time-correlated single-photon counting technique is used to determine the florescence lifetimes of the samples. The measurement is made using an IBH-5000 single photon counting spectrometer. A nano-LED from Spectra Physics with a resolution of 200 ps is used to excite the sample, and a Hamamatsu R 3809 U-50 microchannel plate photomultiplier is used as the detector. The fluorescence decay curves are analyzed using an iterative fitting program provided by IBH. The singlet excitedstate lifetime is determined from the decay profile. The excitation and emission wavelengths are fixed in the setup at 640 and 694 nm in the case of ClSNf and 640 and 736 nm in the case of ClNf. Photostability of the dye molecules embedded in the matrix is investigated by exposing the sample to ambient light for six months and monitoring the optical absorption spectrum. 4. Results and Discussion A. Linear Optical Properties. Figure 3 shows the optical absorption spectra of chloroaluminum phthalocyanine-doped Nafion membrane (denoted as ClNf) and chloroaluminum phthalocyanine-doped silica-Nafion nanocomposite membrane

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Figure 4. Fluorescence emission spectra of ClSNf (triangle) and ClNf (circle) on excitation at 640 nm.

(denoted as ClSNf). There are two bands in the absorption spectra, a broad Q-band in visible and a Soret band in the ultraviolet regions, which are attributed to the π-π* transitions, but these absorption bands are found to be red-shifted with respect to the corresponding bands of ClAlPc in ethanol. The shift in bands is more pronounced in the case of ClNf than in ClSNf due to different chemical environments faced by the molecules within the embedded matrices. Further, the Q-band shows a well defined split in the case of ClNf whereas it is unsplit in the case of ClSNf. This is in agreement with earlier observations that the metal ion gets expelled from the molecule during its incorporation in Nafion, and it gets embedded in the free base form.3,36 The split in the Q-band spectrum of ClNf shown here resembles that reported in the case of the free base of a very similar compound which was attributed to the lowering in symmetry.37 Bare solid Nafion contains hydrophilic/hydrophobic domains distributed in indistinguishable manner, at random. When Nafion is immersed in dye solution, the ClAlPc is expected to localize in the hydrophobic clusters present in structural nanopores of the Nafion membrane. Because of loose packaging of the solid Nafion morphology with inhomogeneous distribution of hydrophilic and hydrophobic domains, the probability of interaction of the ClAlPc with the nearest neighboring sulfonic acid groups (SO3-) is enhanced, which results in protonation of ClAlPc molecule.35 On the other hand, it is interesting to note that no such split is observed in the case of the silica-encapsulated phthalocyanine sample. This indicates that silica encapsulation tends to prevent the expulsion of the metal ligand while being incorporated in Nafion. This result may be of significance while embedding optical materials such as phthalocynanines in solid host membranes for convenience in device applications. The fluorescence emission spectra shown in Figure 4 show maximum emission at 694 nm in the case of ClSNf (shown by circles) and 736 nm in the case of ClNf (shown by triangles) on excitation at 640 nm. The shift in the fluorescence bands toward longer wavelengths can be attributed to the different chemical environments in the matrices. The emission peak in ClSNf is much closer to the corresponding peak in the case of the phthalocyanine in solution form whereas the peak is much shifted in ClNf form, indicating that silica encapsulation helps to preserve the optical properties of the molecule.

Sathiyamoorthy et al.

Figure 5. Infrared spectra of dye-doped polymer, polymer, and dye, respectively.

Figure 6. Energy level diagram for metallophthalocyanine.

B. FTIR. Figure 5 represents the FTIR spectra of polymer matrix, bare dye and dye embedded in the polymer matrix, respectively. The most noticeable change in the spectra of dyedoped polymer is the disappearance of the N-Al vibrations at 910 cm-1 which confirms the protonation of ClAlPc to free base phthalocyanine by the presence of sulfonic groups in the Nafion membrane.38 The same conclusion is confirmed by other changes in the spectra such as the overall change in shape of the group of peaks between 716 and 778 cm-1 which also corresponds to a transition from ClAlPc to H2Pc.39 C. Open Aperture Z-Scan. Nonlinear absorption processes in MPc’s are generally described by a five-level model (Figure 6).25,29 At thermal equilibrium the unperturbed ClAlPc molecules reside in the lowest electronic state. Upon being pumped by a laser pulse at 532 nm, the molecules are promoted to a first singlet state S1 via a one-photon process, or to a second singlet state S2 via a two-photon process. The molecules excited to state S1 would undergo one of the following three processes: (1) fluorescent decay to the ground-state S0, (2) intersystem crossing to the triplet state T1, or (3) one photon excitation to state S2. Those relaxing to T1 can further be excited to T2 if the pulse duration is comparable with TISC or longer. Nonlinear absorption coefficient β is measured independently by the open aperture Z-scan technique. Figure 2b represents

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Figure 7. (a) Open aperture Z-scan trace for ClNf and (b) open aperture Z-scan trace for ClSNf. (Solid line represents the theoretical fit to the experimental data shown by symbols).

the schematic diagram of the open aperture Z-scan. This technique measures the total transmittance through the sample as a function of incident laser intensity while the sample is moved along the optical axis of a convex lens. The irradiance distribution of the beam at the exit surface of the sample is given by35

Ie(z, r, t) )

I(z, r, t)e-RL 1 + q(z, r, t)

(1)

where q(z,r,t) ) βI(z,r,t). In the case of the open aperture Z-scan, the transmittance becomes insensitive to beam distortion and is only a function of nonlinear absorption. The total transmittance fluence in that case (S ) 1) can be obtained by spatially integrating the above expression without having to include the free space propagation process. The normalized transmittance of the open aperture Z-scan as a function of the position along the z axis can be written as27,35

Tnorm(z) )

loge[1 + q0(z)] q0(z)

q0(z) )

q 1 + (z/zo)2

(2) (3)

where z0 is the diffraction length of the beam and q ) βI0Leff, where Leff ) (1 - e-RL)/R, β is the intensity-dependent nonlinear

Figure 8. Fluorescence decays of ClNf (a) and ClSNf (b) on excitation at 640 nm.

absorption coefficient, I0 is the intensity of the light at focus. Leff is known as the effective length of the sample, defined in terms of the linear absorption coefficient, R0, and the true optical path length through the sample, L. Reverse saturable absorption (RSA) generally arises in a molecular system when the excited-state absorption cross-section is larger than the ground-state cross-section. ClAlPc is found to be a RSA material. Moreover it has been found that though ClAlPc has significant singlet-singlet excited-state absorption for sub-nanosecond pulses, triplet-triplet dynamics begin to dominate in the case of excitation with longer pulses. As the nonlinear behavior of the materials is studied at 532 nm using 8 ns pulses, the pulsewidth being much longer than TISC, triplet-triplet dynamics will also have significant influence on β. Hence β in the expression (eq 2) is replaced by the generalized term βeff while performing the theoretical fit. Figure 7a and 7b show the normalized transmittance of ClNf and ClSNf, respectively, measured using the open aperture Z-scan technique. Both the compounds exhibited a reduction in the transmission near the focus of the lens which is indicative of nonlinear absorption. The nonlinear absorption coefficient βeff is calculated from the experimental data by fitting eq 2. Curve fitting is done with βeff as a free running parameter. The solid line represents a theoretical fit of the experimental data

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Figure 9. Stern-Volmer plots of the photoexcited H2Pc in the Nafion matrix on the basis of the relative emission yield (φ/φ0, shown by stars) and the relative emission lifetime (τ0/τ, shown by circles).

obtained from the open aperture Z-scan to the theoretical model given by eq 2. The values of βeff obtained from the fit in the case of ClSNf and ClNf are 7.3 cm/GW and 9.3 cm/GW, respectively. The fluorescence decay curves of ClSNf and ClNf are shown in Figures 8a and 8b, respectively. The data are fitted using a monoexponential function represented by

I(t) ) I0 exp(-t/τ)

(4)

where I0 is the intensity at time zero. The lifetime is given by the inverse of the total decay rate, τ ) (Γ + knr)-1, where Γ is the emissive rate and knr is the nonradiative decay rate. The fluorescence lifetime is obtained from the slope of a plot of log I(t) versus t. In Figure 8, the solid line represents the theoretical fit for the experimental data using eq 4. The measured excited singlet state life times of ClSNf and ClNf are found to be 6.2 and 0.65 ns, respectively. The singlet state lifetime is increased by 1 order of magnitude between the two samples, indicating that the dye is incorporated in different chemical forms in very different local environments in the two different samples. This could be due to the fact that silica encapsulation tends to prevent the expulsion of the metal ligand of the dye while being incorporated in Nafion in the case of ClSNf (unlike the case of ClNf), as was also noted earlier in connection with the interpretation of the optical absorption spectra. The decrease in lifetime of dye-embedded Nafion could be due to quenching effects. It can be either dynamic or static quenching. Dynamic quenching is associated with diffusion of dye molecules and is suppressed to a great extent in a solid matrix44 and hence the dominant mechanism in a solid matrix expected to be a static one. Figure 9 shows the Stern-Volmer plots of the photoexcited ClAlPc in the Nafion matrix on the basis of the relative emission yield and the relative emission lifetime. ClAlPc-doped Nafion film is prepared by dipping 1 cm × 1 cm size Nafion film in the dye solution of known concentration. The concentration of the doped Nafion film is calculated using the expression45

Cs )

(CiVi - CfVf) Vs

(5)

where C and V are concentration and volume, respectively, and the subscripts i, f, and s refer to initial solution, final solution,

Figure 10. (a) Closed aperture Z-scan trace for ClNf and (b) closed aperture Z-scan trace for ClSNf. (Solid line represents the theoretical fit to the experimental data shown by symbols.)

and the embedded sample, respectively. The figure indicates that both static and dynamic mechanisms are operational. If the quenching obeys a dynamic mechanism, the emission lifetime would decrease with an increase in dye concentration. A dynamic mechanism can be represented by τ0/τ ) 1+ κsvc36 where c is the quencher concentration and κsv is the Stern-Volmer constant. The value of κsv is obtained by fitting the above expression and calculating the second-order quenching rate constant κq2 () κsv/τ0). The value is found to be 16 × 109 M-1 s-1. In a static quenching when a quencher molecule is present in a quenching sphere (radius R0) around a luminescent probe, the emission is quenched entirely. Therefore the relative emission ratio (φ/φ0) is equal to the probability that quencher molecule is out of the quenching ∫R∞0 P(r,c)dr ) Φ/Φ0. By integrating the above equation we get the expression Φ/Φ0 ) exp(-4π(R30 - s3)NA10-24c/3), where NA, s, and c are Avogadro’s number, the radius of the excluded sphere in which another molecule center does not exist, and concentration of the dispersed molecule, respectively. Since the contribution of dynamic quenching should also be considered, the above quenching model now takes the form

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Φ/Φ0 ) (1 + κq2τ0c)exp(-4π(R30 - s3)NA10-24c/3) (6) The H2Pc molecule is tentatively assumed to be spherical with the same volume as a phthalocyanine ring with surface area 160 Å and thickness 3.4 Å.40–43 Hence the diameter of the hypothetical sphere of the H2Pc is estimated to be 0.26 nm, which is considered as the contact distance (s) between the molecules. From the plot φ/φ0, we can estimate the value of R and κq2 by fitting eq 6, and the values are found to be 1.7 nm and 3.2 × 107 M-1 s-1, respectively. The value of the secondorder quenching rate constant in the case of static mechanism is found to be 2 orders of magnitude lower than that of dynamic mechanism. Hence the dominant quenching process in the present case appears to be the dynamic mechanism. The singlet state lifetime values indicate that the nonlinear off-resonance irradiation of sample (ClNf) at 532 nm with nanosecond pulses produces the effect of reverse saturable absorption between triplet to triplet states, as the pulse width is much longer than the singlet excited-state lifetime, but in the case of ClSNf both singlet and triplet excited-state absorption contribute to nonlinear absorption as the singlet-state lifetime is comparable with laser pulse width. Hence the value of βeff corresponding to two-photon absorption should be large in the case ClSNf as compared to ClNf, but in contrast, ClNf exhibits a higher value, presumably due to the higher concentration of ClNf. D. Closed Aperture Z-Scan. Figure 10 shows the result of the closed aperture Z-scan experiment where the normalized transmittance of the sample is plotted as a function of on axis distance through which the sample is moved. The sign of closed aperture Z-scan profile indicates a negative nonlinearity as the peak is followed by a valley. This Z-scan trace has been normalized with respect to an open scan trace thereby obtaining the pure refractive data. The index of refraction n is related to nonlinear refractive index n2 by the expression

n ) no + n2Io

(7)

where no is the linear refractive index and Io is the irradiance of the laser beam within the sample. The solid line in Figure 10 represents the theoretical fit on experimental data obtained by Z-scan. The fit is done using the following expression derived by Sheik-Bahae35

T)1+

4X ∆Φ0 (X2 + 9)(X2 + 1)

(8)

where X ) Z/Zo. The nonlinear refractive index of the sample is calculated from the calculated value of the on axis phase shift |∆Φ0| obtained from the theoretical fit using the following expression,

|∆Φ0| ) kn2LeffIo

under nanosecond excitation at the pulse repetition rate of 10 Hz as the response time of such samples is expected to be of the order of a few milliseconds.46,47 E. Photostability. The optical stability of the materials is monitored by recording the optical absorption spectra of the samples as they are kept in the ambient environment for several months. Sample ClNf in which the phthalocyanine is incorporated in pure Nafion is found to get completely photobleached due to ambient light in about a month’s time. On the other hand, sample ClSNf (in which the phthalocyanine is incorporated in the silica-Nafion nanocomposite), continues to be photostable without any bleaching effect even after exposure to ambient conditions for several months. The reason could be the increase in the mobility of the excited dye molecules due to heating effects resulting from nonradiative decay processes during the deexcitation.43,48 This is clear from the earlier discussion on the dominant mechanism being dynamic rather than static which could be the reason for photobleaching of incorporated dye molecules. The optical absorption spectra are routinely recorded a few times before and after each Z-scan experiment to check the optical quality of the film with progress of time, and no changes could be found in the spectral features. The excitation at 532 nm at the power levels used does not seem to cause any thermal or photodegradation in the samples. The absence of photodegrdation could be due to the fact that the dye is embedded in Nafion, a material known to be thermally, chemically, and mechanically very stable.33,34 Thus the new nanocomposite appears to hold promise as an optically stable material with excellent optical nonlinear properties. 5. Conclusion A novel composite material based on ClAlPc molecules incorporated in to SiO2-Nafion nanocomposite membrane is synthesized using the sol gel technique. The sample exhibits large refractive nonlinearity arising from thermal as well as electronic mechanisms. The performance is compared with another sample in which the dye is incorporated in pure Nafion without silica doping. The two samples differ both physically and chemically in terms of the mode of incorporation of the dye molecule in the host environment, leading to interesting differences in their optical properties. Interesting processes of nonlinear refraction and absorption are found to occur in the samples, and the singlet and triplet excited states play an important role in the optical response of the samples. Embedding the optical medium in silica and incorporating it in the nanopores of Nafion membrane with a high damage threshold is found to stabilize the molecule against photobleaching and to render it as a convenient candidate material for photonic device applications.

(9)

where Leff ) (1 - exp(-RL)/R is the effect length of the sample, Io is the on axis irradiance at focus, k is the wave vector, L is the sample length, and R is the linear absorption coefficient. The value of n2 obtained by the experiment is -9.3 × 10-12 cm2/W for ClSNf and -10 × 10-12 cm2/W for ClNf. The higher value of n2 in the case of ClNf is attributed to higher dye concentration. The experiments are repeated with undoped sample (which does not contain the phthalocynanine) which did not exhibit any nonlinear effect at the power levels used. The analysis of the refractive nonlinearity is based on the Kerr effect, and the effect due to thermal nonlinearity is ignored. However, thermal mechanism could also contribute to some extent in such systems

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