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FTO polymeric composites films: A new designed optical system for laser power attenuation
Optics and Laser Technology 121 (2020) 105823
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Optical linearity and bandgap analysis of RhB-doped PMMA/FTO polymeric composites films: A new designed optical system for laser power attenuation M.I. Mohammeda, M.S. Abd El-sadekb, I.S. Yahiac,d,
T
⁎
a
Metallurgical Lab.1., Nanoscience Laboratory for Environmental and Biomedical Applications (NLEBA), Semiconductor Lab., Department of Physics, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt b Nanomaterials Lab, Physics Department, Faculty of Science, South Valley University, Qena, Egypt c Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia d Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia
H I GH L IG H T S
optical system based on RhB-doped PMMA doped with Rhodamine B-dye/FTO has deposited. • The illustrates a uniform and semicrystalline structure in nature for the obtained thin films. • X-ray direct and indirect bandgaps were calculated and analyzed. • Both studied polymeric composites films are highly attenuated for the laser beam of 532 nm. • The • RhB-doped PMMA/FTO composites can be extensively appropriate to the optoelectronic applications.
A R T I C LE I N FO
A B S T R A C T
Keywords: Rhodamine B dye PMMA/FTO films Polymeric composite films Direct and indirect bandgap Laser power attenuations
Thin films of PMMA doped with Rhodamine B-dye have deposited successfully on a fluorine-doped tin oxide (FTO) glass substrate as a new optical system by using a spin coating method. This new system is promising to be used in various technological applications as solar energy cells, electronic apparatuses, devices of optical filter properties and optoelectronic tools. X-ray diffraction (XRD) illustrates a uniform and semicrystalline structure in nature for the obtained thin films. A spectrophotometric investigation was applied on the prepared thin films at the range of (300–2500 nm) for the wavelength. Both linear refractive index and absorption index values were measured to be reduced as the wavelength was raised. Tauc’s plot indicated that direct and indirect transitions were displayed by the thin film. Both direct and indirect bandgaps were calculated and analyzed. Continuous waves of the green laser beam (operating at 532), and He-Ne laser beam (operating at 632.8 nm) were used to study the optical properties of the prepared thin films with different dye concentrations. The results illustrated that the polymeric composites films are highly attenuated for the laser beam of 532 nm. The designed RhB-doped PMMA/FTO composites can be extensively appropriate to the optoelectronic applications.
1. Introduction A great deal of interest has been given for the incorporation of organic dyes into the matrix of polymers owing to the important applications such as sensors, solar cells light concentrators, and nonlinear optical materials [1]. Moreover, polymers have become profitable for sensor technologies, because of their low-cost and their simple fabrication techniques. Considerable interest has been given to the materials of polymer-based sensing because they display a variation in their
properties of absorption and/or fluorescence as a response to an external stimulus. The stimuli as light, heat, chemicals, and deformation which widen the useful application of the sensors in the technology [2]. Polymeric thin films containing luminescent dyes are widely utilized in sensors. The selecting of polymer-dye combinations in which the polymer solvent dissolves the dye leads to enhance the sensor effectiveness[3]. Polymethyl methacrylate (PMMA) is considered one of the most important polymeric media. PMMA is a rigid, hard, transparent polymer, polar material, has a glass transition temperature of
⁎ Corresponding author at: Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia. E-mail address: [email protected] (I.S. Yahia).
https://doi.org/10.1016/j.optlastec.2019.105823 Received 24 May 2019; Received in revised form 14 August 2019; Accepted 6 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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125 °C and has a large dielectric constant. It is a thermoplastic material that can be melted and molded into the desired shape [4]. But, PMMA has a distinct disadvantage that is its low nonlinear refractive index. This work is a try to increase and control the PMMA optical properties by mixing with Rhodamine B dye and deposit it on FTO glass substrate by using chloroform as a solvent to get transparency thin films [5]. The family of Rhodamine dyes was considered as one of the oldest and most commonly synthetic dyes which applied in various fields. Firstly, they were useful for the cloth coloration [6–8]. Due to their individual optical properties, they utilized as water tracing agents, in fluorescent markers for the studies of microscopic structure, in photosensitizers. Also, due to their spectral luminescence properties, they are used as biomarkers which important for the biological systems studies and in sensors [9–13]. FTO substrate is considered as a favorable alternative for the deposition of different organic materials and nanomaterials. It is cheaper than the indium-doped tin oxide (ITO) substrate. FTO substrate has many advantages such as [14]: its high transparency (up to 82%) at the visible region, conductivity up to 7 Ω−1 and optical transmittance cut-off at the near-infrared (NIR) region. FTO glass has high thermal stability and heating cancellation of the incident light at NIR region. RhB-doped PMMA/FTO [15] is considered as a new trend in the optical constant of the recent fabricated thin films. The aim of this work is to study the impact of RhB dye on the optical properties of PMMA/FTO glass substrate thin films. The obtained thin films were characterized by various techniques like structure analysis, transmittance, absorbance, absorption index, and optical energy gap. The optical properties of RhB-doped PMMA composite films as a filler material and those of the main matrix PMMA were also studied. He-Ne laser power of different wavelengths i.e. 532 nm and 632.8 was used to stand up at the applicability of the investigated substances in the laser power attenuation. The deposited films indicated that the fabricated materials are promising and can be used in wide-scale for photonic applications.
Table 1 Samples name with respect to the composition. The concentration of the dye in PMMA
Name of the samples
Pure PMMA 10−5 M 10−4 M 10−3 M 10−2 M 10−1 M
(So) (S1) (S2) (S3) (S4) (S5)
Different concentrations of Rh-B (10−5, 10−4, 10−3, 10−2 and 10−1 M) were added to PMMA. The applied spin coating technique had been adopted for depositing the uniform thin films of the studied materials onto fluorine-doped tin oxide glass substrates. The spin coating apparatus speed was adjusted to be 2000 rpm. Onto the rotating substrate, a drop from each cast was dropped to obtain a uniform thin film. The respective compositions are tabulated in Table 1.
2.3. Devices and measurements Shimadzu Lab XRD-6000 with CuKα radiation of wavelength λ = 1.5406 Å was utilized to investigate the structure of RhB-doped PMMA/FTO substrates. The X-ray analysis conditions were operated at 2θo from 10° to 70°, current = 30 mA, voltage = 30 kV and with a slow speed of scanning (0.02°). To measure the linear absorption spectra of the prepared films, a double beam UV–Vis–NIR, JASCO V-570 spectrophotometer in the wavelength 190–2500 nm was employed. All the optical characteristics were operated at room temperature. The error in the optical parameters such as absorbance a, transmittance and reflectance are ± 1%. The error bar is shown in the transmittance and reflectance spectra. The film thickness was estimated by Alpha-Step IQ surface profiler to be about 150 nm. To estimate the optical limiting of the laser power of RhB-doped PMMA/FTO composite films, a manual Z-scan system was utilized with a fixed sample holder. The predominant characteristic here is that the sample must be put at the focus of the inserted lens between the optical laser power meter and the laser beam. The He-Ne laser operating at 632.8 nm and green laser of 532 nm was used as a continuous wave source for the excitation. The 10 cm focal lens was put on the stand of the settled optical bench. The photodetector model (Newport, model1916R) was used to measure the input/output power. The considered samples were conveyed by an appropriate holder for polymeric films.
2. Experimental techniques 2.1. Materials PMMA (polymethyl methacrylate) (from BDH England) with MW of 145,000 g mol−1 was used as the matrix. Moreover, Rhodamine-B [9(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene] dimethyl ammonium chloride, (with high purity from Aldrich) was added to PMMA. In addition, chloroform, with high purity, was used as a common solvent for PMMA and RhB dye. 2.2. Preparation of RhB-doped PMMA/FTO polymeric composite films
3. Results and discussion
PMMA polymer and Rhodamine-B with molecular formula C28H31ClN2O3, have molecular structures given in Fig. 1. Thin films with the thickness (150 nm) were prepared by using a spin coating method. PMMA and RhB had been dissolved in chloroform separately.
3.1. XRD patterns of RhB-doped PMMA/FTO polymeric films Fig. 2 illustrates the XRD spectra of RhB-doped PMMA/FTO polymeric films with different dye concentrations. From the figure, it is obvious that there is no peak which denotes about an amorphous or semi-crystalline nature of the prepared films. At room temperature, the deposited organic materials on the glass and conductive substrates, mostly display the amorphous nature [15]. A matching was made to the main preferred orientation peaks of Miller indices (1 1 0), (2 0 0), (2 1 1), (2 2 0) (3 1 0), and (3 0 1) with those of FTO film, by using JCDPS data (No. 41-1445). The decrease in the peak intensity of RhBdoped PMMA/FTO film is attributed to the adhesion of the composites (polymer PMMA + RhB dye) on the FTO layer surface. There is a high stack of RhB-doped PMMA on the surface of FTO/glass substrates was supported by XRD measurements[16]. For the newly designed electronic and optoelectronic devices, the FTO substrate had become a principal material in the recent industries. The results for FTO glass substrate were reported by others as Xia et al. [17].
Fig. 1. Molecular structure of (a) PMMA polymer and (b) Rhodamine B dye. 2
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Fig. 2. X-ray diffraction patterns of RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
3.2. Optical properties of RhB-doped PMMA/FTO polymeric films Without any doubt, there is essential and technological importance for the optical property’s investigation of polymer thin films. The optical constants calculations of coatings thin films even organic or inorganic composites dominate plenty of their optical applications. The performance of an optical system or device depends on these constants. Fig. 3a shows the spectrum of the measured transmittance T(λ), as a function of wavelength for RhB-doped PMMA/FTO films. T(λ) of So (pure PMMA) in the visible range of the spectrum is about ≅84% and then its transparency decreases at NIR region to (≅25%). This refers to the low transparency of the film at 1350 nm. There is one absorption band appeared at 555 nm for the high concentration of Rh-B-doped PMMA/FTO (from S3 to S5) which is identical to the absorption band of RhB dye [18]. The presence of one ‘‘valley” region becomes more declared with increasing the concentration of RhB. The composite films showed a full absorptive flat band from 300 to 600 nm at which (T → 0) for (S4 and S5) where the dye concentration equal (10−2 and 10−1 M). The increase of hydrogen bonding between PMMA and H-aggregate layers of RhB dye may be the main cause for the observed decrease by increasing the concentration of dye. For the optical blocking in the band (300–600 nm); it can be reported that the best CUT-OFF optical laser filter properties can be applied for 10−1 RhB-doped PMMA/FTO films [19]. The reflectance R(λ) values for pure PMMA and RhB-doped PMMA/FTO films shown in Fig. 3b are nearly the same and matched with previous work [20]. The decrease of transmittance in the IR region indicates the high values of the reflection of IR signals [18,19]. The reflectance showed multi-oscillations peaks in the studied spectrum region due to the interaction between the light incident and the RhBdoped PMMA. Fig. 4 illustrates the absorption spectra for RhB-doped PMMA/FTO films for all the doping ratios. By increasing the doping ratio of RhB in the PMMA matrix, the absorption intensity was increased due to the evolution of the dye molecules number. There are no peaks in the absorption spectrum for PMMA and RhB-doped PMMA/ FTO films when the dye concentration is lower than 10−3 M (from So to S2). It could be seen that there is an absorption band around 554 nm for S4 and S5. This band results from the π-π* transitions[21–25] from the binding highest occupied molecular orbital (HOMO) to the anti-binding lowest unoccupied molecular orbital (LUMO) along the longest dimension of the conjugated system [26]. The absorption peak values increased with increasing Rh-B contents, due to the increase of the absorbing species number according to Beer’s law [27]. The conventional Tauc,s equation is used to estimate the optical
Fig. 3. (a) Transmittance and (b) reflectance for RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
Fig. 4. Absorption spectra for RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
band gap (Eg) as the following [28]:
αhυ = A (hυ − Eg )r ,
(1)
where A is a constant, α is the absorption coefficient =(2.303*Abs)/t), 3
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extrapolated to hυ = 0 [29] from two different regions of the plots in Figs. 5 and 6. The gained values are recorded in Table 2. The indirect energy band gap values were found to be in the range 3.54–3.39 eV for Eg(1) and 1.94–2.07 eV for Eg(2). While for the direct bandgap the values are in the range 4.09–3.39 eV for Eg(1) and 2.04–2.13 eV for (Eg(2)). The variations of optical band gap emphasize the presence of further energy states motivated by doping concentration [30]. These results indicate that the RhB dye adjusts both the structure and the electronic structure of PMMA because of the formation of defect levels with RhB dye as effective doping. From Figs. 5 and 6 and Table 1, Two-band gaps can be distinguished, indirect and direct optical transitions, the first one can be attributed to the PMMA absorption band edge, and the second is observed in the visible region and it may be caused by the addition of the RhB dye to the PMMA matrix. Also, it is obvious that the indirect optical energy gap of pure PMMA sample is agreed with the reported by other works [31–33]. The energy band gap value for Rh-B/PMMA thin film on the FTO-coated glass substrate was slightly less than those for PMMA thin film on an FTO-coated glass substrate [34]. The decrease in the bandgap value may be attributed to the impact of the groups attached to the organic materials which make them more stable than the PMMA itself. Also; PMMA has direct and indirect energy gaps [29,35]. The values of refractive (n) and absorption (k) indices effect on the electromagnetic wave velocity during its propagation through RhBdoped PMMA thin film on FTO covered glass. By the aid of reflection (R), and the extinction coefficient (k), the refractive index (n) for all films was deduced [36]:
Fig. 5. The relationship between (αhυ)1/2 and hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
k=
αλ , 4π
n=
(1 + R) + (1 − R)
(2)
4R − k 2, (R − 1)2
(3)
The estimation of the refractive index, n is decisive for the materials which can be used in communication such as optical cables, filters, and switches [13]. The refractive index n indicates that the spectral feature of RhB-doped PMMA/FTO thin films shown in Fig. 7, displays both normal and anomalous dispersions with the wavelength increase [37,38]. The normal dispersion in which the refractive index decreased with wavelength is due to the single oscillator model. While the increase of the refractive index with a wavelength in the anomalous dispersion is due to the multi-oscillator model. The resonance effect which occurred between the incident electromagnetic radiations and the polarization of the electrons is considered as anomalous behavior. This leads to the aggregation of an electron in the RhB-doped PMMA/ FTO thin films to the oscillating electric field [39]. Fig. 8 displays the dependence of the absorption index k for RhBdoped PMMA/FTO films on the wavelength in UV–Vis-NIR regions. RhB-doped PMMA/FTO glass absorption spectrum has a peak at 554 nm which characteristic the RhB dye. A remarkable increase in the absorption peak intensity is found with the evolution of RhB dye in PMMA matrix [40]. It can be noticed that the rapid increase was detected above 1500 nm which means that the material transmittance decreased
Fig. 6. Plotting of (αhυ)2 and hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
(Abs) is the measured absorbance and t is the thickness of RhB-doped PMMA/FTO films), r = 2 or 3, 1/2 and 3/2 for direct allowed transition, indirect direct allowed transition, and directly forbidden transitions, respectively. The examined polymeric thin films display both direct and indirect band gaps. Figs. 5 and 6 indicate that both (αhυ)1/2 and (αhυ)2 depend linearly on hυ and both direct and indirect optical transitions are possible for the thin films. The two optical energy band gaps for jointly indirect and direct optical transitions are obtained from the straight-line portions of the curves. The straight lines are
Table 2 The optical band gap (indirect and direct transitions) of RhB-doped PMMA thin film and the comparison of the optical band gap of different systems. Bandgap/composite thin film
Eg1(ind), (eV)
Eg2(ind), (eV)
Eg1(d), (eV)
Eg2(d), (eV)
Pure PMMA (So) 10−5 M (S1) 10−4 M (S2) 10−3 M (S3) 10−2 M (S4) 10−2 M (S5) 2/, 7/dichloro-fluorescein/FTO Fluorescein/FTO CuInS2/FTO V2O5/FTO
3.54 3.5 3.52 3.51 3.48 3.39 3.4 1.7 – 2.05
– – – 1.94 2.07 1.95 2.25 –– –
4.09 4.08 4.07 4.07 4.02 3.9 –4 1.53 –
– – – – 2.13 2.04 –– – – –
4
Present work
[31] [17] [19] [18]
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Fig. 7. The spectral behavior of the refractive index, n, for RhB-doped PMMA/ FTO films with different doping ratio of RhB dye.
Fig. 9. Plots of ε1 versus photon energy hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 10. Plots of ε2, versus photon energy hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 8. The spectral behavior of absorption index, k, for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
constant. Since special emphasis should be placed on the optical behavior of RhB-doped PMMA/FTO films for the application of optoelectronics, that is useful when both values become constant. It is a valid and acceptable behavior for the as-deposited polymeric films. The refractive index, dielectric constant and dielectric loss are the principal characteristics for the designing of optoelectronic and electronic applications. Dielectric loss (tanδ) is a considerable factor to estimate the dispersed and absorbed energy of the as-deposited thin films. Damping could be referred to tanδ that determines the ratio of dissipation to dispersed energy of a thin film (the ratio of loss to the storage of energy). This measurement aids to identify the preferable energy absorption in the prepared film and essentially can be utilized to establish the modulus of the as-deposited thin film. The associated angle to tanδ should be ranged from 0°-90°. The film will display an elastic behavior when the angle tends to zero while the material will be more viscous in behavior if the angle reaches 90°. The tanδ can be calculated as following [44]:
gradually in this region [41]. Dielectric function declares the electron transitions between the bands of a solid structure. Therefore, the dielectric spectrum of the material declares important information about the band structure of a solid. The complex dielectric function of RhBdoped PMMA/FTO films is given by the relation:
ε ∗ = ε1 + iε2,
(4)
where ε1 and ε2 are the real and imaginary parts of the dielectric constant, respectively. The optical constants n and k are employed to calculate the values of the ε1 and ε2 by applying these relations [42,43]:
ε1 = n2 − k 2,
(5)
ε2 = 2nk ,
(6)
Figs. 9 and 10 display a sharp decay of dielectric constant and dielectric loss (ε1 and ε2 ), respectively at about 0.85 eV, which considered as a threshold value for the dielectric constants decay. A sharp decrease of dielectric constant was observed within the small energy range (0.50 to 0.85 eV), then it becomes nearly constant at a range close to 4 eV. However, it is detected a very small difference in the real part of the dielectric constant ε1over the range of 0.85 to 4.5 eV. This variation as compared to its initial value is negligible and it means that it is almost
tanδ =
ε2 , ε1
(11)
where, ε2 is the dielectric constant and ε1 is the dielectric loss. From the 5
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Fig. 11. Plot of tanδ versus photon energy, hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 12. Plots of the normalized power vs. the concentration of RhB in PMMA matrix using green and He-Ne lasers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
equation, the energy dissipation from the studied materials increases when tanδ is larger. When tanδ becomes smaller, the film shows more dielectric behavior and then it can store energy more than dissipates it. This denotes that tanδ is an essential value to deduce the deterioration level of the fabricated thin films. Fig. 11 indicates that there are two maximum values for tanδ at 554 nm and 2500 nm, respectively. Both values have a sharp decrease in the range (0.5–3 eV). Also, a fast increase in the dissipation factor is noticed at photon energy value (4.25 eV) which matches with the FTO coated glass substrate. Finally, we can conclude that the incorporation of RhB in the PMMA matrix led to the formation of new molecular dipole levels, as a point defect inside the HOMO - LOMO gap. In its structure, RhB dye has both Oxygen and Nitrogen atoms (the highest group of electric negative elements). Therefore, it can establish hydrogen bonds with the carbonyl group in the PMMA structure. Such hydrogen bonds can result in extremely polarized films produced from PMMA/RhB composites.
reach zero value. From our previous work, we can conclude that that, the addition of the dye inside the PMMA or PVA polymeric matrix with special dye concertation, we can reach the fully blocked the laser beams of different wavelength according to the CUT-OFF region of the transmittance plots i.e. T tends to zero.
4. Conclusions Uniform Rh-B-doped PMMA films were deposited on FTO conductive substrate with different dye concentrations by a spin coating method. XRD spectra indicate that the as-deposited films have an amorphous nature and the XRD peaks come from the FTO glass substrate. The optical investigation indicates that transmittance was prominently higher (≈82% at visible region) when compared to the reflectance and absorbance. Thin films absorption coefficient was attributed to the direct and indirect bandgap transitions. The indirect energy band gap values were found to be in the range 3.54–3.39 eV for Eg(1) and 1.94–2.07 eV for Eg(2). While for the direct bandgap the values are in the range 4.09–3.39 eV for Eg(1) and 2.04–2.13 eV for (Eg(2)). It is observed that the optical limiting properties of these thin films were found to be sensitive to green laser than He-Ne laser beams. Dielectric constants, dielectric loss, and the dissipation factor were calculated and interpreted in detail. The studied material showed a highly attenuated of the green laser power beam due to the complete absorption band which occurs around 554 nm for RhB doped PMMA. So, the fabricated material is a promising material for the optical limiting of the green laser light with wavelength = 532 nm for CUT-OFF laser filter and the applications of photo-electronic.
3.3. The optical limiting effect of RhB-doped PMMA/FTO polymeric films The search for optical limiting organic substances is a significant matter. The curve of the optical limiting for pure PMMA and RhB-doped PMMA/FTO films is shown in Fig. 12. Two different laser beams are identified as He-Ne laser of wavelength = 632.8 and green laser of wavelength = 532 and nm was employed. The distinctive curve that illustrates the relation between the normalized power (Normalized power is the ratio of output power/input power) against the molar percent of Rh-B-doped PMMA/FTO films. The pure PMMA has a limited restriction of the laser power for the two laser sources. The films are more sensitive to the green laser of 532 nm than 632.8 nm laser. This is due to the complete absorption band which occurs around 554 nm for RhB doped PMMA. This indicates that the thin film samples are good candidates for the optical limiting of the green laser light with wavelength = 532 nm [45–47]. Whereas the addition of RhB to PMMA host matrix causes a partial blocking of the incident laser beam 532 nm (from 0.91 to 0.24). The attenuation of the green laser occurs due to the sample with high RhB dye concentration has more molecules per unit volume. So, this strongly participates in the optical interaction during the nonlinear absorption processes [46,47]. But for the He-Ne laser of 632.8 nm, the optical limiting mostly weaker than the green laser of 532 nm and nearly unchanged, because the transmittance in this region is very high and reached 85%. Even so, the laser light blocking dependence on the transmittance of the laser light through the blocked transmittance region of the polymeric films i.e. CUT-OFF region of the transmittance in which T tends to
Acknowledgement The authors are grateful to The Research Center for Advanced Material Science (RCAMS) at King Khalid University, with grant number (RCAMS-KKU/001-19).
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105823. 6
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7
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Full length article
Optical linearity and bandgap analysis of RhB-doped PMMA/FTO polymeric composites films: A new designed optical system for laser power attenuation M.I. Mohammeda, M.S. Abd El-sadekb, I.S. Yahiac,d,
T
⁎
a
Metallurgical Lab.1., Nanoscience Laboratory for Environmental and Biomedical Applications (NLEBA), Semiconductor Lab., Department of Physics, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt b Nanomaterials Lab, Physics Department, Faculty of Science, South Valley University, Qena, Egypt c Research Center for Advanced Materials Science (RCAMS), King Khalid University, Abha 61413, P.O. Box 9004, Saudi Arabia d Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia
H I GH L IG H T S
optical system based on RhB-doped PMMA doped with Rhodamine B-dye/FTO has deposited. • The illustrates a uniform and semicrystalline structure in nature for the obtained thin films. • X-ray direct and indirect bandgaps were calculated and analyzed. • Both studied polymeric composites films are highly attenuated for the laser beam of 532 nm. • The • RhB-doped PMMA/FTO composites can be extensively appropriate to the optoelectronic applications.
A R T I C LE I N FO
A B S T R A C T
Keywords: Rhodamine B dye PMMA/FTO films Polymeric composite films Direct and indirect bandgap Laser power attenuations
Thin films of PMMA doped with Rhodamine B-dye have deposited successfully on a fluorine-doped tin oxide (FTO) glass substrate as a new optical system by using a spin coating method. This new system is promising to be used in various technological applications as solar energy cells, electronic apparatuses, devices of optical filter properties and optoelectronic tools. X-ray diffraction (XRD) illustrates a uniform and semicrystalline structure in nature for the obtained thin films. A spectrophotometric investigation was applied on the prepared thin films at the range of (300–2500 nm) for the wavelength. Both linear refractive index and absorption index values were measured to be reduced as the wavelength was raised. Tauc’s plot indicated that direct and indirect transitions were displayed by the thin film. Both direct and indirect bandgaps were calculated and analyzed. Continuous waves of the green laser beam (operating at 532), and He-Ne laser beam (operating at 632.8 nm) were used to study the optical properties of the prepared thin films with different dye concentrations. The results illustrated that the polymeric composites films are highly attenuated for the laser beam of 532 nm. The designed RhB-doped PMMA/FTO composites can be extensively appropriate to the optoelectronic applications.
1. Introduction A great deal of interest has been given for the incorporation of organic dyes into the matrix of polymers owing to the important applications such as sensors, solar cells light concentrators, and nonlinear optical materials [1]. Moreover, polymers have become profitable for sensor technologies, because of their low-cost and their simple fabrication techniques. Considerable interest has been given to the materials of polymer-based sensing because they display a variation in their
properties of absorption and/or fluorescence as a response to an external stimulus. The stimuli as light, heat, chemicals, and deformation which widen the useful application of the sensors in the technology [2]. Polymeric thin films containing luminescent dyes are widely utilized in sensors. The selecting of polymer-dye combinations in which the polymer solvent dissolves the dye leads to enhance the sensor effectiveness[3]. Polymethyl methacrylate (PMMA) is considered one of the most important polymeric media. PMMA is a rigid, hard, transparent polymer, polar material, has a glass transition temperature of
⁎ Corresponding author at: Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia. E-mail address: [email protected] (I.S. Yahia).
https://doi.org/10.1016/j.optlastec.2019.105823 Received 24 May 2019; Received in revised form 14 August 2019; Accepted 6 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
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125 °C and has a large dielectric constant. It is a thermoplastic material that can be melted and molded into the desired shape [4]. But, PMMA has a distinct disadvantage that is its low nonlinear refractive index. This work is a try to increase and control the PMMA optical properties by mixing with Rhodamine B dye and deposit it on FTO glass substrate by using chloroform as a solvent to get transparency thin films [5]. The family of Rhodamine dyes was considered as one of the oldest and most commonly synthetic dyes which applied in various fields. Firstly, they were useful for the cloth coloration [6–8]. Due to their individual optical properties, they utilized as water tracing agents, in fluorescent markers for the studies of microscopic structure, in photosensitizers. Also, due to their spectral luminescence properties, they are used as biomarkers which important for the biological systems studies and in sensors [9–13]. FTO substrate is considered as a favorable alternative for the deposition of different organic materials and nanomaterials. It is cheaper than the indium-doped tin oxide (ITO) substrate. FTO substrate has many advantages such as [14]: its high transparency (up to 82%) at the visible region, conductivity up to 7 Ω−1 and optical transmittance cut-off at the near-infrared (NIR) region. FTO glass has high thermal stability and heating cancellation of the incident light at NIR region. RhB-doped PMMA/FTO [15] is considered as a new trend in the optical constant of the recent fabricated thin films. The aim of this work is to study the impact of RhB dye on the optical properties of PMMA/FTO glass substrate thin films. The obtained thin films were characterized by various techniques like structure analysis, transmittance, absorbance, absorption index, and optical energy gap. The optical properties of RhB-doped PMMA composite films as a filler material and those of the main matrix PMMA were also studied. He-Ne laser power of different wavelengths i.e. 532 nm and 632.8 was used to stand up at the applicability of the investigated substances in the laser power attenuation. The deposited films indicated that the fabricated materials are promising and can be used in wide-scale for photonic applications.
Table 1 Samples name with respect to the composition. The concentration of the dye in PMMA
Name of the samples
Pure PMMA 10−5 M 10−4 M 10−3 M 10−2 M 10−1 M
(So) (S1) (S2) (S3) (S4) (S5)
Different concentrations of Rh-B (10−5, 10−4, 10−3, 10−2 and 10−1 M) were added to PMMA. The applied spin coating technique had been adopted for depositing the uniform thin films of the studied materials onto fluorine-doped tin oxide glass substrates. The spin coating apparatus speed was adjusted to be 2000 rpm. Onto the rotating substrate, a drop from each cast was dropped to obtain a uniform thin film. The respective compositions are tabulated in Table 1.
2.3. Devices and measurements Shimadzu Lab XRD-6000 with CuKα radiation of wavelength λ = 1.5406 Å was utilized to investigate the structure of RhB-doped PMMA/FTO substrates. The X-ray analysis conditions were operated at 2θo from 10° to 70°, current = 30 mA, voltage = 30 kV and with a slow speed of scanning (0.02°). To measure the linear absorption spectra of the prepared films, a double beam UV–Vis–NIR, JASCO V-570 spectrophotometer in the wavelength 190–2500 nm was employed. All the optical characteristics were operated at room temperature. The error in the optical parameters such as absorbance a, transmittance and reflectance are ± 1%. The error bar is shown in the transmittance and reflectance spectra. The film thickness was estimated by Alpha-Step IQ surface profiler to be about 150 nm. To estimate the optical limiting of the laser power of RhB-doped PMMA/FTO composite films, a manual Z-scan system was utilized with a fixed sample holder. The predominant characteristic here is that the sample must be put at the focus of the inserted lens between the optical laser power meter and the laser beam. The He-Ne laser operating at 632.8 nm and green laser of 532 nm was used as a continuous wave source for the excitation. The 10 cm focal lens was put on the stand of the settled optical bench. The photodetector model (Newport, model1916R) was used to measure the input/output power. The considered samples were conveyed by an appropriate holder for polymeric films.
2. Experimental techniques 2.1. Materials PMMA (polymethyl methacrylate) (from BDH England) with MW of 145,000 g mol−1 was used as the matrix. Moreover, Rhodamine-B [9(2-carboxyphenyl)-6-diethylamino-3-xanthenylidene] dimethyl ammonium chloride, (with high purity from Aldrich) was added to PMMA. In addition, chloroform, with high purity, was used as a common solvent for PMMA and RhB dye. 2.2. Preparation of RhB-doped PMMA/FTO polymeric composite films
3. Results and discussion
PMMA polymer and Rhodamine-B with molecular formula C28H31ClN2O3, have molecular structures given in Fig. 1. Thin films with the thickness (150 nm) were prepared by using a spin coating method. PMMA and RhB had been dissolved in chloroform separately.
3.1. XRD patterns of RhB-doped PMMA/FTO polymeric films Fig. 2 illustrates the XRD spectra of RhB-doped PMMA/FTO polymeric films with different dye concentrations. From the figure, it is obvious that there is no peak which denotes about an amorphous or semi-crystalline nature of the prepared films. At room temperature, the deposited organic materials on the glass and conductive substrates, mostly display the amorphous nature [15]. A matching was made to the main preferred orientation peaks of Miller indices (1 1 0), (2 0 0), (2 1 1), (2 2 0) (3 1 0), and (3 0 1) with those of FTO film, by using JCDPS data (No. 41-1445). The decrease in the peak intensity of RhBdoped PMMA/FTO film is attributed to the adhesion of the composites (polymer PMMA + RhB dye) on the FTO layer surface. There is a high stack of RhB-doped PMMA on the surface of FTO/glass substrates was supported by XRD measurements[16]. For the newly designed electronic and optoelectronic devices, the FTO substrate had become a principal material in the recent industries. The results for FTO glass substrate were reported by others as Xia et al. [17].
Fig. 1. Molecular structure of (a) PMMA polymer and (b) Rhodamine B dye. 2
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Fig. 2. X-ray diffraction patterns of RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
3.2. Optical properties of RhB-doped PMMA/FTO polymeric films Without any doubt, there is essential and technological importance for the optical property’s investigation of polymer thin films. The optical constants calculations of coatings thin films even organic or inorganic composites dominate plenty of their optical applications. The performance of an optical system or device depends on these constants. Fig. 3a shows the spectrum of the measured transmittance T(λ), as a function of wavelength for RhB-doped PMMA/FTO films. T(λ) of So (pure PMMA) in the visible range of the spectrum is about ≅84% and then its transparency decreases at NIR region to (≅25%). This refers to the low transparency of the film at 1350 nm. There is one absorption band appeared at 555 nm for the high concentration of Rh-B-doped PMMA/FTO (from S3 to S5) which is identical to the absorption band of RhB dye [18]. The presence of one ‘‘valley” region becomes more declared with increasing the concentration of RhB. The composite films showed a full absorptive flat band from 300 to 600 nm at which (T → 0) for (S4 and S5) where the dye concentration equal (10−2 and 10−1 M). The increase of hydrogen bonding between PMMA and H-aggregate layers of RhB dye may be the main cause for the observed decrease by increasing the concentration of dye. For the optical blocking in the band (300–600 nm); it can be reported that the best CUT-OFF optical laser filter properties can be applied for 10−1 RhB-doped PMMA/FTO films [19]. The reflectance R(λ) values for pure PMMA and RhB-doped PMMA/FTO films shown in Fig. 3b are nearly the same and matched with previous work [20]. The decrease of transmittance in the IR region indicates the high values of the reflection of IR signals [18,19]. The reflectance showed multi-oscillations peaks in the studied spectrum region due to the interaction between the light incident and the RhBdoped PMMA. Fig. 4 illustrates the absorption spectra for RhB-doped PMMA/FTO films for all the doping ratios. By increasing the doping ratio of RhB in the PMMA matrix, the absorption intensity was increased due to the evolution of the dye molecules number. There are no peaks in the absorption spectrum for PMMA and RhB-doped PMMA/ FTO films when the dye concentration is lower than 10−3 M (from So to S2). It could be seen that there is an absorption band around 554 nm for S4 and S5. This band results from the π-π* transitions[21–25] from the binding highest occupied molecular orbital (HOMO) to the anti-binding lowest unoccupied molecular orbital (LUMO) along the longest dimension of the conjugated system [26]. The absorption peak values increased with increasing Rh-B contents, due to the increase of the absorbing species number according to Beer’s law [27]. The conventional Tauc,s equation is used to estimate the optical
Fig. 3. (a) Transmittance and (b) reflectance for RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
Fig. 4. Absorption spectra for RhB-doped PMMA/FTO thin films with different doping ratio of RhB.
band gap (Eg) as the following [28]:
αhυ = A (hυ − Eg )r ,
(1)
where A is a constant, α is the absorption coefficient =(2.303*Abs)/t), 3
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extrapolated to hυ = 0 [29] from two different regions of the plots in Figs. 5 and 6. The gained values are recorded in Table 2. The indirect energy band gap values were found to be in the range 3.54–3.39 eV for Eg(1) and 1.94–2.07 eV for Eg(2). While for the direct bandgap the values are in the range 4.09–3.39 eV for Eg(1) and 2.04–2.13 eV for (Eg(2)). The variations of optical band gap emphasize the presence of further energy states motivated by doping concentration [30]. These results indicate that the RhB dye adjusts both the structure and the electronic structure of PMMA because of the formation of defect levels with RhB dye as effective doping. From Figs. 5 and 6 and Table 1, Two-band gaps can be distinguished, indirect and direct optical transitions, the first one can be attributed to the PMMA absorption band edge, and the second is observed in the visible region and it may be caused by the addition of the RhB dye to the PMMA matrix. Also, it is obvious that the indirect optical energy gap of pure PMMA sample is agreed with the reported by other works [31–33]. The energy band gap value for Rh-B/PMMA thin film on the FTO-coated glass substrate was slightly less than those for PMMA thin film on an FTO-coated glass substrate [34]. The decrease in the bandgap value may be attributed to the impact of the groups attached to the organic materials which make them more stable than the PMMA itself. Also; PMMA has direct and indirect energy gaps [29,35]. The values of refractive (n) and absorption (k) indices effect on the electromagnetic wave velocity during its propagation through RhBdoped PMMA thin film on FTO covered glass. By the aid of reflection (R), and the extinction coefficient (k), the refractive index (n) for all films was deduced [36]:
Fig. 5. The relationship between (αhυ)1/2 and hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
k=
αλ , 4π
n=
(1 + R) + (1 − R)
(2)
4R − k 2, (R − 1)2
(3)
The estimation of the refractive index, n is decisive for the materials which can be used in communication such as optical cables, filters, and switches [13]. The refractive index n indicates that the spectral feature of RhB-doped PMMA/FTO thin films shown in Fig. 7, displays both normal and anomalous dispersions with the wavelength increase [37,38]. The normal dispersion in which the refractive index decreased with wavelength is due to the single oscillator model. While the increase of the refractive index with a wavelength in the anomalous dispersion is due to the multi-oscillator model. The resonance effect which occurred between the incident electromagnetic radiations and the polarization of the electrons is considered as anomalous behavior. This leads to the aggregation of an electron in the RhB-doped PMMA/ FTO thin films to the oscillating electric field [39]. Fig. 8 displays the dependence of the absorption index k for RhBdoped PMMA/FTO films on the wavelength in UV–Vis-NIR regions. RhB-doped PMMA/FTO glass absorption spectrum has a peak at 554 nm which characteristic the RhB dye. A remarkable increase in the absorption peak intensity is found with the evolution of RhB dye in PMMA matrix [40]. It can be noticed that the rapid increase was detected above 1500 nm which means that the material transmittance decreased
Fig. 6. Plotting of (αhυ)2 and hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
(Abs) is the measured absorbance and t is the thickness of RhB-doped PMMA/FTO films), r = 2 or 3, 1/2 and 3/2 for direct allowed transition, indirect direct allowed transition, and directly forbidden transitions, respectively. The examined polymeric thin films display both direct and indirect band gaps. Figs. 5 and 6 indicate that both (αhυ)1/2 and (αhυ)2 depend linearly on hυ and both direct and indirect optical transitions are possible for the thin films. The two optical energy band gaps for jointly indirect and direct optical transitions are obtained from the straight-line portions of the curves. The straight lines are
Table 2 The optical band gap (indirect and direct transitions) of RhB-doped PMMA thin film and the comparison of the optical band gap of different systems. Bandgap/composite thin film
Eg1(ind), (eV)
Eg2(ind), (eV)
Eg1(d), (eV)
Eg2(d), (eV)
Pure PMMA (So) 10−5 M (S1) 10−4 M (S2) 10−3 M (S3) 10−2 M (S4) 10−2 M (S5) 2/, 7/dichloro-fluorescein/FTO Fluorescein/FTO CuInS2/FTO V2O5/FTO
3.54 3.5 3.52 3.51 3.48 3.39 3.4 1.7 – 2.05
– – – 1.94 2.07 1.95 2.25 –– –
4.09 4.08 4.07 4.07 4.02 3.9 –4 1.53 –
– – – – 2.13 2.04 –– – – –
4
Present work
[31] [17] [19] [18]
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Fig. 7. The spectral behavior of the refractive index, n, for RhB-doped PMMA/ FTO films with different doping ratio of RhB dye.
Fig. 9. Plots of ε1 versus photon energy hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 10. Plots of ε2, versus photon energy hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 8. The spectral behavior of absorption index, k, for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
constant. Since special emphasis should be placed on the optical behavior of RhB-doped PMMA/FTO films for the application of optoelectronics, that is useful when both values become constant. It is a valid and acceptable behavior for the as-deposited polymeric films. The refractive index, dielectric constant and dielectric loss are the principal characteristics for the designing of optoelectronic and electronic applications. Dielectric loss (tanδ) is a considerable factor to estimate the dispersed and absorbed energy of the as-deposited thin films. Damping could be referred to tanδ that determines the ratio of dissipation to dispersed energy of a thin film (the ratio of loss to the storage of energy). This measurement aids to identify the preferable energy absorption in the prepared film and essentially can be utilized to establish the modulus of the as-deposited thin film. The associated angle to tanδ should be ranged from 0°-90°. The film will display an elastic behavior when the angle tends to zero while the material will be more viscous in behavior if the angle reaches 90°. The tanδ can be calculated as following [44]:
gradually in this region [41]. Dielectric function declares the electron transitions between the bands of a solid structure. Therefore, the dielectric spectrum of the material declares important information about the band structure of a solid. The complex dielectric function of RhBdoped PMMA/FTO films is given by the relation:
ε ∗ = ε1 + iε2,
(4)
where ε1 and ε2 are the real and imaginary parts of the dielectric constant, respectively. The optical constants n and k are employed to calculate the values of the ε1 and ε2 by applying these relations [42,43]:
ε1 = n2 − k 2,
(5)
ε2 = 2nk ,
(6)
Figs. 9 and 10 display a sharp decay of dielectric constant and dielectric loss (ε1 and ε2 ), respectively at about 0.85 eV, which considered as a threshold value for the dielectric constants decay. A sharp decrease of dielectric constant was observed within the small energy range (0.50 to 0.85 eV), then it becomes nearly constant at a range close to 4 eV. However, it is detected a very small difference in the real part of the dielectric constant ε1over the range of 0.85 to 4.5 eV. This variation as compared to its initial value is negligible and it means that it is almost
tanδ =
ε2 , ε1
(11)
where, ε2 is the dielectric constant and ε1 is the dielectric loss. From the 5
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Fig. 11. Plot of tanδ versus photon energy, hυ for RhB-doped PMMA/FTO films with different doping ratio of RhB dye.
Fig. 12. Plots of the normalized power vs. the concentration of RhB in PMMA matrix using green and He-Ne lasers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
equation, the energy dissipation from the studied materials increases when tanδ is larger. When tanδ becomes smaller, the film shows more dielectric behavior and then it can store energy more than dissipates it. This denotes that tanδ is an essential value to deduce the deterioration level of the fabricated thin films. Fig. 11 indicates that there are two maximum values for tanδ at 554 nm and 2500 nm, respectively. Both values have a sharp decrease in the range (0.5–3 eV). Also, a fast increase in the dissipation factor is noticed at photon energy value (4.25 eV) which matches with the FTO coated glass substrate. Finally, we can conclude that the incorporation of RhB in the PMMA matrix led to the formation of new molecular dipole levels, as a point defect inside the HOMO - LOMO gap. In its structure, RhB dye has both Oxygen and Nitrogen atoms (the highest group of electric negative elements). Therefore, it can establish hydrogen bonds with the carbonyl group in the PMMA structure. Such hydrogen bonds can result in extremely polarized films produced from PMMA/RhB composites.
reach zero value. From our previous work, we can conclude that that, the addition of the dye inside the PMMA or PVA polymeric matrix with special dye concertation, we can reach the fully blocked the laser beams of different wavelength according to the CUT-OFF region of the transmittance plots i.e. T tends to zero.
4. Conclusions Uniform Rh-B-doped PMMA films were deposited on FTO conductive substrate with different dye concentrations by a spin coating method. XRD spectra indicate that the as-deposited films have an amorphous nature and the XRD peaks come from the FTO glass substrate. The optical investigation indicates that transmittance was prominently higher (≈82% at visible region) when compared to the reflectance and absorbance. Thin films absorption coefficient was attributed to the direct and indirect bandgap transitions. The indirect energy band gap values were found to be in the range 3.54–3.39 eV for Eg(1) and 1.94–2.07 eV for Eg(2). While for the direct bandgap the values are in the range 4.09–3.39 eV for Eg(1) and 2.04–2.13 eV for (Eg(2)). It is observed that the optical limiting properties of these thin films were found to be sensitive to green laser than He-Ne laser beams. Dielectric constants, dielectric loss, and the dissipation factor were calculated and interpreted in detail. The studied material showed a highly attenuated of the green laser power beam due to the complete absorption band which occurs around 554 nm for RhB doped PMMA. So, the fabricated material is a promising material for the optical limiting of the green laser light with wavelength = 532 nm for CUT-OFF laser filter and the applications of photo-electronic.
3.3. The optical limiting effect of RhB-doped PMMA/FTO polymeric films The search for optical limiting organic substances is a significant matter. The curve of the optical limiting for pure PMMA and RhB-doped PMMA/FTO films is shown in Fig. 12. Two different laser beams are identified as He-Ne laser of wavelength = 632.8 and green laser of wavelength = 532 and nm was employed. The distinctive curve that illustrates the relation between the normalized power (Normalized power is the ratio of output power/input power) against the molar percent of Rh-B-doped PMMA/FTO films. The pure PMMA has a limited restriction of the laser power for the two laser sources. The films are more sensitive to the green laser of 532 nm than 632.8 nm laser. This is due to the complete absorption band which occurs around 554 nm for RhB doped PMMA. This indicates that the thin film samples are good candidates for the optical limiting of the green laser light with wavelength = 532 nm [45–47]. Whereas the addition of RhB to PMMA host matrix causes a partial blocking of the incident laser beam 532 nm (from 0.91 to 0.24). The attenuation of the green laser occurs due to the sample with high RhB dye concentration has more molecules per unit volume. So, this strongly participates in the optical interaction during the nonlinear absorption processes [46,47]. But for the He-Ne laser of 632.8 nm, the optical limiting mostly weaker than the green laser of 532 nm and nearly unchanged, because the transmittance in this region is very high and reached 85%. Even so, the laser light blocking dependence on the transmittance of the laser light through the blocked transmittance region of the polymeric films i.e. CUT-OFF region of the transmittance in which T tends to
Acknowledgement The authors are grateful to The Research Center for Advanced Material Science (RCAMS) at King Khalid University, with grant number (RCAMS-KKU/001-19).
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105823. 6
Optics and Laser Technology 121 (2020) 105823
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