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Film thickness effects on nanorods organic films of azo quinoline derivatives for optical applications
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Original Research
Film thickness effects on nanorods organic films of azo quinoline derivatives for optical applications A.Z. Mahmouda,b,∗, A.A.A. Darwishc,d, Saleem I. Qashoue a
Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt College of Sciences and Art at ArRass -Qassim University, ArRass 51921, Saudi Arabia c Nanotechnology Research Laboratory, Department of Physics, Faculty of Science, University of Tabuk, Tabuk, Saudi Arabia d Department of Physics, Faculty of Education at Al-Mahweet, Sana'a University, Al-Mahweet, Yemen e Department of Physics, Faculty of Science, Zarqa University, Zarqa 13132, Jordan b
ARTICLE INFO
ABSTRACT
Keywords: Thin films Nanorod Quinoline Thickness effect Optical properties
The effect of film thickness on the structural and optical properties of bisbenzimidazo [2,1-a:2′,1′-a′] anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-10,21-dione (BI-diisoQ) films was carried out. The morphology of BIdiisoQ film showed a definite shape of a nanorod with a length of around 5 μm and a diameter of approximately 20 nm. Also, X-ray diffraction patterns demonstrated that BI-diisoQ thin films are characterized by a mixture of amorphous and crystalline structure, whereas the phase-nature crystallization of BI-diisoQ film enhanced as the film thickness increased. The investigation of the absorption coefficient of BI-diisoQ films revealed two indirect allowed band gaps energy. The calculated values of nonlinear optical parameters for BI-diisoQ film were observed to be increased with the increase of film thickness. The optical properties of BI-diisoQ nanorod films indicated that these films have appropriate optical properties, and thus it can be recommended as a promised candidate material in superconductors, optoelectronic and photonic applications.
1. Introduction The organic compounds with small molecules have numerous promising applications in electronic devices like solar cells [1,2] and organic light-emitting diodes [3], because of their simple handling at low temperatures and low-cost preparation comparing with inorganic materials [4,5]. The azo dyes have been exploited significantly in several branches of optoelectronic manufacturing because of their simplicity production, and their promising optical and electrical properties. One of the azo dyes is a quinoline compound which is perceived by its high thermal stability. Quinoline has various applications such as biological activity [6], information storage [7] and optoelectronic devices [8]. The substitution of peripheral groups is a well-known process that has been used widely in changing the chemical and physical properties of organic scaffold molecules. The examination of various substitution groups effect on the optical properties of the quinoline compound has been investigated. For instance, it was found that the variation of substituent groups showed a significant effect on the refractive index and dispersion parameters [9]. Many optical and electrical studies on quinoline and its derivatives
∗
have been reported [10–13]. The optical properties of 2,9-Bis [2-(4chlorophenyl)ethyl] anthrax [2,1,9-def:6,5,10-d0e0f0] diisoquinoline1,3,8,10 (2H, 9H) under the effect of annealing temperature were studied in our previous work [10]. It was observed that the energy of the band gap was reduced under the effect of annealing temperature [10]. Also, the investigation of the optical absorption measurements of 2-Amino-4-(5-bromothiophen-2-yl)-5,6-dihydro-6-methyl-5-oxo-4Hpyrano[3,2-c]quinoline-3-carbonitrile clarified that the electron transitions between the HOMO and LUMO of this organic compound were dominated by indirect allowed transition with an optical band gap of 2.5 eV [11]. Moreover, this material showed convenient photoelectric characteristics and thus can be used as photosensors [11]. Also, it was found that the quinoline ligands display two fluorescence bands in the region of 512–580 nm. Quinoline ligands were realized by similar behavior of conductivity concerning most of organic semiconductors materials [12]. On the other hand, the dependence of optical and structural properties of 4-cyanopyrano-quinoline dione on the film thicknesses was studied by Soliman et al. [13]. In the notation of their work, it was found that the optical constants of this compound did not affect the variation of the film thickness [13]. However, one of the important quinoline derivatives is bisbenzimidazo [2,1-a:2′,1′-a′]
Corresponding author. Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt. E-mail address: [email protected] (A.Z. Mahmoud).
https://doi.org/10.1016/j.pnsc.2019.04.009 Received 13 October 2018; Received in revised form 28 April 2019; Accepted 29 April 2019 1002-0071/ © 2019 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: A.Z. Mahmoud, A.A.A. Darwish and Saleem I. Qashou, Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.04.009
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Fig. 1. FTIR spectra of BI-diisoQ powder and film.
anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-10,21-dione (BIdiisoQ). Nevertheless, the studying of the optical properties of this organic compound correlated with different film thickness has not been reported yet. Indeed, the optical properties of small organic molecules are straightforwardly connected to the basic electronic features. For this reason, the searching for reasonable organic compounds with beneficial optical properties gives a rule to the outline of optoelectronic and photonic cells designing. The growing up of the layer deposited thickness is proposed to modify the homogeneity of thin films, which in turns will reduce the defects inside the films. Also, the increase in film thickness plays an essential role in the agglomeration of stacking particles and thus will promote the accretion of sublevel states number. These rational reasons are sufficient to alter the optical and electrical properties of the materials. On the other hand, the preparation of thin films with various thicknesses by avoiding the oxidation of the deposited material is essential to request for improving the performance of optoelectronic devices. Otherwise, the oxidation layers will lead to un-coveted interactions between any deposited oxide and the organic molecules. For this reason, many of techniques were adopted to minify the oxidation of the deposition material on the substrate. For instance, wet chemical processing [14], atomic hydrogen treatments [15], sulfur and nitrogen passivations, as well as Atomic-Layer-Deposition-Derived Al2O3 passivation layer, have all been applied to remove native oxides and reduce the interaction between any deposited oxide and the semiconductor [16–19]. Hence, the removable of native oxides will improve the performance of optoelectronic devices. In our present work, we adopted the thermal vacuum evaporating technique to attain homogeneous thin films. Of course, in high vacuum, the mean free path of vapor atoms is confirmed to be the same order as the vacuum chamber dimensions. So these particles pass in straight lines from the evaporation source towards the deposition substrate without colliding with the background gas. Consequently, high vacuum for a long time is essential to keep the stoichiometry of the deposited film during film growth [20]. Therefore, the present work aims to prepare homogenous BI-diisoQ thin films with different thickness by thermal vacuum vapor deposition. Also, another important goal of this research is to highlight the effect of the film thickness on the structural and optical properties of BI-diisoQ thin films.
2. Experimental techniques Powder of BI-diisoQ compound was obtained with 98% purity from Sigma Aldrich Company. BI-diisoQ was deposited on a cleaned glass and cleaved KBr by a thermal vapor deposition technique. The substrates at room temperature were fixed onto a spin holder to obtain uniform films. A coating unit model of HHV Auto 306 was used to achieve a vacuum of 2.45 × 10−5 mbar. The system was maintained under vacuum for 3 h to pledge straightforward evaporation of the material with a minimization of the sample oxidation [21]. Afterward, the deposition rate (0.25 nm/s), as well as the film thickness, were controlled using the quartz crystal monitor. The film thicknesses have been checked by using the interferometric technique, while the measurement values of the film thickness were found to be 25, 75, 100 and 150 nm. The X-ray Diffractometer (Philips PW 1710) was used to examine the phase structure of the powder and the obtained films. Scanning electron microscope (SEM) was used to investigate the morphologies of BI-diisoQ film by SEM-JOEL unit. The Fourier-transform infrared (FTIR) in the scope of 400–4000 cm−1 was used to get a comprehensive view of the molecular structure of the powder, as well as, the as-deposited BIdiisoQ thin films. To get a high precision of IR measurements, 1 mg of BI-diisoQ, which was scraped from the as-deposited thin film, was mixed with 50 mg of vacuum dried conventional IR materials (KBr powders). The double beam spectrophotometer (Shimadzu 2101) was utilized to measure the transmittance, T, and reflectance, R, of the films in the wavelength range of 200–2500 nm. 3. Results and discussion 3.1. Structural properties Fig. 1 shows FTIR spectra of BI-diisoQ powder and film, whereas the peaks of the wave number values are listed in Table 1. The consistency spectra of both powder and film assure that thermal evaporation procedure is an excellent method to secure un-dissociated of BI-diisoQ film. The absorption bands observed essentially correspond to both kinds of stretching modes like O–H and C–H. The band at 1440 cm−1 is associated with C–CH3 asymmetrical, while 2849 and 2920 cm−1 absorption bands are related to CH stretching modes and CH3 stretching 2
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Table 1 Infrared frequencies and most probable bands assignment of BI-diisoQ film. Wavenumber (cm−1) Powder
as-prepared film
755 810 850 1023 1159 1240 1294 1437 1449 1509 1595 1698 3055
743 813 847 1026 1160 1240 1291 1441 1445 1505 1596 1691 2043
Table 2 Crystallite size, optical absorption parameters of BI-diisoQ film as a function of film thickness.
Assignment
CH2-Skeletal Vibrations CH out-of-plane deformation C=H bending C=N stretching vibration νCC CH3 asymmetrical vibration CH in-plane deformation CH2 deformation vibration C–CH3 asymmetrical Coupling of pyrrole and aza stretching C=C stretching vibration C=O stretching vibration CH stretching
Thickness (nm)
D (nm)
Eg1 (eV)
Eg2 (eV)
Eu (me V)
25 75 100 150
20.7 30.1 45.2 59.9
1.38 1.47 1.53 1.54
3.58 3.58 3.57 3.58
130.20 90.17 47.79 46.23
Fig. 4. Transmittance, T, and Reflectance, R, for BI-diisoQ film with different thicknesses.
Fig. 2. SEM images of BI-diisoQ film.
Fig. 5. Absorption coefficient, α, as a function of photon energy for BI-diisoQ film with different thicknesses.
modes respectively. Therefore, thermal vacuum evaporating treatment is considered as a perfect performance competitive tool to prepare homogenous stoichiometric thin films of small organic molecules. The morphology of BI-diisoQ film, as illustrated in Fig. 2, shows a regular uniform distribution of nanorod shape with a length of around 5 μm and a diameter of approximately 20 nm. X-ray diffraction patterns (XRD) of BI-diisoQ powder has been made before by our group [22]. It is found that BI-diisoQ bulk material is polycrystalline with a monoclinic structure and the Miller indices (hkl) were indexed [22]. Fig. 3 displays the analysis of XRD for BI-diisoQ thin films with various thicknesses. It is clear from this Figure that all of the films are mostly characterized by a mixture of amorphous and crystalline structure. XRD-pattern of BI-diisoQ films with 25 and 75 nm thickness, shows a single significant peak at 2θ = 12.40° with a perfect
Fig. 3. X-ray diffraction pattern of BI-diisoQ film with different thickness.
3
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Fig. 6. Plots of (αE)1/2 versus photon energy, E, for BI-diisoQ film with different thicknesses.
D=
K x cos
(1)
where K is the Scherrer's constant which has an estimation of 0.95 [21], λx is the X-ray wavelength, β is the width of a peak in radians at the half-maximum. Table 2 gives the variation of D with the film thickness. This Table illustrates the increases of D as the film thickness increases, which proves the enhancement of BI-diisoQ film crystallization as the film thickness increases. This phenomenon can be ascribed to the growth of stacking particles and to the decrease of microstrain inside BI-diisoQ thin film [24]. 3.2. Effect of film thickness on optical properties The various values of T and R versus wavelength are displayed in Fig. 4. This figure reveals that BI-diisoQ films became transparent at λ > 850 nm. In addition, the transmittance in the visible range was significant, and the width of the transparent region was observed to be uniform for all of the films, indicating the homogeneity of the films. On the other hand, the optical transmittance edge shifted towards the UV wavelength with the increase of film thickness, and therefore an increase of optical band gap can be expected. On the other hand, the transmittance in the visible region is noticed to be decreased as the film thickness increases, which may be attributed to the agglomeration of atoms, which in turns will boost the number of sub-level states, and thus increase the opportunity of photons absorption [25]. Overall, it is obvious from Fig. 4, that the plotted curves of T or R are congruent when the film thickness exceeds 100 nm. Using the measured data of T and R, the absorption coefficient, α, has been computed by using [26].
Fig. 7. Dependence lnα on photon energy, E, for various thickness of BI-diisoQ film.
orientation in the (200) plane [22], whereas the background of amorphity appeared in the range of 18–38°. Besides, it was noticed that the (200) peak was preserved even when the film thickness was increased. This could be clarified by the interplanar spacing (dhkl), which is autonomous of the film thickness. It was noticed that the intensity of (200) peak becomes sharper as the film thickness increases, while the background of the amorphity was observed to be diminished. On the other hand, the XRD patterns of the BI-diisoQ film with 100 nm and 150 nm thickness illustrate another two peaks at 2θ = 52.84° and 72.16°, which are corresponding to the orientation planes at (−1 0 8) and (1 3 7) respectively [22]. In particular, the significant intensity of peaks was observed particularly for the film of 150 nm thickness, which indicates that the crystallization of BI-diisoQ film was enhanced by the increase of the film thickness. Indeed, the mean crystallite size, D, of BI-diisoQ film can be accessed by applying the Scherrer's equation [23].
=
1 1 R2 ln + d 2T
R2 +
1
R2 4T 2
(2)
where d is the thickness of the film. Fig. 5 portrays the effect of film thickness on α spectra of BI-diisoQ films. It is clear from this figure that α spectra intensity declines as the film thickness increases, which can be explained due to the rising of packing density [27]. Moreover, α spectra are characterized by two main bands in the visible and ultraviolet 4
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Fig. 8. (a) Variation of refractive index with wavelength; (b) dependence of (n2–1)−1 on E2 for BI-diisoQ film with different thickness.
Fig. 9. Variation of dielectric constants with photon energy for BI-diisoQ film with different thickness.
Table 3 Dispersion and nonlinear optical parameters of BI-diisoQ film as a function of film thickness. Thickness (nm)
Eo (eV)
Ed (eV)
ε∞
χ(3) (10−13 esu)
n2 (10−12 esu)
25 75 100 150
1.58 1.59 1.69 1.71
2.29 4.49 5.33 5.36
2.50 2.53 2.87 2.91
0.30 4.32 6.70 6.67
0.72 8.34 12.40 12.35
(αE)m was plotted as a function of the photon energy (hν), as shown in Fig. 6, and the best fit line was observed for m = 1/2. This result emphasizes that the indirect allowed transition is the suitable adopted transition for BI-diisoQ films. The values of the onset optical band gap (Eg1) and the fundamental band gap (Eg2) were calculated particularly from the intersection of the linear portions with the energy axis. The first band energy value (Eg1), which is terminated as the optical gap of the thin film, is affected by the formation of vacancies, interstitials Frenkel pairs or dislocations inside the film microstructure [31,32]. Whereas, the fundamental gap energy (Eg2) occurs due to the electron transition between (HUMO, π-band) and (LUMO, π*-band) [33]. The values of Eg1 and Eg2 for different film thickness were listed in Table 2. In fact, one can deduce from this Table that the value of Eg1 increases, while the value of Eg2 is approximately invariant as the film thickness increases. The increasing of the optical band gap, Eg1, as the film thickness increases can be dissected as follows: As the film thickness grows up, the phase-nature crystallization of BI-diisoQ films increases, thus defects and disorder will be diminished. The increasing of defects plays an important role in increasing the width of the localized states inside the forbidden band gap energy. In fact, it was found that the localized defects have a significant effect on the creation of electron-hole pairs and excitons inside the film [33]. Conversely, our results verified that the localized states had been decreased as the film
spectrum as seen from Fig. 5. These two distinct bands were observed in many of quinoline derivatives [27–29]. The two bands which found in the UV and visible-spectra are attributed to the electronic transition crosswise over π-π* orbital [29]. The observed two edges in the α spectra unveil that BI-diisoQ film has two distinct energy gaps. The premier one appears in the visible region and is called onset absorption edge, while the other edge appears in the UV-spectrum, which is known as fundamental absorption edge. The energy band gap, Eg, can be evaluated from α spectra in the two regions using this empirical relation [30].
( E )m = B (E
Eg )
(3)
where E is the photon energy, B is proportionality constant, and m is an index which has the probabilities of 2 for direct allowed transition and 1/2 for the indirect allowed transition. To obtain the proper value of m, 5
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Fig. 10. Variation of optical conductivities with photon energy for BI-diisoQ film with different thickness.
thickness increases, which indicates that the recombination of electronholes has occurred. Consequently, the optical band gap energy has increased as the film thickness increases. On the other hand, to shed more of lights on the localized states, the Urbach tail (Eu) can be used as a good an indicator of the defect levels in the forbidden band gap [34]. The following relation was used to calculate the width of Eu near the band edge [34].
=
o
exp
E Eu
n=
1+R 1 R
+
(1
4R R2)
k2
(5)
where k is the absorption index and given as k = αλ/4π. Fig. 8a displays the spectral dependence of n on λ for BI-diisoQ film with different thickness. As reported for many organic films, the dispersion curves have two regions known as anomalous and normal dispersions at the low and high wavelength, respectively. In the normal dispersion range, n values increase as the film thickness increases. This result may be attributed to the intermission of the film deposition in the initial stage of the film formation [37]. The dispersion parameters play an important role in the applications of optical communication and devices designing. These dispersions parameters are classified into two types: Ed “dispersion energy” which estimates the ability of the interband optical transitions, and Eo, “single oscillator energy” which involves with the whole of the electronic excitations created in the thin films. In the transparent region (λ > 800 nm) the refractive index is usually explained by a single oscillator model. This model describes the dielectric interaction of the particles in the region below the optical band gap. In
(4)
where ao is a constant. The value of Eu was determined from the slope of the straight lines of lnα versus photon energy of Fig. 7. The results of Eu are given in Table 2, and its values decrease as the film thickness increases, which is mostly affected by the decreasing of structural disorders [35]. The refractive index, n, is a necessary examination tool used in the upgrade of optical communication devices [36]. The value of n can be estimated from the following relationship [10]: 6
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normal dispersion, Wemple and DiDomenico (WDD) found that the optical spectra of n can be evaluated by Ref. [10].
n2
1=
Eo2
Ed Eo (E )2
decreasing of the dielectric constants and the optical conductivity with the increasing of the film thickness reveals the decreasing of the electron states in the forbidden optical band gap energy. This key result confirms that the increase in the film thickness has a strong impact on decreasing disorder and imperfections inside the films.
(6) 2
−1
2
Fig. 8b shows the dependence of (n -1) on E for BI-diisoQ film with different film thickness. From the slopes and the intersection of the plotted straight lines, the dispersion parameters Eo and Ed can be calculated. The values of Eo and Ed were listed in Table 3. The increase in Ed with the increasing of the film thickness may be attributed to the change in the ionicities which is caused by the increase of the coordination number of cations to the nearest neighbor anions [38]. The obtained values of Ed, Eo and high-frequency dielectric constant (ε∞ 2 =n∞) for various film thickness were given in Table 3. As well known, the evaluation of third-order nonlinear optical susceptibility (χ(3)) is considered an essential principle in the fabrication of several photo-electronic devices [39]. In the regarding of Miller's principles, χ(3) can be expressed by Ref. [40]: (3)
=
A Ed (4 ) 4 Eo
4
=
A (no2 (4 ) 4
4. Conclusion The thermal vapor deposition has been used to prepare BI-diisoQ film with various thicknesses, and the structural and optical properties of BI-diisoQ films as a function of film thickness studied. The results of the scanning electron microscope of BI-diisoQ films show the topography of nanorod thin film. The transmittance spectra of BI-diisoQ film have been characterized by unique optical transmittance edge, confirming that the BI-diisoQ film can be used as an excellent optical filter material. The dispersion parameters are found to be increased with the increase of the film thickness. The nonlinear optical parameters are found to be increased with the increase of the film thickness. While the decreasing of the dielectric constants and the optical conductivity reveals the decreasing of the electron density inside the forbidden energy gaps. Overall, the comprehensive picture of structural and optical behaviors of BI-diisoQ nanorod films confer it the opportunity to be one of the promised candidate materials that can be utilized in the development of optical devices.
4
1)
(7) −10
where A is a constant equal to 1.7 × 10 esu and no is an experimentally refractive index at a longer wavelength (E→0). Using Miller's principle and WDD model, Tichy et al. [41] derived an equation to determine the value of the nonlinear refractive index, n2 [10]:
12 n2 = no
(3)
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The obtained values of χ(3) and n2 for BI-diisoQ film as a function of film thickness were listed in Table 3. It is clear from this figure that χ(3) and n2 values increase with the increase of the film thickness. The increase of χ(3) and n2 for BI-diisoQ film may be attributed to the increase of the nonlinear refractive index [40]. These results indicate that the growth of film thickness contributes to the improvement of the thirdorder optical nonlinearities due to the enhancement of quantum size effects [41]. The complex dielectric constant (ε = ε1 – iε2) is an essential quantity that reveals the interactions between photons and electrons within the material. Indeed, these interactions are related to the real (ε1 = n2 – k2) and imaginary (ε2 = 2nk) parts of the dielectric constant in the energy spectrum [31,32]. Further, the imaginary and real parts of the dielectric constant are related directly to the density of states within the forbidden gap of the films [36]. Fig. 9 displays the dispersion spectra of ε1 and the absorption spectra of ε2. Particularly, the existence of the peaks in the dielectric spectra refers to various interactions produced between photons and electrons in the films [42]. This figure shows that both ε1 and ε2 increase with the increase of photon energy. Meanwhile, it was found that the magnitudes of ε1 and ε2 decreased with the increase of the film thickness. In addition, the optical conductivity (σ) is considered as an important optical constant that can be applied to provide information about the inferior electronic states [43]. To evaluate the values of real (σ1) and imaginary (σ2) parts, the following equations have been applied [43]: 1
=
O 2
(9)
2
=
o 1
(10)
where ω is the angular frequency and εo is the free space dielectric constant. Fig. 10 displays the dependent of σ1 and σ2 on E, respectively. It can be observed from this Figure, that the real and imaginary optical conductivity increase with the increase of photon energy, which can be explained predominately due to the increase of excited electrons by the induced photon energy. Accordingly, the origin of this increase may be attributed to some variation occurred in the film structure which is resulted from the charge ordering effect [42]. Meanwhile, the 7
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A.Z. Mahmoud, et al. 20402. [31] Saleem I. Qashou, S.E. Al Garni, A.A.A. Darwish, M.M. Hawamdeh, Al Allayth Aldrabee, Optik 170 (2018) 540–547. [32] A.A.A. Darwish, Saleem I. Qashou, Z. Khattari, M.M. Hawamdeh, Allayth Aldrabee, S.E. Al Garni, J. Electron. Mater. 47 (2018) 7196–7203. [33] M.M. El-Nahass, A.A.M. Farag, M. El-Metwally, F.S.H. Abu-Samah, E. Elesh, Synth. Met. 195 (2014) 110–116. [34] F. Urbach, Phys. Rev. 92 (1953) 1324. [35] F. Yakuphanoglu, M. Sekerci, A. Balaban, Opt. Mater. 27 (2005) 1369–1372. [36] A.A.A. Darwish, H.A.M. Ali, J. Alloys Compd. 710 (2017) 431–435. [37] M.I. Abd-Elrahman, A.Y. Abdel-Latief, R.M. Khafagy, N. Younis, M.M. Hafiz, Mater. Sci. Semicond. Process. 24 (2014) 21–25.
[38] H. Ticha, L. Tichy, J. Optoelectron. Adv. Mater. 4 (2002) 381–386. [39] D. Cotter, R.J. Manning, K.J. Blow, A.D. Ellis, A.E. Kelly, D. Nesset, I.D. Phillips, A.J. Poustie, D.C. Rogers, Science 286 (1999) 1523–1528. [40] L. Tichy, H. Tichá, P. Nagels, R. Callaerts, R. Mertens, M. Vlcek, Mater. Lett. 39 (1999) 122–128. [41] B. Shanmugavelu, V.V.R.K. Kumar, R. Kuladeep, D.N. Rao, J. Appl. Phys. 114 (2013) 243103. [42] M. Dongol, M.M. El-Nahass, A. El-Denglawey, A.F. Elhady, A.A. Abuelwafa, Curr. Appl. Phys. 12 (2012) 1178–1184. [43] M.M. El-Nahass, H.S. Soliman, B.A. Khalifa, I.M. Soliman, Mater. Sci. Semicond. Process. 38 (2015) 177–183.
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Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Original Research
Film thickness effects on nanorods organic films of azo quinoline derivatives for optical applications A.Z. Mahmouda,b,∗, A.A.A. Darwishc,d, Saleem I. Qashoue a
Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt College of Sciences and Art at ArRass -Qassim University, ArRass 51921, Saudi Arabia c Nanotechnology Research Laboratory, Department of Physics, Faculty of Science, University of Tabuk, Tabuk, Saudi Arabia d Department of Physics, Faculty of Education at Al-Mahweet, Sana'a University, Al-Mahweet, Yemen e Department of Physics, Faculty of Science, Zarqa University, Zarqa 13132, Jordan b
ARTICLE INFO
ABSTRACT
Keywords: Thin films Nanorod Quinoline Thickness effect Optical properties
The effect of film thickness on the structural and optical properties of bisbenzimidazo [2,1-a:2′,1′-a′] anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-10,21-dione (BI-diisoQ) films was carried out. The morphology of BIdiisoQ film showed a definite shape of a nanorod with a length of around 5 μm and a diameter of approximately 20 nm. Also, X-ray diffraction patterns demonstrated that BI-diisoQ thin films are characterized by a mixture of amorphous and crystalline structure, whereas the phase-nature crystallization of BI-diisoQ film enhanced as the film thickness increased. The investigation of the absorption coefficient of BI-diisoQ films revealed two indirect allowed band gaps energy. The calculated values of nonlinear optical parameters for BI-diisoQ film were observed to be increased with the increase of film thickness. The optical properties of BI-diisoQ nanorod films indicated that these films have appropriate optical properties, and thus it can be recommended as a promised candidate material in superconductors, optoelectronic and photonic applications.
1. Introduction The organic compounds with small molecules have numerous promising applications in electronic devices like solar cells [1,2] and organic light-emitting diodes [3], because of their simple handling at low temperatures and low-cost preparation comparing with inorganic materials [4,5]. The azo dyes have been exploited significantly in several branches of optoelectronic manufacturing because of their simplicity production, and their promising optical and electrical properties. One of the azo dyes is a quinoline compound which is perceived by its high thermal stability. Quinoline has various applications such as biological activity [6], information storage [7] and optoelectronic devices [8]. The substitution of peripheral groups is a well-known process that has been used widely in changing the chemical and physical properties of organic scaffold molecules. The examination of various substitution groups effect on the optical properties of the quinoline compound has been investigated. For instance, it was found that the variation of substituent groups showed a significant effect on the refractive index and dispersion parameters [9]. Many optical and electrical studies on quinoline and its derivatives
∗
have been reported [10–13]. The optical properties of 2,9-Bis [2-(4chlorophenyl)ethyl] anthrax [2,1,9-def:6,5,10-d0e0f0] diisoquinoline1,3,8,10 (2H, 9H) under the effect of annealing temperature were studied in our previous work [10]. It was observed that the energy of the band gap was reduced under the effect of annealing temperature [10]. Also, the investigation of the optical absorption measurements of 2-Amino-4-(5-bromothiophen-2-yl)-5,6-dihydro-6-methyl-5-oxo-4Hpyrano[3,2-c]quinoline-3-carbonitrile clarified that the electron transitions between the HOMO and LUMO of this organic compound were dominated by indirect allowed transition with an optical band gap of 2.5 eV [11]. Moreover, this material showed convenient photoelectric characteristics and thus can be used as photosensors [11]. Also, it was found that the quinoline ligands display two fluorescence bands in the region of 512–580 nm. Quinoline ligands were realized by similar behavior of conductivity concerning most of organic semiconductors materials [12]. On the other hand, the dependence of optical and structural properties of 4-cyanopyrano-quinoline dione on the film thicknesses was studied by Soliman et al. [13]. In the notation of their work, it was found that the optical constants of this compound did not affect the variation of the film thickness [13]. However, one of the important quinoline derivatives is bisbenzimidazo [2,1-a:2′,1′-a′]
Corresponding author. Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt. E-mail address: [email protected] (A.Z. Mahmoud).
https://doi.org/10.1016/j.pnsc.2019.04.009 Received 13 October 2018; Received in revised form 28 April 2019; Accepted 29 April 2019 1002-0071/ © 2019 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: A.Z. Mahmoud, A.A.A. Darwish and Saleem I. Qashou, Progress in Natural Science: Materials International, https://doi.org/10.1016/j.pnsc.2019.04.009
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Fig. 1. FTIR spectra of BI-diisoQ powder and film.
anthrax [2,1,9-def:6,5,10-d′e′f′] diisoquinoline-10,21-dione (BIdiisoQ). Nevertheless, the studying of the optical properties of this organic compound correlated with different film thickness has not been reported yet. Indeed, the optical properties of small organic molecules are straightforwardly connected to the basic electronic features. For this reason, the searching for reasonable organic compounds with beneficial optical properties gives a rule to the outline of optoelectronic and photonic cells designing. The growing up of the layer deposited thickness is proposed to modify the homogeneity of thin films, which in turns will reduce the defects inside the films. Also, the increase in film thickness plays an essential role in the agglomeration of stacking particles and thus will promote the accretion of sublevel states number. These rational reasons are sufficient to alter the optical and electrical properties of the materials. On the other hand, the preparation of thin films with various thicknesses by avoiding the oxidation of the deposited material is essential to request for improving the performance of optoelectronic devices. Otherwise, the oxidation layers will lead to un-coveted interactions between any deposited oxide and the organic molecules. For this reason, many of techniques were adopted to minify the oxidation of the deposition material on the substrate. For instance, wet chemical processing [14], atomic hydrogen treatments [15], sulfur and nitrogen passivations, as well as Atomic-Layer-Deposition-Derived Al2O3 passivation layer, have all been applied to remove native oxides and reduce the interaction between any deposited oxide and the semiconductor [16–19]. Hence, the removable of native oxides will improve the performance of optoelectronic devices. In our present work, we adopted the thermal vacuum evaporating technique to attain homogeneous thin films. Of course, in high vacuum, the mean free path of vapor atoms is confirmed to be the same order as the vacuum chamber dimensions. So these particles pass in straight lines from the evaporation source towards the deposition substrate without colliding with the background gas. Consequently, high vacuum for a long time is essential to keep the stoichiometry of the deposited film during film growth [20]. Therefore, the present work aims to prepare homogenous BI-diisoQ thin films with different thickness by thermal vacuum vapor deposition. Also, another important goal of this research is to highlight the effect of the film thickness on the structural and optical properties of BI-diisoQ thin films.
2. Experimental techniques Powder of BI-diisoQ compound was obtained with 98% purity from Sigma Aldrich Company. BI-diisoQ was deposited on a cleaned glass and cleaved KBr by a thermal vapor deposition technique. The substrates at room temperature were fixed onto a spin holder to obtain uniform films. A coating unit model of HHV Auto 306 was used to achieve a vacuum of 2.45 × 10−5 mbar. The system was maintained under vacuum for 3 h to pledge straightforward evaporation of the material with a minimization of the sample oxidation [21]. Afterward, the deposition rate (0.25 nm/s), as well as the film thickness, were controlled using the quartz crystal monitor. The film thicknesses have been checked by using the interferometric technique, while the measurement values of the film thickness were found to be 25, 75, 100 and 150 nm. The X-ray Diffractometer (Philips PW 1710) was used to examine the phase structure of the powder and the obtained films. Scanning electron microscope (SEM) was used to investigate the morphologies of BI-diisoQ film by SEM-JOEL unit. The Fourier-transform infrared (FTIR) in the scope of 400–4000 cm−1 was used to get a comprehensive view of the molecular structure of the powder, as well as, the as-deposited BIdiisoQ thin films. To get a high precision of IR measurements, 1 mg of BI-diisoQ, which was scraped from the as-deposited thin film, was mixed with 50 mg of vacuum dried conventional IR materials (KBr powders). The double beam spectrophotometer (Shimadzu 2101) was utilized to measure the transmittance, T, and reflectance, R, of the films in the wavelength range of 200–2500 nm. 3. Results and discussion 3.1. Structural properties Fig. 1 shows FTIR spectra of BI-diisoQ powder and film, whereas the peaks of the wave number values are listed in Table 1. The consistency spectra of both powder and film assure that thermal evaporation procedure is an excellent method to secure un-dissociated of BI-diisoQ film. The absorption bands observed essentially correspond to both kinds of stretching modes like O–H and C–H. The band at 1440 cm−1 is associated with C–CH3 asymmetrical, while 2849 and 2920 cm−1 absorption bands are related to CH stretching modes and CH3 stretching 2
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Table 1 Infrared frequencies and most probable bands assignment of BI-diisoQ film. Wavenumber (cm−1) Powder
as-prepared film
755 810 850 1023 1159 1240 1294 1437 1449 1509 1595 1698 3055
743 813 847 1026 1160 1240 1291 1441 1445 1505 1596 1691 2043
Table 2 Crystallite size, optical absorption parameters of BI-diisoQ film as a function of film thickness.
Assignment
CH2-Skeletal Vibrations CH out-of-plane deformation C=H bending C=N stretching vibration νCC CH3 asymmetrical vibration CH in-plane deformation CH2 deformation vibration C–CH3 asymmetrical Coupling of pyrrole and aza stretching C=C stretching vibration C=O stretching vibration CH stretching
Thickness (nm)
D (nm)
Eg1 (eV)
Eg2 (eV)
Eu (me V)
25 75 100 150
20.7 30.1 45.2 59.9
1.38 1.47 1.53 1.54
3.58 3.58 3.57 3.58
130.20 90.17 47.79 46.23
Fig. 4. Transmittance, T, and Reflectance, R, for BI-diisoQ film with different thicknesses.
Fig. 2. SEM images of BI-diisoQ film.
Fig. 5. Absorption coefficient, α, as a function of photon energy for BI-diisoQ film with different thicknesses.
modes respectively. Therefore, thermal vacuum evaporating treatment is considered as a perfect performance competitive tool to prepare homogenous stoichiometric thin films of small organic molecules. The morphology of BI-diisoQ film, as illustrated in Fig. 2, shows a regular uniform distribution of nanorod shape with a length of around 5 μm and a diameter of approximately 20 nm. X-ray diffraction patterns (XRD) of BI-diisoQ powder has been made before by our group [22]. It is found that BI-diisoQ bulk material is polycrystalline with a monoclinic structure and the Miller indices (hkl) were indexed [22]. Fig. 3 displays the analysis of XRD for BI-diisoQ thin films with various thicknesses. It is clear from this Figure that all of the films are mostly characterized by a mixture of amorphous and crystalline structure. XRD-pattern of BI-diisoQ films with 25 and 75 nm thickness, shows a single significant peak at 2θ = 12.40° with a perfect
Fig. 3. X-ray diffraction pattern of BI-diisoQ film with different thickness.
3
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Fig. 6. Plots of (αE)1/2 versus photon energy, E, for BI-diisoQ film with different thicknesses.
D=
K x cos
(1)
where K is the Scherrer's constant which has an estimation of 0.95 [21], λx is the X-ray wavelength, β is the width of a peak in radians at the half-maximum. Table 2 gives the variation of D with the film thickness. This Table illustrates the increases of D as the film thickness increases, which proves the enhancement of BI-diisoQ film crystallization as the film thickness increases. This phenomenon can be ascribed to the growth of stacking particles and to the decrease of microstrain inside BI-diisoQ thin film [24]. 3.2. Effect of film thickness on optical properties The various values of T and R versus wavelength are displayed in Fig. 4. This figure reveals that BI-diisoQ films became transparent at λ > 850 nm. In addition, the transmittance in the visible range was significant, and the width of the transparent region was observed to be uniform for all of the films, indicating the homogeneity of the films. On the other hand, the optical transmittance edge shifted towards the UV wavelength with the increase of film thickness, and therefore an increase of optical band gap can be expected. On the other hand, the transmittance in the visible region is noticed to be decreased as the film thickness increases, which may be attributed to the agglomeration of atoms, which in turns will boost the number of sub-level states, and thus increase the opportunity of photons absorption [25]. Overall, it is obvious from Fig. 4, that the plotted curves of T or R are congruent when the film thickness exceeds 100 nm. Using the measured data of T and R, the absorption coefficient, α, has been computed by using [26].
Fig. 7. Dependence lnα on photon energy, E, for various thickness of BI-diisoQ film.
orientation in the (200) plane [22], whereas the background of amorphity appeared in the range of 18–38°. Besides, it was noticed that the (200) peak was preserved even when the film thickness was increased. This could be clarified by the interplanar spacing (dhkl), which is autonomous of the film thickness. It was noticed that the intensity of (200) peak becomes sharper as the film thickness increases, while the background of the amorphity was observed to be diminished. On the other hand, the XRD patterns of the BI-diisoQ film with 100 nm and 150 nm thickness illustrate another two peaks at 2θ = 52.84° and 72.16°, which are corresponding to the orientation planes at (−1 0 8) and (1 3 7) respectively [22]. In particular, the significant intensity of peaks was observed particularly for the film of 150 nm thickness, which indicates that the crystallization of BI-diisoQ film was enhanced by the increase of the film thickness. Indeed, the mean crystallite size, D, of BI-diisoQ film can be accessed by applying the Scherrer's equation [23].
=
1 1 R2 ln + d 2T
R2 +
1
R2 4T 2
(2)
where d is the thickness of the film. Fig. 5 portrays the effect of film thickness on α spectra of BI-diisoQ films. It is clear from this figure that α spectra intensity declines as the film thickness increases, which can be explained due to the rising of packing density [27]. Moreover, α spectra are characterized by two main bands in the visible and ultraviolet 4
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Fig. 8. (a) Variation of refractive index with wavelength; (b) dependence of (n2–1)−1 on E2 for BI-diisoQ film with different thickness.
Fig. 9. Variation of dielectric constants with photon energy for BI-diisoQ film with different thickness.
Table 3 Dispersion and nonlinear optical parameters of BI-diisoQ film as a function of film thickness. Thickness (nm)
Eo (eV)
Ed (eV)
ε∞
χ(3) (10−13 esu)
n2 (10−12 esu)
25 75 100 150
1.58 1.59 1.69 1.71
2.29 4.49 5.33 5.36
2.50 2.53 2.87 2.91
0.30 4.32 6.70 6.67
0.72 8.34 12.40 12.35
(αE)m was plotted as a function of the photon energy (hν), as shown in Fig. 6, and the best fit line was observed for m = 1/2. This result emphasizes that the indirect allowed transition is the suitable adopted transition for BI-diisoQ films. The values of the onset optical band gap (Eg1) and the fundamental band gap (Eg2) were calculated particularly from the intersection of the linear portions with the energy axis. The first band energy value (Eg1), which is terminated as the optical gap of the thin film, is affected by the formation of vacancies, interstitials Frenkel pairs or dislocations inside the film microstructure [31,32]. Whereas, the fundamental gap energy (Eg2) occurs due to the electron transition between (HUMO, π-band) and (LUMO, π*-band) [33]. The values of Eg1 and Eg2 for different film thickness were listed in Table 2. In fact, one can deduce from this Table that the value of Eg1 increases, while the value of Eg2 is approximately invariant as the film thickness increases. The increasing of the optical band gap, Eg1, as the film thickness increases can be dissected as follows: As the film thickness grows up, the phase-nature crystallization of BI-diisoQ films increases, thus defects and disorder will be diminished. The increasing of defects plays an important role in increasing the width of the localized states inside the forbidden band gap energy. In fact, it was found that the localized defects have a significant effect on the creation of electron-hole pairs and excitons inside the film [33]. Conversely, our results verified that the localized states had been decreased as the film
spectrum as seen from Fig. 5. These two distinct bands were observed in many of quinoline derivatives [27–29]. The two bands which found in the UV and visible-spectra are attributed to the electronic transition crosswise over π-π* orbital [29]. The observed two edges in the α spectra unveil that BI-diisoQ film has two distinct energy gaps. The premier one appears in the visible region and is called onset absorption edge, while the other edge appears in the UV-spectrum, which is known as fundamental absorption edge. The energy band gap, Eg, can be evaluated from α spectra in the two regions using this empirical relation [30].
( E )m = B (E
Eg )
(3)
where E is the photon energy, B is proportionality constant, and m is an index which has the probabilities of 2 for direct allowed transition and 1/2 for the indirect allowed transition. To obtain the proper value of m, 5
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Fig. 10. Variation of optical conductivities with photon energy for BI-diisoQ film with different thickness.
thickness increases, which indicates that the recombination of electronholes has occurred. Consequently, the optical band gap energy has increased as the film thickness increases. On the other hand, to shed more of lights on the localized states, the Urbach tail (Eu) can be used as a good an indicator of the defect levels in the forbidden band gap [34]. The following relation was used to calculate the width of Eu near the band edge [34].
=
o
exp
E Eu
n=
1+R 1 R
+
(1
4R R2)
k2
(5)
where k is the absorption index and given as k = αλ/4π. Fig. 8a displays the spectral dependence of n on λ for BI-diisoQ film with different thickness. As reported for many organic films, the dispersion curves have two regions known as anomalous and normal dispersions at the low and high wavelength, respectively. In the normal dispersion range, n values increase as the film thickness increases. This result may be attributed to the intermission of the film deposition in the initial stage of the film formation [37]. The dispersion parameters play an important role in the applications of optical communication and devices designing. These dispersions parameters are classified into two types: Ed “dispersion energy” which estimates the ability of the interband optical transitions, and Eo, “single oscillator energy” which involves with the whole of the electronic excitations created in the thin films. In the transparent region (λ > 800 nm) the refractive index is usually explained by a single oscillator model. This model describes the dielectric interaction of the particles in the region below the optical band gap. In
(4)
where ao is a constant. The value of Eu was determined from the slope of the straight lines of lnα versus photon energy of Fig. 7. The results of Eu are given in Table 2, and its values decrease as the film thickness increases, which is mostly affected by the decreasing of structural disorders [35]. The refractive index, n, is a necessary examination tool used in the upgrade of optical communication devices [36]. The value of n can be estimated from the following relationship [10]: 6
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normal dispersion, Wemple and DiDomenico (WDD) found that the optical spectra of n can be evaluated by Ref. [10].
n2
1=
Eo2
Ed Eo (E )2
decreasing of the dielectric constants and the optical conductivity with the increasing of the film thickness reveals the decreasing of the electron states in the forbidden optical band gap energy. This key result confirms that the increase in the film thickness has a strong impact on decreasing disorder and imperfections inside the films.
(6) 2
−1
2
Fig. 8b shows the dependence of (n -1) on E for BI-diisoQ film with different film thickness. From the slopes and the intersection of the plotted straight lines, the dispersion parameters Eo and Ed can be calculated. The values of Eo and Ed were listed in Table 3. The increase in Ed with the increasing of the film thickness may be attributed to the change in the ionicities which is caused by the increase of the coordination number of cations to the nearest neighbor anions [38]. The obtained values of Ed, Eo and high-frequency dielectric constant (ε∞ 2 =n∞) for various film thickness were given in Table 3. As well known, the evaluation of third-order nonlinear optical susceptibility (χ(3)) is considered an essential principle in the fabrication of several photo-electronic devices [39]. In the regarding of Miller's principles, χ(3) can be expressed by Ref. [40]: (3)
=
A Ed (4 ) 4 Eo
4
=
A (no2 (4 ) 4
4. Conclusion The thermal vapor deposition has been used to prepare BI-diisoQ film with various thicknesses, and the structural and optical properties of BI-diisoQ films as a function of film thickness studied. The results of the scanning electron microscope of BI-diisoQ films show the topography of nanorod thin film. The transmittance spectra of BI-diisoQ film have been characterized by unique optical transmittance edge, confirming that the BI-diisoQ film can be used as an excellent optical filter material. The dispersion parameters are found to be increased with the increase of the film thickness. The nonlinear optical parameters are found to be increased with the increase of the film thickness. While the decreasing of the dielectric constants and the optical conductivity reveals the decreasing of the electron density inside the forbidden energy gaps. Overall, the comprehensive picture of structural and optical behaviors of BI-diisoQ nanorod films confer it the opportunity to be one of the promised candidate materials that can be utilized in the development of optical devices.
4
1)
(7) −10
where A is a constant equal to 1.7 × 10 esu and no is an experimentally refractive index at a longer wavelength (E→0). Using Miller's principle and WDD model, Tichy et al. [41] derived an equation to determine the value of the nonlinear refractive index, n2 [10]:
12 n2 = no
(3)
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The obtained values of χ(3) and n2 for BI-diisoQ film as a function of film thickness were listed in Table 3. It is clear from this figure that χ(3) and n2 values increase with the increase of the film thickness. The increase of χ(3) and n2 for BI-diisoQ film may be attributed to the increase of the nonlinear refractive index [40]. These results indicate that the growth of film thickness contributes to the improvement of the thirdorder optical nonlinearities due to the enhancement of quantum size effects [41]. The complex dielectric constant (ε = ε1 – iε2) is an essential quantity that reveals the interactions between photons and electrons within the material. Indeed, these interactions are related to the real (ε1 = n2 – k2) and imaginary (ε2 = 2nk) parts of the dielectric constant in the energy spectrum [31,32]. Further, the imaginary and real parts of the dielectric constant are related directly to the density of states within the forbidden gap of the films [36]. Fig. 9 displays the dispersion spectra of ε1 and the absorption spectra of ε2. Particularly, the existence of the peaks in the dielectric spectra refers to various interactions produced between photons and electrons in the films [42]. This figure shows that both ε1 and ε2 increase with the increase of photon energy. Meanwhile, it was found that the magnitudes of ε1 and ε2 decreased with the increase of the film thickness. In addition, the optical conductivity (σ) is considered as an important optical constant that can be applied to provide information about the inferior electronic states [43]. To evaluate the values of real (σ1) and imaginary (σ2) parts, the following equations have been applied [43]: 1
=
O 2
(9)
2
=
o 1
(10)
where ω is the angular frequency and εo is the free space dielectric constant. Fig. 10 displays the dependent of σ1 and σ2 on E, respectively. It can be observed from this Figure, that the real and imaginary optical conductivity increase with the increase of photon energy, which can be explained predominately due to the increase of excited electrons by the induced photon energy. Accordingly, the origin of this increase may be attributed to some variation occurred in the film structure which is resulted from the charge ordering effect [42]. Meanwhile, the 7
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