Gamma radiation-induced changes on the optical properties of dibenzthiopheno-perylene-N,N′-dicyclohexylimide thin films

Radiation Physics and Chemistry 97 (2014) 178–183

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Gamma radiation-induced changes on the optical properties of dibenzthiopheno-perylene-N,N′-dicyclohexylimide thin films M.M. El-Nahass a, A.M. Hassanien b,n a b

Department of Physics, Faculty of Education, Ain Shams University, Roxy Square, 11757 Cairo, Egypt Department of Physics, Faculty of Science and Humanity Studies at Al-Quwayiyah, Shaqra University, Al-Quwayiyah 11971, Saudi Arabia

H I G H L I G H T S








The effect of γ-irradiation on the optical properties of perylene-66 films has been reported. From fundamental absorption edge, a picture of the energetic transitions of was described. The γ-ray irradiation has an effect on the fundamental energy gap. The molecular polarizability was used to describe the change in the refractive index. A single-oscillator model and Drude model were used to describe the refractive index.

art ic l e i nf o

a b s t r a c t

Article history: Received 26 April 2013 Accepted 22 November 2013 Available online 2 December 2013

The effect of γ-irradiation (6 kGy) on the optical properties of thermally evaporated perylene-66 (dye content 40%) thin films has been reported. The optical constants (refractive index, n, and absorption index, k) of the as-deposited and γ-irradiated films have been obtained in the wavelength range 200– 2500 nm using spectrophotometric measurements at nearly normal incidence. The obtained optical constants were used to estimate the type of transition for the as-deposited and irradiated films. The single oscillator model and Drude model of free carriers′ absorption were used for the analysis of refractive index dispersion, in the normal dispersion range, before and after γ-irradiation. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Perylene-66 thin films Optical properties γ-irradiation

1. Introduction Perylene derivatives are extensively studied, not only due to their strong electron accepting and fast electron transporting properties, but also due to their remarkable chemical and thermal stability (Bagui et al., 2012). As π-conjugated organic materials with an aromatic core, the perylene derivatives show strong absorption and intense fluorescence in the visible range with brilliant colors (Kufazvinei et al., 2009). An attractive feature of perylene aromatic core is that the structural modifications at either its π-positions and/or the imide positions can be chemically tuned allowing for their extensive use as functional dyes; enabling the optimization of its properties (Herrmann and Müllen, 2006). Beside perylene derivatives have large carrier mobility (Benning et al., 2000), they are suitability for epitaxial growth (Schouwink et al., 2001).

n

Corresponding author. Tel./fax: þ 966565361174. E-mail addresses: [email protected], [email protected] (A.M. Hassanien). 0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.11.020

Rębarz et al. (2008) reported that the perylene derivatives reveal a tendency to aggregation. Therefore, new, completely different excited states appear in its solid state, such as excimers, charge transfer excitons and other aggregate excitations. Such states make dramatic changes in the optical properties of thin films in comparison with single molecules. Also, Kaur et al. (2012) found that the perylene-based pigments exhibit considerable reflectance in the NIR region. The reflectance of a material depends upon a number of factors, such as a particle size, a concentration of the reflective material. Also there is a potential relationship between the nature of the substituent and the expected reflectance in the NIR region. Efforts were being made to study perylene derivatives and boosting their physical properties in order to incorporating potential devices into all plastic integrated circuits for low end and low cost optoelectronics (Kufazvinei et al., 2009). For use these materials in a certain molecular electronic device such as organic sensors (Che et al., 2007, 2010; Huang et al., 2011), photovoltaic organic solar cells (Li et al., 2010; Schmidt-Mende et al., 2001; Zhan et al., 2007); it is desirable to extend its absorption to longer wavelength regions by substitution at the π-positions (Bagui et al., 2012). While substituents at the imide positions have negligible

M.M. El-Nahass, A.M. Hassanien / Radiation Physics and Chemistry 97 (2014) 178–183

ground state electronic interaction with the perylene core due to the existence of a node plane at the imido position (Würthner, 2004). The exposure of solid materials to ionizing radiations such as γ-rays produce changes in the microstructural properties of the material as a result of inducing the structural defects, which in turn affect the optical properties. These changes are strongly depending on the internal structure of the material, the radiation energy and the irradiation dose. Clarification of these changes is quite significant, not only to recognize the physicochemical functions and spectroscopic properties of this material, but also to increase their applicability in different fields and enable information about the induced irradiation defects and their interaction with the matter components (El-Nahass et al., 2010a, 2012; Soliman et al., 2009; Zeyada et al., 2012a). In our previous work on perylene-66 we studied the effect of annealing temperatures on the optical properties of perylene-66 thin films (El-Nahass et al., 2013). It was found that the transmittance and reflectance spectra of perylene-66 films are influenced by heat treatment, and then the optical constants are affected by annealing. Also, it was found that the fundamental energy gap, Eg, of the as deposited film increased by annealing. On the other hand, it was found that the refractive index of the as deposited film decreased with annealing. In the present communication, we have reported the effect of γ-irradiation on the optical properties of perylene-66 thin films.

2. Experimental techniques Dibenzthiopheno-perylene-N,N′-dicyclohexylimide (Preylene66) was supplied from Sigma-Aldrich Chem. Co. (Dye content 40%). and was used without further purification. The schematic diagram of the molecular structure of perylene-66 is shown in Fig. 1. Thin films of preylene-66 were prepared thermal evaporation technique using a high vacuum coating unit “Edward E306A” under vacuum of 10  4 Pa. The films were deposited on fused optically flat quartz substrates for optical measurements, and potassium bromide (KBr) single crystals for FT-IR measurements. Evaporation of the material is carried out using a quartz crucible heated by a tungsten coil. The evaporation rate (2.5 nm/s) as well as the film thickness (150 nm) of the evaporated films were controlled using a quartz crystal monitor FTM6. During the deposition the substrates temperature was kept at room temperature. Film thickness was determined accurately after deposition using multiple-beam Fizeau fringes in reflection (Tolansky, 1948). The chemical structure of the powder and the as-deposited films were investigated by Fourier-transform infrared (FT-IR) technique. The IR spectra for the powder mixed with KBr and the as-deposited film onto optically flat KBr single crystal substrates were determined using a Bruker Vector 22 infrared

Fig. 1. The molecular structure of perylene-66.

179

spectrophotometer in the spectral range 400–4000 cm  1. The spectral resolution of the IR spectrophotometer was 71 cm  1 throughout the experiment. In order to investigate the irradiation effect on the investigated material, the films in the air atmosphere were exposed to γ-rays produced from a Technetium-99 m generator (Bristol-Myers Squibb Pharma Belgium S.A.). The γ-ray dose was 6 kGy. The transmittance, T, and reflectance, R, were determined at normal incidence in the wavelength range 190–2500 nm by means of a double beam spectrophotometer (JASCO model V-570 UV–visNIR). The spectral data obtained directly from the spectrophotometer were transformed to absolute values by making a correction to eliminate the absorbance and reflectance of the substrate. The absolute values of T and R are given by El-Nahass et al. (2010b):
 If t T¼ ð1Þ ð1  Rq Þ; Iq where I f t and I q are the intensities of light passing through the film-quartz system and that passing through the reference quartz, respectively, and Rq is the reflectance of the quartz substrate, and
 If r R ¼ Rm ð1 þ ½1  Rq 2 Þ  T 2 Rq ; ð2Þ Im where I m is the intensity of light reflected from the reference mirror, I f r is the intensity of light reflected from the sample and Rm is the mirror reflectance. In order to calculate the optical constants, refractive index, n, and the absorption index, k, of the films at different wavelengths, we use the following equations (Zeyada et al., 2012b): 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 1 4ð1  RÞ2 ð1  RÞ4 ð3Þ þ α ¼ ln þ R2 5; d 2T 4T 2 k ¼

αλ

4π

;

 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1þR 4R 2 þ n¼ k ; 1R ð1  RÞ2

ð4Þ




ð5Þ

where α is the absorption coefficient and d is the film thickness. The experimental error in measuring the film thickness was taken as 7 2%, in T and R as 71% and in the calculated values of n and k as 73.5% and 74%, respectively.

3. Results and discussion Fig. 2 illustrates FT-IR spectra in the range 400–4000 cm  1 for fresh preylene-66 powder in and for the as-deposited film (thickness 150 nm). Inspection of this figure reveals that the spectrum of the as-deposited film does not change with evaporation indicating that the thermal evaporation technique is a suitable technique to obtain undissociated and stoichiometric preylene-66 films. The observed FT-IR bands and their assignments are listed in Table 1. The spectral behavior of the transmittance, T(λ), and the reflectance, R(λ), for the as-deposited and the irradiated films of thickness 150 nm are plotted in Fig. 3. It can be seen that the transmittance and the reflectance curves systematically affected by γ-irradiation. The transmittance spectrum of the irradiated film is shifted to the shorter-wavelength region (blue shift), and shows greater transmittance than that of the as-deposited film. Such shift is a consequence of a change in the band gap. While the asdeposited film shows greater optical reflectance than irradiated film which decreases the value of the refractive index. Also, it can

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1.0

T 0.8

T , R

d= 150 nm 0.6

T-as R-as T-irrad R-irrad

0.4

0.2

0.0 180

R

500

1000

1500

2000

2500

λ [nm] Fig. 2. FT-IR spectra of perylene-66 in powder form and as-deposited film.

Fig. 3. The spectral dependence of the transmittance, T(λ) and reflectance, R(λ) for the as-deposited and irradiated perylene-66 thin films.

Table 1 IR spectral data in powder form and as-deposited film. Wavenumber (cm  1)

Assignment

3.5 As-deposited film

3447

3447

υ O–H

1767 1731

1766

υ CQO

1663 1591 1507 1234

1639 1593

υ CQC

1186 1150 1121 1024

1289

939

900

860 808 757 733 697

as-deposited irrad.

3.0

n

Powder

2.5

2.0 β C–H

1115 1027 υ C–C þυ C–N þυ C–S

812 γ C–H

1.5 180

500

1000

1500

2000

2500

λ [nm] Fig. 4. The spectral dependence of the real part of refractive index, n(λ), for the as-deposited and irradiated perylene-66 thin films.

739

641 570

620

C–C þβ C–N þ β C–Sβ

436

440

γ C–C þγ C–N þγ C–S

υ, stretching vibration; β, in plane bending vibration; γ, out of plane bending.

be observed that the behavior of T(λ) and R(λ) in the transparent region, λ 4 1000 nm, are quite similar for the films before and after irradiation; this indicated that all films have almost the same thickness and within the experimental errors 72%. Optical properties of materials are described by refractive index n(λ) and extinction coefficient k(λ) that comprises the complex index of refraction. The absolute values of T and R were used to determine the optical constants n and k as mentioned earlier (ElNahass et al., 2010b). Fig. 4 shows the real part of the refractive index, n(λ), for the as-deposited and irradiated films. It is found that the refractive index is strongly dispersive in the wavelength range λ o 1000 nm (this is called an anomalous dispersion), but changes slowly over the wavelength range λ 41000 nm (this is called a normal dispersion). At longer wavelength, hυ-0, the calculated value of the refractive index decreases by an amount Δn¼ 0.14 after irradiation. The spectral dependence of the refractive index shows multi-oscillation peaks in the absorption region.

There are variations in the intensity and slightly in the position of these peaks as a result of irradiation. It is also found that the irradiation leads to film with lower refractive indices which correlated with a decrease of mass density as reported by other workers (Andreas et al., 2005). Also, the change in n(λ) is related to the change in molecular polarizability. The Molecular polarizability can be calculated using the Lorentz model (Ksianzou et al., 2006; Zha et al., 2007). The Lorentz relation takes into account the electric field due to mutual interactions between an atom and its neighbors. Combining the Lorenz approximation of a spherical cavity and the Maxwell equations, one obtains the so-called Lorentz–Lorenz equation which combines refractive index, n, of a homogeneous medium with the ratio P¼ αp/V representing the molecular polarizability, where αp is normalized to the molecular volume V according to the following equation (Ksianzou et al., 2006; Zha et al., 2007): n2  1 4 ¼ πP n2 þ 2 3

ðcgs systemÞ;

ð6Þ

The calculated value of the specific polarizability, P, at λ ¼ 2500 nm is found to be 11.6  10  2 for the as-deposited film which slightly decreases to 10.39  10  2 for the irradiated film.

M.M. El-Nahass, A.M. Hassanien / Radiation Physics and Chemistry 97 (2014) 178–183

The molecular volume can be found by the density ρ, the molar mass M, and Avogadro′s number NA: M

ρN A

;

ðαhυÞ ¼ B ðhυ  Eg Þr ;

1.2

as-deposited irrad.

1.0

0.8

k

1400 1200 1000 800 600 400 200 0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

h ν [eV] Fig. 6. The relation between (αhυ)1/2 and photon energy (hυ) for the as-deposited and irradiated perylene-66 thin films.

ð8Þ

where B is a constant and r ¼1/2 and 3/2 for direct allowed and forbidden transitions, respectively, r ¼2 and 3 for indirect allowed and forbidden transitions, respectively. The dependence of (αhυ)1/r on photon energy (hυ) for onset gap was plotted for different values of r. The best fit was obtained for r ¼2 and illustrated in Fig. 6; this result indicates that the transitions are indirect allowed transitions. The extrapolation of the straight line graphs (αhυ)1/2 ¼0 gives the values of the optical gap for the as-deposited and irradiated films. The as-deposited film has two transitions at 1.55 eV and 3.44 eV corresponding to the optical gap, Eopt g , and the fundamental energy gap, Eg , respectively, while the irradiated films have also two transitions; the first transition at 1.55 eV (Eopt g ) and the second transition at 3.86 eV (E g ). The above results show that the γ-irradiation affects the fundamental energy gap, but no effect on the optical gap (excitonic gap). Blue shift in the band gap was attributed to the annihilation of localized energy bands near the band edges (Sreekumar et al., 2008). These results are opposite the effect of X-ray on FeTPPCl where the X-ray irradiation affects the excitonic gap and impurities levels but no effect on the fundamental gap (El-Nahass et al., 2010a). Although, X-ray and gamma-ray are both ionizing radiation producing similar effects but the effect also depends on the

0.6

0.4

0.2

500

as-deposited irrad.

1600

ð7Þ

With a measured density 1.539 g/cm3 and with a molar mass of perylene-66 molecule ( ¼766.92 g/mol) one can obtain a molecular volume of 827 Å3. Using the values of P and V; αp was found to be 96 Å3 and 86 Å3 for the as-deposited and irradiated films, respectively. Fig. 5 shows the variation of the absorption index, k(λ), with the wavelength, λ, for the as-deposited and irradiated films. It can be seen that a considerable increase in the intensity of the absorption index, k(λ), peaks by irradiation. Also, the variation of absorption coefficient, α (hυ), with energy, hυ, for the as-deposited and the irradiated films is displayed inset Fig. 5. The type of transition and the optical energy gap values can be demonstrated using the equation (El-Nahass et al., 2013).

0.0 180

1800

(αhν)1/2 [eV/cm]1/2

V¼

181

1000

1500

2000

2500

λ [nm] Fig. 5. Spectral behavior of the absorption index, k(λ), for the as-deposited and irradiated perylene-66 thin films; the inset figure shows the spectral behavior of the absorption coefficient, α (hυ).

stability and nature of examine materials. The skeleton of the chemical structure of the Iron(III) chloride tetraphenylporphyrin (FeTPPCl) molecule is differ from dibenzthiopheno-perylene-N,N ′-dicyclohexylimide molecule. The macrocyclic skeleton influences the related irradiation effects of properties of these compounds. The frequency dispersion of ε~ characterizes completely the propagation, reflection and loss of light in multilayer structures. It provides us with information about the electronic structure of the material. Therefore, ε~ is an important quantity for the design of highly efficient optoelectronic devices. The real part of the dielectric constant is associated with the term that describes how much the material will slow down the speed of light, and the imaginary part explains how a dielectric absorbs energy from an electric field due to the dipole motion. The complex dielectric constant is described by Zhokhavets et al. (2003) and Sharma and Katyal (2009):

ε~ ðhυÞ ¼ ε1 ðhυÞ  iε2 ðhυÞ; tan δ ¼ ε2 =ε1

ð9Þ

ð10Þ

where ε1 (¼ n2  k2), ε2 (¼ 2nk) and tan δ are the real, the imaginary parts of the dielectric constant and the loss factor, respectively. The spectra of real and imaginary parts are, respectively, called dispersion and absorption curves, respectively, which are shown in Fig. 7. It is clear that the variation of ε1 follows the same trend as n, whereas the variation of ε2 mainly follows the behavior of k which is related to the variation of α with photon energy. Also, the loss factor, tan δ, for the as-deposited and irradiated preylene-66 films are calculated using Eq. 10. The variation in the loss factor with photon energy is displayed inset Fig. 7(b). The optical properties of matter are usually determined by coupling various types of oscillators to the electromagnetic radiation field. The amplitude of these oscillations depends on the frequency of the incident electromagnetic field, the oscillator frequencies, the different coupling strengths f between the electromagnetic field and the oscillators, and their damping functions (Della Corte et al., 2000). The classic dispersion theory provides for the description of the variation of the refractive index at longer wavelength, that means in the region of very small values of the absorption constant k, under negligible damping. Wemple and DiDomenico (1971) and Wemple (1973) described the wavelength dependence of the refractive index in the transparent region for about 100 different solids using the single-oscillator model of the

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0.45 14

as-deposited irrad.

0.40

as-depsited irrad.

12

0.35

(n - 1)

-1

10

ε1

2

8

0.30

0.25

6

0.20 4

0.15 2

0

0.10 0.0

0.4

0.6

0.8

1.0

2

0

1

2

3

4

5

(hν) [eV]

6

hν [eV]

2

1.2

1.4

1.6

2

1

Fig. 8. The relation between (n  1) and the squared photon energy (hυ)2 for the as-deposited and irradiated perylene-66 thin films.

0.20

Table 2 Optical parameters for the as-deposited and irradiated perylene-66 thin films.

as-deposited irrad.

0.18 0.16

Optical parameters

0.14

E opt g (eV)

As-deposited 1.55 film Irradiated film 1.55

0.12

ε2

0.2

0.10

εL

N/mn (1047 g  1 cm  3)

Eg (eV)

Eo (eV)

Ed (eV)

ε1

3.44

1.59

5.53

4.48 5.99 4.42

3.86

2.02

6.09

4.01 4.96 3.42

0.08 0.06 0.04

9

as-deposited irrad.

0.02 0.00

1

2

3

4

5

8

6

h ν [eV]

n

6

5

form: ðn2  1Þ ¼

2

7

Fig. 7. (a) Plot of (ε1) the real dielectric constant vs (hυ) for the as-deposited and irradiated perylene-66 thin films. (b) Plot of (ε2) the imaginary dielectric constant vs (hυ) for the as-deposited and irradiated perylene-66 thin films; the inset figure shows the variation of the loss factor with photon energy.

Ed Eo E2o  ðhυÞ2

;

ð11Þ

where hυ represents the photon energy, Eo is the energy of the oscillator which gives a quantitative information on the overall band structure of the material “average gap” (Solomon et al., 1988) and corresponds to the distance between the centers of gravity of the valence and conduction bands. It is therefore related to the bond energy of different chemical bonds present in the molecule, as the optical band gap is a bond sensitive property. Ed is the dispersion energy which describes the strength of the electronic transitions. The calculated values of the dispersion parameters as well as the infinite frequency dielectric constant, ε1, obtained by plotting of (n2  1)  1 against (hυ)2 for the as-deposited and irradiated films, as shown in Fig. 8. The obtained data are listed in Table 2. In transparent region, the obtained data of refractive index, n, can be analyzed to obtain the lattice dielectric constant, εL, via a procedure describes the contribution of the free carriers and the lattice vibration modes of the dispersion. The relation between the real dielectric constant, ε1, and the wavelength, λ, in normal

4

3

0

1

2

3 2

λ [μm] 2

4

5

6

2

2

Fig. 9. The relation between n and λ for the as-deposited and irradiated perylene66 thin films.

dispersion region is given by (Palik Edward, 1985):

ε1 ¼ n2 ¼ εL 

e2 N λ2 ; 4π 2 εo mn c2

ð12Þ

where εL is the lattice dielectric constant, e is the elementary charge, εo is the permittivity of free space and, N/mn is the ratio of free carrier concentration to the free carrier effective mass. Fig. 9 shows the relation between n2 and λ2 for the as-deposited and irradiated films. It is observed that the dependence of n2 (ε1) on λ2 is linear at longer wavelengths. Extrapolating these linear parts to zero wavelength gives the value of εL and from the slopes of these

M.M. El-Nahass, A.M. Hassanien / Radiation Physics and Chemistry 97 (2014) 178–183

linear parts the ratios N/mn are obtained. The values of εL and N/mn are listed in Table 2 for the as-deposited and irradiated films. The disagreement between εL and ε1 may be due to free carriers′ contribution (El-Nahass et al., 2010a). 4. Conclusion The infrared absorption spectra of the powder, and the asdeposited film revealed that the main chemical composition of perylene-66 (Dye content 40%) has been preserved by thermal evaporation technique. The optical constants of perylene-66 thin film before and after γ-ray irradiation have been found by means of spectrophotometric measurement. From the fundamental absorption edge, an approximate picture of the energetic transitions of charge carries was described. The transitions in the energy gap have been found to be of indirect type. The γ-ray irradiation has an effect on the fundamental energy gap, but no effect on the optical gap (excitonic gap). Blue shift in band gap was attributed to the annihilation of localized energy bands near the band edges. On the other hand, a significant decreases in the index of refraction is observed by γirradiation. The molecular polarizability was used to describe the change in the refractive index after γ-irradiation. We interpret the refractive index dispersion before and after γ-irradiation using the modified single-oscillator model together with Drude model to differentiate between bound and free charge carries. References Andreas, B., Breunig, I., Buse, D.K., 2005. Chem. Phys. Chem. 6, 1. Bagui, M., Dutta, T., Zhong, H., Li, S., Chakraborty, S., Keightley, A., Peng, Z., 2012. Tetrahedron 68, 2806. Benning, S., Kitzerow, H.S., Bock, H., Archard, M.F., 2000. Liq. Cryst. 27, 901. Che, Y., Datar, A., Yang, X., Naddo, T., Zhao, J., Zang, L., 2007. J. Am. Chem. Soc. 129, 6354.

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