Topological, morphological and optical properties of Gamma irradiated Ni (II) tetraphenyl porphyrin thin films

Optics Communications 285 (2012) 1872–1881

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Optics Communications

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Topological, morphological and optical properties of Gamma irradiated Ni (II)tetraphenyl porphyrin thin films

M.M. El-Nahass a,⁎, H.M. Abd El-Khalek b, Ahmed M. Nawar b

a Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Heliopolis, Roxy, Cairo, Egyptb Thin Film Laboratory, Physics Department, Faculty of Science, Suez Canal University, Ismailia, Egypt

⁎ Corresponding author. Tel.: +20 124168621.E-mail address: [email protected] (M.M. El-N

0030-4018/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.optcom.2011.12.019

a b s t r a c t

Article history:Received 11 June 2011Received in revised form 27 November 2011Accepted 1 December 2011Available online 15 December 2011

Keywords:NiTPP thin filmGamma-irradiationOptical dispersionRoughness parameters

Thermal evaporation technique was used to prepare NiTPP Thin films at room temperature. The preparedfilms were divided into two groups; the first group was as-deposited films, and the second group was irradi-ated in gamma cell type 60Co source at room temperature with total absorbed dose of 150 kGy in air. All filmswere identified by X-ray diffraction (XRD), Fourier-transform infrared (FTIR), scanning electron microscopy(SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM) before and after ex-posed to gamma radiation. The spectrophotometric measurement of transmittance and reflectance wereused to investigate the optical properties at normal incidence of light in the wavelength range200–2500 nm for as-deposited and gamma-irradiated films. Optical constants (refractive index n, and ab-sorption index k) of as-deposited and irradiated films have been obtained in the wavelength range200–2500 nm for all the samples. The single oscillator energy (Eo), the dispersion energy (Ed), the high fre-quency dielectric constant (ε∞), the lattice dielectric constant (εL) and the ratio of the free charge carrier con-centration to the effective mass (N/m⁎) were estimated for each group. The absorption analysis has been alsoperformed to determine the type of electronic transition and the optical energy gap.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Porphyrins are representative of photofunctional organics, and theyshow a remarkable photo-, electro- and biochemical property that con-tributes to light harvesting by their strong absorption in photosynthesis[1,2]. The architectures of meso/nano-scaled porphyrin assemblies orparticles are expected to be promising candidates for use in photonicdevices [3–6]. In order to exploit the properties of porphyrins andmetalloporphyrins, they can be deposited as thin films by using severaltechniques such as solvent casting, Langmuir–Blodgett, spin coating,high vacuum evaporation and glow discharge induced sublimation[7–9]. In order to fabricate organic nano-architectures composed of por-phyrins, it should be recognized that Van derWaals intermolecular andhydrogen-bonding interactions aswell as the electrostatic attraction areresponsible for the specific electronic/optical properties that are funda-mentally different from those of inorganic metals or semiconductors[10–16]. The presence of π-electrons diminishes the probability of a lo-calization of excitation energy at a specific bond. In turns, the excitationenergywill spread over thewhole carbon ring and de-excitation ismorelikely to occur through processes such as collisional transfer rather thanby dissociation. The aromatic ring is considered as the dominant com-ponent in porphyrins, which is mostly responsible for the strength,the high temperature and radiation resistance of the material [17].

ahass).

rights reserved.

Porphyrins and metalloporphyrins exhibit an intensive absorptionband called the Soret band in visible region, and at longer wavelength,there is another series of absorption band called the Q-band. Using fea-tures in color, porphyrin and related compounds can be applied to opti-cal memory [18,19]. The interaction of gamma-rays with material,mainly, occurs by means of electronic excitation, electronic ionization,and, primarily, atomic displacement of the orbital electrons [20]. The in-fluence of radiation on the material depends on dose rate and the pa-rameters of the films, including their thickness and composition. Thedegradation is more severe for the higher dose and the thinner films[21,22]. Numerous efforts have recently been made to investigate theinfluence of gamma radiation on thin films and thin film structures ofdifferent metal oxides and polymers, in order to find out the suitabilityof using thin films and thin film structures of different metal oxides andpolymers as gamma radiation dosimeters [22–24]. The present work,aims to study in detail the induced changes in the topological and themorphological nanostructures of NiTPP thin films and estimate the re-lated optical and dispersion parameters of these films before and afterexposed to gamma radiation of dose 150 kGy. The change in exposuretime and exposure intensity have not considered because they werenot one of the objective of this study.

2. Experimental procedures

A dark purple crystalline powder 5, 10, 15, 20-Tetraphenyl-21H,23H-porphine nickel (II) NiTPP was purchased from Aldrich Chem.

Fig. 1. The molecular structure of NiTPP.

1873M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

Co. andwas used as receivedwithout any further purification. The sche-matic diagram of the NiTPP molecular structure is shown in Fig. 1. Thinfilms of NiTPPwith thickness 153.5 nmwere prepared by thermal evap-oration technique, using a high vacuum coating unit (Edward, E 306 A,England), under a pressure of about 2×10−4 Pa. A suitable quartz cru-cible heated slowly by a tungsten coil was used for evaporating theNiTPP powder. The films were deposited onto well-cleaned opticallyflat fused quartz substrates for optical measurements. The substrateswere maintained at room temperature during deposition. The rate ofdeposition was controlled to be 2 nm/s using a quartz crystal thicknessmonitor (Edwards, Model FTM4). The thickness was also monitored byusing the same thicknessmonitor and subsequently calibrated interfer-omerically by Tolansky's method [25].

The films are divided into two groups after the calibration of filmthickness. The first (as-deposited) group is characterized as prepared.The second group was irradiated in air, at room temperature bygamma-irradiation cell with a dose rate 3.52 kGy/h from 60Co source.

5 10 15 20 250

20000

40000

)211(

5 10 15 20 250

10203040506070

)211(

)011(

)011(

5 10 15 20 250

10203040506070

Inte

nsity

(co

unts

/s)

2θ°

Fig. 2. Shows the diffractograms (XRD) for NiTPP: (a) in powder form, (b) as-deposited fil153.5 nm.

The cell was attached to a cooling system to avoid heating of the sam-ples during irradiation. The used gamma irradiation system is availablein National Center for Radiation Research and Technology (NCRRT),Atomic Energy Authority (AEA), Cairo, Egypt. The exposure time andtotal absorbed dosewere 42.6 h and 150 kGy, respectively. These valueswere chosen for NiTPP thin films as a remarkable effective value for theoptical and structural properties without any stoichiometric changes inNiTPP thin films. The geometrical arrangements of the facility consid-ered the safety aspects and protection rules [26,27].

The structural characterizations were performed using X-ray dif-fraction (XRD) system (model X' pert) with utilized monochromaticCu-Kα radiation (λ=1.54056 Å). The transmittance T(λ) and reflec-tance R(λ) spectra of NiTPP films were measured at normal incidenceof the light at room temperature in the spectral range 200–2500 nmby using a computer-aided double beam spectrophotometer (JASCOmodel V-570 UV–VIS-NIR). A blank quartz substrate identical to theone used for the thin film deposition was used as a reference for thetransmittance scan. An uncertainty of 1% was given by the manufac-turer for the measurements obtained by this spectrophotometer.

The absolute value expressions of the total measured transmit-tance T(λ), and reflectance R(λ) of NiTPP thin films are calculated,after including the corrections due to the substrate absorbance andreflectance, by [28,29]:

T λð Þ¼ IftIq

!1� Rq

� �ð1Þ

R λð Þ¼ IfrIm

� �Rm 1� Rq

� �2þ 1� �

�T2Rq ð2Þ

where Rq is the reflectance of quartz substrate , Ift ,and Iq are the in-tensities of light passing through quartz system and reference quartzsubstrate, respectively, Ifr and Im are the intensities of light reflectedfrom the sample and that from reference Al-mirror reaching the detec-tor, respectively, and Rm is the reflectance of Al-mirror. The absorptioncoefficient α(λ), the absorption index k(λ), and the refractive index

30 35 40 45 50 55

(a)

NiTPP Powder

30 35 40 45 50 55

(b)

As-deposited NiTPP film

30 35 40 45 50 55

(c)gamma irradiatedNiTPPfilm

m, (c) film on glass substrate after being exposed to gamma radiation with thickness

4000 3500 3000 2500 2000 1500 1000 500

90

95

100

(a)

4000 3500 3000 2500 2000 1500 1000 500

85

90

95

100

(b)

4000 3500 3000 2500 2000 1500 1000 500

90

95

100

(c)

Tra

nsm

ittan

ce (

Arb

. Uni

ts)

(wavenumber (cm-1)

Fig.3. FTIR spectra of NiTPP compound :(a) in powder form, (b) as-deposited film, (c) gamma-irradiated film with thickness 153.5 nm.

Table 1IR spectral data in different forms; powder, as-deposited and gamma-irradiated NiTPPfilms.

Wave number (cm−1) Assignment

Powder As-depositedfilm

Gamma-irradiatedfilm

3424 3426 3427 (−OH) is associated, certainly,with the water absorbed byKBr from the atmosphere.

(2919–2329) (2917–2332) (2922–2331) (C-H) (aromatic) StretchingVibration

1636.2 1629.3 1632 ν(Ca−Cm)asym.

1600 1599 1598 Phenyl1436 1436.6 1439 Phenyl1349 1351 1350.4 Phenyl1158.4 1158.2 1157.2 ν(Cm−H)1068.3 1066.6 1069.7 ν(Cb−H)sym.

1000.2 1004 1003 ν(pyr. breathing)789.6 792 790.6 (Pyr. fold)sym.

696.8 697.6 695.5 Phenyl516 515.7 515.3 Phenyl467.7 468.5 465.4 δ (Pyr rot)404.4 405.5 404.8 δ (Pyr rot)

1874 M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

n(λ) of the films at different wavelengths are calculated by using thefollowing equations [28,29]:

n λð Þ¼1þ R1� R

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

4R1� Rð Þ2 �k2

sð3Þ

k λð Þ¼αλ4π

ð4Þ

α λð Þ¼ 1d

� �In

1� Rð Þ22T

þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� Rð Þ44T2

þ R2

s24

35 ð5Þ

where d is the film thickness. The experimental error in measuringthe film thickness was ±2% for T(λ), and ±1% for R(λ).

3. Results and discussion

3.1. Structure characterizations

The X-ray diffraction spectra of NiTPP in powder form, and thinfilms conditions (as-deposited and gamma-irradiated) as the diffrac-tograms (a), (b) and (c) show in Fig. 2. The diffractogram (a) depictsthat NiTPP in powder form is polycrystalline. The diffractogram (b)shows that the as-deposited NiTPP film has two peaks at diffractionangles 2θ (=8.6314° and 20.0149°) which is an indicator of a poly-crystalline nature which is embedded in amorphous microstructure.Also, the two intensities and wide peaks are indicators to fine struc-ture, and is characterizing the formation of nanocrystalline structurewith particle size 42.7 and 17.7 nm at 2θ (=8.6314 and 20.0149 ), re-spectively. The diffractogram (c) shows that gamma radiation of dose150 kGy decreases the polycrystalline nature of the prepared NiTPPfilm and increases the ripples and the amplitude of the hump. Inturn, these results are implying the formation of amorphous nano-structure. The chosen exposure dose of gamma radiation lay in the

absorbing dose region which can affect the refractive index for appli-cation [30,31]. The change in exposure time and exposure intensitywas not under consideration according to the objective of this study.

The chemical structure of the powder, the as-deposited and irradiat-ed films was investigated by Fourier transform infrared (FT-IR) tech-nique. Fig. 3 illustrates FT-IR spectra in the range 400–4000 cm−1

acquired from fresh NiTPP powder in comparison with spectra corre-sponding to as-deposited and gamma-irradiated films of thickness153.5 nm. Inspection of this figure reveals that the thermal evaporationtechnique is a good one to obtain undissociated and stoichiometricNiTPP films. In addition, the spectrum of the gamma-irradiated filmsis also quite similar to that of the powder and the as-deposited film,which revealed that a molecular NiTPP film has a radiation resistance

1875M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

up to the given dose of gamma radiation. The observed FT-IR bands andtheir assignments are listed in Table 1, and they are in a good agreementwith that in literature [32–34].

3.2. Topological and morphological characterizations

Thin film specimens for SEM characterization were fixed withdouble-faced stickers on the specimen holder and sputtered (S150Aspatter Coater, Edward's, England) using a very thin layer of goldabout 10 Å in thickness under vacuum (3×10−1 mbar) of inert gas(Argon). The surface topography for as-deposited and gamma-irradiated NiTPP films was investigated by the scanning electron mi-croscopy (SEM) as shown in Fig. 4. The topograph (a) in Fig. 4 depicts

Fig. 4. (a) SEM micrograph of as-deposited, (b) SEM

the grains characteristics of an ordered polycrystalline as depositedNiTPP film with some porous regions [30]. The gamma radiation has aprofound impact on NiTPP film as the topograph (b) shows in Fig. 4.The grains have well-defined boundaries with smaller nano-sizedparticles of granular shape [30]. The topological distribution ofgamma-irradiated NiTPP film is changed in comparison with that ofas-deposited film. These changes may be due to radiolysis effect ofgamma radiation on the grains in NiTPP film during the irradiation pro-cess of gamma radiation. As a result of this effect, the grain sizes and ag-gregates boundaries are decreased.

Surface topography was studied by contact mode atomic force mi-croscopy AFM in (Engineering and Surface Metrology Department,National Institute for Standards, Egypt). The collected 3D topographical

micrograph of gamma-irradiated NiTPP film.

Table 2Summary of surface morphology parameters from AFM image analyses.

Roughness parameter NiTPP thin film category

Asdeposited

Gammairradiated

Rq (nm) root mean square (RMS) parameter 7.88 6.70R(p-v) (nm) mean peak-to-valley profile roughness 80.80 67.99Rv (nm) maximum valley depth 39.37 37.72Rp (nm) maximum peak height 41.43 30.27Projected area (μm²) 25.00 25.00Shw (nm) mean half wavelength 312.5 238.1

1876 M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

data was analyzed using data analysis software (IP; Image Processingand Data Analysis, V.2.1.15, Thermo Microscopes, CA, USA) and SPIPsoftware (Image Metrology). Nano-roughness data was calculatedover the whole image (area), with cited numbers resulting from atleast three images of 5×5 μm2. Plane correction was applied to eachimage before the roughness calculations. All the recorded parametersin Table 2 are based on two-dimensional standards that are extendedto three dimensions [35].

Topographical images (a) and (b) in Fig. 5 display characteristicAFM images of as-deposited and gamma-irradiated NiTPP films, re-spectively. Topographical image (b) in Fig. 5 depicts the effects ofgamma radiation on the surface topography of NiTPP films. The valuesof Rq and Rp-v are decreased by amounts of 1.77 and 12.81 nm, re-spectively, (may be due to the radiolysis effect of gamma radiation),while the valley depth (Rv) and peak profile (Rp) are decreased by

Fig. 5. AFM 3D-topographical image of typical tapping mode for (a) as-deposited,(b) gamma-irradiated NiTPP films with thickness 153.5 nm.

amounts of 1.65 and 11.16 nm, respectively. In conclusion, gamma ra-diation decreases the surface roughness Rq, mean peak-to-valley pro-file roughness Rp-v and mean half wavelength (Shw) of the as-deposited NiTPP thin films by 14.97%, 15.85% and 23.8% respectively.All analyzed data for gamma-irradiated NiTPP thin films are limited tothe given dose of gamma radiation.

The morphological structure of as-deposited and irradiated NiTPPfilms are investigated by the transmission electron microscopy (TEM)as shown in Fig. 6. (TEM) image (a) of the as-deposited NiTPP film inFig. 6 shows a polycrystalline structure of nanoporous nature and aspatial configuration of well-defined grains boundary [30]. The nano-crystalline particles are embedded in a large scale of amorphousstructure. The (TEM) image (b) depicts that the crystal growth ofthe particles is decreased by the gamma-irradiation process. This re-sult can play roles in the morphologies of the NiTPP film in order tocontrol its optical properties. The (TEM) image also shows thatgamma radiation induces dislocations in the NiTPP film nanostruc-ture. These defects diffuse, agglomerate and eventually accumulateforming defect clusters as small dislocation free loops of black dots[20,27]. The diameter of these “black dots” is ranging from 22 to39 nm as the inset shows in Fig. 6(b). These results are in agreementwith the analysis (XRD) for the as-deposited and gamma-irradiatedNiTPP film.

4. Optical characterizations

4.1. Transmission and reflection spectra

Fig. 7 illustrates the spectral behaviour of the transmittance, T(λ),and the reflectance, R(λ), in the wavelength range 200–2500 nm foras-deposited and gamma-irradiated NiTPP films of thickness 153.5 nmas a representative sample. At shorter wavelengths λb1000 nm, thespectrum of T(λ) and R(λ) implies the existence of absorption, ( T(λ)+R(λ) is less than unity). The given dose of gamma radiation shifts thetransmission edge, slightly, towards longer wavelengths and decreasesthe intensity of transmittance peaks at 329 and 489 nm in violet andblue spectrum, respectively. At longer wavelength, λ>1000 nm, nolight was scattered or absorbed, T(λ)+R(λ)≈1. In non-absorbing re-gion, the given dose of gamma radiation decreases the values of T(λ)and R(λ). All films have almost the same thickness (within ±2% as anexperimental errors).

4.2. Optical dispersion characteristics

Figs. 8 and 9 show the real, n(hν), and imaginary, k(hν), parts ofthe indices of refraction for as-deposited and irradiated NiTPP films.The values of the real part of the indices of refraction exhibit ananomalous dispersion in the range λb1000 nm [36], and a normaldispersion in the range λ>1000 nm [37,38]. At longer wavelength(hν→0) the calculated value of the refractive index decreases by anamount Δn=0.26 after gamma-irradiation. The refractive index in-crease is caused by a radiation-induced chemical alteration of thepolymer composition, while the decrease in the refractive indexvalues is attributed to the decrease of mass density [39]. A strong var-iation in the intensity of peaks at 1.14 and 3.16 eV, and slight varia-tion in the intensity of peaks at 2.22, 2.57, and 3.98 eV as a result ofgamma radiation with red shift in their positions [40,41].

Optical absorption analysis has widely proven to be an importantand efficient tool in exploring and interpreting the various phenome-na of electronic structures and processes in the materials subjected toradiation [42–44]. Fig. 10 shows the variation of the absorption coef-ficient (α) of as-deposited and irradiated NiTPP films with thickness153.5 nm, as a function of photon energy, (hν). It is evident that theoptical absorption spectral distribution is sensitive to gamma radia-tion; a considerable red shift in the absorption was observed. Theintensity of the absorption peaks increases, slightly, for gamma-

Fig. 6. TEM image of (a) as-deposited NiTPP film and (b) gamma-irradiated NiTPP films.

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 24000.00

0.25

0.50

0.75

1.00

R((λ)

T((λ)

T, R

T(λ) as-deposited filmT(λ) gamma irradiated filmR(λ) as-deposited filmR(λ) gamma irradiated film

λ (nm)

Fig. 7. Transmittance, T(λ), and reflectance, R(λ), spectra for as-deposited and gammairradiated NiTPP thin films with thickness 153.5 nm.

0 1 2 3 4 5 6

2

3

4

5

6

3.98

eV

hν (eV)

n

as-deposited film gamma-irradiated film

2.22

eV

3.16

eV

2.57

eV

1.14

eV

Fig. 8. The spectral dependence of the real part of refractive index, n(hν), for the as-deposited and gamma-irradiated NiTPP thin films with thickness 153.5 nm.

1877M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

0 1 2 3 4 5 6

0.2

0.4

0.6

0.8

1.0

as-deposited film gamma irradiated film

hν (eV)

k

Fig. 9. The spectral dependence of the imaginary part of refractive index, k(hν), for theas-deposited and gamma irradiated NiTPP thin films with thickness 153.5 nm.

0 1 2 3 4 5 6

20000

90000

160000

230000

300000 as-deposited film gamma-irradiated film

hν (ev)

α (c

m)-1

Fig. 10. Spectral behavior of absorption coefficient, αhν, for the as-deposited andgamma-irradiated NiTPP thin films with thickness 153.5 nm.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

0.1

0.2

0.3

0.4

0.5

as-deposited filmgamma−irradiated film

(hν)2(eV)2

(n2

-1)-1

Fig. 11. Plots of (n2−1)−1 versus (hν)2 for as-deposited and irradiated NiTPP thinfilms with thickness 153.5 nm.

Table 3the dispersion parameters of the as-deposited and the irradiated NiTTP, FeTPPCl and CoMT

Film condition Eo(eV) Ed(eV) ε∞

NiTPP FeTPPCl CoMTPP NiTPP FeTPPCl CoMTPP NiTPP Fe

As-deposited 1.8 2.68 3.02 10 7.52 7.99 6.4 3.Radiationtype

Gamma 2.1 – – 10.5 – – 5.9 –

X-ray - 2.71 3.4 – 5.85 7.5 – 3.

1878 M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

irradiated NiTPP film. Below the absorption edge, sharp sub-gap ab-sorption bands were observed in the as-deposited film only in the en-ergy range 0.5–1 eV due to vibrational overtones of C–H stretchingand bending vibrations [45,46].

At lower optical frequencies, Wemple and DiDomenico [37,38] in-troduced two parameters based on the single effective oscillatormodel; Ed to describe the dispersion of the refractive index and thesingle oscillator energy Eo to give quantitative information on theoverall band structure of the material “average gap” [47]. The refrac-tive index is expressed in terms of these parameters as in references[37,38]:

n2�1 ¼ EdEoE2o� hυð Þ2 ð6Þ

Fig. 11 shows the plots of (n2−1)−1 versus (hυ)2 for as-depositedand gamma-irradiated NiTPP films. The values of Ed and Eo were de-termined by fitting a straight line to experimental data points. Thecalculated values of the dispersion parameters as well as the infinitefrequency dielectric constant ε∞ were obtained by extrapolating thestraight line towards (hυ=0). The results for as-deposited and irradi-ated NiTPP films are calculated in Table 3.

In transparent region, the relation between the real part of the di-electric function ε1 and the square of the wavelength λ and the re-fractive index can be expressed by [48]:

ε1¼ n2¼ εL–Nm�

� �e2

4π2εoc2

" #λ2 ð7Þ

where εL is the high frequency dielectric constant, e is the elementarycharge, εo is the permittivity of free space and (N/m*) is the ratio offree carrier concentration to the free carrier effective mass. Fig. 12shows the linear relation between n2 and λ2 for the as-deposited andirradiated NiTPP thin films. Extrapolating the linear parts to zero wave-lengths indicated the value 8.5 for εL of the as-deposited film, andthe values 7.1 for εL of the gamma-irradiated NiTPP thin films. Theratio of free carrier concentration could be calculated from the slopeof these linear parts, where the ratio (N/m*) was considered tobe 5.3×1047 g−1 cm−3 for the as-deposited and 5.31×1047 for thegamma-irradiated NiTPP thin films. The disagreement between ε∞ andεL may be due to the differences in free carrier contribution [49].

4.3. Energy gap determination

For a crystalline and amorphous semiconducting material, theanalysis of optical absorption near the absorption band edge is a stan-dard method for determining the types of transitions and calculationof the optical band gap. The energy dependence of the interband ab-sorption coefficient for direct and indirect allowed transitions is givenby the following expression [50]:

For allowed direct transitions

α hνð Þ¼ Adirct hν� Edg� �1=2 ð8:aÞ

PP thin films.

εL N/m⁎ (gm−1.cm−3)

TPPCl CoMTPP NiTPP FeTPPCl CoMTPP NiTPP FeTPPCl CoMTPP

82 3.65 8.5 4.15 4.07 53×1046 7.72×1046 9.82×1046

– 7.1 – – 53.1×1046 - –

16 3.2 – 3.39 3.49 - 4.18×1046 7.36×1046

0 1 2 3 4 5 6

2

4

6

8

10

12

as-deposited filmgamma irradiated film

λ2 (μm)2

n2

Fig. 12. Plots of n2 versus λ2 for as-deposited and irradiated NiTTP thin films withthickness 153.5 nm.

1879M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

For indirect allowed transitions

α hνð Þ¼ Aind hν� Eindg � Eph� �2 ð8:bÞ

0

200

400

600

800

1000

1200

(αhν

)1/2 (

eV/c

m)1/

2

(αhν

)1/2 (e

V/c

m)1/

2

hν

hν (eV)

as-deposited film

gamma-irradiated film

0.5 1.0 1.5 2.0

0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 20

100

200

300

400

500

600

EOnset

g

Fig. 13. The relation between (αhν1/2 and photon energy (hν) for the as-dep

Table 4The optical parameters of indirect transitions for the as-deposited and the irradiated NiTTP

Film condition NiTPP

Egonset

(eV)Eph(meV)

Eg(eV)

Eg(eV)

Egopt

(eV)E(

As-deposited 1.93 – 2.08 2.14 2.58 2Radiation type Gamma 1.65 110 2.02 2.14 2.58 2

X-ray 0.5 kGy1 kGy1.5 kGy2 kGy20 kGy

in the above equations, Edg and Eind

g represent the band gap energy, Bd

and Bind are characteristic constant parameters, independent of pho-ton energy, for direct and indirect transition, respectively, and Eph isthe phonon energy. The graph of (αhν)2 versus (hν) is found not sup-port the interpretation of direct rather than indirect band gap. Fig. 13shows the plots of (αhυ)1/2 versus (hυ) for as-deposited and irradiat-ed NiTPP films. The indirect band gaps Egonset and Egopt for the as-deposited and irradiated films are evaluated from the x-axis interceptsat (αhυ)1/2=0. According to Eq. (8.b), the values of the correspondingenergies were found to be 1.93, 2.08, 2.14, 2.58 and 2.88 eV for the as-deposited and 1.65, 2.02, 2.14, 2.58 and 2.73 eV for the irradiated filmsof thickness 153.5 nm. The inset of Fig. 13 shows a scope view for theregion of low energies. It should be noted that the first energy value isthe optical gap Eg

onset, corresponds to the onset of optical absorptionand formation of vacancies, interstitials Frenkel pairs or dislocationsin the filmmicrostructure [51]. The last energy value is the fundamen-tal energy gap (energy gap between valence band “π-band” and con-duction band “π*-band” [52], and the values between them may beimpurities energy levels). Table 4 shows the comparison betweenthe optical parameters of indirect transition for NiTPP and the previ-ously obtained results for FeTPPCl [29], CoMTPP [53], and TPP [54].The electronic inter-band transition in gamma-irradiated NiTPP filmsis accompanied by phonon absorption. The phonon assisted energywas estimated and recorded for as-deposited and gamma-irradiatedNiTPP films in Table 4.

Eopt

g

(eV)2.5 3.0 3.5 4.0 4.5

.50 2.75 3.00

osited and gamma-irradiated NiTPP thin films with thickness 153.5 nm.

, FeTPPCl and CoMTPP thin films.

FeTPPCl CoMTPP TPP

g

eV)Egopt

(eV)Eg(eV)

Eg(eV)

Eg(eV)

Eg(eV)

Eg(eV)

Eg(eV)

Eg(eV)

Eg(eV)

.88 1.50 1.83 2 2.43 1.68 1.86 2.1 3 1.764

.731.7551.7741.7871.806

1.36 1.58 2 2.43 – 1.7 1.89 3.2 –

0 1 2 3 4 5 6

5

10

15

20

25

30

1.14

eV

as-deposited film gamma-irradiated film

hν (eV)

ε 1

2.22

eV

2.57

eV

3.16

eV

3.98

eV

Fig. 14. Plot of ε1(hν) the real dielectric constant for the as-deposited and irradiatedNiTPP thin films with thickness 153.5 nm.

1880 M.M. El-Nahass et al. / Optics Communications 285 (2012) 1872–1881

4.4. Dielectric characterizations

The complex dielectric, ~ε , constant is an important quantity for thedesign of highly efficient optoelectronic devices, because the dielectricconstant gives amenable characterizing view about the interactions be-tween photons and electrons in a material. These interactions can beobserved on the shapes of the real, ε1, and imaginary, ε2, parts of the di-electric constant in the dielectric spectrum [52]. The complex dielectricconstant is described by:

~ε hνð Þ¼ ε1 hνð Þþ ε2 hνð Þ ð9Þ

where ε1 (=n2−k2) is the real and ε2 (=2nk) is the imaginary parts ofthe dielectric constant. Figs. 14 and 15 show the spectra of real andimaginary parts which are called dispersion and absorption curves, re-spectively. The behaviour of ε1 follows the same trend as (n) where asthe behaviour of ε2 mainly follows the behaviour of (k) which is relatedto the variation of with photon energy.

5. Summary and conclusions

The structural characteristics of NiTPP have been investigated byXRD which indicates polycrystalline and amorphous nature for as-deposited and gamma-irradiated films. The FTIR results showed thatthermal evaporation is a good technique for obtaining un-dissociated

1 2 3 4 5 60

2

4

6

8

10

hν (eV)

as-deposited film gamma-irradiated film

ε 2

Fig. 15. Plot of ε1(hν) the imaginary dielectric constant for the as-deposited and irradi-ated NiTPP thin films with thickness 153.5 nm.

NiTPP thin films and revealed that the molecular structure of NiTPPcompound is stable structure after exposure to gamma radiation. AFMand TEM for as-deposited and gamma-irradiated films show a largescale of nano particles. The optical properties of NiTPP thin films beforeand after gamma-irradiation have been investigated by means of spec-trophotometric measurement. It is found that the refractive index dis-persion data obeyed the single oscillator model. An interpretation ofsingle oscillator parameters and Drude mode of free carriers absorptionhave been described for the analysis of refractive index dispersion be-fore and after irradiation. From fundamental absorption edge, an ap-proximate picture of the energetic transitions of charge carries wasdescribed. The as-depositedfilms have indirectly allowedfive electronictransitions. The onset band gap of 1.93 eV, gamma radiation decreasedthe onset band gap to 1.65 eV and the indirect inter-band electronictransition in this case is accompanied with phonon emission. In addi-tion, gamma-irradiation affects the values of the calculated dispersionparameters such as oscillator energy, dispersion energy and dielectricconstant.

The effect of gamma irradiation on the film morphology can beinterpreted as caused by the formation of clusters and/or defectsnear the film surface causing the change of the morphology of thefilm from polycrystalline to amorphous. The behaviour (peaks posi-tions for n, k and alpha) of the film in the anomalous region beforeand after gamma irradiation has the same behaviour. Also, the infra-red absorption spectra of the powder, the as-deposited and irradiatedfilms revealed that the main chemical composition of NiTPP has beenpreserve.

Acknowledgements

The authors would deeply like to thank Dr. Mohamed A. Amer(Engineering and Surface Metrology Department, National Institutefor Standards El Harem, Giza, Egypt) for using data analysis software(IP; Image Processing and Data Analysis, V.2.1.15, ThermoMicroscopes,CA, USA) to evolve this work. The authors are grateful to Prof. M.K.Abu-Assy and Prof. Magdy EL-ashry (Physics Department, Facultyof Science, Suez Canal University, Ismailia, Egypt) for their valuablediscussion.

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