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Influence of high-energy X-ray irradiation on the optical properties of tetraphenylporphyrin thin films
ARTICLE IN PRESS
Optics & Laser Technology 39 (2007) 347–352 www.elsevier.com/locate/optlastec
Influence of high-energy X-ray irradiation on the optical properties of tetraphenylporphyrin thin films M.M. El-Nahassa, H.M. Zeyadab, M.S. Azizb, M.M. Makhloufc, a b
Department of Physics, Faculty of Education, Ain Shams University, 11757 Cairo, Egypt Department of Physics, Faculty of Science at New Damietta, 34517 New Damietta, Egypt c Department of Radiation Physics, Damietta Cancer Institute, Damietta, Egypt
Received 20 December 2004; received in revised form 18 June 2005; accepted 15 July 2005 Available online 23 September 2005
Abstract Tetraphenylporphyrin (TPP) thermally evaporated films were irradiated by different doses (0.5–2.5 kGy) of X-ray with energy 6 MeV. The optical properties for TPP were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 200 to 1100 nm. The absorption spectra recorded in the UVVIS region of spectra showed different absorption bands, namely four Q-bands in the visible region of the spectrum and a more intense band termed as the Soret band in the near-UV region of the spectrum. Two other bands labeled N and M appear in the UV region. The Soret band showed Davydov splitting. Increasing X-ray irradiation dose influences the optical properties of TPP films. All absorption bands show a continuous blue shift in position and a decrease in intensity with increasing X-ray dose. At 2.5 kGy the B, N, and M bands disappeared. The reduction in the absorbency was calculated as a function of X-ray dose. The energy gap was determined and the type of optical transition was found to be an indirect allowed transition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Tetraphenylporphyrin; Thin film; X-ray irradiation
1. Introduction Porphyrin compounds have attracted much attention because they have enormous potential for applications on the technological front, including electro-luminescent devices [1], solar energy conversion devices [2], organic semiconductor [3], opto-electronic device fabrication [4] and gas sensors [5]. Porphyrin derivatives play an important role in the metabolism and biological activities of living organisms [6], as for instance an iron complex in the hemoproteins, a magnesium complex in the chlorophylls and a cobalt complex in vitamin B12. They have been used in the photodynamic therapy (PDT) of tumors as photosensi-
Corresponding author. Tel.: +20 57 403867; fax: +20 57 354100.
E-mail address: [email protected] (M.M. Makhlouf). 0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.07.004
tizers due to their selective accumulation toward tumor cells and high radiosensitization efficiency [7]. The basic structure of porphyrin consists of four pyrrolic subunits linked by four methane bridges, and the porphyrin skeleton has an extended p-conjugation system with 24-p electrons leading to a wide range of wavelengths for light absorption and p-type properties as an electronic system. Tetraphenylporphyrin (TPP) is the free base porphyrin macrocycle linked with four phenyl groups as shown in Fig. 1. TPP has polymorphic crystal structures such as triclinic, tetragonal and monoclinic forms. The absorption spectra of TPP showed different bands depending on the method of its preparation [8,9]. Composite films [8] produced by vapor deposition of nylon-11 and TPP have absorption spectra, in the visible region (300–800 nm), and showed that the Q-band region is very small and sometimes not clear.
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
NH
N
N
HN
Fig. 1. Molecular structure for tetraphenylporphyrin (TPP).
However, while in the case of Langmuir–Blodgett TPP films, the splitting of the Soret band due to exciton coupling has been reported [9]. Similarly, the position of the Soret band region and the shapes of the spectra in the Q-band region for TPP and its derivatives in solution depend on the type of solvents [10,11]. The absorption spectra of TPP in nematic liquid crystal solution have been studied [10]; the spectrum shows a characteristic Soret band in the blue region (400–500 nm) and small bands in the Q-band region (500–650 nm). The variation of solvent polarity affects the optical absorption features [11], where dramatic red shifts of Soret, B (425 nm) and Q (450 nm) were observed in polar solvents relative to non-polar solvents. To our knowledge, absorption spectra, optical properties and spectral features of thermally evaporated TPP thin films have not been investigated yet. The aim of the present work is to study the optical properties and spectral features for thermally evaporated TPP thin films. The influence of X-rays of high energy (6 MeV) on these parameters was also investigated.
104 Pa. The rate of deposition was controlled at 2.5 nm/ s using a quartz crystal thickness monitor (Model FTM4, Edward Co., England). The thickness was also monitored by using the same thickness monitor. A shutter, fixed near the substrate, was used to avoid any probable contamination on the substrates in the initial stage of the evaporation process and to control the thickness of films accurately. The structural analysis of TPP in powder form and in as-deposited thin film condition was analyzed by an XRD system (model XJ Pert Pro, Philips Co.) equipped ( with Cu target. Filtered Cu Ka radiation (l ¼ 1:5408 A) was used. The X-ray tube voltage and current were 40 kV and 30 mA, respectively. The transmittance, T(l), and reflectance, R(l), spectra of the films were measured at normal incidence of light in the spectral range 200–1100 nm using a double-beam spectrophotometer (JASCO model V-570 UV-VISNIR), for TPP thin films of thickness 735 nm. An uncertainty of 1% was given by the manufacturer for the measurements obtained by this spectrophotometer. A quartz blank substrate identical to the one used for the thin film deposition was used as a reference for the absorption scan. These measurements were also performed for the films after being exposed to high-energy X-ray (6 MeV); the dose irradiation was varied from 0.5 to 2.5 kGy by using a linear accelerator (Philips electronics UK version SL15).
3. Results and discussion The single crystal and molecular structure for TPP were determined and indicated that the crystal system is triclinic and there is only one TPP molecule per unit cell. ( b ¼ 10:481 A, ( The unit cell parameters are a ¼ 6:438 A, ( c ¼ 12:42 A, a ¼ 95:91, b ¼ 99:31, g ¼ 101:21, V ¼ ( 3 and the density is 1.274 g/cm3 [12]. 803 A The XRD pattern of TPP in the powder form, Fig. 2, shows many peaks with different intensities; this
( 012 )
2500
1500 1000 500
( 001)
2000 ( 011 ) ( 010 ) ( 011 ) ( 100 ) ( 101 ) ( 110 ) ( 111 ) - ) ( 111 )( 111 ( 021 ) ( 012 ) ( 021 )( 102 ) ( 120 ) ( 112 ) ( 121 ) -( 103 ) ( 112 ) ( 022 ) ( 121 )
A dark violet TPP powder was used in this work, and thin films of TPP were sublimated by conventional thermal evaporation technique using a high-vacuum coating system (Model 306A, Edward Co., England). The films were deposited onto clean glass substrates for X-ray diffraction (XRD) analysis and onto optical flat quartz substrates for optical measurements. The quartz substrates were carefully cleaned by immersing in chromic acid for 15 min and then rinsed in deionized water. The material was sublimated from a quartz crucible source heated by a tungsten coil in a vacuum of
Intensity, Counts/sec
2. Experimental details
( 121) ( 131 ) ( 104 )
348
0 10
20
30
40
50
2 ° Fig. 2. X-ray diffraction pattern of TPP in powder form.
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
349
1.0
50
as-deposited 0.5 KGy 1.0 KGy 1.5 KGy 2.5 KGy
Transmittance
Intensity, counts/sec
0.8
40 30 20
0.6 0.4 0.2
10 0.0 200 300 400 500 600 700 800 900 1000 1100 λ, nm
0 10
20
30
40
50
(a)
2Θ° 0.45
Fig. 3. X-ray diffraction pattern of TPP thin film.
as-deposited 0.5 KGy 1.0 KGy 1.5 KGy 2.5 KGy
0.40 Reflectance
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
200 300 400 500 600 700 800 900 1000 1100 λ , nm
(b)
Fig. 4. (a) Spectral distribution of transmittance of TPP thin films before and after being exposed to X-ray irradiation. (b) Spectral distribution of reflectance of TPP thin films before and after being exposed to X-ray irradiation.
By
8
as-deposited 0.5 KGray 1 KGray 1.5 KGray 2.5 KGray
Bx Absorbance
result indicates that the material is polycrystalline in assynthesized condition. Fig. 3 shows the diffraction pattern of thermally evaporated TPP of thickness 735 nm. The diffraction pattern exhibits a broad peak around 2y ¼ 231, indicating that the evaporated TPP film is amorphous. The spectral behavior of transmittance, T(l), and reflectance, R(l), measured at normal incidence in the wavelength range 200–1100 nm for as-deposited and irradiated TPP thin films with irradiation doses ranging from 0.5 to 2.5 kGy, is presented in Figs. 4(a) and (b). Fig. 4(a) shows that the T(l) spectrum can be divided into two regions: (a) in the wavelength range 200–720 nm, the total sum of R(l) and T(l) is less than unity (absorption region). In the wavelength range 390–470 nm, the transmittance is zero (except for the case sample exposed to an X-ray irradiation dose of 2.5 kGy), i.e. there is no light transmitted out of the sample and all waves of light are either absorbed or reflected. It is also observed that the peaks position of TðlÞ and RðlÞ are shifted to shorter wavelength with an increase in irradiation dose. (b) At longer wavelength, l4720 nm, all films become transparent, T(l)bR(l), and no light was scattered or absorbed as T þ R 1 (transparent region); therefore, no light was absorbed. The oscillations in the values of T(l) and R(l), in this region, are a result of light interference in the considered wavelength range. We conclude also that the light is not dispersed and this indicates that the films are homogeneous and optically flat. The absorbance (or optical density) could be calculated by using the measured values of T from the formula [13] as
6 M 4
N
Q
2
0 200
300
400
500
600
700
800
λ , nm Fig. 5. Optical absorption spectrum of TPP thin films before and after being exposed to X-ray irradiation.
Absorbance ¼ Optical density ðODÞ ¼ log 10 ð1=TÞ. (1) Fig. 5 illustrates the absorption spectra of the asdeposited and irradiated TPP thin films with different
doses ranging from 0.5 to 2.5 kGy. The solid line refers to the absorbance in both UV and Vis regions of spectra for as-deposited film. It shows an intense absorption
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
7 0.5 1.0 1.5 2.5
6 5
KGy KGy KGy KGy
4 ∆A
termed Soret (or B) band, which appeared in the wavelength range 360–490 nm, and four additional weaker absorptions termed Q bands in the range 500–720 nm. It is also noted that the N and M bands appear at 283 and 210 nm in the UV region. It is shown that there is a decrease in the absorption peaks intensity with increasing irradiation dose. The absorption peaks may be generally interpreted in terms of p–p* transitions between bonding and anti-bonding molecular orbitals [14]. The two interband transitions Q and B are assigned as p–p* type, from the two highest occupied molecular orbitals (HOMO) levels 1a1u(p) and 4a2u(p) to the first excited lowest unoccupied molecular orbital (LUMO) level 5eg(p*). The transition bands in the UV region are assigned as the N band transition from HOMO level 3a2u(p) to LUMO level 5eg(p*) whereas the M band transition is from HOMO level 1b1u(p) to LUMO level 5eg(p*) [15]. The interaction between two or more molecules in the unit cell of the aggregate results in two or more excitonic transitions with high transition moment and the original absorption band splits into two or more by Davydov splitting [16]. The Soret band has two peaks, Bx and By , at 395 and 443 nm; this splitting depends on the distance between the molecules, the angle of the transition dipole moments with the aggregate, the angle of their transition dipole moments between neighboring molecules and the number of interacting molecules [16,17]. The two absorption transitions, Bx and By , due to splitting in the Soret band, have high intensity because the Soret band has parallel electric dipoles [14]. It is also noted that the band B appears with intensities that are considerably higher than those of Q, N, and M bands; this is because absorption transitions in the Q-band region have smaller oscillator strengths due to the opposite direction of the electric dipoles and the cancellation of electric dipoles that occurs leading to low intensity for these bands. Fig. 5 demonstrates the influence of irradiation process on the absorbency spectrum of TPP films. It is obvious that the absorbance of films decreases with increasing radiation dose until it reaches the value 2.5 kGy, and the B, N, and M bands disappear from the spectrum. It is also shown that the absorption spectrum was shifted slightly to shorter wavelength with increasing radiation dose from 0.5 to 2.5 kGy. In order to find out which parts of the absorption spectra were most affected by X-ray irradiation, an absorbance loss, DA, as a function of wavelength has been presented by subtracting ‘‘irradiated’’ spectra from the initial ‘‘unirradiated’’ samples as shown in Fig. 6. The irradiation process caused a decrease in the absorbance in the whole range of the spectrum; the maximum absorbance loss occurred in the B band region between 320 and 470 nm. The reduction in the optical absorption density, DOD, at different peaks of transition bands as a function of
3 2 1 0 200
300
400
500
600
700
λ, nm Fig. 6. Reduction in optical absorption spectrum, DA, of irradiated TPP thin films as a function of wavelength.
7 6
λ( = nm) 211 276 383 438 516 554 595 649
5 ∆OD
350
4 3 2
nm nm nm nm nm nm nm nm
1 0 0.0
0.5
1.0
1.5 2.0 Dose, KGy
2.5
3.0
3.5
Fig. 7. Reduction in optical absorption density, DOD, of irradiated TPP thin films as a function of irradiation dose.
irradiation doses shows that the absorbance decrease with increasing X-ray radiation as shown in Fig. 7. The absorption coefficient a could be calculated by using the measured values of R and T from the formula [18] " 1=2 # 1 ð1 RÞ2 ð1 RÞ4 2 a ¼ ln þ R , (2) d 2T 4T 2 where d is the film thickness for TPP. The types of transition and the value of optical energy gap can be demonstrated by Bardeen et al. [19] as ahn ¼ a0 ðhn E g Þr ,
(3)
where r ¼ 1=2 and 3/2 for direct allowed and forbidden transitions, respectively, r ¼ 2 and 3 for indirect allowed
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
0.5 kGy decreases the energy gap from 1.763 to 1.755 eV and is connected with the formation of dangling bonds [21]. Increasing the amount of irradiation dose steeply up to 2.5 kGy increases the energy gap from 1.755 to 1.801 eV and annihilates the dangling bond-type defect [21].
180 as-deposited 0.5 KGy
140 120
Eg , eV
(αhν)1/2, cm-1/5 .eV1/2
160
100 80
1.81 1.80 1.79 1.78 1.77 1.76 1.75
1.0 KGy 1.5 KGy 2.5 KGy
0.0
0.5
1.0 1.5 2.0 Dose, KGy
351
2.5
60 40
4. Conclusion
20 0 1.65
1.70
1.75
1.80
1.85
1.90
hν, eV Fig. 8. Relation between (ahn)1/2 and photon energy in the region of the absorption edge for as-prepared and irradiated TPP thin films.
Table 1 Optical energy gap and phonon energy for unirradiated and irradiated TPP films Dose (kGy) Eg (eV) Ephonon (meV)
0 1.764 38
0.5 1.755 44
1.0 1.774 36
1.5 1.787 36
2.5 1.806 37
and forbidden transitions, respectively, and a0 is the absorption background. The dependence of (ahn)1/r on photon energy (hn) for onset gap was discussed and plotted for different values of r. The best fit was obtained for r ¼ 2 as illustrated in Fig. 8; this is characteristic behavior of indirect allowed transitions. For indirect transitions, the wave vector k for the lowest energy state in the conduction band is different from that for the highest energy state in the valence band so that the electrons that transfer from the valence band to the conduction band will have a difference in momenta. In order to conserve momentum, one or more phonons are emitted or absorbed at the same time as the photon is absorbed. The relation between (ahn)1/2 and hn for as-deposited films and after irradiating is linear in the region of a strong absorption edge for fundamental and onset bands. The extrapolation of the straight line graphs ðahnÞ1=2 ¼ 0 will give the values of the optical band gaps as shown in Fig. 8. The values of indirect energy gap Eind and the phonon energies Eph for the films before g and after irradiation are listed in Table 1. The calculated energy gap for as-deposited film is 1.764 eV, and this value agrees with the previous work (1.8–1.9 eV) of Shaffer and Gouterman [20]. It is obvious that, there are two processes occurring in irradiated films depending on the amount of irradiation dose, and irradiation up to
The main conclusions can be summarized as follows. The analysis of XRD of TPP proved that the received powder material form polycrystalline patterns with triclinic structure and the as-deposited TPP thin films have amorphous structure. The absorption spectra of thermally evaporated TPP consist of four Q-bands in the region 500–720 nm, an extremely intense Soret band at about 360–490 nm, the Soret have two peaks at 395 and 443 nm and two other bands labeled N and M at shorter wavelength. X-ray irradiation affected the measured values of transmission, reflectance and absorption spectra. Absorption bands show a continuous shift in position and a decrease in intensity with increasing X-ray dose until at 2.5 kGy the B, N, and M bands disappeared. The reduction in the absorbency increases with increasing irradiation dose. The type of electronic transition responsible for optical properties is an indirect allowed transition. The onset energy gap of as-deposited film is 1.764 eV. This value increases as Xray dose increases as a result of increases in the disorder part.
References [1] Baldo MA, Brien DF, You Y, Shostikov A, Sibley S, Thompson ME, et al. Nature 1998;395:151. [2] Edgaro ND, Otero L. Chemistry 1999;4:144. [3] Kobayashi N, Nevin WA, Mizunuma S, Awaji H, Yamaguchi M. Chem Phys Lett 1993;205:51. [4] Wagner RW, Lindsey JS. J Am Chem Soc 1994;116:9759. [5] Di Natale C, Paolesse R, Burgio M, Martinelli E, Pennazza G, Amico A. Anal Chem Acta 2004;513:49. [6] James B. In: Dolphin D, editor. The porphyrins. New York: Academic Press; 1978. p. 205. [7] Gust D, Moore ThA. IEEE Eng Med Biol 1994;13:58. [8] Sayo K, Deki S, Noguchi T, Goto K. Thin Solid Films 1999;349:276. [9] Grieve MB, Hadson AJ, Richardson T, Johnstone RA, Sobral AJ, Rocha AM. Thin Solid Films 1993;243:581. [10] Wro´bel D, Goc J, Ion RM. J Mol Struct 1998;450:239. [11] Wro´bel D, Lukasiewicz J, Goc J, Waszkowiak A, Ion RM. J Mol Struct 2000;555:407. [12] Silvers SJ, Tulinsky A. J Am Chem Soc 1967;89:3331. [13] Fox M. Optical properties of solids. Oxford Master Series in Condensed Matter Physics, 2001. [14] Baerends EJ, Ricciard G, Rosa A, Van Gisbergen SJ. Coord Chem Rev 2002;230:5.
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[15] EL-Nahass MM, Zeyada MH, Aziz MS, Makhlouf MM. Spectrochim Acta A, 2005, in press, available online on www.sciencedirect.com. [16] Davydov AS. Theory of molecular excitons. New York: Plenum Press; 1971. [17] Mcrae EG, Kasha M. J Chem Phys 1958;28:721.
[18] Pankove JI. Optical process in semiconductors. New York: Dover Publications; 1971. p. 103. [19] Bardeen J, Blatt FJ, Hall LH. Photoconductivity conference. New York: Wiley; 1956. p. 146. [20] Shaffer AM, Gouterman M. Theor Chim Acta 1972;25:62. [21] Sharma P, Vashistha M, Jain IP. Radiat Meas 2003;36:663.
Optics & Laser Technology 39 (2007) 347–352 www.elsevier.com/locate/optlastec
Influence of high-energy X-ray irradiation on the optical properties of tetraphenylporphyrin thin films M.M. El-Nahassa, H.M. Zeyadab, M.S. Azizb, M.M. Makhloufc, a b
Department of Physics, Faculty of Education, Ain Shams University, 11757 Cairo, Egypt Department of Physics, Faculty of Science at New Damietta, 34517 New Damietta, Egypt c Department of Radiation Physics, Damietta Cancer Institute, Damietta, Egypt
Received 20 December 2004; received in revised form 18 June 2005; accepted 15 July 2005 Available online 23 September 2005
Abstract Tetraphenylporphyrin (TPP) thermally evaporated films were irradiated by different doses (0.5–2.5 kGy) of X-ray with energy 6 MeV. The optical properties for TPP were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 200 to 1100 nm. The absorption spectra recorded in the UVVIS region of spectra showed different absorption bands, namely four Q-bands in the visible region of the spectrum and a more intense band termed as the Soret band in the near-UV region of the spectrum. Two other bands labeled N and M appear in the UV region. The Soret band showed Davydov splitting. Increasing X-ray irradiation dose influences the optical properties of TPP films. All absorption bands show a continuous blue shift in position and a decrease in intensity with increasing X-ray dose. At 2.5 kGy the B, N, and M bands disappeared. The reduction in the absorbency was calculated as a function of X-ray dose. The energy gap was determined and the type of optical transition was found to be an indirect allowed transition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Tetraphenylporphyrin; Thin film; X-ray irradiation
1. Introduction Porphyrin compounds have attracted much attention because they have enormous potential for applications on the technological front, including electro-luminescent devices [1], solar energy conversion devices [2], organic semiconductor [3], opto-electronic device fabrication [4] and gas sensors [5]. Porphyrin derivatives play an important role in the metabolism and biological activities of living organisms [6], as for instance an iron complex in the hemoproteins, a magnesium complex in the chlorophylls and a cobalt complex in vitamin B12. They have been used in the photodynamic therapy (PDT) of tumors as photosensi-
Corresponding author. Tel.: +20 57 403867; fax: +20 57 354100.
E-mail address: [email protected] (M.M. Makhlouf). 0030-3992/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2005.07.004
tizers due to their selective accumulation toward tumor cells and high radiosensitization efficiency [7]. The basic structure of porphyrin consists of four pyrrolic subunits linked by four methane bridges, and the porphyrin skeleton has an extended p-conjugation system with 24-p electrons leading to a wide range of wavelengths for light absorption and p-type properties as an electronic system. Tetraphenylporphyrin (TPP) is the free base porphyrin macrocycle linked with four phenyl groups as shown in Fig. 1. TPP has polymorphic crystal structures such as triclinic, tetragonal and monoclinic forms. The absorption spectra of TPP showed different bands depending on the method of its preparation [8,9]. Composite films [8] produced by vapor deposition of nylon-11 and TPP have absorption spectra, in the visible region (300–800 nm), and showed that the Q-band region is very small and sometimes not clear.
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
NH
N
N
HN
Fig. 1. Molecular structure for tetraphenylporphyrin (TPP).
However, while in the case of Langmuir–Blodgett TPP films, the splitting of the Soret band due to exciton coupling has been reported [9]. Similarly, the position of the Soret band region and the shapes of the spectra in the Q-band region for TPP and its derivatives in solution depend on the type of solvents [10,11]. The absorption spectra of TPP in nematic liquid crystal solution have been studied [10]; the spectrum shows a characteristic Soret band in the blue region (400–500 nm) and small bands in the Q-band region (500–650 nm). The variation of solvent polarity affects the optical absorption features [11], where dramatic red shifts of Soret, B (425 nm) and Q (450 nm) were observed in polar solvents relative to non-polar solvents. To our knowledge, absorption spectra, optical properties and spectral features of thermally evaporated TPP thin films have not been investigated yet. The aim of the present work is to study the optical properties and spectral features for thermally evaporated TPP thin films. The influence of X-rays of high energy (6 MeV) on these parameters was also investigated.
104 Pa. The rate of deposition was controlled at 2.5 nm/ s using a quartz crystal thickness monitor (Model FTM4, Edward Co., England). The thickness was also monitored by using the same thickness monitor. A shutter, fixed near the substrate, was used to avoid any probable contamination on the substrates in the initial stage of the evaporation process and to control the thickness of films accurately. The structural analysis of TPP in powder form and in as-deposited thin film condition was analyzed by an XRD system (model XJ Pert Pro, Philips Co.) equipped ( with Cu target. Filtered Cu Ka radiation (l ¼ 1:5408 A) was used. The X-ray tube voltage and current were 40 kV and 30 mA, respectively. The transmittance, T(l), and reflectance, R(l), spectra of the films were measured at normal incidence of light in the spectral range 200–1100 nm using a double-beam spectrophotometer (JASCO model V-570 UV-VISNIR), for TPP thin films of thickness 735 nm. An uncertainty of 1% was given by the manufacturer for the measurements obtained by this spectrophotometer. A quartz blank substrate identical to the one used for the thin film deposition was used as a reference for the absorption scan. These measurements were also performed for the films after being exposed to high-energy X-ray (6 MeV); the dose irradiation was varied from 0.5 to 2.5 kGy by using a linear accelerator (Philips electronics UK version SL15).
3. Results and discussion The single crystal and molecular structure for TPP were determined and indicated that the crystal system is triclinic and there is only one TPP molecule per unit cell. ( b ¼ 10:481 A, ( The unit cell parameters are a ¼ 6:438 A, ( c ¼ 12:42 A, a ¼ 95:91, b ¼ 99:31, g ¼ 101:21, V ¼ ( 3 and the density is 1.274 g/cm3 [12]. 803 A The XRD pattern of TPP in the powder form, Fig. 2, shows many peaks with different intensities; this
( 012 )
2500
1500 1000 500
( 001)
2000 ( 011 ) ( 010 ) ( 011 ) ( 100 ) ( 101 ) ( 110 ) ( 111 ) - ) ( 111 )( 111 ( 021 ) ( 012 ) ( 021 )( 102 ) ( 120 ) ( 112 ) ( 121 ) -( 103 ) ( 112 ) ( 022 ) ( 121 )
A dark violet TPP powder was used in this work, and thin films of TPP were sublimated by conventional thermal evaporation technique using a high-vacuum coating system (Model 306A, Edward Co., England). The films were deposited onto clean glass substrates for X-ray diffraction (XRD) analysis and onto optical flat quartz substrates for optical measurements. The quartz substrates were carefully cleaned by immersing in chromic acid for 15 min and then rinsed in deionized water. The material was sublimated from a quartz crucible source heated by a tungsten coil in a vacuum of
Intensity, Counts/sec
2. Experimental details
( 121) ( 131 ) ( 104 )
348
0 10
20
30
40
50
2 ° Fig. 2. X-ray diffraction pattern of TPP in powder form.
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
349
1.0
50
as-deposited 0.5 KGy 1.0 KGy 1.5 KGy 2.5 KGy
Transmittance
Intensity, counts/sec
0.8
40 30 20
0.6 0.4 0.2
10 0.0 200 300 400 500 600 700 800 900 1000 1100 λ, nm
0 10
20
30
40
50
(a)
2Θ° 0.45
Fig. 3. X-ray diffraction pattern of TPP thin film.
as-deposited 0.5 KGy 1.0 KGy 1.5 KGy 2.5 KGy
0.40 Reflectance
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
200 300 400 500 600 700 800 900 1000 1100 λ , nm
(b)
Fig. 4. (a) Spectral distribution of transmittance of TPP thin films before and after being exposed to X-ray irradiation. (b) Spectral distribution of reflectance of TPP thin films before and after being exposed to X-ray irradiation.
By
8
as-deposited 0.5 KGray 1 KGray 1.5 KGray 2.5 KGray
Bx Absorbance
result indicates that the material is polycrystalline in assynthesized condition. Fig. 3 shows the diffraction pattern of thermally evaporated TPP of thickness 735 nm. The diffraction pattern exhibits a broad peak around 2y ¼ 231, indicating that the evaporated TPP film is amorphous. The spectral behavior of transmittance, T(l), and reflectance, R(l), measured at normal incidence in the wavelength range 200–1100 nm for as-deposited and irradiated TPP thin films with irradiation doses ranging from 0.5 to 2.5 kGy, is presented in Figs. 4(a) and (b). Fig. 4(a) shows that the T(l) spectrum can be divided into two regions: (a) in the wavelength range 200–720 nm, the total sum of R(l) and T(l) is less than unity (absorption region). In the wavelength range 390–470 nm, the transmittance is zero (except for the case sample exposed to an X-ray irradiation dose of 2.5 kGy), i.e. there is no light transmitted out of the sample and all waves of light are either absorbed or reflected. It is also observed that the peaks position of TðlÞ and RðlÞ are shifted to shorter wavelength with an increase in irradiation dose. (b) At longer wavelength, l4720 nm, all films become transparent, T(l)bR(l), and no light was scattered or absorbed as T þ R 1 (transparent region); therefore, no light was absorbed. The oscillations in the values of T(l) and R(l), in this region, are a result of light interference in the considered wavelength range. We conclude also that the light is not dispersed and this indicates that the films are homogeneous and optically flat. The absorbance (or optical density) could be calculated by using the measured values of T from the formula [13] as
6 M 4
N
Q
2
0 200
300
400
500
600
700
800
λ , nm Fig. 5. Optical absorption spectrum of TPP thin films before and after being exposed to X-ray irradiation.
Absorbance ¼ Optical density ðODÞ ¼ log 10 ð1=TÞ. (1) Fig. 5 illustrates the absorption spectra of the asdeposited and irradiated TPP thin films with different
doses ranging from 0.5 to 2.5 kGy. The solid line refers to the absorbance in both UV and Vis regions of spectra for as-deposited film. It shows an intense absorption
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
7 0.5 1.0 1.5 2.5
6 5
KGy KGy KGy KGy
4 ∆A
termed Soret (or B) band, which appeared in the wavelength range 360–490 nm, and four additional weaker absorptions termed Q bands in the range 500–720 nm. It is also noted that the N and M bands appear at 283 and 210 nm in the UV region. It is shown that there is a decrease in the absorption peaks intensity with increasing irradiation dose. The absorption peaks may be generally interpreted in terms of p–p* transitions between bonding and anti-bonding molecular orbitals [14]. The two interband transitions Q and B are assigned as p–p* type, from the two highest occupied molecular orbitals (HOMO) levels 1a1u(p) and 4a2u(p) to the first excited lowest unoccupied molecular orbital (LUMO) level 5eg(p*). The transition bands in the UV region are assigned as the N band transition from HOMO level 3a2u(p) to LUMO level 5eg(p*) whereas the M band transition is from HOMO level 1b1u(p) to LUMO level 5eg(p*) [15]. The interaction between two or more molecules in the unit cell of the aggregate results in two or more excitonic transitions with high transition moment and the original absorption band splits into two or more by Davydov splitting [16]. The Soret band has two peaks, Bx and By , at 395 and 443 nm; this splitting depends on the distance between the molecules, the angle of the transition dipole moments with the aggregate, the angle of their transition dipole moments between neighboring molecules and the number of interacting molecules [16,17]. The two absorption transitions, Bx and By , due to splitting in the Soret band, have high intensity because the Soret band has parallel electric dipoles [14]. It is also noted that the band B appears with intensities that are considerably higher than those of Q, N, and M bands; this is because absorption transitions in the Q-band region have smaller oscillator strengths due to the opposite direction of the electric dipoles and the cancellation of electric dipoles that occurs leading to low intensity for these bands. Fig. 5 demonstrates the influence of irradiation process on the absorbency spectrum of TPP films. It is obvious that the absorbance of films decreases with increasing radiation dose until it reaches the value 2.5 kGy, and the B, N, and M bands disappear from the spectrum. It is also shown that the absorption spectrum was shifted slightly to shorter wavelength with increasing radiation dose from 0.5 to 2.5 kGy. In order to find out which parts of the absorption spectra were most affected by X-ray irradiation, an absorbance loss, DA, as a function of wavelength has been presented by subtracting ‘‘irradiated’’ spectra from the initial ‘‘unirradiated’’ samples as shown in Fig. 6. The irradiation process caused a decrease in the absorbance in the whole range of the spectrum; the maximum absorbance loss occurred in the B band region between 320 and 470 nm. The reduction in the optical absorption density, DOD, at different peaks of transition bands as a function of
3 2 1 0 200
300
400
500
600
700
λ, nm Fig. 6. Reduction in optical absorption spectrum, DA, of irradiated TPP thin films as a function of wavelength.
7 6
λ( = nm) 211 276 383 438 516 554 595 649
5 ∆OD
350
4 3 2
nm nm nm nm nm nm nm nm
1 0 0.0
0.5
1.0
1.5 2.0 Dose, KGy
2.5
3.0
3.5
Fig. 7. Reduction in optical absorption density, DOD, of irradiated TPP thin films as a function of irradiation dose.
irradiation doses shows that the absorbance decrease with increasing X-ray radiation as shown in Fig. 7. The absorption coefficient a could be calculated by using the measured values of R and T from the formula [18] " 1=2 # 1 ð1 RÞ2 ð1 RÞ4 2 a ¼ ln þ R , (2) d 2T 4T 2 where d is the film thickness for TPP. The types of transition and the value of optical energy gap can be demonstrated by Bardeen et al. [19] as ahn ¼ a0 ðhn E g Þr ,
(3)
where r ¼ 1=2 and 3/2 for direct allowed and forbidden transitions, respectively, r ¼ 2 and 3 for indirect allowed
ARTICLE IN PRESS M.M. El-Nahass et al. / Optics & Laser Technology 39 (2007) 347–352
0.5 kGy decreases the energy gap from 1.763 to 1.755 eV and is connected with the formation of dangling bonds [21]. Increasing the amount of irradiation dose steeply up to 2.5 kGy increases the energy gap from 1.755 to 1.801 eV and annihilates the dangling bond-type defect [21].
180 as-deposited 0.5 KGy
140 120
Eg , eV
(αhν)1/2, cm-1/5 .eV1/2
160
100 80
1.81 1.80 1.79 1.78 1.77 1.76 1.75
1.0 KGy 1.5 KGy 2.5 KGy
0.0
0.5
1.0 1.5 2.0 Dose, KGy
351
2.5
60 40
4. Conclusion
20 0 1.65
1.70
1.75
1.80
1.85
1.90
hν, eV Fig. 8. Relation between (ahn)1/2 and photon energy in the region of the absorption edge for as-prepared and irradiated TPP thin films.
Table 1 Optical energy gap and phonon energy for unirradiated and irradiated TPP films Dose (kGy) Eg (eV) Ephonon (meV)
0 1.764 38
0.5 1.755 44
1.0 1.774 36
1.5 1.787 36
2.5 1.806 37
and forbidden transitions, respectively, and a0 is the absorption background. The dependence of (ahn)1/r on photon energy (hn) for onset gap was discussed and plotted for different values of r. The best fit was obtained for r ¼ 2 as illustrated in Fig. 8; this is characteristic behavior of indirect allowed transitions. For indirect transitions, the wave vector k for the lowest energy state in the conduction band is different from that for the highest energy state in the valence band so that the electrons that transfer from the valence band to the conduction band will have a difference in momenta. In order to conserve momentum, one or more phonons are emitted or absorbed at the same time as the photon is absorbed. The relation between (ahn)1/2 and hn for as-deposited films and after irradiating is linear in the region of a strong absorption edge for fundamental and onset bands. The extrapolation of the straight line graphs ðahnÞ1=2 ¼ 0 will give the values of the optical band gaps as shown in Fig. 8. The values of indirect energy gap Eind and the phonon energies Eph for the films before g and after irradiation are listed in Table 1. The calculated energy gap for as-deposited film is 1.764 eV, and this value agrees with the previous work (1.8–1.9 eV) of Shaffer and Gouterman [20]. It is obvious that, there are two processes occurring in irradiated films depending on the amount of irradiation dose, and irradiation up to
The main conclusions can be summarized as follows. The analysis of XRD of TPP proved that the received powder material form polycrystalline patterns with triclinic structure and the as-deposited TPP thin films have amorphous structure. The absorption spectra of thermally evaporated TPP consist of four Q-bands in the region 500–720 nm, an extremely intense Soret band at about 360–490 nm, the Soret have two peaks at 395 and 443 nm and two other bands labeled N and M at shorter wavelength. X-ray irradiation affected the measured values of transmission, reflectance and absorption spectra. Absorption bands show a continuous shift in position and a decrease in intensity with increasing X-ray dose until at 2.5 kGy the B, N, and M bands disappeared. The reduction in the absorbency increases with increasing irradiation dose. The type of electronic transition responsible for optical properties is an indirect allowed transition. The onset energy gap of as-deposited film is 1.764 eV. This value increases as Xray dose increases as a result of increases in the disorder part.
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