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Effect of annealing on structural, electrical and optical properties of p-quaterphenyl thin films
Accepted Manuscript Effect of annealing on structural, electrical and optical properties of p-quaterphenyl thin films A.A.A. Darwish PII: DOI: Reference:
S1350-4495(16)30655-7 http://dx.doi.org/10.1016/j.infrared.2017.03.004 INFPHY 2253
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
20 November 2016 6 March 2017 7 March 2017
Please cite this article as: A.A.A. Darwish, Effect of annealing on structural, electrical and optical properties of pquaterphenyl thin films, Infrared Physics & Technology (2017), doi: http://dx.doi.org/10.1016/j.infrared. 2017.03.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of annealing on structural, electrical and optical properties of p-quaterphenyl thin films A.A.A. Darwish1,2 1) Department of Physics, Faculty of Education at Al-Mahweet, Sana’a University, Al-Mahweet, Yemen. 2) Department of Physics, Faculty of Science, Tabuk University, Tabuk, Saudi Arabia *Corresponding author: E-mail: [email protected], Tel: +966-535846573
Abstract Thin films of p-quaterphenyl are deposited by an evaporation technique. IR spectra confirm that the thermal evaporation method is a decent one to acquire p-quaterphenyl films without dissociation. The X-ray diffraction studies demonstrate that the as-deposited and annealed films are polycrystalline with monoclinic structure. The electrical conductivity shows an activated behavior and indicating that p-quaterphenyl behaves as an organic semiconductor. The value of activation energy decreases by annealing, which explains due to the adjustment in the crystallite size. Optical properties of p-quaterphenyl films were performed to determine some optical constants. Dispersion of the refractive index is described utilizing the Wemple-DiDomenico model. In addition, the third order nonlinear susceptibility and the nonlinear refractive index are calculated. The analysis of the absorption coefficient for the as-deposited film showed an allowed direct optical band gap with a value of 2.35 eV, which decreased by annealing to 2.05 eV.
Keywords: Organic compounds; thin films; optical properties; electrical properties.
1. Introduction Conjugated organic materials have ended up a promising contender for different applications in molecular electronics and optoelectronics [1-7]. Amongst different materials, oligomers have been effectively utilized as dynamic materials as a part of organic light- emitting diodes [8] or organic transistors [9]. The conjugated oligomers like p-quaterphenyl and p-sexiphenyl have an extensive potential as materials for organic transistors [9]. They can be utilized for organic light emitting diodes [10], organic lasers [11-13] and solar cells [11]. These oligomers might be of interest on account of their especially high thermal stability and chemical compatibility with the preparing required for incorporated organic devices [9]. 1
Thin films of p-quaterphenyl show a strong tendency to crystallize [9]. Thin films of crystalline organic semiconductors show promising chances for future improvements in electronic and optoelectronic devices. Crystalline organic semiconductors demonstrate an enormous anisotropy in charge transport and additionally in their optical properties [14]. Some work has been done for p-quaterphenyl films concerning the study of the conduction mechanism in p-quaterphenyl [15, 16]. Therefore, in the current work, our point is to examine the properties of p-quaterphenyl thin film identified with the electronic application. In this sense, it is a great importance to reveal new aspects of the annealing effects on the structural, electrical and the optical constants of p-quaterphenyl films. In addition, the third order nonlinear optical susceptibility of the present system is calculated.
2. Experimental details The p-quaterphenyl powder with a purity of 98%, whose molecular structure is schematically represented in Scheme 1, was supplied by Sigma-Aldrich Company. Films of p-quaterphenyl were manufactured using a thermal evaporation technique utilizing a coating unit (HHV Auto 306). The films were grown onto optically and ultrasonically cleaned flat quartz, glass and cleaved KBr single crystal substrates. The glass substrates were used for depositing films for the X-ray diffraction and the electrical measurements. The KBr substrates were used for depositing films for IR measurements and optically flat amorphous quartz substrates were used for depositing films for the optical measurements. The substrate was settled onto a rotatable holder to get homogeneous fabricated films at a separation of 25 cm over the evaporator. The p-quaterphenyl films were fabricated in a vacuum of 3.43×10-5 mbar and the substrates' temperature was preserved at room temperature. The quartz crystal monitor was utilized to control the rate of deposition at 2.3 nm/s and the thickness of the film, d, to be 300 nm. The film thickness was measured after evaporation by Tolansky's technique. Some specimens were thermally heated in air at 200 o
C for two hours in a furnace. Then, the temperature inside the furnace was left to decrease
gradually until achieving the surrounding temperature. The instrument of DT7 Perkin Elmer was utilizing to get differential thermal analysis for the powder of p-quaterphenyl. The heating rate is 20 oC/min. The chemical structure of as-deposited and annealed films was explored by utilizing Fourier-transform infrared (FTIR) in the 400-4000 cm-1 spectral range. The experimental error of the infrared
2
spectrophotometer is ±1 cm-1 during the examination. The structural properties of pquaterphenyl films were checked utilizing Philips diffractometer 1710 with Ni sifted CuKα source (λ = 0.154 nm). For the electrical measurements, the films were in the planar configuration and the Ohmic contacts were made by evaporating Au with high purity through masks on the films. The Ohmic contact was checked by I-V measurements at room temperature. A high impedance electrometer (Keithley, Model 610) measures the electrical resistance of pquaterphenyl films by the two-probe method. The measurements had been taken at various temperatures going from 293 to 423 K utilizing an electric heater and the temperature was measured by NiCr-NiAl thermocouple checked by a microvoltmeter. The double-beam spectrophotometer (JASCO, V-570 UV-VIS-NIR) was utilizing to record the transmittance (T) and reflectance (R) at room temperature. The optical constants (optical absorption coefficient, , and refractive index, n) at every wavelength were computed from the correction estimations of T and R utilizing a private computer program, which is described previously [17]. A computational method is used for estimating the experimental errors [18]. It is found that the experimental error for d estimation was ±2.5% and that for T and R computation was ±1%. Then, the experimental error for n was ±2.3% and for a was ±2.1.
3. Results and discussion 3.1. Structural properties Fig. 1 demonstrates consequences of the differential thermal analysis (DTA) of pquaterphenyl in powder form. The examination was performed in the temperature range 30500 oC and demonstrates that the p-quaterphenyl has a thermal stability up to roughly 230 o
C. In addition, the investigation indicates an endothermic reaction with one of a kind
downward peak illustrating to the melting point of the p-quaterphenyl compound at 308 oC. The outcomes, additionally, demonstrate that there is neither stage change nor crystallization happening in the temperature range 30-230 oC. FTIR technique is based on the absorption by interatomic bonds in organic compounds. Therefore, it can be utilized in the determination of organic structure. The intensity and frequency of the absorbed IR are strongly dependent on the type of the bond where each bond included by organic compound will absorb a specific frequency at different intensity. In this manner, IR spectroscopy includes gathering absorption data and 3
analyzing it in a spectrum form [19]. FTIR spectra of powder, as-deposited and annealed p-quaterphenyl films in the most important range 400-1500 cm-1 are illustrated in Fig. 2. This figure reveals that the FTIR spectrum of the as-deposited film does not change with evaporation showing that the thermal evaporation method is a decent one to gain undissociated and stoichiometric p-quaterphenyl films. Additionally, FTIR spectrum of the annealed film is exceptionally similar to that of the powder and the as-deposited film, which revealed that the p-quaterphenyl film has a thermal stability up to the utilized annealing temperature. Previously, the X-ray diffraction (XRD) pattern of p-quaterphenyl in the powder structure is studied [20, 21]. It is found that the powder of p-quaterphenyl has monoclinic structure. The unit cell parameters and the estimations of Miller indices hkl and lattice spacing dhkl corresponding to every diffraction were calculated [20]. Fig. 3 shows the results of XRD of as-deposited and annealed films p-quaterphenyl. Unmistakably, the thermal evaporation of p-quaterphenyl prompted polycrystalline films. After annealing at 200 oC, the intensity of the peaks increases and other peaks appear. This demonstrates that the annealing increases the crystallinity of the films. The crystal size, L, might be evaluated from the half width estimation of the XRD peak, utilizing the Scherrer's equation [22, 23]. L
Ks cos
(1)
where Ks is the Scherrer’s constant which has a value of 0.95 [22, 23], is the X-ray wavelength ( = 0.15406 nm), is the width of a strong peak in radians at half maximum intensity, and is the relating Bragg angle. The crystal size of the as-deposited and annealed p-quaterphenyl films was ascertained and listed in Table 1. Unmistakably, the crystal size increments by annealing, which demonstrates that the annealing of pquaterphenyl films results in noteworthy changes in the microstructure of these films.
3.3. Electrical properties The Arrhenius equation describes the variation of the conductivity, σ, of semiconductors varies with temperature as [22-24]
4
(2) where σo is the pre-conductivity, ΔE is the activation energy and kB is the Boltzmann’s constant. The variation of electrical conductivity with the temperature of as-deposited and annealed p-quaterphenyl films is illustrated in Fig. 4. From this figure, one can watch that σ increments with temperature demonstrating that p-quaterphenyl behaves as an organic semiconductor. Also, Fig. 4 demonstrates that the conductivity increments with annealing. Table 1 summarizes the variation σo and E with annealing. The values of E are just about the half of value of the energy gap of the p-quaterphenyl films as will describe in the following section. The increasing in conductivity and decreasing in the activation energy with annealing may be resulting in the adjustment of the crystallite size.
3.3. Optical properties The investigation of the optical constants of a material gives an easy technique to clarifying some features concerning the band structure of the materials. In the spectral range of 300 to 2500 nm, T and R spectra of p-quaterphenyl films were measured. The spectral behaviors of T and R of p-quaterphenyl films before and after annealing are illustrated in Fig. 5. It seems that, at a higher wavelength ( > 800 nm), p-quaterphenyl films get to be pellucid and no light is scattered or absorbed. While the region at shorter wavelengths (< 800 nm) known as the absorbing region. This figure demonstrates an optical transmittance edges, which shifts towards higher wavelength for the annealed film. This suggests the decrease of the optical energy band gap with annealing. The spectral behavior of the dispersion curves of p-quaterphenyl films before and annealing are outlined in Fig. 6. It is watched that the value of n has a maximum peak around λ ~ 850 nm. The decrease of n after λ ~ 850 nm demonstrates the normal dispersion behavior of this system. On the other side, n increased by annealing because of the changes in the structure of the films. Wemple-DiDomenico (WDD) dispersion relationship [25] is used to investigate both the single oscillator energy, Eo, and the single-oscillator strength, Ed, of as-deposited and annealed p-quaterphenyl films. The values of n are fitted using the following equation [25]. E E n 2 1 2 o d 2 Eo E
(3)
5
The plotting (n2 - 1)
-1
versus E2 and fitting the lines of as-deposited and annealed p-
quaterphenyl films is illustrated in Fig. 7. The values of Eo and Ed are calculated from the intercept Eo/Ed and from the slope 1/(EoEd) respectively. The values of both Ed and Eo are listed in Table 2. The extrapolating of straight lines to the points of interception at E2 = 0 gives the value of n2∞ = ∞, which is computed and given also in Table 2. From this table, we can notice that Eo, Ed and ∞ increase by annealing because of the growth of the crystallinity. Nonlinear optics clarifies the nonlinear reaction of properties, for example, frequency, polarization, stage or track of incident light. These nonlinear interactions offer rise to a great number of optical phenomena. Third-order nonlinear optical susceptibility, χ(3), is the essential premise of various applications, for example, capacity communication systems [26]. The χ(3) associated with interband transitions and a numerical calculation for the χ(3) of as-deposited and annealed p-quaterphenyl films. The nonlinear susceptibility can be given according to Miller's rules using the following expression [27]: (4) where A is a constant equal to 1.7×10-10 esu and the χ(3) in the limit hν-0 (n = no). Tichy et al [27] have consolidated the Miller principles and static refractive index, which is predestined by WDD model. According to this model, the following relation was used for calculation of the nonlinear refractive index (n2) [27]: (5) The values of χ(3) and n2 of as-deposited and annealed p-quaterphenyl films are recorded in Table 2. The obtained values of (3) and n2 for the p-quaterphenyl film are in the range of several semiconductor organic molecules reported in many previous works [17, 28-32]. Plainly, the calculated values of χ(3) are increasing by annealing. The rise in third request nonlinear properties for the considered specimens might be credited to rising in nonlinear refractive index [33]. The results indicate that the annealing will contribute to the enhancement of the third-order optical nonlinearities due to the increase of the crystallinity. These are good candidates for all-optical switching applications because until now molecules with high χ(3) have had a high loss due to two-photon absorption [33]. The optical absorption at the fundamental edge is analyzed using the theory of band to band transition [25, 31-36]. The absorption spectrum follows the following equation [37] 6
(6) where H is a constant and r is an index which could be values of 1/2, 2, 2/3 or 1/3 for allowed indirect, allowed direct, forbidden indirect and forbidden direct transitions respectively relying upon the nature of electronic transitions. The best fit of the equation (6) indicates that r = 2 related to directly allowed transitions. Fig. 8 shows the relation between (αhν)2 versus hν of as-deposited and annealed pquaterphenyl films. As shown in this figure, the intercept of the straight line with the hν axis at (αhν)2 = 0 yields the direct optical band gap. Table 2 listed the values of the Eg which found to be 2.35 to 2.05 eV for as-deposited and annealed films respectively. Our results for the band gaps are in agreement with the theoretical data (2.20 eV) [38] and the experimental data (2.87 eV) [39] have already been published. Plainly, the optical band gap is decreasing by the annealing may be due to the adjustment in film density and the growing in grain size [40].
4. Conclusions The thermal evaporation method manufactured polycrystalline films of pquaterphenyl. It is found that the annealing upgrades the crystal size. The conductivity shows an activated behavior and indicating that p-quaterphenyl behaves as an organic semiconductor. The optical properties were examined utilizing a spectrophotometric estimation of transmittance and reflectance at normal incidence of light for p-quaterphenyl films. The investigation of the spectral behavior of the absorption coefficient was performed and uncovered direct transitions with an estimation of energy gap equal to 2.35 eV. These investigations suggest that the present films are a worthy force for optoelectronic applications as absorbing materials. The dispersion parameters Ed and Eo are estimated in terms of the WDD model. The nonlinear optical investigations show that the behavior of third-order nonlinear optical susceptibility χ(3) is increasing by annealing. These are good candidates for all-optical switching applications.
Acknowledgement The authors are grateful to Prof. K.F. Abd-El-Rahman, Ain Shams University, for his help and enlightening discussion throughout this work.
7
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[25]M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish, Int. J. Mod. Phys. B 18 (2004) 421 [26] D. Cotter, R.J. Manning, K.J. Blow, A.D. Ellis, A.E. Kelly, D. Nesset, I.D. Phillips, A.J. Poustie, D.C. Rogers, Science 286 (1999) 1528. [27] L. Tichý, H. Tichá, P. Nagels, R. Callaerts, R. Mertens, M. Vlček, Mater. Lett. 39 (1999) 122 [28] M.M. El-Nahass, H.S. Soliman, B.A. Khalifa, I.M. Soliman, Materials Science in Semiconductor Processing 38 (2015) 177 [29] J.-H. Ha, H.J. Kang, J. Anal. Sci. Technol. 2 (2011) 42 [30] M.M. El-Nahass, A.M. Hassanien, N.M. Khusayfan, Solid State Communications 154 (2013) 51 [31] S.R. Alharbi, A.A.A. Darwish, S.E. Al Garni, H.I. ElSaeedy, K.F. Abd El-Rahman, Infrared Physics and Technology 78 (2016) 77 [32] E.M. El-Menyawy, A.A.A. Darwish, I.T. Zedan , Optics and Laser Technology 79 (2016) 158163 [33] B. Shanmugavelu, V.V.R.K. Kumar, R. Kuladeep, D.N. Rao, J. Appl. Phys. 114 (2013) 243103 [34] S. Ambily, C.S. Menon, Thin Solid Films, 347 (1999) 284 [35] B. Bialek, I.G. Kim, J.I. Lee, Synthetic Metals, 129 (2002) 151 [36] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.A. Darwish, Eur. Phys. J. AP 48 (2009) 20402 [37] P.O. Edward, “Hand Book of Optical Constants of Solids”, Academic Press, New York 1985, p.265. [38] P. Puschnig, C.A. Draxl, Phys. Rev. B 60 (1999) 7891 [39] A.A. Attia, M.M. Saadeldin, H.S. Soliman, A.-S. Gadallah, K. Sawaby, Optical Materials 62 (2016) 711 [40] G.D. Kulkarni, A.V. Murugan, A.K. Viswanath, S.C. Gopinath, J. Nanosci. Nanotech. 9 (2009) 371
9
Tables Table 1: Crystal size and electrical parameters of p-quaterphenyl films
as-deposited annealed
L (nm) 63±5 90±3
o (×109-1cm-1)
E (eV)
96.09±0.05 6.80±0.05
1.14±0.03 0.99±0.02
Table 2: Some optical parameters of p-quaterphenyl films
as-deposited annealed
Eo (eV) Ed (eV) 3.77±0.05 9.56±0.05 3.88±0.04 14.21±0.04
∞
χ(3) ×10-13 esu 3.53±0.03 2.62±0.05 4.66±0.02 11.66±0.05
10
n2×10-12 esu 5.29±0.05 20.46±0.05
Eg (eV) 2.35±0.04 2.05±0.05
Figures
Scheme 1: The molecular structure of p-quaterphenyl.
20 15
(uV)
10 5
DTA
0 -5 -10 -15 -20
o
308 C 0
100
200
300
400 o
Temperature ( C) Fig. 1: DTA spectra of p-quaterphenyl powder.
11
500
100 90 80 70
Taransimittance (%)
60
annealed film
100 50 90 80 70 60
as-deposited film
50 100
80
60 4000
Powder 3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Fig. 2: FTIR spectra of p-quaterphenyl in powder and films forms
12
500
(002) (004)
Intensity ( a. u. )
(003) (110)
(201)
(211)
annealed
as-deposited
5
10
15
20
2
25
30
o
Fig. 3: XRD pattern of as-deposited and annealed p-quaterphenyl films
13
35
-8
as-deposited annealed
-1
ln [ ( . cm) ]
-12
-16
-20
-24
-28 2.0
2.2
2.4
2.6
2.8
1000 / T ( K
3.0 -1
3.2
3.4
3.6
)
Fig. 4: Variation of lnσ versus 1000/T of as-deposited and annealed p-quaterphenyl films
14
T , R
1.0
0.8
T
0.6
as-deposited annealed
0.4
0.2
0.0
R
0
500
1000
1500
2000
2500
(nm)
Fig. 5: The transmittance (T) and reflectance (R) of as-deposited and annealed pquaterphenyl films
15
3.0
Refractive index, n
2.8
as-deposited annealed
2.6 2.4 2.2 2.0 1.8 1.6
0
500
1000
1500
2000
2500
(nm)
Fig. 6: Refractive index, n versus wavelength, of as-deposited and annealed p-quaterphenyl films
16
0.40 0.38 0.36
2
(n -1)
-1
0.34 0.32
as-deposited annealed
0.30 0.28 0.26 0.24 0.22 0.0
0.2
0.4
0.6
0.8
E
1.0 2
1.2
(eV)
1.4
1.6
1.8
2.0
2
Fig. 7: Plot of (n2-1)-1 versus E2 of as-deposited and annealed p-quaterphenyl films
17
5x10
9
7x10
9
6x10
9
5x10
9
4x10
9
3x10
9
2x10
9
1x10
9
as-deposited 9
( h )
2
(eV cm )
-1 2
4x10
annealed
3x10
9
2x10
9
1x10
9
0
1
2
3
h
(eV)
4
5
0
1
2
h
3
(eV)
Fig. 8: (αhν) 2 versus hν of as-deposited and annealed p-quaterphenyl films .
18
4
Research Highlights
Polycrystalline p-quaterphenyl films were deposited on quartz substrates by evaporation technique The electrical conductivity and optical constants of p-quaterphenyl films were affected by annealing temperature. The band gaps of the as-deposited film were found to decrease with the annealing temperature.
19
S1350-4495(16)30655-7 http://dx.doi.org/10.1016/j.infrared.2017.03.004 INFPHY 2253
To appear in:
Infrared Physics & Technology
Received Date: Revised Date: Accepted Date:
20 November 2016 6 March 2017 7 March 2017
Please cite this article as: A.A.A. Darwish, Effect of annealing on structural, electrical and optical properties of pquaterphenyl thin films, Infrared Physics & Technology (2017), doi: http://dx.doi.org/10.1016/j.infrared. 2017.03.004
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of annealing on structural, electrical and optical properties of p-quaterphenyl thin films A.A.A. Darwish1,2 1) Department of Physics, Faculty of Education at Al-Mahweet, Sana’a University, Al-Mahweet, Yemen. 2) Department of Physics, Faculty of Science, Tabuk University, Tabuk, Saudi Arabia *Corresponding author: E-mail: [email protected], Tel: +966-535846573
Abstract Thin films of p-quaterphenyl are deposited by an evaporation technique. IR spectra confirm that the thermal evaporation method is a decent one to acquire p-quaterphenyl films without dissociation. The X-ray diffraction studies demonstrate that the as-deposited and annealed films are polycrystalline with monoclinic structure. The electrical conductivity shows an activated behavior and indicating that p-quaterphenyl behaves as an organic semiconductor. The value of activation energy decreases by annealing, which explains due to the adjustment in the crystallite size. Optical properties of p-quaterphenyl films were performed to determine some optical constants. Dispersion of the refractive index is described utilizing the Wemple-DiDomenico model. In addition, the third order nonlinear susceptibility and the nonlinear refractive index are calculated. The analysis of the absorption coefficient for the as-deposited film showed an allowed direct optical band gap with a value of 2.35 eV, which decreased by annealing to 2.05 eV.
Keywords: Organic compounds; thin films; optical properties; electrical properties.
1. Introduction Conjugated organic materials have ended up a promising contender for different applications in molecular electronics and optoelectronics [1-7]. Amongst different materials, oligomers have been effectively utilized as dynamic materials as a part of organic light- emitting diodes [8] or organic transistors [9]. The conjugated oligomers like p-quaterphenyl and p-sexiphenyl have an extensive potential as materials for organic transistors [9]. They can be utilized for organic light emitting diodes [10], organic lasers [11-13] and solar cells [11]. These oligomers might be of interest on account of their especially high thermal stability and chemical compatibility with the preparing required for incorporated organic devices [9]. 1
Thin films of p-quaterphenyl show a strong tendency to crystallize [9]. Thin films of crystalline organic semiconductors show promising chances for future improvements in electronic and optoelectronic devices. Crystalline organic semiconductors demonstrate an enormous anisotropy in charge transport and additionally in their optical properties [14]. Some work has been done for p-quaterphenyl films concerning the study of the conduction mechanism in p-quaterphenyl [15, 16]. Therefore, in the current work, our point is to examine the properties of p-quaterphenyl thin film identified with the electronic application. In this sense, it is a great importance to reveal new aspects of the annealing effects on the structural, electrical and the optical constants of p-quaterphenyl films. In addition, the third order nonlinear optical susceptibility of the present system is calculated.
2. Experimental details The p-quaterphenyl powder with a purity of 98%, whose molecular structure is schematically represented in Scheme 1, was supplied by Sigma-Aldrich Company. Films of p-quaterphenyl were manufactured using a thermal evaporation technique utilizing a coating unit (HHV Auto 306). The films were grown onto optically and ultrasonically cleaned flat quartz, glass and cleaved KBr single crystal substrates. The glass substrates were used for depositing films for the X-ray diffraction and the electrical measurements. The KBr substrates were used for depositing films for IR measurements and optically flat amorphous quartz substrates were used for depositing films for the optical measurements. The substrate was settled onto a rotatable holder to get homogeneous fabricated films at a separation of 25 cm over the evaporator. The p-quaterphenyl films were fabricated in a vacuum of 3.43×10-5 mbar and the substrates' temperature was preserved at room temperature. The quartz crystal monitor was utilized to control the rate of deposition at 2.3 nm/s and the thickness of the film, d, to be 300 nm. The film thickness was measured after evaporation by Tolansky's technique. Some specimens were thermally heated in air at 200 o
C for two hours in a furnace. Then, the temperature inside the furnace was left to decrease
gradually until achieving the surrounding temperature. The instrument of DT7 Perkin Elmer was utilizing to get differential thermal analysis for the powder of p-quaterphenyl. The heating rate is 20 oC/min. The chemical structure of as-deposited and annealed films was explored by utilizing Fourier-transform infrared (FTIR) in the 400-4000 cm-1 spectral range. The experimental error of the infrared
2
spectrophotometer is ±1 cm-1 during the examination. The structural properties of pquaterphenyl films were checked utilizing Philips diffractometer 1710 with Ni sifted CuKα source (λ = 0.154 nm). For the electrical measurements, the films were in the planar configuration and the Ohmic contacts were made by evaporating Au with high purity through masks on the films. The Ohmic contact was checked by I-V measurements at room temperature. A high impedance electrometer (Keithley, Model 610) measures the electrical resistance of pquaterphenyl films by the two-probe method. The measurements had been taken at various temperatures going from 293 to 423 K utilizing an electric heater and the temperature was measured by NiCr-NiAl thermocouple checked by a microvoltmeter. The double-beam spectrophotometer (JASCO, V-570 UV-VIS-NIR) was utilizing to record the transmittance (T) and reflectance (R) at room temperature. The optical constants (optical absorption coefficient, , and refractive index, n) at every wavelength were computed from the correction estimations of T and R utilizing a private computer program, which is described previously [17]. A computational method is used for estimating the experimental errors [18]. It is found that the experimental error for d estimation was ±2.5% and that for T and R computation was ±1%. Then, the experimental error for n was ±2.3% and for a was ±2.1.
3. Results and discussion 3.1. Structural properties Fig. 1 demonstrates consequences of the differential thermal analysis (DTA) of pquaterphenyl in powder form. The examination was performed in the temperature range 30500 oC and demonstrates that the p-quaterphenyl has a thermal stability up to roughly 230 o
C. In addition, the investigation indicates an endothermic reaction with one of a kind
downward peak illustrating to the melting point of the p-quaterphenyl compound at 308 oC. The outcomes, additionally, demonstrate that there is neither stage change nor crystallization happening in the temperature range 30-230 oC. FTIR technique is based on the absorption by interatomic bonds in organic compounds. Therefore, it can be utilized in the determination of organic structure. The intensity and frequency of the absorbed IR are strongly dependent on the type of the bond where each bond included by organic compound will absorb a specific frequency at different intensity. In this manner, IR spectroscopy includes gathering absorption data and 3
analyzing it in a spectrum form [19]. FTIR spectra of powder, as-deposited and annealed p-quaterphenyl films in the most important range 400-1500 cm-1 are illustrated in Fig. 2. This figure reveals that the FTIR spectrum of the as-deposited film does not change with evaporation showing that the thermal evaporation method is a decent one to gain undissociated and stoichiometric p-quaterphenyl films. Additionally, FTIR spectrum of the annealed film is exceptionally similar to that of the powder and the as-deposited film, which revealed that the p-quaterphenyl film has a thermal stability up to the utilized annealing temperature. Previously, the X-ray diffraction (XRD) pattern of p-quaterphenyl in the powder structure is studied [20, 21]. It is found that the powder of p-quaterphenyl has monoclinic structure. The unit cell parameters and the estimations of Miller indices hkl and lattice spacing dhkl corresponding to every diffraction were calculated [20]. Fig. 3 shows the results of XRD of as-deposited and annealed films p-quaterphenyl. Unmistakably, the thermal evaporation of p-quaterphenyl prompted polycrystalline films. After annealing at 200 oC, the intensity of the peaks increases and other peaks appear. This demonstrates that the annealing increases the crystallinity of the films. The crystal size, L, might be evaluated from the half width estimation of the XRD peak, utilizing the Scherrer's equation [22, 23]. L
Ks cos
(1)
where Ks is the Scherrer’s constant which has a value of 0.95 [22, 23], is the X-ray wavelength ( = 0.15406 nm), is the width of a strong peak in radians at half maximum intensity, and is the relating Bragg angle. The crystal size of the as-deposited and annealed p-quaterphenyl films was ascertained and listed in Table 1. Unmistakably, the crystal size increments by annealing, which demonstrates that the annealing of pquaterphenyl films results in noteworthy changes in the microstructure of these films.
3.3. Electrical properties The Arrhenius equation describes the variation of the conductivity, σ, of semiconductors varies with temperature as [22-24]
4
(2) where σo is the pre-conductivity, ΔE is the activation energy and kB is the Boltzmann’s constant. The variation of electrical conductivity with the temperature of as-deposited and annealed p-quaterphenyl films is illustrated in Fig. 4. From this figure, one can watch that σ increments with temperature demonstrating that p-quaterphenyl behaves as an organic semiconductor. Also, Fig. 4 demonstrates that the conductivity increments with annealing. Table 1 summarizes the variation σo and E with annealing. The values of E are just about the half of value of the energy gap of the p-quaterphenyl films as will describe in the following section. The increasing in conductivity and decreasing in the activation energy with annealing may be resulting in the adjustment of the crystallite size.
3.3. Optical properties The investigation of the optical constants of a material gives an easy technique to clarifying some features concerning the band structure of the materials. In the spectral range of 300 to 2500 nm, T and R spectra of p-quaterphenyl films were measured. The spectral behaviors of T and R of p-quaterphenyl films before and after annealing are illustrated in Fig. 5. It seems that, at a higher wavelength ( > 800 nm), p-quaterphenyl films get to be pellucid and no light is scattered or absorbed. While the region at shorter wavelengths (< 800 nm) known as the absorbing region. This figure demonstrates an optical transmittance edges, which shifts towards higher wavelength for the annealed film. This suggests the decrease of the optical energy band gap with annealing. The spectral behavior of the dispersion curves of p-quaterphenyl films before and annealing are outlined in Fig. 6. It is watched that the value of n has a maximum peak around λ ~ 850 nm. The decrease of n after λ ~ 850 nm demonstrates the normal dispersion behavior of this system. On the other side, n increased by annealing because of the changes in the structure of the films. Wemple-DiDomenico (WDD) dispersion relationship [25] is used to investigate both the single oscillator energy, Eo, and the single-oscillator strength, Ed, of as-deposited and annealed p-quaterphenyl films. The values of n are fitted using the following equation [25]. E E n 2 1 2 o d 2 Eo E
(3)
5
The plotting (n2 - 1)
-1
versus E2 and fitting the lines of as-deposited and annealed p-
quaterphenyl films is illustrated in Fig. 7. The values of Eo and Ed are calculated from the intercept Eo/Ed and from the slope 1/(EoEd) respectively. The values of both Ed and Eo are listed in Table 2. The extrapolating of straight lines to the points of interception at E2 = 0 gives the value of n2∞ = ∞, which is computed and given also in Table 2. From this table, we can notice that Eo, Ed and ∞ increase by annealing because of the growth of the crystallinity. Nonlinear optics clarifies the nonlinear reaction of properties, for example, frequency, polarization, stage or track of incident light. These nonlinear interactions offer rise to a great number of optical phenomena. Third-order nonlinear optical susceptibility, χ(3), is the essential premise of various applications, for example, capacity communication systems [26]. The χ(3) associated with interband transitions and a numerical calculation for the χ(3) of as-deposited and annealed p-quaterphenyl films. The nonlinear susceptibility can be given according to Miller's rules using the following expression [27]: (4) where A is a constant equal to 1.7×10-10 esu and the χ(3) in the limit hν-0 (n = no). Tichy et al [27] have consolidated the Miller principles and static refractive index, which is predestined by WDD model. According to this model, the following relation was used for calculation of the nonlinear refractive index (n2) [27]: (5) The values of χ(3) and n2 of as-deposited and annealed p-quaterphenyl films are recorded in Table 2. The obtained values of (3) and n2 for the p-quaterphenyl film are in the range of several semiconductor organic molecules reported in many previous works [17, 28-32]. Plainly, the calculated values of χ(3) are increasing by annealing. The rise in third request nonlinear properties for the considered specimens might be credited to rising in nonlinear refractive index [33]. The results indicate that the annealing will contribute to the enhancement of the third-order optical nonlinearities due to the increase of the crystallinity. These are good candidates for all-optical switching applications because until now molecules with high χ(3) have had a high loss due to two-photon absorption [33]. The optical absorption at the fundamental edge is analyzed using the theory of band to band transition [25, 31-36]. The absorption spectrum follows the following equation [37] 6
(6) where H is a constant and r is an index which could be values of 1/2, 2, 2/3 or 1/3 for allowed indirect, allowed direct, forbidden indirect and forbidden direct transitions respectively relying upon the nature of electronic transitions. The best fit of the equation (6) indicates that r = 2 related to directly allowed transitions. Fig. 8 shows the relation between (αhν)2 versus hν of as-deposited and annealed pquaterphenyl films. As shown in this figure, the intercept of the straight line with the hν axis at (αhν)2 = 0 yields the direct optical band gap. Table 2 listed the values of the Eg which found to be 2.35 to 2.05 eV for as-deposited and annealed films respectively. Our results for the band gaps are in agreement with the theoretical data (2.20 eV) [38] and the experimental data (2.87 eV) [39] have already been published. Plainly, the optical band gap is decreasing by the annealing may be due to the adjustment in film density and the growing in grain size [40].
4. Conclusions The thermal evaporation method manufactured polycrystalline films of pquaterphenyl. It is found that the annealing upgrades the crystal size. The conductivity shows an activated behavior and indicating that p-quaterphenyl behaves as an organic semiconductor. The optical properties were examined utilizing a spectrophotometric estimation of transmittance and reflectance at normal incidence of light for p-quaterphenyl films. The investigation of the spectral behavior of the absorption coefficient was performed and uncovered direct transitions with an estimation of energy gap equal to 2.35 eV. These investigations suggest that the present films are a worthy force for optoelectronic applications as absorbing materials. The dispersion parameters Ed and Eo are estimated in terms of the WDD model. The nonlinear optical investigations show that the behavior of third-order nonlinear optical susceptibility χ(3) is increasing by annealing. These are good candidates for all-optical switching applications.
Acknowledgement The authors are grateful to Prof. K.F. Abd-El-Rahman, Ain Shams University, for his help and enlightening discussion throughout this work.
7
References [1] A. Natalia Azarova, J.W. Owen, C.A. McLellan, M.A. Grimminger, E.K. Chapman, J.E. Anthony, O.D. Jurchescu, Org. Electron. 11 (2010) 1960. [2] S. Nenon, D. Kanehira, N. Yoshimoto, F. Fages, C. Videlot-Ackermann, Thin Solid Films 518 (2010) 5593. [3] D. Gupta, Y. Hong, Org. Electron. 11 (2010) 127. [4] S. Rasouli, S.J. Moeen, J. Alloys Compd. 509 (2011) 1915 [5] M.M. El-Nahass, K.F. Abd El-Rahman, A.A. M. Farag, A.A.A. Darwish, Organic Electronics 6 (2005) 129 [6] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish, Solar Energy Materials and Solar Cells 91 (2007) 1120 [7] S.E. Al Garni and A.A.A. Darwish, Solar Energy Materials and Solar Cells 160 (2017) 335 [8] M. Era, T. Tsutsui, S. Saito, Appl. Phys. Lett. 67 (1995) 2436 [9] D.J. Gundlach, Y.Y. Lin, T.N. Jackson, D.G. Schlom, Appl. Phys. Lett. 71 (1997) 3853 [10] C. Hosokawa, H. Higashi, T. Kusumoto, Appl. Phys. Lett. 62 (1993) 3238 [11] G. Hlawacek, C. Teichert, S. Müllegger, R. Resel, A. Winkler, Synthetic Metals 146 (2004) 383 [12] F. Quochi, J. Opt., 12 (2010) 024003 [13] J. Gierschner, S. Varghese, S. Y. Park, Adv. Opt. Mater. 4 (2016) 348 [14] N. Karl, Charge Carrier Mobility in Organic Crystals in Organic Electronic Materials, In: R. Farchioni, G. Grosse (Eds.) Springer Series in Materials Science, Berlin, 2001 [15] S. Kania, A. Lipinski, W. Mycielski, Thin Solid Films 72 (1980) L11 [16] S.W. Tkaczyk, J. Phys. D: Appl. Phys.41 (2008) 155411 [17] M.M. El-Nahass, F.S.H. Abu-Samaha, S. Menshawy, E. Elesh, Optics & Laser Technology 64 (2014) 28 [18] M.M. El-Nahass, H.S. Soliman, N.El. Kadry, A.Y. Morsy, S. Yaghmour, J. Mater. Sci. Lett. 7 (1988) 1050. [19] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish, Spectrochimica Acta Part A 69 (2008) 205 [20] A.A. Attia, H.S. Soliman, M.M. Saadeldin, K. Sawaby, Synthetic Metals 205 (2015) 139 [21] Y. Delugeard, J. Desuche and J. L. Baudour, Acta Cryst. B 32 (1976) 702 [22] M.M. El-Nahass, K.F. Abd El-Rahman, A.A.M. Farag, A.A.A. Darwish, Physica Scripta 73 (2006) 40 [23] M.M. El-Nahass, H.M. Zeyada, K.F. Abd-El Rahman, A.A.A. Darwish, Eur. Phys. J. AP 62 (2013) 10202 [24] A. Thakur, G.S.S. Saini, N. Goyal, S.K. Tripathi, J. Non-Crystal. Solids 353 (2007) 1326 8
[25]M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.M. Farag, A.A.A. Darwish, Int. J. Mod. Phys. B 18 (2004) 421 [26] D. Cotter, R.J. Manning, K.J. Blow, A.D. Ellis, A.E. Kelly, D. Nesset, I.D. Phillips, A.J. Poustie, D.C. Rogers, Science 286 (1999) 1528. [27] L. Tichý, H. Tichá, P. Nagels, R. Callaerts, R. Mertens, M. Vlček, Mater. Lett. 39 (1999) 122 [28] M.M. El-Nahass, H.S. Soliman, B.A. Khalifa, I.M. Soliman, Materials Science in Semiconductor Processing 38 (2015) 177 [29] J.-H. Ha, H.J. Kang, J. Anal. Sci. Technol. 2 (2011) 42 [30] M.M. El-Nahass, A.M. Hassanien, N.M. Khusayfan, Solid State Communications 154 (2013) 51 [31] S.R. Alharbi, A.A.A. Darwish, S.E. Al Garni, H.I. ElSaeedy, K.F. Abd El-Rahman, Infrared Physics and Technology 78 (2016) 77 [32] E.M. El-Menyawy, A.A.A. Darwish, I.T. Zedan , Optics and Laser Technology 79 (2016) 158163 [33] B. Shanmugavelu, V.V.R.K. Kumar, R. Kuladeep, D.N. Rao, J. Appl. Phys. 114 (2013) 243103 [34] S. Ambily, C.S. Menon, Thin Solid Films, 347 (1999) 284 [35] B. Bialek, I.G. Kim, J.I. Lee, Synthetic Metals, 129 (2002) 151 [36] M.M. El-Nahass, K.F. Abd-El-Rahman, A.A.A. Darwish, Eur. Phys. J. AP 48 (2009) 20402 [37] P.O. Edward, “Hand Book of Optical Constants of Solids”, Academic Press, New York 1985, p.265. [38] P. Puschnig, C.A. Draxl, Phys. Rev. B 60 (1999) 7891 [39] A.A. Attia, M.M. Saadeldin, H.S. Soliman, A.-S. Gadallah, K. Sawaby, Optical Materials 62 (2016) 711 [40] G.D. Kulkarni, A.V. Murugan, A.K. Viswanath, S.C. Gopinath, J. Nanosci. Nanotech. 9 (2009) 371
9
Tables Table 1: Crystal size and electrical parameters of p-quaterphenyl films
as-deposited annealed
L (nm) 63±5 90±3
o (×109-1cm-1)
E (eV)
96.09±0.05 6.80±0.05
1.14±0.03 0.99±0.02
Table 2: Some optical parameters of p-quaterphenyl films
as-deposited annealed
Eo (eV) Ed (eV) 3.77±0.05 9.56±0.05 3.88±0.04 14.21±0.04
∞
χ(3) ×10-13 esu 3.53±0.03 2.62±0.05 4.66±0.02 11.66±0.05
10
n2×10-12 esu 5.29±0.05 20.46±0.05
Eg (eV) 2.35±0.04 2.05±0.05
Figures
Scheme 1: The molecular structure of p-quaterphenyl.
20 15
(uV)
10 5
DTA
0 -5 -10 -15 -20
o
308 C 0
100
200
300
400 o
Temperature ( C) Fig. 1: DTA spectra of p-quaterphenyl powder.
11
500
100 90 80 70
Taransimittance (%)
60
annealed film
100 50 90 80 70 60
as-deposited film
50 100
80
60 4000
Powder 3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm )
Fig. 2: FTIR spectra of p-quaterphenyl in powder and films forms
12
500
(002) (004)
Intensity ( a. u. )
(003) (110)
(201)
(211)
annealed
as-deposited
5
10
15
20
2
25
30
o
Fig. 3: XRD pattern of as-deposited and annealed p-quaterphenyl films
13
35
-8
as-deposited annealed
-1
ln [ ( . cm) ]
-12
-16
-20
-24
-28 2.0
2.2
2.4
2.6
2.8
1000 / T ( K
3.0 -1
3.2
3.4
3.6
)
Fig. 4: Variation of lnσ versus 1000/T of as-deposited and annealed p-quaterphenyl films
14
T , R
1.0
0.8
T
0.6
as-deposited annealed
0.4
0.2
0.0
R
0
500
1000
1500
2000
2500
(nm)
Fig. 5: The transmittance (T) and reflectance (R) of as-deposited and annealed pquaterphenyl films
15
3.0
Refractive index, n
2.8
as-deposited annealed
2.6 2.4 2.2 2.0 1.8 1.6
0
500
1000
1500
2000
2500
(nm)
Fig. 6: Refractive index, n versus wavelength, of as-deposited and annealed p-quaterphenyl films
16
0.40 0.38 0.36
2
(n -1)
-1
0.34 0.32
as-deposited annealed
0.30 0.28 0.26 0.24 0.22 0.0
0.2
0.4
0.6
0.8
E
1.0 2
1.2
(eV)
1.4
1.6
1.8
2.0
2
Fig. 7: Plot of (n2-1)-1 versus E2 of as-deposited and annealed p-quaterphenyl films
17
5x10
9
7x10
9
6x10
9
5x10
9
4x10
9
3x10
9
2x10
9
1x10
9
as-deposited 9
( h )
2
(eV cm )
-1 2
4x10
annealed
3x10
9
2x10
9
1x10
9
0
1
2
3
h
(eV)
4
5
0
1
2
h
3
(eV)
Fig. 8: (αhν) 2 versus hν of as-deposited and annealed p-quaterphenyl films .
18
4
Research Highlights
Polycrystalline p-quaterphenyl films were deposited on quartz substrates by evaporation technique The electrical conductivity and optical constants of p-quaterphenyl films were affected by annealing temperature. The band gaps of the as-deposited film were found to decrease with the annealing temperature.
19