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Electrical conductivity and optical absorption studies on NiPc-substituted borate glass matrix
Materials Chemistry and Physics 80 (2003) 591–594
Electrical conductivity and optical absorption studies on NiPc-substituted borate glass matrix P.R. Binu a , C.M. Joseph a , K. Shreekrishnakumar b , C.S. Menon a,∗ b
a School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686560, India School of Technology and Applied Sciences, Mahatma Gandhi University, Pullarikunnu, Kottayam, Kerala, India
Received 16 February 2002; received in revised form 13 September 2002; accepted 13 September 2002
Abstract Electrical conductivity and optical absorption of nickel phthalocyanine (NiPc)-substituted borate glass is reported here for the first time. We have successfully prepared NiPc-substituted lithium borate glass at 850 ◦ C and some preliminary studies on this glass are reported. Activation energy and optical band gap were determined for the glass from the Arrhenius plot of conductivity and optical absorption spectra, respectively. © 2002 Published by Elsevier Science B.V. Keywords: Nickel phthalocyanine; Glasses; B2 O3 –Li2 O; Conductivity; Activation energy; Optical band gap
1. Introduction Glasses possess unique properties such as hardness, transparency at room temperature, sufficient strength and excellent corrosion resistance. Due to their potential technological applications in various fields, the study of the properties of glasses is significant. Continued effort for the development of new glassy materials and study of their properties is highly relevant. Glassy materials have acknowledged advantages like physical isotropy, the absence of grain boundaries, continuously variable composition and good workability over their crystalline counterparts. The ionic conductivity of alkali halo borate glass has drawn significant interest for a considerable period of time [1–3]. Upon the addition of a modifier such as Li2 O in the B2 O3 network, it has been found to modify the host structure. Here Li+ is chosen as the singly charged cation since it diffuses more easily being small, which can induce structural changes in the glass network by producing negatively charged sites to which the cations are loosely bound [4]. The fast ion-conducting lithium borate glasses have been studied by many researchers due to their potential use in electrochemical devices such as batteries and solid state display devices [5,6]. Besides, phthalocyanines are organic semiconductors having potential use in ∗ Corresponding author. Tel.: +91-481-2731043; fax: +91-481-2731009. E-mail address: [email protected] (C.S. Menon).
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 2 9 - 7
solid-state electronic devices [7–9]. Although the studies on NiPc are rather scarce, it is a suitable active material for the fabrication of highly sensitive gas sensors [10]. The change of optical properties in phthalocyanine-substituted borate glasses is directly related to the structural change, i.e. precipitation of the crystalline phase in the glassy matrix. Hence, phthalocyanine-substituted glasses are highly functional materials in the field of optoelectronics as they can be used as optical memory devices and nonlinear optical materials [11,12]. The present work gives the preparation method and some basic studies on a new series of glasses that can revolutionize the various technological applications.
2. Experimental NiPc-doped lithium borate glass used in the present study was prepared by conventional glass melting and quenching technique. Reagent grade H3 BO3 , Li2 CO3 and pure NiPc were used as the starting materials. 79.9H3 BO3 , 20Li2 CO3 and 0.1NiPc (all in mol%) were mixed in an agate mortar for 1 h and were melted in a porcelain crucible at 850 ◦ C. It was kept at that temperature for 30 min and quenched to room temperature in a steel mould to get a transparent glass (area = 100 mm2 , thickness = 1.5 mm). The glass so formed was annealed at 350 ◦ C for 2 h before their properties were studied. Composition higher than about 30 mol%
592
P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
Fig. 1. Log σ vs. 1000/T graph for a NiPc-substituted lithium borate glass.
alkali oxide causes a disruption of the borate network. It is also found that increasing the NiPc-mol% has not much effect since most of the NiPc vaporizes during the glass preparation process contaminating the furnace used. We attempted only to optimize the melting point of this new series of glasses after several trials. Besides, control experiment with no NiPc added, is already reported by several researchers.
3. Results and discussion 3.1. DC conduction Electrical conductivity measurements were done to determine the activation energy Ea . The resistivity was measured from room temperature up to 220 ◦ C in a vacuum of 10−3 Torr with silver epoxy as contact for the glass.
Fig. 2. Absorbance (Abs) vs. wavelength (λ) for the NiPc-substituted lithium borate glass.
P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
593
Fig. 3. (αhν)2 vs. hν plot.
Electrical measurements were done using a Keithley programmable electrometer (model 617). The Arrhenius plot of conductivity yields different levels for conduction mechanisms. More than one slope in the conductivity graph indicates the presence of different levels in the sample. Conductivity, σ , is related to the temperature, T, according to the relation: −Ea σ = σ0 exp kB T where σ 0 is the pre-exponential factor, Ea is the thermal activation energy and kB is the Boltzmann constant. Arrhenius plot of conductivity of NiPc-substituted lithium borate glass is shown in Fig. 1. It yielded two linear regions that
gave E1 and E2 , the activation energies. E1 arises from holes as the charge carriers and E2 from impurity scattering. We have obtained a value of 0.46 eV for E1 in the intrinsic region and 0.21 eV for E2 in the impurity scattering region. The activation energy values E1 and E2 are the same as that of an air-annealed NiPc film on glass substrate [13]. The activation energy corresponding to the intrinsic generation is associated with the resonant energy involved in a short-lived excited state and that corresponding to the impurity scattering is attributed to a short-lived charge-transfer between impurity and the complex [14]. The conductivity and activation energy values obtained here are very less compared to those of unsubstituted lithium borate glass [15].
Fig. 4. (αhν)1/2 vs. hν plot.
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P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
3.2. Optical studies The optical absorption spectrum in the range 190–900 nm was taken in a Shimadzu 240 UV-Vis spectrophotometer. The UV-Vis spectrum of phthalocyanines originates from molecular orbitals within the aromatic 18 electron system and from overlapping orbitals within the central atom [16]. The direct electronic transitions from to ∗ orbitals in the 300–450 nm results in an intense band called the Soret band, which gives the absorption edge in phthalocyanines. The absorption edge can be analyzed using one electron theory of Bardeen et al. [17]. For photon energies hν just above the fundamental edge, the absorption, α, follows the standard relation, α = (hν − Eg )1/2
A hν
where A is a constant and Eg is defined as the energy gap between the valence band and the conduction band. A plot of absorbance (Abs) versus wavelength is shown in Fig. 2. A graph of (αhν)2 versus hν is shown in Fig. 3. Extrapolation of this plot to α 2 = 0 gives the optical band gap Eg for direct allowed transition. The value of Eg is 3.65 eV. The band gap Eg obtained for the as-deposited NiPc thin film is 3.2 eV that is comparable with the present value for the NiPc-substituted borate glass. Any crystal phase change would affect the gap between the conduction band and valence bands in phthalocyanines, because the orbital overlap between parallel pairs of molecules will be affected [18]. The value of Eg for indirect transition is obtained as 2.24 eV by extrapolating the plot of (αhν)1/2 versus hν to α 2 = 0 as shown in Fig. 4. This behaviour of indirect transition is shown in noncrystalline materials [19].
4. Conclusion NiPc-substituted lithium borate glass was prepared using the melt quenching method. The melting point of the
glass was optimized to a temperature of 850 ◦ C. Activation energies E1 and E2 are calculated as 0.46 and 0.21 eV, respectively, for the glass. Band gap Eg was determined from the optical absorption spectrum as 3.65 eV for direct allowed transition and 2.24 eV for indirect allowed transition. A detailed study on this new series of glasses is underway. References [1] H.L. Tuller, D.P. Button, D.R. Uhlmann, J. Non-Cryst. Solids 40 (1980) 93. [2] A. Levasseur, J.C. Brethous, J.M. Reau, M. Couzi, P. Hagenmuller, Solid State Ionics 1 (1980) 177. [3] M. Balkanski, R.F. Wallis, J. Deppe, M. Massot, Mater. Sci. Eng. B 12 (1992) 281. [4] C.A. Angell, Solid State Ionics 18/19 (1986) 72. [5] A. Levasseur, J.C. Brethous, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 921. [6] A.R. Kulkarni, H.S. Maits, A. Paul, Bull. Mater. Sci. 6 (1984) 201. [7] S.Q. Zhou, X.F. Jin, W.P. Hu, Y.Q. Liu, S.H. Liu, Solid State Commun. 112 (1999) 269. [8] Z.N. Bao, J.A. Rogers, H.E. Katz, J. Mater. Chem. 9 (1999) 1895. [9] N.R. Armstrong, J. Porphyrins Phthalocyanines 4 (2000) 414. [10] K.F. Schoch Jr., T.A. Temoidnte, Thin Solid Films 165 (1988) 83. [11] G. De la torre, P. Vazquez, F. Agullo-lopez, T. Torres, J. Mater. Chem. 8 (1998) 1671. [12] T. Nishida, Y. Takashima, Nucl. Instrum. Methods Phys. Res. Sect. B 76 (1993) 397–402. [13] K.N. Narayanan Unni, C.S. Menon, Mater. Lett. 45 (2000) 326–330. [14] T.G. Abdel-Malik, G.A. Cox, J. Phys. C: Solid State Phys. 10 (1997) 63. [15] R.J. Elliott, L. Perondi, R.A. Barrio, J. Non-Cryst. Solids 168 (1994) 167–178. [16] E.A. Ough, J.M. Stillman, Can. J. Chem. 11 (1993) 1891. [17] J. Bardeen, F.J. Slatt, L.J. Hall, Photoconductivity Conference, Wiley, New York, 1956. [18] S.E. Harrison, J.M. Assour, J. Chem. Phys. 40 (1964) 365. [19] L. Edwards, M. Gouterman, J. Mol. Spectrosc. 33 (1970) 299.
Electrical conductivity and optical absorption studies on NiPc-substituted borate glass matrix P.R. Binu a , C.M. Joseph a , K. Shreekrishnakumar b , C.S. Menon a,∗ b
a School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686560, India School of Technology and Applied Sciences, Mahatma Gandhi University, Pullarikunnu, Kottayam, Kerala, India
Received 16 February 2002; received in revised form 13 September 2002; accepted 13 September 2002
Abstract Electrical conductivity and optical absorption of nickel phthalocyanine (NiPc)-substituted borate glass is reported here for the first time. We have successfully prepared NiPc-substituted lithium borate glass at 850 ◦ C and some preliminary studies on this glass are reported. Activation energy and optical band gap were determined for the glass from the Arrhenius plot of conductivity and optical absorption spectra, respectively. © 2002 Published by Elsevier Science B.V. Keywords: Nickel phthalocyanine; Glasses; B2 O3 –Li2 O; Conductivity; Activation energy; Optical band gap
1. Introduction Glasses possess unique properties such as hardness, transparency at room temperature, sufficient strength and excellent corrosion resistance. Due to their potential technological applications in various fields, the study of the properties of glasses is significant. Continued effort for the development of new glassy materials and study of their properties is highly relevant. Glassy materials have acknowledged advantages like physical isotropy, the absence of grain boundaries, continuously variable composition and good workability over their crystalline counterparts. The ionic conductivity of alkali halo borate glass has drawn significant interest for a considerable period of time [1–3]. Upon the addition of a modifier such as Li2 O in the B2 O3 network, it has been found to modify the host structure. Here Li+ is chosen as the singly charged cation since it diffuses more easily being small, which can induce structural changes in the glass network by producing negatively charged sites to which the cations are loosely bound [4]. The fast ion-conducting lithium borate glasses have been studied by many researchers due to their potential use in electrochemical devices such as batteries and solid state display devices [5,6]. Besides, phthalocyanines are organic semiconductors having potential use in ∗ Corresponding author. Tel.: +91-481-2731043; fax: +91-481-2731009. E-mail address: [email protected] (C.S. Menon).
0254-0584/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 2 9 - 7
solid-state electronic devices [7–9]. Although the studies on NiPc are rather scarce, it is a suitable active material for the fabrication of highly sensitive gas sensors [10]. The change of optical properties in phthalocyanine-substituted borate glasses is directly related to the structural change, i.e. precipitation of the crystalline phase in the glassy matrix. Hence, phthalocyanine-substituted glasses are highly functional materials in the field of optoelectronics as they can be used as optical memory devices and nonlinear optical materials [11,12]. The present work gives the preparation method and some basic studies on a new series of glasses that can revolutionize the various technological applications.
2. Experimental NiPc-doped lithium borate glass used in the present study was prepared by conventional glass melting and quenching technique. Reagent grade H3 BO3 , Li2 CO3 and pure NiPc were used as the starting materials. 79.9H3 BO3 , 20Li2 CO3 and 0.1NiPc (all in mol%) were mixed in an agate mortar for 1 h and were melted in a porcelain crucible at 850 ◦ C. It was kept at that temperature for 30 min and quenched to room temperature in a steel mould to get a transparent glass (area = 100 mm2 , thickness = 1.5 mm). The glass so formed was annealed at 350 ◦ C for 2 h before their properties were studied. Composition higher than about 30 mol%
592
P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
Fig. 1. Log σ vs. 1000/T graph for a NiPc-substituted lithium borate glass.
alkali oxide causes a disruption of the borate network. It is also found that increasing the NiPc-mol% has not much effect since most of the NiPc vaporizes during the glass preparation process contaminating the furnace used. We attempted only to optimize the melting point of this new series of glasses after several trials. Besides, control experiment with no NiPc added, is already reported by several researchers.
3. Results and discussion 3.1. DC conduction Electrical conductivity measurements were done to determine the activation energy Ea . The resistivity was measured from room temperature up to 220 ◦ C in a vacuum of 10−3 Torr with silver epoxy as contact for the glass.
Fig. 2. Absorbance (Abs) vs. wavelength (λ) for the NiPc-substituted lithium borate glass.
P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
593
Fig. 3. (αhν)2 vs. hν plot.
Electrical measurements were done using a Keithley programmable electrometer (model 617). The Arrhenius plot of conductivity yields different levels for conduction mechanisms. More than one slope in the conductivity graph indicates the presence of different levels in the sample. Conductivity, σ , is related to the temperature, T, according to the relation: −Ea σ = σ0 exp kB T where σ 0 is the pre-exponential factor, Ea is the thermal activation energy and kB is the Boltzmann constant. Arrhenius plot of conductivity of NiPc-substituted lithium borate glass is shown in Fig. 1. It yielded two linear regions that
gave E1 and E2 , the activation energies. E1 arises from holes as the charge carriers and E2 from impurity scattering. We have obtained a value of 0.46 eV for E1 in the intrinsic region and 0.21 eV for E2 in the impurity scattering region. The activation energy values E1 and E2 are the same as that of an air-annealed NiPc film on glass substrate [13]. The activation energy corresponding to the intrinsic generation is associated with the resonant energy involved in a short-lived excited state and that corresponding to the impurity scattering is attributed to a short-lived charge-transfer between impurity and the complex [14]. The conductivity and activation energy values obtained here are very less compared to those of unsubstituted lithium borate glass [15].
Fig. 4. (αhν)1/2 vs. hν plot.
594
P.R. Binu et al. / Materials Chemistry and Physics 80 (2003) 591–594
3.2. Optical studies The optical absorption spectrum in the range 190–900 nm was taken in a Shimadzu 240 UV-Vis spectrophotometer. The UV-Vis spectrum of phthalocyanines originates from molecular orbitals within the aromatic 18 electron system and from overlapping orbitals within the central atom [16]. The direct electronic transitions from to ∗ orbitals in the 300–450 nm results in an intense band called the Soret band, which gives the absorption edge in phthalocyanines. The absorption edge can be analyzed using one electron theory of Bardeen et al. [17]. For photon energies hν just above the fundamental edge, the absorption, α, follows the standard relation, α = (hν − Eg )1/2
A hν
where A is a constant and Eg is defined as the energy gap between the valence band and the conduction band. A plot of absorbance (Abs) versus wavelength is shown in Fig. 2. A graph of (αhν)2 versus hν is shown in Fig. 3. Extrapolation of this plot to α 2 = 0 gives the optical band gap Eg for direct allowed transition. The value of Eg is 3.65 eV. The band gap Eg obtained for the as-deposited NiPc thin film is 3.2 eV that is comparable with the present value for the NiPc-substituted borate glass. Any crystal phase change would affect the gap between the conduction band and valence bands in phthalocyanines, because the orbital overlap between parallel pairs of molecules will be affected [18]. The value of Eg for indirect transition is obtained as 2.24 eV by extrapolating the plot of (αhν)1/2 versus hν to α 2 = 0 as shown in Fig. 4. This behaviour of indirect transition is shown in noncrystalline materials [19].
4. Conclusion NiPc-substituted lithium borate glass was prepared using the melt quenching method. The melting point of the
glass was optimized to a temperature of 850 ◦ C. Activation energies E1 and E2 are calculated as 0.46 and 0.21 eV, respectively, for the glass. Band gap Eg was determined from the optical absorption spectrum as 3.65 eV for direct allowed transition and 2.24 eV for indirect allowed transition. A detailed study on this new series of glasses is underway. References [1] H.L. Tuller, D.P. Button, D.R. Uhlmann, J. Non-Cryst. Solids 40 (1980) 93. [2] A. Levasseur, J.C. Brethous, J.M. Reau, M. Couzi, P. Hagenmuller, Solid State Ionics 1 (1980) 177. [3] M. Balkanski, R.F. Wallis, J. Deppe, M. Massot, Mater. Sci. Eng. B 12 (1992) 281. [4] C.A. Angell, Solid State Ionics 18/19 (1986) 72. [5] A. Levasseur, J.C. Brethous, J.M. Reau, P. Hagenmuller, Mater. Res. Bull. 14 (1979) 921. [6] A.R. Kulkarni, H.S. Maits, A. Paul, Bull. Mater. Sci. 6 (1984) 201. [7] S.Q. Zhou, X.F. Jin, W.P. Hu, Y.Q. Liu, S.H. Liu, Solid State Commun. 112 (1999) 269. [8] Z.N. Bao, J.A. Rogers, H.E. Katz, J. Mater. Chem. 9 (1999) 1895. [9] N.R. Armstrong, J. Porphyrins Phthalocyanines 4 (2000) 414. [10] K.F. Schoch Jr., T.A. Temoidnte, Thin Solid Films 165 (1988) 83. [11] G. De la torre, P. Vazquez, F. Agullo-lopez, T. Torres, J. Mater. Chem. 8 (1998) 1671. [12] T. Nishida, Y. Takashima, Nucl. Instrum. Methods Phys. Res. Sect. B 76 (1993) 397–402. [13] K.N. Narayanan Unni, C.S. Menon, Mater. Lett. 45 (2000) 326–330. [14] T.G. Abdel-Malik, G.A. Cox, J. Phys. C: Solid State Phys. 10 (1997) 63. [15] R.J. Elliott, L. Perondi, R.A. Barrio, J. Non-Cryst. Solids 168 (1994) 167–178. [16] E.A. Ough, J.M. Stillman, Can. J. Chem. 11 (1993) 1891. [17] J. Bardeen, F.J. Slatt, L.J. Hall, Photoconductivity Conference, Wiley, New York, 1956. [18] S.E. Harrison, J.M. Assour, J. Chem. Phys. 40 (1964) 365. [19] L. Edwards, M. Gouterman, J. Mol. Spectrosc. 33 (1970) 299.