29 Conductivity Studies On Oligomeric Cobalt Phthalocyanine S. Saravanan, C. Joseph Mathai, M. R. Anantharaman1, S. Venkatachalam2, P. V. Prabhakaran2 Dept. of Physics, Cochin University of Science and Technology, Cochin - 682 022 India 2 PSCD, Vikram Sarabhai Space Centre, Trivandrum - 695 022 India 1

Author for correspondence Ph. No: +91484 577404 Email: [email protected]

Abstract Polymeric metallophthalocyanines find applications in electromechanical and dielectric devices because of their many attractive features such as lightweight, high mechanical flexibility and conformability. Copper (Cu), Nickel (Ni) and Cobalt (Co) polymeric phthalocyanines possess large extended conjugated structures and exhibit high conductivity. Oligomeric cobalt phthalocyanine was prepared by chemical method and dc conductivity was evaluated. Also in order to understand the conduction mechanism Mott’s variable range hopping was applied to the system. The T-1/4 behaviour of the dc conductivity along with the values of Mott’s Temperature (T0), density of states at the Fermi energy N (EF), range of hopping R and hopping energy W indicate that the transport of charge carriers are by three dimensional variable range hopping. The results are presented and correlated.

Keywords: Cobalt Phthalocyanine, DC Conductivity, Conduction Mechanism, Hopping conduction

1.Introduction Metallo and metal free phthalocyanines and their polymers are known for almost a century and have become one of the mos t important semiconducting materials because of their optical, electrical, photoelectric and electrochemical properties [1]. Metallo phthalocyanines are also known for their highly aromatic structure and excellent stability against heat, light, moisture and air [2]. Thermal and chemical stabilities of these polymers attract the attention of researchers. The physico chemical properties of phthalocyanines can be altered significantly via incorporation of different metal atoms like Copper, Nickel and Cobalt etc. at the center of the phthalocyanine ring or by changing the nature of substituents on the periphery of the macrocyclic ring. Electrical conductivity in these systems begins from the mobility of charge carriers along the highly conjugated electron carbon backbone [3]. Hopping or tunneling conduction mechanism is the usual transport of charge carriers found in conducting polymers. This article narrates synthesis of cobalt oligomeric phthalocyanine and the electrical conductivity studies of these oligomers. The electrical conductivity is explained by using Mott’s variable range hopping mechanism.

2.Experimental Phthalocyanine tetramer of cobalt was prepared, purified and characterised by a method reported elsewhere [4,5]. Metal sulphate, pyromellitic dianhydride, excess urea, ammonium chloride and 0 ammonium molybdate were ground well and mixed with nitrobenzene and heated at 180 C for 12 hours. The reaction mixture was cooled and boiled first with distilled water. The crude product was further boiled with 2N sodium hydroxide containing sodium chloride and filtered. The residue was acidified and washed several times and dried. The structure of tetrameric cobalt phthalocyanine and the details of elemental analysis are shown in Fig 1 and Table I respectively. The values shown in table were calculated based on the structure given in Fig 1. The molecular formula is C120H40N32O32Co4.8H 2O.

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212 Table I Elemental Analysis of tetrameric Cobalt Phthalocyanine C

H

N

Co

Calculated

51.0

2.0

15.88

8.35

Observed

44.0

1.96

14.90

8.20

The oligomer cobalt phthalocyanine powder samples were pressed in the form of pellets of diameter 10mm and the thicknesses vary from 1mm – 3mm. The conductivity studies were carried out on these pellet samples using a home made dielectric cell and a Keithley picoammeter from room temperature to -2 393K under dynamic vacuum (10 Torr). The cell was standardized using wax/teflon and the picoammeter was interfaced to a PC via GPIB card. The conductivity measurements were fully automated usin g a package based on virtual instrumentation (VI) commonly known as LabVIEW.

COOH

HOOC

HOOC

N

N

N

N

N

HOOC N

HOOC

M

N

N

N

N

N

M

HOOC

C OOH N

N

N

COOH

N N

N

N N

N

N

N N

M N

N

HOOC

COOH

N

N

HOOC

COOH

M

N

N N

COOH

N

HOOC

COOH N

COOH

Figure 1 Structure of Metallophthalocyanine Oligomers [M-Cobalt]

3. Results and discussion The dc conductivity σdc in the region 300K to 393K was calculated and plotted in Fig 2. Activation energy of the sample was calculated using the relation,

E σ dc = σ o Exp A KT   

(1) -6

The dc conductivity of oligomeric cobalt phthalocyanine at room temperature is of the order of 10 S/cm and the activation energy is 0.32 eV. In general, in phthalocyanine, the electrical conductivity is associated with mobile π electrons of phthalocyanine ring and the conductivity is due to thermal ____________________________________________________________________________________________ International Seminar on Advances in Polymer Technology,Dec13-14,2002,Cochin,India

213 excitation of π electrons from the highest filled to lowest empty π orbitals. This energy difference between these two levels for monomeric phthalocyanine is 1.5eV to 1.7eV [6]. -8

Ln σ dc (S/cm)

-9

-10

-11

-12 2.4

2.6

2.8

3.0

3.2

3.4

103/T (K -1)

Figure 2 Temperature dependence of dc conductivity of oligomeric cobalt phthalocyanine The presence of extended structure in a polymer reduces the bandgap, which governs the intrinsic electrical properties [3]. In polymeric phthalocyanine because of the extended conjugated structure the energy values are much lower as seen by our measurements. The dc conductivity values obtained for -6 -10 the polymer is 10 S/cm, which is much higher than that of monomeric phthalocyanine (10 S/cm). It is found that the ac conductivity is two orders higher than that of dc conductivity. If the charge transport is due to hopping, the ac conductivity will be higher than the dc conductivity [7]. This proves that hopping type of charge transport occurs in the oligomeric cobalt phthalocyanine. In the case of dc conduction, charge carriers have to cross the entire sample and if the hopping sites are randomly distributed, the current path will inevitably include some long distance hops, which have a very small hopping probability. At high frequencies, hopping mainly occurs between close neighbours, because the field is reversed before the long distance hop. In order to understand the conduction mechanism in these conjugated systems a Mott’s variable range hopping model is employed [8]. According to this model, in a disordered material when the charge carriers are localized due to random electric fields, instead of band conduction, charge transport takes place via phonon assisted hopping between localised states. Since the localised states have quantized energies extending over a certain range, activation energy is required for each hop. The mechanism is based upon the idea that carriers tend to hop larger distances to sites, which lie energetically closer than to their neighbours. According to the Mott’s Variable range hopping model, the dc conductivity in three dimensions can be expressed as

σ dc

1  T 4  o = σ o Exp −   T4     

(2)

Where T0 is the Mott’s Characteristics temperature and is given by

T0 =

λα 3 KN (E F )

(3)

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214

Where λ 18.1 is a dimensionless constant [9] α is the inverse rate of fall of the wave function [10], K is the Boltzmann Constant and N (EF) is the density of states at the Fermi energy. -3.5

log σdc (S/cm)

-4.0

-4.5

-5.0

-5.5 0.050

0.051

0.052

0.053

0.054 -1/2

T

0.055

0.056

0.057

0.058

-1

(K )

Figure 3 Plot of log σ dc as a function of T-1/2 -3.5

log σdc (S/cm)

-4.0

-4.5

-5.0

-5.5 0.222

0.224

0.226

0.228

0.230

0.232

0.234

0.236

0.238

0.240

0.242

T-1/4 K-1

Figure 4 Plot of log σ dc as a function of T-1/4 The Range of hopping R [11] and the energy for hopping W in three -dimensional variable range hopping and are expressed as 1

9 4 R =  πα KTN ( E F ) 8 

(4)

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215

and

(

W = K T0 T 3

)

1 4

(5)

The value of Mott’s Characteristics temperature T0 has been calculated from the graph log σ vs T-1/4 [Fig 4] and is found to be 4.5300 X 107 K. From the equations 3,4 and 5 the value of N (EF), R and W were calculated at 300K and the values cited (Table 2) are estimated after assuming the reasonable value of α = 1 Å. Table 2 depicts the Mott temperature and its parameters for oligomeric cobalt phthalocyanine. Low values obtained for the mobility indicates that the electronic states are localised, in accordance with the hopping mechanism. Table 2 Mott temperature, Density of states at the Fermi energy, Range of hopping and energy for hopping of oligomeric cobalt phthalocyanine

Sample

Mott temperature T 0 (K)

Oligomeric cobalt phthalocyanine

4.53 X 10

7

N (EF) -3 -1 cm eV 21

4.63 X 10

Rhop (Å) (300K)

W hop (eV)(300K)

2.87

0.51

The temperature dependence of dc conductivity has been studied and is found that it follows the 3D VRH. From the above discussion it is shown that the best fit to the dc conductivity can be achieved with a T-1/4 behaviour and the values of T0, N (EF), R and W can be found by assuming that the conduction is three dimensional and follows the variable range model. In order to enhance the conductivity of phthalocyanine they be modified either by thermally or chemically [12].

4. Conclusions Cobalt phthalocyanine oligomers were prepared and the structure was given. DC conductivity studies were carried out on these samples. The activation energy for elec trical conductivity is found to be 0.32 eV and the conduction behaviour is found to fit the 3 dimensional variable range hopping model with the values of 4.530 X 107 K, 4.63 X 1021 cm-3eV-1, 2.87 Å and 0.51 eV for Mott temperature T0, density of states at the Fermi energy N (E F), range of hopping R and hopping energy W at 300K respectively.

Acknowledgement MRA and SS thank the Department of Space for financial Assistance received in the form of a project under ISRO-RESPOND, Government of India (File. No. 10/03/354 dtd.23-02-1999)

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