Elsevier

Optics & Laser Technology

Volume 35, Issue 7, October 2003, Pages 523-531
Optics & Laser Technology

Structural and optical studies of thermally evaporated CoPc thin films

https://doi.org/10.1016/S0030-3992(03)00068-9Get rights and content

Abstract

The structure of the thermally evaporated cobalt phthalocyanine (CoPc) thin film in the β-form is investigated, and shows a single strong peak indicating preferential orientation in the (100) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallites per unit surface area are determined.

The spectral parameters are determined by applying the electronic orbital transitions.

But the optical parameters are deduced using band-model consideration for thin films of Pc.

The spectral and optical parameters have also been investigated by using the spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500nm.

The absorption spectra recorded in the UV–VIS region show two absorption bands of phthalocyanine (Pc) molecule, namely the Soret band (B) and the Q-band. The Q-band shows its characteristic splitting (Davydov splitting) with ΔQ=0.23eV.

Some of the important spectral parameters, namely optical absorption coefficient (α), molar extinction coefficient (εmolar), oscillator strength (f), electric dipole strength (q2) and absorption half bandwidth (Δλ) of the principle optical transitions have been evaluated.

The fundamental and the onset indirect energy gaps could be estimated as 2.90+or0.05 and 1.51eV, respectively.

The refractive index showed an anomalous dispersion in the absorption region as well as normal dispersion in the transparent region. From analysis of dispersion curves, the dielectric constants, the dispersion parameters and the molar polarizability were obtained.

All the above parameters were obtained for films as deposited and as annealed. No remarkable annealing effect on many parameters was observed.

Introduction

The phthalocyanine (Pc) polymers have become one of the most studied of all organic functional materials [1] and have recently attracted considerable interest due to their high thermal and chemical stability.

Phthalocyanine exists in several crystalline polymorphs, including the α-, β- and γ-structures [2].

Metal-substituted and metal-free phthalocyanine polymers such as MnPc, CuPc, NiPc, FePc, CoPc, etc. have been studied and have been widely used as gas sensors, optical logic displays, solar energy conversion [3], colour filters and organic laser materials [4], [5], [6], [7], [8].

These materials are generally p-type semiconductors, and they can easily be sublimed, resulting in high purity thin films without decomposition.

The optical properties of CoPc have been investigated previously in vapour phase [9] as single crystals and as films in [10].

The absorption spectra and the structural investigations of the metal phthalocyanine films have been discussed in [10], [11], [12], [13].

In the present work, thin films of cobalt phthalocyanine have been fabricated by thermal evaporation onto quartz substrates. The structural-, spectral-, and optical parameters have been determined and discussed.

Moreover, the visible and near ultraviolet absorption spectra have been measured.

Section snippets

Experimental

The CoPc powder used in this work is obtained from Kodak, UK. Thin CoPc films of different thicknesses (d) are prepared by conventional thermal evaporation technique, using a high vacuum coating unit (Edwards type E 306 A, England). The films are vacuum deposited onto optical thick flat quartz substrates at room temperature from a quartz crucible source heated by a tungesten coil in a vacuum of 10−4Pa during deposition. The deposition rate is controlled at 2.5nms−1 using a quartz crystal

Structural parameters

Fig. 1 shows an X-ray diffraction pattern taken of monoclinic evaporated CoPc powder (a) and the heat treated thin film at 473K with thickness (d)510nm (b). The evaporated CoPc is identified as the β-form.

Indexing was carried out using the recently reported lattice parameters given by [2] as a=1.93nm,b=0.477nm,c=1.454nm and β=120.82°.

By comparing between (a) and (b) in Fig. 1, it is clear that there is only one significant intense peak implying a preferential orientation in the (100) direction

Conclusion

The structure of the thermally evaporated (CoPc) thin films shows only a single strong absorption peak indicating preferential orientation in the (100) direction. Some structural parameters such as crystallite grain size, dislocation density and the number of crystallite per unit surface area are determined.

The optical properties of CoPc thin films before and after annealing have been studied in the spectral range 200–2500nm.

The absorption spectra recorded in the UV–VIS region show two

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