Structural and optical studies of thermally evaporated CoPc thin films
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 during deposition. The deposition rate is controlled at 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 with thickness (b). The evaporated CoPc is identified as the β-form.
Indexing was carried out using the recently reported lattice parameters given by [2] as 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 direction
Conclusion
The structure of the thermally evaporated (CoPc) thin films shows only a single strong absorption peak indicating preferential orientation in the 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–.
The absorption spectra recorded in the UV–VIS region show two
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