Optical Characterization of C 60 Organic Semiconductor and Bilayers
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ABSTRACT Preparation and characterization either by optical absorption, photoluminescence and micro-Raman spectroscopy of individual components as well as bilayers consisting of organic dye semiconductor Zinc Phthalocyanine (ZnPc) and fullerene, C60 , thin films are reported. The layers and structures were deposited in vacuum and some fullerene films were also prepared by casting the C60 solution in benzene. The optical absorption and photoluminescence dependencies on film thickness in bilayers C6 0 /ZnPc were observed and may be discussed in a context of interface induced simmetry reduction of C60 molecules. INTRODUCTION Efficient synthesis of C60 and related fullerenes [11 has led to intense research of structural, electronic and optical properties of these forms of carbon. Noteworthy that films of fullerenes are of interest because they exhibit such properties as photoconductivity [2-4] and luminescence [5,91. In the case of composites of C6 0 and materials with mild donors such as conducting polymers, spontaneous charge transfer does not occur at the ground state. Upon photoexcitation, it is demonstrated that the photoinduced electrons in the polymer migrate to C60 , resulting in a considerable enhancement of photovoltaic effects [10-12]. Metallophthalocyanines are well known for their thermal and chemical stabilities. The phthalocyanine molecule is planar and symmetrical, and provides opportunities for further research: organic dyes like phthalocyanine can be used in photovoltaic energy conversion [13]. This work presents a study of the Raman and Photoluminescence spectra, and optical absorption spectra of individual components, as well as bilayers consisting of ZnPc and C60 thin films. Particular attention is devoted to the preparation methods and thickness dependencies of these films and some structural information which they provide. EXPERIMENT The C60 (99.9 %) powder, obtained from MER Corporation (Tucson,USA), and ZnPc derivative of tetra-tert-butil phthalocyanine (Zn-Pc[C(CH 3 )314 ) from Kodak Corporation were used. The films and structures were fabricated on transparent glasses, and on Au or Al coated glasses by evaporation of compounds in vacuum chamber (2x10- 5 Torr). The C60 and ZnPc powders in the crucibles were heated at 320'C and 210'C, respectively, to deposit the films. A Alpha Step 200 (Tencor Instruments) profilometer was used to estimate the thickness profile (within ± 10 nm) and the average roughness value of the films. Stokes-shifted Raman spectra were measured at room temperature with the Renishaw Raman Microscope-2000 model, at 1 cm- 1 resolution, using a Spectra-Physics-127 He-Ne laser for excitation at 632.8 nm, with a power of 2.5-25 mW at the source. 933
Mat. Res. Soc. Symp. Proc. Vol. 488 0 1998 Materials Research Society
The sample temperature was estimated by the Stokes/anti-Stokes scattering intensity ratio of the 1469 cm- 1 peak of the Raman spectrum of C6 0 film. The temperature obtained for 0 the region illuminated by the laser beam was 31 C for 25 mW input laser power at 6
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