Influence of X-ray irradiation on the optical properties of CoMTPP thin films M.M. El-Nahass, A.H. Ammar, A.A. Atta, A.A.M. Farag, E.F.M. El-Zaidia ⁎ Physics Department, Faculty of Education, Ain Shams University, Roxy Square 11757, Cairo, Egypt abstract article info Article history: Received 4 May 2010 Received in revised form 8 October 2010 Accepted 10 December 2010 Available online 25 December 2010 Keywords: Organic dye (CoMTPP) Optical dispersion parameters Thin films of 5,10,15,20-Tetrakis (4-methoxyphenyl)-21H,23H-porphine cobalt (II), CoMTPP were prepared at room temperature (300K) by the thermal evaporation technique under vacuum pressure about 2 × 10 -4 Pa. The X-ray diffraction patterns showed the amorphous nature for the as-deposited and the irradiated films, whereas the powder has shown a polycrystalline with triclinic structure. Miller's indices, hkl, values for each diffraction peak in the XRD spectrum were calculated. Optical properties of CoMTPP thin films were characterized by using spectrophotometric measurements of transmittance and reflectance in the spectral range from 200 to 2500 nm. The refractive index, n, and the absorption index, k, were calculated. The obtained data were used to estimate the type of transitions and the optical and fundamental gaps before and after X-ray irradiation. In addition, the normal dispersion of the refractive index is discussed in terms of lorentz-lorentz free single oscillator model and modified lorentz Drude model of free carrier contribution. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Porphyrins are a group of organic compounds of which many occur in nature, most well-known as the pigment in red blood cells. They play a crucial role in a number of biological processes such as photosynthesis, dioxygen transport and activation, photodynamic therapy of cancer and other biophotonic processes [1,2]. Porphyrins are a class of conjugated macrocyclic compounds in which four pyrrole rings are linked to each other in a cyclic fashion through meso- carbon bridges. Metalloporphyrin is a two-dimensional (2D) mole- cule, which consists of an identical planer ring of carbon, nitrogen and hydrogen atoms, surrounding one individual atom of a different metallic element. The difference in properties like the geometric, electronic structure, optical and vibrational spectra of porphyrin may be related to the nature of metal atoms at the centre of the porphyrin molecule and the introduction of a substituent in a meso position [3]. Porphyrins are a class of very attractive materials because they have an enormous potential for applications on the technological front, including: electro-luminescent devices [4], photonic devices [5], solar energy conversion devices [6], photodynamic therapy and diagnosis of cancer using laser excitation [7], photochromic recording medium [8], catalysis [9], photo-electro-chemical cell [10], optoelectronic device fabrication [11] and gas sensors [12]. The detailed organiza- tional features of porphyrin molecules are important because functional behavior often depends on π–π stacking, fixing conforma- tions, and attaining optimal orientations and spacing. It is well known that porphyrin molecules can self-associate by stacking in “head to tail” geometry, forming a “face to face” geometry, and forming H-aggregates in both cases; they exhibit large spectral shifts due to level splitting by excitonic effects [13]. Some physical properties of 5,10,15,20-Tetrakis (4-methoxyphenyl)-21H,23H-porphine cobalt (II) (CoMTPP) bulk and thin films have been studied by some researcher group [14–16], but much of the optical property of CoMTPP is unknown. Therefore, the objective of this study is to investigate the structural and optical properties of thermally evaporated thin films, including the refractive index, n, and the extinction coefficient, k, over the spectral range 200–2500 nm, and examine the effect of radiation. 2. Experimental procedures 2.1. Materials and preparation 5,10,15,20-Tetrakis (4-methoxyphenyl)-21H,23H-porphine cobalt (II),(CoMTPP) powder used in this study was obtained from Aldrich company with purity N 96%. The schematic diagram of the molecular structure of CoMTPP is shown in Fig. 1. Thin films were prepared by the thermal evaporation technique; using a high vacuum coating unit (Edwards type E306A). The films are deposited onto both glass and optically flat quartz substrates. The films were deposited from a quartz crucible source heated by a tungsten coil in a vacuum of 10 -4 Pa during deposition. The temperature of substrates was kept at room temperature (300 Kk). The deposition rate (2.5 nms -1 ) was controlled by using a quartz crystal thickness monitor (Edwards, Model FTM4). Thickness was also monitored by using the same thickness monitor and subsequently calibrated by the Tolansky method [17]. Optics Communications 284 (2011) 2259–2263 ⁎ Corresponding author. E-mail address: ad_eman@hotmail.com (E.F.M. El-Zaidia). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.12.032 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom