The 50MeV Si ion irradiation induced modifications on structural, dielectric, optical and mechanical properties of Vertical Bridgman grown Benzimidazole (BMZ) crystals have been studied. The high resolution X-ray diffraction studies show that amorphization increase on increasing fluence of ion irradiation. The dielectric constant and dielectric loss as a function of frequency and temperature are studied. The hardness behaviour of both irradiated and unirradiated crystals has been explained with indentation effects. UV-VIS studies reveal decrease in bandgap values on irradiation, is explained. The scanning electron micrographs show the collapse of the grain boundaries on irradiation.

Optoelectronics and nonlinear Optics are expected to play a major role in photonics, which is emerging as a multidisciplinary new frontier of science and technology, capturing the imagination of scientists and engineers worldwide. High energy heavy ion irradiation produces electronic excitation/ionization in solids, which leads to many changes in their properties. It is known that when an ion beam passes through matter, it causes damage, which depends on the type and the energy of the ion and the properties of the medium. It is generally pictured that atomic displacements are primarily induced by the elastic collisions and not by electronic excitation or ionization. However, slowing down of energetic heavy ions in insulator targets is known to induce structural changes called latent tracks¹, which result from atomic displacements in the material along the path of the ion. These atomic displacements in narrow column along the incident ion path are explained by coulomb expansion² or thermal spike³_. In the present case, single crystals of BMZ were irradiated with 50MeV Si ions, which are capable of generating the modifications in the crystals. BMZ has the molecular formula of C₇H₆N₂ and crystallizes in orthorhombic crystal system. The high degree of symmetry of this crystal makes it suitable for investigation of its optical, mechanical, structural and dielectric properties. BMZ crystal is a potential material for NLO and scintillation detector and other related applications.

2. CRYSTAL GROWTH

The bulk single crystals of BMZ have been successfully grown by Bridgman technique using glass ampoule of length 22cm with wall thickness 1.2mm and necked portion is less than 1.5mm in diameter. The growth was performed with different ampoule configuration and the growth experiment was not successful when the wall thickness is less than the above thickness. The single zone resistive heating quartz furnace was used for growth so that insitu growth observation was possible during growth. The translation speed was 1mm/hr with temperature gradient of 2^oC/cm. The grown crystals were 18cm in length and 1.8cm in diameter and the cut and polished crystal of BMZ is of 3cm in length (Fig.1). The BMZ crystals have been irradiated by 50MeV Si ion beam delivered from 15MV pelletron accelerator at IUAC, Delhi.

3.UV-VIS SPECTRAL STUDIES

Optical absorption studies performed on 50MeV Si ion irradiated BMZ crystals gives the spectral intensity of light transmitted through the specimen that determines the absorption edge, which is a measure of the bandgap. The absorption of light energy by organic materials in the ultraviolet and visible region involves promotions of electrons in σ, П and n-orbital from the ground state to higher energy state. The electronic transitions (→) that are involved in the ultraviolet and visible region are of the following types σ→σ, n→σ, n→Π, and Π→Π^*. The unirradiated BMZ crystal shows its characteristic peak at around 308nm, which is a П- П^* transition of the heteroatomic benzene ring. The characteristics peaks at 312nm and 318nm it are observed for irradiated BMZ crystals at fluences of 5x10¹¹ions/cm² and 5x10¹²ions/cm², respectively. Fig.1 shows the absorbance with fluence on a log-scale at three different characteristic wavelengths of 308, 312 and 318nm. It may be concluded from the plot that the optical absorption increases (wavelength also increases) with increase of irradiation dose delivered to the BMZ crystals, resulting in higher concentration of defects. The broadening of absorption peak may be occurring due to increase in defect sites on SHI irradiation. The increase in absorption towards high wave number is an indication of high dc conductivity of BMZ crystal.

From the wavelength corresponding to the bandgap of material in the absorbance curve a sudden rise in the absorbance is expected after irradiation. The threshold at which the absorption data showing abrupt rise is determined (graphically), this can be an indicative of the bandgap of material. Working on this hypothesis and equation (1) and utilizing the absorbance data (Fig.2), the wavelength threshold was determined by plotting tangents as shown in Fig.2

The values of bandgap determined were 6.07, 3.98 and 3.90 eV for unirradiated, irradiated at fluence of 5x10¹¹ ions/cm² and 5 x 10¹² ions/cm², respectively. The decrease in bandgap energy upon irradiation may be attributed to the creation of some intermediate energy levels due to structural rearrangements⁷, this can be inferred from the dielectric constant. The increase in absorbance can be explained by decrease in absorption co-efficient (α) on increasing the radiation dose. The absorption coefficient (α) of virgin and irradiated BMZ crystals was calculated by Mclean’s formula.

Reference:

(1) Prasad, P.N. and David,J.W, Introduction to nonlinear optical effects in Molecules and Polymers, John Wiley, New York, 1991.

(2) Metha,G.K., Physics Education,1994, 11-3,245.

(3) Lesueur, D. and Dunlop,A. Radiat. Eff. Defects in solids, 1993,126, 163.

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