[Structural, morphological and optical properties of PbI2 thick films]

In the present paper, the structural, morphological and optical properties of PbI2 thick films prepared by close-spaced sublimation technique were investigated. It was found that the thickness of PbI2 films decreased from 1 000 μm to 220 μm with the increase in the sublimation source temperature. X-ray diffraction (XRD) pattern shows that the thick films are polycrystalline hexagonal structure with preferred growth orientation of (002) plane, and their grain size, dislocation density and growth stress are closely related to the source temperature. Images of scanning electron microscopy (SEM) reveal the accumulation of hexagonal plate-like particles which constitute the samples, and the particles with a diameter of 248 μm and a thickness of 32.7 μm, exhibit clearly layered structure. By spectrum fitting using Gauss function, the Raman spectra show a shift of about 147, 169, 217 and 210 cm(-1) respectively, the first three peaks correspond to the longitudinal optical vibrations (LO) mode in 4H-PbI2 crystal, while the last peak originate from a vibration pattern associated with SnO2 in substrate. Raman peak of 147 cm(-1) changes significantly with the increases in source temperature, and a dramatic decrease in peak intensity with broadening peak width occurred when the source temperature increased up to 225 degrees C or more. Under 340 nm excitation at room temperature, several weak photoluminescence peaks of PbI2 samples which associated with defects and exciton recombination near 2.25, 2.57 and 2.64 eV were observed. Given a comprehensive consideration of structural and spectral characterization results, PbI2 thick films with a thickness of about 659 μm deposited at a source temperature of 200 degrees C achieves the best crystalline quality.