Curious Variations in Energy Gap of Polytypic CdI2 and PbI2 Crystals on Laser Irradiation

Abstract

Crystals of CdI2 and PbI2 were grown by zone melting. They were subjected to argon ion laser irradiation. Both time and intensity of radiation were systematically varied. The crystals of CdI2 were of type 4H while those of PbI2 were of type 2H. Their energy gaps were determined by UV spectrophotometry. The CdI2 crystals showed a linear variation of energy gap with intensity of radiation (0 to 125 mW), with the energy gap value decreasing from 3.28 to 3.11 eV, when the exposure time was kept fixed (7 min). When the time of exposure was varied, with the intensity of radiation fixed at 50 mW, the energy gap initially decreased from 3.28 to 3.07 eV and then gradually rose to be restored to the original value, thus showing a nonlinear variation. For the PbI2 crystals, the energy gap steadily decreased from 2.53 to 2.02 eV, when the intensity of exposure was varied. However, the value drastically decreased from 2.53 to 1.04 eV for the case of variation of exposure time. The results have been explained in terms of changes in phonon frequency, amount of absorption and anisotropy of thermal conductivity of the material.

Cadmium clusters in CdI2layered crystals: the influence on the optical properties

“…CdI 2 crystals have been also proposed as a recording media for ultraviolet (UV) light etc [1]. In the last few years, several interesting works have been devoted to the studies of various nanostructures which are formed in doped CdI 2 crystals under irradiation [3,4], in ultrathin layers of cadmium iodide [5][6][7][8][9], or in heterostructures in the case of the evaporation of metal film on the CdI 2 surface [10,11]. It was shown that these systems exhibit unique properties which do not occur in the pure crystals.…”

Laser-induced microstructuring of two-dimensional layered inorganic–organic perovskites

“…Previous studies of CW-UV laser irradiation on PbI 2 have suggested that the laser induced heating effects involve two processes: the slow heat penetration perpendicular to the layers and a fast dissipation parallel to the layers. 35 Such relative thermal dissipation in PbI 2 was claimed due to a difference of two orders of magnitude between perpendicular and parallel components of thermal conductivity (B1 mW cm À1 K À1 and 100 mW cm À1 K À1 respectively). 35,36 Since (R-NH 3 ) 2 PbI 4 layered perovskite crystal packing is not very different from PbI 2 , similar fast heat dissipation parallel to the layers can also be expected in CHPI, which could possibly explain the evaporation of the organic moiety and partial deformation of the layered structure in l-CHPI.…”

“…35 Such relative thermal dissipation in PbI 2 was claimed due to a difference of two orders of magnitude between perpendicular and parallel components of thermal conductivity (B1 mW cm À1 K À1 and 100 mW cm À1 K À1 respectively). 35,36 Since (R-NH 3 ) 2 PbI 4 layered perovskite crystal packing is not very different from PbI 2 , similar fast heat dissipation parallel to the layers can also be expected in CHPI, which could possibly explain the evaporation of the organic moiety and partial deformation of the layered structure in l-CHPI. However, such deformation could be retained by the re-intercalation process, which is supported by the experimental evidence (Fig.…”

Structure and optical properties of pbI2 thin film by thermal evaporation technique

“…The poly types of PbI2 are 2H ,4H, 6H, 8H, 12H, 12R, 14H, 18H, 18R, 20H, 20R, 36H ,42R, 48R. The most common type is 2H, which represents 95% polytopes described for PbI2 structure [7]. Recently many research was published on the development of the method of prepared thin films of PbI2 from solutions, vapor, melts and gels.…”