Vibrational and Electronic Properties of Lead Iodide Crystals

Thiago C. Seniuk¹, Karolline A. S. Araújo^1,2, and Ariete Righi¹

¹ Departamento de Física, Universidade Federal de Minas Gerais, Avenida Antônio Carlos, 6627, Belo Horizonte, Brazil

² Instituto Federal de Educação, Ciência e Tecnologia de Minas Gerais – IFMG, Rua Mato Grosso, 02, Ibirité, Brazil

Lead iodide (PbI₂) is a semiconductor material that has attracted considerable attention from the scientific community due to its interesting physical properties, with applications in high-sensitivity photodetectors and X-ray and gamma-ray detectors [1], [2]. PbI₂ consists of a layer of lead atoms sandwiched between two layers of iodine atoms, which interact through weak van der Waals forces. In this work, we study the vibrational and electronic properties of PbI₂ crystals and 2D flakes through Raman spectroscopy and the resonant Raman effect. The samples were synthesized via solution evaporation methods. Raman spectra were obtained using a wide range of laser excitation energies. A resonant Raman effect was observed at 2.33 eV at room temperature. Photoluminescence (PL) and UV-VIS spectra were acquired to investigate this resonant behaviour, which was associated with a defect level caused by lead vacancies in the PbI₂ crystal. We also investigated the vibrational properties of PbI₂ 2D flakes as a function of their thickness. Atomic Force Microscopy (AFM) was used to determine the thickness of the flakes, and the Raman spectra were analyzed as a function of the number of layers. A strong dependence of the E_g and A_1g Raman modes was observed as the material goes from its bulk form to the few-layer limit. Additionally, the PL spectra showed a blueshift in the emission peak as the number of layers decreased, in agreement with both theoretical and experimental studies reported in the literature [3], [4].

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[2]      J. Zhanget al., “Layered ultrathin PbI2 single crystals for high sensitivity flexible photodetectors,” J Mater Chem C Mater, vol. 3, no. 17, pp. 4402–4406, May 2015, doi: 10.1039/c4tc02712d.

[3]      M. Yagmurcukardeset al., “Electronic and vibrational properties of PbI2: From bulk to monolayer,” Phys Rev B, vol. 98, no. 8, Aug. 2018, doi: 10.1103/PhysRevB.98.085431.

[4]      S. Golovynskyi et al., “Free exciton and bound excitons on Pb and I vacancies and O and I substituting defects in PbI2: Photoluminescence and DFT calculations,” Appl Surf Sci, vol. 624, Jul. 2023, doi: 10.1016/j.apsusc.2023.157128.