Accoding to a research published in Advanced Optical Materials, Prof. DING Junfeng and his team at the Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, reveals that semiconductor PbI₂ undergoes a transition to a semimetallic state when subjected to pressure. This transition is accompanied by the enhancement of photoelectric properties as well as the spectral response range expanded to infrared band.

PbI₂ is a versatile semiconductor with applications in X-ray and gamma ray detection, as well as perovskite solar cell development. By compressing the lattice constants and inducing structural transition, the employment of hydrostatic pressure can be used to modify structural and electronic properties. Therefore, pressure may be an effective method to promote the photoelectric performance of PbI₂.

In this study, researchers made a significant discovery regarding the behavior of PbI₂ under pressure.

The high-pressure absorption spectra of PbI₂ suggested that the electrical band was closed at the transition point, while the charge transport indicated that the sample remained not metallic. Non-metallic transport was well explained by the development of a semimetal phase at high pressure, as determined by first-principles calculations of the photocurrent spurt and infrared band response.
The semiconductor-semimetal transition in PbI₂ was further confirmed by the notable decrease in lifetime to a few picoseconds in ultrafast spectroscopy under pressure. Moreover, the responding band expanded from visible light to the telecom wavelength of at least 1550 nm.

"Our team employed several techniques to systematically investigate the behavior of PbI₂ under high pressure. Through our study, we were able to settle a long-standing controversy surrounding its high-pressure phase and identify inconsistencies between crystal and electronic transitions," explained CHENG Peng, the lead author of the paper. "This research represents a detailed investigation into the electronic structure phase transition of PbI₂ under high pressure."

Pressure-induced semimetallization presented a new strategy for designing a high-performance photodetector with broadband response.

Photocurrent of PbI₂ under visible light. (Image by CHENG Peng)