Quantum well effect in bulk PbI(2) crystals revealed by the anisotropy of photoluminescence and Raman spectra

A photodetector fabricated from 2H-PbI2 micro-crystals recycled from waste lead acid batteries

Recycling Pb from lead acid batteries is rather important in environmental protection, but current strategies need a high temperature or produce secondary pollution. Herein, we present a green reactant recycling method to synthesize PbI₂ micro-crystals by extracting the Pb from waste lead acid batteries. Systematical characterizations indicate that the as-prepared PbI₂ micro-crystals show high purity, high crystal quality with a 2H-hexagonal crystal structure, and excellent optical properties with a bandgap of 2.3 eV. Based on the recycled 2H-PbI₂ micro-crystals, a symmetrically structured ITO/PbI₂/ITO photodetector is fabricated. Under 10 V bias voltage, the device reveals a distinct photo-response to UV-visible light and superior performance, with a dark current of 1.06 nA, an on-off ratio of 103, a responsivity of 15.5 mA/W, and a detectivity of 4.7 × 10¹⁰ Hz^1/2 W^-1. In addition, the photodetector also exhibits relatively rapid response speeds of 69 ms (rise time) and 64 ms (decay time). Our study provides an innovative and green strategy for producing a UV-visible photodetector based on recycled lead acid batteries, which is significant in environmental protection and the recycling economy.

In Situ Growth of PbS/PbI2 Heterojunction and Its Photoelectric Properties

In this paper, PbI₂ thin films with a uniform surface morphology and compact structure were prepared by adjusting the spin coating process parameters. On such a basis, the PbS/PbI₂ heterojunction was fabricated on the PbI₂ surface by the method of in situ chemical replacement growth. The results show that the PbS/PbI₂ heterojunction grown by this method has a clear interface and is closely combined. The introduction of a PbS layer enables its spectral response range to cover the visible and near-infrared regions. Compared with the PbI₂ thin film device, its responsivity is increased by three orders of magnitude, its response time reduced by 42%, and its recovery time decreased by nearly 1/2 under 450 nm illumination. In the case that there is no response for the PbI₂ thin film device under 980 nm illumination, the specific detectivity of the PbS/PbI₂ heterojunction device still amounts to 1.8 × 10⁸ Jones. This indicates that the in situ chemical replacement is a technique that can construct a high-quality heterojunction in a simple process. PbS/PbI₂ heterojunction fabricated by this method has a visible–near-infrared light detection response range, which provides a new idea for creating visible–near-infrared common-path detection systems.

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two-dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface-based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low-temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂ -based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂ /PbI₂ stacks, while a dramatic photoluminescence quenching of  WS₂ and WSe₂ is revealed in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂ /PbI₂ stacks, where fast-transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂ /PbI₂ and WSe₂ /PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂ , WS₂ , and WSe₂ monolayers with similar electronic structures show completely distinct light-matter interactions when interfacing with PbI₂ , providing unprecedented capabilities to engineer the device performance of 2D heterostructures.

Achieving highly uniform two-dimensional PbI2 flakes for photodetectors via space confined physical vapor deposition

Two-dimensional (2D) PbI₂ flakes have been attracting intensive attention as one potential candidate for the modern optoelectronics. However, suffered from the instability of kinetics-driven growth, the fabricated 2D PbI₂ flakes have a wide dimensional distribution even under the same conditions. Herein, a novel facile space confined physical vapor deposition (PVD) process is provided to synthesize uniform triangle PbI₂ flakes with high quality. The confined space provides a relatively stable growth environment that renders more control on the growth kinetics, leading to highly uniform triangle PbI₂ flakes with the average size of 5 µm and thickness of 17 nm. Moreover, as-fabricated PbI₂-based photodetectors show promising stable and flexible optoelectronic performances to 470 nm light, including high responsivity (0.72 A W^-1), large on/off ratio up to 900, fast photoresponse speed (rise time of 13.5 ms and decay time of 20 ms) and high detectivity (1.04 × 10¹⁰ Jones). The well-controllable growth of the uniform triangle PbI₂ flakes and the detailed exploration of their optoelectronic properties are particularly valuable for their further practical applications.