Two-dimensional (2D) materials are showing promise in next-generation optoelectronic devices. Lead iodine (PbI₂) is a well-known transition metal halide with a hexagonal I-Pb-I layered structure. It displays high electron and highly anisotropic hole mobilities. Unlike graphene, a zero-overlap semimetal, PbI₂ possesses a direct band gap in bulk (2.38 eV) and indirect band gap in monolayer (2.64 eV) due to quantum size effects.  

Recently, a collaborative study led by Prof. WANG Jun's group at Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and Prof. SUN Zhenyu's group from Beijing University of Chemical Technology, prepared 2D PbI₂ nanosheets in cyclopentanone and investigated their nonlinear optical properties.

In the cooperative work, the high-quality PbI₂ nanosheets were obtained by employing an ultrasonication-assisted liquid-phase exfoliation process in the polar aprotic solvent (CH₂)₄CO and the resulting dispersions can remain good stability over 30 days.  

The exfoliation kinetics was observed to fit an apparent pseudo-first-order process. Typical scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) measurements have been performed to characterize the prepared samples.  

They further investigated the saturable absorption (SA) properties of PbI₂ nanosheets both at fs pulse of 515 nm and ns pulse of 532 nm, and found that the PbI₂ nanosheets exhibited stronger SA response at 6 ns pulse excitation than that at 340 fs pulse excitation.  

The FOM, σ_es/σ_gs ratio and I_sat values have been compared with some 2D perovskite, graphene, BP and MoS₂ materials. The high stability along with low nonsaturable loss renders PbI₂ useful as an alternative in passive Q-switch, mode locking, and other ultrafast optoelectronic devices in the visible regime.  

This semiconductor has a great potential as a precursor for the synthesis of lead halide organic-inorganic perovskites in solar cells and photodetectors as well as in optoelectronic devices.  

This study, entitled "Liquid exfoliation of two-dimensional PbI₂ nanosheets for ultrafast photonics" has been published online in ACS Photonics on Mar. 25, 2019.

This work was supported by the Chinese National Natural Science Foundation, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Science of CAS, and the Program of Shanghai Academic Research Leader, the State Key Laboratory of Organic-Inorganic Composites, the Beijing Natural Science Foundation and Beijing National Laboratory for Molecular Sciences.  

PbI₂ crystal structure and saturable absorption property(Image by SIOM)