Anomalously ultra-strong anti-Stokes photoluminescence in submicrometer-thick van der Waals layered semiconductor PbI2

Anti-Stokes photoluminescence (ASPL) in low-dimensional van der Waals (vdW) layered materials is becoming increasingly attractive for its potential in advanced applications such as optical cooling, sub-energy band detection and optoelectronic devices. While transition metal dichalcogenides (TMDCs), among the most studied vdW semiconductors for ASPL, exhibit a direct bandgap exclusively in their monolayer form. This characteristic results in a short light-matter interaction distance and thus low ASPL emission efficiency, which seriously impedes the advancement of ASPL in vdW layered materials. In contrast, transition metal halide lead iodide (PbI₂), a vdW semiconductor with a direct bandgap in a wide range of thicknesses (⩾3 layers) superior to TMDCs, has shown promise for ASPL. However, the reported ASPL emission efficiency of PbI₂ is notably low. Moreover, scant research has focused on the rich ASPL emission states in PbI₂, particularly concerning the assignment of these emission states. Here, through a designed thickness selection, we observed more detailed ASPL emissions in submicrometer-thick PbI₂ at room temperature, in addition to a series of previously unreported ASPL emission peaks that emerge at low temperatures. Importantly, the low-temperature ASPL of PbI₂ exhibits an approximate 1000-fold enhancement compared to that observed at room temperature. This significant enhancement is attributed to the transition from phonon-assisted one-photon absorption to two-step photon absorption induced by resonance absorption effect, as well as substantially reduced nonradiative decays at low temperatures. Our findings enhance the comprehensive understanding of ASPL in PbI₂, holding great significance for the development of ASPL applications.