Enhanced Raman scattering on lead iodide film

Surface-enhanced Raman scattering (SERS) on organic-inorganic hybrid perovskites have been proved a promising SERS-active substrate. As its inorganic skeleton, lead iodide (PbI₂) also represents a novel semiconductor substrate material that serves as either a complement or an alternative to conventional metal-based SERS. Here, for the first time, enhanced Raman scattering (ERS) of 1,2-bis(4-pyridyl)ethylene (BPE) adsorbed on PbI₂ film is observed. To our surprise, we obtained the same ERS spectra on the PbI₂ as SERS spectra obtained on the perovskites, which demonstrates the enhancement of substrate comes from inorganic skeleton rather than organic-inorganic hybrid perovskites. Compared with the normal Raman spectrum of BPE molecules, we find the B_u mode of the BPE molecule was selectively enhanced. We demonstrate that it involves a charge transfer (CT) resonance mechanism and takes place between the HOMO of BPE and the conduction band edge of PbI₂. Moreover, the PbI₂ substrate performs good reproducibility, uniformity and good Raman enhancement (EF ≥ 10⁵). It is believed that this unusual discovery not only enables PbI₂ to be an enhanced substrate, which also provides a new advance toward a better understanding of SERS chemical mechanism.