Correlation Between Photooxidation and the Appearance of Raman Scattering Bands in Lead Chalcogenide Quantum Dots

In this report, we carefully study the effects of photooxidation on the Raman spectra of lead chalcogenide (PbX) quantum dots (QDs). Photoexcitation of PbS, PbSe, and PbTe QD films at 488 nm with power densities as low as 30 W/cm² gives rise to several peaks related to both lead(II) oxide and the group VI chalcogenates (PbXO₄). The amplitudes of these peaks are shown to increase with continuous laser illumination in air, but are completely absent for samples illuminated under rigorously air-free conditions. These results suggest that the ∼135 cm^-1 Raman peak often assigned to an intrinsic PbX LO phonon is more likely an artifact arising from photooxidation. The myriad of potential photooxidation products formed quickly in laser-illuminated, air-exposed PbX QDs suggest that caution should be used in the assignment and interpretation of phonon spectra and phonon-mediated exciton relaxation pathways of these materials, unless the processing and experiments are conducted under air-free conditions.