Measuring the Vibrational Density of States of Nanocrystal-Based Thin Films with Inelastic X‑ray Scattering

Knowledge of the vibrational structure of a semiconductor is essential for explaining its optical and electronic properties and enabling optimized materials selection for optoelectronic devices. However, measurement of the vibrational density of states of nanomaterials is challenging. Here, using the example of colloidal nanocrystals (quantum dots), we show that the vibrational density of states of nanomaterials can be accurately and efficiently measured with inelastic X-ray scattering (IXS). Using IXS, we report the first experimental measurements of the vibrational density of states for lead sulfide nanocrystals with different halide-ion terminations and for CsPbBr3 perovskite nanocrystals. IXS findings are supported with ab initio molecular dynamics simulations, which provide insight into the origin of the measured vibrational structure and the effect of nanocrystal surface. Our findings highlight the advantages of IXS compared to other methods for measuring the vibrational density of states of nanocrystals such as inelastic neutron scattering and Raman scattering.