Size Determination of Cadmium Selenide Quantum Dots via Small-Angle X-ray Scattering

The rising demand for quantum dots (QDs) necessitates swift and reliable quality verification methods. This work focuses on the synthesis of organically passivated CdSe core QDs and their comprehensive characterization via UV/Vis spectroscopy, transmission electron microscopy, and small-angle X-ray scattering (SAXS). The aim was to correlate data to ISO/TS 17466 (data standard deviation ~15%) and to validate SAXS as a reliable method for sizing data modeling. This was achieved by obtaining the number-weighted hard-sphere parameter Dn with a relative deviation percentage of 6.9 ± 4.4 %. Additionally, the volume-weighted gyradius Dg, the model-free mode of the pair-distribution function Dmode, and the volume-weighted hard-sphere parameter Dv were derived and expressed as readily applicable tight-binding model functions. This work utilized laboratory SAXS to measure sub-2 nm particles and various bimodal size distributions of mixed particle samples between 2 and 7 nm. The traceable results significantly enhanced the certainty of characterizing such small particles near the instrumental resolution limit. The generalized indirect Fourier transform method consistently identified mean values for bimodal samples, demonstrating SAXS as a viable method. However, individual population modes were only revealed using experimentally weak Lagrange multiplier. Additionally, this work addresses the longevity of colloidal QDs by preserving their size distribution through embedding in epoxy resin, yielding easily manageable casts of great value for various SAXS applications. The method for epoxy resin embedment supports the proof-of-concept of core CdSe nanoparticles as a sub-10 nm standard, although homogenizing cast processing optimizations are necessary to reduce uncertainties. Hence, an alternative design concept for vertical instead of horizontal casting is proposed. Furthermore, it was observed that the surrounding matrix blue-shifts the band edge energy proportional to the number fraction of surface atoms, highlighting the necessity of understanding matrix-dependent sizing data. Surface reconstruction effects are indicated as the main cause, with follow-up experiments proposed for probing the impact of stress by alternating the matrix. Critical factors for establishing a CdSe QD standard from pre-synthesis to post-evaluation are discussed, emphasizing the importance of handling and process unification. These metrological discussions are essential to delimit the size-dependent property range and uncertainty inherent in the quantum dot material itself.