Affordable Determination of the Elemental Composition of Colloidal Quantum Dots with Portable X-Ray Fluorescence Spectroscopy

Semiconductor nanocrystal quantum dots possess unique optoelectronic properties dependent on their size, shape, and elemental composition. As such, determining their chemical composition is essential to understanding their structure-property relationships to enhance current and/or enable new applications of these low-dimensional semiconductor nanomaterials. Current methods used to measure their composition employ large, expensive instrumentation such as energy dispersive x-ray spectroscopy or inductively coupled plasma optical emission spectroscopy (ICP-OES), which is destructive to the sample. While large and expensive laboratory-scale X-ray fluorescence spectrometers have also been employed to investigate the elemental composition of some nanomaterials, cost-effective portable X-ray fluorescence (pXRF) spectrometers have not yet been demonstrated to have the necessary sensitivity to accurately determine the chemical composition of nanomaterials. pXRF spectroscopy is a handheld non-destructive instrumental technique operated on a simple benchtop under ambient conditions with minimal sample preparation. We experimentally measured the chemical composition of multiple sizes of colloidal solutions of CdSe quantum dots with pXRF and determined the elemental Cd/Se ratio of 3.3 nm and 2.3 nm CdSe nanocrystals to be 1.37±0.08 and 1.5±0.1, respectively. These results agree with previous literature, and we further validated the results with ICP-OES. We also determined figures of merit for the pXRF spectrometer and found limits of detection for Cd and Se to be 56 ppm and 54 ppm, respectively. Recently, we have extended these studies to CsPbBr₃ perovskite quantum dots and demonstrated that pXRF can be applied to different colloidal systems to determine elemental composition. This work demonstrates that colloidal nanomaterials of various compositions can be investigated at the atomic level with portable, cost-effective, and non-destructive chemical instrumentation. pXRF is a cheaper alternative to the ‘standard’ analytical techniques currently employed to interrogate elemental composition of colloidal nanomaterials and our work with this affordable technique allows for nanomaterials research to be accessible to research laboratories and institutions that may have limited resources for facilities and instrumentation.