Method for Determining the Elemental Composition and Distribution in Semiconductor Core−Shell Quantum Dots

摘要

ABSTRACT: In the biological sciences, the use of core-shellnquantum dots (QDs) has gained wide usage but analyticalnchallenges still exist for characterizing the QD structure. Thenapplication of energy-dispersive X-ray spectroscopy and X-raynphotoelectron spectroscopy (XPS) to bulk materials is relativelynstraightforward; however, for meaningful applications of surfacenscience techniques to multilayer nanoparticles requires novelnmodifications and analysis methods. To experimentally charac-nterize the elemental composition and distribution in CdSeCdS/ZnS QDs, we first develop a XPS signal subtraction tech-nnique capable of separating the overlapped selenium 3s (core)nand sulfur 2s (shell) peaks (both peaks have binding energiesnnear 230 eV) with higher precision than is typically reported innthe nanoparticle literature. This method is valid for any nano-nparticle containing selenium and sulfur. Then we apply a cor-nrection formula to the XPS data and determine that the 2 nmnstoichiometric CdSe core is surrounded by 2 CdS layers and anstoichimetric ZnS monolayer. These findings and the multiapproach methodology represent a significant advancement in thendetailed surface science study of multilayer nanoparticles. In agreement with recent surprising findings, the time-of-flight secondarynmass spectrometry measurements suggest that the surface sites of the QDs used in this study are primarily covered with a mixture ofnoctadecylphosphonic acid and trioctylphophine oxide.