The implications of grading on the emission line width of core-shell nanocrystals

An excitonic coupling with phonons leads to broadening of the emission line. The narrow excitonic emission line width observed in CdZnS/ZnS alloy core graded shell nanocrystals (NCs) with varying compositions is studied using temperature-dependent photoluminescence measurements. Contrary to the consensus that a narrow emission line width is observable with a reduction in size (due to the increased exciton-acoustic phonon coupling coefficient sigma), an increased value is noted with reduced size. Based on a theoretical approach to graded core-shell NCs, the relationship between the electron-hole wave function overlap and exciton lifetime is invoked to understand this anomaly. Smaller alloy core-shell NCs (CdZnS/ZnS-I) have a longer lifetime than larger NCs (CdZnS/ZnS-II), indicating reduced electron-hole wave function overlap for CdZnS/ZnS-I NCs and hence a larger 'effective size' of NCs, even though the actual size is smaller. The experimental findings demonstrate that graded core-shell NCs reveal an additional functionality, facilitating control of the emission line width of NCs via minimal interaction with the solid state environment.