Single surface charges on aliovalently doped semiconductor nanocrystals determine their photoluminescence properties

Abstract

For decades, semiconductor nanocrystals (NCs) have been studied for their interesting electronic and optical properties, which also make them potentially suitable for a no. of applications, including solar energy conversion, displays and biomedical imaging. Cation exchange, a chem. transformation used to modify a crystal whereby a cation from soln. is replaces a host cation, has recently become a highly effective tool for enabling the synthesis of nanoparticles with novel chem. compns. In particular, aliovalent doping of CdSe nanocrystals (NCs) via cation exchange of cadmium ions for silver ions has become quite popular for manipulating the optical and electronic properties of the doped NCs, such as for producing n- or p-type NCs. However, despite over a decade of study, the relationship between optical properties of the NC and the aliovalent dopants has largely gone unexplained, partially due to sample under characterization. We will discuss how electrostatic force microscopy (EFM) with single electron sensitivity can be used to det. the charges of individual, cation-doped CdSe NCs in order to investigate their net charge as a function of added cations. While there was no direct trend relating the NC charge to the relative amt. of cation per NC, there was a remarkable and unexpected correlation between the av. NC charge and ensemble exciton photoluminescence (PL) intensity, for all dopant cations introduced. We use an effective mass theor. model to conclude that the changes in PL intensity, as tracked also by changes in NC charge, are likely a consequence of changes in the NC radiative rate caused by symmetry breaking of the electronic states of the nominally spherical NC due to the Columbic potential introduced by ionized cations. Further, we show through energy loss spectroscopy on individual NCs that the cation exchange process is highly heterogeneous, which has profound implications for possible future applications of doped NCs.

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