Explaining the Unusual Photoluminescence of Semiconductor Nanocrystals Doped via Cation Exchange

Abstract

Aliovalent doping of CdSe nanocrystals (NCs) via cation exchange processes has resulted in interesting and novel observations for the optical and electronic properties of the NCs. However, despite over a decade of study, these observations have largely gone unexplained, partially due to an inability to precisely characterize the physical properties of the doped NCs. Here, electrostatic force microscopy was used to determine the static charge on individual, cationdoped CdSe NCs in order to investigate their net charge as a function of added cations. While the NC charge was relatively insensitive to the relative amount of doped cation per NC, there was a remarkable and unexpected correlation between the average NC charge and PL intensity, for all dopant cations introduced. We 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 Coulombic potential introduced by ionized cations.

Heterogeneity in Cation Exchange Ag+ Doping of CdSe Nanocrystals

“…1,2,3,4 Specifically, aliovalent doping can introduce free charge carriers through extra electrons or holes offered by the dopant, with the resulting doped material quite useful for electronic applications. In particular, doping NCs aliovalently has been shown to create interesting optical and charge characteristics, 3,5,6,7 including a correlated enhancement in photoluminescence (PL) intensity and increased average charge. 5 A common technique for doping is cation exchange, 8,9,10,11,12,13,14,15,16 including postsynthesis 9,17 and one-pot 6,18 cation exchanges, which are both becoming more common especially for the Ag + -CdSe NC exchange.…”

“…In particular, doping NCs aliovalently has been shown to create interesting optical and charge characteristics, 3,5,6,7 including a correlated enhancement in photoluminescence (PL) intensity and increased average charge. 5 A common technique for doping is cation exchange, 8,9,10,11,12,13,14,15,16 including postsynthesis 9,17 and one-pot 6,18 cation exchanges, which are both becoming more common especially for the Ag + -CdSe NC exchange. 3,5,6 Cation exchange is an extremely important synthesis technique to not only produce doped NCs, but also to facilitate the conversion of one NC to a chemically different NC, assembling structures that cannot be produced directly.…”

“…5 A common technique for doping is cation exchange, 8,9,10,11,12,13,14,15,16 including postsynthesis 9,17 and one-pot 6,18 cation exchanges, which are both becoming more common especially for the Ag + -CdSe NC exchange. 3,5,6 Cation exchange is an extremely important synthesis technique to not only produce doped NCs, but also to facilitate the conversion of one NC to a chemically different NC, assembling structures that cannot be produced directly. [19][20][21][22] The supposed benefits of the post-synthesis cation exchange for doping of NCs include increased control over doping level and the availability of an undoped sample for direct comparison to the doped samples, 3 while the one-pot exchange allows the dopant to be introduced during or immediately following the NC synthesis.…”

“…6,18 While it is clear that doping induces changes in the optical properties of NCs, different configurations of charged impurities cause variations in the resulting NC photophysics. 3,5,6,7,23,24 For example, for Ag + doped CdSe NCs, the post-synthesis exchange resulted in an enhanced exciton PL intensity and growth of a weak dopant peak to the red of the band edge fluorescence. 3,5 Conversely, the in situ exchange has been shown to quench the exciton peak while producing intense, broad, red-shifted dopant PL.…”

“…3,5,6,7,23,24 For example, for Ag + doped CdSe NCs, the post-synthesis exchange resulted in an enhanced exciton PL intensity and growth of a weak dopant peak to the red of the band edge fluorescence. 3,5 Conversely, the in situ exchange has been shown to quench the exciton peak while producing intense, broad, red-shifted dopant PL. 6 Also, it was recently shown that the PL spectra from Ag + doped CdSe NCs is highly sensitive to the exact placement of the impurity atoms around the NC.…”