Temporary Charge Carrier Separation Dominates the Photoluminescence Decay Dynamics of Colloidal CdSe Nanoplatelets

Abstract

Luminescent colloidal CdSe nanoplatelets with atomically defined thicknesses have recently been developed, and their potential for various applications has been shown. To understand their special properties, experiments have until now focused on the relatively short time scales of at most a few nanoseconds. Here, we measure the photoluminescence decay dynamics of colloidal nanoplatelets on time scales up to tens of microseconds. The excited state dynamics are found to be dominated by the slow (∼μs) dynamics of temporary exciton storage in a charge-separated state, previously overlooked. We study the processes of charge carrier separation and exciton recovery in pure CdSe nanoplatelets as well as in core-crown and core-shell CdSe/CdS nanoplatelets with high ensemble quantum yields of 50%, and discuss the implications. Our work highlights the importance of reversible charge carrier trapping and experiments over a wide range of time scales for the understanding of colloidal nanoemitters in general and nanoplatelets in particular.

Emergence of Nanoplatelet Light-Emitting Diodes

“…NPLs or colloidal quantum wells have received enormous research interest since their thickness can be controlled with atomic precision and NPLs can exhibit thickness-tunable emission [18,19,20,21,22]. In 2006, Joo et al reported the first CdSe nanoribbons/NPLs with a wurtzite structure via low-temperature solution-phase synthesis [23].…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Some NCs have a high quantum efficiency (near 100 %), while others have a quantum efficiency near 0 %. The two subpopulations in a NC ensemble are also known as the “bright fraction” and the “dark fraction” [251–256] (see Fig. 9).…”

“…The power-law exponents are around 1.5 for most NCs, and independent or nearly independent of temperature [276], excitation intensity [276], and nature of the excitation laser (continuous wave or pulsed) [296]. Interestingly, the band-edge emission of semiconductor NCs contains a slow “delayed” component with power-law statistics, that extends over time scales from nanoseconds up to (at least) milliseconds [240, 256, 297] (Fig. 10e).…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…Some NCs have a high quantum efficiency (near 100 %), while others have a quantum efficiency near 0 %. The two subpopulations in a NC ensemble are also known as the ''bright fraction'' and the ''dark fraction'' [251][252][253][254][255][256] (see Fig. 9).…”

“…The power-law exponents are around 1.5 for most NCs, and independent or nearly independent of temperature [276], excitation intensity [276], and nature of the excitation laser (continuous wave or pulsed) [296]. Interestingly, the band-edge emission of semiconductor NCs contains a slow ''delayed'' component with power-law statistics, that extends over time scales from nanoseconds up to (at least) milliseconds [240,256,297] (Fig. 10e).…”