Abstract
Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. In this limited class of nanocrystals, CdSe/CdS core/arm tetrapods exhibit the unusual trait of two-colour (red and green) multiexcitonic emission, with green emission from the CdS arms emerging only at high excitation fluences. Here we show that by synthetic shape-tuning, both this multi-colour emission process, and blinking and photobleaching behaviours of single tetrapods can be controlled. Specifically, we find that the properties of dual emission and single-nanostructure photostability depend on different structural parameters—arm length and arm diameter, respectively—but that both properties can be realized in the same nanostructure. Furthermore, based on results of correlated photoluminescence and transient absorption measurements, we conclude that hole-trap filling in the arms and partial state-filling in the core are necessary preconditions for the observation of multiexciton multi-colour emission.
Highlights
Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance
Using 4 nm CdSe zinc-blende QDs17 as seed material, we synthesized a series of tetrapods characterized by differences in both arm thickness and arm length
The resulting tetrapods were as follows: thin/short (TP1: 6.3 nm±0.5 nm/24.8 nm±2.3 nm), thick/short (TP2: 10.9 nm±0.8 nm/27.6 nm±2.9 nm), thin/long (TP3: 8.2 nm±1.3 nm/40.7 nm±3.8 nm) and thick/long (TP4: 10.6 nm±2.4 nm/41.3 nm±4.6 nm) (Fig. 1). (Note: TP diameters were measured at the arm base.) TP1, TP3 and TP4 were prepared exclusively using the recently developed syntheses[2,19,20], whereas TP2 tetrapods were fabricated from TP1 tetrapods using the successive ionic layer adsorption and reaction (SILAR) method to controllably add CdS shell monolayers[21]
Summary
Semiconductor nanostructures capable of emitting from two excited states and thereby of producing two photoluminescence colours are of fundamental and potential technological significance. This constitutes a multi-excitonic emission process as intense optical excitation results in the formation of multiple excitons that contribute together to the observed PL and, the exciton states that produce these red and green emissions are spatially separated between the core and an arm, respectively, they are electronically coupled.
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