Controlled Alloying of the Core–Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination

Abstract

The influence of a CdSexS1-x interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSexS1-x layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSexS1-x alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the core-shell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, e.g., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics.

Interfacial engineering of core/shell heterostructured nanocrystal quantum dots for light-emitting applications

“…The critical shell thickness and the slope for k nr_def over the shell thickness depend on the structural difference, such as the crystal structures and lattice parameters, between the core and the shell. For example, the QYs of CdSe/ZnS NQDs (lattice mismatch = 12%) [19,35] start to drop at a much thinner shell with a much faster rate than the QYs of CdSe/CdS NQDs (lattice mismatch = 3.9%) (Figure 1(d)) [27,28].…”

“…Spectroscopic analysis combined with structural characterization has unveiled that the interfaces of the core/shell heterostructures indeed play important roles in determining the optical properties of NQDs [21][22][23][24][25]28,29]. Specifically, the structural formulation at the core-shell interface affects the formation of misfit defects that are efficient non-radiative recombination centers [21][22][23][24]27,30].…”

“…Specifically, the structural formulation at the core-shell interface affects the formation of misfit defects that are efficient non-radiative recombination centers [21][22][23][24]27,30]. In addition, the potential profiles across the interfaces influence the Auger recombination (AR) rates [28,29,31,32]. Beyond gaining photophysical insights into the structure-property relationship, the advances in interfacial engineering have enabled the control of the optical properties of core/shell heterostructured NQDs and have led to substantial advances in their applications [5,24,33].…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…For more subtle control over Auger processes, hetero-NCs have been developed with designed charge carrier confinement potentials [219,220]. The most commonly studied hetero-NC composition for reduced Auger losses is CdSe/CdS in various shapes and sizes [72,84,206,[221][222][223]. It was first predicted theoretically [224,225] and later confirmed experimentally [84,[221][222][223] that an alloyed coreshell interface leads to suppressed Auger recombination.…”

“…The most commonly studied hetero-NC composition for reduced Auger losses is CdSe/CdS in various shapes and sizes [72,84,206,[221][222][223]. It was first predicted theoretically [224,225] and later confirmed experimentally [84,[221][222][223] that an alloyed coreshell interface leads to suppressed Auger recombination. An alloyed heterointerface creates a smooth confinement potential for charge carriers in which highmomentum components in the wavefunctions are reduced in amplitude (see Fig.…”