Resolving the Triexciton Recombination Pathway in CdSe/CdS Nanocrystals through State-Specific Correlation Measurements

Abstract

As luminescence applications of colloidal semiconductor nanocrystals push toward higher excitation flux conditions, there is an increased need to both understand and potentially control emission from multiexciton states. We develop a spectrally resolved correlation method to study the triply excited state that enables direct measurements of the recombination pathway for the triexciton, rather than relying on indirect extraction of rates. We demonstrate that, for core–shell CdSe–CdS nanocrystals, triexciton emission arises exclusively from the band-edge S-like state. Time-dependent density functional theory and extended particle-in-a-sphere calculations demonstrate that reduced carrier overlap induced by the core–shell heterostructure can account for the lack of emission observed from the P-like state. These results provide a potential avenue for the control of nanocrystal luminescence, where core–shell heterostructures can be leveraged to control carrier separation and therefore maintain emission color purity over a broader range of excitation fluxes.

Excited States and Their Dynamics in CdSe Quantum Dots Studied by Two-Color 2D Spectroscopy

“…Since then, the field has been expanding toward a broad combination of materials, sophisticated structures, and optoelectronic devices, such as light-emitting diodes and microspectrometers . Throughout the years, numerous studies have addressed a broad set of fundamental questions regarding excited states and their dynamics in QDs. − Several recent studies have addressed issues like high-intensity effects − and the influence of charging on excited-state dynamics , all important from the point of view of possible optoelectronic applications of QDs.…”

Photon Correlations in Spectroscopy and Microscopy

“…Technically, this requires extending the number of channels in an HBT setup by adding more beamsplitters and detectors or using a small pixelated detector array . Just as measuring g (2) targets the doubly excited state of the system, probing g ( n ) exclusively observes its n -times excited state. , …”

Simulations of nonradiative processes in semiconductor nanocrystals

“…These considerations have informed our development of the semiempirical pseudopotential model as a sufficiently detailed description of NCs that can also tackle calculations of experimentally relevant systems. For example, a CdSe quantum dot only 4 nm in diameter has over ∼1000 atoms and ∼4000 valence electrons, so the conventional workhorses of quantum chemistry, such as DFT and related methods for excited states, despite making significant progress, 68,69 are still far from being able to tackle this problem. On the other hand, continuum models based on the effective mass approximation have produced successful predictions for simple, linear spectroscopic observables 11 but are unable to capture many of the more complicated dynamic processes that determine the timescales of process like nonradiative exciton relaxation and AR.…”

“…II we describe the atomistic approach we have adopted to calculate quasiparticle excitations and neutral excitations in semiconductor NCs. First principles approaches, such as time-dependent density functional theory (DFT) [67][68][69] or many-body perturbation approximations, 70 are limited to describing excitons in relatively small clusters, typically those with fewer than 100 atoms, due to their steep computational scaling. 71,72 To make meaningful contact with experimental results on NCs that contain thousands of atoms and tens of thousands of electrons, we rely on the semiempirical pseudopotential model 9,25,26,73 to describe quasiparticle excitations.…”