Suppressed Blinking and Auger Recombination in Near-Infrared Type-II InP/CdS Nanocrystal Quantum Dots

Abstract

Non-blinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1–11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes—up to >7 ns—were obtained for the thickest-shell structures, indicating dramatic suppression of non-radiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

Charge Transfer Dynamics and Auger Recombination of CdTe/CdS Core/Shell Quantum Dots

“…5,11 A relatively small lattice mismatch for epitaxial growth and a large conduction-/valence-band offset for band-alignment tuning are two important factors for the manipulation of nanoheterostructures, such as CdS/ZnSe, CdSe/ZnTe, CdSe/CdS, InP/CdS, and CdTe/CdS core/shell QDs. [11][12][13][14][15] The lattice parameters of bulk CdTe and CdS are 6.481 and 5.838 Å, respectively. 16 CdTe conduction-and valence-band energies (about 3.5 and 5.2 eV, respectively, below vacuum) are higher than those of CdS (about 3.7 and 6.3 eV, respectively, below vacuum) by 0.2 and 1.1 eV, respectively.…”

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…The diffraction peaks observed for the InP CQDs agree well with the expected peak positions and relative intensities. Dennis et al 18 reported that for 'giant' InP/CdS CQDs, with shells that were up to 4 nm thick, the CdS forms in its more thermodynamically stable hexagonal phase. However, here the diffraction peaks are little changed on the addition of the shell, and are consistent with the formation of CdS in its cubic phase (zinc blende/Hawleyite).…”

“…13 In this work, we study MEG in dispersions of quasi-Type II CQDs formed by the growth of a CdS shell around a InP core; in this CQD structure, the electron is delocalised over the core and shell whilst the hole is confined to the InP core, 18 as illustrated in Figure 1. InP CQDs form with a cubic crystal structure (lattice constant 5.87 Å) 19 but the thermodynamically stable phase 4 of CdS is hexagonal 18 . However, CdS can also exist in a cubic phase (lattice constant 5.83 Å) 19 which, as we show below, can form when thin shells of CdS grow on to an InP core.…”

“…5 A CdS shell has also been shown to be an effective means of passivating surface traps, as evidenced by the increase in photoluminescence (PL) and reduction in PL intermittency (also known as 'blinking') resulting from the addition of such a shell. 18 Moreover, this surface passivation does not come at the cost of extraction efficiency from the CQD. Wu et al 20 showed that the characteristic electron transfer time from an InP CQD to a methylviologen acceptor molecule was just 11 ps, and was little changed after the addition of a CdS shell at 15 ps.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…In type-I 1/2 (or quasi-type-II) hetero-NCs one carrier is delocalized over the whole volume of the hetero-NC, while the other is localized in one of the segments (e.g., CdSe/CdS, ZnSe/CdSe). This allows the electron-hole spatial overlap to be tailored by controlling the size, shape, and composition of each segment of the hetero-NC, which has a dramatic impact on several properties (viz., quantum yields, stability, PL wavelength [15,16,21,60,61], reabsorption cross section [22,29,[62][63][64], radiative lifetimes [60,[64][65][66], exciton-phonon coupling strength [67][68][69], Auger recombination [66,[70][71][72], hot carrier relaxation [51,73], thermal quenching [74,75]). The general trend is that the exciton lifetime, exciton-phonon coupling, and PL wavelength increase when going from the type-I to the type-II localization regimes, while Auger recombination rates and hot carrier relaxation rates are reduced.…”