Band Gap Engineering Improves the Efficiency of Double Quantum Dot Upconversion Nanocrystals

Abstract

Solution-processed core/multishell semiconductor quantum dots (QDs) could be tailored to facilitate the carrier separation, promotion, and recombination mechanisms necessary to implement photon upconversion. In contrast to other upconversion schemes, upconverting QDs combine the stability of an inorganic crystalline structure with the spectral tunability afforded by quantum confinement. Nevertheless, their upconversion quantum yield (UCQY) is fairly low. Here, design rules are uncovered that enable to significantly enhance the performance of double QD upconversion systems, and these findings are leveraged to fabricate upconverting QDs with increased photon upconversion efficiency and reduced saturation intensities under pulsed excitation. The role of the intra-QD band alignment is exemplified by comparing the upconversion process in PbS/CdS/ZnSe QDs with that of PbS/CdS/CdSe ones with variable CdSe shell thicknesses. It is shown that electron delocalization into the shell leads to a longer-lived intermediate state in the QDs, facilitating further absorption of photons, and enhancing the upconversion process. The performance of these upconversion QDs under pulsed excitation versus continuous pumping is also compared; the reasons for the significant differences between these two regimes are discussed. The results show how one can overcome some of the limitations of previous upconverting QDs, with potential applications in biophotonics and infrared detection.

CdSe/CdS/CdTe Core/Barrier/Crown Nanoplatelets: Synthesis, Optoelectronic Properties, and Multiphoton Fluorescence Upconversion

“…The ternary heterostructure combines an indirect ground state with direct excited-state transitions and should therefore allow for fluorescence upconversion. As reported previously, colloidal nanocrystals are particularly well suited for fluorescence upconversion, − in particular CdSe:Te/CdS/CdSe, PbSe/CdSe/CdS, and PbS/CdS-CdSe/ZnS QDs have already demonstrated relatively high upconversion efficiencies (of the order of several percent) of near-infrared light. Here we excited sample Batch2 with 5 ns pulses at a repetition rate of 10 Hz, probing the CBC NPL transient fluorescence spectrum to investigate the upconversion process.…”

“…In order to assess the upconversion efficiency ϵ UC , a comparison can be made between the number of fluorescence counts per absorbed photon, at saturation intensity for two-photon ( Compared to other colloidal nanocrystals, [44][45][46] this efficiency is quite modest, likely due to the relatively large crown area of 71 nm 2 (compared to the overall NPL area of 338 nm 2 ), and a possible remedy could be to further engineer the band structure to optimize relaxation of the hot hole toward the CdSe core. 46 On the other hand, the 2D shape offers a specific benefit. Due to the high nonlinear absorption coefficient, 18 not only two-photon, but also three-photon upconversion is feasible.…”

“…It gives a ratio of 0.014:1, which means that 36 absorbed NIR photon pairs are equivalent to one absorbed visible photon. Compared to other colloidal nanocrystals, − this efficiency is quite modest, likely due to the relatively large crown area of 71 nm 2 (compared to the overall NPL area of 338 nm 2 ), and a possible remedy could be to further engineer the band structure to optimize relaxation of the hot hole toward the CdSe core . On the other hand, the 2D shape offers a specific benefit.…”

Shell Stabilization of Photocatalytic ZnSe Nanorods

“…Gaining stability, we next addressed the effect of the shell growth on the photocatalytic activity. This is particularly relevant since, in principle, the band‐alignment of the ZnSe/ZnS system is a straddling gap (Type‐I), in which the band‐gap of the ZnS shell is larger than that of the ZnSe core and straddles it (Figure S4) . Type‐I band alignment might prevent the ability to extract the charge carriers from the core to achieve photocatalytic functionality but a thin shell may still allow this.…”

“…We examined the effects of the surface coating, hole acceptor and pH on the photocatalytic efficiency as a means to further optimize the system's photocatalytic functionality. Although the band alignment of the ZnSe/ZnS system is of Type‐I, therefore confining the electrons and holes to the core ZnSe, the use of the thin ZnS shell still enables high photocatalytic efficiencies while greatly enhancing the system stability.…”