Harmonic Quantum Coherence of Multiple Excitons in PbS/CdS Core-Shell Nanocrystals

Coherent electronic coupling in quantum dot solids induces cooperative enhancement of nonlinear optoelectronic responses

Synchronized dynamics of quantum dot (QD) ensembles are essential for generating ultrafast and giant optical responses beyond those of individual QDs. Increasing the strength of the direct electronic coupling between QDs is a key strategy for the realization of cooperative quantum phenomena. Here, we observe a quantum cooperative effect on nonlinear photocurrents caused by the coherent electronic coupling in semiconductor QD solids. We measure quantum interference signals cooperatively generated in QD solids. We control the inter-QD distance with atomic precision using bidentate ligands that strongly link the QDs. The harmonic quantum interference signals are strongly enhanced when shortening the molecular length of the ligand. Furthermore, we clarify that the coherence length of multiexcitons extends to neighbouring QDs. This finding is direct evidence that multiexciton coherent tunnelling assists the ultrafast exciton delocalization. Cooperative enhancement in QD solids may find application in advanced quantum optoelectronics.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Reduction of Optical Gain Threshold in CsPbI3 Nanocrystals Achieved by Generation of Asymmetric Hot-Biexcitons

Lead halide perovskite nanocrystals (NCs) are a class of promising light-emitting materials and have been considered as gain media in lasers. Strong exciton-exciton interactions in NCs cause an energy shift of the lowest optical transition and affect the optical gain threshold. Here, we clarify the dynamics of exciton-exciton interactions in highly photoexcited CsPbI₃ NCs by double-pump transient absorption spectroscopy. This method provides control over the population of each excited state by varying the time interval between the two pump pulses. We find that the band-edge energy shift induced by the formation of asymmetric hot-biexcitons (comprising one ground-state exciton and one hot exciton) is smaller than that induced by hot excitons and hot biexcitons in the ensemble. We demonstrate that the generation of asymmetric hot-biexcitons reduces the optical gain threshold in the CsPbI₃ NC ensemble.

Phase segregated Cu2-x Se/Ni3Se4 bimetallic selenide nanocrystals formed through the cation exchange reaction for active water oxidation precatalysts

Control over the composition and nanostructure of solid electrocatalysts is quite important for drastic improvement of their performance. The cation exchange reaction of nanocrystals (NCs) has been reported as the way to provide metastable crystal structures and complicated functional nanostructures that are not accessible by conventional synthetic methods. Herein we demonstrate the cation exchange-derived formation of metastable spinel Ni₃Se₄ NCs (sp-Ni₃Se₄) and phase segregated berzelianite Cu_2−xSe (ber-Cu_2−xSe)/sp-Ni₃Se₄ heterostructured NCs as active oxygen evolution reaction (OER) catalysts. A rare sp-Ni₃Se₄ phase was formed by cation exchange of ber-Cu_2−xSe NCs with Ni²⁺ ions, because both phases have the face-centered cubic (fcc) Se anion sublattice. Tuning the Ni : Cu molar ratio leads to the formation of Janus-type ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructured NCs. The NCs of sp-Ni₃Se₄ and ber-Cu_2−xSe/sp-Ni₃Se₄ heterostructures exhibited high catalytic activities in the OER with small overpotentials of 250 and 230 mV at 10 mA cm⁻² in 0.1 M KOH, respectively. They were electrochemically oxidized during the OER to give hydroxides as the real active species. We anticipate that the cation exchange reaction could have enormous potential for the creation of novel heterostructured NCs showing superior catalytic performance.

Bimetallic selenide nanocrystals formed by cation exchange reaction work as a precursor of efficient water oxidation electrocatalyst.

Ionization and Neutralization Dynamics of CsPbBr3 Perovskite Nanocrystals Revealed by Double-Pump Transient Absorption Spectroscopy

Ionization of nanocrystals (NCs) causes both photoluminescence intermittency and a reduction in luminescence quantum efficiency and thus plays a critical role in the optoelectronic performance of NC-based devices. Here, we study the ionization and neutralization processes of CsPbBr₃ perovskite NCs under strong photoexcitation by means of double-pump transient absorption spectroscopy. A strong initial pulse is used to generate ionized NCs, and their optical responses are investigated by varying the excitation intensity and delay time of the second pump pulse. We find that charging can occur either via nonradiative Auger recombination of biexcitons or via any possible recombination of trions. The presence of the extra charge inside of an ionized perovskite NC significantly reduces its absorption cross section. The experiments reveal that ionized NCs exhibit two types of neutralization processes with time constants on the order of nanoseconds and microseconds. These results are useful for the optimal design of NC-based photonic devices.

Enhanced Multiple Exciton Generation in PbS|CdS Janus-like Heterostructured Nanocrystals

Generating multiple excitons by a single high-energy photon is a promising third-generation solar energy conversion strategy. We demonstrate that multiple exciton generation (MEG) in PbS|CdS Janus-like heteronanostructures is enhanced over that of single-component and core/shell nanocrystal architectures, with an onset close to two times the PbS band gap. We attribute the enhanced MEG to the asymmetric nature of the heteronanostructure that results in an increase in the effective Coulomb interaction that drives MEG and a reduction of the competing hot exciton cooling rate. Slowed cooling occurs through effective trapping of hot-holes by a manifold of valence band interfacial states having both PbS and CdS character, as evidenced by photoluminescence studies and ab initio calculations. Using transient photocurrent spectroscopy, we find that the MEG characteristics of the individual nanostructures are maintained in conductive arrays and demonstrate that these quasi-spherical PbS|CdS nanocrystals can be incorporated as the main absorber layer in functional solid-state solar cell architectures. Finally, based upon our analysis, we provide design rules for the next generation of engineered nanocrystals to further improve the MEG characteristics.

Quantum coherence of multiple excitons governs absorption cross-sections of PbS/CdS core/shell nanocrystals

Multiple excitons in semiconductor nanocrystals have been extensively studied with respect to unique carrier dynamics including quantized Auger recombination and implementation in optoelectronic devices such as solar cells and photodetectors. However, the generation mechanism of multiple excitons still remains unclear. Here, we study instantaneous and delayed multiple exciton generation processes in PbS/CdS core/shell nanocrystals. The absorption cross-sections of biexcitons and triexcitons are identical to that of single excitons under instantaneous excitation with a single pulse. In contrast, the delayed excitation using double pulses shows a reduction of the biexciton and triexciton absorption cross-sections. Our theoretical analysis reveals that the excitonic coherence assists the generation of multiple excitons and that the reduction of multiple exciton absorption cross-sections is caused by the reduction of coherent excitation pathways. We clarify that exciton coherences play a key role in multiple exciton generation processes and seamlessly connect the identical and reduced multiple exciton absorption cross-sections.

Hot Biexciton Effect on Optical Gain in CsPbI3 Perovskite Nanocrystals

Combining the superior optical properties of their bulk counterparts with quantum confinement effects, lead halide perovskite nanocrystals are unique laser materials with low-threshold optical gain. In such nonlinear optical regimes, multiple excitons are generated in the nanocrystals and strongly affect the optical gain through many-body interactions. Here, we investigate the exciton-exciton interactions in CsPbI₃ nanocrystals by femtosecond transient absorption spectroscopy. From the analysis of the induced absorption signal observed immediately after the pump excitation, we estimated the binding energy for the hot biexcitons that are composed of an exciton at the band edge and a hot exciton generated by the pump pulse. We found that the exciton-exciton interaction becomes stronger for hot excitons with greater excess energies and that the optical gain can be controlled by changing the excess energy of the hot excitons.