Masaki Saruyama

Spontaneous Formation of Wurzite-CdS/Zinc Blende-CdTe Heterodimers through a Partial Anion Exchange Reaction

Ion exchange of ionic semiconductor nanoparticles (NPs) is a facile method for the synthesis of type-II semiconductor heterostructured NPs with staggered alignment of band edges for photoelectric applications. Through consideration of the crystallographic orientation and strain at the heterointerface, well-designed heterostructures can be constructed through ion exchange reactions. Here we report the selective synthesis of anisotropically phase-segregated cadmium sulfide (CdS)/ cadmium telluride (CdTe) heterodimers via a novel anion exchange reaction of CdS NPs with an organic telluride precursor. The wurtzite-CdS/zinc blende-CdTe heterodimers in this study resulted from spontaneous phase segregation induced by the differences in the crystal structures of the two phases, accompanying a centrosymmetry breaking of the spherical CdS NPs. The CdS/CdTe heterodimers exhibited photoinduced spatial charge separation because of their staggered band-edge alignment.

Drastic Structural Transformation of Cadmium Chalcogenide Nanoparticles Using Chloride Ions and Surfactants

In the present work, we studied a unique and facile method for the drastic structural transformation of hydrophobic small CdE (E = S, Se, Te) nanoparticles into large, high-quality pencil-shaped nanoparticles through an Ostwald ripening process induced by Cl(-) and surfactants (oleic acid and oleylamine). This study revealed that Cl(-) is the effective anion for the controlled structural transformation of CdE nanoparticles. This transformation reaction can be readily extended to the formation of various functional materials.

Large‐Scale Synthesis of High‐Quality Metal Sulfide Semiconductor Quantum Dots with Tunable Surface‐Plasmon Resonance Frequencies

High-quality CdS and Cu(7)S(4) quantum dots (QDs) were synthesized with N,N-dibutylthiourea (DBTU) as an organic sulfur source. In this method, nucleation and growth reactions were controlled simply by the heating rate of the reaction. The mild oxidation conditions gave monodisperse CdS QDs exhibiting pure band-edge emission with relatively high photoluminescence quantum yield. During the synthesis of Cu(7)S(4) QDs, the addition of dodecanethiol to the reaction system controlled the reaction rate to give monodisperse spherical or disk-shaped QDs. A hundred-gram scale of copper precursor could be used to generate the high-quality Cu(7)S(4) QDs, indicating that an industrial-scale reaction is achievable with our method. As observed in anisotropic noble-metal nanocrystals, larger disk-shaped Cu(7)S(4) QDs showed lower localized-surface-plasmon resonance energy in the near-infrared region. The disk-shaped Cu(7)S(4) QDs could be used effectively as templates to form cation-exchanged monodisperse disk-shaped CdS QDs.

CdPd sulfide heterostructured nanoparticles with metal sulfide seed-dependent morphologies.

Seed-mediated growth synthesis has provided us with anisotropically phase-segregated CdPd sulfide heterostructured nanoparticles with seed-dependent morphologies and crystal structures.

Dynamics of Charged Excitons and Biexcitons in CsPbBr3 Perovskite Nanocrystals Revealed by Femtosecond Transient-Absorption and Single-Dot Luminescence Spectroscopy

Metal-halide perovskite nanocrystals (NCs) are promising photonic materials for use in solar cells, light-emitting diodes, and lasers. The optoelectronic properties of these devices are determined by the excitons and exciton complexes confined in their NCs. In this study, we determined the relaxation dynamics of charged excitons and biexcitons in CsPbBr3 NCs using femtosecond transient-absorption (TA), time-resolved photoluminescence (PL), and single-dot second-order photon correlation spectroscopy. Decay times of ∼40 and ∼200 ps were obtained from the TA and PL decay curves for biexcitons and charged excitons, respectively, in NCs with an average edge length of 7.7 nm. The existence of charged excitons even under weak photoexcitation was confirmed by the second-order photon correlation measurements. We found that charged excitons play a dominant role in luminescence processes of CsPbBr3 NCs. Combining different spectroscopic techniques enabled us to clarify the dynamical behaviors of excitons, charged excitons, and biexcitons.

Investigation on photo-induced charge separation in CdS/CdTe nanopencils

CdS/CdTe nanopencils were synthesized via anion exchange. The effect of the geometry on the carrier dynamics was investigated. Transient absorption measurements indicated that these factors did not affect the charge separation rate in CdS/CdTe nanopencils. The results provide important insight for designing heterostructured nanocrystals with efficient charge separation and integrated structures.

Crystal structure-selective formation and carrier dynamics of type-II CdS–Cu31S16 heterodimers

Anisotropically phase-segregated CdS–Cu31S16 heterodimers with type-II band alignment were spontaneously formed by selective growth of monoclinic Cu31S16 phases on preformed hexagonal CdS phases. The photo-induced carrier dynamics of the heterodimer was investigated by fluorescence and transient absorption measurements.

Coulomb-Enhanced Radiative Recombination of Biexcitons in Single Giant-Shell CdSe/CdS Core/Shell Nanocrystals

Giant-shell CdSe/CdS core/shell nanocrystals have attracted much attention due to their unique quasi-type-II band alignment, where a large valence band offset confines holes strongly to the core but electrons are delocalized due to a small conduction band offset. Here, we report the observation of the relative enhancement in the radiative recombination rate of a biexciton compared to that of an exciton in giant-shell CdSe/CdS nanocrystals. We found a clear correlation between the shell thickness of the CdSe/CdS nanocrystals and the ratio between the radiative recombination rates of the biexciton and exciton. Our finding can be explained by a picture in which the biexciton emission efficiency is enhanced through electron localization around the core due to the strong Coulomb potential of the two holes confined in the core.

Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals

We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr3, FA = HC(NH2)2) nanocrystals and the impact of postsynthetic surface treatments on the PL intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on-off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross sections, those of flickering nanocrystals show no correlation with the absorption cross sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no significant effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons.

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.