Conspectus Elaborate chemical synthesis methods allow the production of various types of inorganic nanocrystals (NCs) with uniform shape and size distributions. Many single-step synthesis approaches, ...such as the reduction of metal ions, the decomposition of metal complexes, double replacement reactions, and hydrolysis, have been adapted to promote the generation of monodisperse metal and ionic NCs. However, the question has become, how can we synthesize NCs with thermodynamically metastable phases or very complex structures? The transformation of already-synthesized NCs via elemental substitutions, such as ion exchange reactions for ionic NCs and galvanic replacement reactions for metal NCs, can overcome the difficulties facing conventional one-step syntheses. In particular, NC ion exchange reactions have been studied with numerous combinations of foreign ions and ionic NCs with various shapes. They have been investigated extensively because the reactions proceed under relatively mild conditions thanks to the large surface-to-volume ratio of the NCs relative to their bulk form. The functionality of the resulting ionic NCs, including semiconducting and plasmonic properties, can be easily tuned in a wide range, from the visible to near-infrared. Because anions generally have much larger ionic radii than cations within the frameworks of NCs, the cation exchange reactions proceed much faster than the anion exchange reactions. For ionic NCs above a critical size, the anion framework remains intact, and the original shape of the parent NCs is retained throughout the cation exchange reaction. In contrast, the anion exchange reaction often provides the new NCs with unique structures, such as hollow or anisotropically phase-segregated assemblies. This Account focuses on the full and partial ion exchange reactions involving ionic NCs, which have been thoroughly investigated by our group and others while highlighting important aspects such as the preservation of appearance and dimensions. First, we discuss how each type of ion exchange reaction progresses to understand the morphologies and crystal structures of their final products. This discussion is supported by emphasizing important examples, which help to explore the formation of NCs with thermodynamically metastable phases and complex structures, and other significant features of the ion exchange reactions leading to structure-specific functions. As a special case, we examine how the shape-dependent anionic framework (surface anion sublattice and stacking pattern) of polyhedral Cu2O NCs determines the crystalline structure of the anion-exchanged products of hollow Cu x S NCs. In addition, we review the characteristic anion exchange behavior of metal halide perovskite NCs observed in our laboratory and other laboratories. Finally, a general outline of the transformation of NCs via ion exchange reactions and future prospects in this field are provided.
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Abstract Flexible control of the composition and morphology of nanocrystals (NCs) over a wide range is an essential technology for the creation of functional nanomaterials. Cation exchange (CE) is a ...facile method by which to finely tune the compositions of ionic NCs, providing an opportunity to obtain complex nanostructures that are difficult to form using conventional chemical synthesis procedures. However, due to their robust anion frameworks, CE cannot typically be used to modify the original morphology of the host NCs. In this study, we report an anisotropic morphological transformation of Cu 1.8 S NCs during CE. Upon partial CE of Cu 1.8 S nanoplates (NPLs) with Mn 2+ , the hexagonal NPLs are transformed into crescent-shaped Cu 1.8 S–MnS NPLs. Upon further CE, these crescent-shaped NPLs evolve back into completely hexagonal MnS NPLs. Comprehensive characterization of the intermediates reveals that this waxing-and-waning shape-evolution process is due to dissolution, redeposition, and intraparticle migration of Cu + and S 2− . Furthermore, in addition to Mn 2+ , this CE-induced transformation process occurs with Zn 2+ , Cd 2+ and Fe 3+ . This finding presents a strategy by which to create heterostructured NCs with various morphologies and compositions under mild conditions.
Plasmonic enhancement is a versatile and convenient way to enhance the conversion efficiency of various photoenergy conversion systems, such as photocatalysts and solar cells. We refine a plasmonic ...enhancement system by focusing on a carrier blocking layer (between a plasmonic metal and a photoactive layer), which is commonly used to prevent a major quenching channel in a plasmonic enhancement system. The hydrogen evolution reaction (HER) activity is enhanced by 33 times from the introduction of a carrier-selective blocking layer (CSBL) in Ag-CdS nanoparticles. The Ag2S layer, a typical example of a CSBL, synergistically improves the plasmonic enhancement effect of Ag on the photocatalytic HER activity of CdS by both the selective blocking of photoexcited electrons and the effective transfer of holes, which extends the lifetime of the active species (electrons in the conduction band) in the semiconductor photocatalyst (CdS) to accelerate the photocatalytic HER. We propose a new strategy for a further improvement of plasmonic enhancement systems.
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Highly efficient photoenergy conversion in semiconductor nanoparticle heterostructures requires the formation of epitaxial heterointerfaces and band alignment engineering. This requirement has led to ...attention being given to recent advances and prospects in the charge separation properties of type-II semiconductor heterodimers composed of chalcogenide–chalcogenide blends. Type-II semiconductor heterodimers with a staggered alignment of band edges at the heterointerface can be synthesized by seeded growth or ion exchange to promote the spatial charge separation between electrons and holes in different parts of the heterostructure. Special attention has been given to CdS–Cu2–x S (0 ≤ x ≤ 0.0625) and CdS–CdTe combinations where CdS is a commonly used n-type semiconductor and both Cu2–x S and CdTe are proper p-type semiconductors that are used as light absorbers in heterojunction solar cells.
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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.
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Monodisperse cadmium sulphide (CdS) quantum dots (QDs) with a tunable size from 1.4 to 4.3 nm were synthesized by a non-injection method, and their surface states were characterized by ...photoluminescence spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The steady state photoluminescence study identified that the proportion of the trap state emission increased with the QD size decrease, while from the photoluminescence decay study, it appeared that the trap state emission results from the emission via a surface deep trap state. The XPS measurements revealed the existence of surface Cd with sulfur vacancy sites which act as electron trap sites, and the population of these sites increases with the QD size decrease. These results are consistent to conclude that the trap state emission mainly originates from the surface deep trapped electrons at the surface Cd with sulfur vacancy sites.
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.
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The "ligand effect" can be used as a novel strategy for enhancing the catalytic properties of metal clusters. Herein, we report the ligand effect of porphyrin derivatives on gold clusters (AuCs, size ...<2 nm) and gold nanoparticles (AuNPs, size >2 nm) in the electrochemical hydrogen evolution reaction (HER) at pH 6.7. The current density of porphyrin face-coordinated AuCs at -0.4 V
reversible hydrogen electrode (RHE) was 460% higher than that of phenylethanethiol-protected AuCs. X-ray photoemission spectroscopy indicated that the approach of porphyrin to the Au surface induced charge migration from the porphyrin to the Au core, leading to a shift in the 5d state of AuCs that resulted in enhanced HER activities. This ligand effect is pronounced in the cluster region due to the large surface-to-volume ratio. These results pave the way for enhancing catalytic activity of metal clusters using ligand design.
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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.
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Organic ligands on gold nanoclusters play important roles in regulating the structures of gold cores. However, the impact of the number and positions of the protecting ligands on gold-core structures ...remains unclear. We isolated thiolate-protected Au25 cluster anions, Au 25 (SC 2 Ph) 17 (Por) 1 − and Au 25 (SC 2 Ph) 16 (Por) 2 − (SC 2 Ph = 2-phenylethanethiolate), obtained by ligand exchange of Au25(SC2Ph)18− with one or two porphyrinthiolate (Por) ligands as mixtures of regioisomers. The ratio of two regioisomers in Au 25 (SC 2 Ph) 17 (Por) 1 – as measured by 1H NMR spectroscopy revealed that the selectivity could be controlled by the steric hindrance of the incoming thiols. Extended X-ray absorption fine structure studies of a series of porphyrin-coordinated gold nanoclusters clarified that the Au13 icosahedral core in the Au25 cluster was distorted through steric repulsion between porphyrin thiolates and phenylethanethiolates. This paper reveals interesting insights into the importance of the steric structures of protecting ligands for control over core structures in gold nanoclusters.
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