Yasushi Hamanaka

Synthesis of Ag–In binary sulfide nanoparticles—structural tuning and their photoluminescence properties

In this report, we demonstrate the synthesis of binary sulfide Ag–In–S NPs using a Ag–In thiolate complex. Thermal decomposition of the thiolate complex provides Ag/AgInS2 heterostructured nanoparticles (NPs). A metathesis reaction between the thiolate complex and sulfur source leads to the formation of nearly monodispersed AgInS2 NPs with a chalcopyrite-like or orthorhombic structure. AgInS2 NPs with a chalcopyrite-like structure exhibited room temperature photoluminescence (PL). Spectral shift of the PL band depending on the excitation laser intensity and characteristic behavior of the PL decay time varying over a wide energy range within the PL band were observed. These results indicate that the PL of the AgInS2 NPs may be attributed to the donor–acceptor (D–A) pair recombination.

Phase control and its mechanism of CuInS2 nanoparticles.

CuInS(2) nanoparticles (NPs) usually take chalcopyrite-(CP) structure. Recently, CuInS(2) NPs with pseudo-wurtzite (WZ) structure, which is thermodynamically less favored, have been synthesized. However, the formation mechanism of this metastable-phase has not been understood yet. In this report, the key issue of phase selectivity of CuInS(2) (CIS) NPs has been investigated using various metal sources and ligands. Experimental results suggested that the crystalline structure and morphology of CIS NPs were decided by the stability of indium ligand complex; the active ligand reduces the precipitation rate of In(2)S(3), resulting in pre-generation of Cu(2)S seed NPs. Crystallographic analogy and superionic conductivity of Cu(2)S remind us that the formation of WZ CIS NPs is attributed to the pre-generation of Cu(2)S seed NPs and the following cation exchange reaction. In order to confirm this hypothesis, Cu(2-)(x)S seed NPs with various structures have been annealed in indium-ligand solution. This experiment revealed that the crystalline structure of CIS NP was determined by that of pre-generation Cu(2-)(x)S NPs. Our results provide the important information for the phase control and synthesis of ternary chalcogenide NPs with a novel crystalline structure.

Plasmonic enhancement of third-order nonlinear optical susceptibilities in self-doped Cu 2-x S nanoparticles

Heavily doped Cu2-xS nanoparticles with hole densities of ~1021 cm−3 were chemically synthesized and their localized surface plasmon resonance (LSPR) in the near-infrared region was investigated by nonlinear optical spectroscopy. We found that their third-order susceptibility χ(3) exhibits resonant enhancement around LSPR, similar to that in plasmonic noble metal nanoparticles. It was found that the maximum χ(3) value of Cu2-xS nanoparticles was comparable to that of Au nanoparticles with the same dimensions and concentrations. Our results indicate that Cu2-xS nanoparticles can be used as nonlinear optical materials operating in the near-infrared region, even though their near-field enhancement effect is slightly weaker than that of Au nanoparticles.

Structural transformation and photoluminescence modification of AgInS2 nanoparticles induced by ZnS shell formation

AgInS2 nanoparticles were capped by ZnS via a widely used procedure to fabricate core/shell nanoparticles with highly efficient luminescence. The nanoparticle structures were investigated by ultrahigh-resolution analytical electron microscopy. We found that Zn–Ag–In–S nanoparticles were created by ZnS capping at ~480 K, which suggests that the luminescence enhancement reported for such core/shell nanoparticles is not caused by the passivation of surface defects by ZnS shells but by Zn doping. Quasi-core/shell nanoparticles could be obtained by ZnS capping without heating. However, their luminescence efficiency remained unchanged, indicating that surface passivation was ineffective when ZnS shells were formed at room temperature.

Solution-phase synthesis and photoluminescence characterization of quaternary Cu[sub 2]ZnSnS[sub 4] nanocrystals

Cu2ZnSnS4 (CZTS) nanocrystals were synthesized via solution phase route and their lattice defects were investigated by photoluminescence measurements. Ionization energies of the defect levels were estimated to be 10 and 72 meV from thermal quenching behavior of the photoluminescence spectra. These values are quite different from those experimentally estimated for vapor-grown CZTS films and crystals and theoretically calculated for bulk CZTS. The results indicate that the defects are characteristic of CZTS nanocrystals synthesized in the solution phase.