Photoexcited Carrier Transfer in CuInS2 Nanocrystal Assembly by Suppressing Resonant-Energy Transfer.

High-density assemblies or superlattice structures composed of colloidal semiconductor nanocrystals have attracted attention as key materials for next-generation photoelectric conversion devices such as quantum-dot solar cells. In these nanocrystal solids, unique transport and optical phenomena occur due to quantum coupling of localized energy states, charge-carrier hopping, and electromagnetic interactions among closely arranged nanocrystals. In particular, the photoexcited carrier dynamics in nanocrystal solids is important because it significantly affects various device parameters. In this study, we report the photoexcited carrier dynamics in a solid film of CuInS2 nanocrystals, which is one of the potential nontoxic substitutes with Cd- and Pb-free compositions. Meanwhile, these subjects have been extensively studied in nanocrystal solids formed by CdSe and PbS systems. A carrier-hopping mechanism was confirmed using temperature-dependent photoluminescence spectroscopy, which yielded a typical value of the photoexcited carrier-transfer rate of (2.2±0.6)×107  s-1 by suppressing the influence of the excitation-energy transfer.

Ligand-Induced Luminescence Transformation in AgInS2 Nanoparticles: From Defect Emission to Band-Edge Emission.

I-III-VI2 semiconductor nanoparticles are strong candidates for fluorescent materials composed of nontoxic elements substituting highly fluorescent CdSe nanoparticles. Photoluminescence of I-III-VI2 nanoparticles essentially arise due to defect emission characterized by a broad spectral feature. Band-edge emission exhibits radiation with high monochromaticity, which can drastically expand its application range. Hence, numerous studies were conducted to realize band-edge emission. A successful observation of the band-edge emission was reported only when fabricating GaSx or InSx shells around AgInS2 nanoparticles via surface trap site passivation. This study demonstrates a much easier method of providing band-edge emission from AgInS2 nanoparticles using organic ligands of trioctylphosphine (TOP). Along with the TOP ligand formation around AgInS2 nanoparticles, the defect emission increases once and then decreases in conjunction with the appearance of the band-edge emission. Therefore, TOP ligands can passivate carrier trapping sites for radiative recombination as well as fluorescence quenching sites.

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.