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During the synthesis of CdTe quantum dots (QDs) by the hydrothermal method, a CdS shell layer is naturally formed by the thermal decomposition of thiol ligands, and CdTe/CdS core/shell QDs are produced. Herein, we investigate the selective synthesis of CdTe and CdTe/CdS QDs to control the thermal decomposition of thiol ligands by changing the Te/Cd molar ratio of the precursor solutions. ...

Semiconductor quantum dot superlattices (QDSLs) have attracted much attention as key materials for realizing new optoelectronic devices such as solar cells with high conversion efficiency and thermoelectric elements with high electrical conductivity. To improve the charge transport properties of QDSL-based optoelectronic devices, it is important that the QD structures form minibands, which are the coupled electronic states between QDs. A shorter inter-QD distance and a periodic arrangement of QDs are the essential conditions for the formation of minibands. In this study, we use CdTe QDs capped with short ligands of N-acetyl-l cysteine (NAC) to fabricate three-dimensional QD assemblies by utilizing chemical bonding between NACs. Absorption spectra clearly display the quantum resonance phenomenon originating from the coupling of the wave functions between the adjacent QDs in CdTe QD assemblies. Furthermore, we demonstrate the formation of minibands in CdTe QD assemblies by examining both, the excitation energy dependence of photoluminescence (PL) spectra and the detection energy dependence of PL excitation spectra. The fabrication method of QD assemblies utilizing chemical bonding between NACs can be applied to all QDs capped with NAC as a ligand.

We investigated the effects of surface modification on the defect-related photoluminescence (PL) band in colloidal CdS quantum dots (QDs). A size-selective photoetching process and a surface modification technique with a Cd(OH)2 layer enabled the preparation of size-controlled CdS QDs with high PL efficiency. The Stokes shift of the defect-related PL band before and after the surface modification was ∼1.0 eV and ∼0.63 eV, respectively. This difference in the Stokes shifts suggests that the origin of the defect-related PL band was changed by the surface modification. Analysis by X-ray photoelectron spectroscopy revealed that the surface of the CdS QDs before and after the surface modification was S rich and Cd rich, respectively. These results suggest that Cd-vacancy acceptors and S-vacancy donors affect PL processes in CdS QDs before and after the surface modification, respectively.

CdTe and CdTe/CdS quantum dots (QDs) were synthesized via hydrothermal method in one-step processes just by controlling reaction time. A gradual thermal decomposition of thiol ligand N-acetyl-l-cysteine results in formation of a CdS shell on a CdTe core. The experimental results of X-ray diffraction and energy dispersive X-ray spectroscopy indicated that growth of the CdS shell started in a longer reaction time than 30 minutes and that the thickness of the CdS shell increased with increasing the reaction time. The CdTe/CdS QD exhibited a longer photoluminescence-decay time than the CdTe core QD due to the type-II band alignment, and the decay time in the CdTe/CdS QDs increased with an increase in the reaction time. The increase in the decay time in the CdTe/CdS QDs was qualitatively reproduced by a decrease in an overlap integral of electron and hole wave functions caused by an increase of the CdS shell thickness.

We have systematically investigated the temperature dependence of absorption, photoluminescence (PL), and PL decay profiles in CdTe-core and CdTe/CdS type-II quantum dots (QDs). In CdTe/CdS QDs, Stokes shifts and PL decay time become larger with an increase in temperature above 120 K, while those in CdTe-core QDs are almost independent of temperature. The unusual temperature dependence of Stokes shifts and PL decay time in CdTe/CdS QDs is understood by considering carrier localization at the core/shell interface at low temperatures and thermal-energy-assisted detrapping from localized-exciton to type-II exciton states at higher temperatures. Furthermore, a phenomenological rate-equation model is developed to explain the experimentally observed temperature-dependent PL decay time.

We investigated the size dependence of band-edge photoluminescence (PL) dynamics for CdS quantum dots (QDs). The temperature dependence of the PL-decay profiles of CdS QDs with an average diameter of 3.7–6.0 nm was measured. The PL-decay profiles became longer as the temperature increased. Further, it was found that the temperature dependence of the PL-decay profiles depends greatly on the QD size. These experimental results can be understood by considering that the magnitude of the splitting energy between the bright-and dark-exciton states depends on the QD size and becomes larger as the QD size becomes smaller.

The photoluminescence (PL) properties of self-assembled monolayers (SAMs) of CdSe and CdSe/ZnS quantum dots (QDs) were investigated. From measurements of the temperature dependence of PL spectra, it is demonstrated that the thiol group, which is a constituent of the reagent for formation of the SAM, degrades the PL properties during the SAM formation process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH = 10, the band-edge PL intensity is increased remarkably. The improvement of PL properties is attributed to QD surface modification with a Cd(OH)2 layer. On the other hand, the defect-related PL band is not observed at all, even at 10 K, in the SAMs of CdSe/ZnS core/shell QDs. This fact indicates that the ZnS shell layer prevents degradation of the PL properties during the SAM process.

CuInS2 (CIS) nanoparticles (NPs) were hydrothermally prepared by the reaction of Cu2+, In2+, and Na2S in the presence of N-acetyl-L-cysteine as a stabilizer. The X-ray diffraction pattern of the re...

Thin films of colloidal CdS and ZnS−CdS alloy quantum dots (QDs) were fabricated by layer-by-layer (LBL) self-assembly to synthesize blue luminescent QD thin films. The assembly of negatively charged colloidal QDs and positively charged poly(diallyldimethylammonium chloride) results in QD/polymer multilayers. The relative intensity of the band-edge photoluminescence (PL) to that of the defect-related PL was decreased during the LBL assembly process. With an additional dipping treatment in a Cd(ClO4)2 aqueous solution with pH 10, the band-edge PL intensity was recovered. The PL decay profiles before and after the dipping treatment were considerably different. From this fact, we conclude that the enhancement of the band-edge PL intensity originates from the reduction of nonradiative recombination processes at the QD surface.

The title nanoparticles are hydrothermally prepared from aqueous mixtures of N-acetyl-L-cysteine, CuCl2, InCl3, and Na2S (autoclave, 180 °C, 5—20 min).

We have investigated efficient energy transfer (ET) between CdS quantum dots (QDs) measuring photoluminescence dynamics in layer-by-layer (LBL) self-assembled films. The assembly of negatively charged colloidal QDs and positively charged polyelectrolytes results in QD/polymer multilayers. Furthermore, to reveal how the ET rate depends on the distance between CdS QDs, we fabricated bilayer structures consisting of differently sized CdS QDs. It is experimentally verified that ET between the donor and acceptor QDs is conclusively dominated by the dipole-dipole interaction.

Water soluble nanoparticles (NPs) with a high emission property were synthesized via hydrothermal routes. In this report, we chose thiol ligand N-acetyl-L-cysteine as the ideal stabilizer and have successfully employed it to synthesize readily size-controllable CdTe NPs in a reaction of only one step. Hydrothermal synthesis of CdTe NPs has been carried out in neutral or basic conditions so far. We found out that the pH value of precursor solutions plays an important role in the uniformity of the particle size. Actually, high quality CdTe NPs were synthesized under mild acidic conditions of pH 5. The resultant NPs indicated good visible light-emitting properties and stability. Further, the experimental results showed that the reaction temperature influenced significantly the growth rate and the maximum size of the NPs. The CdTe NPs with a high photoluminescence quantum yield (the highest value: 57%) and narrower half width at half maximum (the narrowest value: 33 nm) were attained in very short time, within 40 minutes, reaching diameters of 2.3 to 4.3 nm. The PL intensity was increased with an increase in the reaction time, reflecting the suppression of nonradiative recombination processes. Furthermore, the formation of CdTe/CdS core-shell structures was discussed from the viewpoint of PL dynamics and X-ray diffraction studies.

We have investigated the temperature dependence of photoluminescence (PL) dynamics of self-assembled monolayers (SAMs) of CdSe quantum dots (QDs). The PL decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behavior due to thermal quenching. Such anomalous temperature dependence of the PL-decay profile is explained using a four-state model which introduces a bound-exciton state into a conventional three-state model consisting of a ground state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state. Furthermore, it is proposed that the radiative decay time of QDs is strongly influenced by the presence or absence of the bound-exciton state.