Reversible Transformations at Room Temperature among Three Types of CdTe Magic-Size Clusters.

We report the first observation of the reversible transformations that occur among three types of CdTe magic-size clusters (MSCs) in dispersion at room temperature and discuss our understanding of the transformation pathway. The reversible transformations were achieved with CdTe prenucleation stage samples, which were prepared with reactions of cadmium oleate [Cd(OA)2] and tri-n-octylphosphine telluride in 1-octadecene and were then dispersed in mixtures of toluene and a primary amine at room temperature. Three types of OA-passivated CdTe MSCs evolved, exhibiting sharp optical absorption singlets peaking at 371, 417, and 448 nm. The MSCs and their immediate precursor compounds (PCs; with no sharp optical absorption) are labeled by the MSC absorption peak wavelengths. The transformation between MSC-371 and MSC-417 has a distinct isosbestic point at ∼385 nm and that between MSC-417 and MSC-448 at ∼430 nm. Our findings suggest that these PC-enabled reversible transformations occur through a process of quasi-isomerization, transforming between PCs and their counterpart MSCs, combined with substitution reactions that cause transformation between the two involved PCs.

Room-Temperature Formation Pathway for CdTeSe Alloy Magic-Size Clusters.

Little is known about the pathway of room-temperature formation of ternary CdTeSe magic-size clusters (MSCs) obtained by mixing binary CdTe and CdSe induction period samples containing binary precursor compounds (PCs) of MSCs, monomers (Ms), and fragments (Fs). Also, unestablished are dispersion effects that occur when as-mixed samples (without incubation) are placed in toluene (Tol) and octylamine (OTA) mixtures. The resulting ternary MSCs, exhibiting a sharp optical absorption peak at 399 nm, are labelled CdTeSe MSC-399, and their PCs are referred to as CdTeSe PC-399. When the amount of OTA is relatively small, single-ensemble MSC-399 evolved without either binary CdTe or CdSe MSCs. When the OTA amount is relatively large, CdTe MSC-371 appeared initially and then disappeared, while single-ensemble MSC-399 developed more deliberately. The larger the OTA amount, the more slowly these changes proceeded. The substitution reaction of CdTe PC + CdSe M/F↔CdTeSe PC-399 + CdTe M/F is proposed to be rate-determining for the MSC-399 formation in a Tol and OTA mixture. This study provides further understanding of the transformation pathway between MSCs.

Evolution of CdTe Magic-Size Clusters with Single Absorption Doublet Assisted by Adding Small Molecules during Prenucleation.

An approach is reported for the exclusive production of CdTe magic-size clusters (MSCs) that exhibit an optical absorption doublet peaking at 385/427 nm, with an explanation of the synthesis procedure. The MSCs, defined as dMSC-427, were produced from the reaction of cadmium oleate (Cd(OA)2) and tri-n-octylphosphine telluride in octadecene at 100 °C, with the addition of acetic acid (HOAc) or acetate (M(OAc)2) during the prenucleation stage (40 °C). Without such an addition or when it was performed in the postnucleation stage (100 °C), quantum dots (QDs) developed. The production of dMSC-427 or QDs is hypothesized to be related to the solubility of the Cd precursor, such as Cd(OA)1(OAc)1 or Cd(OA)2, respectively. Also, the reactions that lead to Cd(OA)1(OAc)1 are proposed. The present study provides an in-depth understanding of the two-pathway model proposed for the prenucleation stage of binary colloidal QDs, as well as of the formation of MSCs and/or QDs.

Simple Method to Supply Organic Nanoparticles with Excitation-Wavelength-Dependent Photoluminescence.

Organic fluorescent nanoparticles (FNPs) have become increasingly prevalent in a variety of applications but the creation of organic FNPs using a simple procedure and that possess diverse morphology, multicolor luminescence, and high brightness has been challenging. Herein, a facile strategy to prepare this class of organic FNPs is established by way of preformed organic nanoparticles themselves. It was found that as long as the nanoparticles contained aromatic/heterocyclic rings in their base unit and regardless of morphologies (e.g., small-molecule micelles, polymeric micelles, reverse micelles, solid microspheres, and vesicles), simple UV irradiation can result in the particles exhibiting excitation-wavelength-dependent photoluminescence with considerable quantum yields (∼8.3-16.7% for tested particles). Upon initial investigation of the mechanism, the photoluminescence behavior was attributed to a polycyclic aromatic hydrocarbon (PAH) process. Furthermore, the application of the synthesized organic FNPs in cancer cell imaging is demonstrated as just one of the many potential applications. The straightforward method to supply preformed organic nanoparticles with photoluminescence would be attractive for scientists in both academia and industry.

Energetics of Nonradiative Surface Trap States in Nanoparticles Monitored by Time-of-Flight Photoconduction Measurements on Nanoparticle-Polymer Blends.

Nanoparticles (NPs) have had increasingly successful applications including in emissive or photovoltaic devices; however, trap states associated with the surface of NPs often drastically reduce the efficiency of devices and are difficult to detect spectroscopically. We show the applicability of photoconduction as the means of detecting and quantifying trap states in NPs. We performed time-of-flight (ToF) photoconduction measurements, using semiconducting poly[bis(4-phenyl)(4-butyphenyl)amine] (P-TPD) doped with either core/shell CdSeS/CdS quantum dots (QDs) or perovskite CsPbBr3 NPs, both of which are carefully designed to be energetically matched. In the case of the QDs, a drop in the hole mobility from ∼10-3 to ∼10-4 cm2 V-1 s-1 was observed when compared to a control sample, suggesting the presence of a hole trapping. These trap states were found to be around -5.0 to -4.9 eV from the vacuum level. The presence of the trap states was further supported by a coincident reduction in the photoluminescence (PL), quantum yield (QY), and lifetime of the core/shell QDs after purification. Using the measured reductions in the PL, QY, and lifetime, the surface trap state density was estimated to increase by between 20 and 40%, most likely due to a ligand detachment. In the case of the perovskite NP-doped samples, a mobility of ∼10-3 cm2 V-1 s-1 was measured. Thus, doping with perovskite NPs did not generate any obvious hole trapping from the P-TPD matrix. The absence of a trapping may be related to the reduced surface-to-volume ratio and NP number density of the perovskite NPs compared to the core/shell QDs, since the perovskite NPs are approximately 10 times larger in radius than that of the core/shell QDs. Our results suggest that to minimize the presence of trap states with a view to improving device performance, large-size perovskite NPs appear to be better than small-size QDs.

Photoluminescent Colloidal Nanohelices Self-Assembled from CdSe Magic-Size Clusters via Nanoplatelets.

Reports on photoluminescent colloidal semiconductor two-dimensional (2D) helical nanostructures with one-dimensional quantum confinement are relatively rare. Here, we discuss the formation of such photoluminescent nanostructures for CdSe. We show that when as-synthesized unpurified zero-dimensional (0D) CdSe magic-size clusters (MSCs) (passivated by carboxylate ligands with three-dimensional quantum confinement) are dispersed in a solvent (such as toluene or chloroform) containing hexadecylamine and then subjected to sonication, helical nanostructures are obtained, as observed by transmission electron microscopy. We demonstrate that the formation involves the self-assembly of 0D MSCs into 2D nanoplatelets, which act as intermediates. The CdSe MSCs and their self-assembled 2D nanostructures display almost identical static optical properties, namely, a sharp absorption doublet with peaks at 433 and 460 nm and a narrow emission peak at 465 nm; this is a subject for further study. This study introduces new methods for fabricating photoluminescent helical nanostructures via the self-assembly of photoluminescent MSCs.

One-Step Approach to Single-Ensemble CdS Magic-Size Clusters with Enhanced Production Yields.

We report on the development of a single-step method for synthesizing colloidal semiconductor magic-size clusters (MSCs) with an enhanced production yield in a single-ensemble form and free of the coproduction of conventional quantum dots (QDs). This process eliminates the need for the second step of a lower-temperature incubation used in a two-step approach reported recently for the fabrication of single-ensemble MSCs without QD contamination. We demonstrate that the combined use of a secondary phosphine (HPR2) and an α-methyl carboxylic acid [RCH(CH3)-COOH, MA] promotes the yield of MSCs and suppresses the nucleation and growth of QDs. With CdO and elemental S powder as Cd and S sources, respectively, a single ensemble of CdS MSC-311 (displaying a sharp absorption peak at 311 nm) evolves directly in a reaction in 1-octadecene with an enhanced production yield. This study introduces a one-step avenue for synthesizing effectively and selectively single-ensemble MSCs and improves our understanding of the two-pathway model proposed for the prenucleation stage of QDs.

Effect of Small Molecule Additives in the Prenucleation Stage of Semiconductor CdSe Quantum Dots.

For the addition of small molecules in the prenucleation stage of colloidal CdSe conventional quantum dots (QDs), there is insufficient knowledge regarding what are advantageous circumstances. Here, we present a study about such addition. When CH3COOH or Zn(OOCCH3)2 is added in the prenucleation stage (at 120 °C) of a reaction consisting of cadmium myristate (Cd(OOC(CH2)12CH3)2, Cd(MA)2 made from CdO) and Se powder in 1-octadecene (ODE), CdSe magic-size clusters (MSCs) exhibiting a single sharp absorption doublet at 433/460 nm are synthesized in a single-ensemble form (around 220 °C). We demonstrate that such small molecule addition suppresses the nucleation and growth of QDs and thus directs a competitive process to the formation of MSCs. The present study provides insight into the individual but linked pathways to forming CdSe QDs and MSCs and introduces new avenues to improve the production of MSCs through the addition of small molecules in the prenucleation stage.

Room temperature synthesis of ReS2 through aqueous perrhenate sulfidation.

In this study, a direct sulfidation reaction of ammonium perrhenate (NH4ReO4) leading to a synthesis of rhenium disulfide (ReS2) is demonstrated. These findings reveal the first example of a simplistic bottom-up approach to the chemical synthesis of crystalline ReS2. The reaction presented here takes place at room temperature, in an ambient and solvent-free environment and without the necessity of a catalyst. The atomic composition and structure of the as-synthesized product were characterized using several analysis techniques including energy dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The results indicated the formation of a lower symmetry (1T') ReS2 with a low degree of layer stacking.

Giant magnetic coercivity in Fe3C-filled carbon nanotubes.

One of the major challenges in the synthesis of ferromagnetically filled carbon nanotubes is the achievement of high coercivities. Up to now the highest coercivity has been shown to be 2200 Oe at 2 K ranging down to 500 Oe at temperatures of 300 K. Here we show that the anomalously large coercivity of 3440 Oe is observed in comparable samples. By comparing our result to those reported in previous studies no correlation is found between coercivity and the shape anisotropy or the crystal-diameter. Instead we suggest that the main parameter which controls the coercivity of these structures is the interplay of the grain size and shape anisotropy. We attribute the anomalous coercivity to the grain size being below the calculated single magnetic domain limit.