Natalia E. Mordvinova

Structural analysis and thermoelectric properties of mechanically alloyed colusites

We describe here a new, easy and scalable route for synthesising colusites by using mechanical-alloying and reactive spark plasma sintering, together with the thermoelectric behaviour of zinc-substituted derivatives, Cu26−xZnxV2Sn6S32 (0 ≤ x ≤ 2). X-ray diffraction analysis coupled with transmission electron microscopy evidences the high crystallinity of the as-synthesized samples. In the pristine compound, an intrinsic exsolution phenomenon leads to the formation of two distinguishable colusite phases. Additional neutron powder diffraction results support the substitution of Zn in the tetrahedral Cu sites and 119Sn Mossbauer spectroscopy analyses prove the presence of only Sn4+ in colusite irrespective of the Zn content (x ≤ 2). The mechanical properties denote a wide homogeneity of the samples despite a significant impact of the exsolution on the microstructure. The Zn for Cu substitution provides electron doping, decreasing the holes concentration. High temperature thermoelectric properties in the p-type series Cu26−xZnxV2Sn6S32 are reported. The highest power factor of 0.92 mW m−1 K−2 at 700 K is found for x = 1, with a corresponding ZT value of 0.4.

Addition of Zn during the phosphine-based synthesis of indium phospide quantum dots: doping and surface passivation

Summary Zinc-doped InP(Zn) colloidal quantum dots (QDs) with narrow size distribution and low defect concentration were grown for the first time via a novel phosphine synthetic route and over a wide range of Zn doping. We report the influence of Zn on the optical properties of the obtained quantum dots. We propose a mechanism for the introduction of Zn in the QDs and show that the incorporation of Zn atoms into the InP lattice leads to the formation of Zn acceptor levels and a luminescence tail in the red region of the spectra. Using photochemical etching with HF, we confirmed that the Zn dopant atoms are situated inside the InP nanoparticles. Moreover, doping with Zn is accompanied with the coverage of the QDs by a zinc shell. During the synthesis Zn myristate covers the QD nucleus and inhibits the particle growth. At the same time the zinc shell leads to an increase of the luminescence quantum yield through the reduction of phosphorous dangling bonds. A scenario for the growth of the colloidal InP(Zn) QDs was proposed and discussed.

Surface processes during purification of InP quantum dots

Summary Recently, a new simple and fast method for the synthesis of InP quantum dots by using phosphine as phosphorous precursor and myristic acid as surface stabilizer was reported. Purification after synthesis is necessary to obtain samples with good optical properties. Two methods of purification were compared and the surface processes which occur during purification were studied. Traditional precipitation with acetone is accompanied by a small increase in photoluminescence. It occurs that during the purification the hydrolysis of the indium precursor takes place, which leads to a better surface passivation. The electrophoretic purification technique does not increase luminescence efficiency but yields very pure quantum dots in only a few minutes. Additionally, the formation of In(OH)3 during the low temperature synthesis was explained. Purification of quantum dots is a very significant part of postsynthetical treatment that determines the properties of the material. But this subject is not sufficiently discussed in the literature. The paper is devoted to the processes that occur at the surface of quantum dots during purification. A new method of purification, electrophoresis, is investigated and described in particular.

The Effect of CdSe and InP Quantum Dots on the Interaction of ZnO with NO 2 under Visible Light Irradiation

Nanocomposites based on nanocrystalline ZnO and CdSe and InP nanocrystals (quantum dots) have been synthesized by chemical precipitation and high-temperature colloidal synthesis. The microstructure parameters of the oxide matrix and the size of the CdSe and InP nanocrystals have been determined. A correlation was established between the spectral dependence of the photoconductivity of nanocomposites and the optical absorption spectra of quantum dots. The influence of CdSe and InP quantum dots on the interaction of ZnO with NO2 under visible light irradiation has been studied. It has been shown that the synthesized nanocomposites can be used to detect NO2 under illumination with green light without additional thermal heating.

Diffusion doping route to plasmonic Si/SiOx nanoparticles

Semiconductor nanoparticles (SNPs) are a valuable building block for functional materials. Capabilities for engineering of electronic structure of SNPs can be further improved with development of techniques of doping by diffusion, as post-synthetic introduction of impurities does not affect the nucleation and growth of SNPs. Diffusion of dopants from an external source also potentially allows for temporal control of radial distribution of impurities. In this paper we report on the doping of Si/SiOx SNPs by annealing particles in gaseous phosphorus. The technique can provide efficient incorporation of impurities, controllable with precursor vapor pressure. HRTEM and X-ray diffraction studies confirmed that obtained particles retain their nanocrystallinity. Elemental analysis revealed doping levels up to 10%. Electrical activity of the impurity was confirmed through thermopower measurements and observation of localized surface plasmon resonance in IR spectra. The plasmonic behavior of etched particles and EDX elemental mapping suggest uniform distribution of phosphorus in the crystalline silicon cores. Impurity activation efficiencies up to 34% were achieved, which indicate high electrical activity of thermodynamically soluble phosphorus in oxide-terminated nanosilicon.

Ultra-high sensitivity of luminescent ZnCr2O4 nanoparticles toward nitroaromatic explosives sensing

Here, we report the luminescence based sensing of trace amounts of nitroaromatic explosive organic compounds. The luminescence emission of nanosized spinel oxide ZnCr2O4 with high chemical and thermal stabilities has been used as a potential probe to detect such organic explosives. Low temperature solution combustion synthesized ZnCr2O4 oxide with an average particle size of ∼9 nm exhibits strong luminescence emission at 410 nm upon excitation at 260 nm in an aqueous suspension. The presence of nitroaromatics in ZnCr2O4 suspension dramatically suppresses the luminescence emission providing an opportunity to detect it quantitatively. The detection limit for 2,4,6-trinitrophenol (TNP) is as low as 23 ppb. A number of organic compounds have been investigated for a comprehensive understanding. The astonishing sensitivity of ZnCr2O4 nanoparticles towards nitro explosives is appealing for sensing application. A plausible explanation of such luminescence quenching has been ascribed to a two-fold mechanism. The underling mechanism is further substantiated by a similar study on ZnO nanoparticles.

Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light

Sub-nanometer silver clusters that exhibit discrete electronic structure with molecular-like properties are highly desirable in various technologies. However, the methods for their preparation suffer from limitations related with the reproducibility and particles uniformity and/or the possibility of the scale-up. Another critical drawback is that free sub-nanometer silver clusters tend to aggregate into larger particles. In this work, a new approach that successfully overcomes the above limitations is developed. It allows, for the first time, an ultrafast preparation of sub-nanometer silver particles with high abundance, uniformity (7 Å), and stability into the cages of nanosized zeolite crystals. The new method consists of UV excitation of a water suspension of nanozeolite containing photoactive vanadate clusters in the presence of ethanol (as an electron donor) and silver precursor. The characteristic features of sub-nanometer silver particles are presented, and the mechanism of their formation is discussed. Sub-nanometer Ag clusters exhibit exceptional photocatalytic activity and selectivity in the reforming of formic acid to H2 and CO2 under visible light.

AgCl-doped CdSe quantum dots with near-IR photoluminescence

We report the synthesis of colloidal CdSe quantum dots doped with a novel Ag precursor: AgCl. The addition of AgCl causes dramatic changes in the morphology of synthesized nanocrystals from spherical nanoparticles to tetrapods and finally to large ellipsoidal nanoparticles. Ellipsoidal nanoparticles possess an intensive near-IR photoluminescence ranging up to 0.9 eV (ca. 1400 nm). In this article, we explain the reasons for the formation of the ellipsoidal nanoparticles as well as the peculiarities of the process. The structure, Ag content, and optical properties of quantum dots are also investigated. The optimal conditions for maximizing both the reaction yield and IR photoluminescence quantum yield are found.

Influence of La(III) on the reactivity and sensor properties of nanocrystalline SnO2

Synthesis of Sn(IV) and La(III) based nanocomposites has been effectuated. According to the data obtained by XRD and TPR-H2 methods, it is supposed that La in nanocompites is located in amorphous La2Sn2O7 segregation. The effect of La(III) on the adsorption properties of SnO2 surface and concentration of chemisorbed oxygen was determined. Sensor properties of obtained materials towards 10 ppm of CO in air were studied by in situ DC conductance measurements. It is shown that La introduction allows to increase sensor response of SnO2 during CO detection in air.