Protima Rauwel

Composition and the thermoelectric performance of β-Zn4Sb3

β-Zn4Sb3 is a promising thermoelectric material due to the abundance of zinc and antimony and reports of high efficiency in bulk samples. This work establishes the high temperature properties of β-Zn4Sb3 across the phase stability window. By controlling the stoichiometry, the Hall carrier concentration can be tuned from 6–9 × 1019 cm−3 without requiring extrinsic dopants. The trend in Seebeck coefficient on carrier concentration is rationalized with a single, parabolic band model. Extremely low lattice thermal conductivity (0.4–0.6 W m−1 K−1) coupled with a moderate effective mass (1.2 me) and mobility leads to a large figure of merit (zT of 0.8 by 550 K). The single parabolic band model is used to obtain the carrier concentration dependence of the figure of merit and an optimum carrier concentration near 5 × 1019 cm−3 is predicted.

A Review on the Green Synthesis of Silver Nanoparticles and Their Morphologies Studied via TEM

Silver has been recognized as a nontoxic, safe inorganic antibacterial/antifungal agent used for centuries. Silver demonstrates a very high potential in a wide range of biological applications, more particularly in the form of nanoparticles. Environmentally friendly synthesis methods are becoming more and more popular in chemistry and chemical technologies and the need for ecological methods of synthesis is increasing; the aim is to reduce polluting reaction by-products. Another important advantage of green synthesis methods lies in its cost-effectiveness and in the abundance of raw materials. During the last five years, many efforts were put into developing new greener and cheaper methods for the synthesis of nanoparticles. The cost decrease and less harmful synthesis methods have been the motivation in comparison to other synthesis techniques where harmful reductive organic species produce hazardous by-products. This environment-friendly aspect has now become a major social issue and is instrumental in combatting environmental pollution through reduction or elimination of hazardous materials. This review describes a brief overview of the research on green synthesis of silver metal nanoparticles and the influence of the method on their size and morphology.

Enhanced electrocatalytic activity of nitrogen-doped multi-walled carbon nanotubes towards the oxygen reduction reaction in alkaline media

In this work multi-walled carbon-nanotubes (MWCNTs) were doped with nitrogen using cyanamide (CM) or dicyandiamide (DCDA). To incorporate nitrogen into the CNT structure, high-temperature pyrolysis in an inert atmosphere was performed. For surface characterisation of nitrogen-doped CNTs (NCNTs) X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used. According to the results of XPS analysis, nitrogen was successfully incorporated into the carbon nanotube network. The electrocatalytic activity of NCNT catalysts for oxygen reduction reaction (ORR) in alkaline media was examined using the rotating disk electrode (RDE) and linear sweep voltammetry (LSV) measurements. The NCNT-DCDA material showed a better ORR performance than the NCNT-CM catalyst. The RDE results reveal that the NCNT materials studied could be considered as interesting alternatives to Pt-based catalysts in alkaline membrane fuel cells.

One step synthesis of pure cubic and monoclinic HfO2 nanoparticles: Correlating the structure to the electronic properties of the two polymorphs

Hafnium dioxide is a wide band-gap, high-κ material, and Hafnium based compounds have already been integrated into micro-electronic devices. The pure cubic HfO2 phase is promising as it presents a higher permittivity (κ > 25), but needs to be stabilized by addition of divalent or trivalent dopants, which in turn modify the electronic properties of HfO2. Here, we employ a one-pot synthesis approach to produce undoped cubic and monoclinic HfO2 nanoparticles by choice of solvent alone. The average size of these nanoparticles from transmission electron microscopy studies was estimated to be around 2.6 nm. We present a study of the morphology and microstructure and also demonstrate the presence of a strong visible photoluminescence linked to the nanosize of the particles. Furthermore, the synthesis in equivalent conditions of these two phases of HfO2 provides means for direct comparison of the chemical composition and electronic structures of the two polymorphs. This has therefore allowed us to experimentally ...

The work function of n-ZnO deduced from heterojunctions with Si prepared by ALD

Highly doped n-type ZnO films have been grown on n-type and p-type Si substrates by atomic layer deposition (ALD). Transmission electron microscopy shows columnar growth of the ZnO films with randomly oriented grains and a very thin interfacial layer of SiOx(x ≤ 2) with a thickness below 0.4 nm to the Si substrate. Current–voltage and capacitance–voltage measurements performed at temperatures from 50 to 300 K reveal a strong rectifying behaviour on both types of substrates with an ideality factor close to unity between 180 and 280 K. Using the classical approach of thermionic emission, the barrier heights of the ZnO/n-Si and ZnO/p-Si junctions have been deduced and consistent values are obtained yielding a work function of n-type ZnO close to 4.65 eV.

Polymer encapsulation effects on the magnetism of EuS nanocrystals

Surface mediated phenomena are important in controlling the magnetism of nanocrystals. Here, we provide a versatile chemical method which allows the formation of EuS nanocrystals encapsulated in poly(styrene) (PS). These materials are the first example of a polymer based EuS nanocomposite prepared in situ. Besides their practical interest, these materials are of general relevance because they allow fundamental knowledge on the magnetism of EuS, which is commonly used as a model of an insulating ferromagnet, to be acquired. Comparatively to the starting powders of EuS nanocrystals (8 nm), in the final nanocomposites there is a decrease of the blocking temperature TB and a change in the shape of the field-cooled susceptibility curve, indicating that the intensity of dipolar interactions decreases when the nanoparticles are incorporated into PS. Differences in the magnetic properties of the EuS nanocrystals and the EuS/PS nanocomposites are also due to the appearance of a peak in the susceptibility at 60 K, in the latter sample, ascribed to surface phenomena occurring in EuS nanocrystals dispersed in the polymer matrix.

Photoluminescent cubic and monoclinic HfO2 nanoparticles: effects of temperature and ambient

We report on the one-pot synthesis and characterization of pure cubic and monoclinic phase HfO2 nanoparticles. An average particles size of 2.6 nm was estimated from transmission electron microscopy studies. The equivalent synthesis conditions of the two polymorphs provided means for direct comparison of their optical absorption and emission properties, which are found to exhibit different behaviors as a function of temperature and ambient environment. The origin of the strong visible luminescence observed from both cubic and monoclinic HfO2 is discussed in terms of surface-defects and nanosize of the particles. The comparative analysis of the PL responses of the two polymorphs suggests that emission at 3.1 eV originates from the oxygen vacancy related defects.

A Review of the Synthesis and Photoluminescence Properties of Hybrid ZnO and Carbon Nanomaterials

Photoluminescent ZnO carbon nanomaterials are an emerging class of nanomaterials with unique optical properties. They each, ZnO and carbon nanomaterials, have an advantage of being nontoxic and environmentally friendly. Their cost-effective production methods along with simple synthesis routes are also of interest. Moreover, ZnO presents photoluminescence emission in the UV and visible region depending on the synthesis routes, shape, size, deep level, and surface defects. When combined with carbon nanomaterials, modification of surface defects in ZnO allows tuning of these photoluminescence properties to produce, for example, white light. Moreover, efficient energy transfer from the ZnO to carbon nanostructures makes them suitable candidates not only in energy harvesting applications but also in biosensors, photodetectors, and low temperature thermal imaging. This work reviews the synthesis and photoluminescence properties of 3 carbon allotropes: carbon quantum or nanodots, graphene, and carbon nanotubes when hybridized with ZnO nanostructures. Various synthesis routes for the hybrid materials with different morphologies of ZnO are presented. Moreover, differences in photoluminescence emission when combining ZnO with each of the three different allotropes are analysed.

Silver Nanoparticles: Synthesis, Properties, and Applications

1Department of Physics, University of Oslo, 24 Sem Saelandsvei, 316 Oslo, Norway 2Institute of Physics, University of Tartu, Ravila 14c, 51014 Tartu, Estonia 3Tartu College, Tallinn University of Technology, Puiestee 78, 51008 Tartu, Estonia 4Department of Physics, University of Minho, 4800-058 Guimaraes, Portugal 5Department of Physics, Brock University, St. Catharines, ON, Canada L2S 3A1 6CWIT Capital Inc., 8 Baker Drive, St. Catharines, ON, Canada L2N 2V8

Biocidal properties study of silver nanoparticles used for application in green housing

We report on the study of surfactant-free silver nanoparticles synthesized using non-hydrolytic sol–gel methods for applications in straw bale constructions. Micro-organism infestation in green constructions is of concern as their proliferation tends to induce health problems. We demonstrate the biocidal properties of these Ag nanoparticles and their efficacy against fungi. Outdoor tests with Ag nanoparticles have demonstrated the effective protection of straw against micro-organisms. Indoor tests using broth liquid are compared with a method of testing we recently developed where the possible nature of the biocidal properties of the silver nanoparticles are further probed. In contrast to the commonly reported results, this study shows that Ag nanoparticles synthesized using non-hydrolytic sol–gel methods have antifungal properties against common fungi in outdoor conditions which demonstrate high potential in related applications.