Possible Routes for Efficient Thermo-Electric Energy Conversion in a Molecular Junction

In the context of designing an efficient thermoelectric energy-conversion device at nanoscale level, we suggest several important tuning parameters to enhance the performance of thermoelectric converters. We consider a simple molecular junction, which is always helpful to understand the basic mechanisms in a deeper way, where a benzene molecule is coupled to two external baths having unequal temperatures. The key component responsible for achieving better performance is associated with the asymmetric nature of transmission function, and in the present work, we show that it can be implemented in different ways by regulating the physical parameters involving the system. Employing a tight-binding framework we calculate electrical and thermal conductances, thermopower, and figure of merit (FOM) by using Landauer integrals, and thoroughly examine the critical roles played by molecule-to-lead (ML) interface geometry, magnetic field, chemical substituent group, ML coupling, and the direct coupling between the two leads. Our results show that a reasonably large FOM (≫1) can be obtained and lead to a possibility of regulating the efficiency by selectively tuning the physical parameters. We believe that the present analysis will enhance the understanding of designing efficient thermoelectric devices, and can be verified in a laboratory.     

In situ TREXS Observation of Surface Reduction Reaction of NiO Film with ∼2 nm Surface Sensitivity

X‐ray absorption fine structure (XAFS) spectroscopy is one of the most widely used methods at synchrotron radiation facilities. XAFS gives us information on chemical states and local structures. Fundamentally, XAFS is bulk sensitive, not surface sensitive. If a surface sensitive XAFS method was available, surface chemical reactions can be observed under realistic conditions. Here, we report the development and present status of a type of surface sensitive x‐ray spectroscopy, which is named total reflection x‐ray spectroscopy, TREXS.     

Numerical analysis of spatial evolution of the small signal gain in a chemical oxygen–iodine laser operating without primary buffer gas

A chemical oxygen–iodine laser (COIL) that operates without primary buffer gas has become a new way of facilitating the compact integration of laser systems. To clarify the properties of spatial gain distribution, three-dimensional (3-D) computational fluid dynamics (CFD) technology was used to study the mixing and reactive flow in a COIL nozzle with an interleaving jet configuration in the supersonic section. The results show that the molecular iodine fraction in the secondary flow has a notable effect on the spatial distribution of the small signal gain. The rich iodine condition produces some negative gain regions along the jet trajectory, while the lean iodine condition slows down the development of the gain in the streamwise direction. It is also found that the new configuration of an interleaving jet helps form a reasonable gain field under appropriate operation conditions.     

Preparation, characterization and photocatalytic activity of multi-walled carbon nanotube-supported tungsten trioxide composites

Multi-walled carbon nanotube (MWCNT)-supported tungsten trioxide (WO₃) composite catalysts were prepared by liquid-phase process. WO₃ nanoparticles grew on the inner and outer surface of MWCNTs. Their photocatalytic activities in the degradation of the Rhodamine B Dye were studied. The effects of mass ratio of MWCNTs to WO₃ were discussed. X-ray diffraction, field emission transmission electron microscopy, thermogravimetric-differential thermal analysis and ultraviolet-visible light absorption spectra were carried out to characterize the composite catalysts. The results indicated that the optimum mass ratio of MWCNTs to WO₃ is 5:100.     

Physical properties of ZnS thin films prepared by chemical bath deposition

Zinc sulphide thin films are deposited on SnO₂/glass using the chemical bath deposition technique. X-ray diffraction and atomic force microscopy are used to characterize the structure of the films; the surface composition of the films is studied by Auger electrons spectroscopy, the work function and the photovoltage are investigated by the Kelvin method. Using these techniques, we specify the effect of pH solution and heat treatment in vacuum at 500 °C. The cubic structure corresponding to the (1 1 1) planes of β-ZnS is obtained for pH equal to 10. The work function (Φ_material − Φ_probe) for ZnS deposited at pH 10 is equal to −152 meV. Annealing at 500 °C increases Φ_m (by about 43 meV) and induces the formation of a negative surface barrier. In all cases, Auger spectra indicate that the surface composition of zinc sulphide thin films exhibits the presence of the constituent elements Zn and S as well as C and O as impurity elements.     

The growth of carbon nanotubes on montmorillonite and zeolite (clinoptilolite)

Synthesis of carbon nanotubes described in the present work is based on activation of methane in a hot filament CVD reactor and subsequent creation of nanostructures on a catalyst pre-treated polished surface of silicon. An essential step of the synthesis is the use of natural minerals as catalysts. We have studied the catalyst parameters, the way of its application and the amount of Fe³⁺ cations on the surface of aluminosilicates on the quality of the grown nanotube layers. The growth of carbon nanotubes catalyzed by montmorillonite and zeolite (clinoptilolite) was confirmed by scanning electron microscopy and Raman spectroscopy.     

The nonlinear refraction and nonlinear absorption in 4-(4,6-diaminopyrimidin-2-ylthio) substituted double-decker Lu(III) phthalocyanine

A 4-(4,6-diaminopyrimidin-2-ylthio) substituted double-decker Lu(III) phthalocyanines (4) have been prepared and characterized by elemental analysis, IR, UV-vis and ¹H NMR spectroscopies. The nonlinear refractive index, nonlinear absorption and the optical limiting (OL) performance of the compound 4 in a 0.5 mm spectroscopic cell in DMF solution were investigated by using 4 ns pulse laser at 532 nm. Z-scan experiments have been conducted between 0.24 and 2.39 GW/cm² peak intensities for 10 Hz repetition rate and also between 2.39 and 23.89 GW/cm² peak intensities for 1 Hz repetition rate. The thermal effect contributes to the nonlinear response of the material higher than 0.72 GW/cm² peak intensity at 10 Hz. We measured the effective nonlinear refractive index of the material as 1.2×10⁻¹¹ esu at 3.5×10⁻⁴ M concentration with the peak intensity less than 0.72 GW/cm² and we found that nonlinear absorption was very small. On the other hand, when concentration is increased to 2.4×10⁻³ M material's nonlinear absorption becomes dominant mechanism for the nonlinear response and the compound 4 indicates OL behavior at 2.4×10⁻³ M concentration.     

Improved dielectric properties of (Pb,Ca)TiO3 thin films prepared by metal-organic decomposition method

(Pb,Ca)TiO₃ (PCT) thin films have been deposited on Pt/Ti/SiO₂/Si substrate by metal-organic decomposition (MOD) technique. The film processing parameters such as drying and annealing temperatures have been optimized to obtain good-quality PCT films. Compositional analysis of the film has been studied by X-ray photoelectron spectroscopy (XPS). The effect of the annealing temperature on the crystalline structure, microstructure and electrical properties have been investigated by X-ray diffraction, atomic force microscopy (AFM) and impedance analyzer, respectively. Amorphous PCT films form at 350 °C and crystallize in the perovskite phase following the isothermal annealing at ?650 °C for 3 h in oxygen ambient. Typical tetragonal structure of the PCT film is evidenced from X-ray diffraction pattern. The grain size in the PCT films increases with an increase in annealing temperature. Significant improvement in the dielectric constant value is observed as compared to other reported work on PCT films. The observed dielectric constant and dissipation factor at 100 kHz for 650 °C annealed PCT films are 308 and 0.015, respectively. The correlation of the film microstructural features and electrical behaviors is described.     

Adiabatic small polaron-hopping conduction in Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) perovskites

Polycrystalline bulk ferromagnetic insulating (FMI) Ln_0.85Ca_0.15MnO₃ (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θ_D/2, θ_D being the Debye temperature) dc conductivity (σ_dc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σ_dc data at low temperature (T<θ_D/2).     

Synthesis and electrochemical properties of Li4Ti5O12/C composite by the PVB rheological phase method

Spinel Li₄Ti₅O₁₂/C powders were synthesized successfully by a simple rheological phase method using polyvinylbutyral (PVB) as both template and carbon source. The structure and morphology characteristics of the composite were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy and transmission electron microscopy. The XRD results showed that the composite had a good crystallinity. Its average particle size was about 2.1 μm with a narrow size distribution as a result of homogeneous mixing of the precursors. The in situ carbon coating produced by decomposition of PVB played an important role in improving electrical conductivity, thereby enhancing the rate capacity of Li₄Ti₅O₁₂ as anode material in Li-ion batteries. The Li₄Ti₅O₁₂/C composite, synthesized at 800 °C for 15 h under argon, containing 0.98 wt% of carbon, exhibited better electrochemical properties in comparison with the pristine Li₄Ti₅O₁₂, which could be attributed to the enhanced electrical conductive network of the carbon coating on the particle surface.