Jeremiah T. Abiade

Robust carbon dioxide reduction on molybdenum disulphide edges

Electrochemical reduction of carbon dioxide has been recognized as an efficient way to convert carbon dioxide to energy-rich products. Noble metals (for example, gold and silver) have been demonstrated to reduce carbon dioxide at moderate rates and low overpotentials. Nevertheless, the development of inexpensive systems with an efficient carbon dioxide reduction capability remains a challenge. Here we identify molybdenum disulphide as a promising cost-effective substitute for noble metal catalysts. We uncover that molybdenum disulphide shows superior carbon dioxide reduction performance compared with the noble metals with a high current density and low overpotential (54 mV) in an ionic liquid. Scanning transmission electron microscopy analysis and first principle modelling reveal that the molybdenum-terminated edges of molybdenum disulphide are mainly responsible for its catalytic performance due to their metallic character and a high d-electron density. This is further experimentally supported by the carbon dioxide reduction performance of vertically aligned molybdenum disulphide.

Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid

Small and salty CO2 reduction scheme Most artificial photosynthesis approaches focus on making hydrogen. Modifying CO2, as plants and microbes do, is more chemically complex. Asadi et al. report that fashioning WSe2 and related electrochemical catalysts into nanometer-scale flakes greatly improves their activity for the reduction of CO2 to CO. An ionic liquid reaction medium further enhances efficiency. An artificial leaf with WSe2 reduced CO2 on one side while a cobalt catalyst oxidized water on the other side. Science, this issue p. 467 Nanostructuring tungsten diselenide enhances catalytic activity for carbon dioxide conversion to carbon monoxide in an ionic liquid medium. Conversion of carbon dioxide (CO2) into fuels is an attractive solution to many energy and environmental challenges. However, the chemical inertness of CO2 renders many electrochemical and photochemical conversion processes inefficient. We report a transition metal dichalcogenide nanoarchitecture for catalytic electrochemical CO2 conversion to carbon monoxide (CO) in an ionic liquid. We found that tungsten diselenide nanoflakes show a current density of 18.95 milliamperes per square centimeter, CO faradaic efficiency of 24%, and CO formation turnover frequency of 0.28 per second at a low overpotential of 54 millivolts. We also applied this catalyst in a light-harvesting artificial leaf platform that concurrently oxidized water in the absence of any external potential.

Ferromagnetism in Cu-doped ZnO films: Role of charge carriers

We report the observation of room temperature ferromagnetism in Cu-doped (5%) ZnO films grown on c-plane sapphire substrates. Films were prepared by pulsed laser deposition technique and were thoroughly characterized using several state-of-the-art characterization techniques. Hall measurements showed that the films are of n-type with a carrier concentration of 3×1017cm−3. Magnetization measurements showed that the films exhibit room temperature ferromagnetism with a saturation magnetization of ∼1.45μB∕Cu atom. When additional carriers were introduced in the films, ferromagnetism was completely vanished. Our results show that the p-type nature of the film is not essential for realizing ferromagnetic characteristics; however, the concentration of n-type carriers should not exceed a critical value.

Enhancement of flux pinning in YBa2Cu3O7−δ thin films embedded with epitaxially grown Y2O3 nanostructures using a multi-layering process

Nanodot arrays of Y2O3 were dispersed in thin films of YBa2Cu3O7−δ (YBCO) by growing alternating layers of these two species using a pulsed laser deposition method. As a result, critical current density Jc both in applied magnetic field and self-field is enhanced by as much as an order of magnitude, along with a significant increase in the irreversibility field Hirr. High-resolution scanning transmission electron microscopy (STEM) and Z -contrast STEM show that the nanoparticles are crystalline and coherent with the YBCO matrix. Whereas in most other studies pinning has been attributed to the strain fields around the nanoparticles, in this case pinning may actually be due to the nanoparticles themselves ,s incethe delineation between the two species is very sharp and STEM reveals no discernible strain fields in the superconducting material around the nanoparticles. (Some figures in this article are in colour only in the electronic version)

Effect of Slurry Ionic Salts at Dielectric Silica CMP

The effects of alkaline ionic salts on silica chemical mechanical polishing ~CMP! have been studied. Particle size, zeta potential, and stability via turbidity tests have been characterized. Particle size and size distributions have been found to increase with ionic strength for three types of alkaline ionic salts due to the decrease in the magnitude of the zeta potential of silica slurry due to the addition of alkaline ionic salts. Slurry stability measured by turbidity tests showed two regimes of slurry stability ~i.e., stable regime and unstable regime!. For the stable slurry regime, the increase in ionic strength leads to an increase in friction force and material removal rate; however, for the unstable slurry regime, the addition of ionic salts results in a decrease in the measured friction force and material removal rate. Surface root-mean-square roughness and maximum depth of surface damage (Rmax) are shown to increase with particle size and size distribution. Investigation into the effect of ionic salts on the polishing mechanism reveals both a chemical and mechanical aspect to polishing silica wafers with silica slurries containing alkaline ionic salts.

Effects of Slurry Particles on Silicon Dioxide CMP

force during polishing. 14,15 This paper presents further validation of the polishing mechanism using sol-gel silica slurries. The effects of particle size and solids loading on friction force and surface finish were studied to delineate the dynamic contact of particles during polishing.

A Tribochemical Study of Ceria-Silica Interactions for CMP

Shallow trench isolation (STI) allows tighter device packing and reduced chip area for isolation. STI is critically dependent on the global planarity that is only possible using chemical mechanical polishing (CMP). Ceria-based slurries are considered the most promising candidates for STI CMP. Despite decades of use in glass polishing, the unique characteristics of ceria slurries are not well understood. In this study, we have conducted force measurements and tribological tests using an atomic force microscope (AFM) and a scanning electron microscope to investigate pH-dependent ceria-silica and silica-silica interactions that occur during CMP. Our studies confirm the effect of hydrolysis at high pH during silica-silica abrasion. An additional physicochemical contribution to ceria-silica polishing is identified and discussed. Furthermore, a strong correlation was observed between the AFM based studies and in situ friction force measurements during CMP.

Effect of pH on ceria-silica interactions during chemical mechanical polishing

To understand the ceria–silica chemical mechanical polishing (CMP) mechanisms, we studied the effect of ceria slurry pH on silica removal and surface morphology. Also, in situ friction force measurements were conducted. After polishing; atomic force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy were used to quantify the extent of the particle–substrate interaction during CMP. Our results indicate the silica removal by ceria slurries is strongly pH dependent, with the maximum occurring near the isoelectric point of the ceria slurry.

Thermal conductivity and interface thermal conductance of amorphous and crystalline Zr47Cu31Al13Ni9 alloys with a Y2O3 coating

We examine the thermal conductivity k and interface thermal conductance G for amorphous and crystalline Zr47Cu31Al13Ni9 alloys in contact with polycrystalline Y2O3. Using time-domain thermoreflectance, we find k=4.5 W m−1 K−1 for the amorphous metallic alloy of Zr47Cu31Al13Ni9 and k=5.0 W m−1 K−1 for the crystalline Zr47Cu31Al13Ni9. We also measure G=23 MW m−2 K−1 for the metallic glass/Y2O3 interface and G=26 MW m−2 K−1 for the interface between the crystalline Zr47Cu31Al13Ni9 and Y2O3. The thermal conductivity of the crystalline Y2O3 layer is found to be k=5.0 W m−1 K−1, and the conductances of Al/Y2O3 and Y2O3/Si interfaces are 68 and 45 MW m−2 K−1, respectively.

The effect of matrix and substrate on the coercivity and blocking temperature of self-assembled Ni nanoparticles

We have shown that the magnetic properties of nanoparticles may be tuned from superparamagnetic to ferromagnetic by changing the substrate or thin film matrix in which they are embedded. Nickel nanoparticles were embedded into alumina, titanium nitride, and cerium oxide matrices on both silicon and sapphire substrates via pulsed laser deposition. The laser ablation time on the nickel target was kept constant. Only nickel nanoparticles in cerium oxide showed characteristics of ferromagnetism (room temperature coercivity and remanence). Ni nanoparticles, in either alumina or titanium nitride, possessed blocking temperatures below 200 K. Detailed scanning transmission electron microscopy analysis has been conducted on the samples embedded into cerium oxide on both substrates and related to the magnetic data.