Static and dynamic experiments in cryo-electron microscopy: comparative observations using high-vacuum, low-voltage and low-vacuum SEM

We carried out a unique comparative study between three modes of cryo‐scanning electron imaging: high‐vacuum, low‐voltage and low‐vacuum, using ice cream as a model system. Specimens were investigated both with and without a conductive coating (Au/Pd) and at temperatures for which ice either remains fully frozen (< ?110 °C) or undergoes sublimation (?110 to ?90 °C). At high magnification, high‐vacuum imaging of coated specimens gave the best results for ‘static’ specimens (i.e. containing fully frozen ice). Low voltages, such as 1 kV, could be used for imaging uncoated specimens at high vacuum, although slight ‘classical’ charging artefacts remained an issue, and the reduced electron beam penetration tended to decrease the definition between different microstructural features. However, this mode was useful for observing in situ sublimation from uncoated specimens. Low‐vacuum mode, involving small partial pressures of nitrogen gas, was particularly suited to in situ sublimation work: when sublimation was carried out in low vacuum in the absence of an anti‐contaminator plate, sublimation rates were significantly reduced. This is attributed to a small partial pressure of sublimated water vapour remaining near the specimen surface, enhancing thermodynamic stability.     

Dealing with lead in Broken Hill--trends in blood lead levels in young children 1991-2003

The objective of the study was to investigate trends in blood lead concentrations in preschool children between 1991 and 2003, as part of the evaluation strategy of a public health lead management program in Broken Hill, Australia. Since 1991, all Broken Hill children aged 1-4 years have been offered at least annual blood lead screening as part of a community-wide lead management program. Recruitment of children was promoted throughout the period using local media and distribution of promotional material from health care centres and preschool, childcare, and educational facilities around the city. Venous blood samples were collected using standard procedures and analyses were subjected to internal and external quality control programs. Because the frequency distribution of blood lead levels are skewed, geometric rather than arithmetic means were used for comparative purposes. Trend analysis was based on age and sex standardised mean blood lead levels. The number of 1- to 4-year-old children screened ranged between 496 and 948 in any one year and response rates varied between 39% and 73%. The age-sex standardised mean blood lead level decreased from 16.3 microg/dL to 7.1 microg/dL between 1991 and 2003. Overall, blood lead levels declined by 56% over 13 years. These reductions were consistently observed irrespective of age or where a child lived in the town. The rate of decline has slowed since 1997. We conclude that substantial progress has been made in dealing with the lead problem in Broken Hill children, although the rate of decline of blood lead levels has slowed. Continued public health action is still needed to bring the proportion of young children with significantly elevated blood lead levels (>15 microg/dL) down from the 2003 figure of 12% to the NHMRC community-based target for lead in young Australians of 5%.     

Measurement of far-infrared laser frequencies from optically pumped CHD/sub 2/OH

A three-laser heterodyne system was used to frequency measure 11 previously observed optically pumped far-infrared (FIR) laser emissions of CHD/sub 2/OH. These newly measured frequencies have fractional uncertainties of /spl plusmn/2/spl times/10/sup -7/ and correspond to laser wavelengths ranging from 47.8 to 238.0 /spl mu/m. The pump laser offset frequency was measured for 15 CHD/sub 2/OH FIR laser emissions.     

The effect of small-signal AC voltages on C–V characterization and parameter extraction of SiO2 thin films

The effect of different small-signal ac voltage amplitudes on C–V curves characterized by thin SiO₂ based p-type MOS capacitor with aluminum gate is reported. When the small-signal ac voltage is comparable to the gate bias, the thickness of SiO₂ thin films extracted from the accumulation capacitance is found to be independent of small-signal ac voltage amplitudes, but the flat band voltage shift and interface state density associated with the variation of depletion layer capacitance are dependent on small-signal ac voltage amplitudes. They all increase with the small-signal ac voltage amplitudes. The experimental results reveal that the optimum small-signal ac voltage should be less than 100 mV. The mechanisms involving the depletion layer changes with small-signal ac voltages in SiO₂ thin films are also discussed in this paper.     

Copper Sulfide (CuxS) Nanowire‐in‐Carbon Composites Formed from Direct Sulfurization of the Metal‐Organic Framework HKUST‐1 and Their Use as Li‐Ion Battery Cathodes

Li‐ion batteries containing cost‐effective, environmentally benign cathode materials with high specific capacities are in critical demand to deliver the energy density requirements of electric vehicles and next‐generation electronic devices. Here, the phase‐controlled synthesis of copper sulfide (Cu_xS) composites by the temperature‐controlled sulfurization of a prototypal Cu metal‐organic framework (MOF), HKUST‐1 is reported. The tunable formation of different Cu_xS phases within a carbon network represents a simple method for the production of effective composite cathode materials for Li‐ion batteries. A direct link between the sulfurization temperature of the MOF and the resultant Cu_xS phase formed with more Cu‐rich phases favored at higher temperatures is further shown. The Cu_xS/C samples are characterized through X‐ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy, and energy dispersive X‐ray spectroscopy (EDX) in addition to testing as Li‐ion cathodes. It is shown that the performance is dependent on both the Cu_xS phase and the crystal morphology with the Cu_1.8S/C‐500 material as a nanowire composite exhibiting the best performance, showing a specific capacity of 220 mAh g^?1 after 200 charge/discharge cycles.     

Effect of NiTe Nanoinclusions on Thermoelectric Properties of Bi2Te3

Metal nanoinclusions in bulk thermoelectric matrix create metal?Csemiconductor interfaces, which can result in improvement in the thermoelectric power factor due to low-energy electron filtering and a simultaneous reduction in lattice thermal conductivity due to increased phonon scattering at grain boundaries. The combined effect results in enhancement of the thermoelectric figure of merit. We report the effect of NiTe nanoinclusions in a Bi₂Te₃ matrix. The Bi₂Te₃/NiTe nanocomposite was synthesized by planetary ball milling. Different volume fractions of NiTe nanoinclusions were incorporated into the bulk (Bi₂Te₃) matrix and uniaxially hot pressed at 100?MPa and 500°C. The presence of nanoinclusions was confirmed by x-ray diffraction and transmission electron microscopy. The Seebeck coefficient, electrical conductivity, and thermal diffusivity were measured from room temperature to 150°C. The carrier concentration of the matrix (Bi₂Te₃) and the nanocomposites (NiTe/Bi₂Te₃) at room temperature were deduced from Hall-effect measurements. Addition of NiTe decreased the carrier concentration, and the power factor increased in the 1?vol.% NiTe/Bi₂Te₃ compared with inclusion-free Bi₂Te₃ matrix due to an increase in mobility.     

Investigation of radiative tunneling in GaN/InGaN single quantum well light-emitting diodes

The mechanisms of carrier injection and recombination in a GaN/InGaN single quantum well light-emitting diodes have been studied. Strong defect-assisted tunneling behavior has been observed in both forward and reverse current–voltage characteristics. In addition to band-edge emission at 400 nm, the electroluminescence has also been attributed to radiative tunneling from band-to-deep level states and band-to-band tail states. The approximately current-squared dependence of light intensity at 400 nm even at high currents indicates dominant nonradiative recombination through deep-lying states within the space-charge region. Inhomogeneous avalanche breakdown luminescence, which is primarily caused by deep-level recombination, suggests a nonuniform spatial distribution of reverse leakage in these diodes.