ALBERT

Notes: Optical properties of ZnO quantum dots (QDs) capped with polyvinyl pyrrolidone (PVP) molecules have been investigated. It is demonstrated that surface modification by PVP can dramatically change the emission spectra of the ZnO QDs. At the optimized condition with a PVP/Zn2+ ratio of 3:5, the photoluminescence (PL) spectrum of ZnO QDs shows a strong ultraviolet (UV) emission while the low energy green emission is fully quenched. This is a result of the surface passivation of the ZnO QDs by the PVP molecules. The origin of the green emission is attributed to the surface states associated with oxygen vacancies. Temperature and excitation power dependent PL studies suggest that the UV emission is associated with localized states. © 2001 American Institute of Physics.

Notes: Chemically hybridized CdS-poly(N-vinylcarbazole) (PVK) nanocomposites with different molar ratios of CdS to PVK were prepared. A variety of techniques including optical absorption, photoluminescence (PL), photocurrent spectroscopy, and time-resolved PL were employed to investigate the properties of CdS–PVK nanocomposites. A significant enhancement of photoconductivity was observed in CdS–PVK nanocomposites as compared to pure PVK and CdS/PVK nanoblends. We interpret this result as the improved interface quality between CdS and PVK in CdS–PVK nanocomposites, which facilitates fast interfacial carrier transfer and thus increases the charge generation efficiency. © 2001 American Institute of Physics.

Notes: Photoluminescence from a GaN0.015As0.985/GaAs quantum well has been measured at 15 K under hydrostatic pressure up to 9 GPa. Both the emissions from the GaNAs well and GaAs barrier are observed. The GaNAs-related peak shows a much weaker pressure dependence compared to that of the GaAs band gap. A group of new peaks appear in the spectra when the pressure is beyond 2.5 GPa, which is attributed to the emissions from the N isoelectronic traps in GaAs. The pressure dependence of the GaNAs-related peaks was calculated using the two-level model with the measured pressure coefficients of the GaAs band gap and N level as fitting parameters. It is found that the calculated results deviate seriously from the experimental data. An increasing of the emission intensity and the linewidth of the GaNAs-related peaks was also observed and briefly discussed. © 2001 American Institute of Physics.

Notes: Intense wide-band photoluminescence (PL) was observed at room temperature from both SiC thin films grown on silicon substrates by hot-filament chemical vapor deposition and anodized SiC thin films formed by electrochemical anodization in HF–ethanol solution. It was found that prolonged irradiation with ultraviolet light from a He–Cd laser (325 nm, 10 mW) generally enhanced the PL intensity of as-grown SiC but induced a new PL band in anodized SiC at room temperature. The light-induced PL emission in anodized SiC was centered at the energy between 2.1 and 2.2 eV in comparison with the initial peak position of about 1.9 eV. These effects were also temperature dependent. © 1999 American Institute of Physics.

Notes: The density and size of short cracks on the surface of 1Cr18Ni9Ti stainless steel smooth specimens during low cycle fatigue are investigated using a replica technique. The density and size data are analysed from two different observation policies, i.e. Policy I pays attention to the whole specimen test piece and Policy II is related to an ‘effective short fatigue crack criterion’, which pays attention to the dominant crack (DC) initiation zone and the zones ahead of the DC tips. The results reveal that both the crack density and crack size evolution exhibit a specific character during the microstructural short crack (MSC) and physical short crack (PSC) stages. The Policy I-based observations exhibit an increasing density and little scatter of the density data. The increasing density violates the general test observation of decreasing collective crack effects in the PSC stage. The little scatter is too small to reflect the intrinsic scatter of fatigue properties. Both the crack density and crack size evolution from this policy show little relationship with the intrinsic localization of fatigue damage. However, Policy II-based observations show an increasing crack density and an increasing density scatter in the MSC stage. The density and scatter reach their maximum values at the transition point between the MSC and PSC stages. Then, they decrease with fatigue cycling in the PSC stage and tend to their saturation values when the DC size is above about 500 μm. This behaviour shows a good agreement with the general test observations of decreasing collective crack effects and growth rate scatter in the PSC stage. Further, both approaches exhibit an evolutionary positively skewed crack size distribution, and an increasing difference between the average crack length and the DC length in the PSC stage, indicative of decreasing collective crack effects. A three-parameter Weibull distribution (3-PWD) is appropriately used to describe the crack sizes and a 6.5 to 7.6 μm value of location parameter of the distribution is obtained to reflect a minimum value for the initial cracks. It is worth noting that Policy I-based observations show an increasing positively skewed crack size distribution, an increasing scatter of the size data and a decreasing shape parameter of the 3-PWD. This represents an increasing collective crack effect and an increasing irregularity of interactive cracks, which violates the general test observations. In contrast, Policy II-based observations exhibit a decreasing positively skewed size distribution shape and an increasing (from 〈1 gradually to 〉1) shape parameter of the 3-PWD that is in agreement with the general test observations. The increasing shape parameter indicates that the collective crack effects act as an evolutionary process from an initial non-ordered (chaotic) random state gradually to an independent random state at the transition point between the MSC and PSC stages and then, to a loading history-dependent random state. This behaviour is in accordance with the evolutionary DC growth behaviour. Therefore, the evolutionary short crack behaviour associated with the intrinsic localization of fatigue damage should be appropriately revealed from the ‘effective short fatigue crack criterion’-based observations.

Notes: Abstract Some previous studies on hot-pressed and sintered SiC polycrystalline materials have been reexamined. Mechanical data and microstructures strongly suggest that the Newtonian creep behaviour observed in these SiC materials was induced by a dislocation process operating in Harper-Dorn creep, rather than by diffusional creep as concluded before. The supporting evidence for this suggestion includes extensive development of dislocation substructures, no dependence of creep rate upon grain size, and the measured creep rates being far faster than those predicted by the model of diffusion creep, but consistent with those estimated by the model of Harper-Dorn creep.