Structural instability of uniaxially compressed α-quartz

We report a molecular dynamics simulation study of stress–strain response of single crystal α-quartz under uniaxial compression using the interatomic potential of van Beest et al. [Phys. Rev. Lett. 64 (1990) 1955]. The results at 1073 K shows an abrupt onset of large deformation of 14 GPa and subsequent time-dependent evolution of an “amorphous” structure with near-planar character. We regard this to be a lattice instability leading to structural failure, to be distinguished from a similar instability observed under hydrostatic loading which results in a phase change. The simulation results allow us to suggest new interpretations of experimental observations with regard to the formation of thin zones of isotropic (glassy) material along faults and a possible mechanism for planar fracture.     

Biodegradable quantum dot nanocomposites enable live cell labeling and imaging of cytoplasmic targets

Semiconductor quantum dots (QDs) offer great promise as the new generation of fluorescent probes to image and study biological processes. Despite their superior optical properties, QDs for live cell monitoring and tracking of cytoplasmic processes remain limited due to inefficient delivery methods available, altered state or function of cells during the delivery process and the requirement of surface-functionalized QDs for specific labeling of subcellular structures. Here, we present a noninvasive method to image subcellular structures in live cells using bioconjugated QD nanocomposites. By incorporating antibody-coated QDs within biodegradable polymeric nanospheres, we have designed a bioresponsive delivery system that undergoes endolysosomal to cytosolic translocation via pH-dependent reversal of nanocomposite surface charge polarity. Upon entering the cytosol, the polymer nanospheres undergo hydrolysis thus releasing the QD bioconjugates. This approach facilitates multiplexed labeling of subcellular structures inside live cells without the requirement of cell fixation or membrane permeabilization. As compared to conventional intracellular delivery techniques, this approach allows the high throughput cytoplasmic delivery of QDs with minimal toxicity to the cell. More importantly, this development demonstrates an important rational strategy for the design of a multifunctional nanosystem for biological applications.     

Kinetic Monte Carlo approach to modeling dislocation mobility

We describe a kinetic Monte Carlo (kMC) approach to modeling dislocation motion, directly linking the energetics of dislocation kink nucleation and migration on the atomistic scale with the experimental data on the microscale. A study of planar glide of screw dislocation in Si, an ideal test-bed for our method is first discussed, followed by preliminary results for a more complicated problem, three-dimensional motion of screw dislocation in BCC metals. We find that accuracy of the model predictions, even in the favorable case of Si, cannot claim to be quantitative because of uncertainties in the atomistic results for kink energetics. On the other hand, the kMC method is useful for qualitatively probing the mechanisms controlling dislocation motion, and it is capable of providing plausible explanation of some puzzling features of the experimental data.     

Opaline backreflector coating in dye-sensitized solar cell

Photonic crystals are ordered nanostructures that are designed to manipulate the propagation of light. The periodicity of a photonic crystal can be engineered to be highly reflective at selected wavelengths. In this work, a mono-layer and double-layer colloidal photonic crystal film were self-assembled on a glass substrate to be used as backreflectors in a dye-sensitized solar cell (DSSC). The colloidal photonic crystal film consists of different polystyrene monodispersed particles with sizes between 200 nm and 290 nm. Making use of flow controlled vertical deposition (FCVD) method, opaline films of Bragg's reflection wavelength between 450 nm to 750 nm were achieved. These wavelengths were designed to match the absorption spectrum of the Ruthenium-complex dye used in DSSC. An enhancement in incident photon-to-current conversion efficiency (IPCE) of the opaline backreflector DSSC of about 30% at Bragg's peak wavelength has been achieved.     

Ultra-Stable oligonucleotide-gold and -silver nanoparticle conjugates prepared by a facile silica reinforcement method

We report a simple method of enhancing the chemical stability of monothiol-modified oligonucleotide-gold and -silver nanoparticle conjugates by a thin silica reinforcement coating. Conventional conjugates prepared by chemisorption of monothiol-modified oligonucleotides onto nanoparticle surfaces undergo rapid aggregation in the presence of thiol-containing small molecules (e.g., dithiothreitol) due to ligand exchange reactions. When the conjugates are treated with (3-mercaptopropyl)trimethoxysilane, a thin silica layer is formed on the nanoparticle surface, thereby entrapping and reinforcing the thiol-gold/-silver linkage. These silica-modified oligonucleotide-gold and -silver nanoparticle conjugates become much more stable toward dithiothreitol as compared to the unmodified conjugates. Moreover, the silica layer significantly hinders the gold/silver core from oxidative dissolution by sodium cyanide. Importantly, the unique hybridization-induced color change property of the oligonucleotide-gold and -silver nanoparticle conjugates is preserved even under harsh condition (i.e., high concentrations of dithiothreitol). Taken together, these ultra-stable oligonucleotide-nanoparticle conjugates hold promise for new diagnostics and therapeutics.      

An energy‐based damage parameter for the life prediction of AISI 304 stainless steel subjected to mean strain

Abstract

This study extends the plastic strain energy approach to predict the fatigue life of AISI 304 stainless steel. A modified energy parameter based on the stable plastic strain energy density under tension conditions is proposed to account for the mean strain and stress effects in a low cycle fatigue regime. The fatigue life curve based on the proposed energy parameter can be obtained directly by modifying the parameters in the fatigue life curve based on the stable plastic strain energy pertaining to fully reversed cyclic loading. Hence, the proposed damage parameter provides a convenient means of evaluating fatigue life on the mean strain or stress effect. The modified energy parameter can also be used to explain the combined effect of alternating and mean strain/stress on the fatigue life. In this study, the mean strain effects on the fatigue life of AISI 304 stainless steel are examined by performing fatigue tests at different mean strain levels. The experimental results indicate that the combination of an alternating strain and a mean strain strongly influences the fatigue life. Meanwhile, it is found that the change in fatigue life is sensitive to changes in the proposed damage parameter under the condition of a constant strain amplitude at various mean strain levels. A good agreement is observed between the experimental fatigue life and the fatigue life predicted by the proposed damage parameter. The damage parameter proposed by Smith et al. (1970) is also employed to quantify the mean strain effect. The results indicate that this parameter also provides a reasonable estimate of the fatigue life of AISI 304 stainless steel. However, a simple statistical analysis confirms that the proposed damage parameter provides a better prediction of the fatigue life of AISI 304 stainless steel than the SWT parameter.     

Magnetic Properties of a Superconductor with no Inversion Symmetry

We study the magnetic properties of a superconductor in a crystal without symmetry, in particular how the lack of this symmetry exhibits itself. We show that, though the penetration depth itself shows no such effect, for suitable orientation of magnetic field, there is a magnetic field discontinuity at the interface which shows this absence of symmetry. The magnetic field profile of a vortex in the x − y plane is shown to be identical to that of an ordinary anisotropic superconductor to second order in a small parameter . For a vortex along z, there is an induced magnetization along the radial direction.     

Efficient polarization converter for projection displays

In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.