Stability-instability transitions in silicon crystal growth

In order for a crystal to grow, source atoms must be incorporated into the underlying lattice. Typically, this process occurs on the surface in one of two modes: either through island nucleation or through step flow. However, a third, morphologically unstable growth mode has been predicted. Monitoring the surface of ultraflat substrates with an in situ scanning electron microscope, we prove that for the (111) face of silicon there is a transition from stable step flow to morphological instability and then to island nucleation.     

Pressure dependence of the superconducting state parameters of metallic glass superconductor Mg70Zn30

Explicit expressions have been derived for the volume dependence of electron-phonon coupling strength (λ) and the Coulomb pseudopotential (μ^*) considering the variation of Fermi momentum (κ _F) and Debye temperature (θ _D) with volume. Ashcroft’s model pseudopotential and RPA form of dielectric screening have been used for obtaining pressure dependence of transition temperature (T _C) and the logarithmic volume derivative (Φ) of the effective interaction strength (N ₀ V) for metallic glass superconductor Mg₇₀Zn₃₀. It has been observed that T _C of the metallic glass Mg₇₀Zn₃₀ decreases rapidly with increase of pressure and the superconducting phase disappears at about 30% decrease of volume, for which the μ^* curve shows a minimum and an elbow is formed in the Φ graph.     

Structure and dehydration of the pyrochlore system NaW2−yMoyO6+δ·nH2−zO between 10 and 675 K

The temperature dependence of the structure of the pyrochlore NaW_2−yMo_yO_6+δ·nH_2−zO has been investigated using a variety of diffraction and spectroscopic methods. The positions of OH⁻/H₂O molecules in the structure have been determined. Increases in temperature induce small lattice parameter changes, which are thought to result from movement of the H₂O molecules in the pyrochlore lattice.     

Polarized photoluminescence excitation spectroscopy of single-walled carbon nanotubes

The polarized photoluminescence excitation spectra of twenty-five single-walled carbon nanotube species are reported. For light polarized along the nanotube axis, the main absorption resonance at E22 shows sidebands attributed to phonon assisted absorption. Sidebands to E11 have a diameter dependent energy and are assigned to excited excitonic states. Along with longitudinal excitations, several transverse excitations are identified. The transverse E12 resonance has a specific family pattern with energy close to E22. Comparison with theory provides an estimate of the strength of the Coulomb interaction.     

Electrostatic self-assembly of PEG copolymers onto porous silica nanoparticles

A critical requirement toward the clinical use of nanocarriers in drug delivery applications is the development of optimal biointerfacial engineering procedures designed to resist biologically nonspecific adsorption events. Minimization of opsonization increases blood residence time and improves the ability to target solid tumors. We report the electrostatic self-assembly of polyethyleneimine-polyethylene glycol (PEI-PEG) copolymers onto porous silica nanoparticles. PEI-PEG copolymers were synthesized and their adsorption by self-assembly onto silica surfaces were investigated to achieve a better understanding of structure-activity relationships. Quartz-crystal microbalance (QCM) study confirmed the rapid and stable adsorption of the copolymers onto silica-coated QCM sensors driven by strong electrostatic interactions. XPS and FT-IR spectroscopy were used to analyze the coated surfaces, which indicated the presence of dense PEG layers on the silica nanoparticles. Dynamic light scattering was used to optimize the coating procedure. Monodisperse dispersions of the PEGylated nanoparticles were obtained in high yields and the thin PEG layers provided excellent colloidal stability. In vitro protein adsorption tests using 5% serum demonstrated the ability of the self-assembled copolymer layers to resist biologically nonspecific fouling and to prevent aggregation of the nanoparticles in physiological environments. These results demonstrate that the electrostatic self-assembly of PEG copolymers onto silica nanoparticles used as drug nanocarriers is a robust and efficient procedure, providing excellent control of their biointerfacial properties.     

An experimental investigation of crack-path directional stability

Using the criterion that a crack will extend perpendicular to the maximum circumferential stress,σ _θ, we show that the directional stability of crack growth is governed by the location of microcrack initiation ahead of the crack tip. At distances greater than a geometrical radiusr _o, the maximum value ofσ _θ deviates from the position of symmetry. Thus, if we assume that the physical processes involved in fracture lead to crack initiation at a distancer _c ahead of the crack tip, the criterion for directional stability isr _o>r _c. Experimental and theoretical values ofr _o verify that, asr _o becomes small, the crack's directional stability deteriorates. Observing that a lengthwise compressive stress increasesr _o, a center-cracked specimen was developed which allows the application of controlled lengthwise compression independently of the opening-mode load. A detailed photoelastic analysis of the specimen has provided the value ofr _o as a function of the crack length. The value ofr _o is then compared with the expected microcrack initiation distances in ductile fracture. By applying sufficient lengthwise compression, we are able to make the crack grow straight and obtain numerous data points from this specimen which would otherwise be directionally unstable. The results indicate that, as the total lengthwise tensile stress at the crack tip increases, the fracture toughness also increases. Using this information we can then adjustK _Ic for zero lengthwise loading and obtain a geometry independent fracture toughness.     

Uniaxial stress-strain curves from a bending test

A method which can be used to obtain both tension and compression stress-strain curves from a single bend test was first proposed in 1910. Curiously, this apparently useful technique has been very little application since that time. For this reason, the present investigation was conducted to examine the merits and shortcomings of the test. Three materials which show different stress-strain curves in tension and compression, beryllium, cast iron and prestrained copper, were studied to test the versatility of the method. A special four-point bend fixture, designed to minimize loading errors, was designed for this study and is described. Generally, the stress-strain curves obtained from bending and conventional tests showed good agreement for strains less than about five percent. Finally, some advantages and disadvantages of the bending tests are pointed out.     

Two modes of electroluminescence from single‐walled carbon nanotubes

The electroluminescence from single‐walled carbon nanotube field effect transistors is spectrally resolved, and shows two distinct modes of light emission. The vast majority of nanotubes have spectrally broad emission consistent with the spectrum of blackbody radiation. Much more rarely, superposed on the broad emission is a single narrow (<50 meV) peak which is consistent with expectation for electron–hole recombination. The narrow emission is strong even at lower biases and in general has greater peak intensity than the broadband emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)