In a recent work, Chen et al. [L-Y. Chen et al., J. Mater. Res.24, 3116 (2009)] presented microstructural observation on a plastic Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass (BMG) reported in Liu et al. [Y.H. Liu et al., Science315, 1385 (2007)] by using transmission electron microscopy (TEM) and anomalous small-angle x-ray scattering experiments. Based on their observation, they draw a conclusion that there are no micrometer-sized or nanometer-sized structural heterogeneities in the BMG, and the large plasticity of the BMG cannot be ascribed to the structural heterogeneities. In this comment, we show that their assessment and analysis of their observation are problematic, and it is not evident and precise to use their observation to claim that the BMG is homogeneous and the structural heterogeneity in the glass is an artifact.

Mechanical and thermodynamical properties of bulk polyethylene have been scrutinized using coarse-grained (CG) molecular dynamics simulations. Entangled but cross-link-free polymer clusters are generated by the semicrystalline lattice method for a wide range chain length of alkane modeled by CG beads, and tested under compressive and tensile stress with various temperature and strain rates. It has been found that the specific volume and volumetric thermal expansion coefficient decrease with the increase of chain length, where the specific volume is a linear function of the bond number to all bead number ratios, while the thermal expansion coefficient is a linear rational function of the ratio. Glass-transition temperature, however, does not seem to be sensitive to chain length. Yield stress under tension and compression increases with the increase of the bond number to all bead number ratio and strain rate as well as with decreasing temperature. The correlation found between chain length and these physical parameters suggests that the ratio dominates the mechanical properties of the present CG-modeled linear polymer material.

Natural materials display a wealth of structures and fulfill a variety of functions. Hierarchical structuring is one of the keys to providing multifunctionality and to adapting to varying needs of an organism. As a consequence, the natural environment represents not only a direct and renewable source of useful materials, such as wood, plant fibers, or even proteins of pharmaceutical importance, but also an enormous “database” of structures with exceptional mechanical, optical, or magnetic properties. Rather than focusing on the direct use of natural materials, this article discusses the use of structures that appeared in evolution and have been implemented in artificial materials of an entirely different type and chemical composition. This may be done either by directly copying the structure (biotemplating) or by extracting the design principles encoded in them for the fabrication of novel bioinspired materials.

The drive for greater use of renewable materials is one that has recently gained momentum due to the need to rely less heavily on petroleum. These renewable materials are defined as such since they are derived from plant-based sources. Some renewable materials also offer properties that conventional materials cannot provide: hierarchical structure, environmental compatibility, low thermal expansion, and the ability to be modified chemically to suit custom-made applications. Nature's materials, particularly from plant- and animal-based polysaccharides and proteins, have hierarchical structures, and these structures can be utilized for conventional applications via biomimetic approaches. This issue begins with an article covering renewable polymers or plastics that can be used to generate block copolymers (where two polymers with specific functions are combined) as an alternative to conventional materials. Applications of renewable polymers, such as cellulose from plants, bacteria, and animal sources, are also covered. Also presented are the use of bacterial cellulose and other plant-based nanofibers for transparent electronic display screens and, in a wider sense, the use of cellulose nanofibers for composite materials, where renewable resources are required to generate larger amounts of material. Finally, this issue shows the use of biomimetic approaches to take the multifunctional properties of renewable materials and use these concepts, or the materials themselves, in conventional materials applications.

The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.

A novel colloidal co-casting process was developed to fabricate laser quality, multisegment composite ceramic laser gain materials. The approach was demonstrated for a three segment transparent composite rod 62 mm long by 3 mm diameter consisting of undoped yttrium aluminus garnet (YAG), 0.25% Er:YAG, and 0.5% Er:YAG. The Er concentration profile in the composite has steep, controllable gradients at the segment interfaces, while maintaining constant dopant concentrations within each segment. The composite rod has 84% transmittance at 1645 nm (the lasing wavelength) with a scatter loss of 0.4% cm−1. Laser operation of such a composite Er:YAG ceramic rod was demonstrated for the first time, with nearly equivalent lasing behavior to an Er:YAG single crystal rod.

CdS nanocrystals embedded in sodium borosilicate glass were synthesized through sol-gel process. The CdS nanocrystals were usually 10 to 20 nm in size. The microstructure of CdS nanocrystals was determined to be of the hexagonal phase. The morphology and microstructure of the glass were examined using diverse techniques including scanning-probe microscopy (SPM), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), energy dispersion x-ray spectra (EDAX), and high-resolution TEM (HRTEM). The linear optical absorption spectrum of the glass showed a blue shift as a result of quantum-size effect. Furthermore, the third-order optical nonlinearities of the glass were studied by Z-scan technique at a wavelength of 770 nm. The results showed that the third-order optical nonlinear refractive index γ, absorption coefficient β, and susceptibility χ(3) were determined to be −2.16 × 10−16 m2/W, 6.32 × 10−11 m/W, and 1.20 × 10−10 esu, respectively, which were greater than those reported previously for CdS nanocrystals embedded in different matrices.

A solid state mechanochemical reaction (MCR) method for synthesizing AlN powder with aluminum and melamine powders as the reactants was proposed and put into practice. It was found that the solid state MCR between aluminum and melamine is an instantaneous and exothermic reaction. For a certain charge ratio, a critical ball milling time is needed for the MCR to occur. The higher the charge ratio, the faster the MCR. Cryogenic environments help to accelerate the MCR between Al and melamine. In addition to the direct one-step MCR synthesis approach mentioned above, AlN powder can also be synthesized by pre-ball-milling Al and melamine powders followed by heat treatment. Using this two-step approach, the heat treatment temperature is only about 638 °C, which is much lower than that used in other ways for synthesizing AlN powder. The lower heat treatment temperature can be attributed to the combined effect of both the adoption of melamine and the high reactivity of powders caused by ball milling. Comparatively, the present solid state MCR method for synthesizing AlN powder may be more cost-effective and hence more promising to be used to industrially produce both AlN powder and in situ AlNP reinforced aluminum matrix composites.

A pulsed direct current (dc) reactive ion beam sputtering system has been used to synthesize highly c-axis oriented aluminum nitride (AlN) thin films on (0002)-oriented 200-nm thin titanium layers deposited on a Si-(111) substrate. After a systematic study of the processing variables, high-quality polycrystalline films with preferred c-axis orientation have been grown successfully on the Ti (0002) layer using an Al target under a N2/(N2 + Ar) ratio of 70%, a 2 mTorr processing pressure, and keeping the temperature of the substrate holder at ambient temperature (no substrate heating). The crystalline quality of the AlN and the underlaying Ti thin films was characterized by high-resolution x-ray diffraction. Best ω- full width at half maximum values of the (0002) reflection for 1-μm thin AlN layers are 0.56°. Hence, the AlN layers show a high degree of orientation in the (0002) direction, which directly translates into a high Q value piezoelectric response. Atomic force microscopy measurements were used to study the surface morphology of the Ti layer in an attempt to understand its impact on the quality of the AlN films deposited on top of them. Transmission electron microscopy cross-section analysis has been carried out to investigate the AlN/Ti interface. Our observations reveal the presence of crack-free layers with a smooth surface and extremely low defect density. Even local epitaxy phenomena have been identified at the AlN/Ti interface. The processing conditions used to synthesize AlN layers on Ti at room temperature are efficient in reducing the dislocation density and in-plane residual strain. Such AlN/Ti bilayers can be applied to manufacture novel electroacoustic device structures (such as bulk acoustic wave filters) on silicon substrates in further investigations.

We report temperature-dependent electrical resistivity (or dc conductivity, σdc) down to 4 K for pristine and gamma-irradiated microwave plasma-assisted chemical vapor-deposited boron-doped diamond films with [B]/[C]gas = 4000 ppm to gain insights into the nature of conduction mechanism, distribution, and kinetics of point defects generated due to gamma irradiation prompted by the article [Gupta et al., J. Mater. Res.24, 1498 (2009)]. The pristine samples exhibit typical metallic conduction up to 50 K and with reduction in temperature to 25 K, the σdc decreases monotonically followed by saturation at 4 K, suggesting “disordered” metal or “localized” behavior. For irradiated films, continuous increasing resistivity with decreasing temperature demonstrates semiconducting behavior with thermal activation/hopping conduction phenomena. It is intriguing to propose that irradiation leads to substantial hydrogen redistribution leading to unexpected low-temperature resistivity behavior. Scanning tunneling microscopy/spectroscopy helped to illustrate local grain and grain boundary effects.

Besides biological and chemical cues, cellular behavior has been found to be affected by mechanical cues such as traction forces, surface topology, and in particular the mechanical properties of the substrate. The present study focuses on completely characterizing the bulk linear mechanical properties of such soft substrates, a good example of which are hydrogels. The complete characterization involves the measurement of Young's modulus, shear modulus, and Poisson's ratio of these hydrogels, which is achieved by manipulating nonspherical magnetic microneedles embedded inside them. Translating and rotating these microneedles under the influence of a known force or torque, respectively, allows us to determine the local mechanical properties of the hydrogels. Two specific hydrogels, namely bis-cross-linked polyacrylamide gels and DNA cross-linked polyacrylamide gels were used, and their properties were measured as a function of gel concentration. The bis-cross-linked gels were found to have a Poisson's ratio that varied between 0.38 and 0.49, while for the DNA-cross-linked gels, Poisson's ratio varied between 0.36 and 0.49. The local shear moduli, measured on the 10 μm scale, of these gels were in good agreement with the global shear modulus obtained from a rheology study. Also the local Young's modulus of the hydrogels was compared with the global modulus obtained using bead experiments, and it was observed that the inhomogeneities in the hydrogel increases with increasing cross-linker concentration. This study helps us fully characterize the properties of the substrate, which helps us to better understand the behavior of cells on these substrates.

Hafnium oxide films doubly doped with CeCl3 and TbCl3 and triply doped with CeCl3, TbCl3, and MnCl2 were deposited at 300 °C with the ultrasonic spray pyrolysis technique. The green and yellow emissions of Tb3+ ions and the yellow-red emission of Mn2+ ions can be generated upon ultraviolet (UV) excitation via a nonradiative energy transfer from Ce3+ to Tb3+ and Ce3+ to Mn2+. In the doubly doped film Ce3+ → Tb3+ energy transfer via an electric dipole–quadrupole interaction appears to be the most probable transfer mechanism; the efficiency of this transfer is about 81% upon excitation at 270 nm. In the HfO2 films activated with Ce3+, Tb3+, and Mn2+ the efficiency of energy transfer from Ce3+ to Tb3+ and Mn2+ ions is enhanced by increasing the Mn2+ concentration, up to about 76% for the film with the highest manganese content (1.6 at.%). In addition, it is demonstrated that these triply doped films can generate cold white light emission upon excitation at 270 nm (peak emission wave length of an AlGaN/GaN-based LEDs).

The morphology of the dark and bright regions observed by transmission electron microscopy for the Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass strongly depends on the ion beam parameters used for ion milling. This indicates that the ion beam could introduce surface fluctuation to metallic glasses during ion milling.

In situ residual gas analyzer techniques were used to identify process-property relationships that regulate microstructure evolution in chemical solution-deposited BaTiO3 films. In situ analysis of furnace exhaust gasses enabled quantitative exploration of thermolysis and crystallization reactions and an ability to identify processing parameters that influence the temperature ranges over which they occur. The atmospheric analysis was instrumental in identifying heat treatments that produced optimally consolidated precursor gels that crystallized into BaTiO3 layers with optimized structure and properties. Slow ramp rates resulted in higher porosity, larger grain size, and a dramatic drop in the capacitor yield. Fast ramp rates produced similar trends; however, the mechanisms were distinct. The effects of oxygen partial pressure were also explored. BaTiO3 grain size increased with increasing pO2, whereas there was no appreciable influence on density and capacitor yield. Optimal firing parameters, i.e., 20 °C/min ramp rate at a pO2 of 10−13 atm, were identified as those that produced an overlap in the temperature ranges of thermolysis and crystallization reactions and thus a precursor gel with a density and compliance that supports crystallization and densification while tolerating the associated volume contraction. This in situ approach to analyze downstream furnace gas is shown to be a generically applicable means to understand synthesis methods that are complicated by simultaneous mechanisms of precursor decomposition, extraction of volatile components, and crystallization.

Molecular dynamics simulations are used to evaluate the influence of Sb dopant atoms at the grain boundaries on plastic deformation of nanocrystalline Cu. Deformation is conducted under uniaxial tensile loading, and Sb atoms are incorporated as substitutional defects at the grain boundaries. The presence of randomly dispersed Sb atoms at the grain boundaries does not appreciably influence the mechanisms associated with dislocation nucleation in nanocrystalline Cu; grain boundary ledges and triple junctions still dominate as partial dislocation sources. However, the magnitude of the tensile stress associated with the partial dislocation nucleation event does increase with increasing Sb concentration and also with increasing grain size. The flow stress of nanocrystalline Cu increases with increasing Sb concentration up to 1.0 at.% Sb, with a maximum observed at a grain size of 15 nm for all Sb concentrations (0.0–2.0 at.% Sb).