Thin films of GaNBi alloys with up to 12.5 at.% Bi were grown on sapphire using low-temperature molecular beam epitaxy. The low growth temperature and incorporation of Bi resulted in a morphology of nanocrystallites embedded in an amorphous matrix. The composition and optical absorption shift were found to depend strongly on the III:V ratio controlled by the Ga flux during growth. Increasing the incorporation of Bi resulted in an increase in conductivity of almost five orders of magnitude to 144 Ω-cm−1. Holes were determined to be the majority charge carriers indicating that the conductivity most likely results from a GaNBi-related phase. Soft x-ray emission and x-ray absorption spectroscopies were used to probe the modification of the nitrogen partial density of states due to Bi. The valence band edge was found to shift abruptly to the midgap position of GaN, whereas the conduction band edge shifted more gradually.

Half-Heusler (HH) phases, a versatile class of alloys with promising functional properties, have recently gained attention as emerging thermoelectric materials. These materials are investigated from the perspective of thermal and electronic transport properties for enhancing the dimensionless figure of merit (ZT) at 800–1000 K. The electronic origin of thermopower enhancement is reviewed. Grain refinement and embedment of nanoparticles in HH alloy hosts were used to produce fine-grained as well as nanocomposites and monolithic nanostructured materials. Present experiments indicated that n-type Hf0.6Zr0.4NiSn0.995Sb0.005 HH alloys and p-type Hf0.3Zr0.7CoSn0.3Sb0.7/nano-ZrO2 composites can attain ZT = 1.05 and 0.8 near 900–1000 K, respectively. The observed ZT enhancements could be attributed to multiple origins; in particular, the electronic origin was identified. The prospect for higher ZT was investigated in light of a recently developed nanostructure model of lattice thermal conductivity. Tests performed on p–n couple devices from the newly developed HH materials showed good power generation efficiencies—achieving 8.7% efficiency for hot-side temperatures of about 700 °C.

We describe experiments on self-heating and melting of nanocrystalline silicon microwires using single high-amplitude microsecond voltage pulses, which result in growth of large single-crystal domains upon resolidification. Extremely high current densities (>20 MA/cm2) and consequent high temperatures (1700 K) and temperature gradients (1 K/nm) along the microwires give rise to strong thermoelectric effects. The thermoelectric effects are characterized through capture and analysis of light emission from the self-heated wires biased with lower magnitude direct current/alternating current voltages. The hottest spot on the wires consistently appears closer to the lower potential end for n-type microwires and to the higher potential end for p-type microwires. The experimental light emission profiles are used to verify the mathematical models and material parameters used for the simulations. Good agreement between experimental and simulated profiles indicates that these models can be used to predict and design optimum geometry and bias conditions for current-induced crystallization of microstructures.

Predicting the dislocation nucleation rate as a function of temperature and stress is crucial for understanding the plastic deformation of nanoscale crystalline materials. However, the limited time scale of molecular dynamics simulations makes it very difficult to predict the dislocation nucleation rate at experimentally relevant conditions. We recently develop an approach to predict the dislocation nucleation rate based on the Becker–Döring theory of nucleation and umbrella sampling simulations. The results reveal very large activation entropies, which originated from the anharmonic effects, that can alter the nucleation rate by many orders of magnitude. Here we discuss the thermodynamics and algorithms underlying these calculations in greater detail. In particular, we prove that the activation Helmholtz free energy equals the activation Gibbs free energy in the thermodynamic limit and explain the large difference in the activation entropies in the constant stress and constant strain ensembles. We also discuss the origin of the large activation entropies for dislocation nucleation, along with previous theoretical estimates of the activation entropy.

The densification kinetics of a blend of unalloyed Ti and Al-40V master alloy powders are measured during uniaxial hot pressing under 3–10 MPa pressure for thermal cycling (860–1020 °C) or isothermal (1020 °C) conditions. Subsequent heat treatment for 4–16 h at 1020 °C results in a homogeneous Ti-6Al-4V microstructure. This process provides a low-cost alternative to hot isostatic pressing of prealloyed Ti-6Al-4V powders.

Calcium phosphates form a vast family of biominerals, which have attracted much attention in fields like biology, medicine, and materials science, to name a few. Solid state Nuclear Magnetic Resonance (NMR) is one of the few techniques capable of providing information about their structure at the atomic level. Here, examples of recent advances of solid state NMR techniques are given to demonstrate their suitability to characterize in detail synthetic and biological calcium phosphates. Examples of high-resolution 31P, 1H (and 17O), solid state NMR experiments of a 17O-enriched monocalcium phosphate monohydrate-monetite mixture and of a mouse tooth are presented. In both cases, the advantage of performing fast Magic Angle Spinning NMR experiments at high magnetic fields is emphasized, notably because it allows very small volumes of sample to be analyzed.

An array of 32 sensor elements with single-walled carbon nanotubes (SWCNTs) as the sensing medium has been fabricated. The microfabrication approach used allows reduction of the chip size and increases the number of sensor elements in a chip and is amenable for large wafer scale-up. The sensor array chip is designed as an electronic nose for use with the aid of a pattern recognition algorithm. The sensor chips were tested for NO2 sensing and interfering effects from humidity and a background of chlorine. The results indicate that NO2 can be detected at low concentration levels of 0.5 ppm in the presence of chlorine at 30 times higher concentrations. The sensor response is affected by humidity, which implies that the training data set for NO2 detection needs to be generated for multiple humidity levels for interpolation purposes during field use.

We report the results of constant strain rate experiments on electroplated, single crystalline copper pillars with diameters between 75 and 525 nm. At slow strain rates, 10−3 s−1, pillar diameters with 150 nm and above show a size-dependent strength similar to previous reports. Below 150 nm, we find that the size effect vanishes as the strength transitions to a relatively size-independent regime. Strain rate sensitivity and activation volume are determined from uniaxial compression tests at different strain rates and corroborate a deformation mechanism change. These results are discussed in the framework of recent in situ transmission electron microscopy experiments observing two distinct deformation mechanisms in pillars and thin films on flexible substrates: partial dislocation nucleation from stress concentrations in smaller structures and single arm source operation in larger samples. Models attempting to explain these different size-dependent regimes are discussed in relation to these experiments and existing literature revealing further insights into the likely small-scale deformation mechanisms.

The preparation and characterization of Fe3O4 microtubes by a polymer-based template approach were described. Fe3O4 tubes with diameter of 600 ± 50 nm and an average tube thickness of about 50 nm were fabricated after removing the electrospun polystyrene fiber template. The microtubes were composed of individual Fe3O4 nanocrystals. The synthesis process was ambient, generalizable, inexpensive, and nontoxic. The magnetite tubes thus fabricated behave with a saturation magnetization of 37 emu/g measured in the vibrating sample magnetometer. The microtubes prepared in this way might find potential applications in catalysis, magnetic fluid, and biological field.

The possibility and suitability of micro-Raman spectroscopy as a noncontact, in-line measurement technique for boron (B) concentration in ultrathin (20~35 nm thick) Si1–xGex layers epitaxially grown on 300 mm diameter p−-Si(100) wafers, by ultrahigh vacuum chemical vapor deposition, was investigated. Raman spectra from Si1–xGex/Si(100) wafers were measured under 363.8, 457.9, 488.0, and 514.5 nm excitation. Strong correlation was found between B content and characteristics of the Si–Si Raman peak from the Si1–xGex films. As B concentration increased from undoped to 9.1 × 1020 atoms/cm3, the Si–Si Raman peak broadened and the peak height became smaller for a given Ge content. The B concentration in Si1–xGex film estimated from Raman measurement was in good agreement with secondary ion mass spectroscopy analysis results. Boron concentration as low as 8.7 × 1017 atoms/cm3 can be detected by Raman spectroscopy, which is ~30 times more sensitive than the detection limit (2.7 × 1019 atoms/cm3) of high-resolution x-ray diffraction.

The objective of this study was to identify the microstructural mechanisms controlling Ω precipitates’ contribution to the high strength and ductility of Al–Cu–Mg–Ag alloys subjected to high impact loading conditions. Three interrelated approaches were used: (i) HRTEM imaging of deformed Ω precipitates in ballistically impacted Al–Cu–Mg–Ag plates, (ii) microstructurally based finite element (FE) analysis based on specialized crystalline plasticity formulations, and (iii) molecular dynamics (MD) simulations of dislocation nucleation and emission. The FE and MD simulations detail the evolution of dislocation densities and dislocations at the Al/Ω interface, which are consistent with the experimentally observed multiplicity of shear cutting of thin Ω precipitates. Furthermore, the FE results indicate that unrelaxed tensile strains at the Al/Ω interface can inhibit localized deformation in the alloy.

High-quality Zn3P2 nanowires are synthesized at a temperature as low as 350 °C using Zn foil and trioctylphosphine by chemical reflux method. Scanning electron microscopy and transmission electron microscopy (TEM) images show their diameters vary from ∼15 to 70 nm. Energy dispersive x-ray spectroscopy and nanobeam electron diffraction patterns in combination with structure factor simulation reveal that the nanowires have tetragonal α-Zn3P2 structure. Based on high-resolution TEM images and their fast Fourier transform patterns, Zn3P2 nanowires are considered to grow on a vicinity of the possibly highest surface energy plane of (101) with a growth direction parallel to [101].

We demonstrate that the size and crystallinity of grains in individual ZnO nanofibers greatly influence their sensing properties for CO. The sensing properties, including sensitivity, response and recovery times of a sensor fabricated with ZnO nanofibers composed of large grains are much superior to those of a sensor fabricated with nanofibers of small grains. The crystallinity, improved by the longer calcination time, is likely to be responsible for the higher sensitivity in the large-grained nanofibers. The facilitated occupancy and desorption of CO molecules at grain boundaries in the large-grained nanofibers are the most probable causes of the shorter response and recovery times in detecting CO, respectively. This work suggests not only that electrospinning-synthesized ZnO nanofibers hold promise for realizing sensitive and reliable gas sensors but also that the size as well as the crystallinity of the grains existing in individual nanofibers need to be optimized to obtain the best sensing properties.

Mechanisms and magnitudes of the large piezoelectric response observed in lead-free (1–x)BiFeO3–xBaTiO3 (BFBT) ceramics are investigated. Preceding studies reported significant strain hysteresis and hard ferroelectric behavior in BFBT leading to a small low-field piezoelectric coefficient, instability of the poled domain state, and rapid degradation of piezoelectric properties. The current investigation shows that under application of a suitable direct current (dc) bias to stabilize the ferroelectric phase low- and high-field piezoelectric coefficients (d33) of 150 pC/N and 250 pC/N are observed for the composition 0.67BiFeO3–0.33BaTiO3 + 0.1 wt% MnO with a Curie temperature of 605 °C. Such enhancement of electromechanical properties under dc bias is in contrast to the expected behavior in traditional piezoelectric materials such as soft lead zirconate titanate (PZT). The large piezoelectric coefficients confirm strong intrinsic and extrinsic contributions to the piezoelectric response in BFBT, which coupled with high ferroelectric Curie temperature TC > 500 °C, suggests BFBT-based materials as promising lead-free alternatives to PZT piezoceramics.

Skutterudites are among the most exciting thermoelectric (TE) materials that could be used for various intermediate temperature applications. This study summarized our recent work on n-type partially filled skutterudites. By combining theoretical and experimental approaches, we revealed the underlying mechanism of void filling in the intrinsic lattice voids in CoSb3. With that, the electronegativity selection rule is established for the current stable filled skutterudites and further used for the discovery of a few novel filled CoSb3 compounds. The correlation between the thermal/electrical transport properties and impurity fillers in n-type partially filled skutterudites was also carefully investigated. Our results provide fundamental understanding to how those filler impurities affect electronic structures and lattice dynamics. Based on these basic understanding on transport mechanisms and sophisticated strategy in materials synthesis, TE figure of merit for n-type materials were continually increased from 1.1 to 1.4 and then to 1.7 for single-, double-, and triple-filled skutterudites.

Tin dioxide (SnO2) is an important base material for a variety of gas sensors and catalysts. However, there is a lack of experimental data on the energetics of SnO2 surfaces and their water adsorption. In this work, the surface energies of anhydrous and hydrated SnO2 nanoparticles were measured by combining high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The SnO2 nanoparticles were synthesized through oxidation of metallic tin using nitric acid followed by heat treatment at different temperatures to achieve surface areas ranging from 4000 to 10,000 m2·mol−1(25–65 m2·g−1). The enthalpy of the anhydrous surface is 1.72 ± 0.01 J·m−2, and that of the hydrated surface is 1.49 ± 0.01 J·m−2. The integral heat of water adsorption is −75 kJ·mol−1, with a chemisorbed maximum coverage of ∼5 H2O·nm−2. SnO2 has a lower surface energy and less exothermic enthalpy of water adsorption than the isostructural TiO2 (rutile) reported previously. This comparison suggests that the excellent sensing properties of SnO2 may be a consequence of its relatively low affinity for surface H2O molecules that compete with other gases for adsorption.

We review the position-controlled growth of III-V nanowires (NWs) by selective-area metal-organic vapor-phase epitaxy (SA-MOVPE). This epitaxial technique enables the positioning of the vertical NWs on (111) oriented surfaces with lithographic techniques. Core-shell structures have also been achieved by controlling the growth mode during SA-MOVPE. The core-shell III-V NW-based devices such as light-emitting diodes, photovoltaic cells, and vertical surrounding-gate transistors are discussed in this article. Nanometer-scale growth also enabled the integration of III-V NWs on Si regardless of lattice mismatches. These demonstrated achievements should have broad applications in laser diodes, photodiodes, and high-electron mobility transistors with functionality on Si not made possible with conventional Si-CMOS techniques.

The current work describes the simple solution process of ball-chain polycrystalline Fe nanofibers with aspect ratios (λ) and diameters (D) of over 1−65 and 30−95 nm. Static magnetic and microwave electromagnetic property studies demonstrated that such properties strongly depend on the λ and D of the Fe nanofibers. As the λ and D increase, the U-shape of the saturation magnetization (Ms) reaches a maximum of 180.0 emu·g−1, owing to the cooperative action of nanoeffects and magnetic interactions, whereas the coercivity (Hc) gradually increases due to aspect ratio variations. In contrast, the change in trends of the permittivity (ε′, ε″) and the dielectric loss (tgδE) are represented as an inverted U-shape; the permeability (μ′, μ″) and magnetic loss (tgδM) increase at low-frequency ranges and decrease at high frequency ranges. Stronger absorption and broader bandwidths of Fe nanofibers compared with Fe nanoparticles are ascribed to higher dielectric losses. The prepared Fe nanofibers have high potential in light-weight and broad bandwidth microwave absorbing applications.

The optical properties of N-ion-implanted diamond are evaluated. The color of implanted layer became glossy black with metallic luster, which was further enhanced after postimplantation annealing at 600 °C for 2 h in vacuum or inert gas atmosphere. Raman spectroscopy revealed that the crystalline diamond became completely disordered after irradiation, but surprisingly the crystalline nature was restored to a mixture of well-defined diamond and diamond-like carbon after annealing. When it was annealed in air at the same temperature, however, the black color disappeared, indicating a removal of the disordered or graphitized layer by oxidation. X-ray photoelectron spectroscopy and Raman analyses indicate that the black color of as-implanted diamond is associated mainly with the disordered carbon and modified band structure. Fourier transform infrared (FTIR) analysis shows that the implanted nitrogen atoms are in N–N and symmetrical 4N-vacancy bonding states, which are commonly found in the natural diamonds with yellow and brown tint.

The metal nanoparticles dispersed in matrices of composite material are able to apply in different technologies based on their peculiarity. This article reports the preparation of Ag/epoxy nanocomposite film with aligned Ag nanowires coated on glass substrate by multistep processing including synthesis of Ag nanowires by seed-mediated method, dispersion of Ag nanowires in the epoxy resin, and stretching to form the Ag/epoxy nanocomposite film. The results showed that Ag nanowires had been well aligned in the direction of stretching, both in the surface layer and in the internal of the film. Meanwhile, the Ag/epoxy nanocomposite film showed an obviously infrared polarization property in a broad wavelength range from 1600 to 2600 nm, with transmittance over 70%. The mechanisms for the orientation of Ag nanowires and the generation of polarization property of the films were discussed, respectively.