We present an experimental study on the epitaxy and orientational relationships of WO2 and NbO2 films on (0001) Al2O3, (111) MgAl2O4, and (111) MgO substrates, as well as WO2 on (111) SrTiO3. The higher symmetry of the substrate planes compared to the film planes leads to the formation of epitaxial structural variants, and they are related by the surface rotational symmetry elements of the substrates. WO2 and NbO2 crystallize in distorted versions of the rutile structure, and we discuss our findings in context of the rutile unit cell. Our results are applicable to other compounds that occur in (distorted) rutile structures. For the case of NbO2 thin films, we also demonstrate that they can be grown epitaxially on (10$\bar 1$2) and (10$\bar 1$0) Al2O3, lower symmetry surfaces; in these cases, surface symmetry does not induce the formation of epitaxial rotational variants, though domains related by glide symmetry are possible.

A new approach has been developed for nanoscale conductance mapping (NCM) based on multidimensional atomic force microscopy (AFM) to efficiently investigate the nanoscale electronic properties of heterogeneous surfaces. The technique uses a sequence of conductive AFM images, all acquired in a single area but each with incrementally higher applied voltages. This generates a matrix of current versus voltage (I–V) spectra, providing nanoscale maps of conductance and current nonlinearities with negligible spatial drift. For crystalline and amorphous phases of a GeSe chalcogenide phase change film, conductance and characteristic amorphous phase “turn-on” voltages are mapped with results providing traditional point-by-point I–V measurements, but acquired hundreds of times faster. Although similar to current imaging tunneling spectroscopy in a scanning tunneling microscope, the NCM technique does not require conducting specimens. It is therefore a promising approach for efficient, quantitative electronic investigations of heterogeneous materials used in sensors, resistive memories, and photovoltaics.

In this article, we present novel sample preparation methods using a helium ion microscope (HIM). We report the possibility of reshaping, at room temperature, thin metal lines on an electron-transparent membrane: A set of platinum bridges with standard geometry (300 × 200 × 15 nm) was modified at room temperature into different shapes using focused helium (He)-ion beam. Also the applicability of the HIM as a tool for precise modification of silicon (Si) and strontium titanate (SrTiO3) lamellae is shown and discussed. We demonstrated that in situ heating (e.g., at 600 °C) of the samples during He-beam illumination by use of a specially developed heating stage enables production of thin Si and SrTiO3 samples without significant artifacts. The quality of such cuts was inspected by transmission electron microscopy with high-resolution imaging, and the diffraction patterns were analyzed.

Carbon nanotube (CNT)-reinforced magnesium (Mg) matrix composites were synthesized using a powder metallurgical method and tested compressively along the plane normal and in-plane orientations. Yield strengths of composites were significantly increased by 35–129% compared with that of pure Mg. With the increase of CNT weight percentage, yield strength first increased until reaching a critical CNT weight percentage and then decreased. Twinning operated in the in-plane samples when CNT weight percentage was less than or equal to 0.5%, whereas twinning operation was not observed in all plane normal samples and the in-plane samples with 1% or higher CNT weight percentage. Severe plastic deformation was exhibited in fracture surface images with low magnification, whereas intrinsic brittle fracture feature was observed under high magnification. A theoretical model incorporating the Orowan strengthening and the thermal expansion mismatch strengthening was utilized and made good yield strength predictions.

We determine the nonequilibrium grain size distribution (GSD) during the crystallization of a solid in d-dimensions under fixed thermodynamic conditions, for the random nucleation and growth model, and in the absence of grain coalescence. Two distinct generalizations of the theory established earlier are considered. A closed analytic expression of the GSD useful for experimental studies is derived for anisotropic growth rates. The main difference from the isotropic growth case is the appearance of a constant prefactor in the distribution. The second generalization considers a Gaussian source term: nuclei are stable when their volume is within a finite range determined by the thermodynamics of the crystallization process. The numerical results show that this generalization does not change the qualitative picture of our previous study. The generalization only affects quantitatively the early stage of crystallization when nucleation is dominant. The remarkable result of these major generalizations is that the nonequilibrium GSD is robust against anisotropic growth of grains and fluctuations of nuclei sizes.

Understanding the physical origin of threshold switching and resistance drift phenomena is necessary for making a breakthrough in the performance of low-cost nanoscale technologies related to nonvolatile phase-change memories. Even though both phenomena of threshold switching and resistance drift are often attributed to localized states in the band gap, the distribution of defect states in amorphous phase-change materials (PCMs) has not received so far, the level of attention that it merits. This work presents an experimental study of defects in amorphous PCMs using modulated photocurrent experiments and photothermal deflection spectroscopy. This study of electrically switching alloys involving germanium (Ge), antimony (Sb) and tellurium (Te) such as amorphous germanium telluride (a-GeTe), a-Ge15Te85 and a-Ge2Sb2Te5 demonstrates that those compositions showing a high electrical threshold field also show a high defect density. This result supports a mechanism of recombination and field-induced generation driving threshold switching in amorphous chalcogenides. Furthermore, this work provides strong experimental evidence for complex trap kinetics during resistance drift. This work reports annihilation of deep states and an increase in shallow defect density accompanied by band gap widening in aged a-GeTe thin films.

The irradiation damage behaviors of single crystal (SC), coarse-grained (CG), and nanograined (NG) copper (Cu) films were investigated under Helium (He) ion implantation at 450 °C with different ion fluences. In irradiated SC films, plenty of cavities are nucleated, and some of them preferentially formed on growth defects or dislocation lines. In the irradiated CG Cu, cavities formed both in grain interior and along grain boundaries; obvious void-denuded zones can be identified near grain boundaries. In contrast, irradiation-induced cavities in NG Cu were observed mainly gathering along grain boundaries with much less cavities in the grain interiors. The grains in irradiated NG Cu are significantly coarsened. The number density and average radius of cavities in NG Cu was smaller than that in irradiated SC Cu and CG Cu. These experiments indicate that grain boundaries are efficient sinks for irradiation-induced vacancies and highlight the important role of reducing grain size in suppressing radiation-induced void swelling.

Order and porosity of block copolymer membranes have been controlled by solution thermodynamics, self-assembly, and macrophase separation. We have demonstrated how the film manufacture with long-range order can be up-scaled with the use of conventional membrane production technology.

The epitaxial growth of graphene on hexagonal silicon carbide polytypes on both the silicon-terminated (0001) and carbon-terminated ($000\bar 1$) faces has shown promise in the development of large area graphene production. It is important during these growth procedures to ensure that the underlying silicon carbide substrate is well ordered before the graphene growth. Regularly, this involves the use of a hydrogen etching procedure before graphene growth to remove polishing scratches and other defects from the substrate surface. Here, we present evidence that annealing silicon carbide substrates in argon gas at atmospheric pressure suppresses the onset of graphitization up to a temperature of 1500 °C and allows for regularly stepped terraces and removes surface defects. This allows substrate preparation and subsequent graphitization (by increasing the annealing temperature) to be carried out within a single process under an inert gas atmosphere.

Gold surfaces exhibit most interesting frictional properties on the nanometer scale. They can be studied in detail by means of friction force microscopy. Atomic-scale variations of the lateral force allow investigation of microscopic mechanisms of sliding. Friction force microscopy even reveals surface reconstruction of the gold surface as a modulation of the lateral force signal. Experiments indicate that the mobility of surface atoms at room temperature and plastic deformation mechanisms give rise to neck formation between gold and microscopic asperities in sliding contact. The frictional properties of gold surfaces change dramatically at temperatures below 150 K, where the surface diffusion is greatly reduced. Insight into the lubrication properties of self-assembled monolayers is provided by molecular-scale modulations of frictional forces. Molecular-scale maps of the friction force also allow identification of the relevant surface structure in experiments on electrochemically modified gold surfaces. Variation of the electrochemical potential is a means to reversibly switch between low and high friction states on gold surfaces.

Differentiation of the energy-based power function used to represent the nanoindentation unloading response at the peak indentation load generally overestimates the contact stiffness. This is mainly because of the larger curvature associated with this function and the proximity between the contact and maximum penetration depths. Using the nanoindentation data from ceramics and metals, we have shown that these two errors can be eliminated if the derivative is multiplied by the geometric and stiffness correction factors, respectively. The stiffness correction factor is found to be a function of the elastic energy constant and is independent of the peak indentation load. The contact stiffness evaluated by the proposed method is in excellent agreement with that obtained from the power law derivative for a wide range of elastoplastic materials and peak indentation loads. The relationship between the elastic recovery ratio and elastic energy constant developed in this study further simplifies the proposed procedure.

Three-dimensional (3D) nanostructures and nanodevices have attracted tremendous interest in the past few years due to their special mechanical and physical properties. Nanodevices using 3D nanostructures as the building blocks have been demonstrated to exhibit multifunctionality and functions that conventional planar devices cannot achieve. In this article, we report and review focused ion beam techniques for direct site-specific growth of 3D nanostructures and postgrowth shape modification of freestanding nanostructures by ion beam-induced chemical vapor deposition and ion-beam-irradiation-induced plastic bending, respectively. Such techniques have shown nanometer-scale resolution and accuracy in the fabrication of metallic nanoelectrodes, 3D pickup coils, nanogaps, and multibranched structures. Characterization of the resulting nanostructures shows that focused ion beam techniques allow conducting and superconducting freestanding 3D structures to be tailored in size, geometry, and integrated with planar electronic, mechanical, and superconducting nanodevices, potentially enabling lab-on-a-chip experiments.

We show that, by changing and tuning the direction of the As flux on a rippled substrate, at temperatures higher than 530 °C and high As/In flux ratio, a selective growth of InAs dots can be obtained on GaAs. This is an undisclosed effect related to the Arsenic flux in the molecular beam epitaxial growth of InAs quantum dots (QDs) on GaAs(001). This effect cannot be explained by a shadowing effect, due to the gentle slopes of the mounds (1–3°), and reveals instead that As plays a fundamental role at these growth conditions. We have developed a kinetic model, which takes into account the coupling between cations and anions, and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of QDs.

The fabrication methods and the basic properties of the metal-oxide nanostructures referred as nanowires are presented and reviewed in this paper, with particular emphasis to the electrical and optical properties and their useful implementation for chemical and biochemical sensing. The field of chemical sensors has benefited by the wealth of highly crystalline nanostructures produced by physical and chemical methods. Large variation in bulk electrical conductivity, structural stability upon high temperature operation, high degree of crystalline ordering, large impact of point defects and surface states have unveiled the potential for the sensing field and have opened up new perspectives of application and for the realization of novel device architectures. This paper will summarize various techniques for preparation and characterization; then, the growth mechanisms and working principles will be discussed. Finally, the challenges that this field is currently facing are presented to signify the perspectives of expansion.

Metal oxide optoelectronics is an emerging field that exploits the intriguing properties of the ns orbital-derived isotropic band structure as a replacement for traditional silicon-based electronics in advanced active-matrix information displays. Although the device performance of metal oxide thin film transistors (TFTs) has been substantially improved, the device reliability against external light and gate bias stress remains a critical issue. This paper provides a literature review of light-induced gate bias stress instability in metal oxide TFTs and explain the importance of photo-bias instability in the applications of metal oxide TFTs to optoelectronic device. The rationale of threshold voltage (Vth) instability under the negative bias illumination stress (NBIS) condition is discussed in detail. The charge trapping/injection model, oxygen vacancy photoionization model, and ambient interaction model are described as plausible degradation mechanisms. Finally, the possible approaches to prevent NBIS-induced Vth instability are proposed based on an understanding of the NBIS instability.