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Rapid crystallization behavior of amorphous Tex Se 1 − x thin films limits the use of these alloys as coatings and in optoelectronic devices. Understanding the short- and medium-range ordering of the amorphous structure and the fundamental physics governing the crystallization of the films is crucial. Although the lack of long range crystalline order restricts the characterization of the amorphous films, electron microscopy offers a way to extract information about the nanoscale ordering. In this paper, the local ordering of amorphous Tex Se 1 − x thin films with x = 0.22 , 0.61 , 0.70 , 0.90 , and 1 grown by thermal evaporation is investigated using radial distribution function (RDF) and fluctuation electron microscopy (FEM) analysis. RDF results show that the nearest-neighbor distances of selenium (Se) and tellurium (Te) in their crystalline structure are preserved, and their bond lengths increase with the addition of Te. Density functional theory (DFT) calculations predict structures with interatomic distances similar to those measured experimentally. Additionally, fluctuations in atomic coordination are analyzed. Medium range order (MRO) analysis obtained from FEM and DFT calculations suggests that there are at least two populations within the chain network structure, which are close to the Se–Se and Te–Te intrachain distances. For the binary alloy with x > 0.61, Tex Se 1 − x , Te–Te like populations increase and Te fragments might form, suggesting that the glass forming ability decreases rapidly. [ABSTRACT FROM AUTHOR]

Classical nucleation theory is used to consider the solidification of a melt confined between two planar surfaces. The critical nucleus shapes and the associated nucleation energy barriers are computed as a function of the thickness of the film and the film's relevant bulk and interface energies. The analysis is then repeated for the melting transition, and expressions for the depression and elevation of the melting temperature, relative to the thermodynamic bulk melting temperature of the film material, are found. A nucleus morphology diagram is constructed. This diagram presents the lowest energy morphology of the nuclei, as well as melting points, as a function of the system parameters. Using the nucleus morphology diagram, experimental and system parameters that allow for the desired nucleation behavior can be identified. Furthermore, the nucleus morphology diagram illustrates a region of parameter space where the film is predicted to solidify above its thermodynamic bulk melting temperature, a behavior termed presolidification. The theory is used to predict the temperature at which the nucleation of the solid phase and liquid phase is expected for Ge between two glass substrates. Furthermore, a possible route for controlling the orientation of the film is identified. By controlling the growth temperature, certain orientations may not be able to nucleate, thereby reducing the possible number of orientations within a film. [ABSTRACT FROM AUTHOR]

A simple mathematical argument explains a recently identified route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained. [ABSTRACT FROM AUTHOR]

Pulsed-laser melting (PLM) is commonly used to achieve a fast quench rate in both thin films and nanoparticles. A model for the size evolution during PLM of nanoparticles confined in a transparent matrix, such as those created by ion-beam synthesis, is presented. A self-consistent mean-field rate equations approach that has been used successfully to model ion beam synthesis of germanium nanoparticles in silica is extended to include the PLM process. The PLM model includes classical optical absorption, multiscale heat transport by both analytical and finite difference methods, and melting kinetics for confined nanoparticles. The treatment of nucleation and coarsening behavior developed for the ion beam synthesis model is modified to allow for a nonuniform temperature gradient and for interacting liquid and solid particles with different properties. The model allows prediction of the particle size distribution after PLM under various laser fluences, starting from any particle size distribution including as-implanted or annealed simulated samples. A route for narrowing the size distribution of embedded nanoparticles is suggested, with simulated distribution widths as low as 15% of the average size. [ABSTRACT FROM AUTHOR]

Isotopically pure 74Ge nanocrystals were formed in a sapphire matrix by the ion beam synthesis method. In contrast to those embedded in amorphous silica, sapphire-embedded nanocrystals are clearly faceted and are preferentially oriented with respect to the crystalline matrix. In situ transmission electron microscopy of heated samples reveals that the nanocrystals melt at 955±15 °C, very near to the bulk Ge melting point. The Raman spectra indicate that the sapphire-embedded Ge nanocrystals are under compressive stress in the range of 3–4 GPa. The magnitude of the stress is consistent with that expected for hydrostatic pressure arising from solidification. Stress relaxation was not observed for sapphire-embedded Ge nanocrystals; this is attributed to the slow self-diffusion rate of the alumina matrix atoms at temperatures below the nanocrystal melting point. [ABSTRACT FROM AUTHOR]

Freestanding Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5 nm are grown in amorphous SiO2 by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain freestanding nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Freestanding nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For freestanding as opposed to embedded Ge nanocrystals, an additional amorphouslike contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms. [ABSTRACT FROM AUTHOR]

Due to their unique boundary conditions, nanowire heterostructures may exhibit defect-free interfaces even for systems with large lattice mismatch. Heteroepitaxial material integration is limited by lattice mismatches in planar systems, but we use a variational approach to show that nanowire heterostructures are more effective at relieving mismatch strain coherently. This is an equilibrium model based on the Matthews critical thickness in which the lattice mismatch strain is shared by the nanowire overlayer and underlayer, and could as well be partially accomodated by the introduction of a pair of misfit dislocations. The model is highly portable to other nanowire material systems and can be used to estimate critical feature sizes. We find that the critical radius of this system is roughly an order of magnitude larger than the critical thickness of the corresponding thin film/substrate material system. Finite element analysis is used to assess some aspects of the model; in particular, to show that the variational approach describes well the decay of the strain energy density away from the interface. [ABSTRACT FROM AUTHOR]

Presents a study that investigated the magnetic structure of stacking faults in nickel utilizing a fully self-consistent, layered Korringa-Kohn-Rostocker approach. Complications of alloy and overlayer systems; Discussion of the Korringa-Kohn-Rostocker approach; Calculations of the electronic structures of the close-packed bulk systems.

An important theoretical ductility criterion for group V and VI metal-based refractory alloys in body-centered cubic (bcc) lattices is the mechanical failure mode of their perfect crystals under tension along the weakest direction [100]. Pure Mo and W fail by cleavage and are deemed intrinsically brittle. However, first-principles calculations show that alloying with group IV or V transition metals can transform these materials into ones that display intrinsically ductile behavior, failing in shear under [100] tension. Remarkably, this transition can be understood as an electron filling effect with the intrinsically ductile response the manifestation of a Jahn-Teller distortion. [ABSTRACT FROM AUTHOR]

We apply state-of-art first principle calculations to study the polymorphism and electronic structure of three previously reported phases of TlBr. The calculated band structures of NaCl-structure phase and orthorhombic-structure phase have different features than that of commonly observed CsCl-structure phase. We further interpret photoluminescence spectra based on our calculations. Several peaks close to calculated band gap values of the NaCl-structure phase and the orthorhombic-structure phase are found in unpolished TlBr samples. [ABSTRACT FROM AUTHOR]

A continuum theory is formulated to describe dislocations within graphene. The bending modes and defect formation energies predicted by the theory are in good agreement with atomic scale total energy calculations. Dislocations induce out-of-plane buckling that screens their strain fields and enables the identification of a dislocation formation energy. Further, the theory provides an efficient means to model nanoidentation of suspended graphene. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations within the isoenthalpic-isotension-isobaric ensemble are employed to compute the elastic properties and relevant thermodynamic functions of freestanding monolayer graphenes in order to explore the influence of thermal ripples on the elastic properties of graphene. Thermal ripples soften the elastic constants, and lead to the negative thermal expansion coefficient observed experimentally at low temperatures. [ABSTRACT FROM AUTHOR]

Nanocrystalline materials offer very high strength but are typically limited in their strain to failure, and efforts to improve deformability in these materials are usually found to be at the expense of strength. Using a combination of quantitative in situ compression in a transmission electron microscope and finite-element analysis, we show that the mechanical properties of nanoparticles can be directly measured and interpreted on an individual basis. We find that nanocrystalline CdS synthesized into a spherical shell geometry is capable of withstanding extreme stresses (approaching the ideal shear strength of CdS). This unusual strength enables the spherical shells to exhibit considerable deformation to failure (up to 20% of the sphere’s diameter). By taking into account the structural hierarchy intrinsic to novel nanocrystalline materials such as this, we show it is possible to achieve and characterize the ultrahigh stresses and strains that exist within a single nanoparticle during deformation. [ABSTRACT FROM AUTHOR]

We develop elasticity theory to predict the energies of topological defects in carbon nanostructures. The theory is a simple, quantitatively accurate and transferable continuum approach to predicting defect formation energies that obviates the need for computationally expensive quantum mechanical methods. Thus the theory has the potential to serve as the basis for thermodynamic and multi-scale modelling of the structural properties of carbon nanostructures. [ABSTRACT FROM AUTHOR]

During the past 50 years, transmission electron microscopy (TEM) has evolved from an imaging tool to a quantitative method that approaches the ultimate goal of understanding the atomic structure of materials atom by atom in three dimensions both experimentally and theoretically. Today's TEM abilities are tested in the special case of a Ga-terminated 30° partial dislocation in GaAs:Be where it is shown that a combination of high-resolution phase contrast imaging, scanning TEM (STEM), and local electron energy loss spectroscopy (EELS) allows for a complete analysis of dislocation cores and associated stacking faults. We find that it is already possible to locate atom column positions with picometre precision in directly interpretable images of the projected crystal structure and that chemically different elements can already be identified together with their local electronic structure. In terms of theory, the experimental results can be quantitatively compared with ab initio electronic structure total energy calculations. By combining elasticity theory methods with atomic theory, an equivalent crystal volume can be addressed. Therefore, it is already feasible to merge experiments and theory on a picometre length scale. Whereas current experiments require the utilization of different, specialized instruments, it is foreseeable that the rapid improvement of electron optical elements will soon generate a next generation of microscopes with the ability to image and analyze single atoms in one instrument with deep sub-angstrom spatial resolution and an energy resolution better than 100  meV. [ABSTRACT FROM AUTHOR]

We present direct observations of room temperature dislocation plasticity in single crystalline silicon. Previous studies have shown that phase transformation and fracture are the relevant mechanisms of deformation in silicon. In contrast, using in-situ nanoindentation in a transmission electron microscope we find dislocation nucleation and metal-like flow. The results of finite-element modelling suggest that the presence of free surfaces in our unique sample geometry leads to preferential surface nucleation of dislocations and the suppression of phase transformation. [ABSTRACT FROM AUTHOR]

The cohesive energy of Fe as a function of structure, strain and magnetic state has been computed using the full potential linearized augmented-plane-wave method within the framework of density functional theory and the generalized gradient approximation. Calculations corresponding to uniaxial stress in the <100> direction reveal that the ideal tensile strength of bcc Fe is about 14.2GPa and is determined by instability with respect to transformation into an unstable ferromagnetic fcc structure. The low-energy fcc phase is a modulatedantiferromagnetic fcc structure that is connected to the bcc phase via a first-order magnetic transformation and does not compromise its ideal strength. [ABSTRACT FROM AUTHOR]

A simple statistical analysis of the structure or superdislocations in L1[sub 2] compounds displaying the yield strength anomaly is developed. This analysis is based on the superkink model of dislocation dynamics, where it is assumed that a superdislocation propagates through the lateral motion of superkinks. The analysis is used to assess the probability that a superdislocation is immobile, and this probability is found to depend exponentially on the nominal length of the superdislocation. The statistical analysis requires knowledge of the global distribution of superkink lengths to obtain quantitative relative immobilization rates; this information is extracted from published experimental work and also from previous simulations. The implications of both superkink distributions are explored. The predictions of the analysis appear consistent with the results from simulations. [ABSTRACT FROM AUTHOR]

One explanation of the Hall-Petch relationship is that dislocation pile-ups serve to enhance the stress felt at grain boundaries. This dislocation pile-up model results in a d-1/2 dependence of the yield strength, with d the grain size. In fact, this dependence is observed often. However, the traditional pile-up picture neglects two important aspects of pile-up formation: firstly the existence of a threshold stress for dislocation production, and secondly the necessity of a finite-sized dislocation-free region in which a source may operate. In this paper, both of these aspects are addressed within a continuum model of the dislocation pile-ups. A closed-form expression is obtained for the dependence of yield stress on grain size and source characteristics. The continuum model agrees closely with the corresponding discrete dislocation model even when the pile-up contains as few as ten dislocations. [ABSTRACT FROM AUTHOR]

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge-Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge-Au. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) of Er-doped silica films containing Ge nanoclusters synthesized by ion implantation was investigated. The area of the 1540 nm Er3+ PL peak was enhanced by up to a factor of 200 by the addition of Ge nanoclusters. The PL enhancement was found to be proportional to the concentration of Ge atoms. Control experiments with argon ion implantation were used to show that the enhancement is due to the presence of Ge and not radiation damage. Furthermore, the Er3+ PL was found to be strongly influenced by the postgrowth annealing and the crystallinity of the Ge nanoclusters. [ABSTRACT FROM AUTHOR]

Ion beam synthesis of nanocrystals is explored using a recently developed kinetic Monte Carlo model for the process. The model suggests that temperature can be used to engineer nanocrystal size distributions. Specifically, by initiating implants at low temperature and then ramping the temperature upward, one can both tune the average size of the nanocrystals and restrict size distribution widths to less than 20% of the average size. [ABSTRACT FROM AUTHOR]

The equilibrium structure of embedded nanocrystals formed from strongly segregating binary alloys is considered within a simple thermodynamic model. The model identifies two dimensionless interface energies that dictate the structure and allows prediction of the stable structure for any choice of these parameters. The resulting structure map includes three distinct nanocrystal morphologies: core/shell, lobe/lobe, and completely separated spheres. [ABSTRACT FROM AUTHOR]

Photo-oxidation of Ge nanocrystals illuminated with visible laser light under ambient conditions was investigated. The photo-oxidation kinetics were monitored by in situ measurement of the crystalline Ge volume fraction by Raman spectroscopy. The effects of laser power and energy on the extent of oxidation were measured using both in situ and ex situ Raman scattering techniques. A mechanistic model in which the tunneling of photoexcited carriers to the oxide surface for electron activated molecular oxygen dissociation is proposed. This quantitative model describes all experimental photo-oxidation observations using physical parameters. [ABSTRACT FROM AUTHOR]

Ion-beam-synthesized 74Ge nanocrystals embedded in an amorphous silica matrix exhibit large compressive stresses in the as-grown state. The compressive stress is determined quantitatively by evaluating the Raman line shift referenced to the line position of free-standing nanocrystals. Postgrowth thermal treatments lead to stress reduction. The stress relief process is shown to be governed by the diffusive flux of matrix atoms away from the local nanocrystal growth region. A theoretical model that quantitatively describes this process is presented. [ABSTRACT FROM AUTHOR]