Nanocrystalline Systems Research Articles

Revealing the impact of sink strength, injected interstitial and grain growth on the temperature-dependent depth distribution of voids in nanocrystalline Ni

The application of electrodeposited (ED) Ni coating on structural materials of molten salt reactors has emerged as an attractive option for improving the corrosion resistance of the base alloy. However, the radiation response of the Ni coating, especially the void swelling, is an important aspect to be considered for potential in-pile applications. The study investigates depth-dependent void swelling behavior in 20 nm grain-sized ED nanocrystalline (NC) Ni, irradiated with 1.4 MeV Ni+ ion irradiation at 350–550°C to a peak displacement damage of 18.5 dpa. The injected interstitial effect is absent at 350°C; however, it becomes prominent at 450°C and less at 550°C. This is atypical of ion irradiations in coarse-grained Ni, where the effect becomes more pronounced at lower temperatures. The occurrence of voids at all temperatures goes deeper than TRIM predictions; this effect is more significant at 350°C, surpassing any previous findings in coarse-grain systems. Depending on the irradiation temperature, various types of void distributions about grain boundaries are observed: grain boundaries with no void denudations or one-sided and double-sided denudations. Here, we demonstrate that the instability of NC grains due to the combined influence of temperature and irradiation and the high fractions of internal sinks in NC Ni are responsible for void distributions that may be unique to NC systems.

Magnetic orderings in BaTi1−xFexO3-δ (with x = 0.1)

We report here various magnetic orderings and Para magnetism in Iron (both56Fe and enriched 57Fe) doped BaTi1−xFexO3-δ (x = 0.1) pristine and Nano-crystalline systems. We tried to establish that oxygen vacancies in different charge states are the key factors for the magnetic behavior of these systems. The samples were prepared by the solid state reaction method and were formed predominantly in tetragonal phase with a trace amount of hexagonal phase. The nascent sizes of the grains were in the micron order. The grains of the pristine sample were refined by mechanical milling. The presence of oxygen vacancies in different charge states and their relative intensities in the pristine and milled samples were investigated by the Photoluminescence Spectroscopic technique in the visible range. Coexistence of ferromagnetic and antiferromagnetic orderings among Fe3+ ions mediated via oxygen vacancies in different charge states was observed through clear sextet patterns in the Mössbauer spectra which was corroborated by SQUID measurements. Whereas increase in oxygen vacancies increased the overall magnetization in these materials, the decrease in grain sizes made the material softer and eventually resulted in single domain formation.

Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core-Shell In1-xGaxP/ZnS Colloidal Quantum Dots.

Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal momentum (k) space. It has, however, been debated if k is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III-V semiconductor alloys like In1-xGaxP, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of x. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III-V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In1-xGaxP/ZnS core-shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In1-xGaxP cores. The radiative decay lifetime for In1-xGaxP/ZnS monotonically increases with higher gallium content. Transient absorption studies on In1-xGaxP/ZnS nanocrystals demonstrate signatures of direct- and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III-V/II-VI core-shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In1-xGaxP/ZnS QD emitters with desirable optoelectronic properties.

Grain-boundary segregation and grain growth in nanocrystalline substitutional solid solution alloys

A model for describing solute segregation at grain boundaries has been developed for substitutional solid solution alloys, which integrates multiple factors from atomic to microstructural scales. A concept of molar Gibbs free energy of segregation was introduced to evaluate the segregating capability of the solute elements in a closed system, through which the influences of grain boundary structure, grain size, material composition, and external conditions were described. Based on the evaluation of various energy forms related to solute segregation and grain growth processes, the nature of the thermal stabilization of nanograin structures by solute segregation was disclosed. A criterion for the destabilization of nanostructures, which is determined by the competition of the change rates between the molar Gibbs free energy of segregation and the total energy of grain boundaries with grain size, has been proposed. This study provided guideline to achieve high-temperature stability of nanograin structures of solid solution alloys even for the weakly segregating nanocrystalline systems.

Effect of constrained inter-granular regions on the inverse Hall-petch phenomena

The paper makes an effort to lay out a critical assessment of grain size and its role in limiting strength gains in stable nanocrystalline (NC) metals. In particular, this study uses copper-tantalum binary alloys, in which the predominate mechanisms of grain boundary sliding and rotation are shut down, to decipher the breakdown of classical Hall-Petch behavior and its underlining mechanisms. Through varying grain sizes and tantalum concentrations, the results show Cu–Ta's strength exceeds the traditional strength limits anticipated when strictly applying smaller grain size translates to greater strength than that from the Hall-Petch relationship for NC Cu. Within this work, we also highlight the consistent linear behavior holding up to the point of losing full crystallinity given this alloy's ability to constrain failures occurring within other non-stabilized NC systems.

Predictive Removal of Interfacial Defect-Induced Trap States between Titanium Dioxide Nanoparticles via Sub-Monolayer Zirconium Coating.

First principles modeling of anatase TiO2 surfaces and their interfacial contacts shows that defect-induced trap states within the band gap arise from intrinsic structural distortions, and these can be corrected by modification with Zr(IV) ions. Experimental testing of these predictions has been undertaken using anatase nanocrystals modified with a range of Zr precursors and characterized using structural and spectroscopic methods. Continuous-wave electron paramagnetic resonance (EPR) spectroscopy revealed that under illumination, nanoparticle-nanoparticle interfacial hole trap states dominate, which are significantly reduced after optimizing the Zr doping. Fabrication of nanoporous films of these materials and charge injection using electrochemical methods shows that Zr doping also leads to improved electron conductivity and mobility in these nanocrystalline systems. The simple methodology described here to reduce the concentration of interfacial defects may have wider application to improving the efficiency of systems incorporating metal oxide powders and films including photocatalysts, photovoltaics, fuel cells, and related energy applications.

Magnetic relaxation in nanocrystalline systems: linking Monte Carlo steps with time

Abstract The magnetic relaxation of a noninteracting two-dimensional ensemble of magnetic nanoparticles is simulated as a function of temperature using a Monte Carlo technique. By properly fitting the decay of magnetization using real parameters it is possible to make, at any finite temperature, a clear correspondence between Monte Carlo steps and time measured in seconds. The results allow one to visualize the intrinsic problems related to the simulation of nonequilibrium systems, and to understand the limits and range of validity of a particular system.

Характеристики электрических свойств нанокристаллических систем селенидов цинка и кадмия

The physical properties of nanocrystalline systems of cadmium and zinc selenides have been studied experimentally and theoretically. The effect of temperature and various gaseous media on the specific conductivity of films of cadmium and zinc selenides has been experimentally studied. The values of the Fermi energy, the concentration of donor charge carriers in the conduction band, the activation energy of donors, and the temperature coefficient of resistance are theoretically calculated. It is shown that the calculated values correlate with the experimental ones, which will make it possible to predict the stability of the operation of semiconductor devices.

Pyroelectricity of Lead Sulfide (PbS) Quantum Dot Films Induced by Janus-Ligand Shells.

Asymmetry is an essential property to control. To do that in nanocrystalline systems we have developed methods to produce Janus-ligand shells on otherwise symmetric PbS quantum dots (QDs). Here, we demonstrate that control by constructing a system that exhibits pyroelectricity built from spherical PbS QDs. We observed a pyroelectric current in two different configurations. In one configuration, the QDs are self-assembled into close-packed arrays while in the second configuration, the QDs are dispersed into an electro-inactive polymer, polydimethylsiloxane. Both exhibit a pyroelectric response. In the first configuration we estimate a lower limit of the pyroelectric coefficient to be 1.97 × 10-7 C/m2·K, which is likely limited by the degree of QD alignment during film formation but is already on par with common pyroelectric systems. Compared with inorganic ceramic-like and polymeric pyroelectric materials, pyroelectric films self-assembled from polar QDs are easier to prepare, responsive to light with different energies based on QD exciton energy, and the polarization of each QD could be easily tuned by constructing different Janus-ligand shells.

Polycrystalline Ni nanotubes under compression: a molecular dynamics study

Mechanical properties of nanomaterials, such as nanowires and nanotubes, are an important feature for the design of novel electromechanical nano-architectures. Since grain boundary structures and surface modifications can be used as a route to modify nanostructured materials, it is of interest to understand how they affect material strength and plasticity. We report large-scale atomistic simulations to determine the mechanical response of nickel nanowires and nanotubes subject to uniaxial compression. Our results suggest that the incorporation of nanocrystalline structure allows completely flexible deformation, in sharp contrast with single crystals. While crystalline structures at high compression are dominated by dislocation pinning and the multiplication of highly localized shear regions, in nanocrystalline systems the dislocation distribution is significantly more homogeneous. Therefore, for large compressions (large strains) coiling instead of bulging is the dominant deformation mode. Additionally, it is observed that nanotubes with only 70% of the nanowire mass but of the same diameter, exhibit similar mechanical behavior up to 0.3 strain. Our results are useful for the design of new flexible and light-weight metamaterials, when highly deformable struts are required.

On the reduction and effect of non-metallic impurities in mechanically alloyed nanocrystalline Ni-W alloys

Non-metallic contamination is a practically unavoidable byproduct of commercial synthesis processes; however, non-metallic impurities are often overlooked when considering material design and performance of nanostructured materials. Importantly, there is disputing evidence as to whether non-metallic contamination stabilize or destabilize nanocrystalline materials against grain growth. Furthermore, it is unclear if non-metallic contamination has a significant impact on hardness of nanocrystalline systems. In this work, two nanocrystalline Ni-28at%W alloys with different impurity concentrations were produced via mechanical alloying to directly evaluate the effect of contamination on thermal stability and hardness of nanostructured materials. The alloys were isothermally annealed at several temperatures, and the microstructures were compared by applying atom probe tomography and aberration-corrected scanning transmission electron microscopy. It was determined that impurity concentrations can be substantially reduced by using specific milling media and pre-milling reduction processes. Furthermore, the clean and contaminated alloys exhibited similar thermal stabilities against grain growth, but the clean alloy displayed a 100% improvement in hardness despite a similar grain size and distribution of second phases. Impurity phases including CrOx, Ni6W6C, and trapped Ar pores were also identified and likely contributed to the thermal stability and mechanical properties observed in this study. Overall, this work suggests that impurities my not always be detrimental to thermomechanical behavior of nanocrystalline systems.

Enhanced thermoelectric efficiency in nanocrystalline bismuth telluride nanotubes

We report on the thermal and thermoelectric properties of individual nanocrystalline Bi2 Te3 nanotubes synthesized by the solution phase method using 3ω method and a microfabricated testbench. Measurements show that the nanotubes offer improved ZT compared to bulk Bi2Te3 near room temperature due to an enhanced Seebeck coefficient and suppressed thermal conductivity. This improvement in ZT originates from the nanocrystalline nature and low dimensionality of the nanotubes. Domain boundary filtering of low-energy electrons provides an enhanced Seebeck coefficient. The scattering of phonons at the surface of the nanotube leads to suppressed thermal conductivity. These have been theoretically analyzed using the Boltzmann equation based on the relaxation time approximation and Landauer approach. This work clearly demonstrates the possibility of achieving enhancement in thermoelectric efficiency by combining nanocrystalline and low-dimensional systems.

Microstructural heterogeneity in the electrodeposited Ni: insights from growth modes

Microstructures of electrodeposited Ni were studied from the perspective of growth modes during electrodeposition. The electrodeposited Ni had a heterogeneous microstructure composed of nanocrystalline- and microcrystalline-grains. Electron backscatter diffraction analyses showed that nanocrystalline- and microcrystalline-grains were preferentially oriented to specific planes. Secondary ion mass spectrometry also revealed that coarse-grained regions had higher S content than that of finer-grained regions. Hence, microstructural heterogeneity in electrodeposited Ni is reflected by the overlap of inhibited and free growth modes. Our discussion surrounding microstructural heterogeneity also provides insight into other electrodeposited nanocrystalline systems.

Heat transport in disordered network forming materials: Size effects and existence of propagative modes

We show that in nanoscale disordered networks the thermal conductivity reduces with respect to the bulk value in a way remindful of the analogous behavior in nanocrystalline systems. Our rationale is based on the analogy with nanostructured crystalline silicon and is substantiated by results obtained (experimentally, analytically and by atomic scale modelling) on crystalline Si, amorphous Si, glassy SiO2 and glassy GeTe4. We point out the implication of such findings on the potential performances of nanoscale heat devices.

Orientational competition in quantum dot growth in Si-Ge heteroepitaxy on pit-patterned Si(001) substrates.

Towards precisely controlled nanostructure growth, patterned substrates are used as templates to direct heteroepitaxial self-assembly. This affects the size, shape and ordering of nanostructures, which are formed as a consequence of the mismatch in strain. In the well-studied case of Si-Ge heteroepitaxy on Si, the lattice mismatch leads to spontaneous formation of quantum dots. On patterned substrates, the competition between the length scale of the pattern and the intrinsic quantum dot size leads to rich behavior, where the localization of dots can be modified with respect to the features of the patterns. We show by continuum modeling that, in cubic elastic materials such as silicon and germanium, there is also a competition between the pattern orientation and the elastically soft directions of the film, which affects the precise location of quantum dots on the surface. When the pattern is between the elastically soft directions, the quantum dots can form purely in the narrow region directly between two neighboring pits, referred to as the saddle region. On the other hand, when the pattern is along the elastically soft directions, the quantum dots prefer to form in the region at the centre of four pits, referred to as the crown region. This resolves a discrepancy between theory and experiments and gives another dimension to control quantum dot formation in strained nanocrystalline systems.

Elimination of Extreme Boundary Scattering via Polymer Thermal Bridging in Silica Nanoparticle Packings: Implications for Thermal Management

Recent advances in our understanding of thermal transport in nanocrystalline systems are responsible for the integration of many technologies into advanced energy systems, including thermoelectric refrigeration systems and renewable energy platforms. However, there is little understanding of heat energy transport mechanisms that govern the thermal properties of disordered nanocomposites. In this work, we explore thermal transport mechanisms in disordered packings of amorphous nanoparticles with and without a polymer filling the interstices in order to quantify the impact of thermal boundary scattering introduced at nanoparticle edges in an already amorphous system and within the context of a minimum thermal conductivity approximation. By fitting a modified minimum thermal conductivity model to temperature-dependent measurements of thermal conductivity from 80 to 300 K, we find that the interstitial polymer eliminates boundary scattering in the disordered nanoparticle packing, which surprisingly leads to a...

Phase transformations in the W–Cr system at the nanoscale

In order to enhance the thermal stability of nanomaterials, it is important to understand phase transformations at the nanoscale in high detail. In this study, the phase transformations occurring in the nanocrystalline W–Cr system were investigated using nanometallic multilayers to shift the kinetics of nucleation and to increase the diffusivities of W and Cr. The initial metastable state of the multilayers was tailored to favor the formation of solutions rich in W. In addition, the higher diffusivities in the system made it possible to observe the nucleation of the W3Cr intermetallic at 1000 °C, a temperature lower than that reported in previous studies. In this manuscript, the use of nanometallic multilayers is proposed as an approach to modify the thermal evolution of a nanocrystalline systems and study phase transformations at the nanoscale.

Structure and atomic correlations in molecular systems probed by XAS reverse Monte Carlo refinement

The Reverse Monte Carlo (RMC) algorithm for structure refinement has been applied to x-ray absorption spectroscopy (XAS) multiple-edge data sets for six gas phase molecular systems (SnI2, CdI2, BBr3, GaI3, GeBr4, GeI4). Sets of thousands of molecular replicas were involved in the refinement process, driven by the XAS data and constrained by available electron diffraction results. The equilibrated configurations were analysed to determine the average tridimensional structure and obtain reliable bond and bond-angle distributions. Detectable deviations from Gaussian models were found in some cases. This work shows that a RMC refinement of XAS data is able to provide geometrical models for molecular structures compatible with present experimental evidence. The validation of this approach on simple molecular systems is particularly important in view of its possible simple extension to more complex and extended systems including metal-organic complexes, biomolecules, or nanocrystalline systems.

Modelling thermodynamics of nanocrystalline binary interstitial alloys

Grain boundary (GB) segregation in nanocrystalline alloys can cause reduction of GB energy, which leads to thermodynamic stabilization of nanostructures. This effect has been modelled intensively. However, the previous modelling works were limited to substitutional alloy systems. In this work, thermodynamics of nanocrystalline binary interstitial alloy systems was modelled based on a two-sublattice model proposed by Hillert [M. Hillert, et al. Acta Chem. Scand., 24 (1970) 3618] and an atomic configuration for nanocrystalline systems proposed by Trelewicz and Schuh [J.R. Trelewicz, et al. Physical Review B, 79 (2009) 094112]. The modelling calculations agree with the reported experimental data, indicating that the current thermodynamic model is capable of accounting for the alloying effect in the nanocrystalline binary interstitial alloys.

Exploring the microstructural evolution of Hf-Ti: From nanometallic multilayers to nanostructures

Microstructural changes in nanocrystalline materials are driven by their high grain boundary density and greater mobility at elevated temperatures. For instance, segregation, precipitation, and grain growth become active as the temperature increases. The potential of nanometallic multilayers as a route to synthesize nanostructures is explored examining the microstructural evolution of Hf-Ti nanometallic multilayers through differential scanning calorimetry and characterization at different temperatures. Thermodynamic and kinetic calculations were performed to understand the observed transitions. This study presents a guide to microstructural changes in nanometallic multilayers which can be used to develop new nanocrystalline systems.