High-pressure studies of size dependent yield strength in rhenium diboride nanocrystals

The superhard ReB2 system is the hardest pure phase diboride synthesized to date. Previously, we have demonstrated the synthesis of nano-ReB2 and the use of this nanostructured material for texture analysis using high-pressure radial diffraction. Here, we investigate the size dependence of hardness in the nano-ReB2 system using nanocrystalline ReB2 with a range of grain sizes (20-60 nm). Using high-pressure X-ray diffraction, we characterize the mechanical properties of these materials, including bulk modulus, lattice strain, yield strength, and texture. In agreement with the Hall-Petch effect, the yield strength increases with decreasing size, with the 20 nm ReB2 exhibiting a significantly higher yield strength than any of the larger grained materials or bulk ReB2. Texture analysis on the high pressure diffraction data shows a maximum along the [0001] direction, which indicates that plastic deformation is primarily controlled by the basal slip system. At the highest pressure (55 GPa), the 20 nm ReB2 shows suppression of other slip systems observed in larger ReB2 samples, in agreement with its high yield strength. This behavior, likely arises from an increased grain boundary concentration in the smaller nanoparticles. Overall, these results highlight that even superhard materials can be made more mechanically robust using nanoscale grain size effects.

Antithermal Quenching Upconversion Luminescence via Suppressed Multiphonon Relaxation in Positive/Negative Thermal Expansion Core/Shell NaYF4:Yb/Ho@ScF3 Nanoparticles

Thermal quenching (TQ) has been naturally entangling with luminescence since its discovery, and lattice vibration, which is characterized as multiphonon relaxation (MPR), plays a critical role. Considering that MPR may be suppressed under exterior pressure, we have designed a core/shell upconversion luminescence (UCL) system of α-NaYF4:Yb/Ln@ScF3 (Ln = Ho, Er, and Tm) with positive/negative thermal expansion behavior so that positive thermal expansion of the core will be restrained by negative thermal expansion of the shell when heated. This imposed pressure on the crystal lattice of the core suppresses MPR, reduces the amount of energy depleted by TQ, and eventually saves more energy for luminescing, so that anti-TQ or even thermally enhanced UCL is obtained.

3D microscopy at the nanoscale reveals unexpected lattice rotations in deformed nickel

In polycrystalline metals, plastic deformation is accompanied by lattice rotations resulting from dislocation glide. Following these rotations in three dimensions requires nondestructive methods that so far have been limited to grain sizes at the micrometer scale. We tracked the rotations of individual grains in nanograined nickel by using three-dimensional orientation mapping in a transmission electron microscope before and after in situ nanomechanical testing. Many of the larger-size grains underwent unexpected lattice rotations, which we attributed to a reversal of rotation during unloading. This inherent reversible rotation originated from a back stress-driven dislocation slip process that was more active for larger grains. These results provide insights into the fundamental deformation mechanisms of nanograined metals and will help to guide strategies for material design and engineering applications.

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Inhibiting creep in nanograined alloys with stable grain boundary networks

Creep, the time-dependent deformation of materials stressed below the yield strength, is responsible for a great number of component failures at high temperatures. Because grain boundaries (GBs) in materials usually facilitate diffusional processes in creep, eliminating GBs is a primary approach to resisting high-temperature creep in metals, such as in single-crystal superalloy turbo blades. We report a different strategy to inhibiting creep by use of stable GB networks. Plastic deformation triggered structural relaxation of high-density GBs in nanograined single-phased nickel-cobalt-chromium alloys, forming networks of stable GBs interlocked with abundant twin boundaries. The stable GB networks effectively inhibit diffusional creep processes at high temperatures. We obtained an unprecedented creep resistance, with creep rates of ~10 –7 per second under gigapascal stress at 700°C (~61% melting point), outperforming that of conventional superalloys.

Ultra-incompressible High-Entropy Diborides

Transition metal borides are commonly hard and incompressible, offering great opportunities for advanced applications under extreme conditions. Recent studies show that the hardness of high-entropy borides may exceed that of their constituent simple borides due to the "cocktail effect". However, how high-entropy borides deform elastically remains largely unknown. Here, we show that two newly synthesized high-entropy diborides are ultra-incompressible, attaining ∼90% of the incompressibility of single-crystalline diamond and exhibiting a 50-60% enhancement over the density functional theory predictions. This unusual behavior is attributed to a Hall-Petch-like effect resulting from nanosizing under high pressure, which increases the bulk moduli through dynamic dislocation interactions and creation of stacking faults. The exceptionally low compressibility, together with their high phase stabilities, high hardness, and high electric conductance, renders them promising candidates for electromechanics and microelectronic devices that demand strong resistance to environmental impacts, in addition to traditional grinding and abrading.

Controllably Introducing Exposed Surfaces to Nanocrystalline CeO₂ Catalysts by High-Pressure Treatment

Controllably introducing highly active exposed surfaces into catalysts is a promising way to improve their properties. In addition to the widely used bottom-up method by limited crystal growth and topdown method by etching, in this study, a high-pressure treatment method is used to introduce fully crystalline clean, highly active exposed planes on submicrometer- or tens of nanometer-sized brittle catalysts. This treatment is based on a mechanism at the submicrometer or tens of nanometer scale, in which the catalysis materials are still brittle (they become ductile only when reaching the size of a couple of nanometers by the strong size effect) but do not crack randomly under high pressure like macrosized materials do. In fact, the catalyst displays a predominant cracking orientation, which is likely a highly active exposed plane, in the predominant dislocation orientation under high pressure. In this work, we used a CeO₂ catalyst as a model system to show the mechanism that leads to an obvious photocatalytic property enhancement. Currently, since most catalysts have already been prepared at the submicrometer or tens of nanometer level, we believe that our findings provide a potential route to further improve their properties through a high-pressure treatment.

Exploring nanomechanics with high-pressure techniques

For around three decades, high-pressure techniques have been used to study nanomaterials. In most studies, especially the early ones, x-ray diffraction and Raman and infrared spectroscopy were used to investigate the structural transition and equation of state. In recent years, the exploration has been extended to the plastic deformation of nanomaterials by using radial diamond-anvil-cell x-ray diffraction and transmission electron microscopy. Compared with the traditional techniques, high-pressure techniques are more advantageous in applying mechanical loads to nanosized samples and characterizing the structural and mechanical properties either in situ or ex situ, which could help to unveil the mysteries of mechanics at the nanoscale. With such knowledge, more-advanced materials could be fabricated for wider and specialized applications. This paper provides a brief review of recent progress.

Atomically Precise Nanocrystals

Nanocrystals are a state-of-matter in the border area between molecules and bulk materials. Unlike bulk materials, nanocrystals have size-dependent properties, yet the question remains whether nanocrystal properties can be analyzed, understood, and controlled with atomic precision, a key characteristic of molecules. Acknowledging the inclination of nanocrystals to form defect structures, we first outline the prospects of atomically precise analysis. A broad spectrum of analytical methods has become available over the last five years, such that for heterogeneous nanocrystal ensembles, a single, atomically precise representative structure can be determined to explore structure-property relations. Atomically precise synthesis, on the other hand, remains an outstanding challenge that may well face fundamental limitations. However, to amplify properties and prepare nanocrystals for specific applications, full atomic precision may not be needed. Examples of an atomic precision light approach, focusing on exact thickness or facet control, exist and can inspire scientists to explore atomic precision in nanocrystal research further.

Nanoscale Mapping of Heterogeneous Strain and Defects in Individual Magnetic Nanocrystals

We map the three-dimensional strain heterogeneity within a single core-shell Ni nanoparticle using Bragg coherent diffractive imaging. We report the direct observation of both uniform displacements and strain within the crystalline core Ni region. We identify non-uniform displacements and dislocation morphologies across the core–shell interface, and within the outer shell at the nanoscale. By tracking individual dislocation lines in the outer shell region, and comparing the relative orientation between the Burgers vector and dislocation lines, we identify full and partial dislocations. The full dislocations are consistent with elasticity theory in the vicinity of a dislocation while the partial dislocations deviate from this theory. We utilize atomistic computations and Landau–Lifshitz–Gilbert simulation and density functional theory to confirm the equilibrium shape of the particle and the nature of the (111) displacement field obtained from Bragg coherent diffraction imaging (BCDI) experiments. This displacement field distribution within the core-region of the Ni nanoparticle provides a uniform distribution of magnetization in the core region. We observe that the absence of dislocations within the core-regions correlates with a uniform distribution of magnetization projections. Our findings suggest that the imaging of defects using BCDI could be of significant importance for giant magnetoresistance devices, like hard disk-drive read heads, where the presence of dislocations can affect magnetic domain wall pinning and coercivity.

Origin of Plasticity in Nanostructured Silicon

The mechanism of plasticity in nanostructured Si has been intensively studied over the past decade but still remains elusive. Here, we used in situ high-pressure radial x-ray diffraction to simultaneously monitor the deformation and structural evolution of a large number of randomly oriented Si nanoparticles (SiNPs). In contrast to the high-pressure β-Sn phase dominated plasticity observed in large SiNPs (∼100  nm), small SiNPs (∼9  nm) display a high-pressure simple hexagonal phase dominated plasticity. Meanwhile, dislocation activity exists in all of the phases, but significantly weakens as the particle size decreases and only leads to subtle plasticity in the initial diamond cubic phase. Furthermore, texture simulations identify major active slip systems in all of the phases. These findings elucidate the origin of plasticity in nanostructured Si under stress and provide key guidance for the application of nanostructured Si.

Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure

As circuitry approaches single nanometer length scales, it has become important to predict the stability of single nanometer-sized metals. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity.

Synthesis and High-Pressure Mechanical Properties of Superhard Rhenium/Tungsten Diboride Nanocrystals

Rhenium diboride is an established superhard compound that can scratch diamond and can be readily synthesized under ambient pressure. Here, we demonstrate two synergistic ways to further enhance the already high yield strength of ReB₂. The first approach builds on previous reports where tungsten is doped into ReB₂ at concentrations up to 48 at. %, forming a rhenium/tungsten diboride solid solution (Re_0.52W_0.48B₂). In the second approach, the composition of both materials is maintained, but the particle size is reduced to the nanoscale (40-150 nm). Bulk samples were synthesized by arc melting above 2500 °C, and salt flux growth at ∼850 °C was used to create nanoscale materials. In situ radial X-ray diffraction was then performed under high pressures up to ∼60 GPa in a diamond anvil cell to study mechanical properties including bulk modulus, lattice strain, and strength anisotropy. The differential stress for both Re_0.52W_0.48B₂ and nano ReB₂ (n-ReB₂) was increased compared to bulk ReB₂. In addition, the lattice-preferred orientation of n-ReB₂ was experimentally measured. Under non-hydrostatic compression, n-ReB₂ exhibits texture characterized by a maximum along the [001] direction, confirming that plastic deformation is primarily controlled by the basal slip system. At higher pressures, a range of other slip systems become active. Finally, both size and solid-solution effects were combined in nanoscale Re_0.52W_0.48B₂. This material showed the highest differential stress and bulk modulus, combined with suppression of the new slip planes that opened at high pressure in n-ReB₂.

Comparative Study on Microstructural Stability of Pre-annealed Electrodeposited Nanocrystalline Nickel During Pack Rolling

Microstructural stability is an important issue for nanocrystalline materials to be practically used in many fields. The present work shows how microstructure evolves with rolling strain in pre-annealed electrodeposited nanocrystalline nickel containing an initial strong fiber texture, on the basis of X-ray diffraction line profile analysis as well as transmission electron microscopy observation. The influence of shear strain on microstructural stability of the metal/roll contact interface is compared with that of the metal/metal contact interface; the latter would be closer to deformation in plane strain compression. From the statistical microstructural information, together with experimentally observed microstructure of deformed grains after the final rolling pass, it seems fair to conclude that the microstructure of the metal/metal contact interface is more stable during pack rolling than that of the metal/roll interface.

The effect of core-shell engineering on the energy product of magnetic nanometals

Solution-based growth of magnetic FePt-FeCo (core-shell) nanoparticles with a controllable shell thickness has been demonstrated. The transition from spin canting to exchange coupling of FePt-FeCo core-shell nanostructures leads to a 28% increase in the coercivity (12.8 KOe) and a two-fold enhancement in the energy product (9.11 MGOe).

High pressure phase transformations revisited

High pressure phase transformations play an important role in the search for new materials and material synthesis, as well as in geophysics. However, they are poorly characterized, and phase transformation pressure and pressure hysteresis vary drastically in experiments of different researchers, with different pressure transmitting media, and with different material suppliers. Here we review the current state, challenges in studying phase transformations under high pressure, and the possible ways in overcoming the challenges. This field is critically compared with fields of phase transformations under normal pressure in steels and shape memory alloys, as well as plastic deformation of materials. The main reason for the above mentioned discrepancy is the lack of understanding that there is a fundamental difference between pressure-induced transformations under hydrostatic conditions, stress-induced transformations under nonhydrostatic conditions below yield, and strain-induced transformations during plastic flow. Each of these types of transformations has different mechanisms and requires a completely different thermodynamic and kinetic description and experimental characterization. In comparison with other fields the following challenges are indicated for high pressure phase transformation: (a) initial and evolving microstructure is not included in characterization of transformations; (b) continuum theory is poorly developed; (c) heterogeneous stress and strain fields in experiments are not determined, which leads to confusing material transformational properties with a system behavior. Some ways to advance the field of high pressure phase transformations are suggested. The key points are: (a) to take into account plastic deformations and microstructure evolution during transformations; (b) to formulate phase transformation criteria and kinetic equations in terms of stress and plastic strain tensors (instead of pressure alone); (c) to develop multiscale continuum theories, and (d) to couple experimental, theoretical, and computational studies of the behavior of a tested sample to extract information about fields of stress and strain tensors and concentration of high pressure phase, transformation criteria and kinetics. The ideal characterization should contain complete information which is required for simulation of the same experiments.

New twinning route in face-centered cubic nanocrystalline metals

Twin nucleation in a face-centered cubic crystal is believed to be accomplished through the formation of twinning partial dislocations on consecutive atomic planes. Twinning should thus be highly unfavorable in face-centered cubic metals with high twin-fault energy barriers, such as Al, Ni, and Pt, but instead is often observed. Here, we report an in situ atomic-scale observation of twin nucleation in nanocrystalline Pt. Unlike the classical twinning route, deformation twinning initiated through the formation of two stacking faults separated by a single atomic layer, and proceeded with the emission of a partial dislocation in between these two stacking faults. Through this route, a three-layer twin was nucleated without a mandatory layer-by-layer twinning process. This route is facilitated by grain boundaries, abundant in nanocrystalline metals, that promote the nucleation of separated but closely spaced partial dislocations, thus enabling an effective bypassing of the high twin-fault energy barrier.

Metal nanoparticle induced hormetic activation: a novel mechanism of homeopathic medicines

BACKGROUND

High-potency homeopathic remedies, 30c and 200c have enormous dilution factors of 10⁶⁰ and 10⁴⁰⁰ respectively. Therefore, the presence of physical entities in them is inconceivable. As a result, their efficacy is highly debated and often dismissed as a placebo. Despite several hypotheses postulated to explain the claimed homeopathic efficacy, none have satisfactorily answered the qualms of the sceptics. Against all beliefs and principles of conventional dilution, we have shown that nanoparticles (NPs) of the starting metals are unequivocally found in the 30c and 200c remedies at concentrations of a few pg/ml. In this paper, our aim was to answer the important question of whether such negligible metal concentrations elicit a biological response.

METHODS

Metal-based homeopathic medicines (30c and 200c) were analysed at doses between 0.003%v/v and 10%v/v in in-vitro HepG2 cell-line. Upon treatment, cell response was estimated by MTT assay, FACS and total intracellular protein. Experiments were performed to discern whether the hormesis was a cell-activation or a proliferation effect.

RESULTS

Remedies at doses containing a few femtograms/ml levels of the starting metals induced a proliferation-independent hormetic activation by increasing the intracellular protein synthesis. The metal concentrations (at fg/ml) were a billion-fold lower than the studies with synthetic NPs (at μg/ml). Further, we also highlight a few plausible mechanisms initiating a hormetic response at a billion-fold lower dose.

CONCLUSIONS

Hormetic activation has been shown for the first time with standard homeopathic high-potency remedies. These findings should have a profound effect in understanding these extreme dilutions from a biological perspective.

Reversal in the Size Dependence of Grain Rotation

The conventional belief, based on the Read-Shockley model for the grain rotation mechanism, has been that smaller grains rotate more under stress due to the motion of grain boundary dislocations. However, in our high-pressure synchrotron Laue x-ray microdiffraction experiments, 70 nm nickel particles are found to rotate more than any other grain size. We infer that the reversal in the size dependence of the grain rotation arises from the crossover between the grain boundary dislocation-mediated and grain interior dislocation-mediated deformation mechanisms. The dislocation activities in the grain interiors are evidenced by the deformation texture of nickel nanocrystals. This new finding reshapes our view on the mechanism of grain rotation and helps us to better understand the plastic deformation of nanomaterials, particularly of the competing effects of grain boundary and grain interior dislocations.

Assembly and Electronic Applications of Colloidal Nanomaterials

Artificial solids and thin films assembled from colloidal nanomaterials give rise to versatile properties that can be exploited in a range of technologies. In particular, solution-based processes allow for the large-scale and low-cost production of nanoelectronics on rigid or mechanically flexible substrates. To achieve this goal, several processing steps require careful consideration, including nanomaterial synthesis or exfoliation, purification, separation, assembly, hybrid integration, and device testing. Using a ubiquitous electronic device - the field-effect transistor - as a platform, colloidal nanomaterials in three electronic material categories are reviewed systematically: semiconductors, conductors, and dielectrics. The resulting comparative analysis reveals promising opportunities and remaining challenges for colloidal nanomaterials in electronic applications, thereby providing a roadmap for future research and development.

High-energy X-ray focusing and applications to pair distribution function investigation of Pt and Au nanoparticles at high pressures

We report development of micro-focusing optics for high-energy x-rays by combining a sagittally bent Laue crystal monchromator with Kirkpatrick-Baez (K-B) X-ray focusing mirrors. The optical system is able to provide a clean, high-flux X-ray beam suitable for pair distribution function (PDF) measurements at high pressure using a diamond anvil cell (DAC). A focused beam of moderate size (10-15 μm) has been achieved at energies of 66 and 81 keV. PDF data for nanocrystalline platinum (n-Pt) were collected at 12.5 GPa with a single 5 s X-ray exposure, showing that the in-situ compression, decompression, and relaxation behavior of samples in the DAC can be investigated with this technique. PDFs of n-Pt and nano Au (n-Au) under quasi-hydrostatic loading to as high as 71 GPa indicate the existence of substantial reduction of grain or domain size for Pt and Au nanoparticles at pressures below 10 GPa. The coupling of sagittally bent Laue crystals with K-B mirrors provides a useful means to focus high-energy synchrotron X-rays from a bending magnet or wiggler source.

Pressure-induced stiffness of Au nanoparticles to 71 GPa under quasi-hydrostatic loading

The compressibility of nanocrystalline gold (n-Au, 20 nm) has been studied by x-ray total scattering using high-energy monochromatic x-rays in the diamond anvil cell under quasi-hydrostatic conditions up to 71 GPa. The bulk modulus, K0, of the n-Au obtained from fitting to a Vinet equation of state is ~196(3) GPa, which is about 17% higher than for the corresponding bulk materials (K0: 167 GPa). At low pressures (<7 GPa), the compression behavior of n-Au shows little difference from that of bulk Au. With increasing pressure, the compressive behavior of n-Au gradually deviates from the equation of state (EOS) of bulk gold. Analysis of the pair distribution function, peak broadening and Rietveld refinement reveals that the microstructure of n-Au is nearly a single-grain/domain at ambient conditions, but undergoes substantial pressure-induced reduction in grain size until 10 GPa. The results indicate that the nature of the internal microstructure in n-Au is associated with the observed EOS difference from bulk Au at high pressure. Full-pattern analysis confirms that significant changes in grain size, stacking faults, grain orientation and texture occur in n-Au at high pressure. We have observed direct experimental evidence of a transition in compressional mechanism for n-Au at ~20 GPa, i.e. from a deformation dominated by nucleation and motion of lattice dislocations (dislocation-mediated) to a prominent grain boundary mediated response to external pressure. The internal microstructure inside the nanoparticle (nanocrystallinity) plays a critical role for the macro-mechanical properties of nano-Au.

Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles

NaYF4:Yb(3+),Er(3+) nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.

Theory for plasticity of face-centered cubic metals

The activation of plastic deformation mechanisms determines the mechanical behavior of crystalline materials. However, the complexity of plastic deformation and the lack of a unified theory of plasticity have seriously limited the exploration of the full capacity of metals. Current efforts to design high-strength structural materials in terms of stacking fault energy have not significantly reduced the laborious trial and error works on basic deformation properties. To remedy this situation, here we put forward a comprehensive and transparent theory for plastic deformation of face-centered cubic metals. This is based on a microscopic analysis that, without ambiguity, reveals the various deformation phenomena and elucidates the physical fundaments of the currently used phenomenological correlations. We identify an easily accessible single parameter derived from the intrinsic energy barriers, which fully specifies the potential diversity of metals. Based entirely on this parameter, a simple deformation mode diagram is shown to delineate a series of convenient design criteria, which clarifies a wide area of material functionality by texture control.

Detecting grain rotation at the nanoscale

It is well-believed that below a certain particle size, grain boundary-mediated plastic deformation (e.g., grain rotation, grain boundary sliding and diffusion) substitutes for conventional dislocation nucleation and motion as the dominant deformation mechanism. However, in situ probing of grain boundary processes of ultrafine nanocrystals during plastic deformation has not been feasible, precluding the direct exploration of the nanomechanics. Here we present the in situ texturing observation of bulk-sized platinum in a nickel pressure medium of various particle sizes from 500 nm down to 3 nm. Surprisingly, the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials in a few-nanometer length scale.