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The ability to gradually modify the atomic structures of nanomaterials and directly identify such structural variation is important in nanoscience research. Here, we present the first example of a high-pressure single-crystal X-ray diffraction analysis of atomically precise metal nanoclusters. The pressure-dependent, subangstrom structural evolution of an ultrasmall gold nanoparticle, Au

Upconverting nanoparticles (UCNPs) are an emerging platform for mechanical force sensing at the nanometer scale. An outstanding challenge in realizing nanometer-scale mechano-sensitive UCNPs is maintaining a high mechanical force responsivity in conjunction with bright optical emission. This Letter reports mechano-sensing UCNPs based on the lanthanide dopants Yb(3+) and Er(3+), which exhibit a strong ratiometric change in emission spectra and bright emission under applied pressure. We synthesize and analyze the pressure response of five different types of nanoparticles, including cubic NaYF(4) host nanoparticles and alkaline-earth host materials CaLuF, SrLuF, SrYbF, and BaLuF, all with lengths of 15 nm or less. By combining optical spectroscopy in a diamond anvil cell with single-particle brightness, we determine the noise equivalent sensitivity (GPa/√Hz) of these particles. The SrYb(0.72)Er(0.28)F@SrLuF particles exhibit an optimum noise equivalent sensitivity of 0.26 ± 0.04 GPa/√Hz. These particles present the possibility of robust nanometer-scale mechano-sensing.

Optical and confocal microscopy is used to image the self-assembly of microscale colloidal particles. The density and size of self-assembled structures is typically quantified by hand, but this is extremely tedious. Here, we investigate whether machine learning can be used to improve the speed and accuracy of identification. This method is applied to confocal images of dense arrays of two-photon lithographed colloidal cones. RetinaNet, a deep learning implementation that uses a convolutional neural network, is used to identify self-assembled stacks of cones. Synthetic data is generated using Blender to supplement experimental training data for the machine learning model. This synthetic data captures key characteristics of confocal images, including slicing in the z-direction and Gaussian noise. We find that the best performance is achieved with a model trained on a mixture of synthetic data and experimental data. This model achieves a mean Average Precision (mAP) of ∼85%, and accurately measures the degree of assembly and distribution of self-assembled stack sizes for different cone diameters. Minor discrepancies between machine learning and hand labeled data is discussed in terms of the quality of synthetic data, and differences in cones of different sizes.

Natural materials exhibit emergent mechanical properties as a result of their nanoarchitected, nanocomposite structures with optimized hierarchy, anisotropy, and nanoporosity. Fabrication of such complex systems is currently challenging because high-quality three-dimensional (3D) nanoprinting is mostly limited to simple, homogeneous materials. We report a strategy for the rapid nanoprinting of complex structural nanocomposites using metal nanoclusters. These ultrasmall, quantum-confined nanoclusters function as highly sensitive two-photon activators and simultaneously serve as precursors for mechanical reinforcements and nanoscale porogens. Nanocomposites with complex 3D architectures are printed, as well as structures with tunable, hierarchical, and anisotropic nanoporosity. Nanocluster-polymer nanolattices exhibit high specific strength, energy absorption, deformability, and recoverability. This framework provides a generalizable, versatile approach for the use of photoactive nanomaterials in additive manufacturing of complex systems with emergent mechanical properties.

Gold nanoclusters with near-infrared (NIR) photoluminescence (PL) have great potential as sensing and imaging materials in biomedical and bioimaging applications. In this work, Au21(S-Adm)15 and Au38S2(S-Adm)20 are used to unravel the underlying mechanisms for the improved quantum yields (QY), large Stokes shifts, and long PL lifetimes in gold nanoclusters. Both nanoclusters show decent PL QY. In particular, the Au38S2(S-Adm)20 nanocluster shows a bright NIR PL at 900 nm with QY up to 15% in normal solvents (such as toluene) at ambient conditions. The relatively lower QY for Au21(S-Adm)15 (4%) compared to that of Au38S2(S-Adm)20 is attributed to the lowest-lying excited state being symmetry-disallowed, as evidenced by the pressure-dependent antispectral shift of the absorption spectra compared to PL, yet Au21(S-Adm)15 maintains some emissive properties due to a nearby symmetry-allowed excited state. Furthermore, our results show that suppression of nonradiative decay due to the surface "lock rings", which encircle the Au kernel and the surface "lock atoms" which bridge the fundamental Au kernel units (e.g., tetrahedra, icosahedra, etc.), is the key to obtaining high QYs in gold nanoclusters. The complicated excited-state processes and the small absorption coefficient of the band-edge transition lead to the large Stokes shifts and the long PL lifetimes that are widely observed in gold nanoclusters.

Surfaces can significantly alter the optical properties of nanomaterials, but they are difficult to control and their roles are hard to understand in highly reactive materials such as silicon nanomaterials. In this work, we investigate the role of the surface in controlling the optical transitions in highly luminescent silicon-derived nanoparticles. By combining high-pressure and low-temperature experiments, we experimentally correlate the anomalously intense and narrow transitions in the UV range with the surface oxides, while the visible transition and the photoluminescence (PL) are verified to originate from the Si-ligand charge transfer band. We find that the high-pressure absorption and PL depends on the rigidity of the surface ligand. This indicates that the surface plays a dominant role on the optical properties of these silicon-derived nanoparticles, and is different than other semiconductor nanomaterials, in which pressure-dependent optical transitions depend on lattice strain or phase transformations. This work presents a comprehensive understanding of the optical transitions and the effect of surface ligands and surface oxidation in these highly luminescent Si-derived nanoparticles. The new insight into the oxidation-activated and ligand-mediated transitions, and the pressure-dependent PL may help with engineering the band structure of other highly-reactive optical nanomaterials.

Solid-state electrolytes are an attractive option to increase the chemical stability and energy density of rechargeable lithium-metal batteries. However, they are susceptible to lithium-metal short-circuiting during charging, especially under high current densities. In this work, using a force-controlled electrical microprobe inside a focused-ion beam scanning electron microscope, we performed 48 experiments of electroplating lithium metal onto Li6.6La3 Ta0.4Zr1.6O12 (LLZO) and observed the initiation of lithium penetration (termed intrusions) into the solid electrolyte, with and without externally applied stress. Employing a statistical approach, we find that the cumulative probability of intrusion as a function of lithium metal diameter follows a two-parameter Weibull distribution, implying defect-governed fracture behavior for the intrusion initiation. Upon applying a contact force of 5 mN with a 5-10 µm microprobe, the characteristic failure diameter of lithium metal decreases by a factor of 2.6 due to the generation of nanoscale cracks as suggested by nanoindentation and finite element simulations. Furthermore, we introduce in-plane compressive strain to LLZO through a cantilever bending experiment and demonstrate that strain as small as 0.067% strongly influences the direction of intrusion propagation immediately following initiation. Overall, our results suggest that both mechanically-generated and pre-existing defects as well as mechanical strain dominate the intrusion behavior in LLZO, a phenomenon that could extend to other solid electrolytes.

Radiation damage can cause significantly more surface damage in metallic nanostructures than bulk materials. Structural changes from displacement damage compromise the performance of nanostructures in radiation environments such as nuclear reactors and outer space, or used in radiation therapy for biomedical treatments. As such, it is important to develop strategies to prevent this from occurring if nanostructures are to be incorporated into these applications. Here, in situ transmission electron microscope ion irradiation was used to investigate whether a metallic glass (MG) coating mitigates sputtering and morphological changes in metallic nanostructures. Dislocation-free Au nanocubes and Au nanocubes coated with a Ni-B MG were bombarded with 2.8 MeV Au4+ ions. The formation of internal defects in bare Au nanocubes was observed at a fluence of 7.5 × 1011 ions/cm2 (0.008 dpa), and morphological changes such as surface roughening, rounding of corners, and formation of nanofilaments began at 4 × 1012 ions/cm2 (0.04 dpa). In contrast, the Ni-B MG-coated Au nanocubes (Au@NiB) showed minimal morphological changes at a fluence of 1.9 × 1013 ions/cm2 (0.2 dpa). The MG coating maintains its amorphous nature under all irradiation conditions investigated.

Currently, a comprehensive understanding of the relationship between atomic structures and optical properties of ultrasmall metal nanoclusters with diameters between 1 and 3 nm is lacking. To address this challenge, it is necessary to develop tools for perturbing the atomic structure and modulating the optical properties of metal nanoclusters beyond what can be achieved using synthetic chemistry. Here, we present a systematic high-pressure study on a series of atomically precise ligand-protected metal nanoclusters. A diamond anvil cell is used as a high-pressure chamber to gradually compress the metal nanoclusters, while their optical properties are monitored

The design of ductile metallic glasses has been a longstanding challenge. Here, we use colloidal synthesis to fabricate nickel-boron metallic glass nanoparticles that exhibit homogeneous deformation at room temperature and moderate strain rates. In situ compression testing is used to characterize the mechanical behavior of 90-260 nm diameter nanoparticles. The force-displacement curves consist of two regimes separated by a slowly propagating shear band in small, 90 nm particles. The propensity for shear banding decreases with increasing particle size, such that large particles are more likely to deform homogeneously through gradual shape change. We relate this behavior to differences in composition and atomic bonding between particles of different size using mass spectroscopy and XPS. We propose that the ductility of the nanoparticles is related to their internal structure, which consists of atomic clusters made of a metalloid core and a metallic shell that are connected to neighboring clusters by metal-metal bonds.

Deciphering the complicated excited-state process is critical for the development of luminescent materials with controllable emissions in different applications. Here we report the emergence of a photo-induced structural distortion accompanied by an electron redistribution in a series of gold nanoclusters. Such unexpected slow process of excited-state transformation results in near-infrared dual emission with extended photoluminescent lifetime. We demonstrate that this dual emission exhibits highly sensitive and ratiometric response to solvent polarity, viscosity, temperature and pressure. Thus, a versatile luminescent nano-sensor for multiple environmental parameters is developed based on this strategy. Furthermore, we fully unravel the atomic-scale structural origin of this unexpected excited-state transformation, and demonstrate control over the transition dynamics by tailoring the bi-tetrahedral core structures of gold nanoclusters. Overall, this work provides a substantial advance in the excited-state physical chemistry of luminescent nanoclusters and a general strategy for the rational design of next-generation nano-probes, sensors and switches., Excited-state structural and electronic changes, observed in molecules, are hampered in nanomaterials. Here the authors identify structural distortion and electron redistribution in three photoexcited gold nanoclusters, connecting molecular and nanocrystal regimes, enabled by flexibility of the tetrahedral core units.

Precipitate hardening is a key strengthening mechanism in metallic alloys. Classical models for precipitate hardening are based on the average behaviour of an ensemble of precipitates, and fail to ...

Grain size and shape and the structure of grain boundaries greatly influence the mechanical, electrical, and optical properties of polycrystalline metals and alloys. In this issue of Chem, Nagaoka et al. take a bottom-up approach to grain boundary engineering by consolidating colloidal nanocrystals into millimeter-scale polycrystalline and metallic glass samples.

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co3O4 nanocube core that carries a Mn3O4 shell on each facet. The individual shells are symmetry-related interconnected grains6, and the large geometric misfit between adjacent tetragonal Mn3O4 grains results in tilt boundaries at the sharp edges of the Co3O4 nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase7. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.

The strength of single-crystalline nanoscale metals is controlled by dislocation nucleation from free surfaces. Surface properties such as crystallographic orientation, surface stress, and surface diffusion have been proposed as key parameters that control dislocation surface nucleation, but have not been confirmed experimentally. To investigate the influence of surface parameters, in situ scanning electron microscope mechanical testing is used to compress defect-free Ag and Cu nanocubes that are passivated with organic surfactants in order to tune their surface properties. Comparison between passivated nanocubes indicates that yield strength may depend on surfactant binding energy, but is also dependent on intrinsic material properties.

Fluorophores with high quantum yields, extended maximum emission wavelengths, and long photoluminescence (PL) lifetimes are still lacking for sensing and imaging applications in the second near-infrared window (NIR-II). In this work, a series of rod-shaped icosahedral nanoclusters with bright NIR-II PL, quantum yields up to ≈8%, and a peak emission wavelength of 1520 nm are reported. It is found that the bright NIR-II emission arises from a previously ignored state with near-zero oscillator strength in the ground-state geometry and the central Au atom in the nanoclusters suppresses the non-radiative transitions and enhances the overall PL efficiency. In addition, a framework is developed to analyze and relate the underlying transitions for the absorptions and the NIR-II emissions in the Au nanoclusters based on the experimentally defined absorption coefficient. Overall, this work not only shows good performance of the rod-shaped icosahedral series of Au nanoclusters as NIR-II fluorophores, but also unravels the fundamental electronic transitions and atomic-level structure-property relations for the enhancement of the NIR-II PL in gold nanoclusters. The framework developed here also provides a simple method to analyze the underlying electronic transitions in metal nanoclusters.

The interaction of crystalline defects leads to strain hardening in bulk metals. Metals with high stacking fault energy (SFE), such as aluminum, tend to have low strain hardening rates due to an inability to form stacking faults and deformation twins. Here, we use in situ SEM mechanical compressions to find that colloidally synthesized defect-free 114 nm Al nanocubes combine a high linear strain hardening rate of 4.1 GPa with a high strength of 1.1 GPa. These nanocubes have a 3 nm self-passivating oxide layer that has a large influence on mechanical behavior and the accumulation of dislocation structures. Post-compression TEM imaging reveals stable prismatic dislocation loops and the absence of stacking faults. MD simulations relate the formation of dislocation loops and strain hardening to the surface oxide. These results indicate that slight modifications to surface and interfacial properties can induce enormous changes to mechanical properties in high SFE metals., Comment: 10 pages, 7 figures

Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation., 32 pages, 14 figures, and 1 movie (please open pdf with Adobe Acrobat Reader to see the embedded movie)

Summary The long alkyl ligands on nanocrystal (NC) surfaces must be exchanged with short ligands to render NC films semiconducting for optoelectronic applications. It has become the consensus that through this process, it is unavoidable that detrimental cracks are induced due to the ligand volume loss. In this work, we demonstrate an advanced solid-state ligand exchange strategy to achieve crack-free PbS NC film in a single deposition step, through managing NC packing state and appropriate processing solvent selection. This strategy exhibits remarkable versatility for films with various thicknesses (up to 1 μm), substrates, and areas. The quality of the NC film is confirmed by solar cells as a representative application. The photovoltaic performance is on par with the reported state-of-the-art PbS NC solar cells. Our approach possesses the advantages of simplicity, “greener” processing, and lower materials costs, showing potential as a facile and reliable way to produce high-quality NC films for practical optoelectronic applications.

Porous, nano-architected metals with dimensions down to ~10 nm are predicted to have extraordinarily high strength and stiffness per weight, but have been challenging to fabricate and test experimentally. Here, we use colloidal synthesis to make ~140 nm length and ~15 nm wall thickness hollow Au-Ag nanoboxes with smooth and rough surfaces. In situ scanning electron microscope and transmission electron microscope testing of the smooth and rough nanoboxes show them to yield at 130 ± 45 MPa and 96 ± 31 MPa respectively, with significant strain hardening. A higher strain hardening rate is seen in rough nanoboxes than smooth nanoboxes. Finite element modeling is used to show that the structure of the nanoboxes is not responsible for the hardening behavior suggesting that material mechanisms are the source of observed hardening. Molecular dynamics simulations indicate that hardening is a result of interactions between dislocations and the associated increase in dislocation density., Fabricating and mechanically testing nanoarchitected materials remains a challenge. Here, the authors use colloidal synthesis to fabricate Au-Ag hollow nanoboxes and investigate the effect of either a rough or a smooth nanobox surface on the mechanical properties.

Grain boundaries have complex structural features that influence strength, ductility and fracture in metals and alloys. Grain boundary misorientation angle has been identified as a key parameter that controls their mechanical behavior, but the effect of misorientation angle has been challenging to isolate in polycrystalline materials. Here, we describe the use of bicrystal Au thin films made using a rapid melt growth process to study deformation at a single grain boundary. Tensile testing is performed on bicrystals with different misorientation angles using in situ TEM, as well as on a single crystalline sample. Plastic deformation is initiated through dislocation nucleation from free surfaces. Grain boundary sliding is not observed, and failure occurs away from the grain boundary through plastic collapse in all cases. The failure behavior in these nanoscale bicrystals does not appear to depend on the misorientation angle or grain boundary energy but instead has a more complex dependence on sample surface structure and dislocation activity.

This paper presents a high-resolution, in-situ material testing system that integrates acoustic emission (AE) testing with a nanoindentation system for crack generation and detection in thin film stack structures. This is used to find the critical contact load during wafer probing of crack-sensitive backend-of-line (BEOL) structures in semiconductor integrated circuits. Scanning electron microscopy (SEM) and load–displacement curve analysis were used to confirm the formation and propagation of cracks in the multilayer structures. In order to improve the manual classification performance and understand the physical meaning of AE signals, this paper introduces a machine learning based signal processing approach based on a k-means clustering algorithm applied on collected AE signals. To obtain the optimal number of k-means clusters, Davies–Bouldin, Dunn, and Silhouette indices were calculated, and the individual ratings were cumulated based on a voting scheme. Multiple feature extraction methods, including raw time-domain AE signals, conventional AE extracted parameters, short-term signal energy, and representation features learned by the autoencoder, were used and evaluated by manually labeled clusters and binary confusion matrices. A supervised learning technique, the k-nearest neighbors algorithm, was also utilized on different AE signal datasets using different loading rates to further investigate the damage processes during nanoindentation and the physical meaning of different AE signals. The influences of loading rates on AE signals have been investigated, and loading rate effects on the critical load were observed – higher loading rates led to higher critical loads. This integrated test system and signal processing approach provides a high-resolution mechanical testing platform for studying and enabling automatic crack detection in wafer probing.

Nanoprecipitates play a significant role in the strength, ductility, and damage tolerance of metallic alloys through their interaction with crystalline defects, especially dislocations. However, the difficulty of observing the action of individual precipitates during plastic deformation has made it challenging to conclusively determine the mechanisms of the precipitate-defect interaction for a given alloy system and presents a major bottleneck in the rational design of nanostructured alloys. Here, we demonstrate the in situ compression of core-shell nanocubes as a promising platform to determine the precise role of individual precipitates. Each nanocube with a dimension of ∼85 nm contains a single spherical precipitate of ∼25 nm diameter. The Au-core/Ag-shell nanocubes show a yield strength of 495 MPa with no strain hardening. The deformation mechanism is determined to be surface nucleation of dislocations which easily traverses through the coherent Au-Ag interface. On the other hand, the Au-core/Cu-shell nanocubes show a yield strength of 829 MPa with a pronounced strain hardening rate. Molecular dynamics and dislocation dynamics simulations, in conjunction with TEM analysis, have demonstrated the yield mechanism to be the motion of threading dislocations extending from the semicoherent Au-Cu interface to the surface, and strain hardening to be caused by a single-armed Orowan looping mechanism. Nanocube compression offers an exciting opportunity to directly compare computational models of defect dynamics with in situ deformation measurements to elucidate the precise mechanisms of precipitate hardening.

Colloidal nanocrystals that consist of an inorganic core and an organic ligand shell can be assembled into architected nanocomposites. A wide range of mechanical properties can be achieved by tuning structural parameters such as sample dimensions, ligand length, and nanocrystal packing density. This review article describes the fabrication, structural characterization and nanomechanical testing of solid assemblies of nanocrystals. Self-assembly and solution processing techniques can be used to form freestanding membranes, thin films and 3D crystals that consist of ordered arrays, or disordered aggregates of nanocrystals. Membrane deflection, nano-indentation, and thin film buckling have been used to determine elastic properties, hardness and fracture toughness. Nanocrystal solids have elastic moduli of

Nanomaterials with multiscale porosity are attractive as lightweight structural materials because unique deformation mechanisms can be programmed at different length scales. Here, we explore this concept by investigating the mechanics of hollow nanoboxes, as well as films self-assembled from nanoboxes. Hollow CoS nanoboxes with lengths of ∼ 900 nm and ∼ 15 MPa and loading modulus of ∼ 240 MPa regardless of strain rate. The nanoboxes deform through ductile, plastic bending of side walls at 0.001 s−1 strain rate, while brittle fracture occurs at the higher strain rates. Self-assembly of these nanoboxes results in films with relative density of ∼ 15 MPa and ∼ 230 kPa, respectively. The mechanical behavior of the film is compared to that of the nanobox building blocks, cellular foams and granular materials.

Nanoarchitected Cu–Si core–shell lattices were fabricated via two-photon lithography and tested as mechanically robust Li-ion battery electrodes which accommodate ∼250% Si volume expansion during lithiation. The superior mechanical performance of the nanolattice electrodes is directly observed using an in situ scanning electron microscope, which allows volume expansion and morphological changes to be imaged at multiple length scales, from single lattice beam to the architecture level, during electrochemical testing. Finite element modeling of lithiation-induced volume expansion in a core–shell structure reveals that geometry and plasticity mechanisms play a critical role in preventing damage in the nanolattice electrodes. The two-photon lithography-based fabrication method combined with computational modeling and in situ characterization capabilities would potentially enable the rational design and fast discovery of mechanically robust and kinetically agile electrode materials that independently optimize geometry, feature size, porosity, surface area, and chemical composition, as well as other functional devices in which mechanical and transport phenomena are important.

As circuitry approaches single nanometer length scales, it is important to predict the stability of metals at these scales. 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., Comment: 33 pages, 12 figures

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials

Pseudoelasticity in metals is typically associated with phase transformations (e.g., shape memory alloys) but has recently been observed in sub-10 nm Ag nanocrystals that rapidly recovered their original shape after deformation to large strains. The discovery of pseudoelasticity in nanoscale metals dramatically changes the current understanding of the properties of solids at the smallest length scales, and the motion of atoms at surfaces. Yet, it remains unclear whether pseudoelasticity exists in different metals and nanocrystal sizes. The challenge of observing deformation at atomistic to nanometer length scales has prevented a clear mechanistic understanding of nanoscale pseudoelasticity, although surface diffusion and dislocation-mediated processes have been proposed. We further the understanding of pseudoelasticity in nanoscale metals by using a diamond anvil cell to compress colloidal Au nanocrystals under quasihydrostatic and nonhydrostatic pressure conditions. Nanocrystal structural changes are measured using optical spectroscopy and transmission electron microscopy and modeled using electrodynamic theory. We find that 3.9 nm Au nanocrystals exhibit pseudoelastic shape recovery after deformation to large uniaxial strains of up to 20%, which is equivalent to an ellipsoid with an aspect ratio of 2. Nanocrystal absorbance efficiency does not recover after deformation, which indicates that crystalline defects may be trapped in the nanocrystals after deformation.

3-dimensional solid Cu octet meso-lattices with characteristic features on the micron-scale were fabricated and mechanically tested under uniaxial compression. These architected cellular materials were fabricated by a three-step process: (1) direct laser writing of the lattice pattern into a polymer template, (2) electroplating of Cu into the template, and (3) removal of the polymer matrix. The microstructure of the electroplated Cu mainly consists of polycrystalline grains with average diameters of 2μm such that cross-sections of lattice beams mostly consist of a single grain. We discovered that the compressive yield strengths of the open-cell Cu meso-lattices can exceed the yield strength of monolithic bulk Cu as measured from a Cu thin film made with identical conditions. Meso-lattices with relative density of 0.8 had a strength of 332 MPa, which surpassed the bulk yield strength by 80%. This is diametrically opposite to predictions from structural mechanics theory, which states that strength scales linearly with relative density for the octet structure. We attribute the ability of solid Cu meso-lattices to attain such high strengths to the “smaller is stronger” size effect present in single crystalline metals with sub-micron dimensions. This work demonstrates the use and proliferation of the size-dependent strengthening unique to nanostructures in an architected structural material.

Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.

We report fabrication and nanomechanical tension experiments on as-fabricated and helium-implanted 130 nm diameter Ni_(73)P_(27) metallic glass nanocylinders. The nanocylinders were fabricated by a templated electroplating process and implanted with He+ at energies of 50, 100, 150, and 200 keV to create a uniform helium concentration of 3 atom % throughout the nanocylinders. Transmission electron microscopy imaging and through-focus analysis reveal that the specimens contained 2 nm helium bubbles distributed uniformly throughout the nanocylinder volume. In situ tensile experiments indicate that helium-implanted specimens exhibit enhanced ductility as evidenced by a 2-fold increase in plastic strain over as-fabricated specimens with no sacrifice in yield and ultimate tensile strengths. This improvement in mechanical properties suggests that metallic glasses may actually exhibit a favorable response to high levels of helium implantation.

We report the synthesis, mechanical properties, and deformation mechanisms of polycrystalline, platinum nanocylinders of grain size d = 12 nm. The number of grains across the diameter, D/d, was varied from 5 to 80 and 1.5 to 5 in the experiments and molecular dynamics simulations, respectively. An abrupt weakening is observed at a small D/d, while the strengths of large nanopillars are similar to bulk. This "smaller is weaker" trend is opposite to the "smaller is stronger" size effect in single crystalline nanostructures. The simulations demonstrate that the size-dependent behavior is associated with the distinct deformation mechanisms operative in interior versus surface grains.

Useful properties of structural materials generally depend on their bulk microstructure. For centuries, improvements in structural materials relied heavily on processing, which in turn determined the resulting microstructure and properties. Materials sciences are entering an era in which specific properties of a material are obtained not only from its processing but also by controlling of the architecture of its constituents, often with sub-micron dimensions. To utilize this newly achievable nanoscale engineering precision in structural applications, it is imperative to quantify the deformation processes at each relevant scale, with special attention focusing on the importance of internal and external heterogeneities, for example grain boundaries, bi-material interfaces, phase boundaries, etc., on mechanical loading. It has been shown for single crystals that yield (and fracture) strengths increase with power-law dependence on sample size reduction when the micron scale is reached, and therefore, can no longer be inferred from bulk response or from the literature. Although these studies provide a powerful foundation for fundamental deformation processes operating at small scales, they are far from representing real materials used in structural applications, whose microstructure is often complex, containing boundaries and interfaces. Both homogeneous (i.e. grain and twin boundaries) and heterogeneous (i.e. phase and precipitate–matrix boundaries) interfaces in size-limited features are crucial aspects of the structural reliability of most modern materials. They are also of particular importance to damage initiation. This article provides a comprehensive overview of the state-of-the-art experimental and computational methods used to investigate mechanical behavior and microstructural evolution in small-scale metallic systems, deformation of which depends on intricate interactions of defects with internal interfaces and with free surfaces. Attention is focused on the effects of multiple grain boundaries spanning the sample volume (nanocrystalline and polycrystalline metals). This overview sheds light on the relative importance of intrinsic versus extrinsic length scale limitations on deformation mechanisms in nanostructured metals, which has significant implications for the development of new materials with tunable mechanical properties.

Semiconducting nanowires frequently have enhanced properties and unique functionality compared to their bulk counterparts. Controlling the geometry of nanowires is crucial for their integration into nanoscale devices because the shape of a device component can dictate its functionality, such as in the case of a mechanical spring or an antenna. We demonstrate a novel synthetic method for making polycrystalline CdSe nanowires with controlled geometries by using self-organized single-walled carbon nanotubes as a template for the selective electrodeposition of nanowires. Nanowires of up to hundreds of micrometers in length are formed as high-density straight arrays, as well as in the shape of serpentines and loops. These nanowires exhibit significant photoluminescence and photoconductivity applicable to photodetectors and respond to illumination up to 2 orders of magnitude faster than single crystalline CdSe.

Silicon has been intensely studied as an anode material in Li batteries because of its high storage capacity. A major drawback is that Si suffers from mechanical degradation because of the large volumetric expansion during lithiation. Nano-structuring Si can alleviate the problem for each nanoscale element such as nanowires and nanoparticles [1, 2] because of the size-induced ductility and the availability of open space for Si to expand into. However, incorporating such nanoscale elements into larger-sized electrodes is difficult because of Si’s poor electronic conductivity and the loss of electrical contact after cycling [3]. Si particles tend to lose contact with surrounding particles or binding materials during lithiation and delithiation cycles. Meanwhile, due to the large surface to volume ratio of nanoparticles, solid-electrolyte interface (SEI) formation on particle surface reduces the amount of electrochemically active materials. Therefore, we propose 3D architected Si-Cu core-shell nanolattices as interconnected, binder-free Li-ion battery electrodes with the combined benefits of size-induced ductility, fast electron and ion transport, and mechanically robust structural geometries. To fabricate architected Si-Cu core-shell electrodes, a polymer nanolattice mold is first made from positive photoresist on a glass substrate covered by metal thin film via two-photon lithography. Cu is then electroplated into the mold, and free-standing solid Cu nanolattices are made by removing the remaining photoresist. Amorphous Si is then deposited over Cu by plasma-enhanced chemical vapor deposition (PECVD). The Si layer is kept to hundreds of nanometers in thickness in order to take advantage of the enhanced ductility in Si. Amorphous Si is used to reduce internal fracture due to anisotropic stress during Si expansion. For in situ battery testing, an electrochemical half-cell is built inside of a scanning electron microscope (SEM) using a lithium electrode and the nano-architected Si-Cu electrode. First, Li is brought into contact with Si-Cu nanolattices directly with an oxidized layer of Li2O on Li as a solid electrolyte, and a constant voltage bias is applied during electrochemical cycling. In situ SEM observation shows that during the first lithiation a reaction front propagates from the top of the electrode where it is contacting solid Li2O electrolyte to the bottom as each lattice beam expands in volume and bows out. During delithiation, no clear reaction front is observed as the structure shrinks in volume entirely. Si volume expansion is calculated to be about 200% yet the total expansion of the electrode structure is minimal. To better study how the electrode will be cycled in real batteries where electrode is fully submerged in electrolyte, 10wt% LiTFSI in P14TFSI ionic liquid electrolyte is used to connect the Li electrode and the Si-Cu lattices inside SEM. Cyclic voltammetry confirms the electrochemical reaction occurs between Si and Li. Galvanostatic cycling is being conducted to examine capacity and cyclability performance. Solid mechanics analysis of the volume expansion and stress evolution of the Si-Cu core-shell nanolattices during lithitation is also being conducted using finite element analysis (FEA) software. The core-shell configuration significantly reduces the shear interfacial stress at the Si-Cu interface comparing to Si thin film on Cu current collector. Electrochemically facilitated plastic flow of Si expansion bonded by the outermost lithiated Si shell [4] induces a compressive normal stress at the Si-Cu interface, which further eliminates the risk of Si delamination, a key mechanism for Si electrode failure. Future work will be put into optimizing nanolattice geometries with different unit cell types, unit cell parameters, and Cu/Si thicknesses and predicting the most favorable geometry to withstand dramatic volume expansion during lithiation. Reference: Chan, C. K., Peng, H., Liu, G., McIlwrath, K., Zhang, X. F., Huggins, R. a, & Cui, Y. (2008). High-performance lithium battery anodes using silicon nanowires. Nature Nanotechnology, 3(1), 31–35. McDowell, M. T., Lee, S. W., Nix, W. D., & Cui, Y. (2013). 25th anniversary article: Understanding the lithiation of silicon and other alloying anodes for lithium-ion batteries. Advanced Materials, 25(36), 4966–4985. Cui, L.-F., Hu, L., Wu, H., Choi, J. W., & Cui, Y. (2011). Inorganic Glue Enabling High Performance of Silicon Particles as Lithium Ion Battery Anode. Journal of The Electrochemical Society, 158(5), A592-A596. Zhao, K., Pharr, M., Wan, Q., Wang, W. L., Kaxiras, E., Vlassak, J. J., & Suo, Z. (2012). Concurrent Reaction and Plasticity during Initial Lithiation of Crystalline Silicon in Lithium-Ion Batteries. Journal of the Electrochemical Society, 159(3), A238–A243.