Crystallography-Derived Young's Modulus and Tensile Strength of AlN Nanowires as Revealed by in Situ Transmission Electron Microscopy

In Situ TEM Characterization and Modulation for Phase Engineering of Nanomaterials

Solid-state phase transformation is an intriguing phenomenon in crystalline or noncrystalline solids due to the distinct physical and chemical properties that can be obtained and modified by phase engineering. Compared to bulk solids, nanomaterials exhibit enhanced capability for phase engineering due to their small sizes and high surface-to-volume ratios, facilitating various emerging applications. To establish a comprehensive atomistic understanding of phase engineering, in situ transmission electron microscopy (TEM) techniques have emerged as powerful tools, providing unprecedented atomic-resolution imaging, multiple characterization and stimulation mechanisms, and real-time integrations with various external fields. In this Review, we present a comprehensive overview of recent advances in in situ TEM studies to characterize and modulate nanomaterials for phase transformations under different stimuli, including mechanical, thermal, electrical, environmental, optical, and magnetic factors. We briefly introduce crystalline structures and polymorphism and then summarize phase stability and phase transformation models. The advanced experimental setups of in situ techniques are outlined and the advantages of in situ TEM phase engineering are highlighted, as demonstrated via several representative examples. Besides, the distinctive properties that can be obtained from in situ phase engineering are presented. Finally, current challenges and future research opportunities, along with their potential applications, are suggested.

Anisotropic crystal orientations dependent mechanical properties and fracture mechanisms in zinc blende ZnTe nanowires

The orientations of crystal growth significantly affect the operating characteristics of elastic and inelastic deformation in semiconductor nanowires (NWs). This work uses molecular dynamics simulation to extensively investigate the orientation-dependent mechanical properties and fracture mechanisms of zinc blende ZnTe NWs. Three different crystal orientations, including [100], [110], and [111], coupled with temperatures (100 to 600 K) on the fracture stress and elastic modulus, are thoroughly studied. In comparison to the [110] and [100] orientations, the [111]-oriented ZnTe NW exhibits a high fracture stress. The percentage decrease in fracture strength exhibits a pronounced variation with increasing temperature, with the highest magnitude observed in the [100] direction and the lowest magnitude observed in the [110] direction. The elastic modulus dropped by the largest percentage in the [111] direction as compared to the [100] direction. Most notably, the [110]-directed ZnTe NW deforms unusually as the strain rate increases, making it more sensitive to strain rate than other orientations. The strong strain rate sensitivity results from the unusual short-range and long-range order crystals appearing due to dislocation slipping and partial twinning. Moreover, the {111} plane is the principal cleavage plane for all orientations, creating a dislocation slipping mechanism at room temperature. The {100} plane becomes active and acts as another fundamental cleavage plane at increasing temperatures. This in-depth analysis paves the way for advancing efficient and reliable ZnTe NWs-based nanodevices and nanomechanical systems.

Crystal orientation-dependent tensile mechanical behavior and deformation mechanisms of zinc-blende ZnSe nanowires

Crystal deformation mechanisms and mechanical behaviors in semiconductor nanowires (NWs), in particular ZnSe NWs, exhibit a strong orientation dependence. However, very little is known about tensile deformation mechanisms for different crystal orientations. Here, the dependence of crystal orientations on mechanical properties and deformation mechanisms of zinc-blende ZnSe NWs are explored using molecular dynamics simulations. We find that the fracture strength of [111]-oriented ZnSe NWs shows a higher value than that of [110] and [100]-oriented ZnSe NWs. Square shape ZnSe NWs show greater value in terms of fracture strength and elastic modulus compared to a hexagonal shape at all considered diameters. With increasing temperature, the fracture stress and elastic modulus exhibit a sharp decrease. It is observed that the {111} planes are the deformation planes at lower temperatures for the [100] orientation; conversely, when the temperature is increased, the {100} plane is activated and contributes as the second principal cleavage plane. Most importantly, the [110]-directed ZnSe NWs show the highest strain rate sensitivity compared to the other orientations due to the formation of many different cleavage planes with increasing strain rates. The calculated radial distribution function and potential energy per atom further validates the obtained results. This study is very important for the future development of efficient and reliable ZnSe NWs-based nanodevices and nanomechanical systems.

Surface-topology-controlled mechanical characteristics of triply periodic carbon Schwarzite foams

Bulky sp²-carbon Schwarzites with negative Gaussian curvature are promising structures for practical applications due to their unique properties such as high surface area, large porosity, and stability against graphitization. Herein, a comprehensive study on the tension, compression and shear mechanical characteristics of seven triply periodic carbon Schwarzite foams with distinct topologies is performed using reactive molecular dynamics (MD) simulations. All carbon Schwarzites exhibit unique thermal and mechanical properties that are markedly dictated by the topology. One of the structures presents a negative thermal expansion coefficient. Under uniaxial tension, the temperature is able to play a positive or negative role in the tensile stiffness, and there is no apparent positive relationship between tensile strength and mass density. Subjected to compression and shear loads, carbon Schwarzites can fail due to brittle fracture, and uniform and stepwise structural instabilities. Both compression- and tension-negative Poisson's ratios are revealed to originate from a curvature-flattening deformation mechanism. Analysis of the crush force efficiency, the stroke efficiency and the energy-absorption demonstrates that carbon Schwarzites are effective energy-absorbers. This study provides a fundamental understanding of the relationship between the topology and mechanical properties of carbon Schwarzites for designing 3D graphitic nanostructures with good mechanical performances.

Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials

In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing physical and chemical process dynamics at atomic resolutions. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O₂ , Na-O₂ , Li-S, etc., fuel cells, thermoelectrics, photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, electrical biasing, light illumination, and liquid and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.

Surface softening regulates size-dependent stiffness of diamond nanowires

Diamond nanowires (NWs) belong to an important class of nanoscale materials for their outstanding potential in mechanical, electrical, and thermal applications. However, their mechanical behavior under pristine and defective conditions remains less understood. This paper reveals a comprehensive understanding of the effective elastic behavior of diamond NWs, and it uncovers surface-softening as the dominant mechanism that regulates their effective behavior. We applied the force-based and energy-based approaches and constructed a comparative analysis to reveal the atomistic basis behind the diameter-dependent elastic properties of the nanowires. Our findings suggest the energy-based approach to produce physically meaningful results, whereas the widely used force-based scheme produces inconsistent size-dependent behavior. Results show that, with increasing diameter, the softening of the surface and the defective regimes decreases. As a direct consequence of the alteration in the softening state, the first-order elastic modulus increases with increasing diameter, whereas the second-order modulus decreases. Also, vacancy defects, even in very dilute concentrations, are found to substantially affect the elastic behavior of the nanowire. Furthermore, surface, core, and defective regimes exhibit very different roles in nanowires of different diameters: the surface regime acts as a softer regime and the core as stiffer, regardless of the diameter. Their cumulative effect is however dominated by the surface in smaller-diameter nanowire-but in wider diameter nanowires it is dominated by the core. As a result, the size-dependent behavior is strictly controlled by the softening state of the surface. The diameter-dependent elastic moduli show a power-law relation, which deviates substantially from the simple surface-to-volume ratio. These findings suggest surface-engineering as an important tool for modulating the effective behavior of brittle nanowires.