Latent track formation and recrystallization in swift heavy ion irradiation.

Swift heavy ions (SHI) irradiation is a complex coupled multiphysics phenomenon with applications in studying the effects of fission fragments, nano-patterning, and material modification. However, existing models are oversimplified, e.g., inelastic thermal spike, or complicated with inherent size limitations, e.g., hybrid Monte Carlo molecular dynamic. Here, we present a phase-field inelastic thermal spike (PF-iTS) approach to predict the amorphization, recrystallization, and stress generation in a single-impact SHI scenario. Our model quantitatively predicts the latent track radius, which agrees with experiments. Also, the PF-iTS calculates superheating temperature, stress generation, and recrystallization that cannot be obtained with classical i-TS. The appearance of mean compressive stress at the center of the latent track explains the hillock and void formation in TiO2, showing the importance of mechanical stresses in forming latent tracks during SHI irradiation.

Defect Engineering: A Path toward Exceeding Perfection.

Moving to nanoscale is a path to get perfect materials with superior properties. Yet defects, such as stacking faults (SFs), are still forming during the synthesis of nanomaterials and, according to common notion, degrade the properties. Here, we demonstrate the possibility of engineering defects to, surprisingly, achieve mechanical properties beyond those of the corresponding perfect structures. We show that introducing SFs with high density increases the Young's Modulus and the critical stress under compressive loading of the nanowires above those of a perfect structure. The physics can be explained by the increase in intrinsic strain due to the presence of SFs and overlapping of the corresponding strain fields. We have used the molecular dynamics technique and considered ZnO as our model material due to its technological importance for a wide range of electromechanical applications. The results are consistent with recent experiments and propose a novel approach for the fabrication of stronger materials.

Atomic Defects Influenced Mechanics of II-VI Nanocrystals.

Mechanical properties of nanocrystals are influenced by atomic defects. Here, we demonstrate the effect of planar defects on the mechanics of ZnO nanorods using atomic force microscopy, high-resolution transmission electron microscopy, and large-scale atomistic simulation. We study two different conditionally grown single nanorods. One contains extended I1-type stacking fault (SF) and another is defect free. The SF containing nanorods show buckling behaviors with reduced critical loading, whereas the other kinds show linear elastic behavior. We also studied the size dependence of elastic modulus and yield strength. The elastic modulus in both nanorods is inversely proportional to their size. Similar trend is observed for yield strength in the SF containing nanorods; however, the opposite is observed in the SF-free nanorods. This first experimental and theoretical study will guide toward the development of reliable electromechanical devices.

Structural transformation in monolayer materials: a 2D to 1D transformation.

Reducing the dimensions of materials to atomic scales results in a large portion of atoms being at or near the surface, with lower bond order and thus higher energy. At such scales, reduction of the surface energy and surface stresses can be the driving force for the formation of new low-dimensional nanostructures, and may be exhibited through surface relaxation and/or surface reconstruction, which can be utilized for tailoring the properties and phase transformation of nanomaterials without applying any external load. Here we used atomistic simulations and revealed an intrinsic structural transformation in monolayer materials that lowers their dimension from 2D nanosheets to 1D nanostructures to reduce their surface and elastic energies. Experimental evidence of such transformation has also been revealed for one of the predicted nanostructures. Such transformation plays an important role in bi-/multi-layer 2D materials.

Electromechanical properties of 1D ZnO nanostructures: nanopiezotronics building blocks, surface and size-scale effects.

One-dimensional (1D) zinc oxide nanostructures are the main components of nanogenerators and central to the emerging field of nanopiezotronics. Understanding the underlying physics and quantifying the electromechanical properties of these structures, the topic of this research study, play a major role in designing next-generation nanoelectromechanical devices. Here, atomistic simulations are utilized to study surface and size-scale effects on the electromechanical response of 1D ZnO nanostructures. It is shown that the mechanical and piezoelectric properties of these structures are controlled by their size, cross-sectional geometry, and loading configuration. The study reveals enhancement of the piezoelectric and elastic modulus of ZnO nanowires (NW) with diameter d > 1 nm, followed by a sudden drop for d < 1 nm due to transformation of NWs to nanotubes (NTs). Degradation of mechanical and piezoelectric properties of ZnO nanobelts (NBs) followed by an enhancement in piezoelectric properties occurs when their lower dimension is reduced to <1 nm. The latter enhancement can be explained in the context of surface reconfiguration and formation of hexagon-tetragon (HT) pairs at the intersection of (21[combining macron]1[combining macron]0) and (011[combining macron]0) planes in NBs. Transition from a surface-reconstructed dominant to a surface-relaxed dominant region is demonstrated for lateral dimensions <1 nm. New phase-transformation (PT) kinetics from piezoelectric wurtzite to nonpiezoelectric body-centered tetragonal (WZ → BCT) and graphite-like phase (WZ → HX) structures occurs in ZnO NWs loaded up to large strains of ∼10%.

Anisotropic compositional expansion and chemical potential for amorphous lithiated silicon under stress tensor.

Si is a promising anode material for Li-ion batteries, since it absorbs large amounts of Li. However, insertion of Li leads to 334% of volumetric expansion, huge stresses, and fracture; it can be suppressed by utilizing nanoscale anode structures. Continuum approaches to stress relaxation in LixSi, based on plasticity theory, are unrealistic, because the yield strength of LixSi is much higher than the generated stresses. Here, we suggest that stress relaxation is due to anisotropic (tensorial) compositional straining that occurs during insertion-extraction at any deviatoric stresses. Developed theory describes known experimental and atomistic simulation data. A method to reduce stresses is predicted and confirmed by known experiments. Chemical potential has an additional contribution due to deviatoric stresses, which leads to increases in the driving force both for insertion and extraction. The results have conceptual and general character and are applicable to any material systems.