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Development of high energy density solid-state batteries with Li metal anodes has been limited by uncontrollable growth of Li dendrites in liquid and solid electrolytes (SEs). This, in part, may be caused by a dearth of information about mechanical properties of Li, especially at the nano- and microlength scales and microstructures relevant to Li batteries. We investigate Li electrodeposited in a commercial LiCoO2/LiPON/Cu solid-state thin-film cell, grown in situ in a scanning electron microscope equipped with nanomechanical capabilities. Experiments demonstrate that Li was preferentially deposited at the LiPON/Cu interface along the valleys that mimic the domain boundaries of underlying LiCoO2 (cathode). Cryogenic electron microscopy analysis of electrodeposited Li revealed a single-crystalline microstructure, and in situ nanocompression experiments on nano-pillars with 360–759 nm diameters revealed their average Young's modulus to be 6.76 ± 2.88 GPa with an average yield stress of 16.0 ± 6.82 MPa, ~24x higher than what has been reported for bulk polycrystalline Li. We discuss mechanical deformation mechanisms, stiffness, and strength of nano-sized electrodeposited Li in the framework of its microstructure and dislocation-governed nanoscale plasticity of crystals, and place it in the parameter space of existing knowledge on small-scale Li mechanics. The enhanced strength of Li at small scales may explain why it can penetrate and fracture through much stiffer and harder SEs than theoretically predicted.
The reversible switching between the amorphous and crystalline phases of Ge2Sb2Te5 (GST) is investigated with ab initio molecular dynamics. We apply different quench rates (-16 K/ps, -5 K/ps, -2 K/ps, and -0.45 K/ps) and different annealing temperatures (500 K, 600 K, 700 K, and 800 K) to amorphize and crystallize GST respectively. Results show that the generated amorphous is strongly dependent on the quench rate. For -16 K/ps and -5 K/ps, generated amorphous samples have different density of crystal seeds, higher in the later. The amorphous structure formed at -2 K/ps contains a single crystalline cluster, while that formed at the quench rate of -0.45 K/ps had sufficient time to completely crystallize the amorphous phase. Annealing the amorphous systems formed at different rates shows that crystallization depends both on the annealing temperature and on the structure of the initial system (i.e., whether or not it contains crystalline clusters or crystal seeds). At 500 K, formation of crystalline clusters occurs readily within a few ps while the rate at which they grow is slow, taking 0.9 ns to complete the crystallization. In contrast, crystalline cluster formation is inhibited at 800 K. In the intermediate temperature range, both crystalline cluster formation and growth occur within a few hundred ps indicating that these temperatures leads to the fastest crystallization. The crystallization of a 63-atom at ∼900 K resulted in a highly relaxed crystal structure showing a clear tendency for separation of Ge and Sb species in layers. This model also indicates a tendency of segregation of vacancies, suggesting that vacancy layering may play a key role in the crystallization process.
When thin nanomaterials spontaneously deform into nonflat geometries (e.g., nanorods into nanohelices, thin sheets into ruffled forms), their properties may change by orders of magnitude. We discuss this phenomenon in terms of a formal mathematical concept: codimension c = D − d, the difference between the dimensionality of space D, and that of the object d. We use several independent examples such as the edge stress of graphene nanoribbons, the elastic moduli of nanowires, and the thermal expansion of a modified bead-chain model to demonstrate how this framework can be used to generically understand some nanomaterial properties and how these properties can be engineered by using mechanical constraints to manipulate the codimension of the corresponding structure.
When materials are very thin in one or more dimensions, their equilibrium shapes are often curved/bent. Such shapes commonly represent a compromise between elastic strain energy and other thermodynamic forces (e.g. related to surface stresses, electrostatic interactions, or adsorption). Examples include ZnO and SnO2 nanobelts, silica/carbonate helicoids, and graphene sheets and nanoribbons. Here, we demonstrate that when the equilibrium shape of a nanomaterial is not flat/straight, important fundamental material properties may be orders of magnitude different from their bulk counterparts. We focus here primarily on the graphene edges. Graphene in three dimensions is a codimension c = 1 material; the codimension is c = D – d = 3 – 2 = 1, where D is the dimensionality of the space in which the material is embedded and d is the dimensionality of the object. By contrast, a flat graphene sheet has c = 2 – 2 = 0. We use the REBO-II interatomic potential to calculate the edge orientation dependence of the edge energy and edge stresses of graphene with c = 0 and c = 1. The edge stress for all edge orientations is compressive with c = 0. Both edge energy and stresses are in reasonable agreement with DFT calculations. The compressive edge stresses in c = 0 lead to edge buckling (out-of-the-plane of the graphene sheet) for all edge orientations (c = 1). The edge buckling in c = 1 reduces all edge energies and dramatically reduces all edge stresses to near zero (more than an order of magnitude drop). We also report the effect of codimension on the free energy and entropy of a graphene sheet and the elastic properties of ZnO nanohelices.
Recent research has shown that biologically inspired approaches to materials synthesis and self-assembly, hold promise of unprecedented atomic level control of structure and interfaces. In particular, the use of organic molecules to control the production of inorganic technological materials has the potential for controlling grain structure to enhance material strength; controlling facet expression for enhanced catalytic activity; and controlling the shape of nanostructured materials to optimize optical, electrical and magnetic properties. In this work, we use organic molecules to modify silver crystal shapes towards understanding the metal-organic interactions that lead to nanoparticle shape control.
Using in situ electrochemical AFM (EC-AFM) as an in situ probe, we study the influence of a cationic surfactant cetyltrimethylamminobromide (CTAB) on Ag growth during electrochemical deposition on Ag(100). The results show that the organic surfactant leads to a unique crystal growth habit. With CTAB present in the growth solution, Ag islands grow in a truncated pyramidal shape. To understand the shape evolution of the Ag islands, we utilize electron backscatter diffraction (EBSD) in conjunction with microscopic ellipsometry to characterize the facet-specific binding of the organic molecules to large-grained polycrystalline Ag substrates.
We examined the influence of the boundary plane on grain-boundary diffusion in Ni through a series of molecular dynamics simulations. A series of 〈010〉 ∑5 tilt boundaries, including several high symmetry and low symmetry boundary planes, were considered. The self-diffusion coefficient is a strong function of boundary inclination at low temperature but is almost independent of inclination at high temperature. At all temperatures, the self-diffusion coefficients are low when at least one of the two grains has a normal with low Miller indices. The grain boundary self-diffusion coefficient is an Arrhenius function of temperature. The logarithm of the pre-exponential factor in the Arrhenius expression was shown to be nearly proportional to the activation energy for diffusion. The activation energy for self-diffusion in a (103) symmetric tilt boundary is much higher than in boundaries with other inclinations. We discuss the origin of the boundary plane density–diffusion coefficient correlation.
As-deposited thin films grown by vapor deposition often exhibit large intrinsic stresses that can lead to film failure. While this is an “old” materials problem, our understanding has only recently begun to evolve in a more sophisticated fashion. Sensitive real-time measurements of stress evolution during thin-film deposition reveal a generic compressive–tensile–compressive behavior that correlates with island nucleation and growth, island coalescence, and postcoalescence film growth. In this article, we review the fundamental mechanisms that can generate stresses during the growth of Volmer–Weber thin films. Compressive stresses in both discontinuous and continuous films are generated by surface-stress effects. Tensile stresses are created during island coalescence and grain growth. Compressive stresses can also result from the flux-driven incorporation of excess atoms within grain boundaries. While significant progress has been made in this field recently, further modeling and experimentation are needed to quantitatively sort out the importance of the different mechanisms to the overall behavior.
Two dimensional, non-equilibrium molecular dynamics simulations have been performed to examine the microstructures of both homoepitaxial and heteroepitaxial thin films grown on single crystal substrates. The principal microstructural features to develop within these films are small voids and edge dislocations. Voids form near the surface of the growing film as surface depressions between microcolumns pinch off to become closed volumes. These voids often form in such a way as to introduce dislocations into the crystal with their cores positioned within the voids. Dislocations are also formed during heteroepitaxy at the interface between the substrate and film. These dislocations tend to be mobile. When voids are present in the film and when the lattice misfit is low, dislocations tend to be trapped in the voids or pulled toward them due to dislocation image interactions. Once attached to voids, dislocations are effectively pinned there. When voids are absent or when the misfit is high, dislocations are restricted to the film-substrate interface. In the case of heteroepitaxy, dislocations are found to relieve either tensile or compressive misfit stresses. Misfit stresses may also be accommodated, to some extent, merely by the free volume of the voids themselves.
Two dimensional non-equilibrium molecular dynamics simulations are performed to study microstructural evolution during the growth of polycrystalline thin films. Attention is focused on the interaction between grain boundaries and voids which form during deposition, and on the development of a preferred, crystallographic texture during film growth. In an intermediate temperature regime, where the film is cold enough to allow void formation but hot enough to allow grain boundary motion, boundaries move such as to attach themselves to voids as the voids form from depressions in the film surface. At lower temperatures, the boundaries have insufficient mobility to migrate toward the voids. At higher temperatures, films grow in the absence of voids. At low deposition kinetic energies, there is no tendency for polycrystalline films to develop a preferred texture. At moderate or high energy deposition kinetic energies, however, as in the case of magnetron sputtering, significant texture formation can result due to preferential (re)sputtering of atoms from the surface of grains with low-binding-energy exposed surfaces. Such preferential (re)sputtering provides a height advantage for grains possessing high-binding-energy exposed surfaces. The taller grains are seen to widen as deposition continues, resulting in the development of a preferred crystallographic orientation.
We have calculated the work of adhesion (i.e. energy for rigid fracture) and peak interfacial stress for NiAl, Cr, and NiAl–Cr using self-consistent density functional calculations to obtain the complete energy vs. separation curve for each system. Our calculations indicate that the work of adhesion is largest for Cr and smallest for NiAI while those for interfaces of NiAI with Cr are intermediate. We have also estimated that segregation processes could alter the work of adhesion for the AlNi/Cr interface by up to 20% since Al tends to segregate to the free NiAl surface while Ni tends to segregate to the AlNi/Cr interface.
A detailed comparison between the experimental evolution of a two-dimensional soap froth and the large Q state Potts model is presented. The pattern evolution starting from identical initial conditions will be compared as well as a variety of distribution functions and correlations of the two systems. Simulations on different lattices show that the discrete lattice of the Potts model causes deviations from universal domain growth by weakening the vertex angle boundary conditions that form the basis of von Neumann's law. We show that the anisotropy inherent in a discrete lattice simulation, which masks the underlying ‘universal’ grain growth, can be overcome by increasing the range of the interaction between spins or increasing the temperature. Excellent overall agreement between the kinetics, topological distributions and domain size distributions between the low lattice anisotropy Potts-model simulations and the soap froth suggests that the Potts model is useful for studying domain growth in a wide variety of physical systems.
The correlation between surface cross-hatched morphology and interfacial misfit dislocations in strained III-V semiconductor heteroepitaxy has been studied. The surface pattern is clearly seen on samples grown at high temperature (520°C) and with lattice mismatch f < 2%. A poorly defined cross-hatched morphology is found on layers grown at low temperature (400°C). For f > 2%, a rough textured surface morphology is observed in place of cross hatching. Few threading dislocations are observed in the strained layer when cross hatch develops. Cross hatch occurs after most interfacial misfit dislocations are generated. The results suggest that surface cross hatch is directly related to the generation and glide of interfacial misfit dislocations.
Thin films can break up into islands only if they are perturbed by substrate-intersecting perturbations. Grain boundary grooves and vertex pits are typical defects which nucleate holes in these films. Holes which exceed a critical size - proportional to the ratio of the film thickness to the equilibrium contact angle - grow, eventually disconnecting the film.
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