Amorphous shear bands in crystalline materials as drivers of plasticity.

Traditionally, the formation of amorphous shear bands in crystalline materials has been undesirable, because shear bands can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently were shear bands found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit shear-band deformation, and by varying the composition, we were able to switch from ductile to brittle behaviour. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing the toughness of nominally brittle materials.

Uncertainty and Irreproducibility of Triboelectricity Based on Interface Mechanochemistry.

Triboelectrification mechanism is still not understood, despite centuries of investigations. Here, we propose a model showing that mechanochemistry is key to elucidate triboelectrification fundamental properties. Studying contact between gold and silicate glasses, we observe that the experimental triboelectric output is subject to large variations and polarity inversions. First principles analysis shows that electronic transfer is activated by mechanochemistry and the tribopolarity is determined by the termination exposed to contact, depending on the material composition, which can result in different charging at the macroscale. The electron transfer mechanism is driven by the interface barrier dynamics, regulated by mechanical forces. The model provides a unified framework to explain several experimental observations, including the systematic variations in the triboelectric output and the mixed positive-negative "mosaic" charging patterns, and paves the way to the theoretical prediction of the triboelectric properties.

Reconfiguration of Amorphous Complex Oxides: A Route to a Broad Range of Assembly Phenomena, Hybrid Materials, and Novel Functionalities.

Reconfiguration of amorphous complex oxides provides a readily controllable source of stress that can be leveraged in nanoscale assembly to access a broad range of 3D geometries and hybrid materials. An amorphous SrTiO3 layer on a Si:B/Si1- x Gex :B heterostructure is reconfigured at the atomic scale upon heating, exhibiting a change in volume of ≈2% and accompanying biaxial stress. The Si:B/Si1- x Gex :B bilayer is fabricated by molecular beam epitaxy, followed by sputter deposition of SrTiO3 at room temperature. The processes yield a hybrid oxide/semiconductor nanomembrane. Upon release from the substrate, the nanomembrane rolls up and has a curvature determined by the stress in the epitaxially grown Si:B/Si1- x Gex :B heterostructure. Heating to 600 °C leads to a decrease of the radius of curvature consistent with the development of a large compressive biaxial stress during the reconfiguration of SrTiO3 . The control of stresses via post-deposition processing provides a new route to the assembly of complex-oxide-based heterostructures in 3D geometry. The reconfiguration of metastable mechanical stressors enables i) synthesis of various types of strained superlattice structures that cannot be fabricated by direct growth and ii) technologies based on strain engineering of complex oxides via highly scalable lithographic processes and on large-area semiconductor substrates.

Control of Surface Chemical Reactions through Solid Stiffness.

Control of surface reactions is commonly achieved by modification of surface electronic structures. Here, we discover an alternative pathway for controlling surface reactions by tuning the mechanical stiffness of the underlying material. We find that in addition to the typically assumed surface electronic contribution right at the reactive site, the contribution from the deformation of the bulk region plays a vital role in controlling surface reactions. The underlying mechanism is an elastic relaxation of the solid, which depends on the material's stiffness and can be modified by tuning bulk stoichiometry. The effect of bulk stiffness on surface reactions has been demonstrated by considering hydrogen scission reaction and oxygen incorporation reaction during corrosion of amorphous SiC in water and air, respectively. Our results imply that tuning of bulk stiffness by modifying stoichiometry can provide an effective method for controlling surface reactions.

Radiation-induced segregation in a ceramic.

Radiation-induced segregation is well known in metals, but has been rarely studied in ceramics. We discover that radiation can induce notable segregation of one of the constituent elements to grain boundaries in a ceramic, despite the fact that the ceramic forms a line compound and therefore has a strong thermodynamic driving force to resist off-stoichiometry. Specifically, irradiation of silicon carbide at 300 °C leads to carbon enrichment near grain boundaries, whereas the enrichment diminishes for irradiation at 600 °C. The temperature dependence of this radiation-induced segregation is different from that shown in metallic systems. Using an ab initio informed rate theory model, we demonstrate that this difference is introduced by the unique defect energy landscapes present in the covalent system. Additionally, we discover that grain boundaries in unirradiated silicon carbide grown by chemical vapour deposition are intrinsically carbon-depleted. The inherent grain boundary chemistry and its evolution under radiation are both critical for understanding the many properties of ceramics associated with grain boundaries.

Physical Origin of the Mechanochemical Coupling at Interfaces.

We used density functional theory calculations to investigate the physical origin of the mechanochemical response of material interfaces. Our results show that the mechanochemical response can be decomposed into the contribution from the interface itself (deformation of interfacial bonds) and a contribution from the underlying solid. The relative contributions depend on the stiffness of these regions and the contact geometry, which affects the stress distribution within the bulk region. We demonstrate that, contrary to what is commonly assumed, the contribution to the activation volume from the elastic deformation of the surrounding bulk is significant and, in some case, may be dominant. We also show that the activation volume and the mechanochemical response of interfaces should be finite due to the effects on the stiffness and stress distribution within the near-surface bulk region. Our results indicate that the large range of activation volumes measured in the previous experiments even for the same material system might originate from the different degrees of contributions probed from the bulk vs interface.

Linear Aging Behavior at Short Timescales in Nanoscale Contacts.

Nanoscale silica-silica contacts were recently found to exhibit logarithmic aging for times ranging from 0.1 to 100 s, consistent with the macroscopic rate and state friction laws and several other aging processes. Nanoscale aging in this system is attributed to progressive formation of interfacial siloxane bonds between surface silanol groups. However, understanding or even data for contact behavior for aging times <0.1 s, before the onset of logarithmic aging, is limited. Using a combination of atomic force microscopy experiments and kinetic Monte Carlo simulations, we find that aging is nearly linear with aging time at short timescales between ∼ 5 and 90 ms. We demonstrate that aging at these timescales requires the existence of a particular range of reaction energy barriers for interfacial bonding. Specifically, linear aging behavior consistent with experiments requires a narrow peak close to the upper bound of this range of barriers. These new insights into the reaction kinetics of interfacial bonding in nanoscale aging advance the development of physically based rate and state friction laws for nanoscale contacts.

Massive Vacancy Concentration Yields Strong Room-Temperature Ferromagnetism in Two-Dimensional ZnO.

Two-dimensional (2D) ZnO nanosheets with highly concentrated Zn vacancies (VZn) of up to approximately 33% were synthesized by ionic layer epitaxy at the water-toluene interface. This high cation vacancy concentration is unprecedented for ZnO and may provide unique opportunities to realize exotic properties not attainable in the conventional bulk form. After annealing, the nanosheets showed characteristic magnetic hysteresis with saturation magnetization of 57.2 emu/g at 5 K and 50.9 emu/g at room temperature. This value is 1 order of magnitude higher than other ZnO nanostructures and comparable to the conventional ferrimagnetic Fe3O4. Density functional theory calculations, with the support of experimental results, suggest that a high concentration of VZn (approximately one-third of the Zn sites) can form spontaneously during synthesis when stabilized by H ions, and the formation of VZn could be further facilitated by the presence of grain boundaries. It is essential to remove the H for the nanosheets to show ferromagnetism. The mechanisms identified for the origin of the high magnetism in ZnO nanosheets presents an intriguing example of a kinetically stabilized, non-equilibrium, highly defective 2D nanomaterial with a significantly enhanced physical property.

Load and Time Dependence of Interfacial Chemical Bond-Induced Friction at the Nanoscale.

Rate and state friction (RSF) laws are widely used empirical relationships that describe the macroscale frictional behavior of a broad range of materials, including rocks found in the seismogenic zone of Earth's crust. A fundamental aspect of the RSF laws is frictional "aging," where friction increases with the time of stationary contact due to asperity creep and/or interfacial strengthening. Recent atomic force microscope (AFM) experiments and simulations found that nanoscale silica contacts exhibit aging due to the progressive formation of interfacial chemical bonds. The role of normal load (and, thus, normal stress) on this interfacial chemical bond-induced (ICBI) friction is predicted to be significant but has not been examined experimentally. Here, we show using AFM that, for nanoscale ICBI friction of silica-silica interfaces, aging (the difference between the maximum static friction and the kinetic friction) increases approximately linearly with the product of the normal load and the log of the hold time. This behavior is attributed to the approximately linear dependence of the contact area on the load in the positive load regime before significant wear occurs, as inferred from sliding friction measurements. This implies that the average pressure, and thus the average bond formation rate, is load independent within the accessible load range. We also consider a more accurate nonlinear model for the contact area, from which we extract the activation volume and the average stress-free energy barrier to the aging process. Our work provides an approach for studying the load and time dependence of contact aging at the nanoscale and further establishes RSF laws for nanoscale asperity contacts.

Unit Cell Level Thickness Control of Single-Crystalline Zinc Oxide Nanosheets Enabled by Electrical Double-Layer Confinement.

Ionic layer epitaxy (ILE) has recently been developed as an effective strategy to synthesize nanometer thick 2D materials with a nonlayered crystal structure, such as ZnO. The packing density of the amphiphilic monolayer is believed to be a key parameter that controls the nanosheet nucleation and growth. In this work, we systematically investigated the growth behavior of single-crystalline ZnO nanosheets templated at the water-air interface by an anionic oleylsulfate monolayer with different packing densities. The thicknesses of ZnO nanosheets were tuned from one unit cell to four unit cells and exhibited good correlation with the width of Zn2+ ion concentration zone (the Stern layer) underneath the ionized surfactant monolayer. Further analysis of the nanosheet sizes and density revealed that the nanosheet growth was dominated by the steric hindrance from the surfactant monolayer at lower surface pressure, while the nucleation density became the dominating factor at higher surface pressure. The ZnO nanosheets exhibited a decreasing work function as the thickness reduced to a few unit cells. This research validated a critical hypothesis that the nanosheet growth is self-limited by the formation of a double layer of ionic precursors. This work will open up a new way toward controlled synthesis of novel 2D nanosheets from nonlayered materials with a thickness down to one unit cell.

Investigation of the Role of Polysaccharide in the Dolomite Growth at Low Temperature by Using Atomistic Simulations.

Dehydration of water from surface Mg(2+) is most likely the rate-limiting step in the dolomite growth at low temperature. Here, we investigate the role of polysaccharide in this step using classical molecular dynamics (MD) calculations. Free energy (potential of mean force, PMF) calculations have been performed for water molecules leaving the first two hydration layers above the dolomite (104) surface under the following three conditions: without catalyst, with monosaccharide (mannose), and with oligosaccharide (three units of mannose). MD simulations reveal that there is no obvious effect of monosaccharide in lowering the dehydration barrier for surface Mg(2+). However, we found that there are metastable configurations of oligosaccharide, which can decrease the dehydration barrier of surface Mg(2+) by about 0.7-1.1 kcal/mol. In these configurations, the molecule lies relatively flat on the surface and forms a bridge shape. The hydrophobic space near the surface created by the nonpolar -CH groups of the oligosaccharide in the bridge conformation is the reason for the observed reduction of dehydration barrier.

Friction laws at the nanoscale.

Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

Amorphization driven by defect-induced mechanical instability.

Using ab initio molecular dynamics simulations, we perform a comparative study of the defect accumulation process in silicon carbide (SiC) and zirconium carbide (ZrC). Interestingly, we find that the fcc Si sublattice in SiC spontaneously and gradually collapses following the continuous introduction of C Frenkel pairs (FPs). Above a critical amorphization dose of ~0.33 displacements per atom (dpa), the pair correlation function exhibits no long-range order. In contrast, the fcc Zr sublattice in ZrC remains structurally stable against C sublattice displacements up to the highest dose of 1.0 dpa considered. Consequently, ZrC cannot be amorphized by the accumulation of C FPs. We propose defect-induced mechanical instability as the key mechanism driving the amorphization of SiC under electron irradiation.

Chemical origins of frictional aging.

Although the basic laws of friction are simple enough to be taught in elementary physics classes and although friction has been widely studied for centuries, in the current state of knowledge it is still not possible to predict a friction force from fundamental principles. One of the highly debated topics in this field is the origin of static friction. For most macroscopic contacts between two solids, static friction will increase logarithmically with time, a phenomenon that is referred to as aging of the interface. One known reason for the logarithmic growth of static friction is the deformation creep in plastic contacts. However, this mechanism cannot explain frictional aging observed in the absence of roughness and plasticity. Here, we discover molecular mechanisms that can lead to a logarithmic increase of friction based purely on interfacial chemistry. Predictions of our model are consistent with published experimental data on the friction of silica.

Friction and slip at the solid/liquid interface in vibrational systems.

Molecular dynamics simulations have been performed to study frictional slip and its influence on energy dissipation and momentum transfer at atomically smooth solid/water interfaces. By modifying the surface chemistry, we investigate the relationship between slip and the mechanical response of a vibrating solid for both hydrophilic and hydrophobic surfaces. We discover physical phenomena that emerge at high frequencies and that have significant contributions to energy dissipation. A new analytical model is developed to describe the mechanical response of the resonators in this high-frequency regime, which is relevant in such applications as microelectromechanical-system-based biosensors. We find a linear relationship between the slip length and the ratio of the damping rate shift to the resonant frequency shift, which provides a new way to obtain information about the slip length from experiments.

Large Area, High Resolution Mapping of Approximate Rotational Symmetries in a Pd77.5Cu6Si16.5 Metallic Glass Thin Film.

Densely spaced four-dimensional scanning transmission electron microscopy (4D STEM) analyzed using correlation symmetry coefficients enables large area mapping of approximate rotational symmetries in amorphous materials. Here, we report the effects of Poisson noise, limited electron counts, probe coherence, reciprocal space sampling, and the probe-sample interaction volume on 4D STEM symmetry mapping experiments. These results lead to an experiment parameter envelope for high quality, high confidence 4D STEM symmetry mapping. We also establish a direct link between the symmetry coefficients and approximate rotational symmetries of nearest-neighbor atomic clusters using electron diffraction simulations from atomic models of a metallic glass. Experiments on a Pd77.5Cu6Si16.5 metallic glass thin film demonstrate the ability to image the types, sizes, volume fractions, and spatial correlations amongst local rotationally symmetry regions in the glass.

Enhancing the phase stability of ceramics under radiation via multilayer engineering.

In metallic systems, increasing the density of interfaces has been shown to be a promising strategy for annealing defects introduced during irradiation. The role of interfaces during irradiation of ceramics is more unclear because of the complex defect energy landscape that exists in these materials. Here, we report the effects of interfaces on radiation-induced phase transformation and chemical composition changes in SiC-Ti3SiC2-TiC x multilayer materials based on combined transmission electron microscopy (TEM) analysis and first-principles calculations. We found that the undesirable phase transformation of Ti3SiC2 is substantially enhanced near the SiC/Ti3SiC2 interface, and it is suppressed near the Ti3SiC2/TiC interface. The results have been explained by ab initio calculations of trends in defect segregation to the above interfaces. Our finding suggests that the phase stability of Ti3SiC2 under irradiation can be improved by adding TiC x , and it demonstrates that, in ceramics, interfaces are not necessarily beneficial to radiation resistance.

Plasticity without dislocations in a polycrystalline intermetallic.

Dislocation activity is critical to ductility and the mechanical strength of metals. Dislocations are the primary drivers of plastic deformation, and their interactions with each other and with other microstructural features such as grain boundaries (GBs) lead to strengthening of metals. In general, suppressing dislocation activity leads to brittleness of polycrystalline materials. Here, we find an intermetallic that can accommodate large plastic strain without the help of dislocations. For small grain sizes, the primary deformation mechanism is GB sliding, whereas for larger grain sizes the material deforms by direct amorphization along shear planes. The unusual deformation mechanisms lead to the absence of traditional Hall-Petch (HP) relation commonly observed in metals and to an extended regime of strength weakening with grain refinement, referred to as the inverse HP relation. The results are first predicted in simulations and then confirmed experimentally.

Memory Distance for Interfacial Chemical Bond-Induced Friction at the Nanoscale.

Macroscale rate and state friction (RSF) laws include a memory distance, Dc, which is considered to be the distance required for a population of frictional contacts to renew itself via slip, counteracting the effects of aging in slow or static contact. This concept connects static friction and kinetic friction. Here, we use atomic force microscopy to study interfacial chemical bond-induced kinetic friction and the memory distance at the nanoscale for single silica-silica nanocontacts. We observe a logarithmic trend of decreasing friction with sliding velocity (i.e., velocity-weakening) at low velocities and a transition to increasing friction with velocity at higher velocities (i.e., velocity-strengthening). We propose a physically based kinetic model for the nanoscale memory effect, the "activation-passivation loop" model, which accounts for the activation and passivation of chemical reaction sites and the formation of new chemical bonds from dangling bonds during sliding. In the model, we define the memory distance to be the average sliding distance that accrues before an activated reaction site becomes passivated. Results from numerical simulations based on this model match experimental friction data well in the velocity-weakening regime and show that Dc is sensitive to the surface chemistry, and nearly independent of sliding velocity. The simulations also show values of Dc that are consistent with those obtained from the experiments. We propose a semiquantitative physical explanation of the observed logarithmic velocity-weakening behavior based on the conservation of the number of interfacial bonds during sliding. We also extract from the experimental data physically reasonable values of the energy barriers to the activation of reaction sites. Our results provide one possible physical mechanism for the nanoscale memory distance.