wileyonlinelibrary.comwill specify exactly the conditions under which the formation of nanostructures is reversible after bending the nanowire.Reversibility is typically related to the shape memory effect and pseudo-elasticity in shape memory alloys (SMAs). [5][6][7] The shape recovery is achieved by thermoelastic martensitic phase transformations and domain switching. The driving force for the shape recovery arises from the free energy difference between the martensite and parent phase. This effect is strongly size-dependent and it is not obvious how SMAs operate in thin wires. [ 8,9 ] The fundamental question is whether a different shape memory effect exists at the nanoscale and if so by which mechanism. This is important because many traditional SMAs fail under nanoscale bending and it becomes important to search for alternative functional materials to replace the traditional SMAs for such nanoscale applications. We show by molecular dynamics simulations that α-Fe, which is not a shape memory alloy, also shows shape recovery (or pseudo-elasticity) after bending. Large bending in α-Fe occurs via the formation of interfaces between domains of different orientation and twinning. The deformed nanowire completely recovers under unloading. The mechanism is shown to be very different from the classic pseudo-elasticity, and is related to high-energy interfaces in the nanowire and not the martensite-austenite phase transformation. This result has implications more widely for shape-dependent SMAs that are used in microelectromechanical systems (MEMS) technology. This is also an example of the emerging fi eld of domain boundary engineering where functionality (namely the shape recovery) is linked to domain boundaries and interfaces, but not to bulk properties (such as the martensite-austenite phase transformation). [10][11][12] Previous molecular dynamics (MD) simulations have identifi ed a class of metallic nanowires with both face-centered cubic (fcc) and body-centered cubic (bcc) structures that show pseudo-elasticity and shape memory effects. [13][14][15][16][17][18][19] This pseudoelastic behavior was achieved under uniaxial tension while little is known whether such unique behavior can still exist when a wire is bent. Under tension, the shape recovery relates to the reversibility of conventional twinning. The driving force for the recoverable deformation stems from the minimization of the surface energy. [ 14,15,17,18 ] The total recoverable strain is very large and can exceed 40%. A large inelastic deformation mediated by conventional twinning has been confi rmed experimentally in fcc palladium and bcc tungsten. [ 20,21 ] Here we also show that the shape recovery effect under bending in α -Fe relates to the formation of nonconventional interfaces between domains of different orientations. Unlike conventional <111>/{112}-type Interface Driven Pseudo-Elasticity in α-Fe Nanowires Yang Yang , Suzhi Li , * Xiangdong Ding , * Jun Sun , and Ekhard K. H. Salje * Molecular dynamics simulations of bent [100] α-Fe nanowires...

We studied the torsion behavior of -Fe nanowires seeded with twin boundaries (TBs) using molecular dynamics simulations. Twisting the wire generates topological defects in the twin walls, namely kinks inside the twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting the kink motion is jerky and uncorrelated at small twist angles. The probability density function (PDF) of jerk energies follows approximately a Gaussian distribution, indicating a mild deformation mode. The kink dynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high density of junctions are generated. The collective motion of kinks now shows avalanche behavior with the energy being power-law distributed. The wildness, which measures the proportion of strain energy relaxed through such avalanches, is correlated with the junction density, and controlled by the external length scale (wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverability is achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolution of twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanical properties governed by twin boundary motion in nanowires.

Induced inhomogeneous strain also enables the concept proposal and experimental realization of exciton funnel. [7][8][9] Remarkably, the flexural mode also leads to the formation of ripples, [10] a class of intrinsic structural defects in 2D materials. Atomistically, ripples are quantized, referred to as ripplocations, and the ripplocations carrying the same sign exhibit an intriguing attractive behavior which is in direct contrast to the well-known repulsive behavior for dislocations in bulk with the same sign. [11] Recently, ripples were found to be layer-dependent, which gives rise to an evolving friction behavior in few-layer graphene. [12,13] Flexural mode induced ripples add an additional dimension in 2D materials with potential impact on the physical properties. [14,15] In parallel, 2D ferroic (e.g., strongly coupled ferroelastic-ferroelectric) and multiferroic [16][17][18][19] materials recently sparked much interest owing to their potential applications in emerging 2D functional devices. For example, long-range ferromagnetic order was recently discovered in semiconducting CrI 3 [20] and Cr 2 Ge 2 Te 6 [21] as well as van der Waals heterostructure [22] originating from magnetic anisotropy, which may pave the way for realizing 2D spintronics. Layered transition metal dichalcogenide WTe 2 were found to hold semimetallicity, ferroelectricity, [23] and ferroelasticity [24] simultaneously, responsible for ferroelectric-switchable nonlinear anomalous Hall effect in few-layer WTe 2 with potential application for nonlinear quantum electronics. [25,26] Moreover, van der Waals layered compound In 2 Se 3 was experimentally demonstrated to possess in-plane ferroelectricity, [27][28][29][30] and group IV monochalcogenides hold strongly coupled ferroelastic-ferroelectric orders with great promise for developing ultrathin memories and actuators. [31][32][33] However, the stability and kinetics of ferroic orders are closely related to the external magnetic, electrical, or stress field, which may be affected by the localized field generated by the intrinsic ripples in 2D materials, suggesting the great importance to understand the role of ripples on 2D ferroicity. Surprisingly, the role of ripples has been largely unexplored in the past, their impact on the physical properties consequently remains largely elusive.Monolayer group IV monochalcogenides represent a class of strongly coupled ferroelastic-ferroelectric 2D materials with highly anisotropic, strongly coupled, and externally switchable physical properties will engender a wide variety of ultrathin mechano-opto-electronic applications. Among them, GeSe is Ripples are a class of native structural defects widely existing in 2D materials. They originate from the out-of-plane flexibility of 2D materials introducing spatially evolving electronic structure and friction behavior. However, the effect of ripples on 2D ferroics has not been reported. Here a molecular dynamics study of the effect of ripples on the temperature-induced ferroic phase transition and stress-induced...

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Ferroic phase transformation in monolayer nanosheets or nanoribbons endows 2D nanoelectronic devices with novel functionalities. However, less is known how the phase transformation behaves with the system size. Combined with molecular dynamic simulations and a machine learning model, we systematically investigate the temperature induced ferroic phase transformation in monolayer GeSe nanoribbons, which exhibits remarkable size effect. Specifically, the transformation hysteresis is found continuously decreased with ribbon width at the investigated scales. In contrast, the transformation temperature of monolayer GeSe nanoribbons shows non-monotonic size-dependency, i.e., it is first increased and then decreased as we narrow the GeSe nanoribbons. We attribute this to a competition between the enhanced ripple deformation, which will promote phase transformation upon cooling, and the stronger edge effect that can suppress phase transformation. In addition, the two factors are well captured by the Landau model, which will deepen our understanding of phase transformation behaviors in 2D ferroic materials.