Giant pyroelectricity in nanomembranes.

Pyroelectricity describes the generation of electricity by temporal temperature change in polar materials1-3. When free-standing pyroelectric materials approach the 2D crystalline limit, how pyroelectricity behaves remained largely unknown. Here, using three model pyroelectric materials whose bonding characters along the out-of-plane direction vary from van der Waals (In2Se3), quasi-van der Waals (CsBiNb2O7) to ionic/covalent (ZnO), we experimentally show the dimensionality effect on pyroelectricity and the relation between lattice dynamics and pyroelectricity. We find that, for all three materials, when the thickness of free-standing sheets becomes small, their pyroelectric coefficients increase rapidly. We show that the material with chemical bonds along the out-of-plane direction exhibits the greatest dimensionality effect. Experimental observations evidence the possible influence of changed phonon dynamics in crystals with reduced thickness on their pyroelectricity. Our findings should stimulate fundamental study on pyroelectricity in ultra-thin materials and inspire technological development for potential pyroelectric applications in thermal imaging and energy harvesting.

Phonon thermal conductivity of the stanene/hBN van der Waals heterostructure.

We use classical non-equilibrium molecular dynamics (NEMD) simulations to investigate the phonon thermal conductivity (PTC) of hexagonal boron nitride (hBN) supported stanene. At first, we examine the length dependent PTCs of bare stanene and hBN, and the stanene/hBN heterostructure and realize the dominance of the hBN layer to dictate the PTC in the heterostructure system. Afterward, we assess the length-independent bulk PTCs of these materials. The bulk PTCs at room temperature are found as ∼15.20 W m-1 K-1, ∼550 W m-1 K-1, and ∼232 W m-1 K-1 for bare stanene and hBN, and stanene/hBN, respectively. Moreover, our simulations reveal that bare stanene exhibits a substantially lower PTC compared to bare hBN, and the predicted PTC of stanene/hBN lies between those of stand-alone stanene and hBN. We also found that the PTC obtained for the stanene/hBN system from NEMD simulations nicely agrees with the theoretical formula developed to predict the PTC of heterostructures of two distinct materials. Temperature studies suggest that the PTC of the stanene/hBN heterostructure system follows a decreasing trend with increasing temperature. Additionally, corresponding phonon density of states (PDOS) and phonon dispersion data are provided to comprehensively understand the phonon properties of bare stanene and hBN, and stanene/hBN. Overall, this NEMD study would offer a deep understating towards the PTC of the stanene/hBN heterostructure and would widen the scope of its successful operations in future nanoelectronic, spintronic, and thermoelectric devices.

Nanomechanics of antimonene allotropes under tensile loading.

Monolayer antimonene has drawn the attention of research communities due to its promising physical properties. However, the mechanical properties of antimonene have remained largely unexplored. In this work, we investigate the mechanical properties and fracture mechanisms of two stable phases of monolayer antimonene - ß-antimonene (puckered structure) and α-antimonene (buckled structure) - through molecular dynamics (MD) simulations. Our simulations reveal that a stronger chiral effect results in a greater anisotropic elastic behavior in α-antimonene than in ß-antimonene. We focus on crack-tip stress distribution using local volume averaged virial stress definition and derive the fracture toughness from the crack-line stress. Our calculated crack tip stress distribution ensures the applicability of linear elastic fracture mechanics (LEFM) for cracked antimonene allotropes with considerable accuracy up to a pristine structure. We evaluate the effect of temperature, strain rate, crack-length, and point-defect concentration on the strength and elastic properties. The tensile strength of antimonene degrades significantly with the increase of temperature, crack length and defect concentration. The elastic modulus is found to be less susceptible to temperature variation but is largely affected by the increase in defects. The strain rate exhibits a power law relationship between strength and fracture strain. Finally, we discuss the fracture mechanisms in the light of crack propagation and establish the relationship between the fracture mechanism and the observed anisotropic properties.

Tuning the mechanical properties of functionally graded nickel and aluminium alloy at the nanoscale.

In this article, Molecular Dynamics (MD) simulation is used to investigate the tensile mechanical properties of functional graded Ni-Al (Ni3Al) alloy with Ni coating. The grading profile, temperature, crystallographic direction, and concentration of vacancy defects have been varied and corresponding changes in the tensile properties are reported. In general, it has been revealed that functional grading may reduce the ultimate tensile strength (UTS) of this homogeneous alloy but increase Young's modulus (YM). Furthermore, MD simulations suggest that elliptically graded Ni-Al alloy has the highest UTS at low temperature while at high temperature, the largest UTS is recorded for the parabolic grading. Besides, at any temperature, the parabolically graded Ni-Al alloy shows the largest YM, followed by linear grading and elliptical grading. Moreover, it is also observed that the [111] crystallographic direction for this alloy demonstrates the highest UTS and YM. At extremely low temperatures, lattice mismatch is also observed to exert a significant impact on the failure characteristics of functional graded Ni-Al alloys. This investigation also suggests that the vacancy defects introduced via removing either Al or Ni atoms degrades the UTS and YM of FGM alloys remarkably. Besides, it is also found that the UTS and YM of Ni-Al FGM alloys are very sensitive to Ni vacancies compared to Al vacancies. Parabolic grading demonstrates more resilience against vacancy defects, followed by linear and elliptical grading. This paper provides a comprehensive understanding of the mechanical properties of Ni-Al FGM alloys at the atomic level as a potential substitute for homogeneous alloys.

Atomic-scale analysis of the physical strength and phonon transport mechanisms of monolayer ß-bismuthene.

Bismuthene has opened up a new avenue in the field of nanotechnology because of its spectacular electronic and thermoelectric features. The strong spin-orbit-coupling enables its operation as the largest nontrivial bandgap topological insulator and quantum spin hall material at room temperature, which is unlikely for any other 2D material. It is also known to be the most promising thermoelectric material due to its remarkable thermoelectric properties, including a substantially high power factor. However, an in-depth understanding of the mechanical and thermal transport properties of bismuthene is crucial for its practical implementation and efficient operation. Employing the Stillinger-Weber potential, we utilized molecular dynamics simulations to inspect the mechanical strength and thermal conductivity of the monolayer ß-bismuthene for the first time. We analyzed the effect of temperature on the tensile mechanical properties along the armchair and zigzag directions of bismuthene nanosheets and found that increasing temperature causes a significant deterioration in these properties. The material shows superior fracture resistance with zigzag loading, whereas the armchair direction exhibits an improved elasticity. Next, we showed that increasing vacancy concentration and crack length notably reduce the fracture stress and strain of ß-bismuthene. Under all these conditions, ß-bismuthene showed a strong chirality effect under tensile loading. We also explored the fracture phenomena of a pre-cracked ß-bismuthene, which reveal that the armchair-directed crack possesses a higher fracture resistance than the zigzag-directed crack. Interestingly, branching phenomena occurred during crack propagation for the armchair crack; meanwhile, the crack propagates perpendicular to loading for the zigzag crack. Afterward, we investigated the effect of loading rate on the fracture properties of bismuthene along the armchair and zigzag directions. Finally, we calculated the thermal conductivity of bismuthene under the influence of temperature and vacancy and recorded a substantial decrement in thermal conductivity with increasing temperature and vacancy. The obtained results are comprehensively discussed in the light of phonon density of states, phonon dispersion spectrum, and phonon group velocities. It is also disclosed that the thermal conductivity of ß-bismuthene is considerably lower than that of other analogous honeycomb structures. This study can add a new dimension to the successful realization of bismuthene in future (opto)electronic, spintronic, and thermoelectric devices.

Investigation on the mechanical properties and fracture phenomenon of silicon doped graphene by molecular dynamics simulation.

Silicon doping is an effective way to modulate the bandgap of graphene that might open the door for graphene to the semiconductor industries. However, the mechanical properties of silicon doped graphene (SiG) also plays an important role to realize its full potential application in the electronics industry. Electronic and optical properties of silicon doped graphene are well studied, but, our understanding of mechanical and fracture properties of the doped structure is still in its infancy. In this study, molecular dynamics (MD) simulations are conducted to investigate the tensile properties of SiG by varying the concentration of silicon. It is found that as the concentration of silicon increases, both fracture stress and strain of graphene reduces substantially. Our MD results also suggest that only 5% of silicon doping can reduce the Young's modulus of graphene by ∼15.5% along the armchair direction and ∼13.5% along the zigzag direction. Tensile properties of silicon doped graphene have been compared with boron and nitrogen doped graphene. The effect of temperature, defects and crack length on the stress-strain behavior of SiG has also been investigated. Temperature studies reveal that SiG is less sensitive to temperature compared to free stranding graphene, additionally, increasing temperature causes deterioration of both fracture stress and strain of SiG. Both defects and cracks reduce the fracture stress and fracture strain of SiG remarkably, but the sensitivity to defects and cracks for SiG is larger compared to graphene. Fracture toughness of pre-cracked SiG has been investigated and results from MD simulations are compared with Griffith's theory. It has been found that for nano-cracks, SiG with larger crack length deviates more from Griffith's criterion and the degree of deviation is larger compared to graphene. Fracture phenomenon of pre-cracked SiG and the effect of strain rate on the tensile properties of SiG have been reported as well. These results will aid the design of SiG based semiconducting nanodevices.

Inherent mechanical properties of bilayer germanene coupled by covalent bonding.

Germanene, a two-dimensional buckled hexagonal structure of germanium atoms, has attractive mechanical, optical, thermal and electronic features. Recently it has been reported that covalent bonding between two monolayer germanene sheets leads to the integration of intrinsic magnetism and band gap opening that makes it attractive to future nanoelectronics. In order to use the captivating features of this structure, its mechanical characterization needs to be studied. In this study, molecular dynamics simulations have been performed using optimized Tersoff potential to analyze the effect of chirality, temperature and strain rate on the uniaxial tensile properties of this structure. This study suggests that bonded bilayer germanene shows higher mechanical strength compared to monolayer germanene. Uniaxial loading in the armchair direction shows higher fracture strength and strain compared to the zigzag direction which is contrary to the monolayer germanene. It also reports that with increasing temperature, both the fracture strength and strain of the structure decrease. It has been found that at a higher strain rate, the material exhibits higher fracture strength and strain. Mechanical properties and fracture mechanisms of defected structures have also been reported below the curie temperature. Moreover, the interlayer shear characteristics of the bilayer structure have been looked into. These results will provide significant insight to the investigation of this structure as a potential nano-electronics substitute.