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Two-dimensional lateral heterostructures exhibit novel electronic and optical properties that are induced by their in-plane interface for which the mechanical properties of the interface are important for the stability of the lateral heterostructure. Therefore, we performed molecular dynamics simulations and developed a convolutional neural network-based machine learning model to study the fracture properties of the interface in a graphene/hexagonal boron nitride lateral heterostructure. The molecular dynamics (MD) simulations show that the shape of the interface can cause an 80% difference in the fracture stress and the fracture strain for the interface. By using 11 500 training samples obtained with help of high-cost MD simulation, the machine learning model is able to search out the strongest interfaces with the largest fracture strain and fracture stress in a large sample space with over 150 000 structures. By analyzing the atomic configuration of these strongest interfaces, we disclose two major factors dominating the interface strength, including the interface roughness and the strength of the chemical bond across the interface. We also explore the correlation between the fracture properties and the thermal conductivity for these lateral heterostructures by examining the bond type and the shape of the graphene/hexagonal boron nitride interface. We find that interfaces comprised of stronger bonds and smoother zigzag interfaces can relieve the abrupt change of the acoustic velocity, leading to the enhancement of the interface thermal conductivity. These findings will be valuable for the application of the two-dimensional lateral heterostructure in electronic devices. [ABSTRACT FROM AUTHOR]

Group theory is a powerful tool to explore fundamental symmetry constraints for the physical properties of crystal structures, e.g. it is well-known that only a few components of the elastic constants are independent due to the symmetry constraint. This work further applies group theory to derive constraint relationships for high-order elastic constants with respect to the orientation angle, where the constraint relationships are more explicit than the traditional tensor transformation law. These analytic symmetry constraints are adopted to explain the molecular dynamics simulation results, which disclose that the high-order elastic constants are highly anisotropic with an anisotropy percentage of up to 25% for the hexagonal boron nitride monolayer. The elastic constant is a basic quantity in the mechanics field, so its high anisotropy shall cause strong anisotropy for other mechanical properties. Based on the anisotropic high-order elastic constants, we demonstrate that Poisson's ratio is highly anisotropic for the hexagonal boron nitride at large strains. These findings provide fundamental insights into the symmetry dependence of high-order elastic constants and other mechanical properties. [ABSTRACT FROM AUTHOR]

In this study, we conducted molecular dynamic simulations to investigate the thermal expansion behavior of Janus MoSSe nanotubes. We focused on understanding how the intrinsic strain in these nanotubes affects their thermal expansion coefficient (TEC). Interestingly, we found that Janus MoSSe nanotubes with sulfur (S) on the outer surface (MoSeS) exhibit a different intrinsic strain compared to those with selenium (Se) on the outer surface (MoSSe). In light of this observation, we explored the influence of this intrinsic strain on the TEC of the nanotubes. Our results revealed distinct trends for the TEC along the radial direction (TEC- r) and the axial direction (TEC- l x ) of the MoSSe and MoSeS nanotubes. The TEC- r of MoSeS nanotubes was found to be significantly greater than that of MoSSe nanotubes. Moreover, the TEC- l x of MoSeS nanotubes was smaller than that of MoSSe nanotubes. Further analysis showed that the TEC- r of MoSeS nanotubes decreased by up to 37% as the radius increased, while that of MoSSe nanotubes exhibited a slight increase with increasing radius. On the other hand, the TEC- l x of MoSeS nanotubes increased by as much as 45% with increasing radius, whereas that of MoSSe nanotubes decreased gradually. These opposite tendencies of the TECs with respect to the radius were attributed to the presence of intrinsic strain within the nanotubes. The intrinsic strain was found to play a crucial role in inducing thermally induced bending and elliptization of the nanotubes' cross-section. These effects are considered key mechanisms through which intrinsic strain influences the TEC. Overall, our study provides valuable insights into the thermal stability of Janus nanotubes. By understanding the relationship between intrinsic strain and the thermal expansion behavior of nanotubes, we contribute to the broader understanding of these materials and their potential applications. [ABSTRACT FROM AUTHOR]

This paper proposes a Whipple structure to enhance the impact resistance of graphene/aluminum alloy composites by varying the interlayer spacing between graphene and aluminum alloy. The increased interlayer spacing provides more deformation space for the graphene to absorb more deformation energy, and enables the formation of a debris cloud from the bullet fragments and graphene fragments, significantly reducing the impact energy per unit area of the next material. The impact limit serves as a critical metric for assessing the impact resistance of the Whipple structure. Based on molecular dynamics simulations, we developed a machine learning model to predict the protection of aluminum alloy, and quickly determined the impact limits of velocity, bullet radius, and interlayer spacing by using the machine learning model. An empirical equation for the impact limit of interlayer spacing was established. The results showed that non-zero interlayer spacing can significantly improve the impact resistance of the hybrid structure; to fully exploit the superior impact resistance of this Whipple structure, the number of graphene layers should be at least 3. Furthermore, at high impact velocities and large bullet radii, the impact limit of the interlayer spacing exhibits a substantial correlation with the number of graphene layers. These results provide valuable information for the design of the impact resistance of the graphene/aluminum alloy composites. [ABSTRACT FROM AUTHOR]

The heat transport properties of van der Waals layered structures are crucial for ensuring the reliability and longevity of high-performance optoelectronic equipment. Owing to the two-dimensional nature of atomic layers, the presence of bubbles is commonly observed within these structures. Nevertheless, the effect of bubbles on the interfacial thermal conductance remains unclear. Based on the elastic membrane theory and the improved van der Waals gas state equation, we develop an analytical formula to describe the influence of bubble shape on the interfacial thermal conductance. It shows that the presence of bubbles has a considerable impact on reducing the interfacial thermal conductance across graphene/graphene interfaces. More specifically, the presence of nanobubbles can result in a reduction of up to 53% in the interfacial thermal conductance. The validity of the analytical predictions is confirmed through molecular dynamic simulations. These results offer valuable insights into the thermal management of van der Waals layered structures in the application of next-generation electronic nanodevices. [ABSTRACT FROM AUTHOR]

Exploration of novel carbon allotropes has been a central subject in materials science, in which carbon peapods hold great potential as a precursor for the development of new carbon allotropes. To enable precise large-scale molecular dynamics simulations, we develop a high-accurate and low-cost machine-learned potential (MLP) for carbon materials using the neuroevolution potential framework. Based on the MLP, we conduct an investigation into the structural transitions of peapod arrays under high-temperature and high-pressure conditions and disclose the impact of vacancy defects. Defects promote the transition from the ordered crystalline structure to the disordered amorphous structure in peapods at low temperatures, while impeding the transition to the ordered diamond structure. Benefiting from the accurate MLP, we are able to reproduce the experimentally observed carbon structures in numerical simulations. We build a diagram summarizing all the structures that appear in the compression simulation of peapod arrays at various temperatures. The present work not only discloses the underlying mechanism of structural transitions from carbon peapods into various functional carbon materials, but also provides a high-accurate and low-cost interatomic potential that shall be valuable in the exploration of novel carbon allotropes. [ABSTRACT FROM AUTHOR]

Because of their advanced properties inherited from their constituent atomic layers, van der Waals heterostructures such as graphene/MoS2 are promising candidates for many optical and electronic applications. However, because heat tends to be generated during the operation of nanodevices, thermal expansion is an important phenomenon to consider for the thermal stability of such heterostructures. In the present work, molecular dynamics simulations are used to investigate the thermal expansion coefficient of the graphene/MoS2 heterostructure, and how the unavoidable misfit strain affects that coefficient is revealed. The misfit strain can tune the thermal expansion coefficient by a factor of six, and this effect is quite robust in the sense that it is insensitive to the size or direction of the heterostructure. Further analysis shows that the misfit strain offers an efficient means of engineering thermally induced ripples, this being the key mechanism for how the misfit strain affects the thermal expansion coefficient. These findings provide valuable information about the thermal stability of van der Waals heterostructures and offer help for practical applications of nanodevices based on such heterostructures. [ABSTRACT FROM AUTHOR]

Van der Waals heterostructures inherit many novel electronic and optical properties from their constituent atomic layers. Mechanical stability is key for realizing high-performance nanodevices based on van der Waals heterostructures. However, buckling instability is a critical mechanical issue for heterostructures associated with its two-dimensional nature. Using molecular dynamics simulations of graphene/MoS2 heterostructures, we demonstrate the relationship between buckling instability and the misfit strain that arises inevitably in such heterostructures. The impact of misfit strain on buckling depends on its magnitude: (1) A negative misfit strain causes a pre-compression of the graphene layer, which in turn initiates and accelerates buckling in this layer and reduces the buckling stability in the heterostructure as a whole. (2) A small positive misfit strain enhances the buckling stability of the graphene/MoS2 heterostructure by pre-stretching and hence decelerating the buckling of the graphene layer (where heterostructure buckling is initiated). (3) In the case of a large positive misfit strain, the graphene layer is pre-stretched while the MoS2 layer is significantly pre-compressed, so that heterostructure buckling is initiated by the MoS2 layer. Consequently, the buckling stability of the graphene/MoS2 heterostructure is reduced by increasing the large positive misfit strain. These findings are valuable for understanding the mechanical properties of van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Because of their advanced properties inherited from their constituent atomic layers, van der Waals heterostructures such as graphene/MoS 2 are promising candidates for many optical and electronic applications. However, because heat tends to be generated during the operation of nanodevices, thermal expansion is an important phenomenon to consider for the thermal stability of such heterostructures. In the present work, molecular dynamics simulations are used to investigate the thermal expansion coefficient of the graphene/MoS 2 heterostructure, and how the unavoidable misfit strain affects that coefficient is revealed. The misfit strain can tune the thermal expansion coefficient by a factor of six, and this effect is quite robust in the sense that it is insensitive to the size or direction of the heterostructure. Further analysis shows that the misfit strain offers an efficient means of engineering thermally induced ripples, this being the key mechanism for how the misfit strain affects the thermal expansion coefficient. These findings provide valuable information about the thermal stability of van der Waals heterostructures and offer help for practical applications of nanodevices based on such heterostructures.

Forming heterostructures is a well-established technique to utilize different constituent materials to achieve novel properties like efficient light emission and high-quality electron tunneling. Recent experiments have successfully synthesized one-dimensional van der Waals heterostructures and have discovered plenty of superior properties benefiting from the dimension reduction. Inspired by the success of the van der Waals counterparts, we propose a one-dimensional lateral heterostructure based on transition metal dichalcogenide nanotubes. Molecular simulations show that the misfit strain is restricted to the radial direction due to the one-dimensional tubular confined structure, and the regular exponential distribution of the radial misfit strain can be well interpreted by a mechanics model. Besides the normal exponential distribution, there also exists an abnormal strain distribution within a narrow domain nearby the interface, in which the structure of the larger lattice constant is stretched instead of compressed by the misfit strain. The abnormal misfit strain is due to the interplay between several bending interactions and the stretching interaction. Possible experiments to synthesize this new type of heterostructure are discussed based on current experimental techniques.

Van der Waals heterostructures inherit many novel electronic and optical properties from their constituent atomic layers. Mechanical stability is key for realizing high-performance nanodevices based on van der Waals heterostructures. However, buckling instability is a critical mechanical issue for heterostructures associated with its two-dimensional nature. Using molecular dynamics simulations of graphene/MoS 2 heterostructures, we demonstrate the relationship between buckling instability and the misfit strain that arises inevitably in such heterostructures. The impact of misfit strain on buckling depends on its magnitude: (1) A negative misfit strain causes a pre-compression of the graphene layer, which in turn initiates and accelerates buckling in this layer and reduces the buckling stability in the heterostructure as a whole. (2) A small positive misfit strain enhances the buckling stability of the graphene/MoS 2 heterostructure by pre-stretching and hence decelerating the buckling of the graphene layer (where heterostructure buckling is initiated). (3) In the case of a large positive misfit strain, the graphene layer is pre-stretched while the MoS 2 layer is significantly pre-compressed, so that heterostructure buckling is initiated by the MoS 2 layer. Consequently, the buckling stability of the graphene/MoS 2 heterostructure is reduced by increasing the large positive misfit strain. These findings are valuable for understanding the mechanical properties of van der Waals heterostructures., (© 2021 IOP Publishing Ltd.)