Heterostrain and temperature-tuned twist between graphene/h-BN bilayers

Universal scaling laws on the rotational energy landscape for twisted van der Waals bilayers

The emerging field of twistronics utilizes the interfacial twist angle between two-dimensional materials to design and explore unconventional electronic properties. However, recent investigations revealed that not every twist angle is stable. Understanding and predicting preferred twist angles are therefore of vital importance and have received considerable attention; however, general analytical theories that can feasibly address the stability of twist angles have not yet been developed. In this work, we reveal the existence of universal analytical scaling laws that delineate the interface rotational energy landscape, enabling the determination of both stable angles and interlayer rotational torque. The universality of our theoretical results is fundamentally based on the evolution of moiré geometry, which is applicable across many material interface systems. Our results not only unify experimental observations and literature atomistic simulations, but also provide new perspectives for the rational design of nanoscale rotation-tunable electronic devices. Our theories can potentially inspire a deeper understanding of moiré-correlated interface mechanics.

Double-Walled Carbon Nanotubes with Dynamic Strength of over 90 GPa Enhanced by Intershell Friction

Low-dimensional ultra-strong nanomaterials have attracted great anticipation for applications under extreme dynamic conditions. A photocatalytic method is developed to selectively cut off the outer shell of double-walled carbon nanotubes (DWCNTs), achieving non-contact measurement of intershell friction with both high temporal and spatial resolutions at high sliding velocities under optical microscope. The intershell friction linearly increases with the sliding velocity, with a slope related to intershell distance and chirality of DWCNTs. The maximum measured friction reaches 194.1 ± 7.3 nN at a sliding velocity of 977 mm s-1, a value comparable to the tensile force (≈450 nN) for breaking the outer shell. Molecular dynamics simulations indicate that the velocity-dependent intershell friction is related to dynamic localized commensurate contacts. The friction-induced "intershell locking" enhances the effective dynamic strength of DWCNTs from 64.8 ± 3.4 GPa to 90.1 ± 4.0 GPa at a tensile strain rate of 3300 s-1. This study reveals anomalous friction mechanisms at nanoscale and demonstrates promising application of DWCNTs as ultra-strong materials.

A new approach in generating stable crack propagation at twisted bilayer graphene/hBN heterostructures

Thermodynamic theory suggests that the obvious mechanical behavior caused by temperature and interlayer angle will affect the physical properties of materials, such as mechanical properties and transportation behavior, and it is different from the behavior in three-dimensional bulk materials. We observe an abnormal physical effect of bilayer graphene/hexagonal boron nitride (G/BN)-carbon nanotube (CNT) heterostructures, with a normalized out-of-plane deformation and normalized bond angle percentage to almost several times higher those of pristine G/BN heterostructures (without CNT) at 700-800 K. Our combined finite element theory and molecular dynamics simulations confirmed that the combination of CNT and interlayer angle diverted and bridged the propagating crack and provided a stable crack propagation path and crack tip opening displacement, resulting in the stress fields to be controlled around the CNT at high temperature. It offers an ideal design for two-dimensional (2D) materials that can maintain exceptional mechanical properties in flexible device applications.

Carbon materials and their metal composites for biomedical applications: A short review

Carbon materials and their hybrid metal composites have garnered significant attention in biomedical applications due to their exceptional biocompatibility. This biocompatibility arises from their inherent chemical stability and low toxicity within biological systems. This review offers a comprehensive overview of carbon nanomaterials and their metal composites, emphasizing their biocompatibility-focused applications, including drug delivery, bioimaging, biosensing, and tissue engineering. The paper outlines advancements in surface modifications, coatings, and functionalization techniques designed to enhance the biocompatibility of carbon materials, ensuring minimal adverse effects in biological systems. A comprehensive investigation into hybrid composites integrating carbon nanomaterials is conducted, categorizing them as fullerenes, carbon quantum dots, carbon nanotubes, carbon nanofibers, graphene, and diamond-like carbon. The concluding section addresses regulatory considerations and challenges associated with integrating carbon materials into medical devices. This review culminates by providing insights into current achievements, challenges, and future directions, underscoring the pivotal role of carbon nanomaterials and their metal composites in advancing biocompatible applications.

Mechanism of Controllable Growth of Large-Area Single-Crystal Hexagonal Boron Nitride on Preoxidized Copper Substrate

Two-dimensional (2D) hexagonal boron nitride (h-BN) exhibits promising properties for electronic and photoelectric devices, while the growth of high-quality h-BN remains challenging. Here we theoretically explored the mechanism of epitaxial growth of high-quality h-BN by using the preoxidized and hydrogen-annealed copper substrate, i.e., Cu2O. It is revealed thermodynamically that the unidirectional nucleation of h-BN can be rationalized on the symmetry-matched Cu2O(111) surface rather than the antiparallel nucleation on the Cu(111) surface. Kinetically, the dehydrogenation of feedstock of h-BN on the Cu2O(111) surface is also much easier than that on the Cu(111) surface. Both the B and N atoms are energetically more preferred to stay on the surface rather than inside the body of Cu2O, which leads to a surface-diffusion-based growth behavior on the Cu2O(111) surface instead of the precipitation-diffusion mixed case on the Cu(111) surface. Our work may guide future experimental design for the controllable growth of wafer-scale single-crystal h-BN.

Emerging Characteristics and Properties of Moiré Materials

In recent years, scientists have conducted extensive research on Moiré materials and have discovered some compelling properties. The Moiré superlattice allows superconductivity through flat-band and strong correlation effects. The presence of flat bands causes the Moiré material to exhibit topological properties as well. Modulating electronic interactions with magnetic fields in Moiré materials enables the fractional quantum Hall effect. In addition, Moiré materials have ferromagnetic and antiferromagnetic properties. By tuning the interlayer coupling and spin interactions of the Moiré superlattice, different magnetic properties can be achieved. Finally, this review also discusses the applications of Moiré materials in the fields of photocurrent, superconductivity, and thermoelectricity. Overall, Moiré superlattices provide a new dimension in the development of two-dimensional materials.

One-Dimensional Moiré Physics and Chemistry in Heterostrained Bilayer Graphene

Twisted bilayer graphene (tBLG) has emerged as a promising platform for exploring exotic electronic phases. However, the formation of moiré patterns in tBLG has thus far been confined to the introduction of twist angles between the layers. Here, we propose heterostrained bilayer graphene (hBLG), as an alternative avenue for accessing twist angle-free moiré physics via lattice mismatch. Using atomistic and first-principles calculations, we demonstrate that the uniaxial heterostrain can promote isolated flat electronic bands around the Fermi level. Furthermore, the heterostrain-induced out-of-plane lattice relaxation may lead to a spatially modulated reactivity of the surface layer, paving the way for moiré-driven chemistry and magnetism. We anticipate that our findings can be readily generalized to other layered materials.

Superlubricity of Materials: Progress, Potential, and Challenges

This review paper provides a comprehensive overview of the phenomenon of superlubricity, its associated material characteristics, and its potential applications. Superlubricity, the state of near-zero friction between two surfaces, presents significant potential for enhancing the efficiency of mechanical systems, thus attracting significant attention in both academic and industrial realms. We explore the atomic/molecular structures that enable this characteristic and discuss notable superlubric materials, including graphite, diamond-like carbon, and advanced engineering composites. The review further elaborates on the methods of achieving superlubricity at both nanoscale and macroscale levels, highlighting the influence of environmental conditions. We also discuss superlubricity's applications, ranging from mechanical systems to energy conservation and biomedical applications. Despite the promising potential, the realization of superlubricity is laden with challenges. We address these technical difficulties, specifically those related to achieving and maintaining superlubricity, and the issues encountered in scaling up for industrial applications. The paper also underscores the sustainability concerns associated with superlubricity and proposes potential solutions. We conclude with a discussion of the possible future research directions and the impact of technological innovations in this field. This review thus provides a valuable resource for researchers and industry professionals engaged in the development and application of superlubric materials.