Defects in Graphene/h-BN Planar Heterostructures: Insights into the Interfacial Thermal Transport Properties

Interlayer sp3 Bonds and Chirality at Bilayer Graphene Oxide/Calcium Silicate Hydrate Abnormally Enhance Its Interlayer Stress Transfer

Graphene oxide (GO) is an ideal reinforcing material with super design capability, which can achieve the combination of strength and toughness. However, the actual effect of GO is far below the theoretical prediction. This is mainly due to the weak interface between the nanofiller and the matrix. In this paper, a controllable method for improving interlayer stress transfer of double-layer graphene oxide/C-S-H (D-GO-CSH)-layered nanostructures is proposed by using interlayer sp3 bond and chirality. The results show that, compared with the control group, the normalized shear stress and normalized pull-out energy of the OH-sp3 model are increased by 44.93 and 49.25%, respectively, while those of the OO-sp3 model are increased by 32.26 and 31.03%, respectively. The interlayer sp3 bonds lead to a great enhancement (more than 3 times) in normalized interlayer stress transfer of D-GO-CSH-layered nanostructures while exerting a little opposite effect (about 5%). The improvement effects induced by the interlayer sp3 bonds are also strongly dependent on their distributions and the chirality of GO. According to the fracture mechanic theory and molecular dynamics results, the strain energy percentage difference (bond length and bond angle) of the zigzag-cen model is 34.8% lower than that of the control group model, which proves that the interlayer sp3 bonds have a remarkably positive effect on the interlayer stress transfer of D-GO-CSH-layered nanostructures. This provides a new way to further improve the interlayer stress transfer, pull-out energy, and interlayer shear stress of D-GO-CSH-layered nanostructures.

Prediction and Control of Thermal Transport at Defective State Gr/h-BN Heterojunction Interfaces

The control of interfacial thermal conductivity is the key to two-dimensional heterojunction in semiconductor devices. In this paper, by using non-equilibrium molecular dynamics (NEMD) simulations, we analyze the regulation of interfacial thermal energy transport in graphene (Gr)/hexagonal boron nitride (h-BN) heterojunctions and reveal the variation mechanism of interfacial thermal energy transport. The calculated results show that 2.16% atomic doping can effectively improve interfacial heat transport by more than 15.6%, which is attributed to the enhanced phonon coupling in the mid-frequency region (15-25 THz). The single vacancy in both N and B atoms can significantly reduce the interfacial thermal conductivity (ITC), and the ITC decreases linearly with the increase in vacancy defect concentration, mainly due to the single vacancy defects leading to an increased phonon participation rate (PPR) below 0.4 in the low-frequency region (0-13 THz), which shows the phonon the localization feature, which hinders the interfacial heat transport. Finally, a BP neural network algorithm is constructed using machine learning to achieve fast prediction of the ITC of Gr/h-BN two-dimensional heterogeneous structures, and the results show that the prediction error of the model is less than 2%, and the method will provide guidance and reference for the design and optimization of the ITC of more complex defect-state heterogeneous structures.

Adsorption of water on C sites vacancy defected graphene/h-BN: First-principles study

Heterostructures (HS), vacancy defects in HS, and molecular adsorption on defected HS of 2D materials are fervently inspected for a profusion of applications because of their aptness to form stacked layers that confer approach to an amalgamation of favorable electronic and magnetic properties. In this context, graphene (Gr), hexagonal boron nitride (h-BN), HS of graphene/h-BN (Gr/h-BN), and molecular adsorption on Gr/h-BN offer promising prospects for electronic, spintonic, and optoelectronic devices. In this study, we investigated the structural, electronic, and magnetic properties of C sites vacancy defects in Gr/h-BN HS and adsorption of water molecule on defected Gr/h-BN HS materials by using first-principles calculations based on spin-polarized density functional theory method within van der Waals (vdW) corrections DFT-D2 approach. We found that these considered materials are stable 2D vdW HS. Based on band structure calculations, they are semimetallic, and on density of states and partial density of states analysis, they are magnetic materials. The magnetic moment developed in these defected systems is due to the unpaired up-spin and down-spin states in the orbitals of atoms present in the materials created by the vacancy defects.

Temperature Dependence of Interfacial Bonding and Configuration Transition in Graphene/Hexagonal Boron Nitride Containing Grain Boundaries and Functional Groups

The quasi-three-dimensional effect induced by functional groups (FGo) and the in-plane stress and structural deformation induced by grain boundaries (GBs) may produce more novel physical effects. These physical effects are particularly significant in high-temperature environments and are different from the behavior in bulk materials, so its physical mechanism is worth exploring. Considering the external field (strain and temperature field), the internal field (FGo and GBs) and the effect of distance between FGs and GBs on the bonding energy, configuration transition, and stress distribution of graphene/h-BN with FGo and GBs (GrO-BN-GBs) in the interface region were studied by molecular dynamics (MD). The results show that the regions linked by hydroxyl + epoxy groups gradually change from honeycomb to diamond-like structures as a result of a hybridization transition from sp² to sp³. The built-in distortion stress field generated by the coupling effect of temperature and tension loading induces the local geometric buckling of two-dimensional materials, according the von Mises stresses and deflection theory. In addition, the internal (FGo and GBs) and external field (strain and temperature field) have a negative chain reaction on the mechanical properties of GrO-BN-GBs, and the negative chain reaction increases gradually with the increase in the distance between FGo and GBs. These physical effects are particularly obvious in high-temperature environments, and the behavior of physical effects in two-dimensional materials is different from that in bulk materials, so its physical mechanism is worth exploring.

Defects in graphene-based heterostructures: topological and geometrical effects

The combination of graphene (Gr) and graphene-like materials provides the possibility of using two-dimensional (2D) atomic layer building blocks to create unprecedented architectures. The most attractive characteristics are strongly dependent on the various spatial structures, mainly including in-plane heterostructures butt-joined at the side of an atomic monolayer through covalent bonds, van der Waals (vdW) heterostructures involving a vertically stacked hybrid structure, and their combinations. Heterostructures can not only overcome the limitations inherent to each material but may also obtain new features by appropriate material combination. However, heterostructures made of vdW force superposition or covalent bond splicing are prone to defects. The introduction of external and internal defects causes local deformation and stress in the material, thereby affecting the physical properties of the material, such as its transport properties and mechanical properties. Therefore, research, utilization and control of these defects are highly critical. This paper reviews the vacancy, topological and geometrical effects of defects in modulating the structures and mechanical responses of Gr-based heterostructures. Moreover, the coupling effects of various defects on the Gr-based heterostructures in multi-physics fields are also discussed. This work aims to improve the understanding of the physical mechanism of defective configurations and their association in low dimensions, so as to realize various configurations and to aid the search for new usages.

The combination of graphene (Gr) and graphene-like materials provides the possibility of using two-dimensional (2D) atomic layer building blocks to create unprecedented architectures.

An insight into thermal properties of BC3-graphene hetero-nanosheets: a molecular dynamics study

Simulation of thermal properties of graphene hetero-nanosheets is a key step in understanding their performance in nano-electronics where thermal loads and shocks are highly likely. Herein we combine graphene and boron-carbide nanosheets (BC3N) heterogeneous structures to obtain BC3N-graphene hetero-nanosheet (BC3GrHs) as a model semiconductor with tunable properties. Poor thermal properties of such heterostructures would curb their long-term practice. BC3GrHs may be imperfect with grain boundaries comprising non-hexagonal rings, heptagons, and pentagons as topological defects. Therefore, a realistic picture of the thermal properties of BC3GrHs necessitates consideration of grain boundaries of heptagon-pentagon defect pairs. Herein thermal properties of BC3GrHs with various defects were evaluated applying molecular dynamic (MD) simulation. First, temperature profiles along BC3GrHs interface with symmetric and asymmetric pentagon-heptagon pairs at 300 K, ΔT = 40 K, and zero strain were compared. Next, the effect of temperature, strain, and temperature gradient (ΔT) on Kaptiza resistance (interfacial thermal resistance at the grain boundary) was visualized. It was found that Kapitza resistance increases upon an increase of defect density in the grain boundary. Besides, among symmetric grain boundaries, 5-7-6-6 and 5-7-5-7 defect pairs showed the lowest (2 × 10-10 m2 K W-1) and highest (4.9 × 10-10 m2 K W-1) values of Kapitza resistance, respectively. Regarding parameters affecting Kapitza resistance, increased temperature and strain caused the rise and drop in Kaptiza thermal resistance, respectively. However, lengthier nanosheets had lower Kapitza thermal resistance. Moreover, changes in temperature gradient had a negligible effect on the Kapitza resistance.