查看更多>>摘要：Motivated by the successful preparations and excellent properties of Janus monolayers, we construct the van der Waals (vdW) heterostructures HfSe2/Ga2SeS, and HfSeS/GaSe. Their electronic features and the type of band alignment are investigated. We find that the HfSe2/Ga2SeS exhibits the similar electronic properties to HfSe2/GaSe. Both heterostructures own type-I band alignment, in which both valence band maximum (VBM) and conduction band minimum (CBM) are provided by HfSe2 layer. However, the HfSeS/GaSe heterostructure owns type-II band alignment, in which the electrons and holes are located within the HfSeS layer and the layer of GaSe, respectively. This means that the HfSeS/GaSe heterostructure can facilitate the effective separation of the photogenerated hole and electron pairs. The band offsets of valence bands are 0.09 eV, 0.26 eV and 0.17 eV for HfSe2/GaSe, HfSe2/Ga2SeS, and HfSeS/GaSe heterostructures. The small band offsets indicate the band alignment can easily be modulated by some external means. Thus, the external electric field and interlayer coupling effect are considered to modulate the electronic properties of those heterostructures. They can simultaneously tune the band gap and induce the band alignment transition between type-I and type-II. These results indicate that HfSe2/GaSe and Janus HfSe2/GaSe heterostructures have great applications in nanoelectronic device.
查看更多>>摘要：Helical graphenes (HGs) have attracted much attention due to its excellent electrical and mechanical properties, and provide an ideal nanofiller for the preparation of novel functional epoxy resin matrix composites. The mechanical properties of HGs/epoxy resin composites (marked as HGs/epoxy) are investigated by molecular dynamics (MD) simulations and compared with those of pure epoxy resin systems (referred to as epoxy). The results show that HGs/epoxy systems have higher Young's modulus and yield strength than pure epoxy systems with the same crosslinking degree. Interestingly, the strengthening effect of HGs on the epoxy systems with low crosslinking degree is more obvious. The Young's modulus of the HGs/epoxy system with 40% crosslinking degree increase by 55.73%, 53.82% and 48.61% along the X, Y, Z directions, respectively. For the HGs/epoxy system with 60% crosslinking degree, the yield strengths increase by 36.64%, 26.69% and 43.39% along three directions, respectively. The enhancement of the overall mechanical properties for the HGs/epoxy systems is mainly attributed to the entanglement between molecular chains and HGs. Especially when the molecular chains are wound around adjacent coils of HGs, the binding energy between the matrix and reinforcement is greater, and the system exhibits better mechanical properties and more obvious mechanical anisotropy. These results provide the theoretical support for the preparation of novel epoxy resin matrix composites.
查看更多>>摘要：The recently synthesised one-dimensional (1D) double-helix nanofibers that are self-assembled from negatively curved nanographenes, i.e., 1-H nanofibers are expected to have new applications in materials science and biology. In this paper, the mechanical behaviours of 1-H nanofibers are investigated by using molecular dy-namics simulations. 1-H nanofibers are found to possess a Young's modulus comparable to that of conventional cup-stacked carbon nanofibers. Meanwhile, a large plastic deformation is observed in the axially stretched 1-H nanofibers before their final breaking. In addition, 1-H nanofibers possess a bending stiffness significantly larger than that of many other 1D nanomaterials, making 1-H nanofibers behave more like an elastic rod or beam. More importantly, a unique compression-torsion coupling behaviour is observed in 1-H nanofibers, which is strongly dependent on the rotation direction. Specifically, when 1-H nanofibers are rotated along the helix direction, a residual tensile stress or, equivalently, an axial contraction is generated. However, no significant compression-torsion coupling behaviour is observed in 1-H nanofibers rotated along the direction reverse to helix. Further-more, due to the compression-torsion coupling effect, a unique helical buckling behaviour can occur in 1-H nanofibers under pure axial compression, which is totally different from the conventional Euler buckling behaviour observed in many other 1D nanomaterials.
NSTL
Elsevier