Insight into the stacking and the species-ordering dependences of interlayer bonding in SiC/GeC polar heterostructures

Two-dimensional (2D) polar materials experience an in-plane charge transfer between different elements due to their electron negativities. When they form vertical heterostructures, the electrostatic force triggered by such charge transfer plays an important role in the interlayer bonding beyond van der Waals (vdW) interaction. Our comprehensive first principle study on the structural stability of the 2D SiC/GeC hybrid bilayer heterostructure has found that the electrostatic interlayer interaction can induce the π-π orbital hybridization between adjacent layers under different stacking and out-of-plane species ordering, with strong hybridization in the cases of Si-C and C-Ge species orderings but weak hybridization in the case of the C-C ordering. In particular, the attractive electrostatic interlayer interaction in the cases of Si-C and C-Ge species orderings mainly controls the equilibrium interlayer distance and the vdW interaction makes the system attain a lower binding energy. On the contrary, the vdW interaction mostly controls the equilibrium interlayer distance in the case of the C-C species ordering and the repulsive electrostatic interlayer force has less effect. Interesting finding is that the band structure of the SiC/GeC hybrid bilayer is sensitive to the layer-layer stacking and the out-of-plane species ordering. An indirect band gap of 2.76 eV (or 2.48 eV) was found under the AA stacking with Si-C ordering (or under the AB stacking with C-C ordering). While a direct band gap of 2.00 eV – 2.88 eV was found under other stacking and species orderings, demonstrating its band gap tunable feature. Furthermore, there is a charge redistribution in the interfacial region leading to a built-in electric field. Such field will separate the photo-generated charge carriers in different layers and is expected to reduce the probability of carrier recombination, and eventually give rise to the electron tunneling between layers.

Layer-dependent Fracture Strength of Few-layer WS2 Induced by Interlayer Sliding: A Molecular Dynamics Study

Understanding the relationship of interlayer interaction with mechanical properties and behaviors of two-dimensional layered materials (2DLMs) is critical in favoring the development of related nanodevices, nevertheless, still challenging due to the difficulties in experiments. In this work, nanoindentation simulations on few-layer WS2 were conducted by varying tip radius, suspended membrane radius and membrane size using molecular dynamics simulation. Consistent with our previous experimental results, few-layer WS2 exhibited layer-dependent reduction in fracture strength owing to the uneven stress distribution among individual layer induced by interlayer sliding under out-of-plane deformation. Besides, apparent curve hysteresis was observed due to interlayer sliding in the supported region when large tip radius and membrane radius were employed. However, instead of the supported part, the interlayer sliding within the suspended part resulted in the reduced fracture strength with the increase of layer number. These findings not only provide an in-depth comprehension on the influence of interlayer sliding on the fracture strength of few-layer WS2, but also suggest that the role of interlayer interaction should be seriously considered during nanodevice design.

Flexible Spin Valves: Interlayer Interaction and Deformation Sensitivity

Exchange-coupled spin valves based on ferromagnetic alloys CoFeNi and antiferromagnetic alloy FeMn are obtained on flexible polyimide substrates by magnetron sputtering. The magnetoresistive properties of films and microstrips of spin valves are measured at various degrees of bending deformation of the sample. The behavior of the dependence of the deformation sensitivity of the spin valve on the interaction between the magnetic layers and on the arrangement of the anisotropy axes with respect to the deformation vector is characterized. It is found that the deformation sensitivity decreases with an increase in the interval between the fields of magnetization reversal of the free and fixed layers in the spin valve.