Unraveling Electronic Structure, Magnetic States, and Topological Phenomena in Pristine, Defected, and Strained Ti2N MXene

We unravel the evolution of structural, electronic, magnetic, and topological properties of graphene-like pristine, defected, and strained titanium nitride MXene with different functional groups (-F, -O, -H, and -OH) employing first-principles calculations. The formation and cohesive energies reveal their chemical stability. The MAX phase and defect free functionalized MXenes are metallic in nature except for oxygen terminated one, which is 100% spin polarized half-metallic. Additionally, the bare MXene is nearly half-metallic ferromagnet. The spin-orbit coupling significantly influences the bare MXene possessing band inversion. The strain effect sways the Fermi level thereby shifting it toward lower energy state under compression and toward higher energy state under tensile strain in Ti2NH2. These properties are reversed in Ti2N, Ti2NF2, and Ti2N(OH)2. The half-metallic nature changes to semi-metallic under 1% compression and is completely destroyed under 2% compression. In single vacancy defect, the band structure of Ti2NO2 remarkably transforms from half-metallic to semi-conducting (with large band gap of 1.73 eV) in 12.5% Ti, weakly semi-conducting in 5.5% Ti, and topological semi-metal in 12.5% oxygen. The 25% N defect changes the half-metallic to the metallic with certain topological features. Further, the 12.5% Co substitution in Ti2NO2 preserves the half-metallic character, whereas Mn substitution allows to convert half-metallic into weak semi-metallic preserving ferromagnetic character. However, Cr substitution converts half-metallic ferromagnetic to half-metallic anti-ferromagnetic state. The understanding made here on collective structural stability, and magnetic and topological phenomena in novel 2D MXenes open up their possibility in designing them for synthesis and thereby taking to applications.

Atomic Simulations of Packing Structures, Local Stress and Mechanical Properties for One Silicon Lattice with Single Vacancy on Heating

The effect of vacancy defects on the structure and mechanical properties of semiconductor silicon materials is of great significance to the development of novel microelectronic materials and the processes of semiconductor sensors. In this paper, molecular dynamics is used to simulate the atomic packing structure, local stress evolution and mechanical properties of a perfect lattice and silicon crystal with a single vacancy defect on heating. In addition, their influences on the change in Young’s modulus are also analyzed. The atomic simulations show that in the lower temperature range, the existence of vacancy defects reduces the Young’s modulus of the silicon lattice. With the increase in temperature, the local stress distribution of the atoms in the lattice changes due to the migration of the vacancy. At high temperatures, the Young’s modulus of the silicon lattice changes in anisotropic patterns. For the lattice with the vacancy, when the temperature is higher than 1500 K, the number and degree of distortion in the lattice increase significantly, the obvious single vacancy and its adjacent atoms contracting inward structure disappears and the defects in the lattice present complex patterns. By applying uniaxial tensile force, it can be found that the temperature has a significant effect on the elasticity–plasticity behaviors of the Si lattice with the vacancy.

Interfacial Strengthening of Graphene/Aluminum Composites through Point Defects: A First-Principles Study

The relationship between point defects and mechanical properties has not been fully understood yet from a theoretical perspective. This study systematically investigated how the Stone–Wales (SW) defect, the single vacancy (SV), and the double vacancy (DV) affect the mechanical properties of graphene/aluminum composites. The interfacial bonding energies containing the SW and DV defects were about twice that of the pristine graphene. Surprisingly, the interfacial bonding energy of the composites with single vacancy was almost four times that of without defect in graphene. These results indicate that point defects enhance the interfacial bonding strength significantly and thus improve the mechanical properties of graphene/aluminum composites, especially the SV defect. The differential charge density elucidates that the formation of strong Al–C covalent bonds at the defects is the most fundamental reason for improving the mechanical properties of graphene/aluminum composites. The theoretical research results show the defective graphene as the reinforcing phase is more promising to be used in the metal matrix composites, which will provide a novel design guideline for graphene reinforced metal matrix composites. Furthermore, the sp3-hybridized C dangling bonds increase the chemical activity of the SV graphene, making it possible for the SV graphene/aluminum composites to be used in the catalysis field.