Since the advent of graphene, two-dimensional nanomaterials have gained wide attention due to their excellent properties. In 2011, Drexel University proposed a new type of two-dimensional nanomaterial MXene, which was obtained by hydrofluoric acid stripping phase layer material M n+1 AX n phase. Here, M is a pre-transition metal element, A means a group A element, such as Al, Si, etc., X is C or N, and n is a natural number 1-3. Due to etching in a solution such as hydrofluoric acid, the surface of MXene is often covered by functional groups such as O, F, and OH. Up to now, more than 20 kinds of MXene materials have been successfully synthesized, showing application prospects in many fields such as energy storage, electromagnetic shielding, catalysis, sensors, and nuclides adsorption.
Sc 2 CT x MXene (T x represents a surface functional group) contains the lightest pre-transition metal Sc, although it has not been synthesized experimentally, but a large amount of theoretical work has shown that Sc 2 CTx MXene has various excellent properties. For example, Sc 2 C(OH) 2 is a direct band gap semiconductor with ultra-high electron mobility (about 2.0×10 3 cm 2 V -1 s -1 ) and an ultra-low work function (about 1.6 eV). It is an excellent optical and semiconductor electronic device material. Sc 2 CO 2 is an indirect bandgap semiconductor, and its indirect band gap can be adjusted to a direct band gap under external strain and electric field. In addition, the material system is also the only structure with eigen dipole moment in the currently studied MXene system, which can be used as a ferroelectric material. The structure and electron band diagrams of Sc 2 C(OH) 2 and Sc 2 CO 2 are shown in Fig. 1. Sc 2 C(OH) 2 is mainly composed of ionic bonds, and the space group number of the structure is 164. The Sc 2 CO 2 intermediate layer carbon atom and one side oxygen atom have partial covalent bonds, and the space group number is 156. The existing experimental work shows that the oxygen-functionalized MXene structure is mostly obtained by high temperature dehydrogenation of hydroxyl groups. To study how Sc 2 C(OH) 2 is dehydrogenated into Sc 2 CO 2 structure, and how its direct band gap is transformed into the indirect band gap of Sc 2 CO 2 , for the practical application of basic physical materials science and Sc-based MXene materials. Both have important significance.
Based on the theory of first-principles theory, the research team of the Nuclear Energy Materials Engineering Laboratory of the Institute of Materials Technology and Engineering, Chinese Academy of Sciences first examined the surface functional group transformation mechanism of the 2×2×1 supercellular configuration. That is, the number of hydrogen atoms on the surface of the supercellular configuration is regulated, and all possible seventy-four intermediate configurations are studied. The results show that during the conversion of Sc 2 C(OH) 2 to Sc 2 CO 2 , the neighboring positions on both sides are dehydrogenated. Further studies have found that this rule is equally applicable to the conversion of other various MXene surface functional groups, such as the conversion of Ti 2 C(OH) 2 to Ti 2 CO 2 and the conversion of Ti 2 CF 2 to Ti 2 C(OH) 2 . When the hydrogen content is reduced to a certain extent, the intermediate layer C atoms and Sc atoms undergo structural rearrangement. The p orbital of the C atom and the d orbital of the Sc atom form a pd orbital hybrid, and the covalent bond component increases. This is mainly due to the fact that as the hydrogen atoms decrease, the electron-donating Sc and H atoms are insufficient to provide sufficient electrons to the C and O atoms of the electrons to be obtained, so that a covalent bond is formed between the C and Sc atoms to promote structural stability. In the rearranged structure, the researchers found that when the hydrogen atoms on both sides of the layer are not equal, the structure is prone to the characteristics of the Bipolar Magnetic System (BMS state), that is, the valence band top and the bottom of the conduction band are opposite in the spin direction. The electronic state, as shown in Figure 2a, thus has a higher electron spin polarizability. Since the Sc system MXene is composed of light elements, the spin-orbit coupling effect of the system is weak, so it is expected that the material will obtain a Curie temperature higher than room temperature. Therefore, this type of material is an excellent spintronic device material. In order to verify the reliability of the results, the researchers further investigated the configuration of Sc 2 C(OH) x O 2 -x in the intermediate state of (3 × 3 × 1 and 4 × 4 × 1) under different supercellular structures. The hydrogen content of the BMS state appears. The interval is roughly at 0.188.
The above work has been published online in the Nanoscale Journal ( Nanoscale, 2018, 10, 8763), and has won the National Key Research and Development Program (No. 2016YFB0700100) and the National Natural Science Foundation of China (No. 11604346, No. 21671195, No. 21476247, No. 21473229). , No. 91545121, No. 21603252), supported by the Innovation Crossing Team of the Chinese Academy of Sciences and the Lu Jiaxi International Innovation Team of the Chinese Academy of Sciences in 2014.
Figure 1. Schematic diagram of structure, charge distribution, atomic charge and band structure of Sc 2 C(OH) 2 and Sc 2 CO 2 . a, b is a top view and a side view of the structure of Sc 2 C(OH) 2 , the right side of b corresponds to the atomic charge of each layer of atoms, and e is the energy band structure diagram of Sc 2 C(OH) 2 ; c, d, f Corresponds to the structure, charge distribution, atomic charge, and band structure of Sc 2 CO 2
Fig. 2a is an electron density map of BMS based on 2×2×1 supercells, b is the hydrogen content x of the intermediate state Sc 2 C(OH) x O 2-x under different supercellular configurations and the magnetic moment of the system A diagram in which solid black rectangles, red circles, and blue triangles correspond to 2×2×1, 3×3×1, and 4×4×1 super-cell BMS configurations, respectively.
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