Since the advent of graphene, two-dimensional nanomaterials have attracted wide attention due to their excellent properties such as high specific surface area and easy handling. A variety of graphene structures have been synthesized in succession and have shown promising applications in specific fields such as siliconene, black phosphorus, and molybdenum disulfide. In 2011, Michel Barsoum, professor at Drexel University in the US, for the first time eroded the combination of MAX in hydrofluoric acid by etching the MAX phase (M refers to the front transition metal, A refers to the A group element, and X refers to carbon or nitrogen). The A-atomic layer gives a tightly packed MX sheet structure. Analogous to graphene, the resulting sheet is named MXene. Since the known MAX phase has more than 70 species, the corresponding etched product MXene will be very rich. Up to now, more than 15 kinds of MXene have been prepared in the experiment. In addition, due to etching in an acid solution, the MXene surface is often covered with oxygen, fluorine, hydroxyl, and other functional groups. Based on abundant chemical elements and various surface functional groups, the physical properties of MXene with different configurations are significantly different. Therefore, in order to better design and apply MXene materials, the study of intrinsic physical properties is essential. Based on the theoretical research, the Institute of Materials Technology and Engineering of the Chinese Academy of Sciences made a series of related work on the physical property prediction and mechanism analysis of MXene materials.
In order to investigate the effects of common functional groups oxygen, fluorine, and hydroxyl on the physical properties of MXene, the researchers first systematically studied M2CT2 (M=Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W; T=O , F, OH) MXene's intrinsic structure, mechanical strength, and electron energy band diagram. Studies have shown that for the same M metal element, the oxygen-functionalized MXene has a smaller molar volume, a higher mechanical strength, and more semiconducting members than the corresponding fluorine- or hydroxyl-containing system. The stable configuration and mechanical constant c11 of most M2CT2 are shown in Figure 1. Analysis of the microstructure shows that the oxygen functional group has a stronger electron-accepting ability than fluorine and a hydroxyl group, and forms a stronger ionic bond with the surface metal atom M, resulting in a difference in the above properties. In addition, in the semiconductor type MXene, the energy band gaps of Sc2CT2 (T=F, OH) and M2CO2 (M=Ti, Zr, Hf) are in the range of 0.85 to 1.8 eV, satisfying the moderate energy band of semiconductor electronic devices. Bandgap requirements. This work was published in the European Physical Letters (EPL 111, 26007 (2015)).
In order to verify the application of semiconductor MXene with moderate bandgap in electronic devices, researchers examined the critical properties of Sc2CT2 (T=F, OH) and M2CO2 (M=Ti, Zr, Hf) for carrier mobility and thermal conductivity. . The results show that Sc2CF2 and Sc2C(OH)2 have high electron mobility, and the mobility of zirconium in Sc2CF2 is as high as 5.03×103 cm2V-1s-1, which is about 4 times higher than that of electron mobility of current electronic devices. . In addition, the electron mobility exhibits a strong anisotropy, which is mainly determined by the spatial distribution of the electron wave function at the bottom of the conduction band, as shown in Fig. 2(a). The thermal conductivity of semiconductor materials is mainly contributed by the lattice thermal conductivity, and the room temperature thermal conductivity (5um size) of Sc2CF2 is as high as 472 Wm-1K-1. As the sample size increases, the thermal conductivity can still be further increased, as shown in Figure 2(c). Related work was published in Nanoscale 8, 6110 (2016). The surface oxygen-functionalized M2CO2 (M=Ti, Zr, Hf) MXenes systems all exhibited extremely high hole transport considerations, all with a magnitude of 104, which is in agreement with the reported Ti2COx carrier mobility in the experiment. The thermal conductance increases with the increase of the atomic number of the metal atom M, which is mainly due to the fact that the metal atom in the same group increases in chemical activity with increasing atomic number and forms a stronger chemical bond with the surface oxygen functional group. The thermal conductivity of Hf2CO2 is close to that of metallic iron, while the thermal conductivity of Ti2CO2 is approximately 21 Wm-1K-1. This work was published in the journal Scientific Reports 6, 27971 (2016). Based on the above data, semiconductor-type MXene is expected to be applied to semiconductor electronic device materials.
In addition, researchers have conducted a comprehensive study of their electrical, thermal, and mechanical properties for Mo2C configurations that have been synthesized by vapor deposition and have no functional groups on the surface. The results show that Mo2C has a small molar volume, good electrical and thermal conductivity, and has excellent structural stability under applied strain and temperature. Its conductance value is in the order of 106Ω-1m-1. The thermal conductivity at room temperature is 48.4 Wm-1K-1, which can be further increased by increasing the temperature or doping. The coefficient of thermal expansion at room temperature was 2.26 × 10-6 K-1, and the Young's modulus in the plane was 312 GPa. Based on the above performance parameters, Mo2C has good application prospects in electrode materials and film base materials. This work was published in the Journal of Physical Chemistry C 120, 15082 (2016).
The above work has obtained the national key R&D plan (No. 2016YFB0700100), the Youth Employees Plan of the Central Organization Department, the National Natural Science Foundation of China (11604346, 51173046, 51479037, 91226202), the Natural Science Foundation of Ningbo (2016A610272, 2014A610006) and the 2014 Chinese Academy of Sciences. Support for projects such as cross innovation teams.
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