| Outline of Annual Research Achievements |
Recently, a group of new MAX phases with in-plane ordered double transition metals have been synthesized. Experimentally, some of these MAX phases have also been chemically exfoliated into 2D MXenes. In this fiscal year, firstly, I could show that their pristine, F, and OH-terminated 2D MXenes are metallic. However, upon O termination, they turn into semiconducting. Owing to the absence of centrosymmetry, these semiconducting MXenes may find applications in piezoelectricity. My calculations reveal that the newly discovered semiconducting MXenes possess giant piezoelectric coefficie. Secondly, considering the recent breakthroughs in the synthesis of novel two-dimensional (2D) materials from layered bulk structures, ternary layered transition metal borides, known as MAB phases, have received attention to obtain novel 2D transition metal borides, so-called MBene. By comparing the formation energies of many MAB phases with those of their available competing binary M－B and M－Al, and ternary M－Al－B phases, I found that some of the Sc-, Ti-, V-, Cr-, Mo-, W-, Mn-, Tc-, and Fe- based MAB phases can be favorably synthesized in an appropriate experimental condition. In addition, by examining the strengths of various bonds in MAB phases, I could find that the B－B and then M－B bonds are stiffer than the M－Al and Al－B bonds. The different strength between these bonds implies the etching possibility of Al atoms from MAB phases, consequently formation of various 2D MB, M2B3, and M3B4 MBenes.
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