Abstract
Piezoelectricity is a property of a material that converts mechanical energy into electrical energy or vice versa. It is known that group-III monochalcogenides, including GaS, GaSe, and InSe, show piezoelectricity in their monolayer form. Piezoelectric coefficients of these monolayers are the same order of magnitude as the previously discovered two-dimensional (2D) piezoelectric materials such as boron nitride (BN) and molybdenum disulfide (MoS2) monolayers. Considering a series of monolayer monochalcogenide structures including boron and aluminum (MX, M =B, Al, X = O, S, Se, Te), we design a series of derivative Janus structures (AlBX2, X = O, S, Se, Te). Ab-initio density functional theory (DFT) and density functional perturbation theory (DFPT) calculations are carried out systematically to predict their structural, electronic, electromechanical and phonon dispersion properties. The electronic band structure analysis indicate that all these 2D materials are semiconductors. The absence of imaginary phonon frequencies in phonon dispersion curves demonstrate that the systems are dynamically stable. In addition, this study shows that these materials exhibit outstanding piezoelectric properties. For AlBO2 monolayer with the relaxed-ion piezoelectric coefficients, d11=15.89(15.87) pm/V and d31=0.52(0.44) pm/V, the strongest piezoelectric properties were obtained. It has large in-plane and out-of-plane piezoelectric coefficients that are comparable to or larger than those of previously reported non-Janus monolayer structures such as MoS2 and GaSe, and also Janus monolayer structures including: In2SSe, Te2Se, MoSeTe, InSeO, SbTeI, and ZrSTe. These results, together with the fact that a lot of similar 2D systems have been synthesized so far, demonstrate the great potential of these materials in nanoscale electromechanical applications.
Phonon dispersion of a two-dimensional boron sheet on Ag(111)