Nanocomposites and nanomaterials
Ab initio study of the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeSe, GeS)
Balabai R.M., Zadorozhnii V.M.
Kryvyi Rih State Pedagogical University, 54 Gagarina Ave., Kryvyi Rih, 50086, Ukraine. E-mail: balabai@i.ua , vitaliy_zadorozhniy@ukr.net

The piezoelectric effect in nanostructured semiconductor attracts attentions in recent years. Since Zhong Lin Wang group observed piezoelectric generation in a ZnO nanowire and coined a new term in 2006, which is piezotronics [1]. The piezotronics takes advantage of the coupling properties of piezoelectric effect and semiconductor characteristic. The rapid development of piezoelectric applications, including wearable electronics and human–machine interfacing, put forward a new requirement for piezoelectric materials, which must be highly flexible and atomically thin. However, conventional piezoelectric materials, such as piezoelectric ceramics and piezoelectric semiconductors suffer from fragile and bulk characteristics; therefore, 2D materials with atomic layer thickness and mechanical flexibility have been regarded as a new generation of piezoelectric materials. Existing 2D materials are piezoelectric if they fulﬁll two criteria: they must have a non-symmetric crystal structure and they must be non-metallic [2]. The family of 2D semiconductors, group IV monochalcogenides is non piezoelectric in the bulk form as they are centro-symmetric. However, their inversion centers disappear in mono-layers, resulting in piezoelectricity [3]. The strong in-plane covalent bonding and weak interlayer van der Waals interaction in these 2D materials provides high in-plane stability allowing  to produce it freestanding in nature. 
In this paper the piezoelectric effects of the 2D semiconductors of IV group monochalcogenides (GeS, GeSe) are studied by methods of the density functional theory and first-principles pseudopotential based on own program code. The spatial distribution of valence electron density, density of states, and Coulomb potential along transverse direction are calculated. The redistribution of the valence electrons in films with an even number of mono-layers (centro-symmetric) and with an odd number (non centro-symmetric) under mechanical action are discussed.

1. Ronan Hinchet et al. Piezoelectric properties in two-dimensional materials: Simulations and experiments // Materials Today.-2018.-21.- P.611.
2. Feng Li et al.  Advances in Strain‑Induced Piezoelectric and Piezoresistive Effect-Engineered 2D Semiconductors for Adaptive Electronics and Optoelectronics // Nano-Micro Lett.- 2020.- 12.- P.106.
3. Ruixiang Fe et al. Giant piezoelectricity of monolayer group IV monochalcogenides: SnSe, SnS, GeSe, and GeS // Appl. Phys. Lett.- 2015.- 107.- P. 173104.