[Paper Review] Layer dependence of geometric, electronic and piezoelectric properties of SnSe

Wuzhang Fang, Lichuan Zhang+4 more|arXiv (Cornell University)|Mar 6, 2016

2D Materials and Applications3 citations

TL;DR

This first-principles study investigates the layer-dependent geometric, electronic, and piezoelectric properties of SnSe. It reveals that increasing layer count reduces lattice constants and band gap toward bulk values, while odd-layer SnSe exhibits strong piezoelectricity due to broken inversion symmetry, outperforming known piezoelectric materials.

ABSTRACT

By means of first-principles calculations, we explore systematically the geometric, electronic and piezoelectric properties of multilayer SnSe. We find that these properties are layer-dependent, indicating that the interlayer interaction plays an important role. With increasing the number of SnSe layers from 1 to 6, we observe that the lattice constant decreases from 4.27 $\mathring{A}$ to 4.22 $\mathring{A}$ along zigzag direction, and increases from 4.41 $\mathring{A}$ to 4.51 $\mathring{A}$ along armchair direction close to the bulk limit (4.21 $\mathring{A}$ and 4.52 $\mathring{A}$, respectively); the band gap decreases from 1.45 eV to 1.08 eV, approaching the bulk gap 0.95 eV. Although the monolayer SnSe exhibits almost symmetric geometric and electronic structures along zigzag and armchair directions, bulk SnSe is obviously anisotropic, showing that the stacking of layers enhances the anisotropic character of SnSe. As bulk and even-layer SnSe have inversion centers, they cannot exhibit piezoelectric responses. However, we show that the odd-layer SnSe have piezoelectric coefficients much higher than those of the known piezoelectric materials, suggesting that the odd-layer SnSe is a good piezoelectric material.

Motivation & Objective

• To systematically investigate the layer-dependent geometric, electronic, and piezoelectric properties of multilayer SnSe.
• To understand the role of interlayer interactions in modulating structural and electronic behavior.
• To identify conditions under which SnSe exhibits piezoelectric response, particularly focusing on layer symmetry.
• To evaluate the potential of few-layer SnSe as a high-performance piezoelectric material.

Proposed method

• First-principles density functional theory (DFT) calculations were employed to study SnSe across 1 to 6 layers.
• Structural optimization was performed to determine equilibrium lattice constants and atomic positions.
• Electronic band structures and density of states were computed to analyze band gap evolution with layer count.
• Piezoelectric tensor components were calculated using finite displacement methods to assess piezoelectric response.
• Symmetry analysis was conducted to determine the presence or absence of inversion centers in different layer configurations.
• Convergence tests and structural relaxation ensured accuracy in the computed properties.

Experimental results

Research questions

• RQ1 How do the lattice constants of SnSe vary with increasing layer count from monolayer to bulk?
• RQ2 How does the electronic band gap of SnSe evolve as the number of layers increases?
• RQ3 What is the origin of anisotropy in SnSe, and how does it change with layer thickness?
• RQ4 Why do even-layer and bulk SnSe lack piezoelectric response, while odd-layer SnSe exhibits strong piezoelectricity?
• RQ5 How do the piezoelectric coefficients of odd-layer SnSe compare to those of known piezoelectric materials?

Key findings

• The lattice constant along the zigzag direction decreases from 4.27 Å (monolayer) to 4.22 Å (6 layers), approaching the bulk value of 4.21 Å.
• The lattice constant along the armchair direction increases from 4.41 Å (monolayer) to 4.51 Å (6 layers), approaching the bulk limit of 4.52 Å.
• The band gap of SnSe decreases from 1.45 eV in monolayer to 1.08 eV in 6 layers, converging toward the bulk band gap of 0.95 eV.
• Bulk and even-layer SnSe possess inversion symmetry and thus exhibit no piezoelectric response.
• Odd-layer SnSe lacks inversion symmetry and displays piezoelectric coefficients significantly higher than those of conventional piezoelectric materials.
• The anisotropic character of SnSe increases with layer stacking, with monolayer SnSe being nearly symmetric but bulk SnSe strongly anisotropic.