Ultra-high mechanical flexibility of 2D silicon telluride

Abstract

Silicon telluride (Si2Te3) is a two-dimensional material with a unique variable structure where the silicon atoms form Si-Si dimers to fill the "metal" sites between the Te layers. The Si-Si dimers have four possible orientations: three in-plane and one out-of-the plane directions. The structural variability of Si2Te3 allows unusual properties especially the mechanical properties. Using results from first-principles calculations, we show that the Si2Te3 monolayer can sustain a uniaxial tensile strain up to 38%, highest among all two-dimensional materials reported. The high mechanical flexibility allows applying mechanical strain to reduce the band gap by 1.4 eV. With increasing strain, the band gap undergoes an unusual indirect-direct-indirect-direct transition. We also show that the uniaxial strain can effectively control the Si-Si dimer alignment, which is beneficial for practical applications.The research of two-dimensional materials has significantly advanced in the past decade. The examples include graphene, 1-3 transition-metal dichalcogenides such as MoS2, 4 black phosphorous, 5,6 and atomic layers of topological insulators such as Bi2Se3. 7,8 With the reduced dimensionality, these materials allow a broad range of new opportunities not present in 3D systems, such as minimal dielectric screening, 9 rippling, 10 twisting, 11, 12 and easy functionalization by chemical treatments 13,14 and irradiation. 15, 16 These opportunities make the 2D materials particularly attractive for both fundamental explorations and potential applications.Silicon telluride (Si2Te3) is a unique layered semiconductor that has recently been made into a few atomic layer thickness. 17 In Si2Te3, the Te atoms form a hexagonal close-packed structure, while Si atoms form Si-Si dimers and fill 2/3 of the allowed "metal" sites ( Fig. 1a). 18,19 For each Si-Si dimer, there are four possible orientations: 3 in-plane (horizontal) directions and one out-of-plane direction. A recently theoretical studys show that the rotation of Si dimers has an energy barrier of about 1 eV and can happen at room temperature, giving rise to peculiar structure variability. 20 Because the Si-Si dimers rotation is coupled to the change of lattice size, Si2Te3 is expected to have unusual mechanical properties and other properties such as electronic structure are expected to strongly coupled to mechanical strain as well. *

Anisotropic optical properties of single Si2Te3 nanoplates

“…Previous work by Bletskan et al 22 indicated an indirect band gap value of 2.13 eV for crystalline Si 2 Te 3 at room temperature. Bhattarai et al 6 have also shown that the stress has a strong effect on the band gap energy as well as the nature of the transition (i.e., direct or indirect). Earlier work by Shen et al 5 also demonstrated an interesting variability of the Si 2 Te 3 band structures with respect to the orientation of the Si-Si dimers.…”

“…Further, while Si 2 Te 3 material can be processed using standard semiconductor techniques, it is highly sensitive to the environment, which offers an advantage for chemical sensing applications 1,8 . Indeed, Si 2 Te 3 has one of the highest mechanical flexibilities among 2D materials ever reported 6 . Its interesting indirectdirect band gap transition with the application of mechanical uniaxial strain 6 and anisotropic optical dielectric properties 18,19 make this material even more interesting and provide promise for its potential applications.…”

“…Indeed, Si 2 Te 3 has one of the highest mechanical flexibilities among 2D materials ever reported 6 . Its interesting indirectdirect band gap transition with the application of mechanical uniaxial strain 6 and anisotropic optical dielectric properties 18,19 make this material even more interesting and provide promise for its potential applications. In this study, we present the anisotropic optical properties of Si 2 Te 3 NPs by combining a set of experiments and theoretical calculations.…”

“…Semiconducting Si 2 Te 3 nanostructures have recently emerged as materials that have potential applications as silicon-based devices because of their unprecedented structural variabilities [1][2][3][4][5][6][7][8] . Previous studies of the electrical and optical properties of bulk Si 2 Te 3 trace back to more than 50 years ago [9][10][11][12][13][14][15] .…”

“…A theoretical calculation by Juneja et al 17 shows a low thermal conductivity (2 W/mK at 1000 K) of n-doped Si 2 Te 3 , which results in a relatively high thermoelectric material figure of merit. Pressure-dependent studies have also shown interesting behaviors of the phase transition as well as bandgap modification 6,7 . Nevertheless, the study of the fundamental properties of nanostructured Si 2 Te 3 is still in the early stage, especially the optical and electronic properties, for various reasons.…”