Source-linked AI summary
Tinselenidene: a Two-dimensional Auxetic Material with Ultralow Lattice Thermal Conductivity and Ultrahigh Hole Mobility
Li-Chuan Zhang, Guangzhao Qin, Wu-Zhang Fang, Hui-Juan Cui, Qing-Rong Zheng, Qing-Bo Yan, Gang Su
TL;DR
The paper addresses limited knowledge of tinselenidene’s intrinsic transport, thermal, mechanical, and strain-dependent properties. Using DFT-based ab initio calculations, it predicts a semiconductor with high hole mobility, ultralow thermal conductivity, strong flexibility, auxetic behavior, and strain-tunable band structure. These properties indicate potential relevance to thermoelectrics, nanomechanics, and optoelectronics, while the calculated mobility may exceed experimentally observed values because the deformation-potential treatment omits some scattering effects.
Problem
The intrinsic carrier mobility, lattice thermal conductivity, and strain effects of SnSe monolayer, or tinselenidene, remain less known.
Method
The study uses DFT-based ab initio calculations to investigate tinselenidene’s properties and strain response.
Results
1.45 eV is the predicted indirect bandgap, while stress below 0.5 GPa can induce an indirect-direct bandgap transition and hole mobility reaches 11520-14880 cm2V-1S-1 along x.
Takeaways & Limitations
Tinselenidene’s predicted combination of high mobility, low thermal conductivity, flexibility, and auxetic behavior supports potential applications in thermoelectrics, nanomechanics, and optoelectronics.
Takeaways & Limitations
Experimentally observed carrier mobility may be lower because deformation-potential theory considers only longitudinal scattering effects.
Abstract
from arXiv · showhide
By means of extensive ab initio calculations, a new two-dimensional (2D) atomic material tin selenide monolayer (coined as tinselenidene) is predicted to be a semiconductor with an indirect gap (1.45 eV) and a high hole mobility (of order 10000 cm2V-1S-1), and will bear an indirect-direct gap transition under a rather low strain (<0.5 GPa). Tinselenidene has a very small Young's modulus (20-40 GPa) and an ultralow lattice thermal conductivity (<3 Wm-1K-1 at 300 K), making it probably the most flexible and most heat-insulating material in known 2D atomic materials. In addition, tinseleniden has a large negative Poisson's ratio of -0.17, thus could act as a 2D auxetic material. With these intriguing properties, tinselenidene could have wide potential applications in thermoelectrics, nanomechanics and optoelectronics.
Introduction
The study systematically investigates tinselenidene, a SnSe monolayer whose intrinsic properties were previously less known. It predicts unusual combinations of high hole mobility, low thermal conductivity, mechanical flexibility, strain sensitivity, and auxetic behavior.
- DFT-based ab initio calculations systematically investigate the geometrical, mechanical, and electronic properties of tinselenidene.
- 1.45 eV is the predicted indirect bandgap of tinselenidene, which is a semiconductor.
- 11000 cm2V-1S-1 is the reported hole mobility, while lattice thermal conductivity is below 3 Wm-1K-1 and Poisson’s ratio is -0.17.
- Tinselenidene has nearly symmetric phonon and electronic band structures, although its carrier mobility remains highly anisotropic under strain.
- 24.3 GPa and 43.5 GPa are the Young’s moduli along the armchair and zigzag directions, respectively, indicating strong flexibility.
- <0.5 GPa of stress can induce an indirect-direct bandgap transition, while 1.6 GPa along the armchair direction can induce a geometrical phase transition.
- The predicted properties support potential applications in thermoelectrics, nanomechanics, and optoelectronics.
Results and Discussion
Tinselenidene combines a nearly square-like effective bilayer lattice with symmetric electronic and phononic structures, producing isotropic carrier masses and thermal transport. It also exhibits strong mechanical tunability, including auxetic behavior, low stiffness, a strain-induced geometrical phase transition, and indirect-to-direct gap conversion.
- Thermal properties: The lattice thermal conductivities along the two directions are 2.20 and 2.54 Wm-1K-1, much lower than phosphorene’s 14 and 30 Wm-1K-1.The nearly isotropic transport may give tinselenidene the lowest lattice thermal conductivity among studied 2D materials.
- Electronic properties: Tinselenidene is an indirect-gap semiconductor with a 1.45 eV bandgap, while the direct gaps between VX–CX and VY–CY are 1.49 and 1.65 eV.VX is the valence-band top and CY is the conduction-band bottom.
- Structure and bonding: The effective bilayer square-like lattice induces symmetric phononic and electronic band structures and isotropic carrier effective masses.Strong interactions between nonbonding neighboring Sn and Se atoms contribute to this structural character.
- Mechanical properties: A Young’s modulus of 24 GPa and 44 GPa along the two directions makes tinselenidene much more flexible than phosphorene and other isotropic 2D materials.The material may be the most flexible among known 2D materials.
Conclusions
Extensive ab initio calculations identify tinselenidene as a semiconductor with ultralow lattice thermal conductivity, high hole mobility, strain-tunable electronic properties, and auxetic behavior. Its nearly symmetric square-like bilayer structure is associated with nearly isotropic thermal and effective-mass properties, while strain strongly affects its geometry, mechanics, and electronic structure.
- Electronic and transport properties: 1.45 eV indirect bandgap and hole mobility as high as 11000 cm2V-1S-1 characterize tinselenidene as a high-mobility semiconductor.The hole mobility is reported as high as 11000 cm2V-1S-1.
- Electronic and transport properties: Nearly symmetric phonon and electronic band structures lead to nearly isotropic lattice thermal conductivity and charge-carrier effective mass.These properties are attributed to an effectively symmetric square-like bilayer lattice structure.
- Strain effects: A uniaxial strain induces an indirect-direct bandgap transition under stress below 0.5 GPa by shifting different energy valleys asynchronously.A stress of 1.6 GPa along x can also induce a geometrical phase transition.
- Strain effects: Although the carrier effective mass is isotropic, carrier mobility is anisotropic because of the anisotropic response to strain.The reported anisotropy concerns mobility rather than effective mass.
- Mechanical properties and applications: A negative Poisson’s ratio of -0.17 indicates that tinselenidene may be an auxetic material.The paper identifies potential applications in nanomechanics, thermoelectrics, and optoelectronics.
Methods
The study combines plane-wave DFT calculations with structural, electronic, phonon, thermal-transport, and carrier-mobility analyses. Mobility is evaluated using deformation-potential theory, while lattice thermal conductivity is calculated with finite displacements and ShengBTE.
- Electronic-structure calculations: DFT calculations use PBE exchange-correlation, PAW, a 700 eV plane-wave cutoff, and a 15×15×1 Monkhorst-Pack k-mesh.The energy convergence threshold is 10^-5 eV.
- Structural optimization: Cell shape and volume are fully optimized until the maximum atomic force is below 0.002 eV/Å.The optB88-vdW functional accounts for van der Waals interactions.
- Electronic-structure calculations: The modified Becke-Johnson method is used to calculate electronic band structures.This supplements the underlying DFT setup for electronic-structure evaluation.
- Phonon and thermal transport: Phonon dispersion is calculated with PHONOPY, and lattice thermal conductivity is calculated using the finite displacement method and ShengBTE.The workflow therefore combines phonon calculations with a dedicated thermal-transport code.
Caption of Figure 1
Figure 1 depicts tinselenidene’s perspective, side, and top views, including its primitive cell, atomic labeling, and charge-density distribution.
- The perspective, side, and top views show the schematic structure of tinselenidene.
- Yellow and gray balls represent Se and Sn atoms, respectively, with numbers labeling neighboring atoms.
- The primitive cell is outlined with dashed lines, with a and b denoting the armchair x and zigzag y lattice parameters.
- The side view of phosphorene is included for structural comparison.
- The xy-plane charge-density map crosses all Sn atoms and uses red and blue for high and low density, respectively.Its unit is e/bohr3.
Caption of Figure 2
Figure 2 presents tinselenidene’s phonon dispersion, directional lattice thermal conductivity, electronic bands, and carrier effective masses.
- The phonon-dispersion panel identifies three acoustic branches: LA, TA, and ZA.
- The lattice thermal conductivity is shown separately along the armchair x and zigzag y directions.
- The electronic band structure compares mBJ-functional calculations with HSE06 results.Black lines denote mBJ and red lines denote HSE06.
- The conduction- and valence-band extrema are marked as CY, CX, VY, and VX, respectively.
- The final panel reports electron and hole effective masses in tinselenidene.
Caption of Figure 3
Figure 3 characterizes tinselenidene’s mechanical response under uniaxial strain along the x and y directions and defines the associated strain and Poisson-ratio analysis.
- The mechanical response is evaluated under uniaxial strain along x and y.
- Strain is defined as s=(l−l0)/l0, where l denotes the strained lattice parameter and l0 its unstrained value.
- Positive and negative s represent tensile and compressive strain, while s=0 denotes the unstrained case.
- The Poisson’s ratio is obtained by fitting s=−ν1t+ν2t2+ν3t3, with ν1 treated as the Poisson’s ratio.Here, t is the strain along x or y.
- Stress–strain relations for the x and y directions are shown in the upper-right insets.
Caption of Figure 4
Figure 4 examines strain-dependent band structures and band-edge energies, while Table 1 defines the effective-mass, deformation-potential, elastic-modulus, and mobility quantities used for carrier analysis.
- Figure 4: Band structures are calculated with the mBJ method under uniaxial strain from −10% to 10% along x and y.
- Figure 4: The conduction- and valence-band extrema are labeled CY, CX, VY, and VX, whose energies are plotted versus strain.
- Figure 4: The indirect band gap occurs in regions I, III, IV, and VI, whereas the direct band gap occurs in regions II and V.
- Figure 4: The indirect-to-direct gap transition is identified by the crossover of the relevant band-edge energies.
- Table 1: Table 1 lists effective masses and mobilities, with x and y quantities defined along the corresponding directions.
- Table 1: The effective 2D elastic modulus agrees with the Young’s modulus obtained from stress–strain relations.