High-throughput calculation screening for new silicon allotropes with monoclinic symmetry
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-05-31 |
| Journal | IUCrJ |
| Authors | Qingyang Fan, Jie Wu, Yingbo Zhao, Yanxing Song, Sining Yun |
| Institutions | Xiâan University of Architecture and Technology, Xidian University |
| Citations | 21 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research utilizes high-throughput computational screening (RG2 combined with DFT) to identify novel monoclinic silicon (Si) allotropes, focusing on overcoming the indirect band gap limitation of traditional diamond Si.
- Core Achievement: 87 new, mechanically and dynamically stable 3D sp3 monoclinic Si allotropes were discovered and characterized.
- Electronic Breakthrough: 13 of the new allotropes exhibit a direct or quasi-direct band gap, making them highly suitable for optoelectronic applications.
- Photovoltaic Potential: All semiconductor allotropes show strong photon absorption in the visible spectral region, significantly exceeding that of diamond Si.
- Enhanced Mobility: Five allotropes possess electron effective masses (me) substantially smaller than diamond Si (some less than one-third), indicating potential for high carrier mobility.
- Superior Mechanicals: Several new structures demonstrate bulk moduli up to 99 GPa and shear moduli up to 66 GPa, surpassing the mechanical stiffness of diamond Si (88 GPa and 64 GPa, respectively).
- Target Band Gap: The direct band gaps fall within the 1.22-1.91 eV range, which is ideal for efficient solar energy conversion.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Total New Allotropes Identified | 87 | Structures | All monoclinic symmetry (Space Groups 3-15) |
| Direct/Quasi-Direct Gap Structures | 13 | Structures | Identified as promising semiconductors |
| Metallic Structures | 12 | Structures | Remaining 75 are indirect gap semiconductors |
| Maximum Bulk Modulus (B) | 99 | GPa | Allotrope 13-3-12-232508 (Diamond Si: 88 GPa) |
| Maximum Shear Modulus (G) | 66 | GPa | Allotrope 15-3-24-232448 (Diamond Si: 64 GPa) |
| Direct Band Gap Range (HSE06) | 1.22 - 1.91 | eV | Ideal range for solar cell applications (1.0-1.5 eV) |
| Lowest Electron Effective Mass (me) | < 0.317 | m0 | For 11-4-16-232321, 12-2-16-231721, 12-3-12-232409 |
| Formation Energy Range | 85 - 249 | meV/atom | Relative to diamond Si (0 eV/atom) |
| Crystal Density Range | 1.50 - 2.50 | g/cm3 | For all 87 new sp3 monoclinic Si allotropes |
| Plane Wave Energy Cut-off | 340 | eV | Used for DFT calculations (CASTEP) |
| Brillouin Zone Sampling | ~2Ď x 0.025 | A-1 | Monkhorst-Pack meshes used for k-point grids |
Key Methodologies
Section titled âKey MethodologiesâThe study employed a high-throughput computational workflow combining structure prediction and density functional theory (DFT) calculations:
- Structure Generation (RG2 Method): The Random Strategy combined with Group and Graph Theory (RG2) was used to systematically scan the configuration space for 3D sp3 silicon allotropes within the monoclinic symmetry range (Space Groups 3-15).
- Initial Optimization (DFT-GGA): Thousands of geometrically acceptable structures were optimized using DFT implemented in the CASTEP package.
- The Perdew-Burke-Ernzerhof (PBE) functional, a Generalized Gradient Approximation (GGA), was used for geometry optimization.
- The ultrasoft quasipotential (Vanderbilt) described electron-ion interaction.
- Electronic Structure Refinement (DFT-Hybrid): Electronic band structures, including band gap type (direct/indirect) and magnitude, were calculated using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional for improved accuracy.
- Mechanical Stability Check: Elastic constants (Cij) were calculated, and all 87 final allotropes were confirmed to meet the generalized Born mechanical stability criteria.
- Dynamic Stability Check: Phonon spectra were calculated using density functional perturbation theory (DFPT). The absence of negative frequencies confirmed the dynamic stability of all 87 structures.
- Property Calculation: Effective carrier masses (me, mh), elastic moduli (Bulk, Shear, Youngâs), and optical absorption spectra were calculated using MedeA-VASP and CASTEP.
Commercial Applications
Section titled âCommercial ApplicationsâThe discovery of stable silicon allotropes with direct band gaps and superior transport properties opens new avenues for silicon-based device engineering:
- High-Efficiency Photovoltaics (Solar Cells):
- The direct/quasi-direct band gaps (1.22-1.91 eV) are highly efficient for absorbing the solar spectrum.
- The strong visible light absorption capacity, significantly greater than diamond Si, promises thinner, more efficient solar cells.
- Optoelectronic Devices:
- The strong absorption characteristics make these materials ideal for use in photodetectors, optical sensors, and potentially silicon-based light-emitting diodes (LEDs), overcoming the poor light emission efficiency of traditional Si.
- Advanced Microelectronics (Transistors):
- Allotropes with low electron effective masses (me < 1/3 of diamond Si) imply high carrier mobility, which is critical for developing faster, lower-power high-performance transistors and integrated circuits.
- Mechanically Robust Components:
- The superior bulk (up to 99 GPa) and shear moduli (up to 66 GPa) suggest applications in microelectromechanical systems (MEMS) or high-stress semiconductor packaging where structural integrity is paramount.
View Original Abstract
A total of 87 new monoclinic silicon allotropes are systematically scanned by a random strategy combined with group and graph theory and high-throughput calculations. The new allotropes include 13 with a direct or quasi-direct band gap and 12 with metallic characteristics, and the rest are indirect band gap semiconductors. More than 30 of these novel monoclinic Si allotropes show bulk moduli greater than or equal to 80 GPa, and three of them show even greater bulk moduli than diamond Si. Only two of the new Si allotropes show a greater shear modulus than diamond Si. The crystal structures, stability (elastic constants, phonon spectra), mechanical properties, electronic properties, effective carrier masses and optical properties of all 87 Si monoclinic allotropes are studied in detail. The electron effective masses m l of five of the new allotropes are smaller than that of diamond Si. All of these novel monoclinic Si allotropes show strong absorption in the visible spectral region. Taken together with their electronic band gap structures, this makes them promising materials for photovoltaic applications. These investigations greatly enrich the current knowledge of the structure and electronic properties of silicon allotropes.