Six new silicon phases with direct band gaps

Theoretical study on structural and mechanical properties of Si-containing ternary transition metal nitrides M0.5Si0.5N (M = Ti, Zr, Hf)

Si-containing transition-metal nitrides Ti_0.5Si_0.5N, Zr_0.5Si_0.5N and Hf_0.5Si_0.5N with conventional rock salt B1 structure exhibit superior hardness, strength and oxidation resistance. However, the potential phases of the ternary systems at various pressures remain unexplored. In this work, we firstly studied the potential structures of Ti_0.5Si_0.5N, Zr_0.5Si_0.5N and Hf_0.5Si_0.5N in pressures of 0-100 GPa. A hexagonal phase with P6₃/mmc symmetry was uncovered and verified to be quenchable in the ambient conditions. The structural, mechanical and electronic properties were systematically studied and compared with the well-known ordered B1 structure. We surprisingly found that Ti_0.5Si_0.5N within this hexagonal phase displayed much improved ideal indentation shear strength from about 10 GPa for a B1 structure to 30 GPa. The estimated hardness based on the empirical formula is up to 38 GPa, greatly exceeding that of the B1 structure. By the detailed electronic analysis, the underlying atomic mechanism for the outstanding mechanical properties was also studied.

A Review on Metastable Silicon Allotropes

Diamond cubic silicon is widely used for electronic applications, integrated circuits, and photovoltaics, due to its high abundance, nontoxicity, and outstanding physicochemical properties. However, it is a semiconductor with an indirect band gap, depriving its further development. Fortunately, other polymorphs of silicon have been discovered successfully, and new functional allotropes are continuing to emerge, some of which are even stable in ambient conditions and could form the basis for the next revolution in electronics, stored energy, and optoelectronics. Such structures can lead to some excellent features, including a wide range of direct or quasi-direct band gaps allowed efficient for photoelectric conversion (examples include Si-III and Si-IV), as well as a smaller volume expansion as lithium-battery anode material (such as Si₂₄, Si₄₆, and Si₁₃₆). This review aims to give a detailed overview of these exciting new properties and routes for the synthesis of novel Si allotropes. Lastly, the key problems and the developmental trends are put forward at the end of this article.

Six novel carbon and silicon allotropes with their potential application in photovoltaic field

By stacking up five novel cagelike structures, three novel three-dimensional (3D)sp3bonding networks, namedhP24,hP30 andhP36, were predicted in this work for the first time. These three newly discovered structures have trigonal unit cell with the space groups ofP-3m1,P-3m1 andP3m1, respectively. Using first-principle calculations, the physical properties, including structural, mechanical, electronic and optical properties of C and Si inhP24,hP30 andhP36 phases were systematically studied. All these newly discovered carbon and silicon allotropes were proven to be thermodynamically and mechanically stable. The wide indirect bandgap value in range of 3.89-4.03 eV suggests that C inhP24,hP30 andhP36 phases have the potential to be applied in high frequency and high power electronic devices. The direct bandgap value in range of 0.60-1.16 eV, the smaller electron and hole effective mass than diamond-Si, and the significantly better photon absorption characteristics than diamond-Si suggest thathP24-Si,hP30-Si andhP36-Si are likely to have better performance in photovoltaic applications than diamond-Si.hP24-Si also has the potential to be applied in infrared detectors.