First Principles Study On Novel Si And Ge Allotropes And XC (X=Si, Ge) Compounds

Group IV elements Si and Ge as well as their carbides are important semiconductormaterials. Their application values are of great importance in modern science andtechnology. In this dissertation, we explored novel structures of Si and Ge allotropes aswell as monocarbides XC (X=Si, Ge) and predicted their fundamental properties by usingparticle-swarm optimization (PSO) method on structural search combine with firstprinciples calculation. First, four kinds of pressure-driven type Si and Ge allotropes werepredicted, namely oP10, mP10, oP12, and oC8phases. It can be revealed by analyzingtheir thermodynamic stability that they may be thermodynamically accessible, and may beobserved during compression (decompression) of corresponding diamond (β-Sn) phases orunder nonhydrostatic pressure conditions (such as nanoindentation environment). Theelectronic properties calculations suggest the oP10and mP10phases of Si and Ge are bothsemiconductors, whereas the oP12and oC8phases exhibit metallic features. Theabsorption spectrums of oP10Si and mP10Si are higher than that of c-Si within energyrange of1-4.4eV, correspond to the strong region of the solar spectral irradiance of AM1.5. Next, the high pressure phase transition path β-Snâ†'Immaâ†'sh of Ge was studied. Anorthorhombic oP6Ge which is structurally very similar to intermediate Imma phase wasfound. This phase could form between β-Sn and sh phases under the effect ofnonhydrostatic pressure and could transform into a tetragonal tI8structure. The calculatedelectronic band structure of oP6Ge exhibits metallic features, which is due to the distortedatomic arrangements induced imbalanced charge distribution among adjacent atoms. Theelectronic density of state at Fermi surface is originated mainly from the contribution of4p electron.Eight kinds of pressure-driven type Ge allotropes were studied theoretically. These Geallotropes are all capable of stabilize at room conditions, four of which arethermodynamically competitive to the known ST12and BC8Ge. These allotropes are allless dense structures with low bulk modulus, the atoms all forming distorted tetrahedralbondings, and containing complex atomic ring arrangement and stacking sequence.Electronic band structures calculation suggested one of them has metallic feature, theothers are all indirect band gap semiconductors. The calculated band gaps Egrange from 0.08to0.58eV. Different from ST12Ge with high dielectric constant and high refractiveindex, the optical constants of these allotropes are close to those of c-Ge, and infraredtransmittances are approximate to0.6.One kind of sp3-bonded low-energy orthorhombic Si and Ge allotropes (denote asCco-Si8and Cco-Ge8) were explored, and two possible synthetic routes were proposed.One is by decompressing from the corresponding high-pressure β-Sn phases through amultistep approach; the other is by polymerizing the corresponding (6,6) nanotubebundles under nonhydrostatic pressure conditions. The electronic band structurecalculation reveals Cco-Si8and Cco-Ge8are both semiconductors, and the band gap ofCco-Ge8is tunable from zero to narrow band gap with slight variations in latticeparameters. Next, the polymerization of Ge (n,0) and (n, n) nanotubes polymers underhydrostatic and nonhydrostatic pressure conditions suggested these Ge nanotube polymerscould be produced by interlinking adjacent nanotubes along the radial direction under theeffect of compression on Ge nanotube bundles. The atomic bondings transform fromsp2-hybridized nanotube to sp3-hybridized nanotube polymers. Among these nanotubepolymers, some exhibit metallic feature, while others are semiconductors. Thesemiconducting Ge nanotube polymers have low dielectric constant and refractive index.Eight novel XC (X=Si, Ge) nanotube polymers were proposed theoretically. Thesepolymers contain Si-C and Ge-C bonds only. The atoms are all fourfold coordinated andformed even-number rings. Bulk modulus, Vickers hardness and tensile strength of theseare calculated, and the values of GeC nanotube polymers are slightly lower those of SiCnanotube polymers. The electronic band structure calculations show that SiC and GeCnanotube polymers are both wide band gap semiconductors. The calculated band gapsrange from1.6to2.6eV. SiC and GeC nanotube polymers have relatively lower dielectricconstant and refractive index, but higher infrared transmittances (exceed0.8). The intrinsicrelations between refractive index n and bond length d of diamond structured C, Si, Gecrystals and zinc-blende structured SiC were explored, and a semiempirical n-d formulawas proposed.