Silicon Allotropes Research Articles

New candidate to reach Shockley–Queisser limit: The DFT study of orthorhombic silicon allotrope Si(oP32)

In the present study, the unit cell parameters and atomic coordinates are predicted for the Pbcm orthorhombic structure of Si(oP32) modification. This new allotrope of silicon is mechanically stable and stable with respect to the phonon states. The electronic structure of Si(oP32) is calculated for LDA and HSE06 optimized structures. The band gap value Eg = 1.361 eV predicted for Si(oP32) is extremely close to the Shockley–Queisser limit and it indicates that the Si(oP32) modification is a promising material for efficient solar cells. The frequencies of Raman and Infrared active vibrations is calculated for allotrope Si(oP32).

Deformed octagon-hexagon-square structure of group-IV and group-V elements and III-V compounds

We report the prediction of a two-dimensional (2D) allotrope common to group-IV and group-V elements and III-V compounds, which consist of two nonplanar atomic layers connected by vertical bonds and form deformed octagon, hexagon, and squares (dohs) with threefold and fourfold coordinated atoms. Specifically for silicon, it is a semiconductor with cohesion stronger than silicene and can be chemically doped to have localized donor and acceptor states in the band gap. This allotrope can be functionalized to construct quasi-2D clathrates with transition metal atoms and attain spin polarized metallic, half-metallic, or semiconducting states. It is demonstrated that these properties can be maintained, when it is grown on a specific substrate. Stringent tests show that the atomic structure is dynamically stable and can sustain thermal excitation at high temperatures. Additionally, stable bilayer, as well as 3D layeredlike structures, can be constructed by the vertical stacking of single-layer dohs. Surprisingly, C, Ge, AlP, and GaAs can form also similar 2D semiconducting structures. In contrast to semiconducting black and blue phosphorene, P-dohs is a semimetal with band inversion. While the premise of using well-developed silicon technology in 2D electronics has been hampered by the semimetallic silicene, the realization of this 2D, semiconducting allotrope of silicon and compounds can constitute a productive direction in 2D nanoelectronics/spintronics.

Si-Cmma: A silicon thin film with excellent stability and Dirac nodal loop

Silicon thin film with both remarkable stability and graphenelike properties attract many research interests in materials science and condensed matter physics. In this paper, two new two-dimensional silicon allotropes (Si-Cmma and Si-Pmma) with excellent energetic stability and positive dynamical stability are proposed and investigated by first-principles calculations. Si-Pmma possesses the energy of about 203 meV/atom lower than that of silicene and it is a normal metallic phase. Si-Cmma is of about 198 meV/atom lower than silicene in energy and it is confirmed to be a nodal loop semimetallic material. Symmetry analysis reveals that the nodal loop in Si-Cmma is protected by the glide mirror and it belongs to class AI $+$ R of Altland-Zirnbauer symmetry classes. A further three-band $k\ifmmode\cdot\else\textperiodcentered\fi{}\mathrm{p}$ model is constructed to understand the formation of the nodal loop and its strong anisotropy.

Prediction of a new direct-gap silicon phase: T36 silicon

Elemental silicon plays an extremely important role in technologically relevant fields. Owing to its abundance, silicon is the preferred solar-cell materials in spite of the fact that many silicon allotropes have indirect band gaps. Looking for silicon materials with direct band gap is still a significant field in material science. Here, we propose a new silicon allotrope possessing a tetragonal cell with 36 silicon atoms (termed as T36 silicon), which displays P42nmc symmetry. By first-principles calculations, we have systematically studied the structural stabilities, electronic and optical properties of this novel silicon. Our calculations reveal that T36 silicon is thermally, dynamically and mechanically stable, and is a semiconductor with direct band gap of 0.591 eV. The absorption coefficients of T36 silicon in the visible light range are significantly enhanced compared with diamond-Si. The T36 silicon can be expected to be useful for practical applications in optoelectronics, photo catalysts, and so on.

Synthesis of new Si9 material with a direct bandgap and its unique physical properties

Even though economic advantages of silicon still keep it as the dominant material for the solar cell industry in the near future, crystal silicon in the diamond structure (d-Si) is an indirect bandgap semiconductor which prevents to consider it as a next-generation platform for optical material technologies. Here, we report the formation of a new allotrope of silicon on surface, Si9, using a novel two-step synthesis methodology. First, a film of amorphous silicon was produced by using pulsed laser deposition method, and second, new Si9 was synthesized under irradiation of coherent electron beam on the amorphous Si film. It is important that the structure of Si9, forming six-membered sp3 silicon rings and involving 9 silicon atoms in one unit, possesses a direct bandgap near 1.59 eV, around which we have measured the emission peak in photoluminescence spectra on the pure Si9. It is discovered that Si9 can be easily doped as both p- and n-type on surface, where boron and nitrogen are demonstrated as the most promising elements for the p-type and n-type doping in Si9, respectively, due to their low formation energies and reductions in the band gap. These properties suggest great potential in constructing a novel Si9-based p-n junction which is highly desired for future industrial application of optoelectronic technologies and photovoltaic devices.

Elastic anisotropy and thermal conductivity of silicon allotropes

Silicon is the most basic and commonly used semiconductor material in the modern microelectronics industry. Because diamond silicon is an indirect band gap semiconductor, its application in the optoelectronic industry is limited. Searching for new silicon allotropes with direct band gaps has been a hot topic in the fields of condensed matter and materials science. In this work, the elastic anisotropy and thermal conductivity of seven silicon allotropes with direct band gaps (D76, D63, D135, D243, D12, D239, and D979), a silicon allotrope with quasi-direct band gap (Q135) and a silicon allotrope with indirect band gap (I926) are investigated. The elastic anisotropy of shear modulus, Young’s modulus and Poisson’s ratio for Q135, D76, D63, D135, D243, D12, I926, D239, and D979 silicon allotropes are investigated by surface constructions of the 2D representation. The I926 phase has the greatest thermal conductivity among the nine silicon allotropes, and it is greater than that of the oC12 phase. In addition, the anisotropy of elastic moduli and the anisotropy of thermal conductivity in each main plane for Q135, D76, D63, D135, D243, D12, I926, D239, and D979 are studied for the first time.

Relative stability of diamond and graphite as seen through bonds and hybridizations.

The relative stability of the two most important forms of elemental carbon, diamond and graphite, is readdressed from a newly developed perspective as derived from historically well-known roots. Unlike other theoretical studies mostly relying on numerical methods, we consider an analytical model to gain fundamental insight into the reasons for the quasi-degeneracy of diamond and graphite despite their extremely different covalent bonding patterns. We derive the allotropes' relative energies and provide a qualitative picture predicting a quasi-degenerate electronic ground state for graphite (graphene) and diamond at zero temperature. Our approach also gives numerical estimates of the energy difference and interatomic separations in good agreement with experimental data and recent results of hybrid DFT modeling, although obtained with a much smaller numerical but highly transparent effort. An attempt to extend this treatment to the lowest energy allotropes of silicon proves to be successful as well.

Crystal structures of silicon-rich lithium silicides at high pressure

Silicon (Si) is one of the most essential elements, as it is indispensable for modern electronic technology. The standard Si structure at ambient conditions is the cubic diamond structure, and it has an indirect band gap, which prevents it from being considered as a next-generation platform for semiconductor technologies. Therefore, the search for new allotropes of silicon has attracted great attention. Herein, first principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium containing compounds, LiSix (x=4–8). The LiSi4 stoichiometry was predicted to be stable, and it was found to assume one of the following space groups, P4/mnc, Cmmm, and C2/m within the pressure range of 0 to 50 GPa. By removing the Li atoms from these compounds, three silicon allotropes were obtained that were metastable at ambient pressures. Our work illustrates how novel silicon allotropes can be predicted using the CALYPSO method.

All-Silicon Topological Semimetals with Closed Nodal Line.

Because of the natural compatibility with current semiconductor industry, silicon allotropes with diverse structural and electronic properties provide promising platforms for next-generation Si-based devices. After screening 230 all-silicon crystals in the zeolite frameworks by first-principles calculations, we disclose two structurally stable Si allotropes (AHT-Si24 and VFI-Si36) containing open channels as topological node-line semimetals with Dirac nodal points forming a nodal loop in the k z = 0 plane of the Brillouin zone. Interestingly, their nodal loops protected by inversion and time-reversal symmetries are robust against SU(2) symmetry breaking because of the very weak spin-orbit coupling of Si. When the nodal lines are projected onto the (001) surface, flat surface bands can be observed because of the nontrivial topology of the bulk band structures. Our discoveries extend the topological physics to the three-dimensional Si materials, highlighting the possibility of realizing low-cost, nontoxic, and semiconductor-compatible Si-based electronics with topological quantum states.

T-Si64 : A Novel Silicon Allotrope.

Utilizing first principle calculations, a novel Si64 silicon allotrope in the I41 /amd space group with tetragonal symmetry (denoted as t-Si64 below) is proposed in this work. In addition, also its structural, anisotropic mechanical, and electronic properties along with its minimum thermal conductivity κmin were predicted. The mechanical and thermodynamic stability of t-Si64 were evaluated by means of elastic constants and phonon spectra. The electronic band structure indicates that t-Si64 is an indirect band gap semiconductor with a band gap: 0.67 eV (primitive cell) compared to a direct band gap of 0.70 eV with respect to a conventional cell. The minimum thermal conductivity of t-Si64 (0.74 W cm-1 K-1 ) is much smaller than that of diamond silicon (1.13 W cm-1 K-1 ). Therefore, Si-Ge alloys in the I41 /amd space group are potential thermoelectric materials.

Nanomechanical Properties of Epitaxial Silicene Revealed by Noncontact Atomic Force Microscopy

AbstractSilicene, the graphene‐like allotrope of silicon, has a great potential for the development of novel silicon‐based nanotechnologies. Its unique buckling makes the atomic structure of this silicon 2D material particularly flexible and tunable. The nanomechanical properties of silicene are investigated by probing epitaxial silicene on ZrB2 by atomic‐resolution noncontact atomic force microscopy. Δf–distance curve and scan direction dependence of topographic images point out, respectively, the out‐of‐plane and in‐plane flexibility of the Si honeycomb structure. A contrast visible only in dissipation images reveals the stress distribution across the periodic domain structure texturing the silicene sheet.

First-principles prediction of two hexagonal silicon crystals as potential absorbing layer materials for solar-cell application

Based on first-principles calculations, the structures, stabilities, electronic properties, and mechanical properties as well as optical properties of two new hexagonal silicon allotropes (Hex-193 and Hex-194) were theoretically investigated. Both Hex-193 and Hex-194 are confirmed to be meta-stable phases with energetic stability exceeding the previously proposed Si-20 and the experimentally viable Si-24. They are also confirmed to be dynamically and elastically stable silicon phases according to their calculated phonon band structures and mechanical properties. The HSE06-based band structures of Hex-193 and Hex-194 indicate that they are quasi-direct bandgap semiconductors with fundamental bandgaps of 1.275 eV (direct bandgap is 1.277 eV) and 1.200 eV (direct bandgap is 1.302 eV), respectively. Further investigation of the optical properties shows that both Hex-193 and Hex-194 exhibit a stronger absorption coefficient in comparison to that of diamond silicon, which indicates that both are potential materials for solar-cell application.

Thermoelectric properties of four typical silicon allotropes

M585-Si, mC12-Si, oC12-Si and Pbam24-Si are four typical silicon allotropes which are predicted to possess direct or quasi-direct band gap and better optical absorption coefficient than that of diamond Si (d-Si), foreshowing they are promising candidates for future thin-film photovoltaic devices. For realizing the recombination of photovoltaic and thermoelectric technologies, in this paper, we systematically investigate the thermoelectric properties by combining first-principles calculation and Boltzmann transport theory. The calculations show that the four typical silicon allotropes possess higher Seebeck coefficient (which is almost three to four times as large as that of d-Si), meanwhile anisotropic electronic conductivity and electronic thermal conductivity of these silicon allotropes are found as well. For the thermal transport properties, the four typical silicon allotropes present lower lattice thermal conductivity. At room temperature, the average thermal lattice conductivity of the four silicon allotropes is about 49.6 W m−1 K−1 (M585-Si), 54.7 W m−1 K−1 (mC12-Si), 45.1 W m−1 K−1 (oC12-Si) and 52.2 W m−1 K−1 (Pbam24-Si), respectively. Moreover, the characteristic phonon mean free path of these four silicon allotropes was also predicted. Based on the empirical electronic scattering time, the room temperature maximum thermoelectric figure of merit (ZT) of the M585-Si, mC12-Si, oC12-Si and Pbam24-Si can be as high as 0.68, 0.45, 0.34 and 0.42, respectively, which is substantially larger than that of d-Si (ZT ≈ 0.01). Better thermoelectric performance could be obtained in high temperature region (700 K), and the M585-Si along yy crystal direction is observed as large as 2.3 (1.6) with n (p)-type doping. The results presented in this work elucidate the thermoelectric performance of the four typical silicon allotropes and indicate that all of them are potential multifunctional materials for realizing the recombination of photovoltaic and thermoelectric technologies.

Synthesis and Properties of Single-Crystalline Na4Si24

Na4Si24 is the precursor to Si24, a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si24 has potential to transform silicon-based optoelectronics including solar energy conversion. However, the lack of large, pure crystals has prevented the characterization of intrinsic properties and has delayed deposition-based metastable growth efforts. Here, we report an optimized synthesis methodology for single-crystalline Na4Si24 with crystals approaching the millimeter-size scale with conditions near 9 GPa and 1123 K. Single-crystal diffraction was used to confirm the open-framework structure, and Na atoms remain highly mobile within the framework channels, as determined by electrical conductivity and electron energy loss spectroscopy measurements. An epitaxial relationship between Na4Si24 and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy, is proposed to facilitate the growth of high-quality Na4Si24 crystals from DC-Si wafers mix...

Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure.

Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high electric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si24 features highly anisotropic crystal structure with nanometer-sized regular pores. Here, based on first-principles study without any empirical parameter we show that the slightly doped Si24 can provide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.

Effect of sample size, temperature and strain velocity on mechanical properties of plumbene by tensile loading along longitudinal direction: A molecular dynamics study

In recent years, two-dimensional (2D) nanomaterials have received tremendous attention due to their unique structure and extraordinary properties. Graphene, silicene, germanene and stanene- the 2D allotropes of carbon, silicon, germanium and tin have been reported in last few years and their properties have been studied elaborately. Plumbene, the 2D allotrope of lead, is recently reported. It is analogous to graphene, silicene, germanene and stanene in single-layered and hexagonal arrangement of atomic structure. In this paper, molecular dynamics simulations have been carried out to study the effect of sample size, temperature and strain velocity on the mechanical properties of plumbene. The results show superior mechanical properties of the single layer plumbene sheet which is several times higher than bulk lead making it a suitable candidate for using as reinforcement to develop high strength nanocomposites.

Thermal conductivity of hexagonal Si, Ge, and Si1-xGex alloys from first-principles

Hexagonal Si and Ge with a lonsdaleite crystal structure are allotropes of silicon and germanium that have recently been synthesized. These materials as well as their alloys are promising candidates for novel applications in optoelectronics. In this paper, we systematically study the phonon transport and thermal conductivity of hexagonal Si, Ge, and their alloys by using the first-principle-based Peierls-Boltzmann transport equation approach. Both three-phonon and four-phonon scatterings are taken into account in the calculations as the phonon scattering mechanisms. The thermal conductivity anisotropy of these materials is identified. While the thermal conductivity parallel to the hexagonal plane for hexagonal Si and Ge is found to be larger than that perpendicular to the hexagonal plane, alloying effectively tunes the thermal conductivity anisotropy by suppressing the thermal conductivity contributions from the middle-frequency phonons. The importance of four-phonon scatterings is assessed by comparing the results with the calculations without including four-phonon scatterings. We find that four-phonon scatterings cannot be ignored in hexagonal Si and Ge as the thermal conductivity would be overestimated by around 10% (40%) at 300 K (900) K. In addition, the phonon mean free path distribution of hexagonal Si, Ge, and their alloys is also discussed.

Structural impact on the eigenenergy renormalization for carbon and silicon allotropes and boron nitride polymorphs

The phonon-induced renormalization of electronic band structures is investigated through first-principles calculations based on the density functional perturbation theory for nine materials with various crystal symmetries. Our results demonstrate that the magnitude of the zero-point renormalization (ZPR) of the electronic band structure is dependent on both crystal structure and material composition. We have performed analysis of the electron-phonon-coupling--induced renormalization for two silicon (Si) allotropes, three carbon (C) allotropes, and four boron nitride (BN) polymorphs. Phonon dispersions of each material were computed, and our analysis indicates that materials with optical phonons at higher maximum frequencies, such as graphite and hexagonal BN, have larger absolute ZPRs, with the exception of graphene, which has a considerably smaller ZPR despite having phonon frequencies in the same range as graphite. Depending on the structure and material, renormalizations can be comparable to the GW many-body corrections to Kohn-Sham eigenenergies and, thus, need to be considered in electronic structure calculations. The temperature dependence of the renormalizations is also considered, and in all materials, the eigenenergy renormalization at the band gap and around the Fermi level increases with increasing temperature.

Few-Layer Silicene Nanosheets with Superior Lithium-Storage Properties.

Silicene, a 2D silicon allotrope with unique low-buckled structure, has attracted increasing attention in recent years due to its many superior properties. So far, epitaxial growth is one of the very limited ways to obtain high-quality silicene, which severely impedes the research and application of silicene. Therefore, large-scale synthesis of silicene is a great challenge, yet urgently desired. Herein, the first scalable preparation of free-standing high-quality silicene nanosheets via liquid oxidation and exfoliation of CaSi2 is reported. This new synthesis strategy successfully induces mild oxidation of the (Si2n )2n- layers in CaSi2 into neutral Si2n layers without damage of pristine silicene structure and promotes the exfoliation of stacked silicene layers. The obtained silicene sheets are dispersible and ultrathin ones with monolayer or few-layer thickness and exhibit excellent crystallinity. As a unique 2D layered silicon allotrope, the silicene nanosheets are further explored as new anodes for lithium-ion batteries and exhibit a nearly theoretical capacity of 721 mAh g-1 at 0.1 A g-1 and an extraordinary cycling stability with no capacity decay after 1800 cycles in contrast to previous most silicon anodes showing rapid capacity decay, thus holding great promise for energy storage and beyond.

Superconducting Open-Framework Allotrope of Silicon at Ambient Pressure.

Diamond Si is a semiconductor with an indirect band gap that is the basis of modern semiconductor technology. Although many metastable forms of Si were observed using diamond anvil cells for compression and chemical precursors for synthesis, no metallic phase at ambient conditions has been reported thus far. Here we report the prediction of pure metallic Si allotropes with open channels at ambient pressure, unlike a cubic diamond structure in covalent bonding networks. The metallic phase termed P6/m-Si_{6} can be obtained by removing Na after pressure release from a novel Na-Si clathrate called P6/m-NaSi_{6}, which is predicted through first-principles study at high pressure. We identify that both P6/m-NaSi_{6} and P6/m-Si_{6} are stable and superconducting with the critical temperatures of about 13 and 12K at ambient pressure, respectively. The prediction of new Na-Si and Si clathrate structures presents the possibility of exploring new exotic allotropes useful for Si-based devices.