Silicon Allotropes Research Articles

Formation of inclusion type silicon phases induced by inert gases

Silicon clathrate, an important allotrope of silicon, has attractive opto-electronic properties for energy applications. However, it remains an experimental challenge to synthesize electrically undoped, intrinsic clathrate. Here we show, through high-throughput computer modeling, that unconventional silicon phases spontaneously nucleate from liquid silicon in the presence of noble gases under high pressure and high temperature. In particular, our results show that a medium-sized noble gas, for example, argon, can trigger the nucleation and growth of inert-gas silicon clathrate, whereas a small noble gas such as helium is able to induce the formation of an unconventional, inclusion-type compound Si2He. The formation of both silicon phases can be attributed to the same thermodynamic and kinetic rationale that explains the crystallization of clathrate hydrate, an isostructural analog. Our findings, along with the gained molecular insights, thus strongly suggest a viable experimental synthesis route for these silicon phases using noble gases at high pressure.

Studying the properties of a predicted tetragonal silicon by first principles

Silicon is a very important material in many technological fields. It also has a complicated phase diagram of scientific interest. Here we reported a new allotrope of silicon obtained from crystal structure prediction. We studied its electronic, vibrational, dielectric, elastic and hardness properties by first-principles calculations. The results indicate that it is an indirect narrow-band-gap semiconductor. It is dynamically stable with a doubly degenerate infrared-active mode at its Brillouin zone center. Born effective charges of the constituent element are very small, resulting in a negligible ionic dielectric contribution. Calculated elasticity-related quantities imply that it is mechanically stable but anisotropic. There exist slowly increasing stages in the stress-strain curves of this crystal, which make it difficult to estimate the hardness of the crystal by calculating its ideal strengths. Taking advantage of the hardness model proposed by Šimůnek, we obtained a value of 12.0 GPa as its hardness. This value is lower than that of the cubic diamond-structural Si by about 5.5%.

Advanced Synthesis of Na4Si24

The recently discovered orthorhombic allotrope of silicon, Si24, is an exciting prospective material for the future of solar energy due to a quasi-direct bandgap near 1.3 eV, coupled with the abundance and environmental stability of silicon. Synthesized via precursor Na4Si24 at high temperature and pressure (∼850 °C, 9 GPa), typical synthesis results have yielded polycrystalline samples with crystallites on the order of 20 μm. Several approaches to increase the crystal size have yielded success, including in-situ thermal spikes and refined selection of the starting materials. Microstructural analysis suggests that coherency exists between diamond silicon (d-Si) and Na4Si24. This hypothesis has led to the successful attempts at single crystal synthesis by selecting large crystals of d-Si along with metallic Na as the precursors rather than powdered and mixed precursor material. The new synthesis approach has yielded single crystals of Na4Si24 greater than 100 μm. These results represent a breakthrough in synthesis that enables further characterization and utility. The promise of Si24 for the future of solar energy generation and efficient electronics is strengthened through these advances in synthesis.

CI24‐Si: a Semiconducting Silicon Phase with an All‐sp3 Bonding Network

AbstractA silicon allotrope which consists of 24 atoms in a body centered cubic cell and displays an symmetry (termed as cI24‐Si) with a mass density of 2.38 g/cm3, is theoretically investigated. This silicon phase has an all‐sp3 network with hexagons. Phonon dispersion confirms its dynamical stability and elastic constant implies it is mechanically stable. The analysis of electronic band structure performing by HSE06 functional shows that cI24‐Si is an indirect semiconductor with a small band gap of 0.83 eV. Additionally, its direct band gap is only 0.02 eV larger than the indirect band gap, indicating that cI24‐Si possesses a quasi‐direct band gap. The calculations of imaginary part of dielectric function and optical absorption for cI24‐Si show that it has better optical properties than diamond‐like Si−I phase as it can capture more sunlight from visible to ultraviolet range. To provide more characterizations for future experimental observations, X‐ray diffraction patterns and Raman spectra are also theoretically simulated. Due to its small band gap, cI24‐Si may possess potential electronic, optical and photovoltaic applications.

Interband π -like plasmon in silicene grown on silver

Silicene, the two-dimensional allotrope of silicon, is predicted to exist in a low-buckled honeycomb lattice, characterized by semimetallic electronic bands with graphenelike energy-momentum dispersions around the Fermi level (represented by touching Dirac cones). Single layers of silicene are mostly synthesized by depositing silicon on top of silver, where, however, the different phases observed to date are so strongly hybridized with the substrate that not only the Dirac cones, but also the whole valence and conduction states of ideal silicene appear to be lost. Here, we provide evidence that at least part of this semimetallic behavior is preserved by the coexistence of more silicene phases, epitaxially grown on Ag(111). In particular, we combine electron energy loss spectroscopy and time-dependent density functional theory to characterize the low-energy plasmon of a multiphase-silicene/Ag(111) sample, prepared at controlled silicon coverage and growth temperature. We find that this mode survives the interaction with the substrate, being perfectly matched with the {\pi}-like plasmon of ideal silicene. We therefore suggest that the weakened interaction of multiphase silicene with the substrate may provide a unique platform with the potential to develop different applications based on two-dimensional silicon systems.

Fvs-Si48: a direct bandgap silicon allotrope.

A structurally stable silicon allotrope is predicted by means of first principles calculations. This new structure is composed of a six-membered ring, a five-membered ring and a three-membered ring with the space group PA3[combining macron] and fvs topology, which is named fvs-Si48. The calculations of geometrical, vibrational, and electronic and optical properties reveal that fvs-Si48 has good mechanical stability with a mass density of 1.86 g cm-3. More importantly, it is a semiconductor with a direct band gap of 2.15 eV. From the analysis of its optical properties, there is the possibility of its synthesis in theory. This fvs-Si48 could have a wide range of applications in photo catalysts, optoelectronics, hydrogen storage and aerospace engineering.

Prediction of another semimetallic silicene allotrope with Dirac fermions

Materials with Dirac point are so amazing since the charge carriers are massless and have an effective speed of light. However, among the predicted two-dimensional silicon allotropes with Dirac point, no one has been directly proved by experiment. This fact motivates us to search for other two-dimensional silicon allotropes. As a result, another stable single atomic layer thin silicon allotrope is found with the help of CALYPSO code in this work. This silicene allotrope is composed of eight-membered rings linked by Si–Si bonds with buckling formation. The electronic calculation reveals that it behaves as a nodal line semimetal with the linear energy dispersion relation near the Fermi surface. Notably, the ab initio molecular dynamics simulations display that the original atomic configuration can be remained even at an extremely high temperature of 1000 K. Additionally, hydrogenation could induce a semimetal-semiconductor transition in this silicene allotrope. We hope this work can expand the family of single atomic layer thin silicon allotropes with special applications.

Exploring silicon allotropy and chemistry by high pressure – high temperature conditions

Silicon is the second abundant element, after oxygen, in the earth crust. It is essential for today’s electronics because of its ability to show various electronic behaviors that allow covering the numerous fields of cutting-edge applications. Moreover, silicon is not a pollutant and, therefore, is an ideal candidate to replace the actual materials in photovoltaics, like compounds based on the arsenic and heavy metals. It has not replaced them so far because Si is an indirect gap semiconductor and cannot absorb directly the solar photons without thermal agitations of crystal lattice (phonons). This puts it apart from the next-generation applications (light diode, high-performance transistor). That justifies the attempts to create silicon materials with direct gap that can absorb and emit light. Our recent high-pressure studies of the chemical interaction and phase transformations in the Na-Si system, revealed a number of interesting routes to new and known silicon compounds and allotropes. The pressure-temperature range of their formation is suitable for large-volume synthesis and future industrial scaling. The variety of properties observed (e.g. quasi-direct bandgap of open-framework allotrope Si24) allows us to suggest future industrial applications.

Integrated form of the Anderson-Grüneisen equation of state for p-V-T data fit: Application to the compounds of boron, carbon, silicon and some metals.

The pressure-volume-temperature equations of state have been constructed by combining experimental data and semiempirical estimations for a number of compounds recently synthesized under extreme pressure-temperature conditions. The solids with various bonding types were considered: covalent hard and superhard boron-rich and diamond-like compounds (B6O, B13N2, BP, cBC5, and nano-cBN), ionic semiconductors (Mg2C and Mg2C3), as well as intercalation compounds (clathrates Na4Si24 and Na24+xSi136), and simple substances (γ-B28 and t’-B52 boron allotropes, Na and Mg metals, and open-framework silicon allotrope o-Si24). We also showed how the reliable p-V-T equations of state may be constructed using different types of available data.

Potential thermoelectric material open framework Si24 from a first-principles study

Open framework Si24 is a new synthesis cage-like silicon allotrope with a quasi-direct bandgap and predicted to exhibit outstanding adsorption efficiency, foreshowing the potential applications in the photovoltaic community. In this paper, the thermoelectric property of such new Si structures is investigated by combining first-principles calculation and semiclassical Boltzmann transport theory. The calculations show that the Si24 possesses a superb Seebeck coefficient, and obviously anisotropic electronic conductivity. Owing to more energy extremums existing in the conduction band region, the power factor of Si24 in the n-type doping is always better than that in p-type samples. Anisotropic phonon transport property is observed as well in Si24 with average lattice thermal conductivity of 45.35 W m−1 K−1 at room temperature. Based on the electron relaxation time estimated from the experiment, the thermoelectric figure of merit of Si24 is found to be as high as 0.69 (n-type doping at 700 K) and 0.51 (p-type doping at 700 K) along the xx crystal direction, which is about two orders of magnitude larger than that of diamond Si (d-Si). The findings presented in this work shed light on the thermoelectric performance of Si24 and qualify that such new Si allotrope is a promising platform for achieving the recombination of photovoltaic and thermoelectric technologies together.

Si24: An Efficient Solar Cell Material

Si24, a recently synthesized allotrope of silicon, has received much attention due to its quasi-direct band gap around 1.3 eV. To explore its potential application as a solar cell device, we investigated the doping effect on the electronic properties of Si24 using first-principles calculations. It is found that Si24 can be easily doped as both p- and n-type, and the dopants are readily ionized. Furthermore, the incorporation of these dopants only reduces the band gap of Si24 slightly, which remains in the ideal region for solar cells. Boron and phosphorus are identified as the most promising elements for the p-type and n-type doping in Si24, respectively, due to their low formation energies, small ionization energies, and small reductions in the band gap. These properties suggest great potential in constructing a novel Si24-based p-n junction which is highly desired for future industrial application of photovoltaic devices.

First-Principles Study of Sodium Intercalation in Crystalline Nax Si24 (0\u2009\u2264\u2009x\u2009\u2264\u20094) as Anode Material for Na-ion Batteries

The search for Si-based anodes capable of undergoing low volume changes during electrochemical operation in rechargeable batteries is ample and active. Here we focus on crystalline Si24, a recently discovered open-cage allotrope of silicon, to thoroughly investigate its electrochemical performance using density functional theory calculations. In particular, we examine the phase stability of NaxSi24 along the whole composition range (0 ≤ x ≤ 4), volume and voltage changes during the (de)sodiation process, and sodium ion mobility. We show that NaxSi24 forms a solid solution with minimal volume changes. Yet sodium diffusion is predicted to be insufficiently fast for facile kinetics of Na-ion intake. Considering these advantages and limitations, we discuss the potential usefulness of Si24 as anode material for Na-ion batteries.

Electronic structure and switching behavior of the metastable silicene domain boundary

Silicene, a silicon allotrope with a buckled honeycomb lattice, has been extensively studied in the search for materials with graphene-like properties. Here, we study the domain boundaries of a silicene 4 × 4 superstructure on an Ag(111) surface at the atomic resolution using scanning tunneling microscopy (STM) and spectroscopy (STS) along with density functional theory calculations. The silicene domain boundaries (β-phases) are formed at the interface between misaligned domains (α-phases) and show a bias dependence, forming protrusions or depressions as the sample bias changes. In particular, the STM topographs of the silicene–substrate system at a bias of ∼2.0 V show brightly protruding domain boundaries, which can be explained by an energy state originating from the Si 3s and 3pz orbitals. In addition, the topographs depicting the vicinity of the domain boundaries show that the structure does not follow the buckled geometry of the atomic ball-and-stick model. Inside the domain, STS data showed a step-up at ∼0.4 V, which originated from the Si 3p orbitals. We found this step-up to have shifted, which may be attributed to the strain effect at the interface regions between silver and silicene and between the domain and its boundary upon performing spatially resolved STS measurements. The metastable characteristic of the domain boundary (β-phase) causes changes, such as creation or annihilation, in the buckling structures (switching behavior). The observed low activation energy for the buckling change between distinct states may find applications in the electronic control of properties related to domain boundary structures in silicene.

A Novel Silicon Allotrope in the Monoclinic Phase.

This paper describes a new silicon allotrope in the P2/m space group found by first-principles calculations using the Cambridge Serial Total Energy Package (CASTEP) plane-wave code. The examined P2/m-Si belongs to the monoclinic crystal system. P2/m-Si is an indirect band-gap semiconductor with a band gap of 1.51 eV, as determined using the HSE06 hybrid functional. The elastic constants, phonon spectra and enthalpy indicate that P2/m-Si is mechanically, dynamically, and thermodynamically stable. P2/m-Si is a low-density (2.19 g/cm3) silicon allotrope. The value of B/G is less than 1.75, which indicates that the new allotrope is brittle. It is shown that the difference in the elastic anisotropy along different orientations is greater than that in other phases. Finally, to understand the thermodynamic properties of P2/m-Si, the thermal expansion coefficient α, the Debye temperature ΘD, and the heat capacities CP and CV are also investigated in detail.

Multilayer silicene: clear evidence of Ag-terminated bulk silicon

The existence of silicite, a new allotrope of silicon based on a stacking of hexagonal silicene planes, is one of the most discussed topics in the field of two-dimensional materials. Using grazing incidence x-ray diffraction (GIXD), we have followed the in situ growth of Si films on Ag(1 1 1) in the low-temperature growth regime (510–520 K). GIXD experiments demonstrate that Si films have a diamond-like structure, with an average lattice constant slightly different from that of bulk Si. The diffracted intensities associated with Si films are well reproduced by the Ag/Si(1 1 1) honeycomb chain model, whereas models with Ag-free Si surfaces fail to reproduce the experimental data.

A new family of multifunctional silicon clathrates: Optoelectronic and thermoelectric applications

To develop Si structures for multifunctional applications, here we proposed four new low-density silicon clathrates (Si-CL-A, Si-CL-B, Si-CL-C, and Si-CL-D) based on the same bonding topologies of clathrate hydrates. The electronic and thermal properties have been revealed by first-principles calculations. By computing their equation of states, phonon dispersion, and elastic constants, the thermodynamic, dynamic, and mechanical stabilities of Si-CL-A, Si-CL-B, Si-CL-C, and Si-CL-D allotropes are confirmed. In the low-density region of the phase diagram, Si-CL-B, Si-CL-D, and Si-CL-C would overtake diamond silicon and type II clathrate (Si-CL-II) and emerge as the most stable Si allotropes successively. Among them, the two direct semiconductors with bandgaps of 1.147 eV (Si-CL-A) and 1.086 eV (Si-CL-D) are found. The suitable bandgaps close to the optimal Shockley-Queisser limit result in better absorption efficiency in solar spectrum than conventional diamond silicon. Owing to the unique cage-based framework, the thermal conductivity of these Si allotropes at room temperature are very low (2.7–5.7 Wm−1 K−1), which are lower than that of diamond structured Si by two orders of magnitude. The suitable bandgaps, small effective masses, and low thermal conductivity of our new silicon allotropes are anticipated to find applications in photovoltaic and thermoelectric devices.

Ab initioprediction of a new allotrope of two-dimensional silicon

Silicene, a promising candidate for future electronic devices, has been fabricated only on supporting substrates as silicon atom prefers to form the sp3 hybridization structure. Therefore, it's important to search more stable two-dimensional (2D) silicon allotropes and several 2D silicon allotropes have been proposed recently. In this work, we predict a new type of 2D silicon allotrope (called OTDS) based on ab initio structure, phonon-mode and molecular dynamics calculations. OTDS has the in-plane octagonal tiling (OT) pattern with dumbbell-like structures and silicon atoms in OTDS are four- and three-coordinated. OTDS is a semiconductor with a large band gap (about 1.5 eV by HSE calculation) and the band gap can be tuned effectively by the in-plane strain. Perspective and side views of the atomic structure of OTDS.

Interaction of nucleobases with silicene nanoribbon: A density functional approach

Silicene, a hexagonal buckled 2D allotrope of silicon, shows potential for numerous new applications, and may allow for easier integration with existing silicon-based microelectronics than graphene. The possibility of the application of silicene as a biosensor is discussed. The interaction between a hydrogen-passivated zigzag silicene nanoribbon (ZSiNR) and the nucleic acid bases, adenine (A), guanine (G), cytosine (C) and thymine (T), is investigated by density functional theory. The binding energy of the nucleobase-stacked nanoribbons shows the following order: G>C>A>T.

Predicting 2D silicon allotropes on SnS2

A first principles study on the stability and structural and electronic properties of two-dimensional silicon allotropes on a semiconducting layered metal-chalcogenide compound, namely SnS2, is performed. The interactions between the two-dimensional silicon layer, commonly known as silicene, and the layered SnS2 template are investigated by analyzing different configurations of silicene. The calculated thermodynamic phase diagram suggests that the most stable configuration of silicene on SnS2 belongs to a family of structures with Si atoms placed on three different planes; so-called dumbbell silicene. This particular dumbbell silicene structure preserves its atomic configuration on SnS2 even at a temperature of 500 K or as a “flake” layer (i.e., a silicene cluster terminated by H atoms), thanks to the weak interactions between the silicene and the SnS2 layers. Remarkably, an electric field can be used to tune the band gap of the silicene layer on SnS2, eventually changing its electronic behavior from semiconducting to (semi)metallic. The stability of silicene on SnS2 is very promising for the integration of silicene onto semiconducting or insulating substrates. The tunable electronic behavior of the silicene/SnS2 van der Walls heterostructure is very important not only for its use in future nanoelectronic devices, but also as a successful approach to engineering the bang-gap of layered SnS2, paving the way for the use of this layered compound in energy harvesting applications.

Dispersion interactions in silicon allotropes.

van der Waals interactions are known to play a key role in the formation of weakly bound solids, such as molecular or layered crystals. Here we show that the correct quantum-chemical description of van der Waals dispersion is also essential for a correct description of the relative stability between purely covalently-bound solids like silicon allotropes. To this end, we apply periodic local MP2 and DFT with Grimme's empirical -D3 correction to 11 experimentally determined or yet hypothetical crystalline silicon structures, including the most recently discovered silicon allotropes. Both methods provide similar energy ordering of the polymorphs, which, at the same time, noticeably deviate from the order predicted by standard DFT without an appropriate description of the van der Waals dispersion.