Silicon Allotropes Research Articles

Quantum nuclear motion in silicene: Assessing structural and vibrational properties through path-integral simulations

This paper explores the interplay between quantum nuclear motion and anharmonicity, which causes nontrivial effects on the structural and dynamical characteristics of silicene, a two-dimensional (2D) allotrope of silicon with interesting electronic and mechanical properties. Employing path-integral molecular dynamics (PIMD) simulations, we investigate the quantum delocalization of nuclei, unraveling its impact on the behavior of silicene at the atomic scale. Our study reveals that this delocalization induces significant deviations in the structural parameters of silicene, influencing in-plane surface area, bond lengths, angles, compressibility, and overall lattice dynamics. Through extensive simulations, we delve into the temperature-dependent behavior between 25 and 1200 K, unveiling the role of quantum nuclear fluctuations in dictating thermal expansion and phonon spectra. The extent of nuclear quantum effects is assessed by comparing results of PIMD simulations using an efficient tight-binding Hamiltonian, with those obtained from classical molecular dynamics simulations. The observed quantum effects showcase non-negligible deviations from classical predictions, emphasizing the need for accurate quantum treatments in understanding the material’s behavior at finite temperatures. At low T, the 2D compression modulus of silicene decreases by a 14% due to quantum nuclear motion. We compare the magnitude of quantum effects in this material with those in other related 2D crystalline solids, such as graphene and SiC monolayers.

Magnetic interactions in chromium adsorption on silicene

Silicon is a material with well established technological applications. Thus, obtaining this material in nanostructured form increases its possibility of integration in the current technology. Silicene, a two-dimensional allotrope of silicon, has a natural compatibility with current silicon-based technology. In this work we use density-functional theory to identify the electronic and magnetic properties of chromium atoms on monolayer and bilayer silicene. We investigate several properties of chromium in silicene, such as dopant solubility limits, site preference for adsorption and charge transfer. We find that magnetization depends on key parameters such as buckling, interlayer distance and adsorption site. Chromium intercalated in silicene bilayers showed a very interesting relation between the buckling of silicene and the total magnetic moment. The larger is the buckling the smaller is the magnetic moment. Our results demonstrate that it is possible to manipulate the magnetic properties of silicene by changing chromium concentration, adsorption site and structural parameters of the host material.

Discovery of novel silicon allotropes with optimized band gaps to enhance solar cell efficiency through evolutionary algorithms and machine learning

In the pursuit of advancing solar energy technologies, this study presents 20 direct and quasi-direct band gap silicon crystalline semiconductors that satisfy the Shockley-Queisser limit, a benchmark for solar cell efficiency. Employing two evolutionary algorithm-based searches, we optimize structures and calculate fitness function using the DFTB method and Gaussian approximation potential. Following the preselection of structures based on energy considerations, we further optimize them using PBEsol DFT. Subsequently, we screen the structures based on their band gap, employing a DFTB method tailored for band gap calculation of silicon crystals. To ensure accurate band gap determination, we employ HSE and GW methods. To validate the structural stability, we employ phonon analysis via linear regression algorithm applied to PBEsol DFT data. Significantly, the structures unveiled in this study are of great importance due to their proven stability from both mechanical and dynamic perspectives. Furthermore, the ductility and low density of certain structures enhance their potential application. We examine the optical properties by studying the imaginary part of the dielectric function by solving the Bethe–Salpeter Equation on top of GW approximation. By calculating the SLME, we achieve an efficiency of 32.7% for Si22 at a thickness of 500 nm. Moreover, the study harnesses various machine learning algorithms to develop a predictive model for the band gap energy of these silicon structures. Input data for machine learning models are derived from structural MBTR and SOAP descriptors, as well as DFT outputs. Notably, the results reveal that features extracted from DFT outperform the MBTR and SOAP descriptors.

Stable novel silicon allotropes in space group P2/m with various band gap structures by high-throughput screening

Silicon is the fundamental material for the semiconductor and microelectronics industries, but controlling the electronic band gap structure of diamond-type silicon remains a huge challenge to adapt to growing applications. Here, we have predicated 23 new silicon allotropes in space group P2/m from 279 possible structures by high-throughput screening accompanied by graph and group theory based on random strategy (RG2) code. The mechanical, electronic and optical properties of these structures were studied in detail. These novel silicon allotropes demonstrate various electronic structures, including metal, direct/quasi direct bandgap structure, and indirect bandgap structures. These new silicon allotropes demonstrate various electronic structures, including metal, direct/quasi direct bandgap structure, and indirect bandgap structures. Besides different electronic bandgap structures, all 23 structures exhibit strong absorption in the visible light region and P2/m-15 demonstrates the excellent mechanical properties (Bulk modulus beyond 80 GPa). Based on their nice stability, good mechanical, electronic and optical properties validated by the ab inito molecular dynamics simulation, phonon spectra and density functional theoretical calculations, these predicted silicon allotropes provide not only ideas for the synthesis of new silicon allotropes but also dawn for expanding the application of semiconductor materials.

Computational insights into popsilicene as a new planar silicon allotrope composed of 5–8–5 rings

Silicon-based two-dimensional (2D) materials have garnered significant attention due to their unique properties and potential applications in electronics, optoelectronics, and other advanced technologies. Here, we present a comprehensive investigation of a novel silicon allotrope, Popsilicene (Pop-Si), derived from the structure of Popgraphene. Using density functional theory and ab initio molecular dynamics simulations, we explore the thermal stability, mechanical and electronic properties, and optical characteristics of Pop-Si. Our results demonstrate that Pop-Si exhibits good thermal stability at 1000 K. Electronic structure calculations reveal that Pop-Si is metallic, with a high density of states at the Fermi level. Furthermore, our analysis of the optical properties indicates that Pop-Si has pronounced UV–Vis optical activity, making it a promising candidate for optoelectronic devices. Mechanical property assessments show that Pop-Si has Young’s modulus ranging from 10 to 92 GPa and a Poisson’s ratio of 0.95. These results combined suggest its potential for practical applications.

Two novel silicon allotropes in monoclinic phase

Two new monoclinic silicon allotropes P2/m Si54 and V-Silicon are proposed. Mechanical, thermal and dynamic stability of P2/m Si54 and V-Silicon were verified by elastic constants, (AIMD) simulations and phonon spectra. The enthalpy of V-Silicon exceeds that of diamond Si by only 0.04 eV/atom, which indicates that it has excellent stability. V-Silicon has an indirect band gap with a band gap of 1.00 eV, P2/m Si54 has a quasi-direct band gap with a band gap of 0.60 eV. Additionally, P2/m Si54 and V-Silicon show excellent light absorption performance in different wavelength ranges in the visible region.

Flexural and acoustic phonon-drag thermopower and electron energy loss rate in silicene

We have performed a comprehensive numerical and analytical examination of two crucial transport aspects in silicene: the phonon-drag thermopower, Sp , and the electron’s energy loss rate, Fe . Specifically, our investigation is centered on their responses to out-of-plane flexural phonons and in-plane acoustic phonons in silicene, a two-dimensional allotrope of silicon as a function of electron temperature, T, and electron concentration, n, upto the room temperature. It is found that the calculated quantities have a non-monotonic dependence for the phonon modes for both parameters (T and n) considered while analytical results predict definite dependencies up to the complete low-temperature Bloch–Gruneisen (BG) regime. To provide a more comprehensive picture, we contrast the complete numerical outcomes with the approximated analytical BG results, revealing a convergence within a specific range of temperature and carrier concentration. In light of this convergence, we put forth suggestions to elucidate the underlying factors responsible for this behavior. A comparison based on the magnitude of the calculated quantities can be made from the figures between the two considered phonon modes, which clearly shows that the out-of-plane flexural phonons are effective throughout the considered temperature range. This observation leads us to posit that the dominating contribution of the out-of-plane flexural phonon modes hinges upon the deformation potential constant and phonon energy associated with the phonon mode. Our study carries significant implications for estimating the electrical and thermal properties of silicene and provides valuable insights for the development of devices based on silicene-based technologies.

A new superior electronic properties Si allotrope for power electronic device applications

A new I−4 space group silicon allotrope is proposed in this paper. The electronics properties, mechanical properties and Ag(100)/I4Si(100) interface properties are studied using first principle calculations method. The results of the phonon show that I−4 Si is dynamically stable. Elastic constants reveal I−4 Si is dynamical stable. Electronics properties calculations reveal that the CBM and VBM of I−4 Si are at X and M point, which indicates that I−4 Si is an indirect band gap semiconductor with a high band gap of 1.95 eV. To satisfy the demands for fabricating electronic devices, the N-type doping, P-type doping and Ohmic contact are studied, too. The fermi energy level of N-type and P-type I−4 Si move into conduction band and valence band, respectively. The Schottky barrier of Ag/I-4 interface is 0.65 eV. Meanwhile, the current-voltage curve becomes highly symmetric, suggesting an Ohmic behavior of the Ag(100)/I4Si(100) interface. Critical breakdown field calculations results show that the critical breakdown field of I−4 Si is 9.05 × 105 V cm−1, which is 3.02 times that of the diamond Si. Because band gap and critical breakdown field of I-4 Si are much greater than that of diamond Si, I-4 Si is potential electronic semiconductor material. Thus, I−4 Si can be applied in the field of modern power electronic device applications due to its superior electronic properties.

Theoretical prediction of a novel hexagonal narrow-gap silicon allotrope under high pressures

Silicon material plays a vital role in contemporary technology-related fields, including electronics and the photovoltaics. There is a growing demand for exploring new silicon structures with potential applications, and numerous metastable structures have been reported. In this study, we present the prediction of a novel stable sp 3 hybridized silicon allotrope using particle swarm optimization global structure search. The predicted Si allotrope is a semiconductor with an indirect band gap of approximately 0.21 eV. It possesses three Si basis atoms in the unit cell, and we named it Si3. Interestingly, when subjected to strain, it undergoes a transition from a semiconductive state to a metallic state. Furthermore, moderate tensile strain enhances the interactions between silicon and lithium atoms, suggesting its potential for Li-ion batteries. Additionally, Si3 exhibits exceptional sunlight absorption across a wide range of wavelengths, with a significantly higher light absorption intensity than cubic diamond silicon. These findings have important implications for photovoltaic applications.

Full-landscape selection rules of electrons and phonons and temperature-induced effects in 2D silicon and germanium allotropes

The electron-phonon (el-ph) and phonon-phonon interactions play crucial roles in determining the electronic and thermal transport properties of materials. Using the group theory and ab-initio calculations, we have derived the full-landscape selection rules for intravalley and intervalley scattering for carriers in 2D silicon and germanium allotropes with low buckled (LB) and large honeycomb dumbbell (LHD) structures, showing dominant role of optical modes in total el-ph coupling in LHD Si/Ge due to most el-ph scattering channels via acoustic modes blocked by symmetries. Remarkably, we show that due to the relatively large optical phonon bandwidth and “bunching effect" of acoustic phonon bands, the selection rules of three-phonon interactions reveal the dominant ooo and aoo channels in LHD and LB Si/Ge, respectively, and the temprature-induced effects significantly increase the ratio between mode-resolved four-phonon interactions and three-phonon interactions in most of the phonon-frequency region, finally leading to the low thermal conductivities in both LB and LHD Si/Ge. Furthermore, we observe the anomalous temperature-dependent thermal conductivities in LHD Ge, resulted from the vanishing quasi-acoustic-optical phonon gap under the temperature-induced effects. By considering full el-ph coupling and higher-order anharmonic phonon-phonon interactions, the maximal thermoelectric figures of merits in LHD Si and LB Ge are found to reach 1.06 and 0.66 at 700 K, respectively, significantly surpassing their bulk counterparts. Our work is poised to stimulate wide-ranging exploration into phonon transport across diverse materials, and benefits both fundamental knowledge and advanced technologies of 2D Si/Ge allotropes.

Two-dimensional silicene-based technologies in oncology: an emerging avenue.

Silicene, a two-dimensional allotrope of silicon, has attracted considerable attention due to its distinctive electronic, mechanical, and biochemical properties. This review critically examines the emerging applications of silicene in oncology, emphasising its potential roles in cancer therapy and research. Silicene exhibits exceptional biocompatibility and surface reactivity, positioning it as a promising candidate for oncological applications. This review addresses the current challenges and limitations in the clinical translation of silicene-based technologies, including issues of stability, toxicity, and scalable production. By synthesizing recent research findings, this review aims to provide an assessment of silicene's potential contributions to oncology and delineate future research trajectories in this innovative field.

Transferable equivariant graph neural networks for the Hamiltonians of molecules and solids

This work presents an E(3) equivariant graph neural network called HamGNN, which can fit the electronic Hamiltonian matrix of molecules and solids by a complete data-driven method. Unlike invariant models that achieve equivariance approximately through data augmentation, HamGNN employs E(3) equivariant convolutions to construct the Hamiltonian matrix, ensuring strict adherence to all equivariant constraints inherent in the physical system. In contrast to previous models with limited transferability, HamGNN demonstrates exceptional accuracy on various datasets, including QM9 molecular datasets, carbon allotropes, silicon allotropes, SiO2 isomers, and BixSey compounds. The trained HamGNN models exhibit accurate predictions of electronic structures for large crystals beyond the training set, including the Moiré twisted bilayer MoS2 and silicon supercells with dislocation defects, showcasing remarkable transferability and generalization capabilities. The HamGNN model, trained on small systems, can serve as an efficient alternative to density functional theory (DFT) for accurately computing the electronic structures of large systems.

High-throughput calculation screening for new silicon allotropes with monoclinic symmetry.

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 ml 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.

Lattice thermal conductivity of silicon monolayer in biphenylene network

Recently, the two-dimensional carbon sheet in a biphenylene network has been successfully fabricated by experiment [Fan et al., Science 372, 852 (2021)], promoting the study of silicon allotropes with similar structures. In this work, we investigate the lattice thermal conductivity of a silicon monolayer in a biphenylene network through first-principles calculations. It is found that the thermal conductivity is anisotropic and much lower than that of carbon sheets with a similar structure. At 300 K, the thermal conductivity is 2.46 and 3.25 W m−1 K−1 along the two crystallography directions, respectively. The phonon group velocity, relaxation time, and the contribution of each mode to total thermal conductivity are analyzed, to understand the underlying physical mechanisms of the low thermal conductivity. Our work provides fundamental insights into thermal transport in the silicon monolayer in the biphenylene network and should stimulate further experimental exploration of these materials for possible thermoelectric and thermal management applications.

The penta-hexa silicene: A promising candidate for intrinsic room temperature magnetic semiconductor

Performing ab initio calculations, we investigate electronic and magnetic properties of a silicon allotrope (PH-silicene) composed entirely by six silicon pentagons and two silicon hexagons. The dynamically and mechanically stable PH-silicene hosts two-dimensional honeycomb spin structures, which can be antiferromagnetic, ferromagnetic, or ferrimagnetic depending on the applied tensile strain and/or number of stacked layers. In particular, the transition temperature of an in-plane antiferromagnetic ground state and a strain-induced ferromagnetic state of monolayer PH-silicene is found to be around 533 and 80 K, respectively. This unusual metal-free magnetism can be explained by the d0 charge transfer mechanism. On the other hand, we show that the PH-silicene is an indirect semiconductor with the bandgap of 0.585 eV. When stacking up to 4-layers, they vary from the semiconductor, the semimetal to the normal metal. Our findings suggest PH-silicene as a promising candidate for the room temperature magnetic semiconductor and will pave a way for silicon based spintronic devices.

All-around encapsulation of silicene.

Silicene or the two-dimensional (2D) graphene-like silicon allotrope has recently emerged as a promising candidate for various applications in nanotechnology. However, concerns on the silicene stability still persist to date and need to be addressed aiming at the fabrication of competing and durable silicene-based devices. Here, we present an all-around encapsulation methodology beyond the current state-of-the-art silicene configuration, namely silicene sandwiched in between a capping layer (e.g., Al2O3) and the supporting substrate (e.g., Ag). In this framework, the insertion of one or two sacrificial 2D Sn layers enables the realization of different atomically thin encapsulation schemes, preserving the pristine properties of silicene while decoupling it from the growth template. On one hand, the epitaxy of a 2D Sn layer before silicene allows for the removal of the Ag substrate with no effect on silicene which in turn can be easily gated, for example, with an oxide layer on its top face. On the other hand, a full 2D encapsulation scheme, where top and bottom faces of silicene are protected by 2D Sn layers, gives rise to an atomically thin and cm2-scaled membrane preventing degradation of silicene for months. Both schemes thus constitute an advancement for the silicene stability and encapsulation in ambient conditions, paving the way to further exploitation in flexible electronics and photonics.

A Novel Two-Dimensional Allotrope of Silicon Grown on Al(111): A Case Study of the Interface Effect

Based on density functional calculations, a novel planar allotrope of silicon on Al(111) is rationally designed by using the lowest energy principle, which is also predicted to be stable above room temperature to suggest the possibility for its fabrication. The corresponding free-standing sheet would be buckled though the structural characteristics still remain, whose stability has also been evaluated in detail. It has been found to have strong anisotropic properties to be as hard as silicene in a specific direction and to be very soft in the perpendicular direction. Surprisingly, a peculiar negative Poisson’s ratio showing unusual mechanical properties is also found. The indirect bandgap of 0.14 eV of the free-standing sheet could be closed by applying tensile strain. Interestingly, the Dirac cone of the orthogonal lattice allotrope sheet can be obtained by applying biaxial strain, for which the velocity of the charge carriers could reach 5.97 × 105 m/s. In addition, the hydrogenated sheet is also quite stable and may find usage for sunlight absorption, field-effect transistor, etc., which may be decorated with Ti, Fe, or Co adsorptions to have half-metallic conducting properties for spin filtering.

Structure, Stability, and Electronic Properties of Hydrogenated Monolayer 2D Silicon Allotropes by First‐Principles Calculation

Due to silicon having a sp3 hybridization configuration in nature, most monolayer 2D silicon allotropes have to be proposed based on the dumbbell‐like units to ensure their structural stability. Herein, hydrogenated monolayer 2D silicon allotropes are predicted, and the sp3 configuration of structures is guaranteed by covering the surface with hydrogen. Compared with the corresponding hydrogen‐free structures and the hydrogen‐free structures on Ag(111) substrate, these predicted structures have lower formation energies, indicating that these monolayer 2D silicon allotropes can be obtained by hydrogenation reactions. These predicted structures are all dynamically stable and structures SCH, HPH, and HDH are thermodynamically stable. The structure SLH is the transition‐state structure of structure SCH and structures OTQTH and HHKH are metastable structures. The stable and metastable structures are all semiconductors and can be applied to semiconductor devices. All of these monolayer 2D silicon allotropes have high gravimetric hydrogen contents and may have applications for chemical hydrogen storage or water‐enabled H2 production. It is believed that the results of this work have a good reference value for the experimental synthesis of hydrogenated monolayer 2D silicon allotropes, and have reference significance for understanding the diversity and electronic properties of hydrogenated monolayer 2D silicon allotropes.

First-principle prediction of one-dimensional silicon allotropes: Promising new candidate for chemical and electrochemical hydrogen storage

Silicon is an excellent candidate for efficient hydrogen-storage medium, but hydrogen is mainly stored on the surface of silicon structure and the large volume expansion and contraction of structure during hydrogen storage/release seriously damages its structural stability. In order to increase hydrogen concentration values in silicon nanostructures and enhance the cyclic stability of structure, we predicted 1D silicon allotropes at different ambient hydrogen concentration conditions, and investigated the structural stability and electronic properties of these allotropes. The results show that the spaces around these structures give them a larger degree of freedom and the H2 filled in the interspaces also continuously provides them with sufficient hydrogen sources for supplement and replacement. Most of the generated 1D silicon allotropes are both dynamically and thermodynamically stable and most of them are semiconductors. The low structural dimension ensures that these 1D silicon allotropes have high chemical hydrogen storage capacity, and the structural stability at ambient hydrogen concentration provides a strong guarantee for the cyclic stability of chemical and electrochemical hydrogen storage for these allotropes. Moreover, hydrogen atoms can diffuse not only on the surface of 1D silicon allotropes but also between two closely spaced 1D silicon allotropes, which provides more convenience for the application of 1D silicon allotropes in chemical and electrochemical hydrogen storage. Our results can enrich the library of 1D silicon nanostructures used for hydrogen storage and provide more understanding of their structural stability and electronic properties, which can provide theoretical reference for their future application.

Zeolite framework silicon allotropes with direct band gap

It is a significant issue in the field of semiconductor devices and optoelectronic devices to find silicon allotropes with high mobility, direct band gap and high light absorption to replace traditional diamond silicon (d-Si). By constructing a zeolite framework, fifteen silicon allotropes with a direct band gap of 0.47–1.66 eV were screened from hundreds of zeolite framework silicon allotropes by ab initio calculations. The crystal structures, stability, effective mass, mechanical, electronic and optical properties were comprehensively studied. Compared with diamond silicon, several allotropes showed easy doping, low carrier effective mass and high absorption of the solar spectrum, which indicate promising candidates for adoption in photovoltaic applications.