Spherical Si Research Articles

Low pressure preparation of spherical Si@C@CNT@C anode material for lithium-ion batteries

Si@C@CNT@C composite is prepared by preliminary low-pressure forming Si@C and Si@C@CNT precursors from Si powder. After pyrolysis from glucose, acetylene and pitch, Si@C@CNT@C shows a spherical multi-phase composite structure. By using as lithium storage material, Si@C@CNT@C shows an initial discharge capacity of 620.5mAhg−1 with an initial coulombic efficiency of 82.2%. After 60 cycles, this spherical sample can maintain a reversible capacity of 563.5mAhg−1 at 100mAg−1, corresponding to a capacity retention of 90.8%. For comparison, the reversible capacities for Si powder, Si@C and Si@C@CNT are 10.9, 380.3 and 494.6mAhg−1, respectively. Even cycled at 400mAg−1, Si@C@CNT@C can deliver a reversible lithium storage capacity of 389.2mAhg−1. It indicates that spherical Si@C@CNT@C can be used as a high performance anode material for lithium-ion batteries.

Nanostructured silicon spheres prepared by a controllable magnesiothermic reduction as anode for lithium ion batteries

In this work, nanostructured, monodisperse Si spheres have been prepared by a facile, controllable magnesiothermic reduction method using monodisperse SiO2 spheres as the precursor. The reaction conditions for magnesiothermic reduction have been optimized, yielding desired product and byproduct. Carbon coating on Si nanocrystals is performed by an easy chemical vapor deposition (CVD) method. The effect of carbon coating on electrochemical performance of Si is investigated. The results show that the original monodisperse spheric morphology of SiO2 can be preserved during magnesiothermic reduction and the cycling stability of Si can be enhanced by carbon coating. This work provides a facile, scalable method to synthesize nanostructured Si and Si/C composites.

Porous Si spheres encapsulated in carbon shells with enhanced anodic performance in lithium-ion batteries

A novel type of porous Si–C micro/nano-hybrids, i.e., porous Si spheres encapsulated in carbon shells (porous Si@C spheres), has been constructed through the pyrolysis of polyvinylidene fluoride (PVDF) and subsequent magnesiothermic reduction methodology by using SiO2 spheres as precursors. The as-synthesized porous Si@C spheres have been applied as anode materials for lithium-ion batteries (LIBs), and exhibit enhanced anodic performance in term of cycling stability compared with bare Si spheres. For example, the porous Si@C spheres are able to exhibit a high reversible capacity of 900.0mAhg−1 after 20 cycles at a current density of 0.05C (1C=4200mAg−1), which is much higher than that of bare Si spheres (430.7mAhg−1).

Microstructures, optical and photoelectric conversion properties of spherical silicon solar cells with anti-reflection SnOx:F thin films

The microstructures and optical and photoelectric conversion properties of spherical silicon (Si) solar cells were investigated and discussed. The surface of the spherical Si with a pn junction provided high crystallinity, and the lattice constant of the center of Si spheres is larger than that of the surface, which would be due to the lattice distortion by defect structures at the center of Si. The conversion efficiencies of spherical Si solar cells coated with SnOx:F anti-reflection thin films were improved by annealing. The optical absorption and fluorescence of the solar cells increased, and the lattice constants of SnOx:F anti-reflection layers decreased after annealing. The mechanisms of chemical reactions at the Si/metal interface were also discussed.

In situ observation of melting and crystallization of Si on porous Si3N4 substrate that repels Si melt

High temperature in situ observation of melting and crystallization of spherical Si droplets on a substrate with a porous surface was carried out for the first time using an original in situ observation apparatus. The contact angle between the Si melt and the substrate was measured to be 160°, with the Si melt forming spherical droplets on the substrate. During crystallization, a ring-like pattern was observed on the surface of the spherical Si melt droplets due to crystal growth at low levels of supercooling. The solidified spherical Si crystals consisted of single or twin grains. This demonstrates that high-quality spherical Si crystals can be prepared easily and stably by using a Si melt-repelling substrate.

Three-Dimensional Interconnected Network of Graphene-Wrapped Porous Silicon Spheres: In Situ Magnesiothermic-Reduction Synthesis and Enhanced Lithium-Storage Capabilities

A novel type of 3D porous Si-G micro/nanostructure (i.e., 3D interconnected network of graphene-wrapped porous silicon spheres, Si@G network) was constructed through layer-by-layer assembly and subsequent in situ magnesiothermic-reduction methodology. Compared with bare Si spheres, the as-synthesized Si@G network exhibits markedly enhanced anodic performance in terms of specific capacity, cycling stability, and rate capability, making it an ideal anode candidate for high-energy, long-life, and high-power lithium-ion batteries.

Growth of spherical Si crystals on porous Si3N4 substrate that repels Si melt

Porous ceramic substrates that repel Si melt have been developed, and spherical Si crystals have been easily grown using these substrates. Spherical Si crystals grown at cooling rates of less than 100K/h consist of either single crystals or fewer than three grains. In each of the spherical Si crystals obtained in repeated growth experiments using the same substrate, the carbon and oxygen concentrations were consistently below the specification values for Si in solar cells. Degradation of the porous substrate was also examined to check whether it is possible to reuse the substrate.

Microstructure analysis and properties of spherical silicon solar cells with anti‐reflection thin films

AbstractStructure and properties of spherical silicon solar cells with anti‐reflection thin films were investigated and discussed. Conversion efficiencies of spherical Si solar cells coated with F‐doped SnO2 anti‐reflection films were improved by annealing. Optical absorption and fluorescence of the solar cells increased after annealing. Lattice constants of F‐doped SnO2 anti‐reflection layers, which were investigated by X‐ray diffraction, decreased after annealing. Mechanisms of atomic diffusion in SnO2 and reactions at the Si/metal interface were discussed. The present work indicated a guideline for spherical silicon solar cells with higher efficiencies. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nanostructure of the Si–Al eutectic and its use in lithium batteries

Abstract

Revised transmission line model for electromagnetic characterization of metasurfaces

We have revised the transmission line model with the purpose of simplifying the electromagnetic characterization of metasurfaces with both electric and magnetic responses. The revised model is an efficient tool for the case when a metasurface is located on top of a finite-thickness substrate or inside a finite-thickness slab. We have derived analytical expressions for the grid series impedance and shunt admittance of a substrated metasurface. The presented theory is illustrated by a numerical example of an array consisting of Mie-resonant silicon nanospheres. We have retrieved individual polarizabilities of a single sphere in a uniform host medium and compared them with the predictions of classical Mie theory. Further, we have studied a more practical case when the array of Si spheres is located on top of a finite-thickness glass substrate.

Designing photonic structures of nanosphere arrays on reflectors for total absorption

By means of full electrodynamic simulations, we investigate structures that can totally absorb light minimizing all reflections. Such efficient absorbers of visible and infrared light are useful in photovoltaic and sensor applications. Our study provides a simple and transparent analysis of the optical properties of structures comprising a resonant cavity and a reflector, which are the basic ingredients of a resonant absorber, based on general principles of scattering theory. We concentrate on periodic arrays of metallic or dielectric spherical particles in front of metallic or dielectric mirrors and show that tuning the material absorption could turn resonances in the structures into total absorption bands. Perfect absorption is predicted in metallic sphere arrays but also for Si spheres on a metallic substrate, moreover, by replacing the substrate below the Si spheres with a lossless dielectric Bragg mirror an all-dielectric-perfect-absorber is designed.

Nanoscale evidence of erbium clustering in Er-doped silicon-rich silica

Photoluminescence spectroscopy and atom probe tomography were used to explore the optical activity and microstructure of Er3+-doped Si-rich SiO2 thin films fabricated by radio-frequency magnetron sputtering. The effect of post-fabrication annealing treatment on the properties of the films was investigated. The evolution of the nanoscale structure upon an annealing treatment was found to control the interrelation between the radiative recombination of the carriers via Si clusters and via 4f shell transitions in Er3+ ions. The most efficient 1.53-μm Er3+ photoluminescence was observed from the films submitted to low-temperature treatment ranging from 600°C to 900°C. An annealing treatment at 1,100°C, used often to form Si nanocrystallites, favors an intense emission in visible spectral range with the maximum peak at about 740 nm. Along with this, a drastic decrease of 1.53-μm Er3+ photoluminescence emission was detected. The atom probe results demonstrated that the clustering of Er3+ ions upon such high-temperature annealing treatment was the main reason. The diffusion parameters of Si and Er3+ ions as well as a chemical composition of different clusters were also obtained. The films annealed at 1,100°C contain pure spherical Si nanocrystallites, ErSi3O6 clusters, and free Er3+ ions embedded in SiO2 host. The mean size and the density of Si nanocrystallites were found to be 1.3± 0.3 nm and (3.1± 0.2)×1018 Si nanocrystallites·cm−3, respectively. The density of ErSi3O6 clusters was estimated to be (2.0± 0.2)×1018 clusters·cm−3, keeping about 30% of the total Er3+ amount. These Er-rich clusters had a mean radius of about 1.5 nm and demonstrated preferable formation in the vicinity of Si nanocrystallites.

In Situ TEM of Two-Phase Lithiation of Amorphous Silicon Nanospheres

To utilize high-capacity Si anodes in next-generation Li-ion batteries, the physical and chemical transformations during the Li-Si reaction must be better understood. Here, in situ transmission electron microscopy is used to observe the lithiation/delithiation of amorphous Si nanospheres; amorphous Si is an important anode material that has been less studied than crystalline Si. Unexpectedly, the experiments reveal that the first lithiation occurs via a two-phase mechanism, which is contrary to previous understanding and has important consequences for mechanical stress evolution during lithiation. On the basis of kinetics measurements, this behavior is suggested to be due to the rate-limiting effect of Si-Si bond breaking. In addition, the results show that amorphous Si has more favorable kinetics and fracture behavior when reacting with Li than does crystalline Si, making it advantageous to use in battery electrodes. Amorphous spheres up to 870 nm in diameter do not fracture upon lithiation; this is much larger than the 150 nm critical fracture diameter previously identified for crystalline Si spheres.

Microstructure Analysis and Properties of Anti-Reflection Thin Films for Spherical Silicon Solar Cells

Structure and properties of anti-reflection thin films of spherical silicon solar cells were investigated and discussed. Conversion efficiencies of spherical Si solar cells coated with F-doped SnO2 anti-reflection films were improved by annealing. Optical absorption and fluorescence of the solar cells increased after annealing. Lattice constants of F-doped SnO2 anti-reflection layers, which were investigated by X-ray diffraction, decreased after annealing. A mechanism of atomic diffusion of F in SnO2 was discussed. The present work indicated a guideline for spherical silicon solar cells with higher efficiencies.

High Temperature In-situ Observation of Si on the Porous Ceramic Substrate Repelling Si Melt for Growth of Spherical Si Crystal

High Temperature In-situ Observation of Si on the Porous Ceramic Substrate Repelling Si Melt for Growth of Spherical Si Crystal

Microstructure and microhardness of melt-spun Al–25Si–5Fe–XCo (X=0, 1, 3, 5) alloys

The microstructure and microhardness evolution of melt-spun Al–25Si–5Fe alloy with Co addition (1, 3 and 5 wt.%) were investigated. Microstructural and spectroscopic analyses demonstrate that Co could refine primary Si grains and change their morphology because it causes higher constitutional undercooling and has large mixing enthalpy with Si. Especially, 3 wt.% Co addition causes homogeneously distributed fine spherical Si particles in the rapidly solidified Al–25Si–5Fe alloy. The size of the spherical silicon particles was from around 200 to 600 nm near the wheel side region, and it varied from 800 nm to 1.3 μm at the air side. The optimum ratio of Si and Co should be between 6 and 8.3 to form spherical Si grains in Al–25/30Si–5Fe alloys. A considerable improvement in microhardness value (from 211 to 370 HV) was obtained with the addition of Co. ► Microstructure of melt-spun Al–Si–Fe alloy with Co addition was studied. ► Co modifies the primary Si in rapidly solidified Al–Si–Fe alloy. ► Formation of spherical Si particles in Al–Si alloy by Co addition was reported. ► An optimum ratio of Si and Co for formation of spherical Si particles was suggested.

Effect Of Barrier Height On Linear And Nonlinear Photoabsorption and Refractive Index Change In Si Quantum Dots

The effect of barrier height on the photoabsorption process and refractive index changes are studied for the intraband transitions in spherical single electron Si quantum dots embedded in dielectric matrix. We use the effective mass approximation and consider (i) the finite, and (ii) infinite barrier height at the interface of the dot and matrix material. The results obtained for the dipole allowed S-P transition show that the increase in barrier height leads to blue shift in peak positions of absorption coefficient and refractive index change. We also investigate the effect of intensity and dot radius on the above parameters.

Modeling of a Room-Temperature Silicon Quantum Dot-Based Single-Electron Transistor and the Effect of Energy-Level Broadening on Its Performance

In this work, we have modeled silicon quantum dot (QD)-based single-electron transistors (SETs) operating at room temperature and investigated the effect of the QD’s energy-level broadening on the performance of the SET. First we obtained the energy levels and corresponding wave functions for spherical Si QDs by solving the coupled Schrodinger–Poisson equations in three dimensions. Then, we demonstrated different tunneling current rates for separated energy levels by considering nonequal energy-level broadenings. Accordingly, an expression for the corresponding tunneling rates in the quantum Coulomb blockade regime was derived. In the next step, the transconductance characteristics of the Si QD SET device with Coulomb oscillations were simulated, and their differences from previously investigated metal-based SETs were demonstrated. Finally, by applying different bias voltages, we determined the effect of temperature variations on the transconductance characteristics.

Third harmonic generation in intraband transitions of spherical silicon quantum dots

A theoretical study of the third harmonic generation (THG) is reported involving intraband transitions in the conduction band of spherical Si semiconductor quantum dot surrounded by SiO2, Si3N4, and SiC matrix. The wave function and energies of a singly charged Si dot are calculated using the effective mass approximation. A finite barrier height is considered at the interface of the dot and the surrounding matrix. The results show that the THG coefficient of the silicon quantum dot strongly depends on the radius of the dot and the surrounding matrix.

Demonstration of Magnetic Dipole Resonances of Dielectric Nanospheres in the Visible Region

Strong resonant light scattering by individual spherical Si nanoparticles is experimentally demonstrated, revealing pronounced resonances associated with the excitation of magnetic and electric modes in these nanoparticles. It is shown that the low-frequency resonance corresponds to the magnetic dipole excitation. Due to high permittivity, the magnetic dipole resonance is observed in the visible spectral range for Si nanoparticles with diameters of ∼200 nm, thereby opening a way to the realization of isotropic optical metamaterials with strong magnetic responses in the visible region.