Amorphous Si Matrix Research Articles

Interface and Morphology Engineered Amorphous Si for Ultrafast Electrochemical Lithium Storage.

Ultrafast high-capacity lithium-ion batteries are extremely desirable for portable electronic devices, where Si is the most promising alternative to the conventional graphite anode due to its very high theoretical capacity. However, the low electronic conductivity and poor Li-diffusivity limit its rate capability. Moreover, high volume expansion/contraction upon Li-intake/uptake causes severe pulverization of the electrode, leading to drastic capacity fading. Here, interface and morphology-engineered amorphous Si matrix is being reported utilizing a few-layer vertical graphene (VG) buffer layer to retain high capacity at both slow and fast (dis)charging rates. The flexible mechanical support of VG due to the van-der-Waalsinteraction between the graphene layers, the weak adhesion between Si and graphene, and the highly porous geometry mitigated stress, while the three-dimensional mass loading enhanced specific capacity. Additionally, the high electronic conductivity of VG boosted rate-capability, resulting in a reversible gravimetric capacity of ≈1270mAhg-1 (areal capacity of ≈37µAhcm-2) even after 100 cycles at an ultrafast cycling rate of 20C, which provides a fascinating way for conductivity and stress management to obtain high-performance storage devices.

Facile co-electrodeposition of amorphous silicon anode with embedded nickel nanoparticles for enhancing rate capability in lithium-ion batteries

• Potentiostatic co-electrodeposition enables incorporation of Ni nanoparticles into Si matrix. • Crystalline Ni nanoparticles enhances total electrical conductivity of Si anode. • Ni particles effectively buffers the volume change of Si matrix during charge/discharge. • A high discharge capacity of 1633 mA h g −1 is retained after 100 charge-discharge cycles. Many efforts have been invested to improve the electrical conductivity and stability of Si anode, such as alloying and adding conductive additives, which unfortunately, are difficult for large scale manufacturing due to the complicated and expensive preparation techniques. In this work, a Si–Ni composite anode is prepared by a facile potentiostatic co-electrodeposition approach from organic solution. Correspondingly, it exhibits a capacity as high as 1633 mA h g − 1 after 100 charge-discharge cycles, whereas, Si–Ni composite anodes synthetized by previous approaches show specific capacity less than 1200 mA h g − 1 . It is believed that Ni nanoparticles with a diameter from 3 to 10 nm embedded in amorphous Si not only result in a lower resistance of Si–Ni anode compared with pristine Si anode, thus enhancing the rate performance, but also stabilize the integrity of the amorphous Si matrix, and buffer the volume change of Si anode during the lithiation–delithiation processes, thus improving the cycling performance.

Thermostructural Characterization of Silicon Carbide Nanocomposite Materials via Molecular Dynamics Simulations

In this paper, we investigate the thermostructural properties of a type of silicon-based nanomaterials, which we refer to as SiC@Si nanocomposites, formed by SiC crystalline nanoparticles (with the cubic phase), embedded within an amorphous Si matrix. We have followed an in silico approach to characterize the mechanical and thermal behaviour of these materials, by calculating the elastic constants, uniaxial stress-strain curves, coefficients of thermal expansion, and specific heats, at different temperatures, using interatomic potential calculations. The results obtained from our simulations suggest that this type of material presents enhanced thermal resistance features, making it suitable to be used in devices subjected to big temperature changes, such as heat sinks in micro and nanoelectronics, solar energy harvesters at high temperatures, power electronics, or in other applications in which good thermomechanical properties are required.

Amorphous Si1-yCy composite anode materials: ab initio molecular dynamics for behaviors of Li and Na in the framework.

Although amorphous Si/C composite anode materials with various types of nanostructures Si/C materials have been experimentally proposed for rechargeable ion batteries for their structural durability, the atomistic mechanism primarily suggesting Li and Na monovalent ion intercalation into an amorphous Si/C composite matrix has not theoretically been understood to explore the thermodynamic and kinetic features of the a-Si/C composite phase regarding the effects on the carbon addition to an amorphous Si matrix. In this work, systematic ab initio molecular dynamics calculations (AIMDs) were conducted to identify electrochemical intercalation reactions involved in nanostructure evolutions, which correspond to favorable ion-intercalated formations, volume expansions, pair correlations, charge transfers, and diffusion behaviors of metals in a-MxSi1-yCy (Mx: Lix and Nax) alloys with increasing x contents of atomic concentrations. AIMDs using the a-Si1-yCy composite phase might allow one to have an atomic-level understanding of the composite phase and further insightful comprehension of any implementations such as the controlled ratio of the Si1-yCy composite and multivalent ions inserted into the framework.

Surface Effects and Optical Properties of Self-Assembled Nanostructured a-Si:Al.

We present a study of the surface effects and optical properties of the self-assembled nanostructures comprised of vertically aligned 5 nm-diameter Al nanowires embedded in an amorphous Si matrix (a-Si:Al). The controlled (partial) removal of Al nanowires in a selective etching process yielded nanoporous a-Si media with a variable effective surface area. Different spectroscopy techniques, such as X-ray photoelectron spectroscopy (XPS), UV-Vis spectrophotometry and photoluminescence (PL), have been combined to investigate the impact of such nanostructuring on optical absorption and emission properties. We also examine long-term exposure to air ambient and show that increasing level of surface oxidation determines the oxide defect-related nature of the dominant PL emission from the nanoporous structures. The role of bulk, nanosize and surface effects in optical properties has been separated and quantified, providing a better understanding of the potential of such nanoporous a-Si:Al structures for future device developments.

Low‐frequency dielectric performance of polyimide composites containing amorphous polymer‐derived SiCN ceramics

AbstractHigh‐dielectric‐constant polymer‐matrix composites have potential applications as embedded capacitors. In this paper, an amorphous polymer‐derived SiCN ceramic was incorporated into a polyimide (PI) matrix by in situ polymerization for the first time. The resulting SiCN/PI composite showed good thermal stability below 500°C, with the SiCN filler being homogeneously dispersed in the PI matrix. The dielectric properties of the SiCN/PI composites were investigated over a broad frequency range (0.1‐106 Hz). The dielectric properties of the SiCN/PI composites at low frequencies revealed a weak percolation phenomenon similar to that shown by conductive/polymer composites and the calculated percolation threshold was 22.8 vol%. At SiCN loadings of 25 vol%, the SiCN/PI composites showed a high dielectric constant of 190 at 0.1 Hz, which is 57 times higher than that of pure PI (3.3). The conductivity of the 25 vol% SiCN/PI composite was as low as 8.5 × 10−10 S·cm−1. These results could be explained by the unique microstructure of the polymer‐derived SiCN ceramic, containing an amorphous Si matrix and a free carbon phase. This study revealed SiCN/PI composites as promising dielectric materials with promising applications in energy storage fields.

Meandering growth of in-plane silicon nanowire springs

Despite the fundamental difference in material systems and temporal evolution, self-oscillating growth of silicon nanowires (SiNWs), led by metal droplets, resembles very much natural river meanders in terms of their sinuosity, fractal dimensions, and scaling law. Both of them are driven by the release of higher potential energy stored in the disorder hydrogenated amorphous Si (a-Si:H) matrix or at highlands, tailored by a streamwise flow mechanism and subject to an erodible boundary constraint imposed by the a-Si:H thin film or the soil banks, respectively. Under specific conditions, the cross-droplet/stream velocity difference can be magnified, during the in-plane growth of SiNWs, to stimulate regular swaggering dynamics that produce continuous and smooth SiNW meanders. This interesting phenomenon indicates a rather simple and highly efficient strategy to shape complex elastic channels with only a few control parameters. A kinetic model has been established to explain the underlying mechanism of the self-oscillating meandering growth, which has unique potential to transform rigid SiNW channels into elastic forms for flexible or stretchable electronic applications.

Vertically aligned Si nanocrystals embedded in amorphous Si matrix prepared by inductively coupled plasma chemical vapor deposition (ICP-CVD)

Abstract Vertically-aligned nanostructured silicon films are deposited at room temperature on p-type silicon wafers and glass substrates by inductively-coupled, plasma-enhanced chemical vapor deposition (ICPCVD). The nanocrystalline phase is achieved by reducing pressure and increasing RF power. The crystalline volume fraction ( X c ) and the size of the nanocrystals increase with decreasing pressure at constant power. Columnar growth of nc-Si:H films is observed by high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). The films exhibit cauliflower-like structures with high porosity that leads to slow but uniform oxidation after exposure to air at room temperature. Films deposited at low pressures exhibit photoluminescence (PL) signals that may be deconvoluted into three distinct Gaussian components: 760–810, 920–935, and 990–1000 nm attributable to the quantum confinement and interface defect states. Hydrogen dilution is manifested in significant enhancement of the PL, but it has little effect on the nanocrystal size and X c .

A novel Si film with Si nanocrystals embedded in amorphous matrix on Cu foil as anode for lithium ion batteries

A novel Si film with Si nanocrystals embedded in amorphous Si matrix was grown on Cu foil via an inductively coupled plasma-enhanced chemical vapor deposition and subsequent electron beam irradiation. The Si nanocrystals show spherical morphology with rich defects and the mean size is about 6nm. The electrochemical performance of the obtained Si as anode for lithium ion batteries was studied via conventional charge/discharge test, which shows distinct improvement on cycle performance compared with that of amorphous Si obtained without electron beam irradiation.

Stabilization mechanism of electrodeposited silicon thin films.

Amorphous composite silicon thin films electrodeposited in tetrahydrofuran, containing up to 80 at% of Si and exhibiting an homogeneous dispersions of O, C and Cl in the amorphous Si matrix, have been successfully stabilized against oxidation using a post-annealing step in inert atmosphere. In order to understand the impact of the annealing step on their stabilization against oxidation, their composition and structure have been investigated upon heat treatments. It has been shown that the presence of impurities such as O, C and Cl does not have any impact on the stabilization process, which is rather linked to the presence of hydrogen in the Si composites. This conclusion has been drawn after a detailed analysis of the bonding structure of films annealed at different temperatures and dwell times by the mean of Raman spectroscopy. It has been shown that annealing the as-deposited films at 350 °C for a couple of hours or at higher temperatures induced a hydrogen evolution, characterized by the breaking of Si-H bonds and the formation of Si-Si bonds, which stabilized the silicon network. The understanding and the reproducibility of this stabilization process of silicon thin film electrodeposited in organic solvent paves the way for their use for many applications.

Deposition and characterization of silicon–aluminum non-conductive vacuum metallization coatings

This study has examined the microstructure, electrical and optical properties of Si–Al thin films that have an Al content of 15–50wt% and can be used as non-conductive vacuum metallization coatings. The films were magnetron sputtered using composite targets at ambient temperatures. A dual-phase microstructure composed of nano-sized Al particles dispersed in an amorphous Si matrix was obtained. The Si–Al films exhibited a high reflectance of 70–80% in the visible region and a high electric resistivity of 25–1000Ωcm at Al content of 15–30wt%.

Characterizations and electrochemical behaviors of milled Si with a degree of amorphization and its composite for Li-ion batteries

Electrochemical behaviors of milled Si using high energy mechanical milling (HEMM) process are characterized with a degree of amorphization. The amount of amorphous Si increases with milling time, and Si crystallites are embedded in the amorphous Si matrix. Unlike crystallite Si, partially amorphous Si starts to react with Li near 0.32 V. An intermediate LiSi phase is identified at 0.17 V during the first discharge of amorphous Si with Li. The milled Si/graphite (m-Si/G) composite prepared by using the simple HEMM process shows excellent electrochemical performance with a reversible capacity of 800 mAh g−1 at a rate of 0.5 A g−1 when cycled between 0.11 and 2.0 V.

Synthesis of embedded Si nano-particles using swift heavy ions and its optical properties

ABSTRACTEmbedded Si nano-particles of average size around 5nm were synthesized in an amorphous Si matrix by two stage ion implantation processes. It has been observed that amorphous Si (a-Si) layers were recrystallized using 50 MeV Au ions with enhanced regrowth rate with activation energy in the range of 0.29 eV. During the crystallization process Si nanocrystals were formed in the a-Si layers due to sudden quenching of the molten tracks created by MeV Au ions. The recrystalizations were confirmed by Rutherford backscattering spectrometry-Channeling (RBSC) technique. The structural modification and nanocluster creation that emerged during recrystallization process was observed in high-resolution transmission electron microscopy and photoluminescence (PL) spectroscopy. The PL emission was observed over a broad band of 2.8 – 3.4 eV and centered at 3.25 eV. The Si nano-crystal formation can be explain by a mechanism combining the melting within the ion tracks by thermal spike process and the subsequent recrystallization nucleated from the crystalline sides at the interface.

Study of annealing induced redistribution of implanted Au in Si: Fluence dependence

One hour 500 °C air annealing induced movement of implanted Au in Si have been studied for 32 keV Au implantation in Si, in the fluence range of 1 × 10 15 – 1 × 10 17 ions cm - 2 . Samples were characterized using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy. The results indicate that, depending on the initial state of Au in the matrix, there is a clear difference in the diffusion behaviour of Au in Si. When Au is precipitated as gold-silicide nanoclusters inside the Si matrix, annealing is found to cause diffusion of Au into the bulk Si. Compared to this, for a random atomic distribution of Au in an amorphous Si matrix, annealing is found to result in out-diffusion of Au towards the surface.

Nanoindentation-induced phase transformations in silicon at elevated temperatures

The nanoindentation-induced phase transformation behavior of silicon at elevated temperatures(25–150 °C) has been studied. Nucleation of Si-III/Si-XII on unloading is enhanced withincreasing temperature and at the highest temperatures in an amorphous Si matrix,occurs in a continuous fashion without a pop-out event. Interestingly, for slowunloading at the highest temperatures, formation of Si-III/Si-XII in a crystalline Simatrix was not observed. Elevated temperatures enhance the nucleation of Si-IIIand Si-XII during unloading but the final composition of the phase transformedzone is also dependent on the thermal stability of the phases in their respectivematrices.

Phase separated AlSi thin films prepared by filtered cathodic arc deposition

Phase separated AlSi films composed of Al cylinders embedded in an amorphous Si matrix were prepared on conducting Si substrates by filtered cathodic arc deposition. The compositional dependence of AlSi films on a negative substrate bias showed a different trend depending on the cathode composition because of the self-sputtering process during the deposition. The porous structure was obtained from the phase separated AlSi film after removal of Al cylinders by wet etching in an ammonia solution. Scanning electron microscope images of the etched AlSi films showed that the average diameter of pores was increased from 3 nm to 7 nm by applying a negative substrate bias voltage during the deposition. The honeycomb ordered arrangement of pores was observed at 0 V and − 25 V substrate bias. The substrate temperature during the depositions had almost the same effect on the film morphologies as the negative substrate bias.

Surface Plasmon Resonance of Ag Nanoparticles Embedded in Partially Oxidized Amorphous Si Matrix

Nanocomposite films containing Ag nanoparticles embedded in partially oxidized amorphous Si matrix were deposited on silica glass substrates by co-sputtering of Ag and Si with 1.5 keV neutral Ar atoms. The Ag content and thickness of the nanocomposite films was determined by Rutherford backscattering spectrometry. Optical absorption studies revealed the presence of surface plasmon resonance (SPR) indicating the formation of Ag nanoparticles in the as-deposited films. The position, width and strength of SPR have been found to be strongly dependent on the Ag content of the films. For annealing in oxidizing atmosphere, a significant red shift in the SPR along with a drastic reduction in the resonant absorption has been observed. The amount of red shift has been found to be dependent on the Ag content of the films. Transmission electron microscopy was used to study the size distribution, shape and crystal structure of Ag nanoparticles in the nanocomposite films. TEM analysis of annealed sample revealed the formation of silver oxide nanoshells surrounding Ag nanoparticles.

Stress and Crystallization of Plasma Enhanced Chemical Vapour Deposition Nanocrystalline Silicon Films

Nanocrystalline Si films were prepared with a RF-PECVD system using different SiH4/H2 ratios, plasma powers, substrate temperatures and annealing conditions. The film's intrinsic stress was characterized in relation to the crystallization fraction. Results show that an increasing H2 gas ratio, plasma power or substrate temperature can shift the growth mechanism across a transition point, past which nanocrystalline Si is dominant in the film structure. The film's intrinsic stress normally peaks during this transition region. Different mechanisms of stress formation and relaxation during film growth were discussed, including ion bombardment effects, hydrogen induced bond-reconstruction and nanocomposite effects (nanocrystals embedded in an amorphous Si matrix). A three-parameter schematic plot has been proposed which is based on the results obtained. The film structure and stress are presented in relation to SiH4 gas ratio, plasma power and temperature.

Substrate bias effect on Al–Si and Al–Ge thin film structure

Phase-separated Al–Si and Al–Ge films, composed of Al cylinders embedded in an amorphous Si and Ge matrix, respectively, were investigated by sputtering method with substrate bias voltage. In the Al–Si system, the period of Al cylinders was increased by applying a negative bias voltage to the substrate, which is consistent with the simulation results. On the other hand, the diameter of Al cylinders increased up to a bias voltage of − 80 V, and then it began decreasing beyond − 100 V accompanied by the appearance of a chain-like arrangement of Al cylinders. In the Al–Ge system, the period and diameter of Al cylinders increased by applying a negative bias voltage. However, in the Ge-rich film prepared at a high bias voltage, relatively large dots with low area density were observed. Transmission electron microscope observation revealed that the film was composed of AlGe γ 2 phase as a matrix and Al-rich amorphous dots.

Low temperature thermal annealing-induced α-FeSi2 derived phase in an amorphous Si matrix

Thermal annealing-induced recrystallisation in Fe ion-implanted Si was investigated by transmission electron microscopy. Single crystals of Si(111) were implanted with 120 keV Fe ions to a fluence of 1.0×1017 cm-2 at cryogenic temperature. A buried amorphous Fe-Si layer in an amorphous Si matrix was formed in the as-implanted sample. Nanobeam electron diffraction revealed that metastable α-FeSi2 precipitates embedded in the amorphous Si matrix were formed after annealing at 350 °C for 8 h. The formation of this α-FeSi2-derived phase was unusual, because it has been observed only in epitaxially grown thin films. Based on the Fe1-xSi (0<x<0.5) phase with the CsCl structure, which is another metastable phase in the Fe-Si binary system, we discuss the formation process of the metastable α-FeSi2 in the amorphous matrix.