Amorphous Si Nanowires Research Articles

Revealing Silicon’s Delithiation Behaviour through Empirical Analysis of Galvanostatic Charge–Discharge Curves

The galvanostatic charge–discharge (GCD) behaviour of silicon (Si) is known to depend strongly on morphology, cycling conditions and electrochemical environment. One common method for analysing GCD curves is through differential capacity, but the data processing required necessarily degrades the results. Here we present a method of extracting empirical information from the delithiation step in GCD data for Si at C-rates above equilibrium conditions. We find that the function is able to quickly and accurately determine the best fit to historical half-cell data on amorphous Si nanowires and thin films, and analysis of the results reveals that the function is capable of distinguishing the capacity contributions from the Li3.5Si and Li2Si phases to the total capacity. The method can also pick up small differences in the phase behaviour of the different samples, making it a powerful technique for further analysis of Si data from the literature. The method was also used for predicting the size of the reservoir effect (the apparent amount of Li remaining in the electrode), making it a useful technique for quickly determining voltage slippage and related phenomena. This work is presented as a starting point for more in-depth empirical analysis of Si GCD data.

Empirical potential optimization for the investigation of lithiation-delithiation cycles of amorphous Si nanowires

The atomistic mechanisms during lithiation and delithiation of amorphous Si nanowires ($a$-SiNW) have been investigated over cycles by molecular dynamics simulations. First, the Modified Embedded Atom Method (MEAM) potential from Cui et al. [J. Power Sources. 2012, (207) 150] has been further optimized on static (Li$_x$Si alloy phases and point defect energies) and on dynamic properties (Li diffusion) to reproduce the lithiation of small crystalline Si nanowires calculated at the {\it ab initio} level. The lithiation of $a$-SiNW reveals a two-phase process of lithiation with a larger diffusion interface compared to crystalline Si lithiation. Compressive axial stresses are observed in the amorphous Si$_x$Li alloy outer shell. They are easily released thanks to the soft glassy behavior of the amorphous alloy. Conversely, in crystalline SiNW, the larger stress in the narrow crystalline lithiated interface is hardly released and requires a phase transformation to amorphous to operate, which delays the lithiation. The history of the charge-discharge cycles as well as the temperature appear as driving forces for phase transformation from amorphous Li$_x$Si alloy to the more stable crystalline phase counterpart. Our work suggest that a full delithiation could heal the SiNWs to improve the life cycles of Li-ion batteries with Si anode.

Tunable Anelasticity in Amorphous Si Nanowires.

In situ bending tests of amorphous Si nanowires (a-Si NWs) found different elastic behavior depending on whether they were straight or curved to begin with. The axially straight NWs exhibit pure elastic deformation; however, the axially curved NWs exhibit obvious anelastic behavior when they are bent in the direction of original curvature. On the basis of STEM-EELS analysis, we propose that the underlying mechanism for this anelastic behavior is a bond-switching assisted redistribution of the nonuniform density (structure) in the curved NWs under the inhomogeneous stress field. This mechanism was further supported by the fact that the originally straight a-Si NWs also display similar anelasticity with the as-grown curved NWs after focused ion beam irradiation that can cause nonuniform structure distribution. As compared to what has been reported in other 1D materials, the anelasticity of a-Si NWs can be tuned by modifying their morphology, controlling the loading direction, or irradiating them via ion beam. Our findings suggest that a-Si NWs could be a promising material in the nanoscale damping systems, especially the semiconductor nanodevices.

Fast growth of amorphous Si nanowires by direct current arc plasma jet chemical vapor deposition

Silicon (Si) nanowires were synthesized by direct current arc plasma jet chemical vapor deposition. It was found that growth temperature and growth time played important roles in the morphologies of the Si nanowires. The possible formation mechanism of Si nanowire was also proposed according to the corresponding results. In addition, the Ni particles are dispersed on the Si substrate with the help of alkaline etching, which enables the lateral growth of Si nanowires on a Si substrate. This offers a simple and practical way of synthesizing and positioning the Si nanowires on a substrate.

An alternative route for the synthesis of silicon nanowires via porous anodic alumina masks.

Amorphous Si nanowires have been directly synthesized by a thermal processing of Si substrates. This method involves the deposition of an anodic aluminum oxide mask on a crystalline Si (100) substrate. Fe, Au, and Pt thin films with thicknesses of ca. 30 nm deposited on the anodic aluminum oxide-Si substrates have been used as catalysts. During the thermal treatment of the samples, thin films of the metal catalysts are transformed in small nanoparticles incorporated within the pore structure of the anodic aluminum oxide mask, directly in contact with the Si substrate. These homogeneously distributed metal nanoparticles are responsible for the growth of Si nanowires with regular diameter by a simple heating process at 800°C in an Ar-H2 atmosphere and without an additional Si source. The synthesized Si nanowires have been characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman.

Synthesis of amorphous Si nanowires from solid Si sources using microwave irradiation

Si nanowires were synthesized from Si wafers and from thin Si films deposited on various substrates by microwave irradiation. The power and time were key determinants of the diameter and morphology of the synthesized Si nanowires. The nanowires had an amorphous structure due to the extremely high heating rate. Carbon coating of the Si nanowires was easily achieved by introducing acetylene after synthesizing the nanowires. Carbon-coated Si nanowires are potential candidates for use as the anode material in next generation Li-ion batteries.

Polymer-assisted synthesis of aligned amorphous silicon nanowires and their core/shell structures with Au nanoparticles

Aligned amorphous Si nanowires (SiNWs) were synthesized directly from Si substrates with the assistance of a new carbon-based network polymer, poly(phenylcarbyne), during the heat-treatment in Ar atmosphere at 1120 °C. A core/shell structure of SiNWs wrapped with Au nanoparticles was simply fabricated as well. The analytic results of the morphology and microstructure confirmed the orientation and the amorphous nature of the SiNWs, and the high dispersion of Au nanoparticles on the surface of the SiNWs without any aggregation. The formation of the SiNWs was explained on the basis of the reaction of carbon with the native silica layer covering Si substrates.

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H2 atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model.