Xiaotang Lu

Lithium Ion Battery Peformance of Silicon Nanowires with Carbon Skin

Silicon (Si) nanomaterials have emerged as a leading candidate for next generation lithium-ion battery anodes. However, the low electrical conductivity of Si requires the use of conductive additives in the anode film. Here we report a solution-based synthesis of Si nanowires with a conductive carbon skin. Without any conductive additive, the Si nanowire electrodes exhibited capacities of over 2000 mA h g(-1) for 100 cycles when cycled at C/10 and over 1200 mA h g(-1) when cycled more rapidly at 1C against Li metal. In situ transmission electron microscopy (TEM) observation reveals that the carbon skin performs dual roles: it speeds lithiation of the Si nanowires significantly, while also constraining the final volume expansion. The present work sheds light on ways to optimize lithium battery performance by smartly tailoring the nanostructure of composition of materials based on silicon and carbon.

Monodisperse silicon nanocavities and photonic crystals with magnetic response in the optical region

It is generally accepted that the magnetic component of light has a minor role in the light-matter interaction. The recent discovery of metamaterials has broken this traditional understanding, as both the electric and the magnetic field are key ingredients in metamaterials. The top-down technology used so far employs noble metals with large intrinsic losses. Here we report on a bottom-up approach for processing metamaterials based on suspensions of monodisperse full dielectric silicon nanocavities with a large magnetic response in the near-infrared region. Experimental results and theory show that silicon-colloid-based liquid suspensions and photonic crystals made of two-dimensional arrays of particles have strong magnetic response in the near-infrared region with small optical losses. Our findings might have important implications in the bottom-up processing of large-area low-loss metamaterials working in the near-infrared region.

Colloidal Luminescent Silicon Nanorods

Silicon nanorods are grown by trisilane decomposition in hot squalane in the presence of tin (Sn) nanocrystals and dodecylamine. Sn induces solution-liquid-solid nanorod growth with dodecylamine serving as a stabilizing ligand. As-prepared nanorods do not luminesce, but etching with hydrofluoric acid to remove residual surface oxide followed by thermal hydrosilylation with 1-octadecene induces bright photoluminescence with quantum yields of 4-5%. X-ray photoelectron spectroscopy shows that the ligands prevent surface oxidation for months when stored in air.

Silicon nanoparticles as Raman scattering enhancers

In this communication we demonstrate the large amplification values of the Raman signal of organic molecules attached to silicon nanoparticles (SiNPs). Light induced Mie resonances of high refractive index particles generate strong evanescent electromagnetic (EM) fields, thus boosting the Raman signal of species attached to the nanoparticles. The interest of this process is justified by the wide range of experimental configurations that can be implemented including photonic crystals, the sharp spectral resonances easily tuneable with the particle size, the biocompatibility and biodegradability of silicon, and the possibility of direct analysis of molecules that do not contain functional groups with high affinity for gold and silver. Additionally, silicon nanoparticles present stronger field enhancement due to Mie resonances at larger sizes than gold.

A single-step reaction for silicon and germanium nanorods

A single-step reaction has been developed for colloidal quantum-size silicon (Si) and germanium (Ge) nanorods. The nanorods are formed by solution-liquid-solid (SLS) growth from tin (Sn) seed particles prepared by in situ reduction of a molecular tin(II) complex by trisilane, the reactant for Si nanorod growth. Using the same procedure, Ge nanorods can be grown by including a diphenyl germane reactant. The nanorod length could be adjusted from several nanometers to more than a micrometer without significant increase of diameter by manipulating reactant concentrations.

Enhancing the lithiation rate of silicon nanowires by the inclusion of tin

Silicon (Si) has a very high lithium storage capacity and is being explored as a negative electrode material in lithium-ion batteries (LIBs). Si nanowires can exhibit relatively stable performance for many cycles of charging; however, conductive carbon must often be added to the electrode layer to improve the rate capability due to the relatively low electrical conductivity of Si. The added carbon lowers the capacity of the electrode. Here, we show that the rate capability of Si in LIBs can be substantially enhanced by incorporating tin (Sn) into Si nanowires. The solubility of Sn in Si is very low (0.015 at%); yet, Sn used as a seed for supercritical fluid–liquid–solid (SFLS) growth can be trapped in Si nanowires with relatively high concentration (10 at%). Such Sn-containing Si nanowires and no added conductive carbon in the electrode layer, could be cycled in LIBs with high capacity (∼1000 mA h g−1 over 100 cycles) at a current density of 2.8 A g−1 (1 C). Capacities exceeding that of graphite could still be reached at cycle rates as high as 2 C. Real-time in situ transmission electron microscopy (TEM) revealed that lithiation occurs five times faster in Si nanowires with significant amounts of Sn than in the Si nanowires without Sn, and twice as fast as in nanowires that were coated with carbon.

Efficient Carrier Multiplication in Colloidal Silicon Nanorods

Auger recombination lifetimes, absorption cross sections, and the quantum yields of carrier multiplication (CM), or multiexciton generation (MEG), were determined for solvent-dispersed silicon (Si) nanorods using transient absorption spectroscopy (TAS). Nanorods with an average diameter of 7.5 nm and aspect ratios of 6.1, 19.3, and 33.2 were examined. Colloidal Si nanocrystals of similar diameters were also studied for comparison. The nanocrystals and nanorods were passivated with organic ligands by hydrosilylation to prevent surface oxidation and limit the effects of surface trapping of photoexcited carriers. All samples used in the study exhibited relatively efficient photoluminescence. The Auger lifetimes increased with nanorod length, and the nanorods exhibited higher CM quantum yield and efficiency than the nanocrystals with a similar band gap energy Eg. Beyond a critical length, the CM quantum yield decreases. Nanorods with the aspect ratio of 19.3 had the highest CM quantum yield of 1.6 ± 0.2 at 2.9Eg, which corresponded to a multiexciton yield that was twice as high as observed for the spherical nanocrystals.