Guangwen Zhou

The crystal structural evolution of nano-Si anode caused by lithium insertion and extraction at room temperature

Abstract The crystal structure and morphology of nanosized Si particles and wires after Li-insertion/extraction electrochemically have been studied by ex-situ XRD, Raman spectroscopy and electronic microscopy. It is confirmed that the insertion of lithium ions at room temperature destroys the crystal structure of Si gradually and leads to the formation of metastable amorphous Li–Si alloy. Furthermore, local ordered structure of Si can be restored after the partial extraction of lithium ions, which indicates the extraction of lithium ions promoting the recrystallization of amorphous Li-inserted Si. It was also observed that nanosized Si particles and wires were merged together after the insertion/extraction of lithium ions.

Synthesis of boron nitride nanotubes by means of excimer laser ablation at high temperature

Boron nitride nanotubes (BN-NTs) were synthesized by using excimer laser ablation at 1200 °C in different carrier gases. The main characteristic of the BN-NTs produced by this method is that nanotubes are of only one to three atomic layers thick, which could be attributed to the dominance of the axial growth rate over the radial growth rate. The diameter of the BN-NTs ranged from 1.5 to 8 nm. The tips of the BN-NTs are either a flat cap or of polygonal termination, in contrast to the conical ends of carbon nanotubes. The atomic ratio of boron to nitrogen as measured by means of parallel electron energy loss spectroscopy is 0.8, which is within the experimental error of the stoichiometry of hexagonal BN structure.

Synthesis of nano-scale silicon wires by excimer laser ablation at high temperature

We report below synthesis of nano-scale silicon wires by using laser ablation at high temperature. By this approach we have been able to produce silicon nano wires (SiNWs) with a very high yield, a uniform diameter distribution and a high purity. The structure, morphology and chemical composition of the SiNWs have been characterized by using high resolution X-ray diffraction (XRD), high resolution electron microscopy (HREM), as well as spectroscopy of energy dispersive X-ray fluorescence (EDAX). Our results should be of great interest to researchers working on mesoscopic physical phenomena, such as quantum confinement effects related to materials of reduced dimensions and should lead to the development of new applications for nano-scale devices, together with providing a powerful method for synthesis of similar one-dimensional conducting and semi-conducting wire.

CONTROLLED LI DOPING OF SI NANOWIRES BY ELECTROCHEMICAL INSERTION METHOD

Si nanowires (NWs) were doped with large amounts of Li+ ions by an electrochemical insertion method at room temperature. Si NWs with different doping levels were obtained by controlling the discharging/charging of Li/Si NWs cell. The microstructures of Si NWs with different doses of Li+ ions were investigated by high-resolution electron microscopy. The crystalline structure of the Si NWs was destroyed gradually with the increasing of Li+ ion dose. When the Li+ ions were extracted from the amorphous Li-doped Si NWs by the same electrochemical method, local ordering of atoms occurred and recrystallization was observed. The photoluminescence peak and intensity of Li+-doped Si NWs are closely related to the doping dose.

Growth mechanism of Ni0.3Mn0.7CO3 precursor for high capacity Li-ion battery cathodes

Transition metal carbonate (Ni0.3Mn0.7CO3) was co-precipitated as the precursor for Li- and Mn-enriched composite materials used as advanced cathodes for lithium-ion batteries. The optimal pH range for synthesis of Ni0.3Mn0.7CO3 in a continuous stirred tank reactor (CSTR) at the pilot scale was predicted by taking into account the chemical equilibriums between the products and reactants. The nucleation and growth of precursor particles were investigated during the CSTR process by monitoring particle size distributions, particle morphologies, chemical compositions, and structures with time. It was found that in the early stage of co-precipitation both the particle size distribution and the chemical composition were not homogeneous; a lead time of about 5 hours under our experiment conditions was necessary to achieve the uniformity in particle shape and chemical composition. The latter was not altered during extended times of co-precipitation; however, a continuous growth of particles resulted in relatively large particles (D50 > 30 μm). The electrochemical performance of the final lithiated cathode materials is reported.

Initial oxidation kinetics of Cu(100), (110), and (111) thin films investigated by in situ ultra-high-vacuum transmission electron microscopy

The initial oxidation stages of Cu(100), (110), and (111) surfaces have been investigated by using in situ ultra-high-vacuum transmission electron microscopy (TEM) techniques to visualize the nucleation and growth of oxide islands. The kinetic data on the nucleation and growth of oxide islands shows a highly enhanced initial oxidation rate on the Cu(110) surface as compared with Cu(100), and it is found that the dominant mechanism for the nucleation and growth is oxygen surface diffusion in the oxidation of Cu(100) and (110). The oxidation of Cu(111) shows a dramatically different behavior from that of the other two orientations, and the in situ TEM observation reveals that the initial stages of Cu(111) oxidation are dominated by the nucleation of oxide islands at temperatures lower than 550 °C, and are dominated by two-dimensional oxide growth at temperatures higher than 550 °C. This dependence of the oxidation behavior on the crystal orientation and temperature is attributed to the structures of the oxygen-chemisorbed layer, oxygen surface diffusion, surface energy, and the interfacial strain energy.

Transmission electron microscopy study of Si nanowires

Microstructures of Si nanowires (SiNW’s) synthesized using laser ablation were investigated by transmission electron microscopy. The SiNW’s have a high density of structural defects, which may play an important role in the formation of SiNW’s and in the determination of the morphology of the nanowires. A model for the growth mechanism of the SiNW’s was discussed on the basis of the observation.

Mechanical Peeling of Free‐Standing Single‐Walled Carbon‐Nanotube Bundles

An in situ electron microscopy study is presented of adhesion interactions between single-walled carbon nanotubes (SWNTs) by mechanically peeling thin free-standing SWNT bundles using in situ nanomanipulation techniques inside a high-resolution scanning electron microscope. The in situ measurements clearly reveal the process of delaminating one SWNT bundle from its originally bound SWNT bundle in a controlled-displacement manner and capture the deformation curvature of the delaminated SWNT bundle during the peeling process. A theoretical model based on nonlinear elastica theory is employed to interpret the measured deformation curvatures of the SWNTs and to quantitatively evaluate the peeling force and the adhesion strength between bundled SWNTs. The estimated adhesion energy per unit length for each pair of neighboring tubes in the peeling interface based on our peeling experiments agrees reasonably well with the theoretical value. This in situ peeling technique provides a potential new method for separating bundled SWNTs without compromising their material properties. The combined peeling experiments and modeling presented in this paper will be very useful to the study of the adhesion interactions between SWNTs and their nonlinear mechanical behaviors in the large-displacement regime.

Catalyst effects on formation of boron nitride nano-tubules synthesized by laser ablation

The effect of catalysts on the growth of boron nitride nano-tubules (BN-NTs) synthesized using laser ablation was investigated by high resolution electron microscopy (HREM). It was revealed that BN-NTs fabricated with or without catalysts have some differences in atomic layers of the wall, tubule length, surface smoothness, and tip morphology. Based on the experimental evidences, a model is proposed to account for the growth of BN-NTs.

Growth morphology and micro-structural aspects of Si nanowires synthesized by laser ablation

Si nanowires (SiNWs) synthesized by laser ablation method exhibit different morphological characteristics such as straight, curved, kink, braided, and coiled shapes. These morphologies and their corresponding micro-structural aspects were examined in some details by transmission electron microscopy (TEM). It is found that the formation of various morphologies of SiNWs is closely related to the insertion of twins or the spatial twisting along the growth direction during the growth process of SiNWs.