Wan Neng

The in situ TEM observation of rapid lithium encapsulation and release in LiCl nanoshells and nanotubes

In this work, we performed detailed in situ TEM observations of the rapid encapsulation and release of lithium in nanometric structures under e-beam irradiation. With fast kinetics, the e-beam irradiation induced the formation of a LiCl shell due to the surface reaction between lithium and the surrounding chloride. In parallel, a rapid lithium release from the LiCl surface shell underwent as a competitive process. The detailed dynamic encapsulation and releasing processes were revealed with enhanced time and spatial resolution. Lithium nanoparticles, nanorods and Li/LiCl core–shell nanoparticles and nanotubes were observed during these processes. Due to the important application of lithium materials in the energy storage field, where lithium encapsulation and releasing processes play the fundamentally important roles, our findings may trigger new application methods for lithium based energy storage, in which e-beam irradiation may be used to enhance the ultra-rapid energy harvesting or releasing process.

Graphene-based gas sensor.

Graphene, a honeycomb structure with single layer sp2 hybridization carbon atom, has extraordinary mechanical and electrical properties, exhibiting great potential in sensing area. The ultrahigh electron mobility in room temperature as well as the ultrahigh specific surface area, endows graphene a promising candidate for ultrasensitive gas sensor. As a typical two dimensional material, every atom in graphene can be regarded as surface atom. Therefore, every atom will be able to interact with gas molecule, which provides an ultrahigh sensitivity and the ultralow detection limit (as low as for single molecule detection). The current researches for improvement of gas sensing mainly focus on two aspects: (1) Design of different working principle devices; (2) surface modification on graphene and composite with other materials (i.e . metal, metal oxide and organic polymer). The specific adsorption sites can be achieved, resulting the improvement of selectivity. This review will present and summarize recent achievements of graphene-based gas sensor in both aspects and also predict the potential research direction in the future. According to different working mechanism, graphene-based gas sensor can be classified as resistive type, field-effect-transistor (FET) type, mass sensitive type, and micro-electromechanical system (MEMS) type. Each kind has its advantages. Thanks to the simple manufacture process, resistive gas sensor is investigated broadly. Compared to passive resistive sensor, the active FET sensor exhibits better performance in sensitivity and stability generally. As for mass sensitive sensor, the unique mechanism can provide a new device structure different from the other three types. However, concerning the compatibility with integrated circuits (IC) manufacture, MEMS sensor can be a good choice. Materials with high specific surface area are able to provide more adsorption sites. Consequently, sensor performance can be improved sharply. Therefore, surface modification on graphene and composite with other materials attracts broad attention recently. By changing the density and the type of functional groups on graphene surface, specific adsorption sites can be achieved. Hence, the ability for selective gas detection is enhanced. Besides, graphene-based composite with other materials (i.e. metal, metal oxide and organic polymer) is another effective strategy to optimize sensor performance. In conclusion, lots of achievements and big breakthroughs for graphene-based gas sensor have been made in recent years. Especially the obvious enhancement for sensitivity (as low as for single molecule detection) and gas selective detection, graphene-based gas sensor exhibits great advantage compared with traditional sensors. However, the time-consuming process of gas adsorption and desorption hampers the real-time measurements due to the long response and recover time. Therefore, improvement in response and recover performance will be a potential research direction in the future. In addition, the integration of different kinds of gas sensors even other types of sensors will be a trend. Also, it will be a desirable fundamental research to give explanations for the sensing mechanism as well as to investigate the dynamic interaction between gas molecules and graphene. Recently, it is believed that with the help of special in - situ holder, researchers can launch experiments in transmission electron microscope (TEM) to explore this fundamental study.

A 'jump-to-coalescence' mechanism during nanoparticle growth revealed by in situ aberration-corrected transmission electron microscopy observations.

In this work, we used in situ aberration-corrected transmission electron microscopy (AC-TEM) to observe the coalescence of gold nanoparticles. We observed a critical edge-to-edge distance d(c)(e) ~ 0.5 nm below which the two particles will coalesce rapidly (jump-to-coalescence). A model based on the single-atom-triggered rapid particle contraction was proposed and verified by first-principles calculations, in which evident energy decrease was detected when adding a gold atom between two gold nanoparticles. Our ex situ TEM study of sputtering-deposited gold nanoparticles on different substrates with varied time also supports the jump-to-contact mechanism. This observation afforded physical insight into the fundamental growth mechanism during dynamic particle coalescence processes.

Surface energy guided sub-10 nm hierarchy structures fabrication by direct e-beam etching

It is found that surface energy plays the dominant role in direct e-beam etching of single crystal ZnO and ITO nanostructures through a surface etching (sputtering) process, especially for electron energies smaller than the characteristic energy for bulk atoms knock-off. This mechanism can be used for the fabrication of sub-10 nm hierarchical structures on single crystal nanostructure precursors. We performed detailed in situ HRTEM observations of the surface energy guided etching (SEGE) process on ZnO nanowires. The etching of (0001) crystal planes, that have the lowest surface energy, was found to dominate the etching process. It is also shown that the SEGE mechanisms can be extended to single crystal ITO nanowires for sub-10 nm hierarchical structure fabrication. The hierarchical structures fabricated by the SEGE process show increased specific surface areas (SSA), which may be useful in fields such as sensors, catalysts and solar cells.