In Situ Monitored Self-Assembly of Three-Dimensional Polyhedral Nanostructures

The self-assembly of 3D nanostructures is a promising technology for the fabrication of next generation nanodevices and the exploration of novel phenomena. However, the present techniques for assembly of 3D nanostructures are invisible and have to be done without physical contact, which bring great challenges in controlling the shapes with nanoscale precision. This situation leads to an extremely low yield of self-assembly, especially in 3D nanostructures built with metal and semiconductor materials. Here, an in situ self-assembly process using a focused ion beam (FIB) microscopy system has been demonstrated to realize 3D polyhedral nanostructures from 2D multiple pieces. An excited ion beam in the FIB microscopy system offers not only a visualization of the nanoscale self-assembly process but also the necessary energy for inducing the process. Because the beam energy that induces the self-assembly can be precisely adjusted while monitoring the status of the self-assembly, it is possible to control the self-assembly process with sub-10 nm scale precision, resulting in the realization of diverse 3D nanoarchitectures with a high yield. This approach will lead to state-of-the-art applications utilizing properties of 3D nanostructures in diverse fields

Enhanced Lithium Ion Battery Cycling of Silicon Nanowire Anodes by Template Growth to Eliminate Silicon Underlayer Islands

It is well-known that one-dimensional nanostructures reduce pulverization of silicon (Si)-based anode materials during Li ion cycling because they allow lateral relaxation. However, even with improved designs, Si nanowire-based structures still exhibit limited cycling stability for extended numbers of cycles, with the specific capacity retention with cycling not showing significant improvements over commercial carbon-based anode materials. We have found that one important reason for the lack of long cycling stability can be the presence of milli- and microscale Si islands which typically form under nanowire arrays during their growth. Stress buildup in these Si island underlayers with cycling results in cracking, and the loss of specific capacity for Si nanowire anodes, due to progressive loss of contact with current collectors. We show that the formation of these parasitic Si islands for Si nanowires grown directly on metal current collectors can be avoided by growth through anodized aluminum oxide templates containing a high density of sub-100 nm nanopores. Using this template approach we demonstrate significantly enhanced cycling stability for Si nanowire-based lithium-ion battery anodes, with retentions of more than ∼1000 mA·h/g discharge capacity over 1100 cycles

Silicon Nanowire Degradation and Stabilization during Lithium Cycling by SEI Layer Formation

Silicon anodes are of great interest for advanced lithium-ion battery applications due to their order of magnitude higher energy capacity than graphite. Below a critical diameter, silicon nanowires enable the ∼300% volume expansion during lithiation without pulverization. However, their high surface-to-volume ratio is believed to contribute to fading of their capacity retention during cycling due to solid-electrolyte-interphase (SEI) growth on surfaces. To better understand this issue, previous studies have examined the composition and morphology of the SEI layers. Here we report direct measurements of the reduction in silicon nanowire diameter with number of cycles due to SEI formation. The results reveal significantly greater Si loss near the nanowire base. From the change in silicon volume we can accurately predict the measured specific capacity reduction for silicon nanowire half cells. The enhanced Si loss near the nanowire/metal current collector interface suggests new strategies for stabilizing nanowires for long cycle life performance

Tunable Optical Transparency in Self-Assembled Three-Dimensional Polyhedral Graphene Oxide

The origami-like self-folding process is an intellectually stimulating technique for realizing three-dimensional (3D) polyhedral free-standing graphene oxide (GO) structures. This technique allows for easy control of size, shape, and thickness of GO membranes, which in turn permits fabrication of free-standing 3D microscale polyhedral GO structures. Unlike 2D GO sheets, the 3D polyhedral free-standing GO shows a distinct optical switching behavior, resulting from a combination of the geometrical effect of the 3D hollow structure and the water-permeable multilayered GO membrane that affects the optical paths

Lithium-Assisted Electrochemical Welding in Silicon Nanowire Battery Electrodes

From in situ transmission electron microscopy (TEM) observations, we present direct evidence of lithium-assisted welding between physically contacted silicon nanowires (SiNWs) induced by electrochemical lithiation and delithiation. This electrochemical weld between two SiNWs demonstrates facile transport of lithium ions and electrons across the interface. From our in situ observations, we estimate the shear strength of the welded region after delithiation to be approximately 200 MPa, indicating that a strong bond is formed at the junction of two SiNWs. This welding phenomenon could help address the issue of capacity fade in nanostructured silicon battery electrodes, which is typically caused by fracture and detachment of active materials from the current collector. The process could provide for more robust battery performance either through self-healing of fractured components that remain in contact or through the formation of a multiconnected network architecture