5 research outputs found
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