Vertically aligned
silicon nanowire (VA-SiNW) arrays can significantly
enhance light absorption and reduce light reflection for efficient
light trapping. VA-SiNW arrays thus have the potential to improve
solar cell design by providing reduced front-face reflection while
allowing the fabrication of thin, flexible, and efficient silicon-based
solar cells by lowering the required amount of silicon. Because their
interaction with light is highly dependent on the array geometry,
the ability to control the array morphology, functionality, and dimension
offers many opportunities. Herein, after a short discussion about
the remarkable optical properties of SiNW arrays, we report on our
recent progress in using chemical and electrochemical methods to structure
and pattern SiNW arrays in three dimensions, providing substrates
with spatially controlled optical properties. Our approach is based
on metal-assisted chemical etching (MACE) and three-dimensional electrochemical
axial lithography (3DEAL), which are both affordable and large-scale
wet-chemical methods that can provide a spatial resolution all the
way down to the sub-5 nm range.
Gold nanoparticle/silicon composites are canonical substrates
for
sensing applications because of their geometry-dependent physicochemical
properties and high sensing activity via surface-enhanced Raman spectroscopy
(SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized
via wet-chemistry on functionalized flat silicon (Si) and vertically
aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective
approach to prepare such substrates. Herein, we report on the critical
parameters that influence nanoparticle coverage, aggregation, and
assembly sites in two- and three-dimensions to prepare substrates
with homogeneous optical properties and SERS activity. We show that
the degree of AuNP aggregation on flat Si depends on the silane used
for the Si functionalization, while the AuNP coverage can be adjusted
by the incubation time in the AuNP solution, both of which directly
affect the substrate properties. In particular, we report the reproducible
synthesis of nearly touching AuNP chain monolayers where the AuNPs
are separated by nanoscale gaps, likely to be formed due to the capillary
forces generated during the drying process. Such substrates, when
used for SERS sensing, produce a uniform and large enhancement of
the Raman signal due to the high density of hot spots that they provide.
We also report the controlled self-assembly of AuNPs on VA-SiNW arrays,
which can provide even higher Raman signal enhancement. The directed
assembly of the AuNPs in specific regions of the SiNWs with a control
over NP density and monolayer morphology (i.e., isolated vs nearly
touching NPs) is demonstrated, together with its influence on the
resulting SERS activity.
As a result of the global demand for sustainable products, a suitable alternative to the resorcinol-formaldehyde aerogels, which are frequently used as precursors for carbon aerogels, is searched for. In this study, the replacement of petroleum-derived formaldehyde with a natural, biobased crosslinker, namely 5-(hydroxymethyl)furfural (5-HMF) is shown, and the synthesis of renewable, monolithic tannin aerogels is demonstrated. Compared to well-known tannin-formaldehyde aerogels, this green alternative shows lower reactivity of the crosslinker associated with lower gelation times as well as lower specific surface areas at the organic stage. Nonetheless, the morphologies and synthesis-structure relationships follow similar trends for both tannin-based aerogels, e.g., the pore size is influenced by the initial pH in the same manner. The turnover to carbon aerogels by a carbothermal treatment results in enhanced high-specific surface areas of the tannin-5-HMF-based carbon aerogels, which are similar and even slightly outperform those obtained from tannin-formaldehyde aerogels. This suggests that they are a convenient alternative for carbon aerogel applications.
Graphical Abstract
EELS, SERS and electromagnetic simulations demonstrate large E-field enhancements at Rh segments located in the gap region of AuRh_Au nanorod heterodimers.
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