12 research outputs found
Two-Fluid Wetting Behavior of a Hydrophobic Silicon Nanowire Array
The
two-fluid wetting behavior of surfaces textured by an array of silicon
nanowires is investigated systematically. The Si nanowire array is
produced by a combination of colloidal patterning and metal-catalyzed
etching, with control over its roughness depending upon the wire length.
The nanowires are made hydrophobic and oleophobic by treatment with
hydrocarbon and fluorinated self-assembled monolayers, respectively.
Static, advancing, and receding contact angles are measured with water,
hexadecane, and perfluorotripentylamine in both single-fluid (droplet
on solid in an air environment) and two-fluid (droplet on solid in
a liquid environment) configurations. The single-fluid measurements
show wetting behavior similar to that expected by the Wenzel and Cassie–Baxter
models, where the wetting or non-wetting behaviors are amplified with
increasing roughness. The two-fluid systems on the rough surface exhibit
more complex configurations because either the droplet or the environment
fluid can penetrate the asperities depending upon the wettability
of each fluid. It is observed that, when the Young contact angles
are significantly increased or reduced from single-liquid to two-liquid
systems, the effect of roughness is relatively minimal. However, when
the Young contact angles are similar, roughness has almost identical
influence on apparent contact angles in single- and two-liquid systems.
The Wenzel and Cassie–Baxter models are modified to describe
various two-fluid wetting states. In cases where metastable behavior
is observed for the droplet, advancing and receding measurements are
performed to suggest the equilibrium state of the droplet
Demonstration of Hexagonal Phase Silicon Carbide Nanowire Arrays with Vertical Alignment
SiC
nanowire based electronics hold promise for data collection
in harsh environments wherein conventional semiconductor platforms
would fail. However, the full adaptation of SiC nanowires as a material
platform necessitates strict control of nanowire crystal structure
and orientation for reliable performance. Toward such efforts, we
report the growth of hexagonal phase SiC nanowire arrays grown with
vertical alignment on commercially available single crystalline SiC
substrates. The nanowire hexagonality, confirmed with Raman spectroscopy
and atomic resolution microscopy, displays a polytypic distribution
of predominantly 2H and 4H. Employing a theoretical growth model,
the polytypic distribution of hexagonal phase nanowires is accurately
predicted in the regime of high supersaturation. Additionally, the
reduction of disorder-induced phonon density of states is achieved
while maintaining nanowire morphology through a postgrowth anneal.
The results of this work expand the repertoire of SiC nanowires by
implementing a low-temperature method that promotes polytypes outside
the well-studied cubic phase and introduces uniform, vertical alignment
on industry standard SiC substrates
Atomic-Scale Electronic Characterization of Defects in Silicon Carbide Nanowires by Electron Energy-Loss Spectroscopy
The atomic-level
resolution of scanning transmission electron microscopy
(TEM) is used for structural characterization of nanomaterials, but
the resolution afforded by TEM also enables electronic characterization
of defects in these materials through electron energy-loss spectroscopy
(EELS). Here, the power of EELS is harnessed to characterize the local
band gap of inclusion defects in hexagonal silicon carbide nanowires
with a high density of stacking faults. The band gaps we extract from
the EELS data align within 0.1 eV of expected values for hexagonal
silicon carbide and stacking faults within hexagonal silicon carbide.
These experiments show that individual cubic phase inclusions in hexagonal
silicon carbide significantly alter the local electronic structure,
in particular, the band gap, in contrast to the polarizability tensor
that retains the characteristic signature of the global hexagonal
crystal structure
In Situ Localized Growth of Ordered Metal Oxide Hollow Sphere Array on Microheater Platform for Sensitive, Ultra-Fast Gas Sensing
A simple
and versatile strategy is presented for the localized on-chip synthesis
of an ordered metal oxide hollow sphere array directly on a low power
microheater platform to form a closely integrated miniaturized gas
sensor. Selective microheater surface modification through fluorinated
monolayer self-assembly and its subsequent microheater-induced thermal
decomposition enables the position-controlled deposition of an ordered
two-dimensional colloidal sphere array, which serves as a sacrificial
template for metal oxide growth via homogeneous chemical precipitation;
this strategy ensures control in both the morphology and placement
of the sensing material on only the active heated area of the microheater
platform, providing a major advantage over other methods of presynthesized
nanomaterial integration via suspension coating or printing. A fabricated
tin oxide hollow sphere-based sensor shows high sensitivity (6.5 ppb
detection limit) and selectivity toward formaldehyde, and extremely
fast response (1.8 s) and recovery (5.4 s) times. This flexible and
scalable method can be used to fabricate high performance miniaturized
gas sensors with a variety of hollow nanostructured metal oxides for
a range of applications, including combining multiple metal oxides
for superior sensitivity and tunable selectivity
Selective Ultrathin Carbon Sheath on Porous Silicon Nanowires: Materials for Extremely High Energy Density Planar Micro-Supercapacitors
Microsupercapacitors are attractive
energy storage devices for
integration with autonomous microsensor networks due to their high-power
capabilities and robust cycle lifetimes. Here, we demonstrate porous
silicon nanowires synthesized via a lithography compatible low-temperature
wet etch and encapsulated in an ultrathin graphitic carbon sheath,
as electrochemical double layer capacitor electrodes. Specific capacitance
values reaching 325 mF cm<sup>–2</sup> are achieved, representing
the highest specific ECDL capacitance for planar microsupercapacitor
electrode materials to date
Templated 3D Ultrathin CVD Graphite Networks with Controllable Geometry: Synthesis and Application As Supercapacitor Electrodes
Three-dimensional ultrathin graphitic
foams are grown via chemical
vapor deposition on templated Ni scaffolds, which are electrodeposited
on a close-packed array of polystyrene microspheres. After removal
of the Ni, free-standing foams composed of conjoined hollow ultrathin
graphite spheres are obtained. Control over the pore size and foam
thickness is demonstrated. The graphitic foam is tested as a supercapacitor
electrode, exhibiting electrochemical double-layer capacitance values
that compare well to those obtained with the state-of-the-art 3D graphene
materials
Direct Organization of Morphology-Controllable Mesoporous SnO<sub>2</sub> Using Amphiphilic Graft Copolymer for Gas-Sensing Applications
A simple
and flexible strategy for controlled synthesis of mesoporous metal
oxide films using an amphiphilic graft copolymer as sacrificial template
is presented and the effectiveness of this approach for gas-sensing
applications is reported. The amphiphilic graft copolymer polyÂ(vinyl
chloride)-<i>g</i>-polyÂ(oxyethylene methacrylate) (PVC-<i>g</i>-POEM) is used as a sacrificial template for the direct
synthesis of mesoporous SnO<sub>2</sub>. The graft copolymer self-assembly
is shown to enable good control over the morphology of the resulting
SnO<sub>2</sub> layer. Using this approach, mesoporous SnO<sub>2</sub> based sensors with varied porosity are fabricated in situ on a microheater
platform. This method reduces the interfacial contact resistance between
the chemically sensitive materials and the microheater, while a simple
fabrication process is provided. The sensors show significantly different
gas-sensing performances depending on the SnO<sub>2</sub> porosity,
with the highly mesoporous SnO<sub>2</sub> sensor exhibiting high
sensitivity, low detection limit, and fast response and recovery toward
hydrogen gas. This printable solution-based method can be used reproducibly
to fabricate a variety of mesoporous metal oxide layers with tunable
morphologies on various substrates for high-performance applications
Air-Stable n‑Doping of WSe<sub>2</sub> by Anion Vacancy Formation with Mild Plasma Treatment
Transition
metal dichalcogenides (TMDCs) have been extensively
explored for applications in electronic and optoelectronic devices
due to their unique material properties. However, the presence of
large contact resistances is still a fundamental challenge in the
field. In this work, we study defect engineering by using a mild plasma
treatment (He or H<sub>2</sub>) as an approach to reduce the contact
resistance to WSe<sub>2</sub>. Material characterization by X-ray
photoelectron spectroscopy, photoluminescence, and Kelvin probe force
microscopy confirm defect-induced n-doping, up to degenerate level,
which is attributed to the creation of anion (Se) vacancies. The plasma
treatment is adopted in the fabrication process flow of WSe<sub>2</sub> n-type metal-oxide–semiconductor field-effect transistors
to selectively create anion vacancies at the metal contact regions.
Due to lowering the metal contact resistance, improvements in the
device performance metrics such as a 20Ă— improvement in ON current
and a nearly ideal subthreshold swing value of 66 mV/dec are observed.
This work demonstrates that defect engineering at the contact regions
can be utilized as a reliable scheme to realize high-performance electronic
and optoelectronic TMDC devices
In Situ Localized Growth of Porous Tin Oxide Films on Low Power Microheater Platform for Low Temperature CO Detection
This
paper reports a facile method for creating a nanostructured
metal oxide film on a low power microheater sensor platform and the
direct realization of this structure as a gas sensor. By fast annealing
the deposited liquid precursors with the microheater, a highly porous,
nanocrystalline metal oxide film can be generated in situ and locally
on the sensor platform. With only minimal processing, a high performance,
miniaturized gas sensor is ready for use. A carbon monoxide sensor
using the in situ synthesized porous tin oxide (SnO<sub>2</sub>) sensing
film is made as a demonstration of this technique. The sensor exhibits
a low detection limit and fast response and recovery time at a low
operating temperature. This facile fabrication method is highly flexible
and has great potential for large-scale gas sensor fabrication
Programming Nanoparticles in Multiscale: Optically Modulated Assembly and Phase Switching of Silicon Nanoparticle Array
Manipulating and
tuning nanoparticles by means of optical field
interactions is of key interest for nanoscience and applications in
electronics and photonics. We report scalable, direct, and optically
modulated writing of nanoparticle patterns (size, number, and location)
of high precision using a pulsed nanosecond laser. The complex nanoparticle
arrangement is modulated by the laser pulse energy and polarization
with the particle size ranging from 60 to 330 nm. Furthermore, we
report fast cooling-rate induced phase switching of crystalline Si
nanoparticles to the amorphous state. Such phase switching has usually
been observed in compound phase change materials like GeSbTe. The
ensuing modification of atomic structure leads to dielectric constant
switching. Based on these effects, a multiscale laser-assisted method
of fabricating Mie resonator arrays is proposed. The number of Mie
resonators, as well as the resonance peaks and dielectric constants
of selected resonators, can be programmed. The programmable light-matter
interaction serves as a mechanism to fabricate optical metasurfaces,
structural color, and multidimensional optical storage devices