16 research outputs found
Polymeric Nanofibers with Ultrahigh Piezoelectricity <i>via</i> Self-Orientation of Nanocrystals
Piezoelectricity
in macromolecule polymers has been gaining immense
attention, particularly for applications in biocompatible, implantable,
and flexible electronic devices. This paper introduces coreâshell-structured
piezoelectric polyvinylidene fluoride (PVDF) nanofibers chemically
wrapped by graphene oxide (GO) lamellae (PVDF/GO nanofibers), in which
the polar ÎČ-phase nanocrystals are formed and uniaxially self-oriented
by the synergistic effect of mechanical stretching, high-voltage alignment,
and chemical interactions. The ÎČ-phase orientation of the PVDF/GO
nanofibers along their axes is observed at atomic scale through high
resolution transmission electron microscopy, and the ÎČ-phase
content is found to be 88.5%. The piezoelectric properties of the
PVDF/GO nanofibers are investigated in terms of piezoresponse mapping,
local hysteresis loops, and polarization reversal by advanced piezoresponse
force microscopy. The PVDF/GO nanofibers show a desirable out-of-plane
piezoelectric constant (<i>d</i><sub>33</sub>) of â93.75
pm V<sup>â1</sup> (at 1.0 wt % GO addition), which is 426%
higher than that of the conventional pure PVDF nanofibers. The mechanism
behind this dramatic enhancement in piezoelectricity is elucidated
by three-dimensional molecular modeling
Ultrathin Coaxial Fiber Supercapacitors Achieving High Energy and Power Densities
Fiber-based supercapacitors
have attracted significant interests because of their potential applications
in wearable electronics. Although much progress has been made in recent
years, the energy and power densities, mechanical strength, and flexibility
of such devices are still in need of improvement for practical applications.
Here, we demonstrate an ultrathin microcoaxial fiber supercapacitor
(ÎŒCFSC) with high energy and power densities (2.7 mW h/cm<sup>3</sup> and 13 W/cm<sup>3</sup>), as well as excellent mechanical
properties. The prototype with the smallest reported overall diameter
(âŒ13 ÎŒm) is fabricated by successive coating of functional
layers onto a single micro-carbon-fiber via a scalable process. Combining
the simulation results via the electrochemical model, we attribute
the high performance to the well-controlled thin coatings that make
full use of the electrode materials and minimize the ion transport
path between electrodes. Moreover, the ÎŒCFSC features high bending
flexibility and large tensile strength (more than 1 GPa), which make
it promising as a building block for various flexible energy storage
applications
Biomimetic, Flexible, and Self-Healable Printed Silver Electrode by Spontaneous Self-Layering Phenomenon of a Gelatin Scaffold
Organicâinorganic
hybrid layer-by-layer (LBL) composite
structures can not only increase the strength and ductility of materials
but also well disperse nanomaterials for better-conducting pathways.
Here, we discovered the self-assembly process of an organic and silver
(Ag) LBL hybrid structure having excellent sustainability during the
long-term bending cycle. During the assembly process, the organic
and Ag hybrid structure can be self-assembled into a layered structure.
Unlike other conventional LBL fabrication processes, we applied the
hydrogel scaffold of a biological polymer, which can spontaneously
phase separate into an LBL structure in a water/alcohol solvent system.
This new hydrogel-based Ag LBL patterns can successfully be printed
on a flexible polyimide film without nozzle-clogging problem. Although
these Ag LBL patterns cracked during the bending cycle, carbonized
organic compounds between the Ag layers help to self-heal within few
minutes at a low temperature (<80 °C). On the basis of our
new hydrogel-based Ag ink, we could fabricate a fully printed reliable
microscale flexible heater. We expect that our self-layering phenomenon
can expand to the broad research field of flexible electronics in
the near future
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Cavitation-Induced Stiffness Reductions in Quantum DotâPolymer Nanocomposites
The elastic stiffness
of two polymer nanocomposite systems is investigated.
The nanoscale fillers comprise cadmium selenide (CdSe, âŒ4 nm)
and cadmium selenide/cadmium sulfide (CdSe/CdS, âŒ13 nm) quantum
dots (QDs). The QDs are embedded within an electrospun structural
block copolymer, polyÂ(styrene-ethylene-butylene-styrene) (SEBS). Tensile
testing shows a monotonic decrease in the tensile Youngâs modulus
with increasing partially phase-separated QD concentration; this is
to be compared to corresponding nanocomposites reinforced with nanorod
(NR) and tetrapod (TP)-SEBS nanocomposites which show a monotonic
increase with particle loading. While most studies to date emphasize
the increase in Youngâs modulus in polymer nanocomposites at
higher reinforcement loadings, few focus on the tunability of the
modulus from <i>reductions</i> in stiffness. The present
work reveals up to an âŒ80% reduction in tensile Youngâs
modulus with the addition of 5 vol % of QDs to electrospun SEBS. In
this study, we sought mechanistic insight into this reduction in composite
stiffness using a 2D lattice spring model. Simulation results reveal
that the stiffness decrease with the addition of QD reinforcements
is likely due to cavitation in the polymer in the vicinity of the
QD aggregates arising from polymer debonding under tension. We anticipate
that this study, performed with a commonly used structural rubber,
may find use in designing polymerâmatrix nanocomposite fibers
with specific Youngâs moduli for applications requiring a tunable
lower stiffness material
Cu<sub>2</sub>Te Incorporation-Induced High Average Thermoelectric Performance in <i>p</i>âType Bi<sub>2</sub>Te<sub>3</sub> Alloys
p-Type (Bi, Sb)2Te3 alloys
are attractive materials for near-room-temperature thermoelectric
applications due to their high atomic masses and large spinâorbit
interactions. However, their narrow band gaps originating from spinâorbit
interactions lead to bipolar excitation, thereby limiting average
thermoelectrics within a local temperature region (300â400
K). Here, we introduce Cu2Te into the Bi0.3Sb1.7Te3 (BST) lattice to implement high thermoelectrics
over a wide temperature range. The carrier concentration is synergistically
modulated via Cu substitution and the evolution of intrinsic point
defects (antisites and vacancies). Furthermore, the chain effect caused
by Cu2Te incorporation in BST is reflected in the improvement
of the weighted mobility ÎŒW, thereby enhancing the
power factor in the whole temperature range. Extrinsic and intrinsic
defects due to the incorporation of Cu2Te lead to a significant
reduction in the lattice thermal conductivity ÎșL, which is further demonstrated by Raman spectroscopy.
Combining ÎșL and ÎŒW, the quantity factor B increases from 0.5 to 1
with increasing Cu2Te content due to not only the reduction
of ÎșL but also a significant improvement
in electrical properties. Eventually, a peak figure of merit (zT) of âŒ1.15 at 423 K is achieved in BST-Cu2Te samples, and an average figure of merit (zTave) of âŒ1.12 (350â500 K) surpasses other excellent p-type Bi2Te3-based thermoelectrics.
Such a synergistic effect can facilitate near-room-temperature thermoelectric
applications of Bi2Te3-based alloys and provide
chances for the technology space in thermoelectrics
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Encapsulation of Perovskite Nanocrystals into Macroscale Polymer Matrices: Enhanced Stability and Polarization
Lead
halide perovskites hold promise for photonic devices, due to their
superior optoelectronic properties. However, their use is limited
by poor stability and toxicity. We demonstrate enhanced water and
light stability of high-surface-area colloidal perovskite nanocrystals
by encapsulation of colloidal CsPbBr<sub>3</sub> quantum dots into
matched hydrophobic macroscale polymeric matrices. This is achieved
by mixing the quantum dots with presynthesized high-molecular-weight
polymers. We monitor the photoluminescence quantum yield of the perovskiteâpolymer
nanocomposite films under water-soaking for the first time, finding
no change even after >4 months of continuous immersion in water.
Furthermore, photostability is greatly enhanced in the macroscale
polymer-encapsulated nanocrystal perovskites, which sustain >10<sup>10</sup> absorption events per quantum dot prior to photodegradation,
a significant threshold for potential device use. Control of the quantum
dot shape in these thin-film polymer composite enables color tunability
via strong quantum-confinement in nanoplates and significant room
temperature polarized emission from perovskite nanowires. Not only
does the high-molecular-weight polymer protect the perovskites from
the environment but also no escaped lead was detected in water that
was in contact with the encapsulated perovskites for months. Our ligand-passivated
perovskite-macroscale polymer composites provide a robust platform
for diverse photonic applications
Self-Assembly of Silver Nanowire Ring Structures Driven by the Compressive Force of a Liquid Droplet
In a nanowire dispersed
in liquid droplets, the interplay between
the surface tension of the liquid and the elasticity of the nanowire
determines the final morphology of the bent or buckled nanowire. Here,
we investigate the fabrication of a silver nanowire ring generated
as the nanowire encapsulated inside of fine droplets. We used a hybrid aerodynamic and electrostatic atomization method
to ensure the generation of droplets with scalable size in the necessary
regime for ring formation. We analytically calculate the compressive
force of the droplet driven by surface tension as the key mechanism
for the self-assembly of ring structures. Thus, for potential large-scale
manufacturing, the droplet size provides a convenient parameter to
control the realization of ring structures from nanowires
High Stability Induced by the TiN/Ti Interlayer in Three-Dimensional Si/Ge Nanorod Arrays as Anode in Micro Lithium Ion Battery
Three-dimensional (3D) Si/Ge-based
micro/nano batteries are promising
lab-on-chip power supply sources because of the good process compatibility
with integrated circuits and Micro/Nano-Electro-Mechanical System
technologies. In this work, the effective interlayer of TiN/Ti thin
films were introduced to coat around the 3D Si nanorod (NR) arrays
before the amorphous Ge layer deposition as anode in micro/nano lithium
ion batteries, thus the superior cycling stability was realized by
reason for the restriction of Si activation in this unique 3D matchlike
Si/TiN/Ti/Ge NR array electrode. Moreover, the volume expansion properties
after the repeated lithium-ion insertion/extraction were experimentally
investigated to evidence the superior stability of this unique multilayered
Si composite electrode. The demonstration of this wafer-scale, cost-effective,
and Si-compatible fabrication for anodes in Li-ion micro/nano batteries
provides new routes to configurate more efficient 3D energy storage
systems for micro/nano smart semiconductor devices
ZIFâ8 Cooperating in TiN/Ti/Si Nanorods as Efficient Anodes in Micro-Lithium-Ion-Batteries
Zeolite imidazolate framework-8 (ZIF-8)
nanoparticles embedded
in TiN/Ti/Si nanorod (NR) arrays without pyrolysis have shown increased
energy storage capacity as anodes for lithium ion batteries (LIBs).
A high capacity of 1650 ÎŒAh cm<sup>â2</sup> has been
achieved in this ZIF-8 composited multilayered electrode, which is
âŒ100 times higher than the plain electrodes made of only silicon
NR. According to the electrochemical impedance spectroscopy (EIS)
and <sup>1</sup>H nuclear magnetic resonance (NMR) characterizations,
the improved diffusion of lithium ions in ZIF-8 and boosted electron/Li<sup>+</sup> transfer by the ZIF-8/TiN/Ti multilayer coating are proposed
to be responsible for the enhanced energy storage ability. The first-principles
calculations further indicate the favorable accessibility of lithium
with appropriate size to diffuse in the open pores of ZIF-8. This
work broadens the application of ZIF-8 to silicon-based LIBs electrodes
without the pyrolysis and provides design guidelines for other metalâorganic
frameworks/Si composite electrodes
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Mechanisms of Local Stress Sensing in Multifunctional Polymer Films Using Fluorescent Tetrapod Nanocrystals
Nanoscale
stress-sensing can be used across fields ranging from detection of
incipient cracks in structural mechanics to monitoring forces in biological
tissues. We demonstrate how tetrapod quantum dots (tQDs) embedded
in block copolymers act as sensors of tensile/compressive stress.
Remarkably, tQDs can detect their own composite dispersion and mechanical
properties with a switch in optomechanical response when tQDs are
in direct contact. Using experimental characterizations, atomistic
simulations and finite-element analyses, we show that under tensile
stress, densely packed tQDs exhibit a photoluminescence peak shifted
to higher energies (âblue-shiftâ) due to volumetric
compressive stress in their core; loosely packed tQDs exhibit a peak
shifted to lower energies (âred-shiftâ) from tensile
stress in the core. The stress shifts result from the tQDâs
unique branched morphology in which the CdS arms act as antennas that
amplify the stress in the CdSe core. Our nanocomposites exhibit excellent
cyclability and scalability with no degraded properties of the host
polymer. Colloidal tQDs allow sensing in many materials to potentially
enable autoresponsive, smart structural nanocomposites that self-predict
impending fracture