9 research outputs found
Scalable Production of Few-Layer Boron Sheets by Liquid-Phase Exfoliation and Their Superior Supercapacitive Performance
Although two-dimensional
boron (B) has attracted much attention
in electronics and optoelectronics due to its unique physical and
chemical properties, in-depth investigations and applications have
been limited by the current synthesis techniques. Herein, we demonstrate
that high-quality few-layer B sheets can be prepared in large quantities
by sonication-assisted liquid-phase exfoliation. By simply varying
the exfoliating solvent types and centrifugation speeds, the lateral
size and thickness of the exfoliated B sheets can be controllably
tuned. Additionally, the exfoliated few-layer B sheets exhibit excellent
stability and outstanding dispersion in organic solvents without aggregates
for more than 50 days under ambient conditions, owing to the presence
of a solvent residue shell on the B sheet surface that provides excellent
protection against air oxidation. Moreover, we also demonstrate the
use of the exfoliated few-layer B sheets for high-performance supercapacitor
electrode materials. This as-prepared device exhibits impressive electrochemical
performance with a wide potential window of up to 3.0 V, excellent
energy density as high as 46.1 Wh/kg at a power density of 478.5 W/kg,
and excellent cycling stability with 88.7% retention of the initial
specific capacitance after 6000 cycles. This current work not only
demonstrates an effective strategy for the synthesis of the few-layer
B sheets in a controlled manner but also makes the resulting materials
promising for next-generation optoelectronics and energy storage applications
Facile Synthesis of Millimeter-Scale Vertically Aligned Boron Nitride Nanotube Forests by Template-Assisted Chemical Vapor Deposition
There
is an increasing amount of research interest in synthesizing
boron nitride nanotubes (BNNTs) as well as BN coatings to be used
for various applications due to their outstanding mechanical, electrical,
and thermal properties. However, vertically aligned (VA) BNNTs are
difficult to synthesize and the longest VA-BNNTs achieved to date
are up to several tens of microns. Here, we report the synthesis of
over millimeters long multiwalled BN coated carbon nanotubes (BN/CNT)
and BNNT forests via a facile and effective two-step route involving
template-assisted chemical vapor deposition at a relatively low temperature
of 900 °C and a subsequent annealing process. The as-prepared
BN/CNTs and BNNTs retain the highly ordered vertically aligned structures
of the CNT templates as identified by scanning electron microscopy.
The structure and composition of the resulting products were studied
using transmission electron microscopy, electron energy-loss spectroscopy,
X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform
infrared spectroscopy, and thermogravimetric analysis. This versatile
BN coating technique and the synthesis of millimeter-scale BN/CNTs
and BNNTs pave the way for new applications especially where the aligned
geometry of the NTs is essential such as for field-emission, interconnects,
and thermal management
Supercompressible Coaxial Carbon Nanotube@Graphene Arrays with Invariant Viscoelasticity over −100 to 500 °C in Ambient Air
Vertically
aligned carbon nanotube (CNT) arrays have been recognized
as promising cushion materials because of their superior thermal stability,
remarkable compressibility, and viscoelastic characteristics. However,
most of the previously reported CNT arrays still suffer from permanent
shape deformation at only moderate compressive strains, which considerably
restricts their practical applications. Here, we demonstrate a facile
strategy of fabricating supercompressible coaxial CNT@graphene (CNT@Gr)
arrays by using a two-step route involving encapsulating polymer layers
onto plastic CNT arrays and subsequent annealing processes. Notably,
the resulting CNT@Gr arrays are able to almost completely recover
from compression at a strain of up to 80% and retain ∼80% recovery
even after 1000 compression cycles at a 60% strain, demonstrating
their excellent compressibility. Furthermore, they possess outstanding
strain- and frequency-dependent viscoelastic responses, with storage
modulus and damping ratio of up to ∼6.5 MPa and ∼0.19,
respectively, which are nearly constant over an exceptionally broad
temperature range of −100 to 500 °C in ambient air. These
supercompressibility and temperature-invariant viscoelasticity together
with facile fabrication process of the CNT@Gr arrays enable their
promising multifunctional applications such as energy absorbers, mechanical
sensors, and heat exchangers, even in extreme environments
Trimethylamine Borane: A New Single-Source Precursor for Monolayer h‑BN Single Crystals and h‑BCN Thin Films
Due to their exceptional chemical
and thermal stabilities as well
as electrically insulating property, atomically thin hexagonal boron
nitride (h-BN) films have been identified as a promising class of
dielectric substrate and encapsulation material for high-performance
two-dimensional (2D) heterostructure devices. Herein, we report a
facile chemical vapor deposition synthesis of large-area atomically
thin h-BN including monolayer single crystals and C-doped h-BN (h-BCN)
films utilizing a relatively low-cost, commercially available trimethylamine
borane (TMAB) as a single-source precursor. Importantly, pristine
2D h-BN films with a wide band gap of ∼6.1 eV can be achieved
by limiting the sublimation temperature of TMAB at 40 °C, while
C dopants are introduced to the h-BN films when the sublimation temperature
is further increased. The h-BCN thin films displayed band gap narrowing
effects as identified by an additional shoulder at 205 nm observed
in their absorbance spectra. Presence of N–C bonds in the h-BCN
structures with a doping concentration of ∼2 to 5% is confirmed
by X-ray photoelectron spectroscopy. The inclusion of low C doping
in the h-BN films is expected to result in constructive enhancement
to its mechanical properties without significant alteration to its
electrically insulating nature. This study provides new insights into
the design and fabrication of large-area atomically thin h-BN/h-BCN
films toward practical applications and suggests that the range of
precursors can be potentially extended to other anime borane complexes
as well
Control of Nanoplane Orientation in voBN for High Thermal Anisotropy in a Dielectric Thin Film: A New Solution for Thermal Hotspot Mitigation in Electronics
High anisotropic thermal materials,
which allow heat to dissipate in a preferential direction, are of
interest as a prospective material for electronics as an effective
thermal management solution for hot spots. However, due to their preferential
heat propagation in the in-plane direction, the heat spreads laterally
instead of vertically. This limitation makes these materials ineffective
as the density of hot spots increases. Here, we produce a new dielectric
thin film material at room temperature, named vertically ordered nanocrystalline
h-BN (voBN). It is produced such that its preferential thermally conductive
direction is aligned in the vertical axis, which facilitates direct
thermal extraction, thereby addressing the increasing challenge of
thermal crosstalk. The uniqueness of voBN comes from its h-BN nanocrystals
where all their basal planes are aligned in the direction normal to
the substrate plane. Using the 3ω method, we show that voBN
exhibits high anisotropic thermal conductivity (TC) with a 16-fold
difference between through-film TC and in-plane TC (respectively 4.26
and 0.26 W·m<sup>–1</sup>·K<sup>–1</sup>).
Molecular dynamics simulations also concurred with the experimental
data, showing that the origin of this anisotropic behavior is due
to the nature of voBN’s plane ordering. While the consistent
vertical ordering provides an uninterrupted and preferred propagation
path for phonons in the through-film direction, discontinuity in the
lateral direction leads to a reduced in-plane TC. In addition, we
also use COMSOL to simulate how the dielectric and thermal properties
of voBN enable an increase in hot spot density up to 295% compared
with SiO<sub>2</sub>, without any temperature increase
High-Density 3D-Boron Nitride and 3D-Graphene for High-Performance Nano–Thermal Interface Material
Compression
studies on three-dimensional foam-like graphene and
h-BN (3D-C and 3D-BN) revealed their high cross-plane thermal conductivity
(62–86 W m<sup>–1</sup> K<sup>–1</sup>) and excellent
surface conformity, characteristics essential for thermal management
needs. Comparative studies to state-of-the-art materials and other
materials currently under research for heat dissipation revealed 3D-foam’s
improved performance (20–30% improved cooling, temperature
decrease by Δ<i>T</i> of 44–24 °C)
Growth of Large Single-Crystalline Two-Dimensional Boron Nitride Hexagons on Electropolished Copper
Hexagonal-boron
nitride (h-BN) or “white graphene”
has many outstanding properties including high thermal conductivity,
high mechanical strength, chemical inertness, and high electrical
resistance, which open up a wide range of applications such as thermal
interface material, protective coatings, and dielectric in nanoelectronics
that easily exceed the current advertised benefits pertaining to the
graphene-based applications. The development of h-BN films using chemical
vapor deposition (CVD) has thus far led into nucleation of triangular
or asymmetric diamond shapes on different metallic surfaces. Additionally,
the average size of the triangular domains has remained relatively
small (∼0.5 μm<sup>2</sup>) leading to a large number
of grain boundaries and defects. While the morphology of Cu surfaces
for CVD-grown graphene may have impacts on the nucleation density,
domain sizes, thickness, and uniformity, the effects of the decreased
roughness of Cu surface to develop h-BN films are unknown. Here, we
report the growth and characterization of novel large area h-BN hexagons
using highly electropolished Cu substrate under atmospheric pressure
CVD conditions. We found that the nucleation density of h-BN is significantly
reduced while domain sizes increase. In this study, the largest hexagonal-shape
h-BN domain observed is 35 μm<sup>2</sup>, which is an order
of magnitude larger than a typical triangular domain. As the domains
coalesce to form a continuous film, the larger grain size offers a
more pristine and smoother film with lesser grain boundaries induced
defects
Biocompatible Hydroxylated Boron Nitride Nanosheets/Poly(vinyl alcohol) Interpenetrating Hydrogels with Enhanced Mechanical and Thermal Responses
Poly(vinyl alcohol)
(PVA) hydrogels with tissue-like viscoelasticity,
excellent biocompatibility, and high hydrophilicity have been considered
as promising cartilage replacement materials. However, lack of sufficient
mechanical properties is a critical barrier to their use as load-bearing
cartilage substitutes. Herein, we report hydroxylated boron nitride
nanosheets (OH-BNNS)/PVA interpenetrating hydrogels by cyclically
freezing/thawing the aqueous mixture of PVA and highly hydrophilic
OH-BNNS (up to 0.6 mg/mL, two times the highest reported so far).
Encouragingly, the resulting OH-BNNS/PVA hydrogels exhibit controllable
reinforcements in both mechanical and thermal responses by simply
varying the OH-BNNS contents. Impressive 45, 43, and 63% increases
in compressive, tensile strengths and Young’s modulus, respectively,
can be obtained even with only 0.12 wt% (OH-BNNS:PVA) OH-BNNS addition.
Meanwhile, exciting improvements in the thermal diffusivity (15%)
and conductivity (5%) can also be successfully achieved. These enhancements
are attributed to the synergistic effect of intrinsic superior properties
of the as-prepared OH-BNNS and strong hydrogen bonding interactions
between the OH-BNNS and PVA chains. In addition, excellent cytocompatibility
of the composite hydrogels was verified by cell proliferation and
live/dead viability assays. These biocompatible OH-BNNS/PVA hydrogels
are promising in addressing the mechanical failure and locally overheating
issues as cartilage substitutes and may also have broad utility for
biomedical applications, such as drug delivery, tissue engineering,
biosensors, and actuators
Concentric and Spiral Few-Layer Graphene: Growth Driven by Interfacial Nucleation vs Screw Dislocation
Spiral growth of various nanomaterials including some two-dimensional (2D) transition metal dichalcogenides had recently been experimentally realized using chemical vapor deposition (CVD). However, such growth that is driven by screw dislocation remained elusive for graphene and is rarely discussed because of the use of metal catalysts. In this work, we show that formation of few-layer graphene (FLG) with a spiral structure driven by screw dislocation can be obtained alongside FLG having a concentric layered structure formed by interfacial nucleation (nucleation at the graphene/Cu interface) using Cu-catalyzed ambient pressure CVD. Unlike commonly reported FLG grown by interfacial nucleation where the second layer is grown independently beneath the first, the growth of a spiral structure adopts a top growth mechanism where the top layers are an extension from the initial monolayer which spirals around an axial dislocation in self-perpetuating steps. Since the same atomic orientation is preserved, the subsequent spiraling layers are stacked in an oriented AB-stacked configuration. This contrasts with FLG formed by interfacial nucleation where turbostratic stacking of the entire adlayer may exist. In both growth scenarios, the second layer (either top or bottom) can grow across the grain boundaries of the initial monolayer domains, forming partial regions with turbostratic stacking configuration due to weak interlayer van der Waals interactions. The unique interlayer coupling of FLG spirals, which enable superior conductivity along the normal of the 2D crystal with spiraling trajectories, are expected to have new and interesting nanoscale applications