12 research outputs found
Phase Transformation-Induced Tetragonal FeCo Nanostructures
Tetragonal FeCo nanostructures are
becoming particularly attractive
because of their high magnetocrystalline anisotropy and magnetization
achievable without rare-earth elements, . Yet, controlling their metastable
structure, size and stoichiometry is a challenging task. In this study,
we demonstrate AuCu templated FeCo shell growth followed by thermally
induced phase transformation of AuCu core from face-centered cubic
to L1<sub>0</sub> structure, which triggers the FeCo shell to transform
from the body-centered cubic structure to a body-centered tetragonal
phase. High coercivity, 846 Oe, and saturation magnetization, 221
emu/g, are achieved in this tetragonal FeCo structure. Beyond a critical
FeCo shell thickness, confirmed experimentally and by lattice mismatch
calculations, the FeCo shell relaxes. The shell thickness and stoichiometry
dictate the magnetic characteristics of the tetragonal FeCo shell.
This study provides a general route to utilize phase transformation
to fabricate high performance metastable nanomagnets, which could
open up their green energy applications
Surface-Stress-Induced Phase Transformation of Ultrathin FeCo Nanowires
Ultrathin
metal nanowires have attracted wide attention becau se
oftheir unique anisotropy and surface-to-volume effects. In this study,
we use ultrathin Au nanowires as the templating core to epitaxially
grow magnetic iron–cobalt (FeCo) shell through metal-redox
with the control on their thickness and stoichiometry. Large surface-stress-induced
phase transformation in Au nanowires triggers and stabilizes metastable
tetragonal FeCo nanostructure to enhance its magnetic anisotropy and
coercivity. Meanwhile, under illumination, plasmon-induced hotspot
in ultrathin Au nanowires enables the light-control on magnetic characteristics
of FeCo shell. This study demonstrates the feasibility of surface-stress-induced
phase transformation to stabilize and control metastable nanostructures
for enhanced magnetic anisotropy, which is one of the key properties
of functional magnetic materials
Synergistic Strain Engineering Effect of Hybrid Plasmonic, Catalytic, and Magnetic Core–Shell Nanocrystals
Hybrid core–shell nanocrystals,
consisting of distinct components, represent an emerging functional
material system, which could facilitate synergistic coupling effects
via integrating drastically different functionalities. Here we report
a unique strain engineering effect induced by phase transformation
between plasmonic core and magnetic shell materials, which leads to
a facile surface reconstruction of bimetallic core–shell nanocrystals
to enhance their synergistic magnetic and catalytic properties. This
advancement dramatically results in two orders of magnitude enhancement
in magnetic coercivity and significant improvement in catalytic activity.
Mechanistic studies involving the kinetic measurement and theoretical
modeling uncover a structural distortion and surface rearrangement
mechanism during the core–shell phase transformation pathway.
This facile methodology could potentially open up the new design of
multifunctional artificial hybrid nanostructures by the combination
of phase transformation and surface engineering for emerging technological
applications
Metal-Redox Synthesis of MnBi Hard Magnetic Nanoparticles
High
coercivity MnBi alloy is a promising candidate as earth abundant
permanent magnet for energy-critical technologies. We report here
a new metal-redox method to synthesize colloidal MnBi nanoparticles,
exhibiting a saturation magnetization of 49 emu/g and coercivity of
15 kOe. It is shown that the magnetic properties of the MnBi nanoalloys
can be readily modified by precursor stoichiometry, temperature ramp
rate, and reaction temperature, making it a versatile scalable strategy
for generation of MnBi
Quantum Dots-Facilitated Printing of ZnO Nanostructure Photodetectors with Improved Performance
A nanocomposite
ink composed of zinc oxide precursor (ZnOPr) and
crystalline ZnO quantum dots (ZnOPrQDs) has been explored for printing
high-performance ultraviolet (UV) photodetectors. The performance
of the devices has been compared with their counterparts’ printed
from ZnOPr ink without ZnO QDs. Remarkably, higher UV photoresponsivity
of 383.6 A/W and the on/off ratio of 2470 are observed in the former,
which are significantly better than 14.7 A/W and 949 in the latter.
The improved performance is attributed to the increased viscosity
in the nanocomposite ink to enable a nanoporous structure with improved
crystallinity and surface-to-volume ratio. This is key to enhanced
surface electron-depletion effect for higher UV responsivity and on/off
ratio. In addition, the QD-assisted printing provides a simple and
robust method for printing high-performance optoelectronics and sensors
Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection
Inorganic/biomolecule
nanohybrids can combine superior electronic and optical properties
of inorganic nanostructures and biomolecules for optoelectronics with
performance far surpassing that achievable in conventional materials.
The key toward a high-performance inorganic/biomolecule nanohybrid
is to design their interface based on the electronic structures of
the constituents. A major challenge is the lack of knowledge of most
biomolecules due to their complex structures and composition. Here,
we first calculated the electronic structure and optical properties
of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC) using ab initio
OLCAO method, which was followed by experimental confirmation using
ultraviolet photoemission spectroscopy. For the first time, the highest
occupied molecular orbital and lowest unoccupied molecular orbital
energy levels of Cyt c, a well-known electron transport chain in biological
systems, were obtained. On the basis of the result, pairing the Cyt
c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted
to have a favorable band alignment and built-in electrical field for
exciton dissociation and charge transfer across the s-SWCNT/Cyt c
heterojunction interface. Excitingly, photodetectors based on the
s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary
ultra-broadband (visible light to infrared) responsivity (46–188
A W<sup>–1</sup>) and figure-of-merit detectivity <i>D</i>* (1–6 × 10<sup>10</sup> cm Hz<sup>1/2</sup> W<sup>–1</sup>). Moreover, these devices can be fabricated on transparent flexible
substrates by a low-lost nonvacuum method and are stable in air. These
results suggest that the s-SWCNT/biomolecule nanohybrids may be promising
for the development of CNT-based ultra-broadband photodetectors
Room Temperature Multiferroicity of Charge Transfer Crystals
Room temperature multiferroics has been a frontier research field by manipulating spin-driven ferroelectricity or charge-order-driven magnetism. Charge-transfer crystals based on electron donor and acceptor assembly, exhibiting simultaneous spin ordering, are drawing significant interests for the development of all-organic magnetoelectric multiferroics. Here, we report that a remarkable anisotropic magnetization and room temperature multiferroicity can be achieved through assembly of thiophene donor and fullerene acceptor. The crystal motif directs the dimensional and compositional control of charge-transfer networks that could switch magnetization under external stimuli, thereby opening up an attractive class of all-organic nanoferronics
Printable Nanocomposite FeS<sub>2</sub>–PbS Nanocrystals/Graphene Heterojunction Photodetectors for Broadband Photodetection
Colloidal nanocrystals are attractive
materials for optoelectronics
applications because they offer a compelling combination of low-cost
solution processing, printability, and spectral tunability through
the quantum dot size effect. Here we explore a novel nanocomposite
photosensitizer consisting of colloidal nanocrystals of FeS<sub>2</sub> and PbS with complementary optical and microstructural properties
for broadband photodetection. Using a newly developed ligand exchange
to achieve high-efficiency charge transfer across the nanocomposite
FeS<sub>2</sub>–PbS sensitizer and graphene on the FeS<sub>2</sub>–PbS/graphene photoconductors, an extraordinary photoresponsivity
in exceeding ∼10<sup>6</sup> A/W was obtained in an ultrabroad
spectrum of ultraviolet (UV)-visible-near-infrared (NIR). This is
in contrast to the nearly 3 orders of magnitude reduction of the photoresponsivity
from ∼10<sup>6</sup> A/W at UV to 10<sup>3</sup> A/W at NIR
on their counterpart of FeS<sub>2</sub>/graphene detectors. This illustrates
the combined advantages of the nanocomposite sensitizers and the high
charge mobility in FeS<sub>2</sub>–PbS/graphene van der Waals
heterostructures for nanohybrid optoelectronics with high performance,
low cost, and scalability for commercialization
High-Sensitivity Light Detection via Gate Tuning of Organometallic Perovskite/PCBM Bulk Heterojunctions on Ferroelectric Pb<sub>0.92</sub>La<sub>0.08</sub>Zr<sub>0.52</sub>Ti<sub>0.48</sub>O<sub>3</sub> Gated Graphene Field Effect Transistors
Organometallic
perovskite (OMP) CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> doped
with [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM)
has been shown to form bulk heterojunction (OMP-PCBM BHJ) for improved
charge separation. In this work, the OMP-PCBM BHJ photosensitizer
is combined with graphene field effect transistors (GFETs) with a
ferroelectric Pb<sub>0.92</sub>La<sub>0.08</sub>Zr<sub>0.52</sub>Ti<sub>0.48</sub>O<sub>3</sub> gate of high gating efficiency. A remarkable
gate tunability via shifting the Fermi energy of graphene with respect
to the valence band maximum and conduction band minimum of the OMP
was observed, which is critical for facilitating efficient charge
transfer across the OMP-PCBM BHJ/GFET interface. The combination of
the high-efficiency charge separation by BHJ and charge transfer by
high gate tunability leads to achievement of high photoresponsivity
up to 7 × 10<sup>6</sup> A/W and detectivity exceeding 7 ×
10<sup>12</sup> Jones at 550 nm at a small gate voltage of 1.0 V.
These results represent almost 2 orders of magnitude improvement over
that without a gate tuning under the similar experimental condition,
illustrating the importance of the interface electronic structure
in optimizing the optoelectronic performance of the OMP-PCBM BHJ/GFET
devices
All-Printable ZnO Quantum Dots/Graphene van der Waals Heterostructures for Ultrasensitive Detection of Ultraviolet Light
In ZnO quantum dot/graphene
heterojunction photodetectors, fabricated
by printing quantum dots (QDs) directly on the graphene field-effect
transistor (GFET) channel, the combination of the strong quantum confinement
in ZnO QDs and the high charge mobility in graphene allows extraordinary
quantum efficiency (or photoconductive gain) in visible-blind ultraviolet
(UV) detection. Key to the high performance is a clean van der Waals
interface to facilitate an efficient charge transfer from ZnO QDs
to graphene upon UV illumination. Here, we report a robust ZnO QD
surface activation process and demonstrate that a transition from
zero to extraordinarily high photoresponsivity of 9.9 × 10<sup>8</sup> A/W and a photoconductive gain of 3.6 × 10<sup>9</sup> can be obtained in ZnO QDs/GFET heterojunction photodetectors, as
the ZnO QDs surface is systematically engineered using this process.
The high figure-of-merit UV detectivity <i>D*</i> in exceeding
1 × 10<sup>14</sup> Jones represents more than 1 order of magnitude
improvement over the best reported previously on ZnO nanostructure-based
UV detectors. This result not only sheds light on the critical role
of the van der Waals interface in affecting the optoelectronic process
in ZnO QDs/GFET heterojunction photodetectors but also demonstrates
the viability of printing quantum devices of high performance and
low cost