13 research outputs found
Atomic Resolution Imaging of Nanoscale Structural Ordering in a Complex Metal Oxide Catalyst
The determination of the atomic structure of a functional
material
is crucial to understanding its âstructure-to-propertyâ
relationship (e.g., the active sites in a catalyst), which is however
challenging if the structure possesses complex inhomogeneities. Here,
we report an atomic structure study of an important MoVTeO complex
metal oxide catalyst that is potentially useful for the industrially
relevant propane-based BP/SOHIO process. We combined aberration-corrected
scanning transmission electron microscopy with synchrotron powder
X-ray crystallography to explore the structure at both nanoscopic
and macroscopic scales. At the nanoscopic scale, this material exhibits
structural and compositional order within nanosized âdomainsâ,
while the domains show disordered distribution at the macroscopic
scale. We proposed that the intradomain compositional ordering and
the interdomain electric dipolar interaction synergistically induce
the displacement of Te atoms in the MoâVâO channels,
which determines the geometry of the multifunctional metal oxo-active
sites
Defects and Surface Structural Stability of MoTe<sub>2</sub> Under Vacuum Annealing
Understanding
the structural stability of transition-metal dichalcogenides
is necessary to avoid surface/interface degradation. In this work,
the structural stability of 2H-MoTe<sub>2</sub> with thermal treatments
up to 500 °C is studied using scanning tunneling microscopy and
scanning transmission electron microscopy. On the exfoliated sample
surface at room temperature, atomic subsurface donors originating
from excess Te atoms are observed and presented as nanometer-sized,
electronically-induced protrusions superimposed with the hexagonal
lattice structure of MoTe<sub>2</sub>. Under a thermal treatment as
low as 200 °C, the surface decomposition-induced cluster defects
and Te vacancies are readily detected and increase in extent with
the increasing temperature. Driven by Te vacancies and thermal energy,
intense 60° inversion domain boundaries form resulting in a âwagon
wheelâ morphology after 400 °C annealing for 15 min. Scanning
tunneling spectroscopy identified the electronic states at the domain
boundaries and the domain centers. To prevent extensive Te loss at
higher temperatures, where Mo<sub>6</sub>Te<sub>6</sub> nanowire formation
and substantial desorption-induced etching effects will take place
simultaneously, surface and edge passivation with a monolayer graphene
coverage on MoTe<sub>2</sub> is tested. With this passivation strategy,
the structural stability of MoTe<sub>2</sub> is greatly enhanced up
to 500 °C without apparent structural defects
Atomic Bonding between Metal and Graphene
To understand structural and chemical properties of metalâgraphene
composites, it is crucial to unveil the chemical bonding along the
interface. We provide direct experimental evidence of atomic bonding
between typical metal nano structures and graphene, agreeing well
with density functional theory studies. Single Cr atoms are located
in the valleys of a zigzag edge, and few-atom ensembles preferentially
form atomic chains by self-assembly. Low migration barriers lead to
rich dynamics of metal atoms and clusters under electron irradiation.
We demonstrate no electron-instigated interaction between Cr clusters
and pristine graphene, though Cr has been reported to be highly reactive
to graphene. The metal-mediated etching is a dynamic effect between
metal clusters and pre-existing defects. The resolved atomic configurations
of typical nano metal structures on graphene offer insight into modeling
and simulations on properties of metal-decorated graphene for both
catalysis and future carbon-based electronics
Atomic Bonding between Metal and Graphene
To understand structural and chemical properties of metalâgraphene
composites, it is crucial to unveil the chemical bonding along the
interface. We provide direct experimental evidence of atomic bonding
between typical metal nano structures and graphene, agreeing well
with density functional theory studies. Single Cr atoms are located
in the valleys of a zigzag edge, and few-atom ensembles preferentially
form atomic chains by self-assembly. Low migration barriers lead to
rich dynamics of metal atoms and clusters under electron irradiation.
We demonstrate no electron-instigated interaction between Cr clusters
and pristine graphene, though Cr has been reported to be highly reactive
to graphene. The metal-mediated etching is a dynamic effect between
metal clusters and pre-existing defects. The resolved atomic configurations
of typical nano metal structures on graphene offer insight into modeling
and simulations on properties of metal-decorated graphene for both
catalysis and future carbon-based electronics
Site-Specific Growth of AuâPd Alloy Horns on Au Nanorods: A Platform for Highly Sensitive Monitoring of Catalytic Reactions by Surface Enhancement Raman Spectroscopy
Surface-enhanced Raman scattering (SERS) is a highly sensitive
probe for molecular detection. The aim of this study was to develop
an efficient platform for investigating the kinetics of catalytic
reactions with SERS. To achieve this, we synthesized a novel AuâPd
bimetallic nanostructure (HIF-AuNR@AuPd) through site-specific epitaxial
growth of AuâPd alloy horns as catalytic sites at the ends
of Au nanorods. Using high-resolution electron microscopy and tomography,
we successfully reconstructed the complex three-dimensional morphology
of HIF-AuNR@AuPd and identified that the horns are bound with high-index
{11<i>l</i>} (0.25 < <i>l</i> < 0.43) facets.
With an electron beam probe, we visualized the distribution of surface
plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal
surface plasmon resonance concentrated at the rod ends. This unique
crystal morphology led to the coupling of high catalytic activity
with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an
excellent bifunctional platform for <i>in situ</i> monitoring
of surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol
as a model reaction, we demonstrated that its first-order reaction
kinetics could be accurately determined from this platform. Moreover,
we clearly identified the superior catalytic activity of the rod ends
relative to that of the rod bodies, owing to the different SERS activities
at the two positions. In comparison with other reported AuâPd
bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic
activity and greater detection sensitivity
Chiral Transformation: From Single Nanowire to Double Helix
We report a new type of water-soluble ultrathin AuâAg alloy nanowire (NW), which exhibits unprecedented behavior in a colloidal solution. Upon growth of a thin metal (Pd, Pt, or Au) layer, the NW winds around itself to give a metallic double helix. We propose that the winding originates from the chirality within the as-synthesized AuâAg NWs, which were induced to untwist upon metal deposition
Atomic Bonding between Metal and Graphene
To understand structural and chemical properties of metalâgraphene
composites, it is crucial to unveil the chemical bonding along the
interface. We provide direct experimental evidence of atomic bonding
between typical metal nano structures and graphene, agreeing well
with density functional theory studies. Single Cr atoms are located
in the valleys of a zigzag edge, and few-atom ensembles preferentially
form atomic chains by self-assembly. Low migration barriers lead to
rich dynamics of metal atoms and clusters under electron irradiation.
We demonstrate no electron-instigated interaction between Cr clusters
and pristine graphene, though Cr has been reported to be highly reactive
to graphene. The metal-mediated etching is a dynamic effect between
metal clusters and pre-existing defects. The resolved atomic configurations
of typical nano metal structures on graphene offer insight into modeling
and simulations on properties of metal-decorated graphene for both
catalysis and future carbon-based electronics
Ru Nanoframes with an fcc Structure and Enhanced Catalytic Properties
Noble-metal
nanoframes are of great interest to many applications due to their
unique open structures. Among various noble metals, Ru has never been
made into nanoframes. In this study, we report for the first time
an effective method based on seeded growth and chemical etching for
the facile synthesis of Ru nanoframes with high purity. The essence
of this approach is to induce the preferential growth of Ru on the
corners and edges of Pd truncated octahedra as the seeds by kinetic
control. The resultant PdâRu coreâframe octahedra could
be easily converted to Ru octahedral nanoframes of âŒ2 nm in
thickness by selectively removing the Pd cores through chemical etching.
Most importantly, in this approach the face-centered cubic (fcc) crystal
structure of Pd seeds was faithfully replicated by Ru that usually
takes an hcp structure. The fcc Ru nanoframes showed higher catalytic
activities toward the reduction of <i>p</i>-nitrophenol
by NaBH<sub>4</sub> and the dehydrogenation of ammonia borane compared
with hcp Ru nanowires with roughly the same thickness
Doping Monolayer Graphene with Single Atom Substitutions
Functionalized graphene has been extensively studied
with the aim
of tailoring properties for gas sensors, superconductors, supercapacitors,
nanoelectronics, and spintronics. A bottleneck is the capability to
control the carrier type and density by doping. We demonstrate that
a two-step process is an efficient way to dope graphene: create vacancies
by high-energy atom/ion bombardment and fill these vacancies with
desired dopants. Different elements (Pt, Co, and In) have been successfully
doped in the single-atom form. The high binding energy of the metal-vacancy
complex ensures its stability and is consistent with in situ observation
by an aberration-corrected and monochromated transmission electron
microscope
Sub-10 nm Tunable Hybrid Dielectric Engineering on MoS<sub>2</sub> for Two-Dimensional Material-Based Devices
The
successful realization of high-performance 2D-materials-based
nanoelectronics requires integration of high-quality dielectric films
as a gate insulator. In this work, we explore the integration of organic
and inorganic hybrid dielectrics on MoS<sub>2</sub> and study the
chemical and electrical properties of these hybrid films. Our atomic
force microscopy, X-ray photoelectron spectroscopy (XPS), Raman, and
photoluminescence results show that, aside from the excellent film
uniformity and thickness scalability down to 2.5 nm, the molecular
layer deposition of octenyltrichlorosilane (OTS) and Al<sub>2</sub>O<sub>3</sub> hybrid films preserves the chemical and structural
integrity of the MoS<sub>2</sub> surface. The XPS band alignment analysis
and electrical characterization reveal that through the inclusion
of an organic layer in the dielectric film, the band gap and dielectric
constant can be tuned from âŒ7.00 to 6.09 eV and âŒ9.0
to 4.5, respectively. Furthermore, the hybrid films show promising
dielectric properties, including a high breakdown field of âŒ7.8
MV/cm, a low leakage current density of âŒ1 Ă 10<sup>â6</sup> A/cm<sup>2</sup> at 1 MV/cm, a small hysteresis of âŒ50 mV,
and a top-gate subthreshold voltage swing of âŒ79 mV/dec. Our
experimental findings provide a facile way of fabricating scalable
hybrid gate dielectrics on transition metal dichalcogenides for 2D-material-based
flexible electronics applications