223 research outputs found
Surface Rearrangement and Evaporation Kinetics of Supported Gold Nanoparticle Catalysts
Heterogeneous catalysts consisting of supported metallic nanoparticles
typically derive exceptional catalytic activity from their large proportion of
under-coordinated surface sites which promote adsorption of reactant molecules.
Simultaneously, these high energy surface configurations are unstable, leading
to nanoparticle growth or degradation, and eventually a loss of catalytic
activity. Surface morphology of catalytic nanoparticles is paramount to
catalytic activity, selectivity, as well as degradation rates, however, it is
well-known that harsh reaction conditions can cause the surface structure to
change. Still, limited research has focused on understanding the link between
nanoparticle surface facets and degradation rates or mechanisms. Here, we study
a model Au supported catalyst system over a range of temperatures using a
combination of \textit{in situ} Transmission Electron Microscopy, kinetic Monte
Carlo simulations, and density functional theory calculations to establish an
atomistic picture of how variations in surface structures and atomic
coordination environments lead to shifting evolution mechanisms as a function
of temperature. By combining experimental results, which yield direct
observation of dynamic shape changes and particle evaporation rates, with
computational techniques, which enable understanding the fundamental
thermodynamics and kinetics of nanoparticle evolution, we illustrate a two-step
evolution mechanism in which mobile adatoms form through desorption from
low-coordination facets and subsequently evaporate off the particle surface. By
understanding the role of temperature in the competition between surface
diffusion and evaporation, we are able to show how individual atomic movements
lead to particle-scale morphological changes, and rationalize why evaporation
rates vary between particles in a system of nearly identical nanoparticles
Gas mixing system for imaging of nanomaterials under dynamic environments by environmental transmission electron microscopy
A gas mixing manifold system that is capable of delivering a stable pressure stream of a desired composition of gases into an environmental transmission electron microscope has been developed. The system is designed to provide a stable imaging environment upon changes of either the composition of the gas mixture or upon switching from one gas to another. The design of the system is described and the response of the pressure inside the microscope, the sample temperature, and sample drift in response to flow and composition changes of the system are reported. (C) 2014 AIP Publishing LLC
Large-scale Graphitic Thin Films Synthesized on Ni and Transferred to Insulators: Structural and Electronic Properties
We present a comprehensive study of the structural and electronic properties
of ultrathin films containing graphene layers synthesized by chemical vapor
deposition (CVD) based surface segregation on polycrystalline Ni foils then
transferred onto insulating SiO2/Si substrates. Films of size up to several
mm's have been synthesized. Structural characterizations by atomic force
microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional
transmission electron microscopy (XTEM) and Raman spectroscopy confirm that
such large scale graphitic thin films (GTF) contain both thick graphite regions
and thin regions of few layer graphene. The films also contain many wrinkles,
with sharply-bent tips and dislocations revealed by XTEM, yielding insights on
the growth and buckling processes of the GTF. Measurements on mm-scale
back-gated transistor devices fabricated from the transferred GTF show
ambipolar field effect with resistance modulation ~50% and carrier mobilities
reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with
phase coherence length over 0.2 m from the observation of 2D weak
localization in low temperature magneto-transport measurements. Our results
show that despite the non-uniformity and surface roughness, such large-scale,
flexible thin films can have electronic properties promising for device
applications.Comment: This version (as published) contains additional data, such as cross
sectional TEM image
Non-Volatile Control of Valley Polarized Emission in 2D WSe2-AlScN Heterostructures
Achieving robust and electrically controlled valley polarization in monolayer
transition metal dichalcogenides (ML-TMDs) is a frontier challenge for
realistic valleytronic applications. Theoretical investigations show that
integration of 2D materials with ferroelectrics is a promising strategy;
however, its experimental demonstration has remained elusive. Here, we
fabricate ferroelectric field-effect transistors using a ML-WSe2 channel and a
AlScN ferroelectric dielectric, and experimentally demonstrate efficient tuning
as well as non-volatile control of valley polarization. We measured a large
array of transistors and obtained a maximum valley polarization of ~27% at 80 K
with stable retention up to 5400 secs. The enhancement in the valley
polarization was ascribed to the efficient exciton-to-trion (X-T) conversion
and its coupling with an out-of-plane electric field, viz. the quantum-confined
Stark effect. This changes the valley depolarization pathway from strong
exchange interactions to slow spin-flip intervalley scattering. Our research
demonstrates a promising approach for achieving non-volatile control over
valley polarization and suggests new design principles for practical
valleytronic devices.Comment: Manuscript (22 pages and 5 figures), supporting informatio
Ultra-fast Vacancy Migration: A Novel Approach for Synthesizing Sub-10 nm Crystalline Transition Metal Dichalcogenide Nanocrystals
Two-dimensional materials, such as transition metal dichalcogenides (TMDCs),
have the potential to revolutionize the field of electronics and photonics due
to their unique physical and structural properties. This research presents a
novel method for synthesizing crystalline TMDCs crystals with < 10 nm size
using ultra-fast migration of vacancies at elevated temperatures. Through
in-situ and ex-situ processing and using atomic-level characterization
techniques, we analyze the shape, size, crystallinity, composition, and strain
distribution of these nanocrystals. These nanocrystals exhibit electronic
structure signatures that differ from the 2D bulk i.e., uniform mono and
multilayers. Further, our in-situ, vacuum-based synthesis technique allows
observation and comparison of defect and phase evolution in these crystals
formed under van der Waals heterostructure confinement versus unconfined
conditions. Overall, this research demonstrates a solid-state route to
synthesizing uniform nanocrystals of TMDCs and lays the foundation for
materials science in confined 2D spaces under extreme conditions.Comment: MS+S
- …