72 research outputs found
Point Defects and Localized Excitons in 2D WSe2
Identifying the point defects in 2D materials is important for many
applications. Recent studies have proposed that W vacancies are the predominant
point defect in 2D WSe2, in contrast to theoretical studies, which predict that
chalcogen vacancies are the most likely intrinsic point defects in transition
metal dichalcogenide semiconductors. We show using first principles
calculations, scanning tunneling microscopy (STM) and scanning transmission
electron microscopy experiments, that W vacancies are not present in our
CVD-grown 2D WSe2. We predict that O-passivated Se vacancies (O_Se) and O
interstitials (Oins) are present in 2D WSe2, because of facile O2 dissociation
at Se vacancies, or due to the presence of WO3 precursors in CVD growth. These
defects give STM images in good agreement with experiment. The optical
properties of point defects in 2D WSe2 are important because single photon
emission (SPE) from 2D WSe2 has been observed experimentally. While strain
gradients funnel the exciton in real space, point defects are necessary for the
localization of the exciton at length scales that enable photons to be emitted
one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations,
we predict that only Oins defects give localized excitons within the energy
range of SPE in previous experiments, making them a likely source of previously
observed SPE. No other point defects (O_Se, Se vacancies, W vacancies and Se_W
antisites) give localized excitons in the same energy range. Our predictions
suggest ways to realize SPE in related 2D materials and point experimentalists
toward other energy ranges for SPE in 2D WSe2
Controlled alignment of supermoir\'e lattice in double-aligned graphene heterostructures
The supermoir\'e lattice, built by stacking two moir\'e patterns, provides a
platform for creating flat mini-bands and studying electron correlations. An
ultimate challenge in assembling a graphene supermoir\'e lattice is in the
deterministic control of its rotational alignment, which is made highly
aleatory due to the random nature of the edge chirality and crystal symmetry of
each component layer. Employing the so-called golden rule of three, here we
present an experimental strategy to overcome this challenge and realize the
controlled alignment of double-aligned hBN/graphene/hBN supermoir\'e lattice,
where graphene is precisely aligned with both top hBN and bottom hBN.
Remarkably, we find that the crystallographic edge of neighboring graphite can
be used to better guide the stacking alignment, as demonstrated by the
controlled production of 20 moir\'e samples with an accuracy better than 0.2
degree. Finally, we extend our technique to other strongly correlated electron
systems, such as low-angle twisted bilayer graphene and ABC-stacked trilayer
graphene, providing a strategy for flat-band engineering in these moir\'e
materials.Comment: 20 pages, 4 figure
Evidence for metallic 1T phase, 3d1 electronic configuration and charge density wave order in molecular-beam epitaxy grown monolayer VTe2
We present a combined experimental and theoretical study of monolayer VTe2
grown on highly oriented pyrolytic graphite by molecular-beam epitaxy. Using
various in-situ microscopic and spectroscopic techniques, including scanning
tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved
photoemission, and X-ray absorption, together with theoretical analysis by
density functional theory calculations, we demonstrate direct evidence of the
metallic 1T phase and 3d1 electronic configuration in monolayer VTe2 that also
features a (4 x 4) charge density wave order at low temperatures. In contrast
to previous theoretical predictions, our element-specific characterization by
X-ray magnetic circular dichroism rules out a ferromagnetic order intrinsic to
the monolayer. Our findings provide essential knowledge necessary for
understanding this interesting yet less explored metallic monolayer in the
emerging family of van der Waals magnets.Comment: 21 pages, 5 figure
Strain-Controlled Spin Wave Excitation and Gilbert Damping in Flexible Co2FeSi Films Activated by Femtosecond Laser Pulse
The dynamic response of magnetic order to optical excitation at sub-picosecond scale has offered an intriguing alternative for magnetism manipulation. Such ultrafast optical manipulation of magnetism has become a fundamental challenging topic with high implications for future spintronics. Here, this study demonstrates such manipulation in Co2FeSi films grown on flexible polyimide substrate, and demonstrates how the magneto-optical interaction can be modified by using strain engineering which in turn triggers the excitation of both dipolar and exchange spin waves modes. Furthermore, Gilbert damping and spin-orbit coupling in Co2FeSi can both be tuned significantly by altering the magnitude and type of applied strain, suggesting an appealing way to manipulate spin wave propagation. These results develop the optical manipulation magnetism into the field of spin wave dynamics, and open a new direction in the application of spin orbitronics and magnonics devices using strain engineering
Irreversible Denaturation of Proteins through Aluminum-Induced Formation of Backbone Ring Structures
The molecule-metal interface
Reviewing recent progress in the fundamental understanding of the molecule-metal interface, this useful addition to the literature focuses on experimental studies and introduces the latest analytical techniques as applied to this interface.The first part covers basic theory and initial principle studies, while the second part introduces readers to photoemission, STM, and synchrotron techniques to examine the atomic structure of the interfaces. The third part presents photoelectron spectroscopy, high-resolution UV photoelectron spectroscopy and electron spin resonance to study the electron
Method to form MOS transistors with shallow junctions using laser annealing
US6335253Granted Paten
On-Surface Synthesis and Applications of 2D Covalent Organic Framework Nanosheets
Covalent organic framework nanosheets (COF nanosheets) are two-dimensional crystalline porous polymers with in-plane covalent bonds and out-of-plane Van der Waals forces. Owing to the customizable structure, chemical modification, and ultra-high porosity, COF nanosheets show many fascinating properties unique to traditional two-dimensional materials, and have shown potential applications in gas separation, sensors, electronic, and optoelectronic devices. This minireview aims to illustrate recent progress on two-dimensional covalent organic framework nanosheets, from two aspects of on-surface synthesis and potential applications. We first review the synthesis of COF nanosheets at the gas–solid interface. On-surface synthesis under ultrahigh vacuum and on-surface synthesis under vapor are highlighted. In addition, we also review the liquid–solid interface synthesis of COF nanosheets at various substrates, i.e., both crystalline and amorphous substrates. Beyond the synthesis, we highlight state-of-the-art applications of the COF nanosheets, particularly in charge transport, chemical sensors, and gas separation
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