5 research outputs found
Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures
Single
layer boron-doped graphene layers have been grown on polycrystalline
copper foils by chemical vapor deposition using methane and diborane
as carbon and boron sources, respectively. Any attempt to deposit
doped layers in one-step has been fruitless, the reason being the
formation of very reactive boron species as a consequence of diborane
decomposition on the Cu surface, which leads to disordered nonstoichiometric
carbides. However, a two-step procedure has been optimized: as a first
step, the surface is seeded with pure graphene islands, while the
boron source is activated only in a second stage. In this case, the
nonstochiometric boron carbides formed on the bare copper areas between
preseeded graphene patches can be exploited to easily release boron,
which diffuses from the peripheral areas inward of graphene islands.
The effective substitutional doping (of the order of about 1%) has
been demonstrated by Raman and photoemission experiments. The electronic
properties of doped layers have been characterized by spatially resolved
photoemission band mapping carried out on single domain graphene flakes
using a photon beam with a spot size of 1 Ī¼m. The whole set
of experiments allow us to clarify that boron is effective at promoting
the anchoring carbon species on the surface. Taking the cue from this
basic understanding, it is possible to envisage new strategies for
the design of complex 2D graphene nanostructures with a spatially
modulated doping
Twist-Induced Modification in the Electronic Structure of Bilayer WSe<sub>2</sub>
The recent discovery of strongly correlated phases in
twisted transition-metal
dichalcogenides (TMDs) highlights the significant impact of twist-induced
modifications on electronic structures. In this study, we employed
angle-resolved photoemission spectroscopy with submicrometer spatial
resolution (Ī¼-ARPES) to investigate these modifications by comparing
valence band structures of twisted (5.3Ā°) and nontwisted (AB-stacked)
bilayer regions within the same WSe2 device. Relative to
the nontwisted region, the twisted area exhibits pronounced moireĢ
bands and ā¼90 meV renormalization at the Ī-valley, substantial
momentum separation between different layers, and an absence of flat
bands at the K-valley. We further simulated the effects of lattice
relaxation, which can flatten the Ī-valley edge but not the
K-valley edge. Our results provide a direct visualization of twist-induced
modifications in the electronic structures of twisted TMDs and elucidate
their valley-dependent responses to lattice relaxation
Reactivity of Carbon in LithiumāOxygen Battery Positive Electrodes
Unfortunately,
the practical applications of LiāO<sub>2</sub> batteries are
impeded by poor rechargeability. Here, for the first time we show
that superoxide radicals generated at the cathode during discharge
react with carbon that contains activated double bonds or aromatics
to form epoxy groups and carbonates, which limits the rechargeability
of LiāO<sub>2</sub> cells. Carbon materials with a low amount
of functional groups and defects demonstrate better stability thus
keeping the carbon will-oā-the-wisp lit for lithiumāair
batteries
Laterally Selective Oxidation of Large-Scale Graphene with Atomic Oxygen
Using
X-ray photoemission microscopy, we discovered that oxidation
of commercial large-scale graphene on Cu foil, which typically has
bilayer islands, by atomic oxygen proceeds with the formation of the
specific structures: though relatively mobile epoxy groups are generated
uniformly across the surface of single-layer graphene, their concentration
is significantly lower for bilayer islands. More oxidized species
like carbonyl and lactones are preferably located at the centers of
these bilayer islands. Such structures are randomly distributed over
the surface with a mean density of about 3Ć 10<sup>6</sup> cm<sup>ā2</sup> in our case. Using a set of advanced spectromicroscopy
instruments including Raman microscopy, X-ray photoelectron spectroscopy
(Ī¼-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved
photoelectron spectroscopy (Ī¼-ARPES), we found that the centers
of the bilayer islands where the second layer nucleates have a high
defect concentration and serve as the active sites for deep oxidation.
This information can be potentially useful in developing lateral heterostructures
for electronics and optoelectronics based on graphene/graphene oxide
heterojunction
Gold Dispersion and Activation on the Basal Plane of Single-Layer MoS<sub>2</sub>
Gold
islands are typically associated with high binding affinity
to adsorbates and catalytic activity. Here we present the growth of
dispersed nanoscale gold islands on single layer MoS<sub>2</sub>,
prepared on an inert SiO<sub>2</sub>/Si support by chemical vapor
deposition. This study offers a combination of growth process development,
optical characterization, photoelectron spectroscopy at submicron
spatial resolution, and advanced density functional theory modeling
for detailed insight into the electronic interaction between gold
and single-layer MoS<sub>2</sub>. In particular, we find the gold
density of states in Au/MoS<sub>2</sub>/SiO<sub>2</sub>/Si to be far
less well-defined than Au islands on other 2-dimensional materials
such as graphene, for which we also provide data. We attribute this
effect to the presence of heterogeneous Au adatom/MoS<sub>2</sub>-support
interactions within the nanometer-scale gold cluster. Theory predicts
that CO will exhibit adsorption energies in excess of 1 eV at the
Au cluster edges, where the local density of states is dominated by
Au 5d<sub><i>z</i></sub>2 symmetry