36 research outputs found
Single-layer -MoS under electron irradiation from molecular dynamics
Irradiation with high-energy particles has recently emerged as an effective
tool for tailoring the properties of two-dimensional transition metal
dichalcogenides. In order to carry out an atomically-precise manipulation of
the lattice, a detailed understanding of the beam-induced events occurring at
the atomic scale is necessary. Here, we investigate the response of
-MoS to the electron irradiation by molecular dynamics
means. Our simulations suggest that an electron beam with energy smaller than
75 keV does not result in any knock-on damage. The displacement threshold
energies are different for the two nonequivalent sulfur atoms in -MoS
and strongly depend on whether the top or bottom chalcogen layer is considered.
As a result, a careful tuning of the beam energy can promote the formation of
ordered defects in the sample. We further discuss the effect of the electron
irradiation in the neighborhood of a defective site, the mobility of the sulfur
vacancies created and their tendency to aggregate. Overall, our work provides
useful guidelines for the imaging and the defect engineering of -MoS
using electron microscopy.Comment: 8 pages, 5 figure
Electronic transport across quantum dots in graphene nanoribbons: Toward built-in gap-tunable metal-semiconductor-metal heterojunctions
The success of all-graphene electronics is severely hindered by the
challenging realization and subsequent integration of semiconducting channels
and metallic contacts. Here, we comprehensively investigate the electronic
transport across width-modulated heterojunctions consisting of a graphene
quantum dot of varying lengths and widths embedded in a pair of armchair-edged
metallic nanoribbons, of the kind recently fabricated via on-surface synthesis.
We show that the presence of the quantum dot enables the opening of a
width-dependent transport gap, thereby yielding built-in one-dimensional
metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity
of the band edges, the conductance is subject to a smooth transition from an
antiresonant to a resonant transport regime upon increasing the channel length.
These results are rationalized in terms of a competition between
quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work
establishes graphene quantum dot nanoarchitectures as appealing platforms to
seamlessly integrate gap-tunable semiconducting channels and metallic contacts
into an individual nanoribbon, hence realizing self-contained carbon-based
electronic devices
One-Dimensional Moir\'e Physics and Chemistry in Heterostrained Bilayer Graphene
Twisted bilayer graphene (tBLG) has emerged as a promising platform to
explore exotic electronic phases. However, the formation of moir\'e patterns in
tBLG has thus far been confined to the introduction of twist angles between the
layers. Here, we propose heterostrained bilayer graphene (hBLG), as an
alternative avenue to access twist-angle-free moir\'e physics via lattice
mismatch. Using atomistic and first-principles calculations, we demonstrate
that uniaxial heterostrain can promote isolated flat electronic bands around
the Fermi level. Furthermore, the heterostrain-induced out-of-plane lattice
relaxation may lead to a spatially modulated reactivity of the surface layer,
paving the way for the moir\'e-driven chemistry and magnetism. We anticipate
that our findings can be readily generalized to other layered materials
Unveiling and Manipulating Hidden Symmetries in Graphene Nanoribbons
Armchair graphene nanoribbons are a highly promising class of semiconductors
for all-carbon nanocircuitry. Here, we present a new perspective on their
electronic structure from simple model Hamiltonians and
calculations. We focus on a specific set of nanoribbons of width ,
where is the number of carbon atoms across the nanoribbon axis and is a
positive integer. We demonstrate that the energy-gap opening in these
nanoribbons originates from the breaking of a previously unidentified hidden
symmetry by long-ranged hopping of -electrons and structural distortions
occurring at the edges. This hidden symmetry can be restored or manipulated
through the application of in-plane lattice strain, which enables continuous
energy-gap tuning, the emergence of Dirac points at the Fermi level, and
topological quantum phase transitions. Our work establishes an original
interpretation of the semiconducting character of armchair graphene nanoribbons
and offers guidelines for rationally designing their electronic structure
Dirac half-semimetallicity and antiferromagnetism in graphene nanoribbon/hexagonal boron nitride heterojunctions
Half-metals have been envisioned as active components in spintronic devices
by virtue of their completely spin-polarized electrical currents. Actual
materials hosting half-metallic phases, however, remain scarce. Here, we
predict that recently fabricated heterojunctions of zigzag nanoribbons embedded
in two-dimensional hexagonal boron nitride are half-semimetallic, featuring
fully spin-polarized Dirac points at the Fermi level. The half-semimetallicity
originates from the transfer of charges from hexagonal boron nitride to the
embedded graphene nanoribbon. These charges give rise to opposite energy shifts
of the states residing at the two edges while preserving their intrinsic
antiferromagnetic exchange coupling. Upon doping, an
antiferromagnetic-to-ferrimagnetic phase transition occurs in these
heterojunctions, with the sign of the excess charge controlling the spatial
localization of the net magnetic moments. Our findings demonstrate that such
heterojunctions realize tunable one-dimensional conducting channels of
spin-polarized Dirac fermions that are seamlessly integrated into a
two-dimensional insulator, thus holding promise for the development of
carbon-based spintronics
Edge Disorder in Bottom-Up Zigzag Graphene Nanoribbons: Implications for Magnetism and Quantum Electronic Transport
We unveil the nature of the structural disorder in bottom-up zigzag graphene
nanoribbons along with its effect on the magnetism and electronic transport on
the basis of scanning probe microscopies and first-principles calculations. We
find that edge-missing m-xylene units emerging during the cyclodehydrogenation
step of the on-surface synthesis are the most common point defects. These
"bite'' defects act as spin-1 paramagnetic centers, severely disrupt the
conductance spectrum around the band extrema, and give rise to spin-polarized
charge transport. We further show that the electronic conductance across
graphene nanoribbons is more sensitive to "bite" defects forming at the zigzag
edges than at the armchair ones. Our work establishes a comprehensive
understanding of the low-energy electronic properties of disordered bottom-up
graphene nanoribbons
Electronic excitations and spin interactions in chromium trihalides from embedded many-body wavefunctions
Although chromium trihalides are widely regarded as a promising class of
two-dimensional magnets for next-generation devices, an accurate description of
their electronic structure and magnetic interactions has proven challenging to
achieve. Here, we quantify electronic excitations and spin interactions in
Cr (~Cl, Br, I) using embedded many-body wavefunction calculations and
fully generalized spin Hamiltonians. We find that the three trihalides feature
comparable -shell excitations, consisting of a high-spin
ground state lying 1.51.7 eV below the first excited
state (). CrCl exhibits a single-ion anisotropy
meV, while the Cr spin-3/2 moments are ferromagnetically
coupled through bilinear and biquadratic exchange interactions of
meV and meV, respectively. The corresponding values for CrBr
and CrI increase to meV and meV
for the single-ion anisotropy, meV, meV and meV, meV for the exchange couplings, respectively. We find
that the overall magnetic anisotropy is defined by the interplay between
and due to magnetic dipole-dipole interaction that
favors in-plane orientation of magnetic moments in ferromagnetic monolayers and
bulk layered magnets. The competition between the two contributions sets
CrCl and CrI as the easy-plane () and
easy-axis () ferromagnets, respectively. The
differences between the magnets trace back to the atomic radii of the halogen
ligands and the magnitude of spin-orbit coupling. Our findings are in excellent
agreement with recent experiments, thus providing reference values for the
fundamental interactions in chromium trihalides.Comment: 9 pages, 2 figures, 5 tables, Supporting Information included as
ancillary fil
Quantum Electronic Transport Across "Bite" Defects in Graphene Nanoribbons
On-surface synthesis has recently emerged as an effective route towards the
atomically precise fabrication of graphene nanoribbons of controlled topologies
and widths. However, whether and to which degree structural disorder occurs in
the resulting samples is a crucial issue for prospective applications that
remains to be explored. Here, we experimentally identify missing benzene rings
at the edges, which we name "bite" defects, as the most abundant type of
disorder in armchair nanoribbons synthesized by the bottom-up approach. First,
we address their density and spatial distribution on the basis of scanning
tunnelling microscopy and find that they exhibit a strong tendency to
aggregate. Next, we explore their effect on the quantum charge transport from
first-principles calculations, revealing that such imperfections substantially
disrupt the conduction properties at the band edges. Finally, we generalize our
theoretical findings to wider nanoribbons in a systematic manner, hence
establishing practical guidelines to minimize the detrimental role of such
defects on the charge transport. Overall, our work portrays a detailed picture
of "bite" defects in bottom-up armchair graphene nanoribbons and assesses their
effect on the performance of carbon-based nanoelectronic devices