34 research outputs found
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Mechanical and Electronic Properties of MoS Nanoribbons and Their Defects
We present our study on atomic, electronic, magnetic and phonon properties of
one dimensional honeycomb structure of molybdenum disulfide (MoS) using
first-principles plane wave method. Calculated phonon frequencies of bare
armchair nanoribbon reveal the fourth acoustic branch and indicate the
stability. Force constant and in-plane stiffness calculated in the harmonic
elastic deformation range signify that the MoS nanoribbons are stiff quasi
one dimensional structures, but not as strong as graphene and BN nanoribbons.
Bare MoS armchair nanoribbons are nonmagnetic, direct band gap
semiconductors. Bare zigzag MoS nanoribbons become half-metallic as a
result of the (2x1) reconstruction of edge atoms and are semiconductor for
minority spins, but metallic for the majority spins. Their magnetic moments and
spin-polarizations at the Fermi level are reduced as a result of the
passivation of edge atoms by hydrogen. The functionalization of MoS
nanoribbons by adatom adsorption and vacancy defect creation are also studied.
The nonmagnetic armchair nanoribbons attain net magnetic moment depending on
where the foreign atoms are adsorbed and what kind of vacancy defect is
created. The magnetization of zigzag nanoribbons due to the edge states is
suppressed in the presence of vacancy defects.Comment: 11 pages, 5 figures, first submitted at November 23th, 200
Extraordinary room-temperature photoluminescence in WS2 monolayers
Individual monolayers of metal dichalcogenides are atomically thin
two-dimensional crystals with attractive physical properties different from
their bulk layered counterpart. Here we describe the direct synthesis of WS2
monolayers with triangular morphologies and strong room-temperature
photoluminescence (PL). Bulk WS2 does not present PL due to its indirect band
gap nature. The edges of these monolayers exhibit PL signals with extraordinary
intensity, around 25 times stronger than the platelets center. The structure
and composition of the platelet edges appear to be critical for the PL
enhancement effect. Electron diffraction revealed that platelets present zigzag
edges, while first-principles calculations indicate that sulfur-rich zigzag WS2
edges possess metallic edge states, which might tailor the optical response
reported here. These novel 2D nanoscale light sources could find diverse
applications including the fabrication of flexible/transparent/low-energy
optoelectronic devices
Unconventional molecule-resolved current rectification in diamondoid-fullerene hybrids
The unimolecular rectifier is a fundamental building block of molecular electronics. Rectification in single molecules can arise from electron transfer between molecular orbitals displaying asymmetric spatial charge distributions, akin to p-n junction diodes in semiconductors. Here we report a novel all hydrocarbon molecular rectifier consisting of a diamantane-C60 conjugate. By linking both sp3 (diamondoid) and sp2 (fullerene) carbon allotropes, this bybrid molecule opposingly pairs negative and positive electron affinities. The single-molecule conductances of self-assembled domains on Au(111), probed by low-temperature scanning tunneling microscopy and spectroscopy, reveal a large rectifying response of the molecular constructs. This specific electronic behaviour is postulated to originate from the electrostatic repulsion of diamantane-C60 molecules due to positively charged terminal hydrogen atoms on the diamondoid interacting with the top electrode (scanning tip) at various bias voltages. Density functional theory computations scrutinize the electronic and vibrational spectroscopic fingerprints of this unique molecular structure and corroborate the unconventional rectification mechanism
Ultrasensitive gas detection of large-area boron-doped graphene
Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely lowconcentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG