41 research outputs found
Cumulene Molecular Wire Conductance from First Principles
We present first principles calculations of current-voltage characteristics
(IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire
junctions with realistic contacts. The transport properties are calculated
using full self-consistent ab initio NEGF-DFT methods under external bias. The
conductance of the cumulene wires shows oscillatory behavior depending on the
number of carbon atoms (double bonds). Among all conjugated oligomers, we find
that cumulene wires with odd number of carbon atoms yield the highest
conductance with metallic-like ballistic transport behavior. The reason is the
high density of states in broad LUMO levels spanning the Fermi level of the
electrodes. The transmission spectrum and the conductance depend only weakly on
applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V.
Cumulene wires are therefore potential candidates for metallic connections in
nanoelectronic applications.Comment: Accepted in Phys. Rev. B; 5 pages and 6 figure
Mechano-switching devices from carbon wire-carbon nanotube junctions
Well-known conductive molecular wires, like cumulene or polyyne, provide a
model for interconnecting molecular electronics circuit. In the recent
experiment, the appearance of carbon wire bridging two-dimensional electrodes -
graphene sheets - was observed [PRL 102, 205501 (2009)], thus demonstrating a
mechanical way of producing the cumulene. In this work, we study the structure
and conductance properties of the carbon wire suspended between carbon
nanotubes (CNTs) of different chiralities (zigzag and armchair), and
corresponding conductance variation upon stretching. We find the geometrical
structure of the carbon wire bridging CNTs similar to the experimentally
observed structures in the carbon wire obtained between graphene electrodes. We
show a capability to modulate the conductance by changing bridging sites
between the carbon wire and CNTs without breaking the wire. Observed current
modulation via cumulene wire stretching/elongation together with CNT junction
stability makes it a promising candidate for mechano-switching device for
molecular nanoelectronics.Comment: 8 pages, 8 figure
Transverse Electronic Transport through DNA Nucleotides with Functionalized Graphene Electrodes
Graphene nanogaps and nanopores show potential for the purpose of electrical
DNA sequencing, in particular because single-base resolution appears to be
readily achievable. Here, we evaluated from first principles the advantages of
a nanogap setup with functionalized graphene edges. To this end, we employed
density functional theory and the non-equilibrium Green's function method to
investigate the transverse conductance properties of the four nucleotides
occurring in DNA when located between the opposing functionalized graphene
electrodes. In particular, we determined the electrical tunneling current
variation as a function of the applied bias and the associated differential
conductance at a voltage which appears suitable to distinguish between the four
nucleotides. Intriguingly, we observe for one of the nucleotides a negative
differential resistance effect.Comment: 19 pages, 7 figure
DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore
We propose two-terminal devices for DNA sequencing which consist of a
metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its
interior through which the DNA molecule is translocated. Using the
nonequilibrium Green functions combined with density functional theory, we
demonstrate that each of the four DNA nucleotides inserted into the nanopore,
whose edge carbon atoms are passivated by either hydrogen or nitrogen, will
lead to a unique change in the device conductance. Unlike other recent
biosensors based on transverse electronic transport through DNA nucleotides,
which utilize small (of the order of pA) tunneling current across a nanogap or
a nanopore yielding a poor signal-to-noise ratio, our device concept relies on
the fact that in ZGNRs local current density is peaked around the edges so that
drilling a nanopore away from the edges will not diminish the conductance.
Inserting a DNA nucleotide into the nanopore affects the charge density in the
surrounding area, thereby modulating edge conduction currents whose magnitude
is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not
limited to ZGNRs and it could be realized with other nanowires supporting
transverse edge currents, such as chiral GNRs or wires made of two-dimensional
topological insulators.Comment: 6 pages, 6 figures, PDFLaTe
Crystal structure of the pressure-induced metallic phase of SiH4 from ab initio theory
Metallization of pure solid hydrogen is of great interest, not least because it could lead to high-temperature superconductivity, but it continues to be an elusive goal because of great experimental challenges. Hydrogen-rich materials, in particular, CH4, SiH4, and GeH4, provide an opportunity to study related phenomena at experimentally achievable pressures, and they too are expected to be high-temperature superconductors. Recently, the emergence of a metallic phase has been observed in silane for pressures just above 60 GPa. However, some uncertainty exists about the crystal structure of the discovered metallic phase. Here, we show by way of elimination, that a single structure that possesses all of the required characteristics of the experimentally observed metallic phase of silane from a pool of plausible candidates can be identified. Our density functional theory and GW calculations show that a structure with space group P4/nbm is metallic at pressures >60 GPa. Based on phonon calculations, we furthermore demonstrate that the P4/nbm structure is dynamically stable at >43 GPa and becomes the ground state at 97 GPa when zero-point energy contributions are considered. These findings could lead the way for further theoretical analysis of metallic phases of hydrogen-rich materials and stimulate experimental studies
In search of a two-dimensional material for DNA sequencing
We analyze the transmission of narrow semiconducting nanoribbons designed from two-dimensional (2D) layered materials such as graphene, silicene, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS2). The Fano resonance driven dips in the transmission, when nucleobases stack with graphene nanoribbon, are known to be useful for DNA sequencing. For graphene and hBN nanoribbons the transmission dips are distinct for each nucleobase, but with a larger band gap for the latter case. For silicene nanoribbon the dips due to different nucleobases are somehow less clear. The transmission of the MoS2 nanoribbon is unpromising for DNA sequencing as the dip in the transmission is not useful to identify any of the nucleobase. The dip positions in the transmission shift linearly with bias voltage. This shift depends on the nanoribbon used and the orientation of the DNA base. Hence, edge-modified hBN nanoribbons with a reduced band gap could be an alternative to graphene nanoribbon (GNR) for DNA sequencing and recognition of other adsorbents.close1