21 research outputs found
Tailoring the atomic structure of graphene nanoribbons by STM lithography
The practical realization of nano-scale electronics faces two major
challenges: the precise engineering of the building blocks and their assembly
into functional circuits. In spite of the exceptional electronic properties of
carbon nanotubes only basic demonstration-devices have been realized by
time-consuming processes. This is mainly due to the lack of selective growth
and reliable assembly processes for nanotubes. However, graphene offers an
attractive alternative. Here we report the patterning of graphene nanoribbons
(GNRs) and bent junctions with nanometer precision, well-defined widths and
predetermined crystallographic orientations allowing us to fully engineer their
electronic structure using scanning tunneling microscope (STM) lithography. The
atomic structure and electronic properties of the ribbons have been
investigated by STM and tunneling spectroscopy measurements. Opening of
confinement gaps up to 0.5 eV, allowing room temperature operation of GNR-based
devices, is reported. This method avoids the difficulties of assembling
nano-scale components and allows the realization of complete integrated
circuits, operating as room temperature ballistic electronic devices.Comment: 8 pages text, 5 figures, Nature Nanotechnology, in pres
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
Defect symmetry influence on electronic transport of zigzag nanoribbons
The electronic transport of zigzag-edged graphene nanoribbon (ZGNR) with local Stone-Wales (SW) defects is systematically investigated by first principles calculations. While both symmetric and asymmetric SW defects give rise to complete electron backscattering region, the well-defined parity of the wave functions in symmetric SW defects configuration is preserved. Its signs are changed for the highest-occupied electronic states, leading to the absence of the first conducting plateau. The wave function of asymmetric SW configuration is very similar to that of the pristine GNR, except for the defective regions. Unexpectedly, calculations predict that the asymmetric SW defects are more favorable to electronic transport than the symmetric defects configuration. These distinct transport behaviors are caused by the different couplings between the conducting subbands influenced by wave function alterations around the charge neutrality point
First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers
We review a unified approach for computing: (i) spin-transfer torque in
magnetic trilayers like spin-valves and magnetic tunnel junction, where
injected charge current flows perpendicularly to interfaces; and (ii)
spin-orbit torque in magnetic bilayers of the type
ferromagnet/spin-orbit-coupled-material, where injected charge current flows
parallel to the interface. Our approach requires to construct the torque
operator for a given Hamiltonian of the device and the steady-state
nonequilibrium density matrix, where the latter is expressed in terms of the
nonequilibrium Green's functions and split into three contributions. Tracing
these contributions with the torque operator automatically yields field-like
and damping-like components of spin-transfer torque or spin-orbit torque
vector, which is particularly advantageous for spin-orbit torque where the
direction of these components depends on the unknown-in-advance orientation of
the current-driven nonequilibrium spin density in the presence of spin-orbit
coupling. We provide illustrative examples by computing spin-transfer torque in
a one-dimensional toy model of a magnetic tunnel junction and realistic
Co/Cu/Co spin-valve, both of which are described by first-principles
Hamiltonians obtained from noncollinear density functional theory calculations;
as well as spin-orbit torque in a ferromagnetic layer described by a
tight-binding Hamiltonian which includes spin-orbit proximity effect within
ferromagnetic monolayers assumed to be generated by the adjacent monolayer
transition metal dichalcogenide.Comment: 22 pages, 9 figures, PDFLaTeX; prepared for Springer Handbook of
Materials Modeling, Volume 2 Applications: Current and Emerging Material
The Edge Stresses and Phase Transitions for Magnetic BN Zigzag Nanoribbons
The edge states are of particular importance to understand fundamental properties of finite two-dimensional (2D) crystals. Based on first-principles calculations, we investigated on the bare zigzag boron nitride nanoribbons (zzBNNRs) with different spin-polarized states well localized at and extended along their edges. Our calculations examined the edge stress, which is sensitively dependent on the magnetic edge states, for either B-terminated edge or N-terminated edge. Moreover, we revealed that different magnetic configurations lead to a rich spectrum of electronic behaviors at edges. Using an uniaxial tensile strain, we proposed the magnetic phase transitions and thereby obtained the metallic to half-metallic (or reverse) phase transitions at edges. It suggests zzBNNR as a promising candidate for potential applications of non-metal spintronic devices.</p