44 research outputs found
Tunable bandgaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons
Doping of semiconductors is essential in modern electronic and photonic
devices. While doping is well understood in bulk semiconductors, the advent of
carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications
raises some key questions about the role and impact of doping at low
dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps
and exciton binding energies can be dramatically reduced upon experimentally
relevant doping, and can be tuned gradually over a broad range of energies in
contrast to higher dimensional systems. The later feature is made possible by a
novel mechanism involving strong dynamical screening effects mediated by
acoustic plasmons.Comment: 5 pages, 4 figures, published in Phys. Rev. Lett
Atomistic study of an ideal metal/thermoelectric contact: the full-Heusler/half-Heusler interface
Half-Heusler alloys such as the (Zr,Hf)NiSn intermetallic compounds are
important thermoelectric materials for converting waste heat into electricity.
Reduced electrical resistivity at the hot interface between the half-Heusler
material and a metal contact is critical for device performance, however this
has yet to be achieved in practice. Recent experimental work suggests that a
coherent interface between half-Heusler and full-Heusler compounds can form due
to diffusion of transition metal atoms into the vacant sublattice of the
half-Heusler lattice. We study theoretically the structural and electronic
properties of such an interface using a first-principles based approach that
combines {\it ab initio} calculations with macroscopic modeling. We find that
the prototypical interface HfNiSn/HfNiSn provides very low contact
resistivity and almost ohmic behavior over a wide range of temperatures and
doping levels. Given the potential of these interfaces to remain stable over a
wide range of temperatures, our study suggests that full-Heuslers might provide
nearly ideal electrical contacts to half-Heuslers that can be harnessed for
efficient thermoelectric generator devices.Comment: 8 pages, 8 figure
Ab initio calculations of low-energy quasiparticle lifetimes in bilayer graphene
Motivated by recent experimental results we calculate from first-principles
the lifetime of low-energy quasiparticles in bilayer graphene (BLG). We take
into account the scattering rate arising from electron-electron interactions
within the approximation for the electron self-energy and consider several
p-type doping levels ranging from to
holes/cm. In the undoped case we find that the average inverse lifetime
scales linearly with energy away from the charge neutrality point, with values
in good agreement with experiments. The decay rate is approximately three times
larger than in monolayer graphene, a consequence of the enhanced screening in
BLG. In the doped case, the dependence of the inverse lifetime on quasiparticle
energy acquires a non-linear component due to the opening of an additional
decay channel mediated by acoustic plasmons.Comment: 15 pages, 3 figures, accepeted for publication in Applied Physics
Letter
Diameter and Chirality Dependence of Exciton Properties in Carbon Nanotubes
We calculate the diameter and chirality dependences of the binding energies,
sizes, and bright-dark splittings of excitons in semiconducting single-wall
carbon nanotubes (SWNTs). Using results and insights from {\it ab initio}
calculations, we employ a symmetry-based, variational method based on the
effective-mass and envelope-function approximations using tight-binding
wavefunctions. Binding energies and spatial extents show a leading dependence
with diameter as and , respectively, with chirality corrections
providing a spread of roughly 20% with a strong family behavior. Bright-dark
exciton splittings show a leading dependence. We provide analytical
expressions for the binding energies, sizes, and splittings that should be
useful to guide future experiments