274 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
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
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