245 research outputs found
Lattice thermal conductivity of graphene with conventionally isotopic defects
The thermal conductivity of doped graphene flake of finite size is
investigated with emphasis on the influence of mass of substituting atoms on
this property. It is shown that the graphene doping by small concentrations of
relatively heavy atoms results in a disproportionately impressive drop of
lattice thermal conductivity.Comment: 12 pages, 3 figure
Low-temperature thermal conductivity in polycrystalline graphene
The low-temperature thermal conductivity in polycrystalline graphene is
theoretically studied. The contributions from three branches of acoustic
phonons are calculated by taking into account scattering on sample borders,
point defects and grain boundaries. Phonon scattering due to sample borders and
grain boundaries is shown to result in a -behaviour in the thermal
conductivity where varies between 1 and 2. This behaviour is found to
be more pronounced for nanosized grain boundaries.
PACS: 65.80.Ck, 81.05.ue, 73.43.C
Phonons and thermal transport in Si/SiO multishell nanotubes: Atomistic study
Thermal transport in the Si/SiO multishell nanotubes is investigated
theoretically. The phonon energy spectra are obtained using the atomistic
Lattice Dynamics approach. Thermal conductivity is calculated using the
Boltzmann transport equation within the relaxation time approximation.
Redistribution of the vibrational spectra in multishell nanotubes leads to a
decrease of the phonon group velocity and the thermal conductivity as compared
to homogeneous Si nanowires. Phonon scattering on the Si/SiO interfaces is
another key factor of strong reduction of the thermal conductivity in these
structures (down to 0.2 W/mK at room temperature). We demonstrate that phonon
thermal transport in Si/SiO nanotubes can be efficiently suppressed by a
proper choice of nanotube's geometrical parameters: lateral cross-section,
thickness and number of shells.Comment: 14 pages, 4 figure
Excitons in the wurtzite AlGaN/GaN quantum-well heterostructures
We have theoretically studied exciton states and photoluminescence spectra of
strained wurtzite AlGaN/GaN quantum-well heterostructures. The electron and
hole energy spectra are obtained by numerically solving the Schr\"odinger
equation, both for a single-band Hamiltonian and for a non-symmetrical 6-band
Hamiltonian. The deformation potential and spin-orbit interaction are taken
into account. For increasing built-in field, generated by the piezoelectric
polarization and by the spontaneous polarization, the energy of size
quantization rises and the number of size quantized electron and hole levels in
a quantum well decreases. The exciton energy spectrum is obtained using
electron and hole wave functions and two-dimensional Coulomb wave functions as
a basis. We have calculated the exciton oscillator strengths and identified the
exciton states active in optical absorption. For different values of the Al
content x, a quantitative interpretation, in a good agreement with experiment,
is provided for (i) the red shift of the zero-phonon photoluminescence peaks
for increasing the quantum-well width, (ii) the relative intensities of the
zero-phonon and one-phonon photoluminescence peaks, found within the
non-adiabatic approach, and (iii) the values of the photoluminescence decay
time as a function of the quantum-well width.Comment: 32 pages, 9 figure
PHONON ENGINEERING OF THERMAL PROPERTIES OF AMORPHOUS SILICON NANOWIRES
Research has been conducted to determine the quantum states of the vibrational motion of atoms in amorphous silicon nanowires. The effect of a strong drop in thermal conductivity is observed in such nanocompounds, explained by size quantization of the phonon spectrum due to a disordering of atomic bonds that form quasi-one-dimensional nanostructures. The investigated nanowires can be used as semiconductor thermoelectric cells to convert thermal energy into electrical energy.Благодарность: за финансовую поддержку проведённых исследований авторы выражают благодарность научным проектам Республики Молдова 15.817.02.29F и ASM-STCU-5937
Thermal Conductivity and Thermal Rectification in Graphene Nanoribbons: a Molecular Dynamics Study
We have used molecular dynamics to calculate the thermal conductivity of
symmetric and asymmetric graphene nanoribbons (GNRs) of several nanometers in
size (up to ~4 nm wide and ~10 nm long). For symmetric nanoribbons, the
calculated thermal conductivity (e.g. ~2000 W/m-K @400K for a 1.5 nm {\times}
5.7 nm zigzag GNR) is on the similar order of magnitude of the experimentally
measured value for graphene. We have investigated the effects of edge chirality
and found that nanoribbons with zigzag edges have appreciably larger thermal
conductivity than nanoribbons with armchair edges. For asymmetric nanoribbons,
we have found significant thermal rectification. Among various
triangularly-shaped GNRs we investigated, the GNR with armchair bottom edge and
a vertex angle of 30{\deg} gives the maximal thermal rectification. We also
studied the effect of defects and found that vacancies and edge roughness in
the nanoribbons can significantly decrease the thermal conductivity. However,
substantial thermal rectification is observed even in the presence of edge
roughness.Comment: 13 pages, 5 figures, slightly expanded from the published version on
Nano Lett. with some additional note
- …