144 research outputs found
Exciton interference in hexagonal boron nitride
In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite , and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call and
from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system
Characterization methods dedicated to nanometer-thick hBN layers
Hexagonal boron nitride (hBN) regains interest as a strategic component in
graphene engineering and in van der Waals heterostructures built with two
dimensional materials. It is crucial then, to handle reliable characterization
techniques capable to assess the quality of structural and electronic
properties of the hBN material used. We present here characterization
procedures based on optical spectroscopies, namely cathodoluminescence and
Raman, with the additional support of structural analysis conducted by
transmission electron microscopy. We show the capability of optical
spectroscopies to investigate and benchmark the optical and structural
properties of various hBN thin layers sources
Excitonic recombinations in hBN: from bulk to exfoliated layers
Hexagonal boron nitride (h-BN) and graphite are structurally similar but with
very different properties. Their combination in graphene-based devices meets
now a huge research focus, and it becomes particularly important to evaluate
the role played by crystalline defects in them. In this work, the
cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are
reported and compared to those of nanosheets mechanically exfoliated from them.
First the link between the presence of structural defects and the recombination
intensity of bound-excitons, the so-called D series, is confirmed. Low
defective h-BN regions are further evidenced by CL spectral mapping
(hyperspectral imaging), allowing us to observe new features in the
near-band-edge region, tentatively attributed to phonon replica of exciton
recombinations. Second the h-BN thickness was reduced down to six atomic
layers, using mechanical exfoliation, as evidenced by atomic force microscopy.
Even at these low thicknesses, the luminescence remains intense and exciton
recombination energies are not strongly modified with respect to the bulk, as
expected from theoretical calculations indicating extremely compact excitons in
h-BN
Entropy driven stability of chiral single-walled carbon nanotubes
Single-walled carbon nanotubes are hollow cylinders, that can grow
centimeters long by carbon incorporation at the interface with a catalyst. They
display semi-conducting or metallic characteristics, depending on their
helicity, that is determined during their growth. To support the quest for a
selective synthesis, we develop a thermodynamic model, that relates the
tube-catalyst interfacial energies, temperature, and the resulting tube
chirality. We show that nanotubes can grow chiral because of the
configurational entropy of their nanometer-sized edge, thus explaining
experimentally observed temperature evolutions of chiral distributions. Taking
the chemical nature of the catalyst into account through interfacial energies,
structural maps and phase diagrams are derived, that will guide a rational
choice of a catalyst and growth parameters towards a better selectivity
Measuring many-body effects in carbon nanotubes with a scanning tunneling microscope
Electron-electron interactions and excitons in carbon nanotubes are locally
measured by combining Scanning tunneling spectroscopy and optical absorption in
bundles of nanotubes. The largest gap deduced from measurements at the top of
the bundle is found to be related to the intrinsic quasi-particle gap. From the
difference with optical transitions, we deduced exciton binding energies of 0.4
eV for the gap and 0.7 eV for the second Van Hove singularity. This provides
the first experimental evidence of substrate-induced gap renormalization on
SWNTs
Near Band Edge excitation in 2D materials by Transmission Electron Microscopy
International audienc
Exciton and interband optical transitions in hBN single crystal
Near band gap photoluminescence (PL) of hBN single crystal has been studied
at cryogenic temperatures with synchrotron radiation excitation. The PL signal
is dominated by the D-series previously assigned to excitons trapped on
structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV
and 5.804 eV has been observed using time-window PL technique. The S-series
excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton
absorption. Complementary photoconductivity and PL measurements set the band
gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger
than 380 meV
Spectroscopy on Black Phosphorus exfoliated down to the monolayer
International audienc
- …