129 research outputs found
Probing electronic excitations in mono- to pentalayer graphene by micro-magneto-Raman spectroscopy
We probe electronic excitations between Landau levels in freestanding
layer graphene over a broad energy range, with unprecedented spectral and
spatial resolution, using micro-magneto Raman scattering spectroscopy. A
characteristic evolution of electronic bands in up to five Bernal-stacked
graphene layers is evidenced and shown to remarkably follow a simple
theoretical approach, based on an effective bilayer model. -layer
graphene appear as appealing candidates in the quest for novel phenomena,
particularly in the quantum Hall effect regime. Our work paves the way towards
minimally-invasive investigations of magneto-excitons in other emerging
low-dimensional systems, with a spatial resolution down to 1m.Comment: to appear in Nano Letter
Raman scattering excitation spectroscopy in monolayer WS
Resonant Raman scattering is investigated in monolayer WS at low
temperature with the aid of an unconventional spectroscopy technique, ,
Raman scattering excitation (RSE). The RSE spectrum is made up by sweeping the
excitation energy, when the detection energy is fixed in resonance with
excitonic transitions related to neutral and/or charged excitons. We
demonstrate that the shape of the RSE spectrum strongly depends on a selected
detection energy. The out-going resonance with the neutral exciton leads to an
extremely rich RSE spectrum displaying several Raman scattering features not
reported so far, while no clear effect on the associated background
photoluminescence is observed. Instead, a strong enhancement of the emission
due to the negatively charged exciton is apparent when the out-going photons
resonate with this exciton. Presented results show that the RSE spectroscopy
can be a useful technique to study electron-phonon interactions in thin layers
of transition metal dichalcogenides.Comment: 9 pages, 5 figure
Single Photon Emitters in Boron Nitride: More Than a Supplementary Material
We present comprehensive optical studies of recently discovered single photon
sources in boron nitride, which appear in form of narrow lines emitting
centres. Here, we aim to compactly characterise their basic optical properties,
including the demonstration of several novel findings, in order to inspire
discussion about their origin and utility. Initial inspection reveals the
presence of narrow emission lines in boron nitride powder and exfoliated flakes
of hexagonal boron nitride deposited on Si/SiO2 substrates. Generally rather
stable, the boron nitride emitters constitute a good quality visible light
source. However, as briefly discussed, certain specimens reveal a peculiar type
of blinking effects, which are likely related to existence of meta-stable
electronic states. More advanced characterisation of representative stable
emitting centres uncovers a strong dependence of the emission intensity on the
energy and polarisation of excitation. On this basis, we speculate that rather
strict excitation selectivity is an important factor determining the character
of the emission spectra, which allows the observation of single and
well-isolated emitters. Finally, we investigate the properties of the emitting
centres in varying external conditions. Quite surprisingly, it is found that
the application of a magnetic field introduces no change in the emission
spectra of boron nitride emitters. Further analysis of the impact of
temperature on the emission spectra and the features seen in second-order
correlation functions is used to provide an assessment of the potential
functionality of boron nitride emitters as single photon sources capable of
room temperature operation
Excitonic resonances in thin films of WSe2: From monolayer to bulk material
We present optical spectroscopy (photoluminescence and reflectance) studies
of thin layers of the transition metal dichalcogenide WSe2, with thickness
ranging from mono- to tetra-layer and in the bulk limit. The investigated
spectra show the evolution of excitonic resonances as a function of layer
thickness, due to changes in the band structure and, importantly, due to
modifications of the strength of Coulomb interaction as well. The observed
temperature-activated energy shift and broadening of the fundamental direct
exciton are well accounted for by standard formalisms used for conventional
semiconductors. A large increase of the photoluminescence yield with
temperature is observed in WSe2 monolayer, indicating the existence of
competing radiative channels. The observation of absorption-type resonances due
to both neutral and charged excitons in WSe2 monolayer is reported and the
effect of the transfer of oscillator strength from charged to neutral exciton
upon increase of temperature is demonstrated.Comment: 12 pages, 5 figure
Magneto-optical probing of weak disorder in a two-dimensional hole gas
In two-beam magneto-photoluminescence spectra of a two-dimensional valence
hole gas we identify the three-level energy spectrum of a free positive trion
with a field-induced singlet-triplet transition. The recombination spectrum of
acceptor-bound trions is also detected, including a cyclotron replica
corresponding to the hole shake-up process. The emergence of a shake-up peak at
low temperature is shown to be a sensitive probe of the presence of a small
number of impurities inside the high-mobility quantum well, and its relative
position is directly related to the hole cyclotron mass.Comment: 4 pages, 5 figure
Tuning carrier concentration in a superacid treated MoS monolayer
The effect of bis(trifluoromethane) sulfonimide (TFSI, superacid) treatment
on the optical properties of MoS monolayers is investigated by means of
photoluminescence, reflectance contrast and Raman scattering spectroscopy
employed in a broad temperature range. It is shown that when applied multiple
times, the treatment results in progressive quenching of the trion
emission/absorption and in the redshift of the neutral exciton
emission/absorption associated with both the A and B excitonic resonances.
Based on this evolution, a trion complex related to the B exciton in monolayer
MoS is unambiguously identified. A defect-related emission observed at low
temperatures also disappears from the spectrum as a result of the treatment.
Our observations are attributed to effective passivation of defects on the
MoS monolayer surface. The passivation reduces the carrier density, which
in turn affects the out-of-plane electric field in the sample. The observed
tuning of the carrier concentration strongly influences also the Raman
scattering in the MoS monolayer. An enhancement of Raman scattering at
resonant excitation in the vicinity of the A neutral exciton is clearly seen
for both the out-of-plane A and in-plane E modes. On the
contrary, when the excitation is in resonance with a corresponding trion, the
Raman scattering features become hardly visible. These results confirm the role
of the excitonic charge state plays in the resonance effect of the excitation
energy on the Raman scattering in transition metal dichalcogenides.Comment: 8 pages, 4 figure
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