65 research outputs found
ZnPSe as ultrabright indirect bandgap system with microsecond excitonic lifetimes
We report an optical characterization of ZnPSe crystals that demonstrates
indirect band gap characteristics in combination with unusually strong
photoluminescence. We found evidence of interband recombination from excitonic
states with microsecond lifetimes. Through optical characterization, we
reconstructed the electronic band scheme relevant for fundamental processes of
light absorption, carrier relaxation and radiative recombination. The
investigation of the radiative processes in the presence of magnetic field
revealed spin polarization of fundamental electronic states. This observation
opens a pathway towards controlling the spin of excitonic states in
technologically relevant microsecond timescales
Breathing modes in few-layer MoTe activated by h-BN encapsulation
The encapsulation of few-layer transition metal dichalcogenides (TMDs) in
hexagonal boron nitride (h-BN) is known to improve significantly their optical
and electronic properties. However, it may be expected that the h-BN
encapsulation may affect also vibration properties of TMDs due to an atomically
flat surface of h-BN layers. In order to study its effect on interlayer
interactions in few-layer TMDs, we investigate low-energy Raman scattering
spectra of bi- and trilayer MoTe. Surprisingly, three breathing modes are
observed in the Raman spectra of the structures deposited on or encapsulated in
h-BN as compared to a single breathing mode for the flakes deposited on a
SiO/Si substrate. The shear mode is not affected by changing the MoTe
environment. The emerged structure of breathing modes is ascribed to the
apparent interaction between the MoTe layer and the bottom h-BN flake. The
structure becomes visible due to a high-quality surface of the former flake.
Consequently, the observed triple structure of breathing modes originates from
the combination modes due to interlayer and layer-substrate interactions. Our
results confirm that the h-BN encapsulation affects substantially vibration
properties of layered materials.Comment: 10 pages, 9 figure
Neutral and charged dark excitons in monolayer WS
Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS encapsulated in hexagonal BN (hBN). We find
that this system is an -type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be -8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia
Raman scattering from the bulk inactive out-of-plane B mode in few-layer MoTe
Raman scattering from the out-of-plane vibrational modes
(A/A'), which originate from the bulk-inactive out-of-plane
B mode, are studied in few-layer MoTe.
Temperature-dependent measurements reveal a doublet structure of the
corresponding peaks in the Raman scattering spectra of tetralayer and
pentalayer samples. A strong enhancement of their lower energy components is
recorded at low temperature for 1.91 eV and 1.96 eV laser excitation. We
discuss the attribution of the peaks to the inner modes of the respective
Raman-active vibrations. The temperature evolution of their intensity strongly
suggests a resonant character of the employed excitation, which leads to the
mode enhancement at low temperature. The resonance of the laser light with the
singularity of the electronic density of states at the point of the
Brillouin zone in MoTe is proposed to be responsible for the observed
effects.Comment: 10 pages, 5 figure
Pressure-driven phase transitions in bulk HfS
The effect of hydrostatic pressure up to 27 GPa on the Raman scattering (RS)
in bulk HfS is investigated. There are two transformations of RS spectra,
which take place during compression at pressure between 5.7 GPa and 9.8 GPa as
well as between 12.8 GPa and 15.2 GPa. Seven vibrational modes can be observed
after the transformation, as compared to four modes before the transformation.
The observed change suggests structural change in the material of yet unknown
nature. The frequencies of the RS modes observed above the transformation
change linearly with pressure and corresponding pressure coefficients have been
determined. The other transition manifests itself as a change in the RS
lineshape. While a series of well-defined RS modes are observed under pressure
below the transition, broad spectral bands can be seen at higher pressure. The
overall lineshape of the spectra resembles that of disordered materials. The
lineshape does not change during decompression, which suggests permanent nature
of the high-pressure transition.Comment: 4 pages, 4 figure
Raman scattering excitation in monolayers of semiconducting transition metal dichalcogenides
Raman scattering excitation (RSE) is an experimental technique in which the
spectrum is made up by sweeping the excitation energy when the detection energy
is fixed. We study the low-temperature (=5~K) RSE spectra measured on four
high quality monolayers (ML) of semiconducting transition metal dichalcogenides
(S-TMDs), MoS, MoSe, WS, and WSe, encapsulated in
hexagonal BN. The outgoing resonant conditions of Raman scattering reveal an
extraordinary intensity enhancement of the phonon modes, which results in
extremely rich RSE spectra. The obtained spectra are composed not only of
Raman-active peaks, in-plane E and out-of-plane A, but the
appearance of 1, 2, and higher-order phonon modes is recognised.
The intensity profiles of the A modes in the investigated MLs resemble the
emissions due to neutral excitons measured in the corresponding PL spectra for
the outgoing type of resonant Raman scattering conditions. Furthermore, for the
WSe ML, the A mode was observed when the incoming light was in
resonance with the neutral exciton line. The strength of the exciton-phonon
coupling (EPC) in S-TMD MLs strongly depends on the type of their ground
excitonic state, bright or dark, resulting in different shapes of the
RSE spectra. Our results demonstrate that RSE spectroscopy is a powerful
technique for studying EPC in S-TMD MLs.Comment: 9 pages, 6 figures, ES
Valley polarization of singlet and triplet trions in WS monolayer in magnetic fields
The spectral signatures associated with different negatively charged exciton
complexes (trions) in a WS monolayer encapsulated in hBN, are analyzed from
low temperature and polarization resolved reflectance contrast (RC) and
photoluminescence (PL) experiments, with an applied magnetic field. Based on
results obtained from the RC experiment, we show that the valley Zeeman effect
affects the optical response of both the singlet and the triplet trion species
through the evolution of their energy and of their relative intensity, when
applying an external magnetic field. Our analysis allows us to estimate a free
electron concentration of cm. The observed
evolutions based on PL experiments on the same sample are different and can
hardly be understood within the same simple frame highlighting the complexity
of relaxation processes involved in the PL response.Comment: 7 pages, 4 figures; source file correcte
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