23 research outputs found
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
The effect of temperature and excitation energy on Raman scattering in bulk HfS
Raman scattering (RS) in bulk hafnium disulfide (HfS) is investigated as
a function of temperature (5 K 350 K) with polarization resolution and
excitation of several laser energies. An unexpected temperature dependence of
the energies of the main Raman-active (A and E)
modes with the temperature-induced blueshift in the low-temperature limit is
observed. The low-temperature quenching of a mode (134 cm)
and the emergence of a new mode at approx. 184 cm, labeled Z, is
reported. The optical anisotropy of the RS in HfS is also reported, which
is highly susceptible to the excitation energy. The apparent quenching of the
A mode at =5 K and of the E mode at =300 K
in the RS spectrum excited with 3.06~eV excitation is also observed. We discuss
the results in the context of possible resonant character of light-phonon
interactions. Analyzed is also a possible effect of the iodine molecules
intercalated in the van der Waals gaps between neighboring HfS layers,
which inevitably result from the growth procedure.Comment: 9 pages, 7 figures +S
Temperature induced modulation of resonant Raman scattering in bilayer 2H-MoS
The temperature evolution of the resonant Raman scattering from high-quality
bilayer 2H-MoS encapsulated in hexagonal BN flakes is presented. The
observed resonant Raman scattering spectrum as initiated by the laser energy of
1.96 eV, close to the A excitonic resonance, shows rich and distinct
vibrational features that are otherwise not observed in non-resonant
scattering. The appearance of 1 and 2 order phonon modes is
unambiguously observed in a broad range of temperatures from 5 K to 320 K. The
spectrum includes the Raman-active modes, E()
and A() along with their Davydov-split counterparts,
E() and B(). The temperature
evolution of the Raman scattering spectrum brings forward key observations, as
the integrated intensity profiles of different phonon modes show diverse
trends. The Raman-active A() mode, which dominates the Raman
scattering spectrum at =5~K quenches with increasing temperature.
Surprisingly, at room temperature the B() mode, which is
infrared-active in the bilayer, is substantially stronger than its nominally
Raman-active A() counterpart.Comment: 7 pages, 3 figure
Excitation-Dependent High-Lying Excitonic Exchange via Interlayer Energy Transfer from Lower-to-Higher Bandgap 2D Material
High light absorption (~15%) and strong photoluminescence (PL) emission in
monolayer (1L) transition metal dichalcogenide (TMD) make it an ideal candidate
for optoelectronic applications. Competing interlayer charge (CT) and energy
transfer (ET) processes control the photocarrier relaxation pathways in TMD
heterostructures (HSs). In TMDs, long-distance ET can survive up to several
tens of nm, unlike the CT process. Our experiment shows that an efficient ET
occurs from the 1Ls WSe2-to-MoS2 with an interlayer hBN, due to the resonant
overlapping of the high-lying excitonic states between the two TMDs, resulting
in enhanced HS MoS2 PL emission. This type of unconventional ET from the
lower-to-higher optical bandgap material is not typical in the TMD HSs. With
increasing temperature, the ET process becomes weaker due to the increased
electron-phonon scattering, destroying the enhanced MoS2 emission. Our work
provides new insight into the long-distance ET process and its effect on the
photocarrier relaxation pathways.Comment: 5 figures and SI include
Analogy and dissimilarity of excitons in monolayer and bilayer of MoSe
Excitons in thin layers of semiconducting transition metal dichalcogenides
are highly subject to the strongly modified Coulomb electron-hole interaction
in these materials. Therefore, they do not follow the model system of a
two-dimensional hydrogen atom. We investigate experimentally and theoretically
excitonic properties in both the monolayer (ML) and the bilayer (BL) of
MoSe encapsulated in hexagonal BN. The measured magnetic field evolutions
of the reflectance contrast spectra of the MoSe ML and BL allow us to
determine -factors of intralayer A and B excitons, as well as the -factor
of the interlayer exciton. We explain the dependence of -factors on the
number of layers and excitation state using first principles calculations.
Furthermore, we demonstrate that the experimentally measured ladder of
excitonic states in the ML can be reproduced using the
approach with the Rytova-Keldysh potential that describes the electron-hole
interaction. In contrast, the analogous calculation for the BL case requires
taking into account the out-of-plane dielectric response of the MoSe BL.Comment: 10 pages, 4 figures, + S
Localisation-to-delocalisation transition of moir\'{e} excitons in WSe/MoSe heterostructures
Moir\'{e} excitons (MXs) are electron-hole pairs localised by the periodic
(moir\'{e}) potential forming in two-dimensional heterostructures (HSs). MXs
can be exploited, , for creating nanoscale-ordered quantum emitters and
achieving or probing strongly correlated electronic phases at relatively high
temperatures. Here, we studied the exciton properties of a WSe/MoSe HS
from =6 K to room temperature using time-resolved and continuous-wave
micro-photoluminescence, also under magnetic field. The exciton dynamics and
emission lineshape evolution with temperature show clear signatures that MXs
de-trap from the moir\'{e} potential and turn into free interlayer excitons
(IXs) at 120 K. The MX-to-IX transition is also apparent from the
exciton magnetic moment reversing its sign when the moir\'{e} potential is not
capable to localise excitons at elevated temperatures. Concomitantly, the
exciton formation and decay times reduce drastically. Thus, our findings
establish the conditions for a truly confined nature of the exciton states in a
moir\'{e} superlattice with increasing temperature
Excitons and trions in WSSe monolayers
The possibility of almost linear tuning of the band gap and of the electrical
and optical properties in monolayers (MLs) of semiconducting transition metal
dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with
on-demand characteristics. By making use of photoluminescence spectroscopy, we
investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited
on SiO/Si substrate and encapsulated in hexagonal BN flakes. Similarly to
the WS and WSe MLs, we assign the WSSe MLs to the ML family
with the dark ground exciton state. We find that, in addition to the neutral
bright A exciton line, three observed emission lines are associated with
negatively charged excitons. The application of in-plane and out-of-plane
magnetic fields allows us to assign undeniably the bright and dark (spin- and
momentum-forbidden) negative trions as well as the phonon replica of the dark
spin-forbidden complex. Furthermore, the existence of the single photon
emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to
enlarge the wavelength range for potential future quantum applications of
S-TMDs.Comment: 6 pages, 5 figures, +ES
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Księga poświęcona Profesorowi Hieronimowi Kubiakowi w 75. rocznicę urodzin.
Tom ofiarowany przez przyjaciół i uczniów