982 research outputs found
Observation of forbidden phonons and dark excitons by resonance Raman scattering in few-layer WS
The optical properties of the two-dimensional (2D) crystals are dominated by
tightly bound electron-hole pairs (excitons) and lattice vibration modes
(phonons). The exciton-phonon interaction is fundamentally important to
understand the optical properties of 2D materials and thus help develop
emerging 2D crystal based optoelectronic devices. Here, we presented the
excitonic resonant Raman scattering (RRS) spectra of few-layer WS excited
by 11 lasers lines covered all of A, B and C exciton transition energies at
different sample temperatures from 4 to 300 K. As a result, we are not only
able to probe the forbidden phonon modes unobserved in ordinary Raman
scattering, but also can determine the bright and dark state fine structures of
1s A exciton. In particular, we also observed the quantum interference between
low-energy discrete phonon and exciton continuum under resonant excitation. Our
works pave a way to understand the exciton-phonon coupling and many-body
effects in 2D materials.Comment: 14 pages, 11 figure
Determining layer number of two dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrate
Transition-metal dichalcogenide (TMD) semiconductors have been widely studied
due to their distinctive electronic and optical properties. The property of TMD
flakes is a function of its thickness, or layer number (N). How to determine N
of ultrathin TMDs materials is of primary importance for fundamental study and
practical applications. Raman mode intensity from substrates has been used to
identify N of intrinsic and defective multilayer graphenes up to N=100.
However, such analysis is not applicable for ultrathin TMD flakes due to the
lack of a unified complex refractive index () from monolayer to bulk
TMDs. Here, we discuss the N identification of TMD flakes on the SiO/Si
substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm)
SiO/Si substrates underneath TMD flakes and that from bare SiO/Si
substrates. We assume the real part of of TMD flakes as that of
monolayer TMD and treat the imaginary part of as a fitting
parameter to fit the experimental intensity ratio. An empirical ,
namely, , of ultrathin MoS, WS and WSe
flakes from monolayer to multilayer is obtained for typical laser excitations
(2.54 eV, 2.34 eV, or 2.09 eV). The fitted of MoS has
been used to identify N of MoS flakes deposited on 302-nm SiO/Si
substrate, which agrees well with that determined from their shear and
layer-breathing modes. This technique by measuring Raman intensity from the
substrate can be extended to identify N of ultrathin 2D flakes with N-dependent
. For the application purpose, the intensity ratio excited by
specific laser excitations has been provided for MoS, WS and
WSe flakes and multilayer graphene flakes deposited on Si substrates
covered by 80-110 nm or 280-310 nm SiO layer.Comment: 10 pages, 4 figures. Accepted by Nanotechnolog
Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material
Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit
remarkable electronic and optical properties. The 2D features, sizable
bandgaps, and recent advances in the synthesis, characterization, and device
fabrication of the representative MoS, WS, WSe, and MoSe TMDs
make TMDs very attractive in nanoelectronics and optoelectronics. Similar to
graphite and graphene, the atoms within each layer in 2D TMDs are joined
together by covalent bonds, while van der Waals interactions keep the layers
together. This makes the physical and chemical properties of 2D TMDs layer
dependent. In this review, we discuss the basic lattice vibrations of
monolayer, multilayer, and bulk TMDs, including high-frequency optical phonons,
interlayer shear and layer breathing phonons, the Raman selection rule,
layer-number evolution of phonons, multiple phonon replica, and phonons at the
edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in
investigating the properties of TMDs are discussed, such as interlayer
coupling, spin--orbit splitting, and external perturbations. The interlayer
vibrational modes are used in rapid and substrate-free characterization of the
layer number of multilayer TMDs and in probing interface coupling in TMD
heterostructures. The success of Raman spectroscopy in investigating TMD
nanosheets paves the way for experiments on other 2D crystals and related van
der Waals heterostructures.Comment: 30 pages, 23 figure
Confined Acoustic Phonons in Colloidal Nanorod Heterostructures Investigated by Nonresonant Raman Spectroscopy and Finite Elements Simulations
Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core–shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by nonresonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5–50 cm–1, we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architectu..
Monolayer Molybdenum Disulfide Nanoribbons with High Optical Anisotropy
Two-dimensional Molybdenum Disulfide (MoS2) has shown promising prospects for
the next generation electronics and optoelectronics devices. The monolayer MoS2
can be patterned into quasi-one-dimensional anisotropic MoS2 nanoribbons
(MNRs), in which theoretical calculations have predicted novel properties.
However, little work has been carried out in the experimental exploration of
MNRs with a width of less than 20 nm where the geometrical confinement can lead
to interesting phenomenon. Here, we prepared MNRs with width between 5 nm to 15
nm by direct helium ion beam milling. High optical anisotropy of these MNRs is
revealed by the systematic study of optical contrast and Raman spectroscopy.
The Raman modes in MNRs show strong polarization dependence. Besides that the
E' and A'1 peaks are broadened by the phonon-confinement effect, the modes
corresponding to singularities of vibrational density of states are activated
by edges. The peculiar polarization behavior of Raman modes can be explained by
the anisotropy of light absorption in MNRs, which is evidenced by the polarized
optical contrast. The study opens the possibility to explore
quasione-dimensional materials with high optical anisotropy from isotropic 2D
family of transition metal dichalcogenides
Interlayer Interactions in Anisotropic Atomically-thin Rhenium Diselenide
Recently, two-dimensional (2D) materials with strong in-plane anisotropic
properties such as black phosphorus have demonstrated great potential for
developing new devices that can take advantage of its reduced lattice symmetry
with potential applications in electronics, optoelectronics and
thermoelectrics. However, the selection of 2D material with strong in-plane
anisotropy has so far been very limited and only sporadic studies have been
devoted to transition metal dichalcogenides (TMDC) materials with reduced
lattice symmetry, which is yet to convey the full picture of their optical and
phonon properties, and the anisotropy in their interlayer interactions. Here,
we study the anisotropic interlayer interactions in an important TMDC 2D
material with reduced in-plane symmetry - atomically thin rhenium diselenide
(ReSe2) - by investigating its ultralow frequency interlayer phonon vibration
modes, the layer dependent optical bandgap, and the anisotropic
photoluminescence (PL) spectra for the first time. The ultralow frequency
interlayer Raman spectra combined with the first study of polarization-resolved
high frequency Raman spectra in mono- and bi-layer ReSe2 allows deterministic
identification of its layer number and crystal orientation. PL measurements
show anisotropic optical emission intensity with bandgap increasing from 1.26
eV in the bulk to 1.32 eV in monolayer, consistent with the theoretical results
based on first-principle calculations. The study of the layer-number dependence
of the Raman modes and the PL spectra reveals the relatively weak van der Waals
interaction and 2D quantum confinement in atomically-thin ReSe2.Comment: 17 pages, 5 figures, supplementary informatio
The ultra-low-frequency shear modes of 2-4 layer graphenes observed in their scroll structures at edges
The in-plane shear modes between neighbor-layers of 2-4 layer graphenes (LGs)
and the corresponding graphene scrolls rolled up by 2-4LGs were investigated by
Raman scattering. In contrast to that just one shear mode was observed in
3-4LGs, all the shear modes of 3-4LGs were observed in 3-4 layer scrolls (LSs),
whose frequencies agree well with the theoretical predication by both a
force-constant model and a linear chain model. In comparison to the broad width
(about 12cm) for the G band in graphite, all the shear modes exhibit an
intrinsic line width of about 1.0 cm. The local electronic structures
dependent on the local staking configurations enhance the intensity of the
shear modes in corresponding 2-4LSs zones, which makes it possible to observe
all the shear modes. It provides a direct evidence that how the band structures
of FLGs can be sensitive to local staking configurations. This result can be
extended to n layer graphene (n > 4) for the understanding of the basic phonon
properties of multi-layer graphenes. This observation of all-scale shear modes
can be foreseen in other 2D materials with similar scroll structures.Comment: 14 pages, 5 figure
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