6 research outputs found
Composition-Tunable Synthesis of Large-Scale Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> Alloys with Enhanced Photoluminescence
Alloying
two-dimensional transition metal dichalcogenides (2D TMDs)
is a promising avenue for band gap engineering. In addition, developing
a scalable synthesis process is essential for the practical application
of these alloys with tunable band gaps in optoelectronic devices.
Here, we report the synthesis of optically uniform and scalable single-layer
Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> alloys by a two-step chemical vapor deposition (CVD)
method followed by a laser thinning process. The amount of W content
(<i>x</i>) in the Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> alloy is systemically
controlled by the co-sputtering technique. The post-laser process
allows layer-by-layer thinning of the Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> alloys down to
a single-layer; such a layer exhibits tunable properties with the
optical band gap ranging from 1.871 to 1.971 eV with variation in
the W content, <i>x</i> = 0 to 1. Moreover, the predominant
exciton complexes, trions, are transitioned to neutral excitons with
increasing W concentration; this is attributed to the decrease in
excessive charge carriers with an increase in the W content of the
alloy. Photoluminescence (PL) and Raman mapping analyses suggest that
the laser-thinning of the Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> alloys is a self-limiting
process caused by heat dissipation to the substrate, resulting in
spatially uniform single-layer Mo<sub>1–<i>x</i></sub>W<sub><i>x</i></sub>S<sub>2</sub> alloy films. Our findings
present a promising path for the fabrication of large-scale single-layer
2D TMD alloys and the design of versatile optoelectronic devices
Observation of Charge Transfer in Heterostructures Composed of MoSe<sub>2</sub> Quantum Dots and a Monolayer of MoS<sub>2</sub> or WSe<sub>2</sub>
Monolayer transition metal dichalcogenides
(TMDs) are atomically
thin semiconductor films that are ideal platforms for the study and
engineering of quantum heterostructures for optoelectronic applications.
We present a simple method for the fabrication of TMD heterostructures
containing MoSe<sub>2</sub> quantum dots (QDs) and a MoS<sub>2</sub> or WSe<sub>2</sub> monolayer. The strong modification of photoluminescence
and Raman spectra that includes the quenching of MoSe<sub>2</sub> QDs
and the varied spectral weights of trions for the MoS<sub>2</sub> and
WSe<sub>2</sub> monolayers were observed, suggesting the charge transfer
occurring in these TMD heterostructures. Such optically active heterostructures,
which can be conveniently fabricated by dispersing TMD QDs onto TMD
monolayers, are likely to have various nanophotonic applications because
of their versatile and controllable properties
Impeding Exciton–Exciton Annihilation in Monolayer WS<sub>2</sub> by Laser Irradiation
Monolayer
(1L) transition metal dichalcogenides (TMDs) are two-dimensional
direct-bandgap semiconductors with promising applications of quantum
light emitters. Recent studies have shown that intrinsically low quantum
yields (QYs) of 1L-TMDs can be greatly improved by chemical treatments.
However, nonradiative exciton–exciton annihilation (EEA) appears
to significantly limit light emission of 1L-TMDs at a nominal density
of photoexcited excitons due to strong Coulomb interaction. Here we
show that the EEA rate constant (γ) can be reduced by laser
irradiation treatment in mechanically exfoliated monolayer tungsten
disulfide (1L-WS<sub>2</sub>), causing significantly improved light
emission at the saturating optical pumping level. Time-resolved photoluminescence
(PL) measurements showed that γ reduced from 0.66 ± 0.15
cm<sup>2</sup>/s to 0.20 ± 0.05 cm<sup>2</sup>/s simply using
our laser irradiation. The laser-irradiated region exhibited lower
PL response at low excitation levels, however at the high excitation
level displayed 3× higher PL intensity and QY than the region
without laser treatment. The shorter PL lifetime and lower PL response
at low excitation levels suggested that laser irradiation increased
the density of sulfur vacancies of 1L-WS<sub>2</sub>, but we attribute
these induced defects, adsorbed by oxygen in air, to the origin for
reduced EEA by hindering exciton diffusion. Our laser irradiation
was likewise effective for reducing EEA and increasing PL of chemically
treated 1L-WS<sub>2</sub> with a high QY, exhibiting the general applicability
of our method. Our results suggest that exciton–exciton interaction
in 1L-TMDs may be conveniently controlled by the laser treatment,
which may lead to unsaturated exciton emission at high excitation
levels
Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides
The photocarrier
relaxation between direct and indirect band gaps
along the high symmetry K−Γ line in the Brillion zone
reveals interesting electronic properties of the transition metal
dichalcogenides (TMDs) multilayer films. In this study, we reported
on the local strain engineering and tuning of an electronic band structure
of TMDs multilayer films along the K−Γ line by artificially
creating one-dimensional wrinkle structures. Significant photoluminescence
(PL) intensity enhancement in conjunction with continuously tuned
optical energy gaps was recorded at the high strain regions. A direct
optical band gap along K–K points and an indirect optical gap
along Γ–K points measured from the PL spectra of multilayer
samples monotonically decreased as the strain increased, while the
indirect band gap along Λ–Γ was unaffected owing
to the same level of local strain in the range of 0%–2%. The
experimental results of band gap tuning were in agreement with the
density functional theory calculation results. Local strain modified
the band structure in which K-conduction band valley (CBV) was aligned
below the Λ-CBV, and this explained the observed local PL enhancement
that made the material indirect via the K−Γ transition.
The study also reported experimental evidence for the funneling of
photogenerated excitons toward regions of a higher strain at the top
of the wrinkle geometry
Atomic Observation of Filling Vacancies in Monolayer Transition Metal Sulfides by Chemically Sourced Sulfur Atoms
Chemical
treatment using bis(trifluoromethane) sulfonimide (TFSI)
was shown to be particularly effective for increasing the photoluminescence
(PL) of monolayer (1L) MoS<sub>2</sub>, suggesting a convenient method
for overcoming the intrinsically low quantum yield of this material.
However, the underlying atomic mechanism of the PL enhancement has
remained elusive. Here, we report the microscopic origin of the defect
healing observed in TFSI-treated 1L-MoS<sub>2</sub> through a correlative
combination of optical characterization and atomic-scale scanning
transmission electron microscopy, which showed that most of the sulfur
vacancies were directly repaired by the extrinsic sulfur atoms produced
from the dissociation of TFSI, concurrently resulting in a significant
PL enhancement. Density functional theory calculations confirmed that
the reactive sulfur dioxide molecules that dissociated from TFSI can
be reduced to sulfur and oxygen gas at the vacancy site to form strongly
bound SMo. Our results reveal how defect-mediated nonradiative
recombination can be effectively eliminated by a simple chemical treatment
method, thereby advancing the practical applications of monolayer
semiconductors
Enhanced Light Emission from Monolayer Semiconductors by Forming Heterostructures with ZnO Thin Films
Monolayer
transition-metal dichalcogenides (1L-TMDs) are atomically thin direct
band gap semiconductors, from which the emission of light is determined
by optical transitions of exciton complexes such as neutral excitons
and trions. While the quantum yields of 1L-TMDs are quite low, the
ability to control the populations of exciton complexes in 1L-TMDs
through various doping processes is an interesting advantage, and
provides ample possibilities for engineering the optical properties
of these semiconductor monolayers. Here we demonstrate a simple method
of controlling the populations of excitons and trions to enhance the
light emission of 1L-TMDs by having them form heterostructures with
ZnO thin films (TFs). 1Ls of MoS<sub>2</sub> or MoSe<sub>2</sub> showed
up to 17-fold increases in photoluminescence (PL) when they were placed
on ∼50 nm thick ZnO TFs. This enhancement of the PL was due
to charge exchanges occurring through the 1L-TMD/ZnO interface. The
PL enhancements and changes in the PL spectra of the 1L-TMDs were
greater when the 1L-TMD/ZnO heterostructures were subjected to 355
nm wavelength laser excitation than when they were excited with a
514 nm wavelength laser, which we attributed to the onset of energy
transfer by photoexcited excitons and/or the additional p-doping by
photoexcited holes in ZnO. The p-doping phenomenon and the enhanced
light emission of 1L-TMD/ZnO heterostructures were unambiguously visualized
in spatially resolved PL and Raman spectral maps. Our approach using
the 1L-TMD/ZnO TF heterostructure suggests that a rich variety of
options for engineering the optical properties of 1L-TMDs may be made
available by carrying out simple and intuitive manipulations of exciton
complexes, and these endeavors may yield practical applications for
1L-TMDs in nanophotonic devices