4 research outputs found
Efficient Energy Transfer (EnT) in Pyrene- and Porphyrin-Based Mixed-Ligand Metal–Organic Frameworks
Designing and synthesizing
the ordered light-harvesting systems,
possessing complementary absorption and energy-transfer process between
the chromophores, are essential steps to accomplish successful mimicking
of the natural photosynthetic systems. Metal–organic frameworks
(MOFs) can be considered as an ideal system to achieve this due to
their highly ordered structure, superior synthetic versatility, and
tailorable functionality. Herein, we have synthesized the new light-harvesting
mixed-ligand MOFs (MLMs, MLM-1–3) via solvothermal reactions
between a Zr<sub>6</sub> cluster and a mixture of appropriate ratio
of 1,3,6,8-tetrakis(<i>p</i>-benzoic acid)pyrene and [5,10,15,20-tetrakis(4-carboxy-phenyl)porphyrinato]-Zn(II)
ligands. The identical symmetry and connectivity of the two ligands
of the MLMs was the key parameter of successful synthesis as a single
MOF form, and the ample overlap between the emission spectrum of pyrene
and the absorption spectrum of porphyrin provided the ideal platform
to design an efficient-energy transfer (EnT) process within the MLMs.
We obtained the nanoscale maps of the fluorescence intensities and
lifetimes of microsize MLM grains for unambiguous visualization of
EnT phenomena occurring between two ligands in MLMs. Moreover, due
to complementary absorption and energy transfer between the two ligands
in the MLMs, our MLMs performed as superior photoinduced singlet-oxygen
generators, verifying the enhanced light-harvesting properties of
the pyrene- and porphyrin-based MLMs
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
Simultaneous Hosting of Positive and Negative Trions and the Enhanced Direct Band Emission in MoSe<sub>2</sub>/MoS<sub>2</sub> Heterostacked Multilayers
Heterostacking
of layered transition-metal dichalcogenide (LTMD)
monolayers (1Ls) offers a convenient way of designing two-dimensional
exciton systems. Here we demonstrate the simultaneous hosting of positive
trions and negative trions in heterobilayers made by vertically stacking
1L MoSe<sub>2</sub> and 1L MoS<sub>2</sub>. The charge transfer occurring
between the 1Ls of MoSe<sub>2</sub> and MoS<sub>2</sub> converted
the polarity of trions in 1L MoSe<sub>2</sub> from negative to positive,
resulting in the presence of positive trions in the 1L MoSe<sub>2</sub> and negative trions in the 1L MoS<sub>2</sub> of the same heterostacked
bilayer. Significantly enhanced MoSe<sub>2</sub> photoluminescence
(PL) in the heterostacked bilayers compared to the PL of 1L MoSe<sub>2</sub> alone suggests that, unlike other previously reported heterostacked
bilayers, direct band transition of 1L MoSe<sub>2</sub> in heterobilayer
was enhanced after the vertical heterostacking. Moreover, by inserting
hexagonal BN monolayers between 1L MoSe<sub>2</sub> and 1L MoS<sub>2</sub>, we were able to adjust the charge transfer to maximize the
MoSe<sub>2</sub> PL of the heteromultilayers and have achieved a 9-fold
increase of the PL emission. The enhanced optical properties of our
heterostacked LTMDs suggest the exciting possibility of designing
LTMD structures that exploit the superior optical properties of 1L
LTMDs
Simple Chemical Treatment to n‑Dope Transition-Metal Dichalcogenides and Enhance the Optical and Electrical Characteristics
The
optical and electrical properties of monolayer transition-metal
dichalcogenides (1L-TMDs) are critically influenced by two dimensionally
confined exciton complexes. Although extensive studies on controlling
the optical properties of 1L-TMDs through external doping or defect
engineering have been carried out, the effects of excess charges,
defects, and the populations of exciton complexes on the light emission
of 1L-TMDs are not yet fully understood. Here, we present a simple
chemical treatment method for n-dope 1L-TMDs, which also enhances
their optical and electrical properties. We show that dipping 1Ls
of MoS<sub>2</sub>, WS<sub>2</sub>, and WSe<sub>2</sub>, whether exfoliated
or grown by chemical vapor deposition, into methanol for several hours
can increase the electron density and also can reduce the defects,
resulting in the enhancement of their photoluminescence, light absorption,
and the carrier mobility. This methanol treatment was effective for
both n- and p-type 1L-TMDs, suggesting that the surface restructuring
around structural defects by methanol is responsible for the enhancement
of optical and electrical characteristics. Our results have revealed
a simple process for external doping that can enhance both the optical
and electrical properties of 1L-TMDs and help us understand how the
exciton emission in 1L-TMDs can be modulated by chemical treatments