Boosting quantum yields in two-dimensional semiconductors via proximal metal plates

Validated enhancement and temperature modulated absorbance of a WS2 monolayer based on a planar structure

Transition-metal dichalcogenides (TMDCs), as emerging optoelectronic materials, necessitate the establishment of an experimentally viable system to study their interaction with light. In this study, we propose and analyze a WS2/PMMA/Ag planar Fabry-Perot (F-P) cavity, enabling the direct experimental measurement of WS2 absorbance. By optimizing the structure, the absorbance of A exciton of WS2 up to 0.546 can be experimentally achieved, which matches well with the theoretical calculations. Through temperature and thermal expansion strain induced by temperature, the absorbance of the A exciton can be tuned in situ. Furthermore, temperature-dependent photocurrent measurements confirmed the consistent absorbance of the A exciton under varying temperatures. This WS2/PMMA/Ag planar structure provides a straightforward and practical platform for investigating light interaction in TMDCs, laying a solid foundation for future developments of TMDC-based optoelectronic devices.

Ultralow Auger-Assisted Interlayer Exciton Annihilation in WS2/WSe2 Moiré Heterobilayers

Transition metal dichalcogenide (TMD) heterobilayers have emerged as a promising platform for exploring solid-state quantum simulators and many-body quantum phenomena. Their type II band alignment, combined with the moiré superlattice, inevitably leads to nontrivial exciton interactions and dynamics. Here, we unveil the distinct Auger annihilation processes for delocalized interlayer excitons in WS2/WSe2 moiré heterobilayers. By fitting the characteristic efficiency droop and bimolecular recombination rate, we quantitatively determine an ultralow Auger coefficient of 1.3 × 10-5 cm2 s-1, which is >100-fold smaller than that of excitons in TMD monolayers. In addition, we reveal selective exciton upconversion into the WSe2 layer, which highlights the significance of intralayer electron Coulomb interactions in dictating the microscopic scattering pathways. The distinct Auger processes arising from spatial electron-hole separation have important implications for TMD heterobilayers while endowing interlayer excitons and their strongly correlated states with unique layer degrees of freedom.

Strategic Assessment of Boron-Enriched Carbon Dots/Naproxen: Diagnostic, Toxicity, and In Vivo Therapeutic Evaluation

Cancer is a significant global public health concern, ranking as the leading cause of mortality worldwide. This study thoroughly explores boron-doped carbon dots (B-CDs) through a simple/rapid microwave-assisted approach and their versatile applications in cancer therapy. The result was highly uniform particles with an average diameter of approximately 4 nm. B-CDs exhibited notable properties, including strong fluorescence with a quantum yield of 33%. Colloid stability tests revealed their robustness within a pH range of 6-12, NaCl concentrations up to 0.5 M, and temperatures ranging from 30 to 60 °C. The study also delved into the kinetics of naproxen release from B-CDs as a drug delivery system. The loading efficacy of naproxen exceeded 55.56%. Under varying pH conditions, the release of naproxen from B-CDs conformed to the Peppas-Sahlin model, demonstrating the potential of Naproxen-loaded CDs for cancer drug delivery. In vitro cytotoxicity assessments, conducted using the CCK-8 Assay and flow cytometry, consistently indicated low toxicity with average cell viability exceeding 80%. An in vivo toxicity test on female mice administered 20 mg/kg of B-CDs for 31 days revealed reversible histological changes in the liver and kidneys, while the pancreas remained unaffected. Importantly, B-CDs did not impact the mice's physical behavior, body weight, or survival. In vivo experiments targeting benzo(a)pyrene-induced fibrosarcoma demonstrated the efficacy of B-CDs as naproxen carriers in the treatment of cancer. This in vivo study provides a thorough comprehension of B-CDs synthesis and toxicity and their potential applications in cancer therapy and drug delivery systems.

Synergetic Enhancement of Quantum Yield and Exciton Lifetime of Monolayer WS2 by Proximal Metal Plate and Negative Electric Bias

The efficiency of light emission is a critical performance factor for monolayer transition metal dichalcogenides (1L-TMDs) for photonic applications. While various methods have been studied to compensate for lattice defects to improve the quantum yield (QY) of 1L-TMDs, exciton-exciton annihilation (EEA) is still a major nonradiative decay channel for excitons at high exciton densities. Here, we demonstrate that the combined use of a proximal Au plate and a negative electric gate bias (NEGB) for 1L-WS2 provides a dramatic enhancement of the exciton lifetime at high exciton densities with the corresponding QY enhanced by 30 times and the EEA rate constant decreased by 80 times. The suppression of EEA by NEGB is attributed to the reduction of the defect-assisted EEA process, which we also explain with our theoretical model. Our results provide a synergetic solution to cope with EEA to realize high-intensity 2D light emitters using TMDs.

Interfacial Effect on the Transient Dielectric Function and Charge Transfer in a Monolayer WS2/Si Heterojunction

Monolayer tungsten disulfide (WS2) is a highly promising material for silicon photonics. Thus, the WS2/Si interface plays a very important role due to the interfacial complex effects and abundant states. Among them, the effect of charge transfer on exciton dynamics and the optoelectronic property is determined by the dielectric function, which is very crucial for the performance of optoelectronic devices. However, research on the exciton dynamics or the transient dielectric function of WS2 in such WS2/Si junctions is still rare. In this work, both the transient dielectric function and charge transfer of WS2/Si heterojunctions are analyzed based on the transient reflectance spectra measured by the pump-probe spectrometer. The dynamic processes of the A exciton, affected by charge transfer within the WS2/Si heterojunction, are interpreted. Moreover, the transient dielectric function of WS2 is quantitatively analyzed. The dielectric function of WS2 exhibits a notable 19% change, persisting for more than 180 ps within the WS2/Si heterojunction. These findings can pave the way for the advancement of silicon photonic devices based on WS2.

Photoluminescence Enhancement of Monolayer WS2 by n-Doping with an Optically Excited Gold Disk

In nanophotonics and quantum optics, we aim to control and manipulate light with tailored nanoscale structures. Hybrid systems of nanostructures and atomically thin materials are of interest here, as they offer rich physics and versatility due to the interaction between photons, plasmons, phonons, and excitons. In this study, we explore the optical and electronic properties of a hybrid system, a naturally n-doped monolayer WS2 covering a gold disk. We demonstrate that the nonresonant excitation of the gold disk in the high absorption regime efficiently generates hot carriers via localized surface plasmon excitation, which n-dope the monolayer WS2 and enhance the photoluminescence emission by regulating the multiexciton population and stabilizing the neutral exciton emission. The results are relevant to the further development of nanotransistors in photonic circuits and optoelectronic applications.

Enhanced interactions of excitonic complexes in free-standing WS2

Excitonic complexes, bound states of electrons and holes, provide a promising platform in monolayer transition-metal dichalcogenide (TMDC) semiconductors for investigating diverse many-body interaction phenomena. The surrounding dielectric environment has been found to strongly influence the excitonic properties of the TMDC monolayers. While the impact of different dielectric surroundings on two-dimensional semiconductor materials and their strong correlations have been well studied, the effects on exciton formation and its properties resulting from a further reduction in dielectric screening remain elusive. In this study, we examined free-standing tungsten disulfide (WS2) monolayers, where the efficient generation of higher-order correlated excitonic complexes is readily observed. This phenomenon arises from the effective mutual interactions among excitons and internal carriers, attributed to the modulated exciton dynamics generated by the further reduced dielectric screening effect in the freestanding structure. The formation efficiency of excitonic complexes is enhanced and the multiple biexciton species (five particles such as charged biexcitons and acceptor/donor-bound biexcitons) are successfully induced under low excitation intensity and moderate temperature conditions. Our findings offer valuable insights into the influence of the dielectric environment on exciton interactions and enable a productive avenue for exploring fundamental many-body interactions, providing new possibilities for dielectric engineering of atomic thin semiconductors.

Ultrafast carrier dynamics in vanadium-doped MoS2 alloys

Substitutional doping is a most promising approach to manipulate the electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). In addition to inducing magnetism, vanadium (V) doping can lead to semiconductor-metal transition in TMDCs. However, the dynamics of charge carriers that governs the optoelectronic properties of doped TMDCs has been rarely revealed. In this work, we have investigated the dynamics of photocarriers in pristine and V-doped monolayer (ML) MoS2. Comparison of the transient absorption (TA) spectra of ML MoS2 with lightly (≤1%) and heavily (3.62%) V-doped MoS2 infers the induction of additional energy states in the doped materials giving rise to new low energy bleach features in the TA spectra. The quasiparticle band structure of MoS2 is found to disappear at sufficiently high V doping due to the presence of impurity bands. An attempt has also been made to study the manipulation of the carrier lifetime with V doping in MoS2. Our TA kinetic measurements suggest that the decay kinetics of the carriers becomes slower with increasing doping percentage and at a higher doping level the carriers survive for a much longer time compared to pristine MoS2. Furthermore, we have identified a new electronic transition (NET) in heavily V-doped MoS2 at high pump fluences.

Green and Red Photoluminescent Manganese Bromides with Aminomethylpyridine Isomers

Two positional isomers, 4-amino-3-methylpyridine and 3-amino-5-methylpyridine, produce 4-amino-3-methylpyridinium and 5-methylpyridin-3-aminium, respectively, under acidic conditions. The two protonated isomers create different hydrogen bonding networks, resulting in different coordination environments of the [MnX₄]^2- unit embedded in molecular compounds such as 4-amino-3-methylpyridinium manganese bromide, [(C₆H₉N₂)₂MnBr₄] and 5-methylpyridin-3-aminium manganese bromide, [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)]. Both compounds can be prepared using the slow evaporation method or mechanochemical synthetic procedures. Single-crystal structure analysis of [(C₆H₉N₂)₂MnBr₄] and [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)] revealed different manganese halide units, including tetrahedral and tetrahedral with distorted trigonal bipyramidal structures, which emit photoluminescence in the green (527 nm) and red (607 nm) regions, respectively. Electronic structure calculations were conducted to support the validity and interpretation of the UV-vis and photoluminescence (PL) spectral data. Thin films deposited using the [(C₆H₉N₂)₂MnBr₄] precursor also exhibit PL properties. The diverse pseudo-three-dimensional networks can be constructed by using positional isomers with different hydrogen bonding pathways and π-π stacking of organic units, in which the design strategy successfully enables the tuning of various optical properties.

Temperature Dependence of Excitonic Auger Recombination in Excitonic-Complex-Free Monolayer WS2 by Considering Auger Broadening and Generation Efficiency

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their optoelectronic properties and applications. However, even at moderate exciton densities, their light-emitting capability is severely limited by Auger-type exciton-exciton annihilation (EEA). Previous work on EEA used oversimplified models in the presence of excitonic complexes, resulting in seriously underestimated values for the Auger coefficient. In this work, we transferred monolayer WS₂ on a gold substrate with hBN encapsulation, where excitons persist as the main species at 3-300 K via metal proximity. We numerically solved the rate equation for excitons to accurately determine the Auger coefficient as a function of temperature by considering laser pulse width and spatially inhomogeneous exciton distribution. We found that the Auger coefficient consists of temperature-dependent and independent terms, consistent with a theoretical model for direct and exchange processes, respectively. We believe that our results provide a guide for enhancing the luminescence quantum yield of TMDs.

Probing Anisotropic Deformation and Near-Infrared Emission Tuning in Thin-Layered InSe Crystal under High Pressure

Indium selenide (InSe) exhibits high lattice compressibility and an extraordinary capability of tailoring the optical band gap under pressure beyond other 2D materials. Herein, by applying hydrostatic pressure via a diamond anvil cell, we revealed an anisotropic deformation dynamic and efficient manipulation of near-infrared light emission in thin-layered InSe strongly correlated to layer numbers (N = 5-30). As N > 20, the InSe lattice is compressed in all directions, and the intralayer compression leads to widening of the band gap, resulting in an emission blue shift (∼120 meV at 1.5 GPa). In contrast, as N ≤ 15, an efficient emission red shift is observed from band gap shrinkage (rate of 100 meV GPa^-1), which is attributed to the predominant uniaxial interlayer compression because of the high strain resistance along the InSe-diamond interface. These findings advance the understanding of pressure-induced lattice deformation and optical transition evolution in InSe and could be applied to other 2D materials.

Engineering Exciton Recombination Pathways in Bilayer WSe2 for Bright Luminescence

Exciton-exciton annihilation (EEA) in counterdoped monolayer transition metal dichalcogenides (TMDCs) can be suppressed by favorably changing the band structure with strain. The photoluminescence (PL) quantum yield (QY) monotonically approaches unity with strain at all generation rates. In contrast, here in bilayers (2L) of tungsten diselenide (WSe₂) we observe a nonmonotonic change in EEA rate at high generation rates accompanied by a drastic enhancement in their PL QY at low generation rates. EEA is suppressed at both 0% and 1% strain, but activated at intermediate strains. We explain our observation through the indirect to direct transition in 2L WSe₂ under uniaxial tensile strain. By strain and electrostatic counterdoping, we attain ∼50% PL QY at all generation rates in 2L WSe₂, originally an indirect semiconductor. We demonstrate transient electroluminescence from 2L WSe₂ with ∼1.5% internal quantum efficiency for a broad range of carrier densities by applying strain, which is ∼50 times higher than without strain. The present results elucidate the complete optoelectronic photophysics where indirect and direct excitons are simultaneously present and expedite exciton engineering in a TMDC multilayer beyond indirect-direct bandgap transition.