High degree of circular polarization in WS2 spiral nanostructures induced by broken symmetry.
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PROVIDER: S-EPMC6391472 | BioStudies |
REPOSITORIES: biostudies
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Project description:Single-monolayer transition metal dichalcogenides exhibit exceptionally strong photoluminescence (PL), dominated by a combination of distinct neutral and charged exciton contributions. We show here that the surface charge associated with ferroelectric domains patterned into a lead zirconium titanate film with an atomic force microscope laterally controls the spatial distribution of neutral and charged exciton populations in an adjacent WS<sub>2</sub> monolayer. This is manifested by the intensity and spectral composition of the PL measured in air at room temperature from the areas of WS<sub>2</sub> over a ferroelectric domain with a polarization dipole pointed either out of the surface plane or into the surface plane. This approach enables spatial modulation of PL intensity and trion/neutral exciton populations and fabrication of lateral quantum dot arrays in any geometry, with potential applications in nonvolatile optically addressable memory or optical quantum computation.
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Project description:Herein, we have investigated the tunability of the photoluminescence (PL) of the monolayer MoS<sub>2</sub> (1L-MoS<sub>2</sub>) by decorating it with WS<sub>2</sub> quantum dots (WS<sub>2</sub> QD). The direct bandgap 1L-MoS<sub>2</sub> and WS<sub>2</sub> QDs are grown by chemical vapor deposition and liquid exfoliation methods, respectively. The room temperature PL spectrum of bare 1L-MoS<sub>2</sub> is systematically quenched with its decoration with WS<sub>2</sub> QDs at different concentrations. A decrease in the work function of 1L-MoS<sub>2</sub> with the decoration of WS<sub>2</sub> QDs was established from the Kelvin probe force microscopy analysis. A detailed quantitative analysis using the four-energy level model involving coupled charge transfer was employed to explain the redshift and the systematic decrease in the intensity of the PL peak in 1L-MoS<sub>2</sub>/WS<sub>2</sub> QD heterostructure. The modulation of the PL in the heterostructure is attributed to the increase in the formation of negative trions through the charge transfer from WS<sub>2</sub> QD to the 1L-MoS<sub>2</sub> and thus making the 1L-MoS<sub>2</sub> heavily n-type doped, with increase in the electron density by ~1.5?×?10<sup>13</sup>?cm<sup>-2</sup>. This study establishes the contribution of defects in the coupled charge transfer dynamics in 1L-MoS<sub>2</sub>, and it lays out a convenient strategy to manipulate the optical and electrical properties of 1L-MoS<sub>2</sub> for various optoelectronic applications.
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Project description:We investigate the origin of emission lines apparent in the low-temperature photoluminescence spectra of n-doped WS<sub>2</sub> monolayer embedded in hexagonal BN layers using external magnetic fields and first-principles calculations. Apart from the neutral A exciton line, all observed emission lines are related to the negatively charged excitons. Consequently, we identify emissions due to both the bright (singlet and triplet) and dark (spin- and momentum-forbidden) negative trions as well as the phonon replicas of the latter optically inactive complexes. The semidark trions and negative biexcitons are distinguished. On the basis of their experimentally extracted and theoretically calculated <i>g</i>-factors, we identify three distinct families of emissions due to exciton complexes in WS<sub>2</sub>: bright, intravalley, and intervalley dark. The <i>g</i>-factors of the spin-split subbands in both the conduction and valence bands are also determined.
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Project description:Two-dimensional (2D) materials are being used widely for chemical sensing applications due to their large surface-to-volume ratio and photoluminescence (PL) emission and emission exciton band tunability. To better understand how the analyte affects the PL response for a model 2D platform, we used atomic force microscopy (AFM) and co-localized photoluminescence (PL) and Raman mapping to characterize tungsten disulfide (WS<sub>2</sub>) flakes on template-stripped gold (TSG) under acetone challenge. We determined the PL-based response from single- and few-layer WS<sub>2</sub> arises from three excitons (neutral, A<sup>0</sup>; biexciton, AA; and the trion, A<sup>-</sup>). The A<sup>0</sup> exciton PL emission is the most strongly quenched by acetone whereas the A<sup>-</sup> PL emission exhibits an enhancement. We find the PL behavior is also WS<sub>2</sub> layer number dependent.
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Project description:Tungsten disulfide (WS<sub>2</sub>) has tunable bandgaps, which are required for diverse optoelectronic device applications. Here, we report the bandgap modulation in WS<sub>2</sub> monolayers with two-dimensional core-shell structures formed by unique growth mode in chemical vapor deposition (CVD). The core-shell structures in our CVD-grown WS<sub>2</sub> monolayers exhibit contrasts in optical images, Raman, and photoluminescence spectroscopy. The strain and doping effects in the WS<sub>2</sub>, introduced by two different growth processes, generate PL peaks at 1.83 eV (at the core domain) and 1.98 eV (at the shell domain), which is distinct from conventional WS<sub>2</sub> with a primary PL peak at 2.02 eV. Our density functional theory (DFT) calculations explain the modulation of the optical bandgap in our core-shell-structured WS<sub>2</sub> monolayers by the strain, accompanying a direct-to-indirect bandgap transition. Thus, the core-shell-structured WS<sub>2</sub> monolayers provide a practical method to fabricate lateral heterostructures with different optical bandgaps, which are required for optoelectronic applications.
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Project description:Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. To date, most have focused on charge transfer and energy transfer from QDs to TMDs, that is, from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulfide (WS<sub>2</sub>) to near-infrared emitting lead sulfide-cadmium sulfide (PbS-CdS) QDs. The high absorption cross section of WS<sub>2</sub> in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems makes this an interesting donor-acceptor system that can effectively use the WS<sub>2</sub> as an antenna and the QD as a tunable emitter, in this case, downshifting the emission energy over hundreds of millielectron volts. We study the energy transfer process using photoluminescence excitation and PL microscopy and show that 58% of the QD PL arises due to energy transfer from the WS<sub>2</sub>. Time-resolved photoluminescence microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the WS<sub>2</sub>, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tunable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light-emitting technologies or artificial light-harvesting systems or be used to read out the state of TMD devices optically in various logic and computing applications.
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Project description:Many potential applications of monolayer transition metal dichalcogenides (TMDs) require both high photoluminescence (PL) yield and high electrical mobilities. However, the PL yield of as prepared TMD monolayers is low and believed to be limited by defect sites and uncontrolled doping. This has led to a large effort to develop chemical passivation methods to improve PL and mobilities. The most successful of these treatments is based on the nonoxidizing organic "superacid" bis(trifluoromethane)sulfonimide (TFSI) which has been shown to yield bright monolayers of molybdenum disulfide (MoS<sub>2</sub>) and tungsten disulfide (WS<sub>2</sub>) but with trap-limited PL dynamics and no significant improvements in field effect mobilities. Here, using steady-state and time-resolved PL microscopy we demonstrate that treatment of WS<sub>2</sub> monolayers with oleic acid (OA) can greatly enhance the PL yield, resulting in bright neutral exciton emission comparable to TFSI treated monolayers. At high excitation densities, the OA treatment allows for bright trion emission, which has not been demonstrated with previous chemical treatments. We show that unlike the TFSI treatment, the OA yields PL dynamics that are largely trap free. In addition, field effect transistors show an increase in mobilities with the OA treatment. These results suggest that OA serves to passivate defect sites in the WS<sub>2</sub> monolayers in a manner akin to the passivation of colloidal quantum dots with OA ligands. Our results open up a new pathway to passivate and tune defects in monolayer TMDs using simple "wet" chemistry techniques, allowing for trap-free electronic properties and bright neutral exciton and trion emission.
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Project description:The inherently low photoluminescence (PL) yields in the as prepared transition metal dichalcogenide (TMD) monolayers are broadly accepted to be the result of atomic vacancies (<i>i.e.</i>, defects) and uncontrolled doping, which give rise to non-radiative exciton decay pathways. To date, a number of chemical passivation schemes have been successfully developed to improve PL in sulphur based TMDs <i>i.e.</i>, molybdenum disulphide (MoS<sub>2</sub>) and tungsten disulphide (WS<sub>2</sub>) monolayers. Studies on solution based chemical passivation schemes for improving PL yields in selenium (Se) based TMDs are however lacking in comparison. Here, we demonstrate that treatment with oleic acid (OA) provides a simple wet chemical passivation method for monolayer MoSe<sub>2</sub>, enhancing PL yields by an average of 58-fold, while also improving spectral uniformity across the material and reducing the emission linewidth. Excitation intensity dependent PL reveals trap-free PL dynamics dominated by neutral exciton recombination. Time-resolved PL (TRPL) studies reveal significantly increased PL lifetimes, with pump intensity dependent TRPL measurements also confirming trap free PL dynamics in OA treated MoSe<sub>2</sub>. Field effect transistors show reduced charge trap density and improved on-off ratios after treatment with OA. These results indicate defect passivation by OA, which we hypothesise as ligands passivating chalcogen defects through oleate coordination to Mo dangling bonds. Importantly, this work combined with our previous study on OA treated WS<sub>2</sub>, verifies OA treatment as a simple solution-based chemical passivation protocol for improving PL yields and electronic characteristics in both selenide and sulphide TMDs - a property that has not been reported previously for other solution-based passivation schemes.
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Project description:We report a comprehensive theory to describe exciton and biexciton valley dynamics in monolayer Mo<sub>1-x</sub>W<sub>x</sub>Se<sub>2</sub> alloys. To probe the impact of different excitonic channels, including bright and dark excitons, intravalley biexcitons, intervalley scattering between bright excitons, as well as bright biexcitons, we have performed a systematic study from the simplest system to the most complex one. In contrast to the binary WSe<sub>2</sub> monolayer with weak photoluminescence (PL) and high valley polarization at low temperatures and the MoSe<sub>2</sub>, that presents high PL intensity, but low valley polarization, our results demonstrate that it is possible to set up a ternary alloy with intermediate W-concentration that holds simultaneously a considerably robust light emission and an efficient optical orientation of the valley pseudospin. We find the critical value of W-concentration, x<sub>c</sub>, that turns alloys from bright to darkish. The dependence of the PL intensity on temperature shows three regimes: while bright monolayer alloys display a usual temperature dependence in which the intensity decreases with rising temperature, the darkish alloys exhibit the opposite behavior, and the alloys with x around x<sub>c</sub> show a non-monotonic temperature response. Remarkably, we observe that the biexciton enhances significantly the stability of the exciton emission against fluctuations of W-concentration for bright alloys. Our findings pave the way for developing high-performance valleytronic and photo-emitting devices.
| S-EPMC6418264 | BioStudies
Project description:Photoluminescent zero-dimensional (0D) quantum dots (QDs) derived from transition metal dichalcogenides, particularly molybdenum disulfide, are presently in the spotlight for their advantageous characteristics for optoelectronics, imaging, and sensors. Nevertheless, up to now, little work has been done to synthesize and explore photoluminescent 0D WS<sub>2</sub> QDs, especially by a bottom-up strategy without using usual toxic organic solvents. In this work, we report a facile bottom-up strategy to synthesize high-quality water-soluble tungsten disulfide (WS<sub>2</sub>) QDs through hydrothermal reaction by using sodium tungstate dihydrate and L-cysteine as W and S sources. Besides, hybrid carbon quantum dots/WS<sub>2</sub> QDs were further prepared based on this method. Physicochemical and structural analysis of QD hybrid indicated that the graphitic carbon quantum dots with diameters about 5 nm were held onto WS<sub>2</sub> QDs via electrostatic attraction forces. The resultant QDs show good water solubility and stable photoluminescence (PL). The excitation-dependent PL can be attributed to the polydispersity of the synthesized QDs. We found that the PL was stable under continuous irradiation of UV light but can be quenched in the presence of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>). The obtained WS<sub>2</sub>-based QDs were thus adopted as an electrodeless luminescent probe for H<sub>2</sub>O<sub>2</sub> and for enzymatic sensing of glucose. The hybrid QDs were shown to have a more sensitive LOD in the case of glucose sensing. The Raman study implied that H<sub>2</sub>O<sub>2</sub> causes the partial oxidation of QDs, which may lead to oxidation-induced quenching. Overall, the presented strategy provides a general guideline for facile and low-cost synthesis of other water-soluble layered material QDs and relevant hybrids in large quantity. These WS<sub>2</sub>-based high-quality water-soluble QDs should be promising for a wide range of applications in optoelectronics, environmental monitoring, medical imaging, and photocatalysis.
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