Enhanced Light-Matter Interactions in Self-Assembled Plasmonic Nanoparticles on 2D Semiconductors.

Two-dimensional (2D) transition-metal dichalcogenide (TMD) monolayers of versatile material library are spotlighted for numerous unexplored research fields. While monolayer TMDs exhibit an efficient excitonic emission, the weak light absorption arising from their low dimensionality limits potential applications. To enhance the light-matter interactions of TMDs, while various plasmonic hybridization methods have been intensively studied, controlling plasmonic nanostructures via self-assembly processes remains challenging. Herein, strong light-matter interactions are reported in plasmonic Ag nanoparticles (NPs) hybridized on TMDs via an aging-based self-assembly process at room temperature. This hybridization is implemented by transferring MoS2 monolayers grown via chemical vapor deposition onto thin-spacer-covered Ag films. After a few weeks of aging in a vacuum desiccator, the Ag atoms in the heterolayered film diffuse to the MoS2 layers through a SiO2 spacer and self-cluster onto MoS2 point defects, resulting in the formation of Ag-NPs with an estimated diameter of ≈50 nm. The photoluminescence intensities for the Ag-NP/MoS2 hybrids are enhanced up to 35-fold compared with bare MoS2 owing to the local field enhancement near the plasmonic Ag-NPs. The localized surface plasmon resonances modes of this hybrid are systematically investigated via numerical simulations and dark-field scattering microscopy.

Optically active charge transfer in hybrids of Alq3 nanoparticles and MoS2 monolayer.

Organic/inorganic hybrid structures have been widely studied because of their enhanced physical and chemical properties. Monolayers of transition metal dichalcogenides (1L-TMDs) and organic nanoparticles can provide a hybridization configuration between zero- and two-dimensional systems with the advantages of convenient preparation and strong interface interaction. Here, we present such a hybrid system made by dispersing π-conjugated organic (tris (8-hydroxyquinoline) aluminum(III)) (Alq3) nanoparticles (NPs) on 1L-MoS2. Hybrids of Alq3 NP/1L-MoS2 exhibited a two-fold increase in the photoluminescence of Alq3 NPs on 1L-MoS2 and the n-doping effect of 1L-MoS2, and these spectral and electronic modifications were attributed to the charge transfer between Alq3 NPs and 1L-MoS2. Our results suggested that a hybrid of organic NPs/1L-TMD can offer a convenient platform to study the interface interactions between organic and inorganic nano objects and to engineer optoelectronic devices with enhanced performance.

Hydropower capacity factors trending down in the United States.

The United States hydropower fleet has faced increasing environmental and regulatory pressures over the last half century, potentially constraining total generation. Here we show that annual capacity factor has declined at four fifths of United States hydropower plants since 1980, with two thirds of decreasing trends significant at p < 0.05. Results are based on an analysis of annual energy generation totals and nameplate capacities for 610 plants (>5 megawatt), representing 87% of total conventional hydropower capacity in the United States. On aggregate, changes in capacity factor imply a fleetwide, cumulative generation decrease of 23% since 1980 before factoring in capacity upgrades-akin to retiring a Hoover Dam once every two to three years. Changes in water availability explain energy decline in only 21% of plants, highlighting the importance of non-climatic drivers of generation, including deterioration of plant equipment as well as changes to dam operations in support of nonpower objectives.

Molybdenum Disulfide Nanoribbons with Enhanced Edge Nonlinear Response and Photoresponsivity.

MoS2 nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS2 nanoribbons and triangular crystals formed by the reaction between films of MoOx (2

A facile strategy for the growth of high-quality tungsten disulfide crystals mediated by oxygen-deficient oxide precursors.

Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the precursor composition's role on the CVD process remains largely unknown and remains to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage approach to tune the oxygen content in the tungsten oxide (WO3-x) precursors and demonstrate the presence of oxygen vacancies in the oxide films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab initio density functional theory (DFT) calculations and experimental observations, we unravel that the oxygen vacancies in WO3-x can serve as niches through which sulfur atoms enter the lattice and facilitate an efficient conversion into WS2 crystals. By solely modulating the precursor stoichiometry, the photoluminescence emission of WS2 crystals can be significantly enhanced. Atomic resolution scanning transmission electron microscopy imaging (STEM) reveals that tungsten vacancies are the dominant intrinsic defects in mono- and bilayers WS2. Moreover, bi- and multilayer WS2 crystals derived from oxides with a high V0 content exhibit dominant AA'/AB or AA(A…) stacking orientations. The atomic resolution images reveal local strain buildup in bilayer WS2 due to competing effects of complex grain boundaries. Our study provides means to tune the precursor composition to control the lateral growth of TMDs while revealing insights into the different pathways for forming grain boundaries in bilayer WS2.

Doping-Mediated Lattice Engineering of Monolayer ReS2 for Modulating In-Plane Anisotropy of Optical and Transport Properties.

ReS2 exhibits strong anisotropic optical and electrical responses originating from the asymmetric lattice. Here, we show that the anisotropy of monolayer (1L) ReS2 in optical scattering and electrical transport can be practically erased by lattice engineering via lithium (Li) treatment. Scanning transmission electron microscopy revealed that significant strain is induced in the lattice of Li-treated 1L-ReS2, due to high-density electron doping and the resultant formation of continuous tiling of nanodomains with randomly rotating orientations of 60°, which produced a nearly isotropic response of polarized Raman scattering and absorption of Li-treated 1L-ReS2. With Li treatment, the in-plane conductance of 1L-ReS2 increased by an order of magnitude, and its angle dependence became negligible. Our result that the asymmetric phase was converted into the isotropic phase by electron injection could significantly expand the optoelectronic applications of polymorphic two-dimensional transition metal dichalcogenides.

Probing Multiphased Transition in Bulk MoS2 by Direct Electron Injection.

Structural phase transitions in layered two-dimensional (2D) materials are of significant interest owing to their ability to exist in multiple metastable states with distinctive properties. However, phase transition in bulk MoS2 by nondestructive electron infusion has not yet been realized. In this study, we report the 2H to 1T' phase transition and in-between intermediates in bulk MoS2 using MoS2/[Ca2N]+·e- heterostructures, in which kinetic free electrons were directly injected into MoS2. We observed various phases in MoS2 ranging from heavily doped 2H to a distorted lattice state and then on to a complete 1T' state. Snapshots of the multiphase transition were captured by extraordinary Raman shift and bandgap reduction and were further elucidated by theoretical calculations. We also observed a weakening in interlayer coupling in the vicinity of the metallic regime, which led to an unusually strong photoluminescence emission, suggesting light-efficient bulk MoS2. Our results thus suggest the optoelectronic applications that can fully utilize the multiphase transition of bulk 2D materials.

Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero-Bilayers.

Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe2 /MoS2 hetero-bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band-to-band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices.