Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure Deposited on Monolayer WS2 at Room Temperature

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides (TMDCs) with strong many-body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, we explore the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles (AuNTs) and monolayer WS2. We observe a narrow Fano resonance when the hybrid system is surrounded by water, and we attribute the narrowing of the spectral Fano linewidth to the plasmon-enhanced decay of dark K-K excitons. Our results reveal that dark excitons in monolayer WS2 can strongly modify Fano resonances in hybrid plasmon-exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices

Tuning Multipolar Mie Scattering of Particles on a Dielectric-Covered Mirror

Optically resonant particles are key building blocks of many nanophotonic devices such as optical antennas and metasurfaces. Because the functionalities of such devices are largely determined by the optical properties of individual resonators, extending the attainable responses from a given particle is highly desirable. Practically, this is usually achieved by introducing an asymmetric dielectric environment. However, commonly used simple substrates have limited influences on the optical properties of the particles atop. Here, we show that the multipolar scattering of silicon microspheres can be effectively modified by placing the particles on a dielectric-covered mirror, which tunes the coupling between the Mie resonances of microspheres and the standing waves and waveguide modes in the dielectric spacer. This tunability allows selective excitation, enhancement, and suppression of the multipolar resonances and enables scattering at extended wavelengths, providing new opportunities in controlling light-matter interactions for various applications. We further demonstrate with experiments the detection of molecular fingerprints by single-particle mid-infrared spectroscopy, and, with simulations strong optical repulsive forces that could elevate the particles from a substrate.Comment: 16 pages, 4 figure

Thermodynamic synthesis of solution processable ladder polymers

The synthesis of a carbazole-derived, well-defined ladder polymer was achieved under thermodynamic control by employing reversible ring-closing olefin metathesis. This unique approach featured mild conditions and excellent efficiency, affording the ladder polymer backbone with minimum levels of unreacted defects. Rigorous NMR analysis on a C-13 isotope-enriched product revealed that the main-chain contained less than 1% of unreacted precursory vinyl groups. The rigid conformation of the ladder-type backbone was confirmed by photophysical analysis, while the extended rod-like structure was visualized under scanning tunneling microscope. Excellent solubility of this polymer in common organic solvents allowed for feasible processing of thin films using solution-casting techniques. Atomic force microscopy and grazing incident X-ray scattering revealed a uniform and amorphous morphology of these films, in sharp contrast to the polycrystalline thin films of its small molecular counterpart.National Priorities Research Program award from the Qatar National Research Fund NPRP7-285-1-045U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences DE-AC02-06CH11357Mechanical Engineerin

Room-temperature observation of near-intrinsic exciton linewidth in monolayer WS2

The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light-matter interactions in two-dimensional transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control has rarely been achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS2, the reported RT linewidth was typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, we show that a 7.18 meV near-intrinsic linewidth can be observed at RT when monolayer WS2 is coupled with a moderate-refractive-index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS2 through the nanosphere-supported Mie resonances, we have managed to tune the coherent linewidth from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD-based nanophotonics and optoelectronics.J.F., K.Y., and Y.Z. acknowledge the financial support of the National Aeronautics and Space Administration Early Career Faculty Award (80NSSC17K0520), the National Science Foundation (NSF-ECCS-2001650), and the National Institute of General Medical Sciences of the National Institutes of Health (DP2GM128446). M.W. and A.A. acknowledge the financial support of the Air Force Office of Scientific Research MURI program (FA9550-17-1-0002), the Vannevar Bush Faculty Fellowship, and the Simons Foundation. T.Z. and M.T. acknowledge the financial support of the Air Force Office of Scientific Research (FA9550-18-1-0072). T.J. and B.A.K. acknowledge the financial support of the Robert A. Welch Foundation (F-1464), and the Center for Dynamics and Control of Materials (CDCM), Materials Research Science and Engineering Center (MRSEC) (DMR-1720595).Center for Dynamics and Control of Material

Light-driven C-H bond activation mediated by 2D transition metal dichalcogenides

C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials