Atomically thin transition metal dichalcogenides like MoS monolayers exhibit unique luminescent properties. However, weak quantum yield and low light absorption hinder their practical applications in two-dimensional light emitting devices. Here, we report 1300 times enhancement in photoluminescence emission from a MoS monolayer via simultaneous Fano resonances in a dielectric photonic crystal. The spatially extended double Fano resonance scheme allows resonant enhancement of both the MoS absorption and emission. We also achieve unidirectional emission within a narrow divergence angle of 5° by engineering the Fano resonance angular dispersion. Our approach provides a new platform for efficient light sources with high directionality based on emerging two-dimensional materials.

redox reactions in fuel cells and metaloxygen batteries as well as water-splitting devices. [1][2][3][4] Including the initial notable works in the 1980s, [5][6][7] many experimental and theoretical studies on the ABO 3 -type perovskite catalysts have dealt with the number of d-electrons of B-site cations, the bond strength of BOH, the position of the O 2p band center, the effect of oxygen vacancies, and the role of lattice oxygen redox in efforts to account for the crucial contributions that correlate with the high catalytic activity during the oxygen evolution reaction (OER). [6][7][8][9][10][11][12] By extensive comparison of more than ten perovskite oxides, recent studies also suggested activity descriptors on the basis of the number of e g -orbital electrons of the 3d transition metals [13] and the charge transfer energy between metal and oxygen electrons [14] to understand the OER activity differences among the perovskite family. Although various structural and electronic factors should be taken into account to precisely depict the catalytic behavior of perovskites [15][16][17] and other transitionmetal oxides, [18][19][20][21][22][23] it appears to be accepted in general that relative difference of energy level between the 3d orbitals of B-site cations and the 2p orbitals of oxygen anions play an important role in significant promotion of charge transfer between [BO 6 ] at the surface and adsorbates, resulting in much higher OER efficiency at a lower overpotential. [9,[11][12][13][14][24][25][26][27][28] In stark contrast to the [BO 6 ] octahedra in the bulk, most [BO 6 ] units at grain boundaries (GBs) in polycrystals have severely distorted geometric configurations of B and O. [29,30] Note that the atomic displacement from this distortion at GBs is usually much greater than the displacement that occurs at the crystal surface for relaxation [31] and/or is induced by the coherent strain via using the epitaxial thin-film growth. [26,32] As a result, it can be reasonably anticipated that the electronic states of transition-metal 3d and oxygen 2p orbitals at GBs considerably differ from those in the bulk. In this regard, a combination of selective analysis of the OER characteristics at GBs and observation of the grain-boundary atomic structure can provide direct and insightful information that has not been offered in previous studies to elucidate the correlation between A grain boundary forms as an internal interface when two crystalline grains with mutually different crystallographic orientations are in direct contact with each other. As a result, atomic arrangement at grain boundaries differs from that of the bulk, showing serious displacements deviating from the original symmetric positions. As these symmetry-broken configurations are difficult to achieve in the bulk crystals, grain boundaries are considered distinctive platforms that can exhibit new physical properties. By using both sintered polycrystals with various grain sizes and thin films on bicrystal substrates, it is directly verified that surfac...

Active tunability of photonic resonances is of great interest for various applications such as optical switching and modulation based on optoelectronic materials. Manipulation of charged excitons in atomically thin transition metal dichalcogenides (TMDCs) like monolayer MoS offers an unexplored route for diverse functionalities in optoelectronic nanodevices. Here, we experimentally demonstrate the dynamic photochemical and optoelectronic control of the photonic crystal Fano resonances by optical and electrical tuning of monolayer MoS refractive index via trions without any chemical treatment. The strong spatial and spectral overlap between the photonic Fano mode and the active MoS monolayer enables efficient modulation of the Fano resonance. Our approach offers new directions for potential applications in the development of optical modulators based on emerging 2D direct band gap semiconductors.

Graphene is an attractive material for all-optical modulation because of its ultrafast optical response and broad spectral coverage. However, all-optical graphene modulators reported so far require high pump fluence due to the ultrashort photo-carrier lifetime and limited absorption in graphene. We present modulator designs based on graphene-metal hybrid plasmonic metasurfaces with highly enhanced light-graphene interaction in the nanoscale hot spots at pump and probe (signal) wavelengths. Based on this design concept, we have demonstrated high-speed all-optical modulators at near and mid-infrared wavelengths (1.56 μm and above 6 μm) with significantly reduced pump fluence (1–2 orders of magnitude) and enhanced optical modulation. Ultrafast near-infrared pump-probe measurement results suggest that the modulators’ response times are ultimately determined by graphene’s ultrafast photocarrier relaxation times on the picosecond scale. The proposed designs hold the promise to address the challenges in the realization of ultrafast all-optical modulators for mid-and far-infrared wavelengths.

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We synthesized three dendron-coil-dendron block copolymers consisting of ionophilic poly(ethylene oxide) (PEO) coils and mesogenic dendrons with four octadecyl peripheries via stepwise click reactions.The obtained polymers were doped with lithium triflate, whose concentration per ethylene oxide unit was 0.05. As characterized by optical polarized microscopy (POM) and X-ray scattering techniques, polymer 1 with the shortest PEO coil (M n ¼ 2000 g mol À1 ) did not show the liquid crystalline (LC) phase, while its ionic sample (1-Li + ) exhibited a hexagonal columnar LC phase. On the other hand, 2 and 3 (with PEOs of M n ¼ 4000 and 8000 g mol À1 , respectively) displayed identical LC morphologies to 2-Li + and 3-Li + , respectively, although the phase transition temperatures increased upon salt doping.For 2 and 2-Li + , gyroid and lamellar LC phases were observed with increasing temperature, while 3 and 3-Li + showed only a lamellar LC phase. The observed ion-transporting behavior was strongly dependent upon the connectivity of ion-conducting domain structures. The investigation of the morphology-conductivity correlation using a normalized conductivity (s* ¼ s/f, where s and f are the original conductivity and the PEO volume fraction) indicated that the 3-D gyroid LC phase showed the highest value, while the lowest conductivity was found in the 1-D columnar structure at identical temperatures. Additionally, the gyroid to lamellar phase transition temperature of 2-Li + could be determined by the s*, which was consistent with the X-ray data. Consequently, the results can be explained by the fact that the ionic pathway becomes complicated when going from higher to lower dimensional structures in polygrain samples.

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