Visualization of Band Shifting and Interlayer Coupling in WxMo1-xS2 Alloys Using Near-Field Broadband Absorption Microscopy.

Beyond-diffraction-limit optical absorption spectroscopy provides in-depth information on the graded band structures of composition-spread and stacked two-dimensional materials, in which direct/indirect bandgap, interlayer coupling, and defects significantly modify their optoelectronic functionalities such as photoluminescence efficiency. We here visualize the spatially varying band structure of monolayer and bilayer transition metal dichalcogenide alloys by using near-field broadband absorption microscopy. The near-field spectral and spatial information manifests the excitonic band shift that results from the interplay of composition spreading and interlayer coupling. These results enable us to identify, notably, the top layer of the bilayer alloy as pure WS2. We also use the aberration-free near-field transmission images to demarcate the exact boundaries of alloyed and pure transition metal dichalcogenides. This technology can offer valuable insights on various layered structures in the era of "stacking science" in the quest of quantum optoelectronic devices.

Near-field spectroscopic imaging of exciton quenching at atomically sharp MoS2/WS2 lateral heterojunctions.

Heterojunctions made by laterally stitching two different transition metal dichalcogenide monolayers create a unique one-dimensional boundary with intriguing local optical properties that can only be characterized by nanoscale-spatial-resolution spectral tools. Here, we use near-field photoluminescence (NF-PL) to reveal the narrowest region (105 nm) ever reported of photoluminescence quenching at the junction of a laterally stitched WS2/MoS2 monolayer. We attribute this quenching to the atomically sharp band offset that generates a strong electric force at the junction to easily dissociate excitons. Besides the sharp heterojunction, a model considering various widths of the alloying interfacial region under low or high optical pumping is presented. With a spatial resolution six times better than that of confocal microscopy, NF-PL provides an unprecedented spectral tool for non-scalable 1D lateral heterojunctions.

A horizontal-type scanning near-field optical microscope with torsional mode operation toward high-resolution and non-destructive imaging of soft materials.

We design and build a horizontal-type aperture based scanning near-field optical microscope (a-SNOM) with superior mechanical stability toward high-resolution and non-destructive topographic and optical imaging. We adopt the torsional mode in AFM (atomic force microscopy) operation to achieve a better force sensitivity and a higher topographic resolution when using pyramidal a-SNOM tips. The performance and stability of the AFM are evaluated through single-walled carbon nanotube and poly(3-hexyl-thiophene) nanowire samples. An optical resolution of 93 nm is deduced from the a-SNOM imaging of a metallic grating. Finally, a-SNOM fluorescence imaging of soft lipid domains is successfully achieved without sample damage by our horizontal-type a-SNOM instrument with torsional mode AFM operation.

Large-Area and Strain-Reduced Two-Dimensional Molybdenum Disulfide Monolayer Emitters on a Three-Dimensional Substrate.

Atomically thin membranes of two-dimensional (2-D) transition-metal dichalcogenides (TMDCs) have distinct emission properties, which can be utilized for realizing ultrathin optoelectronic integrated systems in the future. Growing a large-area and strain-reduced monolayer 2-D material on a three-dimensional (3-D) substrate with microstructures or nanostructures is a crucial technique because the electronic band structure of TMDC atomic layers is strongly affected by the number of stacked layers and strain. In this study, a large-area and strain-reduced MoS2 monolayer was fabricated on a 3-D substrate through a two-step growth procedure. The material characteristics and optical properties of monolayer TMDCs fabricated on the nonplanar substrate were examined. The growth of monolayer MoS2 on a cone-shaped sapphire substrate effectively reduced the tensile strain induced by the substrate by decreasing the thermal expansion mismatch between the 2-D material and the substrate. Monolayer MoS2 grown on the nonplanar substrate exhibited uniform strain reduction and luminescence intensity. The fabrication of monolayer MoS2 on a nonplanar substrate increased the light extraction efficiency. In the future, large-area and strain-reduced 2-D TMDC materials grown on a nonplanar substrate can be employed as novel light-emitting devices for applications in lighting, communication, and displays for the development of ultrathin optoelectronic integrated systems.

Near-Field Plasmonic Probe with Super Resolution and High Throughput and Signal-to-Noise Ratio.

Near-field scanning optical microscopy (NSOM) enables observation of light-matter interaction with a spatial resolution far below the diffraction limit without the need for a vacuum environment. However, modern NSOM techniques remain subject to a few fundamental restrictions. For example, concerning the aperture tip (a-tip), the throughput is extremely low, and the lateral resolution is poor; both are limited by the aperture size. Meanwhile, with regard to the scattering tip (s-tip), the signal-to-noise ratio (SNR) appears to be almost zero; consequently, one cannot directly use the measured data. In this work, we present a plasmonic tip (p-tip) developed by tailoring subwavelength annuli so as to couple internal radial illumination to surface plasmon polaritons (SPPs), resulting in an ultrastrong, superfocused spot. Our p-tip supports both a radial symmetric SPP excitation and a Fabry-Pérot resonance, and experimental results indicate an optical resolution of 10 nm, a topographic resolution of 10 nm, a throughput of 3.28%, and an outstanding SNR of up to 18.2 (nearly free of background). The demonstrated p-tip outperforms state-of-the-art NSOM tips and can be readily employed in near-field optics, nanolithography, tip-enhanced Raman spectroscopy, and other applications.

Multiple hydrogen bonds tuning guest/host excited-state proton transfer reaction: its application in molecular recognition.

A molecular recognition concept exploiting multiple-hydrogen-bond fine-tuned excited-state proton-transfer (ESPT) was conveyed using 3,4,5,6-tetrahydrobis(pyrido[3,2-g]indolo)[2,3-a:3',2'-j]acridine (1a). The catalytic type 1a/carboxylic acids hydrogen-bonding (HB) complexes undergo ultrafast ESPT, resulting in an anomalously large Stokes shifted tautomer emission (lambdamax approximately 600 nm). Albeit forming a quadruple HB complex, ESPT is prohibited in the noncatalytic-type 1a/urea complexes (lambdamax approximately 430 nm). The HB configuration tuning ESPT properties lead to a feasible design for sensing multiple-HB-site analytes of biological interest.

2,7-Bis(1H-pyrrol-2-yl)ethynyl-1,8-naphthyridine: an ultrasensitive fluorescent probe for glucopyranoside.

[structure: see text] A push-pull conjugated molecule, 2,7-bis(1H-pyrrol-2-yl)ethynyl-1,8-naphthyridine (BPN), has been designed to bind selectively with octyl glucopyranoside (OGU). The BPN/OGU quadruple hydrogen-bonding complex adopts a rigid BPN conformation in which the proton donor (d) and acceptor (a) relays (daad) are resonantly conjugated through the ethynyl bridge, inducing pi-electron delocalization, i.e., a charge transfer effect. The excellent photophysical properties make BPN a highly sensitive probe for monitoring glucopyranoside to a detection limit of approximately 100 pM.