Self-Assembly of "Chalcone" Type Push-Pull Dye Molecules into Organic Single Crystalline Microribbons and Rigid Microrods for Vis/NIR Range Photonic Cavity Applications.

A novel supramolecular fluorescent donor-acceptor type dye molecule, (2E,4E)-1-(2-hydroxyphenyl)-5-(pyren-1-yl)penta-2,4-dien-1-one (HPPD) self-assembles in a mixture of ethanol/chloroform through intermolecular π-π stacking (distance ca. 3.384 Å) to form J-aggregated single-crystalline microribbons displaying Fabry-Pèrot (F-P) type visible-range optical resonance. The corresponding borondifluoride dye (HPPD-BF), with a reduced HOMO-LUMO gap, self-assembles into crystalline microrods acting as an F-P type resonator in the near-infrared (NIR) range.

Single-particle to single-particle transformation of an active type organic µ-tubular homo-structure photonic resonator into a passive type hetero-structure resonator.

Self-assembled hexagonal organic submicrotubes, upon electronic excitation with an UV laser, display an active type polarized whispering gallery mode (WGM) resonance in the visible (Vis) range (400-600 nm). Due to the photonic cavity effect the tubes show fluorescence (FL) signal intensity 5× greater than the corresponding powder state. Furthermore, the same tubes, which are passive to a visible laser, produce yellow-orange emitting carbonaceous lumps when burnt with an intense laser beam (42 mW) forming a chemically binary heterogeneous structure. The hetero-structure upon excitation with a visible laser at the passive tubular part showed emission in the Vis-Near infrared (NIR) range (500-800 nm) with WGMs thus producing a passive/active type hetero-structure photonic resonator.

Nanoironing van der Waals Heterostructures toward Electrically Controlled Quantum Dots.

Assembling two-dimensional van der Waals (vdW)-layered materials into heterostructures is an exciting development that sparked the discovery of rich correlated electronic phenomena. vdW heterostructures also offer possibilities for designer device applications in areas such as optoelectronics, valley- and spintronics, and quantum technology. However, realizing the full potential of these heterostructures requires interfaces with exceptionally low disorder which is challenging to engineer. Here, we show that thermal scanning probes can be used to create pristine interfaces in vdW heterostructures. Our approach is compatible at both the material- and device levels, and monolayer WS2 transistors show up to an order of magnitude improvement in electrical performance from this technique. We also demonstrate vdW heterostructures with low interface disorder enabling the electrical formation and control of quantum dots that can be tuned from macroscopic current flow to the single-electron tunneling regime.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications.

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Liquid-Metal-Printed Ultrathin Oxides for Atomically Smooth 2D Material Heterostructures.

Two-dimensional (2D) semiconductors are promising channel materials for continued downscaling of complementary metal-oxide-semiconductor (CMOS) logic circuits. However, their full potential continues to be limited by a lack of scalable high-k dielectrics that can achieve atomically smooth interfaces, small equivalent oxide thicknesses (EOTs), excellent gate control, and low leakage currents. Here, large-area liquid-metal-printed ultrathin Ga2O3 dielectrics for 2D electronics and optoelectronics are reported. The atomically smooth Ga2O3/WS2 interfaces enabled by the conformal nature of liquid metal printing are directly visualized. Atomic layer deposition compatibility with high-k Ga2O3/HfO2 top-gate dielectric stacks on a chemical-vapor-deposition-grown monolayer WS2 is demonstrated, achieving EOTs of ∼1 nm and subthreshold swings down to 84.9 mV/dec. Gate leakage currents are well within requirements for ultrascaled low-power logic circuits. These results show that liquid-metal-printed oxides can bridge a crucial gap in dielectric integration of 2D materials for next-generation nanoelectronics.

Mechanophotonics: precise selection, assembly and disassembly of polymer optical microcavities via mechanical manipulation for spectral engineering.

The advancement of nanoscience and technology relies on the development and utility of innovative techniques. Precise manipulation of photonic microcavities is one of the fundamental challenges in nanophotonics. This challenge impedes the construction of optoelectronic and photonic microcircuits. As a proof-of-principle, we demonstrate here that an atomic force microscopy cantilever and confocal microscopy can be used together to mechanically micromanipulate polymer-based whispering gallery mode microcavities or microresonators into well-ordered geometries. The micromanipulation technique efficiently assembles or disassembles resonators and also produces well-ordered dimer, trimer, tetramer, and pentamer assemblies of resonators in linear and bent geometries. Interestingly, an intricate L-shaped coupled-resonator optical waveguide (CROW) comprising a pentamer assembly effectively transduces light through a 90° bend angle. The presented new research direction, which combines mechanical manipulation and nanophotonics, is also expected to open up a plethora of opportunities in nano and microstructure-based research areas including nanoelectronics and nanobiology.

Photonic Microresonators from Charge Transfer in Polymer Particles: Toward Enhanced and Tunable Two-Photon Emission.

Novel photonic microresonators with enhanced nonlinear optical (NLO) intensity are fabricated from polymer particles. As an additional advantage, they offer band gap tunability from the visible to near-infrared regions. A special protocol including (i) copolymerization of 4-(1-pyrenyl)-styrene, styrene, and 1,4-divinylbenzene, (ii) extraction of a dispersible and partly dissolvable, lightly cross-linked polymer network (PN), and (iii) treatment of the blue-emitting PN with electron acceptor (A) molecules such as 1,2,4,5-tetracyanobenzene (TCNB) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) furnishes orange- and red-emitting D-A charge-transfer (CT) complexes with the pendant pyrene units. These complexes, here named PN-TCNB and PN-TCNQ, respectively, precipitate as microparticles upon the addition of water and subsequent ultrasonication. Upon electronic excitation, these spherical microparticles act as whispering-gallery-mode resonators by displaying optical resonances in the photoluminescence (PL) spectra because of light confinement. Further, the trapped incident light increases the light-matter interaction and thereby enhances the PL intensity, including the two-photon luminescence. The described protocol for polymer-based CT microresonators with tunable NLO emissions holds promise for a myriad of photonic applications.

Two-Photon Luminescence and Second-Harmonic Generation in Organic Nonlinear Surface Comprised of Self-Assembled Frustum Shaped Organic Microlasers.

An ultrathin nonlinear optical (NLO) organic surface composed of numerous self-assembled frustum-shaped whispering-gallery-mode resonators displays both two-photon luminescence and second-harmonic-generation signals. A super-second-order increase of the NLO intensity with respect to pump power confirms the microlasing action and practical usefulness of the NLO organic surfaces.

Visible-Near-Infrared Range Whispering Gallery Resonance from Photonic µ-Sphere Cavities Self-Assembled from a Blend of Polystyrene and Poly[4,7-bis(3-octylthiophene-2-yl)benzothiadiazole-co-2,6-bis(pyrazolyl)pyridine] Coordinated to Tb(acac)3.

A novel red emitting copolymer (P1) was prepared (Mn ∼ 10.7 kDa) by copolymerizing tridentate ligand, namely 2,6-bis(pyrazolyl)pyridine (BPP) with 4,7-bis(2-ethynyl-5-thienyl)-2,1,3-benzothiadiazole. This copolymer readily formed an orange yellow emitting metal containing conjugated polymer (P1.Tb) with Tb(acac)3. Further, a judicial blend of P1.Tb with polystyrene and its subsequent self-assembly in THF/water produced microspheres with smooth surface area. Interestingly, continuous wave laser excitation of a single microsphere displayed whispering-gallery-mode (WGM) resonance modes over a broad wavelength range covering visible (Vis) and near-infrared (NIR) regions (0.550-0.875 µm). The estimated Q factor was up to 700, which is very high for a metal containing conjugated polymer (MCCP)-based optical gain medium.