Controlled Self-Assembly of Organic Microcrystals for Laser Applications

The small organic molecule <i>p</i>-distyrylbenzene (DSB) has been controllably prepared into one-dimensional microwires (1D-MWs) and 2D rhombic microdisks (2D-RMDs) by modulating the growth kinetics in the process of morphology growth. These as-prepared organic microcrystals, 1D-MWs and 2D-RMDs, exhibit a shape-dependent microcavity effect in that the single 1D-MW can act as a Fabry-Pérot (FP) mode lasing resonator while the individual 2D-RMD functions as the whispering-gallery-mode (WGM) microcavity. Moreover, as compared with the 1D FP resonators, there exists a higher quality factor (<i>Q</i>) in the WGM lasing resonator under the identical optical path length. Significantly, the lasing threshold, <i>E</i>_th = 1.02 μJ/cm², of 2D-RMDs is much lower than <i>E</i>_th = 2.57 μJ/cm² of 1D-MWs. Our demonstration can give the direction for the development of the organic solid-state microlasers

Epitaxial Self-assembly of Binary Molecular Components into Branched Nanowire Heterostructures for Photonic Applications

We report a sequential epitaxial growth to prepare organic branched nanowire heterostructures (BNwHs) consisting of a microribbon trunk of 1,4-dimethoxy-2,5-di­[4′-(cyano)­styryl]­benzene (COPV) with multiple nanowire branches of 2,4,5-triphenylimidazole (TPI) in a one-pot solution synthesis. The synthesis involves a seeded-growth process, where COPV microribbons are grown first as a trunk followed by a seeded-growth of TPI nanowire branches at the pregrown trunk surfaces. Selected area electron diffraction characterizations reveal that multiple hydrogen-bonding interactions between TPI and COPV components play an essential role in the epitaxial growth as a result of the structural matching between COPV and TPI crystals. A multichannel optical router was successfully realized on the basis of the passive waveguides of COPV green photoluminescence (PL) along TPI nanowire branches in a single organic BNwH

Colorimetric Signal Amplification Assay for Mercury Ions Based on the Catalysis of Gold Amalgam

Mercury is a major threat to the environment and to human health. It is highly desirable to develop a user-friendly kit for on-site mercury detection. Such a method must be able to detect mercury below the threshold levels (10 nM) for drinking water defined by the U.S. Environmental Protection Agency. Herein, we for the first time reported catalytically active gold amalgam-based reaction between 4-nitrophenol and NaBH₄ with colorimetric sensing function. We take advantage of the correlation between the catalytic properties and the surface area of gold amalgam, which is proportional to the amount of the gold nanoparticle (AuNP)-bound Hg²⁺. As the concentration of Hg²⁺ increases until the saturation of Hg onto the AuNPs, the catalytic performance of the gold amalgam is much stronger due to the formation of gold amalgam and the increase of the nanoparticle surface area, leading to the decrease of the reduction time of 4-nitrophenol for the color change. This sensing system exhibits excellent selectivity and ultrahigh sensitivity up to the 1.45 nM detection limit. The practical use of this system for Hg²⁺ determination in tap water samples is also demonstrated successfully

Organic–Inorganic Hybrid Perovskite Nanowire Laser Arrays

Fabrication of semiconductor nanowire laser arrays is very challenging, owing to difficulties in direct monolithic growth and patterning of III–V semiconductors on silicon substrates. Recently, methylammonium lead halide perovskites (MAPbX₃, X = Cl, Br, I) have emerged as an important class of high-performance solution-processed optoelectronic materials. Here, we combined the “top–down” fabricated polydimethylsiloxane rectangular groove template (RGT) with the “bottom-up” solution self-assembly together to prepare large-scale perovskite nanowire (PNW) arrays. The template confinement effect led to the directional growth of MAPbX₃ along RGTs into PNWs. We achieved precise control over not only the dimensions of individual PNWs (width 460–2500 nm; height 80–1000 nm, and length 10–50 μm) but also the interwire distances. Well-defined dimensions and uniform geometries enabled individual PNWs to function as high-quality Fabry–Perot nanolasers with almost identical optical modes and similarly low-lasing thresholds, allowing them to ignite simultaneously as a laser array. Optical tests demonstrated that PNW laser arrays exhibit good photostabillity with an operation duration exceeding 4 × 10⁷ laser pulses. Precise placement of PNW arrays at specific locations makes our method highly compatible with lithographic techniques, which are important for integrating PNW electronic and photonic circuits

Cluster-Mediated Nucleation and Growth of J- and H‑Type Polymorphs of Difluoroboron Avobenzone for Organic Microribbon Lasers

Controlled fabrication of organic polymorphisms with well-defined dimensions and tunable luminescent properties plays an important role in developing optoelectronic devices, sensors, and biolabeling agents but remains a challenge due to the weak intermolecular interactions among organic molecules. Herein, we developed a two-step solution self-assembly method for the controlled preparation of blue-emissive or green-emissive microribbons (MRs) of difluoroboron avobenzone (BF₂AVB) by adjusting the cluster-mediated nucleation and subsequent one-dimensional growth processes. We found that blue-emissive MRs belong to the monoclinic phase, in which BF₂AVB molecules form slipped π-stacks, resulting in J-aggregates with the solid-state photoluminescence efficiency φ = 68%. Meanwhile, green-emissive MRs are ascribed to the orthorhombic phase and exhibit cofacial π-stacks, which lead to H-aggregates with φ = 24%. Furthermore, these as-prepared MRs can both act as polymorph-dependent Fabry–Pérot resonators for lasing oscillators. The strategy described here might offer significant promise for the coherent light source of optoelectronic devices

Exciton-Polaritons with Size-Tunable Coupling Strengths in Self-Assembled Organic Microresonators

Self-assembled nano/microcrystals of organic semiconductors with regular faces can serve as optical microresonators, which hold a promise for studying the light confinement and the light-matter interaction. Here, single crystalline microribbons of 1,4-bis­(2-(4-(<i>N</i>,<i>N</i>-di­(<i>p</i>-tolyl)­amino)­phenyl)-vinylbenzene (DPAVB) are synthesized with well-controlled sizes by a facile solution-exchange method. We find that individual microribbon can work as Fabry-Pérot (FP) resonator along its width (<i>w</i>), in which strong coupling of optical modes with excitons results in the formation of exciton polaritons (EPs). The dispersion relation of <i>E</i> ∼ <i>k</i>_<i>z</i> of EPs is constructed by extracting the energies (<i>E</i>) of FP resonances at integer multiples of π/<i>w</i> in the wavevector (<i>k</i>_<i>z</i>) space. By simulating the significantly curved dispersion of EPs with a two coupled harmonic oscillator model, a coupling strength between 0.48 and 1.09 eV are obtained. Two coupling regimes are classified: in regime I, the coupling strength is constant at 0.48 eV for microribbons with the cavity length of <i>w</i> ≥ 2.00 μm; in regime II, the coupling strength increases dramatically from 0.48 to about 1 eV with decreasing the resonator length from <i>w</i> = 2.00 to 0.83 μm. More significantly, our results suggest that the exciton-photon coupling strength could be modulated by varying the size of microribbon cavities, providing an effective method for engineering the light–matter interaction in organic single crystalline microstructures

Organic Phosphorescence Nanowire Lasers

Organic solid-state lasers (OSSLs) based on singlet fluorescence have merited intensive study as an important class of light sources. Although the use of triplet phosphors has led to 100% internal quantum efficiency in organic light-emitting diodes (OLEDs), stumbling blocks in triplet lasing include generally forbidden intersystem crossing (ISC) and a low quantum yield of phosphorescence (Φ_P). Here, we reported the first triplet-phosphorescence OSSL from a nanowire microcavity of a sulfide-substituted difluoroboron compound. As compared with the unsubstituted parent compound, the lone pair of electrons of sulfur substitution plus the intramolecular charge transfer interaction introduced by the nitro moiety lead to an highly efficient T₁ (π,π*) ← S₁ (n,π*) ISC (Φ_ISC = 100%) and a moderate Φ_P (10%). This, plus the optical feedback provided by nanowire Fabry–Perot microcavity, enables triplet-phosphorescence OSSL emission at 650 nm under pulsed excitation. Our results open the door for a whole new class of laser materials based on previously untapped triplet phosphors

Shape-Dependent Optical Waveguides and Low-Threshold Lasers from Polymorphic Two-Dimensional Organic Single Crystals

Organic single crystals (OSCs) with uniform morphologies and highly ordered molecular aggregations are promising for high-performance optoelectronic devices, such as organic solid-state lasers (OSSLs), organic light-emitting transistors (OLETs), and organic light-emitting diodes (OLEDs). However, manipulating OSC morphologies and aggregation is challenging. In this study, we synthesized two-dimensional (2D) OSCs of 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) in hexagonal and parallelogram microplate (H-MP and P-MP) forms. Both types exhibit H-aggregation in the 2D plate plane but with different molecular transition dipole moment (TDM) orientations. This leads to different photon coupling modes with H-MP and P-MP microcavities. H-MPs enable isotropic 2D-waveguiding, forming whispering gallery mode (WGM) resonators, while P-MPs create unidirectional waveguiding, forming Fabry-Pérot mode (FP) resonators. These resonators can generate low-threshold laser emissions at 467 and 473 nm, respectively, and exhibit superior lasing stability with a half-life exceeding 2 h. Our BSBCz microplate OSCs are attractive candidates to combine controlled organic microcavities with photon transporting for realizing future integrated optoelectronic devices

Tuning the Doping Types in Graphene Sheets by N Monoelement

The doping types of graphene sheets are generally tuned by different dopants with either three or five valence electrons. As a five-valence-electrons element, however, nitrogen dopants in graphene sheets have several substitutional geometries. So far, their distinct effects on electronic properties predicted by theoretical calculations have not been well identified. Here, we demonstrate that the doping types of graphene can be tuned by N monoelement under proper growth conditions using chemical vapor deposition (CVD), characterized by combining scanning tunneling microscopy/spectroscopy, X-ray/ultraviolet photoelectron spectroscopy, Hall effect measurement, Raman spectroscopy, and density functional theory calculations. We find that a relatively low partial pressure of CH₄ (mixing with NH₃) can lead to the growth of dominant pyridinic N substitutions in graphene, in contrast with the growth of dominant graphitic N substitutions under a higher partial pressure of CH₄. Our results unambiguously confirm that the pyridinic N leads to the p-type doping, and the graphitic N leads to the n-type doping. Interestingly, we also find that the pyridinic N and the graphitic N are preferentially separated in different domains. Our findings shed light on continuously tuning the doping level of graphene monolayers by using N monoelement, which can be very convenient for growth of functional structures in graphene sheets

Near-Infrared Lasing from Small-Molecule Organic Hemispheres

Near-infrared (NIR) lasers are key components for applications, such as telecommunication, spectroscopy, display, and biomedical tissue imaging. Inorganic III–V semiconductor (GaAs) NIR lasers have achieved great successes but require expensive and sophisticated device fabrication techniques. Organic semiconductors exhibit chemically tunable optoelectronic properties together with self-assembling features that are well suitable for low-temperature solution processing. Major blocks in realizing NIR organic lasing include low stimulated emission of narrow-bandgap molecules due to fast nonradiative decay and exciton–exciton annihilation, which is considered as a main loss channel of population inversion for organic lasers under high carrier densities. Here we designed and synthesized the small organic molecule (<i>E</i>)-3-(4-(di-<i>p</i>-tolylamino)­phenyl)-1-(1-hydroxynaphthalen-2-yl)­prop-2-en-1-one (DPHP) with amphiphilic nature, which elaborately self-assembles into micrometer-sized hemispheres that simultaneously serves as the NIR emission medium with a photoluminescence quantum efficiency of ∼15.2%, and the high-<i>Q</i> (∼1.4 × 10³) whispering gallery mode microcavity. Moreover, the radiative rate of DPHP hemispheres is enhanced up to ∼1.98 × 10⁹ s^–1 on account of the exciton-vibrational coupling in the solid state with the J-type molecular-coupling component, and meanwhile the exciton–exciton annihilation process is eliminated. As a result, NIR lasing with a low threshold of ∼610 nJ/cm² is achieved in the single DPHP hemisphere at room temperature. Our demonstration is a major step toward incorporating the organic coherent light sources into the compact optoelectronic devices at NIR wavelengths