Lattice-Matched Epitaxial Growth of Organic Heterostructures for Integrated Optoelectronic Application

Epitaxial Growth of Two-Dimensional Organic Crystals with In-Plane Heterostructured Domain Regulation

Complex organic lateral heterostructures (OLHs) with spatial distribution of two or more chemical components are crucial for designing and realizing unique structure-dependent optoelectronic applications. However, the precise design of well-defined OLHs with flexible domain regulation remains a considerable challenge. Herein, we present a stepwise solution self-assembly method to synthesize two-dimensional (2D) OLHs with a central rhombus domain and a lateral region featuring tunable blue and green emission based on the sequential nucleation and growth of 2D crystals. By controlling the initial crystallization time of 2,6-diphenylanthracene, the rhombic length ratio (α) of the multicolor-emissive part of the 2D OLHs is precisely modified. Furthermore, a third lateral layer is constructed on the resulting OLHs, demonstrating scalable lateral regulation. Significantly, these prepared 2D OLHs exhibit great excitation position-dependent waveguide characteristics and enable a 0.06 dB/μm low-loss waveguiding, which are conducive to photon transport and conversion for photonic integrated circuits. This work provides a stepwise strategy for the accurate fabrication of 2D OLHs, fabricating the developments of next-generation optoelectronics devices.

Capturing the Interplay Between TADF and RTP Through Mechanically Flexible Polymorphic Optical Waveguides

Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TPA-CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM-tip-based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal-based photonic circuit technologies.

Hierarchical Integration of Organic Core/Shell Microwires for Advanced Photonics

The combination of multiple components or structures into integrated micro/nanostructures for practical application has been pursued for many years. Herein, a series of hierarchical organic microwires with branch, core/shell (C/S), and branch C/S structures are successfully constructed based on organic charge transfer (CT) cocrystals with structural similarity and physicochemical tunability. By regulating the intermolecular CT interaction, single microwires and branch microstructures can be integrated into the C/S and branch C/S structures, respectively. Significantly, the integrated branch C/S microwires, with multicolor waveguide behavior and branch structure multichannel waveguide output characteristics, can function as an optical logic gate with multiple encoding features. This work provides useful insights for creating completely new types of organic microstructures for integrated optoelectronics.

A new strategy: realization of organic heteroepitaxy and organic alloys based on the similarity of CC and NN

The similarity of NN and CC in trans-4,4′-azobis(pyridine) and trans-1,2-bis(pyridin-4-yl)ethene offers an innovative approach for creating controllable and versatile organic heterostructure and organic alloy.

Organic Branched Heterostructures with Optical Interconnects for Photonic Barcodes

Optical interconnects exhibit superior potential in the precise regulation of photon transmission for organic photonic circuits. However, the rational design of well-defined organic heterostructures toward active optoelectronics remains challenging. Herein, we designed organic branched heterostructures (OBHs) with accurate spatial organization for optical interconnection. Notably, the precise regulation of OBHs has been controllably achieved including the trunk morphologies and the branched microwire number. Significantly, these as-prepared OBHs inherently exhibit the multichannel coupling outputs and the excitation position-dependent waveguide characteristics, leading to various outcoupling signals with tunable intensity and emission colors. The optical interconnects are realized due to the occurrence of exciton conversion and photon propagation between branch and trunk at the heterojunction, benefiting the application possibilities of two-dimensional (2D) optical barcodes.

Micromechanically-Powered Rolling Locomotion of a Twisted-Crystal Optical-Waveguide Cavity as a Mobile Light Polarization Rotor

We demonstrate mechanically-powered rolling locomotion of a twisted-microcrystal optical-waveguide cavity on the substrate, rotating the output signal's linear-polarization. Self-assembly of (E)-2-bromo-6-(((4-methoxyphenyl)imino)methyl)-4-nitrophenol produces naturally twisted microcrystals. The strain between several intergrowing, orientationally mismatched nanocrystalline fibres dictates the pitch lengths of the twisted crystals. The crystals are flexible, perpendicular to twisted (001) and (010) planes due to π⋅⋅⋅π stacking, C-H⋅⋅⋅Br, N-H⋅⋅⋅O and C-H⋅⋅⋅O interactions. The twisted crystals in their straight and bent geometries guide fluorescence along their body axes and display optical modes. Depending upon the degree of mechanical rolling locomotion, the crystal-waveguide cavity correspondingly rotates the output signal polarization. The presented twisted-crystal cavity with a combination of mechanical locomotion and photonic attributes unfolds a new dimension in mechanophotonics.

Hyperbranched Microwire Networks of Organic Cocrystals with Optical Waveguiding and Light-Harvesting Abilities

We report the synthesis of hyperbranched organic microwire (MW) networks comprising 1,4-bis(pentafluorostyryl)benzene (10Ft) and 9,10-bis(phenylethynyl)anthracene (BA) using a simple solution co-assembly route. Pure 10Ft or BA assemblies cannot produce such complex MW networks; in contrast with a binary cocrystal of 10Ft and BA with a 2:1 molar ratio ((2:1)10Ft:BA), which is formed via intermolecular arene-perfluoroarene (AP) interactions. A new generation of multiple MWs grow epitaxially on the previous generation of MWs to form MW arrays in which BA may also act as an intermediate product to facilitate the regeneration of (2:1)10Ft:BA. Highly aligned and well-connected MW networks enable superior optical waveguiding ability. Moreover, a red-emitting dopant, 5,12-bis(phenylethynyl)naphthacene (BN) was incorporated into (2:1)10Ft:BA host MWs, giving light-harvesting hierarchical MW networks via an energy-transfer (ET) process. The realization of the hyperbranched MWs provides us with deep insight into the rational creation of complex branched arrays from functional organic cocrystals.

Polarised Optical Emission from Organic Anisotropic Microoptical Waveguides Grown by Ambient Pressure Vapour-deposition

Ambient pressure chemical vapour deposition of 5,5'-bis((2-(trifluoromethyl)phenyl)ethynyl)-2,2'-bithiophene provides ultrapure needle-shaped crystals. The crystal's supramolecular structure consists of an array of hydrogen bonds and π-π interactions leading to anisotropic arrangements. The cyan emitting crystals exhibit an optical waveguiding tendency with guided polarised optical emissions due to anisotropic molecular arrangements.

Mechanophotonics-Mechanical Micromanipulation of Single-Crystals toward Organic Photonic Integrated Circuits

The advent of molecular crystals as "smart" nanophotonic components namely, organic waveguides, resonators, lasers, and modulators are drawing wider attention of solid-state materials scientists and microspectroscopists. Crystals are usually rigid, and undeniably developing next-level crystalline organic photonic circuits of complex geometries demands using mechanically flexible crystals. The mechanical shaping of flexible crystals necessitates applying challenging micromanipulation methods. The rise of atomic force microscopy as a mechanical micromanipulation tool has increased the scope of mechanophotonics and subsequently, crystal-based microscale organic photonic integrated circuits (OPICs). The unusual higher adhesive energy of the flexible crystals to the surface than that of crystal shape regaining energy enables carving intricate crystal geometries using micromanipulation. This perspective reviews the progress made in a key research area developed by my research group, namely mechanophotonics-a discipline that uses mechanical micromanipulation of single-crystal optical components, to advance nanophotonics. The precise fabrication of photonic components and OPICs from both rigid and flexible microcrystal via AFM mechanical operations namely, moving, lifting, cutting, slicing, bending, and transferring of crystals are presented. The ability of OPICs to guide, split, couple, and modulate visible electromagnetic radiation using passive, active, and energy transfer mechanism are discussed as well with recent literature examples.

Organic multicomponent microparticle libraries

Multimetallic nanostructures can be synthesized by integrating up to seven or eight metallic elements into a single nanoparticle, which represent a great advance in developing complex multicomponent nanoparticle libraries. Contrary, organic micro- and nanoparticles beyond three π-conjugated components have not been explored because of the diversity and structural complexity of molecular assemblies. Here, we report a library of microparticles composed of an arbitrary combination of four luminescent organic semiconductors. We demonstrate that the composition and emission color of each domain as well as its spatial distribution can be rationally modulated. Unary, binary, ternary, and quaternary microparticles are thus realized in a predictable manner based on the miscibility of the components, resulting in mixed-composition phases or alloyed or phase separated heterostructures. This work reports a simple yet available synthetic methodology for rational modulation of organic multicomponent microparticles with complex architectures, which can be used to direct the design of functional microparticles.

The synthesis of alloyed complex multicomponent micro- and nanoparticles composed of organic molecules is challenging. Here, the authors demonstrate a library of mixed composition organic microparticles in comprising up to four compounds in their structure.

Mechanical Processing of Naturally Bent Organic Crystalline Microoptical Waveguides and Junctions

Precise mechanical processing of optical microcrystals involves complex microscale operations viz. moving, bending, lifting, and cutting of crystals. Some of these mechanical operations can be implemented by applying mechanical force at specific points of the crystal to fabricate advanced crystalline optical junctions. Mechanically compliant flexible optical crystals are ideal candidates for the designing of such microoptical junctions. A vapor-phase growth of naturally bent optical waveguiding crystals of 1,4-bis(2-cyanophenylethynyl)benzene (1) on a surface forming different optical junctions is presented. In the solid-state, molecule 1 interacts with its neighbors via CH⋅⋅⋅N hydrogen bonding and π-π stacking. The microcrystals deposited at a glass surface exhibit moderate flexibility due to substantial surface adherence energy. The obtained network crystals also display mechanical compliance when cut precisely with sharp atomic force microscope cantilever tip, making them ideal candidates for building innovative T- and Δ-shaped optical junctions with multiple outputs. The presented micromechanical processing technique can also be effectively used as a tool to fabricate single-crystal integrated photonic devices and circuits on suitable substrates.

Mechanophotonics: Flexible Single-Crystal Organic Waveguides and Circuits

We present the one-dimensional optical-waveguiding crystal dithieno[3,2-a:2',3'-c]phenazine with a high aspect ratio, high mechanical flexibility, and selective self-absorbance of the blue part of its fluorescence (FL). While macrocrystals exhibit elasticity, microcrystals deposited at a glass surface behave more like plastic crystals due to significant surface adherence, making them suitable for constructing photonic circuits via micromechanical operation with an atomic-force-microscopy cantilever tip. The flexible crystalline waveguides display optical-path-dependent FL signals at the output termini in both straight and bent configurations, making them appropriate for wavelength-division multiplexing technologies. A reconfigurable 2×2-directional coupler fabricated via micromanipulation by combining two arc-shaped crystals splits the optical signal via evanescent coupling and delivers the signals at two output terminals with different splitting ratios. The presented mechanical micromanipulation technique could also be effectively extended to other flexible crystals.

Hierarchical Self-assembly of Well-Defined Louver-Like P-Doped Carbon Nitride Nanowire Arrays with Highly Efficient Hydrogen Evolution

Self-assembled nanostructure arrays integrating the advantages of the intrinsic characters of nanostructure as well as the array stability are appealing in advanced materials. However, the precise bottom-up synthesis of nanostructure arrays without templates or substrates is quite challenging because of the general occurrence of homogeneous nucleation and the difficult manipulation of noncovalent interactions. Herein, we first report the precisely manipulated synthesis of well-defined louver-like P-doped carbon nitride nanowire arrays (L-PCN) via a supramolecular self-assembly method by regulating the noncovalent interactions through hydrogen bond. With this strategy, CN nanowires align in the outer frame with the separation and spatial location achieving ultrastability and outstanding photoelectricity properties. Significantly, this self-assembly L-PCN exhibits a superior visible light-driven hydrogen evolution activity of 1872.9 μmol h-1 g-1, rendering a ~ 25.6-fold enhancement compared to bulk CN, and high photostability. Moreover, an apparent quantum efficiency of 6.93% is achieved for hydrogen evolution at 420 ± 15 nm. The experimental results and first-principles calculations demonstrate that the remarkable enhancement of photocatalytic activity of L-PCN can be attributed to the synergetic effect of structural topology and dopant. These findings suggest that we are able to design particular hierarchical nanostructures with desirable performance using hydrogen-bond engineering.

Hierarchical self-assembly of organic heterostructure nanowires

Organic heterostructures (OHSs) integrating the intrinsic heterostructure characters as well as the organic semiconductor properties have attracted intensive attention in material chemistry. However, the precise bottom-up synthesis of OHSs is still challenging owing to the general occurrence of homogeneous-nucleation and the difficult manipulation of noncovalent interactions. Herein, we present the rational synthesis of the longitudinally/horizontally-epitaxial growth of one-dimensional OHSs including triblock and core/shell nanowires with quantitatively-manipulated microstructure via a hierarchical self-assembly method by regulating the noncovalent interactions: hydrogen bond (−15.66 kcal mol⁻¹) > halogen bond (−4.90 kcal mol⁻¹) > π-π interaction (−0.09 kcal mol⁻¹). In the facet-selective epitaxial growth strategy, the lattice-matching and the surface-interface energy balance respectively facilitate the realization of triblock and core/shell heterostructures. This hierarchical self-assembly approach opens up avenues to the fine synthesis of OHSs. We foresee application possibilities in integrated optoelectronics, such as the nanoscale multiple input/out optical logic gate with high-fidelity signal.

Organic heterostructures attract attention in material chemistry but the precise bottom-up synthesis is still challenging. Herein the authors present a hierarchical self-assembly approach to synthesize one-dimensional organic heterostructures by regulating the noncovalent interactions.

Epitaxial growth of dual-color-emitting organic heterostructures via binary solvent synergism driven sequential crystallization

The controlled construction of organic heterostructured architectures derived from molecules with similar nucleation thresholds and concentrations has been rare and remains a great challenge. Herein, we report a sequential epitaxial growth to synthesize dual-color-emitting organic heterostructures with 9,10-bis(phenylethynyl)anthracene (BPEA) microwire trunks and tris-(8-hydroxyquinoline)aluminium (Alq3) microstructure branches by an anti-solvent induced sequential crystallization strategy. During the epitaxial growth process, the hydrogen-bonding interactions of the anti-solvent and solvent cause a large change in the solubility and crystallization rate of BPEA and Alq3 molecules in the mixed system, which facilitates sequential crystallization of organic molecule pairs with similar nucleation thresholds and concentrations into desired heterostructures by manipulating the synergism of anti-solvents and solvents. The Förster resonant energy transfer process in heterostructures could be modulated by varying the structure of heterostructures, such as the shape, amount and angles of the branches. The present synthesis strategy provides a unique insight into the detailed formation mechanism of complex organic heterostructures, further guiding the construction of more functional heterostructure materials.

In Situ Construction of One-Dimensional Component-Interchange Organic Core/Shell Microrods for Multicolor Continuous-Variable Optical Waveguide

The core/shell micro-/nanostructures with versatility, tunability, stability, dispersibility, and biocompatibility are widely applied in optics, biomedicine, catalysis, and energy. Organic micro-/nanocrystals have significant applications in miniaturized optoelectronics because of their controllable self-assembly behavior, tunable optical properties, and tailor-made molecular structure. Nevertheless, the advanced organic core/shell micro-/nanostructures, which possess multifunctionality, flexibility, and higher compatibility, are rarely demonstrated because of the dynamic nature of molecular self-assembly and the complex epitaxial relationship of material combination. Herein, we demonstrate the one-dimensional organic core/shell micro-/nanostructures with component interchange, which originates from the 4,4'-((1 E,1' E)-(2,5-dimethoxy-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine (DPEpe) single-crystal microrods or the DPEpe-HCl single-crystal microrods after a reversible protonation or deprotonation process. Notably, the DPEpe/DPEpe-HCl core/shell microrods display vivid visualizations of tunable emission color via an efficient energy-transfer process during the stepwise formation of a shell layer. More significantly, these DPEpe/DPEpe-HCl and DPEpe-HCl/DPEpe core/shell microrods cooperatively demonstrate the multicolor optical waveguide properties continuously adjusted from green [CIE (0.326, 0.570)], to yellow [CIE (0.516, 0.465)], and to red [CIE (0.614, 0.374)]. Our investigation provides a new strategy to fabricate the organic core/shell micro-/nanostructures, which can eventually contribute to the advanced organic optoelectronics at the micro-/nanoscale.

Light-Harvesting Organic Nanocrystals Capable of Photon Upconversion

Harvesting and converting low energy photons into higher ones through upconversion have great potential in solar energy conversion. A light-harvesting nanocrystal assembled from 9,10-distyrylanthracene and palladium(II) meso-tetraphenyltetrabenzoporphyrin as the acceptor and the sensitizer, respectively effects red-to-green upconversion under incoherent excitation of low power density. An upconversion quantum yield of 0.29±0.02 % is obtained upon excitation with 640 nm laser of 120 mW cm^-2 . The well-organized packing of acceptor molecules with aggregation-induced emission in the nanocrystals dramatically reduces the nonradiative decay of the excited acceptor, benefits the triplet-triplet annihilation (TTA) upconversion and guides the consequent upconverted emission. This work provides a straightforward strategy to develop light-harvesting nanocrystals based on TTA upconversion, which is attractive for energy conversion and photonic applications.

Two-Dimensional Seeded Self-Assembly of a Complex Hierarchical Perylene-Based Heterostructure

A complex two-dimensional (2D) hierarchical heterostructure was fabricated by a sequential two-dimensional seeded self-assembly, which consisted of laterally grown nanotubes from one perylene monomer and terminally elongated nanocoils from a similar perylene monomer on microribbon seeds from a third perylene. Because the nanotube and nanocoil monomers can form kinetically trapped off-pathway aggregates to prevent self-nucleation and have similar molecular organizations to different facets of the seeds, the nanotube and nanocoil monomers preferentially nucleate and grow on the seed sides and terminal ends, respectively, to form a complex 2D hierarchical heterostructure. The strategy used in this work can be extended to fabricate other complex nanoarchitectures from small molecules.