Hongbing Fu

Solvent-assisted self-assembly of fullerene into single-crystal ultrathin microribbons as highly sensitive UV-visible photodetectors.

The size, shape, and crystallinity of organic nanostructures play an important role in their physical properties and are mainly determined by the self-assembling kinetics of molecular components often involving the solvent conditions. Here, we reported a kinetically controlled self-assembly of C60 assisted by the solvent carbon bisulfide (CS2) into single-crystal ultrathin microribbons of 2C60·3CS2, upon mixing the poor solvent isopropyl alcohol with a C60/CS2 stock solution. Surface energy calculations reveal that these microribbons represent a kinetically favored high-energy state as compared with the thermodynamically stable shape of prismatic rods. High-resolution transmission electron microscopy observations clarify that association of CS2 at the nucleation stage helps to guide and rigidify the formation of π-π stacking 1D chains of C60 through the surrounding CS2 cage-like structures, which further act as glue, boosting lateral assembly of as-formed 1D chains into untrathin 2D microribbon single crystals. Precise control over the thickness, width, and length of 2C60·3CS2 microribbons was achieved by manipulation of the growth kinetics through adjusting the solvent conditions. Upon heating to 120 °C, sublimation of CS2 components results in fcc C60 microribbons. We found that both microribbons of solvated monoclinic 2C60·3CS2 and pure fcc C60 exhibit highly sensitive photoconductivity properties with a spectral response range covering UV to visible. The highest on/off ratio of two-terminal photodetectors based on single ribbons reaches around 250, while the responsitivity is about 75.3 A W(-1) in the UV region and 90.4 A W(-1) in the visible region.

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.

Fullerene Hollow Microspheres Prepared by Bubble-Templates as Sensitive and Selective Electrocatalytic Sensor for Biomolecules

We developed an electrocatalytic sensor based on C(60) hollow microspheres for highly sensitive and selective detection of dopamine (DA) in the presence of ascorbic acid (AA), and uric acid (UA) in the presence of l-cysteine (RSH). The hollow microspheres of C(60) with a diameter controllable in the range of 0.5 to 1.5 μm and a thickness of 200 nm are synthesized by a high-temperature reprecipitation method with the assistance of alcohol bubbles. The superhydrophobicity of C(60) hollow microspheres makes them capable of forming a compact thin film at air/water interface, which can be readily transferred on the surface of gold or glassy carbon electrodes. This porous C(60) film made from C(60) hollow microspheres shows a specific surface area as high as 107 m(2) g(-1). In order to obtain a conducting film, the C(60)-modified electrode is pretreated by scanning the potential range from 0.0 to -1.5 V in 1 M KOH followed by potential cycling between 550 to -50 mV in a pH 7.2 phosphate buffer solution. On the basis of XPS and IR measurements, we found that surface oxides, such as -OH and C═O groups, are introduced on the surfaces of the conducting C(60) film. This, combined with the porosity that enhances the adsorption activity of C(60)-modified electrodes, enable the electrocatalytic analysis of target biomolecules with detection limit as low as 0.1 nM for DA in the presence of AA, and 1 μM for UA in the presence of RSH.

Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon

Singlet fission (SF), in which one singlet exciton (S1 ) splits into two triplets (T1 ) on adjacent molecules through a correlated triplet-pair 1 (TT) state, requires precise but difficult tuning of exciton energetics and intermolecular electronic couplings in the solid state. Antiaromatic 4nπ dibenzopentalenes (DPs) are demonstrated as a new class of single-chromophore-based intramolecular SF materials that exhibit an optically allowed S2 state with E(S2 )>2×E(T1 ) and an optically forbidden S1 state. Ultrafast population transfer from a high-lying S2 state to a 1 (TT) state was observed in monomeric solution of styryl-substituted DP (SDP) on a sub-picosecond timescale. There is evidence of exciton diffusion (ED) of the 1 (TT) state to yield two individual long-lived triplets in SDP thin film. The overall triplet yield via intramolecular SF and subsequent triplet-pair diffusion can be as high as 142±10u2009% in thin film.

Tuning the organic microcrystal laser wavelength of ESIPT-active compounds via controlling the excited enol* and keto* emissions

The excited-state intramolecular proton transfer (ESIPT) process provides a real four-level system, which forms the working basis of many laser dyes in solutions but remains largely unexplored in the solid-state gain medium. Herein, we modulated the keto* and enol* emissions by switching the intramolecular hydrogen-bond to the intermolecular hydrogen-bond in the solid-state for tuning the emission colors of ESIPT-microcrystal lasers. Both model compounds of 2′-hydroxychalcone derivatives of M1 and M2 exhibit very similar dual enol* and keto* emissions in the aprotic solvents, typical for ESIPT-active molecules. When aggregated into hexagonal-plate microcrystals (HPMCs), the intramolecular hydrogen-bond ensures the photoinduced proton tautomerization from enol* to keto* tautomers, resulting in intense red fluorescence of M1 HPMCs at 647 nm from the keto* form. In sharp contrast, the introduction of an extra hydroxyl group into M2 leads to the formation of an intermolecular hydrogen bond between two adjacent molecules, which suppresses the ESIPT process in the solid-state and therefore leads to intense green emission of M2 HPMCs at 537 nm from the enol* form. The solid-state photoluminescence efficiencies of HPMCs of M1 and M2 are as high as 42% and 51%, respectively. Moreover, well-faceted HPMCs of both M1 and M2 can function as whispering-gallery mode microresonators, enabling microlasers with very low laser thresholds of 10.8 μJ cm−2 for red-emissive M1-HPMCs and 9.4 μJ cm−2 for green emissive M2-HPMCs.

Polymorph-Dependent Green, Yellow, and Red Emissions of Organic Crystals for Laser Applications

Color tuning of organic solid-state luminescent materials remains difficult and time-consuming through conventional chemical synthesis. Herein, we reported highly efficient polymorph-dependent green (P1), yellow (P2), and red (P3) emissions of organic crystals made by the same molecular building blocks of 4-(2-{4-[2-(4-diphenylamino-phenyl)-vinyl]-phenyl}-vinyl)-benzonitrile (DOPVB). Single-crystal X-ray diffraction (XRD) and spectroscopic data reveal that all three polymorphs follow the herringbone packing motif in H-type aggregations. On the one hand, from P1, P2 to P3, the reduced pitch translation along π stacks increases the intermolecular interactions between adjacent molecules, therefore leading to gradually red-shifted emissions from 540, 570 to 614u2005nm. On the other hand, the edge-to-face arrangement and large roll translations avoid strong π-π overlap, making P1, P2 and P3 highly emissive with record-high solid-state fluorescence quantum yields of 0.60, 0.98, and 0.68, respectively. Furthermore, the optically allowed 0-1 transitions of herringbone H-aggregates of P1, P2 and P3 naturally provide a four-level scheme, enabling green and yellow amplified spontaneous emissions (ASE) with very low thresholds.

Organic-Nanowire–SiO2 Core–Shell Microlasers with Highly Polarized and Narrow Emissions for Biological Imaging

Development of luminescence probes with polarized and narrow emissions simultaneously is helpful for removing multiply scattered light and enables multiplexing detection, but it remains challenging to use conventional organic dyes, fluorescence proteins, and quantum dots. Here, we demonstrated smart one-dimensional microlaser probes (MLPs) by coating a thin layer of silica shell on the surface of organic nanowires (ONWs) of 1,4-dimethoxy-2,5-di[4-(methylthio)styryl]benzene (TDSB), namely, ONW@SiO2 core-shell structures. Different from the Fabry-Pérot (FP) cavity formed between two end-faces of semiconductor nanowires, whispering gallery mode (WGM) microresonators are built within the rectangular cross section of ONW@SiO2 MLPs. This enables a lasing threshold as low as 1.54 μJ/cm2, above which lasing emissions are obtained with a full width at half-maximum (fwhm) < 5 nm and a degree of polarization (DOP) > 83%. Meanwhile, small dimensions of ONW@SiO2 MLPs with a side-length of ca. 500 nm and a length of 3-8 μm help to reduce their perturbations in living cells. With the help of mesoporous silica shells, which provide both high biocompatibility and good photostability, ONW@SiO2 MLPs can be easily introduced into the cell cytoplasm through natural endocytosis. Using their narrow and highly polarized lasing emissions in vitro, we demonstrate that it is possible to tag individual cells using ONW@SiO2 MLPs with high stability.

Complex assembly from planar and twisted π-conjugated molecules towards alloy helices and core-shell structures

Integrating together two dissimilar π-conjugated molecules into controlled complex topological configurations remains a largely unsolved problem owing to the diversity of organic species and their respective different assembly features. Here, we find that two structurally similar organic semiconductors, 9,10-bis(phenylethynyl)anthracene (BA) and 5,12-bis(phenylethynyl)naphthacene (BN), co-assemble into two-component helices by control of the growth kinetics as well as the molar ratio of BA/BN. The helical superstructures made of planar and twisted bis(phenylethynyl) derivatives can be regarded as (BA)x(BN)1−x alloys, which are formed due to compatible structural relationship between BA and BN. Moreover, epitaxial growth of (BA)x(BN)1−x alloy layer on the surface of BA tube to form BA@(BA)x(BN)1−x core-shell structure is also achieved via a solute exchange process. The precise control over composition and morphology towards organic alloy helices and core-shell microstructures opens a door for understanding the complex co-assembly features of two or more different material partners with similar structures.Manipulating the assembly of π-conjugated organic molecules into alloys to control composition and shape remains a largely unsolved problem. Here the authors show the co-assembly of two structurally similar organic semiconductors into two-component helices by control of their growth kinetics as well as the molar ratio of the building blocks.

The effect of 1D- and 2D-polymorphs on organic single-crystal optoelectronic devices: lasers and field effect transistors

Rational design and fabrication of organic microcrystal polymorphs with controlled molecular-stacking arrangements provides an effective way to optimize the optical and electronic properties, but remains challenging owing to weak van der Waals’ intermolecular interactions associated with the kinetic process of crystal growth. In this paper, we synthesized a novel compound, 1,4-dimethoxy-2,5-di[bithiophenestyryl]benzene (TPDSB), which combines the high optical-gain of the oligomeric phenylene vinylene (OPV) core-moiety with enhanced charge transportation offered by bithiophene end-groups. Controlled fabrication of one-dimensional microwires (1D-MWs) and two-dimensional microdisks (2D-MDs) was achieved by controlling the growth temperature: 1D-MWs at 30 °C as a kinetically favored morphology and 2D-MDs at 2 °C as a thermodynamically stable product. We found that though both 1D-MWs and 2D-MDs belong to the monoclinic phase, their different molecular packing arrangements caused by different molecular conformations result in distinctly different optical and electrical properties. In particular, well-faceted 1D-MWs and 2D-MDs can function as yellow-emissive Fabry–Perot (FP) and red-emissive whispering-gallery-mode (WGM) microlasers, respectively. Furthermore, single-crystal data reveal that TPDSB molecules adopt a planar conformation in 1D-MWs with relatively loose molecular-packing into rolled π-stacks, while twist molecules in 2D-MDs exhibit tight molecular-packing into slipped π-stacks. This makes 2D-MDs have a higher PL quantum yield (Φ), a lower lasing threshold and a higher carrier mobility than 1D-MWs. Our results demonstrated that these easily solution-processed organic microcrystal polymorphs might be optimized on the way to realizing electrically driven organic lasers.

A Two‐Dimensional Ruddlesden–Popper Perovskite Nanowire Laser Array based on Ultrafast Light‐Harvesting Quantum Wells

Miniaturized nanowire nanolasers of 3D perovskites feature a high gain coefficient; however, room-temperature optical gain and nanowire lasers from 2D layered perovskites have not been reported to date. A biomimetic approach is presented to construct an artificial ligh-harvesting system in mixed multiple quantum wells (QWs) of 2D-RPPs of (BA)2 (FA)n-1 Pbn Br3n+1 , achieving room-temperature ASE and nanowire (NW) lasing. Owing to the improvement of flexible and deformable characteristics provided by organic BA cation layers, high-density large-area NW laser arrays were fabricated with high photostability. Well-controlled dimensions and uniform geometries enabled 2D-RPPs NWs functioning as high-quality Fabry-Perot (FP) lasers with almost identical optical modes, high quality (Q) factor (ca. 1800), and similarly low lasing thresholds.