Yuanxiang Xu

Phenanthro[9,10-d]imidazole as a new building block for blue light emitting materials

The blue light emitting compound, BPPI, based on a phenanthro[9,10-d]imidazole group, is prepared by a facial synthetic process and exhibits excellent thermal stability, highly efficient fluorescence and balanced carrier injection. The double-layered device based on BPPI shows a higher maximum luminance and a lower turn-on voltage than the multi-layered one with an independent electron injection layer. The results suggest that the phenanthroimidazole unit is an excellent building block for tuning the carrier injection properties as well as blue emission.

Electrical properties and stability of p-type ZnO film enhanced by alloying with S and heavy doping of Cu

Single wurtzite p-type Zn1−yCuyO1−xSx alloy films with 0.081≤x≤0.186 and 0.09≤y≤0.159 were grown on quartz reproducibly by magnetron sputtering. The alloys show very stable p-type conductivity with a hole concentration of 4.31–5.78×1019 cm−3, a resistivity of 0.29–0.34 Ω cm and a mobility of 0.32–0.49 cm2 V−1 s−1. The p-type conductivity is attributed to substitution of Cu+1 for the Zn site, and the ionization energy of the Cu+1 acceptor is measured to be 53 meV, much less than that of Cu-doped ZnO reported previously. The small ionization energy is due to Cu heavy doping and increase in valence band maximum of ZnO induced by alloying with S.

Supramolecular interaction-induced self-assembly of organic molecules into ultra-long tubular crystals with wave guiding and amplified spontaneous emission

Ultra-long tubular crystals (length up to 10 mm and diameter of 70 μm) of a small organic functional molecule, 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB), are successfully prepared through a physical vapor transport (PVT) method. On the basis of crystal structural analysis and density functional theory calculations, we show that the formation of such tubular crystals is drawn by the branched hydrogen bonds between the BCPEB molecules. High crystalline quality, highly ordered molecular orientation, and hollow-like topological structure enable the crystal to exhibit optical wave guided emission behaviors with low optical loss (3 dB mm−1) and highly polarized emission (the polarized ratio is about 20). Amplified spontaneous emission (ASE) characteristics of the tubular crystals were also studied; the net gain coefficients at the peak wavelength and the threshold are 91.7 cm−1 and 36 kW cm−2, respectively.

Fluorescence mutation and structural evolution of a π-conjugated molecular crystal during phase transition

Two thermodynamically stable crystalline phases (B- and G-phases) were found for a twistable π-conjugated molecule, CN-DSB, condensed from its solution. We investigated the structural evolution at the molecular and supramolecular levels as the crystalline phase transforms from the B-phase to G-phase under varied temperature or pressure. The intermolecular interactions were undermined before phase transition as the B-phase crystal was stimulated with an external energy. Heating the B-phase crystal up to 175 °C or applying stress up to its critical pressure (0.75 GPa) initially resulted in mixture phases or disordered state. At this stage, the molecules slightly adjust from a twisting configuration to a planar configuration, corresponding to the gradual red shift of the fluorescence spectra. Above the phase transition point, the initial intermolecular interaction of the B-phase is broken down, and the CN-DSB molecules re-assemble to the new phase—a new thermodynamic equilibrium state—corresponding to the sudden change of the emission color. Furthermore, the property of thermal-induced phase transition can be used to fabricate patterns on the CN-DSB crystal surface, and a uniform raster has been prepared by femtosecond laser direct writing (FsLDW) on the B-phase. The investigations provide new insight and understanding for the crystal phase transition and may contribute to process innovation in optical devices.

Self-cavity laser oscillations with very low threshold from a symmetric organic crystal waveguide

A symmetric waveguide structure of air/crystal/air comprising a slab crystal of cyano substituted oligo(p-phenylenevinylene) (OPV) has been developed for organic crystal lasers. The OPV crystals exhibit high photoluminescence efficiency of 30% and have smooth surfaces and edges, which ensures strong self-cavity optical confinement and provides optical feedback for the laser oscillation. A single-mode laser oscillation is observed around 573 nm with a threshold of 12.8 kW/cm2 that is one of the lowest values for organic crystal lasers. The leaky loss caused by the substrate is prevented by the air/crystal/air waveguide structure.

Conformational polymorphism of a twisted conjugated molecule: controllable torsional motion for crystal growth selectivity

Controlling the emergence of the desired polymorphic form in crystal growth is of importance for both understanding the phenomenon of polymorphism and practical applications, but remains a great challenge as well. Herein, two polymorphs of cyano substituted distyrylbenzene (CN-DSB) are obtained as the crystals grow from chlorobenzene solution (Form I, blue fluorescence) and from its vapor (Form II, green fluorescence). Single-crystal X-ray analysis for the two crystal forms shows unambiguous evidence that the distinctive feature of molecules in both crystal forms is their difference in torsion angles of peripheral phenyl segments. A computational study addresses that the controllable growth of polymorphs can be rationalized on the basis of structural relationships with torsion potential under different external conditions. Our demonstration of single-crystal growth by tuning subtle external conditions illustrates a controllable process of obtaining conformational polymorphs.

Cross-linked luminescent films via electropolymerization of multifunctional precursors for highly efficient electroluminescence

To fabricate stable organic light-emitting diodes, a cross-linked network that firmly fixes the position of the chromophore is an ideal structure, because quenching caused by aggregation and/or phase separation effects can be effectively restrained in such robust films. Herein, in situ electropolymerization (EP) of fluorene-based precursors containing multi-electroactive carbazole units (multi-functionality) is utilized to construct highly cross-linked, smooth and compact luminescent films for highly efficient and stable electroluminescent devices. Cyclic voltammetry (CV), UV and atomic force microscopy (AFM) studies of the series of precursors with varied functionality (2, 4 and 8) reveal that increasing the functionality of the EP precursor can significantly increase the deposition rate, cross-linking degree and packing density as well as the luminous efficiency and stability of the fabricated EP films. The electrode kinetic parameters, determined by CV and chronocoulometry (CC), indicate that such improvements in EP film properties could be attributed to the enhanced interfacial electron-transfer rate. These observations could potentially offer a new approach to adjust the microstructures of EP films and provide a simple way of enhancing the performance of organic luminescent devices.