Xianjie Li

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.

Electron emission of carbon nitride films and mechanism for the nitrogen-lowered threshold in cold cathode

Carbon nitride films have been deposited by rf reactive magnetron sputtered graphite carbon in an N2 discharge. The process parameters, viz., nitrogen partial pressure (PN2), substrate temperature (Ts), and substrate bias (Vb) were varied in order to investigate their influence on the field emission properties. The effective work function for carbon nitride films determined using the Fowler–Nordheim equation is in the range of 0.01–0.1 eV. Insight is presented into the nitrogen-lowered threshold of cold cathode electron emission of carbon from the perspective of nitride tetrahedron bond formation. The involvement of nonbonding (lone pair) and lone-pair-induced antibonding (dipole) states is suggested to be responsible for lowering the work function and hence the electron emission threshold. It is found that the substrate temperature of 200 °C, floating potential at the substrate, and nitrogen partial pressure of 0.3 Pa are favorable to promote the reaction that lowers the work function.

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.

Investigation of the magnetic field effects on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices

We investigated the magnetic field effects (MFEs) on electron mobility in tri-(8-hydroxyquinoline)-aluminum based light-emitting devices by the transient electroluminescence (EL) method upon application of various offset voltages (Voffset). It is found that the rising edges of the EL overlap closely but the falling edges are separated by the magnetic field both when Voffset = 0 V and Voffset > Vturnon. The results suggest the bipolaron model and the triplet-polaron interaction model related to the carriers’ mobility are not the dominant mechanisms for explaining the MFEs under our experimental conditions, and the external magnetic field is confirmed to affect the carriers’ recombination process.

Large organic single crystal sheets grown from the gas–liquid and gas–liquid–solid interface

The growth of high-quality organic single crystals with controlled size is crucial to both the study of the fundamental physical properties of the materials, and to their practical utilization. Here, a novel gas–liquid interface technique is presented for the growth of anthracene (AN) and its doped single crystals. The AN single crystals showed sheet shape, large size of up to centimeter range, and neat surface morphology. The results of X-ray diffraction and optical measurements indicated that such laminated films exhibited higher crystalline quality and photoluminescence (PL) efficiency than those obtained by physical vapor transport (PVT) method or original solution growth (SG) method. The crystal formation process is explored by quoting the principle of Langmuir–Blodgett film and the terrace–ledge–kink crystal growth model. Such a crystal growth technique can be expanded to the gas–liquid–solid interface to directly grow AN single crystals on electrodes (such as silicon wafer and golden substrate), which may be valuable for the preparation of organic single crystal based devices including light-emitting diodes and field effect transistors.

Time-resolved spin-dependent processes in magnetic field effects in organic semiconductors

We investigated the time-resolved magnetic field effects (MFEs) in tri-(8-hydroxyquinoline)-aluminum (Alq3) based organic light-emitting diodes (OLEDs) through the transient electroluminescence (EL) method. The values of magneto-electroluminescence (MEL) decrease with the time, and the decreasing slope is proportional to the driving voltage. Specifically, negative MELs are seen when the driving voltage is high enough (V > 11 V). We propose a model to elucidate the spin-dependent processes and theoretically simulate the time-resolved MELs. In particular, this dynamic analysis of time-resolved MELs reveals that the intersystem crossing between singlet and triplet electron-hole pairs and the triplet-triplet annihilation are responsible for the time-resolved MELs at the beginning and enduring periods of the pulse, respectively.