This paper reports a facile fabrication of N‐doped graphene quantum dots (N‐GQDs) showing controllable chemical properties through a hydrothermal treatment. The N‐GQDs have a uniform size of 3.06 ± 0.78 nm and prefer the equilibrium shapes of circle and ellipse due to the minimization of edge free energy. The N/C atomic ratio in N‐GQDs can be precisely tailored in a range from 8.3 at% to 15.8 at% by simply controlling the concentration of N source (ammonium hydroxide). One order of magnitude quantum yield of 34.5% is achieved by N‐GQDs, compared with the N‐free GQDs, as the substitutional N has an essential role in more effective radiative emission. Excessive N dopants in N‐GQDs can lead to photoluminescence quenching, through nonradiative transition back to the ground state. The N‐GQDs are further found to be suitable as photocurrent conversion materials due to benign energy matching with anatase nanofibers, the ultrafast electron injection at their interface, and efficient electron transfer. This work provides an efficient and inspiring approach to engineering both chemical components and physical properties of N‐GQDs, and will therefore promote their basic research and applications in energy conversion.

A novel bipolar host material, which meets the requirements of high triplet energy, good charge carrier transport properties, high solubility, and film-forming ability at the same time, has been designed and synthesized. Utilizing a new compound as host material, high-efficiency solution-processed blue and white phosphorescent organic light-emitting diodes (PHOLEDs) have been achieved.

We have studied the magnetic properties of free cobalt clusters in two semi-independent Stern-Gerlach deflection experiments at temperatures between 60 and 307 K. We find that clusters consisting of 13 to 200 cobalt atoms exhibit behavior that is entirely consistent with superparamagnetism, though complicated by finite-system fluctuations in cluster temperature. By fitting the data to the Langevin function, we report magnetic moments per atom for each cobalt cluster size and compare the results of our two measurements and all those performed previously. In addition to a gradual decrease in moment per atom with increasing size, there are oscillations that appear to be caused by geometrical shell structure. We discuss our observations in light of the two competing models for Langevin-like magnetization behavior in free clusters, superparamagnetism and adiabatic magnetization, and conclude that the evidence strongly supports the superparamagnetic model.

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A novel series of solution-processible carbazole-based host materials, 3,6-bis(N-carbazolyl)-Nphenylcarbazole (BCC-36), 3,6-bis(3,6-di-tert-butyl-9-carbazolyl)-N-phenylcarbazole (BTCC-36), 2,7-bis(N-carbazolyl)-N-phenylcarbazole (BCC-27), and 2,7-bis(3,6-di-tert-butyl-9-carbazolyl)-Nphenylcarbazole (BTCC-27), is designed and synthesized. Owing to the highly twisted configuration, these hosts exhibit high triplet energy levels (2.90-3.02 eV) and high glass transition temperatures (147-210 C). They also exhibit appropriate HOMO energy levels (À5.21-À5.36 eV), resulting in an improved hole-injection property. These novel compounds are employed to fabricate phosphorescent organic lighting-emitting diodes (OLEDs) as the host materials doped with the guests of iridium(III) bis (4,6-difluorophenylpyridinato)-picolinate (FIrpic) and iridium(III) bis(4 0 ,6 0 -difluorophenylpyridinato) tetrakis(1-pyrazolyl)borate ( FIr6) by spin coating. The best device performance of FIrpic based blueemitting devices has a rather low turn-on voltage of 3.9 V, a maximum efficiency of 27.2 cd A À1 (11.8 lm W À1 ), and a maximum external quantum efficiency of 14.0%. Moreover, the best device performance of FIr6 based deep-blue-emitting devices exhibits a turn-on voltage of 4.9 V, a maximum efficiency of 11.5 cd A À1 (4.9 lm W À1 ), and a maximum external quantum efficiency 6.8%. The performance data are outstanding for solution-processed blue phosphorescent OLEDs.