A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.

In this study, we introduce a plasmonic TiO2–Fe2O3 cocatalyst photoelectrode to improve the water-splitting process. The absorption of incident photons and the separation rate of photogenerated electron–hole pairs are enhanced due to the broadband absorption and strong electric field of the composite formed from these two metal oxide semiconductors and plasmonic silver nanoparticles (Ag NPs). Plasmonic TiO2–Fe2O3 cocatalyst photoelectrodes were fabricated using a precipitation and solution processing method. Under visible light irradiation, a photocurrent that is 20 times higher than that of pure Fe2O3 was observed using an optimized ratio of the plasmonic TiO2–Fe2O3/Ag cocatalyst. The mechanism for this enhancement in the plasmonic cocatalyst system was investigated using different structural configurations of the photoelectrode. Both the crystallinity and absorption band edge of the TiO2–Fe2O3 cocatalyst were characterized using X-ray diffraction (XRD) and ultraviolet–visible absorption spectroscopy (UV–vis). Furthermore, the spatial distribution of the photocurrent was investigated using this plasmonic cocatalyst system.

Several superconducting transition temperatures in the range of 30–46 K were reported in the recently discovered intercalated FeSe system (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe1+δTe.

Supercritical CO 2 (SCCO 2 ) is used to synthesize MnO 2 . This process is found to be promising for producing oxide nanorods ($5 nm in diameter) with little agglomeration. With suitable SCCO 2 pressure and temperature, nanocrystalline a-MnO 2 with an extremely high surface area of 245 m 2 g À1 (versus 80 m 2 g À1 for the oxide prepared in ambient air) was obtained. Introduction of graphene nanosheets and carbon nanotubes into MnO 2 with and without the aid of SCCO 2 is compared. SCCO 2 can help debundle the graphene nanosheets and uniformly disperse the MnO 2 nanorods in between, thus preventing graphene restacking. Since the oxide particles are sandwiched between the highly conductive sheets, a high electrochemical utilization is obtained. Accordingly, SCCO 2 -MnO 2 /graphene shows superior supercapacitor properties. The feasibility of using an ionic liquid (IL) as a conductive wrapping agent to improve the performance of the MnO 2 /graphene electrode is demonstrated.Because the intimate contact between the oxide and graphene can be ensured and the interfacial double-layer capacitance can be optimized, the electrode wrapped with an IL shows a higher capacitance, improved high-rate capability, and higher cyclic stability compared to those obtained without IL wrapping.

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