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.

No abstract

This study analyses empirically how built environment affects school travel with a focus on independence from adults and travel mode. Students in three elementary schools—Chinan, Jingmei and Xinhwa—in Taipei’s Wenshan District are analysed after questionnaire surveys. The survey data are analysed using nested logit models at two decision levels. Analytical results indicate that high shade-tree density and high sidewalk coverage encourage children to walk to school independently, while large block sizes and increased intersection numbers discourage children from walking to school independently. Furthermore, although high building density, high vehicle density and diversified mode option encourage children to travel home after school by walking, bus or vanpool, block size and road width discourage children from so doing. These results are mostly similar to the findings of previous studies, although they also have some differences. Based on the empirical evidence presented in this study, three strategies are recommended for reshaping the built environment in Taipei: compact structure, pedestrian-friendly design and frequent bus services.

No abstract

No abstract