Preparation of N-doped graphite oxide for supercapacitors by NH3 cold plasma

In this work, N-doped graphite oxide (GO-P) was prepared by cold plasma treatment of GO using a mixture of NH3 and Ar as the working gas. When the ratios of NH3:Ar were 1:2, 1:3, and 1:4, the specific capacitances of the GO-P(NH3:Ar1:2), GO-P(NH3:Ar1:3), and GO-P(NH3:Ar1:4) were 124.5, 187.7, and 134.6 Fg−1, respectively, which were 4.7, 7.1, and 5.1 times that of GO at the current density of 1 Ag−1. The capacitance retention of the GO-P(NH3:Ar1:3) was 80% when it was cycled 1000 times. The characterization results showed that the NH3 cold plasma could effectively produce N-doped GO and generate more active defects. The N/C ratio and the contents of pyridinic nitrogen and graphitic nitrogen of the GO-P(NH3:Ar1:3) were the highest. These were conducive to providing pseudocapacitance and reducing the internal resistance of the electrode. In addition, the ID/IG of the GO-P(NH3:Ar=1:3) (1.088) was also the highest, indicating the highest number of defects. The results of discharge parameters measurement and in situ optical emission spectroscopy diagnosis of NH3 plasma showed that the discharge is the strongest when the ratio of NH3:Ar was 1:3, thereby the generated nitrogen active species can effectively promote N-doping. The N-doping and abundant defects were the keys to the excellent electrochemical performance of the GO-P(NH3:Ar1:3). NH3 cold plasma is a simple and rapid method to prepare N-doped GO and regulate the N-doping to prepare high-performance supercapacitors.

Impact of n-Doping Mechanisms on the Molecular Packing and Electron Mobilities of Molecular Semiconductors for Organic Thermoelectrics

Electrical conductivity is one of the key parameters for organic thermoelectrics and depends on both the concentration and mobility of charge carriers. To increase the carrier concentration, molecular dopants have to be added into organic semiconductor materials, whereas the introduction of dopants can influence the molecular packing structures and hence carrier mobility of the organic semiconductors. Herein, we have theoretically investigated the impact of different n-doping mechanisms on molecular packing and electron transport properties by taking N-DMBI-H and Q-DCM-DPPTT respectively as representative n-dopant and molecular semiconductor. The results show that when the doping reactions and charge transfer spontaneously occur in the solution at room temperature, the oppositely charged dopant and semiconductor molecules will be tightly bound to disrupt the semiconductor to form long-range molecular packing, leading to a substantial decrease of electron mobility in the doped film. In contrast, when the doping reactions and charge transfer are activated by heating the doped film, the molecular packing of the semiconductor is slight affected and hence the electron mobility remains quite high. This work indicates that thermally-activated n-doping is an effective way to achieve both high carrier concentration and high electron mobility in n-type organic thermoelectric materials.

Electronic properties of TaAs2 topological semimetal investigated by transport and ARPES

We have performed electron transport and ARPES measurements on single crystals of transition metal dipnictide TaA$s_{2}$ cleaved along the ($\overline{2}$ 0 1) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [$\overline{2}$ 0 1] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved ($\overline{2}$ 0 1) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level is increased, which is due to a small n-doping of the samples. This shifts the Fermi level closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.