Evolution of Raman spectra in nitrogen doped graphene

We present systematical Raman studies of nitrogen doped graphene (NG). Defective graphene by Ar⁺ ion bombardment was also studied for comparison. It was found that the defects/nitrogen dopants in NG are not homogenous. Our results also suggest that the G peak position and I_2D/I_G ratio cannot be simply used as fingerprint of doping concentration in NG. Both doping and compressive strain (as verified by transmission electron microscope) contribute to the shift of Raman peaks, while both doping and lattice defects contribute to the attenuation of 2D peak. Finally, the nature of defects in NG was probed and found that they are boundary defects. The detail analysis of the evolution of Raman spectra in NG would greatly help on the characterization and future application of this novel material.     

Platinum on nitrogen doped graphene and tungsten carbide supports for ammonia electro-oxidation reaction

Ammonia electrooxidation reaction involving multistep electron-proton transfer is a significant reaction for fuel cells, hydrogen production and understanding nitrogen cycle. Platinum has been established as the best electrocatalyst for ammonia oxidation in aqueous alkaline media. In this study, Pt/nitrogen-doped graphene (NDG) and Pt/tungsten monocarbide (WC)/NDG are synthesized by a wet chemistry method and their ammonia oxidation activities are compared to commercial Pt/C. Pt/NDG exhibits a specific activity of 0.472 mA∙cm^–2, which is 44% higher than commercial Pt/C, thus establishing NDG as a more effective support than carbon black. Moreover, it is demonstrated that WC as a support also impacts the activity with further 30% increase in comparison to NDG. Surface modification with Ir resulted in the best electrocatalytic activity with Pt-Ir/WC/NDG having almost thrice the current density of commercial Pt/C. This work adds insights regarding the role of NDG and WC as efficient supports along with significant impact of Ir surface modification.     

Ab-initio calculation of electronic and optical properties of nitrogen and boron doped graphene nanosheet

Graphene nanosheet has been doped with nitrogen, boron and nitrogen–boron pair of different concentrations. Modifications of electronic and optical properties due to nitrogen, boron and nitrogen–boron codoping in graphene nanosheet have been explored in the frame work of ab-initio density functional theory. Band gap opening has been observed and besides, its magnitude increases with the doping concentration of three different species of adatoms. The static dielectric constant in the long wave length limit for parallel polarization of electric field increases with the doping concentration, whereas for perpendicular polarization it remains almost constant with respect to the doping concentration and specific types. Moreover, in case of nitrogen doped systems, a new electron energy loss spectra peak emerges around ∼2.4 eV for parallel polarization of applied external electric field vector. This peak height increases with the doping concentration. The maximum value of the reflectivity is enhanced with nitrogen concentration while for boron and nitrogen–boron pair concentration, a decreasing tendency is noticed.     

掺杂型石墨烯基纳米复合材料的研究进展

简述了石墨烯具有独特的结构、优异的性能以及制备方法;着重探讨了石墨烯基纳米复合材料的主要掺杂方法,如元素掺杂法,主要包括非金属元素和金属元素掺杂;化合物掺杂法以及碳素材料掺杂法。这些掺杂法制备出的纳米复合材料应用广泛,主要在超级电容器、传感器、储氢方面以及生物医学等领域突出。最后进一步提出了在石墨烯探索过程中的一些问题,如其易产生褶皱以及分散性能不稳定等。同时也阐述了其未来可能发展趋势,如探讨磁性、光学性能等。     

Rapid and controllable synthesis of nitrogen doped reduced graphene oxide using microwave-assisted hydrothermal reaction for high power-density supercapacitors

Nitrogen doped reduced graphene oxide (N-RGO) is synthesized using microwave-assisted hydrothermal (MAHA) reaction. The proper configurations of nitrogen atoms in graphene sheets considerably increase the intrinsic electrical properties of N-RGO resultantly improving its capacitance and other kinetic properties in supercapacitor. Here, under the controlled MAHA reaction, we adjusted the ratio of nitrogen configurations (pyridinic-N, pyrrolic-N and quaternary-N) for the most optimum supercapacitor performances of N-RGOs in the shortest time ever reported, and clarified that its enhanced electrical conductivity and supercapacitor performances are attributed to its enlarged concentration of quaternary-N. With this MAHA reaction, we present a supercapacitor based on N-RGO, which is capable of displaying the promising electrochemical properties.     

Tuning the curing behavior of fluoroelastomer (FKM) by incorporation of nitrogen doped graphene nanoribbons (CNx-GNRs)

Graphene nanoribbons (GNRs), obtained by different methods from carbon nanotubes (CNTs) or graphene, are attractive materials for polymer nanocomposites due to their considerably high interfacial area, as compared to CNTs. Consequently, a better adhesion with a polymer matrix is anticipated for GNRs. Also, surface modification of these nanofillers, such as nitrogen doping, is known to be an efficient method to improve their properties. In this work, fluoroelastomers (FKM) were used as the polymer matrix to host GNRs. Undoped and nitrogen doped GNRs were synthesized from the parent multiwall carbon nanotubes (MWCNTs). MWCNT/FKM and GNR/FKM nanocomposites were prepared via a solution mixing/melt mixing protocol.     

Spin negative differential resistance in edge doped zigzag graphene nanoribbons

The nonlinear spin-dependent transport properties in zigzag graphene nanoribbons (ZGNRs) edge doped by an atom of group III and V elements are studied systematically using density functional theory combined with non-equilibrium Green’s functions. The dopant type, acceptor or donor, and the geometrical symmetry, odd or even, are found critical in determining the spin polarization of the current and the current–voltage characteristics. For ZGNRs substitutionally doped on the lower-side edge, the down (up) spin current dominates in odd-(even-) width ZGNRs under a bias voltage around 1 V. Remarkably, in even-width ZGNRs, doped by group III elements (B and Al), negative differential resistance (NDR) occurs only for down spins. The bias range of the spin NDR increases with the width of ZGNRs. The clear spin NDR is not observed in any odd-width ZGNRs nor in even-width ZGNRs doped by group V elements (N and P). This peculiar spin NDR of edge doped ZGNRs suggests potential applications in spintronics.     

Structural characterization and chemical reactivity of dual doped graphene

Herein, we employed first-principle calculations to study the structure and reactivity of dual-doped graphene. The new materials were derived from graphene by replacing two carbon atoms with one 2p element (B, N or O), and one 3p element (Al, Si, P or S). A total of 12 dual-doped graphenes were used to perform a comparative study and identify the most promising materials for the development of new catalysts and anchoring nanoparticles. The structural analysis indicated that in all cases, except SiB, the dopants prefer to replace a CC bond. Yet, not always the dopants are bonded, presenting an edge-like bonding. As regards chemical reactivity, in general the introduction of two atoms remarkably increases reactivity as compared to graphene. The most prominent examples are Al–O, S–N, P–O and Si–B(para) codoped graphene which present reactivity higher than perfect and monodoped graphenes. This effect was attributed to the strong charge redistribution induced by the dopants. In most cases the heteroatoms are the most reactive sites, but in others the carbon atoms are more reactive, as in Al–O and Si–B codoped graphene. The combination of 2p and 3p dopants offers the possibility of adjusting over a wide range the reactivity of graphene.     

High frequency dielectric characterization of graphene doped flexible ceramics multilayers

This paper explores the tape casting technique to produce flexible heterostructured multilayers composed of graphene nanoplatelets doped Alumina and Ceria doped Nickel Oxide-Gadolinium. Given the excellent mechanical and electrical properties of graphene, multilayers were structured to explore the dielectric properties at a high-frequency regime. The stability of the suspensions, measured by rheological characterization, showed a pseudoplastic behavior. The structural properties of the flexible tapes were analyzed by X-ray diffraction and scanning electron microscopy. Electrical properties obtained through I ? V curves showed an insulating behavior. The dielectric characterization was measured at a high-frequency regime (0.01 up to 1.5 GHz). The findings reflect the strong dependence of the dielectric behavior with the multilayer structure and graphene nanoplatelets doping concentration in the Alumina. The results open new perspectives of multifunctionalization flexible ceramics tapes for high-frequency applications.     

Iodine doped graphene as anode material for lithium ion battery

I-doped graphene is synthesized by a facile heat treatment method and used as anode material for lithium ion battery. The doped graphene exhibits high reversible capacity (1690 mAh g⁻¹ at 100 mA g⁻¹), good cyclability (retaining 92.6% reversible capacity after 200 cycles) and excellent rate performance compared with undoped graphene. The superior electrochemical performance of the I-doped graphene is explained by the change of graphene lattice, defects and positive charge density introduced by the doping of I atoms.     

Piezoelectric Anisotropy and Polarization Sublattice Coupling in Perovskite Crystals

Anisotropic piezoelectricity has been intensively investigated in PbZrO₃- or PbTiO₃-based perovskite ceramics, in comparison with the isotropic characteristic in the intermediate composition of the solid-solution Pb(Zr, Ti)O₃ system. In order to explain the piezoelectric anisotropy, a phenomenological treatment is proposed for a perovskite sublattice system, including electromechanical coupling terms. Electrostrictive couplings, especially between the two sublattices, provide significant contribution to the piezoelectric anisotropy.     

Boron doped graphene cathode for capacitor via a new one-step method

The graphene cathode was doped with boron via a new and fast method of plasma enhanced chemical vapor deposition (PECVD) at room temperature. Various plasma species of BH_x (x?=?0–3) with high reactivity reacted with graphene electrode via surface re-reactions and gas-interface intersection. The cathode made of boron doping into graphene (BG) exhibited excellent electrochemical performances in Li-ion capacitors, including a large discharge capacity of 140?mAh?g^?1 (voltage range: 1.5–4.2?V vs. Li/Li⁺, current density: 100?mA?g^?1) and the coulombic efficiency of more than 99.6% within 1000 circles. The capacity, coulombic efficiency and circle performance of the BG electrode were more superior to the undoped graphene electrode owing to the uniform doping of boron plasma species. The PECVD method has the advantages of being simple, is conducted at room-temperature, is time efficient and uniform, thus making it a fast and effective way for doping hetero-atoms into the electrode.     

氧化石墨烯掺杂反渗透混合基质膜制备及性能

由于芳香族聚酰胺反渗透膜在抗污染性以及耐氯性方面存在不足,限制了其在海水淡化等方面的应用。采用往油相中添加氧化石墨烯（GO）的二次界面聚合法改性了商业反渗透膜,评价了GO掺杂反渗透混合基质膜的分离性能和耐氯性能,并用接触角仪、Zeta电位仪、扫描电镜和原子力显微镜等仪器表征了膜的亲水性能、荷电性能以及膜表面形貌。结果表明,GO的添加提高了膜的分离性能、耐氯性能和亲水性能;当GO添加量为30 mg·L^-1时,膜的通量为（77.7±0.9） L·m^-2·h^-1,膜的截留率为97.6%±0.5%,相比商业膜分别提高了38.4%和4.5%。当氯化强度低于4800 mg·L^-1·h时,膜的水通量和盐截留率变化不明显。     

功能化石墨烯掺杂熔盐的制备及性能研究

通过物理混合方法,在由硝酸钾、硝酸钠、硝酸锂组成的熔盐中加入不同量的氧化石墨烯（GO）、钠化石墨烯（Na-GO）和钾化石墨烯（K-GO）,制得3个系列熔盐复合材料,考察功能化石墨烯的含量,氧化、钠化、钾化对熔盐复合材料熔点、热分解温度、热传导率、黏度等物化性质的影响。结果发现在添加量为0.1%~5%范围内,随着功能化石墨烯添加量的增加,熔盐复合材料的熔点先逐渐下降,后下降趋平缓,其中Na-GO对熔盐熔点下降作用最为明显;熔盐复合材料的起始分解温度随着功能化石墨烯添加量的增加而提高。GO、Na-GO和K-GO的加入提高了熔盐复合材料的热传导率,降低了其流动性能。综合考虑各种因素,在熔盐中添加0.5%~1%的Na-GO,所得熔盐复合材料的综合性能较好。     

High capacity Li storage in sulfur and nitrogen dual-doped graphene networks

Three-dimensional (3D) networks composing of S and N dual-doped graphene (SNG) were synthesized by a chemical vapor deposition approach using MgSO₄-containing whiskers as templates and S source and NH₃ as N source. Energy dispersive spectrometer mapping and X-ray photoelectron spectroscopy coupled with Raman analysis have revealed that S and N atoms with concentrations of 5.2 and 1.8 atom%, respectively, have been substitutionally incorporated into the graphene networks via covalent bonds. The SNG, as an anode material for lithium ion batteries (LIBs), exhibits extremely high capacity (3525 mAh/g at the current density of 50 mA/g) and superior rate capability (870 mAh/g at 1000 mA/g) with excellent cycling stability (remaining a reversible capacity of 400 mAh/g at 10 A/g after 2500 cycles). The enhanced conductivity, the 3D porous network with many disorders and the intrinsically high Li storage capacity of S and N-doped carbon segments have led to the excellent electrode performance of the SNG networks. The effects of binder content and calendaring pressure on the electrode performance have been investigated. The full LIB with SNG as anode and LiCoO₂ as cathode can afford a high reversible capability (164 mAh/g at 0.2 C) and good cycling stability.     

Thermal conductivity of graphene nanoribbons with defects and nitrogen doping

Thermal conductivity of defective graphene nanoribbons doped with nitrogen for different distributions around the defect edge at nanoscale is investigated using the reverse non-equilibrium molecular dynamics (RNEMD) method, which explores ways to improve thermal management. In addition, thermal conductivity of graphene nanoribbons with both defects and nearby nitrogen doping is investigated in comparison to that of nanoribbons with defects alone. The simulation results are analyzed from three perspectives: phonon match, concentration of N doping, and distribution of N doping. This approach reveals that a coupling effect is the cause of the observed results. Nitrogen doped graphene nanoribbons (both perfect and defective variants) perform better with thermal management than do graphene nanoribbons with defects alone, which is of considerable interest. Based on these investigations, a guide for graphene-interconnected circuits design is implied.     

Photoluminescence and positron annihilation measurements of nitrogen doped CVD diamond

A range of techniques have been used, to determine the nitrogen levels in a series of polycrystalline diamonds. The crystals were grown by the chemical vapour deposition (CVD) technique and EPR measurements indicate that they have a single substitutional nitrogen (N_s) concentration between 10 ppb and 50 ppm. Photoluminescence (PL) spectroscopy provides an extremely sensitive way of detecting the nitrogen vacancy complex, in both the neutral (N–V)⁰ and negative charge states (N–V)⁻, down to the 10 ppb range and below. It has been observed that for diamonds with a single substitutional nitrogen content of approximately (0.5–2.0) ppm, the nitrogen vacancy complex exists in both charge states, with an almost equal abundance. Below this level, the complex exists with greater regularity in the neutral charge state, with the (N–V)⁻ centre dominating at (N_s) levels greater than 2 ppm. Electron irradiation of so-called ‘high nitrogen’ films, with an (N_s) value in excess of 15 ppm, show an incredibly high abundance of the interstitial related 3H centre. Such samples show little sign of the neutral vacancy, GR1, although some absorption is evident in the tail of the (N–V)⁻ centre. No evidence of the negative vacancy, the ND1 centre, was observed in PL spectra. For equivalent electron doses on ‘low N’ samples, typically with an (N_s) level of below 1 ppm, the GR1 centre completely dominates the PL spectra. For all diamonds the intensity of the nitrogen vacancy complexes are seen to reduce within the irradiated region. Annealing studies of high N samples reveal that by 900 °C all the vacancies are annealed with many being trapped at the nitrogen to form a large concentration of the (N–V)⁰ centre. Positron annihilation measurements have been performed on the samples. As positron annihilation is sensitive to vacancy concentrations, it is expected to yield lifetime measurements that are strongly dependent on the bulk (N–V) concentration. Doppler broadening spectra have been recorded as the samples were illuminated with light varying from 325 to 785 nm. The shape ‘S’ parameter was found to increase upon illumination and relax back to its ground state, post-illumination. The relaxation time was found to vary between 30 and 200 hours and be strongly dependent on both the nitrogen concentration and the wavelength of light used. Work is currently on-going, and includes angular correlation of annihilation experiments. Lifetime measurements show clear evidence of the formation of positronium in the voids that are present in the CVD material.     

Growth of homoepitaxial diamond doped with nitrogen for electron emitter

Although we had reported the remarkable low threshold emission from polycrystalline diamond heavily doped with nitrogen (N) [Nature 381 (1996) 140], the problems caused by polycrystallinity still remain for understanding the electron emission mechanism. This paper describes the growth of N-doped homoepitaxial diamond film {100}, {111} and {110}, and their electron emission properties. N-doped homoepitaxial diamond is grown on synthetic diamond by hot filament chemical vapor deposition. Urea [(NH₂)₂CO] is used as a dopant for N. Atomic force microscope (AFM) observations indicate that the relatively smooth surface morphologies are obtained for all the films. The epitaxial growth of all the film is confirmed using reflective high energy electron diffraction (RHEED) patterns. Reflective electron energy loss spectra (REELS) indicate that the very surfaces of {100} and {111} are diamond while {110} is graphite rather than diamond. Raman spectra suggest that the bulk of the obtained films are diamond. The resistivities of the films are found to be much higher than the detection limit of the system. The relatively low threshold emission was observed even from the smooth surface and the threshold voltage is confirmed to depend on the crystal orientation. It is speculated from the film characterizations and the electron emission properties that the low threshold emission is due to high resistance rather than rough surface and/or grain boundaries.     

Photoluminescence studies of type IIa and nitrogen doped CVD diamond

Photoluminescence (PL) studies were carried out on CVD, type IIa and high purity HPHT diamond samples irradiated with electrons of energies between 150 and 300 KeV; near threshold energies for carbon displacement. The majority of PL spectra are obtained using a 488-nm lasing line, with samples cooled to approximately 7 K. Of particular interest is the behaviour of the self-interstitial related centre, 3H, at 503.5 nm. The centre is particularly sensitive; its formation varies significantly with dose and dose rate and is severely quenched with incident laser power in excess of 10 mW. 3H is the dominant centre in highly doped (50–100 ppm) nitrogen samples, for doses between (10¹⁹–10²⁰) el/cm², but reduces with higher doses. In lower nitrogen (few ppm) samples, the centre is considerably weaker after equivalent doses, comparable to the Raman line. In type IIa crystals, creation of 3H varies considerably from sample to sample. Upon annealing, 3H is at an optimum between 310 and 330 °C for type IIa diamonds and vanishes by 400 °C. Indications show these temperatures increase slightly as nitrogen content is increased. Migration of the centre well outside the irradiated area is frequent, tens of microns after irradiation and hundreds of microns post annealing. Other centres of interest include GR1, the neutral diamond vacancy, which is found to be created linearly with dose and be rate independent. Using 325 and 457.9 nm lines the TR12 centre was studied. It has a strong dose rate dependence, growing as dose rate raised to a power of approximately 2 and is unaffected by annealing up to 700 °C. A 244-nm line was used to study the 5RL centre and contrary to some reports was observed in samples containing approximately 0.1 ppm of nitrogen. PL provides an extremely sensitive way of measuring the nitrogen concentration in diamond, to levels of less than 0.1 ppm. The problem remains how to obtain an accurate measurement.     

氧化石墨烯的制备及其掺杂的炭/炭复合材料导热性能研究

设定两种不同配比强酸氧化剂,以鳞片石墨为原料,采用Hummers法,制备了氧化石墨烯,再经过高温炭化得到热处理氧化石墨烯。并分别以中间相沥青为基体炭前驱体,炭纤维为增强相,氧化石墨烯及其热处理物为热疏导功能体,制备出掺杂氧化石墨烯的炭/炭复合材料。TEM、SEM等表征表明,选用强酸氧化剂组合配比用量较少的制备出的氧化石墨烯,其形貌整体上要优于用量较多的,具有独特的褶皱结构;相比于氧化石墨烯,掺杂其热处理物的复合材料界面覆盖均匀平滑且结合更优良,且其导热系数可达到60 W.m-1.K-1,是无掺杂的纯复合材料两倍多,导热系数得到了较大幅度提高。