Nujiang Tang

Synthesis and upconversion luminescence of N-doped graphene quantum dots

A hydrothermal approach was developed for the synthesis of N-doped graphene quantum dots (N-GQDs) by cutting N-doped graphene. The N-GQDs obtained have a N/C atomic ratio of ca. 5.6% and diameter of 1–7 nm. The photoluminescence (PL) properties of the N-GQDs were investigated. It was found that the N-GQDs possess bright blue PL and excellent upconversion PL properties.

Synthesis and photoluminescence of fluorinated graphene quantum dots

Fluorinated graphene quantum dots (F-GQDs) were synthesized by cutting fluorinated graphene through a hydrothermal approach. The F-GQDs with oxygen-rich functional groups have a F/C atomic ratio of ca. 23.68% and diameter of 1-7 nm. The photoluminescence (PL) properties of the F-GQDs were investigated. The results showed that besides exhibiting bright blue PL, the F-GQDs display a clear upconversion PL.

Helical carbon nanotubes: catalytic particle size-dependent growth and magnetic properties.

The exact knowledge of helical carbon nanotube (HCNT) growth mechanism has not yet been completely clarified, and effective synthesis of high-purity helical carbon nanotubes in high yield still remains a tremendous challenge. In this study, HCNTs were synthesized via a catalytic chemical vapor deposition method using Fe nanoparticles as catalysts. We performed systematic experiments to investigate the specific effect of catalytic particle size (CPS) on the selective growth of HCNTs, such as on the morphology, yield, mobility of carbon atoms, and HCNT purity of carbon products. Our study showed that the CPS plays a key role in the selectivity to HCNTs, yield, and morphology of the carbon products, and a small catalytic particle is favorable to HCNT formation. We hope that this result may give a beneficial suggestion to obtain highly pure HCNTs. A CPS-dependent growth mechanism for HCNTs was finally proposed. Magnetic measurements demonstrated that the HCNTs are ferromagnetic properties and show high magnetization at room temperature.

Elemental superdoping of graphene and carbon nanotubes.

Doping of low-dimensional graphitic materials, including graphene, graphene quantum dots and single-wall carbon nanotubes with nitrogen, sulfur or boron can significantly change their properties. We report that simple fluorination followed by annealing in a dopant source can superdope low-dimensional graphitic materials with a high level of N, S or B. The superdoping results in the following doping levels: (i) for graphene, 29.82, 17.55 and 10.79 at% for N-, S- and B-doping, respectively; (ii) for graphene quantum dots, 36.38 at% for N-doping; and (iii) for single-wall carbon nanotubes, 7.79 and 10.66 at% for N- and S-doping, respectively. As an example, the N-superdoping of graphene can greatly increase the capacitive energy storage, increase the efficiency of the oxygen reduction reaction and induce ferromagnetism. Furthermore, by changing the degree of fluorination, the doping level can be tuned over a wide range, which is important for optimizing the performance of doped low-dimensional graphitic materials.

Obtaining High Localized Spin Magnetic Moments by Fluorination of Reduced Graphene Oxide

Fluorination was confirmed to be the most effective route to introduce localized spins in graphene. However, adatoms clustering in perfect graphene lead to a low efficiency. In this study, we report experimental evidence of the generation of localized spin magnetic moments on defective graphene (reduced graphene oxide) through fluorination. More interstingly, the result shows that defects help increase the efficiency of the fluorination with regard to the density of magnetic moments created. Fluorinated reduced graphene oxide can have a high magnetic moment of 3.187 × 10(-3) μB per carbon atom and a high efficiency of 8.68 × 10(-3) μB per F atom. It may be attributed to the many vacancies, which hinder the clustering of F atoms, and introduce many magnetic edge adatoms.

Realization of ferromagnetic graphene oxide with high magnetization by doping graphene oxide with nitrogen

The long spin diffusion length makes graphene very attractive for novel spintronic devices, and thus has triggered a quest for integrating the charge and spin degrees of freedom. However, ideal graphene is intrinsic non-magnetic, due to a delocalized π bonding network. Therefore, synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance. Here we report that N-doping can be an effective route to obtain a very high magnetization of ca. 1.66 emu/g, and can make graphene oxide (GO) to be ferromagnetism with a Curie-temperature of 100.2 K. Clearly, our findings can offer the easy realization of ferromagnetic GO with high magnetization, therefore, push the way for potential applications in spintronic devices.

Coil-in-Coil Carbon Nanocoils: 11 Gram-Scale Synthesis, Single Nanocoil Electrical Properties, and Electrical Contact Improvement

Coil-in-coil carbon nanocoils (CNCs) were synthesized by means of acetylene decomposition using nickel nanoparticles as catalysts. The investigations revealed that there are often several CNCs self-assembled in one nanospring. The yield of coil-in-coil CNCs was high up to 11 g in each run at the decomposition temperature of 450 degrees C. CNC nanodevices were fabricated for systematical examinations of charge conduction in the single CNC and in the electrical contacts. A focused laser beam of about 70 mum in diameter was applied for selective annealing CNC nanodevices so as to improve the electrical contacts to the CNC. Our study showed that the selective focused laser annealing technique is an effective route to improve the electrical contacts to the nanodevice. Temperature-dependent CNC resistances are analyzed with the Mott-variable range hopping (VRH) and Efros-Shklovskii VRH model, revealing electron hopping conduction in the disordered CNCs with a characteristic length of about 5-50 nm.

Quenching of fluorescence of reduced graphene oxide by nitrogen-doping

N-doped graphene (NG) has been prepared by annealing reduced graphene oxide (RGO) in ammonia in atmosphere and in vacuum, respectively. The photoluminescence properties of RGO and NG have been examined systematically. The results showed that doping RGO with N can quench its fluorescence, and the fluorescence quenching of NG obtained in vacuum is more efficient than that prepared in atmosphere

Enhanced ultraviolet photoluminescence from SiO2/Ge:SiO2/SiO2 sandwiched structure

SiO2/Ge:SiO2/SiO2 sandwiched structure was fabricated for exploring efficient light emission. After annealed in N2 (O2<1%), this structure shows three photoluminescence (PL) bands at 293, 395, and 780 nm. The intensity of the 395 nm band is largely enhanced in comparison with that from the monolayered Ge:SiO2 film. Spectral analyses suggest that the three PL bands originate from S1→S0, T∑(T∏)→S0, and T∏′→S0 optical transitions in GeO color centers, respectively. The improvement of the GeO density resulting from the confinement on Ge diffusion is responsible for the enhanced ultraviolet PL. This structure is expected to have important applications in optoelectronics.

Nitrogen-Superdoped 3D Graphene Networks for High-Performance Supercapacitors

An N-superdoped 3D graphene network structure with an N-doping level up to 15.8 at% for high-performance supercapacitor is designed and synthesized, in which the graphene foam with high conductivity acts as skeleton and nested with N-superdoped reduced graphene oxide arogels. This material shows a highly conductive interconnected 3D porous structure (3.33 S cm-1 ), large surface area (583 m2 g-1 ), low internal resistance (0.4 Ω), good wettability, and a great number of active sites. Because of the multiple synergistic effects of these features, the supercapacitors based on this material show a remarkably excellent electrochemical behavior with a high specific capacitance (of up to 380, 332, and 245 F g-1 in alkaline, acidic, and neutral electrolytes measured in three-electrode configuration, respectively, 297 F g-1 in alkaline electrolytes measured in two-electrode configuration), good rate capability, excellent cycling stability (93.5% retention after 4600 cycles), and low internal resistance (0.4 Ω), resulting in high power density with proper high energy density.