Da Zhan

Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction

We present two different ways to fabricate nitrogen-doped graphene (N-graphene) and demonstrate its use as a metal-free catalyst to study the catalytic active center for the oxygen reduction reaction (ORR). N-graphene was produced by annealing of graphene oxide (G-O) under ammonia or by annealing of a N-containing polymer/reduced graphene oxide (RG-O) composite (polyaniline/RG-O or polypyrrole/RG-O). The effects of the N precursors and annealing temperature on the performance of the catalyst were investigated. The bonding state of the N atom was found to have a significant effect on the selectivity and catalytic activity for ORR. Annealing of G-O with ammonia preferentially formed graphitic N and pyridinic N centers, while annealing of polyaniline/RG-O and polypyrrole/RG-O tended to generate pyridinic and pyrrolic N moieties, respectively. Most importantly, the electrocatalytic activity of the catalyst was found to be dependent on the graphitic N content which determined the limiting current density, while the pyridinic N content improved the onset potential for ORR. However, the total N content in the graphene-based non-precious metal catalyst does not play an important role in the ORR process.

Magnetism in MoS2 induced by proton irradiation

Molybdenum disulphide, a diamagnetic layered dichalcogenide solid, is found to show magnetic ordering at room temperature when exposed to a 2 MeV proton beam. The temperature dependence of magnetization displays ferrimagnetic behavior with a Curie temperature of 895 K. A disorder mode corresponding to a zone-edge phonon and a Mo valence higher than +4 has been detected in the irradiated samples using Raman and x-ray photoelectron spectroscopy, respectively. The possible origins of long-range magnetic ordering in irradiated MoS2 samples are discussed.

Engineering the Electronic Structure of Graphene

Graphene exhibits many unique electronic properties owing to its linear dispersive electronic band structure around the Dirac point, making it one of the most studied materials in the last 5-6 years. However, for many applications of graphene, further tuning its electronic band structure is necessary and has been extensively studied ever since graphene was first isolated experimentally. Here we review the major progresses made in electronic structure engineering of graphene, namely by electric and magnetic fields, chemical intercalation and adsorption, stacking geometry, edge-chirality, defects, as well as strain.

Thermal Dynamics of Graphene Edges Investigated by Polarized Raman Spectroscopy

In this report, we present Raman spectroscopy investigation of the thermal stability and dynamics of graphene edges. It is found that graphene edges (both armchair and zigzag) are not stable and undergo modifications even at temperature as low as 200 °C. On the basis of polarized Raman results, we provide possible structural models on how graphene edges change during annealing. The zigzag edges rearrange and form armchair segments that are ±30° relative to the edge direction, while armchair edges are dominated by armchair segments even at annealing temperature as high as 500 °C. The modifications of edge structures by thermal annealing (zigzag segments rearrange in form of armchair segments) provide a flexible way to control the electronic properties of graphene and graphene nanostructures.

Plasma Modified MoS2 Nanoflakes for Surface Enhanced Raman Scattering

Though the SERS effect based on pristine MoS2 is hardly observed, however, the plasma treated MoS2 nanoflakes can be used as an ideal substrate for surface enhanced Raman scattering. It is proved that the structural disorder induced generation of local dipoles and adsorption of oxygen on the plasma treated MoS2 nanosheets are the two basic and important driven forces for the enhancement of Raman signals of surface adsorbed R6G molecules.

Mega-electron-volt proton irradiation on supported and suspended graphene: A Raman spectroscopic layer dependent study

Graphene samples with 1, 2, and 4 layers and 1 + 1 folded bi-layers and graphite have been irradiated with 2 MeV protons at fluences ranging from 1 × 1015 to 6 × 1018 ions/cm2. The samples were characterized using visible and UV Raman spectroscopy and Raman microscopy. The ion-induced defects were found to decrease with increasing number of layers. Graphene samples suspended over etched holes in SiO2 have been fabricated and used to investigate the influence of the substrate SiO2 for defect creation in graphene. While Raman vibrational modes at 1460 cm−1 and 1555 cm−1 have been observed in the visible Raman spectra of substantially damaged graphene samples, these modes were absent in the irradiated-suspended monolayer graphene.

Low temperature edge dynamics of AB-stacked bilayer graphene: naturally favored closed zigzag edges.

Closed edges bilayer graphene (CEBG) is a recent discovered novel form of graphene structures, whose regulated edge states may critically change the overall electronic behaviors. If stacked properly with the AB style, the bilayer graphene with closed zigzag edges may even present amazing electronic properties of bandgap opening and charge separation. Experimentally, the CEBG has been confirmed recently with HRTEM observations after extremely high temperature annealing (2000 °C). From the application point of view, the low temperature closing of the graphene edges would be much more feasible for large-scale graphene-based electronic devices fabrication. Here, we demonstrate that the zigzag edges of AB-stacked bilayer graphene will form curved close structure naturally at low annealing temperature (< 500 °C) based on Raman observation and first principles analysis. Such findings may illuminate a simple and easy way to engineer graphene electronics.

Repeated microwave-assisted exfoliation of expandable graphite for the preparation of large scale and high quality multi-layer graphene

The increasing demand for graphenes industrial application requires a new route for its mass production with high quality. Here, we report a facile, green, highly efficient and cost effective method for preparing a large amount of high quality graphene flakes, which is by the repeated microwave assisted exfoliation of expandable graphite (EG). The successful exfoliation of graphite is realized through the intercalation and decomposition of eco-friendly chemicals with the assistance of a microwave source. The chemical morphology and electrocatalytic performance analyses reveal that the graphene flakes are of high quality with little degradation.

Ultrafast carrier dynamics in pristine and FeCl3-intercalated bilayer graphene

Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and BLG intercalated with FeCl3 (FeCl3–G), were studied using time-resolved transient differential reflection (ΔR/R). Compared to BLG, the FeCl3–G data showed an opposite sign of ΔR/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting in the Fermi level in FeCl3–G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and BLG intercalated with FeCl3 (FeCl3–G), were studied using time-resolved transient differential reflection (ΔR/R). Compared to BLG, the FeCl3–G data showed an opposite sign of ΔR/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting in the Fermi level in FeCl3–G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.

Microwave-assisted production of giant graphene sheets for high performance energy storage applications

A simple, low-cost and energy-effective method has been developed in this work to fabricate giant graphene sheets by double microwave assisted exfoliations of expandable graphite. The graphene sheets, which are large (10–100 microns) in size and thin (1–10 nm) in thickness, manifest superior electrochemical performance for energy storage, achieving 221 F g−1 capacitance after 5000 cycles at 10 mV s−1 in a symmetric capacitor measurement and 400 mA h g−1 in a coin-type Li-ion battery after 300 cycles at 0.5 A g−1.