Sublattice imbalance of substitutionally doped nitrogen in graphene

Motivated by the recently observed sublattice asymmetry of substitutional nitrogen impurities in CVD grown graphene, we show, in a mathematically transparent manner, that oscillations in the local density of states driven by the presence of substitutional impurities are responsible for breaking the sublattice symmetry. While these oscillations are normally averaged out in the case of randomly dispersed impurities, in graphene they have either the same, or very nearly the same, periodicity as the lattice. As a result, the total interaction energy of randomly distributed impurities embedded in the conduction-electron-filled medium does not vanish and is lowered when their configuration is sublattice-asymmetric. We also identify the presence of a critical concentration of nitrogen above which one should expect the sublattice asymmetry to disappear. This feature is not particular to nitrogen dopants, but should be present in other impurities.     

Resonant Raman spectra of graphene with point defects

The Raman spectra of graphene with three different types of point defects, namely, a mono-vacancy, a di-vacancy, and a Stone-Wales defect, was calculated within a non-orthogonal tight-binding model using supercells of graphene with a single defect. The defects were found to modify the electronic structure and the phonons of graphene giving rise to new optical transitions and defect-related phonons. Based on the calculated Raman spectra, we determined the Raman lines that can serve as signatures of the specific defects. The comparison of the calculated Raman intensity of the graphitic (G-) band of perfect graphene and graphene with defects shows that the intensity can be enhanced up to one order of magnitude by the presence of defects.     

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.     

Increased chemical enhancement of Raman spectra for molecules adsorbed on fluorinated reduced graphene oxide

Fluorinated reduced graphene oxide (F-RGO) is prepared by CF₄ plasma treatment of RGO. The fluorine (F) doping is confirmed by X-ray photoelectron spectroscopy and its content is directly related to the plasma exposure time. A modest p-doping effect of the fluorination is observed from the electrical measurements. It is found that the F-RGO is an even better substrate for surface enhanced Raman spectroscopy of molecules than RGO. The relative enhancement factor can be tuned by manipulating the F content in the F-RGO. The effect is attributed to the presence of the strong local electric field induced by the local dipoles of F-containing groups on the F-RGO surface. This shows that the formation of well-designed surface dipoles could be a general way to increase the chemical enhancement of molecular Raman spectra.     

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。     

Fano resonance in Raman scattering of graphene

Fano resonances and their strong doping dependence are observed in Raman scattering of single-layer graphene (SLG). As the Fermi level is varied by a back-gate bias, the Raman G band of SLG exhibits an asymmetric line shape near the charge neutrality point as a manifestation of a Fano resonance, whereas the line shape is symmetric when the graphene sample is electron or hole doped. However, the G band of bilayer graphene (BLG) does not exhibit any Fano resonance regardless of doping. The observed Fano resonance can be interpreted as interferences between the phonon and excitonic many-body spectra in SLG. The absence of a Fano resonance in the Raman G band of BLG can be explained in the same framework since excitonic interactions are not expected in BLG.     

Raman spectra,photoluminescence and ferromagnetism of pure,Co and Fe doped SnO2 nanoparticles

The pure and transition metal (Co and Fe = 3 and 5 mol%) doped SnO₂ nanoparticles have been synthesized by a chemical route using polyvinyl alcohol as surfactant. These nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) and magnetic measurements. The XRD patterns show that all the samples have tetragonal rutile structure without any extra phase and the value of average particle size using FWHM lies within 12–29 nm is also confirmed by TEM. FTIR spectrum has been used to confirm the formation of SnO bond. Raman spectroscopy shows the intensity loss of classical cassiterite SnO₂ vibration lines which is an indication of significant structural modifications. From PL, an intense blue luminescence centered at a wavelength ～530 nm is observed in the prepared SnO₂ nanoparticles, which is different from the yellow-red light emission observed in SnO₂ nanostructures prepared by other methods. The strong blue luminescence from the as-grown SnO₂ nanoparticles is attributed to oxygen-related defects that have been introduced during the growth process. These Co and Fe-doped SnO₂ nanoparticles exhibit room temperature ferromagnetism and the value of their magnetic moment and phase transition temperature are sensitive to their size and stoichiometric ratio.     

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.     

Surface-enhanced Raman scattering of suspended monolayer graphene

Abstract

The interactions between phonons and electrons induced by the dopants or the substrate of graphene in spectroscopic investigation reveal a rich source of interesting physics. Raman spectra and surface-enhanced Raman spectra of supported and suspended monolayer graphenes were measured and analyzed systemically with different approaches. The weak Raman signals are greatly enhanced by the ability of surface-enhanced Raman spectroscopy which has attracted considerable interests. The technique is regarded as wonderful and useful tool, but the dopants that are produced by depositing metallic nanoparticles may affect the electron scattering processes of graphene. Therefore, the doping and substrate influences on graphene are also important issues to be investigated. In this work, the peak positions of G peak and 2D peak, the I_2D/I_G ratios, and enhancements of G and 2D bands with suspended and supported graphene flakes were measured and analyzed. The peak shifts of G and 2D bands between the Raman and SERS signals demonstrate the doping effect induced by silver nanoparticles by n-doping. The I_2D/I_G ratio can provide a more sensitive method to carry out the doping effect on the graphene surface than the peak shifts of G and 2D bands. The enhancements of 2D band of suspended and supported graphenes reached 138, and those of G band reached at least 169. Their good enhancements are helpful to measure the optical properties of graphene. The different substrates that covered the graphene surface with doping effect are more sensitive to the enhancements of G band with respect to 2D band. It provides us a new method to distinguish the substrate and doping effect on graphene.

PACS

78.67.Wj (optical properties of graphene); 74.25.nd (Raman and optical spectroscopy); 63.22.Rc (phonons in graphene)     

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.     

Raman spectroscopic imaging of graphene dispersion in polymer composites

Polymer composites reinforced by graphene platelets (GPLs) have the advantage of enhanced mechanical properties at low loadings. However the planar geometry of GPLs and their entanglement complicate the dispersion of graphene in the polymer matrix and limit the improvement of the mechanical properties. In this study, confocal Raman spectroscopic imaging was used to map the dispersion of GPLs in an epoxy-based composite and was shown to be a suitable technique to study the dispersion of GPLs in the matrix. The Raman images were then used to explain the observed mechanical behavior of graphene–epoxy composites.     

Raman spectra of ground natural graphite

Structural changes in Ceylon natural graphite with grinding were studied by Raman spectroscopy along with X-ray diffraction. The natural graphite shows a single Raman band at 1580 cm^?1, but the ground graphite samples exhibit two Raman bands at 1360 and 1620 cm^?1 in addition to the 1580 cm^?1 graphite band. The 1360 cm^?1 band increases in intensity with increasing grinding time, and becomes much stronger than the 1580 cm^?1 band after 200-hr grinding. Raman results are compared with structural parameters such as effective Debye parameter and C₀ spacing obtained from X-ray diffraction measurements, and discussed in terms of structural defects introduced into the crystal lattice of natural graphite. A linear relationship was obtained for the ground graphite when the relative intensity of the 1360 cm^?1 band is plotted as a function of effective Debye parameter. The slope of the linear plot is different for the ground graphite from that for the graphitized cokes, indicating a difference in the type of structural defects involved.     

Raman spectra of Graphon carbon black

The Raman spectrum of Graphon carbon black has been recorded using rotating cell techniques. Angular dependence of scattering at 1360, 1580 and 2700 cm^?1 are reported and these data suggest that the 1360 cm^?1 line is associated with non-planar microstructure distortions. The excitation frequency dependence of the intensity ratio of the bands at 1360 (D) and 1580 cm^?1 (G) is interpreted in terms of resonance (vibronic) interaction. This dependence is primarily the result of an increase in the intensity of the 1360 cm ^?1 line. The disorder-associated line (D) exhibits a significant excitation-dependent shift from 1378 cm^i?1 (457.9 nm Ar⁺) to 1330 cm^?1 (647.1 nm Kr⁺). The “graphite” (G) line position is less sensitive to changes in excitation frequency. The spectral features are discussed in terms of factor group, C_6v⁴, and layer site symmetry, C_3v. Also the possible role of localized alkene-like structure in zones of structural distortion is considered.     

Raman spectroscopy at the edges of multilayer graphene

Individual graphene layers in a multilayer graphene sample contribute their own edges. The edge of a graphene layer laid on an n layer graphene (nLG) is a building block for the edges of multilayer graphenes. We found that the D band observed from the edge of the top graphene layer laid on the nLG exhibits an identical line shape to that of disordered (n + 1)LG. Based on the spectral features of the D and 2D bands, we identified two types of alignment configurations at the edges of bilayer and trilayer graphenes, whose edges are well-aligned from their optical images.     

Quantifying ion-induced defects and Raman relaxation length in graphene

Raman scattering is used to study disorder in graphene subjected to low energy (90 eV) Ar⁺ ion bombardment. The evolution of the intensity ratio between the G band (1585 cm⁻¹) and the disorder-induced D band (1345 cm⁻¹) with ion dose is determined, providing a spectroscopy-based method to quantify the density of defects in graphene. This evolution can be fitted by a phenomenological model, which is in conceptual agreement with a well-established amorphization trajectory for graphitic materials. Our results show that the broadly used Tuinstra-Koenig relation should be limited to the measure of crystallite sizes, and allows extraction of the Raman relaxation length for the disorder-induced Raman scattering process.     

The origin of sub-bands in the Raman D-band of graphene

In Raman spectroscopy investigations of defective suspended graphene, splitting in the D band is observed. Four double resonance Raman scattering processes: the outer and inner scattering processes, as well as the scattering processes with electrons first scattered by phonons (“phonon-first”) or by defects (“defect-first”), are found to be responsible for these features of the D band. The D sub-bands associated with the outer and inner processes merge with increasing defect concentration. However a Stokes/anti-Stokes Raman study indicates that the splitting of the D band due to the separate “phonon-first” and “defect-first” processes is valid for suspended graphene. For graphene samples on a SiO₂/Si substrate, the sub-bands of D band merge due to the increased Raman broadening parameter resulting from the substrate doping. Moreover, the merging of the sub-bands shows excitation energy dependence, which can be understood by considering the energy dependent lifetime and/or scattering rate of photo-excited carriers in the Raman scattering process.     

Nitrogen doped multi walled carbon nanotubes produced by CVD-correlating XPS and Raman spectroscopy for the study of nitrogen inclusion

High purity aligned nitrogen doped multi walled carbon nanotubes were synthesized by the catalytic chemical vapor deposition method using pyridine and Fe/Co (2:1 volume ratio) as the single C/N precursor and catalyst material. The average diameter of the synthesized tubes ranges between 29 nm and 57 nm and the nitrogen content of the tubes reaches a maximum of 9.2 (at.)% nitrogen. The effect of nitrogen doping on the Raman scattering of doped tubes and its correlation with X-ray photoelectron spectra (XPS) was investigated. The analysis is based on the investigation of the I_D/I_G (integrated area ratio), other nitrogen characteristic Raman modes and the type of nitrogen inclusion interpreted from the N 1s electron bonding energies in XPS. At doping levels higher than 5% the nitrogen inclusion takes place through another mechanism than at low nitrogen doping levels. Most significant is that pyridinic defects are relatively readily incorporated at low nitrogen doping levels while at nitrogen content higher than 5% the major incorporation mechanism is dominated by pyridinic and pyrrolic defects on an equal basis. Our study gives further insight into nitrogen doping effects and the relation between type of nitrogen inclusion and nitrogen doping levels.