Appearance of Flat Bands and Edge States in Boron-Carbon-Nitride Nanoribbons

Presence of flat bands and edge states at the Fermi level in graphene nanoribbons with zigzag edges is one of the most interesting and attracting properties of nanocarbon materials but it is believed that they are quite fragile states and disappear when B and N atoms are doped at around the edges. In this paper, we theoretically investigate electronic and magnetic properties of boron-carbon-nitride (BCN) nanoribbons with zigzag edges where the outermost C atoms on the edges are alternately replaced with B and N atoms using the first principles calculations. We show that BCN nanoribbons have the flat bands and edge states at the Fermi level in both H_2 rich and poor environments. The flat bands are similar to those at graphene nanoribbons with zigzag edges, but the distributions of charge and spin densities are different between them. A tight binding model and the Hubbard model analysis show that the difference in the distribution of charge and spin densities is caused by the different site energies of B and N atoms compared with C atoms.Comment: 5 pages; 3 figure

"Narrow" Graphene Nanoribbons Made Easier by Partial Hydrogenation

It is a challenge to synthesize graphene nanoribbons (GNRs) with narrow widths and smooth edges in large scale. Our first principles study on the hydrogenation of GNRs shows that the hydrogenation starts from the edges of GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the number of carbon-carbon $\pi$-$\pi$ bonds. Furthermore, the partially hydrogenated wide GNRs have similar electronic and magnetic properties as those of narrow GNRs. Therefore, it is not necessary to directly produce narrow GNRs for realistic applications because partial hydrogenation could make wide GNRs "narrower"

Designing all-graphene nanojunctions by covalent functionalization

We investigated theoretically the effect of covalent edge functionalization, with organic functional groups, on the electronic properties of graphene nanostructures and nano-junctions. Our analysis shows that functionalization can be designed to tune electron affinities and ionization potentials of graphene flakes, and to control the energy alignment of frontier orbitals in nanometer-wide graphene junctions. The stability of the proposed mechanism is discussed with respect to the functional groups, their number as well as the width of graphene nanostructures. The results of our work indicate that different level alignments can be obtained and engineered in order to realize stable all-graphene nanodevices

Electronic and Magnetic Properties of Partially-Open Carbon Nanotubes

On the basis of the spin-polarized density functional theory calculations, we demonstrate that partially-open carbon nanotubes (CNTs) observed in recent experiments have rich electronic and magnetic properties which depend on the degree of the opening. A partially-open armchair CNT is converted from a metal to a semiconductor, and then to a spin-polarized semiconductor by increasing the length of the opening on the wall. Spin-polarized states become increasingly more stable than nonmagnetic states as the length of the opening is further increased. In addition, external electric fields or chemical modifications are usable to control the electronic and magnetic properties of the system. We show that half-metallicity may be achieved and the spin current may be controlled by external electric fields or by asymmetric functionalization of the edges of the opening. Our findings suggest that partially-open CNTs may offer unique opportunities for the future development of nanoscale electronics and spintronics.Comment: 6 figures, to appear in J. Am. Chem. So

Mapping of functionalized regions on carbon nanotubes by scanning tunneling microscopy

Scanning tunneling microscopy (STM) gives us the opportunity to map the surface of functionalized carbon nanotubes in an energy resolved manner and with atomic precision. But this potential is largely untapped, mainly due to sample stability issues which inhibit reliable measurements. Here we present a simple and straightforward solution that makes away with this difficulty, by incorporating the functionalized multiwalled carbon nanotubes (MWCNT) into a few layer graphene - nanotube composite. This enabled us to measure energy resolved tunneling conductance maps on the nanotubes, which shed light on the level of doping, charge transfer between tube and functional groups and the dependence of defect creation or functionalization on crystallographic orientation.Comment: Keywords: functionalization, carbon nanotubes, few layer graphene, STM, CITS, ST

Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons.

Published versio

Paramagnetic centers in graphene nanoribbons prepared from longitudinal unzipping of carbon nanotubes

Electron spin resonance (ESR) investigation of graphene nanoribbons (GNRs) prepared through longitudinal unzipping of multiwalled carbon nanotubes (MWCNTs) indicates the presence of C-related dangling bond centers, exhibiting paramagnetic features. ESR signal broadening from pristine or oxidized graphene nanoribbons (OGNRs) is explained in terms of unresolved hyperfine structure, and in the case of reduced GNRs (RGNRs), the broadening of ESR signal can be due to enhancement in conductivity upon reduction. The spin dynamics observed from ESR linewidth-temperature data reflect a variable range hopping (VRH) mechanism through localized states, consistent with resistance-temperature data.Comment: 17 pages, 4 Figure

Ferromagnetism in graphene nanoribbons: split versus oxidative unzipped ribbons

Two types of graphene nanoribbons: (a) potassium-split graphene nanoribbons (GNRs), and (b) oxidative unzipped and chemically converted graphene nanoribbons (CCGNRs) were investigated for their magnetic properties using the combination of static magnetization and electron spin resonance measurements. The two types of ribbons possess remarkably different magnetic properties. While the low temperature ferromagnet-like feature is observed in both types of ribbons, such room temperature feature persists only in potassium-split ribbons. The GNRs show negative exchange bias, but the CCGNRs exhibit a 'positive exchange bias'. Electron spin resonance measurements infer that the carbon related defects may responsible for the observed magnetic behaviour in both types of ribbons. Furthermore, proton hyperfine coupling strength has been obtained from hyperfine sublevel correlation experiments performed on the GNRs. Electron spin resonance provides no indications for the presence of potassium (cluster) related signals, emphasizing the intrinsic magnetic nature of the ribbons. Our combined experimental results may infer the coexistence of ferromagnetic clusters with anti-ferromagnetic regions leading to disordered magnetic phase. We discuss the origin of the observed contrast in the magnetic behaviours of these two types of ribbons

Thickness-Dependent Morphologies of Gold on N-Layer Graphenes

We report that gold thermally deposited onto n-layer graphenes interacts differently with these substrates depending on the number layer, indicating the different surface properties of graphenes. This results in thickness-dependent morphologies of gold on n-layer graphenes, which can be used to identify and distinguish graphenes with high throughput and spatial resolution. This technique may play an important role in checking if n-layer graphenes are mixed with different layer numbers of graphene with a smaller size, which cannot be found by Raman spectra. The possible mechanisms for these observations are discussed

Physicochemical Characterization, and Relaxometry Studies of Micro-Graphite Oxide, Graphene Nanoplatelets, and Nanoribbons

The chemistry of high-performance magnetic resonance imaging contrast agents remains an active area of research. In this work, we demonstrate that the potassium permanganate-based oxidative chemical procedures used to synthesize graphite oxide or graphene nanoparticles leads to the confinement (intercalation) of trace amounts of Mn2+ ions between the graphene sheets, and that these manganese intercalated graphitic and graphene structures show disparate structural, chemical and magnetic properties, and high relaxivity (up to 2 order) and distinctly different nuclear magnetic resonance dispersion profiles compared to paramagnetic chelate compounds. The results taken together with other published reports on confinement of paramagnetic metal ions within single-walled carbon nanotubes (a rolled up graphene sheet) show that confinement (encapsulation or intercalation) of paramagnetic metal ions within graphene sheets, and not the size, shape or architecture of the graphitic carbon particles is the key determinant for increasing relaxivity, and thus, identifies nano confinement of paramagnetic ions as novel general strategy to develop paramagnetic metal-ion graphitic-carbon complexes as high relaxivity MRI contrast agents