Graphene has emerged as an exciting material because of the novel properties associated with its two-dimensional structure. [1,2] Single-layer graphene is a one-atom thick sheet of carbon atoms densely packed into a two-dimensional honeycomb lattice. It is the mother of all graphitic forms of carbon, including zero-dimensional fullerenes and one-dimensional carbon nanotubes.[1] The remarkable feature of graphene is that it is a Dirac solid, with the electron energy being linearly dependent on the wave vector near the vertices of the hexagonal Brillouin zone. It exhibits a room-temperature fractional quantum Hall effect [3] and an ambipolar electric field effect along with ballistic conduction of charge carriers.[4] It has been reported recently that a top-gated single-layer graphene transistor is able to reach electron-or hole-doping levels of upto 5 Â 10 13 cm À2 . The doping effects are ideally monitored by Raman spectroscopy. [5][6][7][8][9][10] Thus, the G-band in the Raman spectrum stiffens for both electron-and hole-doping, and the ratio of the intensities of the 2D-and G-band varies sensitively with doping. Doping graphene through molecular charge-transfer caused by electron-donor and -acceptor molecules also gives rise to significant changes in the electronic structure of graphenes composed of a few layers, as evidenced by changes in the Raman and photoelectron spectra. [6,7] Charge-transfer by donor and acceptor molecules soften and stiffen the G-band, respectively. The difference between electrochemical doping and doping through molecular charge-transfer is noteworthy. It is of fundamental interest to investigate how these effects compare with the effects of doping graphene by substitution with boron and nitrogen and to understand dopant-induced perturbations of the properties of graphene. Secondly, opening the bandgap in graphene is essential for facilitating its applications in electronics, and graphene bilayers [11] are an attractive option for this. With this motivation, we prepared, for the first time, B-and N-doped graphene (BG and NG) bilayer samples by employing different strategies and investigated their structure and properties. We also carried out first-principles density functional theory (DFT) calculations to understand the effect of substitutional doping on the structure of graphene as well as its electronic and vibrational properties.To prepare BGs and NGs, we exploited our recent result in which it was determined that arc discharge between carbon electrodes in a hydrogen atmosphere yields graphenes (HG) composed of two to three layers.[12] The method makes use of the fact that in the presence of hydrogen, graphene sheets do not readily roll into nanotubes. In the case of BG, we carried out the arc discharge using graphite electrodes in the presence H 2 þ B 2 H 6 (BG1) or using boron-stuffed graphite electrodes (BG2). We prepared NG by carrying out the arc discharge in the presence of H 2 þ pyridine (NG1) or H 2 þ ammonia (NG2). We also performed the transformation of nanodiamond in th...

We report the effect of carboxyl functionalization of graphene in pacifying its strong hydrophobic interaction with cells and associated toxic effects. Pristine graphene was found to accumulate on the cell membrane causing high oxidative stress leading to apoptosis, whereas carboxyl functionalized hydrophilic graphene was internalized by the cells without causing any toxicity.

Arc discharge between graphite electrodes under a relatively high pressure of hydrogen yields graphene flakes generally containing 2-4 layers in the inner wall region of the arc chamber. The graphene flakes so obtained have been characterized by X-ray diffraction, atomic force microscopy, transmission electron microscopy, and Raman spectroscopy. The method is eminently suited to dope graphene with boron and nitrogen by carrying out arc discharge in the presence of diborane and pyridine respectively.Graphene is one of the most exciting materials being explored today. 1-3 It is a one-atom thick sheet of carbon atoms forming six-membered rings and is the basic building block of all other graphitic carbon materials. 1 The charge carriers in graphene behave as massless relativistic particles that are described on the basis of Dirac equation rather than the Schrödinger equation. Graphene exhibits fascinating properties such as quantum Hall effect at room temperature, 4-6 ballistic conduction with high mean free path, 1 tunable band gap, 7 and high elasticity. 8 Singlelayer graphene is generally prepared by micromechanical cleavage of highly ordered pyrolytic graphite (HOPG). 9 While this method may suffice for certain physical measurements, it cannot be employed for large-scale preparations and chemical studies. Single-layer graphene has been prepared and deposited on solid substrate by other methods as well, which include heating SiC, 10,11 intercalation followed by sonication 12 and interaction with polar solvents. 13,14 Bilayer graphene has been prepared by plasma enhanced chemical vapor deposition. 15,16 Various types of graphene samples have been prepared by thermal exfoliation of graphite oxide. 17,18 The number of layers in this preparation is 5-6 or more. Furthermore, graphene prepared by this method contain carboxyl and other functional groups. Another method is the conversion of nanodiamond in an inert atmosphere at high temperatures. 18,19 This method generally yields samples containing 5-6 layers with the inclusion of some graphitic particles. Few-layer graphene films have been grown on polycrystalline Ni employing chemical vapor deposition technique. 20 We have described a new method of preparing graphene containing 2-4 layers on a relatively large-scale. The procedure involves arc evaporation of graphite electrodes in a hydrogen atmosphere, and makes use of the knowledge that the presence of H 2 during the arc discharge process terminates the dangling carbon bonds with hydrogen and prevents the formation of closed structures. 21,22 It appears that H 2 plays a key role in the formation of graphene by preventing the rolling of sheets into nanotubes and graphitic polyhedral particles. We have noticed a report on the arc discharge of carbon electrodes in the presence of H 2 that reports the formation of multiwalled carbon nanotubes (MWNTs) in the central part of the cathode and petal-like graphite sheets consisting of interlaced graphene sheets in the outside region surrounding the cathode. 23,24 In the proce...

Graphene and its derivatives are being proposed for several important biomedical applications including drug delivery, gene delivery, contrast imaging, and anticancer therapy. Most of these applications demand intravenous injection of graphene and hence evaluation of its hemocompatibility is an essential prerequisite. Herein, both pristine and functionalized graphene are extensively characterized for their interactions with murine macrophage RAW 264.7 cells and human primary blood components. Detailed analyses of the potential uptake by macrophages, effects on its metabolic activity, membrane integrity, induction of reactive oxygen stress, hemolysis, platelet activation, platelet aggregation, coagulation cascade, cytokine induction, immune cell activation, and immune cell suppression are performed using optimized protocols for nanotoxicity evaluation. Electron microscopy, confocal Raman spectral mapping, and confocal fluorescence imaging studies show active interaction of both the graphene systems with macrophage cells, and the reactive oxygen species mediated toxicity effects of hydrophobic pristine samples are significantly reduced by surface functionalization. In the case of hemocompatibility, both types of graphene show excellent compatibility with red blood cells, platelets, and plasma coagulation pathways, and minimal alteration in the cytokine expression by human peripheral blood mononuclear cells. Further, both samples do not cause any premature immune cell activation or suppression up to a relatively high concentration of 75 μg mL(-1) after 72 h of incubation under in vitro conditions. This study clearly suggests that the observed toxicity effects of pristine graphene towards macrophage cells can be easily averted by surface functionalization and both the systems show excellent hemocompatibility.

The use of reduced graphene oxide (RGO) and graphene nanoribbons (GNRs) as infrared photodetectors is explored, based on recent results dealing with solar cells, light-emitting devices, photodetectors, and ultrafast lasers. IR detection is demonstrated by both RGO and GNRs in terms of the time-resolved photocurrent and photoresponse. The responsivity of the detectors and their functioning are presented.

Field emission studies have been carried out on undoped as well as N- and B-doped graphene samples prepared by arc-discharge method in a hydrogen atmosphere. These graphene samples exhibit very low turn-on fields. N-doped graphene shows the lowest turn-on field of 0.6 V/μm, corresponding to emission current density of 10 μA/cm2. These characteristics are superior to the other types of nanomaterials reported in the literature. Furthermore, emission currents are stable over the period of more than 3 h for the graphene samples. The observed emission behavior has been explained on the basis of nanometric features of graphene and resonance tunneling phenomenon.

Sensing characteristics of few-layer graphenes for NO 2 and humidity have been investigated with graphene samples prepared by the thermal exfoliation of graphitic oxide, conversion of nanodiamond (DG) and arc-discharge of graphite in hydrogen (HG). The sensitivity for NO 2 is found to be highest with DG. Nitrogen-doped HG (n-type) shows increased sensitivity for NO 2 compared with pure HG. The highest sensitivity for humidity is observed with HG. Sensing characteristics of graphene have been examined for different aliphatic alcohols and the sensitivity is found to vary with the chain length and branching.

Double-walled carbon nanotubes (DWNTs) doped with nitrogen and boron have been prepared by the decomposition of a CH(4) + Ar mixture along with pyridine (or NH(3)) and diborane, respectively, over a Mo(0.1)Fe(0.9)Mg(13)O catalyst, prepared by the combustion route. The doped DWNTs bave been characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. The dopant concentration is around 1 atom % for both boron and nitrogen. The radial breathing modes in the Raman spectra have been employed along with TEM to obtain the inner and outer diameters of the DWNTs. The diameter ranges for the undoped, N-doped (pyridine), N-doped (NH(3)), and B-doped DWNTs are 0.73-2.20, 0.74-2.30, 0.73-2.32, and 0.74-2.36 nm, respectively, the boron-doped DWNTs giving rise to a high proportion of the large diameter DWNTs. Besides affecting the G-band in the Raman spectra, N- and B-doping affect the proportion of semiconducting nanotubes.