Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high-temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half-integer quantum Hall effect and ballistic electron transport. This two-dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two-layer and few-layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.

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...

Most of recent research on layered chalcogenides is understandably focused on single atomic layers. However, it is unclear if single-layer units are the most ideal structures for enhanced gas-solid interactions. To probe this issue further, we have prepared large-area MoS2 sheets ranging from single to multiple layers on 300 nm SiO2/Si substrates using the micromechanical exfoliation method. The thickness and layering of the sheets were identified by optical microscope, invoking recently reported specific optical color contrast, and further confirmed by AFM and Raman spectroscopy. The MoS2 transistors with different thicknesses were assessed for gas-sensing performances with exposure to NO2, NH3, and humidity in different conditions such as gate bias and light irradiation. The results show that, compared to the single-layer counterpart, transistors of few MoS2 layers exhibit excellent sensitivity, recovery, and ability to be manipulated by gate bias and green light. Further, our ab initio DFT calculations on single-layer and bilayer MoS2 show that the charge transfer is the reason for the decrease in resistance in the presence of applied field.

Field effect transistors using ultrathin molybdenum disulfide (MoS(2)) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS(2) devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS(2) field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS(2). Uniform encapsulation of MoS(2) transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.

Room-temperature ferromagnetism has been observed in nanoparticles ͑7 -30 nm diam͒ of nonmagnetic oxides such as CeO 2 , Al 2 O 3 , ZnO, In 2 O 3 , and SnO 2 . The saturated magnetic moments in CeO 2 and Al 2 O 3 nanoparticles are comparable to those observed in transition-metal-doped wideband semiconducting oxides. The other oxide nanoparticles show somewhat lower values of magnetization but with a clear hysteretic behavior. Conversely, the bulk samples obtained by sintering the nanoparticles at high temperatures in air or oxygen became diamagnetic. As there were no magnetic impurities present, we assume that the origin of ferromagnetism may be the exchange interactions between localized electron spin moments resulting from oxygen vacancies at the surfaces of nanoparticles. We suggest that ferromagnetism may be a universal characteristic of nanoparticles of metal oxides.Integration of semiconductor with ferromagnetic functionality of electrons has been the focus of recent research in the area of spintronics because of the difficulties associated with the injection of spins into nonmagnetic semiconductors in conventional spintronic devices. Ferromagnetism in semiconductors and insulators is rare, the well-known ferromagnetic semiconductors being the chalcogenides EuX ͑X =O, S, and Se͒ ͑T C Ͻ 70 K͒ and CdCr 2 X 4 ͑X = S and Se͒ ͑T C Ͻ 142 K͒ with the rocksalt and spinel structure, respectively. 1,2 Following the theoretical prediction of Dietl et al. that Mn-doped ZnO and GaN could exhibit ferromagnetism above room temperature, 3 several studies have focused on films and bulk samples of metal oxides such as TiO 2 , ZnO, In 2 O 3 , SnO 2 , and CeO 2 doped with Mn, Co, and other transition metal ions. [4][5][6][7][8] While the existence of ferromagnetism in transitionmetal-doped semiconducting oxides remains controversial, 9 thin films of the band insulator HfO 2 have been reported to exhibit ferromagnetism at room temperature in the absence of any doping. 10 This is puzzling, since pure HfO 2 does not have any magnetic moment and the bulk sample is diamagnetic. Similar ferromagnetism has been reported in other nonmagnetic materials such as CaB 6 , CaO, and SiC where the origin of ferromagnetism is believed to be due to intrinsic defects. 11-13 It has been suggested that ferromagnetism in thin films of HfO 2 may be related to anion vacancies. 14 It has been reported very recently that thin films of undoped TiO 2 and In 2 O 3 also show ferromagnetism at room temperature, 15 the corresponding bulk forms of these materials being diamagnetic. Thin films of these oxides might have defects or oxygen vacancies that could be responsible for the observed ferromagnetism. Ab initio electronic structure calculations using density functional theory in HfO 2 have shown that isolated halfnium vacancies lead to ferromagnetism. 16 Meanwhile, there is a conflicting report attributing the ferromagnetism in HfO 2 to possible iron contamination while using stainless-steel tweezers in handling thin films. 17 In this Rapid Communication, ...