K. S. Subrahmanyam

Graphene: the new two-dimensional nanomaterial.

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, the new nanocarbon

Graphene is a fascinating new nanocarbon possessing, single-, bi- or few- (≤ ten) layers of carbon atoms forming six-membered rings. Different types of graphene have been investigated by X-ray diffraction, atomic force microscopy, transmission electron microscopy, scanning tunneling microscopy and Raman spectroscopy. The extraordinary electronic properties of single-and bi-layer graphenes are indeed most unique and unexpected. Other properties of graphene such as gas adsorption characteristics, magnetic and electrochemical properties and the effects of doping by electrons and holes are equally noteworthy. Interestingly, molecular charge-transfer also markedly affects the electronic structure and properties of graphene. Many aspects of graphene are yet to be explored, including synthetic strategies which can yield sufficient quantities of graphene with the desired number of layers.

A study of graphenes prepared by different methods: characterization, properties and solubilization

Graphene has been prepared by different methods: pyrolysis of camphor under reducing conditions (CG), exfoliation of graphitic oxide (EG), conversion of nanodiamond (DG) and arc evaporation of SiC (SG). The samples were examined by X-ray diffraction (XRD), transmission electron microscopy, atomic force microscopy, Raman spectroscopy and magnetic measurements. Raman spectroscopy shows EG and DG to exhibit smaller in-plane crystallite sizes, but in combination with XRD results EG comes out to be better. The CG, EG and DG samples prepared by us have BET surface areas of 46, 925 and 520 m2 g−1 respectively and exhibit significant hydrogen uptake up to 3 wt%. EG also exhibits a high CO2 uptake (34.7 wt%). Electrochemical redox properties of the graphene samples have been examined in addition to their use in electrochemical supercapacitors. Functionalization of EG and DG through amidation has been carried out with the purpose of solubilizing them in non-polar solvents. Water-soluble graphene has been produced by extensive acid treatment of EG or treatment with polyethylene glycol.

Chemical storage of hydrogen in few-layer graphene

Birch reduction of few-layer graphene samples gives rise to hydrogenated samples containing up to 5 wt % of hydrogen. Spectroscopic studies reveal the presence of sp3 C-H bonds in the hydrogenated graphenes. They, however, decompose readily on heating to 500 °C or on irradiation with UV or laser radiation releasing all the hydrogen, thereby demonstrating the possible use of few-layer graphene for chemical storage of hydrogen. First-principles calculations throw light on the mechanism of dehydrogenation that appears to involve a significant reconstruction and relaxation of the lattice.

Femtosecond carrier dynamics and saturable absorption in graphene suspensions

Nonlinear optical properties and carrier relaxation dynamics in graphene, suspended in three different solvents, are investigated using femtosecond (80 fs pulses) Z-scan and degenerate pump-probe spectroscopy at 790 nm. The results demonstrate saturable absorption property of graphene with a nonlinear absorption coefficient, beta of (similar to 2-9) x 10(-8) cm/W. Two distinct time scales associated with the relaxation of photoexcited carriers, a fast one in the range of 130-330 fs (related to carrier-carrier scattering) followed by it slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are observed

A study of the synthetic methods and properties of graphenes

Abstract Graphenes with varying number of layers can be synthesized by using different strategies. Thus, single-layer graphene is prepared by micromechanical cleavage, reduction of single-layer graphene oxide, chemical vapor deposition and other methods. Few-layer graphenes are synthesized by conversion of nanodiamond, arc discharge of graphite and other methods. In this article, we briefly overview the various synthetic methods and the surface, magnetic and electrical properties of the produced graphenes. Few-layer graphenes exhibit ferromagnetic features along with antiferromagnetic properties, independent of the method of preparation. Aside from the data on electrical conductivity of graphenes and graphene-polymer composites, we also present the field-effect transistor characteristics of graphenes. Only single-layer reduced graphene oxide exhibits ambipolar properties. The interaction of electron donor and acceptor molecules with few-layer graphene samples is examined in detail.

Unusual magnetic properties of graphene and related materials

High-temperature ferromagnetism in graphene and other graphite-derived materials reported by several workers has attracted considerable interest. Magnetism in graphene and graphene nanoribbons is ascribed to defects and edge states, the latter being an essential feature of these materials. Room-temperature ferromagnetism in graphene is affected by the adsorption of molecules, especially hydrogen. Inorganic graphene analogues formed by layered materials such as BN and MoS2 also show such ferromagnetic behaviour. Magnetoresistance observed in graphene and graphene nanoribbons is of significance because of the potential applications.

GRAPHENE PRODUCED BY RADIATION-INDUCED REDUCTION OF GRAPHENE OXIDE

Effect of irradiation on graphene oxide by sunlight, UV light and KrF excimer laser has been investigated in detail. Both sunlight and ultraviolet light reduce graphene oxide well after prolonged irradiation, but laser irradiation produces graphene with negligible oxygen functionalities within a short time. Laser irradiation is also useful for one-step synthesis of metal particle decorated graphene. Laser irradiation of graphene oxide appears to be an efficient procedure for large-scale synthesis of graphene.

Photoluminescence, white light emitting properties and related aspects of ZnO nanoparticles admixed with graphene and GaN

ZnO nanoparticles exhibit a broad band centred around 530 nm in the photoluminescence (PL) spectrum due to the presence of oxygen vacancies. Composites of ZnO nanoparticles with graphenes show marked changes in the PL spectrum with broad bands covering the entire visible region, making them candidates for solid state lighting, while graphene prepared by arc discharge of graphite in a hydrogen atmosphere (HG) containing 2-3 layers as well as boron-doped (BHG) and nitrogen-doped (NHG) samples of HG give white light when admixed with ZnO nanoparticles; excellent results are obtained with the addition of just 7 wt% of BHG to the ZnO nanoparticles. Mixtures of ZnO and GaN nanoparticles also exhibit white light emission. The quantum yields of these ZnO nanoparticle based white light sources are in the 4-6% range. Photoconductivity characteristics of ZnO nanoparticles are affected by the addition of even a small amount of graphene (<0.5 wt%).

Synthesis and Selected Properties of Graphene and Graphene Mimics

Graphene has generated great excitement in the last few years because of its novel properties with potential applications. Graphene exhibits an ambipolar electric field effect, ballistic conduction of charge carriers, and the quantum Hall effect at room temperature. Some of the other interesting characteristics of graphene include high transparency toward visible light, high elasticity and thermal conductivity, unusual magnetic properties, and charge transfer interactions with molecules. In this Account, we present the highlights of some of our research on the synthesis of graphene and its properties. Since the isolation and characterization of graphene by micromechanical cleavage from graphite, several strategies have been developed for the synthesis of graphene with either a single or just a few layers. The most significant contribution from our laboratory is the synthesis of two to four layer graphene by arc-discharge of graphite in a hydrogen atmosphere. Besides providing clean graphene surfaces, this method allows for doping with boron and nitrogen. UV and laser irradiation of graphene oxide provides fairly good graphene samples, and laser unzipping of nanotubes produces graphene nanoribbons. We have exploited Raman spectroscopy to investigate the charge-transfer interactions of graphene with electron-donor and -acceptor molecules, as well as with nanoparticles of noble metals. Graphene quenches the fluorescence of aromatics because of electron transfer or energy transfer. Notable potential applications of the properties of graphene are low turn-on field emission and radiation detection. High-temperature ferromagnetism is another intriguing feature of graphene. Although incorporation of graphene improves the mechanical properties of polymers, its incorporation with nanodiamond or carbon nanotubes exhibits extraordinary synergy. The potential of graphene and its analogues as adsorbents and chemical storage materials for H(2) and CO(2) is noteworthy.