We report controlled doping in graphene monolayers through charge-transfer interaction by trapping selected organic molecules between graphene and underneath substrates. Controllability has been demonstrated in terms of shifts in Raman peaks and Dirac points in graphene monolayers. Under field effect transistor geometry, a shift in the Dirac point to the negative (positive) gate voltage region gives an inherent signature of n- (p-)type doping as a consequence of charge-transfer interaction between organic molecules and graphene. The proximity of organic molecules near the graphene surface as a result of trapping is evidenced by Raman and infrared spectroscopies. Density functional theory calculations corroborate the experimental results and also indicate charge-transfer interaction between certain organic molecules and graphene sheets resulting p- (n-)type doping and reveals the donor and acceptor nature of molecules. Interaction between molecules and graphene has been discussed in terms of calculated Mulliken charge-transfer and binding energy as a function of optimized distance.

Recently, increased attention has been drawn to application of graphene and its derivatives for construction of biosensors, since they can be used to rapidly detect the presence of bio-analytes.

High yield production of high quality graphene is essential for its application in electronics, optoelectronics and energy storage devices. Liquid phase exfoliation based methods for obtaining graphene are becoming popular because of their versatility and scalability. These advantages are absent with other growth methods such as mechanical exfoliation using scotch tape and chemical vapor deposition. Here we present a sonication assisted, surfactant free method for liquid phase exfoliation of graphene using solvents with varying dielectric constants. We have shown that the method presented here is capable of producing high yields (1.22 wt%), and exceptionally large sizes (30-50 microns) with a high carrier mobility of 10 000 cm 2 Vs À1 in monolayer graphene. Moreover, it is possible to obtain pristine as well as doped monolayer or bilayer or multilayer graphene with extreme controllability, on any solid substrate. It has been shown that choice of a solvent of a particular dielectric constant and sonication time are key parameters for liquid phase exfoliation. It is further shown that the exfoliation efficiency can be enhanced using solvents with high dielectric constant due to functionalization which has also been supported by density functional theory based electronic structure calculations. We have also tested this fact by using different solvents with similar dielectric constant. This method promises high-end industrial scale synthesis for potential applications in different types of devices, graphene based composites and liquid phase chemistry as well. † Electronic supplementary information (ESI) available: Method to calculate yield of graphene monolayers, associated chart and a brief discussion on electronic structure calculations. See Fig. 4 Atomic force microscopic images of graphene layers obtained by exfoliation of HOPG in (a) toluene and (b) PC. Respective height profiles are also given and indicated by arrows. (c) Histogram showing size distribution of graphene flakes as a function of their respective thickness as observed by AFM. Data corresponds to dispersions sonicated for 12 hours. (d) Size of monolayer graphene as a function of sonication time. It indicates that average size of the graphene flakes varies inversely with the sonication time. Y-axis error bars denote variation in flake sizes over 10-20 flakes. Circles denote the data points and solid line is the linear fit.This journal is

Excitonic transitions in graphene monolayers embedded in different dielectric environments have been investigated using combined absorption and transmission spectroscopy techniques. To vary the dielectric environment, graphene monolayer has been exfoliated in liquid medium. It has been shown that exciton binding energy decreases with increase in the dielectric constant of exfoliating solvents due to the screening of electron-electron and electron-hole interactions in graphene. The typical line shape of the excitonic peak in the absorption spectra is explained by the Fano resonance between the excitonic state and band continuum. Further it has been shown that, there exists a scaling relationship between the dielectric constant of the liquid and the exciton binding energy.

The development of an efficient test-bed for biosensors requires stable surfaces, capable of interacting with the functional groups present in bioentities. This work demonstrates the formation of highly stable electrochemically reduced graphene oxide (ERGO) thin films reproducibly on indium tin oxide (ITO)-coated glass substrates using a reliable technique through 60 s chronoamperometric reduction of a colloidal suspension maintained at neutral pH containing graphene oxide in deionized water. Structural optimization and biocompatible interactions of the resulting closely packed and uniformly distributed ERGO flakes on ITO surfaces (ERGO/ITO) are characterized using various microscopic and spectroscopic tools. Lipase enzyme is immobilized on the ERGO surface in the presence of ethyl-3-[3-(dimethylamino)propyl]carbodimide and N-hydroxysuccinimide for the detection of triglyceride in a tributyrin (TBN) solution. The ERGO/ITO surfaces prepared using the current technique indicate the noticeable detection of TBN, a source of triglycerides, at a sensitivity of 37 pA mg dL(-1) cm(-2) in the linear range from 50 to 300 mg dL(-1) with a response time of 12 s. The low apparent Michaelies-Menten constant of 0.28 mM suggests high enzyme affinity to TBN. The currently developed fast, simple, highly reproducible, and reliable technique for the formation of an ERGO electrode could be routinely utilized as a test bed for the detection of clinically active bioentities.

An electronically segmented amphiphile was created by conjugating two π-functional units hydroxyquinoline and naphthalenediimide (HQ/NDI) for the first time. The differential electrostatic potential of the π-surfaces, H-bonding units, etc. trigger a manifold response and direct the assembly of a unique collection of seven diverse nano-architectures. Chiral assembly, distinct classes of fibers, 3-D sheets, and metallo-spheres/fibrils with μM levels of Co/Cu/Zn(ii) ions emerged from this new approach of assorted morphosynthesis under ambient conditions.

We present a simple, non-oxidative and controlled method to synthesize graphene monolayers by exfoliation in water from different solid carbon sources, such as highly ordered pyrolytic graphite and low density graphite. Any water based method is highly desirable due to several attractive features, such as environmental friendliness, low cost and wide compatibility with other water based processes. We show that thin graphene layers can be exfoliated controllably and reproducibly by varying different parameters during exfoliation in aqueous medium. It has been possible to obtain high quality graphene monolayers with a yield of ∼2.45 wt%, which can be increased up to 16.6 wt% by recycling the sediments. Field effect transistors based on exfoliated graphene monolayers have shown n-type doping and a high carrier mobility of 4500 cm(2) V(-1) s(-1) at room temperature and ∼20 000 cm(2) V(-1) s(-1) at low temperature. Density functional calculations corroborate the infrared spectroscopic results and also indicate that the charge transfer preferentially occurs from water molecules to the graphene sheets resulting in n-type doping. We anticipate that exfoliation of high quality graphene layers in aqueous medium would open up a pathway for various graphene based electronic and biological applications.

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