Tharangattu N. Narayanan

Artificially stacked atomic layers: Toward new van der Waals solids

Strong in-plane bonding and weak van der Waals interplanar interactions characterize a large number of layered materials, as epitomized by graphite. The advent of graphene (G), individual layers from graphite, and atomic layers isolated from a few other van der Waals bonded layered compounds has enabled the ability to pick, place, and stack atomic layers of arbitrary compositions and build unique layered materials, which would be otherwise impossible to synthesize via other known techniques. Here we demonstrate this concept for solids consisting of randomly stacked layers of graphene and hexagonal boron nitride (h-BN). Dispersions of exfoliated h-BN layers and graphene have been prepared by liquid phase exfoliation methods and mixed, in various concentrations, to create artificially stacked h-BN/G solids. These van der Waals stacked hybrid solid materials show interesting electrical, mechanical, and optical properties distinctly different from their starting parent layers. From extensive first principle calculations we identify (i) a novel approach to control the dipole at the h-BN/G interface by properly sandwiching or sliding layers of h-BN and graphene, and (ii) a way to inject carriers in graphene upon UV excitations of the Frenkell-like excitons of the h-BN layer(s). Our combined approach could be used to create artificial materials, made predominantly from inter planar van der Waals stacking of robust bond saturated atomic layers of different solids with vastly different properties.

Probing the biocompatibility of MoS2 nanosheets by cytotoxicity assay and electrical impedance spectroscopy.

Transition metal dichalgogenides such as MoS2 have recently emerged as hot two-dimensional (2D) materials due to their superior electronic and catalytic properties. Recently, we have reported the usefulness of MoS2 nanosheets toward the electrochemical detection of neurotransmitters and glucose (Narayanan et al 2014 Nanotechnology 25 335702). Furthermore, there are reports available in the literature that demonstrate the usefulness of MoS2 nanosheets for biosensing and energy storage applications (Zhu et al 2013 J. Am. Chem. Soc. 135 5998-6001; Pumera and Loo 2014 Trends Anal. Chem. 61 49-53; Lee et al 2014 Sci. Rep. 4 7352; Stephenson et al 2014 Energy Environ. Sci. 7 209-31). Understanding the cytotoxic effect of any material is very important prior to employing them for any in vivo biological applications such as implantable sensors, chips, or carriers for drug delivery and cell imaging purposes. Herein, we report the cytotoxicity of the MoS2 nanosheets based on the cytotoxic assay results and electrical impedance analysis using rat pheochromocytoma cells (PC12) and rat adrenal medulla endothelial cells (RAMEC). Our results indicated that the MoS2 nanosheets synthesized in our work are safe 2D nanosheets for futuristic biomedical applications.

Fluorinated graphene oxide for enhanced S and X-band microwave absorption

Here we report the microwave absorbing properties of three graphene derivatives, namely, graphene oxide (GO), fluorinated GO (FGO, containing 5.6 at. % Fluorine (F)), and highly FGO (HFGO, containing 23 at. % F). FGO is known to be exhibiting improved electrochemical and electronic properties when compared to GO. Fluorination modifies the dielectric properties of GO and hence thought of as a good microwave absorber. The dielectric permittivities of GO, FGO, and HFGO were estimated in the S (2 GHz to 4 GHz) and X (8 GHz to 12 GHz) bands by employing cavity perturbation technique. For this, suspensions containing GO/FGO/HFGO were made in N-Methyl Pyrrolidone (NMP) and were subjected to cavity perturbation. The reflection loss was then estimated and it was found that −37 dB (at 3.2 GHz with 6.5 mm thickness) and −31 dB (at 2.8 GHz with 6 mm thickness) in the S band and a reflection loss of −18 dB (at 8.4 GHz with 2.5 mm thickness) and −10 dB (at 11 GHz with 2 mm thickness) in the X band were achieved for 0.01 wt. % of FGO and HFGO in NMP, respectively, suggesting that these materials can serve as efficient microwave absorbers even at low concentrations.

Selective and efficient electrochemical biosensing of ultrathin molybdenum disulfide sheets

Atomically thin molybdenum disulfide (MoS₂) sheets were synthesized and isolated via solvent-assisted chemical exfoliation. The charge-dependent electrochemical activities of these MoS₂ sheets were studied using positively charged hexamine ruthenium (III) chloride and negatively charged ferricyanide/ferrocyanide redox probes. Ultrathin MoS₂ sheet-based electrodes were employed for the electrochemical detection of an important neurotransmitter, namely dopamine (DA), in the presence of ascorbic acid (AA). MoS₂ electrodes were identified as being capable of distinguishing the coexistence of the DA and the AA with an excellent stability. Moreover, the enzymatic detection of the glucose was studied by immobilizing glucose oxidase on the MoS₂. This study opens enzymatic and non-enzymatic electrochemical biosensing applications of atomic MoS₂ sheets, which will supplement their established electronic applications.

Large enhanced dielectric permittivity in polyaniline passivated core-shell nano magnetic iron oxide by plasma polymerization

Commercial samples of Magnetite with size ranging from 25–30u2009nm were coated with polyaniline by using radio frequency plasma polymerization to achieve a core shell structure of magnetic nanoparticle (core)–Polyaniline (shell). High resolution transmission electron microscopy images confirm the core shell architecture of polyaniline coated iron oxide. The dielectric properties of the material were studied before and after plasma treatment. The polymer coated magnetite particles exhibited a large dielectric permittivity with respect to uncoated samples. The dielectric behavior was modeled using a Maxwell–Wagner capacitor model. A plausible mechanism for the enhancement of dielectric permittivity is proposed.

Functionalized boron nitride porous solids

Hexagonal boron nitride (h-BN), also known as white graphene, is well known for its chemical inertness. Recent studies indicate that functionalization of h-BN can tune its physico-chemical properties, including its electrical conductivity. Here we propose a method for the functionalization of h-BN flakes with various oxygen functionalities to make a graphite oxide analogue of h-BN, with a view to develop cross-linked, low-density (∼40 mg cm−3), and porous h-BN solids, as have been recently well cited for graphene and graphite oxide. For the first time, a macro-porous low density h-BN monolith foam is developed via a single step template free chemical route followed by a lyophilisation process. h-BN is known for its high thermal stability, and here oil adsorption by the foam (∼2 g g−1) and complete burning of the adsorbed oil without disrupting the h-BN skeleton were demonstrated indicating the flexibility of tuning the morphology of the h-BN in bulk, like graphite, without losing its inherent physical properties, opening new avenues for h-BN in the energy and environment related fields.

A single-step room-temperature electrochemical synthesis of nitrogen-doped graphene nanoribbons from carbon nanotubes

Multiwalled carbon nanotubes (MWCNTs) were transformed into nitrogen-doped graphene/graphitic nanoribbons (N-doped GNRs) in a single-step electrochemical process at room temperature in formamide, which acts as a solvent and a source of nitrogen. The N-doped GNRs, with about 4 at% pyridinic and pyrrolic nitrogens, were characterized by transmission electron microscopy, atomic force microscopy, Raman spectrometry, infra-red spectrophotometry, X-ray photoelectron spectroscopy and cyclic voltammetry. Nitrogen doping can be regulated by varying the applied electric field or the duration of the reaction, wherein the kinetics of decomposition of formamide plays a critical role. N-doped GNRs show enhanced charge storage ability, attributed mainly to the increased surface area resulting from a change in morphology when cylindrical MWCNTs are sequentially unzipped as well as to a possible pseudocapacitance contribution from pyridinic and pyrrolic nitrogens. This unprecedented synthetic route provides a room-temperature method for the production of high quality, N-doped GNRs with specific nitrogen types for a variety of applications.

Temperature assisted shear exfoliation of layered crystals for the large-scale synthesis of catalytically active luminescent quantum dots

Edge state manipulations of transition metal dichalcogenides of ultra-small sizes are receiving tremendous scientific interest due to their applications in electronics, optoelectronics and energy conversion technologies. Here, we report a novel single step route for the large-scale production of luminescent quantum dots (QDs) of layered materials using a temperature assisted shear exfoliation method. The syntheses of molybdenum disulfide (MoS2) and tungsten disulfide (WS2) QDs are demonstrated, and enhanced hydrogen evolution reaction catalytic activities of QDs in comparison to their bulk and layered counterparts are demonstrated. This synthesis strategy is generalized to other layered structures such as graphite, leading to a bulk production of luminescent monodispersed QDs, enabling their status to the technology readiness level 9.

Contact potential induced enhancement of magnetization in polyaniline coated nanomagnetic iron oxides by plasma polymerization

The present work derives motivation from the so called surface/interfacial magnetism in core shell structures and commercial samples of Fe3O4 and γ Fe2O3 with sizes ranging from 20 to 30u2009nm were coated with polyaniline using plasma polymerization and studied. The High Resolution Transmission Electron Microscopy images indicate a core shell structure after polyaniline coating and exhibited an increase in saturation magnetization by 2u2009emu/g. For confirmation, plasma polymerization was performed on maghemite nanoparticles which also exhibited an increase in saturation magnetization. This enhanced magnetization is rather surprising and the reason is found to be an interfacial phenomenon resulting from a contact potential.

Electric field induced transformation of carbon nanotube to graphene nanoribbons using Nafion as a solid polymer electrolyte

We report a remarkable transformation of multiwalled carbon nanotubes (MWCNTs, average diameter 40u2009nm) to graphene nanoribbons (GNRs) in response to a field gradient of ∼25u2009V/cm, in a sandwich configuration using a solid state proton conducting polymer electrolyte like a thin perfluorosulphonated membrane, Nafion. In response to the application of a constant voltage for a sustained period of about 24 h at both room temperature and elevated temperatures, an interesting transformation of MWCNTs to GNRs has been observed with reasonable yield. GNRs prepared by this way are believed to be better for energy storage applications due to their enhanced surface area with more active smooth edge planes. Moreover, possible morphological changes in CNTs under electric field can impact on the performance and long term stability of devices that use CNTs in their electronic circuitry.