Bipin Kumar Gupta

Graphene Quantum Dots Derived from Carbon Fibers

Graphene quantum dots (GQDs), which are edge-bound nanometer-size graphene pieces, have fascinating optical and electronic properties. These have been synthesized either by nanolithography or from starting materials such as graphene oxide (GO) by the chemical breakdown of their extended planar structure, both of which are multistep tedious processes. Here, we report that during the acid treatment and chemical exfoliation of traditional pitch-based carbon fibers, that are both cheap and commercially available, the stacked graphitic submicrometer domains of the fibers are easily broken down, leading to the creation of GQDs with different size distribution in scalable amounts. The as-produced GQDs, in the size range of 1-4 nm, show two-dimensional morphology, most of which present zigzag edge structure, and are 1-3 atomic layers thick. The photoluminescence of the GQDs can be tailored through varying the size of the GQDs by changing process parameters. Due to the luminescence stability, nanosecond lifetime, biocompatibility, low toxicity, and high water solubility, these GQDs are demonstrated to be excellent probes for high contrast bioimaging and biosensing applications.

High permittivity polyaniline–barium titanate nanocomposites with excellent electromagnetic interference shielding response

Organic conductive polymers are at the forefront of materials science research because of their diverse applications built around their interesting and unique properties. This work reports for the first time a correlation between the structural, electrical, and electromagnetic properties of polyaniline (PANI)-tetragonal BaTiO3 (TBT) nanocomposites prepared by in-situ emulsion polymerization. XRD studies and HRTEM micrographs of these nanocomposites clearly revealed the incorporation of TBT nanoparticles in the conducting PANI matrix. EPR and XPS measurements reveal that increase in loading level of BaTiO3 results in a reduction of the doping level of PANI. The Ku-Band (12.4-18 GHz) network analysis of these composites shows exceptional microwave shielding response with absorption dominated total shielding effectiveness (SET) value of -71.5 dB (blockage of more than 99.99999% of incident radiation) which is the highest value reported in the literature. Such a high attenuation level, which critically depends on the fraction of BaTiO3 is attributed to optimized dielectric and electrical attributes. This demonstrates the possibility of using these materials in stealth technology and for making futuristic radar absorbing materials (RAMs).

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.

Encapsulation of γ-Fe2O3 decorated reduced graphene oxide in polyaniline core–shell tubes as an exceptional tracker for electromagnetic environmental pollution

The ultimate goal of the development of a new material γ-Fe2O3 decorated reduced graphene oxide (rGO)–polyaniline (PANI) core–shell tubes has been done for absorbing electromagnetic interference (EMI) pollution. Herein, we report on the synthesis and characterization of PANI tubes consisting of rGO decorated with iron oxide nanoparticles (RF). The intercalated RF was synthesized by thermal decomposition of ferric acetyl acetonate in a reducing atmosphere. Furthermore, RF was encapsulated through oxidative polymerization of aniline in the presence of β-naphthalene sulphonic acid which results in RF–PANI core–shell morphology. Scanning electron microscopy results confirm the formation of tubular core–shell morphology having 5–15 μm length and 1–5 μm diameter. The presence of rGO–γ-Fe2O3 in PANI core enhances the interfacial polarization and the effective anisotropy energy of the composite which contributes to more scattering and leads to high shielding effectiveness (SET ∼ 51 dB) at a critical thickness of 2.5 mm. Additionally, the effective complex permeability and permittivity parameters of the composites have been evaluated from the experimental scattering parameters (S11 & S21) using theoretical calculations given in Nicholson–Ross and Weir algorithms.

Synthesis and characterization of ultra-fine Y2O3:Eu3+ nanophosphors for luminescent security ink applications

We report a simple method for the synthesis of ultra-fine Eu(3+)-doped yttria (Y(2)O(3)) nanophosphors with an average diameter of approximately 5 nm for development of a transparent colloid that could be used as a luminescent security ink. This has been achieved by suitably substituting Eu(3+) ions at the favorable C(2) symmetry sites of Y(3+) ions and quantum mechanically confining the growth of the nanophosphor using a novel acid-catalyzed sol-gel technique. This is one of the few reports that depict the development of a transparent aqueous-stable Y(2)O(3):Eu(3+) colloidal solution for strategic applications related to security codes. High resolution transmission electron microscopy images showed excellent lattice fringes that in turn support the presence of better crystal quality and enhanced photoluminescence (PL) emission from the Y(1.9)O(3)Eu(0.1)(3+) nanophosphor system. Time resolved emission spectroscopy measurement indicated a PL decay time in the range of a few milliseconds, suitable for making luminescent security ink and other advanced applications in optoelectronic devices and bio-labeling.

Optical bifunctionality of europium-complexed luminescent graphene nanosheets.

Graphene is an intriguing two-dimensional material, which could be modified for achieving tunable properties with many applications. Photoluminescence of graphene due to plasmonic emission is well-known, however, attempts to develop strong luminescent graphene have been difficult. Synthesis of a graphene-based material with a dual optical functionality, namely quenching the fluorescence of organic dyes while maintaining its own self-luminescence, is an interesting and challenging proposition. Here, we demonstrate this optical bifunctionality in a lattice-modified luminescent graphene, where europium(III) cations are complexed with graphene through oxygen functionalities. After excitation at 314 nm, a hypersensitive red emission is observed at 614 and 618 nm showing the complexation of europium(III) with graphene. We demonstrate dual functionality of this graphene by the quenching of luminescence of Rhodamine-B while displaying its own hypersensitive red emission. The decay lifetime observed through the time-resolved spectroscopy confirms its potential for applications in biosensing as well as optoelectronics.

Facile synthesis and morphogenesis of superparamagnetic iron oxide nanoparticles for high-performance supercapacitor applications

A facile method has been developed for the synthesis of nearly mono-dispersed iron oxide nanocrystals. The structural analysis of the synthesized iron oxide nanocrystals reveals the magnetite phase of Fe3O4. The average particle size of the iron oxide was estimated to be 8 ± 2 nm. The observed particle size is in good correlation with the particle size estimated by magnetic measurements. Furthermore, these nanocrystals showed bi-functional ferromagnetic and superparamagnetic behavior below and above the blocking temperature, respectively. The potential use of these nanocrystals as an electrode for supercapacitors was examined by investigating the electrochemical behavior of iron oxide using cyclic voltammetry (CV) and galvanostatic charge–discharge tests. The CV characteristics of the iron oxide electrode showed a typical pseudocapacitive behavior in 3 M KOH solution. Moreover, the specific capacitance of 185 F g−1 at the current of 1 mA was observed with excellent cyclic stability, which is much higher than the reported value for iron oxide. The higher specific capacitance is due to the uniform nano-size of iron oxide. This work provides an ultimate facile method to synthesize nanostructured iron oxide for application in next generation energy storage materials.

Highly luminescent dual mode rare-earth nanorod assisted multi-stage excitable security ink for anti-counterfeiting applications

In this paper, the synthesis of multi-stage excitable (379 nm, 980 nm and 1550 nm), highly luminescent Y2O3:Yb3+/Er3+ nanorods by a hydrothermal method which can emit both hypersensitive green (562 nm) and strong red (660 nm) in a single host lattice and which can be used for a transparent security ink application is reported. Furthermore, these luminescent nanorods also exhibit both down-shift (1127 nm) as well as upconversion (562 nm) features at a single excitation near infrared wavelength of 980 nm. The characterization of these luminescent nanorods was explored by studying their structure/microstructure, using photoluminescence and time-resolved spectroscopic techniques. Furthermore, these luminescent nanorods exhibited the tuning of emission colours from red to green by controlling the sintering temperature. It was demonstrated that these novel luminescent nanorods offer new opportunities for making high-end, multi-stage excitable transparent security ink. The state-of-art production process of ink is easy and it is most suitable for bulk production at an economical cost. Thus, the security inks could be used globally in applications for protection against counterfeiting.

Hybrid 2D nanomaterials as dual-mode contrast agents in cellular imaging

The design of multifunctional nanofluids is highly desirable for biomedical therapy/cellular imaging applications.[1–4] The emergence of hybrid nanomaterials with specific properties, such as magnetism and fluorescence, can lead to an understanding of biological processes at the biomolecular level.[1] Various hybrid systems have been analyzed in the recent past for several possible biomedical applications.[5–9] Carbon-based hybrid systems such as carbon nanotubes with various nanoparticles are being widely tested for their biological applications because of their ability to cross cell membranes and their interesting thermal and electrical properties.[10,11] Graphene oxide (GO) is a fairly new graphene-based system with a 2D carbon honeycomb lattice decorated with numerous functional groups attached to the backbone: these functional groups make it an excellent platform for further attachment of nanoparticles and synthesis of hybrid materials. Cell viability studies on GO have been recently attempted, showing biocompatibility. [12,13] Moreover, the intrinsic photoluminescence (PL) properties of GO can be utilized for cellular imaging.[13] The large surface area and non-covalent interactions with aromatic molecules make GO an excellent system for biomolecular applications and drug attachment.

Highly luminescent-paramagnetic nanophosphor probes for in vitro high-contrast imaging of human breast cancer cells.

Highly luminescent-paramagnetic nanophosphors have a seminal role in biotechnology and biomedical research due to their potential applications in biolabeling, bioimaging, and drug delivery. Herein, the synthesis of high-quality, ultrafine, europium-doped yttrium oxide nanophosphors (Y(1.9)O(3):Eu(0.1)(3+)) using a modified sol-gel technique is reported and in vitro fluorescence imaging studies are demonstrated in human breast cancer cells. These highly luminescent nanophosphors with an average particle size of ≈6 nm provide high-contrast optical imaging and decreased light scattering. In vitro cellular uptake is shown by fluorescence microscopy, which visualizes the characteristic intense hypersensitive red emission of Eu(3+) peaking at 610 nm ((5)D(0)-(7)F(2)) upon 246 nm UV light excitation. No apparent cytotoxicity is observed. Subsequently, time-resolved emission spectroscopy and SQUID magnetometry measurements demonstrate a photoluminescence decay time in milliseconds and paramagnetic behavior, which assure applications of the nanophosphors in biomedical studies.