Venkata Sai Kiran Chakravadhanula

Design of a Perfect Black Absorber at Visible Frequencies Using Plasmonic Metamaterials

The design and fabrication of a plasmonic black absorber with almost 100% absorbance spanning a broad range of frequencies from ultraviolet (UV) to the near infrared (NIR) is demonstrated. The perfect plasmonic absorber is achieved by a combination of a metal film with suitable metal/dielectric nanocomposites. Our fabrication technique is simple, versatile, cost-effective, and compatible with current industrial methods for solar absorber production.

Crystal growth behaviour in Au-ZnO nanocomposite under different annealing environments and photoswitchability

The growth of gold nanoparticles and ZnO nanorods in atom beam co-sputtered Au-ZnO nanocomposite (NC) system by annealing at two different ambient conditions is demonstrated in this work. Annealing in a furnace at 600 °C (air environment) confirmed the formation of ZnO nanorods surrounded with Au nanoparticles. In-situ annealing inside a transmission electron microscope (TEM) led to the formation of gold nanocrystals with different polygonal shapes. TEM micrographs were obtained in real time at intermediate temperatures of 300 °C, 420 °C, and 600 °C under vacuum. The growth mechanisms of Au nanocrystals and ZnO nanorods are discussed in the framework of Au-Zn eutectic and Zn-melting temperatures in vacuum and air, respectively. Current-voltage responses of Au-ZnO NC nanorods in dark as well as under light illumination have been investigated and photoswitching in Au-ZnO NC system is reported. The photoswitching has been discussed in terms of Au-ZnO band-diagram.

Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes

Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt₉Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the targets stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26 nm. The zeta potential of -45 mV indicates high stability of the colloid with a hydrodynamic diameter of 63 nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr.

Green chemistry and nanofabrication in a levitated Leidenfrost drop

Green nanotechnology focuses on the development of new and sustainable methods of creating nanoparticles, their localized assembly and integration into useful systems and devices in a cost-effective, simple and eco-friendly manner. Here we present our experimental findings on the use of the Leidenfrost drop as an overheated and charged green chemical reactor. Employing a droplet of aqueous solution on hot substrates, this method is capable of fabricating nanoparticles, creating nanoscale coatings on complex objects and designing porous metal in suspension and foam form, all in a levitated Leidenfrost drop. As examples of the potential applications of the Leidenfrost drop, fabrication of nanoporous black gold as a plasmonic wideband superabsorber, and synthesis of superhydrophilic and thermal resistive metal–polymer hybrid foams are demonstrated. We believe that the presented nanofabrication method may be a promising strategy towards the sustainable production of functional nanomaterials.

Functional Polymer Nanocomposites

While extensive research has been carried out in the field of structural polymer-based nanocomposites much less investigations have been concerned with polymer nanocomposites for functional applications. Among the functional nanomaterials, nanocomposites consisting of metal nanoparticles dispersed in a dielectric matrix are of particular interest due to their novel functional properties offering hosts of new applications. Here, polymers are attractive as matrix, and several approaches have been reported to incorporate metal nanoparticles into polymers. The present review is concerned with the preparation of polymer-based nanocomposites by vapor phase co-and tandem deposition and the resulting functional properties. The techniques involve evaporation and sputtering, respectively, of metallic and organic components and inter alia allow the preparation of composites which contain alloy clusters of well defined composition. Emphasis is placed on soft-magnetic high frequency materials with cut-off frequencies well above 1 GHz and on optical composites with tuned plasmon resonances suitable for ultra thin color filters, Bragg reflectors, and other devices. In addition, antibacterial coatings and sensors for organic vapors are addressed. The latter take advantage of the steep drop of the electrical resistivity at the percolation threshold. First results are also reported on the incorporation of photo-switchable molecules into nanocomposites near the percolation threshold. Moreover, a novel approach to produce magnetic nanorods is presented.

Assembling Photoluminescent Silicon Nanocrystals into Periodic Mesoporous Organosilica

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH(2)CH(2)Si(OEt)(3))(n)H, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications.

An omnidirectional transparent conducting-metal-based plasmonic nanocomposite.

A transparent conducting metal (TCM) composed of a stack of a gold film and silver/polymer nanocomposite fabricated by sputtering onto a glass substrate is presented. The plasmonic metamaterial shows an omnidirectional optical transmission up to 80 in the visible spectrum, which is comparable to that of ITO, and the electrical conductivity is one order of magnitude higher than that of ITO.

Performance study of magnesium–sulfur battery using a graphene based sulfur composite cathode electrode and a non-nucleophilic Mg electrolyte

Here we report for the first time the development of a Mg rechargeable battery using a graphene-sulfur nanocomposite as the cathode, a Mg-carbon composite as the anode and a non-nucleophilic Mg based complex in tetraglyme solvent as the electrolyte. The graphene-sulfur nanocomposites are prepared through a new pathway by the combination of thermal and chemical precipitation methods. The Mg/S cell delivers a higher reversible capacity (448 mA h g(-1)), a longer cyclability (236 mA h g(-1) at the end of the 50(th) cycle) and a better rate capability than previously described cells. The dissolution of Mg polysulfides to the anode side was studied by X-ray photoelectron spectroscopy. The use of a graphene-sulfur composite cathode electrode, with the properties of a high surface area, a porous morphology, a very good electronic conductivity and the presence of oxygen functional groups, along with a non-nucleophilic Mg electrolyte gives an improved battery performance.

Fe3O4 Anchored onto Helical Carbon Nanofibers as High‐Performance Anode in Lithium‐Ion Batteries

Lithium-ion batteries (LIBs) have been receiving increasing attention as attractive power sources, motivated especially by the rapid development of electric vehicles and portable devices. 2] As anode materials for LIBs, transition metal oxides utilize all of a metal’s redox potentials by the formation of metal through a chemical conversion mechanism, resulting in high theoretical capacities (500–1000 mAhg ) compared to commercially used graphite based on an intercalation mechanism (372 mAhg ). Among the oxides, Fe3O4 has gained considerable attention due to its low cost, abundance in nature, the fact that it is environmentally benign, and its high theoretical capacity of 926 mAhg 1 through the reaction Fe3O4+8Li + + 8e

Solvent-surface interactions control the phase structure in laser-generated iron-gold core-shell nanoparticles

3Fe+4Li2O. However, the application of Fe3O4 is hampered by a large volume expansion and structural rearrangements upon electrochemical cycling (problems that are common for conversion materials). To circumvent these problems, several hybrid nanostructures have been designed by mixing Fe3O4 with carbon. [4–6] Carbon plays a dual role in the electrodes: it can increase the electronic conductivity ; furthermore, it can work as a structural buffering material to accommodate the strain caused by the large volume change during the charge–discharge process. Various carbon materials, such as carbon nanotubes, carbon fibers, graphene, and others, have been successfully utilized to host Fe3O4 nanoparticles. Several approaches, such as hydrothermal, co-precipitation, as well as template syntheses, have been developed. Most strategies involve time-consuming procedures, usually requiring a separate step to, for example, deposit the carbon onto the surface of the presynthesized iron oxide or to fill or disperse iron oxide nanoparticles into pores or onto the carbon’s surface. Multistep processes or the use of several reagents might be required. It is thus desirable to develop a facile synthetic route to generate such functional composite materials. The present work describes a simple solvent-free process to obtain Fe3O4–C nanocomposites. In these composites Fe3O4 nanoparticles are anchored onto self-developed helical carbon fibers. Ferrocene, as a single precursor, acts as both iron and carbon source. As illustrated in Scheme 1, the synthesis is accomplished by a pyrolysis–oxidation route: pyrolysis of ferrocene in a closed stainless-steel reactor produces a fine black powder, referred to as “Fe–C” composite, consisting of two iron-rich phases: Fe and Fe3C; then, further mild oxidization of Fe–C under CO2, yields the final oxide product, referred to as “Fe3O4–C”. It is known that Fe and Fe3C can catalyze the formation of helical carbon. In the present procedure, Fe and Fe3C formed during pyrolysis of ferrocene and act as catalysts for the growth of helical carbon. Upon oxidation, both Fe and Fe3C are fully converted into Fe3O4, while the carbon morphology remains helical. Fe or Fe3C and later their transformation product (Fe3O4) are firmly embedded in the helical carbon structure. The thus-obtained Fe3O4–C composite shows a high reversible capacity, and good cycling and rate capability. Synergistic effects, through combining the redox reaction of metal oxide and carbon nanofibers, are discussed. X-ray diffraction (XRD) patterns of the material before and after oxidation (Figure 1a) show that upon pyrolysis of ferrocene, two distinct iron-rich phases of Fe3C (Joint Committee on Powder Diffraction Standards (JCPDS) card number 0350772) and a-Fe (JCPDS 006-0696) form in the Fe–C composite. The formation of Fe3C was due to the partial dissolution of carbon atoms into Fe. Upon further oxidation of the Fe–C composite, both Fe and Fe3C were oxidized to Fe3O4 (JCPDS 019-0629). In the Fe3O4–C composite, the narrow and sharp peaks suggest that the obtained Fe3O4 is of a highly crystalline nature. The mean crystallite size of Fe3O4 was calculated as 44 nm, according to the Scherrer equation. A strong peak due to graphitic carbon (2q=26.58) was observed in both Fe–C and Fe3O4–C composites. That the carbon morphology was well-maintained was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). N2 isotherms of the Fe3O4–C composite showed type IV curves with an H3 hysteresis loop (Figure 1b), indicating a mesoporous structure. The sample had a high Brunauer–Emmett– Teller (BET) surface area of 126 mg . The pore-size distribution (inset of Figure 1b) indicated the coexistence of both micropores (0.026 cmg ) and mesopores (0.08 cmg ). The relatively large specific surface area and high porosity offer a large material/electrolyte contact area and promote the diffusion of Li ions. Scheme 1. Fabrication of Fe–C and Fe3O4–C composites.