Rakesh K. Joshi

Precise and Ultrafast Molecular Sieving Through Graphene Oxide Membranes

Graphene oxide membranes allow only very small hydrated molecules and ions to pass with an accelerated transport rate. [Also see Perspective by Mi] Graphene-based materials can have well-defined nanometer pores and can exhibit low frictional water flow inside them, making their properties of interest for filtration and separation. We investigate permeation through micrometer-thick laminates prepared by means of vacuum filtration of graphene oxide suspensions. The laminates are vacuum-tight in the dry state but, if immersed in water, act as molecular sieves, blocking all solutes with hydrated radii larger than 4.5 angstroms. Smaller ions permeate through the membranes at rates thousands of times faster than what is expected for simple diffusion. We believe that this behavior is caused by a network of nanocapillaries that open up in the hydrated state and accept only species that fit in. The anomalously fast permeation is attributed to a capillary-like high pressure acting on ions inside graphene capillaries. On the Fast Track Membranes based on graphene can simultaneously block the passage of very small molecules while allowing the rapid permeation of water. Joshi et al. (p. 752; see the Perspective by Mi) investigated the permeation of ions and neutral molecules through a graphene oxide (GO) membrane in an aqueous solution. Small ions, with hydrated radii smaller than 0.45 nanometers, permeated through the GO membrane several orders of magnitude faster than predicted, based on diffusion theory. Molecular dynamics simulations revealed that the GO membrane can attract a high concentration of small ions into the membrane, which may explain the fast ion transport.

Pd Nanoparticles and Thin Films for Room Temperature Hydrogen Sensor

We report the application of palladium nanoparticles and thin films for hydrogen sensor. Electrochemically grown palladium particles with spherical shapes deposited on Si substrate and sputter deposited Pd thin films were used to detect hydrogen at room temperature. Grain size dependence of H2sensing behavior has been discussed for both types of Pd films. The electrochemically grown Pd nanoparticles were observed to show better hydrogen sensing response than the sputtered palladium thin films. The demonstration of size dependent room temperature H2sensing paves the ways to fabricate the room temperature metallic and metal–metal oxide semiconductor sensor by tuning the size of metal catalyst in mixed systems. H2sensing by the Pd nanostructures is attributed to the chemical and electronic sensitization mechanisms.

Electron transfer mechanism of cytochrome c at graphene electrode

We report the direct electron transfer of cytochrome c (Cyt c) observed at graphene electrodes. Graphene nanosheets were chemically synthesized and immobilized on to a glassy carbon electrode. Cyclic voltammetry of Cyt c in phosphate buffered saline was performed at these electrodes. Results indicated a pair of reversible redox waves with a peak-to-peak separation value of 0.07 V in a diffusion controlled electrochemical process. Furthermore, the voltammetric response of these electrodes in Cyt c were found to be stable over time with negligible electrode fouling toward Cyt c.

Influence of Ag particle size on ethanol sensing of SnO1.8:Ag nanoparticle films: A method to develop parts per billion level gas sensors

The influence of Ag particle size on ethanol sensing of SnO1.8:Ag films composed of size-selected nanoparticles with independently controlled size and concentration of Ag is reported in the present study. The study shows that Ag nanoparticles are acting as catalyst for chemical sensitization through a spillover effect. The catalyst particles are observed to be more active on decreasing their size, resulting into an improved sensor response. A response time of 2s for 1000ppm ethanol has been achieved. Detection of 100ppb ethanol in air has been demonstrated using this well-defined technique.

Size dependence of optical properties in solution-grown Pb1?xFexS nanoparticle films

The optical band gap (Eg) of Pb1−xFexS solution-grown nanoparticle films was varied from 2.65 to 2.22 eV with an increase in iron concentration 0.25 ≤ x ≤ 0.75 in films grown at fixed pH and temperature by the chemical bath deposition method. The presence of excitonic structure in the ternary Pb1−xFexS for x ≥ 0.50 suggests increasing binding energy with increase in iron concentration in the films. A shift of excitonic peak towards higher energy with an increase in iron concentration is also observed.

Fermi level shifting of TiO2 nanostructures during dense electronic excitation

Scanning Kelvin probe microscopy has been used to understand the modification of work function of TiO2 with swift heavy ion irradiation. The observed increase in contact potential difference (CPD) indicates a shift in Fermi level towards the valence band, which is due to the development of defects during the bombardment of high energy heavy ions. The change in CPD values on ion irradiation is attributed to electronic excitation induced defect concentration and surface roughness.

Structure, conductivity and Hall effect study of solution grown Pb1?xFexS nanoparticle films

Nanoparticle films of ternary Pb1?xFexS alloys were grown on glass and quartz substrates using a chemical bath deposition technique. Critical control of pH, temperature and dilution of the chemical bath was necessary for growth of the nanoparticle films. The ternary films were observed to grow as single phase with a lattice identical to that of PbS. Analysis of temperature-dependent dc conductivity data suggests variable-range hopping to be the conduction mechanism. Room-temperature Hall mobility of the films is found to increase with an increase in x and has a temperature dependence of ? T?0.4, which suggests predominant surface scattering. Majority carriers were found to be holes in the 0.25 ? x ? 0.50 films and electrons in the x = 0.66 and 0.75 films.

Structural and electrical characteristics of nitrogen-doped nanocrystalline diamond films prepared by CVD

Nitrogen-doped nanocrystalline diamond (N-NCD) films were grown using microwave plasma enhanced chemical vapour deposition method. Films were characterized by Raman spectroscopy, x-ray diffraction and scanning electron microscopy. Perfectly ohmic contacts for the N-doped NCD film and p-type Si substrate were obtained using silver (Ag). Silver deposited at a temperature of 250 °C with the pulsed laser deposition method was found to show ohmic behaviour with a high degree of reproducibility. Current–voltage (I–V) characteristics are presented for various Ag/N-NCD and Ag–Si. Rectifying I–V behaviour for the interface N-NCD/p-Si suggests that the majority carriers for N-NCD are electrons.

Room temperature gas detection using silicon nanowires

Gas detection using silicon nanowires should only be possible at high temperatures. So how can sensing at ambient conditions be explained?

Graphene-based ultra-sensitive gas sensors

In the present work, we report the performance of grapheme-based gas sensors. Monolayer quality graphene was synthesized using thermal chemical vapor deposition. Graphene was characterized using Raman spectroscopy and Transmission electron microscopy. In addition, Hall Effect experiments were conducted to study the electronic properties of graphene layers. These graphene layers were tested to sense NO2, NH3, and Ethanol at room temperature. The responses of the grapheme-based gas sensor are highly reproducible.