Browsing by Author "Ajayan P.M."

Now showing 1 - 6 of 6

Covalently Connected Carbon Nanotubes as Electrocatalysts for Hydrogen Evolution Reaction through Band Engineering
(2017) Pal S.; Sahoo M.; Veettil V.T.; Tadi K.K.; Ghosh A.; Satyam P.; Biroju R.K.; Ajayan P.M.; Nayak S.K.; Narayanan T.N.
Controlled assembly of mesoscopic structures can bring interesting phenomena because of their interfaces. Here, carbon nanotubes (CNTs) are cross-coupled via a C-C bonding through Suzuki reaction resulting in three-dimensional (3D) CNT sponges, and these 3D CNTs are studied for their efficacy toward the electrocatalytic hydrogen evolution reaction (HER) in acidic medium - one of the promising methods for the production of a renewable energy source, hydrogen. Both single and multiwall CNTs (SWCNTs and MWCNTs) are studied for the development of 3DSWCNTs and 3DMWCNTs, and these 3D CNTs are found to be HER active with small reaction onset potentials and low charge-transfer resistances unlike their uncoupled counterparts. First-principle density functional calculations show that the combination of electron acceptor and donor bonded to the CNT network can provide a unique band structure modulation in the system facilitating the HER reaction. This study can provide possibilities for band engineering of CNTs via functionalization and cross-coupling reactions. (Graph Presented). � 2017 American Chemical Society.
Electron Transfer Directed Antibacterial Properties of Graphene Oxide on Metals
(2018) Panda S.; Rout T.K.; Prusty A.D.; Ajayan P.M.; Nayak S.
Nanomaterials such as silver nanoparticles and graphene-based composites are known to exhibit biocidal activities. However, interactions with surrounding medium or supporting substrates can significantly influence this activity. Here, it is shown that superior antimicrobial properties of natural shellac-derived graphene oxide (GO) coatings is obtained on metallic films, such as Zn, Ni, Sn, and steel. It is also found that such activities are directly correlated to the electrical conductivity of the GO-metal systems; the higher the conductivity the better is the antibacterial activity. GO-metal substrate interactions serve as an efficient electron sink for the bacterial respiratory pathway, where electrons modify oxygen containing functional groups on GO surfaces to generate reactive oxygen species (ROS). A concerted effect of nonoxidative electron transfer mechanism and consequent ROS mediated oxidative stress to the bacteria result in an enhanced antimicrobial action of naturally derived GO-metal films. The lack of germicidal effect in exposed cells for GO supported on electrically nonconductive substrates such as glass corroborates the above hypothesis. The results can lead to new GO coated antibacterial metal surfaces important for environmental and biomedical applications. � 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
First-principles study of a vertical spin switch in atomic scale two-dimensional platform
(2019) Sahoo M.R.; Kushwaha A.K.; Pati R.; Ajayan P.M.; Nayak S.K.
High in-plane charge carrier mobility and long spin diffusion length makes graphene a unique material for spin-based devices. However, in a vertical graphene junction, the 2pz orbitals of carbon atoms in graphene can be tuned via suitable magnetic substrates; this would affect the spin injection into graphene. Here, a vertical spin switch has been designed by embedding a single layer of graphene as a tunnel layer between the Ni (1 1 1) substrate. Periodic density functional approach in conjunction with Julliere's model is used to calculate the tunnel magnetoresistance (TMR). Further, single-layered hexagonal Boron Nitride (h-BN) is sandwiched between the graphene and Ni (1 1 1) substrate to understand the role of hybridization at the interface on TMR. Our calculation shows that in contrast to the graphene junction, a much higher TMR value is obtainable in the case of the graphene/h-BN multi-tunnel junction (MTJ). The TMR in graphene junction is found to decrease with the increase of an externally applied electric field, and drops to zero for a field greater than equal to 0.16 V/�. Similar phenomenon was observed in the case of h-BN/graphene MTJ, where TMR value remains unchanged for electric field up to 0.1 V/� beyond which it drops to zero. The change in hybridization and charge-carrier-population at the interface modifies the magnetic exchange interaction and magnetic anisotropy resulting in a spin flip at interface, leads to rapid drop in TMR after a threshold electric field. The high and tunable TMR value suggests h-BN assisted high performance graphene based vertical spin switch. � 2019
Giant quasiparticle bandgap modulation in graphene nanoribbons supported on weakly interacting surfaces
(2013) Jiang X.; Kharche N.; Kohl P.; Boykin T.B.; Klimeck G.; Luisier M.; Ajayan P.M.; Nayak S.K.
In general, there are two major factors affecting bandgaps in nanostructures: (i) the enhanced electron-electron interactions due to confinement and (ii) the modified self-energy of electrons due to the dielectric screening. While recent theoretical studies on graphene nanoribbons (GNRs) report on the first effect, the effect of dielectric screening from the surrounding materials such as substrates has not been thoroughly investigated. Using large-scale electronic structure calculations based on the GW approach, we show that when GNRs are deposited on substrates, bandgaps get strongly suppressed (by as much as 1 eV) even though the GNR-substrate interaction is weak. � 2013 AIP Publishing LLC.
Temperature-dependent phonon shifts in monolayer MoS2
(2013) Lanzillo N.A.; Glen Birdwell A.; Amani M.; Crowne F.J.; Shah P.B.; Najmaei S.; Liu Z.; Ajayan P.M.; Lou J.; Dubey M.; Nayak S.K.; O'Regan T.P.
We present a combined experimental and computational study of two-dimensional molybdenum disulfide and the effect of temperature on the frequency shifts of the Raman-active E 2 g and A 1 g modes in the monolayer. While both peaks show an expected red-shift with increasing temperature, the frequency shift is larger for the A 1 g mode than for the E 2 g mode. This is in contrast to previously reported bulk behavior, in which the E 2 g mode shows a larger frequency shift with temperature. The temperature dependence of these phonon shifts is attributed to the anharmonic contributions to the ionic interaction potential in the two-dimensional system. � 2013 AIP Publishing LLC.
Tuning the Electrocatalytic Activity of Co3O4 through Discrete Elemental Doping
(2019) Swaminathan J.; Puthirath A.B.; Sahoo M.R.; Nayak S.K.; Costin G.; Vajtai R.; Sharifi T.; Ajayan P.M.
To gain constructive insight into the possible effect of doping on the electrocatalytic activity of materials, a catalytic framework with a discrete distribution of dopants is an appropriate model system. Such a system assures well-defined active centers, maximum atom utilization efficiency, and hence enhanced selectivity, catalytic activity, and stability. Herein, a comprehensive investigation of the electrocatalytic activity of iron-doped cobalt oxide (Fe-Co3O4) nanosheets is presented. In order to understand the contribution of dopants, a series of materials with controlled doping levels are investigated. By controlled iron inclusion into the structure of Co3O4, an apparent improvement in the oxygen evolution reaction activity which is reflected in the decrease of 160 mV in the overpotential to reach the current density of 10 mA/cm2 is manifested. Additionally, it is shown that there exists an optimum doping content above which the catalytic activity fades. Further investigation of the system with density functional calculations reveals that, along with the optimization of adsorption energy toward the reaction intermediates, substantial downshift of the Fermi level and delocalization of electron density occurs on introducing iron ions into the structure. Copyright � 2019 American Chemical Society.