Amrita Singh

Electrochemical fabrication of ultralow noise metallic nanowires with hcp crystalline lattice

We experimentally demonstrate that low-frequency electrical noise in silver nanowires is heavily suppressed when the crystal structure of the nanowires is hexagonal closed pack (hcp) rather than face centered cubic (fcc). Using a low-potential electrochemical method we have grown single crystalline silver nanowires with hcp crystal structure, in which the noise at room temperature is two to six orders of magnitude lower than that in the conventional fcc nanowires of the same diameter. We suggest that motion of dislocations is probably the primary source of electrical noise in metallic nanowires, which is strongly diminished in hcp crystals.

Tracking random walk of individual domain walls in cylindrical nanomagnets with resistance noise.

The stochasticity of domain-wall (DW) motion in magnetic nanowires has been probed by measuring slow fluctuations, or noise, in electrical resistance at small magnetic fields. By controlled injection of DWs into isolated cylindrical nanowires of nickel, we have been able to track the motion of the DWs between the electrical leads by discrete steps in the resistance. Closer inspection of the time dependence of noise reveals a diffusive random walk of the DWs with a universal kinetic exponent. Our experiments outline a method with which electrical resistance is able to detect the kinetic state of the DWs inside the nanowires, which can be useful in DW-based memory designs.

Efficient electrochemical water oxidation catalysis by nanostructured Mn2O3

Prompted by recent research studies on the higher catalytic efficiency of Mn2O3 over MnO2, efforts have been made to develop nanostructured MnO2 and Mn2O3 and explore their comparative electrocatalytic response for water oxidation.

Field-tunable stochasticity in the magnetization reversal of a cylindrical nanomagnet

The nature of magnetization reversal in an isolated cylindrical nanomagnet has been studied employing time-resolved magnetoresistance measurement. We find that the reversal mode is highly stochastic, occurring either by multimode or single-step switching. Intriguingly, the stochasticity was found to depend on the alignment of the driving magnetic field to the long axis of the nanowires, where predominantly multimode switching gives way to single-step switching behavior as the field direction is rotated from parallel to transverse with respect to the nanowire axis.

Resistivity noise in crystalline magnetic nanowires and its implications to domain formation and kinetics

We have investigated the time-dependent fluctuations in electrical resistance, or noise, in high-quality crystalline magnetic nanowires within nanoporous templates. The noise increases exponentially with increasing temperature and magnetic field, and has been analyzed in terms of domain wall depinning within the Neel–Brown framework. The frequency-dependence of noise also indicates a crossover from nondiffusive kinetics to long-range diffusion at higher temperatures, as well as a strong collective depinning, which need to be considered when implementing these nanowires in magnetoelectronic devices.

Structure, Interactions and Action of Mycobacterium tuberculosis3-Hydroxyisobutyric Acid Dehydrogenase.

Biochemical and crystallographic studies on Mycobacterium tuberculosis 3-hydroxyisobutyric acid dehydrogenase (MtHIBADH), a member of the 3-hydroxyacid dehydrogenase superfamily, have been carried out. Gel filtration and blue native PAGE of MtHIBADH show that the enzyme is a dimer. The enzyme preferentially uses NAD+ as the cofactor and is specific to S-hydroxyisobutyric acid (HIBA). It can also use R-HIBA, l-serine and 3-hydroxypropanoic acid (3-HP) as substrates, but with much less efficiency. The pH optimum for activity is ∼11. Structures of the native enzyme, the holoenzyme, binary complexes with NAD+, S-HIBA, R-HIBA, l-serine and 3-HP and ternary complexes involving the substrates and NAD+ have been determined. None of the already known structures of HIBADH contain a substrate molecule at the binding site. The structures reported here provide for the first time, among other things, a clear indication of the location and interactions of the substrates at the active site. They also define the entrance of the substrates to the active site region. The structures provide information on the role of specific residues at the active site and the entrance. The results obtained from crystal structures are consistent with solution studies including mutational analysis. They lead to the proposal of a plausible mechanism of the action of the enzyme.

Domain wall dynamics in cylindrical nanomagnet

The stochasticity associated with domain wall nucleation and propagation in a cylinderical nanowire has been studied using time resolved resistance measurement in presence of magnetic field. We have shown that the propagation stochasticity of domain wall in a cylindrical nanowire is reflected in the magnetic field dependent velocity distribution whereas the stochasticity involved in the domain wall nucleation can be effectively tuned by varying the angle between the direction of applied magnetic field and the long axis of the cylinder.

Ultra‐low conductivity noise in metallic nanowires

By modifying the electrodeposition technique, we have stabilized the silver nanowires (AgNWs) in high‐energy hexagonal closed packed (hcp) structure. The conductivity noise measurements show that the noise magnitude in hcp silver nanowires is several orders of magnitude smaller than that of face centered cubic (fcc) silver nanowires, which is obtained by standard over potential electrodeposition (OPD) technique. The reduction of noise can be attributed to the restricted dislocation dynamics in hcp AgNWs due to the presence of less number of slip systems. Temperature dependent noise measurements show that the noise magnitude in hcp AgNWs is weakly temperature dependent while in fcc AgNWs it is strong function of temperature.

Tailoring of Structural Morphology of Silver Nanowires in Electrochemical Growth

Noble metal such as Ag normally exists in an fcc crystal structure. However as the size of the material is decreased to nanometer lengthscales, a structural transformation from that of its bulk state can be expected with new atomic arrangements due to competition between internal packing and minimization of surface energy. In many previous studies, it has been shown that silver nanowires (AGNWs) grown inside anodic alumina (AAO) templates by ac or dc electrochemical deposition from silver salts or complexes, adopt fcc structure and below some critical diameter ∼ 20 nm they may acquire hcp structure at low temperature. This is, however, critically dependant on the nature of confinement, as AgNWs grown inside nanotube confinement with subnanometer diameter have been reported to have fcc structure. Hence the question of the crystal structure of metal nanowires under combined influence of confinement, temperature and deposition condition remains open. In this abstract we show that the alternative crystal structures of AGNWs at room temperature can be achieved with electrochemical growth processes under specific conditions determined by the deposition parameters and nature of confinement. We fabricated AgNWs of 4H hexagonal structure with diameters 30 – 80 nm inside polycarbonate (PC) templates with a modified dc electrodeposition technique, where the nanowires were grown at deposition potentials as low as 10 mV in 2 M silver nitrate solution[1]. We call this low-potential electrodeposition (LPED) since the electrodeposition process occurs at potential much less than the standard Nernst potential (770 mV) of silver. Two types of electrodes were used – stainless steel and sputtered thin Pt film, neither of which had any influence on the crystal structure of the nanowires. EDS elemental analysis showed the nanowires to consist only of silver. Although the precise atomic dynamics during the LPED process is unclear at present, we investigated this with HRTEM (high-resolution transmission electron microscopy) characterization of nanowires grown over various deposition times, as well as electrical conductivity measurements. These experiments indicate that nanowire growth does not occur through a three-dimensional diffusion controlled process, as proposed for conventional over-potential deposition, but follow a novel instantaneous linear growth mechanism. Further experiments showed that, (a) conventional electrochemical growth at a small over-potential in a 2 mM AgNO 3 solution yields nanowires with expected fcc structure inside the same PC templates, and (2) no nanowire was observed under the LPED conditions inside hard AAO templates, indicating that LPED-growth process, and hcp structure of the corresponding nanowires depend on deposition parameters, as well as nature of confinement.