C. V. Dharmadhikari

Linear Superclusters of Colloidal Gold Particles by Electrostatic Assembly on DNA Templates

The organization of nanoparticles into superstructures of predefined geometry is an important challenge in the area of nanoscale architecture. Attractive Coulombic interaction between positively charged amine groups on gold particle surfaces and negatively charged phosphate backbones of DNA molecules drives the self-assembly of gold nanoparticles into linear supercluster structures.

DNA-mediated electrostatic assembly of gold nanoparticles into linear arrays by a simple drop-coating procedure

The assembly of nanoparticles in topologically predefined superstructures is an important problem in the area of nanoscale architecture. In this letter, we demonstrate the electrostatic assembly of lysine-capped colloidal gold particles on drop-coated DNA films. Electrostatic interaction between the positive charges on the gold nanoparticles and the negative charges on the phosphate groups of the DNA template molecules leads to the assembly of the gold nanoparticles in linear superstructures. The use of DNA as templates for the assembly of nanoparticles shows promise for extension to more complex geometries through rational design of the DNA base sequences as well as in the realization of nanowires by stringing together metal nanoparticles.

Investigation of interparticle interactions of larger (4.63 nm) monolayer protected gold clusters during quantized double layer charging

In this article, the effect of interparticle interactions of 4.63 nm sized monolayer protected gold clusters (Au MPCs) during quantized double layer (QDL) charging has been investigated using electrochemical techniques. Voltammetry and scanning tunneling microscopy have been used to compare their electron transfer behavior. Furthermore, since the QDL process is diffusion controlled, the diffusion coefficient values have been estimated at various charge steps using two independent electroanalytical techniques, viz. chronoamperometry and impedance. These results show that higher core charge facilitates higher diffusion coefficient values, and indicate that repulsive interactions dominate for charged MPCs compared to those of its neutral analogue, which are mainly attractive in nature. Additionally, the electron transfer rate constants at various charge steps have been estimated from the impedance results, showing comparatively faster electron transfer rate at higher charge states.

Evidence for layered growth of (100) textured diamond films

The growth of diamond films synthesized by the hot filament chemical vapor deposition method has been studied by scanning tunneling microscopy (STM). The smooth (100) faces of micron-sized diamond crystallites were found to exhibit pyramidal shaped architecture at the nano level. The STM data for chemical vapor deposited diamond is analyzed in the light of recently developed dynamical scaling approach. The value of the scaling exponent α≅0.85 signifies a growth primarily driven by the transport of reacting species on the surface rather than molecular diffusion in gas phase.

Directed organization of gold nanoclusters on silver nanowires: a step forward in heterostructure assembly

We investigate the directed assembly of tridecylamine protected gold nanoclusters of 4–5nm size on functionalized silver nanowires of 55–60nm diameter and the electron transfer behavior of this integrated structure using transmission electron microscopy, non-contact atomic force microscopy, and scanning tunneling microscopy/spectroscopy. Linear I-V for bare silver nanowire suggests metallic behavior but high tunnel resistance indicates presence of insulating layer on the surface. Identical I-Vs obtained for isolated gold nanoparticle and heterostructure suggests that electron transport across nanowires in the latter is governed by gold nanoparticles in contrast to expected ballistic or diffusive transport along their length.

Pencil lead tips: a field ion and field electron emission microscopic study

Pencil lead tips composed of graphite flakes were subjected to field ion and field emission microscopic investigations. The ion micrographs showed elongated images of ledge atoms of the graphite flakes due to uneven magnification over the layers of the flake. The gross features of the field evaporated tip surface were observed by scanning electron microscopy. The field emission pattern showed emitting lobes which displayed intensity fluctuations consisting of a combination of emission spots turning on and off randomly and a localized flicker of individual spots. These effects gave rise to noise in the emission current involving isolated spikes of rapid rise time and trains of digital pulses of constant height. The variation of noise with residual gas pressure, emission current, and temperature has also been investigated. The results are discussed in view of the microtopography of the pencil lead tips and the nature of the emitting sites on the surface.

Observation of electron tunneling induced photon emission in gallium (Ga) doped (1%) zinc oxide (ZnO) sample using scanning tunneling microscopy

Electron tunneling induced photon emission in gallium doped (1%) zinc oxide was detected by scanning tunneling microscope in ambient condition. Simultaneously acquired topography and photon maps reveal interesting correlation. Photon maps depict few intense emission spots. Existence of a threshold tunneling current (6 nA) and applied bias (1.8 V), for detectable photon emission was observed. Further analysis of the results suggests Fowler-type photon emission which is ascribed to radiative electron-hole recombination. Localized photon emission spectroscopy exhibits prominent peaks at hν=2.96, 3.43, and 3.80 eV, which match well with the results obtained from photoluminescence studies.

Study of direct‐current, pulsed, and temperature–field emission from LaB6/tungsten field emitter

This paper reports dc, temperature–field (T–F), and pulsed emission from a lanthanum hexaboride (LaB6)–tungsten (W) field emitter. Various parameters are drived from the Fowler–Nordheim plots for the three configurations of the emitter, viz., clean tungsten, LaB6 overgrowth on W(111) plane (‘‘two spot’’), and LaB6 equilibrated W surface (‘‘well spread’’). The current stability is observed at 300, 730, and 850 K, (and 1065 K for well spread) under T–F operation. A comparison is made with various types of field emitting cathodes and the results are discussed from the point of view of LaB6/W field emitting cathodes.

Electrical characterization of zinc oxide/aluminum nitride thin film precursor field effect transistor structures: A conducting atomic force microscopy and density functional theoretical study

Electrical transport across pulsed laser deposited zinc oxide (ZnO)/aluminum nitride (AlN)/Si(100) thin film structures has been studied using conducting atomic force microscopy. Current versus voltage spectroscopy performed on the samples with varying AlN layer thickness (t), revealed asymmetric nonlinear behavior with a finite zero current region. The effective barrier height of the system is found to be (∼0.2 eV). The width of the zero current region was found to decrease exponentially with respect to t finally attaining the bulk band gap value. Density functional theory based calculations were carried out on the AlN and AlN–ZnO composite surface to investigate the band gap variation and ZnO adsorption on AlN. Results obtained by these calculations are in harmony with the experimental findings. Calculated values of the bulk cohesive energy explain the growth of the ZnO in an axis perpendicular to the surface supporting the experimentally observed results. Constant voltage current scans studies on the s...

Direct observation/characterization of spatial distribution of current leakage spots in zinc oxide/aluminum nitride thin film precursor field effect transistor structures using conducting atomic force microscopy

Charge transport across pulsed laser deposited zinc oxide (ZnO)/aluminum nitride (AlN)/Si(100) thin film structures has been studied using conducting atomic force microscopy at different stages of sample preparation. The spatial coverage of current leakage spots could be directly imaged, characterized, and shown to exhibit hysteresis against applied bias voltage. Current-voltage (I-V) measurements on both AlN and ZnO/AlN/Si(100) structure exhibited asymmetric nonlinear behavior with a large zero current region. Further analysis of I-V and current-force data suggests Fowler–Nordheim like behavior under Hertzian contact as a dominant mechanism for electron transport.