Bikas C. Das

Core-shell hybrid nanoparticles with functionalized quantum dots and ionic dyes: growth, monolayer formation, and electrical bistability.

We report growth, monolayer formation, and (electrical bistability and memory phenomenon) properties of hybrid core-shell nanoparticles. While inorganic quantum dots, such as CdS or CdSe, act as the core, a monolayer of ionic organic dye molecules, electrostatically bound to the surface of functionalized quantum dots, forms the shell. We form a monolayer of the core-shell hybrid nanoparticles via a layer-by-layer electrostatic assembly process. Growth and monolayer formation of the organic-inorganic hybrid nanoparticles have been substantiated by usual characterization methods, including electronic absorption spectroscopy of dispersed solution and atomic force microscope images of scratched films. Devices based on the hybrid nanoparticles have exhibited electrical bistability and memory phenomena. From the comparison of these properties in core-shell nanoparticles and in its components, we infer that the degree of conductance switching or on/off ratio is substantially higher in the hybrid nanoparticles. Also, they (core-shell particles) provide routes to tune the bistability and memory phenomena by choosing either of the components. A monolayer of hybrid nanoparticles has been characterized by a scanning tunneling microscope tip as the other electrode. We show that a single core-shell hybrid nanoparticle can exhibit bistability with an associated memory phenomenon. Charge confinement, as evidenced by an increase in the density of states, has been found to be the mechanism of electrical bistability.

Enhancement of electrical bistability through semiconducting nanoparticles for organic memory applications

We report that an enhancement in electrical bistability in devices based on organic molecules can be achieved by the introduction of semiconducting nanoparticles. Here, devices based on alternate layers of a dye in the xanthene class and CdSe nanoparticles have been compared with devices based on the individual components. Results from dye/CdSe devices have yielded an appreciable enhancement in electrical bistability compared with those based on the dye or the nanoparticles. The enhancement is due to augmented carrier transport through the nanoparticles to the dye that consequently undergoes a change in its conformation, having a higher conductivity. We have evidenced read-only and random-access memory applications in the dye/nanoparticle hybrid system.

To change transport gap of semiconducting nanoparticles without disturbing the optical one: Core-shell approach

We show that transport gap of semiconducting nanoparticles can be changed without disturbing the optical gap. This is achieved through inorganic-organic hybrid core-shell approach. Different inorganic nanoparticles with a bandgap in the UV to NIR range are used as the core; as a shell to the nanoparticles, a monolayer of different organic molecules is used. With the inclusion of the shell layer, optical gap of the nanoparticles does not change. Transport gap, as obtained from current-voltage characteristics of a single nanoparticle with scanning tunneling microscope tip, changes to that of the shell-material irrespective of the bandgap of core nanoparticles.

Transport gap of nanoparticle-passivated silicon substrates.

The transport gap of nanoparticle-passivated Si substrates is measured by scanning tunneling microscopy. Passivation is achieved using a monolayer of CdSe nanoparticles. It is shown that the transport gap and conduction-band edge of the system change upon passivation. The size of the nanoparticles that passivate the Si substrate is varied to study its effect on the transport gap of the system. Plots of the tunneling current versus voltage show that the transport gap of the system can be tuned by the binding of just a monolayer of suitable nanoparticles. From the normalized density of states, it is shown that the conduction-band edge of the system responds to the size of the nanoparticles. Here, a monolayer of the nanoparticles, which were capped with suitable functional groups, has been formed via electrostatic adsorption with the substrate.