Sudeshna Chattopadhyay

Tunable optical properties in atomic layer deposition grown ZnO thin films

ZnO thin films with very low surface roughness and bulklike electron density were grown on Si and SiO2 by atomic layer deposition. The real and imaginary parts of the complex dielectric function of ZnO on Si show monotonically decreasing values with decreasing film thickness at and below a threshold of about 20 nm. On the other hand, x-ray reflectivity results show that the electron density of our ZnO films is close to that of bulk ZnO and does not vary considerably with film thickness. While the reduction of the dielectric function cannot be explained by the electron density of our ZnO films, the Tanguy–Elliott amplitude prefactor governing the strength of optical interband transitions can explain our results consistently through the lowering of the electron–hole overlap factor at the ZnO/Si interface. In the case of ZnO/Si, a staggered type-II (spatially indirect) quantum well, holes are scattered into the Si substrate, causing a lowering of the electron–hole overlap factor and thus the reduction of exc...

Role of Interfaces in Growth Dynamics of Nanostructures

Interfaces between nanostructures and the respective matrices have multiple roles in the growth dynamics and stability of the nanostructures. They can serve as confining barriers deciding the structure and orientation of growth, as in the case of air-water interfaces. They can redistribute the excess free energy, generated through different processing protocols, and give rise to specific growth centres. They can also play the role of exchanging energy (and material) between a nano-domain and a bulk, as seen in microscopic phase transitions. We have presented these specific aspects of interfaces in relation to nanoscale dynamics.

Morphology of Au nanoparticles formed by magnetron sputtering: ellipsoids and rings

Abstract Ultraviolet (UV) visible spectroscopy, atomic force microscopy and grazing incidence X-ray reflectivity have been used to study morphology of Au nanoparticles grown by direct current (DC) magnetron sputter deposition on hard (glass) and soft amorphous (polystyrene films on quartz) substrates. Au nanoparticles are found to be ellipsoidal showing an increase in ellipticity ϵ [≡a/b, a(b)=semimajor (semiminor) axis] with decrease in polystyrene film thickness from 250 to 20 nm, where b remains almost invariant around 3 nm. They sit on top of the film with the semimajor axes roughly parallel to film surface. On glass, the Au film was probed at different stages of growth. After an initial period (1 min) of spheroid nanoparticle formation by dewetting, the coverage was complete (as observed from Au optical spectra) and partially wetting islands appeared after 2 min on the Au covered glass surface. After 5 min, these islands formed rings resembling quantum rings. The rings broke up again into islands after 10 min.