Abstract

Abstract

Thin Solid films find ubiquitous applications, but among them, the formation of band-gap engineered superlattices and epitaxial self-assembled nanostructures hold great promise in electronics, opto-electronics and catalysis. The need to tailor-make thin films and nanostructures of desired properties has motivated attempts to understand the initial stages of interface formation, since the morphology of the growing film is the result of a delicate interplay of the thermodynamic and kinetic processes involved. Thus, along with the material properties, growth conditions and quantum effects influence the final outcome of the low dimensional structures and their self-assembly. With the advent of ultra-high vacuum technology and surface sensitive probes, several interesting growth phenomena have been revealed at the atomistic level. I shall present my experimental experience of the formation of model systems in the sub-monolayer regime and in-situ characterization by electron spectroscopic, diffraction and imaging techniques. The deterministic role of lattice mismatch, surface free energy, dangling bonds, etc., in the evolution of the interfaces, will be elucidated. The formation of epitaxial self-assembled nanostructures and surface-phases by employing surface morphologies and reconstructions, as templates, will be demonstrated. The role of Edge Barriers and Quantum Size Effects on growth modes in a metal-semiconductor system, will be discussed. Our attempts to chemically modify ultra-thin films for materials related to GaN band-engineering applications will be demonstrated.

Abstract

Heavy ion reaction dynamics has been studied using the Dynamical Cluster-decay Model (DCM) of Gupta and collaborators. DCM have been applied to study the decay of hot and rotating compound systems (i.e. having angular momentum, l ≠0 and temperature, T ≠0). The DCM is a reformulation of Preformed Cluster-decay Model PCM for ground-state decays (i.e. l = 0 and T = 0). Both DCM and PCM are based on Quantum Mechanical Fragmentation Theory (QMFT), first proposed by Gupta and Collaborators at Frankfurt, and then extended at Chandigarh. In DCM, decay of excited compound nuclei is studied as a collective clusterization process for emissions of the light particles LPs (n, p, α) and γ-rays, as well as the intermediate mass fragments IMFs (with 2 < Z < 10 and 5≤A≤20) and the symmetric and near-symmetric mass fragments SF and nSF (20≤A≤A/2), in contrast to the statistical models in which each type of emission is treated on different footing. Another advantage of using the DCM is that the structure effect of compound nuclei (CN) can be addressed via the preformation probability of the fragments, an information missing in the statistical fission models. Keeping in mind the temperature-dependence of collective potential used in DCM, first the temperature-dependent binding energies are calculated, which is an important step in the study of the decay of excited compound systems. The DCM have been applied successfully to study the decay of light (48Cr*) and heavy (202Pb*, 246Bk*) mass nuclear systems formed in different entrance channels at a number of centre-of-mass energies Ec.m.. The role of excitation energy, entrance channel effects together with the effects of deformations and orientations in the fusion-fission process have been investigated extensively. The calculated DCM cross-section for all possible decay paths are found to be in excellent comparison with the available experimental data. Moreover, the possible role of deformed and oriented nuclei in cluster radioactivity has been investigated using the PCM, besides the well established shell effects. For this purpose we have chosen only those cluster decays for which daughter is always spherical and doubly magic (N = 126, Z = 82) 208Pb. The effects of nuclear structure and deformation together with orientations are thus established both in (radioactive) nuclei and excited compound systems via their cluster decay studies.

Abstract

Abstract