Characteristics Of Ultrathin Films Research Articles

Development of Ultrathin Polymer Films and Their Characteristics in Two Dimensions

Abstract Nanostructures and properties of ultrathin polymer films prepared by repeated deposition of monolayers have been investigated with emphasis on single chain morphology in two dimensions. The particular characteristics of ultrathin films were successfully revealed by novel optical microscopy and fluorescence spectroscopy techniques. A single polymer chain, which is confined into a two-dimensional plane, takes a contracted conformation, being segregated from other chains in the monolayer. Unlike three dimensional bulk polymers, there is little entanglement of polymer chains even for large molecular weight samples. These characteristics at the molecular level result in the entropy relaxation of individual chains with a small activation energy of diffusion, when the segmental motions are allowed at elevated temperatures. These understandings of structures and properties in two dimensions are indispensable for fabrication and application of polymeric thin films because the fine structures in low dimensions play critical roles in the production of advanced functions.

Josephson junction array type I–V characteristics of quench-condensed ultra thin films of Bi

In this communication we report studies of dc current–voltage (I–V) characteristics of ultra thin films of Bi, quench condensed on single crystal sapphire substrates at T=15K. The hysteretic I–V characteristics are explained using a resistively and capacitively shunted junction (RCSJ) model of Josephson junction arrays. The Josephson coupling energy (EJ) and the charging energy (Ec) are calculated for different thickness (d) values. A low resistance state is found in the low current regime below the critical current, Ic. This resistance R0 is found to have a minimum at a particular thickness (dc) value. Reflection high energy electron diffraction (RHEED) studies are done on these films. A distinct appearance of a diffuse ring near dc is observed in the diffraction images, consistent with the recent STM studies (Ekinci and Valles, Phys. Rev. Lett., 82 (1999) 1518). These films show an irreversible annealing when temperature is increased. The annealing temperature (Ta) also has a maximum at the same thickness. Although the Rs vs T of quench-condensed Bi films suggest that the films are uniform, our results indicate that even in thick films, the order parameter is not fully developed over the complete area of the film. These results are discussed qualitatively.