Fabrication and Device Applications of Self Assembled Nanostructures

Abstract

FABRICATION AND DEVICE APPLICATIONS OF SELF ASSEMBLED NANOSTRUCTURES By Bhargava Kanchibotla A Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2009 Major Director: Supriyo Bandyopadhyay Professor, Department of Electrical and Computer Engineering and Department of Physics The spin dynamics of electrons in inorganic and organic semiconducting nanostructures has become an area of interest in recent years. The controlled manipulation of an electron’s spin, and in particular its phase, is the primary requirement for applications in quantum information processing. The phase decoheres in a time known as the transverse relaxation time or T2 time. We have carried out a measurement of the ensemble-averaged transverse spin relaxation time (T 2 *) in bulk and few molecules of the organic semiconductor tris-(8-hydroxyquinolinolato aluminum) or Alq3. The Alq3 system exhibits two characteristic T 2 * times: the longer of which is temperature independent and the shorter is temperature dependent, indicating that the latter is most likely limited by spinphonon interaction. Based on the measured data, we infer that the single-particle T2 time xi in Alq3 is probably long enough to meet Knill's criterion for fault-tolerant quantum computing even at room temperature. Alq3 is also an optically active organic, and we propose a simple optical scheme for spin qubit readout. Moreover, we found that the temperature-dependent T 2 * time is considerably shorter in bulk Alq3 powder than in few molecules confined in 1–2-nm-sized cavities. Because carriers in organic molecules are localized over individual molecules or atoms but the phonons are delocalized, we believe that this feature is caused by a phonon bottleneck effect. Organic fluorophore molecules, electrosprayed within nanometer sized pores of an anodic alumina film, exhibit unusually large molecule-specific redor blue-shifts in the fluorescence peak. This molecular specificity allows us to resolve different constituents in a mixture optically, providing a unique new technology for bioand chemical sensing. We have also observed that the fluorescence efficiency progressively increases with decreasing pore diameter. This trend cannot be explained by the usual photo carrier confinement model since the photo carriers are localized over individual molecules (or atoms) which are much smaller than the pore diameter. A more likely explanation is the metal enhanced fluorescence caused by the plasmon resonance of nanotextured aluminum lying at the bottom of the pores.