R. Turan; "Nanocrystals&Nanoparticles for Electro-...",
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Faculty of Engineering and Natural Sciences



  Nanocrystals and Nanoparticles for Electro-Optical Applications


Rasit Turan, Department of Physics, Middle East Technical University

Quantum confined Stark effect (QCSE) in Si nanocrystals can be utilized for the production of Si based electro-optical devices sucs as optical modulators  and waveguides, since light absorption and emission can be controlled by using an external bias. Optical sources based on waveguide devices such as ring resonators can be fabricated by making use of QCSE that can provide tunable light for optically functional circuits. Successful fabrication of such devices may lead to brakethroughs in the fabrication of integrated electo-optical systems.
  In this work, Quantum confined Stark effect (QCSE) on excitons confined in Si nanocrystals embedded in SiO2 matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures.  PL peak shifts to lower energies with increasing electric field are recorded as expected from carrier polarization within the quantum dot. Reducing the measurement temperature further enhances the QCSE due to improved localization at the lowest energy states of the quantum dot. The variation of the PL intensity with applied voltage and temperature are also studied to understand the effect of other mecahisms such as carrier escape from the nanocrystals and Auger recombination. It is shown that the emission intensity remains constant for a wide range of applied voltage indicating that the observed energy shift is related to QCSE rather than carrier population of the nanocrystal. We have also found that the effect depends on the polarity of the applied voltage. This dependence is studied through C-V measurements in which the charge injection into the oxide from the substrate can be observable.   The optical behavior of metal nanoparticles with different geometries is of great interest for the plasmonic resonances in the visible and infrared region. Recently this interest has grown due to a wide range of potential applications including biosensing and photovoltaics where the nanoparticles behave like an optical antenna which couples the incoming electromagnetic wave into the underlying device very effectively. Electron Beam Lithography (EBL), being an excellent nanomanufacturing tool, enables us to fabricate well defined network of nanoparticles with varying sizes.  In this study, the EBL was used to fabricate and study a wide range of properties of local plasmonic oscillations induced in Au and Ag nanoparticles on different substrates. We also focused on the fabrication issues such as the optimization of parameters affecting the resulting particle geometry. Both transmission and reflection spectra of nanoparticle arrays were studied and discussed in terms of expected resonance conditions.  The variation of the resonance peak position with the particle size and the lattice constant of the network of was determined and tested against the theoretical results generated by the DDA model. Bio: Prof. Raşit Turan received his PhD from University of Oslo, Norway in 1990. He worked as a Postdoctoral Research Fellow at Oslo for 2 years before  returning to Turkey in 1991. Since then he is a member of the Physics faculty at Middle East Technical University.Dr. Turan has led several national and international projects on semiconductor materials and devices. He is  interested in solar cells, PV technologies, production, characterization and applications of semiconductor and metal nanocrystals. He is the coordinator of DPT project GÜNAM to establish the Center for Solar Energy Research and Application on METU Campus. He is coordinator for international project 2+2, and was a coordinator of EU FP6 STREP SEMINANO, and SSA project aiming at increasing the research capacity of Turkey. He is a partner of FP7 project Co-nanomet.   March 3, 2010, 13:40, FENS L035