High-Yield Synthesis of Luminescent Silicon Quantum Dots in a Continuous Flow Non thermal Plasma Reactor
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High-Yield Synthesis of Luminescent Silicon Quantum Dots in a Continuous Flow Nonthermal Plasma Reactor L. Mangolini, E. Thimsen and U. Kortshagen Department of Mechanical Engineering High Temperature and Plasma Laboratory University of Minnesota, Minneapolis, MN 55455 ABSTRACT Light-emission from silicon based on quantum confinement in nanoscale structures has sparked intense research into this field ever since its discovery about 15 years ago. The lack of a simple high-yield synthesis approach for luminescent silicon nanocrystals has so far hampered their widespread application in such diverse areas as opto-electronics, solid-state lighting for general illumination, and fluorescent agents for biological applications. In this paper we discuss a nonthermal plasma process for the synthesis of luminescent silicon nanocrystals. The particle size is mainly controlled by the residence time in the plasma region. The system is capable of producing several tens of milligrams of luminescent powder per hour. INTRODUCTION Silicon is a material with rather poor optical emission and absorption properties based on its indirect band gap, which requires that photon emission and absorption involve a momentum balancing phonon. This fact has long hampered the development of silicon opto-electronic devices, which, if realized, would enable a new level of integration of silicon electronics with optical devices on one chip. Hence first reports of room temperature light-emission from quantum confined silicon structures were met with great enthusiasm. Initial processes for the production of luminescent silicon nanostructures used magnetron sputtering of silicon in a hydrogen atmosphere [1] and the production of porous silicon [2, 3]. Other reports of luminescence from quantum confined silicon nanostructures such as surface oxidized nanocrystals [4, 5] and silicon/insulator superlattices [6] soon followed. Since then some first steps towards silicon optoelectronic devices were demonstrated with silicon based light emitting diodes [7], and the observation of optical gain from silicon nanocrystals [8]. While initially light emission from silicon was studied mostly with regards to the role of quantum confinement in enhancing the overlap of the electron and hole wave function and in reducing the rate of non-radiative three body Auger processes [9], more recently increased attention has been paid to the important influence of the surface conditions of silicon nanocrystals [10]. A wide range of synthesis approaches has been proposed for silicon nanocrystals both in the liquid and in the gas phase. Liquid phase approaches range from the synthesis in inverse micelles [11], to the synthesis in high temperature supercritical solutions [12], to the oxidation of metal silicide [13], to the reduction of silicon tetrahalides and other alkylsilicon halides [14]. However, liquid phase approaches are often time-consuming and sometimes have a low yield of crystalline material. Gas phase approaches range from the pyrolysis of silane in furnace flow re
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