Single Particle and Ensemble Spectroscopy of Silicon Nanoparticles
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Single Particle and Ensemble Spectroscopy of Silicon Nanoparticles Lindsay E. Pell, Zhonghua Yu, Douglas S. English, Don O’Connor, Paul F. Barbara, Brian A. Korgel1 Department of Chemical Engineering and Texas Materials Institute Center for Nano-and-Molecular Science and Technology, University of Texas at Austin Austin, TX 78712, USA Telephone: (+1)512-471-5633; Fax: (+1)512-471-7060; 1 Email: [email protected] ABSTRACT Octanethiol-stabilized Si nanocrystals, ranging from 2 to 8 nm in diameter, were synthesized in cyclohexane heated and pressurized above its critical point at temperatures ranging from 400ºC to 500ºC. The nanocrystals exhibit crystalline cores and photoluminesce with relatively high efficiencies. These nanocrystals are suitable for single particle spectroscopic measurements that reveal optical information about the individual chromophores that are buried in ensemble measurements. The sterically-stabilized Si nanocrystals emit with relatively narrow emission spectra at room temperature, characteristic of molecules. INTRODUCTION Due to their unique size-tunable optical and electronic properties, semiconductor nanocrystals could provide the basis for a variety of new technologies. Much progress has been made in the synthesis and study of direct band gap semiconductor nanoparticles such as CdSe and InAs [1-5]. A multitude of high temperature arrested precipitation synthetic routes have been reported to make monodisperse CdSe quantum dots with quantum efficiencies as high as 90% 1,2, along with extensive characterization of the chemical and optical properties of individual dots 2,3, monolayers and 3D superlattices [1-4]. On the other hand, the synthesis of silicon nanocrystals is not well-developed, and much of our current understanding of the effects of quantum confinement on the optical properties of Si—an indirect band gap semiconductor—has been gained through the study of porous Si (por-Si), which consists of a network of Si nanostructures that is difficult to properly characterize structurally and chemically [5,6]. Recently, we developed a new wet chemical approach for synthesizing sterically-stabilized Si nanocrystals that relies on the use of organic solvents heated and pressurized above their critical points. This method allows temperature ranging from 400 to 500ºC to be reached, which are necessary to obtain highly crystalline Si nanocrystals. The particle size can range from 2 nm up to 8 nm in diameter. In this size range, the nanocrystals emit visible light with size-tunable color, ranging from blue to red. Here, we present optical spectra for both ensemble and individual Si nanocrystals stabilized with octanethiol. EXPERIMENTAL DETAILS Silicon nanoparticles were synthesized via a thermal decomposition of 250mM silicon precursor (diphenylsilane (DPS) – Aldrich Chemical) in supercritical hexane. The synthesis conditions and experimental apparatus employed were similar to that described in Ref. 9. Octanethiol, in W3.4.1
molar ratio to DPS of 1:10, served as the steric stabilizer for th
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