Artificial Atoms of Silicon
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Atoms
of
Silicon
Justin D. Holmes, Kirk J. Ziegler, Keith P. Johnston, R. Chris Doty, Brian A. Korgel Department of Chemical Engineering and Texas Materials Institute The University of Texas at Austin Austin, TX 7 8 7 1 2 - 1 0 6 2 Email: [email protected] ABSTRACT Size-monodisperse, stable 15 Å diameter silicon nanocrystals were synthesized in significant quantities using supercritical octanol as a capping ligand. The silicon nanocrystals exhibit a n indirect band gap with discrete electronic transitions in t h e absorbance and photoluminescence excitation (PLE) spectra. T h e octanol-capped clusters show efficient blue b a n d - e d g e photoemission with a luminescence quantum yield of 23 % at r o o m temperature. INTRODUCTION Semiconductor cluster properties depend on size. For example, quantum confinement effects lead to unique electronic and optical properties, such as size-tunable excitation a n d luminescence energies with an overall loss of energy level degeneracy [1]. These distinct material p r o perties might b e exploited in a variety of new technologies—including, electronic, optical, medical, coatings, catalytic, memory and s e n s o r applications. Because of the appearance of discrete energetic states, semiconductor clusters have been called "artificial a t o m s " and provide the opportunity to study semiconductor properties a s they evolve from atoms to small clusters to a bulk crystal. Examples of size-dependent discrete optical transitions exist f o r clusters of direct band gap semiconductors, such a s CdSe [2] a n d InAs [3]. Artificial atom behavior, however, has not previously been observed for nanocrystals of the indirect semiconductor, silicon—the most technologically important semiconductor [4-8]. In this study, we present the important finding t h at 15 Å diameter silicon clusters, synthesized in significant quantities using a supercritical (sc) solvent wet chemical approach, exhibit artificial atom qualities at room t e m p e r a t u r e .
EXPERIMENTAL
DETAILS
Organic surface-passivated silicon nanocrystals were p r e p a r e d by thermally degrading diphenylsilane in octanol (T c = 385 ° C, Pc = 34.5 bar) well above its critical point at 500 °C and 345 bar i n an inconnell high-pressure cell under nitrogen, as described i n detail elsewhere [9]. The presence of silicon particles was observed by the formation of a yellow solution; no color change was observed in the absence of diphenylsilane. When diphenylsilane was degraded in the presence of sc-ethanol rather than sc-octanol, the solution quickly turned from orange to brown and then clear a s polydisperse micron-sized silicon particles formed and settled o n the walls of the reaction vessel. This result suggests that, u n l i k e ethanol, the bound octanol chains provide sufficient steric stabilization to prevent aggregation. The sc-octanol quenches t h e reaction and passivates the silicon nanocrystal surface. Chloroform was used to extract the silicon nanoparticles from t h e cell upon cooling and depressurization.
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