Magic Family of Discretly Sized Ultrabright Si Nanoparticles
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MAGIC FAMILY OF DISCRETLY SIZED ULTRABRIGHT SI NANOPARTICLES G. BELOMOIN, J. THERRIEN, A. SMITH, S. RAO, R. TWESTEN, S. CHAIEBb), AND M. H. NAYFEH Department of Physics and Department of Theoretical and Applied Mechanicsb), University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, Illinois 61801 USA L. WAGNER, AND L. MITAS Department of Physics, North Carolina State University, 127 Stinson Rd., Raleigh, NC 27695 ABSTRACT We describe a procedure for dispersion bulk Si into a family discretely sized ultrasmall ultrabright nanoparticles. We demonstrate that electrochemically etched, hydrogen capped SinHx clusters with n larger than 20 are obtained within a family of discrete sizes. These sizes are 1.0 (Si29), 1.67 (Si123), 2.15, 2.9, and 3.7 nm diameter. We characterize the particles via direct electron imaging, excitation and emission optical spectroscopy, chromatography, and colloidal crystallization. The band gaps and emission bands are measured. The smallest four are ultrabright blue, green, yellow, and red luminescent particles. The availability of discrete sizes and distinct emission in the red, green and blue range is useful for biomedical tagging, RGB displays, and flash memories. INTRODUCTION Semiconductor clusters, especially silicon clusters, is currently one of the most active frontiers in physics and chemistry [1-20]. Their unique structures, stability, and optical and electronic and chemical reactivity, both in free space and on surface have been examined [3]. Fabricating size-, shape-, and orientation-controlled fluorescent Si nanoparticles in the range 1-3 nm (30-1000 atoms), with reproducibility would be critical to the understanding of nanostructures and would be of significant interest to the microelectronics, optoelectonics and biomedical industries. Averaging over size or shape inhomogeneously masks spectroscopic properties, hindering testing of theory. Size uniformity is important for applications that require superlattices, high quality films, or single nanoparticle-based devices, such as fluorescent imaging and tagging. The availability of discrete sizes with distinct emission in the red, green and blue range is useful for biomedical tagging and in RGB display applications. In flash memory, nanoparticles are embedded in a MOSFET as floating gates to store electric charge. Since the shift in the device's threshold depends on the particle size, a wide size distribution would wash the operating threshold. Several efforts have been directed to develop procedures to control the size, and shape of Si nanoparticles. Only recently progress towards control over the crystallography orientation has been achieved [12]. Self-terminated clusters Sin [6-8] with n < 10 exhibit discrete magic numbers, whereas the abundance spectrum of clusters Sin with n >20 exhibits neither special features nor discrete magic numbers [9]. The shape of those large ones, however, changes from prolate to more spherical in the narrow range between n = 24 and n =30 [10]. In the prolate regime, atoms are arranged in o
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