Colorful Light-Emitting-Diodes via Modulation of the Concentration of Red-Emitting Silicon Nanocrystal Phosphors

The research activity into the development of luminescent forms of silicon has now spanned two decades, driven by scientific interest, commercial potential, and technological advancement [1]. It is now well established that silicon crystallites of reduced dimensions, typically below 5 nm, emit light with high efficiency due to quantum confinement effect, with respect to inefficient light emission of bulk crystalline silicon [2]. Canham [2] was the first to demonstrate in 1990 room- temperature photoluminescence from nanocrystallites of silicon (i.e. porous silicon) that were obtained by electrochemical erosion of crystalline silicon in acidic electrolytes. Many other methods were subsequently developed for the synthesis of luminescent nanostructured forms of silicon, including annealing of SiOx powder followed by etching in HF, plasma synthesis, solution reduction of SiCl4, plasma etching of silicon and subsequent thermal oxidation [3-6]. Very recently silicon nanocrystals (SiNs) with high PL quantum yield (about 17%), obtained by low-cost electrochemical erosion of crystalline silicon substrate, has been proposed as non-toxic phosphors for wavelength conversion in ultraviolet/blue LEDs. [7, 8]. Here we show that SiN phosphors with strong and broad photoluminescence in the red portion of the spectrum, obtained by electrochemical erosion of silicon, can be used as non-toxic phosphors for effectively tuning the color of commercial blue-LEDs from blue to violet, magenta, and red via wavelength conversion, by coating the LED with polydimethylsiloxane (PDMS) encapsulating different SiN concentrations [16]. Good reliability of the tuning process, with respect to SiN fabrication and concentration, and excellent stability of the tuning color, with respect to LED operation current, is demonstrated through simultaneous electrical/optical characterization of a number of SiN-modified commercial LEDs. In spite of the huge research effort that has been paid so far on the use of SiNs for LED applications, the possibility of efficiently tuning the color of LEDs via wavelength-conversion by modultating the concentration of red-emitting SiN phosphors has never been reported.New exciting perspectives in the field of light-emitting applications of SiNs are envisaged by building on these results. [1] L. Mangolini, Journal of Vacuum Science and TechnologyB 31, 020801 (2013). [2] L. T. Canham, Applied Physics Letters 57, 1046 (1990). [3] Shu-Man Liu, Yang Yang, Seiichi Sato, and Keisaku Kimura, Chemistry of Materials 18, 637 (2006). [4] X. D. Pi, R. W. Liptak, J. Deneen Nowak, N. P. Wells, C. B. Carter, S. A. Campbell, and U. Kortshagen, Nanotechnology 19, 245601 (2008). [5] J. Zou, P. Sanelle, K. A. Pettigrew, and S. M. Kauzlarich, Journal of Cluster Science 17, 565 (2006). [6] S. S. Walavalkar, C. E. Hofmann, A. P. Homyk, M. D. Henry, H. A. Atwate, and A. Scherer, Nano Letters 10, 4423 (2010). [14] C.-C. Tu, Q. Zhang, L. Y. Lin, and G. Cao, Optics Express 20, A69 (2011). [15] C.-C. Tu, J. H. Hoo, K. F. Bohringer, L. Y. Lin, and G. Cao, Optics Letters 37, 4771 (2012). [16] G. Barillaro, L. M. Strambini, Applied Physics Letters 104, 091102 (2014