Photonic crystals in biology - NanoTR-VI

Poster Session, Thursday, June 17Theme F686 - N1123Silicon Nanocrystal Hybridized Visible LEDs: A Low-Cost Path for Global LightingŞ. Burç Eryılmaz, Onur Tidin, Alper Yeşilyurt, Ali K. Okyay*Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, TurkeyAbstract- We report UV LEDs coated with Silicon nanocrystals that emit in the visible spectrum. A broad emission from nanocrystals withpeak at 547 nm is observed when pumped with 375 nm LED light. Our results are promising for application of Silicon nanocrystals aswavelength converters in light emitting diodes for multicolor light generation.Semiconductor based solid state lighting has long surpassedfluorescence lamps and incandescent light bulbs in terms ofefficiency, safety and energy saving. Future roadmapsenvisage dramatic improvements in solid state lighting for thenext 10 years. By the year 2025, electricity consumptionwould decrease by at least 50%, and this corresponds toelectricity savings in United States around 525 TWh/year.This would also result in a worldwide reduction of greenhousegas emissions that is produced during the generation ofelectricity by about 87 Mt [1]. These reveal the reason for thegreat interest in semiconductor lighting technology.Research in semiconductor lighting includes theinvestigation of multicolor light emitting diodes. One methodfor producing these devices is using a wavelength convertingemitting layer on top of a primary light source. These devicesutilize electroluminescence of primary light source andphotoluminescence of wavelength converter layer.Nanocrystals, due to their tunable emission properties, havebeen preferable emitters. Due to quantum confinement effects,CdSe/ZnS core/shell nanocrystals were demonstrated to beefficient wavelength converters for multicolor light emittingdiodes [2]. Despite the efficiency and quality of the colorgenerated by this device, Cd based materials have high toxiceffects on the human and the environment, in addition to highmaterials costs. Silicon, being the second most abundantmaterial on the Earth’s crust and its low toxicity, makes it alow cost attractive. Recent advances have made it possible toproduce silicon nanocrystals with indirect-to-direct gaptransition with light emission in visible wavelength [3].In this work, we report light emission from siliconnanocrystals used as wavelength conversion layer on top ofultraviolet light emitting diode. UV light emitted from theLED pumps the nanocrystals, and light emission in visiblewavelength from the nanocrystals is observed.We purchased commercial UV LEDs from NitrideSemiconductors, Co., Ltd. with 375 nm peak as shown in Fig.1.a. Micrograph image of our LED is given in Fig. 1.b. Wedeposited silicon nanocrystals in solution of tetrahydrofuran(THF) on top of the UV LED by drop-casting. The device wasbiased using electrical probes and the electroluminescencespectrum was obtained using a fiber connected to aspectrometer. After this measurement, we repeated the dropcastingprocess to see the effect of increasing the amount ofnanocrystals. Electroluminescence spectra corresponding tomultidrop samples are given in Fig. 1.c including the bareLED as a reference.Luminescence of silicon nanocrystals are observed to have apeak emission wavelength at 547 nm when excited with 375nm light source. FWHM value is measured approximately 140nm. This broad emission can be useful for multicolor lightapplications. The reference UV LED has no measurableemission around these wavelengths, and the emission intensityincreases with the number of nanocrystal solution drops caston the LED.Figure 1: (a) UV LED emission spectrum (b) UV LED micrographimage (c) Electroluminescence measurementsIn summary, we demonstrated the application of Siliconnanocrystal on UV LEDs and obtained visible emission in theyellow-red region. Emitted light can be tuned by changing thesize of nanocrystals in production. This work has promisingresults for opening the way for low cost Silicon material to beused as wavelength converters in the form of nanocrystals. Forthis purpose, increasing the efficiency of these nanocrystalsshould be studied. This can be achieved by blending them intoa polymer and applying as a film. Recent results in plasmonicsto increase the fluorescence efficiency of silicon nanocrystalsalso stands as an alternative.This work was supported by TUBITAK 108E163, 109E044,EU FP7 PIOS.*aokyay@ee.bilkent.edu.tr[1] J. Y. Tsao, IEEE Circuits and Devices Magazine, 28,May/June 2004.[2] S. Nizamoglu et al., Nanotechnology, 18, 405702 (2007)[3] M. H. Nayfeh et al., Appl. Phys. Lett., 80, 842-843 (2002)6th Nanoscience and Nanotechnology Conference, zmir, 2010 641