Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Silicon Nanocrystal Hybridized Visible LEDs: A Low-Cost Path for Global Lighting
Ş. Burç Eryılmaz, Onur Tidin, Alper Yeşilyurt, Ali K. Okyay*
Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey
Abstract- We report UV LEDs coated with Silicon nanocrystals that emit in the visible spectrum. A broad emission from nanocrystals with
peak at 547 nm is observed when pumped with 375 nm LED light. Our results are promising for application of Silicon nanocrystals as
wavelength converters in light emitting diodes for multicolor light generation.
Semiconductor based solid state lighting has long surpassed
fluorescence lamps and incandescent light bulbs in terms of
efficiency, safety and energy saving. Future roadmaps
envisage dramatic improvements in solid state lighting for the
next 10 years. By the year 2025, electricity consumption
would decrease by at least 50%, and this corresponds to
electricity savings in United States around 525 TWh/year.
This would also result in a worldwide reduction of greenhouse
gas emissions that is produced during the generation of
electricity by about 87 Mt [1]. These reveal the reason for the
great interest in semiconductor lighting technology.
Research in semiconductor lighting includes the
investigation of multicolor light emitting diodes. One method
for producing these devices is using a wavelength converting
emitting layer on top of a primary light source. These devices
utilize electroluminescence of primary light source and
photoluminescence of wavelength converter layer.
Nanocrystals, due to their tunable emission properties, have
been preferable emitters. Due to quantum confinement effects,
CdSe/ZnS core/shell nanocrystals were demonstrated to be
efficient wavelength converters for multicolor light emitting
diodes [2]. Despite the efficiency and quality of the color
generated by this device, Cd based materials have high toxic
effects on the human and the environment, in addition to high
materials costs. Silicon, being the second most abundant
material on the Earth’s crust and its low toxicity, makes it a
low cost attractive. Recent advances have made it possible to
produce silicon nanocrystals with indirect-to-direct gap
transition with light emission in visible wavelength [3].
In this work, we report light emission from silicon
nanocrystals used as wavelength conversion layer on top of
ultraviolet light emitting diode. UV light emitted from the
LED pumps the nanocrystals, and light emission in visible
wavelength from the nanocrystals is observed.
We purchased commercial UV LEDs from Nitride
Semiconductors, Co., Ltd. with 375 nm peak as shown in Fig.
1.a. Micrograph image of our LED is given in Fig. 1.b. We
deposited silicon nanocrystals in solution of tetrahydrofuran
(THF) on top of the UV LED by drop-casting. The device was
biased using electrical probes and the electroluminescence
spectrum was obtained using a fiber connected to a
spectrometer. After this measurement, we repeated the dropcasting
process to see the effect of increasing the amount of
nanocrystals. Electroluminescence spectra corresponding to
multidrop samples are given in Fig. 1.c including the bare
LED as a reference.
Luminescence of silicon nanocrystals are observed to have a
peak emission wavelength at 547 nm when excited with 375
nm light source. FWHM value is measured approximately 140
nm. This broad emission can be useful for multicolor light
applications. The reference UV LED has no measurable
emission around these wavelengths, and the emission intensity
increases with the number of nanocrystal solution drops cast
on the LED.
Figure 1: (a) UV LED emission spectrum (b) UV LED micrograph
image (c) Electroluminescence measurements
In summary, we demonstrated the application of Silicon
nanocrystal on UV LEDs and obtained visible emission in the
yellow-red region. Emitted light can be tuned by changing the
size of nanocrystals in production. This work has promising
results for opening the way for low cost Silicon material to be
used as wavelength converters in the form of nanocrystals. For
this purpose, increasing the efficiency of these nanocrystals
should be studied. This can be achieved by blending them into
a polymer and applying as a film. Recent results in plasmonics
to increase the fluorescence efficiency of silicon nanocrystals
also 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