Microelectronics-Photonics Graduate Program
Research Experience for Undergraduates
Summer 2007
Final Research Report
A Polychromatic Approach to
White Light Emission Utilizing
Nanocrystals
Tariq Bond
Dr Omar Manasreh, Electrical Engineering
Eric DeCuir Jr., Electrical Engineering
July 26, 2007
This work was supported by National Science Foundation award EEC-0097714.
Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the author and do not necessarily reflect the views of the
National Science Foundation.
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Table of Contents
List of Tables (Pg 3-4)
List of Figures (Pg 5)
Introduction (Pg 5)
Background (Pg 6-7)
Research Goals (Pg 7)
Description Experiment (Pg 7)
Experimental Results/Conclusions (Pg 8)
Future Research Suggested by this research (Pg 8)
References (Pg 9)
Impact of REU on personal goals and plans (Pg 9)
Acknowledgements (Pg 9)
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List of Tables
These are my results with each sample at room temperature of 300K, and also
after being cooled down to 77K. In each of the graphs we noticed the common
characteristic of having much more pronounced and smoother photoluminescence
response when the temperature decreased to 77K. Other interesting features were a blue
shift that we noticed when the entre curve shifted approximately 15-20nm, and a much
better photoluminescence response from the nanocrystals when cooled.
Mix 1
T=300K
T=77K
1600
Intensity (arbit. units)
1400
1200
1000
800
600
LED Emission
411 nm
400
200
0
350
400
450
500
550
600
650
700
750
800
650
700
750
800
Wavelength (nm)
Intensity (arbit. units)
500
Mix 2
300K
77K
400
300
200
100
0
350
400
450
500
550
600
Wavelength (nm)
3
600
Mix 3
300K
77K
Intensity (arbit. units)
500
400
300
200
100
0
350
1100
450
500
550
600
Wavelength
(nm)650
700
750
800
Mix 4
300K
77K
1000
900
Intensity (arbit. units)
400
800
700
600
500
400
300
200
100
0
350
400
450
500 Wavelength
550
600(nm)650
4
700
750
800
List of Figures
All the following nanocrystal samples are stimulated with an Ultraviolet 405nm LED:
(Utilizing 493, 514, & 614nm nanocrystals at 77K)
(Utilizing 493, 514, 530, 575 & 614nm nanocrystals at 77K)
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(Utilizing 514, 530, 575, & 614nm nanocrystals @ 77K)
(Utilizing 493, 575 & 614nm nanocrystals @ 77K)
Introduction
An LED is a semiconductor diode which consists of a chip of semi-conducting
treated material to create a structure known as a p-n junction. When connected to a source
of power, current flows from the p-side, which is known as the anode, to the n-side,
which is known as cathode. However, current will not flow in the reverse direction.
Electrons and electron holes flow into the junction from electrodes, and when an electron
finds a hole, it falls into a lower energy level and releases energy as emission by releasing
a photon. This is how LEDs are different from traditional light sources in the way they
emit light. In an incandescent lamp, a filament is heated by electrical current until it
stimulates light. In fluorescent lamps electricity excites mercury atoms, which in turn,
emit ultraviolet radiation, but it is not until after the radiation reacts with the phosphor
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coating on the inside of glass tubes that the ultraviolet radiation is converted into visible
light.
Background
Nichia was the first to market gallium nitride, also known as GaN, based blue
LEDs in the early 90s. They then revealed a white LED using a blue LED die in 1996,
and the company has been actively working every since then to raise the efficiency of its
InGaN LEDs. The company since then has increased the efficiency of their LED’s from
20lm/W to more 60lm/W since 2004.
In general, there have been breakthroughs in LEDs and advances in solid-state
lighting in which have currently been displacing technologies in certain applications such
as traffic signals, automotive lighting, exit signs, and flashlights for the past years. With
such dramatic power savings and with such practicality, commercial usage of LEDs is
almost inevitable. Further technology advances will drive the development of white-light
sources with the needed attention that will eventually replace incandescent and
fluorescent lamps in common every day applications. While most types of incandescent
bulbs are improving the quality of light and lumen output, they are also using more and
more power, and do not seem to show many advances toward becoming more efficient.
The key areas that most researchers are focusing on are longevity, cost efficiency,
packaging, stability and even quantum efficiency. Once technology advances are created
in these key areas it will result in an extremely competitive market for all solid-state
lighting sources that provide superior light and conserve energy.
When it comes to LED materials, to this date, color converting phosphors have
been extensively used in the generation of white light. It has not been until lately in
which there has been an interest developing in the usage of nanocrystal technologies.
Research Goals
My research goals were to find a combination of different nanocrystals which of
different wavelengths in order to create white light. I find this research important because
it is an alternative method to the current, and most popular, phosphor techniques.
Although phosphorus techniques are working well, they still can be considered somewhat
limited when you consider the possible precision of colors emitted from the LED. My
goal is to prove the nanocrystal and LED method to be just as emission efficient and be
much more variable when it involves color of tint within the white range.
Description Experiment
During the experiment several things had to be accounted for to ensure that my
results were both accurate and comparable. Each of the four samples were made using
different wavelengths of nanocrystals and even different methods. As far as the LED was
concerned, it was always receiving approximately 19-20mA which was the peak
efficiency amperage range in which the LED was designed and rated for. Since the
custom cold finger held the LED in the same position every time the experiment was run,
it was accurately in place every time. After the Bomem Fourier Transform Infrared
Spectrometer (FTIR) was placed under full vacuum it was then cooled to 77K using
liquid nitrogen. For the number of scans I used sixty four to ensure an accurate graph
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from testing. When the LED shines onto the nanocrystals within the sample it absorbs
some or most of the Ultraviolet wavelength and the nanocrystals reemit the light in their
own wavelength which gives me the photoluminescence response. After that point the
results just have to be recorded and saved for later comparison and analysis.
Experimental Results/Conclusions
In conclusion, my results were much to my approval and I see a lot of potential in
continuing to improve upon this experiment in different ways to produce white light
using nanocrystals. With each of the mixed nanocrystal samples, each of them showed
some form of white light with a difference in the shade, or either a very close potential
for white light. When compared to a blue LED with a yellow phosphorous coating we
had a few graphs that were already comparable before any fine tuning or measuring of
the amounts that we placed on each slide. Ultimately I have found this method to be just
as effective as the blue LED phosphorus coated method and also hold much more
potential for various color generation.
Future Research Suggested by this research
For the experiment to continue to evolve there would just need to have more ways
of measuring both the concentration of the nanocrystals and the purity. Using the
nanocrystals from Evident proved to be less effective then a local company by the name
of Ocean Nanotech and I would strongly recommend that as a first change to continue
development. As far as making the samples, a more refined and carefully measured
method in order to fine tune results would be a good start. Also most of our slides could
have had a much higher concentration of nanocrystals without being as thick if we have
alternate methods of layering such as spin coating. Later projects would then involve
using the results of our experiments and finding another way of efficiently encapsulating
the nanocrystals directly into the LED’s lens.
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References
(1) Nakamura S and Fasol G 1997 The Blue Laser Diode
(2) Schubert E F 2006 Light-Emitting Diodes (Cambridge: Cambridge University
Press)
(3) Yamada M Narukawa Y, Tamaki H, Murazaki Y and Mukai T 2005 IEICE Trans.
Electron. E88—C 1860-71
(4) H Mueller, M.A Petruska , M. Achermann, D.J. Werder, E.A. Akhadov, D.D.
Koleske, M.A. Hoffbauer and V.I. Klimov. Nano Lett 2005 5 1039
(5) A. Moazzam, S. Chattopadhyay, A. Nag, A.Kumar, S.Sapra, S. Chakraborty and
D.D. Sarma, Nanotech 2007 18 075401
(6) M.J. Bowers, J.R. McBride and S.J. Rosenthal. J. Chem. Soc. 2005 127 15378.
(7) H.S Chena, S.J.J. Wang, C.J.Lo and J.Y. Chi. Appl. Phys.Lett. 2005 86 131905.
(8) J.H.Park, J.Y.Kim, B.D. Chin, Y.C Kim, J.K. Kim and O.Ok. Park. Nanotech
2004 15 1217.
Impact of REU on personal goals and plans
Really has influenced my outlook on the technology, the time and effort that goes
into materials research. Before this program I did not quite understand how materials
were developed and what there uses could be, so it was definitely help me open my eyes
and change my perspective on necessary materials and how they can be used. If not a
graduate level of education I feel that I will always still want to be on the cutting edge of
material research in order to keep ahead of most of the new technology on the market.
Acknowledgements
I would definitely like to thank Dr. Arlene Maclin for allowing me the
opportunity to come here in the first place, because she knows plenty of intelligent
students that she could have selected but I was one of them. I would also like to thank
Ken Vickers for making this program such a successful and remembering experience.
This summer is one that I could almost tolerate forever but it is time to use what we have
learned and move forth. I would also like to thank Renee Hearon for her hard work and
dedication throughout the summer keeping things in order and making sure everything
comes together wonderfully. As far as my research I would like to thank Dr. Omar
Manasreh for making me feel just like one of his own graduate students the entire
summer, and allowing me to use his labs at my own will. Thanks to Eric DeCuir Jr. for
helping me with everything the entire summer and making this experiment a huge
success. I would not have been able to chose, or complete this project with out him.
I really appreciate everybody’s hand in making this summer a complete success
and would not be hesitant to do it all over again. Thanks a lot!
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Tariq Bond, Norfolk State University
Faculty Mentor: Dr Omar Manasreh, Electrical Engineering
Graduate Student Mentor: Eric DeCuir Jr., Microelectronics Photonics PhD
A Polychromatic Approach to White Light Emission Utilizing Nanocrystals
Findings: With each of the mixed nanocrystal samples, each of them showed some form
of white light with a difference in the shade, or either a very close potential for white
light. When compared to a blue LED with a yellow phosphorous coating we had a few
graphs that were already comparable before any fine tuning or measuring of the amounts
that we placed on each slide. Ultimately I have found this method to be just as effective
as the blue LED phosphorus coated method and also hold much more potential for
various color generation.
Planned Publications
1. DOE ESCoR Proceedings
2. Poster presentation at EPSCoR conference
Key Illustration/Figure
This is an important picture of mine with mixed nanocrystals of wavelengths 493, 575 &
614nm (Sample A). This produced white light using a 405nm ultraviolet LED. This
finding along with Sample “B” below, allowed me to rest on the conclusion that I could
successfully create white light as well as adjust the tint/hue of that white light as well as
shown by the second picture by the white light along with the bright orange tint.
Sample A with nanocrystals 493, 575, & 614nm
Sample B with nanocrystals 514, 530, 575, & 614nm
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