small is
BIG
Studying
Silicon
Nanotechnology
at UCSD
by Gha Young Lee
Near the end of my freshman summer, I was looking for Sailor. All of these things contributed to my growing underresearch opportunities when I discovered the Sailor Research standing of porous silicon—from how it’s created to the vast
Group at the University of California, San Diego. Headed by range of applications it can have.
Dr. Michael Sailor, the group focuses on porous silicon (pSi)
nanotechnology, which has wide applications in chemical and Porous Silicon: A Primer
biochemical sensors, medical diagnostics, and drug delivery.
Porous silicon is a manmade form of the chemical element
According to his website, Dr. Sailor had already accepted a silicon that has nanoporous holes in its microstructure. The
couple of high school interns in his lab.
pores are created using an electrochemiDrawn to the relatively new technology
cal etch, in which current is applied to
and its promise of the opportunity to
oxidize the silicon. The pore size can be
grow with the field, I emailed my brief
controlled by the density of the applied
résumé to him, describing my interest
current, and the sinusoidal waves of the
in his research and my desire to intern
current can create something called a
in his lab. A few days later, I received a
rugate structure, which has pores that
Cross-section showing the pores of a
reply: Dr. Sailor wanted to meet with me. rugate structure etched into a slicon wafer increase and decrease in diameter as a
At the beginning of my sophomore
function of depth in the porous layer.
year, I met with Dr. Sailor. During our
While porous silicon etched with
meeting, we agreed that I would spend six to nine hours a more uniform pores appears dull brown, the oscillating porosweek in the lab, where I would shadow graduate students and ity of rugate structures produces bright colors in the visible light
postdocs until I came up with a project idea.
range. The rugate structure is a type of optical device known as
I started by learning about safety precautions and training a photonic crystal. More familiar examples of photonic crystals
on various pieces of equipment. For the first few months, I are peacock feathers and abalone shells. When viewed with a
watched the grad students conduct their research, asked lots spectrophotometer—an instrument that measures the intenof questions, and read a book on porous silicon written by Dr. sity and wavelength of reflected light—the vibrant colors of a
www.cty.jhu.edu/imagine
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rugate structure produce a spectrum with one or more sharp
peaks. The spectrum of a normal pSi layer, in contrast, has no
dominant peaks.
The porosity of the rugate structure controls the spectral
properties of the peaks, meaning that a rugate structure’s spectrum can be used to characterize its porosity as well as what is
in the pores. Thus, the spectrum allows scientists to determine
the refractive index of the material in the pores, whether it be
air or liquid, which can in turn facilitate the process of identifying an unknown chemical substance.
RACHEL BISIEWICZ, BROWN UNIVERSITY CLASS OF 2014
Promising Applications
When the pores are filled with materials with a higher refractive index than air, the position of the spectral peak changes.
Therefore, pSi can be used to detect gases, liquids, or any
substance other than air, for that matter. For example, by
measuring the elevated levels of acetone in a diabetic’s breath,
pSi could potentially be used to detect or possibly monitor
diabetes. It can also be used to detect proteins and enzymatic
activity with great precision.
Change in the reflectance peak of the pSi rugate structure
in the presence of air (top) and ethanol (bottom)
imaged and located. Accordingly, silicon nanoparticles are an
aspect of silicon nanotechnology that is emerging as a leader
in medical diagnostics and therapy.
Demonstration of the change in color of pSi filled with air (left)
and ethanol (right)
Another application I found fascinating was the replication
of pSi. Because silicon is brittle, replicating its structure with
stronger, more flexible materials can allow for a wider variety of
applications. For example, the pores can be filled with flexible
plastics. When the silicon template is removed, the plastic is
left, providing a negative of the original. If biodegradable polymers are used to replicate the porous structure, drugs can be
loaded and sealed. This drug-loaded polymer can be implanted
in the body to provide controlled release as the polymer gradually degrades, allowing for targeted, time-release therapies.
In another significant etching technique called a lift-off, the
etched portion of the silicon chip—approximately 100 micrometers thick—is separated from the bulk silicon wafer. This thin,
brittle film has diverse applications. It can be broken down into
porous nanoparticles which can then be loaded with drugs
that release over time as the silicon degrades in the body. The
nanoparticles can also be modified to target specific regions in
the body, or even fluoresce so that the targeted region can be
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A Research Opportunity of My Own
I’d worked in Dr. Sailor’s lab for about five months when I
learned about the Summer School for Silicon Nanotechnology
(SSSiN), funded by the National Science Foundation. This
selective summer program, held at UCSD, is open to students
from all over the world, from high schoolers to postdocs. I had
learned a lot by shadowing in Dr. Sailor’s lab and was eager for
the opportunity to conduct research. I applied to the program
and was one of 25 participants accepted.
The five-week summer program features lectures by Dr.
Sailor and a mentored group project that focuses on a specific
research topic. The program allows younger students to work
with a scientist on a topic of their choosing. Since I was particularly interested in the polymer replication of pSi, I chose
this topic and, along with the four other high school students
in the summer school, worked with PhD student Joanna Wang.
Unlike plastics, which are generally durable and stable,
silicon is brittle and chemically unstable. Our goal was to come
up with procedures to create a stable polystyrene replica that
could have the sensing properties of porous silicon, but with
Mar/Apr 2014
JOANNA WANG, UNIVERSITY OF CALIFORNIA, SAN DIEGO
more structural and chemical stability. Although some previous
attempts had been made to replicate polymers, the method used
produced inconsistent results and poor replication, and this area
remained largely unexplored. Not only would a perfected procedure have real-life applications such as the sensing of harmful
gases, but it would also open the door for creating replicas of
biocompatible polymers for possible drug delivery. While the
application itself is similar to those of porous silicon, using the
polymer to replicate the nanostructure was unique.
At our first meeting, our group discussed the direction of the
project. We would optimize the replication by varying infiltration methods, or ways of inserting the polymer in the pores. We
chose two: melting the polymer with heat, and putting polymer
dissolved in a solvent in the pores and evaporating the solvent out.
For the next five weeks, we tested the parameters. It required
much patience and constant hypothesis, feedback, discussion,
and verification, using equipment that included a spectrophotometer and a scanning electron microscope. Ultimately, we
found that melting the polymer into a partially oxidized lift-off
replicated the structure best. We also found that the ability of
polystyrene to replicate and retain the nanostructure depends
on its viscosity and molecular weight.
The polystyrene replica in air (left) and wetted with ethanol
(right)
Invaluable Information
The summer program gave me the opportunity to participate
in nanotechnology research, discuss ideas with professionals
as we planned the next steps in the project, and gain a deeper
understanding of the emerging field of porous silicon. It was
also a chance to socialize with international participants with
diverse backgrounds.
Throughout the program, the groups ate lunch together and
discussed their projects and past experiences in college, graduate school, and other research environments. Their wisdom and
honest opinions about the field provided me with invaluable
information and insight about the impact and beauty of STEM
and the careers associated with it.
We concluded the program with a series of presentations
in which the groups shared their accomplishments and talked
about their future plans. It was astounding to see how far each
www.cty.jhu.edu/imagine
group got, and how much the field
was able to expand in one summer: One group came up with a
novel method of controlling the
size of silicon nanoparticles during the manufacturing of them;
another optimized the conditions for
manufacturing silicon nanoparticlecontaining polymers for drug delivery.
Beyond SSSiN
After SSSiN ended and the participants returned
to their respective institutions, I was fortunate to be able to
continue working in the lab with Joanna on our polystyrene
project. We finalized our data showing the different peak shifts
over time for each molecular weight and temperature. We also
analyzed the peaks to better clarify the mechanisms of polymer
infiltration of the rugate nanopores and identify important factors that affect successful replication of the rugate structure. Our
project has been accepted for presentation at the 2014 Porous
Semiconductors Science and Technology (PSST) conference
that will be held in Spain in March. We also plan to submit a
paper on the research in the near future. I am very proud that
the five-plus weeks indeed paid off.
I still intern at the Sailor lab with Joanna as my mentor.
Currently, I’m working on a project involving replication of
acrylic cross-linking polymers that will be even more stable than
polystyrene, both chemically and mechanically. It represents a
novel approach to polymer replication and, if successful, will
provide an excellent alternative to the brittle silicon rugate. The
acrylic polymer replicas, being structurally stronger, will be able
to be used to sense a wider variety of materials than the polymer
replicas manufactured thus far. I plan to present the project at
the 2014 Greater San Diego Science and Engineering Fair.
Researching pSi in Dr. Sailor’s lab and through the summer
program not only provided me with amazing opportunities to
explore nanotechnology. It confirmed my determination to
pursue a career in science.
Testing the
parameters
required
much patience
and constant
hypothesis,
feedback,
discussion, and
verification,
using equipment
that included
a spectrophotometer and
a scanning
electron
microscope.
Gha Young Lee is a junior at Torrey Pines
High School in San Diego, CA. She won
first place at the San Diego Science Fair
for her project on dielectric constants, and
with her team, won the award for Best
Experimental Measurements at the
International iGEM Jamboree. At school,
Gha Young founded the Inventions Club
to encourage creativity in her fellow students.
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