Melanie Shaul
Writing 340, Section 66818
Illumin Paper
Layer-by-layer: Engineering Surfaces
Abstract: Technologies that block infrared rays from entering through windows, based on the
possibility of making cars and rooms more energy efficient by eliminating the need for air
conditioners, are explored. In particular, layer-by-layer deposition of thin films is investigated
and the great benefits of a simple principle of “opposites attract” is revealed in making films with
many remarkable applications, including energy-efficient infrared ray reflecting films.
Key words: nanotechnology, layer-by-layer, thin films
Prepared by: Melanie Shaul
Author Bio: Melanie Shaul is a sophomore from Sunnyvale, CA. She is currently attaining her
degree in chemical engineering from the University of Southern California, and she hopes to
pursue an emphasis and a master’s degree in either Nanotechnology or Petroleum Engineering.
Author Contact: email: mshaul@usc.edu
Prepared on February 25, 2013 for Illumin Magazine “Call for Papers”
Layer-by-Layer: Engineering Surfaces
Introduction
You are enjoying a warm mid-summer day at the beach, soaking up the rays from the sun
high in the sky. The time is approaching for you to leave, and as you gather your towel and
sandals and walk toward your car, your good mood suddenly fades. It dawns on you that before
you can consider getting in your car to drive home, you will need to open all of the car doors and
blast the air conditioner to avoid a heat stroke. And, suddenly, you regret your choice of black
leather seats and the beautifully sunny parking spot you selected. Not only is it going to cost you
time and a little discomfort, it is also going to cost you money to run the air conditioner. The
summer heat was enjoyable out on the sandy beach, but inside your car the intense heat can be
uncomfortable, dangerous and expensive.
Imagine a car that doesn’t overheat, even on the hottest of summer days. You could
leave the beach and get into your car without a worry of the seatbelt burning your bare skin or
the hot, stuffy air suffocating you. Sure, you could put a foldable heat shield on the dashboard of
the car and on every window, but what if the heat was kept outside regardless of if you took the
time and effort to put up a heat shield? This idea
might be possible with the right technology integrated
into car windows—Layer by Layer (LbL) film
technology has the potential to block the sun’s rays
Figure 1: The electromagnetic spectrum
shows all forms of light and their
wavelengths. The only region of the
spectrum that is visible to humans is the
visible light region, pictured in a zoomed
view to show the colors of the visible light
region. Infrared rays are among the forms
of light that cannot be seen.
(http://www.yorku.ca/eye/spectru.htm)
from entering your car.
Before we can explore the solution, however,
we must explore the nature of the problem, which
arises from heat. The heat that enters our car windows, creating a human oven inside, comes
directly from the sun in the form of “infrared radiation.” Infrared radiation is waves of light with
wavelengths of 700 nanometers to 100 millimeters [1]. Light at this wavelength cannot be seen
by our human eyes, but we can feel it as heat (Fig. 1). The light we can see is called visible light,
which ranges in wavelengths from 380 to 750 nanometers, and corresponds to the colors of the
rainbow from red to violet [1].
The sun continually radiates infrared rays, so we can always feel its heat, especially in
summer. But why is the beach a pleasant temperature while our car interior gets significantly
hotter? When heat comes into contact with objects, such as the leather seats inside our cars, it is
absorbed. The objects then slowly re-emit, or release the heat
back into the air. In a closed environment such as a car, the
temperature can reach dangerous highs as the heat is absorbed
and re-emitted by leather seats and other objects (Fig. 2). If car
windows were able to reflect the near infrared waves of heat
and prevent them from entering the car, the dangers and
inconveniences of a hot day will no longer be a daily worry.
An article by the American Dermatology Association [2]
explains that infrared radiation from the sun is “readily
Figure 2: The infrared rays radiating
from the sun (shown in yellow)
enter a car and are absorbed and reemitted by objects and seats in the
car (rays shown in red), making the
inside temperature much hotter
than the outside temperature due to
this trapping and recirculation of
the heat inside the car.
(http://www.ggweather.com/heat)
transmitted through standard glass” but protective glass should
block the infrared radiation while still allowing visible light
through. If car windows had such protective glass, a
comfortable, controlled, and safe environment could be maintained in the interior of cars.
Such a solution seems obvious. Why, until now with the promise of LbL films, hasn’t
technology like this been developed? The truth is, it has been developed but it lacks optimum
efficiency. Window coatings that reduce heat from the sun have been used on home windows
for over 30 years.
An Existing Solution
Introduced in the 1980’s, low-emissivity technology is still implemented into home
windows today [3]. “Low-emissivity” means less heat entering or leaving a space [4]. The
purpose of this technology is to maintain a cost efficient home by reducing the need for air
conditioning. Low-emissivity coatings, commonly referred to as “low-e” coatings, are an extra
plastic layer applied to windows that reduce the amount of infrared radiation that can enter the
home through the windows. This is accomplished by vacuum-depositing a layer of silver onto a
plastic sheet and putting it between two window panes [3]. The result is a transparent coating on
the window. A ray of infrared heat that comes into contact with a standard window transmits
directly through the window and continues travelling on the other side of the window, typically
inside a home. Imagine now a ray of infrared radiation that hits a window with a low-e coating.
The ray of heat is absorbed by the low-e layer, and it is prevented from entering the home. The
technology is “low” emissivity because it lowers the amount of heat entering the home; however,
it does not eliminate the entering heat completely. Commercial low-e coatings claim to reduce
about 87% of near infrared radiation [5].
While the “low-e” technology was a great development in the 1980’s, energy costs have
gone up and we are still cringing at the thought of sitting on black leather seats in the middle of
July. This ultimately leads us to our anticipated solution; a more recent technology, LbL
deposition – first introduced in 1966 but evolved as a feasible process only 20 years ago – may
lead to a more efficient, cost effective solution to the problem [6, 7, 8].
Layer-By-Layer
Layer-by-layer deposition (LbL) is a considerable advancement in the nanotechnology
world that creates thin films on the nanoscale (the scale of one billionth of a meter). While the
overall size of an LbL film varies and can be made large enough to coat an entire window, the
typical width of an LbL film is anywhere from 100 to 700 nanometers thick [8]. Paula
Hammond, a professor at MIT, puts things in perspective for us: “If you were to take a piece of
your hair and lay it lengthwise… [and] cut that piece of hair into 10,000 even slices, one of those
slices would be a nanometer” [9]. The exceptionally thin films made by the LbL process can
reflect infrared rays, and may be a revolutionizing advancement.
The process of constructing LbL films for a surface utilizes the simple rule of ‘opposites
attract.’ A film is created by alternately layering two solutions that are attracted to each other,
and therefore “stick” together, to cumulatively create a film. The component of these solutions
that allows them to stick together is ions. An ion, which is a tiny charged particle, is either
positive (a cation) or negative (an anion). Cations and anions are attracted to each other because
their charges are opposite; a concept called electrostatic forces that mimics ‘opposites attract’.
Cationic solutions (solutions rich in cations) and anionic
solutions (solutions rich in anions) are the main ingredients
of LbL films.
Consider a film being created on a small glass
surface. To construct the film, the anionic solution is
sprayed evenly onto the glass to deposit a layer of anions.
Figure 3: A LbL film is made by alternately
spraying the cationic solution and anionic
solution onto the substrate. (The figure implies
the substrate is being dipped alternately into
solution, but this is an older method. Spraying
is the more advanced and widely used
method.). As the alternating layers build up,
the film is made [10].
Next, the cationic solution is sprayed, and the cations stick to the anions that were previously
applied. This leaves one uniform layer of anions and one uniform layer of cations that are bound
to each other [10]. Fig. 3 shows a glass surface with one layer of anions and one layer of cations
that were sprayed on to start the process of making a LbL film [10]. The process is not unlike
spreading peanut butter on a piece of bread, and then spreading jelly on top of the peanut butter.
Figure 4: Formation of
bilayers (one anionic and
one cationic layer) via LbL
assembly [7].
The only difference is the ingredients
are applied with a spray rather than
being spread, and the cations and
anions are not as appetizing as peanut
butter and jelly! An anion layer and a cation layer (the peanut butter and the jelly) together are
called a bilayer. The repetition of spraying anionic solution and then cationic solution on a
surface results in a thin film consisting of several bilayers (Fig. 4) [7]. We can think of a LbL
film as a multi-decker peanut butter and jelly sandwich, although certainly not as tasty!
LbL technology becomes useful for heat reflecting applications when films are made by
alternating two different bilayers that transmit light differently; these films are called Bragg
Stacks. Since light does not travel at the same rate through everything, every medium has a
refractive index that tells how fast light travels through it. Spaces that allow light to penetrate
very quickly, such as air, have a low refractive index while spaces that allow light through more
slowly, such as water, have a higher refractive index. A Bragg Stack is comprised of two
alternating bilayers, one with a high refractive index and one with a low refractive index [11]. In
the case of our peanut butter and jelly, one bilayer could be made out of a layer of smooth peanut
butter and a layer of grape jelly, which, let’s pretend have a high refractive index, and another
bilayer consisting of a layer of chunky peanut butter and a layer of strawberry jelly, which we
will pretend have a low refractive index.
The effect of alternating bilayers of high and low refractive indices causes constructive
and destructive interference of incident light [12, 13]. This
essentially means that light of a certain wavelength will be
reflected off of the film’s surface, while all other light will
be transmitted through the film. The true beauty of LbL
Figure 5: This graph shows a linear
relationship between film thickness
and the number of bilayers that make
up the film. As the number of bilayers
is increased, the film thickness
increases predictably [14].
films is that the wavelength of light that will be rejected by
the film can be chosen. The thickness of the film is what
determines which wavelength of light will be reflected off
the film [12], and the thickness of the film can easily be
controlled. The thickness of the film corresponds directly to the number of bilayers in the film,
just as the height of a multi-decker peanut butter and jelly sandwich is directly dependent upon
the number of layers of peanut butter and jelly you have added; choosing the reflected
wavelength is as easy as decreasing or increasing the amount of bilayers that are sprayed onto the
film. Because there is high control over the thickness of the films (Fig. 5) [14], the films can be
engineered to reflect any wavelength of light, which opens up many practical applications of the
films. Thus, the unwanted infrared rays that threaten the temperature of our car interior can be
reflected when the appropriate number of bilayers is incorporated into a film [13], and our beachday blues can come to an end.
Future
Aside from its potential to create infrared reflecting films, LbL is also being used for a
variety of other applications, including color applications. This is because the films can also
reflect any wavelength of visible light, from red to violet. If the film is engineered to reflect the
wavelength of a specific color, the film will actually appear to be that color. This opens the door
to many color applications because the color created by LbL films is unique in that it exhibits
“optical interference effects” [12], and thus
imitates the three-dimensional color seen in
nature on the wings of butterflies (Fig. 6), on the
Figure 6: Butterfly wings exhibit threedimensional color found in nature. A close-up
shows the color is unlike the one-dimensional
color seen on man-made objects; the color on the
butterfly wing has more dimensions of color and
can change based on the angle of viewing.
(www.phys.org)
feathers of peacocks, and in opalescent pearls—
an accomplishment ordinary paint has not yet
seen [8, 12].
The LbL films can also be engineered for
use as anti-reflective screens. Anti-reflective screens would enhance viewing on mobile devices
or TV screens by eliminating light reflection [15].
Conclusion
Regarding the desire for temperature-controlled interior spaces, the method of LbL
proves to be much more effective than low-e coatings in reducing the amount of heat entering a
home or car through a window. Recalling that the low-e coatings prevent 87% of heat from
entering a home, LbL films are near 100% effective in reflecting heat. The process of
assembling LbL films is also much more cost effective and efficient than other processes of
creating films, such as that of low-e coatings. The process not only produces product at a low
cost and uses a simple technique, it is also very quick and environmentally friendly; unlike other
chemical processes, the waste of LbL assembly is primarily water.
LbL assembly is thus very desirable for commercial processes, and efforts are being
made to implement the process on an industrial-scale. LbL deposition is currently limited to
small-scale depositions, meaning a limited square-inch film can be made at one time. This is
because a good film should be perfectly uniform, which is easy to do on a small scale, but is
more complicated on a large scale.
At the realization and launching of the industrial-scale LbL deposition, we can count on
seeing (but not to the naked eye!) this technology everywhere – car makers could choose to put
the films directly in the glass they use for windows; homes and commercial buildings could use
the infrared reflecting films to maintain energy efficient and cost-effective rooms; we may be
able to watch TV and surf the web on our computers outside on a sunny day; and paint
companies could use the films to create vivid, three-dimensional paint color. From car windows
to paint cans, LbL will revolutionize the world one nano-step at a time.
References
[1] Matthes, R. "Chapter 49 Infrared Radiation." N.p.: n.p., n.d. N. pag. Web. 21 Feb. 2013.
<http://www.ilo.org/safework_bookshelf/english?content&nd=857170571>.
[2] Unknown, “Protect yourself from the sun,” The Berkshire Eagle, May. 2006.
[3] Wilson, Alex. "Window Performance and the Magic of Low-e Coatings." Building Green.
N.p., 27 Mar. 2012. Web. 21 Feb. 2013.
[4] Audin, Lindsay. "A Brief Lesson in Thermodynamics." Building and Construction 53.3
(2006): 46. ABI/INFORM. Web. 21 Feb. 2013.
[5] Thermaflect: Low "E" Glazing System. N.p.: CertainTeed Corporation, 2001. Print.
[6] Iler, R. K. J. Colloid Interface Sci. 1966, 21, 569−594.
[7] Kyung, KyuHong. "Control of Nano-Structure of Thin Film by Spray Layer-by-Layer
Method and the Optical Applications." Thesis. Keio University, 2011. Mar. 2011. Web. 21 Feb.
2013
[8] Lee, Daeyeon, Zekeriyya Gemici, Michael F. Rubner, and Robert E. Cohen. "Multilayers of
Oppositely Charged SiO2 Nanoparticles: Effect of Surface Charge on Multilayer Assembly."
Langmuir 23.17 (2007): 8833-837. ACS Publications. American Chemical Society, 12 July 2007.
Web. 21 Feb. 2013.
[9] Paula Hammond: Fighting Cancer. WCVB: Boston Channel. Boston, Massachusetts, 18
Dec. 2012. 18 Dec. 2012. Web. 21 Feb. 2013.
[10] Hammond, Paula T. "Engineering Materials Layer-by-layer: Challenges and Opportunities
in Multilayer Assembly." AIChE Journal (2011): N/a. Wiley Online Library. 3 Oct. 2011. Web.
21 Feb. 2013.
[11] Choi, Sung Yeun, Marc Mamak, Georg Von Freymann, Naveen Chopra, and Geoffrey A.
Ozin. "Mesoporous Bragg Stack Color Tunable Sensors." Nano Letters 6.11 (2006): 2456-461.
ACS Publications. American Chemical Society, 4 Oct. 2006. Web. 21 Feb. 2013.
[12] Wu, Zhizhong, Daeyeon Lee, Michael F. Rubner, and Robert E. Cohen. "Structural Color in
Porous, Superhydrophilic, and Self-Cleaning SiO2/TiO2 Bragg Stacks." Small 3.8 (2007): 1445451. Wiley Online Library. WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 22 June 2007.
Web. 21 Feb. 2013.
[13] Heiss, W., T. Schwarzl, J. Roither, G. Springholz, M. Aigle, H. Pascher, K. Biermann, and
K. Reimann. "Epitaxial Bragg Mirrors for the Mid-infrared and Their Applications." Progress in
Quantum Electronics 25 (2001): 193-228. Elsevier Science Ltd., 2002. Web. 21 Feb. 2013.
[14] Krogman, K. C., N. S. Zacharia, S. Schroeder, and P. T. Hammond. "Automated Process for
Improved Uniformity and Versatility of Layer-by-Layer Deposition." Langmuir 23.6 (2007):
3137-141. 2007. Web. 21 Feb. 2013.
[15] Zhang, Lianbin, Yang Li, Junqi Sun, and Jiacong Shen. "Layer-by-layer Fabrication of
Broad-band Superhydrophobic Antireflection Coatings in Near-infrared Region." Journal of
Colloid and Interface Science 319.1 (2008): 302-08. Science Direct. Elsevier Science Ltd., 1
Mar. 2008. Web. 21 Feb. 2013.