The Light Waves
The Light Waves
by Tracy M. Mattox
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Did you know that particle size
can control color when
nanoparticles are really small?
Let me show you how
Molecular Foundry scientists
are making waves in science.
Here is a quick refresher from the last book that might be useful:
• World’s smallest building blocks
• Nucleus = center containing
neutral neutrons and positive
• Moving around nucleus =
negatively charged electrons
Molecules Water is H2O
• Two or more atoms joined
• Can be written in many different
ways, as you see here with water
OH NOOOO! I was chatting with some
friends about photovoltaics and forgot to wear
Well, let’s use my pain as a teachable moment.
I’m Nancy Nano, and today I’ll tell you a little
bit about light waves and what it means to study the electromagnetic
Q: What do heat, light, colors, x-rays and
microwaves all have in common?
A: They are all part of the “electromagnetic
The electromagnetic spectrum is the energy that travels through space,
and it’s usually measured as a wavelength. You can think of it like the
waves in the ocean.
The tops of the wave are called “peaks,” and the distance between these
peaks is the “wavelength.” When the peaks are far apart they have less
energy (like you taking long easy steps), and when they are closer
together they have more energy (like when you run with tiny steps). The
frequency is how quickly the wave moves and smaller wavelengths have
a higher frequency (it takes more short running steps to cover the same
distance as long slow steps).
Every color you see has a different wavelength. When you look at a
rainbow you remember the color order by saying “ROY-G-BIV” (red,
orange, green, blue, indigo, and violet). Well, you can also use ROYGBIV
to remember the energy of the color spectrum, where red has the lowest
energy and violet the highest.
What will really blow your mind is that all energy has wavelengths just like
colors do, and most of the energy around you can’t be seen with your eyes.
This is a picture of the electromagnetic spectrum:
The high energy of ultraviolet (UV) waves is what causes your sunburn.
Thankfully scientists have invented sunscreen to block the UV rays to help
keep your skin safe!
Nanoparticles are less than 100 nanometers (nm) in size, and when they are
this crazy small they can act differently than when they are big, like
becoming flammable or magnetic.
Particles called quantum dots are especially interesting because the same
material can be a different color when you change the size (so the
wavelength you see depends on the nanoparticle size).
2 nm These are cadmium selenide
Particle Size Increases (CdSe) quantum dots of
increasing size. The tiny 2
nm particles are pale yellow
while the larger 4 nm
4 nm particles are red.
The color you see depends on which wavelength the particles absorb. In
other words, the particles keep the energy of a certain wavelength and you
only see the complementary color.
It’s easy to think about using a color
wheel. For example, if the particles
absorb green you will see red.
Scientists use special machines called
“spectrometers” to measure absorbance. The
data (or “spectra”) looks like this, and for
quantum dots the wavelength you see depends
entirely on the particle size.
Some quantum dots are made with a wavelength that can be measured
using a special scanner. The picture below is of a rat with a shot of
quantum dots injected into its tail. The dots move through the body in the
blood and using a scanner to detect the right wavelength, you can see
where they end up (in the heart and liver).
Imagine where this science can go! What if we could design quantum dots
that could stick to cancer cells? Doctors would be able to use quantum dots
to understand diseases and develop better treatments.
Sometimes nanoparticles are “plasmonic,” which means you can study
how the electromagnetic field influences free (unbound) electrons in a
Let’s put it this way… nanoparticles can be so tiny that they are smaller
than the wavelength of light. (Now THAT is small!)
If the particles have electrons that are alone and not needed to connect
atoms together they are called “free electrons,” meaning they are free to
move around the molecule. Basically, free electrons don’t have a job!
free electrons e- e-
The wavelength of light oscillates (or moves) up and down, like a slinky.
As it moves the free electrons come together in an “electron cloud” and
pull away from the electric field (or light wave).
e- e-e- electron cloud
So this e- looks more like this
electric field e- e- e-
(movement of light wave)
e- e- e-
Just like quantum dots, changing the particle size changes the wavelengths
that can be absorbed. In other words, changing the particle size changes
what wavelength is needed to move the electron cloud. Large particles
can’t fit under small waves, so they need a longer wavelength.
Even more interesting is when the plasmonic particle is influenced by more
than one wavelength. For example, a disk is longer in one direction than
another, so two different wavelengths move the electron cloud. As a result,
there are two absorbance peaks. Particle shape is important, too!
e- e- e- e-
electric e- e- e-
e- e- e-
Imagine how complicated this gets when you have a 3D particle with a lot
of different shaped surfaces! Below is a real example of cesium tungsten
oxide (CsWO3). A hexagonal disk (the red picture) can be laid flat to give
you two different types of surfaces (rectangles and hexagons) so there are
two absorbance peaks. When CsWO3 is a cube with flat edges, there are
even more surfaces and the extra peaks overlap or smoosh together.
How many peaks does a
sphere of CsWO3 have?
Reprinted with permission from Chem. Mater., Why?
2014, 26(5), 1779-1784. Copyright 2014, If you lay a sphere flat you
American Chemical Society. get an infinite number of
surfaces… and they all
squish so close together it
looks like one peak.
It’s possible to make plasmonic nanoparticles that will absorb infrared (IR)
wavelengths but not the visible, so they can be used as window coatings
that will block heat (IR radiation) from the sun while still letting colored
light shine through.
This model is the real deal! The blue cubes
are glass (indium tin oxide or ITO) and the
green cubes are the plasmonic
nanoparticles (niobium oxide or NbO). The
nanoparticles stick to the glass and absorb
Reprinted with permission from Nature, 2013, 500, 323-326.
Copyright 2013, Nature Research.
Another really neat thing about plasmonic materials is that they can also be
“electrochromic,” which means that you can change their color simply by
adding or removing electrons.
This is a polymer stuck on glass.
When you remove electrons it turns
green, and when you add electrons it
Images reprinted (adapted) with permission from ACS Sustainable Chem. Eng., 2016, 4(5), 2797–2805.
Copyright 2016, American Chemical Society.
Electrochromics are a hot topic (pun intended) in science to make Smart
Windows. Imagine being able to flip a switch and make your window cool
down a room (stop the heat from IR waves) without changing the lighting
in a room. It’s not just smart, it’s brilliant!
Now imagine wanting to watch a movie or play on your computer and
realizing it’s too bright to see the screen. What if you could just flip a
switch and make the window darker?
That’s what scientists are working on! It’s all about using electrons to
control how nanoparticles absorb different wavelengths.
I could go on for days talking about nanoparticles, but I want to enjoy a
cool drink of water in a nice cool house so will stop here.
I hope this book has been en-light-ening. If anyone asks how smarter
windows of the future can cool a room or darken with the flip of a switch,
now you know what to tell them!
Stay tuned for new
“The Molecular Foundry is a Department of Energy-funded nanoscience
research facility that provides users from around the world access to
cutting-edge expertise and instrumentation in a collaborative,
What does this really mean? Our job is to help people from all over the
world accomplish their scientific goals with nanomaterials. Every floor in
our building is dedicated its own field of science: Electron Microscopy,
Imaging, Nanofabrication, Theory, Inorganic Chemistry, Biology, and
The amazing thing about the Molecular Foundry is that experts in all of
these different fields are under one roof, so it’s really easy to work
together. Scientists in completely different fields don’t usually have a
chance to interact with one another, but here we just walk upstairs and have
a conversation or sit at the lunch table and talk science. We are an
incredibly collaborative group, and because we work so well together it’s
possible to accomplish things in science that are beyond the reach of most
researchers. It is truly an incredible place to be.
Science references for this book
Image of double rainbow above the Molecular Foundry
Photo of Cadmium Selenide quantum dots showing particle size controlling
Particles made by Haoran Yang using WANDA (the first nanoparticle
synthesis robot of its kind!)
Quantum dots for imaging in live cells (rats)
Michalet, X; Pinaud, F F; Bentolila, L A; et al., Quantum Dots for Live
Cells, in Vivo Imaging, and Diagnostics, Science, 2005, 307, 538-1179.
L.A. Bentolila; X. Michalet; F.F. Pinaud; et al., Quantum Dots for
Molecular Imaging in Cancer Medicine, Discov. Med. 2005, 5(26),
Plasmonic CsWO3 nanoparticles
Mattox, Tracy M; Bergerud, Amy; Agrawal, Ankit; Milliron, Delia J,
Influence of Shape on the Surface Plasmon Resonance of Tungsten
Bronze Nanocrystals, Chem. Mater., 2014, 26(5), 1779-1784.
Smart windows with NbO
Llordes, Anna; Garcia, Guillermo; Gazquez, Jaume; Milliron, Delia J,
Tunable near-infrared and visible-light transmittance in nanocrystal-in-
glass composites., Nature, 2013, 500, 323-326.
He, Bo; Chen, Teresa L; Klivansky, Liana M; Tan, Tianwei; Teat,
Simon J; Liu, Yi, Low Bandgap Conjugated Polymers Based on a
Nature-Inspired Bay-Annulated Indigo (BAI) Acceptor as Stable
Electrochromic Materials, ACS Sustainable Chem. Eng., 2016, 4(5),
J. Kim; G.K. Ong; Y. Wang; et al., Nanocomposite Architecture for
Rapid, Spectrally-Selective Electrochromic Modulation of Solar
Transmittance, Nano Lett, 2015, 15, 5574-5579.
Williams, Teresa E; Chang, Christina M; Rosen, Evelyn L; et al., NIR-
Selective electrochromic heteromaterial frameworks a platform to
understand mesoscale transport phenomena in solid-state
electrochemical devices, J. Mater. Chem. C, 2014, 2, 3328-3335.
Runnerstrom, Evan L; Llordes, Anna; Lounis, Sebastian D.; Milliron,
Delia J, Nanostructured electrochromic smart windows traditional
materials and NIR-selective plasmonic nanocrystals, Chem. Comm.,
2014, 50, 10555-10572.