| Introduction |
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| Quantum dots are tiny crystals that have been carefully crafted to control the movement of electrical charge
within them. Each quantum dot is extremely small; several million quantum dots could fit into the width of a person's thumb.
Quantum dots can be produced in many different shapes and with many different chemicals, but they all share the ability to
glow, or fluoresce, when light is shined on them. Quantum dots with different structures will glow in different colors, and scientists
can now make quantum dots in almost every color of the rainbow including white. Since quantum dots are a fairly new
technology, they are not yet widely used in many areas, but they show a lot of promise for applications in a wide range of industries. In the
near future, quantum dots will likely be used in fields as diverse as medicine, home lighting, and generating electricity. |
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| How Quantum Dots Work |
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At the heart of how a quantum dot works is its ability to constrain the movement of electrical charges within it. The "electrical
charges" being constrained can be free electrons, or they can be holes in a large group of electrons that behave like positively
charged particles. Regardless of what the electrical charge is in any particular case, the quantum dot will trap it in a very small
region. This is most commonly accomplished by making quantum dots with two layers: a core made of one material, and a
surrounding shell made of a different material. Charges get stuck where the layers meet, and they are confined to the tiny
space in between the layers.
When a charge is trapped like this, something strange happens: the charge's energy becomes quantized, meaning that
there are only a certain number of possible values it can have. For a visible object, like a baseball, energy is not quantized. The
harder you throw a baseball, the faster it will go, and adding just a little bit more force to your throw will always slightly increase
the speed of the ball, which in turn will give it just a little more energy. In a quantum dot, things work differently. If you give
a tiny amount of energy to one of the electrical charges in a quantum dot, nothing will happen. Give a little bit more, and still
nothing will happen. You can keep giving and giving and the charge won't do anything until, at some predetermined amount of
energy, the charge will suddenly jump up to its next energy level. The next energy level works the same way; if you give the
charge more energy than it needs to make the first jump, it will still only make the first jump and stop there. Give it enough
extra energy, though, and it can make two jumps, and so on. You may have heard about the electrons orbiting an atom doing
the same sort of thing. The two situations are very much the same - in fact, quantum dots behave so much like atoms that
they are sometimes referred to as "artificial atoms".
When incoming light or electricity gives one of the charges in a quantum dot enough energy to jump up to the next energy level,
that charge is said to be excited. When a bunch of quantum dots are exposed to a strong light, many of their charges
become excited very quickly. The charges don't stay excited for long, though - charges like to be in the lowest energy level they
can, and an excited charge will soon fall back to its original energy level without anything keeping it excited. In order to get
back down, the charge needs to get rid of all the energy it soaked up when it became excited in the first place. The released
energy takes the form of a photon, a particle of light quite similar to the one that excited the charge in the first place. However,
the charge will not always release all of its energy in a single fall from high to low energy. More often, it will get caught in several intermediate
energy levels on the way down. Each transition will release a separate photon, so by the time it reaches the lowest energy
level, it will have given off one photon for each time it got caught during its descent.
Interestingly enough, no matter how high on the energy ladder a charge climbs, the distance it falls each time it emits a photon
stays roughly the same. This means that regardless of how much or how little energy was used to excite a charge in the
first place, all of the photons it gives off will have about the same amount of energy - exactly how much energy this is depends
on what kind of quantum dot we are looking at. The color of light given off is determined by the amount of energy in each
photon, so any given structure of quantum dot has a characteristic "glow color" that does not depend on the color of light used
to illuminate the dot. (You can check the Links section for a video of purple light being used to make quantum dots glow yellow.)
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| How Quantum Dots Are Made |
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Since quantum dots are so small, manufacturing them is no easy task. Most methods rely on laser etching to get the job done.
First, one of a variety of techniques are used to get an ultra-thin sheet or film made out of the material that will be used to
create the quantum dot. For multi-layered quantum dots, a second film is often deposited on top of the first. One the film is
ready, lasers etch nano-sized shapes into it where the quantum dots are going to be. When pressure is applied, the etched
shapes pop out from the flat film and collapse into quantum dots.
An alternate method, called colloidal synthes, takes a different approach. Instead of creating a thin film of semiconducting
material, it starts by dissolving large amounts of whatever the quantum dots are going to be made of. By carefully controlling
the conditions of the dissolved ingredients, engineers can make them assemble themselves into quantum dots while still in
solution. The finished dots can then be pulled out and purified using electromagnets. This method is easier and less expensive
than laser etching, but it is more difficult to control and thus tends to produce lower-quality quantum dots.
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| What are Quantum Dots Used For? |
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Since quantum dots are so new, there are not a lot of areas in which they are widely used. However, they do look promising
for a number of future applications. One of the best-developed uses for quantum dots right now is as chemical markers in the
medical industry. To do research and diagnoses, doctors often need to be able to "tag" certain cell groups or chemicals with
colorful dyes that will make them easier to see. The dyes most often used today usually work well, but they are far from
perfect. For one thing, many dyes are rather picky when it comes to the colors of light that will make them show up, so great
care must be taken to use the proper illumination when trying to find them. For another, some dyes give off light of many
different colors at once. This is not a problem if only one dye is being used, but if multiple dyes are being used to keep track
of different things, there can be confusing overlap in the colors of light they give off. Quantum dots avoid both of these problems.
Lots of different colors of light will cause them to glow, and they give off light in a way that makes it easy to keep different colors
of quantum dots separate.
Quantum dots can also do a few things that conventional dyes normally can't. For example, quantum dots with certain chemical
properties are sensitive to the temperature and acidity of their surroundings. Not only can these dots be used to tag things for
later identification, they can change color to let a researcher know if and when their surrounding conditions change. Also,
quantum dots with special organic coatings can be made to stick to certain types of cells and not others. For an example of
this, scroll down to the bottom of this section and check out the picture of cancer cells tagged orange against a green background;
the orange quantum dots used here were able to find and stick to the cancer cells because of an organic coating. There are
some conventional dyes that can do these things as well, but none of them are as flexible or come in as many different colors
as quantum dots.
Another promising use for quantum dots is in LEDs (Light Emitting Diodes). LEDs are compact light sources that many
scientists are saying will replace the lightbulb in the near future. They are reliable, long-lasting, and extremely efficient; in fact,
some estimates say that the United States could reduce its energy consumption by up to 29% if it switched over entirely to
LED lighting! Unfortunately, there are a few problems that stand in the way of LEDs becoming the standard. One of the biggest
is that it is difficult to get LEDs in many different colors. An LED's color largely depends on what elements it is made of, so
to produce a lot of colors of LEDs, a manufacturer would need a lot of different raw materials as well. This would make production
rather expensive and difficult to coordinate. LEDs also suffer from the fact that it is extremely tricky to get a white LED. For a
long time, LED manufacturers could only make white LEDs by putting red, blue, and green LEDs together. More recently, white
LEDs have been produced by painting purple and ultraviolet LEDs with fluorescent chemicals. This tends to reduce the efficiency
of the LED, though, and it can lead to undesirable hues in the resulting white light that make it look less natural.
Quantum dots offer a solution to both of these problems. In the hybrid "Quantum LED", a high-energy LED is coated with quantum
dots. The LED gives off light which excites the charges is the quantum dots. The charges then drop back down to lower energy
states, giving off light in the process. At first, it may seem silly to have quantum dots that just soak up light from the LED and
then give off more light. However, recall that the color of light a quantum dot gives off depends only on the structure of the dot
itself and not on the color of light causing it to glow. It turns out that changing the color of quantum dots is much easier that
changing the color of an LED, and unlike changing an LED it can be done without any different raw materials. This means that
all sorts of colors of LEDs could be made just by putting different colors of quantum dots on the same kind of LED.
Surprisingly, quantum dots can also be used to make white light. When quantum dots get very VERY small (containing less
than 70 atoms total), they undergo an abrupt change. Rather than producing a single color of light like most quantum dots do,
they produce a blend of colors from all over the visible spectrum that adds up to make white. Scientists are still not quite sure
why this happens, but regardless of the underlying physics behind it, it can certainly be used in conjunction with LEDs. By
coating an ultraviolet LED with white quantum dots, researchers have successfully produced a white LED. Unlike the
LEDs that use other fluorescent chemicals to produce white, white quantum LEDs are very efficient and lose relatively little
energy when soaking up light and re-emitting it. What's more, white quantum LEDs produce more blue and purple light than
virtually any other white light source, which makes their light seem warmer and more natural than most.
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| Links |
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A Good Glossary of Some Quantum Dot Terminology
Video of Yellow Quantum Dots in Action
Science Daily - Chock Full of Science Articles on Quantum Dots and More
A Good, if a Bit Old, Article on Quantum LEDs
Quantum Dots in Solar Panels
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