Quantum Dots: An Overview
By Avinash Agarwal
UPSC GENERAL STUDIES PAPER III (Awareness in Nanotechnology)
Table
of Content
What are QDs?
What do QDs do?
How is it that QDs are made to emit light of
specific colour?
What can you use QDs for?
* In place of pigments and dyes
* Solar Cells
* In Computer Screens and Displays
* Quantum Computing
* Medical Applications
* As tiny lightbulbs
Environmental Concerns in the use of QDs
QDs in News in India (Links)
Bibliography
Wouldn't it be great
if we could control individual atoms? Just imagine if we could "turn"
them on and off to store bits of information, make them light up with different
colors, or control them in all kinds of other ways. Unfortunately, that's not
possible—but scientists have discovered how to do the next best thing with
quantum dots, which are sometimes known as "artificial atoms." Simply
speaking, they're examples of nanotechnology: groups of atoms made from
semiconductor materials that promise to revolutionize everything from home
lights and computer displays to solar cells and biological warfare detectors.
What are they and how do they work? Let's take a closer look!
What are QDs? Quantum
dots (QD) are very small semiconductor particles, only several nanometres
in size, so small that their optical and electronic properties differ from
those of larger particles. They are a central theme in nanotechnology.
(Note: What is a semiconductor? A semiconductor is a
substance, usually a solid chemical element or compound, that can conduct
electricity under some conditions but not others, making it a good medium for
the control of electrical current.)
What do QDs do? Many types of quantum dot
will emit light of specific frequencies if electricity or light is
applied to them, and these frequencies can be precisely tuned by changing the
dots' size, shape and material, giving rise to many applications.
 |
| Relationship between Wavelength and the Colour of Light Emitted: Recall that as the wavelength of the light increases, the colour changes from V to I to B to G to Y to O to R. |
 |
How atoms make light? After absorbing energy (1), an
electron inside an atom is promoted to a higher energy level further from the
nucleus (2). When it returns, the energy is given out as a photon of light (3).
The color of the light depends on the energy levels and varies from one atom to
another.
|
How is it that QDs are made to emit light of specific
colour?
Quantum dots can be precisely controlled to do
all kinds of useful things.
School-level physics tells us that if you give an
atom energy, you can "excite" it: you can boost an electron
inside it to a higher energy level. When the electron returns to a lower
level, the atom emits a photon of light with the same energy that the
atom originally absorbed. The color (wavelength and frequency) of light an atom
emits depends on what the atom is; iron looks green when you excite its
atoms by holding them in a hot flame, while sodium looks yellow, and
that's because of the way their energy levels are arranged. The rule is that
different atoms give out different colors of light. All this is possible
because the energy levels in atoms have set values; in other words, they are
quantized.
Quantum dots do the
same trick—they also have quantized energy levels—but dots made from the same
material (say, silicon) will give out different colors of light depending on
how big they are.
The biggest quantum dots produce the longest
wavelengths (and shortest frequencies), while the smallest dots make shorter
wavelengths (and higher frequencies); in practice, that means big dots make
red light and small dots make blue, with intermediate-sized dots
producing green light (and the familiar spectrum of other colors too).
The explanation for this is (fairly) simple. A small
dot has a bigger band gap (crudely speaking, that's the minimum energy it takes
to free electrons so they'll carry electricity through a material), so it takes
more energy to excite it; because the frequency of emitted light is
proportional to the energy, smaller dots with higher energy produce higher
frequencies (and shorter wavelengths). Larger dots have more (and more closely)
spaced energy levels, so they give out lower frequencies (and longer
wavelengths).
What can
you use Quantum Dots for?
So far, quantum dots have attracted most interest
because of their interesting optical properties: they're being used for all
sorts of applications where precise control of colored light is important.
In place of pigments and dyes: Quantum dots can also be used instead of pigments and
dyes. Embedded in other materials, they absorb incoming light of one color and
give out light of an entirely different color; they're brighter and more
controllable than organic dyes (artificial dyes made from synthetic
chemicals).
Solar Cells: Quantum
dots are being hailed as a breakthrough technology in the development of more
efficient solar cells. In a traditional solar cell, photons of sunlight
knock electrons out of a semiconductor into a circuit, making useful electric
power, but the efficiency of the process is quite low. Quantum dots produce
more electrons (or holes) for each photon that strikes them, potentially
offering a boost in efficiency of perhaps 10 percent over conventional
semiconductors.
In Computer Screens and Displays: Quantum dots offer three important advantages over
LCDs and LEDs.
* First, in a typical LCD (liquid crystal display
screen), the image you see is made by tiny combinations of red, blue, and green
crystals (effectively color filters that switch on and off under electronic
control) that are illuminated from behind by a very bright backlight. Quantum
dots can be tuned to give off light of any color, so the colors of a quantum
dot display are likely to be much more realistic.
* Second, quantum dots produce light themselves so they
need no backlight, making them much more energy efficient (an important
consideration in portable devices such as cellphones where battery life is very
important).
* Third, quantum dots are much smaller than liquid
crystals so they'd give a much higher-resolution image. Quantum dots are
also brighter than a rival technology known as organic LEDs (OLEDs) and
could potentially make OLED displays (which have yet to catch on) obsolete.
Quantum Computing: Computers get faster and smaller every year,
but a time will come when the physical limits of materials prevent them
advancing any further—unless we develop entirely different technologies. One
possibility would be to store and transmit information with light instead of
electrons—a technology broadly known as photonics. Optical
computers could use quantum dots in much the same way that electronic
computers use transistors (electronic switching devices)—as the basic
components in memory chips and logic gates.
Medical Applications: QDs have potential to be used in cancer treatment. Dots
can be designed so they accumulate in particular parts of the body and
then deliver anti-cancer drugs bound to them. Their big advantage is
that they can be targeted at single organs, such as the liver, much more
precisely than conventional drugs, so reducing the unpleasant side
effects that are characteristic of untargeted, traditional chemotherapy.
As tiny lightbulbs: Quantum dots are also being used in place of organic
dyes in biological research; for example, they can be used like nanoscopic
light bulbs to light up and color specific cells that need to be
studied under a microscope.
Environmental
Concerns in the use of Quantum Dots
Some quantum dots also contain cadmium, which is toxic
at high levels—think “factory emission” levels rather than “sealed tube or film
in your TV” levels. Cadmium is superior with respect to delivering
higher-quality color, meaning a broader color gamut.
Still, there are health and environmental concerns,
especially if a bunch of quantum-dot TVs end up in landfills.
Quantum
Dot in News in India
Samsung launches 44 new TV models in India
Samsung launches SUHD TV with Quantum Dot display
starting at Rs 1,79,000
Bibliography
Semiconductor
Quantum Dots
What are quantum dots?
What Are Quantum Dots, and Why Do I Want Them in My
TV?
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