Quantum Dots
In our current understanding of nanotechnology, quantum dots
reign supreme as the most functional and reproducible nanostructures available
to researchers. Quantum dots are very
small by nature. They are the smallest
objects that we can synthesize on the nanoscale. Like the name suggests, its structure is much like a small
dot. Common shapes include pyramids,
cylinders, lens shapes, and spheres.
Different synthesis routes create different kinds of quantum dots.
The reason why quantum dots are so important is because they
confine electrons in three dimensions.
The total diameter of a quantum dot varies between 3-60 nm depending on
its application. The reason 'quantum'
prefixes the name is because the dots exhibit quantum confinement properties in
all three dimensions. This means that
electrons within a dot can't freely move around in any direction. The only thing that behaves like this in
nature is the atom. Unlike an atom, a
quantum dot is at least ten times bigger.
This has a lot of important consequences for researchers. First of all, they exhibit quantized energy
levels like an atom. For a given input
energy, for instance, a quantum dot will only emit specific spectra of light. Quantum theory predicts that with decreasing
diameters of quantum dots, there will be a corresponding increase in energy of
emitted light.
This element of control over a quantum dot's emission
properties has huge implications for both lasers and medical tags. Due to excellent confinement properties not
seen in nanowires or quantum wells (in all modern lasers), quantum dots are
extremely efficient at emitting light.
They have been the source of some of the world's most powerful lasers
produced to date, though the practicality of a quantum dot laser is still being
improved. In medical studies, quantum
dots are already in practice as tags that can be inserted into patients. These tags can be seen under most medical
scanning technologies and can help pinpoint biological processes as they occur.
Quantum dots can be fabricated with either a top-down
technique or bottom-up technique. Top
down techniques are great for generating a uniform distribution of diameters. This is crucial if researchers wish to
create large arrays of dots that will emit the same wavelength of light. Unfortunately, top down approaches like
lithography are limited by the diffraction limit (that we previous discussed)
and cannot create dense networks of quantum dots. Furthermore, a top down approach inherently implies material damage
and many quantum dots produced with these techniques have defects that reduce
their effectiveness.
The most common way to produce a quantum dot is through a
bottom up approach. This can be done
either with chemical vapor deposition or molecular beam epitaxy on a highly
mismatched substrate. By layering a
desired material that doesn't fit properly with the lattice of the substrate,
high strain occurs at the interface and that layer will start nucleating into
small quantum dots. Bottom up
approaches are a proven way to create quantum dots in dense arrays that will
self-assemble in an orderly manner.
Unfortunately, the uniformity of their size distribution isn't as tight
as with a top down approach mainly because it's impossible to control their
formation as strictly.
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