Transcript Quantum Dot LED

Quantum Dot Led
by
Ignacio Aguilar
Introduction
Quantum dots are nanoscale semiconductor particles that possess
optical properties.
Their emission color can be tuned throughout the visible and infrared
spectrum.
This means the quantum dot enabled LEDs can emit at almost any
color.
This provides more color options and better quality white LEDs.
How Quantum Dots Work
Quantum dots, also known as nanocrystals, are a special class of
materials known as semiconductors, which are crystals composed of
periodic groups of II-VI, III-V, or IV-VI materials.
Quantum dots are unique class of semiconductor because they are so
small, ranging from 2-10 nanometers (10-50 atoms) in diameter.
At these small sizes materials behave differently, giving quantum dots
unprecedented tunability and enabling never before seen applications
to science and technology.
How Quantum Dots Work
cont.
The usefulness of quantum dots comes from their peak emission
frequency's extreme sensitivity to both the dot's size and composition.
The electrons in bulk (much bigger than 10 nm) semiconductor
material have a range of energies. One electron with a different
energy than a second electron is described as being in a different
energy levels.
In bulk, energy levels are very close together, so close that they are
described as continuous, meaning there is almost no energy
difference between them. It is also established that some energy
levels are simply off limits to electrons; this region of forbidden
electron energies is called the bandgap.
How Quantum Dots Work
cont.
How Quantum Dots Work
cont.
In natural bulk semiconductor material, an extremely small
percentage of electrons occupy the conduction band the
overwhelming majority of electrons occupy the valence band, filling it
almost completely.
The only way for an electron in the valence band to jump to the
conduction band is to acquire enough energy to cross the bandgap,
and most electrons in bulk simply do not have enough energy to do
so.
Applying a stimulus such as heat, voltage, or photon flux can induce
some electrons to jump the forbidden gap to the conduction band.
The valence location they vacate is referred to as a hole.
How Quantum Dots Work
cont.
How Quantum Dots Work
cont.
A sufficiently strong stimulus will cause a valence band electron to
take residence in the conduction band, causing the creation of a
positively charged hole in the valence band. The raised electron and
the hole taken as a pair are called an exciton.
There is a minimum energy of radiation that the semiconductor bulk
can absorb towards raising electrons into the conduction band,
corresponding to the energy of the bandgap.
It is established that because of the continuous electron energy levels
as well as the number of atoms in the bulk, the bandgap energy of
bulk semiconductor material of a given composition is fixed.
How Quantum Dots Work
cont.
It is also established that electrons in natural semiconductor bulk that
have been raised into the conduction band will stay there only
momentarily before falling back across the bandgap to their natural,
valence energy levels.
As the electron falls back down across the bandgap, electromagnetic
radiation with a wavelength corresponding to the energy it loses in the
transition is emitted.
Great majority of electrons, when falling from the conduction band
back to the valence band, tend to jump from near the bottom of the
conduction band to the top of the valence band- in other words, they
travel from one edge of the bandgap to the other
emission frequencies. Quantum dots offer the unnatural ability to tune
How Quantum Dots Work
cont.
The bandgap of the bulk is fixed, this transition results in fixed
emission frequencies. Quantum dots offer the unnatural ability to tune
the bandgap and hence the emission wavelength.
Quantum Dots - A
tunable range
The addition or subtraction of just a few atoms to the quantum dot has
the effect of altering the boundaries of the bandgap.
Changing the geometry of the surface of the quantum dot also
changes the bandgap energy, owing again to the small size of the dot,
and the effects of quantum confinement.
The bandgap in a quantum dot will always be energetically larger;
therefore, we refer to the radiation from quantum dots to be "blue
shifted" reflecting the fact that electrons must fall a greater distance in
terms of energy and thus produce radiation of a shorter, and therefore
"bluer" wavelength.
Quantum Dots - A
tunable range
Quantum Dots - A
tunable range
Control of Bandgap in
Quantum Dots
As with bulk semiconductor material, electrons tend to make
transitions near the edges of the bandgap. However, with quantum
dots, the size of the bandgap is controlled simply by adjusting the size
of the dot.
Because the emission frequency of a dot is dependent on the
bandgap, it is therefore possible to control the output wavelength of a
dot with extreme precision.
In effect, it is possible to tune the bandgap of a dot, and therefore
specify its "color".
Control of Bandgap in
Quantum Dots
DEMO
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