Quantum Dot Ledby

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 the bandgap and hence the emission wavelength.

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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