Quantum dots are manmade nanoparticles of semiconducting material comprising only a
few thousand atoms.
Because of the small number of atoms, a quantum dot's properties lie between those of single
atoms or molecules and bulk material with a huge number of atoms.
By changing the nanoparticles' size and shape, it is possible to fine-tune their electronic
and optical properties - how electrons bond and move through the material, and how light
is absorbed and emitted by it.
Thanks to increasingly refined control of the nanoparticles' size and shape, the number
of commercial applications has grown.
Those already available include lasers, LEDs, and TVs with quantum dot technology.
However, there is a problem that can impair the efficiency of devices or appliances using
this nanomaterial as an active medium.
When light is absorbed by a material, the electrons are promoted to higher energy levels,
and when they return to their fundamental state, each one can emit a photon back to
the environment.
In conventional quantum dots the electron's return trip to its fundamental state can be
disturbed by various quantum phenomena, delaying the emission of light to the exterior.
The imprisonment of electrons in this way, known as the "dark state", retards the emission
of light, in contrast with the path that lets them return quickly to the fundamental state
and hence to emit light more efficiently and directly ("bright state").
This delay can be shorter in a new class of nanomaterial made from perovskite, which is
arousing considerable interest among researchers in materials science as a result.
A study conducted by researchers in the Chemistry and Physics Institutes of the University of
Campinas (UNICAMP) in the state of São Paulo, Brazil, in collaboration with scientists at
the University of Michigan in the United States, made strides in this direction by providing
novel insights into the fundamental physics of perovskite quantum dots.
An article on the study is published in the journal Science Advances.
The researchers say they used coherent spectroscopy, which enabled them to analyze separately the
behavior of the electrons in each nanomaterial in an ensemble of tens of billions of nanomaterials.
The study is groundbreaking insofar as it combines a relatively new class of nanomaterials
- perovskite - with an entirely novel detection technique.
The researchers were able to verify the energy alignment between the bright state and the
dark state, indicating how this alignment depends on the size of the nanomaterial.
They also made discoveries regarding the interactions between these states, opening up opportunities
for the use of these systems in other fields of technology, such as quantum information.
Owing to the crystal structure of perovskite, the level of bright energy divides into three,
forming a triplet.
This provides various paths for excitation and for the electrons to return to the fundamental
state.
The most striking result of the study was that by analyzing the lifetimes of each of
the three bright states and the characteristics of the signal emitted by the sample the researchers
obtained evidence that the dark state is present but located at a higher energy level than
two of the three bright states.
This means that when light is shone on the sample the excited electrons are trapped only
if they occupy the highest bright level and are then shifted to the dark state.
If they occupy the lower bright levels, they return to the fundamental state more efficiently.
To study how electrons interact with light in these materials, the group used multidimensional
coherent spectroscopy (MDCS), in which a burst of ultrashort laser pulses is beamed at a
sample of perovskite chilled to minus 269 degrees Celsius.
The MDCS technique can be used to analyze billions of nanoparticles at the same time
and to distinguish between different families of nanoparticles present in the sample.
The researchers say the results obtained are very important since knowledge of the optical
properties of the material and how its electrons behave opens up opportunities for the development
of new technologies in semiconductor optics and electronics.
The incorporation of perovskite is highly likely to be the most distinctive feature
of the next generation of television sets.