Technical / research - Page 3

Guest post by: Fraunhofer IAP & Fraunhofer CAN

"People push towards the light, not to see better, but to shine better" - Friedrich Wilhelm Nietzsche (1844 - 1900)

Quantum dots (QDs) represent the latest generation of hybrid inorganic-organic nanomaterials. They form a triad of inorganic nanotechnology, organic semiconductor technology and solution-based processability. The emission properties of inorganic, luminescent nanoparticles depend directly on the particle size. This size quantization effect makes it possible to control the band gap and thus the emission color of semiconductor materials. The target parameters are a high quantum yield of the luminescence as well as high stability and environmental compatibility.

A team of researchers, which included scientists from SLAC, Stanford, the University of California, Berkeley and DOE’s Lawrence Berkeley National Laboratory, recently explained a process that interferes with making quantum dots brighter - when attempting to increase the intensity of emitted light, heat is generated instead - reducing the dots’ light-producing efficiency. The results of this new work could have broad implications for developing future quantum and photonics technologies.

Image credit: Nature Communications/SLAC

In a QLED TV screen, dots absorb blue light and turn it into green or red. At the low energies where TV screens operate, this conversion of light from one color to another is virtually 100% efficient. But at the higher excitation energies required for brighter screens and other technologies, the efficiency drops sharply. Researchers theorized about why this happens, but no one had ever observed it at the atomic scale until now.

Scientists at 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, have provides insights into the fundamental physics of perovskite quantum dots.

Nanomaterials of perovskite dispersed in hexane and irradiated by laser. Light emission by these materials is intense thanks to resistance to surface defects (photo: Luiz Gustavo Bonato)

"We used coherent spectroscopy, which enabled us 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," Lázaro Padilha Junior, principal investigator for the project on the Brazilian side, explained.

Japan-based material developer Green Science Alliance developed a new composite material made from a combination of graphene quantum dots and silica, useful to create white LEDs from blue LEDs (440-470 nm).

The company says that this is the first adoption of such a material for LED applications. The company says the adoption of the QD and Silica offers superior performance to the currently-used phosphor as the QDs do not suffer from light scattering, and the white LED is more efficient. The material is also not expensive as the graphene QDs can be produced on the cheap. GS Alliance believes the new white LED will be cheaper than current white LEDs on the market.
 

A new study by researchers from the University of Illinois Urbana-Champaign and the University of Delaware, Baltimore County, in a collaborative project through the Beckman Institute for Advanced Science and Technology at Illinois, used ultrafast nanometric imaging and found good and bad emitters among populations of carbon dots. This observation suggests that by selecting only super-emitters, carbon nanodots can be purified to replace toxic metal quantum dots in many applications, according to the researchers.

“Coming into this study, we did not know if all carbon dots are only mediocre emitters or if some were perfect and others were bad,” said Illinois chemistry professor Martin Gruebele, who led the study. “We knew that if we could show that there are good ones and bad ones, maybe we could eventually find a way to pick the perfect ones out of the mix.”