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Understand the best process for preparing coupled nanocrystalline solids



Understand the best process for preparing coupled nanocrystalline solids

In a solvent-based system, the dots are covered with long oleic acid molecules, which hinders the flow of current. After the transition, these are replaced by connecting molecules, making these points effectively conduct electricity. From left to right are solvent-led systems, transition-state and linker-led systems. Image source: Ahmad R. Kirmani

A better understanding of the science underpinning the development of quantum dots (tiny semiconductor nanocrystals) can help reduce speculation in current practice, as materials scientists use them to make better solar panels and digital displays.


Quantum dots with a width of one billionth of a meter are usually prepared in a solution, and then coated or sprayed as ink to form a thin conductive film for manufacturing equipment. Materials scientist Ahmad R. Kirmani said: “But finding the best method is a matter of trial and error. Now, together with KAUST and colleagues at the University of Toronto, Canada, he revealed Why certain well-known technologies can significantly improve the performance of movies.

Quantum dots absorb and emit light of different wavelengths according to their size. This means that by simply increasing or decreasing the size of the crystals, they can be adjusted to high-efficiency absorbers in solar panels, or to give different colors to displays.

These points are usually grown from the lead and sulfur in the solution. Since the nature of a dot depends on its size, its growth must be stopped at the correct location, which can be achieved by adding special molecules to limit its growth. Engineers often use oleic acid molecules, each with 1

8 carbon atoms. These molecules attach to the surface of the crystal (such as hair), which prevents its growth.

This produces a solution suitable for coating to form the dots of the film. However, because long acid molecules hinder the flow of electrons between nanocrystals, the film is not good at conducting electricity. So engineers added shorter molecules. These “linkers” have only about two carbon atoms per molecule. The linker replaces the long-cap molecule, thereby increasing the conductivity. Kirmani said: “This method has been used for decades, but no one has investigated exactly what will happen.”

To find out, Kirmani’s team used a microbalance to monitor the exchange of oleic acid to the joint during the transition. They measured the distance between the points by X-rays between the scattered points, and also recorded the changing thickness, density and light absorption characteristics of the film.

They did not see a smooth change in film characteristics, but saw a sudden jump-marking a phase change. When almost all acid molecules are replaced by linkers, these points suddenly come close together and the conductivity rises sharply.

Kirmani hopes that other teams will be inspired to carry out further research, which may be to stop the transition process halfway and introduce various molecules to the point surface to see what novel features appear. He said: “There is great potential for bringing this understanding to new paradigms of new technologies.”


Researchers synthesized silicon-based quantum dots


More information:
Ahmad R. Kirmani et al., “Optimizing solid ligand exchange for colloidal quantum dot optoelectronics: how much is enough?”, ACS Applied Energy Materials (2020). DOI: 10.1021 / acsaem.0c00389

Provided by King Abdullah University of Science and Technology

Citations: Understand the best method for manufacturing coupled nanocrystalline solids (July 9, 2020), retrieved from https://phys.org/news/2020-07-optimal-fabricating-coupled-nanocrystal-solids.html to 2020 July 10.

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