Quantum dots are groups of around 1,000 atoms that behave like a single large ‘super-atom.’ Just by altering the size of these dots, researchers can accurately engineer their electronic properties. To build practical devices, a large amount of dots have to be integrated into a new material. However, while carrying out this process, the properties of the dots tend to get lost.
Colloidal quantum dots with a truncated cube shape and their original ligands (organic molecules) assemble into an ordered superlattice after the ligand exchange. Image Credit: Jacopo Pinna
Currently, a team guided by Maria Antonietta Loi, a professor of Photophysics and Optoelectronics at the University of Groningen, has successfully made an extremely conductive optoelectronic metamaterial via self-organization. The metamaterial has been illustrated in the October 29th issue of the journal Advanced Materials.
Quantum dots of lead selenide (PbSe) or lead sulfide (PbS) can turn shortwave infrared light into an electrical current. This is a beneficial property for creating detectors, or a switch for telecommunications.
However, a single dot does not make a device. And when dots are combined, the assembly often loses the unique optical properties of individual dots, or, if they do maintain them, their capacity to transport charge carriers becomes very poor. This is because it is difficult to create an ordered material from the dots.
Maria Antonietta Loi, Study Lead Author and Professor of Photophysics and Optoelectronics, University of Groningen
Ordered Layer
Working with contemporaries from the Zernike Institute for Advanced Materials at the Faculty of Science and Engineering, University of Groningen, Loi tested a technique that permits the making of a metamaterial from a colloidal solution of quantum dots.
These dots, each approximately 5-6 nanometers in size, exhibit an extremely high conductivity when gathered in an ordered array while preserving their optical properties.
“We knew from the literature that dots can self-organize into a two-dimensional, ordered layer. We wanted to expand this to a 3D material,” says Loi.
To accomplish this, they filled small vessels with a liquid that served as a ‘mattress’ for the colloidal quantum dots. “By injecting a small amount onto the surface of the liquid, we created a 2D material. Then, adding a bigger volume of quantum dots turned out to produce an ordered 3D material.”
Superlattice
The dots are not immersed in the liquid but tend to self-orient on the surface to attain a low-energy state.
The dots have a truncated cubic shape, and when they are put together, they form an ordered structure in three dimensions; a superlattice, where the dots act like atoms in a crystal.
Maria Antonietta Loi, Study Lead Author and Professor of Photophysics and Optoelectronics, University of Groningen
This superlattice that is made up of the quantum dot super-atoms exhibits maximum electron mobility recorded for quantum dot assemblies.
Detectors
It took dedicated equipment to determine what the new metamaterial resembles. The researchers used an electron microscope with atomic resolution to pick out the material’s details. They also ‘imaged’ the material’s large-scale structure using a method known as grazing-incidence small-angle X-Ray scattering.
“Both techniques are available at the Zernike Institute, thanks to my colleagues Bart Kooi and Giuseppe Portale, respectively, which was a great help,” says Loi.
Measurements of the material’s electronic properties exhibit that it is similar to that of a bulk semiconductor, but possessing the optical properties of the dots. In consequence, the experiment forms a route for the creation of new metamaterials established on quantum dots.
The sensitivity of the dots used in the current study to infrared light could be used to make optical switches for telecommunication devices. “And they might also be used in infrared detectors for night-vision and autonomous driving.”
ERC Grant
Loi is very satisfied with the outcomes of the experiments.
People have been dreaming of achieving this since the 1980s. That is how long attempts have been made to assemble quantum dots into functional materials. The control of the structure and the properties we have achieved was beyond our wildest expectations.
Maria Antonietta Loi, Study Lead Author and Professor of Photophysics and Optoelectronics, University of Groningen
Loi is presently involved in understanding and enhancing the technology to construct lengthy superlattices from quantum dots, but is also aiming to do so using other building blocks, for which she is bestowed with an Advanced Grant from the European Research Council.
Our next step is to improve the technique in order to make the materials more perfect and fabricate photodetectors with them.
Maria Antonietta Loi, Study Lead Author and Professor of Photophysics and Optoelectronics, University of Groningen
Journal Reference
Pinna, J., et al. (2022) Approaching Bulk Mobility in PbSe Colloidal Quantum dots 3D Superlattices. Advanced Materials. doi.org/10.1002/adma.202207364.