'Metamaterials' from quantum dots show promise for optoelectronics

Quantum dots are clusters of some 1000 atoms that act as one large ‘super-atom’. It is possible to design the electronic properties of these dots just by changing their size. However, to create functional devices, a large number of dots have to be combined into a new material and the properties of these dots are often lost during this process. Now, a team of researchers from the University of Groningen has made a highly conductive optoelectronic metamaterial through self-organisation. The research findings were published in the journal Advanced Materials.

Quantum dots of PbSe (lead selenide) or PbS (lead sulphide) can convert shortwave infrared light into an electrical current. This is a useful property for making detectors, or a switch for telecommunications. Maria Antonietta Loi, Professor of Photophysics and Optoelectronics at the University of Groningen, said that a single dot does not make a device, and when dots are combined, their assembly often loses the unique optical properties of individual dots. Or, if they are maintained, their capacity to transport charge carriers becomes very poor. “This is because it is difficult to create an ordered material from the dots,” Loi said.

Working with colleagues from the Zernike Institute for Advanced Materials at Groningen University, Loi experimented with a method that allows the production of a metamaterial from a colloidal solution of quantum dots. These dots, each about five to six nanometres in size, show a high conductivity when assembled in an ordered array, while maintaining their optical properties. “We knew from the literature that dots can self-organise into a two-dimensional, ordered layer. We wanted to expand this to a 3D material,” Loi said.

To achieve this, researchers filled small containers with a liquid that acted 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,” Loi said.

The dots are not submersed in the liquid, but self-orient on the surface to achieve a low energy state. Loi said that 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. This superlattice, composed by the quantum dot super atoms, displays a high electron mobility for quantum dot assemblies. The team of researchers used an electron microscope which had an atomic resolution to ascertain what the new metamaterial looked like. They also ‘imaged’ the large-scale structure of the material using a technique called 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,” Loi said.

Measurement of the electronic properties of the material show that it closely resembles that of a bulk semiconductor, with the optical properties of the dots. Thus, the experiment could lead to the creation of new metamaterials based on quantum dots. The sensitivity of the dots used in the present study to infrared light could be used to create optical switches for telecommunication devices. According to Loi, they could also be used in infrared detectors for night-vision and autonomous driving. “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,” Loi said.

Loi is now working on understanding and improving the technology to build extended superlattices from quantum dots and also plans to do so with other building blocks. “Our next step is to improve the technique in order to make the materials more perfect and fabricate photodetectors with them,” Loi said.

Image credit: Jacopo Pinna