Extra sulphur improves electronic structure of quantum dots

said braka 8:41 AM
Quantum dots are clusters of some 1,000 atoms which

act as one large "super-atom." The dots, which are synthesized as colloids, i.e. suspended in a liquid like a sort of paint, can be organized into thin films with simple solution-based processing techniques. These thin films can turn light into electricity. However, scientists have discovered that the electronic properties are a bottleneck. "Especially the conduction of holes, the positive counterpart to negatively charged electrons," explains Daniel Balazs, Ph.D. student in the Photophysics and Optoelectronics group of Prof. Maria A. Loi at the University of Groningen Zernike Institute for Advanced Materials.
Stoichiometry
Loi's group works with lead-sulphide  dots. When light produces an electron-hole pair in these dots, the electron and hole do not move with the same efficiency through the assembly of dots. When the transport of either is limited, the holes and electrons can easily recombine, which reduces the efficiency of light-to-energy conversion. Balazs therefore set out to improve the poor hole conductance in the quantum dots and to find a toolkit to make this class of materials tunable and multifunctional.
"The root of the problem is the lead- stoichiometry," he explains. In quantum dots, nearly half the atoms are on the surface of the super-atom. In the lead-sulphur system, lead atoms preferentially fill the outer part, which means a ratio of lead to sulphur of 1:3 rather than 1:1. This excess of lead makes this quantum dot a better conductor of electrons than holes.


Read more at: https://phys.org/news/2017-09-extra-sulphur-electronic-quantum-dots.html#jCp