A new method for manufacturing small semiconductor materials could help improve applications that rely on converting light and energy. The Los Alamos research team has introduced magnetic dopants into specially designed colloidal quantum dots – nanometer size semiconductor crystals – and can achieve effects that can enable solar cell technology, photodetectors and devices based on light to trigger chemical reactions.
Victor Klimov, leader of the Los Alamos Nanotechnology Team, said, “In a quantum core consisting of a lead selenide core and a cadmium selenide shell, the manganese ion acts as a small magnetic field that interacts with the core and the quantum shell dot,” said Victor Klimov, head of the Los Alamos Nanotechnology Team and the principal investigator of the project.
“During these interactions, energy can be transferred to and from the manganese ion by rotating it, a process commonly known as transition change.” In most spin transfer materials, a single absorbed photon produces not one or two electron pairs, also called excitons, which occur as a result of the ion’s spin transition Manganese is happy.
Due to the extremely fast rate of exchange interactions, magnetic doped quantum dots show a threefold increase in the efficiency of many charge carriers compared to undoped quantum dots of the same structure. Importantly, the vast improvement in the photon energy range in different types of solar, leads to the acceleration of photo conversion technology.
The benefits of multiple carriers
Normally, a photon absorbed by a semiconductor creates an electron in the conduction band and an opening in the valence band called a “vacancy”. This process is based on the work of photodiodes, image sensors and solar cells that release the charge carriers. as a photocurrent. These photo-generated electrons and holes can also be useful in chemistry as they can facilitate what is called a redox reaction that involves the transfer of electrons from one substance to another.
All types of photo-conversion schemes will benefit from carrier amplification, a process triggered by high-energy photons creating a “hot” carrier with high kinetic energy. This energy is removed during the collision with the electron of the valence band by exciting it towards the conduction band. As a result, a new electron-hole path is added to the original path created by the concentrated photon.
Due to competing energy losses due to interactions with lattice sounds (commonly called phonons), many charge carriers are ineffective in large solids. However, as Los Alamos researchers first demonstrated in 2004, this effect was enhanced with chemically engineered colloidal quantum dots. The small size of the colloidal quantum dots increases the frequency of electron collisions and thus facilitates the majority of carriers.
However, even with quantum dots, the efficiency of carrier multiplication is not high enough to have a positive effect on the performance of useful photo-conversion schemes. As in the case of bulk crystals, the main limitation is energy loss due to fast phonon radiation leading to “unproductive” heating of the crystal lattice.
Change the exchange rate to increase the carrier rate
Manganese dopants help to solve the problem of fast phonon emission. Building on previous research that showed the sub-picosecond time of the exchange interaction – which is much faster than the phonon – the researchers found that using these interactions will increase the performance of many charge carriers.
“In order to accept the exchange of charge carriers, well-organized quantum dots are required,” said Clément Livache, a postdoctoral researcher and spectroscopy specialist in the nanotechnology group. “The band gap of these dots must be less than half the energy of the rotation of the manganese, and furthermore, the rotation order of the dot must match that of the excited manganese ion.”
Hin Jo, the project’s chief chemist, said, “The energy requirement can be met with quantum doped manganese doped with a selenide core and a cadmium selenide shell.” “In these buildings, the increase of the carrier occurs through two evolutionary steps.
First, the double energy of the electron hole, which is absorbed by the photon in the cadmium-selenide layer, is transferred to the manganese ion. Then, the manganese ion undergoes relaxation back to the unexcited state producing two excitons in the selenide nucleus pair.
The increase of charge through inversion can be particularly useful in electron/hole reactions that require multiple reduction and oxidation processes. One of the obstacles in this is the waiting time between the reduction process and the oxidation process.
The increase of the carrier removes this punch by creating pairs of charge carriers (two electrons and two holes) connected together in the field of time and space. The research is published in the journal Nature Materials.
Source: Los Alamos National Laboratory
