Apr 21, 2011
Auger effect boosts doped devices
New experiments by researchers at the University of Washington, Seattle, have looked at the roles of dopants in the photoluminescence of electrically active quantum dot films for the first time and have revealed remarkably large effects never previously observed. Daniel Gamelin and colleagues have found that an "Auger process" involving Mn2+ dopants in the quantum dots is much more effective than that in undoped dots – a result that bodes wells for various device technologies, such as field-effect transistors or solar cells.
Auger processes generally begin with the removal of an inner shell atomic electron to form a vacancy. There are many ways to produce this vacancy, with the most common being bombardment with an electron beam. The inner shell vacancy is then filled by a second atomic electron from a higher shell and energy is released at the same time in this step. Finally, a third electron, known as an Auger electron, escapes, so carrying off the excess energy in a "radiationless" process.
The Auger processes studied in Gamelin and colleagues' work are similar, radiationless de-excitation processes in which an excited state is quenched by transferring its energy to an electron. In previous studies on quantum dots, Auger processes have involved excitons (electron-hole pairs) coupling with other excitons, or excitons coupling with electrons.
"In our study, the process involves a Mn2+ excited state coupling with an electron," explained Gamelin. "The energy of the Mn2+ excited state is rapidly transferred to an electron at the bottom of the quantum dot conduction band, promoting that electron to a much higher 'hot electron' level, which can then cool back to the bottom of the conduction band by giving off heat."
A million times longer
The researchers found that the Auger process involving Mn2+ dopants in quantum dots are much more effective than those of their undoped counterparts. This difference arises because the excited state lifetime of Mn2+ is about a million times longer than that of the undoped quantum dot. This gives electrons more time to diffuse around in the doped nanocrystal film and find the excited nanocrystal before the Mn2+ decays
This Auger process is in fact the microscopic reverse of the so-called impact excitation that forms the basis of many electroluminescent devices. These devices generally run at the highest possible current densities to be as bright as possible but their performance is ultimately believed to be limited by Auger processes like the one that Gamelin's team has observed. "By studying these Auger processes, we hope to learn more about how they may impact electroluminescence device performance, and about how to exploit them for other device applications where they may actually be beneficial."
For their measurements, the researchers began by depositing a film of Mn2+-doped CdS quantum dots on top of a transparent conductive oxide. They then used this as the working electrode in an electrochemical cell. When they subsequently applied a potential to the cell, electrons were transferred into the quantum dots.
"We measured the absorption and photoluminescence of the quantum dots as we performed the electrochemical experiments – something that allowed us to determine how the added electrons changed the absorption and photoluminescence properties of the quantum dots," Gamelin told nanotechweb.org.
The work could be fundamentally important for interpreting various photoluminescence and electroluminescence results obtained for doped semiconductor nanocrystals. "Generally, scientists collect data and then try to interpret the data based on models they build to account for all known processes," said Gamelin. "By demonstrating that this Auger process can be extremely effective in doped nanocrystals, we believe that we are alerting the research community to the fact that they must now include the possibility of this process when analysing their data."
For example, a review of the literature has already helped the team identify several examples where this Auger process may provide a more plausible explanation for the results observed over the conclusions published.
For the practical side of things, analysing this Auger process could help researchers understand fundamental performance limits of doped quantum dot electroluminescence devices.
The team is now performing similar measurements to probe electron mobilities in quantum dot films at low carrier densities (where traditional measurements are limited). For example, quantum-dot Schottky junction solar cells are an important class of device where such information would be very welcome. "We are also currently working on exploiting the Auger effect to quench Mn2+ photoluminescence at specific times after a laser excitation pulse, which would allow the Mn2+ luminescence to be modulated on timescales faster than its intrinsic lifetime of around 2 ms," revealed Gamelin. "We would also like to capture the hot electrons produced by the Auger process."
These future experiments will teach the researchers something new about Auger processes in general, and about Auger processes in quantum dots in particular. "They will certainly advance our ability to harness the physical properties of doped semiconductor nanocrystals in future device technologies."
The work was detailed in ACS Nano.
About the author
Belle Dumé is contributing editor at nanotechweb.org