MIT study suggests a rethink on energy transfer between quantum dots
Measured energy-transfer rates10 times more than values expected from Förster theory
Energy transfer in light-sensitive materials such as quantum dots is of interest for better solar cells, LEDs, and other devices. In a recent paper in the Journal of Physical Chemistry, a team from MIT chemistry has shown that fast energy transfers from one quantum dot to another do not meet the expected Förster theory results for calculating the rate of energy transfer for molecules.
"The standard assumption is that it's centre-to-centre distance that matters. So we think the edge-to-edge distance may also matter," says Jolene Mork, lead author off the paper.
When a quantum dot absorbs a photon of light - from a laser, for example - it converts the light energy to an excited state in the form an exciton, which is a paired electron and hole that are attracted to each other by their opposite charges. In contrast to a material such as silicon, which allows an electron and a hole to separate, in excitons created by quantum dots, because they are so small, the electron and hole can't move apart very far and are tightly bound together.
"At some point, the quantum dot will want to relax to its ground state because that's the overall lowest energy, and the way that it relaxes is either by transferring energy to another quantum dot, and the quantum dot that transferred the energy away from itself relaxes back to its ground state, or the quantum dot emits a photon and then also relaxes back down," Mork explains. "So each quantum dot is trying to achieve its lowest energy, and it can achieve that in a couple of different ways."
Each quantum dot has an emission or absorption spectrum that determines what wavelength of light energy it can emit or absorb. "Förster theory describes the rate of energy transfer between these two from the donor to acceptor, and that rate is determined by this spectral overlap and by the distance between the two," Mork says.
Under standard assumptions, an exciton is modeled as a tiny dipole in the centre of the quantum dots. This works for molecules because molecules are very small, but quantum dots are thousands of atoms, and they are much larger than molecules. Modeling just as a dipole in the center of the quantum dot may not be an appropriate assumption in these cases. Measured energy-transfer rates in the study were more than 10 times larger than values expected from Förster theory, the study reports.
The MIT group used spectrally resolved transient photoluminescence quenching to measure the magnitude of the Förster radius in blended donor-acceptor QD assemblies. For blends of CdSe/CdZnS core/shell QDs consisting of 4.0 nm diameter donors (wavelength ≈ 550 nm) and 5.5 nm acceptors (wavelength ≈ 590 nm), they measured energy transfer rates per donor-acceptor pair that were 10-100 times faster than the predictions of Förster theory. These rates correspond to an effective Förster radius of 8-9 nm, compared to a theoretical Förster radius of 5-6 nm.
The team thinks that possible sources for the discrepancy between theory and experiment include the magnitude of the absorption cross section, dipole orientation, and dipole-multipole coupling, and suggest that several common assumptions should be considered carefully before applying Förster theory to solid QD films.
