Quantum wells (QWs) are thin semiconductor layers in which charge carriers (electrons and holes) are confined in one dimension but free to move in the other two dimensions. Such confinement means that these structures have tunable optical band gaps and can strongly absorb and emit light, which makes them good as optical modulators, photodetectors and solar cells. QWs are also used as gain media in semiconductor lasers.

The problem is that QWs are relatively difficult to synthesize and must be made using expensive crystal growth methods, such as molecular beam and metal-organic vapour-phase epitaxy. Colloidal nanoplatelets, on the other hand, are much easier to make – in a chemical solution.

Nanoplatelets are thin, flat, semiconductor nanocrystals that are only a few atomic layers thick but several hundreds of nanometres across. Charge carriers in these structures should thus also behave as they would in a quantum well. Although previous studies have shown that this is the case, some of the results obtained in these experiments have been conflicting.

Charge carrier relaxation

The Illinois team, led by Matthew Pelton at Argonne’s Center for Nanoscale Materials and Dmitri Talapin in Chicago, obtained its results by measuring how quickly charge carriers “relax” in the nanoplatelets using time- and frequency-resolved photoluminescence measurements. This is a good method to distinguish between quantum dots and quantum wells, explains Pelton.

“When a QD or a QW absorbs high-energy photons, high-energy (or “hot”) electrons and holes are created,” he said. “In QDs, these high-energy carriers are restricted to discrete, quantum-confined energies, and can lose their energy only by passing through other discrete energy levels. In QWs, on the other hand, the carriers can take on a continuous range of energies, and so cool in a much different way.”

The measurements by the Illinois team showed that nanoplatelets cool in the same way as QWs – by rapid relaxation.

The experiments

The researchers did their experiments on CdSe nanoplatelets whose thicknesses could be controlled down to the single-atom level. They excited the nanoplatelets using ultrafast laser pulses and measured the light that was subsequently emitted as a function of time and the emitted photon frequency. “The frequency (or energy) of the emitted photons directly corresponds to the energy of the carriers in the platelets that produced the photons,” said Pelton, “so by monitoring how the photon energies are distributed as a function of time, we are also able to monitor the distribution of carrier energies with time.”

“QWs have been widely used in optoelectronics applications for several decades now, so our new results imply that colloidal platelets should be equally as useful for such applications,” he told nanotechweb.org. “The added advantage is that nanoplatelets can easily be produced at low cost and in large quantities.”

One immediate application could be a new type of low-cost semiconductor laser, he says, that would work just like a QW laser relying on rapid relaxation of high-energy carriers.

The team is now busy looking more closely into how the structure of nanoplatelets affects high-energy carrier relaxation. “We also plan to verify that the platelets are indeed good laser materials,” added Pelton.

The current work is detailed in Nano Letters.