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Colloidal quantum dots offer laser promise

11 Jun 2007

US researchers have achieved amplified spontaneous emission from colloidal semiconductor nanocrystals, which could pave the way for a new breed of quantum-dot lasers.

Laser engineers have long been eager to use colloidal nanocrystals as a gain medium, but so far have reported only limited success. Now, a team led by Victor Klimov at the Los Alamos National Laboratory in New Mexico has found a way to achieve amplified spontaneous emission from type-II colloidal semiconductor nanocrystals.

The Los Alamos team exploited so-called core-shell quantum dots, in which a small region of one semiconductor is enclosed within a shell of a different semiconductor. These colloidal nanocrystals are typically 2–10 nm in size, and are known to fluoresce with a colour that depends on the size of the nanocrystal: larger crystals produce longer wavelength light, while small crystals emit light towards the blue end of the spectrum.

Core-shell quantum dots are further classified into two kinds, type-I and type-II, based on the alignment of the conduction and valence band of the core and shell material. "In type-I nanocrystals, both charge carriers (electron and hole) reside either in the core or in the shell, whereas in type-II nanocrystals one charge carrier is primarily localized in the core and the other in the shell," explained Los Alamos researcher Sergei Ivanov.

In the most common type-I nanocrystals, the lowest excited energy level can only hold two electron-hole pairs (excitons), and the energy required to generate each exciton is roughly the same. This makes it impossible to achieve optical amplification using single-exciton states. If only one exciton becomes excited, an incident photon can either be absorbed to generate the second exciton or can cause the first exciton to recombine, producing the second photon.

The net effect of these two processes is known as "optical transparency" – there is no absorption, but no optical gain either. In order to produce optical amplification at least a fraction of the nanocrystals in the sample must contain two excitons (or biexcitons).

The problem is that when two excitons are generated, the population inversion cannot be sustained for more than about 100 picoseconds. This is because of Auger recombination, a process in which the recombination energy of one exciton is rapidly transferred to the other. As a result, stimulated emission in type-I nanocrystals can only be produced using expensive femtosecond laser pump sources.

"Auger recombination is only a problem when there are multiple excitons to contend with," Ivanov told optics.org. " So we looked for ways to achieve population inversion and obtain optical gain with only one exciton." According to Ivanov, the key lies in careful engineering of the nanostructure. "We fabricated core-shell nanocrystals using a cadmium sulfide (CdS) core and zinc selenide (ZnSe) shell," he explained. "Type-II band alignment of these two materials causes the exciton to 'split up' after formation: the electron gets localized in the core, while the hole resides in the shell."

This means that the energy required to generate the first exciton is no longer the same as that required for the second exciton "The second one requires about 100 meV more due to Coulomb repulsion between the same electrons and holes. In this case, nanocrystals excited with single excitons do not contribute to absorption at the emission wavelength." The end resut, says Ivanov, is that optical gain is possible using single-exciton states. This means no Auger effects, and hence a longer optical-gain lifetime of about 2 ns.

The Los Alamos team now hopes to extend the exciton lifetimes still further, and to improve both the emission efficiency and photostability of the nanocrystals. "Electrical pumping of the nanocrystal medium is another problem," commented Ivanov. "It is difficult even in the regime of conventional (spontaneous) emission and it will be more complicated in the case of stimulated emission and optical gain."

• In a related development, Uri Banin's research group at the Hebrew University in Isreal has also highlighted the properties of multiple excitations in type-II semiconductor nanocrystals. "Using a core-shell nanostructure made from cadmium telluride and cadmium selenide (CdTe/CdSe), we observed increased biexciton lifetimes from 50 ps in the type-I CdTe cores to 2 ns in the type II core/shell system," Banin said. Banin's group reported its work in Phyical Review B earlier this year.

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