Strain Engineering Produces Highly Stable Light From Individual QDs

New research at Los Alamos National Laboratory suggests that asymmetrically strained colloidal quantum dots (QDs) can provide stable, blink-free light emission comparable to the light produced by QDs made through more complex processes. The strained QDs were found to emit spectrally narrow light with a highly stable intensity and a nonfluctuating emission energy.

The Los Alamos team applied strain engineering to demonstrate that spectral fluctuations in single-dot emission could be almost completely suppressed. The team combined two semiconductors with a large, directionally anisotropic lattice mismatch, which resulted in strong asymmetric compression of the emitting core. This modified the structure of the electronic states of the QD and thereby its light-emitting properties. These modifications implied the realization of local charge neutrality of the emitting state, which would reduce fluctuations in the electrostatic environment, a key to suppressing fluctuations in the emitted spectrum. The modified electronic structures also caused the narrowing of the emission linewidth.

The individual asymmetrically strained dots exhibited highly stable emission energy and a subthermal room-temperature linewidth of about 20 meV, concurrent with nearly nonblinking behavior, high emission quantum yields, and a widely tunable emission color. The strained QDs could provide a viable alternative to the nanoscale light sources currently used for optical quantum circuits, ultrasensitive sensors, and medical diagnostics.

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Novel colloidal quantum dots are formed of an emitting cadmium/selenium (Cd/Se) core enclosed in a compositionally graded CdxZn1-xSe shell wherein the fraction of zinc versus cadmium increases toward the dot’s periphery. Due to a directionally asymmetric lattice mismatch between CdSe and ZnSe, the core, at top right, is compressed more strongly perpendicular to the crystal axis than along it. This leads to modifications of the electronic structure of the CdSe core, which beneficially affects its light-emission properties. Bottom image: Experimental traces of emission intensity from a conventional quantum dot (upper panel) and a novel asymmetrically compressed quantum dot (lower panel) resolved spectrally and temporally. The emission from the conventional quantum dot shows strong spectral fluctuations (“spectral jumps” and “spectral diffusion”). The emission from the asymmetrically compressed quantum dots is highly stable in both intensity and spectral domains. In addition, it shows a much narrower linewidth, which is below the room-temperature thermal energy (25 meV). Courtesy of Los Alamos National Laboratory.

“In addition to exhibiting greatly improved performance over traditionally produced quantum dots, these new strained dots could offer unprecedented flexibility in manipulating their emission color, in combination with the unusually narrow, ‘subthermal’ linewidth,” said lead researcher Victor Klimov. “The squashed dots also show compatibility with virtually any substrate or embedding medium as well as various chemical and biological environments.”

The research was published in Nature Materials (https://doi.org/10.1038/s41563-018-0254-7).