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Nonblinking quantum dots

Quantum dots are tiny structures made of semiconductor materials of size 15 nm and when a photon of light knocks an electron into the conduction band an electron/hole pair is created and the pair cannot escape from the dot. This confinement means that the wavelengths of the wave functions of both the electron and the hole are forced to be significantly smaller and more on top of each other inside a quantum dot than they would be in ordinary semiconductor material.
Mechanism of blinking
When electron/hole pairs recombine energy is released as light. The energy released in this merger kicks the second electron out of the quantum dot onto the dots surface where it is trapped. The trapped electron creates a huge electric field across the dot which prevents it from emitting photons until the trapped electron can finally tunnel back into the dot and recombine with its hole.
When continuously illuminated by a laser, quantum dots blink on and off randomly from a microsecond to several minutes. Blinking is caused by an electron that, upon photo excitation, escapes the core and becomes trapped in the shell, forming a charged QD. At that point, non-radiative relaxation is favored and, for an instant, emission ceases.
Blinking quantum dots have high brightness, well-defined monochromaticity and superior photo stability. Blinking quantum dots randomly blink even under steady irradiation, a peculiarity that has prevented wider use.
Uses of quantum dots
Quantum dots are being intensively investigated for applications such as light-emitting diodes, solid-state lighting, lasers, solar cells, fluorescent labels for biological imaging and as dyes capable of emitting a wider range of colors with greater stability.
Nonblinking quantum dots
Researchers of the University of Rochester, Cornell University, Kodak Eastman and the Naval Research Laboratory in the USA, have synthesized a QD which emits a constant stream of light and are "nonblinking," in that they emit light steadily. The new quantum dots have multiple peaks in their emission spectra which mean that emission spectra of different quantum dots overlap and remain in excited state before emitting a photon in much shorter than that of traditional CdSe nanocrystals.
Unlike in a typical QD, the core (CdZnSe) and the shell (ZnSe) fade into each other, so there is no abrupt boundary between the two regions. The potential energy surface declines steadily towards the center of the QD. In this way, the non-radiative mechanism becomes highly unfavorable, even when a charged QD is formed. Irradiation remains steady for hours.

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