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Volume 89 Issue 50 | p. 31
Issue Date: December 12, 2011

Why Quantum Dots Blink

Study uncovers two mechanisms that control spontaneous emission fluctuations
Department: Science & Technology
News Channels: Materials SCENE, Nano SCENE, Analytical SCENE
Keywords: quantum dots, core-shell nanocrystals, photoluminescnce, Auger
A microscopy method catches individual nanocrystals (quantum dots) of CdSe randomly fluctuating in emission intensity. Los Alamos researchers have now uncovered the fundamental causes of this “blinking” phenomenon.
Credit: Victor I. Klimov, LANL

Random fluctuations in light emission from semiconductor nanocrystals, also known as quantum dots, are driven by two photoluminescence mechanisms, according to researchers at Los Alamos National Laboratory (LANL). The findings, which were revealed by a new spectroelectrochemical technique, uncover the causes of the “blinking,” a well-known phenomenon that limits the stability of quantum-dot-based devices such as solar cells and light-emitting diodes.

In addition to providing a foundation for developing a comprehensive theoretical model of this detrimental phenomenon, the study details an electrochemical method for suppressing it. The findings were presented at the Materials Research Society (MRS) meeting in Boston last month and were published in Nature (DOI: 10.1038/nature10569).

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Klimov
Credit: Mitch Jacoby/C&EN
Victor I. Klimov of Los Alamos National Laboratory
 
Klimov
Credit: Mitch Jacoby/C&EN

As methods to synthesize nanocrystals with well-controlled compositions and structures have abounded in recent years, the variety of applications that exploit nanocrystals’ optical and electronic properties have grown too. The list includes photovoltaic cells and other energy conversion devices as well as diodes, lasers, and other specialized light emitters. Quantum dots are also used for labeling and tracking cells and in other bioimaging applications.

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Credit: Victor I. Klimov/LANL
By applying a fluorescence spectroscopy technique to CdSe-CdS core-shell nanocrystals (model), Galland (seen here) and coworkers at Los Alamos have uncovered the blinking mechanisms that control light emission from the particles.
 
Credit: Victor I. Klimov/LANL
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TWINKLE TWINKLE
Credit: Victor I. Klimov/LANL
By applying a fluorescence spectroscopy technique to CdSe-CdS core-shell nanocrystals (model), Galland (seen here) and coworkers at Los Alamos have uncovered the blinking mechanisms that control light emission from the particles.
 
TWINKLE TWINKLE
Credit: Victor I. Klimov/LANL

By applying a fluorescence spectroscopy technique to CdSe-CdS core-shell nanocrystals (depicted in model), Galland (seen here with samples in hand) and coworkers at LANL have uncovered the mechanisms that cause the particles to blink.

By applying a fluorescence spectroscopy technique to CdSe-CdS core-shell nanocrystals (depicted in model), Galland (seen here with samples in hand) and coworkers at LANL have uncovered the mechanisms that cause the particles to blink.

All of these uses for quantum dots are adversely affected by their tendency to blink. The underlying causes of that process have been the subject of lively debate because researchers have been unable to design telltale experiments that could pinpoint the origins of the blinking.

Now, Christophe Galland, Victor I. Klimov, and coworkers at LANL have done just that.

“We’ve developed a new spectroelectrochemical technique that allows us to study the effect of controlled charge injection on the intensity and lifetime of light emission from quantum dots,” Klimov said at the annual fall MRS meeting.

To get that information, the team probed the photoluminescence behavior of single quantum dots in solution by exciting the sample with a pulsed laser while they manipulated the charge state of the particles in a working electrochemical cell. For example, they gradually made the particles—CdSe-CdS core-shell nanocrystals—negatively charged (injected with electrons) by applying an increasingly negative potential to the solution, and then they analyzed the light emitted from the particles via microspectroscopy techniques.

By applying those methods to particles with a narrow range of shell thicknesses, the team determined that blinking is caused by two processes. One process results from light-driven charging and discharging of the nanocrystal core. After being irradiated, excited neutral quantum dots relax by emitting photons, which makes the particles appear bright. Charged particles appear dark because they relax by ejecting electrons in a process known as Auger electron emission.

“The majority of quantum dots, however, display blinking due to a different mechanism,” Klimov said. That process is based on charging and discharging of surface electron traps. If these traps are unoccupied, they can intercept energetic or “hot” electrons, thereby preventing the electrons from relaxing to a bright state—one that is characterized by photon emission.

Both blinking mechanisms can be controlled electrochemically, Klimov said, and applying an appropriate potential can completely suppress blinking.

“This study resolves the long-standing controversy concerning the origin of photoluminescence blinking,” says Alexander L. Efros, a Naval Research Laboratory theoretician who specializes in quantum dot physics. The LANL team showed that in addition to a “conventional” charging-discharging mechanism, a second process must be going on, he explains. “The interception mechanism that they propose is the only plausible explanation of these observations,” Efros says.

The team also showed that in nanocrystals with a thick shell, the interception mechanism is suppressed. That finding, Efros adds, “paves the way to designing nanostructures that will combine blinking suppression with extremely high photoluminescence yields.”

 
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