How to Fix Blinking Light-Emitting Nanoparticles

Scientists identified defects responsible for detrimental blinking that limits nanoparticle use in LEDs, solar cells, and lasers.

Image courtesy of Peter Allen and Giulia Galli, University of Chicago

Nanoparticles are promising quantum materials for use in solar cells and in bright, efficient, light-emitting diodes (LEDs). However, they may flash (or blink), a process that limits their efficiency (see ball-and-stick representation of blinking silicon nanoparticles in figure—white light when emitting and yellow when dark due to trapped electrons). Blinking occurs when some nanoparticles do not emit any light (remain dark) after excitation. Now scientists can explain how to eliminate blinking in oxidized silicon nanoparticles. They predict that defects at the interface of the silicon core and the oxidized surface cause the blinking.

The Science

For two decades, scientists have known that unexplained blinking occurs in cadmium selenide nanoparticles and they’ve since observed blinking in silicon nanoparticles. Nanoparticles are promising components of optoelectronic devices such as LEDs, lasers, and solar cells. To add to this mystery, scientists found that nanoparticles obtained with different preparation conditions showed different blinking characteristics. Now first-principles calculations reveal that defects at the surface of oxidized silicon nanoparticles are the root cause of the blinking. In the core of the nanoparticles, silicon atoms form four chemical bonds with neighboring silicon atoms. However, on the surface, some silicon atoms may have missing neighbors, leading to "dangling" bonds. These dangling bonds are the most common defects at the interface between the silicon core and silicon dioxide surface and give rise to electronic states that may lead to bright or dark nanoparticles.

The Impact

This new understanding of blinking may lead to mitigation strategies. These strategies will open the door for the development of efficient devices based on silicon nanoparticles. The devices include LEDs, lasers, and solar cells.

Summary

Blinking, or fluorescence intermittency, limits the usefulness of silicon nanoparticles for optoelectronic applications. Scientists have proposed two mechanisms in the past as being responsible for blinking of silicon nanoparticles: photo-assisted Auger ionization, and phonon-assisted non-radiative recombination occurring at the surface of nanoparticles. Now a research team led by the University of Chicago has used ab initio calculations within density functional theory to identify a defect-induced mechanism. The most common defects at the interface of silicon and silicon dioxide (SiO2) are silicon dangling bonds, which arise when surface silicon atoms lose one of their bonding partners The computational work at the University of Chicago showed that these surface defects can create electronic states that give rise to bright (photon emitting) or dark (non-photon emitting) states. Scientists can use this new understanding to reduce or eliminate blinking (such as varying nanoparticle size). Further, this knowledge can improve the characteristics of the nanoparticle emission for use in LEDs by increasing both the emission brightness and energy conversion efficiency. Understanding the photoexcitation and electron excitation mechanisms in silicon nanoparticles will open the door for the development of high-efficiency LEDs, lasers, and solar cells based on silicon nanoparticles.

Contact

Giulia Galli
University of Chicago
[email protected]  

Funding

Research supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, including support through the Center for Advanced Solar Photophysics (CASP), an Energy Frontier Research Center. Theory calculations used the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility.

Publications

N.P. Brawand, M. Vörös, and G. Galli, "Surface dangling bonds are a cause of B-type blinking in Si nanoparticles." Nanoscale 7, 3737 (2015). [DOI: 10.1039/c4nr06376g]

Related Links

University of Chicago news: Blinking Quantum Dots

National Energy Research Scientific Computing Center science news: What the Blank Makes Quantum Dots Blink?

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