Tecnología Brasil , Brasil, Martes, 27 de octubre de 2015 a las 10:55

Nanoparticles could be a basis for more sensitive radiation detectors

Structures produced by researchers at University of São Paulo and Harvard show potential to enhance efficiency of systems that require highly sensitive detection of light or radiation

AGÊNCIA FAPESP/DICYT Images captured from an electron microscope apparently show a huge number of tiny stars each with a diameter of only a few micrometers (millionths of a meter).

 

The structures, produced by Eder Guidelli and Oswaldo Baffa at the Ribeirão Preto campus of the University of São Paulo (USP) in Brazil, in partnership with David R. Clarke of Harvard University in the United States, have a core of gold and silver particles surrounded by a “shell” of zinc oxide (ZnO). Their properties have the potential to improve the efficiencies of various systems in which it is necessary to detect light or radiation with a high degree of sensitivity.

 

The researchers describe details of the production and properties of these microscopic stars made of precious metals and zinc oxide in an article published in Scientific Reports, an online journal that is part of Nature Publishing Group.

 

The studies that led to the article were performed during Guidelli’s PhD research, supervised by Baffa in Brazil and Clarke at Harvard, with a scholarship from FAPESP. Guidelli is now conducting postdoctoral research with the support of another scholarship.

 

The motivation to create a structure with a gold or silver core and a shell of ZnO arises from the unusual optical properties that derive from the combination of these materials.

 

In response to various types of electromagnetic radiation, such as visible light and X-rays, precious metals and ZnO have common characteristics that enable them to act in harmony, Guidelli explained. “An analogy I like to use is a cell phone and the antenna that amplifies the signal it emits,” he said.

 

Star-like morphology

 

The researchers’ experiments showed exactly how this magnification occurs. An example involves a technique called optically stimulated luminescence (OSL), widely used by geologists and archeologists to date sediments and objects.

 

In this example, the star-like (or thistle-like) structures are bombarded with X-rays. Initially, the electrons in the ZnO are ionized, i.e., stripped out from the position they normally occupy in the molecular structure of ZnO, their valence layer.

 

After the initial bombardment, the stripped-out electrons can be trapped in microscopic flaws within the “points” of the star.

 

“They may be trapped there indefinitely, but a pulse of radiation can make them return to their valence layer. When they return, they emit light,” Guidelli said.

 

In this process, photons (light particles) function as a type of “shuttle” for quantum phenomena: when an electron enters a temporarily excited (abnormally energetic) state, the production of photons enables it to return to its normal energy level.

 

All this could occur only within the ZnO structure, but the presence of gold and silver particles makes the process of de-excitation, when the electrons decay to a less energetic state, take place more rapidly and efficiently.

 

“Hence the analogy with an antenna, which facilitates the transmission and reception of a signal,” Guidelli said.

 

The structure and dimensions of the material affect the details of how this process occurs, so the process of producing star-shaped structures is especially important.

 

The ZnO production process normally creates rod-shaped structures on a glass substrate, resembling a fakir’s bed of nails when seen from above.

 

Adding the right amount of gold and silver particles to the process, however, makes the star-like morphology appear, basically because the “arms” of each star use the particles to fuel the growth of its core.

 

Because the core is spherical, the ZnO arms spread out in all directions to form a shape resembling a star or thistle. The researchers managed to break one of these arms to reveal a nanoparticle of gold nestled in the center of the structure.

 

The process must be precisely controlled because some details, such as the thickness of the material, can influence its optical properties. “Think of the atmosphere of a planet,” Baffa said. “If it’s too thick, sunlight can penetrate to the surface of the planet but can’t bounce back into space.”

 

The structures produced by the Ribeirão Preto research group are so sensitive that they can be used for precise measurement of very low levels of radiation, minimizing the risk inherent in healthcare environments, for example.

 

This same sensitivity means they can also be used to date very small samples from archeological digs, such as bone fragments or shards of pottery.

 

An application has been submitted in Brazil for an umbrella patent to protect a range of possible uses of the technology. “Of course, a huge amount of work will be required to go from a patent application to a future product based on the technology,” Baffa said.