Research throws IR light on plasmonic devices

Researchers found Dy-doping reduces the number of oxygen vacancies in a CdO crystal

Researchers at North Carolina State University have identified and synthesised a material that can be used to create efficient plasmonic devices that respond to light in the mid-infra-red (IR) range.

This is believed to be the first time anyone has demonstrated a material that performs efficiently in response to this light range. Applications range from high-speed computers, to solar energy to biomedical devices.

The phenomenon, surface plasmon resonance, where an interface between a conducting and an insulating material is illuminated, can cause electrons in the conductor to oscillate. This relies on the angle, polarisation, and wavelength of the incoming light. The oscillation creates an intense electric field extending into the insulator that can be used in biomedical sensors, solar cells or opto-electronic devices.

The wavelength of light that causes these oscillations depends on the nature of the conductive material. Materials with a high density of free electrons (e.g. metals) respond to short wavelengths of light, those with lower electron density (e.g. conventional semiconductors) respond to long wavelengths of light, such as those in the far IR. Scientists have been unable to identify materials that could support efficient surface plasmon resonance when targeted with wavelengths of light in the mid-IR range (i.e. between 1,500 and 4,000 wavenumbers).

Identifyng the material could make solar harvesting technology more efficient, by taking advantage of the mid-IR wavelengths of light. It would also allow the development of molecular sensing technology for use in biomedical applications. Finally, it would allow the development of faster, more efficient opto-electronic devices.

The research team added cadmium oxide with dysprosium, a rare earth element, to cadmium oxide without changing the material's crystal structure.

This creates free electrons in the material and increases the mobility of the electrons, making it easier for mid-IR light to induce oscillations in the electrons efficiently.

"Usually when you dope a material, electron mobility goes down," says author Dr Jon-Paul Maria, "but in this case we found the opposite – more dysprosium doping increases this critical characteristic. . . our experiments revealed that Dy-doping reduces the number of oxygen vacancies in a CdO crystal. Oxygen vacancies, which correspond to locations where oxygen atoms are missing, are strong electron scatterers and interfere with electron motion. In the most basic terms, by removing these defects, electrons scatter less and become more mobile."

The paper, Dysprosium doped cadmium oxide: A gateway material for mid-infrared plasmonics, was published in Nature Materials.

Caroline Hayes

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