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Quantum-dot spectrometer might fit inside a mobile phone

05 July 2015

MIT scientists have now shown they can create spectrometers small enough to fit inside a smartphone camera, using tiny semiconductor nanoparticles.

The quantum dot (QD) spectrometer device printing QD filters — a key fabrication step (illustration: Mary O’Reilly)
The quantum dot (QD) spectrometer device printing QD filters — a key fabrication step (illustration: Mary O’Reilly)

Such devices could be used to diagnose diseases, especially skin conditions, or to detect environmental pollutants and food conditions, says Jie Bao, lead author of a paper describing the quantum dot spectrometers in the journal, Nature. This work also represents a new application for quantum dots.

“Using quantum dots for spectrometers is such a straightforward application compared to everything else that we’ve tried to do, and I think that’s very appealing,” says MIT's Professor Moungi Bawendi, the paper’s senior author.

The earliest spectrometers consisted of prisms that separate light into its constituent wavelengths, while current models use optical equipment such as diffraction gratings to achieve the same effect. Replacing that bulky optical equipment with quantum dots has allowed the MIT team to shrink spectrometers to about the size of a coin, and to take advantage of some of the inherent useful properties of quantum dots.

Quantum dots, a type of nanocrystal discovered in the early 1980s, are made by combining metals such as lead or cadmium with other elements including sulphur, selenium, or arsenic. By controlling the ratio f these starting materials, the temperature, and the reaction time, scientists can generate a nearly unlimited number of dots with different bandgaps, which determines the wavelengths of light that each dot will absorb.

Most existing applications for quantum dots exploit their fluorescence — a property that is much more difficult to control. Scientists are also working on solar cells based on quantum dots, which rely on the dots’ ability to convert light into electrons. However, this phenomenon is not well understood, and is difficult to manipulate.

Quantum dots’ absorption properties, on the other hand, are well known and very stable. “If we can rely on these properties, it is possible to create applications that will have a greater impact in the relative short term,” says Bao.

The new quantum dot spectrometer deploys hundreds of quantum dot materials that each filter a specific set of wavelengths of light. The quantum dot filters are printed on a thin film and placed on top of a photodetector - typically a charge-coupled device similar to those used in mobile phone cameras.

The researchers created an algorithm that analyses the percentage of photons absorbed by each filter, then recombines the information from each one to calculate the intensity and wavelength of the original rays of light.

The more quantum dot materials there are, the more wavelengths can be covered and the higher resolution can be obtained. In this case, the researchers used about 200 types of quantum dots spread over a range of about 300 nanometres. With more dots, such spectrometers could be designed to cover an even wider range of light frequencies.

If incorporated into small handheld devices, this type of spectrometer could be used to diagnose skin conditions or analyse urine samples. They could also be used to track vital signs or to measure exposure to different frequencies of ultraviolet light, which vary greatly in their ability to damage skin.

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