Chemists design a quantum-dot spectrometer
New instrument is small enough to function within a smartphone, enabling portable light analysis.
Anne Trafton | MIT News Office
July 1, 2015
In this illustration, the Quantum Dot (QD) spectrometer device is printing QD filters — a key fabrication step. Other spectrometer approaches have complicated systems in order to create the optical structures needed. Here in the QD spectrometer approach, the optical structure — QD filters — are generated by printing liquid droplets. This approach is unique and advantageous in terms of flexibility, simplicity, and cost reduction.
Instruments that measurethe properties of light, known as spectrometers, are widely used in physical,chemical, and biological research. These devices are usually too large to beportable, but MIT scientists have now shown they can create spectrometers smallenough to fit inside a smartphone camera, using tiny semiconductornanoparticles called quantum dots.
Such devicescould be used to diagnose diseases, especially skin conditions, or to detectenvironmental pollutants and food conditions, says Jie Bao, a former MITpostdoc and the lead author of a paper describing the quantum dot spectrometersin the July 2 issue of Nature.
This workalso represents a new application for quantum dots, which have been usedprimarily for labeling cells and biological molecules, as well as in computerand television screens.
“Usingquantum dots for spectrometers is such a straightforward application comparedto everything else that we’ve tried to do, and I think that’s very appealing,”says Moungi Bawendi, the Lester Wolfe Professor of Chemistry at MIT and thepaper’s senior author.
The earliestspectrometers consisted of prisms that separate light into its constituentwavelengths, while current models use optical equipment such as diffractiongratings to achieve the same effect. Spectrometers are used in a wide varietyof applications, such as studying atomic processes and energy levels inphysics, or analyzing tissue samples for biomedical research and diagnostics.
Replacingthat bulky optical equipment with quantum dots allowed the MIT team to shrink spectrometersto about the size of a U.S. quarter, and to take advantage of some of theinherent useful properties of quantum dots.
Quantum dots,a type of nanocrystals discovered in the early 1980s, are made by combiningmetals such as lead or cadmium with other elements including sulfur, selenium,or arsenic. By controlling the ratio of these starting materials, thetemperature, and the reaction time, scientists can generate a nearly unlimitednumber of dots with differences in an electronic property known as bandgap,which determines the wavelengths of light that each dot will absorb.
However, mostof the existing applications for quantum dots don’t take advantage of this hugerange of light absorbance. Instead, most applications, such as labeling cellsor new types of TV screens, exploit quantum dots’ fluorescence — a propertythat is much more difficult to control, Bawendi says. “It’s very hard to makesomething that fluoresces very brightly,” he says. “You’ve got to protect thedots, you’ve got to do all this engineering.”
Scientistsare 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 wellunderstood, and is difficult to manipulate.
On the otherhand, quantum dots’ absorption properties are well known and very stable. “Ifwe can rely on these properties, it is possible to create applications thatwill have a greater impact in the relative short term,” Bao says.
The newquantum dot spectrometer deploys hundreds of quantum dot materials that eachfilter a specific set of wavelengths of light. The quantum dot filters areprinted into a thin film and placed on top of a photodetector such as thecharge-coupled devices (CCDs) found in cellphone cameras.
Theresearchers created an algorithm that analyzes the percentage of photonsabsorbed by each filter, then recombines the information from each one tocalculate the intensity and wavelength of the original rays of light.
The morequantum dot materials there are, the more wavelengths can be covered and thehigher resolution can be obtained. In this case, the researchers used about 200types of quantum dots spread over a range of about 300 nanometers. With moredots, such spectrometers could be designed to cover an even wider range oflight frequencies.
“Bawendi andBao showed a beautiful way to exploit the controlled optical absorption ofsemiconductor quantum dots for miniature spectrometers. They demonstrate aspectrometer that is not only small, but also with high throughput and highspectral resolution, which has never been achieved before,” says Feng Wang, anassociate professor of physics at the University of California at Berkeley whowas not involved in the research.
Ifincorporated into small handheld devices, this type of spectrometer could beused to diagnose skin conditions or analyze urine samples, Bao says. They couldalso be used to track vital signs such as pulse and oxygen level, or to measureexposure to different frequencies of ultraviolet light, which vary greatly intheir ability to damage skin.
“The centralcomponent of such spectrometers — the quantum dot filter array — is fabricatedwith solution-based processing and printing, thus enabling significantpotential cost reduction,” Bao adds.
The research was funded by MIT’s Institute for Soldier Nanotechnologies.