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Fine-tuning quantum dot emissions holds promise for displays

03 June 2013

Tiny particles of matter called quantum dots, which emit light with exceptionally pure and bright colours, have found a prominent role as biological markers.

Vials containing colloidal semiconductor quantum dot nanocrystals (NCs) emit colours that are determined by the exact size of the particles. For a full explanantion of this image (courtesy of Lauren Aleza Kaye) see foot of article.
Vials containing colloidal semiconductor quantum dot nanocrystals (NCs) emit colours that are determined by the exact size of the particles. For a full explanantion of this image (courtesy of Lauren Aleza Kaye) see foot of article.

In addition, they are realizing their potential in computer and television screens, and have promise in solid-state lighting. New research at MIT could now make these quantum dots even more efficient at delivering precisely tuned colours of light.

These materials, called colloidal semiconductor quantum dot nanocrystals, can emit any colour of light, depending on their exact size or composition. But there is some variability in the spread of colours that different batches of nanocrystals produce, and until now there has been no way to tell whether that variability came from within individual particles or from variations among the nanocrystals in a batch. 

That’s the puzzle an MIT team comprising chemistry professor Moungi Bawendi, graduate student Jian Cui, and six others, has now solved using a new observational method.

For many applications — such as flat-panel displays — it’s important to make particles that emit a specific, pure colour of light. So, it’s important to know whether a given process produces nanocrystals with an intrinsically narrow or broad spectrum of colour emission.

“You need to understand how the spectrum of a single particle relates to the spectrum of the whole ensemble,” Cui says. But existing observational methods that detect an entire ensemble produce data that “is blurring the information,” and methods that attempt to extract data from single particles have limitations.

Observing billions at once
The new method, developed in Bawendi’s lab, is a radical departure from conventional means of observing light emissions from single emitters. Normally, this is done by isolating individual emitters, stabilising them on a substrate, and observing them one at a time. 

But this approach has two drawbacks, Bawendi explains: “You only get small numbers, because you’re looking at one at a time, and there’s a selection bias, because you usually look at the bright ones.”

The new method — called photon-correlation Fourier spectroscopy in solution — makes it possible to extract single-particle spectral properties from a large group of particles. While it doesn’t tell you the spectral peak width of a specific particle, it does give you the average single-particle spectral width from billions of particles, revealing whether the individual particles produce pure colours or not.

In addition, Bawendi explains, the particles “are not isolated on a surface, but [are] in their natural environment, in a solution.” With the traditional methods, “There’s always a question: How much does the surface affect the results?”

The method works by comparing pairs of photons emitted by individual particles. That doesn’t tell you the absolute colour of any particular particle, but it does give a representative statistical measure of the whole collection of particles. It does this by illuminating the sample solution with a laser beam and detecting the emitted light at extremely short time scales. So while different particles are not differentiated in space, they can be differentiated in time, as they drift in and out of the narrow laser beam and are turned on by the beam.

“We get the average single-particle line width in the solution, without any selection bias,” Cui says. By applying this method to the production of quantum dot nanocrystals, the MIT team can determine how well different methods of synthesizing the particles work.

Fine-tuning the process
“It was an open question whether the single-dot line widths were variable or not,” Cui says. Now, he and his colleagues can determine this for each variation in the fabrication process, and start to fine-tune the process to produce the most useful output for different applications.

In addition to computer displays, such particles have applications in biomedical research, where they are used as staining agents for different biochemicals. The more precise the colours of the particles are, the greater the number of different coloured particles that can be used at once in a sample, each targeted to a different kind of biomolecule.

Using this method, the researchers were able to show that a widely used material for quantum dots, cadmium selenide, does indeed produce very pure colours. But, they found that other materials that could replace cadmium selenide or produce different colours, such as indium phosphide, can also have intrinsically very pure colours. Previously, this was an open question. 

In the image above, curves in front of each vial show the measurements made by the MIT team. The outer, wider curve shows the spectrum of colours from all the nanocrystals (NCs) in that vial, while the narrower curve shows the average single-particle spectrum within that vial.

Until this new technique was developed, there was no way to tell whether the width of the spectrum in a given batch was caused by different NCs in the batch having slightly different colours, or whether each particle's emissions had a wider spectrum.

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