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LCD screens: towards a revolution in color production?

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In order to obtain LCD screens with high display quality - better defined tones, wider color palette - research on the semiconductor nanocrystals at the origin of the pixels constituting the images is active.

A collaboration between LPEM, INSP, SOLEIL's TEMPO beamline and scientists from Argonne National Laboratory (USA) and Jaume I University (Spain), has led to the development of a new generation of nanocrystals. Thanks to careful engineering, the two-dimensional synthesized nanocrystals can emit both green and red light: an interesting alternative to nanocrystal mixtures for the design of a white light emitter.

Nanocrystals have revolutionized the liquid crystal display (LCD) market, offering an unparalleled color palette. However, the process remains complex, requiring the use of tiny crystals capable of transforming blue light into green or red light. These three colors are combined to generate white light, which is then filtered to produce blue, green and red pixels.

The current strategy relies on the integration of two populations of nanocrystals (green/red), which makes the task difficult.

This problem may have been solved by a French team that has developed a 2D material capable of emitting very pure light in both green and red.

Figure 1: Schematic of a bicolor nanoplatelet

To do so, the scientists synthesized two-dimensional nanocrystals, nanoplatelets, consisting of assemblies of cadmium telluride CdTe and cadmium selenide CdSe crystals (Fig. 1). The chemical and electronic properties of the synthesized materials were studied at the TEMPO beamline. Their chemical composition, the interaction between the different materials and the positioning of the energy bands (energy values of the electrons of these components) were determined. The light emission performance of the materials was then tested.

In the end, due to the chemical nature and the interlocking of the two components within a nanoplatelet - which notably allows the energy bands of the different semiconductors to be aligned - the nanoplatelets were found to be not only capable of emitting in the red and in the green (see figure below), but also tune their color, depending on the laser power received.

Figure 2: Photograph of a test tube containing a dilute solution of bicolor nanoplatelets.

The platelets are illuminated by a blue laser (left of the picture). In response, the nanoplatelets will emit a color that depends on the power of the laser.

Finally, the scientists have also shown that their material can be integrated into electroluminescent LEDs with very good performance.

And they are now working on other, thinner, heterostructures that would emit blue and green light. Indeed, the growth of a shell in thickness, in order to make the particles brighter, is also accompanied by a red shift in the light emitted - hence the interest in reducing the thickness of the material.

With the prospect of replacing a three-source system with a single source, the researchers could well have opened the way to a new generation of much more energy-efficient displays, even if many studies remain to be carried out, particularly with regard to the stability of the materials.