Researchers have manufactured luminous pixels so small that a display could fit 21,000 of them across a length of just 2.54 cm (1 inch).
At the heart of the experiment are quantum dots. These tiny semiconductor crystals, only a few nanometers wide, can emit a precise color when they receive energy. They are already used in some displays, but creating extremely fine patterns with them presents a problem: the necessary treatments can damage their surface and reduce their performance.
Screen pixels.
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To understand why, the surface of a quantum dot is surrounded by small molecules called ligands. They help maintain its properties. Photolithography, a technique that uses light to draw microscopic patterns, can alter these molecules. The pixel then takes the right shape, but the luminous material may function less effectively.
Chang Gu’s team, at the Chinese Academy of Sciences, designed a reversible process. An initial light temporarily links the ligands together, securing the quantum dots in the desired locations. Another wavelength can then break these bonds. The surface of the nanocrystals thereby returns to an arrangement close to its initial state.
This method therefore makes it possible to separate two steps that were previously difficult to reconcile: drawing very fine patterns and then preserving the material’s luminous properties. In their electroluminescent devices, the researchers measured an external quantum efficiency of up to 24.41%. This figure corresponds to the proportion of injected electrical charges that result in photons leaving the device.
The researchers then pushed miniaturization much further. Their nanoscale device reaches 21,000 pixels per inch, or approximately 827 pixels per millimeter. Even at this density, its maximum external quantum efficiency reaches 17.08%.
In practical terms, such a density is of greatest interest for devices where the image must remain detailed on a tiny surface. Microdisplays placed very close to the eye, particularly for certain glasses or headsets, are among the possible applications. The method could also be used to manufacture other optoelectronic components based on quantum dots.
The next challenge is to move from small laboratory devices to larger surfaces while preserving the precision of the process. The authors present their technique as a manufacturing platform, meaning that its compatibility with industrial production lines and different types of quantum dots remains to be studied.