Both attributes stem from the intrinsic properties of the dots, which are just a few nanometres in diameter and emit a narrow emission profile that can be tuned by tweaking their size.

The dots contain a zinc sulphide shell that confines electrons and their positively charged counterparts, holes, to the core of the structure, a cadmium-based compound. Both charge carriers are restricted in all three spatial dimensions on length scales where quantum mechanics come into play. This restricts the energies of electrons and holes to a finite set of values, so that when these carriers reach their lowest energy state and recombine to emit light, the emission profile is very narrow.

Making a colour display with the dots requires their deposition onto a substrate in a well-controlled manner.

Monochrome displays can be made by spin-coating – dropping a dot-containing solution onto a substrate and spinning this around to yield a thin film of material. This approach is unsuitable for making a full-colour display, because it would cross-contaminate red, green and blue pixels.

Patterned rubber stamp

In this new work, the researchers overcame this issue by spin-coating red, green and blue dots onto separate ‘donor’ substrates, before transferring them in turn to the display with a patterned rubber stamp.

To make a 4-inch diameter, 320 by 240 pixel display, the researchers deposited a pair of electron-transporting polymers onto a piece of glass coated in indium tin oxide. Red, green and blue dots were stamped onto this structure, which was then coated in titanium dioxide, a material with good hole transporting properties.

Adding a thin-film transistor array allowed individual biasing of the 46 µm by 96 µm pixels, which emit light when a few volts is applied across them, so that electrons and holes are driven together to recombine radiatively in the dots.

Higher resolution displays are possible by reducing pixel size. "We showed an array of narrow quantum dots stripes of 400 nanometre width [in our paper], which indicates the feasibility of nano-printing quantum dots with extremely high resolution," said Samsung's Byoung Lyong Choi.

One downside of the Korean display is its low efficiency - just a few lumens per Watt, which less than that of an incandescent bulb.

However, Choi believes that far higher efficiencies should be possible by improving hole transport layers and making other modifications to the quantum dot structure.

Commercial promise

Samsung will continue to develop the technology, which it is trying to patent, before deciding whether to manufacture displays with this approach.

"Transfer-printing can be scaled up to roll-to-roll systems for huge size printing onto flat or curved surfaces, such as rolled plastic sheets," explained Choi.

John Rogers, a researcher at the University of Illinois, Urbana Champaign, is very impressed by the Korean effort: "It is, by far, the most complete demonstration of this technology."

However, he points out that substantial progress is needed before this technology can compete with organic light emitting diodes.

"The entrenched technology - backlit liquid crystal displays - continues to get better and better, and cheaper and cheaper."

The team reported its work in the latest edition of Nature Photonics.