It has long been known that certain nanotextured surfaces can interact with light to produce colours without the need for any dyes or pigments. Since the interaction is controlled by the nanoscale structure of the material, such colours are resistant to photobleaching or chemical degradation, and can persist for far longer than those produced by inks.

Unfortunately, forming nanotextures over large surfaces can be a slow process, as Jiang discovered in 2014 when he and his group, led by Bozena Kaminska, tried to use it to print the covers for a run of journals. Jiang explains: "We tried to make a 10 × 10 cm colour image, and needed to make at least 2000 such copies. It turned out to be a very challenging project, because manufacturing nanostructures on the scale of a journal cover can take a few months using traditional nanopatterning techniques such as electron-beam lithography."

Alternative high-throughput methods do exist, but they are not practical for widespread application. "Their prices are on the level of at least $10 m, and these systems also have stringent environmental and material supply requirements," he says. "Therefore, they are mostly expensive pieces of equipment for use inside a cleanroom for manufacturing cutting-edge nano- and micro-devices. To use such systems for recording colours is not practical yet."

Jiang and his team spent the next few years trying a number of different approaches, culminating in an optical technique that they have now described in Nano Futures. The new method, which the researchers term 'NETT' (for nanoimprint, exposure, and thermal treatment), exploits the way in which each wavelength of light is diffracted differently by nanostructures of a certain size and period.

The process begins with a quartz stamp that has been patterned with a periodic arrangement of sub-micron-sized, cone-shaped pits. Three configurations of pits are repeated in bands across the surface of the stamp, with the pits in each band having one of three possible sizes.

When the quartz stamp is pressed against a photoresist layer (SU-8) on a silicon substrate and heated, the photoresist is left with a repeating pattern of three nanocone types, corresponding to red, green and blue for a given set of viewing and incident angles. This imprinted photoresist forms the blank sheet to be recorded on, while the quartz stamp can be used again and again in the same process.

As a negative photoresist, heating SU-8 after exposing it to UV light causes it to become cross-linked, whereas heating unexposed SU-8 causes it to soften and flow. The researchers used a UV laser to control the pattern of cross-linking in the photoresist, and then subjected the surface to a two-step baking process. This preserved the structure of the nanocones in the irradiated areas – corresponding to red, green or blue pixels – while the unexposed nanocones underwent thermoplastic flow and merged into a flat background. These correspond to black or inactive pixels.

Above a certain minimum laser power, there is a relationship between the level of UV exposure and the extent to which the nanocones maintain their structure when heated. As the height of the nanocones in a pixel determines that pixel's brightness, Jiang and his team were able to produce greyscale images by tuning the exposure and therefore the topography. This provided an alternative to the half-tone images that could be created simply by exposing limited areas of each pixel either fully or not at all.

With their NETT procedure, the group has so far achieved an image resolution of approximately 338 pixels per inch (PPI), which is close to the resolution limit of the human eye and so is sufficient for recording eye-readable text and images. At the moment, however, colour fidelity is preserved only for a viewing angle of about 5°, since the colours are produced by the diffraction of light. The team plans to address this issue in future research. "The next phase of development will use plasmonic colours, which are angle-independent, so that the viewing angle will be very wide, at least up to 60°," says Jiang. Plasmonic colours require the fabrication of metallic nanoholes and nanodisks, for which the NETT technique can also be applied.

Jiang envisages the technique being used by organizations such as museums, libraries and governments, implemented in a device "similar to a laser printer, including a mechanical component to move the sheet, a scanning UV laser to write the information, and a heating component to melt and cure the nanostructures".

Full details of the research are presented in Nano Futures.