Until now, researchers have looked at techniques like Langmuir–Blodgett, microtransfer, intaglio, gravure, inkjet and electrohydrodynamic jet printing to pattern QDs on solid substrates. Although the mask-based approaches (Langmuir–Blodgett, microtransfer, intaglio and gravure) are good for producing high-resolution patterns down to the single QD level, they remain relatively expensive and aren’t configurable in real-time. The ink/nozzle-based techniques, although much cheaper, cannot really produce complex structures at sub-micron resolution because of problems like ink spread on the substrate and relatively long post-processing times required for the inks to dry.

Zheng and colleagues have now exploited a light-directed microbubble to capture and print multicolour QDs made from gold nanoparticles in liquid environments. The bubble printing technique, as they have dubbed it, makes use of a single low-power laser to generate a sub-micron-sized bubble at the interface of a colloidal suspension of nanoparticles and a plasmonic substrate containing a network of metallic nanoparticles. “The bubble produced captures and immobilizes the colloidal particles on the substrate, and by programmed movement of a stage and by retaining the sub-micron bubble’s stability, we are able to create various patterns,” explains team member and lead author of the study Bharath Bangalore Rajeev. “What is more, by using multi-step bubble printing, we can integrate multiple QDs on a single substrate.”

Spatial resolution of 650 nm

The technique can produce QD patterns with a spatial resolution of 650 nm. It is also high throughput with a scanning rate of around 10–2 m/s and the QDs adhere firmly to the substrate during printing. Indeed, the researchers have succeeded in printing high-resolution intricate patterns of QDs resembling Charlie Chaplin and the Mona Lisa, and have also printed a fully-functioning microscale QR code (see image) for anti-counterfeiting applications.

And that is not all: they confirm that the printing parameters (optical power, for example) can be optimized in real time to control the fluorescence properties of the QDs, such as emission wavelength and lifetime, and that the technique can be used to print QDs on flexible substrates too. This means that applications such as bendy and foldable displays, optoelectronic circuits and sensors could benefit, says Rajeeva.

New and unconventional way to print nanoparticles

“Bubble printing is a new and unconventional way to print nanoparticles,” he tells nanotechweb.org. “It does not rely on specialized ink formulations and can directly immobilize particles from colloidal suspensions. Moreover, it isn’t plagued by issues such as nozzle clogging, ink spreading and long post-processing times for inks to dry.”

The team, reporting its work in Applied Materials & Interfaces DOI: 10.1021/acsami.7b04881, is now busy looking into printing nanoparticle systems other than QDs to test the limits of its technique in terms of bubble stability and printing throughput. “One critical challenge we need to address is to improve line-roughness and then extend to 3D printing,” says Zheng.