Lasers bond quantum dots into 3D structures

In the first of an occasional series, Physics Today writers add notes and illustrations to an image from a recent study to elucidate the research.

Printing nanoscale objects typically requires polymers, whose residue deteriorates the material properties. Hong-Bo Sun (Tsinghua University) and his colleagues have now devised a nanoprinting method that uses cadmium selenide quantum dots (QDs) to build three-dimensional objects, such as the horse shown in the image above and a buckyball.

In Sun’s experiment, a laser scans over a solution of colloidal semiconducting QDs and bonds them into a desired shape. The trick lies in the influence of laser excitation on the QDs. Photoexcited electrons in semiconducting materials leave behind holes. In QDs, those holes tunnel to the surface to reach energetically favorable ligand molecules. Ligands capture the holes, desorb, and leave behind bonding sites. Ligands from neighboring QDs latch onto those sites. The dots then share those ligands and bond to form a pixel. The luminescence intensity of each pixel depends on its volume, which in turn depends on how quickly the laser scans.

The technique prints lines as thin as about 80 nm, and it works for various QD colors or even combinations of colors. In the future, such nanoprinting may also be possible for metallic nanocrystals. (S.-F. Liu et al., Science 377, 1112, 2022 .)