Researchers led by Michael C. McAlpine of Princeton University have fabricated the first quantum dot light-emitting diodes (LEDs) built using only a three-dimensional printer (Nano Lett. 2014, DOI: 10.1021/nl5033292). Most 3-D printers are used to pattern plastics, metallic inks, and some biological materials, McAlpine says, but not fully printed semiconductor devices. To print an entire LED, the Princeton researchers had to make careful materials decisions. They created a suspension of CdSe-ZnS core-shell quantum dots using a mixture of toluene and dichlorobenzene as the solvent. Their formulation allowed the dots to settle more uniformly as the ink dried, instead of pinning the nanostructures near droplet edges. By stacking a layer of quantum dots on top of printed conductive polymers and between metallic contacts—one of which was printed using a Ga-In liquid metal—the team was able to transport electric current to the luminescent structures. To demonstrate the technique’s flexibility, they printed LEDs in cubic 3-D arrays and atop curved contact lenses. McAlpine hopes these devices are a first step toward using 3-D printing to create fully functional electronic, photonic, and perhaps even bionic devices. “That’s what I’m most excited about,” he says.