Nov.6, 2014

3D printing has so far been limited to specific plastics, passive conductors, and a few biological materials. But team at Princeton has now successfully 3D printed quantum dot-based LEDs, showing that diverse classes of materials can be 3D printed and fully integrated into device components with active properties.

"Achieving seamless integration of diverse materials with 3D printing is a significant challenge," notes Princeton Prof. Michael McAlpine.

In their report in Nano Letters, the researchers describe the seamless interweaving of five different materials, including: 1) emissive semiconducting inorganic nanoparticles, 2) an elastomeric matrix, 3) organic polymers as charge transport layers, 4) solid and liquid metal leads, and 5) a UV-adhesive transparent substrate layer.

The team's approach consists of three key steps. First, it identifies electrodes, semiconductors, and polymers that possess desired functionalities and exist in printable formats. Next, care is taken to ensure that these materials are dissolved in orthogonal solvents so as not to compromise the integrity of underlying layers during the layer-by-layer printing process. Finally, the interwoven patterning of these materials is achieved via direct dispensing in a CAD-designed construct.

Ryan Whitwam of ExtremeTech reported that the team used a custom 3D printer, as "no off-the-shelf printer was going to do the job," said Whitwam. "It took more than six months and $20,000 to get this far."

The bottom layer of each quantum dot LED is composed of silver nanoparticles, which are perfect for connecting the LED to an electronic circuit. On top of that are two polymer layers that push electrical current up toward the next layer. This is where the real "quantum dots" are — they're nanoscale semiconductor crystals, in this case cadmium selenide nanoparticles wrapped in a zinc sulfide shell. Each time an electron hits these nanoparticles, they emit orange or green light. The color can be controlled by changing the size of the particles. The top layer is a comparatively ordinary gallium indium that directs the electrons away from the LED.

3D printed 2 × 2 × 2 multidimensional array of embedded QD-LEDs.

As a proof of concept, they 3D printed quantum dot-based light-emitting diodes (based on CdSe nanoparticles wrapped in ZnS with a top layer of GaIn) that exhibit pure and tunable color emission properties.

By further incorporating the 3D scanning of surface topologies, they went on to show the ability to conformally print devices onto curvilinear surfaces, such as contact lenses.

CAD model showing the QD-LED components and conformal integration onto the curvilinear substrate.

A third example they showed was a 2x 2x 2 cube of encapsulated LEDs, in which every component of the cube and electronics are 3D printed. This was to demonstrate that novel architectures which are not easily achieved using standard microfabrication techniques can be constructed by 3D printing.

Overall, the team says that the results suggest that 3D printing is more versatile than has been demonstrated to date and is capable of integrating many distinct classes of materials.

A low-res picture of a working 3D-printed quantum dot LED

"We anticipate that this general strategy can be expanded to 3D print other classes of active devices, such as MEMS devices, transistors, solar cells, and photodiodes," says McAlpine. "Overall, our results suggest a number of exciting applications, including the generation of geometrically tailored devices containing LEDs and multimodal sensors to provide a new tool for optogenetics for studying neural circuitry."

Co-printing of active electronics with biological constructs could lead to new bionic devices, such as prosthetic implants that optically stimulate nerve cells.

According to the team, Future work will address a number of key challenges. These include: 1) increasing the resolution of the 3D printer such that smaller devices can be printed, 2) improving the performance and yield of the printed devices, and 3) incorporating other classes of nanoscale functional building blocks and devices, including semiconductor, plasmonic, and ferroelectric.

Posted in 3D Printing Company

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