Bench Talk

Using Two-Photon Lithography to Print 3D Nanoscale Structures

(Source: Mihail - stock.adobe.com)

3D printing, otherwise known as additive manufacturing, is an evolving technology. In the early days, designers used it to create small 3D structures because it was a ‘cool’ way of creating custom-made individual pieces using their own printer. It has since moved on to become a powerful manufacturing technique to print geometrically complex parts for a wide range of industries. The scope is further widening because it is now possible to 3D print at the nanoscale.

Many could argue that conventional 3D printing methods involve working at the nanoscale because the structures are built up in atomic layers. While this true, the resulting product—or finished piece—is a bulk structure. Now it is possible to 3D print layers of atoms with the result being a nanomaterial, or a nanosized structure, without the need to produce a bulk structure as the end point. While it is an area that is relatively new, , it could become a useful and versatile nanofabrication technique alongside the other common bottom-up nanofabrication methods—such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and atomic layer deposition (ALD). In this post, we discuss how designers can use two-photon lithography to print 3D nanoscale structures.

Structures can now be printed at the nanoscale using a technique called two-photon lithography. Lithography is a class of fabrication techniques that deposit materials onto a surface with a photoresist mask on top of the surface. The photoresist mask protects certain areas of a surface while other parts are etched into specific patterns and geometries. In general, lithographic techniques are top-down in nature rather than bottom-up.

Two-photon lithography is a little different, and it has gained a lot of use for creating complex nanosized architectures on top of various surfaces. To print at the nanoscale, the materials used need to be in a liquid form and be sensitive to photons of light (preferably for photons near infrared region of the electromagnetic spectrum. From this liquid stating point, a high energy laser (often pulsed) hits the liquid material. The energy from the laser causes the liquid materials to bond together—in many cases by polymerization—by absorbing the near infrared photons, which changes the material from a liquid state to a solid state. This subsequently allows the fabrication of the solid structure. Designers typically use near infrared photons from a pulsed laser because their energy is high enough to induce the phase change and physical bonding of the material in question, but it is not high enough to damage the material or cause chemical changes to the material.

The laser can scan in three dimensions around the liquid starting material, and this enables the architecture of the print to be governed by where the laser scans. Like many other lithographic methods, a photoresistive material i.e., a photoresistive mask, can also be placed on certain areas and can better direct the growth of the structure in three dimensions. The result is a solid nanostructured material built from liquid precursor, much like conventional 3D printing but on a smaller scale. It is a technique that is finding a significant amount of use with both inorganic and organic materials, especially photosensitive polymers, and has started expanding out to metals as well.

3D printing has gained a great deal of interest from the manufacturing space, but it’s use within nanotechnology is relatively new. While polymers are the main material used when printing at the nanoscale, the scope of materials that are viable is increasing. It is an area that will surely grow in the coming years and could become a well-established nanofabrication technique if it grows at the same rate as other 3D printing methods. Some of the future applications could include precise nanoscale patterning, the fabrication of nanomaterials with precise and complex structures, for creating molecular scaffolds, and it even has the potential to be used for fabricating quantum bits (qubits) in quantum computing applications.