Researchers from the University of Dayton have published research describing an enhanced and more cost-effective method of 3D printing nanoscale structures.
The Opto-Thermo-Mechanical (OTM) nano-printing technique was found to be capable of printing on a nanoscale level of fewer than 100 nanometers (nm), or a thousand times smaller than a human hair. What’s more, because it utilizes low-cost laser beams, and doesn’t take place in a vacuum, it’s cheaper than current methods and allows manufacturers to correct any mistakes made during production. As a result, the process could now be used to create products such as computer chips, where tiny errors can leave the product damaged beyond repair.
“Manufacturing error correction is extremely important to reduce manufacturing cost and increase the success rate of a production line,” said Qiwen Zhan, Professor of Electro-optics at Dayton University. “For example, before, if a tiny manufacturing error is found in an electronic chip, the entire chip has to be discarded.”
“This technology will enable us to correct manufacturing errors even after production”
The limitations of existing nanoscale 3D printing
Lasers have often been utilized within 3D printing for rapid prototyping at the macro and microscales due to their excellent directivity for efficient energy to targeted materials. Nonetheless, it has proved a challenge to directly downsize existing macro or microscale printing techniques for nanoscale production. While nanoparticles (NPs) represent ideal 3D printing materials due to their unique electrostatic properties, existing methods such as 2D patterning and optical printing have proved too slow and inaccurate for widespread adoption.
Similarly, electrohydrodynamic printing offers users the ability to print 3D nanostructures using NP solution as ink, but it lacks the capability of individual particle control and requires a conductive surface to work with. To overcome these limitations, the Dayton researchers devised a novel OTM NP production process, which enables both dielectric and metallic particles to be printed onto any substrate.
The Dayton team’s new nanoprinting approach
The researchers’ method begins with the dilution of gold nanoparticle solution, which is summarily drop-casted and naturally dried on a donor substrate consisting of a soft thin layer of polydimethylsiloxane (PDMS) on a glass coverslip. Then, a Continuous Wave (CW) laser is fired into the PDMS substrate, dispersing gold NPs (AUuNPs), which are eventually transferred and printed onto a receiver substrate.
Tests showed that it was possible to additively transfer individual 100nm AuNPs in sequence and onto the same spot, potentially yielding a larger particle depending on the number of NPs printed. What’s more, experiments revealed that adjusting the laser’s intensity could allow gold particles to be printed on top of each other rather than being merged together. To demonstrate this, the research team 3D printed nine NPs that were integrated into one structure, and a tenth which simply landed on top of it.
The material of the donor substrate was also found to play an important role in the OTM-NP technique, and the choice of donor substrate material depended on the optical property of the NPs being printed. If the particles were absorptive to the laser (such as AuNPs), a transparent and flexible substrate such as PDMS must be used. If not, as is the case with dielectric NPs which are transparent to such lasers, an absorptive substrate can be used.
A further 10×10 array of 100-nm AuNPs was later printed on a glass substrate to test this theory. While results showed that 70 percent of the particles were 3D printed with sub 100 nm accuracy, higher printing precision was demonstrated by printing one, two, three, four, and ten individual NPs in the same position. Other experiments didn’t prove as successful, and printing 200 nm imperfect spherical AuNPs proved more difficult due to a misalignment between the thermal expansion of the substrate, and the center-of-mass of the particle.
Pre-heating the 200 nm AuNP with a laser intensity of 4 mW/μm2 and then quickly increasing the intensity to 11 mW/μm2 to desorb the AuNP from the donor substrate, was found to be effective in overcoming this by making the NP more spherical. Moreover, using a circularly-polarized laser beam further improved the process’ printing accuracy, by creating a symmetric focal point that permitted a more even temperature distribution within the particle.
As a result, while the technique is still being developed by the Dayton team to optimize these variables, the researchers did conclude that their OTM NP method could successfully 3D print onto glass or PDMS surfaces without contamination. What’s more, thanks to the commercially available nature of NPs, and the fact the process can take place outside a vacuum in ambient conditions, the process is more affordable than current methods too.
According to the research team, the technique could have potential applications in the fabrication of 2D and 3D electronic and optical devices such as metasurface or even 3D metamaterials. In addition, it could also be used as a nano-repairing tool to correct printing errors that are an inevitable part of the manufacturing process but can be challenging to correct.
“This nano-version 3-D printing technology fills this gap by providing engineers an affordable manufacturing tool for the fabrication of nanostructures and devices,” concluded Chenglong Zhao, an Assistant Professor of Physics and Electro-optics at Dayton University. “This has become increasingly important for applications that are enabled by nanotechnology.”
Nanoscale additive manufacturing
While 3D printing is often used in macro or micro scale production, the technology behind nanoscale manufacturing is still in the early stages of development. As a result, a number of research institutes and universities have developed their own experimental methods in recent years.
Scientists from the Fraunhofer Institute for Microengineering and Microsystems (IMM) in Mainz, Germany for instance, are also developing a novel process to create nanostructures. Using two-photon absorption in combination with commercially available metal precursors, the team has 3D printed in nanoscale directly onto metal printed parts.
Similarly, the researchers from The Greer Group at the California Institute of Technology (Caltech) have used two-photon lithography to 3D print metal structures. By producing dimensions no larger than 100 nm, the group’s technique proved capable of producing metal features that are “an order of magnitude smaller” than any other metal 3D printing method.
Researchers at the Department of Energy’s Oak Ridge National Laboratory meanwhile, have produced a process that could drive nanoscale 3D printing forward. Working with a team from The Graz University of Technology, they developed a new simulation-based technique capable of creating high-fidelity nanostructures predictably and repeatedly.
The researchers’ findings are detailed in their paper titled “Additive Opto-Thermomechanical Nanoprinting and Nanorepairing under Ambient Conditions.” The report was published in the Nano Letters journal and co-authored by Md Shah Alam, Qiwen Zhan, and Chenglong Zhao.
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Featured image shows a schematic of the Dayton team’s OTM-NP process. The arrows indicate the directions of different types of forces acting on the particle. Image via the Nano Letters journal.