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Photochemistry democratizes 3D nanoprinting | Nature Photonics
For 20 years, nanoscale 3D printing has been based on two-photon absorption, requiring expensive pulsed lasers. Now, via a two-step absorption process, such printing has been demonstrated using a low-cost, low-power continuous-wave laser diode, showing the potential for dramatic cost reductions in 3D nanoprinting. You have full access to this article via your institution. Download PDF Download PDF Three-dimensional (3D) printers have penetrated virtually all aspects of life. They range in price from less than US$200 to more than US$1,000,000 and can print everything from toy plastic whistles to rocket motors and tissue scaffolds. Interestingly, with the exception of light-based 3D printers, the critical minimum dimension that the printers can fabricate does not significantly scale with cost. Regardless of cost, the critical minimum dimension that conventional non-light-based printers can form is at least a few 10s of microns. What changes are the variety of materials that can be printed (simple polymers to high-performance metals) and the size of the printed object. However, for light-based 3D printing, the critical dimension is very dependent on the system cost. To 3D print materials at the many-micron scale, low-cost printers are available, while to print at the nanoscale, the low-cost option disappears, even for the simplest material systems. Fundamentally, this is due to the optics and chemistry involved in the printing process. Until now, it has been assumed that nanoscale 3D printing requires a non-linear multiphoton process to confine the photochemistry involved in the writing process to the focal point of the writing laser. The intensity-dependent non-linear multiphoton absorption of the reacting molecules (dyes) explains why the photochemistry only proceeds quickly in the high-intensity region near the focal point of the laser, in contrast to a linear single photon absorption process, where considerable photochemistry occurs away from the focal point. The challenge is that very high peak-power lasers are required to drive two-photon absorption at any reasonable rate, and such high-power lasers are both expensive and bulky. Now, writing in Nature Photonics , Vincent Hahn and colleagues show that two-step absorption (a process where the reacting molecule is excited step-wise into the reactive state, one photon absorption at a time), rather than two-photon absorption can be used to write nanoscale 3D structures 1 . It is hard to overemphasize the importance of moving from a multi-photon to a multi-step absorption process. Moving from multi-photon absorption to multi-step absorption enables use of low-cost highly compact diode lasers that are 2–3 orders of magnitude lower in size and cost than the pico- and femtosecond pulsed lasers required to drive multi-photon absorption 2 . Specifically, using a photoresist system containing a photoinitiator supporting two-step absorption, the team shows that 3D structures with characteristic dimensions of only a few hundred nanometres can be written using only a compact continuous-wave semiconductor laser diode. At first glance, the structures formed look virtually identical to those formed using multi-photon writing, and after careful analysis, might contain even smaller critical dimensions. There are some very important differences between two-step and two-photon absorption that significantly impact the 3D writing process. In two-photon absorption, the intermediate state is a virtual state, and no photons are absorbed unless two photons interact with the dye at effectively the same time 3 . In two-step absorption, as shown in Fig. 1a , the dye is excited into an intermediate triplet-state by the first photon, and then into the final excited state by a second photon that arrives a brief moment later (before the dye relaxes back to the ground state). The first difference is that in two-photon absorption, one can in principle write deep in a photoresist. The writing beam is not attenuated as it passes through the photoresist since no absorption occurs except at the focal point and the primary concern is maintaining temporal coherence. In a two-step absorption process, the writing beam is being continuously absorbed via a linear process, as it passes through the photoresist. Because the absorption is not zero, care is required in experimental design, something Hahn et al. discuss in considerable detail in their report. For example, the benzil dye used has a triplet-state extinction coefficient about 100 times greater than the ground-state extinction coefficient, resulting in relatively low absorption of light away from the focal point, and strong absorption at the focal point 4 . Hahn et al. also add specific chemical agents to the photoresist to minimize reactions between photoinitiator in the ground triplet-state and monomer. It does remain to be seen if careful control of exposure wavelength and resist chemistry can ensure that productive chemical reactions only occur at the focal point of the laser across broad classes of 3D structures, and not limit the diversity or scale of the 3D structure formed. Additional studies will be required to investigate the universality of two-step photochemistry for 3D nanopattern generation. Fig. 1: Two-step 3D nanoprinting photochemistry and enabled nanoscale structures. adapted with permission from ref. 1 , Springer Nature Ltd. a , Jablonski diagram showing how a photon (violet) can take the benzil photoinitator from the ground state S 0 to a singlet vibrational energy state S 1 , which rapidly converts to singlet state S 1 and then a triplet state T 1 (or occasionally to a vibrational ground state S 0 ). A second photon (light violet) then takes the photoinitator from T 1 to T n . T n then generates the radicals R·. which initiate photoresist polymerization. b , Top-view scanning electron micrograph of a woodpile formed via two-step absorption with an in-plane rod-to-rod spacing of 300 nm. c , A chiral 80-μm tall vertical structure similarly formed via two-step absorption with a lattice constant of 16 μm. ISC, intersystem crossing. Full size image What is clear is that when the photochemistry is properly tuned, the results are quite impressive. As shown in Fig. 1b,c , 3D woodpile structures with characteristic dimensions below 200 nm can be formed, as well as other intricate 3D structures. There are already numerous applications in the physical and life sciences of judiciously designed 3D nanostructures. The ability to fashion complex nanostructures at a scale below the wavelength of visible light, using only simple low-power lasers, and new polymer chemistries, has the potential to accelerate research in applications ranging from metamaterials and photonic crystals to nanostructured tissue scaffolds. Where this approach has real potential to shine is in the scalability of the process. As we showed last year, two-photon polymerization can be used to write intricate and highly-functional gradient refractive index optics and 3D waveguides within nanoporous silicon 5 , and as we demonstrated a number of years ago, two-photon polymerization can be employed to form 3D waveguides in photonic crystals 6 . While two-photon polymerization was perfectly acceptable for demonstrating 3D photonic structures, the real-world applications of such structures could never be realized via a serial writing process. For example, one potential application for complex 3D nanopatterns is to form a photonic integrated circuit 7 . Such a structure might contain thousands of optical elements, embedded in a slab of material with a thickness of 100 micrometres and dispersed over square centimetre areas. Creating such structures on the manufacturing scale requires a massively parallel 3D writing process, a possibility for two-step absorption, but likely an impossible challenge for two-photon writing. As the authors suggest, it may be possible to write structures using a machine more akin to a modern laser printer than a conventional 3D printer. At this time, the practical and fundamental limitations of this technique remain unclear. It will be very interesting to see over the next few years how researchers around the globe will leverage this new approach to 3D printing to develop creative new structures with unique properties in volumes of material simply not previously possible. References . 1. Hahn, V. et al. Nat. Photon. https://doi.org/10.1038/s41566-021-00906-8 (2021). Article ? Google Scholar ? 2. Maruo, S. & Fourkas, J. T. Laser Photonics Rev. 2 , 100–111 (2008). ADS ? Article ? Google Scholar ? 3. G?ppert‐Mayer, M. Ann. Phys. 401 , 273–294 (1931). Article ? Google Scholar ? 4. Fang, T.-S., Brown, R. E., Kwan, C. L. & Singer, L. A. J. Phys. Chem. 82 , 2489–2496 (1978). Article ? Google Scholar ? 5. Ocier, C. R. et al. Light Sci. Appl. 9 , 196 (2020). ADS ? Article ? Google Scholar ? 6. Rinne, S., García-Santamaría, F. & Braun, P. Nat. Photon. 2 , 52–56 (2008). ADS ? Article ? Google Scholar ? 7. Richards, C. A., Ocier, C. R., Zhu, J., Goddard, L. L. & Braun, P. V. Appl. Phys. Lett. 119 , 130503 (2021). ADS ? Article ? Google Scholar ? Download references Author information . Affiliations . Department of Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, Il, USA Paul V. Braun Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA Mark L. Brongersma Authors Paul V. Braun View author publications You can also search for this author in PubMed ? Google Scholar Mark L. Brongersma View author publications You can also search for this author in PubMed ? Google Scholar Corresponding author . Correspondence to Paul V. Braun . Ethics declarations . Competing interests . The authors declare no competing interests. Rights and permissions . Reprints and Permissions About this article . Cite this article . Braun, P.V., Brongersma, M.L. Photochemistry democratizes 3D nanoprinting. Nat. Photon. 15, 871–873 (2021). https://doi.org/10.1038/s41566-021-00911-x Download citation Published : 29 November 2021 Issue Date : December 2021 DOI : https://doi.org/10.1038/s41566-021-00911-x Share this article . Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative .
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