New photoresist, listed on Science! Can be used for 3D printing of unprecedentedly complex structures!

Two photon (or multiphoton) lithography technology can achieve polymer nanostructures with almost any geometric shape.
Two photon corrosion inhibitors typically use photo initiators that absorb photons to form reactive species (such as free radicals) that initiate polymerization. High performance two-photon initiators can greatly enhance printing speed and resolution. Currently, almost all two-photon initiators are organic molecules that cannot improve the performance and structural complexity of the final printed product. In addition, each organic photoinitiator is typically only effective for a single type of monomer. Composite nanostructures can be printed by adding metal ions or inorganic particles to existing two-photon resists. But this strategy is ineffective for complex mechanical nanogrids, as the performance of the material is affected when there are small structural defects present. Uncontrollable metal ion reduction and particle growth lead to structural defects and a decrease in characteristic quality, and aggregated particles can interfere with the propagation of light. Therefore, high-quality three-dimensional (3D) nanoprinting is mostly limited to simplicity and currently challenging for the manufacturing of complex systems.

Based on the above challenges, Professor Wendy Gu's research group at Stanford University reported a strategy for rapidly printing complex structured nanocomposites using metal nanoclusters. These ultra small, quantum confined nanoclusters can serve as highly sensitive two-photon initiators, as well as precursors for mechanical strengthening agents and nanoscale pore forming agents. Print nano composite materials with complex 3D architectures, as well as structures with adjustable, layered, and anisotropic nano porosity. Nanocluster polymer nanolattices have high specific strength, energy absorption, deformability, and recoverability. This framework provides a scalable and universal approach for the use of photoactive nanomaterials in additive manufacturing of complex systems with urgent mechanical properties. The related results were published in Science under the title "Mechanical nanolattices printed using nanocluster based photoresists". The first authors are Qi Li and John Kulikowski.

The photoresist based on nanoclusters is composed only of metal nanoclusters, monomers or epoxy monomers (Figure 1A), and solvents. The author found that nanoclusters can serve as photoinitiators for free radical polymerization, photoacid generators for cationic polymerization, and photosensitizers that promote the formation of singlet oxygen to induce protein cross-linking (Figure 1B). In this study, Ag28Pt nanoclusters and rod-shaped Au25 nanoclusters were selected as two-photon initiators. These two nanoclusters have stable and long-lived S1 excited states, which facilitate the generation of free radicals or other active species (Figure 1C). This method is different from organic two-photon initiators, which have a shorter S1 excited state lifetime. In organic two-photon initiators, crossing the triplet state between systems is usually necessary to generate reactive species, which may reduce the overall photo initiation efficiency (Figure 1C, right). More importantly, compared to molecular photoinitiators that typically have limited cleavage pathways, nanoclusters provide more types of bond cleavage (left in Figure 1C), leading to increased reactivity with different reagents.

Then the author evaluated the printing ability of nanoclusters acrylic photoresist. For 5wt% Ag28Pt photoresist, square structures can be fabricated with laser power as low as 4mV and scanning speed as high as 100mm/s (Figure 1D). Under the conditions of laser power as low as 2.5 mW and scanning speed as high as 150 mm/s, 8wt% Au25 photoresist also exhibited similar performance (Figure 1D). The open table, octagonal lattice, and Schwarz primitive (SP) lattice fabricated using Ag28Pt and Au25 photoresist are shown in Figures 1E-I. Figure 1E shows that independent 3D features can be as small as 400 nanometers. The minimum support thickness of an octagonal lattice is 1.27 μ m (Figure 1G). The minimum wall thickness of the SP lattice is 850nm (Figure 1I).

Figure 1. Photochemistry and printability of nanocluster based photoresist

The mechanical properties of the printed nanocluster polymer structure were evaluated using in-situ SEM compression testing (Figure 2). Use 8wt% Au25 PETA photoresist to manufacture cylindrical pillars with a radius of 2.5 μ m and a height of 10 μ m. The cyclic test showed that the sample had a recovery rate of 70% when loaded to 30% strain (Figure 2A). High strength, stiffness, and strain hardening result in energy absorption of up to 110MJ/m3 before the initial crack formation. This mechanical behavior is also evident in honeycomb structures manufactured using 10 wt% Ag28Pt PETA photoresist (Figure 2B). The height of the honeycomb is 8.2 μ m, the cell edge length is 2.6 μ m, and the wall thickness is 800nm (Figure 2C). This results in a density of 0.56g/cm2 and a relative density of~48%. The compressive stress-strain response shows a linear region, followed by a significant yield pressure at 22.1 megapascals (Figure 2B). Octet and Schwarz primary (SP) lattices are made of 8wt% Au25 photoresist. These structures exhibit high energy absorption before densification. The energy absorption capacities of Octet lattice with relative densities of 19% and 27% and SP lattice with relative densities of 20% and 26% reach 7.6 MJ/m3 and 9.7 MJ/m3, respectively. In situ SEM compression testing shows that this optimal performance is due to the characteristic material properties of the nanocluster polymer composite material (Figure 2D-K).

Figure 2. Mechanical behavior of nanocluster polymer nanocrystals

The specific energy absorption of nanoclusters polymer nanolattices and pillars is superior to that of polymer microcrystals and nanolattices with inorganic coatings, as well as traditional material systems (Figure 3A). Nanocluster composite materials also have high compressive strength under high strain (Figure 3B) and high recovery ability under high compressive stress (Figure 3C). The glassy carbon structure, although having considerable strength and hardness, is not included in this comparison due to its brittleness and irreversible behavior. By using nanocluster based photoresist, these mechanical properties can be achieved in a single printing step compared to the multiple manufacturing steps required for core-shell composite lattice formation.

Figure 3. Comparison of mechanical properties

Nanocluster based photoresist can be used to fabricate complex nanoporous structures (Figure 4). At a temperature of 900 ℃ under argon gas flow, the nano cluster polymer composite material is transformed into glassy carbon with complex nano pore characteristics. Figure 4A shows a nanoporous cube printed using 20wt% Ag28Pt photoresist. The average pore size and porosity of the cube surface are~56 ± 23 nm and~50%, respectively (Figure 4B). The nanopores extend into the structure from 500 to 700 nm (Figure 4C) and decrease in size and density further away from the surface. Therefore, larger structures, such as columns with a diameter of approximately 8 μ m as shown in Figure 4D, have graded pores surrounded by a solid core and a nanoporous shell. Small structures with a size less than 2 microns are porous throughout the entire structure (Figure 4E). The octagonal lattice of pyrolysis has two levels of pores: the geometric design space between the lattice pillars and the nanocluster induced nanopores within the lattice pillars (Figure 4, F and G). Subsequently, the authors developed a nanocluster protein photoresist. This photoresist utilizes the efficient singlet oxygen generation and significant photothermal effect of metal nanoclusters under excitation. This induces protein photocrosslinking through the oxidation of tyrosine residues (Figure 1B), as well as the formation of oriented β - sheet crystalline regions through local heating. A water dispersed Au22 nanocluster was synthesized and used for photoresist. The printing speed of silk cellulose structure can reach up to 100mm/s. The printed protein structure is composed of neatly arranged bundles (Figure 4, H-J), indicating that directional self-assembly occurred during the manufacturing process.

Figure 4. Layered, tunable and anisotropic nanoporous structure of glassy carbon and silk protein

Summary: In summary, this work developed photoresists based on metal nanoclusters for the fabrication of nanocluster polymer nanolattices, as well as nanoporous glassy carbon and protein structures with unprecedented structural complexity. The author indicates that nanoclusters are universal and efficient two-photon activators suitable for different types of monomers. Nanoclusters polymer nanocrystals exhibit strain hardening behavior, resulting in a combination of high specific energy absorption, strength, deformability, and recoverability. Based on this simple and universal framework, there is a vast opportunity to directly print more mechanical metamaterials by combining hundreds of available metal nanoclusters with different types of monomers and well-designed three-dimensional topological structures.