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Widening the use of 3D printing: Science
The lab where my son is working on a Ph.D. is involved in this sort of thing, printing small nuclear reactor cores and parts.
A news item in Science: Widening the use of 3D printing
Subtitle:
A light-triggered fabrication method extends the functionality of printable nanomaterials
DANIEL M. BALAZS AND MARIA IBÁÑEZ
Science 28 Sep 2023 Vol 381, Issue 6665 pp. 1413-1414
Some excerpts:
Three-dimensional (3D) printing has reshaped manufacturing paradigms by providing customized on-demand object fabrication. To unlock 3D printing’s full range of applications, material versatility is necessary. Presently, the range of available 3D-printing materials is limited to mostly metals and polymers because of their ease in forming chemical bonds as they convert from ink into a solid object. Recently, complex 3D structures of semiconducting nanoparticles were printed by light-induced reactions between the nanoparticles and molecules at their surface (1). This strategy relies on the properties of the nanoparticle and the surface molecules, limiting the printable materials. On page 1468 of this issue, Li et al. (2) report a material-independent strategy for 3D-printing nanoparticles, which uses an additive that decomposes, upon light irradiation, into a reactive species that binds together two stabilizing molecules connected to the nanoparticle surface (capping molecules). This provides flexibility in the material used, which only requires the ability to form surfactant-stabilized dispersions...
...Li et al. designed a nanoparticle printing strategy that interconnects the ligands between different nanoparticles to create 3D structures. A molecular additive is mixed into the nanoparticle suspension, which then decomposes upon irradiation with light, creating a highly reactive and symmetric species called the binder, which is able to bridge the ligands (see the figure). The bond formation can happen either within two ligands of the same nanoparticle, reducing the local colloidal stability and bringing the particles closer together, or between two ligands of different nanoparticles, indirectly creating interparticle links. In addition, the binding occurs through the ligand hydrocarbon chains, providing complete freedom from the properties of the surface-anchoring group of the ligand. Hence, a large variety of ligands can be used, simplifying the ink preparation. Overall, the light-triggered sequence of reactions results in a build-up of solid material defined by the path of the focal point of the light through the liquid. The 3D-printed structures have remarkable mechanical strength and maintain the functionality of the individual nanoparticles. In some configurations, collective phenomena, such as chiral or plasmon-enhanced photoluminescence, can be observed.
Another advantage of this printing method is the small amount of organic content present in the final 3D structures, especially compared with previous methods in which nanoparticles were encapsulated in polymer matrices (3). Ideally, the final structure should be only made of the solid material intended to be printed, that is, the nanoparticles. The presence of organic molecules is usually detrimental for applications in which the nanoparticles need to be in close proximity to one another, such as those involving charge transfer (electronics, catalysis, among others) (8, 9). The high nanoparticle content used in Li et al.’s approach also allows the removal of the organic network after printing by thermal or chemical stripping without compromising the object’s shape and strength...
...Li et al. designed a nanoparticle printing strategy that interconnects the ligands between different nanoparticles to create 3D structures. A molecular additive is mixed into the nanoparticle suspension, which then decomposes upon irradiation with light, creating a highly reactive and symmetric species called the binder, which is able to bridge the ligands (see the figure). The bond formation can happen either within two ligands of the same nanoparticle, reducing the local colloidal stability and bringing the particles closer together, or between two ligands of different nanoparticles, indirectly creating interparticle links. In addition, the binding occurs through the ligand hydrocarbon chains, providing complete freedom from the properties of the surface-anchoring group of the ligand. Hence, a large variety of ligands can be used, simplifying the ink preparation. Overall, the light-triggered sequence of reactions results in a build-up of solid material defined by the path of the focal point of the light through the liquid. The 3D-printed structures have remarkable mechanical strength and maintain the functionality of the individual nanoparticles. In some configurations, collective phenomena, such as chiral or plasmon-enhanced photoluminescence, can be observed.
Another advantage of this printing method is the small amount of organic content present in the final 3D structures, especially compared with previous methods in which nanoparticles were encapsulated in polymer matrices (3). Ideally, the final structure should be only made of the solid material intended to be printed, that is, the nanoparticles. The presence of organic molecules is usually detrimental for applications in which the nanoparticles need to be in close proximity to one another, such as those involving charge transfer (electronics, catalysis, among others) (8, 9). The high nanoparticle content used in Li et al.’s approach also allows the removal of the organic network after printing by thermal or chemical stripping without compromising the object’s shape and strength...
The full article (Li) to which the news item refers is here:
Fu Li et al., 3D printing of inorganic nanomaterials by photochemically bonding colloidal nanocrystals. Science381,1468-1474(2023).
Some text from the full paper:
Additive manufacturing, also known as 3D printing, has been a revolutionary technology with broad applications (1–3). Despite the substantial progress in the structural complexities and printing scalabilities (4–6), 3D printing is mostly limited to metals and plastics (7), especially for printing with nanoscale resolution. During printing, atomic bonds should form between the building blocks (atoms or molecules) at desired locations to produce three-dimensional (3D) shapes with sufficient mechanical support. For example, strong metallic bonds (e.g., W–W, ~800 kJ/mol) form during the powder bed fusion printing of metals, whereas covalent bonds (e.g., C–C, ~350 kJ/mol) are produced in the photopolymerization of 3D-printed plastics (1)...
The problem is getting certain materials to print, particularly semiconductors.
The authors write:
Such atomic bonding events do not directly occur with comparable resolution for other functional materials, especially inorganic semiconductors and metal oxides. For instance, metal–chalcogen/pnictogen bonding in corresponding II–VI and III–V semiconductors requires elaborate reactions between molecular or ionic building blocks (8, 9), which cannot be triggered at specific locations under regular 3D printing conditions (supplementary text 1). One promising strategy is to use preformed colloidal inorganic nanocrystals (NCs) with compositions covering a broad scope of functional materials (7) and high solution processability as building blocks for 3D printing...
The approach to addressing this issue is described in a technical means.
...Printing mechanism
The core of 3D Pin is to establish strong covalent bonds between colloidal NCs photochemically at desired locations to form 3D structures (Fig. 1A). NCs are composed of inorganic cores coated with a layer of molecules called surface ligands. These ligands are “native” and indispensable because they are introduced before NC synthesis and preserved for providing surface passivation and colloidal solubility to NCs (25, 26). They typically include hydrocarbon chains and terminal anchoring groups that bind to the NC surface (supplementary text 3 and fig. S2). In our work, stable colloidal solutions of various NCs with a small amount of bisazide molecules [e.g., 1,5-bis (4-azido-2,3,5,6-tetrafluorophenyl) penta-1,4-dien-3-one, or BTO; fig. S3] serve as inks (Fig. 1B). BTO has a broad absorption band between 300 and 400 nm (Fig. 1C) and high two-photon absorption cross-section (>103 Goeppert-Mayer units, or GM, at 780 nm, fig. S4). It generates nitrene radicals at both ends that readily bridge NC ligands through nonspecific C–H insertion when triggered by a continuous-wave ultraviolet (UV) laser (fig. S5) or a low-energy fs laser (780 nm, 0.009 nJ; table S1). Printing with a fs laser is preferred as the two-photon process permits longer penetration depth and higher printing resolution (supplementary text 4)...
The core of 3D Pin is to establish strong covalent bonds between colloidal NCs photochemically at desired locations to form 3D structures (Fig. 1A). NCs are composed of inorganic cores coated with a layer of molecules called surface ligands. These ligands are “native” and indispensable because they are introduced before NC synthesis and preserved for providing surface passivation and colloidal solubility to NCs (25, 26). They typically include hydrocarbon chains and terminal anchoring groups that bind to the NC surface (supplementary text 3 and fig. S2). In our work, stable colloidal solutions of various NCs with a small amount of bisazide molecules [e.g., 1,5-bis (4-azido-2,3,5,6-tetrafluorophenyl) penta-1,4-dien-3-one, or BTO; fig. S3] serve as inks (Fig. 1B). BTO has a broad absorption band between 300 and 400 nm (Fig. 1C) and high two-photon absorption cross-section (>103 Goeppert-Mayer units, or GM, at 780 nm, fig. S4). It generates nitrene radicals at both ends that readily bridge NC ligands through nonspecific C–H insertion when triggered by a continuous-wave ultraviolet (UV) laser (fig. S5) or a low-energy fs laser (780 nm, 0.009 nJ; table S1). Printing with a fs laser is preferred as the two-photon process permits longer penetration depth and higher printing resolution (supplementary text 4)...
This approach has certain similarities to the concept of "polymer derived ceramics" which my son considered during his first undergraduate internship in France.
A graphic from the paper:
The caption:
Fig. 1. Mechanism of 3D Pin.
(A) Scheme illustrating 3D printing of inorganic nanomaterials from a solution containing NCs (blue spheres) and bisazides using a fs laser (left). NCs are initially separated at a distance (d) of up to >10 times their sizes (r). Diffusion and accumulation bring NCs to the focal point (top right). At reduced interparticle distance, the bonding photochemistry starts from the two-photon activation of bisazides, which then generate nitrene radicals (·N–N·) and covalently bond adjacent NCs by bridging their ligands (represented by the lines on the blue spheres) (bottom right). The bonded NCs constitute mechanically robust 3D structures that replicate the laser trajectory. (B) Photographs of solutions of various NCs as 3D Pin inks. BQD, GQD, and RQD are II?VI core–shell QDs emitting in blue (CdZnS/ZnS), green (CdSe/CdZnSeS), and red (CdSe/ZnS). (C) UV–visible absorption spectrum and molecular structure of BTO. (D) FTIR and (E) XPS (N1s) spectra of pristine CdSe NC films and printed CdSe structures confirm the underlying photochemistry in 3D Pin. Pristine samples in (D) and (E) refer to spin-coated CdSe NC films with (D) or without (E) BTO linkers. a.u., arbitrary units.
(A) Scheme illustrating 3D printing of inorganic nanomaterials from a solution containing NCs (blue spheres) and bisazides using a fs laser (left). NCs are initially separated at a distance (d) of up to >10 times their sizes (r). Diffusion and accumulation bring NCs to the focal point (top right). At reduced interparticle distance, the bonding photochemistry starts from the two-photon activation of bisazides, which then generate nitrene radicals (·N–N·) and covalently bond adjacent NCs by bridging their ligands (represented by the lines on the blue spheres) (bottom right). The bonded NCs constitute mechanically robust 3D structures that replicate the laser trajectory. (B) Photographs of solutions of various NCs as 3D Pin inks. BQD, GQD, and RQD are II?VI core–shell QDs emitting in blue (CdZnS/ZnS), green (CdSe/CdZnSeS), and red (CdSe/ZnS). (C) UV–visible absorption spectrum and molecular structure of BTO. (D) FTIR and (E) XPS (N1s) spectra of pristine CdSe NC films and printed CdSe structures confirm the underlying photochemistry in 3D Pin. Pristine samples in (D) and (E) refer to spin-coated CdSe NC films with (D) or without (E) BTO linkers. a.u., arbitrary units.
What is really, really, really interesting to me is the nitrogen chemistry involved, since nitrides are a very important type of nuclear ceramic fuels, particularly in connection to the ideas around "Candle reactors".
I'll point this paper out to my son if he's not come across it.
Have a nice weekend.