Printing glass in the nano

A transparent suspension containing colloidal silica is used to additively manufacture three-dimensional structures and photonic devices of silica glass at the nanoscale.

Glass has always possessed a dual nature, oscillating between a long history of creativity and craftsmanship and compelling technological applications. It is an ancient material, but still widely used in modern industry due to its desirable mechanical properties, chemical inertness, thermal stability associated with a low expansion coefficient, electrical insulation and, most of all, excellent optical transparency. It finds wide applications in chemical and pharmaceutical processes, electronic and electrical technologies, high-temperature operations, light and laser technology, and optical system components. A simple example is its use in everyday products such as in smartphone screens and household equipment, another is its less obvious yet revolutionary use in communications as optical fibres.

The qualities of glass are strongly tied to its manufacturing techniques and each advance in technology can open new opportunities for its uses. Silica glass parts can be manufactured, for example by blowing, moulding and casting, from the melt at substantially higher temperatures than that forconventional soda-lime glass, but their short working window significantly limits the structures that can be obtained. Corning developed its multi-step Vycor process in the 1930s as a way to enable the production of (almost) pure silica components with a wider range of shapes using traditional fabrication technologies¹, but challenges in obtaining high precision, small scale and complex architectures still remain. Additive manufacturing, which has developed rapidly and is becoming mature for plastics, enables the fabrication of parts with an almost unlimited variety of geometries, especially those not obtainable with traditional forming technologies. Much higher processing temperatures are required to form glass than to form plastics, therefore glass made a later entrance in the field. Different additive manufacturing techniques have been developed and demonstrated for the manufacturing of glass components, including melt extrusion², filament extrusion³, paste extrusion⁴, stereolithography⁵, digital light processing^6,7 and two-photon polymerization (2PP)⁸. However, the feature resolution for the complex components printed is still limited to the micrometre scale. Now, writing in Nature Materials, Xiewen Wen and colleagues⁹ report the fabrication of silica glass components with a resolution below 200 nm, as shown in Fig. 1, based on the use of a highly transparent two-photon-polymerizable precursor containing colloidal silica nanoparticles.

Fig. 1: Techniques for additive manufacturing of glass at different length scales, from centimetric to nanometric features, with examples of glass components fabricated with each technique.

Top left, light fixture lens. Top right, a gyroid-shaped object at different stages of the vat photopolymerization process. Bottom far left, an europium-doped phosphate glass sample. Bottom second left, cup containing dyed water, highlighting high-aspect-ratio features and merged printing lines. Bottom middle, a micro-rook with a height of ∼2 mm. Bottom second right and far right, scanning electron microscopy images of an octet truss structure and a microtoroid optical resonator. Credit: top left, Liosdesign; top right, reproduced with permission from ref. ⁷, Springer Nature Ltd. Bottom far left, reproduced with permission from ref. ³, Elsevier; bottom second left, reproduced with permission from ref. ⁴, Wiley; bottom middle, reproduced with permission from ref. ⁸ Wiley; bottom second right and far right, reproduced with permission from ref. ⁹, Springer Nature Ltd.

The possibility of using photocurable suspensions based on colloidal silica⁵ or other photocurable systems^6,7 was previously demonstrated in a vat photopolymerization layer-wise process. Technologies such as stereolithography⁵ and digital light processing^6,7, which employ a selective and then subsequent exposure to a light source, can produce complex components but with features still limited in the range of 25 to 100 μm. Instead of crosslinking a suspension layer-by-layer, by limiting the resolution to the thinness of the layer that can be produced, photopolymerization can be activated in a (small) volume of liquid; this is the case of 2PP technology⁸, whose resolution is ideally limited only by the laser spot size. A volumetric additive manufacturing approach such as 2PP requires a very high transparency of the precursor to the radiation to initiate the reaction, which is not a typical feature that a suspension-based feedstock possesses. Transparency can be achieved by careful refractive index matching, and by controlling the amount of dispersed inorganic powders, which should, however, still be maintained above a certain level in order to avoid excessive shrinkage of the printed part and to allow for full densification upon sintering.

Wen and colleagues were able to achieve nanoscale resolution by using a commercially available polyethylene glycol (PEG)-functionalized colloidal silica with an ~10 nm dimension. By matching the PEG functional groups with those of the photocurable polymers, they enhanced the suspension miscibility, obtaining a high loading of silica nanoparticles. Furthermore, the same refractive index possessed by the two feedstock components led to high transparency and limited scattering. This enabled the 3D printing of complex structures with sub-200-nm resolution (Fig. 1). The selection of the firing temperature (1,300 °C or 1,100 °C) led to the production of either crystalline or amorphous silica. The researchers went one step further and employed their approach to fabricate microtoroid whispering gallery resonators with high quality factors, as well as active photonic devices produced by doping the suspension with photoluminescent rare-earth elements.

Two-photon polymerization has been demonstrated to be a suitable manufacturing approach for the production of specialized components, including metamaterials, photonic crystals, opto- and microfluidics, drug delivery devices, bioscaffolds for cell cultivation, freeform lenses and micro-optics. Indeed, the functionality of these structures is determined both by the architecture of the objects as well as by the material properties. The possibility of fabricating nanoscale three-dimensional parts with arbitrary shapes based on transparent silica glass thus greatly extends the range of applications enabled by 2PP technology. Although Wen and co-workers only presented a proof of concept, the exceptional sensitivity and the portability of whispering gallery resonators is of great interest for the production of biosensors and point-of-care diagnostic tests. A whispering gallery resonator made of erbium-doped silica has been applied for the detection of virus particles in air¹⁰, a promising result with potential application for the prevention and control of epidemics such as the COVID-19 pandemic.

Despite the relevance of the work, challenges still remain in the accurate control of the shape resulting from the sintering process, as viscous flow leads to sagging and undesired deformations, as well as in obtaining full density and defect-free parts. Moreover, the use of a substrate, which is required by the 2PP technology, generates additional issues in terms of thermal expansion mismatch with the sample, as well as in limiting the type of components that can be fabricated. The use of a hybrid additive manufacturing approach, in which 2PP is coupled with digital light processing to obtain free-standing parts¹¹, could provide a solution.