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One of the main challenges faced by additive manufacturing (AM) is the limited choice of materials, as the majority of the traditional materials are often not designed or optimised for AM process.

 Ongoing research on materials for additive manufacturing

At CfAM, our current materials research is focussed on polymers, metals, metal nanoparticles, semiconductive nanoparticles, 0D/2D materials, glass and biocompatible materials.

Polymers

In the past years, we have been researching advancements to polymers in additive manufacturing at all levels of development. At the fundamental level, we have researched the influence of monomer and polymer chemistry upon the ability to print polymeric and composite structures. This has involved the synthesis of new monomer species that are designed to impart specific surface effects when converted into polymer articles, for example, the ability to prevent the build-up of biofilms. This has also included the synthesis of new monomeric species derived from sustainable sources. Based on this promising initial work, we plan to increase the sustainability and circular economy footprint of AM by making this a key research area for the Centre for Additive Manufacturing (CfAM) going forward.

We have also investigated the use of pre-polymers with defined three-dimensional molecular structures (that is macromeric, comb, star and hyperbranched structures) in formulating AM inks and resins. Use of these polymers, when compared to the typical linear polymers used up to this point, revealed positive enhancements that these more complex structures impart upon the physical and material properties, molecular reactivity and formulation characteristics of the resultant AM inks and resins, as well as useful functional attributes in the final printed polymers. Thus, we generated a new understanding of how carefully selecting combinations of complex polymers and monomers can control functionality alongside inks and resin viscosity, reactivity and flow properties; all critical aspects defining which blends will be processable by AM techniques. This work also revealed the influence that the structure and concentration of these complex polymers had upon the level of cure, the material properties, print fidelity and surface characteristics of the final printed structures; all properties vital for an AM produced device to be fit-for-purpose for adoption in a specific end-use application.

Metal nanoparticles

Ongoing metal nanoparticle research focusses on three main metals: silver, gold and copper. Each of the metal nanoparticle formulations present their own unique challenges and are at different stages of development. They can be used for a range of functional applications as described below.

• Silver nanoparticles (AgNPs): Silver nanoparticles are commonly used to print conductive elements, and we have been using them in applications such as near-field communications (NFCs) for sensors, a variety of connective circuitry, and producing complex 3D architectures out of AgNPs for use within metamaterial and electronic devices, including pillar geometries and strut-based lattices.
• Gold nanoparticles (AuNPs): We have recently developed photocleavable, ligand-functionalised, gold nanoparticles for UV assisted sintering. This eliminated the need for high-temperature sintering which can be damaging for flexible polymer substrates.This breakthrough approach enables in-situ printing and UV-sintering for a continuous manufacturing process and multimaterial printing of complex conductive circuits. Optimised ink formulation combined with multimaterial co-deposition strategies for inkjetting have been allowing us to progress with the printing of multifunctional structures.
• Copper nanoparticles (CuNPs): CuNPs are highly attractive for deposition via inkjet printing due to their lower cost when compared to silver and gold. However, deposition of electrically conductive structures using CuNP inks has so far been challenging due to their tendency to oxidise during printing and post-processing. Initial studies indicate that the size of the CuNPs used and the composition of the ink need to be further optimised to enable their sintering into conductive inks. To facilitate this process, we are exploring the addition of a small amount of AuNPs into the ink formulations. We have also explored the addition of small amounts of AuNPs into the CuNP inks to improve sintering to achieve high conductivity films.

Semiconductive nanoparticles

In recent years, semiconducting nanoparticles (NPs) have been used extensively in optoelectronic devices such as solar cells, photodetectors, and LEDs because they possess a variety of useful optical properties, such as tuneable band gap energies and high photoluminescent quantum yields. The deposition of semiconducting NPs via inkjet printing enables upscaled manufacturing of optoelectronic devices, controlled deposition of different NPs on a single chip/device and enables the fabrication of devices on flexible substrates.

A variety of semiconducting NP inks with bandgap energies tuneable by the nanocrystal size and composition across the ultraviolet, visible, and near-infrared (UV-Vis-NIR) range, including all-inorganic perovskite nanocrystals (NCs), PbS quantum dots (QDs), lanthanide-doped upconverting NPs, and graphene QDs. Printing of these NPs was demonstrated with both water-based inks and inks based on non-polar solvents, and deposition strategies were optimised to achieve thin printed films which maintained their optical properties after printing and post-processing.

These recently developed inks were used to photosensitise graphene field effect transistors to fabricate high performance photodetectors with tuneable responsive range UV-Vis- NIR range. Fully printed photodetectors were also demonstrated on flexible substrates by inkjet printing graphene, AgNPs and semiconductor NPs as a conductive channel, electrical contacts and an optically active layer, respectively.

2D materials

Graphene inks, containing liquid exfoliated graphene flakes and ethyl cellulose dispersed in a mixture of solvents, had been successfully inkjet-printed onto rigid and flexible substrates. Optimised post-deposition strategies have been developed, including thermal annealing and photonic annealing, as confirmed by high conductivity and signatures in micro-Raman spectroscopy. Alongside deposition strategy, formulation improvements of polymeric stabiliser have been achieved by using hyper-branched polymers to promote dispersion of graphene flakes. Ceaseum lead halide perovskite nanocrystals (PeNCs), lanthanide doped upconverting nanoparticles and PbS quantum dots have been formulated into inks suitable for inkjet printing, with promising optoelectronic applications due to their tunable optical properties, narrow emission bandwidth, and high photoluminescence quantum yield.

Glass

In 2023, a step change in AM glass production using silica nanoparticle-loaded photocurable resins has been achieved. Through digital light processing of silica polymer resins followed by post-process debinding and sintering, highly transparent and crack-free microstructured quartz glass has been produced. Novel doping and surface decoration have also been demonstrated to alter the optical properties and induce additional functionality. This enables miniaturised and integrated systems for micro optics and microfluidics applications, paving the way for integrated multifunctional components and benefiting sectors such as energy, pharmaceuticals, quantum technology, automotive and more.