High-Resolution 3D Fabrication of Glass Fiber-Reinforced Polymer Nanocomposite (FRPN) Objects by Two-Photon Direct Laser Writing

This paper reports on the nanofabrication of a fiber-reinforced polymer nanocomposite (FRPN) by two-photon direct laser writing (TP-DLW) using silica nanowires (SiO2 NWs) as nanofillers, since they feature a refractive index very close to that of the photoresist used as a polymeric matrix. This allows for the best resolution offered by the TP-DLW technique, even with high loads of SiO2 NWs, up to 70 wt %. The FRPN presented an increase of approximately 4 times in Young's modulus (8.23 GPa) and nanohardness (120 MPa) when compared to those of the bare photoresist, indicating how the proposed technique is well-suited for applications with higher structural requirements. Moreover, three different printing configurations can be implemented thanks to the use of silicon chips, on which the SiO2 NWs are grown, as fabrication substrates. First, they can be effectively used as an adhesive layer when the laser beam is focused at the interface with the silicon substrate. Second, they can be used as a sacrificial layer, when the laser beam is focused in a plane inside the SiO2 NW layer. Third, only the outer shell of the object is printed so that the SiO2 NW tangle acts as the internal skeleton for the structure being fabricated in the so-called shell and scaffold printing strategy

Additive Manufacturing of Gold Nanostructures Using Nonlinear Photoreduction under Controlled Ionic Diffusion

Multiphoton photoreduction of photosensitive metallic precursors via direct laser writing (DLW) is a promising technique for the synthesis of metallic structures onto solid substrates at the sub-micron scale. DLW triggered by a two photon absorption process is done using a femtosecond NIR laser (lambda = 780 nm), tetrachloroauric acid (HAuCl4) as a gold precursor, and isinglass as a natural hydrogel matrix. The presence of a polymeric, transparent matrix avoids unwanted diffusive processes acting as a network for the metallic nanoparticles. After the writing process, a bath in deionized water removes the gold precursor ions and eliminates the polymer matrix. Different aspects underlying the growth of the gold nanostructures (AuNSs) are here investigated to achieve full control on the size and density of the AuNSs. Writing parameters (laser power, exposure time, and scanning speed) are optimized to control the patterns and the AuNSs size. The influence of a second bath containing Au3+ to further control the size and density of the AuNSs is also investigated, observing that these AuNSs are composed of individual gold nanoparticles (AuNPs) that grow individually. A fine-tuning of these parameters leads to an important improvement of the created structures' quality, with a fine control on size and density of AuNSs.W.D.C. and M.G. acknowledge the support of the CNR Facility Beyond-Nano-Polo di Cosenza. W.D.C. acknowledges MIUR (Ministero dell'Istruzione, dell'Universita e della Ricerca-Italy) for her industrial PhD grant (PONa3_00362). This work was also funded by Ministry of Science, Innovation and Universities (project TEC2017-86102-C2-2-R) and Junta de Andalucia (Research group INNANOMAT, ref. TEP-946). Co-funding from UE is also acknowledged. A.S.d.L. and M.d.l.M. acknowledge Ministry of Science, Innovation and Universities for their Juan de la Cierva Incorporacion postdoctoral fellowships (IJC2019-041128-I, IJCI-2017-31507). SEM and TEM measurements were carried out at the DME-SC-ICyT-ELECMI-UCA. Documen

Polymer nanocomposites for plasmonics: In situ synthesis of gold nanoparticles after additive manufacturing

A series of nanocomposites containing gold nanoparticles (AuNPs) are prepared by stereolithography (SL) by simply adding a precursor (KAuCl4) to a photoresist. A thermal treatment is performed after manufacturing the nanocomposites, triggering the reduction of KAuCl4 into AuNPs in solid state. In this approach, the photopolymerization of the resin and the formation of the AuNPs occur independently, allowing the optimization of these two processes separately. Advanced electron microscopy analyses reveal the distribution, size and morphology of the AuNPs synthesized within the resin, showing the influence of the gold precursor concentration and different thermal treatments. The localized surface plasmon resonance (LSPR) of the AuNPs modifies the optical properties of the 3D-printed nanocomposites, yielding transparent yet colored materials even for concentrations as low as 0.1 wt% KAuCl4. This behavior can be modelled by the Mie theory, correlating the macroscopic properties of the nanocomposites with the individual AuNPs embedded in the resin. The possibility of tuning the LSPR of the AuNPs together with the ability of manufacturing 3D-structures with sub-millimeter precision by SL, paves the way for the design of advanced platforms for plasmonics, such as sensors for surface enhanced Raman spectroscopy9 página

Materials and processes for the optical additive manufacturing of advanced organic/inorganic nanocomposites for the mask-less plating of insulator and semiconductor substrates, and microfluidic devices

The research presented in this doctoral thesis is carried out in the nanotechnology and soft matter frameworks, under the 4.0 Industry paradigm, inspired by the need to find new strategies for the Additive Manufacturing (AM) and to obtain new processable nanocomposites with enhanced properties. The AM technologies allow to build 3D objects with complex geometries by adding layer-upon-layer of material without any mold and permits to fabricate structured objects and microfluidic systems with particular optical and mechanical properties which cannot be easily made with classical Subtractive Manufacturing (SM) techniques. This paves the way to large improvements in optoelectronics, biotechnology, diagnostic or medicine. Moreover, the combined employment of bottom-up and top-down fabrication approaches could lead to important advances in the field of nanotechnology, widening further the possible applications field, permitting high resolution repeatable nanofabrication of 3D complex objects with the possibility of immediate industrial applications. The first AM technique used in this work is Stereolithography (SL), a vat photopolymerization technique that uses UV light to produce objects with resolution in the range 10-100 µm. Here, the novelty consists in adding a metallic precursor (KAuCl4) to a typical photosensitive resin to produce nanocomposites with gold nanoparticles synthesized in situ via photo- and thermal reduction. Nanocomposites produced are rich in gold NPs and have interesting optical and plasmonic properties. Moreover, a fine tuning of the concentration of the gold salt allows the resin polymerization without suffering any inhibition of the gold precursor. A similar approach, taking advantage of the combination with photoreduction of a gold precursor (HAuCl4), can be achieved using a different technique belonging to the vat photopolymerization category, namely the Direct Laser Writing (TP-DLW). This technique exploits the optical, nonlinear multiphoton absorption process, and allows for the fabrication of 3D objects featuring details below the diffraction limit, down to 100 nm or even less. Here, this multi-photon absorption process is exploited to trigger the photo-reduction of the gold precursor. The use of a transparent hydrogel matrix allows for a fine control of the nanoparticles growth on either transparent or opaque substrates, such as glass or silicon, without the need of using masks or molds. An in-depth study on the diffusive process underlying the created nanoparticles growth and a control of the ionic concentration are done to prove the importance of having a polymeric network to hold the created nanoparticles at their place, which enhances the quality of the created nanostructures. The nanofabrication of fiber reinforced polymer nanocomposites by TP-DLW was also demonstrated. For these experiments, the classical glass or silicon substrates were replaced with a silicon substrate on which silica nanowires (SiO2 NWs) have been previously grown. This research allowed to achieve the best resolution offered by the TP-DLW technique, even with high loads of fillers of SiO2 NWs, up to 70 wt%. This was achieved by matching the refractive indices of the SiO2 NWs and of the photoresist used as polymeric matrix. These nanocomposite materials presented a noticeable improvement of nano-hardness and elastic modulus when compared to the pristine photoresist, indicating how the proposed technique is well-suited for nano-applications with higher structural requirements, as in advanced microfluidics. A final comparison of the AM technologies used in the thesis is done to elucidate the advantages and disadvantages of each one of these techniques to choose the most efficient, easiest and fastest, depending on the materials to be used or the required resolution

Additive Manufacturing of Gold Nanostructures Using Nonlinear Photoreduction under Controlled Ionic Diffusion

Multiphoton photoreduction of photosensitive metallic precursors via direct laser writing (DLW) is a promising technique for the synthesis of metallic structures onto solid substrates at the sub-micron scale. DLW triggered by a two photon absorption process is done using a femtosecond NIR laser (λ = 780 nm), tetrachloroauric acid (HAuCl4) as a gold precursor, and isinglass as a natural hydrogel matrix. The presence of a polymeric, transparent matrix avoids unwanted diffusive processes acting as a network for the metallic nanoparticles. After the writing process, a bath in deionized water removes the gold precursor ions and eliminates the polymer matrix. Different aspects underlying the growth of the gold nanostructures (AuNSs) are here investigated to achieve full control on the size and density of the AuNSs. Writing parameters (laser power, exposure time, and scanning speed) are optimized to control the patterns and the AuNSs size. The influence of a second bath containing Au3+ to further control the size and density of the AuNSs is also investigated, observing that these AuNSs are composed of individual gold nanoparticles (AuNPs) that grow individually. A fine-tuning of these parameters leads to an important improvement of the created structures’ quality, with a fine control on size and density of AuNSs

Polymer nanocomposites for plasmonics: In situ synthesis of gold nanoparticles after additive manufacturing

A series of nanocomposites containing gold nanoparticles (AuNPs) are prepared by stereolithography (SL) by simply adding a precursor (KAuCl4) to a photoresist. A thermal treatment is performed after manufacturing the nanocomposites, triggering the reduction of KAuCl4 into AuNPs in solid state. In this approach, the photopolymerization of the resin and the formation of the AuNPs occur independently, allowing the optimization of these two processes separately. Advanced electron microscopy analyses reveal the distribution, size and morphology of the AuNPs synthesized within the resin, showing the influence of the gold precursor concentration and different thermal treatments. The localized surface plasmon resonance (LSPR) of the AuNPs modifies the optical properties of the 3D-printed nanocomposites, yielding transparent yet colored materials even for concentrations as low as 0.1 wt% KAuCl4. This behavior can be modelled by the Mie theory, correlating the macroscopic properties of the nanocomposites with the individual AuNPs embedded in the resin. The possibility of tuning the LSPR of the AuNPs together with the ability of manufacturing 3D-structures with sub-millimeter precision by SL, paves the way for the design of advanced platforms for plasmonics, such as sensors for surface enhanced Raman spectroscopy (SERS)