Research Progress on Femtosecond Laser 3D Printing Technology of Inorganic Materials(Invited)
Significance Micro and nano manufacturing is increasingly important in today's information society as the demand for integrated manufacturing increases.In the face of complex micro-nano structure manufacturing requirements in the fields of micro-electromechanics,micro-optoelectronics,and micro-nano optics,the traditional two-dimensional(2D)machining processes such as photolithography and nanoimprinting lithography have some limitations.On the one hand,these two-dimensional machining processes are only capable of manufacturing 2D or 2.5-dimensional(2.5D)structures,making it difficult to process elaborate three-dimensional(3D)structures.And on the other hand,due to their high cost,they are not suitable for small batches and personalized processing needs.Laser 3D printing is a mask-less micro-nano manufacturing method.Laser 3D printing methods represented by two-photon polymerization have been able to achieve high-precision complex three-dimensional structure preparation at the hundreds of nanometers level,which greatly meets the demand for three-dimensional micro-nano fabrication.In order to further apply laser printing to functional electronics and optoelectronics,it is crucial to realize three-dimensional micro-nano fabrication of inorganic functional materials.However,including two-photon polymerization,most laser writing three-dimensional fabrication methods rely on organic components as structural scaffolds.These organic components seriously hinder electrical conduction,making it difficult to apply the fabricated structure to optoelectronics and other devices.Therefore,the usual approach is to use heat treatment,etching or other methods to remove organic components in fabricated structures to increase the proportion of inorganic components.Nevertheless,these treatments will also bring about some serious problems,such as:structural shrinkage,surface quality degradation,oxidation,etc.These structural defects bring huge disadvantages to the application of high-performance inorganic functional structures.Thus,the development of laser printing of inorganic functional materials based on non-polymerization is essential.Over the past two decades or so,laser printing three-dimensional micro-nano fabrication of inorganic materials has mostly focused on photoresist doping with functional precursor molecules or nanoparticles.In recent years,the field has been progressing and researchers have gradually moved away from polymer material systems and developed a series of direct-writing processing methods based on non-polymerized systems,such as photo-induced chemical reduction,nanoparticles assembly induced by photo-induced polarity change,photoexcitation-induced chemical bonding,etc.,laying the foundation for the preparation of three-dimensional micro-nano structures of pure inorganic materials.Although great breakthroughs have been achieved,high-performance device exploitation and heterogeneous fabrication still remain challenges.Therefore,it is very important and necessary to provide an overview of the existing research to guide the future development of this field more reasonably.Progress In this review,we first introduce the inorganic materials printed by two-photon polymerization,including functional precursor molecules and inorganic nanoparticles.It also demonstrates the realization of three-dimensional micro and nanostructure printing of metals,metal oxides,glass,semiconductors,ceramic materials from molecular precursors,and the utilization of nanoparticles to realize the processing of silica,magnetic,luminescent,metal and other structures(Figs.2 and 3).Then,we summarize the laser printing based on non-polymerization methods such as metal structures by photo-induced chemical reduction,silica structures by laser printing hydrogen silsesquioxane(HSQ),silicon structures by laser reduction(3-aminopropyl)trimethoxysilane(APTES)and so on(Fig.4).Meanwhile,based on the photo-induced destabilization and chemical bonding of nanoparticles,we summarize the photo-induced nanoparticle ligand desorption,photoexcitation-induced chemical bonding,and chemical cross-linking between nanoparticles based on bis-azido molecules(Figs.6 and 7).Subsequently,from the types of materials and their applications,we present research related to laser printing on metals,semiconductors,dielectrics,silica and heterogeneous materials,and demonstrate their applications in microelectronics,micro-nano optics,optoelectronics,micromachines,micro-nano robots,etc.(Figs.8-12).In the end,the problems faced and the ongoing research trends in this field are discussed,including high-speed,high-throughput processing and nanoparticle-ordered printing(Fig.13).For example,utilizing femtosecond pulses spatio-temporal focusing or light-sheet 3D micro-nano printing via two-colour two-step absorption can achieve more than 1 mm3/h printing throughput and sub-micrometer characteristic resolution.Separately,utilizing colloidal crystals enables nanoparticle periodic coupling properties to realize physical and chemical properties far beyond the intrinsic properties of nanoparticles.Conclusions and Prospects Laser printing inorganic three-dimensional micro-nano printing provides an amazing mask-free three-dimensional micro-nano printing method,and based on inorganic molecular precursors and a wide variety of nanoparticles,it can achieve high inorganic proportion,high precision,high resolution,and heterogeneous fabrication of complex three-dimensional micro-nano structures.At the same time,it is envisioned that laser printing in high-speed,high-throughput printing methods,as well as laser printing colloidal crystals may far exceed the nanoparticle intrinsic properties in terms of electron transport,catalytic activity,photoemission,and absorption under the conditions of nanoparticle periodic alignment coupling.We expect that it can be widely used in the preparation of micro-nano optics,microelectronics,micro-electromechanics and other devices in the future,and provide a universal manufacturing tool for the development of new materials and devices.
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