Research progress of ultrafast laser-induced nanowires joining technology. 超快激光纳米线连接技术研究进展
Significance With the continuous exploration of novel materials, especially nanomaterials, in developing advanced flexible and high-performance micro/nano optical and electrical devices, high-quality nanojoint formation within nanomaterials has become a key issue for device nanofabrication. However, with the restriction of the size and microstructures of nanomaterials, conventional macro-and microjoining technologies cannot achieve highly controlled spatial energy input within the structures, and therefore fail to produce low-damage joints. Optical nanojoining (i.e. plasmonic nanojoining) technology, arising from the surface plasmon resonance generated at the metal-dielectric interface, is advantageous for joining nanomaterials. Specifically, the spatial energy input is confined at the locations with geometric discontinuities, owing to the strong localized plasmonic effect. Therefore, material damage is minimized, even when the entire nanostructure is covered by a large laser beam. This noncontact laser nanojoining technique permits precise and low-damage material interconnection at the nanoscale. Compared with other joining methods, such as nanobrazing and focused ion/electron-beam nanojoining, laser nanojoining can greatly simplify the joining process and reduce the demand for high-precision operation of the energy input. In addition, it is known that the introduction of an ultrafast laser with a pulse duration of femtoseconds or picoseconds can further enhance the electromagnetic field intensity generated by the surface plasmonic effect, which can extend the processed materials selection from metal to oxide/semiconductor. Therefore, ultrafast laser nanojoining can enable heterogeneous material integration, which is of vital importance to the implementation of advanced nanomaterials in micro/nanoelectronic applications.
Progress Since the plasmonic nanowelding of silver nanowires was demonstrated in detail by E. C. Garnett in 2012, the nanojoining of metal nanomaterials, including nanoparticles, nanorods, and nanowires, has been widely studied. Self-limited energy inputs within the nanostructures during the nanojoining process have been observed for low-damage nanojoint formation. Initially, most research work only focused on improving the electrical conduction of the joined metal nanowire networks by using lamps in the visible spectrum. However, because of its low efficiency of energy conversion and high dissipation of incident energy, the lamp was replaced with a laser beam. Based on the processed nanomaterials, laser nanojoining has shown high efficiency and low material damage due to thermal accumulation, even for temperature-and environment-sensitive materials. Therefore, the production rate has been improved by several orders of magnitude, as has the electrical performance. Although laser nanojoining has been used widely in the fabrication of nanowire-based transparent electrodes, the material selection is limited to metals, owing to the low photon-absorption efficiency of dielectric materials (e.g., oxides and semiconductors) under conventional laser-beam irradiation. A. Hu and Y. Zhou proposed applying an ultrafast laser for nanojoining a broad range of materials. Because of the effects of nonlinear photon absorption and intense electromagnetic fields, oxides and semiconductors can be processed accordingly under ultrafast laser irradiation. On this basis, L. C. Lin systematically studied the ultrafast laser nanojoining of materials in different combinations. Specifically, heterogeneous metal and oxide nanowire joining was demonstrated using femtosecond laser irradiation. The as-received heterogeneous nanowire joint shows robust joint strength and improves electrical conduction, further demonstrating the effectiveness of using an ultrafast laser to join metal and oxide materials. Notably, this ultrafast laser nanojoining process is a generic joining technology, in which the joined materials are not limited to metals and oxides, but can also be applied to other metal and dielectric combinations. With the formation of low-damage homogeneous and heterogeneous nanowire joints, the development of nanowires in nanodevices has become possible. Notably, ultrafast laser nanojoining has shown great advantages over the nanosecond laser in the fabrication of transparent electrodes, as the substrate can be protected well during ultrafast laser irradiation (Fig. 10). Further, the robust and stable nanowire joints show various applications in nanoelectronics, including single-nanowire electrical units or even nanowire sensors. However, as summarized, the material combination by ultrafast laser nanojoining is limited presently to metal-metal and metal-oxide/semiconductor applications, which are based on the surface plasmonic effects during laser-matter interaction. Broader combinations (e.g., oxide-oxide) by ultrafast laser nanojoining with low damage have not been studied, as the plasmonic effect no longer exists in such dielectric environments under optical excitation. Therefore, other mechanisms (e.g., nonlinear photon absorption) during laser-matter interaction may be an alternative to extend ultrafast laser nanojoining to dielectric-dielectric material joining.
Conclusion and Prospect Ultrafast laser nanojoining has been used successfully in low-damage nanowire joining with broad material combinations, including metal-metal and metal-oxide/semiconductor. The spatial energy input within the nanowire structures, arising from the localized plasmonic effects, can be confined precisely at the junction area, which greatly simplifies the operation of the laser beam and thus allows mass production of high-quality nanowire joints. By constructing nanoscale homogeneous and heterogeneous joints, ultrafast laser nanojoining can be used not only in fabricating individual functional nanowire devices, but also in scalable material integration, which shows great potential in applications including small-scale additive manufacturing and integrated nanoelectronics manufacturing.