Significance Nanomaterials have been researched and developed in the fields of solar cells, biological detection, sensors, and information storage. However, the interconnection between nanomaterials and external units is limited to simple mechanical contact, and many nanoscale features, such as excellent electrical, optical, and magnetic properties, are not exhibited. The rapid development of nanotechnology has high demands on the joining technology of nanomaterial units to realize complex functional systems. The interconnection of nanomaterials is the basis of nanoscale product integration and will immensely enrich its functionality.

Progress According to the size of the joining materials, if the size is at least in the range 1--100 nm, it is called nanojoining. The essence of nanojoining technology is material interconnection, and conventional joining methods via the force/heat strategy are still applicable in nanojoining. Compared with traditional macro-joining, nanomaterials are melted or interdiffused to obtain effective joints. By using the nanosize effect, the sintering temperature of metallic nanoparticles (NPs) will be much lower than the melting point of the bulk metal, they will be interconnected by sintering at a low temperature, and the metallurgical interface will be formed by diffusion. Surface diffusion is the main sintering mechanism of NPs, while the grain boundary diffusion is the sintering mechanism of large particles.

The metallurgical connection between the metal materials is realized via cold welding without external direct energy input. In situ transmission electron microscopy shows that the joining is almost perfect (Fig. 7). Compared with the traditional joints, the cold-welding joint has the same crystal orientation, strength, and conductivity. For nanowires, the size that can be cold-welded is about 10 nm, whereas that of nanofilms is limited to 2--3 nm.

Laser irradiation is one of the most common joining methods in nanomaterials. This method can avoid the high requirement for mechanical manipulation in cold welding. Surface plasmon heated local nanomaterials, which could achieve cross-scale, cross-material low-damage joining. Owing to surface excitation, the electromagnetic field occurring in the metal nanostructures and the enhanced plasmon contributes to heat and join nanomaterials. In addition to the strong thermal effect of surface plasmon, the electromagnetic field will promote interconnection. If a femtosecond laser with low power density is irradiated, particles will achieve an orderly arrangement. If the laser power density is high, the ends of the nanorod will be arranged under the action of local heat, and the crystal faces will match to realize interconnection.

Numerous studies have been conducted on the interconnection of various metals and nonmetals with the formation of electrical signal connections in the printed electronic products as the main driving force. The interconnection of heterogeneous and homogeneous nanomaterials has the same diffusion mechanism, but the challenge of heterogeneous material interconnection is the lattice matching at the interface. When an ultrafast laser irradiates Ag and Pt NPs, Ag NPs are first melted and interconnected with the surrounding Pt NPs. Ag NPs act as metal solder, and the interface shows good Ag-Pt lattice matching (Fig. 12).

Conclusions and Prospects Nanoscience provides many strategies for building high-performance materials and devices. The bottom-up manufacturing process is conducive to large-scale synthesis, the joining and interconnections, especially heterogeneous nanomaterials, still need further development. The joining between materials should be extended to different systems to ensure the versatility of interconnected nanomaterials and devices and meet the design function requirements. An essential factor in the interconnection of nanomaterials is to precisely control the melting depth to prevent NPs from merging to form a single particle. To avoid excessive damage, space-limited energy input will become necessary. Ultrafast laser-precise irradiation may be an ideal method for joining and interconnection of nanomaterials.