Nanojoining, which is quite new compare to microjoining, is studied mostly on research and university communities. So far, there is still no standard way to define nanojoining. This chapter mainly focuses on nanomaterial. Therefore, the nanojoining will be introduced based on various dimensions of nanobuilding blocks.

Joining of Nanoparticles

Joining of Single Metal Component Nanoparticles

Since nanomaterials have their unique size-dependent properties apart from the bulk material, new joining properties were shown in the nanosystems. Nanoparticles are known as those ultrafine particles sized between 100 and 1 nm. In the bottom-up fabrication of nanostructure and nanodevices, welding of metals at the nanoscale is likely to have a very important role. Au and Ag nanoparticles, as the most typical single metal component nanoparticles, have been widely investigated. Ag nanoparticles, as reported, are able to sinter together at room temperature through a simple process (Wakuda et al. 2007). In that method, bare Ag nanoparticles were produced by removing dodecylamine through rinsing Ag nanoparticle-coated glass substrates in methanol for 10–7,200 s. The Ag nanoparticles started to coalesce as early as 30 s after dipping in the methanol (Fig. 5a, b), and coalesce increased with increasing dipping time (Fig. 5c, d). Clear connections of the coalesced nanoparticles were found in Fig. 5b. After sintering, the resistivity of the Ag nanoparticle film was lowered to 7.3 x 10-7 Ωm. On the other hand, for the purpose of controllable coalesce and stable storage of Ag nanoparticles, different types of materials were coated on the surface of Ag nanoparticles, targeting various sintering temperature. Controllable sintering at 300o C was achieved by coating organic shell on 11 nm Ag nanoparticles (Ide et al. 2005). And the sintering temperature can be tuned to 160o C or even 100o C, when an organic shell of a mixture of Ag2CO3 and citrate was coated on the bare Ag nanoparticles (Hu et al. 2010). These developments are quite promising for flexible electronics packaging on plastic substrates.

Besides thermal sintering, nonthermal processing was also applied in the singlecomponent metal nanoparticle systems. It was reported that, the Au nanoparticles could be welded together with 100 f. laser pulses (Hu et al. 2009). This welding was able to protect the bulk materials by providing welding only on surface with depth in nanoscale. Moreover, mixture of Ag and Pt nanoparticles were easily welded together when irradiated by nanosecond pulsed laser (Mafune´ et al. 2003). As can be seen from Fig. 6, the Au nanoparticles were melted to bridge Pt nanoparticles forming 3D network.

Joining of Multicomponent Nanoparticles

Multicomponent nanoparticles, a big group of nanoparticles, are also involved in the precise control joining in nanorange. As reported, individual CuO nanoparticles were controllably constructed into 3D ordered structure by using polyphenylene dendrimers (G2Td) as template and bridge (Qi et al. 2009). Figure 7a shows the SEM and TEM images of the final polymer-joined 3D ordered structure of CuO nanoparticles (RSA). An RSA having the length of 210 ± 40 nm and the width of 96 ± 30 nm is observed (Fig. 7a). In addition, the TEM image (Fig. 7b) reveals that these RSAs are composed of numerous small primary CuO NPs with an average diameter of 6.2 ± 0.4 nm (Fig. 7c). These primary CuO NPs are separated from each other with an inter-particle distance of ~3 nm that is close to the size of G2Td (Zhang et al. 2000). Noteworthy, irradiation of nanoparticles under energetic particles, electron, for example, is used not only for structure monitoring but also for creating joining in nanorange. The high-energy electron beam of TEM was used as the external force to in situ join the individual CuO nanoparticles and induce structural transformation between CuO and Cu2O (Fig. 8; Qi et al. 2010). Interestingly, the intermediate structural transformation from the coexistence of CuO and Cu2O to pure Cu2O was monitored by the TEM and associated with FFT. The phases (CuO and Cu2O) are proven by indexing and comparing these observed FFT patterns with the typical patterns represented in Fig. 8b', c'. As illustrated in Fig. 9, the phase transformation from CuO to Cu2O under the electron beam irradiation happens gradually; the coexistence of CuO and Cu2O phases is observed in Fig. 9a, a' at exposure time of 44 min. With increasing the irradiation time at intervals of 2 min, the FFT patterns coming from the CuO phase progressively disappear from Fig. 9a'–f'. Finally, the phase is totally transferred into the pure Cu2O phase (Fig. 9f', t = 54 min).

Joining of Nanowires

Joining of Single-Component Metal Nanowire

Nanowires, which are the structure of nanomaterial that have a thickness or diameter constrained to tens of nanometers or even less and an unconstrained length, are one of the most studied 1D nanostructures. It has shown great promise in plenty of applications, including electronics, sensors, optics, and biomedical devices (Shi et al. 2001; Zhan et al. 2005). Single metal component nanowires, such as Au, Ag, and Pt nanowires, were widely investigated in the nanojoining and nanointerconnection.  Besides the common joining method through heating, melting, and solidification, ultrathin Au nanowires were reported to be joined together within seconds by mechanical contact alone at room temperature (Lu et al. 2010). There are several possible welding geometries for nanowires to contact each other, and “head-to-head” and “side-to-side” joining procedures were performed in Fig. 10a, b. A tungsten or gold STM probe was used to manipulate the  movement of one Au nanowire toward another (Fig. 10c). The head-to-head joining orientation was monitored by TEM (Fig. 11). The alignment of the two nanowires was continuously adjusted by the STM probe, until they approached each other head to head (Fig. 11a, b). As shown in Fig. 11c, once the two Au nanowires connected, they were welded instantly within 1.5 s. After welding, the crystal structure at the junction continuously finalize itself till it was in a free standing state when the STM probe was retracted (Fig. 11d). However, this perfect cold welding happened in single crystalline Au nanowires driven by oriented attachment and assisted by surface-atom diffusion, and it is too critical for industrial applications. Thus, other methods about welding nanowires through providing exterior energy are considered to be more operable. Fig. 12a, b shows a setting in the SEM chamber for the head-to-head welding of two Pd nanowires by passage of a welding current (Tohmyoh and Fukui 2009). The joint of the two Pd nanowires was formed by the melting and fusion of the Pt at the tips, driven by the Joule heating provided by the welding current (Fig. 12c, d). Welds driven by Joule heating were also demonstrated in dissimilar metals, such as the formation of heterojunctions through welding Pt nanowires to Au wire (Tohmyoh et al. 2007).

Joining of Multisegmented Metal Nanowires

The fusion and welding manipulated and observed in SEM and TEM display remarkable success in joining nanoparticles and nanowires. However, these methods suffer various inextricable limitations, such as high equipment cost and slow processing ability, which make them less attractive in real industry applications. Thus, new processes, which were inspired by the soldering process in the microjoining, were developed. Multisegmented metal nanowires (such as Sn/Au/Ni) were fabricated. Within which, the low-melting-point soldering materials, such as tin, nickel, and indium, were added as segments in the nanowires and acting as solder to join individual multisegmented metal nanowires. Building nanowire-based device will be very simple, by simply infrared heating or solder reflow. The nanowires soldered together with various structures, as shown in Fig. 13, because of the coalescence of the melted solder and their quick solidification when cooling (Gu et al. 2004; Cui et al. 2009). Besides the randomly dispersed and soldered nanowire networks, large-scale ordered structures were also demonstrated, induced by self-assembly or directed assembly techniques. For example, a magnetic field was used to assist the assembly technique (Hongke et al. 2006). Typically, a piece of patterned silicon substrate with a few drops of ethanol suspension of nanowire on top was placed in a magnetic field for 20–30 min to fabricate the well-orientated nanowires. Moreover the concentration of the nanowires in between the pads was controllable by tuning the concentration of the nanowire suspensions. Therefore, an array of Ni pads would be easily connected by a liner chain of assembled nanowires.

Joining of Carbon Nanotubes

Carbon nanotubes (CNT), which are a series of carbon based nanowires, take a big part and role in the nanojoining of related structure and device fabrication. Similar to metal nanoparticles and nanowires, CNT can be joined by the high-energy particles from TEM (Fig. 14; Terrones et al. 2002). Under electron beam irradiation, the carbon atoms were irradiated and the tube structure was reorganized, so that the two CNT merged together at the overlapping point. High-energy particle can also join CNT together without destroying its structure at the junction; instead solder will be used. As shown in Fig. 15, amorphous carbon was used to solder two CNTs in the SEM (Banhart 2001). CNT strongly attracts hydrocarbon molecules from air, and those insulated hydrocarbons can be easily converted to conductive amorphous carbon under the beam irradiation in SEM chamber. This makes the joining of CNT much simpler and operable.