**3. Structures and properties of DNWs**

In order to establish the diverse applications of DNWs, the structure and properties should be elucidated. The properties such as structural stability, mechanical properties, density and compressibility, photon optical mode and electronic structure, thermal conductivity and electrochemical properties play vital role in their applications. Hence, researchers described the experimental and theoretical investigations on the structure and properties of DNWs as follows.

## **3.1. Structural stability of DNWs**

and an ambient pressure. In the meantime, they developed the aggregated diamond nanorods (ADNRs) from C60 by multi-anvil apparatus [61]. Those ADNRs have the diameter of 5–20 nm

**Figure 5.** (a) Bright-field TEM image of a nanocrystalline aggregate with needle-shaped, elongated crystals diamond nanorods. The crystals can be longer than 1 μm, whereby the needle width is only about 20 nm or less; (b) bright-field image shows a close-up of the elongated crystals. The long edges of the crystals are parallel to the (111) plane, and the

needle axes are approximately parallel (211)\*. Reproduced with permission from [60].

(C) DNWs from diamondoids: Similar to carbon nanotubes and fullerenes, diamondoids may also lead to the formation of DNWs. The 1D diamondoid aggregates confined in CNTs directed to form the DNWs via 'face-fused' reaction. However, these transformations of adamantane into DNWs seem to be energetically not feasible. Contrarily, Zhang et al. explored the theoretical and experimental proof for these fusion reactions by diamantane-4,9-dicarboxylic acid transformation to 1D diamond nanowires inside CNTs [62]. In which, the fusion of

Attributed to the applications of DNWs, numerous efforts have been made by the researchers to synthesize them. Among them, wet chemical route seems to be impressive with respect to cost-effectiveness than that of RIE and CVD techniques. But it is also essential to make them with reproducibility and uniformity. To this footpath, recently, our group report the pH-induced electrostatic self-assembly of novel cysteamine functionalized diamond nanoparticles (**ND-Cys**) to evidence hybrid G-DNW growth [63]. Those G-DNWs are highly stable in respective pH buffers, but if more amount of DI-water is added, the longer nanowires (initially at ~100 μm) break into small wires/rods (few microns). At pH 6, the width of G-DNWs ranges between 20 and 800 nm and the length lies between 200 nm and hundreds of microns with respect to dispersion concentration. Wherein, the DNW formation was initiated through electrostatic forces within the partially graphitized ND-Cys particles. Next, those partially graphitized ND-Cys particles and defects/impurity channels were further promoted to form the graphene shells on the surface of DNPs and sandwiched between the diamond cores. These G-DNWs show exceptional conductivity due to the presence of defects and impurity channels. **Figure 6** illustrates the TEM image of those nanowires with defect or impurity channels.

diamantane-4,9-dicarboxylic acid under the confinement of CNTs yields the DNWs.

and have the length of more than 1 μm.

24 Nanowires - Synthesis, Properties and Applications

**2.4. Wet chemical route to synthesis DNWs**

From theoretical investigations, it has been found that dehydrogenated C(111) octahedral nanodiamond surfaces are structurally unstable. However, cuboctahedral structures of nanodiamond may increase the C(100) surface area and become more stable, which also reduce the surface graphitization. In this light, Barnard et al. investigated three kinds of DNWs including dodecahedral, cubic and cylindrical nanowires and found that nanocrystalline diamonds are structurally stable at one dimension [65]. Moreover, they also demonstrate that stability depends on the surface morphology and crystallographic direction of the principal axis of DNWs. In a similar fashion, Tanskanen and coworkers established the structures of polyicosahedral DNWs derived from diamondoids, C20H20, C20@C80H60, and C20@C80@C180H120. For which they have summarized the HOMO-LUMO gaps, and band gaps via B3LYP calculations [66]. Wherein, the C20@C80@C180H120 structures are energetically favored and the DNWs at 110 direction have the lowest strain energies leading to more stability. This has been experimentally proved by the stability of DNRs (at 110 direction) synthesized through hydrogen plasma posttreatment of multiwalled CNTs [58], whereas the DNWs at 100 direction seem to be unstable as reported earlier [67].
