Nanotechnology is touted as the next logical sequence in technological evolution. This has led to a substantial surge in research activities pertaining to the development and fundamental understanding of processes and assembly at the nanoscale. Both top-down and bottom-up fabrication approaches may be used to realize a range of well-defined nanostructured materials with desirable physical and chemical attributes. Among these, the bottom-up self-assembly process offers the most realistic solution toward the fabrication of next-generation functional materials and devices. Here, we present a comprehensive review on the physical basis behind self-assembly and the processes reported in recent years to direct the assembly of nanoscale functional blocks into hierarchically ordered structures. This paper emphasizes assembly in the synthetic domain as well in the biological domain, underscoring the importance of biomimetic approaches toward novel materials. In particular, two important classes of directed self-assembly, namely, (i) self-assembly among nanoparticle-polymer systems and (ii) external field-guided assembly are highlighted. The spontaneous self-assembling behavior observed in nature that leads to complex, multifunctional, hierarchical structures within biological systems is also discussed in this review. Recent research undertaken to synthesize hierarchically assembled functional materials have underscored the need as well as the benefits harvested in synergistically combining top-down fabrication methods with bottom-up self-assembly.

Nanocrystalline silicon carbide (SiC) thin films were deposited by plasma enhanced chemical vapor deposition technique at different deposition temperatures ${\mathrm{(T}}_d)$ ranging from 80 to 575 °C and different gas flow ratios (GFRs). While diethylsilane was used as the source for the preparation of SiC films, hydrogen, argon and helium were used as dilution gases in different concentrations. The effects of $T_d,$ GFR and dilution gases on the structural and optical properties of these films were investigated using high resolution transmission electron microscope (HRTEM), micro-Raman, Fourier transform infrared (FTIR) and ultraviolet-visible optical absorption techniques. Detailed analysis of the FTIR spectra indicates the onset of formation of SiC nanocrystals embedded in the amorphous matrix of the films deposited at a temperature of 300 °C. The degree of crystallization increases with increasing $T_d$ and the crystalline fraction ${\mathrm{(f}}_c)$ is $\text{65\%±2.2\%}$ at 575 °C. The $f_c$ is the highest for the films deposited with hydrogen dilution in comparison with the films deposited with argon and helium at the same $T_d.$ The Raman spectra also confirm the occurrence of crystallization in these films. The HRTEM measurements confirm the existence of nanocrystallites in the amorphous matrix with a wide variation in the crystallite size from 2 to 10 nm. These results are in reasonable agreement with the FTIR and the micro-Raman analysis. The variation of refractive index (n) with $T_d$ is found to be quite consistent with the structural evolution of these films. The films deposited with high dilution of ${\mathrm{H}}_2$ have large band gap ${\mathrm{(E}}_g)$ and these values vary from 2.6 to 4.47 eV as $T_d$ is increased from 80 to 575 °C. The size dependent shift in the $E_g$ value has also been investigated using effective mass approximation. Thus, the observed large band gap is attributed to the presence of nanocrystallites in the films.