**4. Summary and future prospective**

One-dimensional ultrathin transition metal hydroxides/oxide nanowires are ideal building blocks for the miniaturization of next-generation high-performance integrated circuits (ICs), nanoelectronics and optoelectronic devices due to their quantum confinement and high aspect ratio. Although, we have witnessed the rapid progress and significant achievements of TMOS nanowire fabrication, still there are certain issues that need to be addressed.

1.Lack of better understanding of the guiding principles that control the size and shape of anisotropic nanowires.

It is crucial to develop more efficient, greener, scalable, and reproducible synthesis methods that can control the size of the nanowires. Aggregation of ultrathin nanowires is the major challenge of most available solution-based synthesis methods that are used to fabricate metal hydroxides/oxide nanowires. Fabrication of single ultrathin nanowires using solution-based strategies has not been studied for efficient and cost-effective devices. The controlled synthesis of ultrathin transition metal hydroxides/oxide nanowires is still in the preliminary stage and the guiding principles that control the size and shape are poorly understood and rarely explored. The existing crystal growth kinetic models have explained the crystal growth of nanostructures based on their nanoparticle diameter changes with time. These kinetic models cannot explain the crystal growth of one-dimensional nanostructures along certain specific directions. The attempts describing the kinetics of OA-based elongation of 1D nanostructures are still untapped and understudied. Therefore, exploration on crystal growth processes to generate effective novel kinetic models from experimental data will provide the prospects for the preparation of size and shape-controlled 1D nanostructures.

2.The insufficient yield and quality of NWs for device fabrication.

In general, each synthetic strategy has its advantages and disadvantages. Some synthesis gives high purity ultrathin nanowires, but the yield is poor. Many laboratoryscale synthesis routes and device fabrication are inappropriate for real commercial device fabrication due to this reason. It is challenging to develop strategies that have all the cost-effective features of commercialization. Therefore, more attention needs to pay to the different types of cost-effective fabrication methods of metal hydroxides/oxide nanowires that yield high quality and yield.

3.Lack of sufficient work for optimizing the fabrication process of NWs with desired electronic and optical properties for commercialization.

Optimization of synthesis routes to reduce the reaction time, better yield with desired dimensions of nanowires and their properties is essential to maximize the performance efficiency of NWs based devices. This is the main reason for the inappropriateness of laboratory-scale nanodevices for practical applications. The performance of optoelectronic devices can be improved by tuning the composition, dimensions, and crystal structure of TMOS NWs. These optimization studies are rarely reported for applications and this area can certainly strengthen by having strong collaborations between industries/entrepreneurs and scientists. When exploring the electronic properties of TMOS NWs, understanding their bandgap, electronic structure, electrical conductivity, dielectric constants and electron and hole mobilities are important to accelerate miniaturization and high-density integration of components of the next decade electronic devices. The factors such as light absorption, regeneration and recombination processes and charge carrier mobility of TMOS NWs are necessary to critically evaluate for future optical devices.

#### **5. Conclusions**

The controlled synthesis of anisotropic one-dimensional TMOS nanowires with versatile properties is most fascinating for different applications due to their superior performances based on quantum confinement effects and high aspect ratio. Their ultrathin TMOS nanowires have been attracted recently, however, many nanowires synthesis strategies are not sustainably produced. The fabrication of optoelectronic devices from ultrathin TMOS nanowires is clearly in its infancy. Currently, it is essential to conduct fundamental research on greener synthesis of ultrathin TMOS NWs, and the mechanisms to control the size and shape of 1D nanowires, as well as an evaluation of the optical and electrical properties of these NWs for industrial applications. Augmenting the OA crystal growth mechanism, we successfully fabricated scalable and reproducible sub 6 nm Cu(OH)2 NWs and CuO NWs using a greener sol–gel hydrolysis followed by directed self-assembly and crystal growth of Cu(OH)2 nanocrystals for potential electronic and optical applications. As TMOS NWs are excellent candidates for next-generation optoelectronic devices, their optical and electrical properties need to control by manipulating the fabrication conditions and fundamental understanding of controlling factors that can be optimally designed.

#### **Acknowledgements**

Authors acknowledge the Joint School of Nanoscience and Nanoengineering, a member of the South-eastern Nanotechnology Infrastructure Corridor (SENIC) and National Nanotechnology Coordinated Infrastructure (NNCI), supported by the NSF (Grant ECCS-1542174).

*Ultrathin Metal Hydroxide/Oxide Nanowires: Crystal Growth, Self-Assembly, and Fabrication… DOI: http://dx.doi.org/10.5772/intechopen.101117*
