*7.1.3 Sputtering*

The method relies on using a high-energy plasma or gas to generate nanoparticles that travel and strike the surface to form a layer.

This method is based on the use of high-energy plasma or gas to produce nanomaterials that travels and strikes the surface to form the layer. Sputtering is carried out in different ways: DC sputtering, reactive sputtering, RF sputtering, and magnetron sputtering. In this technique, the target surface is bombarded with highly energetic gas ions, resulting in the ejection of surface molecules or small clusters. Sputtering is advantageous because the composition of the deposited nanofilm is the same as the target source.

#### *7.1.4 Laser ablation*

This method is a complex process in which a laser beam is used to remove microscopic material from a target source. This is a method used to produce highly refined nanoparticles whose properties such as size and distribution depend on laser focusing parameters, laser pulse parameters, and the medium used. Recently, pulsed laser ablation in liquid is an emerging technique used in the synthesis of monodispersed colloidal nanoparticles without the use of complex chemicals. Laser ablation in liquid is advantageous in reducing the thermal effect on the pattern source, reducing preparation time, and being environmentally friendly.

#### *7.1.5 Electrospinning*

This technique is used to develop fibers of metals, ceramics, composites, and polymers of a few microns to the nanoscale range by aligning the fibers, thereby reducing Gibb's free energy. Coaxial electrospinning is used to develop ultrathin fibers up to a length of a few centimeters.

## *7.1.6 Etching*

This method is mainly used in nanotechnology to chemically remove material from a sample surface. The two main types of etching are wet etching (liquid chemicals or etchants are used to remove the layer from the sample surface) and dry etching (etchant gases or plasmas are used to remove the layer from the sample surface). Metal nanoparticles generated after etching the metal surface can be converted into usable material.

#### **7.2 Bottom-up method**

The bottom-up method is to synthesize the nanomaterial from atomic or molecular species via various processes. Chemical vapor deposition, sol-gel, solvothermal and hydrothermal methods, and reverse micelle methods are various methods used for nanoparticle preparation.

### *7.2.1 Chemical vapor deposition (CVD)*

It is a widely used bottom-up method to deposit nanomaterials and thin film on a pre-selected substrate. This is a widely used bottom-up method to deposit nanomaterials and thin film on the preselected substrate. In this technique, the chemical reaction takes place between precursor, gas, or vapor and the preselected substrate at high temperatures. This reaction causes the deposition of desired product on the selected surface. This technique provides nanocrystals with high purity, quality, and minimum defects on the substrate but the disadvantage of this technique is its high production cost and the toxicity of gaseous by-products.

## *7.2.2 Sol-gel method*

This method is mainly based on the precursor hydrolysis and polycondensation reactions of the hydrolyzed products resulting in the formation of the polymeric network. The method derives its name from the process by which a liquid precursor in the preparation of nanoparticles is first transformed into a sol and then into a final structural network known as a gel. The sol-gel method is widely used in the preparation of metal oxides such as ZnO, TiO2, SnO2, and WO3 nanoparticles due to their effective control over the shape and size of the nanoparticles.

#### *7.2.3 Solvothermal and hydrothermal method*

In this method nanoparticles are obtained by heterogeneous reaction in a solvent in a closed vessel at high temperature and pressure near its critical point. This method is carried out in an aqueous medium, whereas the solvothermal method is carried out in a non-aqueous medium. Solvothermal and hydrothermal methods are very helpful in engineering nanomaterials such as nanosheets, nanorods, nanospheres and nanowires.

#### *7.2.4 Reverse micelle method*

In oil-in-water microemulsion, the hydrophilic head region of surfactant molecules orients outwards and hydrophobic tails towards the core trapping the oil droplets while in water-in-oil microemulsion the surfactant inverts its orientation and results in the formation of reverse micelles water droplets. The size of the nanoscale water droplets trapped in the core of reverse micelles, known as the "water-pool", can be changed by changing the ratio of water to surfactants. The type of surfactant in reverse micelles helps in the variation of nanoparticle properties depending on their size and morphology.
