**2. Experimental**

*Zinc Oxide Based Nano Materials and Devices*

**Property Measured value** Crystal structure Hexagonal, wurtzite Molecular weight Zn:65.38, O:16 and ZnO:81.38 Lattice constant a = 3.246 Å, c = 5.207 Å

Cohesive energy Ecoh = 1.89 eV Melting point Tm = 2250 K Heat of fusion 4470 cal/mole Thermal conductivity 25 W/mK at 20°C Bandgap at RT 3.37 eV Refractive index 2.008 Electron and hole effective mass me\* = 0.28, mh\* = 0.59 Dielectric constant ɛo = 8.75, ε∞ = 3.75 Exciton binding energy Eb = 60 meV

Density 5.67 g/cm3

as three dimensional confined systems bridging the gap between bulk materials and molecular compounds. A variety of techniques have been employed for the synthesis of ZnO nanoparticles such as sol-gel synthesis [17], the hydrothermal method [18], the solution combustion method [19] and solid state reactions [20]. Among these, the combustion technique is noteworthy as a fast method to synthesize nanocrystalline materials in as-synthesized form with large surface area without the further need of heat treatment. Nanocrystalline oxides are produced through the redox reaction between an oxidizer containing the metal precursor and anorganic fuel at a moderately low initiation temperature of around 350–600°C within a few minutes [21]. The main advantage of this method is that the high temperature of the exothermic reaction assures high purity and well crystallized powder. In combustion synthesis, the type of fuel and the fuel to oxidizer ratio (F/O) play critical roles in influencing the nature of combustion reaction and the flame temperature. Selection of a suitable fuel and the F/O ratio influences the combustion process and the properties of the product. The F/O ratio of unity is known to produce highest exothermicity with complete combustion. An arbitrary ratio of fuel to oxidizer (F/O—1) sometimes leads to formation of intermediate phases raw materials in the final product [22]. In this regard, various fuels have been tested to synthesize nanocrystalline ZnO. Sousa et al. [23] used metallic nitrate and urea to synthesis ZnO nanopowder with a size about 400–500 nm for various applications. Hwang et al*.* [24] worked on ZnO nanopowder synthesized by a combustion method with glycine

or 4.21 × 1019 ZnO molecules/mm3

as a fuel and metal nitrate mixed in a stoichiometric ratio.

In the present work, we report the synthesis of nanocrystalline ZnO powders by combustion technique using new, eco-friendly and cost-effective organic fuels as urea, glycine and citric acid. The effect of fuel in different ratio of two fuels combinations on the properties of the final product has been studied. The structure and luminescence properties of ZnO nanoparticles are also being studied in

**16**

this work.

**Table 1.**

*Basic properties of ZnO [12].*
