Nickle doped ZnO
Doped zinc oxide (ZnO) has been the subject of much attention because of its potential for important applications such as in opto-electronic and luminescent devices heterojunction solar cells and transparent conductors as well as in chemical and gas sensors. It is also an important material for piezoelectric devices surface acoustic waves anti-reflecting coatings etc. This oxide material is of great importance to several applications such as mentioned above and also phototherapy agents, owing to a wide band gap (3.37 eV), large exciton binding energy (60 meV) and semiconductor properties. It is well known that the changes in optical, electrical, and magnetic properties could occur when impurities were added into a wide gap semiconductor, thus doping of a certain amount in to ZnO matrix has become an important route to optimize its optical, electrical, and magnetic performance. It was reported that transition-metal (TM)-doped ZnO would be a good candidate to achieve Curie temperature above the room temperature and great efforts have been devoted to the investigation of magnetic materials. Nickel is an important dopant in these magnetic materials (Sato et al., 2000).
Furthermore, Ni2+ (0.69Å) has the same valence compared to Zn2+ and its radius is close to Zn2+ (0.74Å), so it is very easy for Ni2+ sub-lattice to replace Zn2+ in ZnO lattice. Some researches on Ni doped ZnO have been reported and several results showed that the various properties of ZnO were changed after inserting Ni into ZnO matrix. By doping Ni into ZnO, a composite material with magnetic and optical properties could be obtained. Magnetic material could be used in magnetic therapy and fluorescence material could be applied in phototherapy agents, so the Ni-doped ZnO would be a new material in medical field. The transition metal doped nanostructure is an effective method to adjust the energy levels and surface states of ZnO, which can further introduce changes in its physical and especially optical properties. In addition to the UV excitonic emission peak, ZnO commonly exhibits the visible luminescence at different emission wavelengths due to the intrinsic or extrinsic defects. Until now, zinc oxide with various shapes was prepared by various methods. Out of these methods of ZnO synthesis, we have used a sol-gel chemical synthesis to prepare the nano-particles. However, it is still a great challenge to synthesize ZnO nano-structures doped with the transition metal element using a simple process with a low cost. The solution growth method is an effective approach and has been a very promising route for synthesizing ZnO nano-materials at a low temperature. Therefore, the solution growth sol-gel chemical method‖ is used to prepare undoped ZnO and Ni-doped nano-particles at a low temperature. The high quality nano-crystalline powders of Zn1-xNixO (x = .00, 0.01, 0.03 and 0.05) are successfully synthesized and their structural and optical absorption and compositional properties are investigated. The present synthesis method is reproducible and ensures the large scale production at a low temperature (Wu et al., 2009).
1.7. Bottom-up approach
Bottom-up syntheses involve the assembly of small (generally atomic or molecular) units into the desired structure. The wide variety of approaches aiming the achieving of this goal can be split into three categories: chemical synthesis, self-assembly and positional assembly. In the bottom-up route, nano to microscale patterned structures are assembled using interactions between molecules or colloidal particles. The strategies for the synthesis of nanomaterials using bottom-up approaches involve the assembly of small units, also called nano building blocks (NBBs) into a nanostructure, where the NBBs are arranged according to a well-defined shape and architecture. Self-assembly is a bottom-up production technique in which atoms or molecules arrange themselves into ordered nanoscale structures by physical or chemical interactions between the units (Gates et al., 2005).
Figure 1.4: Schematic diagram of bottom up approach
1.8. top-down approach
Top-down manufacturing involves starting with a larger piece of material and reduce its dimensions by chemical etching, milling or machining, until a nanomaterial is obtained from it by removing material. This can be done by using techniques such as precision engineering and lithography. Top-down lithographic approaches offer arbitrary geometrical designs and good nanometre-level precision and accuracy. Lithography in general involves the patterning of a surface through exposure to light, ions or electrons, and then subsequent etching and deposition of material on to that surface to produce the desired device (Gates et al., 2005).
Figure 1.5: Schematic diagram of Top down approach
1.9. Synthesis methods of ZnO Nanostructures
The synthesis of nanomaterials is a challenging task toady with the control size and structure which is very much significant for the application of nanopatucels in different areas such as catalysis, medicine, elctronis and industry. The synthesis of nanomaterials usually prepared by the following methods.
- Vapor Phase Transport (VPT)
- Chemical Vapor Deposition (CVD)
- Hydrothermal Method
- Thermal Oxidation
- Sol. Gel. Method
- Electrodeposition (ED)
- Precipitation Method
In this study, we were used precipitation method for fabrication of Ni-doped ZnO nanoparticles. This method involves the formation of a solid from a solution or by the reaction of two or more solids during a chemical reaction or by diffusion in a solid. Un-doped and doped ZnO nanoparticles have been synthesized by this method (Goswami et al., 2013).
Following are some important aims of the present study.
- To obtain and characterize Ni-doped ZnO nanoparticles by co-precipitation method.
- To study the surface morphology of the nanoparticles.
- Electrical conductivity measurement is to be carried out by four probe method.