3. Methodology
3.4 Data collection technique
A stabilizing agent is used to stabilize the rate of reaction between different chemical compounds. Normally in spray pyrolysis deposition technique, the reaction is expected to occur on the surface of the hot glass substrate. Here, the reacting compounds undergo a pyrolytic decomposition on the substrate surface.
However, in the course of this research, it was observed that a mixture of zinc acetate and selenium dioxide reacted immediately they were mixed together in a beaker. The same effect was observed when magnesium acetate and cadmium acetate were mixed separately with selenium dioxide. It was therefore necessary to stabilize the reaction using a stabilizing agent.
Different stabilizing agents such as citric acid, acetic acid and triethanolamine (TEA) were used but did not result in the formation of good films. Finally, concentrated hydrochloric acid (HCl) was used because it is volatile at the decomposition temperature and produced good and adherent films.
48 3.6 Experimental procedure
Experimental procedure entails the description of the method employed in the deposition process. The experimental procedure for each of the compounds will be explained in the subsections that follow.
3.6.1 Experimental procedure of ZnSe films (0% doping)
A mixture of 3 ml of 0.4 M solution of zinc acetate, 0.6 ml of concentrated hydrochloric acid (HCl) and 3 ml of 0.4 M of selenium dioxide were measured into the solution chamber of the expermimental setup and stirred for about 30 seconds. The solution chamber of the spray pyrolysis setup was connected to the spray nozzle. The precursor solution was sprayed onto the heated glass substrates via a syringe pump at a steady flow rate of 400 μL/spray. The nozzle to substrate distance was optimized at 5.67 mm, applied voltage was optimized at 3.5 kV, while temperature was varied between 300 oC and 400 oC at intervals of 25 oC . This range of temperature was preferred because after series of trials, it was observed that good films were obtainable as from 300 oC. Before each spray, the tube carrying the spray solution to the nozzle is flushed with fresh ethanol to avoid contamination of the solution. After variation of temperature, volume of spray solution was varied at 400 μL/spray, 480 μL/spray, 560 μL/spray, 640 μL/spray and 720 μL/spray respectively while the temperature of the substrate was kept constant at 350 oC.
3.6.2 Experimental procedure for ZnSe:Eu films
2.7 ml of 0.4 M of zinc acetate was measured into a beaker while 0.5 ml of HCl was measured and added. 3 ml of selenium dioxide was also added to the solution. The overall mixture was stirred to allow for homogeneity of the solution. Next, 0.3 ml of Eu2O3 (representing 10%
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volume doping concentration due to the fact that the overall volume of the source of zinc ion is 3 ml) was added to the mixture and stirred continuously for 2 mins. Thereafter, the solution was fed into the solution chamber and eventually sprayed on the hot glass substrate by electrostatic spray pyrolysis deposition technique at a deposition temperature of 375 oC and constant flow rate of 400 μL/spray. This process was repeated for doping concentrations of 15%, 20%, 25% and 30% respectively. Applying similar experimental technique, volume of spray solution was varied at 400 μL/spray, 480 μL/spray, 560 μL/spray, 640 μL/spray and 720 μL/spray while keeping temperature and dopant concentration constant at 375 oC and 20%
respectively. Also while keeping volume of spray solution and doping concentration constant at 400 μL/spray and 10% respectively, temperature was varied between 300 oC and 400 oC at 25 oC intervals. Throughout the entire experimental procedure, applied voltage and nozzle-substrate distance were optimized at 3.5 kV and 5.67 mm respectively.
3.6.3 Experimental procedure for CdSe:Eu films
The initial spray solution of CdSe:Eu comprises 2.55 ml of cadmium acetate dehydrate, 0.4 ml of HCl (for stabilizing the solution) and 3 ml of selenium dioxide. Europium trioxide (15%
volume concentration due to the fact that the overall volume of the source of cadmium ion is 3 ml) which served as the dopant was added to the spray solution. The resulting solution was sprayed onto a hot glass substrate using electrostatic assisted aerosol deposition technique.
The flow rate was kept constant at 400 μL/spray. The CdSe:Eu films were sprayed for 10 mins at a deposition temperature of 300 oC with an applied voltage of 7 kV. The nozzle-substrate distance was optimized at 7.98 mm. Different parameters were varied throughout the entire experimental process. At a constant flow rate of 2400 μL/hr, volume of spray solution at 400 μL/spray, doping percentage of 15% and optimized nozzle-substrate distance of 7.98
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mm, temperature was varied between 300 oC and 380 oC at intervals of 20 oC respectively. In likewise manner, volume of spray solution was varied at 400 μL/spray, 480 μL/spray, 560 μL/spray, 640 μL/spray and 720 μL/spray while other parameters were kept constant. Also keeping other parameters constant, doping concentration was varied between 10% and 30% at intervals of 5%.
3.6.4. Experimental procedure for MgSe:Eu films
1.8 ml of magnesium acetate was measured and put into the precursor solution chamber. 6 ml of selenium dioxide was added to the spray solution and mixed continuously with the aid of a magnetic stirrer hot plate. Thereafter, 0.2 ml of europium trioxide (dopant) representing 10%
volume concentration of dopant (the overall volume of the source of magnesium ion is 2 ml) was added to the spray solution and agitated for ten minutes before being sprayed onto the hot glass substrate at substrate temperature of 350 oC, while the volume of spray solution was maintained at 400 μL/spray. This process was repeated at different substrate temperatures of 300 oC, 325 oC, 375 oC and 400 oC, while the volume of spray solution and dopant concentration were kept constant at 400 μL/spray and 10% respectively.
The process described in the above paragraph was also used in varying the volume of spray solution of the films. Volume of spray solution was varied between 400 μL/spray and 720 μL/spray while keeping temperature and dopant concentration constant at 350 oC and 20%
respectively.
For the variation of dopant concentration, substrate temperature and volume of spray solution were kept constant at 350 oC and 400 μL/spray respectively while dopant concentration was varied between 10% and 50% at a step of 10% apart. The voltage and nozzle-substrate
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distance throughout the entire deposition process were optimized at 5.5 kV and 5.82 mm respectively.