3.1.Theoretical simulation results
Theoretically simulated reflectance curves of the SPR phenomenon are obtained by varying the counting of layers as depicted in Fig. 1, i.e. prism/Au/air (black curve), prism/Au/SnO2/air (red curve), and prism/Au/SnO2/PPy/air (blue curve). The angle of resonance moves towards higher values when the counting of layers increases from two (SnO2/air interface) to three (SnO2/PPy/air interface).
3.1.1.Variation of the thickness of SnO2 :
The theoretical simulation results by varying the thickness of SnO2 are presented in Fig. 2.
Resonance angle and minimum reflectance are the coordinates on the x and y-axis respectively for minima in the SPR reflectance curve. There is a continuous increase in resonance angle observed for an increase in thickness of SnO2 where minimum reflectance decreases.
Figure 3 confirms that for a width of 100 nm of SnO2, the reflectance curves of SPR, have the lowest value of Full Wave Half Maximum (FWHM), hence, this is the required value of optimized width of SnO2 layer for optimum functioning.
3.1.2.Variation of the width of the Polypyrrole conducting polymer layer :
The simulation results of the variation in the width of the polypyrrole layer of the prism/Au/SnO2/PPy/air interface are depicted in Fig. 4.
As seen from Fig. 4, The angle of resonance increases with the increase in the thickness of the polypyrrole layer, while the minimum reflectance decrease with the thickness of the polymer polypyrrole layer.
Figure 5, shows the full-wave half maximum (FWHM). It is observed that the FWHM, first increases with the thickness of the polypyrrole layer, reaches a maximum then decreases to a minimum, and again increases with the thickness. The minimum value of FWHM is obtained for 300 nm polypyrrole layer thickness.
3.2 Experimental studies
Figure 6 represents the static mode of measurement of the reflectance curves as recorded for prism/gold/SnO2/PPy multi-layer interfaces when exposed with the varying concentration of NH3 molecules (from 0 to 200 ppm). From Fig. 6, there is a clear indication of the fact that a continuous shift in the minimum resonance angle, θSPR, to the higher angle values, varying from 40.7° to 51.1°, which is due to the variation of the refractive index of SnO2 /PPy nanolayer interface having adsorbed NH3 molecules, when the variation of concentration of inserted gas occurs. This observation can be observed as shown in Fig. 7, when the corresponding graphs of the resonance angle, minimum reflectance, and FWHM, respectively, versus varying concentration of the ammonia gas, is plotted separately.
From Fig. 7, it is observed that there is a sharp increase of values of resonance angle (θSPR) and Minimum Reflectance value (Rmin) for the prism/gold/Tin oxide/Polypyrrole/NH3 adsorbed nanolayer sensing device with the varying concentration of ammonia(NH3) molecules from 0 to 200 ppm. The obtained data of Reflectance values concerning change in the angle of resonance may be the effect of the variation of the effective refractive index of Tin oxide/Polypyrrole interface due to the adsorption of ammonia (NH3)molecules. As evident from Figs. 7 (a) and (b), there is a linear increase in the angle of resonance (θSPR) and the minimum value of reflectance (Rmin) as the concentration reaches 10 ppm. With a further increase in concentration, there is saturation observed for the curves of the angle of resonance and the minimum value of reflectance, respectively. The reason for this observation may be the fact that with a higher concentration of NH3 gas, the absorption of more ammonia molecules does not produce any appreciable variation in the value of the refractive index, hence there is non-linear variation (saturation) of the values of, θSPR and Rmin. Further, the FWHM of the reflectance curves of the sensing system also behaves similar to the behavior of,θSPR and Rmin (Fig. 7 (c)), till 10 ppm concentration.
Tin oxide is an n-type semiconductor, so it gets reduced when a reducing gas, like ammonia, gets adsorbed on its surfaces. Now there is a transfer of an electron from the adsorbed oxygen molecules, hence releasing the trapped electron to the tin oxide semiconductor layer. This, in turn, increases the conductivity of the material which corresponding changes the refractive index of the tin oxide nanolayer.
This change in the effective refractive index of the ammonia adsorbed tin oxide/polypyrrole nanocomposite layer may occur due to two reasons:
(i) Variation of the refractive index of the bulk media from air to the higher refractive index of the ammonia gaseous molecules, and
(ii) Variation of the refractive index of the gas sensing layer of Tin oxide (SnO2)/Polypyrrole nanolayer having adsorbed ammonia molecules on its interface.
The curve fitting of the theoretically calculated values of the refractive indices from the Fresnel coefficients with the experimentally observed reflectance curves data values is done. The variation of the effective refractive index of the gas sensing device of tin oxide/polypyrrole interface having adsorbed ammonia molecules with the changing concentration value is depicted in Fig. 8 (a) (the calibration curve). As observed earlier, The value of the refractive index increases, almost in a linear fashion, to 10 ppm of the concentration, as clearly indicated by Fig. 8(b). A linear fit of the curve is done for the calculation of the Sensitivity by finding the slope of the line from the calibration curve of Fig. 8(b) and the value obtained is 4.5x10− 3 RIU/ppm.
The transient mode of measurement curves are shown in Fig. 9 (a) for the prism/gold/Tin oxide/Polypyrrole/ammonia adsorbed sensing device. As depicted in the graph, at a fixed angle of incidence, θ = 40.7°, the gas sensing device show step like variation of the effective index of refraction with the variation of the concentration of ammonia gas from 0 to 200 ppm. The variation in the reflectance of the sensing system is measured using a CCD. The sensor shows a continuous increase in the response (change in reflectance) with increasing concentration of NH3 gas from 1 to 200 ppm at room temperature (Fig. 9 (a)). The calibration curve of the SPR sensor is also plotted as shown in Fig. 9 (b). Figure 9 (b) shows a linear increase in the response i.e. change in reflectance with an increase in the concentration of NH3 gas over the range of 1 to 200 ppm. The calculated value of the sensitivity of the ammonia gas sensing device is 0.202 /ppm with varying concentrations from 1 ppm to 200 ppm (Fig. 9 (b)). So, the obtained values of sensitivity pave the way to develop an efficient ammonia gas sensor based on the surface plasmon resonance technique.