Table of Contents
- EXPERIMENTAL DETAILS
In the present paper, the influence of 0.1%, 0.2% and 0.3% Ag doping on the photoluminescence property of CdSe-PVA nanocomposite have been studied. Obtained results confirm that with increasing Ag concentration photoluminescence intensity decreases. Photoluminescence quenching mechanism has been explored through Stern-Volmer equation and Langmuir binding isotherm. Stern-Volmer plot exhibits linear relation only upto 0.2%Ag doped CdSe-PVA nanocomposite, afterwards it shows downward curvature. This indicates the surface covering of luminescent CdSe nanoparticles by Ag ions. The calculated value of Stern-Volmer quenching constant is 1.61 × 104 M-1. Langmuir binding isotherm results reveal the binding of more than one Ag ion to the surface of CdSe nanoparticles with increasing Ag concentration.
Photoluminescence property of polymer nanocomposites (PNCs) is gaining great scientific importance due to its applications in various fields such as luminescent screens, light emitting diodes, transducers and optical sensors. PNCs offer excellent optical emission properties such as narrow emission spectra and high photo-stability. Moreover, the emission wavelength of PNCs can be tuned by controlling the particle size during synthesis process. In PNCs, the properties of both nanoparticle filler and polymer matrix combine to generate novel functional materials. As compared to organic dyes such a Rhodamine 6G, II-IV semiconductor quantum dots cover a wide range of light emission spectra from 400 to 1400 nm and are about 20 times more brighter. Metallic doping into PNCs also provides a way to engineer its optical properties. Recently, John et al. had reported the enhanced luminescence properties of ZnO-Poly(2-ethyl 2-oxazoline) polymer nanocomposite doped with Zirconium and Cobalt. Baibarac et al. had demonstrated the photoluminescence quenching of polyaniline due to the incorporation of semiconducting and metallic tubes enriched single-walled carbon nanotubes. Hsieh et al. have also reported that with the metal doping, the PL emission intensity can increase or decrease depending on the size of the semiconductor nanoparticle and the separation between the metal and semiconductor nanoparticle. They demonstrated that the strong local electromagnetic field due to Surface Plasmon waves of metal nanoparticles results in effective semiconductor excitation process. Present work demonstrates the effect of Ag ion doping on the photoluminescence property of CdSe-PVA nanocomposite.
CdSe-PVA and Ag(x):CdSe-PVA (x = 0.1%, 0.2%, 0.3%) nanocomposites were prepared by utilizing in-situ chemical synthesis route as reported in our previous work . For photoluminescence measurements, Shimadzu Spectro-fluorophotometer RF-5301PC spectrometer was used. All the PL measurements were carried out at solution samples and at room temperature.X-Ray Diffraction spectra of the Ag(x):CdSe-PVA nanocomposites confirms the hexagonal wurtzite structure of the PNCs.
The average crystallite sizes calculated from XRD results was 10.4, 12.8 and 17.1 nm for 0.1%, 0.2% and 0.3% Ag doped CdSe-PVA PNCs. With increasing Ag+ ion concentration, crystallite size increases due to the accumulation of Ag clusters on the surface of CdSe nanoparticles. PL emission spectra of Ag(x):CdSe-PVA NCs was recorded at an excitation wavelength of 380 nm. PL emission peak for CdSe-PVA NC was at 585 nm. PL emission peak has been found to be shifted to 589, 595 and 597 nm for Ag(0.1, 0.2, 0.3)%:CdSe-PVA nanocomposites, respectively. The red shift in the photoluminescence emission peak in the presence of Ag+ may be due to the formation of Ag2Se nanoparticles on the surface of CdSe due to the cation exchange reaction. From the plot, it is clear that the emission is quenched with the increase in Ag+ ion concentration. It could be explained due to efficient electron transfer from semiconductors to metal. Nikoobakht et al. also showed that electron transfer from Au nanoparticles to CdSe QD is dominant when they are located close enough to quench the CdSe emission. Thus, similar transfer of electron can be expected between Ag and CdSe affecting the photoluminescence properties. The dependence of photoluminescence intensity on Ag+ ion doping concentration. The PL intensity shows the exponential decreasing behaviour with the increase of Ag+ ion concentration. The mechanism of photoluminescence quenching of CdSe-PVA by Ag+ ions has been studied in detail by using Stern-Volmer equation and Langmuir binding isotherm. Stern-Volmer equation describes the dynamic quenching of PL intensity when the luminescent nanoparticles return to the ground state without emission of a photon on contact with the quencher. The relationship between the PL intensity and quencher ion concentration has been described by the following equation,
o =1+ (1)
where, I and Io are the PL intensities of CdSe-PVA NC in the presence and absence of Ag quencher, respectively. [Q] is the quencher ion concentration and KSV is the Stern-Volmer quenching constant. The Stern-Volmer plot describing the Io/I as a function of Ag+ ion concentration with Io/I = 1 intercept. The Stern-Volmer plot follows linear relation upto 0.2%Ag:CdSe-PVA but after that the curve shows downward curvature.
This indicates that with the increase in Ag concentration, the Ag ions cover the surface of luminescent CdSe nanoparticles thus decreasing or quenching the photoluminescence intensity of the nanocomposites. From the plot, the calculated value of KSV is 1.61 × 104 M-1.To explore the nature of the interaction between Ag and CdSe nanoparticles in more detail, Langmuir binding isotherm has been studied. This isotherm assumed that the surface of nanoparticles consists of fixed number of adsorption sites with monolayer adsorption of molecules on the surface. The relationship between the quencher ion concentration and luminescence intensity is given by the following equation,
[ ] [ ]
where, I and Io are luminescence intensities of the nanoparticles at a given quencher ion concentration and in quencher ion free solution, respectively whereas B is the binding constant. The Langmuir isotherm is feasible, if a plot of [Q]/I as a function of [Q] is linear. The plot of [Ag+] /I vs. [Ag+] for Langmuir binding isotherm. The plot shows the non-Langmuirian behavior with cubic polynomial fit. This suggests that there is a probability of binding more than one Ag ion to the CdSe surface. Thus, based on the above discussion, we could conclude that at given Ag+ ion concentration, binding of more than one Ag ions to the surface of CdSe nanoparticles or the aggregation of nanoparticles result in decrease of photoluminescence intensity. The systematic representation of enhancement and quenching of photoluminescence emission. The enhancement in emission intensity takes place due to the effective excitation of CdSe nanoparticles by the Surface Plasmon waves of Ag whereas the quenching occurs due to electrons transfer from the CdSe nanoparticles to the Ag nanoparticles.
Clearly, it is of scientific and technological importance to understand the effect of Ag ion doping on the photoluminescence properties of CdSe-PVA nanocomposite. The luminescence quenching of CdSe-PVA nanocomposites by Ag doping could be employed as an optical sensor for the detection of Ag+ ions in the aqueous solution.
This work reveals the effect of Ag metal ion doping on the photoluminescence property of a chemically synthesized CdSe-PVA nanocomposite. With increasing Ag ion concentration, the photoluminescence intensity decreases due to the efficient electron transfer from the semiconductor to metal nanoparticles. Quenching mechanism of the PNCs has been studied by using Stern-Volmer equation and Langmuir binding isotherm. Obtained results confirm that the metal ion doping concentration and the separation between the semiconductor and dopant metal have a significant effect on the photoluminescence properties of the PNCs.