For characterization, we synthesized CS with dye molecules, some amount of AuNPs was added and after that, a continuous stirring for 10 hours was done. Finally, the solution of CS and Au NPs werespread over the silicon substrateand left for getting a dry surface in figure 1.
Figure 2, showing scanning electron microscope images, which are typical close packing arrangements. The self-assembly of CS well ordered.In addition, their diameters as 300 nm for CS-Rh6G and 20 nm AuNPs. AuNPs distributed in superlattice plane on CS. Fig.2a showswell-ordered PMMA CS with few defects and fig.2b shows well organizedAuNPs on PCs.Bright tiny particles are AuNPs.
Experiment set-up for laser characteristic
Optical effect was observed by the second harmonicgeneration laser such as Nd:YAG at a repetition rate of 20 kHz and pulse duration of 1ns. The sample was fixed ata circular platformto allowthe detectionofthe luminescence around the particles. A notch filter filtered the collected emission.
The measurements were performedby changing the excitation power from small to large values. The energy transferbetween the dye molecule and metal nanoparticles(Förster resonance energy transfer, FRET) [24, 25]and surface energy transfer (SET) [26-28] was explained using Förster elegant theory.
The value ofE depends on the separation distance {\displaystyle r}r between donor-to-acceptor and varies with an inverse 6th-power law because ofthe dipole-dipole coupling mechanism. R0denotes {\displaystyle R{0}}RrRthe Förster distance of this donor-acceptor pair[29, 30]. However, the conventional FRET technique suffers from many limitations and can be employed only on dye molecules as donor and acceptor. Theenergy-transfer beyond this distance becomes too weak to measure[31, 32]. In this process, dye molecules take part in the resonance energy transfer and affect the optical traits of donor and acceptor molecules.
MNPsare reported to influence the radiative rate of a chromophore [33, 34]. Fluorescence intensity can be enhanced by a modification of the chromophore radiation rate by the AuNPs. Thevariation of the radiative rate of chromophore has already been explained in the context of the coupling of the molecular and nanoparticle dipoles. The rates of decay for radiative and non-radiative decaydependonthe chromophore dipole related to the surface of the particle[35].During the enhancement of laser power, the radiative transition enhanced anda little shift towards the lower side[36]was observed.Timmerman et al. reported the power dependence shift of spectral peaks with different sizes of particles[37].In this paper, laser light interactionwith CS attached with dye molecules, attached with Au-NPsatdifferent powersisstudied.The more is the power of laser; the more energy is absorbed by CS and hence the more is the emissionby the composite. The blue shift in the spectrum is attributed to the absorption at higher energy radiation and emission from the adjacent attached CS and AuNPs.However, for increased power (50 to 275mW) of laser, with Rh 6G dye-doped CS, neither blue shift at high power nor peak broadening was observed (Fig. 4).This effect clearly showstheplasmonic optical effects on CS-Rh6G+AuNPs, peak shift and peak broadening associated withthe attachment of the AuNPswith CS.The peak broadening is recorded to augment first, a reduction after aspecific power of the laser, followed by a saturation[38, 39].
Table 1. Comparativestudy of spontaneous Emission spectrum in colloidal spheres with dye molecules and withoutthe attachment of metal nanoparticles.
Power (X-Axis)
|
FWHM (Y-Axis)
With dye
|
FWHM (Y-Axis)
With Au-NPs
|
50 mW
|
53 nm
|
90 nm
|
75 mW
|
53 nm
|
90 nm
|
100 mW
|
53 nm
|
90 nm
|
125 mW
|
53 nm
|
94 nm
|
150 mW
|
53 nm
|
97 nm
|
175 mW
|
53 nm
|
99 nm
|
200 mW
|
53 nm
|
102 nm
|
225 mW
|
53 nm
|
102 nm
|
250 mW
|
53 nm
|
102 nm
|
275 mW
|
53 nm
|
98 nm
|
In table 1, the variation in FWHM (Y-Axis) with laser power (X-Axis) has been shown. The emission spectrum at different pump powers was recorded, and changes in FWHM of the spectrum was observed.
Colloidal based self-assembly film wasexcited by laser radiation, the probability of radiative transition increases at higher energy edge of the spectrum, creating a blue shift in the recorded spectrum [40], while enhancing the pump power from 50 to 275 mW. It indicates that the laser powers affect the peak broadening and peak shift [41], as has been shown in fig. 3. With the increased pump power, the gain in the proximity of peak emission wavelength increases, attributing to a blue shift towards the maximum of the emission spectrum.This gain, for an inhomogeneous medium, at any wavelength equals to the product of cross-section of stimulated emission and difference of population density, as follows:
CS-RhB+AuNPs are involved in the emission process; hence, the difference in population density is higher for higher pump powers. This will enhance the gain in the proximity of the peak wavelength of the emission, which in turn leads to a relative blue shift in the emission spectrum towards the maximum of emission. The increment in pump power beyond does not produce any further blue shift in the emission spectrum, probably because of gain saturation. In various studies, almost identical relative blueshifts have been observed towards the peak of emission cross-section, while increasing the pump power. Within this study,the CS surface was modified with AuNPs and photoluminescence, higher than without AuNPs was observed (Fig. 5). Gontijo et al. reported that the modification in the density of surface plasmon states is necessary to increase the rate of spontaneous emission andsimultaneously toenhance the quantum efficiency of the semiconductor materials being used in the reaction [38]. The excitation of plasmons can be controlled by surface Plasmon density of states and this configuration is essential to increase the properties of luminescence[42].
The emission spectrum in fig.5a indicates a high PL intensity with and without AuNPsin the CS RH6G surface.This PL enhancement is due to the attachment of AuNPs.Experimental and theoretical compression UV spectrum are shown in fig.5b. Fig. 5d shows, energy level diagram ofCS-Rh 6G AuNPs,excited bythe wavelength of 532 nm.Thelowershift occurs due to the Intersystem crossing (ISC).The peak shifting and peak broadening are attributed tothe plasmonic effect due to AuNPs self-assembly.
Experimental and simulation study of absorption spectrumCS-Rh6G and CS-Rh6G AuNPs (aand b). Simulation studies and experimental data are following each other and show a good fitting. The distinct gold surface plasmonic peak is observed at ~522 nmin gold attached samples(c). Time-resolved fluorescence spectrum of the Rh6G-CS and presence of AuNPs, and energy transfer diagram (d).
The average size of AuNPs was calculated to be 20 nmby SEM. All of the AuNPs were spherically symmetric.Blueshift occurrence is indicative of the capping ofnanoparticles with the stabilization molecule[45]. The plasmonresonance band for gold nanoparticle is not onlydependent onthe particle size but alsothe surrounding medium’s dielectric constant. Whenwe change the stabilization molecule, probably the dielectric constant also changes and therefore, it causes ashift at the band.