Figure 2 represents the scanning electron microscopy (SEM) morphologies (left column) and wavelength dependent angular scattering behaviour (right column) of random and periodic Ag nanoparticles and PS spheres. The random Ag nanoparticles in Fig. 2a were fabricated by annealing a 50 nm thick Ag film for 20 min at a temperature of 500 °C in air. The nanoparticle radii range dominantly from 80 nm to 160 nm (see the size distribution in Fig. 3a) with an averaged spacing of 200 nm. The periodic Ag nanoparticles (Fig. 2b) were prepared using nanosphere lithography [11]: a 30 nm thick Ag film was evaporated onto a hexagonally closely packed monolayer of PS spheres with a radius of 450 nm; subsequently the PS spheres were removed in ultrasonic bath and triangular Ag nanoparticles remained; finally spherical Ag nanoparticles formed after annealing at a temperature of 200 °C for 2 h. Due to the template structure of PS nanospheres, the Ag nanoparticles exhibit a hexagonal order at a uniform radius of 50 nm. Fig. 2c shows the closely packed PS spheres used for the formation of periodic Ag particles in Fig. 2b themselves, which constitute the dielectric nanoparticle sample.
As observed in Fig. 2d, random Ag nanoparticles exhibit a strong scattering ability with a pronounced angular scattering range from 50° to 60°. The scattering is quite broadband and almost covers the whole investigated spectrum, which could be correlated to the broad size and shape distribution of the Ag nanoparticles. Further, as indicated in Fig. 2d, there exits a trend of a moderate increase of scattering angles as the wavelength goes up. To simply explain the scattering behaviour of random Ag nanoparticles, Fig. 3b simulates the angular power distribution of a Ag nanoparticle at air/glass interface using the finite element method as implemented in the software COMSOL [12]. To adapt the simulation geometry to the experimental case, a spherical Ag nanoparticle of R = 140 nm radius was cropped off by 20 nm (C) at the substrate interface. Firstly, as shown in Fig. 3b, the large angle scattering ability (a degree beyond 30°) is demonstrated; additionally, the angle corresponding to the large angle scattering peak is increasing as the wavelength increases. This simulation trend is in agreement with the experimental observation of Fig. 2d. In contrast, the periodic Ag nanoparticles (Fig. 2e) exhibit a distinctive scattering feature. It is characterized by a strong scattering zone where scattering angles are increasing from 40° to 70° as the wavelength goes up from 400 nm to 700 nm. We also observe a similar scattering feature in Fig. 2f for the closely packed PS nanospheres. Treating the periodic nanoparticle arrays as line diffraction gratings for PS spheres with the line distance d and considering the refractive index n = 1.5 of the glass substrate, the zero-order diffraction angle α can be obtained by the equation [13]
$$ {d}^{\ast } sin\alpha =\lambda /n $$
(2)
where λ is the wavelength of incident light. The line distance d according to the shortest spacing is set to 1.15 * R, with R being the radius of a PS sphere, and taking into account a finite spacing between the spheres of the order of 15%.
The diffraction angle curve (dashed line) is plotted as a function of wavelength in both Fig. 2e and f. It can be discovered that the zero-order diffraction angle curve fits very well with the scattering feature for the closely packed PS spheres. This suggests that it is the diffraction which determines the strong scattering behaviour for the closely packed PS spheres. Remarkably, the periodic Ag nanoparticles follow the same trend of increasing scattering angles with wavelengths, but shifted to even larger scattering angles. This behaviour can be correlated to the large-angle scattering ability of plasmonic nanoparticles as shown in Fig. 3b, which is well known for individual metal nanoparticles and less pronounced for dielectric ones [14].
