We investigated the size dependent local field enhancement factor (LFEF) of CdSe@Ag and CdSe@ZnSe@Ag core/shell spherical nanoparticles theoretically and numerically within the framework of quasi-static approximation. From the potential distributions in the core, shell(s), and host medium, and using the modified Drude-Sommerfeld model, we separately obtained the expressions for LFEF of core/shell and core/spacer/shell nanocomposites. By changing the sizes of each of the components of the nanocomposites in these expressions, we found that the LFEF of CdSe@Ag increased as the size of the core is decreased. At the same time, the resonance peaks are red shifted in the inner interface and blue shifted in the outer interface of the shell. The result also reveals that whether the shell radius is kept constant or decreased, increasing the core size produces a lower field enhancement factor showing that the core size is a crucial parameter to change the field enhancement factor of the dielectric core and metal shell nanoparticle (NP). When the spacer (ZnSe) is placed between the core (CdSe) and the shell (Ag), the resonance peaks increased with increase in the size of the core which was not observed in the case of the two layered core/shell nanocomposites having the same core and shell sizes. We found that placing the spacer and varying the sizes of the core, spacer, and shell show different effects on the LFEF of the nanocomposite. The possibility of obtaining size dependent LFEF by adjusting the sizes of nanoparticles makes these nanocomposites attractive for applications in nonlinear optics, photocatalysis, and optoelectronics.
We studied the local field enhancement factor (LFEF), absorption, and extinction cross sections of spherical, cylindrical, oblate, and prolate core–shell nanocomposites (NCs) theoretically and numerically using the quasi-static approach. By solving Laplace’s equations, we obtained expressions for the LFEF, polarizability, absorption, and scattering cross sections for each of the core–shell NCs. We found that the LFEF, absorption, and extinction cross section of spherical and cylindrical core–shell NCs possess two peaks whereas oblate and prolate spheroids show three observable peaks. Moreover, the prolate core–shell spheroid shows greater tunability and larger intensity of the LFEF than its corresponding oblate structure. Furthermore, spherical nanoshells are characterized by the higher LFEF than cylindrical and spheroidal core–shells of the same size and composition. When compared, even the smallest value of the LFEF of the spherical core–shell is 11.42 and 10.09 times larger than the biggest values of oblate and prolate core-shells, respectively. The study also indicated that for spherical and cylindrical NCs, the first two peaks of the LFEF and extinction cross sections are achieved at the same corresponding frequencies. Furthermore, all peaks of the extinction cross sections of the prolate spheroid are found to be the lowest while those of the cylindrical peaks are the highest. Where there are an equal number of peaks of different shapes, the peak values are different, showing that shapes of core–shell NCs determine the intensity, the number, and the positions of peaks of the LFEF and optical cross sections. Such NCs are promising for applications in optical sensing, bio-sensing, and electronic devices. Especially, gold coated core–shell spheroids have good potential applications in multi-channel sensing.
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