Radiation Efficiency of the Spherical Metallic Nanoparticles, Covered with Molecular Adsorbate Layer

The relations for the optical characteristics of the composite nanoparticles of the type “metallic core – molecular aggregate shell” have been obtained in the work. The calculations of the frequency dependencies of the polarizability, absorption and scattering cross-sections, radiation efficiency and size dependencies of the frequencies of the hybrid plasmon-exciton resonances have been performed. An approximate approach to the determination of the size dependencies of the frequencies of the hybrid plasmon-exciton modes, the essence of which is to ignore the relaxation processes in the molecular aggregate shell and in metallic core, has been proposed. It has been demonstrated that the increase in the radius of metallic core results in the decrease in the highest frequency of plasmon-exciton resonance, while the next highest frequency increases. At the same time, the smallest of these frequencies decreases first, and when the content of metallic fraction becomes greater that 50 % of the volume of the entire particle, it begins to increase. The evolution of the maximums of the imaginary part of the polarizability with the change of the geometric parameters of the composite nanoparticle: the radius of metallic core and the thickness of the shell has been observed. It has been established that one of these maximums is situated in the near ultraviolet region, whereas the other two of them are in the visible part of the spectrum. Moreover, it should be pointed out the significant increase in the real and imaginary parts and the module of the polarizability under the increase in the radius of metallic core. The necessity of taking into account both absorption and scattering under the study of the optical properties of nanoparticles of the composition “metallic core – J-aggregate” is proved. It has been shown that the value of radiation efficiency in the optical frequency range is practically independent on the changes of the thickness and material of the shell, while the change of material and radius of metallic core has significant effect on its frequency dependence in the visible part of the spectrum. The feasibility of the practical use of the composite nanoparticles with golden core with the small radius, in which the radiation efficiency is close to one in the entire optical frequency range, has been established.