Optical absorption in array of Ge/Al-shell nanoparticles in an Alumina matrix

Vito Despoja,Lovro Basioli,Maja Mičetić,Jordi Sancho Parramon

doi:10.1038/s41598-019-56673-8

Abstract

The absorption spectra in array of Ge, Al and Ge/Al-shell nanoparticles immersed in alumina (Al2O3) matrix is calculated in framework of ab initio macroscopic dielectric model. It is demonstrated that absorption is strongly enhanced when germanium nanospheres are encapsulated by Al-shell. Two absorption peaks, appearing in the spectra, correspond to low energy ω+ and high energy ω− plasmons which lie in visible and ultraviolet frequency range, respectively. It is demonstrated that in Ge/Al-shell composite the ω+ plasmon exists only because quantum confinement effect which provides larger Ge band gap (Δ ~ 1.5 eV) and thus prevent decay of ω+ plasmon to continuum of interband electron-hole excitation in semiconducting core. Absorption in visible frequency range enhances additional 3 times when alumina is replaced by large dielectric constant insulator, such as SiC, and additional 6 times when Ge core is replaced by wide band-gap insulator, such as Si3N4. Strong enhancement of optical absorption in visible frequency range make this composites suitable for optoelectronic application, such as solar cells or light emitting devices. The simulated plasmon peaks are brought in connection with peaks appearing in ellipsometry measurements.

Highlights

Metallic nanoparticles of subwavelength dimensions due to their reduced dimensionality support dipolar collective electronic modes called localized surface plasmon resonances (LSPR) which can be excited by incident electromagnetic field
In this paper we present the theoretical simulation of optical absorption in lattice of Ge/Al-shell nanoparticles immersed in alumina (Al2O3) matrix
The main aim of this research is to investigate the optical absorption of differently designed Ge/Al nanoparticle arrays immersed in various dielectric environments in order to propose the system with the most favorable optical properties in infra-red (IR) ω ~ 0–1.5 eV, visible (VIS) ω ~ 1.5–3.5 eV and ultraviolet ω ~ 3.5–10 eV frequency ranges

Summary

Introduction

Metallic nanoparticles of subwavelength dimensions (considering the wavelength up to UV) due to their reduced dimensionality support dipolar collective electronic modes called localized surface plasmon resonances (LSPR) which can be excited by incident electromagnetic field. The reason is separation of excited electron and hole in core and shell of the nanoparticle, that is caused by its specific structure These materials are excellent candidate for application in high-efficient photovoltaic devices. In absorption spectra dominates two peaks which correspond to low energy ω+ and high energy ω− plasmons which lie in visible and ultraviolet frequency range, respectively The appearance of these peaks closely resembles the excitation of bonding and anti-bonding modes in metal-dielectric core-shell systems as result of hybridization of sphere and cavity like modes[17] It is demonstrated that in Ge/Al-shell composite the ω+ plasmon exists only because quantum confinement effect which provides larger Ge band gap (Δ ~ 1.5 eV) and prevent decay of ω+ plasmon to continuum of interband electron-hole excitation in semiconducting core. The optical absorption in Ge/Al-shell lattice significantly differ from absorption in pure Ge nanoparticles lattice in the same matrix, and depends on the shell thickness, as predicted by theory

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Conclusion