Comprehensive Band Gap and Electronic Structure Investigations of the Prominent Phosphors M2Si5N8:Eu2+ (M = Ca, Sr, Ba) Determined Using Soft X-ray Spectroscopy and Density Functional Theory

Abstract

The suite of LED phosphors M2Si5N8:Eu2+ (M = Ca, Sr, Ba) emerged as indispensable solid-state light sources for the next-generation lighting industry and display systems due to their unique properties. As such, the structure–property relations underpinning their electronic structures are of prime interest. Their band gaps, electronic structures, optical properties, and energy levels are studied with a combination of soft X-ray spectroscopy and density functional theory (DFT) calculations. A clear trend is found where the tunable band gaps range from 3.70 ± 0.20 to 5.00 ± 0.20 eV (when going from Ba to Sr to Ca), following the predictions of the bonding model. The measurements and calculations are also used to derive a systematic trend between the measured band gaps of M2Si5N8 and are confirmed by our density functional theory calculations employing different exchange correlation functionals. These investigations present a comprehensive picture of the M2Si5N8 system, underlining their broad applicability and importance as one of the preeminent phosphors for LED lighting.