Surface- and volume-related excitation of Eu-doped nanocrystalline GaN powders

The influence of grain sizes on the excitation mechanism of Eu3+ ions doped into GaN powder has been investigated by photoluminescence and photoluminescence excitation spectroscopy. It has been found that depending on the grain sizes, different dopants excitation mechanisms can dominate. In the case of nanograins, where the europium related emission appears at ∼612 nm, it has been concluded that the surface plays a very important role. In this case, the dominant excitation of Eu3+ ions is via the local charge transfer from the surrounding oxygen atoms adsorbed post growth at the GaN nanocrystals surface. This mechanism is not efficient for GaN micro grains, where the europium related emission appears at ∼625 nm, due to small surface-to-volume ratio. In this case it has been concluded that the oxygen atoms at the GaN surface play a negligible role in the total Eu-related emission and the main contribution to Eu excitation comes from the energy transfer from the GAN matrix.