Nanoplasmonics and surface enhanced spectroscopy

Mechano-chemical modification of β-Ga2O3 and β-Ga2O3:Eu micropowders by plasmonic nanoparticles
M. Kushlyk1, D. Slobodzian1, A. Luchechko1, Ya. Shpotiuk1,2, M. Baláž3
1 Ivan Franko National University of Lviv, 79017, 107 Tarnavskoho Str., Ukraine 
2 Institute of Physics, University of Rzeszów, 1, Pigonia str., 35-959 Rzeszów, Poland
3 Institute of Geotechnics SAS, Watsonova 45, 04001 Košice, Slovakia 

In recent years, increasing interest has been given to the synthesis of metal-oxide-semiconductors, such as monoclinic gallium oxide (β-Ga2O3). Especially, β-Ga2O3 semiconductor nanomaterials are attractive candidates as active elements for advanced nanoscale devices due to their unique electronic and optical properties, low effective density, high specific surface area, and shell permeability that are important in many technological applications such as photonics, sensors, solar energy conversion, and electrochemical energy storage, etc. On the other hand, much attention has been paid to the plasmonic effect of metal nanoparticles (NPs) formed in close vicinity to the recombination centres. Combining a thin conversion layer with silver plasmonic nanostructures leads to increased donor absorption and emission efficiency.
The structural, morphological, chemical and optical properties of β-Ga2O3 and β-Ga2O3:Eu (pure and modified by Ag NPs) powder composites were investigated by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy as well as photoluminescence spectroscopy (PL) and Raman spectroscopy. The results show that the properties of obtained β-Ga2O3 composites were strongly influenced by the chamber material and rotation speed. Thus, different crystalline sizes (from 5 um to 300 nm) were obtained, where the bigger particle size show's the greater bulk emission. In the case of β-Ga2O3 powder doped with Eu, an opposite result was obtained. Also, it needs to note, that higher luminescence intensity was obtained for the pure composite prepared in the ZrO2 chamber. It is partially caused by the presence of tungstate and carbon atoms in Ga2O3 powders as well as low content of α-Ga2O3 phase.
It is determined that modification with Ag NPs requires increasing the mechanosynthesis time up to 4 h at a low rotation speed (~300 rpm). The absorption spectra show the existence of two modes of plasmon resonance for silver nanoparticles in the β-Ga2O3 matrix with relatively high intensity. One of them is a quadrupole mode (with λmax near 360-370 nm) and another one is a dipole mode whose maximum is in the range from 450 to 480 nm. It leads to a redshift of the PL band for plasmonically modified Ga2O3 powders in the direction of the plasmon resonance maximum, with 20% of intensity enhancement.