Novel plasmon resonances of nonstoichiometric alumina

Aluminum monoxide (AlO) and a nonstoichiometric alumina (n-alumina) with its chemical composition similar to that of AlO were proposed theoretically for a promising low-loss plasmonic material. However, they were rarely studied due to the high thermodynamic instability. Recently, n-aluminas with their O/Al atomic ratios close to 1 were found to form on Al nanospheres synthesized by electrical explosion. Although a new bulk plasmon resonance was observed below the excitonic transition energy from the n-aluminas, no investigation into surface plasmon resonance – the key component for plasmonics - was conducted yet. Here I report on surface plasmons of the n-aluminas discovered by spatially-resolved electron energy loss spectroscopy. Energy loss spectra collected from local spots of the n-aluminas are analyzed to reveal that there are two types of n-aluminas; one with two surface plasmons (n-alumina A) and the other with one surface plasmon (n-alumina B). Remarkably, the oscillator strengths of the novel surface plasmons are as high as that of surface plasmon of Al. The electronic band structure proposed for the n-alumina A shows a lossless energy zone. Consequently, current study demonstrates that the n-aluminas have the potential as a new low-loss plasmonic material.