Rational synthesis of F-doped iron oxides on Al2O3(0001) single crystals

Abstract

A plasma enhanced-chemical vapor deposition (PE-CVD) route to Fe₂O₃-based materials on Al₂O₃(0001) single crystals at moderate growth temperatures (200–400 °C) is reported. The use of the fluorinated Fe(hfa)₂TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine) molecular precursor in Ar/O₂ plasmas enabled an in situ F-doping of iron oxide matrices, with a fluorine content tunable as a function of the adopted preparative conditions. Variations of the thermal energy supply enabled control of the system phase composition, resulting in γ-Fe₂O₃ at 200 °C and α-Fe₂O₃ nanostructures at higher deposition temperatures. Notably, at 400 °C the formation of highly oriented α-Fe₂O₃ nanocolumns characterized by an epitaxial relation with the Al₂O₃(0001) substrate was observed. Beside fluorine content, phase composition and nano-organization, even the system optical properties and, in particular, energy gap values, could be tailored by proper modifications of processing parameters.