Doping-free bandgap tunability in FeO nanostructured films.

A tunable bandgap without doping is highly desirable for applications in optoelectronic devices. Herein, we develop a new method which can tune the bandgap without any doping. In the present research, the bandgap of FeO nanostructured films is simply tuned by changing the synthesis temperature. The FeO nanostructured films are synthesized on ITO/glass substrates at temperatures of 1100, 1150, 1200, and 1250 °C using the hot filament metal oxide vapor deposition (HFMOVD) and thermal oxidation techniques. The FeO nanostructured films contain two mixtures of Fe and Fe cations and two trigonal (α) and cubic (γ) phases. The increase of the Fe cations and cubic (γ) phase with the elevated synthesis temperatures lifted the valence band edge, indicating a reduction in the bandgap. The linear bandgap reduction of 0.55 eV without any doping makes the FeO nanostructured films promising materials for applications in bandgap engineering, optoelectronic devices, and energy storage devices.