A few years ago, it was claimed that the two-dimensional (2D) feroxyhyte (δ-FeOOH) layer could possess a net magnetic moment and it could be applied for potential spintronics applications because it showed a band gap. However, the exact crystal structure is still unknown. Hereby, we investigate the crystal structure, electronic band structure, and magnetic and optical properties of 2D δ-FeOOH using density functional calculations. On the basis of the experimental observation and dynamical stability calculations, we propose that the 2D δ-FeOOH originates from bulk Fe(OH)2 via oxidation. A perfect antiferromagnetic ground state was observed in the monolayer structure with an indirect band gap of 2.4 eV. On the other hand, the bilayer structure displayed a direct band gap of 0.87 eV, and we obtained a ferrimagnetic state. The net magnetic moment in the bilayer was 1.49 μB per cell. The interlayer distance and film thickness in bilayer δ-FeOOH were 1.68 and 7.37 Å, respectively. This interlayer distance was suppressed to 1.47 Å in a trilayer system, and the band gap of 1.6 eV was found. The trilayer δ-FeOOH had a film thickness of 11.57 Å, and this is comparable to the experimental thickness of 12 Å. To compare with the experimental band gap of 2.2 eV obtained from a UV-visible optical spectrum measurement, we also calculated the absorption spectra, and the onset of the absorption peak in the monolayer, bilayer, and trilayer appeared at 3.2, 2.8, and 2.2 eV, respectively. Overall, considering the magnetic state, optical absorption, and film thickness, we propose that the trilayer structure agrees with the experimentally synthesized structure.
Keywords: 2D material; feroxyhyte; ferrimagnetism; magnetic semiconductor; optical properties.