Optically modulated resistive switching in BiFeO₃ thin film

Figure S1. XRD patterns of (i) aluminium substrate, (ii) and (iii) represents BFO thin films deposited for 10 and 20 repeated cycles during spin coating. Above mentioned substrate and films are annealed at 350 °C for 3 hour. XRD peaks indicate single phase perovskite structure of BFO with diffraction peaks assigned to (012), (112), (202), (116), (112) and (300) crystal planes [1]. The peaks indicated with star mark in the XRD spectrum are for the Al substrate and found in all the samples. The obvious peak splitting shows that the BFO films are rhombohedral in nature (space group: R3c) with lattice constant of a = b = c = 5.63 Å and α = β = γ = 59.4°, which are in good agreement with literature (JCPDS #20-0169).

Figure S2. Optical absorption spectrum of BFO thin film deposited on glass substrate for 20 repeated spin cycles and annealed at 350 °C for 3 h. The inset shows the plot of (ahv)² as a function of photon energy. The absorption cut-off wavelength is about 587 nm, suggesting that the BFO thin film absorb visible light in the wavelength range of 400–587 nm. The inset shows the plot of (ahv)² as a function of photon energy. According to the classical Tauc's approach, the band gap of the BFO thin film was estimated to be about 2.1 eV [2]. This observed absorption edge of the synthesized film clearly indicates that the synthesized BFO is of semiconducting nature with a band gap about 2.1 eV.

Figure S3. FTIR absorption spectra of thin film of BiFeO₃ subscript gel (a) before and (b) after annealing at 350 °C for 3 h. Green, blue and magenta boxes represents region related to modes of Fe-O, C-N/C-O/NO₃ and H–O–H bonds. The FTIR spectrum of BFO thin film collected on (frequency range from 400 to 3000 cm^-1) sample before annealing show a strong peak at ∼460 cm^-1 due to the Fe–O stretching and bending vibration which confirms formation of metal-oxygen bond². In addition to this, absorption bands such as C–N (1,037 cm^-1) vibration, stretching vibration band of NO₃ ions and carbonate groups appeared at 1320, 914 and 730 cm^-1, respectively [3]. These peaks seen to disappear after annealing the film at 350 °C for 3 h (Fig. S3(b)) indicating decomposition of side-chain groups. Moreover, strong absorptive peaks in the frequency range from 400-600 cm^-1 are described to the Fe-O stretching and O–Fe–O bending vibration, which indicates formation of octahedra FeO₆ of the perovskite structure⁴. The Fe-O bonding shows metal-oxide bonding which describes the characteristics of material like perovskite structure. Thus, FTIR study confirms the formation of perovskite structure of as-prepared BiFeO₃.

Figure S4. (a) SEM image and (b) EDS pattern of BFO thin film on Al substrate. The SEM image show nearly flat crack free BFO film surface. The EDS spectrum of BFO thin film on Al substrate is shown in Fig. S4(b). The spectrum collected on the film show characteristics emission lines for Bi M_αand Fe L_α line at E = 2.422 and 0.705 keV, respectively. Along with this, only two extra emission lines for Al K_α and Pt M_α were also observed (Al-substrate and Pt-a conductive coating on the sample). The EDS data confirms high purity of the synthesized BFO thin film.