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. 2022 Aug 11;12(16):2756.
doi: 10.3390/nano12162756.

Studies of the Structure and Optical Properties of BaSrMgWO6 Thin Films Deposited by a Spin-Coating Method

Luciana Punga  1 , Abderrahman Abbassi  2 , Mihaela Toma  1 , Teodor Alupului  1 , Corneliu Doroftei  3 , Marius Dobromir  4 , Daniel Timpu  5 , Florica Doroftei  5 , Laura Hrostea  6 , George G Rusu  1 , Abdelati Razouk  7 , Felicia Iacomi  1
Affiliations

Studies of the Structure and Optical Properties of BaSrMgWO6 Thin Films Deposited by a Spin-Coating Method

Luciana Punga et al. Nanomaterials (Basel). .

Abstract

Highly transparent thin films with the chemical formula BaSrMgWO6 were deposited by spin coating using a solution of nitrates of Ba, Sr, and Mg and ammonium paratungstate in dimethylformamide with a Ba:Sr:Mg:W ratio = 1:1:1:1. XRD, SEM, EDX, and XPS investigations evidenced that annealing at 800 °C for 1 h results in an amorphous structure having a precipitate on its surface, and that supplementary annealing at 850 °C for 45 min forms a nanocrystalline structure and dissolves a portion of the precipitates. A textured double perovskite cubic structure (61.9%) was found, decorated with tetragonal and cubic impurity phases (12.7%), such as BaO2, SrO2, and MgO, and an under-stoichiometric phase (24.4%) with the chemical formula Ba2-(x+y) SrxMgyWO5. From transmittance measurements, the values of the optical band gap were estimated for the amorphous (Egdir = 5.21 eV, Egind = 3.85 eV) and nanocrystalline (Egdir = 4.69 eV, Egind = 3.77 eV) phases. The presence of a lattice disorder was indicated by the high Urbach energy values and weak absorption tail energies. A decrease in their values was observed and attributed to the crystallization process, lattice strain diminution, and cation redistribution.

Keywords: double perovskite; optical properties; structure; thin films; tungstate.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
XRD patterns of thin films deposited by spin coating and annealed at different temperatures: BSMWO−I—black line, 800 °C, 1 h; BSMWO−I—red line, 800 °C, 1 h and 850 °C, 45 min. XRD patterns are compared with three possible impurity phases that may exist. The XRD peaks noted with * and ** may have contributions from the tetragonal BaO2 and SrO2 phases.
Figure 2
Figure 2
SEM images: (a) BSMWO−on quartz substrate; (b) BSMWO−II on quartz substrate; (c) BSMWO−I on (111) Si p substrate; (d) BSMWO−II on (111) Si p substrate.
Figure 3
Figure 3
EDX spectra belonging to 8 different areas of the BSMWO−II SEM image: (a) SEM image; (b) EDX spectra. The investigated area and the corresponding spectra are marked with the same colors.
Figure 4
Figure 4
High-resolution XPS spectra of BSMWO−II thin film: (a) Ba 3d; (b) Sr 3d; (c) Mg 1s; (d) W 4f; (e) O 1s; (f) C 1s. Red line indicates species in cubic coordination in double perovskite structure, blue line indicates surface species, and orange line indicates adsorbed species.
Figure 4
Figure 4
High-resolution XPS spectra of BSMWO−II thin film: (a) Ba 3d; (b) Sr 3d; (c) Mg 1s; (d) W 4f; (e) O 1s; (f) C 1s. Red line indicates species in cubic coordination in double perovskite structure, blue line indicates surface species, and orange line indicates adsorbed species.
Figure 5
Figure 5
Optical properties of BSMWO−I and BSMWO−II thin films: (a) transmittance spectra; (b) absorption coefficient; (c) determination of optical band gap energy for a direct transition; (d) determination of optical band gap energy for an indirect transition; (e) determination of Urbach and UTW energies; (f) thin-film characteristics.
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