Zinc(ii) and cadmium(ii) complexes, [M(iPr2P(X)NC(Y)NC5H10-κ2-X,Y)2] (X and Y = O, S), as single source precursors for metal sulfide thin films

Abstract

Zinc(II) and cadmium(II) complexes of general formula [M(ⁱPr₂P(X)NC(Y)NC₅H₁₀-κ²-X,Y)₂] (M = Zn, X = Y = S (4), X = S, Y = O (5), X = O, Y = S (6), and M= Cd, X = Y = S (7), X = S, Y = O (8), X = O, Y = S (9)) have been synthesized and characterized by using elemental analysis, IR and NMR spectroscopy, and thermogravimetric analysis (TGA). SCXRD molecular structures of complexes 6, 7, and 9 revealed ligand binding to the metal center in a bidentate fashion in an overall distorted tetrahedral coordination geometry. TGA showed that 4–9 decompose in a single step to their corresponding metal sulfides. Aerosol assisted chemical vapor deposition (AACVD) at 450 °C and subsequent annealing at 600 °C were used to examine 4–9 as single source precursors (SSPs) for the deposition of nanostructured metal sulfide thin films (4F–6F unannealed ZnS films and 7F–9F unannealed CdS films). GIXRD confirmed that the resultant films have a nanocrystalline hexagonal structure with the (002) preferential orientation, except for 9F, and the grain size varied from 15 to 24 nm. Scanning electron microscopy and atomic force microscopy (SEM and AFM) images revealed a relatively dense grain columnar structure. The elemental composition and mapping studies showed that the near-stoichiometric MS thin films are homogeneously distributed. The as-deposited ZnS thin films (4F–6F) displayed phosphorus content between 6.4 and 8.2%, while CdS films showed no phosphorus content except for 9F, which showed P 2%. The optical properties of the films such as direct optical energy gap (E_g), and Raman and photoluminescence (PL) spectra displayed some differences related to the employed SSP, being more significant for films obtained from precursors containing the hybrid ligand –P(O)NC(S)– (3), which showed a major phosphorus content (6F and 9F). A sharp excitonic emission at room temperature, as well as multiple Raman overtones, shows the high optical quality of the investigated CdS thin films. The PL spectra of ZnS films showed emissions attributed to the near band edge and different crystalline imperfections such as vacancies and interstitials.