Zinc and cadmium thioamidate complexes: rational design of single-source precursors for the AACVD of ZnS

Abstract

A series of zinc(II) thioamidate complexes [Zn{SC(ⁱPr)NR}₂]_n for R = ⁱPr (n = 2) (2), ^tBu (3) (n = 1), Ph (4) (n = 2) and Cy (5) (n = 2) and one cadmium(II) thioamidate complex [Cd{SC(ⁱPr)N^tBu}₂]₃, (6), were designed and synthesised as single-source precursors for AACVD ZnS and CdS. Solid-state structures of all four zinc(II) compounds revealed distorted tetrahedral or trigonal bipyramidal geometries, with varying tendencies for dimeric association, mediated by {Zn–S} bridging bonds. The thermogravimetric analysis identified the {^tBu} derivertive, 3, as the most promising precursor based on its low decomposition onset (118 °C) and clean conversion to ZnS. This was attributed to the greater availability of β-hydrogen atoms promoting the pyrolysis mechanism. The corresponding cadmium thioamide 6 was found to crystallise as a trimetallic molecule which lacked the thermal stability to be considered viable for AACVD. Hence, 3 was used to deposit ZnS thin films by AACVD at 200–300 °C. Powder X-ray diffraction confirmed phase-pure growth of hexagonal wurtzite ZnS, with approximate crystallite sizes of 15–20 nm. Scanning electron microscopy revealed densely packed spherical nanoclusters. The morphology and crystallinity were most consistent for depositions between 250–300 °C. Energy dispersive X-ray spectroscopy indicated slightly sulfur-deficient stoichiometries.