Soluble Xanthate Compounds for the Solution Deposition of Metal Sulfide Thin Films

A series of soluble metal ethylxanthate compounds was synthesised as precursors for metal sulfide thin films to be deposited using solution‐based techniques. Initially, a range of air‐ and moisture‐stable organic ethylxanthate (EtXn) salts were synthesised (cation=Me4N (1), Et4N (2), Pr4N (3), Ph4P (4), guanidinium (5), NMeH3 (6), NMe2H2 (7), NMe3H (8), NH4 (9)). Thermogravimetric analysis (TGA) was used to examine their decomposition profiles, which in turn informed the decision of which counter cation was best suited for inclusion in the metal xanthate compounds. Periodic NMR spectroscopy studies and single‐crystal X‐ray diffraction (SXRD) were used to determine the role protic ammonium counter cations play in the detrimental conversion of xanthates to dithiocarbamate anions. The organic salts 1 and 4 were used to form the metal ethylxanthate compounds (Me4N)[M(EtXn)x] (x=3 for M=Cd (10 Cd), Ni (10 Ni), Zn (10 Zn); x=4 for M=La (11 La)) and (Ph4P)[M(EtXn)3] (M=Cd (12 Cd), Ni (12 Ni), Zn (12 Zn)). Solubility studies on these compounds were performed using a range of solvents to demonstrate the viability of using these compounds for solution‐based deposition methods for thin‐film formation. TGA of the metal xanthate compounds was used to examine their thermal‐decomposition profiles and the product resulting from thermolysis was found to be the respective metal sulfide. In addition, the coproducts of thermal decomposition were analysed by headspace gas‐chromatography mass spectrometry (HS GC–MS) to probe the decomposition mechanism of the precursors. In situ variable‐temperature synchrotron XRD studies on both bulk and thin‐film samples of 10 Cd and 12 Cd were used to examine metal sulfide crystalline phase formation. Decomposition of both precursors was found to give CdS in a hexagonal phase, with the addition of CdCl2 found to aid in increasing the crystallite size during crystallisation. Film studies: A range of metal ethylxanthate compounds that are highly soluble in benign solvents have been demonstrated to thermolyse cleanly to give metal sulfide thin films (see figure). The evolution of the crystalline metal sulfide thin films deposited by solution‐processable methods were examined by using synchrotron‐based in situ variable‐temperature X‐ray diffraction measurements.