Molecular and Polymer Precursor Routes to Manganese-Doped Zinc Orthosilicate Phosphors

The zinc alkoxy(siloxy) complex {Zn[OSi(OtBu)3]2}2 has been prepared and studied as a molecular precursor to zinc orthosilicate, Zn2SiO4. This complex, prepared by reaction of ZnMe2 with HOSi(OtBu)3, has been characterized by X-ray crystallography as a dimer with two four-coordinate zinc centers. It cleanly looses isobutene, tert-butyl alcohol, and HOSi(OtBu)3 at ca. 100 °C to produce Zn2SiO4·SiO2. Reaction of ZnMe2 with (HO)2Si(OtBu)2 affords the hydrocarbon-soluble polymer [ZnOSi(OtBu)2O]n, which is proposed to have a structure consisting of four-coordinate Zn atoms linked by bridging −O(tBuO)Si(OtBu)O− groups. This polymer undergoes a very clean ceramic conversion over 150−250 °C to give the theoretical yield of carbon-free (by infrared spectroscopy and combustion analyses) Zn2SiO4·SiO2. The pyrolytic elimination products consist of water, tert-butyl alcohol, and isobutene. X-ray diffraction studies on the ceramic products show that Zn2SiO4 is the only zinc-containing, crystalline product formed. Thus this precursor approach appears to offer an advantage over sol-gel processes, which tend to produce ZnO as a side product. Hydrocarbon solutions of the polymer were used to fabricate high-quality thin films. Via pyrolysis of [ZnOSi(OtBu)2O]n/[Mn(CH2SiMe3)2]m mixtures, manganese-doped Zn2SiO4·SiO2 materials were synthesized. These materials exhibit two photoluminescence emission bands centered at 535 (major) and 605 nm (minor). The intensity of the green emission at 535 nm, which is responsible for the cathodoluminescence properties of Zn2SiO4:Mn phosphors, is highly dependent on the level of manganese doping. Concentration quenching is observed at higher manganese contents. The observed photoluminescence decay lifetime of approximately 5 ms is typical for Zn2SiO4:Mn phosphors.