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.