Defect
chemistry in SnO2 is well established for resistive
sensors but remains to be elusive for photoluminescence (PL) sensors.
It demands a comprehensive understanding of the role of cationic and
oxygen defects as well as the creation of abundant such defects to
provide a selective PL signal. To accomplish it, SnO2 quantum
dots (QDs ∼ 2.4 nm) are prepared without a capping agent along
with other dimensions. Then, the relationship of defects with the
blue-emission PL is unfolded by electron energy loss spectroscopy,
lifetime measurements, X-ray absorption, and Raman spectroscopic measurements.
The defects acting as Lewis acid sites are utilized for selective
ammonia detection. Huge enhancements of the obscured blue luminescence
at 2.77 and 2.96 eV from the SnO2 QDs are observed because
of interaction with ammonia. The linear variation of PL intensities
with analyte concentrations and the recovery of the sensor are elaborated
with detection up to 5 ppm. The interplay of defects in SnO2 is further established theoretically for site-specific interactions
with ammonia by density functional theory (DFT) calculations. Thus,
the unique mechanism revealed for the superlative performance of the
PL sensor with uncapped SnO2 QDs provides a novel platform
for defect-engineering-based optoelectronic applications.