Your question is not clear....specially the decrease. I don't know you what do u mean by the decrease in the question.do u talk about the Intensity? please rephrase your question a bit.

if the size of the nanoparticle is bigger than the Bohr radius (r ~ 2.4 nm) then there is no confinement effects. so the emitted wavelength remains the same...But if the nanoparticle's size is less than 2.4 nm, then a blue shift will be observed. so you will see the wavelength shifted towards blue, smaller value, means higher energy.

Decrease means decrease in energy bandgap

So yes, it is possible due to size reduction. The co-dopants may bring new levels with in the band gap..so it will appears as as low energy bands.