Dual behavior or coexistence of trapped and free states in reducible rutile TiO2

Excess electron transport is explored at the reduced $\mathrm{Ti}{\mathrm{O}}_{2}$(110) rutile surface through high-resolution electron energy-loss spectroscopy. Changes in line shape, intensity, and energy position of elastic and phonon peaks are associated to changes in charge-carrier and band-gap state excitations. The dielectric modeling points to a defined inward charge profile and to distinct bulk and surface transport properties. The one-to-one correlation between band-gap state intensity and carrier density favors a dual polaronic behavior of charge carriers over the coexistence of trapped and free states.