Size dependence of carrier dynamics and carrier multiplication in PbS quantum dots

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The time dynamics of the photoexcited carriers and carrier-multiplication efficiencies in PbS quantum dots (QDs) are investigated. In particular, we report on the carrier dynamics, including carrier multiplication, as a function of QD size and compare them to the bulk value. We show that the intraband 1P -> 1S decay becomes faster for smaller QDs, in agreement with the absence of a phonon bottleneck. Furthermore, as the size of the QDs decreases, the energy threshold for carrier multiplication shifts from the bulk value to higher energies. However, the energy threshold shift is smaller than the band-gap shift and, therefore, for the smallest QDs, the threshold approaches 2.35 E(g), which is close to the theoretical energy conservation limit of twice the band gap. We also show that the carrier-multiplication energy efficiency increases with decreasing QD size. By comparing to theoretical models, our results suggest that impact ionization is sufficient to explain carrier multiplication in QDs.