Ultrafast Transient Absorption Spectroscopy of HgTe Quantum Dots

Bulk HgTe is a semi-metal with a small negative band gap (room temperature Eg = -0.14 eV 1) and thus the energies of transitions in HgTe colloidal quantum dots (CQD) are dominated by quantum confinement, which is determined by CQD diameter. This allows the absorption edge of HgTe CQD to be size-tuned across the near-infrared enabling it, for instance, to be optimised for exploitation of the solar spectrum. Recent theoretical work2 has also shown that the process of multiple exciton generation (MEG) should be particularly efficient in semi-metal CQD. MEG is a process by which multiple excitons can be formed following the absorption of a single photon, instead of the usual one3. The additional excitons can contribute to photocurrent and thus improve the efficiency of solar cells based on CQD3. However, the assessment of MEG efficiency requires an understanding of multi-exciton dynamics and interactions and so here we present for the first time a study of these in HgTe CQD using ultrafast transient absorption spectroscopy. We also compare the experimental results with tight-binding calculations of the density of states in HgTe CQD.Figure 1 shows the absorption spectrum of the HgTe QDs used, with the insets showing the second derivative of parts of the spectrum which reveals the positions of the first two energy levels. Typical transmittance transients at the first energy level are shown in the Figure 2, with the changes induced in the absorbance spectrum immediately after the pump pulse shown as an inset.