Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
CdSe-based heteronanocrystals are well-established colloidal nanomaterials, with a high optical stability, wide emission tunability, and enhanced absorption cross section in giant-shell systems. Control over their exciton recombination rate has been mostly achieved via band gap engineering, leading to lifetimes ranging from 20-25 ns in type-I CdSe/ZnS to about 200-500 ns in quasi-type II CdSe/CdS NCs.
In this presentation, we demonstrate a radically different approach to control the optical properties, namely via strain[1] and piezoelectric potentials. We combine the shape-controlled synthesis wurtzite CdSe nanorods with the growth of a giant rod-like shell to form rod-in-rod (RIR) nanocrystals. Photoluminescence spectroscopy reveals exciton lifetimes which increase with the size of the RIR, contrary to usual expectations from quantum confinement engineered nanocrystals. For the largest samples, we measure lifetimes reaching 4.4 µs, several times larger than in spherical giant-shell CdSe/CdS NCs. k·p calculations reveal that the exciton behavior in RIR is governed by the strain-induced piezoelectricity arising at the CdSe/CdS interface. The intense piezo-electric field, along with the weak confinement along the rod axis, steer the hole away from the electron leading to truly type-II ground state excitons, as confirmed by fluorescence line narrowing experiments.
Calculations for biexciton states show however that the piezoelectric potential is too shallow to trap a second exciton, and we predict a blueshifted emission line, with much stronger electron-hole overlap than that of the single exciton. This is consistent with ultrafast spectroscopy at cryogenic temperature, showing blueshifted excited states with lifetimes again in the nanosecond range. Strain engineering may therefore bring about a more complete control over NC optical properties, from spectral position of the fluorescence peak to the corresponding emission lifetime and excited-state carrier dynamics. With the unique RIR band structure in mind, we envision exciting future applications in exciton storage or energy harvesting.
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