Nanocrystals with type-II heterostructures are attractive candidates for applications in which separation of excited charge carriers are exploited, such as photovoltaics [1], upconversion [2], or quantum dot lasing [3]. These properties are in competition with processes involving interfacial trap states, usually associated with the particle surface or strained interfaces. To use semiconductor nanocrystals in technical applications it is of utmost importance to characterise and control these effects.

Here we present a model system made of CdTe-tipped CdSe/CdS seeded nanorods, in which the CdTe particle is separated in space from the CdSe seed while being in electronic contact. The CdTe/CdS interface has type-II character, but charge carrier delocalisation is likely affected by the large lattice mismatch of 10% between CdTe and CdS [4]. We performed ultrafast (femto to picosecond) transient absorption spectroscopy to map carrier dynamics after exciting one or both recombination centres, with seed size and rod length as variable parameters. Conduction band electrons, after exciting the CdTe/CdS interface, are expected to localise in the CdSe seed, which has the lowest energy band edge and acts as a reporter particle. However, while states in the CdS rod get bleached by the electron no signal corresponding to CdSe is detected. We employ electron and hole scavengers to determine the roles of the individual charge carriers, and by this the contribution of Coulomb interactions and interfacial trap states.

This experiment gives fundamental insight into the behaviour of strained nano-heterointerfaces, as well as the effects from size quantisation and mean free path of the carriers. The experimental data is compared to effective mass approximation-based simulations for free carriers. While the basic electronic structure is predicted with sufficient accuracy the comparison highlights the importance to simulate carrier interactions and interfaces on the atomic scale.

[1] Itzhakov, S.; Shen, H.; Buhbut, S.; Lin, H.; Oron, D.; J. Phys. Chem. C 2013, 117 (43), 22203–22210.

[2] Deutsch, Z.; Neeman, L.; Oron, D.; Nat. Nanotech. 2013, 8, 649–653.

[3] Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Achermann, M.; Bezel, I. V.; Mcguire, J. A.; Piryatinski, A.; Nature 2007, 447, 441–446.

[4] Jing, L.; Kershaw, S. V.; Kipp, T.; Kalytchuk, S.; Ding, K.; Zeng, J.; Jiao, M.; Sun, X.; Mews, A.; et al.; J. Am. Chem. Soc. 2015, 137, 2073–2084.