Ultrafast Carrier Relaxation and Extraction in Semiconductor Nanoheterostructures
Klaus Boldt a, Florian Enders a, Peng Zeng b, Trevor A. Smith b
a University of Konstanz, Germany, Universitaetsstr. 10, POB M680, Konstanz, 78457, Germany
b School of Chemistry, The University of Melbourne, Melbourne, Victoria, 3010, Australia
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2017 (NFM17)
SE1: Fundamental Processes in Semiconductor Nanocrystals
Barcelona, Spain, 2017 September 4th - 9th
Organizers: Arjan Houtepen and Zeger Hens
Poster, Klaus Boldt, 003
Publication date: 20th June 2016

Semiconductor nanoheterostructures are of high interest due to the wide range of possibilities they offer for band structure engineering. Confinement or separation of excited charge carriers is employed to optimise photoluminescence, carrier extraction, upconversion, carrier multiplication, or optical gain.[1-3] The nature of the interfaces makes an integral contribution to the resulting electronic structure: while lattice mismatch exerts strain on the crystal lattice alloying between the adjacent materials releases this pressure.[4-7] Both effects may distort the band structure away from the simple step function that is often assumed as first approximation of the system.
We present recent results on the formation of interfaces between two II-VI semiconductors in colloidal nanocrystals. Shell formation or annealing at high temperatures is used to promote mixing of the two phases at the interface. We use transient absorption spectroscopy to observe the effects on charge carrier relaxation and extraction.[8] In core/shell/shell nanocrystals electron scavengers were employed to map the extent of alloying, which has strong implications on synthesis design for specific applications.
In nanorods the potential landscape across an axial heterojunction will dominate carrier relaxation. In these anisotropic systems the axial interface also competes with the potential step at the particle surface. We present evidence that suggests retarded carrier cooling across a CdTe/CdS type-II junction and models that explain these observations. A full characterisation of the electronic state from which the charges relax is highly relevant for applications such as two photon upconversion or as buildung blocks for nanoelectronic devices.[3]

[1]    de Mello Donegá, Chem. Soc. Rev. 2011, 40, 1512-1546.
[2]    Boldt, Kirkwood, Beane, Mulvaney, Chem. Mater. 2013, 25, 4731-4738.
[3]    Deutsch, Neeman, Oron, Nat. Nanotech. 2013, 8, 649-653.
[4]    Christodoulou, Rajadell, Casu, Vaccaro, Grim, Genovese, Manna, Climente, Meinardi, et al., Nat. Commun. 2015, 6, 1-8.
[5]    Boldt, Schwarz, Kirkwood, Smith, Mulvaney, J. Phys. Chem. C 2014, 118, 13276-13284.
[6]    Boldt, Ramanan, Chanaewa, Werheid, Eychmüller, J. Phys. Chem. Lett. 2015, 6, 2590-2597.
[7]    Boldt, Z. Phys. Chem. 2017, 231, 77-92.
[8]    Zeng, Kirkwood, Mulvaney, Boldt, Smith, Nanoscale 2016, 8, 10380-10387.

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