Publication date: 27th June 2014
In this work we combine inductively coupled plasma optical emission spectroscopy (ICP-OES) with UV-VIS-NIR absorption spectroscopy and transmission electron microscopy to calculate the molar extinction coefficient of colloidal CdSe/CdS nanocrystals. These heterostructures allow the engineering of the band alignment, thus achieving a high photoluminescence quantum efficiency in combination with a controllable photoluminescence spectrum and lifetime, making them high-potential materials for diverse photonic applications.[1,2]
Since these materials have a configuration in which the hole wave function is localized in the core, while a small conduction band offset allows the electron wave function to be delocalized into the shell region, they have a peculiar dependence of the molar extinction coefficient on size, composition and aspect ratio. We therefore characterized CdSe/CdS core/shell quantum dots and rods with various core/shell compositions and dimensions. From the molar extinction coefficient we obtained the absorption strength per nanocrystal, both at high energy as well as the band gap, and the concentration of nanocrystals in solution. We investigated the effect of shape on the (internal) electric field profile and compared experimental data with the values calculated in the framework of Maxwell-Garnett theory,[3] to model the molar extinction coefficient dependency on aspect ratio and electron delocalization and obtain general calibration curves.
[1] I. Moreels et al.: Nearly Temperature-Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot-in-Rods, Adv. Mater. 24, OP231–OP235 (2012).
[2] M. Allione et al.: Two-Photon-Induced Blue Shift of Core and Shell Optical Transitions in Colloidal CdSe/CdS Quasi-Type II Quantum Rods, ACS Nano 7, 2443–2452 (2013).
[3] Z. Hens and I. Moreels: Light Absorption by Colloidal Semiconductor Quantum Dots, J. Mater. Chem. 22, 10406-10415 (2012).
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