Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Controlling the surface of colloidal nanocrystals is paramount to maintain colloidal stability and to tune structural and electronic properties, but also for reducing the presence of trap states that deteriorate their optoelectronic performance. Here, we use density functional theory (DFT) to analyze the electronic structure of II-VI and IV-VI nanocrystals by adding trap states “on-prupose”. We find that effectively stoichiometric II-VI nanocrystals present surface traps only when two coordinated chalcogen atoms are formed on the surface, usually after Z-type displacement. Under-coordinated surface metal atoms, on the other hand, produce states within the conduction band and contribute to wavefunction delocalization rather than electron trapping. In IV-VI this type of traps does not occur. Effectively non-stoichiometric nanocrystals, on the other hand, has several flavors of traps that can be classified depending on how they are created (anion or cation vacancy, ligand removal, etc.). We employ a simplified molecular orbital picture to illustrates the pathways in which such mid-gap states form.
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