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2014 MRS Spring Meeting


F8.07 - The Possibility and Implications of Dynamic Nanoparticle Surfaces


Apr 24, 2014 4:00pm ‐ Apr 24, 2014 4:15pm

Description

The current description of the nanoparticle surface often invokes a static population of trap states associated with dangling bonds or defects. These trap states strongly impact interfacial charge transfer and, as a result, nanocrystal performance in lighting, photovoltaic and photocatalysis applications. However, our recent atomic resolution scanning transmission electron microscopy movies show that the surface of semiconductor nanocrystals is in a state of constant fluctuation during imaging. We propose that this is also the case during UV-excitation.

The current description of the nanoparticle surface often invokes a static population of trap states associated with dangling bonds or defects. These trap states strongly impact interfacial charge transfer and, as a result, nanocrystal performance in lighting, photovoltaic and photocatalysis applications. However, our recent atomic resolution scanning transmission electron microscopy movies show that the surface of semiconductor nanocrystals is in a state of constant fluctuation during imaging. We propose that this is also the case during UV-excitation.

Although atomic motion under an electron beam is not unexpected, the images were obtained at low acceleration energies that impart energy to the nanocrystal similar to that of a UV photon. A fluid surface is further made plausible in the context of the drastically lowered melting temperature of nanoparticles, where surface pre-melting has been predicted to occur. The origin of broad spectrum emission from ultrasmall nanocrystals was elucidated using dynamic STEM imaging in conjunction with density functional theory. We showed that the atomic fluctuations can result in white light emission from ultrasmall CdSe nanocrystals.

In this talk, dynamic STEM movies will be presented that show fluxionality is not limited to ultrasmall nanocrystals, but is present for all sizes of nanocrystals. As the diameter of the nanoparticle is increased, a stable crystalline core manifests while a nanometer thick region of fluctuation persist at the surface. The effect of chemical composition (CdS or CdTe) and surface composition (oleic acid, phosphonic acid ligands and ZnS) has on the fluxional surface will be presented as well as possible origins of surface fluxionality including: dynamic surface ligands, surface oxidation and surface pre-melting.

1. Pennycook, T.J.; McBride, J.R.; Rosenthal, S.J.; Pennycook, S.J.; Pantelides, S.T. Dynamic Fluctuations in Ultrasmall Nanocrystals Induce White Light Emission Nano Lett. 2012, 12 (6), 3038-3042.

2. McBride, J.R.; Pennycook, T.J.; Pennycook, S.J.; Rosenthal, S.J. The Possibility and Implications of Dynamic Nanoparticle Surfaces ACS Nano 2013, 7 (10), 8358-8365.

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