Photon upconversion is a process that generates one high energy photon from two low energy photons. It can be potentially applied in the fields of solar energy, biological imaging, and data storage. CdSe nanocrystals (NCs) have been reported as sensitizers to upconvert photons in the visible regime, in combination with 9-anthracene carboxylic acid (9-ACA) transmitters, and diphenylanthracene (DPA) annihilators. It’s been shown that a high surface coverage of transmitter and high photoluminescence quantum yield (PLQY) of sensitizer boost the triplet energy transfer from CdSe sensitizer to 9-ACA transmitter. Therefore, the upconversion quantum yield of the current system is limited by the relatively low PLQY of CdSe NCs and low surface coverage of 9-ACA. In order to address these limitations, we designed a transmitter that can bind to the NC more strongly, in order to better control the surface coverage of the transmitter. As a transmitter, 9-anthracenethiol (9-AT) contains a thiol group that binds more strongly to NCs than the carboxylic group in 9-ACA. For the sensitizer, the PLQY of CdSe NCs is enhanced by an epitaxially grown CdS shell that passivates the surface trap states and prevents hole trapping by thiol groups in the 9-AT. We show that upconversion quantum yields decrease with an increase in CdS shell thickness. This can be explained by Dexter energy transfer from CdSe/CdS NCs to 9-AT, as the efficiency of this energy transfer is exponentially dependent on the distance between CdSe core and 9-AT, i.e. the thickness of CdSe shell. Finally, we show that CdSe/CdS core shell NC sensitizers can be used to upcovert visible photons with a high efficiency.

CdSe/CdS core-shell nanocrystal sensitizers for molecule-nanocrystal photon upconversion

251st National Meeting (2016)

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