Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Infra-red (IR) luminescent quantum dots (QDs) are predominantly based on PbS or PbSe. However, while being attractive materials for solar energy conversion and photodetection, their long radiative lifetime and 8 fold degenerate HOMO and LUMO states make them less interesting for light emission applications. An alternative is offered by the mercury chalcogenides HgX (X=Se, Te). These materials are semi-metals in the bulk, yet when the crystal dimensions are confined to nano-scale regime, a band gap is opened up which can be tuned in the entire infra-red range. While mainly studied in view of photodetection, the lower degeneracy of their band-edge states and their shorter radiative lifetime as compared to PbS and PbSe makes them appealing for light emission too. A key step to make quantum dots fit for lighting application proved to be the formation of core-shell QDs, where the shell provides a better surface passivation and allows for a further tuning of the opto-electronic properties. However, while a broad range of shell growth procedures have been developed for Zn, Cd and Pb chalcogenide QDs, literature shows little examples of HgX based core/shell QDs of use of light emission applications.
Here, we demonstrate the formation of HgSe/CdS and HgSe/CdSe core/shell QDs and report on the opto-electronic properties of the materials thus formed. With lattice parameters of 0.608 (HgSe, CdSe) and 0.582 nm (CdS) and similar zinc blende crystal structures, CdSe and CdS are ideal shell materials to form HgSe-based core/shell structures with minimal lattice strain. Considering the excessive ripening of HgSe QDs at typical shell growth temperatures, we developed a room-temperature colloidal ALD procedure. As confirmed by electron microscopy, this results in the layer-by-layer growth of both CdSe and CdS around the HgSe core, with the QD diameter increasing by ≈0.6 nm per cycle. While growing up to 5 CdS and CdSe shells, we found that especially the first CdSe and CdS layers lead to a considerable redshift of the band-edge transition and eliminate the n-type doping of as-synthesized HgSe QDs. This results in core/shell QDs showing bright photoluminescence in the near infrared, ranging from the telecom wavelengths of 1300 and 1550 nm to the longer wavelengths of interest for IR spectroscopy.
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