Nat. Nanotech. (2017)

The threshold for optical gain in colloidal semiconductor quantum dots (QDs) can be dramatically reduced by the use of interface engineering and charge doping. Kaifeng Wu and co-workers from Los Alamos National Laboratory in New Mexico, USA fabricated QDs consisting of a core of CdSe, an alloy of CdSe x S1–x and a shell of CdS. The QDs have an energy bandgap of ~1.9 eV (which corresponds to a red emission wavelength of 650 nm) and extra electrons are introduced through the use of a photochemical reduction, where a hole from the QD is transferred to lithium triethylborohydride (LiEt3BH) that acts a ‘hole scavenger’. The degree of charge doping can be controlled by the amount of LiEt3BH used. Heavily charged QDs with ~6 electrons were found to have dramatically reduced optical gain thresholds that were about 50 times smaller than neutral QDs. The researchers say that the approach indicates the viability for zero-threshold optical gain. Analysis also shows the laser threshold of optically excited, doubly charged QDs is around 0.5 kW cm–2, which is more than one order of magnitude smaller than neutral or singly charged QDs and potentially compatible with the use of commercial LEDs as a pump source.