Using Alkali Metal Atoms to Tune the Electronic Properties of Narrow-Bandgap Semiconductor Nanocrystals

Infrared-active colloidal quantum dots (CQDs) have recently emerged as a powerful alternative to traditional epitaxially grown semiconductors, notably for photodetection and light harvesting. An essential device structure for these aims is the photodiode, which can extract photogenerated charges thanks to the unique electronic band structure provided by a multi-material stack. Realizing high-performance photodiodes, however, requires the ability to fine tune the band alignment at each material interface.
For traditional semiconductors, this fine tuning can be achieved through the introduction of extrinsic impurities, leading to precise levels of doping. In the case of CQDs, this strategy is usually not viable due to fabrication and stability concerns. Carrier density control is often achieved through surface ligand exchanges instead. These capping molecules generate surface dipoles and charge transfers toward the CQDs, which shift the position of the bands with respect to the Fermi and vacuum levels. However, the most appropriate ligands for a given band alignment are not necessarily compatible with efficient charge conduction. There is thus a need for new strategies.
As an alternative, we explored a method to induce surface dipoles in nanocrystals through the deposition of alkali metal atoms. We applied this strategy to HgTe nanocrystals, one of the most mature infrared-active CQD materials, investigating two different sizes.
We showed that for all studied CQDs, the deposition of potassium on the surface of a nanocrystal film leads to a significant and continuously tunable shift of the material’s work function, reaching 1.3 eV in some cases. We then evidenced that the dipole arises from the polarization of the adatoms without involving any charge transfer toward the CQDs. This seemingly very general approach appears very promising as a way to shift the absolute energy of a band gap, which could ease the integration of colloidal materials into high-performance diodes.