Tuning Electronic Properties of II-VI and III-V Narrow Band Gap Nanocrystals through Exposure to Alkali

The use of nanocrystals in optoelectronics strongly relies on the ability to design photodiodes, which requires advanced knowledge of their electronic structure and offers even greater potential when that structure can be finely tuned. For traditional semiconductors, this degree of freedom is achieved through doping, obtained mostly via the introduction of extrinsic impurities. When it comes to colloidal quantum dots, this capacity is mostly lost and carrier density control is best obtained thanks to surface ligand exchanges. Tuning the capping molecule enables the generation of a surface dipole and a consequent charge transfer, which shifts the relative position of the bands with respect to the Fermi and vacuum level. However, the most efficient ligands (i.e., the one associated with the largest dipole) are not necessarily compatible with charge conduction, which rather prefers short molecules; therefore, new strategies are needed. Here, we explore how such a surface dipole can be obtained through alkali deposition as an alternative approach. We apply this method to a broad range of nanocrystals relevant to infrared optoelectronics, which are HgTe (with two different sizes) and InAs, including a ZnSe shell. Potassium deposition leads to a significant shift of the material work function that can be as large as 1.3 eV. We also bring clear evidence that this dipole arises from the polarization of the adatoms with no charge transfer involved (i.e., no shift in the core levels is measured). This method appears to be quite general and is very promising as a path to shift the absolute energy of a band gap, which may ease future integration of colloidal materials in high-performance diodes.