Tuning Electronic\nProperties of II–VI and III–V\nNarrow Band Gap Nanocrystals through Exposure to Alkali

The use of nanocrystals in optoelectronics strongly relies\non the\nability to design photodiodes, which requires advanced knowledge of\ntheir electronic structure and offers even greater potential when\nthat structure can be finely tuned. For traditional semiconductors,\nthis degree of freedom is achieved through doping, obtained mostly\nvia the introduction of extrinsic impurities. When it comes to colloidal\nquantum dots, this capacity is mostly lost and carrier density control\nis best obtained thanks to surface ligand exchanges. Tuning the capping\nmolecule enables the generation of a surface dipole and a consequent\ncharge transfer, which shifts the relative position of the bands with\nrespect to the Fermi and vacuum level. However, the most efficient\nligands (<i>i.e</i>., the one associated with the largest\ndipole) are not necessarily compatible with charge conduction, which\nrather prefers short molecules; therefore, new strategies are needed.\nHere, we explore how such a surface dipole can be obtained through\nalkali deposition as an alternative approach. We apply this method\nto a broad range of nanocrystals relevant to infrared optoelectronics,\nwhich are HgTe (with two different sizes) and InAs, including a ZnSe\nshell. Potassium deposition leads to a significant shift of the material\nwork function that can be as large as 1.3 eV. We also bring clear\nevidence that this dipole arises from the polarization of the adatoms\nwith no charge transfer involved (<i>i.e</i>., no shift\nin the core levels is measured). This method appears to be quite general\nand is very promising as a path to shift the absolute energy of a\nband gap, which may ease future integration of colloidal materials\nin high-performance diodes.