We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2-8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as ND = 8 × 1020 cm-3 and NA > 5 × 1020 cm-3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (-2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, including hydrogenoid impurities and the presence of a host substrate, in the case of n-type doped NCs. The observed enhancement of the NC-HOPG Fermi-level mismatch can be understood by considering a size-dependent electrostatic contribution attributed to dipolar effects at the NC-ligand interface. The estimated surface dipole density equals a few Debye/nm2 and is increased at smallest NC sizes, which follows the enhancement of ligand densities at small NC sizes previously reported for metallic or semiconducting NCs. The results put forward the role played by the NC-ligand interface electrostatics for electronic applications.
Bibliographical noteFunding Information:
This work has been performed by using the facilities of the French RENATECH network and of the ExCELSiOR Nanoscience Characterization Center at IEMN. We thank H. Diesinger for support with Kelvin probe microscopy experiments and D. Gueŕ in for fruitful discussions and A. Dufrenê for illustrations. We acknowledge financial support from the French National Research Agency under contracts ANR-11- BS10-000, ANR-15-CE09-0004-04, and ANR-13-BS09-0020. U.B. thanks the Alfred & Erica Larisch memorial chair.
© 2019 American Chemical Society.