Semiconductor nanocrystals (NCs) are known as artificial atoms due to the similar electronic properties and the theoretical prediction of atomic-like symmetries for the confined electron wavefunctions (WFs). However, despite recent reports for MBE-grown quantum dots [1], there is no direct experimental evidence of s- and p-like symmetries in nanocrystals WFs yet. Here, we use for the first time WF mapping to obtain direct information about the spatial distribution of WFs in semiconductor NCs.
Specifically, we investigate colloidal InP nanocrystals by scanning tunneling microscopy (STM) and spectroscopy (STS). A size selected NC solution was employed and the NCs were deposited onto a Au(111) surface covered by dithiols to achieve a stable attachment of individual NCs. On the thiolated Au surface, we observed molecularly-ordered domains consisting of thiols self-assembled in a striped phase. Dipping the thiolated Au substrate in the NC solution leads to distinct additional features, having dimensions in good agreement with the nanoparticles' size range (4-6 nm).
To examine the electronic structure of NCs, STS was performed on individual nanoparticles. Discrete peaks in the normalized derivative spectra were observed, which can be associated to a number of confined states grouped in different multiplets/families. For comparison, the STS curves on the dithiol layer are featureless. Peaks belonging to the same peak family exhibit similar separations which can be ascribed to the different charging energies for the different WF symmetries. The spacing between the peaks decreases from s- to p- and d-type states in agreement with the expected decrease of Coulomb interaction when higher orbitals with broader wavefunctions are occupied.
Spatially resolved dI/dV maps on individual NCs were then acquired to determine the symmetry of the squared WFs corresponding to the different confined states resolved in the spectral density. Thanks to our spatial and energy resolution, we were able to distinguish clear s- and p-like WFs. These results are the first direct experimental evidence of s- and p-like symmetries in NC WFs since all previous reports were based on the degeneracies in the energy spectrum.
[1]. Maltezopoulos et al., Phys.Rev.Lett. 91, 196804 |