In condensed matter physics, a stepwise reduction of the dimensionality from bulk to the nanoscale is known to lead to striking electronic and optical properties such as the giant oscillator strength transition for two- dimensional (2D) semiconductor quantum wells and the spin-charge separation in one-dimensional (1D) systems. However, in contrast to metals, the lateral confinement in semiconductor materials competes with the exciton Coulomb interaction, which can be magnified by dielectric mismatch. Due to the complexity of these interactions, the degree of freedom in the electron motion remains poorly understood, whereas its clear circumscription could significantly improve the optoelectronic performances of quasi-2D systems such as CdSe colloidal nanoplatelets (NPLs).

Since the distribution of the quantized states, the so-called density of states (DOS), reflects the restrictions of the electron motion with dimensionality, it can serve as a genuine fingerprint to unambiguously disclose the confinement experienced by the charge carriers. A unique way to probe the DOS consists in measuring single-particle excitation spectra with scanning tunnelling spectroscopy (STS). Ji et al. have demonstrated that the hole DOS is consistent with a free in-plane motion, while the electrons have a more complex DOS that deviates from the simple 2D model. When the width (W) or/and the length (L) are smaller than ten to five times the exciton Bohr radius, the electron behavior, which has a smaller effective mass than the hole, becomes affected, with a significant impact on its interaction with the hole and the dielectric image charges. The already present complexity in understanding electron−hole correlation in these box-shaped nanostructures is further enhanced with the existence of electrical surface traps, which are caused by the sporadic absence of ligands. It makes the electron confinement in the NPLs still unknown, leading to intense debates about their true dimensionality. 

Here we used this technique to study electron confinement in colloidal CdSe nanoplatelets (NPLs), which have a quantized thickness d due to a discrete number of monolayers (MLs) and a rectangular shape with finite length L and width W. The observation of Van Hove singularities in the conduction band implies a paradigm shift on the electronic structure of typical CdSe NPLs considered in the literature. As the electron DOS exhibits a striking modulation that is directly related to the length of the NPLs, delineating the in-plane electron motion at low temperature has important conconsequences for a deeper understanding of the exciton dissociation, diffusive transport, and annihilation in NPLs.
Moreover, it is shown that the side facets of NPLs host electronic deep trap states, which cause a Coulomb blockade in the tunnelling current. As they could be fully removed by the formation of core-crown NPL, our results anticipate a genuine boost for NPL-based lateral heterostructures that will notably allow for the mixing of the dimensionalities to favor specific electronic or optical properties.[1]