# High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

## TL;DR

This study uncovers high-energy exciton transitions in cadmium chalcogenide nanoplatelets originating from points other than the Brillouin zone center, supported by experiments and first-principles calculations, revealing tunable UV optical properties.

## Contribution

It provides experimental evidence and theoretical analysis of high-energy exciton transitions from non-$\Gamma$ points in quasi-2D cadmium chalcogenide nanoplatelets, expanding understanding of their optical properties.

## Key findings

- Observation of UV absorption bands linked to high-energy exciton transitions.
- Transitions originate from $X$ and $M$ points of the 2D BZ, from $L$ and $X$ points of the 3D BZ.
- $E_0$ transitions at $\Gamma$ point are well described by the multiband effective-mass model.

## Abstract

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent $E_0$ series of exciton transitions at the $\Gamma$ point of the Brillouin zone (BZ). In this work, we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the $\Gamma$ point, in the family of cadmium chalcogenide quasi-2D nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the $E_0$ exciton bands in the visible region. These new bands are attributed to transitions analogous to the $E_1$, $E_1+\Delta_1$, and $E_2$ series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from $X$ and $M$ points of the 2D BZ, which originate from $L$ and $X$ points of the 3D BZ. At the same time, the $E_0$ series of transitions at the $\Gamma$ point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.