Size- and Charge-Dependent Optoelectronic Properties of CdSe Clusters: Evolution of the Optical Gap from Molecular to Bulk Behavior

Variation of the optical properties with size and composition of small, isolated Cdx Sey + clusters

Global energy minimum structures and optoelectronic properties are presented for isolated Cd_x Se_y ⁺ clusters with x + y ≤ 26. The compositional- and size-dependent variation of optical, electronic and geometric properties is systematically studied within the framework of ground state and time-dependent density functional theory. The applied methods are justified by benchmarks with experimental data. It is shown that the optical gap can be tuned by more than 2 eV by only changing the composition for a fixed number of atoms. The stoichiometric species reveal an unexpected size-dependent behavior in comparison to larger colloidal CdSe quantum dots, that is, a redshift of the optical gap was observed with decreasing cluster size in contrast to predictions by quantum-size effects. This unexpected result is discussed in detail taking the positive charge of the clusters into account.

From Cd‐Rich to Se‐Rich – The Manipulation of CdSe Nanocluster Structure and Optical Absorption

A combined experimental and quantum‐chemical study of the structure and optoelectronic properties of isolated non‐stoichiometric Cd_xSe_y⁺ clusters with x=3, 4 and y=3, 4 is presented. The consistency of optical response calculations with the measured absorption spectra reveals that the theoretically predicted lowest energy structural isomers have been formed in the molecular beam experiments. The Se‐rich nanocluster Cd₃Se₄⁺ contains a Se₂⁻ unit which is responsible for the enhanced light absorption in the visible spectral range. In contrast, the radical electron of the Cd‐rich species Cd₄Se₃⁺ is localized at a Cd⁺ centre which is attached to a six‐membered Cd₃Se₃ ring. The excess Cd atom is partly responsible for an intense optical absorption in the near ultraviolet. The variation of the geometry and the optical behaviour in dependence of the stoichiometry is discussed with respect to the photocatalytic properties of CdSe nanoparticles