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

Abstract

In the present work, the optical response of isolated (CdSe) n + clusters with n = 3–6 is probed by measuring the photodissociation cross section in the photon energy range ℏω = 1.9–4.9 eV. In this joint experimental and theoretical study, the experimental observations are analyzed with time-dependent density functional theory and equation-of-motion coupled cluster theory. Structural candidates for the time-dependent excited-state calculations are obtained via global optimization by employing a genetic algorithm. The combined experimental and theoretical approach allows the discrimination of cluster geometries in the molecular beam experiments. From n ≥ 5, three-dimensional structures are found. Already for n = 6, light absorption in the red spectral range is observed. This observation is discussed with respect to the size dependence of the optical behavior of finite systems taking experimental and theoretical work on bare and ligated CdSe clusters and nanoparticles into account. Particularly, the influence of the net charge and ligands is considered. This allows a detailed discussion of the size-dependent evolution of the optical properties starting from molecular species over to nanoclusters and nanoparticles and finally to bulk CdSe.

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

“…The reason why Se 4p basis functions dominate the valence MOs of the cationic CdSe cluster species is caused by the positive net charge. As shown previously in a comparison between cationic Cd 3 Se 3 + and neutral Cd 3 Se 3 , [26] the molecular orbitals in the neutral species show an almost identical composition as the ones of the cation. Since the occupation of the MOs for neutral and cationic species is different, the LUMO of the cation resembles the HOMO of the neutral cluster, which is mainly built up of Se 4p basis functions in accordance to the electronic structure of bulk CdSe.…”

“…Nevertheless, the good qualitative agreement between experiment and theory provides strong evidence that the predicted GM structures of the non‐stoichiometric nanoclusters are actually formed in the molecular beam experiment. This also confirms that B3LYP/cc‐pVTZ‐PP as well as PBE0/cc‐pVTZ‐PP are well suited to describe the optical properties of Cd x Se y + clusters [26,28] . Absorption features of B3LYP calculations are more red‐shifted in comparison to PBE0 results.…”

“…With with the exception of Cd 4 Se 3 + , all atoms are two‐fold coordinated. For stoichiometric CdSe clusters it is known that the hexagonal planar Cd 3 Se 3 + structure serves as a building block of larger CdSe NPs [26–28] . This geometry motif can also be found in Cd 4 Se 3 + .…”

“…For the investigated non‐stoichiometric species only one structural isomer in the energy range of 0.10 eV with respect to the predicted global minimum (GM) has been found. The geometric structures of the calculated GMs are depicted in Figure 1 together with the previously examined geometries of Cd 3 Se 3 + and Cd 4 Se 4 + [26] . Additionally, each atom is labelled by its relative electronic spin density.…”

“…An in‐depth presentation of the general methodology and experimental details are described elsewhere, [26,28] therefore, only a brief overview is given here. CdSe clusters are generated by a pulsed laser vaporization source and detected with time‐of‐flight mass spectrometry.…”