Co²⁺-Doping of Magic-Sized CdSe Clusters: Structural Insights via Ligand Field Transitions

Abstract

Magic-sized clusters represent materials with unique properties at the border between molecules and solids and provide important insights into the nanocrystal formation process. However, synthesis, doping, and especially structural characterization become more and more challenging with decreasing cluster size. Herein, we report the successful introduction of Co2+ ions into extremely small-sized CdSe clusters with the intention of using internal ligand field transitions to obtain structural insights. Despite the huge mismatch between the radii of Cd2+ and Co2+ ions (>21%), CdSe clusters can be effectively synthesized with a high Co2+ doping concentration of ∼10%. Optical spectroscopy and mass spectrometry suggest that one or two Co2+ ions are substitutionally embedded into (CdSe)13 clusters, which is known as one of the smallest CdSe clusters. Using magnetic circular dichroism spectroscopy on the intrinsic ligand field transitions between the different 3d orbitals of the transition metal dopants, we demonstrate that the Co2+ dopants are embedded on pseudotetrahedral selenium coordinated sites despite the limited number of atoms in the clusters. A significant shortening of Co–Se bond lengths compared to bulk or nanocrystals is observed, which results in the metastability of Co2+ doping. Our results not only extend the doping chemistry of magic-sized semiconductor nanoclusters, but also suggest an effective method to characterize the local structure of these extremely small-sized clusters.

Magic‐Sized Stoichiometric II–VI Nanoclusters

“…Subsequently, despite a large mismatch of Co 2+ and Cd 2+ radii, Co 2+ has also been doped into (CdSe) 13 nanocluster to form Co x Cd 13‐ x Se 13 clusters, x = 0–2 (Figure 4C–F), enabling the study of structural and optical properties at single‐atom‐difference levels. [ 41 ]…”

“…Unfortunately, the actual structures of stoichiometric metal chalcogenide nanoclusters remain unavailable since the growth of their single crystals suitable for single crystal XRD poses challenges likely due to the weak binding of ligands with nanoclusters and the lack of freely‐soluble, stable and phase‐pure nanoclusters with good yield. [ 21,23,41,46 ] Additionally, it is necessary to design new crystallization methods using appropriate solvents. However, theoretical approach, such as first‐principles electronic structure calculations, alternative to experimental methods, has predicted the atomic structures of several stable magic‐sized (CdSe) n clusters, n = 9, 12, 13, 15, 16, 18, 19, 21, 24, 28, 32, 33, 34, 35, and 36.…”

“…The nanoclusters exhibiting molecule‐like properties lie in between traditional small molecules and quantum‐confined large nanocrystals such as quantum dots (QDs) ( Figure ). [ 27,38,39 ] Addition or removal of constituents at desired atomic levels can significantly modify the optical, photophysical, and catalytic properties, [ 19,23,40–43 ] making nanoclusters even more fascinating. The precise chemical formula, atomic structure, and high homogeneity of the nanoclusters will enable researchers to systematically study the transition of properties from molecular to bulk phase and vice versa.…”

Impurity incorporation and exchange interactions in Co2+-doped CdSe/CdS core/shell nanoplatelets

“…[23][24][25] Doping, i.e., the intentional incorporation of impurities into a crystal lattice, has been employed as a powerful strategy to tune optical, electrical, and magnetic properties of II-VI based zerodimensional colloidal nanostructures, [26][27][28][29][30][31][32][33][34][35] even down to the level of magic-sized or molecular nanocluster. [36][37][38][39][40][41] In contrast to the sophisticated chemical routes reported for the synthesis of doped 0D nanocrystals, the incorporation of impurities into their 2D analogs is at an early stage of development. Via postsynthetic cation exchange, researchers have shown that Cd 2+ can be replaced by copper and silver ions in CdSe NPLs.…”

“…47 More than endowing the host material with new functionalities, impurities can also be used as microscopic tools to investigate fundamental properties such as lattice parameters, thermal expansion, the presence of strain, and electron-phonon coupling. 41,[48][49][50] For this purpose, impurities with a high crystal field sensitivity should be chosen, which are able to disclose small changes of their environment. 51 One example of this type of impurity is Co 2+ , which exhibits well-defined ligand-field transitions in the visible and infrared spectral range.…”