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

Abstract

A combined experimental and quantum‐chemical study of the structure and optoelectronic properties of isolated non‐stoichiometric Cd x Se 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 Cd 3 Se 4 + contains a Se 2 − unit which is responsible for the enhanced light absorption in the visible spectral range. In contrast, the radical electron of the Cd‐rich species Cd 4 Se 3 + is localized at a Cd + centre which is attached to a six‐membered Cd 3 Se 3 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

Ground- and Excited-State Properties of Charged Non-Stoichiometric Quantum Dots

“…Both ground-state and excited-state calculations for all NS-QD structures are performed in vacuum and acetone; for the solvent effects, the conductor-like polarizable continuum model (CPCM) is used. Overall, the applied computational methodology has been thoroughly tested and used for a wide variety of previously modeled stoichiometric ,, and non-stoichiometric QDs. , …”

“…Overall, the applied computational methodology has been thoroughly tested and used for a wide variety of previously modeled stoichiometric 47,65,66 and non-stoichiometric QDs. 24,28 We note that quantitative calculations of trion states, where the charge is coupled to a neutral exciton, require explicit inclusion of three-body interactions, which are not present in the TDDFT. A more accurate approach based on time-dependent density-matrix functional theory (TD-DMFT) for calculating trions in nanostructures has been utilized in refs 54−62.…”

“…These results agree with both experimental and calculated absorption spectra of small positively charged Cd 4 Se 3 clusters demonstrating the appear- ance of red-shifted optically inactive transitions. 28 These charge-related optical transitions can be compared to the positive trion X + state induced by a coupling between +1 charge and the lowest bright exciton. An optically "dark" character of the calculated transitions associated with a positive charge of Cd-rich structures is consistent with respective experimental observations for positively charged CdSe/CdS core/shell QDs: structures with injected holes show the X + trion state with diminishing emission intensity (the dark state), while structures with injected electrons demonstrate the X − trion state with intermediate photoluminescence strength.…”

“…Joined experimental and computational investigations of the optical response of positively charged non-stochiometric CdSe nanostructures have been reported for small clusters of 6−8 atoms in size generated by a pulsed laser vaporization source and detected with time-of-flight mass spectrometry. 28 emission upon irradiation was detected for CdSe NS-QDs of 0.7−2 nm in size and assigned to the photoinduced charging of the NS-QDs surface ligands. 29 However, the mechanisms of such photoinduced charging and the role of perfect vs imperfect ligand passivation in this process are still unclear.…”

“…Several calculations based on density functional theory (DFT) have been recently reported studying the effect of various ligands and solvents on the reduction of surface metal in negatively charged cadmium chalcogenide NS-QDs. , However, these calculations do not include the characterization of the electronically excited states. Joined experimental and computational investigations of the optical response of positively charged non-stochiometric CdSe nanostructures have been reported for small clusters of 6–8 atoms in size generated by a pulsed laser vaporization source and detected with time-of-flight mass spectrometry . Quenching of the emission upon irradiation was detected for CdSe NS-QDs of 0.7–2 nm in size and assigned to the photoinduced charging of the NS-QDs surface ligands .…”