Experimental and Computational Studies of the Structure of CdSe Magic-Size Clusters

Abstract

Experimental and computational studies of resonant Raman spectra of truly monosized (CdSe) 33 and (CdSe) 34 nanoclusters have been performed. Firstprinciples calculations of vibrations are performed to account for the peculiarity of the spectrum and resonant Raman selection rules. The calculation method is based on the analysis of the spatial distribution of the electron density in the ground and excited states and the corresponding displacement of atoms after the electronic transition. The calculated vibrational density of states and resonant Raman spectra of CdSe nanoclusters in a core−cage arrangement are distinctively different from those of small nanocrystals in the bulk fragment model and reasonably agree with the experimentally observed spectral features. The agreement can be considered as experimental evidence for the shell structure of "magic" CdSe nanoclusters. The resonant conditions for the Raman measurements and two different kinds of samples stabilized with decylamine in toluene and with cysteine in water ensure the reliability of our measurements and the minor influence of the stabilizer.

Surface-Induced Deprotonation of Thiol Ligands Impacts the Optical Response of CdS Quantum Dots

“…The conductor-like polarizable continuum model , is used to implicitly include solvation effects for weakly polar PAM and highly polar ACN solvents. This methodology has been previously applied in modeling ground and excited-state properties of cadmium chalcogenide QDs by our group ,,,− , and others , showing qualitative agreement with experimental studies. The ligand–QD binding energy is computed by subtracting the energies of the noninteracting optimized fragments (the pristine QD and the ligand) and the optimized ligand-passivated QD.…”

“…Understanding the conditions governing the thiol/thiolate equilibrium for this model system helps to determine an optimal interplay between the thiolates and thiols that eliminates hole trap states and favors emission in QDs. The initial structures of magic size , (CdS) 33 QD are constructed by cutting quasi-spherical QDs from the bulk CdS wurtzite crystal with a diameter of ∼1.5 nm, as was reported in previous works. ,− This model is used to represent stoichiometric surfaces with an equal number of surface cadmiums and sulfurs that likely prevail over nonstoichiometric Cd-enriched surfaces in larger size QDs . Since in experiments the thiol passivation takes place via ligand exchange in the presence of prime-amine ligands, , a relatively lower concentrations of thiol/thiolate ligands is not expected to result on highly nonstoichiometric structures due to the L-type nature of amine ligands …”

Nature of electronic excitations in small non-stoichiometric quantum dots

“…Several simulation techniques, ranging from the adiabatic ground state to non-adiabatic excited-state calculations, have been applied to understand the photo-chemo-physical properties of QDs in detail. 25–28 However, most of these attempts 29–37 have focused on stoichiometric QDs (with an equal number of cation and anion atoms) that are less frequently obtained from the conventional synthesis. The more common non-stoichiometric QDs, with cation-to-anion ratios deviating from one, that are critical to many optical and photocatalytic applications, remain largely unexplored.…”

Varying the Concentration of Organic Acid and Amine Ligands Allows Tuning between Quantum Dots and Magic-Sized Clusters of CH3NH3PbBr3 Perovskite: Implications for Photonics and Energy Conversion

“…Perovskite quantum dots (PQDs) have received increasing interest due to their novel properties, such as high photoluminescence (PL) quantum efficiency and narrow but tunable emission bandwidth. The emission can be easily tuned over the entire visible spectrum by controlling the crystal size and halide ion composition, which are attractive for light-emitting diodes (LED), − solar cells, − and other optoelectronic applications. − ,,− Compared with perovskite bulk materials, PQDs exhibit tunable properties due to the quantum confinement effect and an extremely large surface-to-volume (S/V) ratio that allows for easy surface functionalization for different applications. − Compared with conventional PQDs, perovskite magic-sized clusters (PMSCs) with single size or narrow size distributions have narrower and bluer optical absorption bands and better-defined structures that are desired for fundamental studies. − For example, they are good model systems for understanding the growth of larger nanostructures including QDs or QD solids. − …”