Absorption Features of CdTe Nanoclusters: Aspect Ratio Dependency of the Singlet/Doublet from First-Principles Calculations

Abstract

Two categories of absorption features, singlet and doublet, have been observed in semiconductive II–VI magic-size clusters (MSCs) experimentally; however, the underlying mechanism resulting in the two absorption features has seldom been addressed. Here, the relationship between morphology and absorption features is studied in CdTe nanoclusters using first-principles calculations. The explorations focus on CdTe isomers and nonisomers at first to simulate the isomerization and layer growth pattern in MSCs. Similar conclusions are reached that the absorption spectra exhibit a gradual transition from singlet to doublet as the aspect ratios increase. We further suppose that the shape of CdTe dMSC-371 may be that of sMSC-371 being stretched in one dimension. Density functional theory absorption spectra show significant agreement with the predicted spectra from the Mie–Gans theory, which indicates that the shape of clusters affects the dielectric function directly and results in different absorption features. The major transition orbitals of these optical excitations are assigned by transition dipole moment, oscillator strengths, and density of states. Furthermore, the aspect-ratio-dependent singlet and doublet absorption features are found to be independent of the structural types, including the zinc blende, hollow nanotubes, and hexagonal wurtzite structures. The present study reveals the aspect-ratio-determined absorption behaviors of II–VI MSCs and provides a promising method for predicting the morphology of MSCs with absorption spectroscopy.

Reversible Facet Reconstruction of CdSe/CdS Core/Shell Nanocrystals by Facet–Ligand Pairing

“…This dissolution, precipitation, centrifugation, and decantation procedure was repeated twice. Finally, the sample was vacuumed for 12 h to remove residual solvents and used for solid-state 13 C NMR experiments. The solid-state 13 C NMR experiments were performed on a Bruker Avance III-HD 400 MHz NMR spectrometer at a resonance frequency of 100.61 MHz, using a 3.2 mm magic-angle spinning (MAS) probe.…”

“…Purification of Polygonal-Nanoplatelets for Solid-State 13 C NMR and FTIR. The reaction solution of the Polygonal-Nanoplatelets (4 mL) was transferred into a centrifuge tube, and 10 mL of acetone was added to precipitate the nanocrystals at room temperature.…”

“…This dissolution, precipitation, centrifugation, and decantation procedure was repeated twice. The precipitate obtained in each purification cycle was vacuumed for 12 h to remove residual solvents for 13 C NMR and FTIR measurements. FTIR spectra were recorded on a Nicolet iS5 FTIR spectrometer.…”

“…Synthesis of colloidal semiconductor nanocrystals, as a foundation of the field, has reached excellent control on their size and shape at the nanometer scale but synthetic control at the atomic scaleequivalent to control of their facet structureremains challenging, leading to a deficiency on exploiting size-tunable properties, eliminating surface-related inhomogeneity, and assembling them into functional superstructures. − For a few successful cases, optical properties of semiconductor nanocrystals are found to be outstandingly homogeneous, such as extremely narrow UV–vis absorption features of cadmium chalcogenide magic-size-clusters with a defined molecular structure − and ultranarrow photoluminescence (PL) peaks of the cadmium chalcogenide nanoplatelets with atomically flat yet very large basal planes, which are all in zinc blende ( fcc ) structures. For II–VI and III–V semiconductor nanocrystals, they can be in the form of either zinc blende ( fcc ) or wurtzite ( hcp ) structures.…”

“…The samples were prepared by drop-casting concentrated hexane dispersions onto calcium fluoride (CaF 2 ) substrates and drying under vacuum at room temperature for 30 min. Solid-State 13 C NMR measurements for the Polygonal-Nanoplatelets were the same as those for the Hexagonal-Nanoplatelets.…”