Brightly Luminescent Core/Shell Nanoplatelets with Continuously Tunable Optical Properties

Abstract

A straightforward, rapid method to create colloidally stable and brightly luminescent core/shell CdSe‐based nanoplatelets (NPLs) with fluorescence quantum yields (QYs) up to 50% is demonstrated. A layer‐by‐layer deposition technique is used which is based on a two‐phase mixture—consisting of a nonpolar phase, which includes the NPLs, and a saturated ionic polar phase—to separate the reagents and hinder the nucleation of the shell material. The deposition of the first sulfur layer leads to a significant redshift (by more than 100 nm) of the optical absorption and emission of the NPLs. Hence, by varying either the sulfur precursor content or the reaction time, one can precisely and continuously tune the absorption and emission maxima from 520 to 630 nm. This evolution of the absorption onset during the shell growth is explained quantitatively using density‐functional theory and atomistic statistical simulations. The emission can be further enhanced by exposure of the NPL solution to ambient sunlight. Finally, it is demonstrated that the core/shell NPLs can be transferred from the organic solution to aqueous media with no reduction of their QY, which opens the door to a broad range of practical applications.

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…In order to overcome limitations of conventional room‐temperature approach for the optical properties of core/shell NPLs, CdSe/CdS core/shell NPLs were here prepared through high‐temperature reactions (300 °C) with the low‐reactivity Cd(oleate) 2 and 1‐octanethiol as precursors. Briefly, we first synthesized four monolayers (≈1.2 nm) CdSe core NPLs with an emission at 513 nm, following a modified procedure reported previously . Figure a shows the schematic diagram of shell growth.…”

“…Synthesis of CdSe/CdS Core/Shell NPLs : CdSe/CdS core/shell NPLs were fabricated by optimizing the typical high‐temperature synthesis approach, the shell thickness could be controlled through the amount of Cd‐ and S‐precursors required for each ML. First, CdSe core NPLs (4 MLs) were synthesized using the reported recipe with slight modifications . Then, the CdS shells were epitaxially grown on the top and bottom surfaces of the CdSe core NPLs at high temperature (300 °C) .…”

“…At the same time, the inorganic shells could help to suppress the rapid nonradiative Auger recombination (AR) of excitons with lower Auger rates and much reduced emission intermittency at single particle level, thereby enabling to realize low‐threshold (≈6 µJ cm −2 ) amplified spontaneous emission (ASE) and obtain high threshold for gain saturation . Generally, the epitaxial growth of semiconductor shells in core/shell hetero‐nanoplatelets is performed via the colloidal atomic layer deposition (c‐ALD) method . However, as c‐ALD usually takes place at room temperature, such synthetic approach allows for CdSe/CdS NPLs with only limited QYs (≈20–40%) and stability .…”

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

“…6,7 They influence surface trap states and thereby control photoluminescence quantum yield. [8][9][10][11] Not every dangling bond can be passivated due to steric hindrance, as the ligand diameter typically exceeds the lattice constant of NC material and due to poor interaction of a facet with the ligands. An electron transfer from the d-shell of a surface atom to the ligand can provide surface magnetism.…”

Polarization‐Sensitive Ultraviolet Detection from Oriented‐CdSe@CdS‐Dot‐in‐Rods‐Integrated Silicon Photodetector

“…According to the previous literatures, the photon energy of 405 nm excitation light is far from the band‐edge of CdSe, where only the dielectric confinement of the electric field contributes to the excitation optical anisotropy . Because the size of CdSe seeds in the CdSe@CdS DIRs was much smaller than the exciton Bohr radius of CdSe (≈4.9 nm), the emission polarization from oriented CdSe@CdS DIRs combined the effects of dielectric and quantum confinement . In addition, we measured the excitation polarization of the composite film (0.2 wt%) at 355 nm and 473 nm (Figure S6, Supporting Information).…”