Ideal CdSe/CdS Core/Shell Nanocrystals Enabled by Entropic Ligands and Their Core Size-, Shell Thickness-, and Ligand-Dependent Photoluminescence Properties

Abstract

This work explored possibilities to obtain colloidal quantum dots (QDs) with ideal photoluminescence (PL) properties, i.e., monoexponential PL decay dynamics, unity PL quantum yield, ensemble PL spectrum identical to that at the single-dot level, single-dot PL nonblinking, and antibleaching. Using CdSe/CdS core/shell QDs as the model system, shell-epitaxy, ligand exchange, and shape conversion of the core/shell QDs were studied systematically to establish a strategy for reproducibly synthesizing QDs with the targeted properties. The key synthetic parameter during epitaxy was application of entropic ligands, i.e., mixed carboxylate ligands with different hydrocarbon chain length and/or structure. Well-controlled epitaxial shells with certain thickness (∼3-8 monolayers of the CdS shells) were found to be necessary to reach ideal photoluminescence properties, and the size of the core QDs was found to play a critical role in determining both photophysical and photochemical properties of the core/shell QDs. Effects of shape of the core QDs were unnoticeable, and shape of the core/shell QDs only affected photophysical properties quantitatively. Surface ligands, amines versus carboxylates, were important for photochemical properties (antiblinking and antibleaching) but barely affected photophysical properties as long as entropic ligands (mixed carboxylate ligands with distinguishable hydrocarbon chain lengths) were applied during epitaxy. Chemical environment (in polymer or in air), coupled with surface ligands, determined photochemical properties of the core/shell QDs with a given core size and shell thickness.

A White Light‐Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile

“…Importantly, WLE QDC exhibited square wave shaped on–off blinking, in time varying fluorescence intensity profile (Figure F; blinking profile was recorded for the particle, which was shown in Movie S1 in the Supporting Information). This is also in support of the single particle nature of the WLE QDC and ruled out the possibility of observation of multiple particles (which generally exhibit flickering in‐between on and dim states, as shown in Figure S7, Supporting Information) . Histogram (based on the blinking data obtained from Figure F) showed the occurrence of more number of on states, compared to off states, in the blinking profile of the WLE QDC and the on/off occurrence ratio was found to be 8.44 (Figure G).…”

“…In our case, the probability distributions of the events (on and off) could be fitted well by a truncated power law through a log–log plot extracted from the obtained data points for 40 single particles (Figure H,I), the details of which are described in the Experimental Section. It is to be mentioned here that the exponent of the power‐law distribution (known as α on and α off ) of the on and off events is an important parameter with regard to the luminescence behavior of a blinking single nanocrystal . Hence, any chemical reaction or physical interaction that alters the emission characteristics of a single nanocrystal may lead to change in the exponents (α on and α off ) and their ratio (α on /α off ) in the blinking statistics .…”

“…This is also in support of the single particle nature of the WLE QDC and ruled out the possibility of observation of multiple particles (which generally exhibit flickering in-between on and dim states, as shown in Figure S7, Supporting Information). [7,[31][32][33][34][35][36] Histogram (based on the blinking data obtained from Figure 1F) showed the occurrence of more number of on states, compared to off states, in the blinking profile of the WLE QDC and the on/off occurrence ratio was found to be 8.44 ( Figure 1G). In our case, the probability distributions of the events (on and off) could be fitted well by a truncated power law [31][32][33][34][35][36] through a log-log plot extracted from the obtained data points for 40 single particles ( Figure 1H,I), the details of which are described in the Experimental Section.…”

“…[7,[31][32][33][34][35][36] Histogram (based on the blinking data obtained from Figure 1F) showed the occurrence of more number of on states, compared to off states, in the blinking profile of the WLE QDC and the on/off occurrence ratio was found to be 8.44 ( Figure 1G). In our case, the probability distributions of the events (on and off) could be fitted well by a truncated power law [31][32][33][34][35][36] through a log-log plot extracted from the obtained data points for 40 single particles ( Figure 1H,I), the details of which are described in the Experimental Section. It is to be mentioned here that the exponent of the power-law distribution (known as α on and α off ) of the on and off events is an important parameter with regard to the luminescence behavior of a blinking single nanocrystal.…”

Sequential Chemistry Toward Core–Shell Structured Metal Sulfides as Stable and Highly Efficient Visible‐Light Photocatalysts

“…[2] Fore xample,i nm etal sulfide coreshell structures,t he larger band gap shell enables the charge carrier confinement within the cores,w hich can greatly improve the photoluminescence quantum yield and stability against photo bleaching.T he staggered alignment shells can promote the spatial charge-carrier separation, leading to reduced charge recombination and thus longer radiative lifetimes. [3] To date,v arious strategies have been developed for the synthesis of metal sulfide core-shell structures including the thermal-cycling coupled single precursor, [4] hot-injection method, [5] successive ion layer adhesion and reaction, [6] and colloidal atomic layer deposition. [7] These methods are majorly dependent on the epitaxial growth process.L attice match between the core and shell is the prerequisite.…”