Article Cite This: J. Am. Chem. Soc. 2017, 139, 16556-16567
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Ideal CdSe/CdS Core/Shell Nanocrystals Enabled by Entropic Ligands and Their Core Size‑, Shell Thickness‑, and Ligand-Dependent Photoluminescence Properties Jianhai Zhou, Meiyi Zhu, Renyang Meng, Haiyan Qin, and Xiaogang Peng* Center for Chemistry of Novel and High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou, 310027, P.R. China S Supporting Information *
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. Wellcontrolled 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.
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INTRODUCTION Colloidal semiconductor nanocrystals with sizes in the quantum confinement regime (quantum dots, QDs) are rapidly being developed as important luminescent materials for advanced technologies.1−7 Especially in the field of displays, color purity, high emission efficiency, stability, solution processability, and flexible excitation warrant QDs as the most desirable emitters.8,9 However, it is a nontrivial task to simultaneously realize all these properties for QDs at present. When luminescent and optoelectronic properties are of concern, key properties of colloidal QDs would be mainly controlled by their excited states. Though size- and shape-control of colloidal QDs has been widely studied, synthetic control of their excited states is in the early stage of understanding and development.10 An optimal synthetic scheme should yield QDs with ideal photophysical properties of their excited states, i.e., including ensemble photoluminescence (PL) peak width matching that of single-dot PL, unity PL quantum yield, and monoexponenital PL decay dynamics. For simplicity, we would use “intrinsic PL peak width” to express identical PL peak width of ensemble and single-dot PL. Simultaneously, photochemical properties of their excited states, mainly antiblinking and antibleaching, should also been synthetically controlled. PL blinking refers to PL intensity of single QD randomly switching between © 2017 American Chemical Society
distinguishable brightness levels under constant excitation, which can be regarded as reversible photochemical processes.11,12 PL bleaching is suspected to be irreversible photochemical etching of the QDs in their excited states.12−15 After tremendous efforts in the past decades, CdSe plain core QDs can reach ideal color purity,16 near unity PL quantum yield,17 and monoexponenital PL decay dynamics.17 However, these ideal photophysical properties have not yet been realized simultaneously. In addition, these CdSe plain core QDs are photochemically very unstable under typical manipulation and application conditions.17 Epitaxial growth of wide bandgap shells18−20 is a favorable solution to their photochemical challenges. CdSe/CdS ones are one of the most studied and best developed core/shell QD systems.19,21−34 CdS and CdSe form type-I band offsets. The top of valence band (bottom of conduction band) of CdS is ∼0.5 eV lower (∼0.3 eV higher) than those of CdSe,19 which provides wave function confinement for the photogenerated electron and hole of CdSe core QDs. However, the potential steps, especially that of the conduction bands, between CdSe and CdS are limited. This implies that optical properties of CdSe/CdS core/shell Received: July 17, 2017 Published: November 2, 2017 16556
DOI: 10.1021/jacs.7b07434 J. Am. Chem. Soc. 2017, 139, 16556−16567
Article
Journal of the American Chemical Society
Figure 1. Schematic illustration of three steps for synthesis of core/shell nanocrystals coated with different ligands and/or in different shapes. Step 1: Epitaxial growth onto core nanocrystals with either spherical (a) or hexahedral (a′) shape to yield hexahedral core/shell nanocrystals (b and b′). Step 2: Surface ligand exchange of as-synthesized hexahedral core/shell nanocrystals (b and b′) to yield core/shell nanocrystals coated with amine ligands (c and c′). Step 3: Conversion of hexahedral core/shell nanocrystals (c and c′) to spherical ones (d and d′) with the same amine ligands.
redesign the synthetic scheme for CdSe/CdS core/shell QDs based on those monodisperse and size/shape-controlled CdSe core QDs. Details of the new scheme in Figure 1 are provided in the Experimental Section. A brief description is provided here as necessary information for discussion. For simplicity, this report adopts a systematic abbreviation for three common types of substances, namely, CdSe QDs, fatty acids and their cadmium salts, and the core/shell QDs. If needed, CdSe core QDs are written as CdSe###s and CdSe###h with “###” representing the first-exciton absorption peak and “s” (or “h”) referring to spherical (or hexahedral) shape. The core/shell QDs are written as CdSe ###s/h/XCdS h and CdSe###s/h/XCdSs with “s” or “h” after “CdS” referring to the final shape of the core/shell QDs and “X” representing X number of the epitaxial CdS monolayers. When the core and core/shell shapes were different from each other, the number of CdS monolayers was defined as the equivalent thickness with the same shape of the core. Fatty acids and fatty acid salts shall be named as HFa and Cd(Fa)2, respectively. Here, “Fa” is the first two letters of the common name of a given fatty acid. The first step of the scheme in Figure 1 was size/shapecontrolled epitaxy of the CdS shells onto the purified CdSe core QDs in octadecene (ODE) using cadmium fatty acid salts and elemental S as the precursors. Similar to the growth of CdSe core QDs,16 the layer-by-layer epitaxy shown in Figure 1 (Step 1) would not occur without a high concentration of carboxylate ligands (Figure S1). Different from growth of the core QDs, free fatty acids needed to be added in doses during the epitaxy. The resulting core/shell QDs in this step were all hexahedral regardless of the shape of the CdSe core QDs (Figures 1b and b′). The CdSe/CdS core/shell QDs were solely coated with carboxylate ligands, which was different from those reported in the literature for CdSe/CdS core/shell QDs. At present, all high-quality CdSe/CdS core/shell QDs in either zinc-blende or wurtzite structure were synthesized with fatty amine as the main ligands though the ligand system would often be a complex mixture of fatty amines with other organic compounds.19,21−25,27−34 Step 2 in Figure 1 replaced the original Cd(Fa)2 ligands of the core/shell QDs from Step 1 by fatty amines, typically oleylamine (NH2Ol). For retaining the hexahedral shape of the
nanocrystals should still be sensitive to the shell thickness, shape of the core/shell QDs, and the surface ligands. Presumably, delocalization of wave functions into the CdS shell should be closely related to the kinetic energy of the excitons in CdSe core QDs. On the basis of quantum confinement,35 the latter is determined by the size of the CdSe core nanocrystals. Though CdSe/CdS core/shell QDs have been widely studied, effects of core size, shape of core and core/shell QDs, shell thickness, and surface ligands on their photophysical and photochemical properties are largely unknown and/or at an empirical level. This report shall describe a new strategy to systematically explore structural effects on photophysical and photochemical properties of CdSe/CdS core/shell QDs. Taking CdSe QDs with nearly intrinsic PL peak width and various sizes as the core for epitaxy, the new synthetic scheme produces CdSe/CdS QDs with monoexponential PL decay dynamics, intrinsic PL peak width, near-unity PL quantum yield, and antiblinking and antibleaching characteristics. The systematic variation of structural parameters included size of CdSe core QDs, shape of the core QDs, shell thickness, shape of the core/shell QDs, and surface ligands.
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RESULTS AND DISCUSSION Design of Reaction Scheme. Recently, monodisperse CdSe QDs with intrinsic PL peak width were reported in either hexahedral or spherical shape,16 which would be applied as the core QDs in this work (Figure 1a and 1a′). These shapecontrolled CdSe QDs were grown with zinc-blende structure, which not only offered the narrowest PL peak width, nearly identical to that at single-dot level, but also were single crystalline.16 Early work found that it was possible to grow 1−2 monolayers of the CdS shells onto the hexahedral CdSe QDs, which retained narrow PL peak width of the core QDs. However, both PL quantum yield and PL decay dynamics of the thin-shell core/shell QDs were quite far away being ideal, and core/shell QDs were photochemically unstable. For instance, single-dot measurements revealed that single hexahedral core/shell QDs with thin CdS shells blinked significantly.16 For the goals of this work to be fulfilled, it is necessary to 16557
DOI: 10.1021/jacs.7b07434 J. Am. Chem. Soc. 2017, 139, 16556−16567
Article
Journal of the American Chemical Society
which were considered as prerequisite for the layer-by-layer growth.16 This was true no matter how long the hydrocarbon chain was for the fatty acid salts and the shape the core QDs. According to the literature, reaction of S under elevated temperatures with ODE should generate H2S,36 which should be very reactive toward cadmium carboxylate either on the surface of QDs or in solution. Excess H2S adsorbed on the surface of the core/shell QDs would become strong hole traps.37 Dropwise addition of the S-ODE solution would benefit the layer-by-layer growth and removal of the H2S hole traps. Temperature significantly deviating from 250 °C would diminish the effects of dropwise addition of the S precursor (Figure S4). Results in Figure S4 revealed that high temperatures would accelerate removal of excess H2S on surface of the core/shell QDs. However, when the temperature was too high, it would broaden the size distribution of the core/ shell QDs by affecting perfection of the hexahedral shape (Figure S4). An excess amount of free acid was found to be necessary for achieving the layer-by-layer growth for either CdSe core or CdSe/CdS core/shell hexahedra. However, instead of addition of all free acid in the beginning for growing the core QDs,16 free acid must be added in doses during the epitaxy. Results (Figure S5) revealed that, either without a sufficient amount of free acid or by addition of all free acid in the beginning, PL peak width of the core/shell QDs would be broadened, the PL quantum yield would be relatively low, and the PL decay dynamics would become multiexponential. In principle, free fatty acids at a relatively high level should assist in removing excess H2S-related species on the QDs, such as -SH,37 and rapidly dissolve tiny CdS nuclei formed by the reaction between the shelling precursors. Addition of all free acid in the initial reaction solution would cause dissolution of the CdSe core QDs, which might not be a problem for growing core nanocrystals but would be a serious issue for the epitaxy. Under the optimized reaction conditions for epitaxy, the size and shape distribution of the core/shell QDs remained monodisperse (Figures 3a and Figure S6). Along with growth of the CdS shells, the absorption and PL spectra continuously shifted to red while remaining narrow and sharp (Figure 3b). The PL peak widths in Figure 3b were found to be similar to the single-dot values reported in the literature16,38 (see details below). The photoluminescence excitation (PLE) at different emission wavelengths (see examples in Figure 3b, inset) almost overlapped with each well consistent with the monodisperse size/shape distribution. The PL decay dynamics of CdSe core QDs used in this work were multiexponential (Figure 3c), and its PL quantum yield was
