Article pubs.acs.org/cm
Monodisperse Hexagonal Pyramidal and Bipyramidal Wurtzite CdSeCdS Core−Shell Nanocrystals Rui Tan,† Yucheng Yuan,† Yasutaka Nagaoka,† Dennis Eggert,‡,§ Xudong Wang,# Sravan Thota,# Peng Guo,∥ Hongrong Yang,†,¶ Jing Zhao,# and Ou Chen*,† †
Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States Max Planck Institute for the Structure and Dynamics of Matter, Hamburg 22761, Germany § Heinrich Pette Institute-Leibniz Institute for Experimental Virology, Hamburg 20251, Germany # Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269, United States ∥ Vascular Biology Program, Children’s Hospital Boston, Harvard Medical School, Boston, Massachusetts 02115, United States ¶ Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, China ‡
S Supporting Information *
ABSTRACT: Heterostructural core−shell quantum dots (hetero-QDs) have garnered a copious amount of research effort for not only scientific advances but also a range of technological applications. Particularly, controlling the heteroshell deposition, which in turn determines the particle morphology, is vital in regulating the photophysical properties and the application potential of the hetero-QDs. In this work, we present the first report on a synthesis of pyramidal shaped (i.e., hexagonal pyramid, HP, and hexagonal bipyramid, HBP) CdSe-CdS hetero-QDs with high morphological uniformity and epitaxial crystallinity through a two-step shell growth method. The stabilization of the exposed (0002) and {101̅1} facets by octadecylphosphonic acid and oleic acid ligands, respectively, is the key for the formation of pyramidal particle shapes. High photoluminescence quantum yield (94%, HP-QDs and 73%, HBP-QDs), minimal inhomogeneous PL line width broadening, and significantly suppressed single-QD blinking are observed. Specifically, the “giant” HBP-QDs showed an average “On” time fraction of 96% with more than 50% of measured particles completely nonblinking. Additionally, high multiexciton emission, prolonged ensemble and single-QD PL lifetimes as compared to their spherical counterparts are also reported. Finally, the HBP-QDs have been successfully transferred into an aqueous solution without aggregation. High cellular uptakes associated with low cytotoxicity render these water-soluble HBPQDs an excellent candidate for intracellular imaging and labeling.
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the first report of the synthesis of high-quality monodispersed CdSe-CdS core−shell QDs by Alivisatos et al. in 1997,23 this particular system has emerged as arguably one of the most studied model systems among all of the known heterostructural QDs (hetero-QDs). The popularity of the CdSe-CdS core− shell system could be due to the established syntheses and extensive knowledge of CdSe QDs24−26 and the common crystal structures (i.e., wurtzite, WZ, and zinc-blende, ZB) with minimal crystal lattice mismatch (3.9%) between core and shell materials.23,27 Moreover, given the band gap of bulk CdSe (∼1.74 eV) and the quasi-Type-II band structural alignment (strongly confined “hole” and loosely confined “electron”) between the CdSe core and CdS shell, the tunable emission color of the CdSe-CdS hetero-QDs covers a large portion of the visible spectrum. The dramatically different excited
INTRODUCTION Colloidal quantum dots (QDs) are semiconductor nanocrystals whose exciton (excited electron−hole pair) wave functions are physically confined in all directions due to their nanoscale spatial dimensions.1 As a consequence, the photophysical properties of the QDs can be precisely tuned, not only by their material composition, but also by the size, shape, and structure of the QDs.2,3 This unique feature offers QD materials exceptional properties, such as high absorption cross sections, tunable absorption and emission profiles, high photoluminescence quantum yields (PL QYs) and stable PL output against photo- and physical-degradations.4,5 These properties together with solution processability and versatile surface functionality of colloidal QDs have been advertised in a range of applications including displays,6−10 lasers,11,12 photodetectors,13,14 biological sensing, tracking and imaging,15−18 etc.19,20 To realize these properties and utilize them in potential applications, QDs with a core−shell heterostructure have proven to be not only beneficial but necessary.5,21,22 Ever since © 2017 American Chemical Society
Received: March 8, 2017 Revised: April 17, 2017 Published: April 18, 2017 4097
DOI: 10.1021/acs.chemmater.7b00968 Chem. Mater. 2017, 29, 4097−4108
Article
Chemistry of Materials
Figure 1. Absorption (a) and PL (b) spectral evolution during the shell growth reaction. (c) Variations of absorption peaks (blue diamond), PL peaks (blue circle) and PL QYs (red square) during the shell growth reaction, empty markers indicate the 2nd growth after purification. TEM images of CdSe-CdS HP-QDs (d) and CdSe-CdS HBP-QDs (e). Distributions of three characteristic dimensions of HP-QDs (f) and HBP-QDs (g) with typical standard deviations of 5−7%. High-resolution TEM (HR-TEM) images of HP-QDs (h) and HBP-QDs (i) in [101̅1] projection labeled with three characteristic dimensions.
core−shell hetero-QDs. Particle size, shape, and structural characterizations by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron tomography revealed high crystallinity and particle uniformity of the resultant QDs and confirmed the proposed HP and HBP shapes. Especially, given the large shell volume, the HBP-QDs can be classified as g-QDs with a high morphological uniformity. Optical characterizations of the HP and HBP hetero-QDs at both single-QD and ensemble levels showed the superior optical properties. In particular, the “giant” HBP-QDs exhibit high PL QYs, enhanced high-energy photon absorption, marginal inhomogeneous PL broadening, significantly suppressed blinking, efficient multiexciton emission, and prolonged PL lifetimes as compared to the spherical g-QDs. In addition, a mechanism of ligand-induced pyramidal QD formation has been proposed based on monitoring the particle shape and surface ligand evolutions at different growth stages. In vitro cytotoxicity tests and cellular uptake experiments showed that the PEGylated HBP-QDs exhibited minimal cytotoxicity and high cellular uptakes (∼3 times as much as for spherical particles) in different cell-lines, illustrating their application potentials in intracellular labeling and imaging.
photocarrier dynamics (delocalized electron vs confined hole) fuel additional interests for this hetero-QD system. Discoveries made in the CdSe-CdS system have played important roles in the hetero-QD field over the past 2 decades. For example, the successive ion layer adsorption and reaction (SILAR) technique was first introduced to the field for growing a CdS shell on a CdSe core in a layer-by-layer manner28 but has since then been applied to a variety of hetero-QD syntheses.29−32 Additionally, shape-controlled syntheses for hetero-QDs taking advantage of different crystal symmetry characteristics were pioneered using CdSe-CdS QDs as a model system.33−36 More recently, despite the controversy on the physical origin of PL “blinking” (single-QD fluorescence intermittency under continuous excitation), chemical synthetic strategies designed to overcome blinking were also initially reported for CdSe-CdS QDs.27,37,38 These seminal discoveries, along with yet to be fully understood, make studies of the CdSe-CdS core−shell QDs persistently intriguing. Within the hetero-QD family, “giant” core−shell QDs (gQDs), typically defined as QDs with a shell thickness equal to or larger than 15-monolayer (ML) of the shell material, have gradually developed into a distinct category since their first demonstration.37,38 Many unique and interesting properties of g-QDs have been discovered including suppressed single-QD blinking,37−40 reduced Auger recombination,41,42 efficient multiexciton emission,43−45 unusual dual-band emission,46,47 large Stokes shift,48,49 etc. These advanced photophysical properties enable g-QDs to have superior performances in applications such as light-emitting diodes and luminescence solar concentrators.41,48−50 However, because of extremely large shell volume deposition, it remains challenging to synthesize g-QDs simultaneously exhibiting high particle uniformity, narrow emission profiles with minimized inhomogeneous line width broadening, and high PL QYs. Additionally, to date, most developments have been focused on studying gQDs with a spherical or quasi-spherical shape. Bals et al. showed a very recent example of synthesizing CdSe-CdS g-QDs with a bullet shape;51 however, very limited ensemble and no single-particle optical data were provided. In this work, we report the first synthesis of hexagonal pyramid (HP) and hexagonal bipyramid (HBP) CdSe-CdS
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RESULTS AND DISCUSSION Synthesis and Characterizations of the HP- and HBPQDs. To form atomically flat crystal facets for the final heteroQDs, three major factors have been taken into consideration for the experimental design: (1) precursors with relatively low reactivity (i.e., Cd-oleate and octanethiol as Cd and S sources) are needed to ensure a slow epitaxial shell formation with a high crystallinity; (2) in contrast to the spherical hetero-QD synthesis using the same combination of the shell precursors, neither oleic acid addition nor high temperature thermal annealing should be performed, both of which will round the developed flat atomic facets and lead to a thermodynamically favored spherical shape;27,49 (3) to form large core−shell QDs with flat crystal facets, an intermediate purification step is critical to remove the unreacted precursors and byproducts accumulated during the shell growth reaction. Indeed, the hetero-QDs synthesized without the intermediate purification step showed polydispersed sizes and shapes (Figure S1). 4098
DOI: 10.1021/acs.chemmater.7b00968 Chem. Mater. 2017, 29, 4097−4108
Article
Chemistry of Materials
Figure 2. HAADF-TEM images, elemental mapping and line-scan for a HP (a, c) and a HBP (b, d) CdSe-CdS core−shell hetero-QD.
On the basis of the above considerations, we have designed a two-step shell growth method separated by an intermediate purification step for synthesizing pyramidal-shaped core−shell QDs (see the Supporting Information for the detailed procedure). The entire shell growth reaction was monitored by absorption and PL spectroscopies. During the reaction, the absorption features and the PL peak shifted to a longer wavelength region (Figure 1a−c) as a result of the quasi-TypeII band structure alignment (weak exciton confinement with largely delocalized electron) of the CdSe-CdS hetero-QDs.27,28 The large increase of the absorbance in the wavelength range below 500 nm during the second-growth step indicates a thick CdS shell formation (Figure 1a), consistent with the band-edge absorption of CdS bulk material (2.42 eV, ∼512 nm).27,28 Interestingly, the high energy photon absorbance (
