Monodisperse Hexagonal Pyramidal and Bipyramidal Wurtzite CdSe-CdS Core−Shell NanocrystalsRui 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 researcheffort for not only scientific advances but also a range oftechnological applications. Particularly, controlling the hetero-shell deposition, which in turn determines the particlemorphology, is vital in regulating the photophysical propertiesand 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 uniformityand epitaxial crystallinity through a two-step shell growthmethod. The stabilization of the exposed (0002) and {101 1} 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-QDsand 73%, HBP-QDs), minimal inhomogeneous PL line width broadening, and significantly suppressed single-QD blinking areobserved. Specifically, the “giant” HBP-QDs showed an average “On” time fraction of 96% with more than 50% of measuredparticles completely nonblinking. Additionally, high multiexciton emission, prolonged ensemble and single-QD PL lifetimes ascompared to their spherical counterparts are also reported. Finally, the HBP-QDs have been successfully transferred into anaqueous solution without aggregation. High cellular uptakes associated with low cytotoxicity render these water-soluble HBP-QDs an excellent candidate for intracellular imaging and labeling.

■ INTRODUCTIONColloidal quantum dots (QDs) are semiconductor nanocrystalswhose exciton (excited electron−hole pair) wave functions arephysically confined in all directions due to their nanoscalespatial dimensions.1 As a consequence, the photophysicalproperties of the QDs can be precisely tuned, not only by theirmaterial composition, but also by the size, shape, and structureof the QDs.2,3 This unique feature offers QD materialsexceptional properties, such as high absorption cross sections,tunable absorption and emission profiles, high photolumines-cence quantum yields (PL QYs) and stable PL output againstphoto- and physical-degradations.4,5 These properties togetherwith solution processability and versatile surface functionality ofcolloidal QDs have been advertised in a range of applicationsincluding displays,6−10 lasers,11,12 photodetectors,13,14 bio-logical sensing, tracking and imaging,15−18 etc.19,20

To realize these properties and utilize them in potentialapplications, QDs with a core−shell heterostructure haveproven to be not only beneficial but necessary.5,21,22 Ever since

the first report of the synthesis of high-quality monodispersedCdSe-CdS core−shell QDs by Alivisatos et al. in 1997,23 thisparticular system has emerged as arguably one of the moststudied model systems among all of the known heterostructuralQDs (hetero-QDs). The popularity of the CdSe-CdS core−shell system could be due to the established syntheses andextensive knowledge of CdSe QDs24−26 and the commoncrystal structures (i.e., wurtzite, WZ, and zinc-blende, ZB) withminimal crystal lattice mismatch (3.9%) between core and shellmaterials.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 emissioncolor of the CdSe-CdS hetero-QDs covers a large portion ofthe visible spectrum. The dramatically different excited

Received: March 8, 2017Revised: April 17, 2017Published: April 18, 2017

Article

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© XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.7b00968Chem. Mater. XXXX, XXX, XXX−XXX

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 growinga CdS shell on a CdSe core in a layer-by-layer manner28 but hassince then been applied to a variety of hetero-QDsyntheses.29−32 Additionally, shape-controlled syntheses forhetero-QDs taking advantage of different crystal symmetrycharacteristics were pioneered using CdSe-CdS QDs as a modelsystem.33−36 More recently, despite the controversy on thephysical origin of PL “blinking” (single-QD fluorescenceintermittency under continuous excitation), chemical syntheticstrategies designed to overcome blinking were also initiallyreported for CdSe-CdS QDs.27,37,38 These seminal discoveries,along with yet to be fully understood, make studies of theCdSe-CdS core−shell QDs persistently intriguing.Within the hetero-QD family, “giant” core−shell QDs (g-

QDs), typically defined as QDs with a shell thickness equal toor larger than 15-monolayer (ML) of the shell material, havegradually developed into a distinct category since their firstdemonstration.37,38 Many unique and interesting properties ofg-QDs have been discovered including suppressed single-QDblinking,37−40 reduced Auger recombination,41,42 efficientmultiexciton emission,43−45 unusual dual-band emission,46,47

large Stokes shift,48,49 etc. These advanced photophysicalproperties enable g-QDs to have superior performances inapplications such as light-emitting diodes and luminescencesolar concentrators.41,48−50 However, because of extremelylarge shell volume deposition, it remains challenging tosynthesize g-QDs simultaneously exhibiting high particleuniformity, narrow emission profiles with minimized inhomo-geneous line width broadening, and high PL QYs. Additionally,to date, most developments have been focused on studying g-QDs with a spherical or quasi-spherical shape. Bals et al.showed a very recent example of synthesizing CdSe-CdS g-QDswith a bullet shape;51 however, very limited ensemble and nosingle-particle optical data were provided.In this work, we report the first synthesis of hexagonal

pyramid (HP) and hexagonal bipyramid (HBP) CdSe-CdS

core−shell hetero-QDs. Particle size, shape, and structuralcharacterizations by powder X-ray diffraction (XRD), trans-mission electron microscopy (TEM), and electron tomographyrevealed high crystallinity and particle uniformity of theresultant QDs and confirmed the proposed HP and HBPshapes. Especially, given the large shell volume, the HBP-QDscan be classified as g-QDs with a high morphologicaluniformity. Optical characterizations of the HP and HBPhetero-QDs at both single-QD and ensemble levels showed thesuperior optical properties. In particular, the “giant” HBP-QDsexhibit high PL QYs, enhanced high-energy photon absorption,marginal inhomogeneous PL broadening, significantly sup-pressed blinking, efficient multiexciton emission, and prolongedPL lifetimes as compared to the spherical g-QDs. In addition, amechanism of ligand-induced pyramidal QD formation hasbeen proposed based on monitoring the particle shape andsurface ligand evolutions at different growth stages. In vitrocytotoxicity tests and cellular uptake experiments showed thatthe PEGylated HBP-QDs exhibited minimal cytotoxicity andhigh cellular uptakes (∼3 times as much as for sphericalparticles) in different cell-lines, illustrating their applicationpotentials in intracellular labeling and imaging.

■ RESULTS AND DISCUSSION

Synthesis and Characterizations of the HP- and HBP-QDs. To form atomically flat crystal facets for the final hetero-QDs, three major factors have been taken into consideration forthe experimental design: (1) precursors with relatively lowreactivity (i.e., Cd-oleate and octanethiol as Cd and S sources)are needed to ensure a slow epitaxial shell formation with a highcrystallinity; (2) in contrast to the spherical hetero-QDsynthesis using the same combination of the shell precursors,neither oleic acid addition nor high temperature thermalannealing should be performed, both of which will round thedeveloped flat atomic facets and lead to a thermodynamicallyfavored spherical shape;27,49 (3) to form large core−shell QDswith flat crystal facets, an intermediate purification step iscritical to remove the unreacted precursors and byproductsaccumulated during the shell growth reaction. Indeed, thehetero-QDs synthesized without the intermediate purificationstep showed polydispersed sizes and shapes (Figure S1).

Figure 1. Absorption (a) and PL (b) spectral evolution during the shell growth reaction. (c) Variations of absorption peaks (blue diamond), PLpeaks (blue circle) and PL QYs (red square) during the shell growth reaction, empty markers indicate the 2nd growth after purification. TEM imagesof CdSe-CdS HP-QDs (d) and CdSe-CdS HBP-QDs (e). Distributions of three characteristic dimensions of HP-QDs (f) and HBP-QDs (g) withtypical standard deviations of 5−7%. High-resolution TEM (HR-TEM) images of HP-QDs (h) and HBP-QDs (i) in [101 1] projection labeled withthree characteristic dimensions.

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On the basis of the above considerations, we have designed atwo-step shell growth method separated by an intermediatepurification step for synthesizing pyramidal-shaped core−shellQDs (see the Supporting Information for the detailedprocedure). The entire shell growth reaction was monitoredby absorption and PL spectroscopies. During the reaction, theabsorption features and the PL peak shifted to a longerwavelength region (Figure 1a−c) as a result of the quasi-Type-II band structure alignment (weak exciton confinement withlargely delocalized electron) of the CdSe-CdS hetero-QDs.27,28

The large increase of the absorbance in the wavelength rangebelow 500 nm during the second-growth step indicates a thickCdS shell formation (Figure 1a), consistent with the band-edgeabsorption of CdS bulk material (2.42 eV, ∼512 nm).27,28

Interestingly, the high energy photon absorbance (<500 nm)for the obtained samples were dramatically higher than theirspherical counterparts containing the same core and shellvolumes (Figure S2). The PL excitation measurement ruled outthe possibility of nonemissive organic ligand contribution,indicating the particle shape (i.e., the shape of the CdS shell)was at play (Figure S2). The PL QY increased monotonicallyduring the first-shell-growth reaction, followed by a slightdecrease during the second-shell-growth process (Figure 1c).The PL QYs for the samples obtained from the first- andsecond-shell-growth reactions were 94% and 73%, respectively.In addition, the PL decay measurements showed an ensemblelifetime of 49.7 ns for the first-shell-growth sample and 242 nsfor the second-shell-growth sample (Figure S2). Both PLdecays were dramatically slower than that of the spherical QDswith the same core and shell volumes (Figure S2).TEM images show that the resultant core−shell QDs display

two-dimensional (2D) outlines of a combination of triangular/truncated triangular (rhombus) and hexagonal shapes after thefirst-shell-growth (2nd-shell-growth) reaction (Figure 1d,e).Considering the different 2D outlines observed in TEM imagesare from different projections of the 3D QDs, we proposed thatthe particle shapes are HP and HBP (Figure 1d,e, insets). Theverification for the HP and HBP shapes as well as the crystalfacet determination of the core−shell QDs are discussed ingreater detail in the following section. Unlike spherical QDs,the size and size distribution of the HP- and HBP-QDs weredetermined by three characteristic dimensions: d, diagonal ofthe hexagonal base; l, the projection of side length; and a, slantheight for the HP-QDs, or the projected distance between twoapexes for the HBP-QDs (Figure 1h,i and Schemes S1 and S2,see detailed analysis in the Supporting Information). The

distributions of these three dimensions for the HP- and HBP-QDs are shown in Figure 1f, g and Table S1, demonstratinghigh uniformities for both samples. The particle volumes ofHP- and HBP-QDs were calculated to be 189 ± 27 nm3 and1481 ± 171 nm3, respectively (Table S2). It is worthmentioning that the shell volume of the HBP-QDs (∼1450nm3) is equivalent to a thickness of ∼16-ML of CdS shell whenconverted to a spherical core−shell QD. Given this large shellvolume, the HBP-QDs can be classified as g-QDs with a highmorphological uniformity.37,52

To verify the core−shell heterostructure of the HP- andHBP-QDs, high-angle annular dark-field scanning TEM(HAADF-STEM) was used to map out the atomic distributionsof Cd, S, and Se atoms. The elemental mapping results clearlyshow that the Cd and S atoms are distributed in the entire QD,while Se atoms are only located at the center of the particle(Figure 2a,b and Figure S3). The line-scans show that the Seatoms distribute across a distance of ∼3.8 nm for both samples(Figure 2c,d), in good agreement with the CdSe core diameterof 3.9 nm used for the shell growth reactions. This resultindicates an epitaxial rather than alloying process occurred inthe growth of the shell material, as also suggested by thecontinuous red-shifts of both absorption and PL features(Figure 1a,b) and a relatively short reaction time. However,slight interdiffusion of anions at the core−shell interface, whichis difficult to identify, may still occur during the hightemperature (i.e., 310 °C) reactions performed here.The XRD measurements unambiguously showed the WZ

crystal structure of both the HP- and HBP-QDs with thefingerprint Bragg peaks of (1012) and (101 3) (Figure 3a),demonstrating epitaxial CdS shell formations on WZ CdSecores. Notably, the (0002) Bragg peaks for both samples arenarrower than the peaks for spherical QD samples with thesame particle volumes, suggesting a larger (0002) domain sizefor the HP- and HBP-QDs (Figure 3b). These results revealthat the fastest growth direction of the HP- and HBP-QDs isalong the (0002) direction (Z-direction of the WZ crystalstructure), consistent with previous reports for WZ nanorodgrowth.33−35,53

3D Shape and Crystal Facet Determinations of theHP- and HBP-QDs. Regular TEM images only display 2Dprojection outlines of the 3D QDs. In order to accuratelydetermine the 3D shape of the QDs, volume reconstructionswere carried out using electron tomography techniques. For the3D electron tomography, a set of TEM images was acquired bytilting the specimens over an angle range of ±65° at a 1°

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.

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increment. The 3D volume reconstructions were performedusing the sequential iterative reconstruction technique (SIRT)algorithm and visualized using a Bitplane Imaris software (seedetails in the Supporting Information). As shown in Figure 4

and Movies S1, S2, S3, and S4, the 3D shapes of the QDsresulted from the first-and second-shell growth reactions wereunambiguously determined to be HP and HBP, respectively.These reconstructed 3D geometries show a high consistencywith the particle shape 2D outlines observed in the regularTEM images (Figure 1d,e and Figures S4 and S5). In order tofully understand the shape, crystal facets, and atomicorientations of these core−shell QDs, detailed crystallographicanalyses based on a series of high-resolution TEM (HR-TEM)measurements have been performed and discussed as follows.HR-TEM measurements of the HP-QDs clearly demonstrate

the existence of different particle projections (Figure 5 andFigure S6). Statistical analysis shows that only ∼5% of the HP-QDs display the projection with a hexagonal-shaped outline,while the majority of the QDs display a truncated-triangular

outline (Figure 1d,h). The center of Figure 5 shows theproposed HP-QD model with four defined directions (i.e., a1,a2, a3, and Z, Figure 5, center left) and a computer generatedatomic model of the HP WZ crystal with one hexagonal (0002)facet as a base and six equivalent {1011} facets on the lateralsides (Figure 5, center right). Each of the four panels in Figure5 (Figure 5a−d) is composed of a representative HR-TEMimage of an individual QD with a certain projection (left), thecorresponding fast-Fourier transform (FFT) pattern (righttop), the atomic model (right bottom), and its simulatedelectron diffraction pattern (right middle) after a one- or two-step rotation along the Z and X axes from the central atomicmodel.Figure 5a shows a clear hexagonal outline and atomic cross-

fringes with a measured d-spacing of 3.6 Å, which is associatedwith the {1010} planes of WZ CdS material, suggesting that theparticle is viewed along the [0001] zone axis. When viewedalong the [11 01] zone axis (Figure 5b), the particle insteadshows characteristic rectangular cross-fringes from its (112 0)and (1104 ) plane families with measured d-spacings of 2.1 and1.5 Å, respectively. Both the outline of the HR-TEM image andthe corresponding FFT pattern perfectly match the computergenerated atomic model (Figure 5b). Furthermore, we alsoobserved a low population (∼4%) of QDs with high-orderprojections (Figure 5c,d and Figure S6). For example, Figure 5cshows the particle viewed along the [2 575 ] zone axis, displayingthe (1 101 ) and (1 013) planes. The cross-fringes in thisprojection exhibit an angle of 80.6° (Figure 5c), which is ingood agreement with the theoretically calculated value of80.50°. In addition, Figure 5d shows the particle viewed fromthe [5 053 ] projection with the cross-fringe angle of 117.2°,matching well to the theoretical value of 117.32° between the(11 03) and (12 10) planes. The measured d-spacings of 1.9 and2.1 Å are in excellent agreement with the calculated d-spacingsof (11 03) and (12 10) planes. These results unequivocallyvalidate the 3D HP-shape, as well as crystal facets and atomicorientations of the hetero-QDs synthesized from the first-shell-growth reaction.Similar crystallographic analyses have been applied to the

HBP-QDs. The model was generated by cutting a bulk WZCdS crystal into a HBP-shape with 12 equivalent {101 1}surfaces merging into two apexes (Figure 6, center). Most ofthe HBP-QDs seen on the TEM substrate display a rhombusoutline, while only a low population (∼1%) show hexagonaloutlines (Figure 6g and Figure S7). Figure 6 shows a series ofHR-TEM images of the HBP-QDs and the correspondingcomputer simulated atomic models. The particles with threecommon projections viewed along the [1 100], [1 101], and[0001] zone axes are shown in Figure 6b,c,g, respectively. Theatomic fringes, FFT patterns, as well as the measured latticedistances can all be precisely matched to the model (Figure 6,center) after completing certain rotations (Figure 6b,c,g). Forexample, the [1 101] projection model can be achieved byrotating 30° along the Z axis, followed by a rotation of 25.11°along the X axis from the center model (Figure 6c). In thisprojection, (011 2), (1012 ), and (112 0) planes can be seen withd(011 2 ) = d(1012 ) = 2.5 Å and d(112 0) = 2.1 Å. The cross-fringe angles of 126.2° between (011 2 ) and (112 0) planes and107.6° between (0112 ) and (1 012 ) planes match well with thetheoretical calculated values of 126.32° and 107.36°,respectively. The HBP-QDs projected from [1011 ] and[1 011 ] directions can also be frequently seen in HR-TEMimages (Figure 6d,h).

Figure 3. (a) XRD spectra of the HP, HBP, and the correspondingspherical CdSe-CdS core−shell QDs with the same particle volumes.The green and blue bars show the positions of standard XRD peaks forbulk WZ-CdSe and WZ-CdS, respectively. (b) The zoomed-in XRDspectra for the highlighted area in part a to show the (101 0), (0002),and [101 1] peaks. Red lines, XRD spectra; black dotted-lines, fittedXRD spectra; blue lines, fitted peaks.

Figure 4. Images of the reconstructed volumes via electrontomography for a HP CdSe-CdS core−shell QD (a) and a HBPCdSe-CdS core−shell QD (b) along three different viewing directions.

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Furthermore, we have also observed a low population (<5%)of the HBP-QDs viewed from high-order projections (Figure6a, e, f and Figure S7). For example, in Figure 6a, the (1013)and (011 1) planes with d(1013) = 1.9 Å and d(011 1) = 3.2 Åwere observed with a measured cross-fringe angle of 81.4°(theoretical calculated angle of 81.48°), suggesting the [43 13]projection. The coexistence of the orthogonal planes of (01 12)and (2110) in Figure 6e and the (1 102) and (101 0) planes withan angle of 70.3° in Figure 6f were observed for the particlesviewed along the [044 7] and [2 425 ] zone axes, respectively.The particles with these unusual high-order projections arelikely due to squeezing by nearby particles or laying on anuneven surface of the TEM substrate.54−56

Taken together, 2D projection outlines, atomic latticefringes, and the corresponding FFT patterns of the HR-TEMimages for individual core−shell hetero-QDs can all beperfectly matched to the computer generated atomic modelsand the corresponding simulated electron diffraction patterns atdifferent viewing projections (Figures 5 and 6 and Figures S6and S7). These results explicitly prove the correctness of theproposed models shown in Figures 5 and 6, demonstrating ahigh degree of agreement between the proposed models andthe shape, facet, and atomic orientation of the obtained hetero-QDs.Ligand-Induced Particle Shape Evolution during the

Shell Growth. The complete characterization and analysis ofthe crystal facets and atomic orientations of the HP- and HBP-QDs allowed us to suggest a mechanism and reconstruct theparticle shape evolution. Figure 7 shows a scheme of theformation mechanism for HP- and HBP-QDs in the two-stepgrowth procedure (Figure 7a) and the TEM images of the QDsamples collected at the corresponding growth stages (Figure7b−f).

It is known that Cd-chalcogenide QDs can be well-passivatedby efficient bonding ligands such as octadecylphosphonic acid(ODPA), oleic acid (OA), and oleylamine (OAm) through theinteractions with surface Cd-sites.57−60 Particularly, the CdSecores used in this study are surface-coated by ODPA ligands(Figure S8).35 During the first-shell-growth reaction, the Cd-rich (0002) facet was stabilized by strong ODPA bonding toCd2+ cations,33,58,59 which led to a slow shell deposition thatconsequently developed into an atomically flat hexagonal base(Figure 7a−c). In contrast, the less stable chalcogenide-rich(0002) facet exhibited a fast epitaxial shell growth thatsubsequently developed to an apex (Figure 7a−c). Concom-itantly, Cd-rich {101 1} facets, which were stabilized by oleateligands (from the Cd precursor) through covalent chelatingbidentate interactions (Figure S8 and Scheme S3), emerged onthe lateral sides of the particle (Figure 7a, Step 1).61,62 Theoverall growth fashion led to a HP particle shape during thefirst-shell-growth (Figure 7a−c). The coexistence of OA andODPA ligands on the HP-QD surface was also proved by 1HNMR and 31P NMR (Figure S9). Unlike CdSe-CdS nanorod ornanobullet syntheses,33−35,51 the absence of the {1010} and{112 0} facets indicates their instability as compared to the{101 1} facets under the present growth conditions. Next, anintermediate purification step is essential for the followingdevelopment of the HBP shape as it removes the excess ODPAligands from the reaction solution (Figure S9). In the second-shell-growth reaction, a particle shape conversion from HP tosphere was observed at the initial stage (Figure 7a,d). A similarQD shape conversion has been observed recently by Peng etal.54 Importantly, during this shape conversion associated withsurface atom rearrangement processes, the remaining surface-bonded ODPA ligands from the first-step were totally removeddue to the presence of a large excess of fresh OAm (solvent)

Figure 5. HR-TEM images for core−shell HP-QDs. Center models define the shape (left) and atomic orientation (right) of the HP-QDs. The[1 120] projection (viewing along the green arrow in the center left model) is set as the starting point of rotations. Each of the four panels (a-d) iscomposed of one representative HR-TEM image (left image) for an individual QD with a certain projection, the corresponding FFT pattern of theHR-TEM image (top right), and the computer generated atomic model (bottom right) and its corresponding simulated electron diffraction pattern(middle right) after a one- or two-step rotation from the center atomic model. The detailed rotation fashions are indicated by the red and bluearrows along the Z and X (the axis normal to both Z and a2 axes) axes, respectively. (a)-(d) show the HP-QDs viewed along [0001], [11 01 ], [2 575 ],and [5053 ] zone axes, respectively.

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and the addition of Cd-oleate and OA in the shell precursorsolution (Figure S8). As a result, the QDs’ surface was coveredby both oleate and OAm but no ODPA as proved by the FT-IRmeasurements (Figure S8). In contrast to the first-shell-growthreaction, the absence of the surface ODPA ligands and thepresence of excess OAm and OA diminished the growthpreference between the (0002) and (0002 ) directions (FiguresS8 and S9).58,63 Thus, the CdS shell grew simultaneously inboth directions during the second-shell-growth (Figure 7). Atthe same time, the oleate stabilized {101 1} facets graduallydominated the particle surface, resulting in first a hexagonalbifrustum shape (Figure 7a,e) and finally a HBP shape (Figure

7a,f). Evidently, the FT-IR characterizations and NMRmeasurements show that the final HBP-QDs’ surface is solelypassivated by bidentate oleate ligand without OAm and ODPA(Figures S8 and S9), consistent with the proposed mechanism.Moreover, if additional ODPA was added to the second-shell-growth solution, instead of developing into the HBP shape,particles preserved the HP shape (Figure S10). This resultstrongly supports the proposed mechanism of ligand-inducedparticle shape evolution.

PL Line Width, PL Lifetime, and Single-Dot PLBlinking Properties of the HP and HBP Core−ShellQDs. To further study the optical properties of these HP and

Figure 6. HR-TEM images for core−shell HBP-QDs. Center models define the shape (left) and atomic orientation (right) of the HBP-QDs. The[1 210] projection (viewing along the green arrow in center left model) is set as the starting point of rotations. Each of the eight panels (a−h) iscomposed of one representative HR-TEM image for an individual QD with a certain projection, the corresponding FFT pattern of the HR-TEMimage, and the computer generated atomic model and its corresponding simulated electron diffraction pattern after a one- or two-step rotation fromthe center atomic model. The detailed rotations are indicated by the red and blue arrows along the Z and X (the axis normal to both Z and a3 axes)axes, respectively. Panels a−h show the HBP-QDs viewed along [431 3], [1 100], [1101], [101 1], [044 7], [2 425 ], [0001], and [1 011 ] zone axes,respectively.

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HBP core−shell QDs, we monitored the PL line width and theglobal Stokes shift evolutions during the CdS shell growth(Figure 8a,b). Both the ensemble PL line width (full width athalf-maximum, fwhm) and global Stokes shift decreased,respectively, from 86.6 to 65.6 meV and 41.2 to 33.6 meVduring the first-shell-growth. The PL measurements at thesingle-QD level show that the PL line width of single-HP-QDsfluctuated around the ensemble PL line width (Figure 8c,d).This suggested the PL line width narrowing effect was due tothe minimization of inhomogeneous broadening with improvedparticle uniformities in size and shape, shell crystallinity, as wellas surface passivation.27 However, during the course of second-shell-growth reaction while developing the HBP shaped QDs,both the PL line width and the Stokes shift increasedsignificantly from 65.6 to 101.5 meV and 33.6 to 66.2 meV,respectively. The PL line width of the final HBP-QDs is ∼23%larger than that of the spherical QDs with the same core sizeand shell volume (101.5 meV vs 78.2 meV, Figure S2). Single-HBP-QD PL measurements revealed nearly overlapped PLprofiles (Figure 8e) and almost identical PL line widths for thesingle-HBP-QD and the ensemble sample (Figure 8f). Theseresults lead to the conclusion that the PL broadening effectduring the second-shell-growth step was solely due to thehomogeneous broadening of the average single-QD PL linewidth rather than the inhomogeneous broadening caused fromlosing particle uniformity.27,64 Given the minimal excitonic finestructure contribution (<8 meV) to line width changes andnegligible spectral diffusion on submillisecond time scales atroom temperature,65,66 this homogeneous broadening can beattributed to two major factors that are directly related to thethick HBP CdS shell: (1) enhanced longitudinal optical (LO)phonon coupling with excitons through increased Frohlichinteractions induced by reduced electron−hole wave functionoverlap during the CdS shell formation;67,68 (2) a stronginternal electrical field and large dipole moment induced by aspontaneous polarization of a WZ crystal structure and theHBP particle shape.69−72 In addition, a ∼43% larger Stokes

shift (66.2 meV vs 46.3 meV) was measured for the HBP-QDsthan for spherical QDs with the same particle volume (∼1480nm3, Figure S11 and Table S2), further providing strong

Figure 7. (a) Mechanism of the HP and HBP shape evolutions of the CdSe-CdS core−shell QDs during the shell formation process. (b−f) Thecorresponding TEM images of the QD samples at different shell-growth stages: starting CdSe cores (b); HP-QDs after the 1st-shell-growth step (c);(d−f) QD samples taken at different time points during the 2nd-shell-growth step, 20 min (d), 120 min (e), and 180 min (f).

Figure 8. Temporal evolutions of the PL line width (a) and Stokesshift (b) of the CdSe-CdS core−shell QDs during the shell growthreactions. The ensemble and single-QD PL spectra of the HP- (c) andHBP-QDs (e). The distributions of single-QD PL line width (fwhm)of HP- (d) and HBP-QDs (f). The ensemble PL line width values areshown as the red lines in parts d and f.

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indication of increased net exciton−phonon coupling for theHBP-QDs.73 Because of the lack of direct experimentalevidence and characterization tools, other factors that mayaffect the single-QD PL line width changes cannot becompletely ruled out, such as localized charges, surfacereconstruction, and the piezoelectric effect on local electricalfields.61,74,75

Since fluorescence intermittency (also known as blinking) ofsingle-QDs was discovered in 1996,76 it has long beenrecognized as a potential limitation of QDs in applications atnot only the single-QD level but also at ensemble scales.77,78

Thus, numerous efforts have been devoted to explore QDs withsuppressed and/or nonblinking PL output.27,37,38,79,80 Tofurther characterize our HP- and HBP-QDs as single-emitters,we studied single-QD blinking behavior of both samples. Figure9 shows representative PL blinking traces, histograms of PLintensity distributions, and the “On” time fraction distributionof the measured HP- and HBP-QDs. The average “On” timefraction is 80% for HP-QDs and 96% for HBP-QDs. More than50% of the HBP-QDs we measured were completelynonblinking (Figure 9). Such high “On” time fractions forthe HBP-QDs can be attributed to the thick and highlycrystalline CdS shell, proven by the XRD and TEMmeasurements (Figure 3, 5, and 6) and also in good accordanceto previous studies with similar growth conditions for sphericalcore−shell QDs.27In addition to the high “On” fraction, we also measured the

PL decay of single HP- and HBP-QDs. A representative decayprofile is shown in Figure 9c,f. The PL decays of single HP-QDs can be fitted to a single exponential function with anaverage single-QD lifetime of ∼36.0 ns, which is consistent withthe ensemble measurements (Figure 9c and Figure S12). Incontrast, the PL decay curve of single HBP-QDs clearlycontains two components and can be well-fitted to a doubleexponential decay function (Figure 9f and Figure S13). Theslow component is attributed to the radiative recombination ofsingle excitons, which is consistent with the slow PL decay fromensemble PL lifetime measurements (∼242 ns, Figure S2). Thefast decay component has an average lifetime of ∼6 ns, which isascribed to the PL decay of biexciton (BX). Since the HBP-QDs have a thick shell and, therefore, high absorption crosssection, there is high probability of generating BX in the QD

even under low power excitation (40 nW).41,81 Moreover, thisfast decay component showed an excitation intensity depend-ence (Figure S14), further proving its origin is due to BXs.41

This long BX decay lifetime indicates a strong suppression ofthe Auger recombination, which has been previously observedin spherical g-QDs (Figure 9d).41,45 Additionally, thissuppression of Auger recombination should result in a highBX QY of the QDs. Previously, Nair et al. showed that the BXQY of a single-QD can be determined from the size of the 0-time feature in the second order photon correlation function(g(2)) of the QD.82 The g(2) function of a single-HBP-QDshowed a high 0-time feature (Figure S15), suggesting theHBP-QDs have a high BX QY. A high BX QY has also beenobserved in conventional g-QDs.44,83 In all, the thick and highcrystalline CdS shell prevents the electron/hole from reachingto the QD surface, thus limiting their accessibility to the defect-and surface-related nonradiative pathways in the HBP-QDs.Furthermore, the reduced spatial overlap due to the large HBPCdS shell formation also results in a suppressed nonradiativeAuger recombination.84 Both factors contribute to the observedhigh QY and long lifetime of BXs in this HBP-QDs.

In Vitro Biological Characterizations for Cytotoxicityand Shape-Dependent Cellular Uptake. Recent discoveriesreveal that the shape of nanomaterials is an importantparameter when interacting with living biological systems(e.g., living cells, organisms, tumors).85−87 To demonstrate theshape effect of pyramidal QDs on in vitro cellular uptake andintracellular imaging, samples of spherical QDs (S-QDs, as areference sample) and HBP-QDs underwent the same ligand-exchange reaction with methoxy-polyethylene-glycol thiol(PEG-SH, MW5000) (details in the Supporting Information).After ligand exchange, both samples were successfully trans-ferred into aqueous solutions and showed similar hydro-dynamic diameters of ∼26.7 nm and ∼25.8 nm for thePEGylated HBP-QDs (PEG-HBP-QDs) and S-QDs (PEG-S-QDs), respectively (Figure S16), suggesting no aggregations inaqueous solution.88,89 No measurable variations in absorptionand PL spectral profiles of the QDs were observed after theligand exchange reaction (Figure S17). A ∼30% PL QYdecrease was measured for both PEG-HBP-QD and PEG-S-QDsamples, consistent with the reported PL quenching effectcaused by a monothiol group.90−92

Figure 9. Single particle PL blinking traces, histogram of PL intensity distributions, “On” time fraction distribution, and lifetimes of the HP-QDs (a−c) and HBP-QDs (d−f).

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The obtained water-soluble QDs were incubated with threedifferent cell lines: Non-neoplastic human mammary epithelialMCF10A cells and two types of human breast cancer cells(MDA-MB-231 and MCF7). Minimal cell toxicities for all threecell lines were detected for both QD samples, indicating robustsurface protection with sufficient PEG coating (Figure S18).89

To quantify the amount of QD cellular uptake, confocalfluorescence imaging was carried out and the relative amount ofcell uptaken QDs were determined based on the fluorescenceintensity quantification method. Interestingly, we found thatthe PEG-HBP-QDs uptake was ∼3 times as much as the PEG-S-QDs under the same experimental conditions for all three celllines tested here (Figure 10, p < 0.02 with Student’s t test, n =3). This is likely caused by a slightly higher hydrodynamicaspect ratio and a dramatically increased contacting curvature(i.e., sharp apexes of the HBP-QDs) of the PEG-HBP-QDscompared to the PEG-S-QDs.86,93,94 When attached on the cellsurface with the apex, the PEG-HBP-QDs could readily“puncture” into the lipid bilayer and break the integrity ofthe cell membrane, leading to a significant increased cellularinternalization.93,95,96

The results provided here manifest potential applications ofthese HBP-QDs in cellular internalization processes andprovide further insights for drug deliveries, therapeutic design,and intracellular activities under fluorescence imaging andtracking. Moreover, the largely prolonged emission lifetime andsignificantly suppressed single-dot PL blinking of the HBP-QDscan further enable more characterization dimensions (e.g.,fluorescence lifetime imaging microscopy, single-particle

sensing and tracking) for biological imaging down to thesingle-molecule scale both in vitro and in vivo.16,97

■ CONCLUSION

In summary, we have demonstrated, for the first time, thesyntheses of HP and HBP CdSe-CdS core−shell hetero-QDs.Through detailed crystallographic analyses using HR-TEM aswell as volume reconstruction via electron tomography, wehave unambiguously identified the HP and HBP shapes as wellas the crystal facets of the obtained hetero-QDs. While the HP-QDs contain one (0002) hexagonal base and six equivalentlow-index {1011} facets on the lateral sides, the HBP-QDs’surface is dominated only by 12 equivalent {1011} facets, whichmerge into two apexes in the (0002) and (0002 ) directions. Weattributed this pyramidal shape development to a ligand-induced growth model during the CdS shell formation process.The resultant HP and HBP core−shell hetero-QDs exhibit highmorphological uniformity, epitaxial WZ crystallinity, andsuperior optical properties. Especially, the “giant” HBP-QDsshow high PL QYs, minimal inhomogeneous PL line widthbroadening, significantly suppressed single-QD blinking, andprolonged ensemble and single-QD PL lifetimes as comparedto spherical QDs with the same core and shell volumes.Furthermore, the HBP-QDs were successfully transferred to anaqueous solution without aggregation while retaining theiroptical properties. The obtained water-soluble HBP-QDs showlow-cytotoxicity and greatly enhanced cellular uptake fordifferent cell-lines, demonstrating their potentials in biologicalintracellular labeling and imaging.

Figure 10. Cellular uptake of PEG-HBP-QDs and PEG-S-QDs. Representative confocal fluorescent microscope images show the cellular uptake ofPEG-S-QDs (a, d, and g) and PEG-HBP-QDs (b, e, h) in MCF10A, MCF7, and MDA-MB-231 cells, respectively. Quantified amount of uptakenPEG-S-QDs and PEG-HBP-QDs in MCF10A (c), MCF7 (f), and MDA-MB-231 (i) cells. (p < 0.02 with Student’s t test, n = 3).

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.chemma-ter.7b00968.

Synthetic methods, characterization methods, UV−vis,PL, PLE, ensemble lifetimes, XRD fittings, HAADF-STEM, TEM, FTIR spectra of the HP and HBP hetero-QDs; blinking traces, lifetimes, g(2) measurements atsingle-QD level; and cytotoxicity and cell uptake tests(PDF)Movie of the reconstructed volumes via electrontomography for a HP CdSe-CdS core−shell QD alongdifferent viewing directions (AVI)Movie of the reconstructed volumes via electrontomography for a HP CdSe-CdS core−shell QD alongdifferent viewing directions (AVI)Movie of the reconstructed volumes via electrontomography for a HBP CdSe-CdS core−shell QDalong different viewing directions (AVI)Movie of the reconstructed volumes via electrontomography for a HBP CdSe-CdS core−shell QDalong different viewing directions (AVI)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: ouchen@brown.edu.ORCIDRui Tan: 0000-0001-8737-6593Jing Zhao: 0000-0002-6882-2196Ou Chen: 0000-0003-0551-090XNotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSO.C. acknowledges the support from the Brown Universitystartup fund and the Salomon award fund. J.Z. acknowledgesthe financial support from the National Science FoundationCAREER award (Grant CHE 1554800). P.G. acknowledges thesupport of the National Natural Science Foundation of China(Grant 81501572). The TEM and XRD measurements wereperformed at the Nano Tools Facility and the ElectronMicroscopy Facility in the Institute for Molecular andNanoscale Innovation (IMNI) at Brown University. TheEDX mapping studies were performed using the facilities inthe UConn/FEI Center for Advanced Microscopy andMaterials Analysis (CAMMA).

■ REFERENCES(1) Brus, L. E. Electron-electron and electron-hole interactions insmall semiconductor crystallites: The size dependence of the lowestexcited electronic state. J. Chem. Phys. 1984, 80, 4403−4409.(2) Alivisatos, A. P. Semiconductor clusters, nanocrystals, andquantum dots. Science 1996, 271, 933−937.(3) Buhro, W. E.; Colvin, V. L. Semiconductor nanocrystals: Shapematters. Nat. Mater. 2003, 2, 138−139.(4) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.;Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.;Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.;Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of nanoscience withnanocrystals. ACS Nano 2015, 9, 1012−1057.

(5) Chen, O.; Wei, H.; Maurice, A.; Bawendi, M.; Reiss, P. Purecolors from core-shell quantum dots. MRS Bull. 2013, 38, 696−702.(6) Bae, W. K.; Park, Y. S.; Lim, J.; Lee, D.; Padilha, L. A.; McDaniel,H.; Robel, I.; Lee, C.; Pietryga, J. M.; Klimov, V. I. Controlling theinfluence of Auger recombination on the performance of quantum-dotlight-emitting diodes. Nat. Commun. 2013, 4, 2661.(7) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-emitting-diodes made from cadmium selenide nanocrystals and a semi-conducting polymer. Nature 1994, 370, 354−357.(8) Mashford, B. S.; Stevenson, M.; Popovic, Z.; Hamilton, C.; Zhou,Z.; Breen, C.; Steckel, J.; Bulovic, V.; Bawendi, M.; Coe-Sullivan, S.;Kazlas, P. T. High-efficiency quantum-dot light-emitting devices withenhanced charge injection. Nat. Photonics 2013, 7, 407−412.(9) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.;Wang, J.; Peng, X. Solution-processed, high-performance light-emittingdiodes based on quantum dots. Nature 2014, 515, 96−99.(10) Niu, Y. H.; Munro, A. M.; Cheng, Y. J.; Tian, Y. Q.; Liu, M. S.;Zhao, J. L.; Bardecker, J. A.; Jen-La Plante, I.; Ginger, D. S.; Jen, A. K.Y. Improved performance from multilayer quantum dot light-emittingdiodes via thermal annealing of the quantum dot Layer. Adv. Mater.2007, 19, 3371−3378.(11) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.;Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.; Bawendi, M. G.Optical gain and stimulated emission in nanocrystal quantum dots.Science 2000, 290, 314−317.(12) Dang, C.; Lee, J.; Breen, C.; Steckel, J. S.; Coe-Sullivan, S.;Nurmikko, A. Red, green and blue lasing enabled by single-excitongain in colloidal quantum dot films. Nat. Nanotechnol. 2012, 7, 335−339.(13) Geyer, S. M.; Scherer, J. M.; Jack, M. D.; Bawendi, M. G.;Jaworski, F. B. Dual-band ultraviolet-short-wavelength infraredimaging via luminescent downshifting with colloidal quantum dots. J.Nanophotonics 2013, 7, 073083.(14) Sukhovatkin, V.; Hinds, S.; Brzozowski, L.; Sargent, E. H.Colloidal quantum-dot photodetectors exploiting multiexciton gen-eration. Science 2009, 324, 1542−1544.(15) Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H.Quantum dot bioconjugates for imaging, labelling and sensing. Nat.Mater. 2005, 4, 435−446.(16) Dahan, M.; Laurence, T.; Pinaud, F.; Chemla, D. S.; Alivisatos,A. P.; Sauer, M.; Weiss, S. Time-gated biological imaging by use ofcolloidal quantum dots. Opt. Lett. 2001, 26, 825−827.(17) Han, M. Y.; Gao, X. H.; Su, J. Z.; Nie, S. Quantum-dot-taggedmicrobeads for multiplexed optical coding of biomolecules. Nat.Biotechnol. 2001, 19, 631−635.(18) Franke, D.; Harris, D. K.; Chen, O.; Bruns, O. T.; Carr, J. A.;Wilson, M. W. B.; Bawendi, M. G. Continuous injection synthesis ofindium arsenide quantum dots emissive in the short-wavelengthinfrared. Nat. Commun. 2016, 7, 12749.(19) Qiu, F.; Han, Z. J.; Peterson, J. J.; Odoi, M. Y.; Sowers, K. L.;Krauss, T. D. Photocatalytic hydrogen generation by CdSe/CdSnanoparticles. Nano Lett. 2016, 16, 5347−5352.(20) Bradshaw, L. R.; Knowles, K. E.; McDowall, S.; Gamelin, D. R.Nanocrystals for luminescent solar concentrators. Nano Lett. 2015, 15,1315−1323.(21) Cui, J.; Beyler, A. P.; Bischof, T. S.; Wilson, M. W.; Bawendi, M.G. Deconstructing the photon stream from single nanocrystals: frombinning to correlation. Chem. Soc. Rev. 2014, 43, 1287−1310.(22) Reiss, P.; Protiere, M.; Li, L. Core/Shell semiconductornanocrystals. Small 2009, 5, 154−168.(23) Peng, X. G.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P.Epitaxial growth of highly luminescent CdSe/CdS core/shell nano-crystals with photostability and electronic accessibility. J. Am. Chem.Soc. 1997, 119, 7019−7029.(24) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis andcharacterization of nearly monodisperse CdE (E = S, Se, Te)semiconductor nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715.

Chemistry of Materials Article

DOI: 10.1021/acs.chemmater.7b00968Chem. Mater. XXXX, XXX, XXX−XXX

J

(25) Peng, Z. A.; Peng, X. G. Formation of high-quality CdTe, CdSe,and CdS nanocrystals using CdO as precursor. J. Am. Chem. Soc. 2001,123, 183−184.(26) Chen, O.; Chen, X.; Yang, Y. A.; Lynch, J.; Wu, H. M.; Zhuang,J. Q.; Cao, Y. C. Synthesis of metal-selenide nanocrystals usingselenium dioxide as the selenium precursor. Angew. Chem., Int. Ed.2008, 47, 8638−8641.(27) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D.K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G.Compact high-quality CdSe-CdS core-shell nanocrystals with narrowemission linewidths and suppressed blinking. Nat. Mater. 2013, 12,445−451.(28) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.;Johnson, M. B.; Peng, X. G. Large-scale synthesis of nearlymonodisperse CdSe/CdS core/shell nanocrystals using air-stablereagents via successive ion layer adsorption and reaction. J. Am.Chem. Soc. 2003, 125, 12567−12575.(29) Li, J. J.; Tsay, J. M.; Michalet, X.; Weiss, S. Wavefunctionengineering: From quantum wells to near-infrared type-II colloidalquantum dots synthesized by layer-by-layer colloidal epitaxy. Chem.Phys. 2005, 318, 82−90.(30) Xie, R. G.; Kolb, U.; Li, J. X.; Basche, T.; Mews, A. Synthesis andcharacterization of highly luminescent CdSe-Core CdS/Zn0.5Cd0.5S/ZnS multishell anocrystals. J. Am. Chem. Soc. 2005, 127, 7480−7488.(31) Nemchinov, A.; Kirsanova, M.; Hewa-Kasakarage, N. N.;Zamkov, M. Synthesis and characterization of type II ZnSe/CdS core/shell nanocrystals. J. Phys. Chem. C 2008, 112, 9301−9307.(32) Yang, Y. A.; Chen, O.; Angerhofer, A.; Cao, Y. C. Radial-position-controlled doping in CdS/ZnS core/shell nanocrystals. J. Am.Chem. Soc. 2006, 128, 12428−12429.(33) Talapin, D. V.; Koeppe, R.; Gotzinger, S.; Kornowski, A.;Lupton, J. M.; Rogach, A. L.; Benson, O.; Feldmann, J.; Weller, H.Highly emissive colloidal CdSe/CdS heterostructures of mixeddimensionality. Nano Lett. 2003, 3, 1677−1681.(34) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.;Sadtler, B.; Alivisatos, A. P. Seeded growth of highly luminescentCdSe/CdS nanoheterostructures with rod and tetrapod morphologies.Nano Lett. 2007, 7, 2951−2959.(35) Carbone, L.; Nobile, C.; De Giorgi, M.; Sala, F. D.; Morello, G.;Pompa, P.; Hytch, M.; Snoeck, E.; Fiore, A.; Franchini, I. R.; Nadasan,M.; Silvestre, A. F.; Chiodo, L.; Kudera, S.; Cingolani, R.; Krahne, R.;Manna, L. Synthesis and micrometer-scale assembly of colloidal CdSe/CdS nanorods prepared by a seeded growth approach. Nano Lett.2007, 7, 2942−2950.(36) Cassette, E.; Mahler, B.; Guigner, J. M.; Patriarche, G.;Dubertret, B.; Pons, T. Colloidal CdSe/CdS dot-in-plate nanocrystalswith 2D-polarized emission. ACS Nano 2012, 6, 6741−6750.(37) Chen, Y.; Vela, J.; Htoon, H.; Casson, J. L.; Werder, D. J.;Bussian, D. A.; Klimov, V. I.; Hollingsworth, J. A. ″Giant″ multishellCdSe nanocrystal quantum dots with suppressed blinking. J. Am.Chem. Soc. 2008, 130, 5026−5027.(38) Mahler, B.; Spinicelli, P.; Buil, S.; Quelin, X.; Hermier, J. P.;Dubertret, B. Towards non-blinking colloidal quantum dots. Nat.Mater. 2008, 7, 659−664.(39) Vela, J.; Htoon, H.; Chen, Y.; Park, Y. S.; Ghosh, Y.; Goodwin,P. M.; Werner, J. H.; Wells, N. P.; Casson, J. L.; Hollingsworth, J. A.Effect of shell thickness and composition on blinking suppression andthe blinking mechanism in ’giant’ CdSe/CdS nanocrystal quantumdots. J. Biophotonics 2010, 3, 706−717.(40) Malko, A. V.; Park, Y. S.; Sampat, S.; Galland, C.; Vela, J.; Chen,Y.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Pump-intensity- andshell-thickness-dependent evolution of photoluminescence blinking inindividual core/shell CdSe/CdS nanocrystals. Nano Lett. 2011, 11,5213−5218.(41) Garcia-Santamaria, F.; Chen, Y.; Vela, J.; Schaller, R. D.;Hollingsworth, J. A.; Klimov, V. I. Suppressed auger recombination in″giant″ nanocrystals boosts optical gain performance. Nano Lett. 2009,9, 3482−3488.

(42) Ji, B. T.; Giovanelli, E.; Habert, B.; Spinicelli, P.; Nasilowski, M.;Xu, X. Z.; Lequeux, N.; Hugonin, J. P.; Marquier, F.; Greffet, J. J.;Dubertret, B. Non-blinking quantum dot with a plasmonic nanoshellresonator. Nat. Nanotechnol. 2015, 10, 170−175.(43) Htoon, H.; Malko, A. V.; Bussian, D.; Vela, J.; Chen, Y.;Hollingsworth, J. A.; Klimov, V. I. Highly emissive multiexcitons insteady-state photoluminescence of individual ″giant″ CdSe/CdSCore/Shell nanocrystals. Nano Lett. 2010, 10, 2401−2407.(44) Park, Y. S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; Garcia-Santamaria, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Near-unity quantum yields of biexciton emission from CdSe/CdSnanocrystals measured using single-particle spectroscopy. Phys. Rev.Lett. 2011, 106, 187401.(45) Nasilowski, M.; Spinicelli, P.; Patriarche, G.; Dubertret, B.Gradient CdSe/CdS quantum dots with room temperature biexcitonunity quantum yield. Nano Lett. 2015, 15, 3953−3958.(46) Brovelli, S.; Bae, W. K.; Galland, C.; Giovanella, U.; Meinardi,F.; Klimov, V. I. Dual-color electroluminescence from dot-in-bulknanocrystals. Nano Lett. 2014, 14, 486−494.(47) Cirloganu, C. M.; Padilha, L. A.; Lin, Q. L.; Makarov, N. S.;Velizhanin, K. A.; Luo, H. M.; Robel, I.; Pietryga, J. M.; Klimov, V. I.Enhanced carrier multiplication in engineered quasi-type-II quantumdots. Nat. Commun. 2014, 5, 4148.(48) Meinardi, F.; Colombo, A.; Velizhanin, K. A.; Simonutti, R.;Lorenzon, M.; Beverina, L.; Viswanatha, R.; Klimov, V. I.; Brovelli, S.Large-area luminescent solar concentrators based on ‘Stokes-shift-engineered’ nanocrystals in a mass-polymerized PMMA matrix. Nat.Photonics 2014, 8, 392−399.(49) Coropceanu, I.; Bawendi, M. G. Core/shell quantum dot basedluminescent solar concentrators with reduced reabsorption andenhanced efficiency. Nano Lett. 2014, 14, 4097−4101.(50) Li, H.; Wu, K.; Lim, J.; Song, H.-J.; Klimov, V. I. Doctor-bladedeposition of quantum dots onto standard window glass for low-losslarge-area luminescent solar concentrators. Nat. Energy. 2016, 1,16157.(51) Bladt, E.; van Dijk-Moes, R. J.; Peters, J.; Montanarella, F.; deMello Donega, C.; Vanmaekelbergh, D.; Bals, S. Atomic structure ofwurtzite CdSe (core)/CdS (giant Shell) nanobullets related to epitaxyand growth. J. Am. Chem. Soc. 2016, 138, 14288−14293.(52) Pal, B. N.; Ghosh, Y.; Brovelli, S.; Laocharoensuk, R.; Klimov, V.I.; Hollingsworth, J. A.; Htoon, H. ’Giant’ CdSe/CdS core/shellnanocrystal quantum dots as efficient electroluminescent materials:strong influence of shell thickness on light-emitting diode perform-ance. Nano Lett. 2012, 12, 331−336.(53) Peng, Z. A.; Peng, X. G. Mechanisms of the shape evolution ofCdSe nanocrystals. J. Am. Chem. Soc. 2001, 123, 1389−1395.(54) Zhou, J.; Pu, C.; Jiao, T.; Hou, X.; Peng, X. A two-step syntheticstrategy toward monodisperse colloidal CdSe and CdSe/CdS core/shell nanocrystals. J. Am. Chem. Soc. 2016, 138, 6475−6483.(55) Ghosh, S.; Gaspari, R.; Bertoni, G.; Spadaro, M. C.; Prato, M.;Turner, S.; Cavalli, A.; Manna, L.; Brescia, R. Pyramid-shaped wurtziteCdSe nanocrystals with inverted polarity. ACS Nano 2015, 9, 8537−8546.(56) Cheng, Y.; Wang, Y.; Bao, F.; Chen, D. Shape control ofmonodisperse CdS nanocrystals: hexagon and pyramid. J. Phys. Chem.B 2006, 110, 9448−9451.(57) Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q. A.; Vantomme,A.; Martins, J. C.; Hens, Z. Binding of phosphonic acids to CdSequantum dots: a solution NMR study. J. Phys. Chem. Lett. 2011, 2,145−152.(58) Rempel, J. Y.; Trout, B. L.; Bawendi, M. G.; Jensen, K. F.Density functional theory study of ligand binding on CdSe (0001),(0001), and (1120) single crystal relaxed and reconstructed surfaces:implications for nanocrystalline growth. J. Phys. Chem. B 2006, 110,18007−18016.(59) Owen, J. S.; Park, J.; Trudeau, P. E.; Alivisatos, A. P. Reactionchemistry and ligand exchange at cadmium-selenide nanocrystalsurfaces. J. Am. Chem. Soc. 2008, 130, 12279−12281.

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DOI: 10.1021/acs.chemmater.7b00968Chem. Mater. XXXX, XXX, XXX−XXX

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(60) Tan, R.; Blom, D. A.; Ma, S. G.; Greytak, A. B. Probing surfacesaturation conditions in alternating layer growth of CdSe/CdS core/shell quantum dots. Chem. Mater. 2013, 25, 3724−3736.(61) Chen, O.; Yang, Y.; Wang, T.; Wu, H.; Niu, C.; Yang, J.; Cao, Y.C. Surface-functionalization-dependent optical properties of II-VIsemiconductor nanocrystals. J. Am. Chem. Soc. 2011, 133, 17504−17512.(62) Wu, N. Q.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P.Interaction of fatty acid monolayers with cobalt nanoparticles. NanoLett. 2004, 4, 383−386.(63) Cirillo, M.; Aubert, T.; Gomes, R.; Van Deun, R.; Emplit, P.;Biermann, A.; Lange, H.; Thomsen, C.; Brainis, E.; Hens, Z. ″Flash″synthesis of CdSe/CdS core-shell quantum dots. Chem. Mater. 2014,26, 1154−1160.(64) Cui, J.; Beyler, A. P.; Marshall, L. F.; Chen, O.; Harris, D. K.;Wanger, D. D.; Brokmann, X.; Bawendi, M. G. Direct probe of spectralinhomogeneity reveals synthetic tunability of single-nanocrystalspectral linewidths. Nat. Chem. 2013, 5, 602−606.(65) Cui, J.; Beyler, A. P.; Coropceanu, I.; Cleary, L.; Avila, T. R.;Chen, Y.; Cordero, J. M.; Heathcote, S. L.; Harris, D. K.; Chen, O.;Cao, J.; Bawendi, M. G. Evolution of the single-nanocrystalphotoluminescence linewidth with size and shell: implications forexciton-phonon coupling and the optimization of spectral linewidths.Nano Lett. 2016, 16, 289−296.(66) Marshall, L. F.; Cui, J.; Brokmann, X.; Bawendi, M. G.Extracting spectral dynamics from single chromophores in solution.Phys. Rev. Lett. 2010, 105, 053005.(67) Chernikov, A.; Bornwasser, V.; Koch, M.; Chatterjee, S.; Bottge,C. N.; Feldtmann, T.; Kira, M.; Koch, S. W.; Wassner, T.;Lautenschlager, S.; Meyer, B. K.; Eickhoff, M. Phonon-assistedluminescence of polar semiconductors: Frohlich coupling versusdeformation-potential scattering. Phys. Rev. B: Condens. Matter Mater.Phys. 2012, 85, 035201.(68) Tessier, M. D.; Mahler, B.; Nadal, B.; Heuclin, H.; Pedetti, S.;Dubertret, B. Spectroscopy of colloidal semiconductor core/shellnanoplatelets with high quantum yield. Nano Lett. 2013, 13, 3321−3328.(69) Rosenthal, S. J.; McBride, J.; Pennycook, S. J.; Feldman, L. C.Synthesis, surface studies, composition and structural characterizationof CdSe, core/shell, and biologically active nanocrystals. Surf. Sci. Rep.2007, 62, 111−157.(70) Shim, M.; Guyot-Sionnest, P. Permanent dipole moment andcharges in colloidal semiconductor quantum dots. J. Chem. Phys. 1999,111, 6955−6964.(71) Rabani, E. Structure and electrostatic properties of passivatedCdSe nanocrystals. J. Chem. Phys. 2001, 115, 1493−1497.(72) Li, L. S.; Alivisatos, A. P. Origin and scaling of the permanentdipole moment in CdSe nanorods. Phys. Rev. Lett. 2003, 90, 097402.(73) Kelley, A. M. Electron-phonon coupling in CdSe nanocrystalsfrom an atomistic phonon model. ACS Nano 2011, 5, 5254−5262.(74) Jiang, Z. J.; Kelley, D. F. Surface charge and piezoelectric fieldscontrol auger recombination in semiconductor nanocrystals. NanoLett. 2011, 11, 4067−4073.(75) Takagahara, T. Electron-phonon interactions and excitonicdephasing in semiconductor nanocrystals. Phys. Rev. Lett. 1993, 71,3577−3580.(76) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.;Trautman, J. K.; Harris, T. D.; Brus, L. E. Fluorescence intermittencyin single cadmium selenide nanocrystals. Nature 1996, 383, 802−804.(77) Frantsuzov, P.; Kuno, M.; Janko, B.; Marcus, R. A. Universalemission intermittency in quantum dots, nanorods and nanowires.Nat. Phys. 2008, 4, 519−522.(78) Brokmann, X.; Hermier, J. P.; Messin, G.; Desbiolles, P.;Bouchaud, J. P.; Dahan, M. Statistical aging and nonergodicity in thefluorescence of single nanocrystals. Phys. Rev. Lett. 2003, 90, 120601.(79) Hohng, S.; Ha, T. Near-complete suppression of quantum dotblinking in ambient conditions. J. Am. Chem. Soc. 2004, 126, 1324−1325.

(80) Fomenko, V.; Nesbitt, D. J. Solution control of radiative andnonradiative lifetimes: A novel contribution to quantum dot blinkingsuppression. Nano Lett. 2008, 8, 287−293.(81) Garcia-Santamaria, F.; Brovelli, S.; Viswanatha, R.;Hollingsworth, J. A.; Htoon, H.; Crooker, S. A.; Klimov, V. I.Breakdown of volume scaling in Auger recombination in CdSe/CdSheteronanocrystals: The role of the core-shell interface. Nano Lett.2011, 11, 687−693.(82) Nair, G.; Zhao, J.; Bawendi, M. G. Biexciton quantum yield ofsingle semiconductor nanocrystals from photon statistics. Nano Lett.2011, 11, 1136−1140.(83) Garcia-Santamaria, F.; Chen, Y. F.; Vela, J.; Schaller, R. D.;Hollingsworth, J. A.; Klimov, V. I. Suppressed auger recombination in″giant″ nanocrystals boosts optical gain performance. Nano Lett. 2009,9, 3482−3488.(84) Oron, D.; Kazes, M.; Banin, U. Multiexcitons in type-II colloidalsemiconductor quantum dots. Phys. Rev. B: Condens. Matter Mater.Phys. 2007, 75, 035330.(85) Florez, L.; Herrmann, C.; Cramer, J. M.; Hauser, C. P.; Koynov,K.; Landfester, K.; Crespy, D.; Mailander, V. How shape influencesuptake: Interactions of anisotropic polymer nanoparticles and humanmesenchymal stem cells. Small 2012, 8, 2222−2230.(86) Chauhan, V. P.; Popovic, Z.; Chen, O.; Cui, J.; Fukumura, D.;Bawendi, M. G.; Jain, R. K. Fluorescent nanorods and nanospheres forreal-time in vivo probing of nanoparticle shape-dependent tumorpenetration. Angew. Chem., Int. Ed. 2011, 50, 11417−11420.(87) Fratila, R. M.; Rivera-Fernandez, S.; de la Fuente, J. M. Shapematters: synthesis and biomedical applications of high aspect ratiomagnetic nanomaterials. Nanoscale 2015, 7, 8233−8260.(88) Popovic, Z.; Liu, W. H.; Chauhan, V. P.; Lee, J.; Wong, C.;Greytak, A. B.; Insin, N.; Nocera, D. G.; Fukumura, D.; Jain, R. K.;Bawendi, M. G. A nanoparticle size series for in vivo fluorescenceimaging. Angew. Chem., Int. Ed. 2010, 49, 8649−8652.(89) Chen, O.; Riedemann, L.; Etoc, F.; Herrmann, H.; Coppey, M.;Barch, M.; Farrar, C. T.; Zhao, J.; Bruns, O. T.; Wei, H.; Guo, P.; Cui,J.; Jensen, R.; Chen, Y.; Harris, D. K.; Cordero, J. M.; Wang, Z. W.;Jasanoff, A.; Fukumura, D.; Reimer, R.; Dahan, M.; Jain, R. K.;Bawendi, M. G. Magneto-fluorescent core-shell supernanoparticles.Nat. Commun. 2014, 5, 5093.(90) Wuister, S. F.; Donega, C. D.; Meijerink, A. Influence of thiolcapping on the exciton luminescence and decay kinetics of CdTe andCdSe quantum. J. Phys. Chem. B 2004, 108, 17393−17397.(91) Munro, A. M.; Jen-La Plante, I.; Ng, M. S.; Ginger, D. S.Quantitative study of the effects of surface ligand concentration onCdSe nanocrystal photoluminescence. J. Phys. Chem. C 2007, 111,6220−6227.(92) Uyeda, H. T.; Medintz, I. L.; Jaiswal, J. K.; Simon, S. M.;Mattoussi, H. Synthesis of compact multidentate ligands to preparestable hydrophilic quantum dot fluorophores. J. Am. Chem. Soc. 2005,127, 3870−3878.(93) Gratton, S. E. A.; Ropp, P. A.; Pohlhaus, P. D.; Luft, J. C.;Madden, V. J.; Napier, M. E.; DeSimone, J. M. The effect of particledesign on cellular internalization pathways. Proc. Natl. Acad. Sci. U. S.A. 2008, 105, 11613−11618.(94) Wang, T.; Wang, X. R.; LaMontagne, D.; Wang, Z. L.; Wang, Z.W.; Cao, Y. C. Shape-controlled synthesis of colloidal superparticlesfrom nanocubes. J. Am. Chem. Soc. 2012, 134, 18225−18228.(95) Shi, X. H.; von dem Bussche, A.; Hurt, R. H.; Kane, A. B.; Gao,H. J. Cell entry of one-dimensional nanomaterials occurs by tiprecognition and rotation. Nat. Nanotechnol. 2011, 6, 714−719.(96) Huang, C. J.; Zhang, Y.; Yuan, H. Y.; Gao, H. J.; Zhang, S. L.Role of nanoparticle geometry in endocytosis: Laying down to standup. Nano Lett. 2013, 13, 4546−4550.(97) Pinaud, F.; Clarke, S.; Sittner, A.; Dahan, M. Probing cellularevents, one quantum dot at a time. Nat. Methods 2010, 7, 275−285.

Chemistry of Materials Article

DOI: 10.1021/acs.chemmater.7b00968Chem. Mater. XXXX, XXX, XXX−XXX

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