CdS Dot-in-Giant-Rod Nanocrystals

Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

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Synthesis of Anisotropic CdSe/CdS Dot-in-Giant-Rod Nanocrystals with Persistent Blue-Shifted Biexciton Emission Anatolii Polovitsyn,†,‡ Ali Hossain Khan,† Ilaria Angeloni,†,§ Joel Q. Grim,†,# Josep Planelles,∥ Juan I. Climente,∥ and Iwan Moreels*,†,⊥ †

Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy § Dipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 33, 16146 Genova, Italy ∥ Departament de Química Física i Analítica, Universitat Jaume I, 12080 Castelló de la Plana, Spain ⊥ Department of Chemistry, Ghent University, Krijgslaan 281-S3, 9000 Ghent, Belgium

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‡

S Supporting Information *

ABSTRACT: Anisotropic single-phase wurtzite CdSe/CdS nanocrystals were synthesized by colloidal chemistry, introducing ZnCl2 to increase the shell growth in the radial direction. As a result, dot-in-giant-rod nanocrystals were obtained, with a core diameter that varied between 3.2 and 7.5 nm and an overall diameter between 15 and 22 nm, corresponding to a 14−26 ML CdS shell. In addition to an extended fluorescence lifetime, typical for CdSe/CdS heteronanocrystals, all samples also yielded a blue-shifted biexciton emission peak. This contrasts with existing data on CdSe/CdS dot-in-rod nanocrystals with a thin shell, which yield a type-I band offset and attractive biexciton interactions for CdSe/CdS with a core larger than about 2.8 nm. However, k·p calculations support the blue shift, with a significant electron delocalization into the CdS shell even for large core diameter. We assign this effect to the influence of strain at the CdSe/CdS interface and associated reduction of the conduction band offset, as well as the buildup of a piezoelectric field along the nanorod long axis. The strain-induced electron−hole separation is particularly effective in large-core nanocrystals, providing a tool to engineer electron and hole wave functions that is complementary to quantum confinement. KEYWORDS: quantum dots, photoluminescence, strain, band offset, piezoelectric field, k·p calculations

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localizes the hole into the CdSe core and a shallow conduction band offset that leads to electron delocalization into the CdS shell, they optimally make use of the possibility to engineer exciton wave functions by varying the core diameter, shell thickness and core/shell interface.2 This gives rise to precise adjustments of the band-edge transition energy, fluorescence lifetime, and excited-state carrier dynamics. Particular properties that render CdSe/CdS NCs highly suitable for photonic applications are a near-unity single-exciton photoluminescence quantum efficiency (PL QE),8,12 strongly reduced PL intermittency,2 a biexciton PL peak that can be precisely tuned to either the red or blue side of the exciton fluorescence by varying the core and shell dimensions,2,13,14 and, through dedicated CdSe/CdS interface engineering,15 a high biexciton QE.16,17 Next to a fundamental interest in their linear and nonlinear optical properties, they are therefore a commonly used material for light-emitting diodes,18−20 lasers,21−23 fluorescent sensors,24,25 and solar concentrators26−28 and as quantum emitters.29

he ability to interface different semiconductors at the nanoscale enables us to create functional nanomaterials with precisely controlled optoelectronic properties. Colloidal nanocrystals (NCs) are a particular class of nanomaterials, with a unique potential to obtain such heterostructures via low-cost solution-processed techniques.1−3 At present, core/shell NCs can be grown using materials of various compositions, crystal structures, and lattice constants and with different geometries. Synthesis protocols for spherical, rod-like, or plate-like core/ shell NCs have all been reported.1−5 Among these systems, the so-called CdSe/CdS giant-shell NCs, CdSe NCs with a CdS shell thickness of typically 4 nm or more, are among the most widely investigated. On one hand, well-established approaches to prepare the NCs are readily available,2,6−9 and the relatively small4% lattice mismatch enables us to grow a thick shell without introducing a large density of crystal defects. The epitaxial growth of the CdS shell and resulting strain in the core/shell heterostuctures have already been investigated in detail for several types of CdSe/CdS giant-shell NCs.7,10,11 From an optoelectronic perspective, they possess a unique band alignment. With a distinct valence band offset that © XXXX American Chemical Society

Received: July 26, 2018 Published: October 8, 2018 A

DOI: 10.1021/acsphotonics.8b01028 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Schematic representation of the two-step growth procedure. (b) TEM image of CdSe/CdxZn1−xS NCs after the first injection step, prepared from 5.7 nm CdSe cores. (c) Corresponding final DiGR NCs.

agreement with recent calculations on CdSe/CdS core/shell NCs of various geometry,34 this is expected to lead to an electron−hole separation via the deformation and piezoelectric potential, resulting in an extended PL lifetime. However, such extended lifetimes can also be induced by temporary shallow trapping of electrons in colloidal NCs, giving rise to so-called delayed emission,37 and on the other hand it is known that in anisotropic NCs the radiative recombination rate is enhanced due to reduced dielectric screening compared to spherical quantum dots (QDs).38 Therefore, to assess the role of interface strain and piezoelectric fields on the charge carrier interactions, instead of analyzing the PL decay time, here we measured nonlinear PL spectra of the DiGR NCs. We observed a persistent blue shift of the biexciton emission for CdSe core diameters up to 7.5 nm, which contrasts with earlier reports of a type-I band alignment and red-shifted biexciton emission in CdSe/CdS DiRs with a thin shell and core diameter above 2.8 nm.13 Theoretical modeling of the exciton−exciton interactions yielded a blue-shifted emission in the presence of strain and a piezoelectric field. Combining strain with quantum confinement thus gives rise to a unique toolset to tune electron and hole wave functions in giant-shell NCs and can lead to a further optimization of the optoelectronic properties of CdSe/CdS NCs toward lowcost, scalable, solution-processed photonic applications.

The large majority of synthetic efforts toward highly fluorescent CdSe/CdS giant-shell NCs have focused on the growth of spherically symmetric NCs, starting from either a zincblende (ZB)2 or a wurtzite (WZ)6,11,30 CdSe core and typically growing a WZ CdS shell. Anisotropic single-phase WZ/WZ nanorods, based on either a spherical8,31,32 or rodshaped CdSe core,7,33 have also been synthesized. For the latter NCs, samples with relatively large core and shell dimensions displayed a strongly red-shifted emission (located at 733 nm, near the bulk band gap transition) and an exceptionally long fluorescence lifetime up to 4.5 μs, more than 2 orders of magnitude larger than the corresponding 20 ns for the CdSe core.7 Results were consistent with the buildup of a piezoelectric field, due to strain at the core/shell interface in NCs with a sufficiently thick CdS shell. This induces a localization of positive and negative charges at opposite ends of the CdSe core nanorod. Importantly, while quantum confinement promotes electron delocalization in CdSe/CdS NCs with a small CdSe core diameter13 due to the increased size of the potential well after shell growth, the electron−hole separation induced by the piezoelectric field is particularly efficient in large-core NCs.34 In this article, we explore this concept further in a novel synthesis of giant-shell WZ CdSe/CdS NCs, focusing on a system with mixed dimensionality: a spherical CdSe core embedded in a thick CdS rod-like shell. The WZ CdSe NC seeds were grown with a diameter from 2.8 up to 7.5 nm, enabling exploration of the role of quantum confinement in such systems. CdSe cores were subsequently coated with a CdxZn1−xS (x < 1%) rod-shaped shell via a two-step injection route, to obtain CdSe/CdS dot-in-giant-rod (DiGR) NCs with a single WZ crystal lattice. Existing syntheses of anisotropic CdSe/CdS NCs typically require the presence of strongly adsorbing organic ligands, such as alkylphosphonic acid, which lead to selective growth of top and bottom facets while limiting the radial increase of the CdS shell.31,32 In contrast, in our procedure we apply chloride precursors,35,36 more specifically ZnCl2, for shape control. By avoiding phosphonic acid ligands, growth in the radial direction was enhanced significantly, and we obtained DiGR NCs with a final diameter up to 22 nm. The thick shell introduced strain at the core/shell interface, and in

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RESULTS AND DISCUSSION The synthesis of CdSe/CdS DiGR NCs follows a two-step growth mechanism (Figure 1a). Initially, we injected a mixture of spherical CdSe QDs, ZnCl2 in oleylamine, cadmium oleate in oleic acid (Cd(OA)2), and sulfur in tri-n-octyl phosphine (TOPS) into 1-octadecene at 300 °C. After maintaining the reaction at this temperature for 40 min, we obtained slightly anisotropic CdSe/CdZnS NCs, as shown with transmission electron microscopy (TEM, Figure 1b). Next, shell dimensions were further increased by a second injection, using only Cd(OA)2 and TOPS. After annealing of the samples at 300 °C for an additional 60 min, we obtained anisotropic DiGR NCs with an average diameter of 20.8 nm and length of 38.5 nm, respectively, resulting in a 1.8:1 aspect ratio (Figure 1c). B


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Optimal conditions for growing well-defined monodisperse DiGR NCs were obtained when using a ZnCl2-to-Cd(OA)2 ratio of 1:1 in the first injection step, together with a molar ratio of TOP to oleylamine of 1.7:1 for both injections. When synthesizing DiGR NCs starting from various core diameters between 3.2 and 7.5 nm, we obtained a final NC length of 35− 55 nm and a typical diameter of 13−22 nm (Table 1, Table 1. CdSe Core (DCdSe) and Overall (DDiGR) Diameter, Core Volume Fraction f CdSe, DiGR Aspect Ratio AR, and Cation Composition, Normalized to Zn = 1, for the Different NCs at Each Injection Step, As Obtained by ICPOES DCdSe (nm)

DDiGR (nm)

f CdSe (%)

AR

Cd:Zn ratio, 1st step

Cd:Zn ratio, 2nd step

3.2 3.8 4.5 5.7 7.5

13.3 21.3 14.4 22.0 16.7

0.4 0.2 0.7 0.5 3.2

2.5 2.5 2.8 2.2 1.9

40:1 42:1 40:1 37:1 33:1

104:1 155:1 110:1 147:1 127:1

Figure 2. (a) XRD patterns of CdSe/CdxZn1−xS intermediate NCs (step 1) and final DiGR NCs, synthesized with a 5.7 nm CdSe core. (b) Observation of Ashby−Brown diffraction contrast on the final DiGR NCs.

Supporting Information, SI, Figure S1). The latter corresponds to 14−26 monolayers (MLs) of CdS, confirming that in all cases we synthesized NCs with a so-called giant shell. Consequently, the final CdSe volume fraction does not exceed 3% (Table 1). DiGRs with a lower aspect ratio of 1.5:1 were obtained by adding oleylamine to both injection mixtures, resulting in a TOP:oleylamine molar ratio of 1:1. We obtained monodisperse NCs with a larger average diameter of 25 nm, yet shorter rod length (Figure S2). Inductively coupled plasma optical emission spectroscopy (ICP-OES) revealed a Zn:Cd ratio in the intermediate DiGRs of about 0.02−0.03:1 (Table 1). As the injected Zn:Cd precursor ratio equaled 1:1, this indicates that most of the Zn is not incorporated into the CdS lattice. After the second injection we measured a Zn:Cd ratio of less than 0.01:1, suggesting that, due to the volume expansion after the second injection when only Cd precursors were used, the Zn ions that were incorporated in step 1 are simply diluted over the final DiGR NCs. At present, the detailed reaction mechanism that leads to the anisotropic growth is not yet fully resolved. It is clear however that, despite the low concentration of Zn incorporated, ZnCl2 plays an important role in the NC shape control, which is likely due to the presence of chloride ions in the synthesis. These ions are known to strongly affect the shape of colloidal NCs,35,36 by adsorbing to specific facets,39 and altering the relative growth rates in these directions. In other experiments, using halides in the colloidal CdSe and CdSe/CdS synthesis has already yielded giant-shell rod-in-rods,7 pyramids,36,39 and octapods.35 The X-ray diffraction (XRD) pattern of a typical CdSe/ CdxZn1−xS NC sample obtained after the first injection step is displayed in Figure 2a. It can be assigned to a WZ crystal structure. The narrower [002] peak at 28.06° 2θ confirms the extension of the DiGR NCs along the c-axis of the WZ crystal. A further peak narrowing is observed following the growth of the CdxZn1−xS shell in the second step, which confirms an epitaxial growth of the shell and the formation of a singlecrystalline structure. Using the Scherrer equation (with shape factor K = 0.9), we obtained a crystallite diameter and length of 21 and 39 nm, respectively, which yields an aspect ratio of

1.9:1. Values are in agreement with those obtained from TEM (Figure 1b). The shift of the diffraction peaks in the final DiGR NCs toward higher angles and the bulk CdS diffraction pattern40 indicate that the second growth step involves a uniform lattice contraction of 0.8% (SI, Table S1) and highlights that in colloidal heterostructured NCs lattice constants and thus strain can vary with size. While a small lattice variation was observed between intermediate and final DiGR NCs, quantifying strain in such NCs via structural analysis is not straightforward. XRD patterns are dominated by the CdS shell volume, and while we measured a change in lattice constant for CdS, strain effects on the CdSe core cannot be discerned. We investigated brightfield TEM images of the NCs, as previously we observed in CdSe/CdS rod-in-rods that strain at the core/shell interface leads to Ashby−Brown diffraction contrast, with two lobes of increased contrast around the CdSe core.7 However, here the effect was only observed in two out of six samples, presumably due to the smaller dimensions of the NCs used, as only the samples with larger core and an overall core/shell diameter exceeding 20 nm displayed a diffraction contrast (Figure 2b, Figure S3). TEM data thus suggest the presence of strain, yet are not conclusive for all samples. We therefore measured the fluency-dependent band-edge emission to provide additional insight. The rationale was that, in the presence of strain, the resulting deformation and piezoelectric potential and associated increased charge separation should have a pronounced effect on the multiexciton Coulomb interactions, providing an optical signature for the influence of strain. We first collected optical data in the linear regime (Figure 3). After normalization of the absorbance spectra at 400 nm (Figure 3a), we observed a similar absorption of the CdS shell in all samples, with a steep decrease and the CdS band edge around 500 nm. This confirms the low Zn concentration in the shell, which would otherwise blue shift the band edge. The sloped absorbance below the CdS band edge suggests that Rayleigh scattering contributes to the absorbance in this range, C


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Effective fluorescence lifetimes are in line with recent data on strained single-phase WZ dot-in-dots34 and markedly longer than CdSe/CdS dot-in-rods NCs with a thin shell38 (15−45 ns) and spherical giant-shell NCs with a 3 nm ZB core, where values up to 150 ns were reported.41 However, the varying PL QE, the multiexponential character of the PL decay traces,38 which may also include components due to shallow electron trapping and delayed emission,37 and even the different DiGR aspect ratios (Table 1) that influence the dielectric screening and thus emission rates in anisotropic NCs38 hamper the unambiguous interpretation of the various decay times. Fluorescence spectroscopy under high excitation fluency, which reveals the biexciton emission of colloidal CdSe/CdS NCs, offers a unique tool to understand their structural properties. It allowed assessing the extent of electron delocalization and electron−hole overlap in quasi-type-II NCs via the spectral shift of the nonlinear emission peak2,13,14 or the importance of interface alloying via a reduced Auger recombination rate and improved biexciton PL QE.15 More recently, it was even employed to evaluate the rigidity of the anion lattice during cation exchange in Te-doped CdSe/ CdS NCs.42 To understand the role of strain and piezoelectric fields in DiGR NCs, we therefore expanded our analysis with a measurement of the multiexciton fluorescence spectra. Closepacked films were cooled to 10 K (to narrow emission lines), and we measured PL spectra with increasing pump fluency (Figure 4a). Here the unique aspects of our WZ CdSe/CdS

Figure 3. Absorbance (a) and PL (b) spectra for typical DiGR NCs with different CdSe core diameter (as indicated). (c) Associated PL decay traces.

yet in all cases we found a strongly suppressed absorption below the CdS band edge, confirming that the shell dominates the core/shell NC optical transitions. In accordance with a quasi-type-II band offset, we measured a red-shifted PL peak position after shell growth, centered at 620−650 nm for the different CdSe core diameters (Figure 3b; see also SI Figure S4 for CdSe core data). The full width at half-maximum (fwhm) of the peaks falls in the range of 28−38 nm (Table 2), or 86− Table 2. PL Spectral Position, Line Width, and Quantum Efficiency, Using Excitation Wavelengths of 450 and 550 nm, Area (tav,Ar), and Amplitude (tav,Am) Averaged Decay Times DCdSe (nm)

PL (nm)

fwhm (nm)

QE450 (%)

QE550 (%)

tav,Ar (ns)

tav,Am (ns)

3.2 3.8 4.5 5.7 7.5

623 637 644 648 653

31 28 38 30 32

5 12 33 15 10

18 51 62 34 22

1971 1342 161 2767 1482

395 216 41 604 266

114 meV, which is comparable to typical CdSe/CdS nanorods in this spectral range.38 The PL QE (Table 2), measured using both CdS shell excitation (λex 450 nm) and CdSe core excitation (λex 550 nm), shows an increase when exciting the samples near the CdSe band edge, which confirms that, likely due to the large shell volume in our samples (Table 1), carriers can be trapped at the CdS surface and recombine nonradiatively before relaxing into the CdSe core. When using λex = 550 nm, we found PL QE values up to 60%, suggesting a small defect density at the CdSe/CdS interface. The time-resolved PL decay is multiexponential for all samples, with a tail that extends into the microsecond time domain. The area-weighted PL lifetime reached, except for the sample with a 4.5 nm core, values above 1 μs (Table 2). As area weighting emphasizes the slower part of the decay, we also include the individual components of the fits (SI, Table S2) and the corresponding amplitude-weighted average (Table 2).

Figure 4. (a) Normalized PL spectra measured with increasing excitation fluency (red to blue) for DiGR NCs with a 5.7 nm core. The rise of the PL at short wavelengths indicates a blue-shifted biexciton emission. (b) Exciton (IX, red circles) and biexciton (IXX, red dots) emission intensity, deduced from a fit to the spectra in panel (a). They follow a linear and quadratic dependence, respectively. (c) Comparison between the DiGR biexciton shift EXX (red dots) and literature values for CdSe/CdS DiR NCs with a thin CdS shell (red circles).13

DiGR NCs become apparent. For example, in the DiGR NCs with a 5.7 nm CdSe core, in addition to the main PL peak at 638 nm we observed a second, blue-shifted emission at high excitation fluency. Fitting the spectra with a sum of two peaks (with a peak shape that was constructed from a sum of Gaussians, as obtained from a fit to low-fluency spectra), we D


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located the second emission at 630 nm, with an intensity that followed a quadratic fluency dependence (Figure 4b). This allowed us to assign it to biexciton emission. The other DiGR NCs displayed a similar behavior. Previously, Sitt et al.13 demonstrated for CdSe/CdS DiR NCs that a blue-shifted biexciton emission peak appears in NCs with a 2−2.5 nm CdSe core, due to the electron delocalization and repulsive interactions between multiple excitons. The shift however evolved rapidly toward a redshifted biexciton emission in samples with a core larger than 2.8 nm, as the electron delocalization is reduced and the system returns to a behavior characteristic for a type-I band offset (Figure 4c). In contrast, here we measured a persistent blue-shifted biexciton PL peak, for all samples including the DiGR NCs with a CdSe core diameter up to 7.5 nm. The associated biexciton binding energy reaches values between 4 and 40 meV and converges to the literature values of DiR NCs in the small-core limit. While the blue shift in our small-core samples (e.g., 3.2 nm) can, in agreement with Sitt et al.,13 be ascribed to an increased electron delocalization, for large-core samples it can no longer be explained by quantum confinement alone, as the electron wave function should be more confined to the CdSe core. However, as recently calculated by Segarra et al.,34 an additional piezoelectric field across a strained CdSe core can strongly reduce the electron−hole overlap in these systems, and this effect increases with core diameter. As such, even in large-core CdSe/CdS a blue-shifted biexciton emission can emerge. In order to substantiate our interpretation of the straininduced repulsive biexciton interaction and blue-shifted emission spectra, we performed effective mass model calculations. A fully 3D DiGR heterostructure was considered, with the inclusion of strain and piezoelectricity calculated as in ref 43. An accurate assessment of the biexciton interaction in large diameter cores is challenging, because the spatial localization of individual carriers and that of excitonic complexes can be completely different. The former is mostly driven by piezoelectric confinement, but the latter includes carrier−carrier Coulomb interactions as well, which are on the same energy order (tens of meV). As a result, the usual basis set formed by single-particle states upon which perturbation theory13 or configuration interaction (CI) methods44 are built, cannot be applied here. A different Hilbert space is required, with sufficient flexibility so that it can describe localized and delocalized states anywhere in the DiGR (core, shell, or core/ shell interface, where the piezoelectric potential well arises) on equal footing. To this end, we have developed a CI method built on a basis of floating spherical Gaussian functions that span across the full DiGR long axis (see SI, Figure S5, for details). To our knowledge, this method has not been employed before in simulations of colloidal NCs, and it provides a robust tool to model the current NCs, where wave function delocalization can vary considerably. Our results, calculated for a typical rod with 15 nm overall diameter and 45 nm length, are summarized in Figure 5. We first considered the case of a biexciton (Figure 5a), where the shift of the emission energy with respect to that of a neutral exciton is plotted as a function of the CdSe core diameter, both for a 0.3 eV (open symbols) and slightly smaller 0.28 eV (closed symbols) conduction band offset. If strain and straininduced piezoelectricity are not considered (blue circles), a large biexciton blue shift of ∼40 meV is observed for 3 nm cores, as expected for CdSe/CdS NCs in the strong

Figure 5. (a) Schematic of a biexciton in a DiGR. (b) Biexciton emission energy shift with respect to that of a neutral exciton, as a function of the core diameter for a conduction band offset of 0.3 eV (open symbols) and 0.28 eV (closed symbols). The inclusion of strain and piezoelectricity (yellow squares) induces an enhancement of the blue shift compared to the unstrained case (blue circles/dots). (c) Biexciton electron (red) and hole (blue) charge density for unstrained (left) and strained (right) DiGR with a core diameter of 8 nm. The conduction band offset equals 0.28 eV. (d−f) Same as before but for a positively charged trion.

confinement limit. With increasing core diameter however the blue shift rapidly decreases, yielding a negligible shift for 6 nm CdSe cores and a small red shift (−0.5 meV) for 8 nm cores. This fast decrease is due to the localization of the electron inside the core, in line with a 0.3 eV CdSe/CdS conduction band offset and the overall type-I band alignment. We note that our calculations do not yield the large experimental red shifts reported by Sitt et al.13 (Figure 4c, red dots), as neither could the calculations in that paper. In fact, within a perturbation theory framework, biexciton interactions in a type-I heteronanocrystal leads to identical interactions between electrons and holes and hence to a zero shift with respect to the exciton emission. A moderate red shift can be induced by Coulomb correlations, but this is demanding even for full CI calculations and the magnitude is usually only a few meV.45 Dielectric confinement may enhance the shift in thin-shell DiRs, but not in DiGRs. We can therefore conclude that, in the absence of strain, the calculated biexciton emission energy in CdSe/CdS DiGRs with a large CdSe core shows a near-zero or small red shift compared to the main exciton peak, in contrast with experimental data. The discrepancy between our unstrained calculations in Figure 5b and the experimental data in Figure 4c is largely resolved if we include strain and piezoelectricity (Figure 5b, yellow squares), which enhance the blue shift and reduce the slope of the curve in the region of small cores. The origin of this behavior can be understood from the biexciton charge densities, illustrated in Figure 5c (see also SI, Figure S6). In the unstrained case, for large cores both electrons and holes localize inside the core (left sketch), which leads to nearly compensated interactions and a small biexciton shift. In the strained case, both hydrostatic strainwhich reduces the conduction band offsetand piezoelectricitywhich creates a potential drop across the CdSe coredrive the electron wave function toward the core/shell interface, while the hole E


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can be related to strain and associated reduction of the conduction band offset and buildup of a piezoelectric field along the NC long axis. Importantly, in contrast with quantum confinement, the latter increases with particle diameter. This enables us to apply strain and piezoelectric fields as an additional tool to engineer electron and hole wave functions and exciton−exciton interactions in order to optimize CdSe/ CdS quantum dots toward photonic applications such as efficient lasers21 with increased bandwidth22 or high-brightness LEDs.46

remains mostly confined to the core (right sketch). This reduces the electron−hole attraction as compared to the electron−electron and hole−hole repulsions, resulting in a net blue shift. We note that the blue shift has additive contributions from both hydrostatic strain through the deformation potential and piezoelectricity, but the latter becomes increasingly important for large core diameter (SI, Figure S7). This is even more apparent when we slightly reduce the conduction band offset by 20 meV to highlight the importance of the piezoelectric field for large-core DiGR NCs, and it explains why, in contrast with DiR NCs with a thin shell, DiGR NCs yield a finite blue-shifted biexciton emission peak even for samples with a CdSe core diameter up to 7.5 nm. As mentioned above, the long PL lifetime (Figure 3c) could also be consistent with shallow electron trapping and delayed emission, rather than a reduction of the electron−hole overlap by strain-induced piezoelectricity. We therefore wanted to understand if electron trapping could also explain the persistent blue-shifted emission. To this end, we considered the origin of the nonlinear emission to be a second excitation of a DiGR that already contains a band-edge hole and a trapped electron (situated for instance at the core/shell interface), thus creating a positive trion with an additional trapped electron (Figure 5d). Our calculations show that such a localized electron charge has only a minor influence on the positive trion emission energy (SI, Figure S8). We can then neglect it and simply study a positive trion. The resulting emission in the absence of strain and piezoelectricity (Figure 5e, blue circles) displays, despite overall stronger repulsive interactions as indicated by the larger blue shift, a qualitatively similar trend to that of a biexciton in the unstrained case, with a blue-shifted emission of about 50 meV in the small-core limit, which rapidly decreases toward a near-zero shift when the core diameter increases. However, when strain and straininduced piezoelectricity are again invoked, the positive trion shows again a persistent blue-shifted emission (Figure 5e, yellow squares). This is also evidenced by the charge densities of the strained case, showing asymmetric electron−hole localization (Figure 5f, right sketch). In any case, we can conclude that trapped charges alone cannot produce significant blue shifts. Even when added to hydrostatic strain, its influence is weaker than that of piezoelectricity (see also SI, Figure S6 and Figure S7).

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METHODS Materials. Trioctylphosphine oxide (TOPO, 99%), trioctylphosphine (TOP, 97%), oleylamine (70%), sulfur (99%), selenium (Se, 99.99%), and cadmium oxide (CdO, 99.999%) were purchased from Strem Chemicals. Octadecylphosphonic acid (ODPA, 99%) and tetradecylphosphonic acid (TDPA, 99%) were purchased from Polycarbon Industries. Toluene was purchased from Carlo Erba. Ethanol (≥99.8%), 1octadecene (ODE, 90%), oleic acid (90%), and zinc chloride (ZnCl2, anhydrous, 99.99%) were purchased from SigmaAldrich. Synthesis of CdSe Quantum Dots. For a typical synthesis,31 3.0 g of TOPO, 3.5 g of TOP, 0.29 g of ODPA, 0.06 g of CdO, and 1.7 g of oleylamine were mixed in a 50 mL three-necked flask and heated to about 150 °C under vacuum for 1 h. Then the system was purged with nitrogen and heated to above 350 °C to dissolve CdO, until a clear and colorless mixture was obtained. Next, the temperature was adjusted to 370 °C, and a TOPSe solution (0.058 g of Se with 0.360 g of TOP) was injected. The injection temperature and the reaction time were modified in order to synthesize CdSe NCs of different size. CdSe cores with a 7.5 nm diameter were prepared with a mixture of TDPA and ODPA (15:85 mass ratio) instead of pure ODPA.22 After cooling the synthesis flask to 80 °C, 3 mL of toluene was added to prevent solidification of the mixture. The NCs were precipitated with ethanol, and unreacted products were further removed by repeated resuspension in toluene and precipitation with ethanol. The NCs were finally dispersed in 3 mL of toluene. Shell Growth, Precursor Preparation. For the Znprecursor, typically 0.136 g of ZnCl2 and 10 mL of oleylamine were loaded in a 25 mL flask. The system was heated to 150 °C under vacuum for 1 h until ZnCl2 was dissolved. Then the solution was cooled to room temperature. For the Cdprecursor, 0.64 g of CdO and 10 mL of oleic acid were mixed in a 25 mL flask and heated to 180 °C under an inert atmosphere, which was maintained until CdO dissolved completely. Then the solution was cooled to room temperature. For the S-precursor, in a 25 mL flask 0.2 g of elemental sulfur was added to 10 mL of trioctylphosphine, and the system was evacuated at room temperature for 30 min and then heated for 200 °C to completely dissolve the sulfur. CdSe/CdS DiGR Synthesis. A CdSe toluene solution, containing 9 × 10−9 mol of CdSe cores, was dried under nitrogen flow and suspended in a mixture of 0.2 mL of ZnCl2 in oleylamine and 0.2 mL of cadmium oleate in oleic acid, together with 0.5 mL of TOPS. This mixture was injected in a three-necked flask containing 7 mL of ODE at 300 °C. After 40 min, a mixture of 0.5 mL of cadmium oleate and 0.5 mL of TOP-S, together with 0.2 mL of oleylamine, was added, after which the reaction continued for 1 h. After cooling the flask to room temperature, the DiGR NCs were precipitated two times

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CONCLUSIONS We have synthesized CdSe/CdS anisotropic core/shell nanorods with an overall diameter up to 22 nm using a colloidal synthesis involving chloride ions. In contrast with CdSe/CdS DiR NCs synthesized with phosphonic acid ligands, we obtained a strong radial growth, yielding a 4.6−8.8 nm thick CdS shell around the different CdSe cores. The DiGR NCs displayed fluorescence QEs up to 62% when using an excitation wavelength of 550 nm, which directly excites the CdSe core. Due to a CdSe volume fraction of 3% or less, a decrease was still observed when exciting the CdS shell; hence further work is needed to improve the passivation of the outer CdS surface. Consistent with previous work, we obtained an extended fluorescence decay rate. However, as emission rates can be influenced by the anisotropic shape as well as delayed emission due to shallow electron trapping, here we also demonstrate that such NCs can yield a persistent biexciton emission blue shift. Calculations of exciton and biexciton eigenstates support the experimental results and show that they F


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with 2-propanol and redispersed in 3 mL of toluene. A third precipitation step was performed with ethanol as nonsolvent, and after centrifugation the NCs were redispersed in 5 mL of chloroform. DiGR Structural Characterization. The DiGR crystal structure was measured with X-ray diffractometry. Samples were prepared by drop casting NCs onto a miscut silicon substrate. Measurements were performed on a Rigaku SmartLab diffractometer with the X-ray source operating at 40 kV and 150 mA, in parallel beam geometry. With TEM we evaluated the diameter and length of the NCs. Samples were prepared by drop casting a dilute solution onto a carboncoated 200 mesh copper grid and were imaged with a JEOL JEM 1011 microscope equipped with a thermoionic source operating at 100 kV. The cation composition of the DiGR NCs was determined by ICP-OES, with a ThermoFisher ICAP 6000 Duo inductively coupled plasma optical emission spectrometer. Samples were prepared by digestion of the NCs with aqua regia for 12 h, followed by dilution with Milli-Q water. Linear DiGR Optical Properties. The NCs were dispersed in chloroform for all measurements. Absorbance spectra were recorded with a Varian Cary 5000 UV−vis−NIR spectrophotometer. The CdSe core diameter was determined from the spectral position of the first absorbance peak using the sizing curve of Jasieniak et al.,47 except for the largest core, which yielded 6 nm via the sizing curve, yet 7.5 nm with TEM. Room-temperature steady-state PL spectra were recorded using an Edinburgh Instruments FLS920 spectrofluorometer, exciting the samples with a xenon lamp, and time-resolved decay traces were collected at the emission maximum by exciting with a 50 ps pulsed laser (λem 405 nm) at a repetition rate of 0.1 MHz. Absolute PL QEs were determined with an integrating sphere, exciting the samples at 450 and 550 nm, respectively. The optical density of the samples was tuned to 0.1 at the excitation wavelength of each respective measurement. Biexciton Emission Spectroscopy. Close-packed films were drop cast onto a sapphire substrate and cooled to 10 K with a closed-cycle helium cryostat (Advanced Research Instruments). The samples were excited by a 400 nm femtosecond pulsed laser, at a repetition rate of 1 kHz, and the PL spectra were measured with a Hamamatsu streak camera.

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U.S. Naval Research Laboratory, Washington, DC 20375, United States. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present publication is realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (IONX-NC4SOL). J.I.C. and J.P. acknowledge support from MINECO project CTQ2017-83781-P and UJI project B2017-59.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b01028.

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REFERENCES

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TEM images and histograms of CdSe/CdS DiGR NCs, detailed analysis of the XRD patterns, CdSe core absorbance spectra, individual components of the PL decay traces, detailed information on the k·p calculations, and additional computational results (PDF)

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Corresponding Author

*E-mail: [email protected]. ORCID

Ali Hossain Khan: 0000-0001-7155-0200 Iwan Moreels: 0000-0003-3998-7618 G


ACS Photonics

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CdS Dot-in-Giant-Rod Nanocrystals

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