Semiconductor Seeded Nanorods with Graded Composition

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Semiconductor seeded nanorods with graded composition exhibiting high quantum-yield, high polarization, and minimal blinking Ido Hadar, John Patrick Philbin, Yossef Efraim Panfil, Shany Neyshtadt, Itai Lieberman, Hagai Eshet, Sorin Lazar, Eran Rabani, and Uri Banin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00254 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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Semiconductor seeded nanorods with graded composition exhibiting high quantum-yield, high polarization, and minimal blinking Ido Hadar1, John P. Philbin2, Yossef E. Panfil1, Shany Neyshtadt3, Itai Lieberman4, Hagai Eshet5,6, Sorin Lazar7, Eran Rabani2,5* and Uri Banin1* 1. The Institute of Chemistry and Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel 2. Department of Chemistry, University of California and Lawrence Berkeley National Laboratory, Berkeley California 94720-1460, USA 3. Qlight Nanotech Ltd. (Merck KGaA), Edmond J. Safra Campus, Danciger Building A, POB: 39082, 9139002 Jerusalem, Israel 4. Merck KGaA, Performance Materials, Advanced Technologies, OLED & Quantum Materials, Frankfurter Straße 250, 64293 Darmstadt, Germany 5. The Sackler Institute for Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv, Israel 69978 6. School of Chemistry, Tel Aviv University, Tel Aviv, Israel 69978

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7. FEI Company, Achtseweg Noord 5, 5651 GG Eindhoven, The Netherlands KEYWORDS

–

Nanorods,

seeded

growth,

synthesis,

heterostructures,

fluorescence

spectroscopy, electronic structure


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ABSTRACT

Seeded semiconductor nanorods represent a unique family of quantum confined materials that manifest characteristics of mixed dimensionality. They show polarized emission with high quantum yield and fluorescence switching under an electric field, features that are desirable for use in display technologies and other optical applications. So far, their robust synthesis has been limited mainly to CdSe/CdS heterostructures, thereby constraining the spectral tunability to the red region of the visible spectrum. Herein we present a novel synthesis of CdSe/Cd Zn S seeded nanorods with radially graded composition that show bright and highly polarized green emission with minimal intermittency, as confirmed by ensemble and single nanorods optical measurements. Atomistic pseudopotential simulations elucidate the importance of the Zn atoms within the nanorods structure, in particular the effect of the graded composition. Thus, the controlled addition of Zn influences and improves the nanorods optoelectronic performance by providing an additional handle to manipulate the degree confinement beyond the common size control approach. These nanorods may be utilized in applications that require the generation of a full, rich spectrum such as energy efficient displays and lighting.


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TEXT Colloidal semiconductor nanocrystals (NCs) are outstanding building blocks for various applications because of the ability to tune their properties by changing their size, shape, composition, and surface characteristics.1–4 Within this family, seeded nanorods (SNRs) composed of a spherical seed from one semiconductor material embedded in a rod-shaped second semiconductor material are particularly interesting, since they show tunable properties manifesting their mixed 0D-1D character.5–8 In these SNRs, control of the electronic properties is further enabled by independently adjusting the size of the seed and/or the rod. However, thus far, the rod composition has been limited to CdS resulting in a limited range of spectral tunability. In particular, accessing green emission and achieving improvements in emission via a graded alloy rod composition, as developed for spherical core/shell quantum dots, has proven difficult for SNRs.9 Herein, we present the synthesis of green emitting CdSe/Cd Zn S SNRs with radially graded rod compositions as proven by atomically resolved structural characterization. These novel seeded rods exhibit intense, highly polarized emission and minimal fluorescence intermittency owing to the localization of the electron in the region of the seed as demonstrated by molecular dynamics and electronic structure calculations. The importance of the graded composition of the nanorod as demonstrated here offers a path for designing high quality nanostructures with targeted optoelectronic properties. In the most common CdSe/CdS SNR, the smaller band gap CdSe is embedded within the larger band gap CdS (figure 1a). Hence, the lowest electron and hole levels are expected to be confined to the seed (type-I band structure), well passivated by the rod surface. These SNRs hence show desirable properties including high emission quantum yield (QY) and polarization, high photo- and chemical-stability, switching response of the fluorescence under an electric field,


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and negligible self-quenching of fluorescence related to low reabsorption and energy transfer losses commonly observed for 0D core/shell quantum dots.5,6 These desirable properties promote their utilization in optical applications such as displays, lighting, and biological tagging with voltage sensing.10,11 However, a small seed is needed for green emission and this leads to a significant portion of the electron spreading into the rod as a result of an interplay of quantum confinement and electron-hole interactions, consistent with a quasi-type-II band-alignment.12,13 Hence, the emission of SNRs is typically limited to the red region of the visible spectrum. In order to utilize SNRs in the above applications there is a real need to tune their emission towards higher energies while maintaining their high quality optical properties. Attempts to synthesize green emitting SNRs by further reducing the seed size have, however, resulted in a reduction in the SNR’s QY and its degree of polarization, which can be traced to the delocalization of the exciton into the rod region.14,15 An alternative route to green emission is to change the composition of the rod to a material with larger band offsets such ZnS (fig. 1a). The synthesis of such SNRs was possible via a cation exchange reaction that results in particles with poor optical properties, likely because of the presence of impurities and defects.16 Growth of an overcoating ZnS shell over the CdS rods was also reported, but was performed via a complex chemical route that involved illumination and yielded only partial improvements in red emitting SNRs.17 A ZnS shell was also grown over CdSe/CdS SNRs to create a double shell structure, resulting in improved optical properties due to better passivation by the ZnS outer layer. However, in this case the effect of the ZnS shell on the electronic structure is minor and cannot be used to push their emission towards higher energies and indeed these SNRs shows emission at the red region of the visible spectrum.18 To the best of our knowledge, the graded shell structure in rods and its implications is reported here by us for the first time.


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The synthesis of green emitting CdSe/Cd Zn S SNRs reported herein successfully addresses these limitations and overcomes the synthetic difficulties related to the much higher reactivity of Cd in comparison to Zn. By introducing a larger band gap material with graded composition into the rod during growth, we maintain the structural integrity and avoid defects. The synthesis of the SNRs is based on the well-established CdSe/CdS seeded growth method, which is a two-step colloidal synthesis.5,6 Briefly, in the first step spherical CdSeS NCs with a diameter of ~1.5 nm and band gap of 460 nm were synthesized. These NCs serve as seeds for the second step, which is the growth of an overcoating rod with a length of ~20 nm and a diameter ranging between 4 and 5 nm. During the rod growth, Zn precursors were added to the solution at known Zn:Cd ratios ranging from 1: 1 to 16: 1, resulting in a growth of a radially graded Cd Zn S overcoating rod. During the initial growth of the rod in the absence of Zn, a red shift in the emission is observed, and upon Zn addition the band gap is shifted back to higher energies and bright green emission at wavelengths of 530 540 nm is obtained with precise wavelength control (figure 1b).


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Figure 1. Optical and electronic characterization of CdSe/CdS and CdSe/Cd Zn S SNRs. (a) Illustration of the band alignment (band gap, conduction and valance bands offsets), for bulk ZnS, CdSe and CdS. (b) Absorption (dashed) and Emission (solid) spectra. A slight blue shift of the emission energy and a noticeable blue shift of the absorption onset upon the addition of Zn can be seen, indicating the stronger influence of the Zn on the rod properties. (c) Comparison of the band edge energies for samples with varying Zn contents; left panel – experimental energy of the first peak of the absorption spectra (red circles were obtained from a calculation performed on a graded nanorod); right panel – calculated band and optical gaps for a uniform alloy. (d) Calculated electron density projected onto the rod axis for the band-gap excitation (ignoring electron-hole interaction, upper panel) and optical exciton (lower panel). The insets show the corresponding 3D electron densities and the seed location. The addition of Zn to the synthesis of green emitting SNRs significantly improved their emission QY, from ~ 50% (without Zn) to ~ 80% (with Zn), as well as enhanced their degree of polarization - both are comparable with the performance of the highest quality red emitting SNRs. Moreover, the Zn does not appear to influence additional properties of the SNRs such as photo- and chemical-stability, which remain high as typical CdSe/CdS SNRs. Adding Zn also resulted in a noticeable shift of the rod absorption onset to above ~460 nm (figure 1b). These shifts of the optical spectra already strongly indicate that the Zn is embedded within the rod. A summary of the measured emission energies as a function of the Zn content in the rods is plotted in the left panel of figure 1c showing a blue shift with increasing Zn concentrations. To analytically determine the amount of Zn in the SNRs, inductively coupled plasma atomic emission spectrometry (ICP-AES) was performed on the different samples, showing that approximately half of the Zn atoms added in the reaction are embedded within the rods. Further


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characterization was carried out by X-Ray powder diffraction, which shows that as the Zn:Cd precursor ratio is increased, the diffraction peaks shift from the hexagonal Wurtzite CdS spectrum towards the ZnS spectrum (figure S1), consistent with the incorporation of Zn into the rod. To better understand the influence of Zn on the optical properties of the SNRs, we have developed an atomistic model of CdSe/Cd Zn S SNR with no adjustable parameters, and computed the fundamental and optical band gaps for SNRs with varying degrees of Zn:Cd alloying (see SI for detailed description of the atomistic model and the electronic structure calculations). The theoretical results are shown in the right panel of figure 1c for a series of CdSe/Cd Zn S SNRs with homogenous (as opposed to graded) composition ranging from = 0 to = 0.5. All calculations were performed for a 1.5 nm diameter CdSe seed and a 20 × 4 nm rod. The fundamental gaps are higher than the optical gaps by ~200 meV. For a neat CdSe/CdS SNR we find that there is an excellent agreement between the measured and computed optical gaps as well as for the value of the exciton binding energy in comparison to experimental results obtained from scanning tunneling spectroscopy measurements.19 This provide additional validation of the model. A comparison of the calculated results for a homogenous alloy composition with the experimental results, show a qualitative agreement with a trend of a blue shift in the absorption onset with increasing Zn content. The simulated optical gaps increase linearly with the Zn:Cd ratio inside the rod with a slope of ~125 meV/ in the relevant range studied while the experimental results show a more moderate increase. This difference can be traced to the effect of a graded shell composition, which is further analyzed and discussed below. The theoretical


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estimate of the optical gap for a graded shell (shown in the left panel of figure 1c) is in good agreement with the experimental measurements. The increase in the fundamental and optical gap energies is explained by a stronger confinement of the electron to the CdSe seed when Zn is added to the rod. This is illustrated in figure 1d, where we plot the electron density projected onto the rod axis for the fundamental (upper panel, ignoring electron-hole interactions) and optical (lower panel) excitations (the insets show the corresponding 3D electron densities as well as the seed location). In both cases, it is clearly seen that the extent of overlap between the electron wave function and the CdSe core increases with the Zn content. The effect is more pronounced for the noninteracting (fundamental) case but is significant also when the electron-hole interactions are included (the width the projected electron density decreases by 20% when Zn is added at a ratio of 2:1, see table S4). The observed localization of the electron near the core as the Zn content increases leads to a quantum confinement effect which results in an increase of the optical gap observed in figure 1c. This electron localization is also intertwined with the improved QY and reduced blinking seen in the Zn containing SNRs, which will be further discussed below. To further reveal the SNR atomistic structure, specifically in relation to the graded radial rod composition, we employed high resolution scanning transmission electron microscopy (STEM, FEI Titan Themis3 300), providing high-resolution structural and elemental imaging. In these measurements, a focused electron beam of relatively low energy (80 kV) probes the specimen and the scattered electrons at each point were used to construct the structural image. Additionally, at each point we performed an energy dispersive X-ray (EDX) measurement that identifies the specific element at the point and enabled us to construct the correlated elemental image. Figure 2a-g presents the correlated images of a few CdSe/Cd Zn S SNRs with a Zn:Cd


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ratio of 2: 1 and dimensions of 23 × 4.5 nm. It can be seen that the S atoms are distributed homogenously throughout the rod, while the Cd (Zn) atom content has a higher (lower) concentration in the inner part and increases (decreases) towards the outer part of the SNR. Remarkably, even the CdSeS seed can be identified for some of the SNRs from the Se signals which emanate from the few tens of Se atoms present (figure 2g). The seeds are located anisotropically, closer to one end of the elongated rods, as was confirmed previously for CdSe/CdS SNRs.20,21 High resolution STEM images of a single SNR (figures 2h-j) enables one to precisely image the atomic lattice with chemical identification and further confirms the S, Cd, Zn distribution extracted from the lower resolution images.


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Figure 2. STEM imaging and elemental maps of CdSe/Cd Zn S SNRs. (a)-(g) Multiple CdSe/Cd Zn S SNRs (scale bar in panel (e)). (a)-(c) - Elemental maps based on EDX scan for Zn, Cd, S respectively. Images show that the Cd is located at the inner part, the Zn mostly at the outer part and the S at the entire rod. (e) High angle angular dark field (HAADF) STEM imaging: Z (atom number) contrast image of the SNRs. (d), (f), (g) Overlay of two elements showing their spatial distribution: (d) Zn vs. Cd; (f) Cd vs S; (g) Zn vs Se shows the inner seed.


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(h)-(j) High resolution STEM image and elemental maps of a single CdSe/Cd Zn S SNR, scale bar 1 nm. (h) HAADF: Z contrast image of the SNR showing the single atoms. (i), (j) EDX elemental maps shows the lattice of the different atoms and the distribution of elements for (i) Zinc vs Sulfur and (j) Cadmium versus Sulfur. Analytical EDX elemental analysis of these SNRs confirms that the total Zn:Cd ratio in the sample is approximately 2: 1, consistent with the ICP-AES measurements. We have analyzed the elemental distribution along the radial and longitudinal axes of the SNRs using a procedure outlined in the SI. The Zn concentration along the radial () direction is the smallest at the center of the SNR and monotonically increases with (figure 3a). The Cd distribution shows an opposite behavior and the S concentration remains fixed. The distribution of all elements along the longitudinal direction ( axis), which is presented in figure 3b, is uniform, except when approaching the tip regions of the SNR, at which point, the Zn content increases and the Cd content decreases rapidly. This analysis clearly proves that the rod has a graded composition of Cd Zn S alloy with growing x values along the radial direction. The outer shell of the rod is nearly completely ZnS.


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Figure 3. Analysis of the STEM EDX measurements. Extracted elemental distribution as a function of distance from the center of the SNR: (a) Along the radial direction, solid lines are exponential fits representing the Zn and Cd radial distributions. (b) Along the longitudinal direction. Markers size represent the estimated experimental error of the EDX measurements. Dashed lines are guide for the eye. See SI for details on the analysis. In order to compare the optical properties of CdSe/Cd Zn S to CdSe/CdS SNRs and analyze their polarization, we carried out a detailed study of single SNRs and of the ensembles. Both samples have a similar seed. The CdSe/CdS sample shell has dimensions of 22 × 5 nm. The CdSe/Cd Zn S (dimensions: 23 × 4.5 nm) has a total Zn:Cd ratio of 2: 1 and a graded Zn:Cd ratio at the radial direction ranging between

0.18 at the center to

0.85 the surface (see

SI for full data).


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For the single SNR study, a dilute sample of SNRs was spin-coated on a microscope cover glass such that the density of the particles was less than 1/!"# , suitable for a single particle fluorescence study. Fluorescence measurements were done in an epi-illumination configuration; the SNRs were excited by un-polarized light at 405 ± 5 nm. The emitted light was measured by a CCD camera, through a polarizing beam displacer, separating the vertical (%& ) and horizontal (%' ) emission components and measuring them simultaneously.22,23 The degree of emission polarization ((), is extracted for the single SNR according to: (=

%& %' %& + %'

The degree of polarization was measured for hundreds of single SNRs from each sample providing the distribution of emission polarizations for each sample (figures 4a-b). For the CdSe/Cd Zn S SNRs we found that the average polarization is 〈(〉 = 0.77, similar to the emission polarization of the highest quality red emitting SNRs and considerably higher than the value for CdSe/CdS green emitting SNRs which was found to be 〈(〉 = 0.67.14,22 The polarization of the same SNRs was also studied in the ensemble by photoselection photoluminescence excitation (PS-PLE). In this method, the anisotropy, a combination of the absorption and emission polarizations, is measured for a sample of SNRs in solution. The SNRs are excited by vertically polarized light that preferably excites the optical transitions aligned in this direction. The emission of the excited SNRs is then measured through vertical (%&& ) and horizontal (%&' ) polarizers and the fluorescence anisotropy () is calculated according to: =

%&& %&' %&& + 2 ⋅ %&'

For a sample in solution the value of anisotropy can range from = 0.4 if both the emission and the absorption are fully linearly polarized in the same direction to = 0.2 if the emission


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and absorption are completely polarized in perpendicular directions.23,24 In PLE, the excitation light energy is scanned while the emission is measured at a specific wavelength. Hence from this measurement information regarding the absorption polarization of higher order optical transitions can be obtained. Specifically, each maximum in the spectrum represents an optical transition polarized parallel to the emission (-polarized), and each minimum represents an optical transition polarized perpendicular to the emission (.-polarized). PS-PLE measurements of CdSe/Cd Zn S SNRs and CdSe/CdS SNRs are presented in figure 4c; it is clear that the anisotropy of the Zn containing SNRs is much higher. The highest value for the anisotropy is obtained when the sample is excited at the band edge in which the absorption and emission arise from the same -polarized transition. The second transition, which cannot be resolved from the absorption spectra, appears clearly in the PS-PLE spectra as a minimum for both samples. This, indicates it is an .-polarized optical transition, in agreement with the calculated electronic structure of the SNRs. Yet, the anisotropy does not reach negative values as expected due to the classical dielectric effect, which reduces the electric field along the xy-direction inside the SNR and induces stronger absorption in the direction.22,23 The magnitude of the absorption polarization related to the electronic transitions is extracted from the anisotropy by calculating the dielectric effect for each sample and accounting for the degree of emission polarization from the single SNRs study (see SI). These spectra are presented in the left panel of figure 4d and show that for the Zn containing SNRs the second transition reaches negative values of polarization, indicating the better separation between the optical transitions for this sample in comparison to the CdSe/CdS sample. The next optical transition that appears is a -polarized transition. At even higher energies, associated mainly with rod transitions, the density of optical transitions becomes very high and there is no preferred


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symmetry and hence the polarization goes to the asymptotic value related to the dielectric effect, as seen in the spectra.

Figure 4. Polarization study of CdSe/CdS and CdSe/Cd Zn S green emitting SNRs. (a), (b) Single SNRs emission polarization distributions, clearly showing the higher emission polarization for the CdSe/Cd Zn S SNRs. (c) Measured ensemble anisotropy of the same samples. (d) Comparison between measured (left panel) and calculated (right panel), absorption degree of polarization as function of energy above the optical gap for the same samples. In our theoretical investigation of the polarization we were able to explicitly calculate the energies of the individual optical transitions along with their intensities, polarization, and radiative lifetimes. In our comparison of CdSe/CdS and CdSe/Cd Zn S SNRs, we found that the Zn being radially distributed was imperative for the improved polarization of the CdSe/ Cd Zn S SNRs. Specifically, the separation between the -polarized transition (which arises from the lowest energy exciton) and the .-polarized transitions were calculated to be 88, 118, and 87 meV for the CdSe/CdS, graded composition CdSe/Cd Zn S, and homogeneously


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distributed Zn CdSe/Cd/.01 Zn/.#1 S SNRs respectively. The calculated absorption polarization spectrum is shown in the right panel of figure 4d and is clearly in agreement with the measured absorption polarization (see SI for additional data). In addition to studying the polarization, we also compared the blinking of CdSe/Cd Zn S SNRs and CdSe/CdS SNRs. Blinking is a typical phenomenon for single particles in which the emission of the particle decreases rapidly to an ‘off’ state and afterwards recovers to the emitting ‘on’ state.25 It is highly desirable to reduce the blinking to a minimum as it sets an upper limit for the ensemble QY. For the blinking study we used the same optical setup as for the single SNR polarization study. The difference is that for this study the emission is measured directly by the camera at the highest frame rate possible (> 200 Hz), in order to extract the blinking kinetics. In figures 5a-b we present a typical emission time trajectory of a single SNR with and without Zn, respectively, showing that for a Zn containing SNR the blinking is reduced and the typical ‘off’ time is shorter. We measured similar emission trajectories for hundreds of SNRs from each sample and for each SNR we extracted the blinking statistics and calculated the fraction of ‘on’ time (56"7 89 (%)). We plot, in figure 5c, the distribution of ‘56"7 89’ for both samples. For the Zn containing SNRs the average 56"7 89 is 87% and the distribution is concentrated at the highest part of the histogram. The distribution for the CdSe/CdS SNRs is shifted to lower values with an average 56"7 89 of 75%. Additionally, we extracted from each trajectory the full ‘on’ / ‘off’ statistics and for each ‘on’ event calculated the ‘56"7 89 ⋅ ?@A’, which is a measure for the number of photons emitted between consecutive blinking events - this is correlated to the probability of entering the ‘off’ state. Figure 5d shows the ‘56"7 89 ⋅ ?@A’ distribution of all the SNRs measured. For the Zn containing SNRs this distribution is shifted towards higher


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values meaning that for these SNRs there is lower probability to enter the ’off’ state. The minimal blinking of the Zn containing SNRs may be explained by the suppression of blinking through the better passivation of the exciton due to its smaller spatial distribution and the increased oscillator strength which reduces the excitonic lifetime (B), both predicted by the computational study (see SI).26,27 Additionally, the graded composition of the shell induces a smooth confinement potential which is predicted to reduce the Auger recombination rate thus further supporting the blinking suppression.28–30

Figure 5. Blinking study of CdSe/CdS and CdSe/Cd Zn S green emitting SNRs. (a), (b) Representative emission time trajectory of (a) CdSe/Cd Zn S SNR and (b) CdSe/CdS SNR, showing the reduced blinking for the CdSe/Cd Zn S SNR. (c) ‘56"7 89’ and (d) ‘56"7 89 ⋅ ?@A’ distributions based on measurements of hundreds of single SNRs, showing the reduced blinking and the lower tendency to enter blinking of the Zn containing SNRs. The introduction of the new synthesis approach for the seeded nanorods has led to formation of a beneficial radially graded composition as revealed by state-of-the-art atomically resolved analytical STEM analysis. Combining experimental and theoretical analysis shows that graded


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shell composition enabled carving out a gradual confining potential for the charge carriers. This provides a class of high quality light emitting rods manifesting tunable emission in particular for green emission, with high QY, minimal blinking and high polarization. The control of the rod’s radial composition thus provides an additional powerful knob to tune the optoelectronic properties. These insights enable further design of SNRs with specific properties (e.g. type-I or quasi-type-II band alignments) for various applications. The new high quality green emitting SNRs may already replace regular green emitting NCs in current applications.

Methods Synthesis. The synthesis of SNRs is based on the seeded growth approach.5,6 CdSeS spherical seeds with diameter of ~ 1.5 nm and bandgap of 460 nm were synthesized according to the published procedure.5 For the seeded growth, elemental sulfur was dissolved in TOP and added to CdSeS cores. The solution of CdSeS cores and sulfur in TOP was swiftly injected into a threeneck flask containing TOPO, ODPA, hexylphosphonic acid, and CdO at 380°C, this is followed by addition of Zn-oleate at defined stoichiometric amounts at few injections. The solution is kept at this temperature for few minutes to allow the growth of the shell SNR, resulting in a growth of an overcoating CdZnS rod with length of ~20 nm and width between 4 and 5 nm. Structural characterization. TEM (FEI, Tecnai T12), was used for sizing of these SNRs. ICP-AES (c x 7500, Agilent), and XRD (Bruker AXS, D8 Advance), were utilized in order to quantify the Zn:Cd ratio in the shell. STEM (FEI, Titan Themis) was utilized to resolve the exact structure (HAADF mode), and composition (EDX mode) of the SNRs. Ensemble optical study. Optical spectroscopy of SNRs was performed using an absorption spectrophotometer (Jasco – V770), fluorescence spectrophotometer (Edinburgh instruments –


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FL920), and absolute quantum yield spectrophotometer (Hamamatsu photonics – QuantaurusQY). The basic characterization for all samples are absorption, emission and QY, more elaborated measurements includes photoluminescence excitation (PLE), photo-selection anisotropy (PS) and PS-PLE which combine both to gain information regarding the symmetry and energy of the optical transitions.22 In addition, time-resolved fluorescence lifetimes (LT) was measured for the samples using time correlated single photon counting (TCSPC) scheme. All measurements were carried at room temperature. Single particle optical study. For single SNRs measurements, a dilute solution was spin coated on a clean glass cover slip such that the average density of SNRs was well below 1 G9H/!"#, suitable for single particle measurement at far field. These samples were measured in epi-fluorescence on an inverted microscope (Nikon Eclipse-Ti), at room temperature. The samples were excited with a blue LED (Prizmatix) centered at 405 nm. Emission was collected by an EM-CCD (Andor iXon3) enabling both high sensitivity and fast imaging. For polarization measurements a wave plate and polarizing beam displacer were entered at the emission path enabling to differentiate between the horizontal and vertical emission polarized components.22 For blinking measurements, time-trajectory of the emission was measured by operating the EMCCD at its maximal frame rate (~200 Hz). Modeling. To briefly describe the computational methodology (see SI and Ref. 13 for more detail), molecular dynamics (MD) was run to relax the configurations, including the seed/shell interface, before quenching to remove thermal fluctuations from the final SNR configurations. MD was followed by calculation of approximately 40 valance and conduction single particle states by the filter-diagonalization technique together with the local version of the semiempirical pseudopotential model.31–34 These states were then used to compute the excitonic states by


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solving the Bethe-Salpeter equation (BSE). The BSE takes into account the Coulomb attraction between the electron and hole which has been shown to be important in SNRs.13,35 These calculations are then used to analyze the spatial distribution of each state and the oscillator strengths of specific optical transitions.


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ASSOCIATED CONTENT Supporting Information. Detailed description of the elemental analysis and quantitative spatial elemental analysis of the graded composition SNRs. Detailed description of the theoretical modeling of SNRs methodology and results. Experimental and theoretical study of the exciton lifetime. Calculation of the absorption polarization based on anisotropy and emission polarization. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * Uri Banin: [email protected] * Eran Rabani: [email protected] Author Contributions I.H., Y.E.P. and U.B. conceived and designed the optical and characterization experiments, I.H. and Y.E.P performed the experiments, J.P.P., H.E. and E.R. conceived, designed and preformed the theoretical study. S.N. and I.L. synthesized the SNRs and performed basic structural characterization. S.L. preformed STEM characterization and supported their interpretation. I.H., Y,E,P., J.P.P., E.R. and U.B. co-wrote the paper. All authors have given approval to the final version of the manuscript. Funding Sources The Israel Science Foundation – ISF, grant No. 811/13 (U.B.).


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Physical Chemistry of Inorganic Nanostructures Program, KC3103, Office of Basic Energy Sciences of the United States Department of Energy, under Contract No. DE-AC02-05CH11231 (E.R.). ACKNOWLEDGMENT We thank Dr. Inna Popov from the Unit for Nanocharacterization at the Hebrew University for assistance in the TEM and STEM measurements. U.B. thanks the Alfred & Erica Larisch memorial chair.


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(11) Park, K.; Deutsch, Z.; Li, J. J.; Oron, D.; Weiss, S. ACS Nano 2012, 6 (11), 10013– 10023. (12) Sitt, A.; Sala, F. Della; Menagen, G.; Banin, U.; Della Sala, F. Nano Lett. 2009, 9 (10), 3470–3476. (13) Eshet, H.; Grünwald, M.; Rabani, E. Nano Lett. 2013, 13 (12), 5880–5885. (14) Diroll, B. T.; Koschitzky, A.; Murray, C. B. J. Phys. Chem. Lett. 2013, 5 (1), 85–91. (15) Vezzoli, S.; Manceau, M.; Leménager, G.; Glorieux, Q.; Giacobino, E.; Carbone, L.; De Vittorio, M.; Bramati, A. ACS Nano 2015, 9 (8), 7992–8003. (16) Li, H.; Brescia, R.; Krahne, R.; Bertoni, G.; Alcocer, M. J. P.; D’Andrea, C.; Scotognella, F.; Tassone, F.; Zanella, M.; De Giorgi, M.; Manna, L. ACS Nano 2012, 6 (2), 1637–1647. (17) Manna, L.; Scher, E. C.; Li, L.-S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124 (24), 7136–7145. (18) Deka, S.; Quarta, A.; Lupo, M. G.; Falqui, A.; Boninelli, S.; Giannini, C.; Morello, G.; De Giorgi, M.; Lanzani, G.; Spinella, C.; Cingolani, R.; Pellegrino, T.; Manna, L. J. Am. Chem. Soc. 2009, 131 (8), 2948–2958. (19) Steiner, D.; Dorfs, D.; Banin, U.; Della Sala, F.; Manna, L.; Millo, O. Nano Lett. 2008, 8 (9), 2954–2958. (20) Menagen, G.; Macdonald, J. E.; Shemesh, Y.; Popov, I.; Banin, U. J. Am. Chem. Soc. 2009, 131 (47), 17406–17411.


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(34) Williamson, A. J.; Zunger, A. Phys. Rev. B 2000, 61 (3), 1978–1991. (35) Shabaev, A.; Rodina, A. V.; Efros, A. L. Phys. Rev. B 2012, 86 (20), 205311.


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TOC:


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Figure 1 - Optical and electronic characterization of CdSe/CdS and CdSe/Cd1-x Znx S SNRs. (a) Illustration of the band alignment (band gap, conduction and valance bands offsets), for bulk ZnS, CdSe and CdS. (b) Absorption (dashed) and Emission (solid) spectra. A slight blue shift of the emission energy and a noticeable blue shift of the absorption onset upon the addition of Zn can be seen, indicating the stronger influence of the Zn on the rod properties. (c) Comparison of the band edge energies for samples with varying Zn contents; left panel – experimental energy of the first peak of the absorption spectra (red circles were obtained from a calculation performed on a graded nanorod); right panel – calculated band and optical gaps for a uniform alloy. (d) Calculated electron density projected onto the rod axis for the band-gap excitation (ignoring electron-hole interaction, upper panel) and optical exciton (lower panel). The insets show the corresponding 3D electron densities and the seed location. Figure 1 297x146mm (300 x 300 DPI)


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Figure 2 - STEM imaging and elemental maps of CdSe/Cd1-x ZnxS SNRs. (a)-(g) Multiple CdSe/Cd1-x ZnxS SNRs (scale bar in panel (e)). (a)-(c) - Elemental maps based on EDX scan for Zn, Cd, S respectively. Images show that the Cd is located at the inner part, the Zn mostly at the outer part and the S at the entire rod. (e) High angle angular dark field (HAADF) STEM imaging: Z (atom number) contrast image of the SNRs. (d), (f), (g) Overlay of two elements showing their spatial distribution: (d) Zn vs. Cd; (f) Cd vs S; (g) Zn vs Se shows the inner seed. (h)-(j) High resolution STEM image and elemental maps of a single CdSe/Cd1-x ZnxS SNR, scale bar 1 nm. (h) HAADF: Z contrast image of the SNR showing the single atoms. (i), (j) EDX elemental maps shows the lattice of the different atoms and the distribution of elements for (i) Zinc vs Sulfur and (j) Cadmium versus Sulfur. Figure 2 316x309mm (300 x 300 DPI)


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Figure 3 - Analysis of the STEM EDX measurements. Extracted elemental distribution as a function of distance from the center of the SNR: (a) Along the radial direction, solid lines are exponential fits representing the Zn and Cd radial distributions. (b) Along the longitudinal direction. Markers size represent the estimated experimental error of the EDX measurements. Dashed lines are guide for the eye. See SI for details on the analysis. Figure 3 83x107mm (300 x 300 DPI)


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Figure 4 - Polarization study of CdSe/CdS and CdSe/Cd1-x ZnxS green emitting SNRs. (a), (b) Single SNRs emission polarization distributions, clearly showing the higher emission polarization for the CdSe/Cd1-x ZnxS SNRs. (c) Measured ensemble anisotropy of the same samples. (d) Comparison between measured (left panel) and calculated (right panel), absorption degree of polarization as function of energy above the optical gap for the same samples. Figure 4 185x139mm (300 x 300 DPI)


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Figure 5 - Blinking study of CdSe/CdS and CdSe/Cd1-x ZnxS green emitting SNRs. (a), (b) Representative emission time trajectory of (a) CdSe/Cd1-x ZnxS SNR and (b) CdSe/CdS SNR, showing the reduced blinking for the CdSe/Cd1-x ZnxS SNR. (c) ‘Time ON’ and (d) ‘Time ON⋅ Counts’ distributions based on measurements of hundreds of single SNRs, showing the reduced blinking and the lower tendency to enter blinking of the Zn containing SNRs. Figure 5 233x92mm (300 x 300 DPI)


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Semiconductor Seeded Nanorods with Graded Composition

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