CdS Dot-in-Rod Heterostructures Using

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A Statistical Description of CdSe/CdS Dot-in-Rod Heterostructures Using Scanning Transmission Electron Microscopy Benjamin T. Diroll, Natalie Gogotsi, and Christopher B. Murray Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00376 • Publication Date (Web): 28 Apr 2016 Downloaded from http://pubs.acs.org on May 1, 2016

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Chemistry of Materials

A Statistical Description of CdSe/CdS Dot-in-Rod Heterostructures Using Scanning Transmission Electron Microscopy Benjamin T. Diroll,a Natalie Gogotsi,b and Christopher B. Murraya,b* a

Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104

b

ABSTRACT:Annular dark field scanning transmission electron microscopy (ADF-STEM) is employed to provide a statistical description of faceting, core location, stacking faults, and polar self-assembly behavior of CdSe/CdS dot-in-rod heterostructures. Applied to dot-in-rod and rod-in-rod heterostructures, STEM enables statistical measurements of core locations which show that the position of the CdSe core lies at ≈ 45 % of the length of the sample, slightly closer to the blunt (001) facet of the CdS nanorod shell. A study of stacking faults reveals a substantially enhanced probability near the epitaxial interface of the core and shell, suggesting the role of epitaxial strain in the formation of defects. Structural analysis is extended to liquid-crystalline monolayers of nanorods and the role of dipolar interactions within lamellae is analyzed using one-dimensional pair-distribution analysis of polarity, showing that the nanorods have a random dipole alignment.

CdSe/CdS dot-in-rod nanoheterostructures have elicited extensive interest as model systems to study catalysis,1 optoelectronic phenomena,2 and self-assembly of nanorod (NR) solids.3,4 The photophysical behavior of these materials is strongly dependent on their structure. The excited state wavefunctions,5–7 quantum yield,8,9 optical anisotropy,10,11 multiexciton generation,12 stimulated emission,13,14 photoluminescence lifetime,9 thermal stability,15 exciton diffusion and storage,16–18 electron mobility,19 spin properties,20,21 band energies,2,22 and catalytic activity1 have all been shown to depend on the size and morphology of the heterostructure. Although descriptions of the core/shell structure, including the core location, polarity, and faceting have been reported in the literature, earlier treatments of these structural parameters, either through real-space microscopy or elemental mapping, has been limited to just a few particles or sometimes only a single particle.4,23,24 Due to the challenges of microscopic imaging, many of the subtle yet potentially important structural parameters of the core/shell NR system remain unquantified. In particular, there is little statistical information about the CdSe core location along the CdS NR length and its relationship (if any) to the photophysical properties of the CdSe/CdS NRs. Here, we employ annular dark field scanning transmission electron microscopy (ADF-STEM), an imaging mode which provides distinguishing contrast between the core and shell of the heterostructure, to generate statistical descriptions of CdSe core location and heterostructure faceting for samples with different core sizes. This analysis is extended to a series of aliquots taken at different times during a single reaction.

Due to the physics of image formation, such high-contrast images can be collected relatively quickly, which limits the effects of specimen misalignment and beam damage as well as enabling improved statistical results through the analysis of large numbers (≈ 100 of each sample) of heterostructures. ADF-STEM measurements allow assignment of the core center location along the NR axis at an average central position between 44 % and 51 % of the entire rod length, defined from the blunt (001) face. Further, these measurements also demonstrate that stacking faults along the c-axis of the NR occur most frequently near the epitaxial interfaces of the CdSe core and CdS NR shell. Analysis of several isolated samples and aliquots from a single reaction demonstrate that the dispersion of growth rates for different facets varies substantially and appears to be subtly affected by the thermal history of a reaction. By simultaneously tracking optical anisotropy and emission energy alongside sizing data, the distinct roles of growth along specific crystallographic axes are correlated with photophysical properties. Extending our analysis to liquid crystalline monolayers selfassembled from CdSe/CdS NRs, the polarity of each NR within smectic assemblies is quantified via microscopy. By mapping the orientation of NRs in lamellar self-assembled structures, the role of dipolar interactions is analyzed by simplifying NR lamellae into Ising-type chains and performing pairdistribution analysis of the NR polarity.25 RESULTS AND DISCUSSION

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Figure 1. ADF-STEM micrographs of CdSe/CdS NRs with 5.1 nm x 6.6 nm, (b) 3.1 nm x 3.9 nm, and (c) 2.4 nm x 2.8 nm CdSe cores. (d) XEDS line-scan of a single CdSe/CdS NR showing prevalence of Se coincident with the high contrast region of the ADF signal; in the same region the sulfur content also decreases. (e) High resolution ADF-STEM image showing the columns of Cd and Se atoms at the core/shell interface. (f) Probability distributions for the location of the CdSe core center as a function of NR length, defined as starting at the blunt (001) facet. Core size increases from top to bottom in (f). Core locations, stacking faults, and faceting. Figures 1a-1c show ADF-STEM images of three samples of dot-in-rod heterostructures synthesized with different core sizes. Several additional micrographs of each sample shown in Figure 1 can be found in Supporting Information Figures S2-S5. An XEDS line-scan across one such particle is shown in Figure 1d to confirm that the region of high contrast does indeed contain the CdSe core. The contrast between the CdSe core region and the CdS shell region decreases with smaller cores, but it is still readily visible in the ADF image in Figures 1b and 1c. At higher magnification, shown in Figure 1e, the core is distinguished from the columns of selenium atoms which have substantially higher contrast than (unseen) columns of sulfur atoms. The probabilities of core center location as a percentage of the total NR length (defined as starting at the blunt (001) face) are reported for four samples in Figure 1f. Notably, the samples show peak probabilities which are quite similar, ranging from 51 % of the length to 44 % for the longest sample that was synthesized with NR seeds. The positions of highest probability are also different than the schematic picture of dot-inrod heterostructures that is commonly deployed in the literature, in which the core seed is presumed to grow in a stronglypreferential manner along the [001̄ ] axis similar to the preferential growth of CdSe. However, the statistical description

provided here is consistent with recent qualitative reports based upon elemental mapping of CdSe/CdS dot-in-rod structures.16 The picture of NRs with cores offset on one side originates in the earliest report by Talapin et al. of dot-in-rod heterostructures synthesized at low temperatures, was based on the appearance of bulbous protrusions.8 High-temperature seeded-growth produces samples (as in Figures 1a-1c) which have comparatively uniform widths, and only in particularly long (~100 nm) NRs does a bulbous protrusion reveal the position of the core near the (100) end of the NR.24 In particular, we found that the appearance of bulbous protrusions depends strongly on the thermal history of the reaction. Their formation is indicative of unstable reaction temperatures, as demonstrated from temperature control experiments reported in Supporting Information Figure S6. The data in Figure 1f suggest that the position of the core shifts slightly closer to the (001) end with elongation of the NR. This should also be understood in the context of the reaction temperature, which first drops substantially upon injection of the CdSe seed stock solution and then recovers over time to the set point of the injection temperature. The slightly increasing preference for growth along the [001̄ ] direction observed for longer NRs may reflect slight changes in the temperaturedependent growth dynamics. (See below and Supporting Information Figures S7 and S8) The core locations and the im-


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plicit growth of the NR along the [001] and [001̄ ] closely agree with previous conclusions reached by studying the distribution of stacking faults in II-VI NRs.26 Hughes and Alivisatos found preferential growth (60% of growth) along the [001̄ ] axis of the NR and a preferential formation of zinc blende stacking faults in growth along the [001] axis. The remaining information captured on the faceting and core location for the four samples shown in Figure 1f is reported in Supporting Information Figure S9. Along the and directions of the wurzite structure both the anion and cation atomic columns can be directly observed enabling determination of the polarity.27 The blunt ends of the CdS crystals are (001) terminated while the tapered ends are (001̄ ) terminated. Tapering of the NR primarily occurs with (101̄ ) facets, confirmed from measurements of the angle between the (001̄ ) and (101̄ ) facets averaging 62.1°±3.1° compared to a expected value for wurtzite CdS of 62.0°. (See Supporting Information Figure S10.) Furthermore, this is in agreement with the exit wave reconstruction results from Bertoni et al.23 In our samples, the distribution of NR lengths is much larger than the distribution of widths, reflecting a greater dispersion of reaction rates. Consistent with uniaxial elongation, the dispersion in rod width in each of the samples is very similar at 0.35 nm to 0.45 nm, representing approximately one unit cell or one monolayer on each side of the NRs. The uniform size of this facet across the samples suggests that there is almost no growth of this face. In contrast, growth along the long axes is substantially faster, but also contains a much broader dispersion of rates reflected in greater polydispersity. Whether the NR growth exhibits a “dynamic distribution” of growth rates, as shown for quantum dots,28 is tested later.

Figure 2. (a) ADF-STEM image of CdSe/CdS dot-in-rod showing two stacking faults highlighted in magenta. (b) Probability distribution of stacking faults along the length of dotin-rod samples. The red box is a guide to the eye marking the approximate position of the CdSe cores. Data derived from 208 samples aligned along the zones of which 129 contained stacking faults with a total of 212 faults. The distribution and location of stacking faults has also been used to describe the growth of colloidal nanomaterials.26,29 Although recent work using X-ray fine structure spectroscopy confirms that the core and shell form high quality crystals (also observed in TEM images),30 planar stacking faults are still commonly observed from cubic zinc blende layers in the hexagonal wurtzite lattice. Figure 2a highlights two stacking faults, in which layers of zinc blende, the cubic polymorph of CdS, occur in growth along the c-axis of the hexagonal wurtzite crystal. Figure 2b is a histogram show-

ing the probability distribution of stacking faults in CdSe/CdS NRs as a function of the distance along from the (001) facet. Contrary to earlier reports on II-VI NRs, more faults (63%) were observed along the [001̄ ] growth direction, but these are not evenly distributed. There is a strong preference for the formation of stacking faults in the region of the CdS shell close to the epitaxial interface with the CdSe core. In all dotin-rod samples, the observed stacking faults are preferentially located near the epitaxial interface, especially along the [001̄ ] axis. The most probable location for stacking faults was at ≈ 60 % of the length from the (001) to (001̄ ) facets. An example of stacking faults bracketing the core location highlighted in Figure 2a. The prevalence of stacking faults was also dependent on the core size. Whereas CdSe/CdS dot-in-rod samples with the largest cores used in this study showed stacking faults in most NRs (90 %), stacking faults were progressively rarer in those made with medium (52 %) and smaller cores (26 %). Normalized to the number of atomic layers along the c-axis, samples with larger cores showed roughly 2.5 times higher probability of faulting than those samples with small cores. (See also Supporting Information Figure S11) The location and apparent size-dependence implicates the CdSe/CdS epitaxial interface in the formation of stacking faults. Although the shell thickness also changes across the samples, it is also possible that these planar defects may be responsible for relatively lower quantum yields, as stacking faults generate band offsets or localized states which may separate charge carriers from recombination.31,32 To investigate the relationship of stacking faults to the epitaxial interface, geometric phase analysis (GPA)33 was performed on a select number of well-aligned NRs to observe the strain tensors associated with the lattice near the core. By comparing the lattice of the NRs near the (nominally unstrained) ends with the region near the cores, GPA suggests that the larger epitaxial interface of the larger CdSe cores generates comparatively more strain. (See Supporting Information Figure S12 and Table S2) This may explain subtle changes in the energy-penalty for faulting that result in the observed differences in stacking fault statistics. Time-Dependent Growth. Figure 3 shows data on the timedependent of growth of a CdSe/CdS NR ensemble measured by analyzing aliquots taken at different times of a single reaction (see Supporting Information Figure S7). The optical red shift, a proxy for the shell thickness, and the optical anisotropy are plotted for the same time points. The optical anisotropy is measured at the band edge and therefore measures anisotropy of the core material, which is strongly enhanced by the anisotropy of the shell. Both the redshift and anisotropy effectively saturated after 120 s of reaction time, which is also reflected in the saturation of the NR width. The continuing growth of the NR with longer times is decoupled from increases in the optical anisotropy because the local anisotropy of the CdSe core is unaffected by the increased length of the NR above ~10 nm. As the NR grows such that the ends of the rod are much further from the core than the Bohr radius of excitons (3.1 nm for CdS34), the electronic structure of the core (particularly the core-localized hole) becomes largely insensitive to further elongation.



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Figure 3. Time-dependent tracking of optical band edge from the maximum of the photoluminescence spectrum (λmax blue dots), optical anisotropy (R, red squares), and facet growth (open symbols) for a dot-in-rod reaction. Time of zero seconds corresponds to the starting CdSe core sample. Time-resolved measurements of growth allow an evaluation of the mechanism of self-focusing of size due to the convergence of dynamic growth rates, as proposed by Talapin.28 In these aliquots, the self-focusing may be facet-specific as suggested above. The (001̄ ) facet of the NRs grew negligibly through the experiment whereas the (101̄ ) continued to grow in length—which accommodated small increases in width without affecting the length of the (001̄ ) facet. Because growth is primarily along the [001] and [001̄ ] directions, these are the most likely to show a convergence in the growth rates. The growth rates of NRs along the c-axis do converge within our ability to measure them: the relative dispersion in NR lengths decreases monotonically from 11 % at 30 s to 5 % at 240 s. The shift of the core to a position slightly closer to the (001) facet with elongation which is apparent in Figure 1 also occurs in the time-dependent series shown in Figure 3 (see also Supporting Information Figure S8). The position of the core center shifts from (48±4) % of the length at 30 s to (43±3) % of the length at 180 s. This suggests that the increase in temperature

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with prolonged reaction times, as the reaction temperature recovers to a set point after decreasing upon injection, increases the rate of growth in the [001̄ ] direction more than the (001) direction, although the change is small. Preferred growth along the [001̄ ] axis has long been considered the cause of anisotropic growth in CdE nanostructures (E = S, Se, Te) due to the termination of that surface with dangling bonds and the additional enhancement of chemical potential due to the dipole of the wurtzite structure.35 The truncation motif observed in CdSe/CdS NRs can similarly be understood as a minimization of the high-energy (001̄ ) face with (101̄ ) faces having lower chemical potential. Nonetheless, it is clear from the core location data, that growth is not strictly along the [001̄ ] axis but more equally distributed between [001̄ ] and [001] axes. Polarity of Self-Assembled NR Liquid Crystals. The polarity of the NR faceting arising from the polar wurtzite CdS crystal structure allows an investigation of the role that dipolar interactions play in NR self-assembly processes. Previously, reports of self-assembled NR liquid crystals have proposed a possible systematic alignment of the NR dipole in the assembly. Specifically, NRs in smectic liquid crystals were proposed to align with alternating dipoles within lamellae but co-aligned with the dipoles of NRs in neighboring lamellar stacks.3 However, more recent evidence of NR self-assembly has shown the formation of the smectic B phase,4,36–38 which is inconsistent with the proposed chains of dipoles. Nonetheless, the role of dipolar attractions39 or subtle shape truncation40,41 effects have been implicated in other nanocrystal self-assembly patterns and their role in NR self-assembly is unclear. To study the role of dipoles in self-assembly of CdSe/CdS NRs, monolayer self-assembled structures were made by dropcasting dilute toluene solutions on to diethylene glycol in a Teflon well.42 The solvent was allowed to evaporate slowly by covering and the floating monolayer film was transferred to TEM grids for further analysis. Although monolayer samples exhibited order over smaller scales than achievable in previously-published thicker films,4,38 they are most amenable to analysis of the polarity of each NR. By taking several images at high magnification and subsequently stitching them together, we were able to generate large area maps with sufficiently high resolution to distinguish the (001) and (001̄ ) ends of the individual NRs and thereby determine the NR polarity. To analyze the relative orientations of NRs in greater detail, we simplified the problem of the NRs with imperfect alignment by assigning NRs within a given lamella a simple binary coding for dipoles pointed with or against the liquid crystal director. This binary classification is demonstrated graphically in Figure 4 and allows for a straightforward determination of the correlation of NR polarization vis-à-vis neighbors.25


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Figure 4. (a) Stitched ADF-STEM Micrographs of NR monolayer with a smectic liquid crystalline order. Dashed squares indicate those areas which are shown at higher magnification in (b-e). The arrows in (b-e) indicate two directors for the possible binary orientations of the NRs in the smectic grain. The NRs are colored red or blue according to the [001̄ ] direction co-aligned with the arrow of the same color. (f) Plot showing the theoretical correlations of NR polarity under the specified assumptions of alignment. Co-aligned (solid blue circles), anti-aligned (open black circles), random (open red triangles), and decaying-alignment (open violet diamonds) are shown in the plot with labels of the same color. (g) Polarization distribution function of the NRs within 1D lamellar chains showing nearly random polarization alignments of 30.0 nm NRs (black circles) and longer 56.3 nm NRs (blue triangles) within smectic lamellae. Figures 4b-4e show inset regions of Figure 4a with color coding to indicate co-alignment (red) and anti-alignment (blue) with the director of the smectic liquid crystal, which is indicated with an arrow in each case. Each of the NRs in a lamella was encoded with a binary value and the strings of binary values were analyzed to determine a pair distribution function for the polarity of NRs. There are three basic possibilities for the correlation of polarization in 1D lamella shown in Figure 4f: co-aligned dipoles which show a perfect correlation at all positions; anti-aligned dipoles, which shows an alternating polarity with position; and unaligned or random dipoles, which should show no correlation at any position. A fourth possibility is that the dipoles may be co- or anti-aligned but this effect decays with distance. The data shown in Figure 4g was collected from two monolayer samples composed of NRs of different average sizes. Each dataset is derived from over 1000 NRs. To study those

samples with medium-range order, only lamellae longer than 10 NRs were analyzed. The correlation obtained from this pair distribution analysis, shown in Figure 4g, hews closely to the pattern expected for random orientations. Both (30.0±2.1) nm × (4.8±0.6) nm NRs and (56.3±6.9) nm × (5.3±0.7) nm showed a negligible correlation at all displacement positions. Under the conditions used for self-assembly (i.e., without fields), neither the permanent dipoles of colloidal NRs nor the subtle differences in shape of the NR tips appear to play a strong role in orienting NRs, indicating behavior closer to hard rods. CONCLUSIONS ADF-STEM is a valuable tool to image the epitaxial interface of CdSe/CdS heterostructures and provides sufficient throughput in measurements to allow statistical descriptions of core/shell dot-in-rod faceting and especially core-locationThe



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core of the dot-in-rod samples is shown to localize somewhat closer to the blunt (001) facet of the heterostructure particularly as the NRs grow longer and stacking faults were observed preferentially near the core/shell interface along the [001̄ ] axis. By taking reaction aliquots, the growth of facets, changes in core position, and the relationship of the physical dimensions to photophysical properties is clearly explicated. From these experiments, it is clear that the optical anisotropy of CdSe/CdS NRs does not arise from anisotropic location of the NR core but rather the local anisotropy of the CdS shell. Using ADF-STEM, we also analyzed the polarity of NRs in liquidcrystalline monolayers and found that the polarity of NRs in self-assembled monolayers was random, indicating that dipolar forces do not strongly direct self-assembly of NRs in the absence of an electric field. EXPERIMENTAL DETAILS Synthesis. A full list of chemicals used in these experiments can be found in the Supporting Information document. Wurtzite CdSe seeds were prepared as prolate ellipsoids or NRs according to literature procedures.4,10 Per literature reports, smaller CdSe seeds were made by hot injection at 380 °C with rapid cooling. Intermediate size seeds were made by injection at 360 °C and maintaining temperature for 3 minutes. Large seeds were made by injecting at 335 °C during rapid heating ramp to 360 °C and maintaining upon reaching the set point temperature for 8 minutes. Following literature protocols, CdSe/CdS NRs were synthesized by seeded growth at 350 to 360 °C using wurtzite CdSe seeds that were either prolate ellipsoids or NRs.4,10 To purify the samples for imaging, each was washed by flocculation with methanol, followed by precipitation with hexanes/ethanol (2x) and hexanes/isopropanol (2x) mixtures. For time-resolved measurements, aliquots were collected at the set time points using a metal needle and glass syringe. For thermal history experiments, the injection temperature for seeded growth was fixed at 360 °C, but reactions were allowed to cool by removing the heating mantle and then reheated to the set point temperature. Scanning Transmission Electron Microscopy. Samples were typically prepared by drop-casting a dilute dispersion of NRs on to carbon-coated Cu TEM grids. Self-assembled monolayer samples were prepared by drop-casting 20 µL of NR dispersions in toluene (≈ 1mg/mL) on to 1.7 mL diethylene glycol (DEG) in a 1.5 cm x 1.5 cm x 1 cm Teflon well. The well was covered with a glass slide to slow solvent evaporation and once dry, the film was scooped up on to a TEM grid and dried under vacuum to remove remaining DEG. ADFSTEM images and EDS data were collected using an aberration-corrected FEI Titan 80-300 operating at 300 kV. The inner and outer collection angles used for ADF imaging were ≈ 35 mrad and ≈ 195 mrad, respectively. The convergence semiangle was ≈ 14 mrad. To minimize the effect of scan distortions some of the images shown here are the combination of several images acquired with short dwell times which were aligned using the StackReg plugin within ImageJ and then integrated.43 To map the alignment of the NRs over large areas mosaics were generated by stitching several images together. The stitching routine was automated as implemented in a ImageJ plugin.44 Statistics data was obtained through analysis of TEM images using ImageJ. Reported data represent the averages of all measurements of a given parameter on a given sample with

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error reported representing the standard deviation of the measurement under an assumption of a normal distribution. Optical Spectroscopy. Fluorescence and fluorescence anisotropy measurements were performed in solution using a Tformat PTI fluorimeter equipped with automated polarizers. Anisotropy measurements were collected by monitoring the peak of the emission curve and exciting the sample at a wavelength 10 nm blue of the emission maximum. The slit width of the instrument was kept to 2 nm to prevent contamination of anisotropy measurements from scattered light. Disclaimer: Certain commercial equipment and materials are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendations by the National Institute of Standards and Technology nor does it imply that the material or equipment identified is necessarily the best available for this purpose.

ASSOCIATED CONTENT Additional statistical data, thermal history experiments, and microscopy can be found in the Supporting Information file. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT We thank Drs. A.C. Johnston-Peck and A.A. Herzing for the ADF-STEM data and comments on the manuscript. This work was supported by the Department of Energy, Office of Basic Sciences, Division of Materials Science, Award No. DE-SC0002158. C.B.M. acknowledges the Richard Perry University Professorship. The authors thank Prof. Yale E. Goldman for use of his instrumentation.

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CdS Dot-in-Rod Heterostructures Using

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